THE PLANNING PROCESS

Formulation of a plan addressing the needs of the study area consists of the following six major steps:

• Identify Problems and Opportunities
• Forecast of Conditions Without a Plan
• Formulate Alternative Plans
• Evaluate Effects
• Compare Alternative Plans
• Plan Selection

This section of the report will identify the problems and opportunities, summarize the existing conditions and forecast future conditions without flood protection in the study area.

STUDY AREA

In reconnaissance, the study area extended from the EABPL to the Mississippi River western guide levee and from Morganza, Louisiana to the Gulf of Mexico, encompassing approximately 4,000 square miles of southern Louisiana. In negotiating the IPMP, both the Federal and non-Federal parties recognized the impact that Atchafalaya River backwater had on flooding in the area west of Thibodaux, Louisiana. That problem will be addressed in the LABR study.   Consequently, the Morganza study limits were revised for the feasibility phase (plate_1).

The Morganza to the Gulf feasibility study is comprised of the area between Louisiana Highway 311 and Bayou du Large to the west, and Bayou Lafourche from Thibodaux southeast toward the Gulf of Mexico. It includes portions of Terrebonne and Lafourche parishes and encompasses approximately 1,700 square miles. The area west of Bayou du Large and Louisiana Highway 311 is being investigated in the LABR study.

The study basin consists of saline and fresh marshes, cypress and tupelogum swamps, bottomland hardwood forests, farmlands, industry (both heavy and light), residential, and other developed areas. Like much of southeast Louisiana, the area contains many abandoned distributaries of the Mississippi River and their associated natural levees.   Historically, this area received fine sand, silts, and clays during flood events from the numerous courses related to different stages in the development of the Mississippi River Deltaic Plain. Bayou Lafourche was a course of the Mississippi River that has long since been abandoned.

Development in the area occurred along alluvial ridges that support numerous communities, agricultural developments, and industries.   Significant developments located in the study area include the cities of Thibodaux and Houma.   The city of Houma is one of the most heavily populated industrial and residential communities in the study area.   Many communities have also developed along the lesser ridges of the aforementioned old distributaries, such as the towns of Dulac, Montegut, Chauvin, and Bourg (figure 1).

Figure 1. Typical development south of Houma, directly adjacent to Bayou Petit Caillou

Elevations in the study area vary.  Near Houma, the largest city in the area, the elevation is about 10 feet National Geodetic Vertical Datum (NGVD).  The elevation along bayou ridges just south of Houma are 4-5 feet NGVD and less than 1 foot NGVD near the Gulf of Mexico.

Rainfall typically runs away from the major streams and collects between the ridges to form bayous, and finally travels southward toward the Gulf of Mexico. The major streams affecting the study area are the Mississippi River and Bayou Lafourche in the east and the Atchafalaya River in the west.

EXISTING CONDITIONS

Existing Flood Control Features. The TLCD currently maintains approximately 20 miles of forced drainage levees in various communities, including flood control structures and drainage pumping stations. The existing levees have a maximum elevation of +7.0 feet NGVD. The levees protect against weak tidal and rainfall events, but not hurricanes.

The TLCD operates and maintains numerous flood control structures, such as: North and South Grand Bayou Units, East and West Montegut Units, Humble Canal, Bayou Terrebonne, Bayou LaCache, Bayou Petit Caillou, and Bayou du Large (plate 2). Four of these floodgates tie into forced drainage levees and provide protection to the communities of Chauvin and Montegut. Two of those four floodgates are sector gates that would become part of the proposed hurricane protection plans.

The Bayou Terrebonne Floodgate is located on Bayou Terrebonne, approximately 1.5 miles south of the Madison Pump Station and 1.5 miles north of the LaCache Pump Station. The Bayou Petit Caillou floodgate is located in Bayou Petit Caillou approximately 500 feet north of the intersection of Boudreaux Canal and Little Caillou Bayou. Both sites are accessible by water and from Louisiana Highway 55 (Terrebonne) or 56 (Petit Caillou). The pile-founded structures are composed of two moveable steel sector gates with a 56-foot wide clear opening for navigation.

Also located in the study area are 12 existing, locally operated and maintained pumping stations of varying capacity. The pump stations are used to drain the small levee systems protecting Montegut, Chauvin, and areas along Bayou du Large.

Storms of Record and Flooding. Hurricanes and tropical storms cause widespread flooding of residential and commercial property in the study area. In the summer and fall of 1998, three tropical storms and two hurricanes posed threats to the Louisiana coast and some minor coastal flooding was experienced. Approximately half of the structures in the study area are adjacent to, or across a roadway, from one of many bayous and waterways that intersect the study area. The existing flood control structures operated by the TLCD are frequently closed two to three days in advance of a storm system reaching landfall due to increased tidal stages that often precede tropical storms and hurricanes.

Hurricane season begins in June and ends in November. A summary of hurricanes affecting the Louisiana coast from 1885 to 1992 is provided in table 1.

Table 1
Number of Hurricanes Affecting the Louisiana Coast
1885-1992

Based on 82 flooding events over a 108-year period of record, the probability of a hurricane affecting the Louisiana coast in any given year is 75.9 percent. Terrebonne and Lafourche have experienced at least three major flood events in the past 15 years.

The total FEMA flood claims paid between 1978 and 1998 by parish and by selected communities within the project area are summarized in table 2. The table includes only those damages that were covered by flood insurance.

On August 26, 1992, Hurricane Andrew (Category 3 Storm) made landfall in St. Mary Parish, 80 miles west of Morgan City. Following its landfall, the storm changed course from northwest to north and battered Lafourche and Terrebonne Parishes as well as the Acadiana Parishes of St. Mary, Iberia, and Lafayette. Figures 2 and 3 show flooding from Hurricane Andrew.

Figure 2. Flooding in Montegut, La. from Hurricane Andrew

Figure 3.  Flooding from Hurricane Andrew

Table 2
FEMA Flood Claims Paid From 1978 to 1998

Following Hurricane Andrew, Terrebonne Parish residents qualified for more than $23 million in FEMA claim settlements.  However, it is estimated that Hurricane Andrew caused $55 million in losses in Terrebonne Parish alone because of damage to crops and other uninsured property. Hurricane Andrew destroyed over 360 homes and damaged approximately 2,900 homes in Terrebonne Parish.  Over 90 percent of the damage occurred in Terrebonne Parish south of Houma, where up to 6 feet of water was reported in residential and commercial structures. Only minor flooding was reported in the city of Houma and areas north of the city.  The unleveed portion of Lafourche Parish near Terrebonne Parish also incurred extensive flood damage.  Most of the agricultural damage in the area occurred as the result of wind damage to sugar cane crops.  Figure 4 shows a structure collapsed by floodwaters during Hurricane Andrew.

Figure 4. Structure collapsed by floodwaters during Hurricane Andrew

In 1985, Hurricane Juan (Category 1 Storm) flooded over 800 homes in Terrebonne Parish and caused extensive flooding throughout southern Louisiana over a 5-day period.  Figure 5 shows flooding near the Humble Canal from Hurricane Juan.

Figure 5.  Flooding near the Humble Canal from Hurricane Juan

Rainfall totals in the area ranged from 5 inches to almost 17 inches.  The storm was responsible for storm surges of 5 to 8 feet and tides of 3 to 6 above normal.  FEMA estimates the residential and commercial flood damage resulting from Hurricane Juan totaled $112.5 million.

Hurricane Juan inundated homes in the lower coast region of Terrebonne Parish, south of Houma. Approximately 40 percent of the homes in the coastal areas of Lafourche Parish, including Pointe au Chien, were also inundated by high tides. Agricultural damages from Juan totaled $175 million, with 24 percent of these damages occurring in the two-parish study area. Excessive rains oversaturated soybean and sugar cane crops causing a reduction in crop yields and the hurricane force winds toppled sugar cane stalks (figure 6).

Figure 6. Sugar cane and soybean crops flooded from Hurricane Juan

Salinity Intrusion and Water Supply. Salinity intrusion has been a recurring problem in the study area. The primary conduits for salinity intrusion in the study area appear to be the HNC, Bayou Grand Caillou, and Bayou Lafourche. In addition to health and pollution concerns, elevated salinity levels in water sources increase the operation and maintenance costs of treatment plants.

The potable fresh water for Terrebonne Parish is furnished by the City of Houma via surface water withdrawal from the GIWW near its confluence with the HNC. The alternate/back-up source is Black Bayou and/or Bayou Lafourche at Terrebonne Parish's Schriever water treatment facilities. Both residential and commercial/industry water users depend on the Terrebonne Parish Consolidated Waterworks District in Houma, Louisiana.

The Houma Water Treatment Plant (HWTP) is located at the intersection of the GIWW and the HNC. From 1961 through 1980, 19 instances of elevated chloride levels were detected at the HWTP.  The duration of the instances ranged from a minimum of 6 days to a maximum of 57 days, with the median number of days of elevated chlorides being 26 days.  From 1981 to 1998, there have been 16 instances that ranged from a maximum of 79 days to a minimum of 3 days with a median of 32 days.  Furthermore, these instances are concentrated in the fall (August to October).

There has also been occasion when Bayou Black, has had elevated salinity levels.  The high organic content of Bayou Black water makes it a less than desirable alternate source of water. The Parish also indicated that Bayou Black, is currently undersized and cannot accommodate extended periods of salinity intrusion in the GIWW. Therefore, water from other plants is used to supplement the freshwater requirements in Houma.

Environmental Habitat. Louisiana contains 40 percent of the coastal wetlands in the United States. Wetlands are certainly a prevalent characteristic in the study area.  They range from swamps and freshwater marshes near the GIWW to brackish marsh around Lake Boudreaux and finally to salt marshes and estuarine bays near the Gulf of Mexico.  There has been no appreciable deltaic development in the Terrebonne Basin for the past 500 years.  Data for the entire Terrebonne Basin (over 1 million acres), which includes the study area, shows that land was lost from 1956-1978 at a rate of 0.79 percent per year. From 1978 to 1990, the land loss rate was 1.2 percent per year. The Terrebonne Parish coastline stretches 70 km from Pointe au Fer to Timbalier Island. Coastal wetland loss in Louisiana has averaged 65 sq km/yr (25 sq mi/yr) with land loss in Terrebonne Parish averaging 27.7 sq km/yr (10.7 sq mi/yr).  Figure 7 shows deteriorating marsh at Bayou Terrebonne.

Few plant species are well adapted to high salinities and frequent flooding.  Therefore, diversity of plants is lowest in the marsh zone nearest the Gulf of Mexico. The major factors that influence the type of wetland community are elevation, hydrology, salinity, and soil type. The major habitat types identified within the study area are described below:

• Bottomland hardwoods (BLH) (e.g. red maple, green ash, oaks, and American elm) cover less than 1.0 percent of the study area due to the low elevations, flat relief, and coastal influences of the area.  BLH areas are not flooded for extended periods.

• Swamp occupies about 4.0 percent of the Terrebonne study area as of 1978 or four times the BLH area.  The two dominant species normally associated with this type of habitat in the area are bald cypress and water-tupelo.  Most of the cypress was clear-cut prior to 1920, which destroyed old-growth cypress forest.

