Morganza To The Gulf of Mexico Risk Reduction Project, Feasibility Study - New Orleans District

Engineering Appendix

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1.0 INTRODUCTION

The purpose of this appendix is to provide a summary of the engineering and design efforts performed during the Morganza, Louisiana to the Gulf of Mexico (Morganza) feasibility study. Additional design details and supplemental information for each project feature are on file at the New Orleans District. This appendix contains all of Volumes 1 and 2. The features are classified as follows:

Volume - 1 Hurricane Protection System Summary

Volume - 2 Hydraulics & Hydrology (H&H) Appendix

Volume - 3 Bayou du Large Floodgate

Volume - 4 Bayou Grand Caillou Floodgate

Volume - 5 Bayou Little Caillou Floodgate

Volume - 6 Bayou Point au Chien Floodgate

Volume - 7 Bayou Petit Caillou & Bayou Terrebonne Floodgate Modification

Volume - 8 Bayou Petit Caillou Floodgate Replacement

Volume - 9 Bayou Terrebonne Floodgate Replacement

Volume - 10 Box Culverts & Sluice Gates

Volume - 11 Bush Canal Floodgate

Volume - 12 Bush Canal, Bayou Little and Bayou Grand Caillou Floodgate Calculations

Volume - 13 Falgout Canal Floodgate

Volume - 14 Falgout Canal Floodgate Calculations

Volume - 15 GIWW Floodgate at Bayou Lafourche

Volume - 16 GIWW Floodgate at Houma

Volume - 17 Grand Bayou Floodgate

Volume - 18 Houma Navigation Canal Lock

Volume - 19 Houma Navigation Floodgate

Volume - 20 Humble Canal Floodgate

Volume - 21 Highway 57 Alignment Levee Design

Volume - 22 Reconnaissance Alignment Levee Design

Volume - 23 Pumping Stations Fronting Protection

Volume - 24 Road Floodgates, Ramps & Relocations

1.1 Location
The study area is located in south Louisiana. The site location map (Plate 1) defines the boundary of the study area. The proposed hurricane protection system would provide protection against flooding from hurricane and tropical events for portions of Terrebonne and Lafourche parishes.

1.2 General
The proposed hurricane protection system is comprised of multiple project features. The features include hurricane protection levees, pumping station fronting protection, road closure floodgates and ramps, channel closure floodgates, box culverts with sluice gates and a lock structure. Three levels of hurricane protection were investigated in this study, 85-year, 100-year and 500-year hurricane events. Two levee alignments were investigated in detail. These alignments were identified as the Highway 57 Alignment and the Reconnaissance Alignment. These alignments are shown on the Project Map plate on the following page. The original Highway 57 Alignment was modified to exclude areas that were not incrementally justified. The area south of the Falgout Canal on the Bayou Du Large channel was eliminated from the alignment along with other minor alignment changes. Changes are indicated on the plates by the word "(Revised)".

1.2.1 Design Assumptions & Compliance with Regulations:

1.2.1.1 Structural Assumptions
In reconnaissance, the sponsor and the Corps recognized that the feasibility cost was escalating due to the size and complexity of the study area and negotiated a lower feasibility study cost. Consequently, design efforts in this feasibility study were to the level of detail required in a pre-1986 Water Resources Development Act (WRDA) feasibility study. When conducting a feasibility study in this manner, the construction cost estimates may not be within 20 percent of the actual construction costs. The Sponsor willingly accepted the risk of having substantial delays in project implementation and higher actual construction costs. Because of the cost constraints, less field data was collected for this feasibility study than is typically gathered in post-WRDA 1986 studies. Existing data from the Parish and local consultants was used to fill in any gaps in the topographic surveys and soil borings collected by the Corps.

Therefore, this does not comply with ER 1110-2-1150, "Engineering and Design for Civil Works Projects". The analysis and results presented in this Draft were arrived at using minimal geotechnical and survey data. The risk and uncertainty associated with this decision may result in the planning, engineering, design and construction cost exceeding the 25 percent contingencies in the cost estimate for the proposed hurricane protection system.

At the request of the local cost share sponsor, Louisiana Department of Transportation and Development (DOTD), an innovative float-in concrete floodgate and a steel float-in floodgate structure designed by DOTD�s A-E consultant were provided to New Orleans District for evaluation and inclusion in Morganza. These design concepts presented by DOTD�s A-E consultant were used to the fullest extent possible in the designs accomplished by the U.S. Army Corps of Engineers, New Orleans and St. Paul Districts. Additional testing and analysis will be performed during the pre-construction engineering and design (PED) phase to determine if the proposed float-in concrete floodgate structures bare further consideration. All proposed float-in floodgates and lock structures in this report that utilize light weight concrete do no comply with current U.S. Army Corps of Engineers Regulation and Design Guidelines. The Steel float-in floodgate structures were determined to be inadequate for inclusion in a hurricane protection project. These structures were replaced with 56� sector-gated structures.