• Scrub shrub habitat forms a thin band between the forested wetlands and marsh. Typical vegetation includes elderberry, wax myrtle, buttonbush, Drummond red maple, and eastern baccharis.

• Marsh occupies the majority of the study area. This study used a four-zone classification system to document conditions and impacts in marsh.  The four zones of marsh are distinguished by salinity zones as fresh, intermediate, brackish, and saline.

• Fresh marsh habitat comprises 12 percent of the study area and is undergoing a precipitous decline.  There are two basic types of fresh marsh in the area, flotant emergent and attached emergent. Approximately 70 percent of the marshes in the Barataria-Terrebonne estuary are flotant emergent. The flotant emergent marsh is actually not attached to the underlying soil although the marsh plants form a dense mat that appears to be solid. The attached emergent fresh marsh is attached to the underlying soil and contains similar species of grasses and brush. Fresh marsh in the eastern half of Terrebonne Parish has been displaced about 1- to 4-km inland.

• Intermediate marsh habitat lies between fresh marsh and brackish marsh and the species of vegetation are not much different from fresh marsh.  Water salinity is higher and averages 3 parts per thousand (ppt) but can range from 0.5 to 8 ppt.

• Brackish marsh covers 16 percent of the study area.  However, intermediate marsh was lumped into this category. Water salinity in brackish marsh average 8 ppt with a range of 1-18 ppt.

• Salt marsh is the last broad category of marsh used in this document. It has the least diverse plant community and represented some 10 percent of the study area in 1978, down considerably from 24 percent in 1955.

Figure 7. Deteriorating marsh at Bayou Terrebonne

Vast acreages of wetlands have been lost and would continue to be lost due to subsidence, salinity intrusion, and hurricane storm surge. It is predicted that 93,792 acres, or 34.5 percent, of remaining vegetated wetlands in the study area would be lost in 50 years without any action to protect these areas.

Navigation. The GIWW intersects the study area in an east-west direction passing through Houma. The HNC extends due south from the GIWW, through Houma across the study area.  Both waterways are major navigational and industrial centers for south Louisiana’s oil, gas, and fishing industries. The Waterborne Commerce Statistics Center (WCSC) reported 5,890 vessel trips on the HNC for the calendar year 1995 (table 3).

An examination of vessel trips by vessel type reveals that most of the traffic was composed of towboats or tugboats and oil field service boats. Together these vessels amount to 77 percent of all traffic on the HNC for calendar year 1995. This explains why 84 percent of the vessel trips can report no tonnage. Through user interviews it was determined that traffic was fairly evenly distributed through the year and rose and fell with the fortunes of the offshore oil industry.

Table 3
Vessel Traffic on HNC by Commodity (1995)

Commercial fishing vessels and oil service crewboats that dock in the study area are frequently damaged by storm events.  Figure 8 shows commercial and recreational boats pushed out of Bayou Petit Caillou by flooding from Hurricane Andrew.

Figure 8.  Commercial and recreational boats pushed out of Bayou Petit Caillou by flooding from Hurricane Andrew

Although very few records are maintained to document the value of these losses, it is estimated that millions of dollars in property damage and production are lost annually due to hurricanes and tropical storms.

The only apparent restriction to navigation along the HNC is the Dulac Pontoon Bridge, with a navigable width of 180 feet (figures 9 and 10).  In feasibility, various enterprises in the study area were contacted to assess storm-related impacts. Offshore operations related to oil and gas production are not typically impeded by high water, but only by high winds. Therefore, once winds associated with a tropical storm or hurricane subside, offshore activity regenerates almost immediately.  The industries desire flood protection for infrastructure and equipment, however any situation that would prevent navigation immediately following subsidence of high winds would be detrimental to these industries. Currently, there are no restrictions along the GIWW through the study area.

Figure 9. Oil drilling platform negotiating the bridge at Dulac

Figure 10. Deep water oil platform at the Dulac pontoon bridge.

Subsidence. One of the primary causes for increased flooding in the study area is apparent subsidence.  Apparent subsidence is defined as the lowering of the land relative to the NGVD. It is the result of numerous factors such as a decrease in sediments reaching the marsh from the Mississippi and Atchafalaya River systems; sea level rise; consolidation of soft, compressible soils; and wave action along the coastline.

In 1996, as part of the feasibility study, T. Baker Smith and Son, Inc. prepared a report entitled “Datum Epochs, Subsidence and Relative Sea Level Rise for Southeastern and South-Central Coastal Louisiana.”  In 1995, the Barataria-Terrebonne National Estuarine Program (BTNEP) gathered elevation data in the Barataria Basin and Terrebonne Parish to evaluate subsidence rates. This data was compared to the 1996 T. Baker Smith and Son report and a 1987 report entitled “Terrebonne Marsh Subsidence Study”, also prepared by T. Baker Smith and Son, Inc for the New Orleans District.

Based on the data available from these sources, for base conditions, apparent subsidence was assumed to be 0.54 ft (0.036 ft/year for 15 years) for all areas except “unhealthy” marsh areas, as identified in the BTNEP. “Unhealthy” marsh was assumed to subside a total of 0.74 ft (0.048 ft/year for 15 years). For future conditions, apparent subsidence was assumed to be 2.34 ft (0.036 ft/year for 65 years) for all areas except for unhealthy marsh areas where a value of 3.12 ft (0.048 ft/year for 65 years) was assumed.  Subsidence is expected to magnify flooding problems for Terrebonne and Lafourche parishes in the future.

Structure Inventory. In 1998, it was estimated that there were 24,768 residential structures in the study area and in 1997; there were 880 non-residential structures in the study area (tables 4 and 5).

Table 4
Number and Types of Residential Structures

Table 5
Number and Types of Non-residential Structures

Approximately 18,796 residential structures were located in the Terrebonne Parish portion of the study area, while 5,972 residential structures were located in the Lafourche Parish portion of the study area. Approximately 740 non-residential structures were located in the Terrebonne Parish portion of the study area, while 140 were located in the Lafourche Parish portion of the study area. The average depreciated replacement cost of residential structures, as determined by the Marshall and Swift Valuation Service, was found to be $63,758 for Terrebonne Parish and $55,156 for Lafourche Parish.  Mobile homes were not included in the determination of these values.

The projected increase in the number of residential and non-residential structures in the study area was based on the projected population growth in the area and employment forecasts prepared by the Bureau of Economic Analysis. Under the Most Likely growth scenario, 7,638 new residential structures and 340 new non-residential structures would be constructed between 1997 and 2057.

Automobiles. The structural equivalent damages due to automobiles and the other structural damage categories are shown in table 6.  As shown on in the table, damages to automobiles only account for approximately 10 percent of the total structural equivalent annual damages and benefits.  Since the study area is very large, an average automobile value was used for all areas.  There was not a noticeable difference reported during field investigations between the cars parked in the lower-income areas than in the upper-income areas and between the cars in the Houma and New Orleans metropolitan areas.  While per capita incomes may be lower in the Houma MSA relative to the New Orleans MSA, a larger portion of the family budget is spent on automobiles.  Since the Houma area is more rural, reliable personal transportation is more important.  While data were not available on the average value of an automobile specifically for this area, historical flood damages to automobiles were verified for the May 1995 flood event that impacted southeast Louisiana.  The Property Claims Service Company estimated that flood claims for automobiles totaled $75 million, while the total residential flood claims paid by FEMA totaled $570 million. Thus, the damage claims for automobiles were about 13 percent of the damage claims for residential property.

Evacuation routes in the study area are woefully inadequate.  The New Orleans Metropolitan Area was evacuated in 1998 in anticipation of Hurricane Georges.  It took approximately 8 hours to travel on the Interstate Highway approximately 80 miles north to Baton Rouge, due to the heavy traffic.  Reports on evacuation behavior in Louisiana show that in most homes one vehicle is evacuated and one is left at the residence.  It is unreasonable to assume that every household would use both automobiles for evacuation, because families prefer to stay together during an evacuation and officials recommend limiting the number of vehicles to reduce congestion on inadequate evacuation routes.

Table 6
Equivalent Annual Damage By Damage Category
Highway 57

EQUIVALENT ANNUAL DAMAGE FOR DAMAGE CATEGORIES

Population. Table 7 summarizes the 1970, 1980, 1990, and 1998 Census Bureau population counts for Terrebonne and Lafourche parishes, as well as for the State of Louisiana and the United States.

Table 7
Population for Two-Parish Area
1970-1998

Using data gathered from the 1990 Census, it was assumed that 2.9 persons reside in each residential structure inventoried in the study area.  The 1998 population in the study area was estimated to be 68,836 persons, with 52,616 persons residing in Terrebonne Parish and 16,220 persons in Lafourche Parish.  These two figures amount to 52 percent of the total population in Terrebonne Parish and 18 percent of the total population in Lafourche Parish. Percentage change in population for the two-parish area, Louisiana and the nation are described in table 8.

Table 8
Percent Change in Population

Population grew more rapidly in the two parishes than the State and national averages during the 1970s due to expansion of the petro-chemical industry.  During the 1980s, the population growth in the two-parish study area, as well as in the State, fell below the national average. During the 1990s, population growth in Lafourche Parish remained slow relative to the national average, while the population growth rate in Terrebonne Parish more than tripled.  Again, this increase can be attributed to oil and gas exploration in the Gulf of Mexico, technological advancements in industry, and improvement in the national economy.

Projections of future population growth in Lafourche and Terrebonne Parishes were developed using data from several agencies, including the Louisiana Population Data Center at Louisiana State University, Woods and Poole Economics, and the Bureau of Economic Analysis.  An average of the population growth rates prepared by the three agencies was used as the Most Likely scenario.  It was assumed that the growth rate in the study area would be the same as the growth rate for the two parishes.  Population in the study area is anticipated to expand to 71,445 in 2008 and 89,853 by the year 2057.

Land Use.  In order to estimate the impacts of development on various land types, the current land cover pattern in the area was defined.  The numbers presented are for the reaches in the study area that presently contain structures. 

Land cover in the study area was classified into the following four categories based on development potential:

• Currently developed: vegetated and non-vegetated urban land. A total of 26,438 acres of currently developed land is present in the study area.

• Highly developable land: There is a total of 71,226 acres of highly developable land in the study area, comprised primarily of 66,313 acres (93 percent) of agricultural cropland.

• Marginally developable land: fresh marsh, wetland, deciduous, evergreen, and mixed forests.  There is a total of 92,900 acres of marginally developable land currently in the study area.

• Non-developable: wetland; water; intermediate, brackish and saline marshes.  There are a total of 84,137 acres of non-developable lands in the study area.

The total land area of the 116 reaches represents 274,701 acres or 429.2 square miles.

Cultural Resources. Elevated natural levees along past and present distributary channels provided elevated dry ground for settlement and temporary subsistence use by prehistoric and historic settlements in the study area. Cultural resource investigations in Terrebonne and Lafourche parishes have identified prehistoric cultural resources dating between 1,500 BC and AD 1,500. Several earthen mound sites are located in the project area and may represent village and/or ceremonial centers. Later historic Indian and Anglo cultural resources are also present in the project area.

Many of the later prehistoric cultural resource sites in the region have ancestral ties to the present day Chitimacha. Another historic group, the Houma, began a migration down Bayou Lafourche in the early nineteenth century and eventually settled along Bayou Terrebonne and Bayou Lafourche. The Houma have remained in this region and are now dispersed among six communities along Bayou du Large, Grand Caillou, Dulac, Lower Montegut, Lower Pointe au Chien, Champs Charles, and Bayou Lafourche.

Historic Anglo settlements associated with the French Colonial (1698-1765), the Spanish Colonial (1765-1803), and Anglo-American (1803-present) periods are located in the region.  Under Spanish rule, Acadian settlements were established throughout the project area and the descendents of these early settlers remain.  Both Terrebonne and Lafourche parishes were established in 1822 from what was formerly known as the Lafourche Settlement. By 1850, several hundred farms and plantations were farming sugar cane.  As the sugar plantations developed, the region became increasingly dependent upon slave labor and by 1860, African-American slaves made up the majority of the region's population. Large sugar plantations such as Ashland (located six miles south of Houma on Bayou Grand Caillou) and Terrebonne (located 18 miles south of Houma on Bayou Terrebonne) flourished well into the early quarter of the twentieth century.

The project area also contains numerous historic shipwrecks. Bayou Terrebonne, du Large, and Grand Caillou were primary routes for waterborne commerce.  Thus, watercraft from all periods could be present within the project area.  In 1997, cultural resource investigations along Bayou du Large recorded and documented six historic vernacular watercrafts.

Cultural resource investigations prior to this study recorded 559 cultural resource sites in the two-parish area.

Modern land-use practices associated with agriculture, the timber industry, oil and gas exploration, urban/commercial development, flood control construction activities and vandalism have contributed to the disturbance and loss of cultural resource sites. Many cultural resource sites are not buffered from damaging tidal surges due to the loss of marsh vegetation caused by natural subsidence, saltwater intrusion and wave action.

Recreation. Most of the study area is brackish and saline marshes with some forested wetlands and uplands.  Recreational facilities include camps for fishing and hunting, marinas, boat launch ramps, and small neighborhood parks.  Recreational fishing in the study area occurs almost entirely from boats.  The number of recreational boats registered in Terrebonne and Lafourche parishes is an indicator of the amount of recreational fishing in the study area. In 1997 within the two parishes, there were 25,430 registered boats, 51,883 resident fishing licenses, and 13,564 resident hunting licenses issued by the State of Louisiana.

PLANNING OBJECTIVES AND CONSTRAINTS

The full range of alternative solutions to a problem including positive and negative impacts should be considered from the outset of the planning process. With and without consequences of each feasible alternative should be determined adequately. The cumulative effects of the plan and other similar activities should be analyzed.  Each alternative plan is to be formulated in consideration of four criteria: completeness, efficiency, effectiveness, and acceptability.

This study required the development of alternatives that addressed the flood protection needs of the community while maintaining navigational passage and protecting the environment, as a minimum.  The Corps, the TLCD and the public through regulatory, planning, and NEPA scoping processes generated numerous flood protection alternatives.  Those alternatives take advantage of existing and permitted forced drainage levees in Terrebonne Parish and utilize existing pump stations and flood control structures, wherever possible.  The Corps analyzed all reasonable alternatives for engineering effectiveness, economic efficiency, and environmental and social acceptability.  The alternatives that satisfied these criteria continued into detailed analysis.

The goal of formulating flood protection alternatives was to maximize the number of residential and commercial structures protected while minimizing the adverse impacts to the environment, local interests, navigation, and industry.  Typically during feasibility a broad array of alternatives is developed, tested in a generic manner against planning objectives, and then further screened using economic criteria to eliminate economically infeasible alternatives.

Six major structural alternatives were considered in this feasibility study.  The alternatives include navigation structures and water control structures of varying sizes for drainage and environmental enhancement. All alternatives considered consist of an earthen levee with flood control features integrated into the design to provide drainage, environmental enhancement, and navigational passage.

Based on the identified flood protection problems, the following planning objectives and constraints were developed:

• Provide increased flood protection for the communities located within the study limits;

• Develop protection and enhancement measures for the environment;

• Ensure that any flood protection measures minimize adverse impacts to local interests, navigation, and industry.

These constraints are related to one another.  For example, improvements in flood protection would include features that may reduce or eliminate the incidence of high salinity levels in the local water supply.  In addition, a hurricane protection levee would protect marine vessels and other commercial facilities from decreased mobilization and clean-up following a storm event.

ALTERNATIVE PLAN FORMULATION

The following flood protection measures were considered to address the stated problems and opportunities:

1. TLCD (Alternative 1). The local sponsor’s proposal would join several existing forced drainage levees and flood control structures around Lower Bayou Du Large, Chauvin and Montegut to form a hurricane protection system extending from Bayou Du Large and Highway 311 to Bayou Lafourche.  The plan consists of earthen levees, 11 flood control structures, and 8 environmental water control structures (plate 3).

2. Modified TLCD (Alternative 2). Same as alternative 1, above, except the levee would continue north from Bayou Pointe au Chien to Montegut and not provide protection to Isle de Jean Charles. The alignment would join several existing forced drainage levees and flood control structures around Lower Bayou Du Large, Chauvin and Montegut to form a hurricane protection system extending from Bayou Du Large and Highway 311 to Bayou Lafourche.  The plan consists of earthen levees, 11 flood control structures, and 8 environmental water control structures (plate 4).

3. Highway 57 (Alternative 3). The proposed levee alternative runs parallel to Louisiana Highway 57 south of Lake Boudreaux.  This plan would also tie together existing forced drainage levees in Lower Bayou Du Large, Chauvin and Montegut, but it would follow a southerly route to cross the Lake Boudreaux Basin.  The plan consists of new earthen levees, 12 navigable flood control structures and 10 environmental water control structures (plate 5).

4. Northern Alignment (Alternative 4).  This alternative was developed as a result of the Corps Regulatory process to leave the Lake Boudreaux Basin as open as possible. The plan would connect existing forced drainage levees similar to the TLCD plan; however, the Lake Boudreaux Basin is traversed with a levee in the extreme northern end.  The plan also includes 8 structures for freshwater and sediment diversion and 11 flood control structures (plate 6).

5. Reconnaissance Alignment (Alternative 5). The Corps developed this plan during the Reconnaissance Phase.  This plan would follow the existing forced drainage levees in Chauvin and Montegut and skirt the Lake Boudreaux Basin as it proceeded north before cutting straight across the HNC. It consists of earthen levees, in conjunction with six-flood control and six environmental structures (plate 7).

6. Modified Highway 57 (Alternative 6).  A modified Highway 57 Alternative was adopted late in the process to save on construction costs.  The modified alternative removed levees in the lower Bayou du Large and lower Bayou Grand Caillou areas (plate 8).The differences between the original Highway 57 Alternative and the Modified Highway 57 Alternative can best be seen by comparing plates 5 and 8).

7. No Action (Alternative 7). No Federal action would be taken to provide improvements for the authorized purposes of this study. The flood protection projects that are currently in-place or permitted for construction in Terrebonne and Lafourche Parishes are identified on plate 2.  The TLCD would continue to operate the forced drainage systems that currently exist, which contains segments and components that were built to be individually self-sufficient.  Due to funding constraints and the size of the study area, the local government cannot provide protection from a 100-year event. Any forced drainage levees already permitted, but not constructed would be built by local entities.  Additional permits may be issued for other self-contained forced drainage systems, but the overall protection system would not be constructed.

8. Non-Structural Alternatives. Section 73 of the WRDA of 1974 (PL 93-251) states a general policy that, during planning, Federal agencies will give consideration to non-structural measures to reduce or prevent flood damage and that the Federal Government may participate in the costs.  Non-structural improvements such as structure raising and relocation out of the floodplain were considered for the study area south of the GIWW.

Modifications to these alternatives were considered during the screening process and in response to technical review comments.  A summary of these modifications is presented in the FINAL ARRAY OF ALTERNATIVE PLANS section of this report.

EVALUATION OF ALTERNATIVE PLANS

Preliminary Screening. In accordance with the IPMP’s Scope of Work, a screening was performed to identify two action alternatives for detailed engineering and design.  A qualitative screening removed those alternatives that did not address the planning objectives. Following the qualitative screening, initial investigations into the engineering, economic, and environmental considerations of the alternatives were performed.  The alternatives providing the greatest economic and environmental benefit for the least cost, while minimizing adverse social and environmental impacts were referred for detailed analysis and design.

The interdisciplinary planning team (IPT) met on March 30, 1998, to evaluate Alternatives 2, 3, and 4.  The purpose was to screen out two of the three options (north, middle, and south) for crossing the Lake Boudreaux Basin.  Preliminary designs and construction costs for levees, navigation structures, and environmental structures were developed using existing data and compared for each of the alternatives around the Lake Boudreaux Basin using the 100-year frequency event for design.  Mitigation costs were developed for each alternative in its entirety because of the interconnectivity of the sub-basins.  The differential impacts between each option and the relative mitigation costs were valid because the remainder of the system outside of the Lake Boudreaux Basin is the same for each of the three alternatives. Therefore, the only differences between Alternatives 2, 3, and 4, are the costs of construction and environmental mitigation.

The least cost option of the three, including environmental structures and environmental mitigation, was Alternative 3 (the southern option following Highway 57).  The relative cost of the Highway 57 option was estimated to be $40.5 million; Alternative 4 (Northern) was estimated at $51.9 million; and Alternative 2 (Modified TLCD) was an estimated $131.6 million.  Therefore, the IPT eliminated Alternative 4 and Alternative 2 from further study.  As a result of the screening, two main alternatives for hurricane protection remained (Highway 57 and Reconnaissance), along with the possibility of non-structural improvements.  Table 9 summarizes the results of the preliminary screening of structural and non-structural plans.

Table 9
Preliminary Screening Results
Structural and Non-structural Plans

Final Array of Alternative Plans.  Based on the screening process, the Reconnaissance Alternative, the Highway 57 Alternative and three non-structural alternatives continued to be studied in addition to the No Action Alternative.  A description of the features included in each of these alternatives follows.

Highway 57 Alternative. The Highway 57 Alternative alignment and approximate locations of all structures is shown on plate 5.  The plan consists of approximately 72-miles of earthen levees, 11 flood control structures, 1 multipurpose lock, and approximately 12 environmental water control structures. The 100-year level of protection levee dimension varies from approximately 40-feet wide at elevation +9.0 NGVD to 200-feet wide at elevation +15.0 NGVD.  New and replacement fish and wildlife structures will be constructed and operated in selected locations in order to maintain tidal exchange. The total project cost for this plan is approximately $680 million, including mitigation.