DOTD provided designs to modify the existing Bayou Petit Caillou and Bayou Terrebonne Floodgates. No geotechnical evaluation was available to determine if the existing floodgate structures could withstand the additional forces imposed on them by the proposed modification. Additional testing and analysis will be required to determine if the proposed modification to the existing floodgate structures bare further consideration in the next phase of design. Replacement floodgate structure designs and costs are presented in the Morganza study in Volumes 8 and 9.

1.2.1.2 Levee Construction MaterialsIn an effort to reduce the study cost the Initial Project Management Plan (IPMP) provided for collection of geotechnical and survey field data. The source of materials for the levees was determined using this data. It was assumed that approximately 50 percent of the materials for the levee construction would be obtained from adjacent borrow area. The risk associated with this decision is that there may be a substantial increase in the cost of materials for construction of the levees if a higher percentage of the material must be hauled in from other areas.

1.2.1.3 Water Quality AssumptionsFor the purpose of preparing an operation plan for the Houma Navigation Canal Lock Complex in the Morganza to the Gulf of Mexico Feasibility Study, it was assumed that the Houma Navigation Canal Lock and the Bayou Grand Caillou floodgate would be operated in tandem. There is no way to accurately evaluate the effects of salinity intrusion from Bayou Grand Caillou without a hydraulic model. This work will not be included in the next phase of design.

1.3 Description of Project Features
The Highway 57 Alignment contains the following features: earthen hurricane protection levees, eleven channel closure floodgates, eleven box culverts with sluice gates, pumping station fronting protection for eight pump stations, road floodgates for five levee road crossings and relocation of pipelines crossing levee alignment.

The Reconnaissance Alignment contains the following features: earthen hurricane protection levees, eight channel closure floodgates, six box culverts with sluice gates, pumping station fronting protection for twelve pumping stations, road floodgates for six levee road crossings and relocations of pipelines crossing levee alignment.

1.4 General Geology

1.4.1 Geology
The study area is located in the central portion of the Mississippi River Deltaic Plain. Dominant physiographic features consist of the abandoned Lafourche-Mississippi course (Bayou Lafourche), abandoned distributary channels and their associated natural levees, swamps, marshes, bays, lakes and tidal channels. Elevations are highest on the natural levee ridges bordering Bayou Lafourche and the major abandoned distributary channels and decrease away from the channels to near 0 feet NGVD in the marsh.

The land surface in the study area is composed of natural levee, swamp, and marsh deposits. Natural levee deposits range in thickness from 25 feet adjacent to Bayou Lafourche to less than 5 feet adjacent to minor distributary channels. A large percentage of the natural levee deposits in the study area have been buried. Natural levees generally consist of medium to stiff, oxidized, silty clay and clay with minor organics. Swamp deposits are located adjacent to natural levee deposits and are generally less than 10 feet thick. They are characterized as very soft to medium clays and silty clays containing silt strata and peat. Swamp deposits transition into fresh, brackish, and salt marshes that are generally less than 10 feet thick. Marsh deposits are generally composed of very soft organic clay and peat with high water content.

Natural levee, swamp, and marsh deposits overlie interdistributary deposits that are deposited in low areas between active distributaries during floods. Interdistributary deposits are up to 300 feet thick in the study area and are composed of very soft to soft clay and silty clay with minor shells and organics. Prodelta deposits commonly underlie interdistributary deposits. Prodelta deposits are usually less than 50 feet thick and are characterized by homogeneous, medium, fat clays with minor silt content. Beneath the prodelta deposits are substratum deposits approximately 150 feet thick, which fill the ancestral Mississippi River Valley. Substratum deposits are made up of fine sand and occasional gravel. The substratum deposits sit directly on Pleistocene deposits of unknown thickness. Pleistocene deposits are characterized as stiff to very stiff, oxidized clay and silty clay with some silt and sand lenses. Plate F1 is a geologic section from Falgout Canal, on Bayou Du Large, northeast to Larose, showing the typical depositional environments and their stratigraphic relationships.

1.4.2 Soils
Soils in the study area consist of Commerce, Mhoon, Sharkey, Swamp, and Marsh series. Commerce soils are level to nearly level, moderately well drained, slightly acid to moderately alkaline silt loam, silty clay loam, and very fine sandy loam on natural levee ridges. The slope on these soils is up to 3 percent and does not require artificial draining for agricultural use. Mhoon soils are level to nearly level, imperfectly drained, slightly acid to moderately alkaline silt loam, silty clay loam, and silty clay on natural levee ridges below the elevation of Commerce soils. The slope is up to 3 percent and these soils usually require draining for agricultural use. Sharkey soils are frequently flooded, slightly acid to moderately alkaline clay and silty clay with moderate amounts of organic material. These clays are from distributaries of the Mississippi River and the soils are adjacent to swamps. These soils can be of agricultural use when drained. Swamp soils are frequently flooded clay and silty clay with organic material forming muck and peat. These soils are landward of the fresh water marshes. Marsh soils are level, poorly drained, frequently flooded mucky clay and clay with organic material forming muck and peat.