The Highway 57 Alternative begins with a 125-foot wide floodgate in the GIWW immediately north of Bayou Lafourche.  The levees begin southeast of the intersection of Bayou Lafourche and the GIWW at the Larose to Golden Meadow Levee, and proceed along the eastern bank of Grand Bayou.  The levee alignment crosses Grand Bayou near Bayou L’eau Blue with a floodgate structure and continues south along the western bank of Grand Bayou to Bayou Pointe au Chien, where it crosses Bayou Pointe au Chien with a floodgate structure.  The alternative then proceeds west-northwest along the southern bank of Bayou Pointe au Chien, skirting along the boundary of the Pointe au Chien wildlife management area.  The levee continues west along the wetland interface then south near the town of Montegut to the Humble Canal, where it intersects a floodgate structure in the Humble Canal.  The levee turns south again at Bayou Terrebonne and follows the eastern bank of the bayou until reaching the existing Bayou Terrebonne floodgate. The alternative then crosses the bayou and continues south along the western bank of Bayou Terrebonne to the Bush Canal where it turns west-northwest and follows the north bank of the Bush Canal.  At the Bush Canal-Bayou Petit Caillou intersection, a floodgate in the Bush Canal allows the alternative to turn south and proceed along the eastern bank of Bayou Petit Caillou until meeting Highway 57 across the bayou.  The alternative crosses Bayou Petit Caillou with a floodgate and proceeds west-southwest parallel to Louisiana Highway 57 until turning west-northwest with the highway.  South of the town of Dulac, the levee turns south along an existing levee before intersecting the HNC at a point approximately 1,600’ south of Bayou Grand Caillou.  The levee alternative crosses the HNC with a 200 foot-wide, 1,200 foot-long lock. The levee turns north and intersects Bayou Grand Caillou and another 56-foot wide floodgate.  The alignment then continues north and follows the western bank of the HNC until intersecting the Falgout Canal, where it turns west and follows the southern bank of the Falgout Canal until it reaches the eastern protection levee along Bayou du Large.  The Highway 57 alignment crosses Bayou du Large with a floodgate and the levee alignment turns abruptly north and crosses the Falgout Canal with another floodgate before proceeding north along the wetland interface and the western bank of Bayou du Large. Continuing north, the levee crosses the Marmande Canal with a set of concrete box culverts.  Continuing north along the wetland interface, approximately parallel to Bayou du Large, the levee turns west-northwest at the Crozier Cemetery, near Bonvillan Canal.  The alternative proceeds along the southern bank of Bonvillan Canal until intersecting the GIWW around mile 42.  The alternative then crosses the GIWW with two 125 foot-wide sector gates and continues on that west-northwest path until it intersects an unnamed drainage canal connecting the GIWW to Minors Canal.  The alternative then follows the southern bank of that drainage canal until reaching Minors Canal, where it crosses by way of an existing structure. The alternative then turns and proceeds along the western bank of Minors Canal and finally ends at Louisiana Highway 90 near the town of Waterproof, La.

Reconnaissance Alternative. The Reconnaissance Alternative alignment and approximate locations of all structures is shown on plate 7. The Reconnaissance Alternative consists of approximately 52-miles of earthen levee, 8 flood control structures, and 7 environmental water control structures. The 100-year level of protection levee dimension varies from approximately 40-feet wide at elevation +8.0 NGVD to 300-feet wide at elevation +16.0 NGVD.  The estimated total project cost of this plan is $460 million.

From the eastern project limit at Bayou Lafourche to Bayou Petit Caillou, the Reconnaissance Alternative is identical to the Highway 57 Alternative.  At Bayou Petit Caillou, the Reconnaissance Alternative crosses Bayou Petit Caillou at an existing floodgate structure and proceeds north, along the wetland interface, parallel to Bayou Petit Caillou.

After crossing Bayou Petit Caillou, the Reconnaissance Alternative follows the existing levee system protecting the town of Chauvin.  The alternative maintains a northerly course following the wetland interface, parallel to the Boudreaux Canal until reaching the St. Louis Canal.  At the St. Louis Canal, the levee begins to turn west toward the town of Ashland.  The levee intersects the HNC with a 200 foot-wide floodgate north of Ashland and continues west until reaching the same location described by the Highway 57 Alternative near the Crozier Cemetery.  The alternative then follows the same route as the Highway 57 plan along Bonvillan Canal, across the GIWW and ending at Highway 90 near Waterproof, La.

Detailed Evaluation of Alternative Plans.  Engineering and design, economic analyses, and an assessment of environmental impacts were performed for both the Highway 57 and Reconnaissance alignments.  A summary of the feasibility evaluation is provided in the following paragraphs.

Hydraulic Analysis.  Several models were used to simulate hydraulic conditions in the study area. They are the Hydrologic Engineering Center-1 (HEC-1) version 4.1, Advanced 3-Dimensional Circulation Model for Shelves, Coasts, and Estuaries (ADCIRC), and 2 One-Dimensional Unsteady Flow Through a Full Network of Open Channels (UNET) models.  One UNET model was used to analyze the overall project area and the second was taken from a CWPPRA project to analyze the impact of proposed floodgates along the GIWW.  A description of the second UNET model is included in the section on STRUCTURAL FEATURES.  This section briefly describes the function of the hydraulic and hydrologic models, engineering assumptions, and modeling results.

HEC Modeling.  Runoff from the study area was determined by using HEC-1 version 4.1.  Four types of watershed were identified in this study area: forced drainage areas (FDA’s), canal and bayou levees, marsh and swamp areas, and upland basins.

UNET Modeling.  Water surface elevation and flow hydrographs were determined using UNET version 3.2.  The UNET model geometry for this study was developed using UNET geometry from two previous studies, new survey data, and apparent subsidence rates.  The UNET model for this study is composed of over 92 reaches (channel segments), 31 forced drainage areas, and 41 ponding areas (e.g., marshes, lakes, etc.) that interconnect and allow transfer of water between them.

The model was used to perform 161 simulations that included the 2-, 5-, 10-, 25-, 50-, and 100-year extra-tropical storms and 17 tropical storms. Table 10 describes the project conditions modeled.

TABLE 10
UNET GEOMETRY FILES

ADCIRC Modeling.  The ADCIRC model was employed to simulate the effects of storm surge at all the boundaries of the study area using the latest numerical model. Seventeen tropical storms were simulated in this model for existing, base, and future conditions.  Verification of storm surge was limited to Hurricanes Carmen (1974), Juan (1985), and Andrew (1992).  These events represent the only storms for which sufficient, reliable storm surge elevation data were available for the study area.  The ADCIRC simulations were used to generate hurricane-specific storm surge hydrographs for input to the UNET model.

Determination of Levee Heights.  For the proposed alternatives, levee heights were determined for the 100-year hurricane for both base and future conditions.  In addition, for comparative purposes levee heights for a project providing 85- and 500-year levels of protection were developed.  For the 85-year level of protection, levee heights were generally 1 foot lower than the 100-year levee heights throughout the alignment.  However, levee heights for the 500-year level of protection are approximately 2 feet higher than the 100-year level of protection.

Levees. The proposed levees would be constructed of uncompacted fill and would require a three-lift construction sequence to attain ultimate design grades for most reaches.  A consolidation period is required between lifts.  Geotextiles will be used at the base of the levee prior to placement of fill material.  Uncompacted fill would be excavated from adjacent borrow areas, where indicated, and placed into cells to reduce moisture content prior to placement in horizontal layers along the alignment not to exceed 3 feet in thickness.  In reaches where adjacent borrow sites are identified the top 4 feet of material are assumed to be unsuitable for construction.  In the remaining levee reaches, the required fill material would be hauled from an off-site source with a one-way haul distance of approximately 10 miles.  Additional investigations will be performed during Pre-construction Engineering and Design (PED) to determine the suitability of on-site material for embankment construction.

Geotechnical Analysis.  Seventeen undisturbed soil borings were obtained along the most likely levee alignments to provide design data for stability analysis, pile capacity calculations, settlement calculations, etc.  Since settlement is a major concern in the study area, one boring was 100 feet deep, two were only 40 feet deep and the remaining borings were 60 feet deep.  Good borrow sources adjacent to the proposed levees are scarce mainly due to the abundance of open water areas.  Some areas contain such a significant amount of organic material that even when mixed with clay, the organic content is high enough to make the layers highly compressible and weak.  Vibracore borings were obtained to augment the data from the undisturbed borings and provide data to determine adequacy of material for borrow.  The data gathered revealed that the foundation along the southern alternative of Lake Boudreaux is relatively stiff and that borrow is acceptable.  These data will be confirmed in the next phase, but the vibracore borings helped to fill in large gaps between the undisturbed boring samples.

Sediments in the natural ridges are stronger than the sediments in the lower surrounding areas, especially sediments that are in open water environments.  Levee construction on the marsh sediments will be challenging from a stability and settlement viewpoint, especially for the levee heights in question.  In order to provide flood protection in a reasonable amount of time, the levees will be built to the 85-year base elevation in one construction period, and subsequent lifts will be performed according to settlement calculations.  Geotextile reinforcement, berms, and flatter than normal side slopes were used to accomplish the task.  The location of suitable borrow material has a significant impact on the construction cost, and in some instances may require modifications in the levee alternative.  Due to the scarcity of land, borrow sources were assumed to be from open water areas, such as canal bottoms and other wetlands, wherever possible.  Where adjacent borrow is used, cost estimates are based on the top four feet being unsuitable due to organic content.  The levees were placed as close as practical to the natural ridges along existing or former distributaries to take advantage of the stronger base and reduce the probability of failure during and after construction.

Three levels of protection were considered in this study.  They are the 85-, 100-, and 500-year for the base and future conditions.  The base year is assumed to be 2008 and the future year, or useful project life is 2058.  In the future condition, the 85-year levee elevation is approximately 1 foot higher than the 85-year base condition.  A second lift should be constructed 3 to 6 years after the initial construction and will provide the 85-year future condition levee elevation.  A third lift to the 85-year future condition plus 1 foot for future maintenance should be constructed according to the time schedule from the settlement analysis.

In the future condition, 100-year levee elevations are 1 to 2 feet higher than the 85-year base condition.  The second lift would be constructed 3 to 6 years after the first construction period and would have a final crown elevation sufficient to provide the 100-year design elevation through 2058.  A third lift to provide for future maintenance is also recommended for this future condition.

The amount of embankment required to provide crown elevations for a 500-year levee exceeds the resistance that is available in the foundation and increases the probability for stability failures to unacceptable levels.  Levee elevations for the 500-year condition are 3 to 4 feet higher than the 85-year base condition levees.  Based on available information, the 500-year level of protection is not recommended and no further analysis was performed.

In order to design the reach of levee between Pointe au Chien and Larose along Grand Bayou and Cutoff Canal, soil borings from an adjacent hurricane protection project known as the Larose to Golden Meadow Hurricane Protection Project were used.  The data revealed that the top layers of clay contain some organic matter in the form of wood and roots, but generally borrow was better than expected.  If the organic areas are discarded, the remaining soil along the bayou and canal is adequate for borrow sources to the depth of the borings.  The geotechnical data provided for the Isle de Jean Charles alternative, by the local sponsor, indicated that the soils in the area are too weak to support a hurricane protection levee and are not a good source for borrow material.  Two borings were obtained along the proposed Harry Bourg Corporation (HBC) alignment for analysis. The HBC alignment is described in greater detail later in this report.

Future geotechnical studies should focus on obtaining additional soil data and surveys that provide adequate knowledge of the construction site.  More in-depth investigations are needed to determine settlement, structural foundation design, and dewatering requirements.

Structural Features.  The feasibility study is based on a comparison of float-in-type structures and traditional cast-in-place type structures.  The Corps gave equal treatment to both.  However, the design details presented in the draft feasibility report (DFR) were limited to the recommended option.  In the case of the Houma Navigation Canal Lock, only the float-in concept is presented.  For the other floodgates in the system, the cast-in-place type structure is recommended and presented.  Additional design of the structural features will be performed in the pre-construction engineering design phase, as indicated by the DFR.  The float-in type structure is preferred for the Houma Navigation Canal Lock.  Detailed design for the lock is currently underway to investigate the float-in application.  So, the appropriate measures are being pursued during PED.  The DFR recommends conventional cast-in place structures for the other floodgates in the hurricane system.

The overall hurricane protection system is designed to maintain tidal ebb and flow in addition to providing flood protection.  This is accomplished in two ways.  The first is by simply leaving all of the navigable floodgates in an open-pass condition until a storm threatens the study area or stages in the channel reach elevation +3 feet NGVD.  The normal high tide rarely reaches elevation +3 feet NGVD therefore; only storm events should initiate operation of the floodgates in a typical year.  The second method that is proposed for maintaining tidal influx requires the construction and operation of environmental water control structures (WCS).

Sector Gate Structures. The top elevation of the existing Bayou Terrebonne flood control structure is +13.0 feet NGVD, 2 feet less than the +15.0 feet NGVD required for hurricane protection at this location. A 2-foot extension may be added to the existing structure to remedy this deficiency.  Existing floodgates and the cable operating system may require replacement to reduce wear and stress on the sector gate, the pivot bearings, and associated gate machinery thereby extending the service life and reducing maintenance costs.  The existing wingwalls of the sector gates were analyzed for the 2-foot extension.  Two of the horizontal members may require plating to carry the additional load.  All other members are sufficient to carry the added hydraulic load of the 2-foot extension to the wingwall.

The Bayou Petit Caillou structure is also pile-supported and composed of two moveable steel sector gates with a 56-foot wide clear opening for navigation.  The top elevation of the existing structure is +12.0 ft NGVD, 3 feet less than the +15.0 feet NGVD required for hurricane protection at this location.  This structure is required for the Reconnaissance Alignment, but not for the Highway 57 Alignment.

Environmental Water Control Structures.  The WCS are concrete box culverts with sluice gates.  For this study, typical designs were prepared for a 2-, 6-, and 9-barrel WCS.  Each barrel is a single 6-foot by 6-foot concrete box culvert.  The culverts are constructed through the levee, to connect wetlands inside the system to wetlands outside the system.  Certain locations will also require a flap gate to prevent saltwater from entering the system.  A UNET model was employed to determine the size of proposed environmental WCS (number of 6-foot by 6-foot box culverts).  The model was configured to monitor the drainage of sensitive wetland areas within two weeks after the onset of storms of varying size and tidal conditions.  All proposed culverts (WCS) were assumed to have inverts at –6.0 feet NGVD.

For simulations involving storm events, where floodgates are operating, all gates were closed at a water surface elevation of +3.0 ft NGVD in the channel.  The gates were opened again when water surface elevations fell below +3.0 ft NGVD, or within 5 days of the storm completion, whichever occurred first.  Model simulations were run using a May and September condition with marsh stages 25 ft above normal.

The District will use Corps standards for all culverts under levees and LDOTD standards for culverts under highways.

GIWW Floodgates.  A UNET model was also used to determine the impact and need for floodgates in the GIWW at each terminus of the levee system.  This precludes the need for any significant improvement to the line of protection that already exists along the stretch of GIWW that runs through the project area.  If floodgates are not placed in the GIWW as part of the project, levees along both banks of the waterway must be elevated to prevent overflow from the design storm event.  In order to provide the same level of protection as the outside levees, the streambank along the GIWW would require an elevation high enough to prevent overtopping from the design flood stage within the GIWW plus wave run-up.  Two cases were considered using the UNET program to run all 17 tropical and 6 extra-tropical events for base and future, and with and without project conditions.

To evaluate the effect of the floodgates in the open-pass condition, a UNET model previously developed for a CWPPRA study was used.  The CWPPRA UNET model extends from Bayou Lafourche to the Terrebonne Marsh area with limited detail regarding overbank storage along the GIWW west of Houma.  Although the model does not accurately represent the prototype floodgate, it can still be used to evaluate the effects.  The magnitude of flow in the GIWW east of Houma appears to be controlled by factors in addition to stage.  As stages on the Lower Atchafalaya River increase, flows in the HNC and in the GIWW east of Houma increase proportionally until the flow in the GIWW east of Houma reaches 4,000 cfs.  The HNC conveys additional flows reaching the Houma area.  As much as 8,000 cfs was measured in the HNC during the Atchafalaya high water season.

The area of two 125-ft wide floodgates west of Houma represents approximately 80 percent of the existing channel area.  Therefore, since head loss is minimal, flow patterns in the adjoining Terrebonne Marsh area (including Bayou Copasaw and Minors Canal) should not be measurably affected.  The CWPPRA UNET model indicated greater channel velocities in the GIWW with the floodgates in-place.  Head loss and flows into the channels connected to the GIWW were not affected.

Houma Navigation Canal Lock.  The engineering assessment involved simulations using the UNET model to represent conditions that would exist during the high water season on the Lower Atchafalaya River with the HNC Lock closed.  The HNC conveys a significant portion of the flow in the study area from the Lower Atchafalaya River, therefore it was important to identify where the water gets stored should the canal be closed for any reason.

Simulations performed with the HNC lock closed and the Grand Caillou structure open indicated that flow was present in the HNC upstream of the lock and water entered and exited the system via Grand Caillou.  Average daily flows and stages during a rainfall event were evaluated for base and future conditions.  For base conditions, the HNC experienced a 20 to 25 percent decrease in flow moving south above the Falgout Canal, but the Falgout Canal experienced a similar percent increase in flow.  There was also an increase in flow into Lake Boudreaux and Bayou Grand Caillou, east of the HNC.

For the economic analysis, the operation was broken down into two conditions: open pass condition and structure operation. The structure will be operated for salinity and flood control primarily during the months of September, October and November and in open pass mode for most of the remaining nine months.  The structure may be operated more or less than this but actual dates are not forecasted and this 3-month period is typical.

Traffic patterns were analyzed to determine whether all of the traffic would traverse the location of the proposed structures. The analysis showed that approximately 60 percent would transit the proposed structure location.  Further analysis showed that all current traffic on the canal would be accommodated by the proposed structure.  These and other data are inputs for a simulation model used to assess the transit impacts to navigation for the lock.  A floodgate was also analyzed. For future with project conditions, both structures would be closed for approximately 78 days for salinity control.

Simulation model runs used traffic pattern data showing frequency of arrivals at the lock and timing data for approach, entry, chambering and exit.  Frequency of arrival time was developed from historical data. Other timing data are based on design specifications for the lock.  Model results indicate that average process time is 18 minutes.  Process time is defined as the sum of delay time plus transit time (approach, entry, chambering, and exit).  Delay time is defined as the elapsed time from arrival to start of lockage.  There is a 1.5-minute average delay time for the lock.  Approximately 860 vessels (total of upbound and downbound) would incur this delay for the estimated three months of lock operation in a typical year.

Operation of the floodgate for flood control in the future would completely stop navigation for an average of 78 days.  All transit at the gate location will halt until the water level conditions improve enough for the gate to be opened.  This could be in excess of the 78-day estimate.

Calculating the cost of impacts to navigation associated with the lock alternative is negligible. Assuming a traffic pattern as described above the 200-foot wide lock alternative would produce an annual average delay cost of $35,000.  Transit time without project is assumed to be zero.  Therefore $35,000 annual average delay cost is an upper bound estimate. Construction of a fixed dimension structure will restrict the size of vessels and/or their cargo that could traverse the structure site both now and in the future.  At present a 200-foot wide lock or floodgate will accommodate virtually all traffic on the HNC.

However the 200-foot wide floodgate, which would be operated for flood control and saltwater intrusion, would halt navigation for an average of 78 days (approximately 2.5 months).  Stopping all navigation for extended periods will cause facility relocation in Houma.  The large oilfield platform manufacturing facilities that have no alternative route to the Gulf of Mexico would relocate first.  This industry is such that the most likely locations for new manufacturing facilities are overseas and there are no domestic facilities that can replace their products due to patent rights.  The National Economic Development (NED) impact of such relocation is approximately $117.9 million.  That translates into an $8.14 million average annual cost associated with the implementation of a 200-foot wide floodgate alternative.

The local industries indicated that construction of a floodgate instead of a lock would result in their immediate relocation.  The structure type (lock vs. floodgate) and the structure width were the primary considerations during the economic and environmental analysis.  The economic impact of a floodgate closing was the critical issue that would prompt the local industries to relocate and not the 78-day duration.  In fact, a 90-day duration was used for the economic analysis, based on a combination of factors.

The 78-day estimate was for a point in time 50 years in the future and was made for the Leon Theriot Lock Study at Golden Meadow.  Based on records from the years 1985 through 1998, the Golden Meadow floodgate is currently closed an average of 4.7 days a year.  At a point 50 years in the future, the floodgate would be closed 78 days in order to maintain the current depth of flooding of the interior.  The flooding of the interior in the future results not from sea level rise, but from subsidence of interior levees, roads and ridges.  In order to maintain the current depth and duration of flooding 50 years in the future, the system would be closed approximately 78 days per year.  This includes the entire system, i.e. all floodgates and culverts.  Salinity or environmental considerations require additional days of closure. In 1999 and 2000, gate closures for salinity intrusion and water supply would have exceeded 100 days.

In the project base year, the HNC Lock Complex (the HNC lock and the Bayou Grand Caillou floodgate) would have to be operated due to weather and water supply, based on historical data, 32 days annually on average.  These days are not expected to be continuous, and generally occur in 4-7 day stretches with similar periods between closures.  Salinity intrusion is most apparent from September through November and occasionally in March and April.  By operating a structure for salinity control, the number of days of closure is estimated to average 90 days annually.  In deriving the operational criteria for the lock complex it was determined that two factors determine whether the lock needs to be shut or not.  The first of these criteria is based on salinity at the USGS Gauge on Bayou Grand Caillou at the Dulac Bridge.  When salinity at this location exceeds 7.5 ppt, the structures need to be closed to prevent contaminating the Houma water supply.  Another situation that necessitates closure of the structure is Atchafalaya River flows less than 100,000 day-second-feet.

The closures due to low Atchafalaya River flows are caused by drought conditions in the Mississippi and Red River basins.  The Atchafalaya River conveys 30 percent of the total flow at the latitude of the Old River Control Structure (100,000 cfs in the Atchafalaya translates to a total latitude flow of approximately 335,000 cfs).  When Atchafalaya River flows decline to below this level, salinity can intrude up the HNC and affect the City of Houma’s water supply.  Since the tides are not affected by wind in this case, significant differences in water surface elevations between the inside of the system and the outside are not expected.

Another situation requiring operation of the Houma Navigation Canal Lock Complex is when salinities at the Bayou Grand Caillou at Dulac Bridge gage exceed 7.5 ppt.  This salinity is largely caused by the numerous frontal passages that produce strong south winds that predominately occur in the fall.  In addition to causing elevated salinities that would affect the Houma Water Treatment Plant the salinity adversely affects the environmental habitat. These sensitive wetlands are located adjacent to the HNC so saltwater reaches those areas well in advance of the Houma Water Treatment facility.  Virtually all instances of the first criteria would fall within these criteria as well.

Since the frequent opening and closing of a floodgate for navigational purposes would allow saltwater intrusion to continue, municipal and industrial water supply benefits were not claimed for the floodgate structure.  In addition, salinity intrusion resulting from floodgate operation provides less environmental benefit than a lock.  The resource agencies represented on the Habitat Evaluation Team are concerned that a gate could not be operated appropriately to reduce salinity intrusion.  Therefore, only the lock alternative realized the full benefit of flood protection and salinity control.

Although the lock and the floodgate provide flood protection, the 200-foot wide lock minimizes adverse impacts to navigation, and maximizes salinity intrusion prevention benefits for the environment and water supply. Therefore, the 200-foot wide lock is recommended for the Highway 57 Alternative.

Salinity Intrusion Analysis. In feasibility, field data was acquired including salinity, conductivity, and discharge at critical points around the junction of the HNC and Bayou Grand Caillou. Additional data was acquired from the United States Geologic Survey (USGS) station on the HNC at Dulac (USGS Station Number 07381328) and from the HWTP. Salinity, discharge and temperature were collected at the intersection of the HNC and Bayou Grand Caillou and stage data was collected at Cocodrie.

A recent USGS study indicates that during late summer/early autumn, high salinity water moves up the HNC into the GIWW and enters Bayou Lafourche north of the GIWW near Larose.  The study recorded salinity intrusion to the Town of Lockport. The USGS discounts the possibility that salinity intrusion comes directly up Bayou Lafourche from the Gulf of Mexico because of the floodgates at Larose and Golden Meadow.

In general, the data collection and analysis indicated that Bayou Grand Caillou upstream of the HNC serves as storage.  While salinity did fluctuate, it appears that either salt or fresh water can be trapped, and prevent salinity levels from returning to normal.  Analysis of this data also indicates that, at least on occasion, salinity levels in Bayou Grand Caillou downstream of the HNC can be greater than in the HNC downstream of Bayou Grand Caillou.

The contribution of these channels based on the cross sectional areas for each waterway is 76 percent -HNC and 24 percent -Bayou Grand Caillou.  However, based upon discharge measurements taken as part of this study, the HNC can contribute up to 97 percent of flows toward Houma, and Bayou Grand Caillou can contribute as much as 35 percent of flows toward Houma, depending upon the tidal cycle. The HNC allows a significant influx of saltwater in the study area, therefore, a gated structure in the HNC would benefit the water supply and environmental resources in the study area.

Road Ramps, Relocations and Floodgates. The project area is traversed by numerous crude oil and natural gas pipelines, and public highways.  These facilities are listed and discussed in detail in the Engineering Summary Appendix (appendix A) and in the “Preliminary Attorney’s Investigation and Report of Compensable Interest,” attached to this report as exhibit B of appendix D--Real Estate Plan.  In summary, neither the Federal or State governments own any existing rights that might be superior to the rights of these facility owners.  However, the TLCD does appear to own some existing levee rights-of-way over some small portions of the project area.  A few of the above listed facilities are located within those existing levee rights-of-way, and TLCD’s rights might be superior to the rights of facility owners.  However, until the project progresses to detailed design, it is uncertain whether the proposed levee will fall within the footprint of those levee rights-of-way.  Due to these uncertainties, those rights-of-way have been disregarded.  Therefore, the feasibility analysis concludes that the relocation of all of the facilities will be compensable.

The local sponsor is responsible for performing all relocations as a part of the project cost-share.  The Government will make a final determination of the relocations necessary for the construction, operation, or maintenance of the project after further analysis, and completion and approval of the Final Attorney’s Opinion of Compensable Interest for each of the impacted utilities and facilities.

In feasibility, several locations were identified where access through the levee will be maintained from the flood side to the protected side requiring State and parish highways and roads to be relocated over and through the hurricane protection levee.  In all cases, information was obtained from the applicable transportation agency to determine the physical characteristics of their facilities as well as traffic requirements.  Estimated costs for earthen ramp and earthen ramp/floodgate options are considered relocation costs.  Estimated costs for floodgate or floodwall/floodgate features are considered as cost shared items because the project is being modified to accommodate the highway or road.

Six roads in the Reconnaissance Alternative and five roads in the Highway 57 Alternative would require relocation.  The road relocation will be accomplished via levee ramps, highway ramps, floodwalls transitioning to vehicular floodgates, and/or some combination thereof.  The roads requiring relocation in the Reconnaissance Alternative and Highway 57 Alternative are contained in table 11.

Table 11
Reconnaissance and Highway 57 Alternatives
Roads Requiring Relocation

The vehicular gates may consist of either a swing or a miter gate system (ranging in width from 25 to 40-feet) and will be closed during flood stages.  Floodgates will be mounted by a hinge and pedestal located on the floodside and will be stored back against the adjacent floodwall when in the open position.  Provisions will be made for locking the floodgates in both the open and closed positions.  Wherever major highways or roads intercept the proposed levees and prove highway ramps most feasible, the existing road will be removed and reconstructed with a ramp over the earthen levee.  One or two lane detours will be utilized during construction to maintain normal operation.

Galvanized steel ladders may be provided at each gate location to allow personnel closing the gates to have access to either side of the protection. At each crossing three options were considered:

Option 1.  Transition from the levee embankment to floodwall and then from floodwall to floodgate at the roadway crossing.

Option 2.  Construct a highway ramp from natural ground to the design elevation of protection at the levee crossing.

Option 3.  Construct a highway ramp from natural ground to the minimum floodwall elevation necessary to transition from the levee crown to the floodgate.  A floodgate will then be located at the intersection of the I-wall and the highway.

Tables 12 and 13 describe the recommended crossing option and floodgate types for the Reconnaissance and Highway 57 Alignments.

TABLE 12
Crossing and Floodgate Recommendations
Reconnaissance Alignment

The location of affected oil and gas pipelines in the study area was obtained from the 1990 Louisiana Parish Pipeline and Industrial Atlas Maps of Terrebonne and Lafourche Parishes, field reconnaissance, and data provided by the facility owners.  Three methods of relocating pipelines were investigated: (1) above ground installation on the levee surface covered by additional fill; (2) permanent pipeline bridge supported by pile-founded piers; and (3) directional drilling.  Substantial subsidence is anticipated for the proposed levee throughout the project life.  Multiple levee lifts required for future elevations would require multiple pipeline relocation.  A permanent pipeline bridge over the levee would impede levee maintenance and construction of future levee lifts.  Therefore, the selected method of relocation for all pipelines is directional drilling prior to construction of the levee. Tables 14 and 15 identify required pipeline relocations for the Reconnaissance and Highway 57 Alignments.

TABLE 14
Reconnaissance Alignment
Pipeline Relocations

Relocation of pipelines due to floatation of floodgates was also considered.  The proposed float-in sector gates would require a flotation channel measuring approximately –10.0 feet NGVD with a 50 to 60 foot width and 1 on 2 side slope.  The proposed dredging may affect various pipelines crossing the bayous and navigable waterways in the study area.  Using survey data, USGS quads, and nautical charts the shortest routes were plotted to each floodgate location from the Gulf of Mexico.  Directional drilling is proposed for relocation of the existing pipelines under the waterways.  In the detailed design phase, additional surveys would be required to verify pipeline locations, and optimize the proposed flotation access routes.  Table 16 summarizes the number of pipelines that require relocation for flotation access.

Table 16
Pipeline Relocation for Flotation Access

* Two pipelines cross the channel twice and require two relocations.

Pump Station Discharge Pipes. Several existing pump stations will be incorporated into either of the proposed hurricane protection levees and the discharge pipes will need upgrades to withstand a hurricane surge.  The pump station discharge pipes will be relocated through the levee using sheet piling, I-walls, or T-walls with seepage collars for each of the discharge pipes.  A total of 10 pump stations are included in the Reconnaissance Alternative and seven pump stations are included in the Highway 57 Alternative (table 17).  No reverse flow protection was developed for the discharge pipes, but should be considered during preparation of plans and specifications.

TABLE 17
Existing Pump Stations

Plan Refinement.  Planning is a dynamic process requiring refinement and re-evaluation throughout the course of the study. In this study three areas were re-evaluated for incremental justification and subsequently removed from the levee alignments under consideration.

Isle De Jean Charles Alignment. Isle de Jean Charles (the Island) contains approximately 100 residential structures.  The Corps investigated two options for the island: (1) provide 100-year flood protection and (2) relocate all residents in accordance with the Uniform Relocation Act (URA).  The average annual benefits associated with flood protection for the Island are estimated to be $900,000, indicating that a project cost of approximately $13 million could be supported.  Due to the isolated location of the island, the estimated construction cost for a 100-year flood protection levee is $190 million; therefore, it is not economically justified.  Relocation of all residents and facilities is estimated to cost approximately $8.0 million.  With a benefit-to-cost ratio of 1.61, relocation is economically justified, but is unacceptable. Chief Albert Naquin’s letter dated December 3, 1999 expresses his displeasure with Isle de Jean Charles being denied the protection of a levee (exhibit 2).  In lieu of relocation, the TLCD opted to construct an earthen levee to elevation +7.0 feet NGVD, and this area was removed from further consideration. The Levee District’s permit request to construct a levee is currently in review at the Corps.

Lower Bayou du Large Alignment.  The feasibility study structure inventory showed that the lower Bayou du Large area contains approximately 250 structures between the existing floodgate structure and the area south of the Falgout Canal.  The proposed improvement would involve construction of approximately 13 miles of earthen levees and one 56-foot wide floodgate at the southern end of the alignment in Bayou du Large. The TLCD constructed a smaller barge-type floodgate at the lower end of Bayou du Large and a levee at elevation +7.0 NGVD along the eastern wetland interface parallel to the bayou. The feasibility analysis determined that the $60 million cost to construct a hurricane protection levee to protect the lower Bayou du Large segment had a benefit to cost ratio of 0.34. Therefore the segment was not incrementally justified and was removed from further consideration. The Levee District has a Corps permit to construct a levee along the western edge of the du Large community.  They are currently constructing that flood protection system at elevation +7.0 feet NGVD.

Harry Bourg Corporation Alignment.  The Harry Bourg Corporation (HBC) is actively involved in oil and gas exploration in the study area and submitted a proposal through the Tidewater District for consideration.  The HBC proposal involves constructing a levee directly from Bayou du Large to the HNC. The initial Highway 57 Alignment involved constructing levees along the eastern bank of Bayou du Large, the southern bank of the Falgout Canal and the western bank of the HNC.  Since the HBC’s proposed modification would not result in additional benefits (there are no additional structures being protected), the proposal was analyzed based on construction costs and environmental impacts.  The construction cost savings associated with the HBC alternative were estimated to be $3,000,000. The savings are minimal and will not make the benefit-to-cost ratio greater than one for the combined lower Bayou du Large-Harry Bourg Corporation area.  Therefore, it was eliminated from further consideration along with the Bayou du Large segment.

Habitat Evaluation Team (HET) Meetings. In 1995, the Corps assembled an interagency team that became known as the HET.  The purpose of the HET was to examine alternatives, provide recommendations, determine habitat impacts, and develop mitigation related to the alternatives formulated for hurricane protection.  The Corps invited members from the FWS, National Marine Fisheries Service (NMFS), Natural Resources Conservation Service (NRCS), Louisiana Department of Wildlife and Fisheries (LDWF), Louisiana Department of Natural Resources (LDNR), Louisiana Department of Transportation and Development (DOTD), the Terrebonne Levee and Conservation District (formerly South Terrebonne Tidewater Management and Conservation District, STTMCD), Louisiana Department of Environmental Quality (DEQ), and the Environmental Protection Agency (EPA).  All agencies except the EPA and DEQ elected to participate in the analyses.  DOTD was represented by the TLCD.  The HET first met on December 13, 1995, to plan the type and direction of habitat analyses to be conducted. Approximately 35 HET meetings were held between 1995 and 1999. The team met throughout the study as necessary and completed its analyses in November 1999.

Environmental Impact Assessment. To document and assess habitat conditions with and without project, the Wetlands Value Assessment (WVA) technique was used by the HET.  See the DEIS - appendix E for details concerning this methodology.  Details of the model results are also provided in DEIS appendix.  A summary of the results of that evaluation is provided in the following text.

The study area was first divided into sub-basins and then subareas within the sub-basins (see DEIS).  Some of the subareas were eventually recombined because they were determined to be similar. The analysis showed that overall impacts from either of the action alternatives would have a positive effect on coastal wetlands in the study area.  This was due to gains in intermediate marsh habitat units.  However, fresh, brackish, and saline marsh would be adversely impacted.  The NMFS and USFWS opposed substituting gains in intermediate marsh for losses in saline, brackish, or fresh marsh because of functional differences between the marsh types. The Corps agreed with the NMFS and USFWS assessment.

Highway 57 Alignment Impacts.  The direct impacts to wetlands, including borrow areas and levee footprint, total an estimated 4,112 acres.  The project would impact about 3,743 acres or 1.4 percent of the approximately 271,551 acres of baseline emergent marsh in the study area. Over the project life, the annual loss ratio of marsh would be decreased slightly because of a reduction in storm and salinity damages to wetlands enclosed by the levee system.  At target year 50, the vegetated marsh acreage lost with this alternative would be approximately 1,422 acres more than with No Action or an increase of 0.8 percent.  The overall Average Annual Habitat Unit (AAHU) would increase during the 50-year analysis period by 837 units.  The discrepancy between acreage of vegetated marsh and AAHU is explained by improvements in habitat quality in remaining wetlands.  Remaining intermediate marsh and swamp would have improved AAHU, but fresh, brackish, and saline marsh would never completely overcome the initial impacts in the 50-year analysis period. It should be noted that the amount of impacts could decrease if this plan is refined in detailed design.

Reconnaissance Alignment Impacts. Direct impacts would be less with the Reconnaissance Alternative compared to the Highway 57 Alternative since it leaves out protection for two of the bayou ridges (du Large and Grand Caillou). The direct impacts to marsh, (including borrow areas and levee footprint), total an estimated 1,332 acres or a 0.5 percent reduction of the approximately 271,551 acres of emergent marsh in the study area at the start of the analysis period.  A summary of the vegetated wetlands impacts is contained in the DEIS.  Over the project life, the annual loss ratio of marsh enclosed by the levees system would be decreased slightly, but less than with the Highway 57 Alternative because of the reduction in storm and salinity damages.  At target year 50, the marsh acreage lost with this alternative would be approximately 968 acres less than with No Action or an increase of 0.55 percent.  The overall AAHU would increase during the 50-year analysis period by 297 units.  The discrepancy between acreage of vegetated marsh and AAHU is explained again by improvements in habitat quality of the enclosed wetlands.  Remaining intermediate marsh and swamp would have improved AAHU, but fresh, brackish, and saline marsh acreage would never completely overcome the initial impacts over the 50-year period.

Cultural Resources. In feasibility, a 1,000-acre cultural resource sample survey of the proposed project right-of-way was conducted. The sample survey tested a model of cultural resource site occurrence that was based upon the regions unique geomorphology. The project right-of-way was divided into low and high potential areas for the survey. Five potentially significant cultural resource sites were identified within the project right-of-way of both project alternatives. A cultural resource survey of these high potential areas would be accomplished through shovel, auger tests, backhoe trenches and/or by metal probes.  The goal of this testing will be to locate buried cultural resource sites, and once located, determine their horizontal and vertical dimensions, and if possible, their cultural affiliation, integrity, and eligibility for nomination to the National Register of Historic Places. Additional significant prehistoric and historic cultural resource sites may be found within the unsurveyed portions of the project area.  The Louisiana State Historic Preservation Officer concurred with the model of cultural resource site occurrence and future recommendation in a letter dated July 19,2000. Dredging activities associated with lock and floodgate construction could result in the disturbance of the significant historic watercraft along Bayou Terrebonne, du Large, and Grand Caillou. Therefore, natural bayous and canals that require dredging would be surveyed for underwater cultural resources.

Highway 57 Alignment Impacts. Approximately 35 percent of the Highway 57 Alignment right-of-way traverses areas that have a low potential for the presence of cultural resources.  Cultural resource surveys are not needed in these areas. Sixty five percent of the project right-of-way is located in areas that have a high potential for the presence of cultural remains. Twenty nine percent of the high potential areas have been previously surveyed.  The remaining 36 percent (2,717 acres) need to be surveyed prior to construction. The modified Highway 57 alignment (without lower Bayou du Large) reduced cultural resource impacts.  The number of high potential acres that require further surveying was reduced by approximately 751 acres.

Fifteen previously recorded cultural resource sites are located within the Highway 57 Alignment project right-of-way.  Ten of the these cultural resource sites are not significant due to past disturbance, lack of cultural evidence and/or they have no research potential. Three sites, 16TR160, 16TR71, and 16TR304, need additional testing to determine their significance and two sites, 16TR19 and 16TR33, are significant National Register eligible prehistoric Indian mounds. Sites 16TR19 and 16TR33 have cultural materials ranging in time from A.D.700 to A.D.1500. Both sites are close to the centerline of the proposed levee, but may be avoided with a slight modification to the levee alignment. Placing the sites inside the levee would help preserve them by reducing storm surge and erosion. If the sites cannot be avoided, then expensive data recovery efforts would be needed to mitigate the construction impacts.  Additionally, prehistoric human remains would likely be encountered during the excavation process.  Under the provisions of the Native American Graves Protection and Repatriation Act of 1990, the Chitimacha tribe would be directly involved in the development and approval of the final mitigation plan. The Corps would investigate the cost effectiveness of modifying the levee alignment to avoid the sites, or include them within the protection levee, in the next phase of the project.

Reconnaissance Alignment Impacts. Approximately 36 percent of the Reconnaissance Alignment right-of-way traverse areas that have a low potential for the presence of cultural resources, therefore, additional surveys are not needed in these areas. Sixty four percent of the Reconnaissance Alignment right-of-way is located in areas that have a high potential for the presence of cultural remains. Forty two percent of these high potential areas have been previously surveyed.  The remaining 58 percent (1,293 acres) would require further surveying.

Four previously recorded cultural resource sites are located within the proposed Reconnaissance Alignment right-of-way.  Three of the these cultural resource sites are not significant due to past disturbance, lack of cultural evidence and/or they have no research potential. One site, 16TR33 is a significant National Register eligible prehistoric Indian mound. Cultural materials ranging in time from A.D.700 to A.D.1500 have been recovered from this site.  The site is located close to the centerline of the proposed levee and can be avoided by a slight modification to the levee alignment.  Placement of this site inside of the levee would help preserve it by reducing storm surge and erosion. The Corps would investigate the cost effectiveness of modifying the levee alignment to avoid the sites, or include them within the protection levee, in the next phase of the project.

Operation, Maintenance, Repair, Replacement and Rehabilitation (OMRR&R) Costs. OMRR&R costs are based on January 2000 price levels. The cost includes mowing levees nine times a year, cleaning and greasing sector gate sills, spot painting, and minor maintenance, as needed.  Each of the sector gates, including the lock, and all sluice gates would be dewatered and refurbished every 10-12 years.

Highway 57 Alignment.  Annual operation and maintenance costs for the GIWW floodgates and the Houma Navigation Canal lock are estimated to be $900,000; and annual operation and maintenance costs for the remaining project features are estimated to be $847,000.

Reconnaissance Alignment. Annual operation and maintenance costs for the GIWW floodgates and the Houma Navigation Canal lock are estimated to be $770,000; and annual operation and maintenance costs for the remaining project features are estimated to be $475,000.

For both alignments, additional staff gages, using a common benchmark are recommended at each of the proposed floodgate and culvert locations to insure proper operation.  Floodgates would be closed in event of a hurricane or severe storm. Several days prior to a severe storm event, monitoring gages may indicate that the outside stage will remain at or above +3.0 feet NGVD for a length of time. Therefore, some floodgates may be operated more frequently to prevent flooding from small tidal events.

The HNC lock and the Bayou Grand Caillou floodgate will be operated in a manner similar to the other flood control structures during storm events, however they serve a dual purpose protecting the study area from storm surge and salinity intrusion.  The most significant predictor of elevated salinity was the USGS monitoring station along the HNC at Dulac and flow on the Atchafalaya River at Simmesport.  For municipal water supply, it is recommended that the HNC lock and Bayou Grand Caillou floodgate be closed if one or both of the following criteria is exceeded:

1. salinity level at the USGS station in Dulac exceeds 7.5 ppt;

2. Atchafalaya River flows drop below 100,000 day-second-feet. 

These operating criteria would have prevented all but 5 of the 111 exceedances of the 250-parts per million (ppm) chloride standard at the Houma Water Treatment Plant from October 1995 through November 1998. In order to capture the full environmental benefit, the salinity levels at the USGS station at Dulac should not exceed 3.8 ppt.  Optimization of the HNC lock and Bayou Grand Caillou floodgate operation will be performed in the detailed design phase.

Real Estate. The real estate acquisition cost for the Highway 57 Alignment is estimated to be $9,450,000 and for the Reconnaissance Alignment is estimated to be $7,000,000.

Highway 57 Alignment.  This alignment would require 1,263 acres of perpetual levee right-of-way, 1,415 acres of borrow area, 292 acres of fee-owned land for the structures and lock, and 433 acres of temporary construction easement.  The project would involve approximately 485 ownerships.

Reconnaissance Alignment. A total of 887 acres of perpetual levee right-of-way, 992 acres of borrow area easement, 33 acres of fee-owned land for the structures and lock, and 317 acres of temporary construction easement would be required.  The project would involve approximately 415 ownerships.

For both alignments, additional real estate acquisition would be required for mitigation (appendix D).

Economic Analysis-Inundation Reduction. Flood protection projects typically provide benefits by reducing actual or potential damage due to flooding of residential and commercial structures, loss of the contents in those structures, and damage to privately owned automobiles.  However, both levee systems will provide additional benefits such as municipal and industrial water supply, agricultural, environmental, emergency cost reduction, and flood insurance benefits.  A detailed description of each benefit category is provided in appendix B of this report.

The plans were evaluated by comparing estimated equivalent annual benefits to estimated equivalent annual project costs.  Benefits were converted to average annual values by use of the current Federal discount rate of 6-5/8 percent and a project life of 50 years.  The project base year (the year in which significant benefits will accrue as a result of project construction) for the 85-year level of protection is the year 2008, and for the 100-year level of protection is the year 2011.

In order to simplify the analysis of such a large geographic region, the study area was divided into 132 reaches (plate 10).  Based on the hydraulic modeling results, 116 of the 132 reaches were inventoried through windshield surveys to identify the number and value of residential and commercial structures that experienced flooding.  Based on the structural data collected, the Marshall and Swift Valuation Service (M&S) was used to calculate an average depreciated replacement cost for residential structures in the study area.  The elevation of the structures above natural ground was obtained through survey of 1,200 randomly selected structures located in the study area. The elevations were entered into a Geographic Information System (GIS) and used to verify the data collected from the windshield survey. The square footage of each residential structure was based on its footprint in the GIS system, which was adjusted downward by 5 percent in order to reflect only the living area and not the overhang of the structure.  The commercial structure values were based upon the structural information collected during windshield surveys. The value of the land for either structure type was not included in the analysis.  Appendix B shows the existing condition inventory of all residential and non-residential structures by reach.

Based on statistics obtained from the Louisiana Motor Vehicle Division and from the 1990 Census, the average residential property owner in Louisiana owns two automobiles. In a flood event, it was assumed that one vehicle per household will be used for evacuation and the other vehicle will remain parked at the residence. Thus, any flood damages were calculated assuming one vehicle per household and an average value of $10,750 per vehicle.  The contents for residential and commercial structures were determined based on limited field surveys and the experience of a panel of building and insurance experts.  The values of the contents of each structure category were totaled and then compared to the total value of a structure in order to develop contents-to-structure value (CSVR) relationships.  Inundation reduction benefits on both existing and future development were considered for project justification.

Economic Analysis – Safe Harbor. Marine vessels in the study area are typically stored in a sheltered location in Terrebonne Parish or further inland when a storm approaches. As of 1998, between 1,344 and 2,035 boats would benefit from the hurricane project, depending upon the time of year in which the storm takes place. During commercial fishing seasons (June through September) the number of boats in the study area is high, but during the winter months the numbers are lower.  An average of 1,689 vessels is assumed present at any given time.  Approximately 54 percent of the boats in the area are commercial fishing vessels, while 24 percent are large recreational vessels. The average net depreciated replacement cost of recreational vessels was $61,000, based on a useful life of 30 years and an average age of 16.5 years.  The average net depreciated replacement cost of commercial fishing vessels was estimated to be $132,000, while passenger boats, oil and gas crew boats, and houseboats were estimated to have an average net depreciated replacement cost of $150,000.

Highway 57 Alignment.  This alignment includes the major bayous in the study area (Bayou du Large, Bayou Grand Caillou, Bayou Petit Caillou, Bayou Terrebonne, Bayou Isle de Jean Charles, and Bayou Pointe au Chien) and would protect boats from storm surge associated with tropical storms and hurricanes.  The reduction in damages to large vessels and a reduction in the costs associated with moving and storing boats further inland are considered benefits attributable to the Project.

Three sets of interviews were conducted in 1998 to determine what actions boat owners currently take to minimize the potential damage to their vessels in advance of approaching storms. The study determined that with the Highway 57 Alignment in place, about 95 percent of boat owners would seek shelter for their vessels at a location that would be protected by a flood protection system.

Reconnaissance Alignment.  The results of the 1998 interviews indicated that the Reconnaissance Alignment would also provide a significant benefit to marine vessels. However, added transportation cost decrease benefits for the Reconnaissance Alignment because of the distance required for vessels to reach safety behind the more northerly levee alignment.  Transportation costs also increase with a floodgate as oil and gas supply vessels will tend to seek shelter out of the study area for fear of being trapped behind a closed floodgate for any length of time following a storm event.

Non-Structural Alternatives.  The flood control problems of the entire study area could not be solved through non-structural means.  However, portions of the study area may receive economic benefit from structure raising or relocation.

A buyout/relocation option and a structure raising option were analyzed for that portion of the study area located south of the GIWW and the city of Houma.  Approximately 5,949 residential structures would be raised above the 100-year stage or relocated out of the 100-year floodplain.  In the structure-raising scenario, nearly 50 percent of the homes would be raised, 6-feet or greater than their existing first floor elevation.  The expected annual cost of this alternative is estimated to be $46,715,109 with a benefit to cost ratio of 0.32.  The cost of purchasing the structures and relocating the residents south of Houma is estimated to be $880,111,000 with a benefit to cost ratio of 0.88.  Neither alternative is economically justified therefore; they were eliminated from further consideration.

A buyout/relocation option was also considered for residents and facilities in the Isle de Jean Charles community. The cost of this alternative is estimated to be $8,000,000 with a benefit to cost ratio of 1.61.

COMPARISON OF ALTERNATIVE PLANS

The difference between average annual benefits and average annual cost represents the net economic benefit. The Federal objective in water resources planning is achieved by maximizing net benefits in plans that are consistent with protecting the nation’s environmental resources.

The number of structures flooded and the damages under the existing condition without project and the with-project 85-year and 100-year levee heights are the same for a 500-year flood event.  The number of residential and commercial structures flooded under existing conditions for the 500-year storm event is shown in table 12 of appendix B.  For this event, 16,676 residential structures, including one automobile per residential structure, and 688 commercial structures will receive flood damage under existing conditions.  The structural damages under for the 500-year flood event total approximately $1.25 billion under without project existing conditions.  The existing condition benefit-to cost-ratio is shown in table 43 of appendix B.

The Corps analyzed the 50-year level of protection for the Highway 57 Alignment.  The 50-year level of protection is approximately 1-ft lower than the 85-year level of protection.  Using the stage-frequency curves available for each of the hydraulic subareas and interpolating where necessary, the Corps developed the with-project flood stages for the 50-year level of protection.  Levee design for this lower level of protection was based on elevation requirements provided from the hydraulic analysis. The 50-year future condition is nearly identical to the 85-year base condition levee, therefore the material quantities are very similar and the cost differential between the two levels of protection is minimal.  Eight floodgate structures in the hurricane alignment would be lowered by approximately 1-ft at the 50-year level of protection compared to the 85-year level of protection.  This lowering is expected to reduce the quantities by 3 percent to 5 percent.  No new design was performed in this analysis of the 50-year plan, however cost estimates for the eight structures were reduced by 5 percent to account for the 1-ft difference in elevation.  Similar relocations would be accomplished for the 50-year levees as for the 85-year levees.  Geotechnical conditions for the 85-year base conditions were also used for the 50-year level of protection.  The cost estimate for the 50-year design was based on January 2000 price levels to maintain consistency with the DFR.

Stage-damage calculations were performed for the 50-year level using the HEC-FDA risk based program.  A Federal discount rate of 6-5/8 was used in the economic analysis, consistent with the DFR.  The safe harbor, agricultural, water supply, and FIA benefit categories were recomputed for the 50-year analysis wherever necessary.  Table 18 compares benefits and costs for the 100-, 85-, and 50- year level of protection for the Highway 57 Levee Alignment.

The 100-year level of protection for the Highway 57 Alignment is the NED plan with a benefit to cost ratio of 1.43 to 1 and equivalent annual net benefits estimated to be $23,300,603.

Tables 19A and 19B summarize the equivalent annual mean (risk-based) benefit for the alternatives presented.  Risk-based analysis was used to calculate the benefits for the following categories: structures and contents, other benefits, protection of large recreational and commercial boat fleets and agricultural crops.  Other benefit categories include emergency cost savings from Parish cleanup, evacuation, and subsistence costs, reoccupation of homes, FIA and commercial building cleanup.  Table 19C lists the unquantifiable effects on environmental quality (EQ) due to gains in AAHU in intermediate marshes and swamps due to the alternatives.  Tables 20A and 20B summarizes the mean equivalent annual project costs and benefits. The base year for the project is 2008 and the current Federal discount rate is 6-5/8 percent.

Table 18
SUMMARY OF MEAN EQUIVALENT ANNUAL COSTS AND BENEFITS
(2000 PRICE LEVEL, 6 5/8 % INTEREST RATE)
HURRICANE PROTECTION ALTERNATIVES
BENEFIT-TO-COST RATIO

Table 19A
Equivalent Annual Mean Benefits Reconnaissance

Table 19B
Equivalent Annual Mean Benefits-Highway 57 (with lock and with floodgate)

Table 19C
Significant Environmental Quality Effects

Table 20A
Reconnaissance Alignment
Summary of Mean Equivalent Costs and Benefits

¹Illustrated in Table 19B
²Illustrated in Table 45

Table 20B
Highway 57 Alignment
Summary of Mean Equivalent Costs and Benefits

¹ Illustrated in table 19B
² Illustrated in table 44

PLAN SELECTION

The plan with the greatest net economic benefit is the Highway 57 Alignment with a 100-year level of protection and a lock structure on the HNC.

Recommended Plan.  After a thorough evaluation of the various hurricane protection alternatives, the Highway 57 Alignment providing 100-year level of protection was selected as the Recommended Plan.  This decision is based on a comprehensive review of economic, social, hydrologic, environmental, and public interest factors relating to the project.  The 100-year level of protection is consistent with the existing Federal hurricane protection in the adjacent areas along the Louisiana coast, including the Larose to Golden Meadow Hurricane Protection Project.  The residents of the communities within the study area expect equal hurricane protection with other nearby areas.  Without this consistency, the required evacuation of only the residents in the study area would become ineffective.  Residents may not have a clear understanding as to the degree of protection provided by the project, thus endangering their lives.

Using the risk-based analysis, a benefit probability distribution curve was developed for the Recommended Plan see figures 11, 12 and 13. The Recommended Plan has a risk based benefit to cost ratio of 1.43 with net benefits of $23.3 million.  The project has greater than or equal to a 91 percent chance of having a benefit to cost ratio greater than 1.0.

Figure 11 Highway 57 Alignment With Lock, 85-Year

Figure 12  Highway 57 Alignment w/Lock, 100-Year NED Plan

Figure 13 Highway 57 Alignment w/Lock, 50-Year

The benefits for the Recommended Plan are slightly lower when traditional (non-risk based) analysis is used, but the traditional benefit to cost ratio is 1.20 with $10.7 million in equivalent annual net benefits.