1.4.3 Groundwater
Ground water is at or near the surface in the study area. Most potable water in the area comes from the surface waters of bayous and the Gulf Intracoastal Waterway but may require considerable treatment. Aquifers at a depth of 150 to 200 feet in the northern part of Terrebonne Parish may contain fresh water but become contaminated with salt water during periods of drought.

1.4.4 Subsidence
One of the primary causes for the increase in water levels in the Morganza to the Gulf of Mexico study area is apparent subsidence. Apparent subsidence is defined as the lowering of the land relative to the mean sea level. An alternative description is relative sea level rise. The potential exists for confusion when discussing subsidence because geologists use the term subsidence for a particular process. Apparent subsidence involves the relationship between water level and land and includes factors considered to be geologic subsidence as well as hydraulic factors such as the rise in sea level.

As part of this 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." This report summarized information on datum and subsidence for areas within the Morganza to the Gulf of Mexico study area. Historical data, public records, various publications, and other pertinent data were acquired through the assistance of the National Geodetic Survey (NGS); the New Orleans District Corps of Engineers, and through independent research of documents and files published or recorded as data bases by various sources.

From the report, subsidence rates within and adjacent to the Morganza to the Gulf of Mexico study area vary widely; documented rates include 4 cm/year, 0.3 to 0.55 cm/year, 1.9 cm/year, 1 cm/year, 2 cm/year, 1.19 cm/year, 1.25 cm/year, 1.06 cm/year, and 0.92 cm/year. The report, however, did not recommend a subsidence rate to be used for the Morganza to the Gulf of Mexico study area.

For the Morganza to the Gulf of Mexico Reconnaissance Study, the New Orleans District utilized information from the Atchafalaya River Delta Study, a study conducted by WES in the 1980s. The WES analysis showed a spatial and temporal variation in apparent subsidence for the period 1962-2030 from 0.7 cm/year in the Houma area to 1.4 cm/year at the mouth of the Lower Atchafalaya River.

The New Orleans District also analyzed water surface elevation data in the Lake Verret area for the reconnaissance study. Some of the stations experienced noticeable subsidence during the period of record, and, subsequently, periodic datum corrections were applied to the historical record. The datum corrections were incorporated back into the data so that the apparent subsidence estimates would not be erroneously biased low. The 50 percent exceedance water surface elevation for the months July through October was computed for each year of the gages� period of record. The months July through October were chosen as generally few flood events occur during this time of year.

In 1995, the Barataria-Terrebonne National Estuarine Program gathered elevation data within Barataria and Terrebonne Parishes to evaluate subsidence rates. Subsidence was also determined in the 1987 "Terrebonne Marsh Subsidence Study", performed by T. Baker Smith and Son, Inc for the New Orleans District. Subsidence rates were determined at three locations within or adjacent to the Morganza to the Gulf of Mexico study area: Houma 0.94 cm/year, Lake de Cade 3.0 cm/year, and Cocodrie, 2.4 cm/year.

Because of the wide variation of documented subsidence rates, further analysis was warranted. An analysis of Corps gage data for three stations within or adjacent to the Morganza to the Gulf of Mexico study area was performed using the same methodology as employed in the Morganza to the Gulf of Mexico Reconnaissance Study. These stations are: Gulf Intracoastal Waterway at Houma, Bayou Lafourche at Leeville, and Bayou Petit Caillou at Cocodrie. (See Plate 2-1). The 50 percent exceedance water surface elevation was plotted against time, and a linear regression line developed. Using this methodology, the following subsidence rates were determined: Houma 0.98 cm/year, Leeville 1.1 cm/year, and Cocodrie 1.46 cm/year.

Some of the subsidence data from the 1987 "Terrebonne Marsh Subsidence Study" was utilized to reset the datum of several Corps gages in the early 1990s. Subsequent GPS work and evaluation of the water surface elevation data by the Corps have demonstrated that the subsidence rates from this report may be overestimated. Questions have also arisen concerning adjustments for curvature of the earth and gravity for the 1995 Barataria-Terrebonne work. Consequently, it is likely that the subsidence rates from these studies may be suspect.

Because of the significant variation in published subsidence rates, questions about the accuracy of subsidence rates from the 1987 Terrebonne Marsh Subsidence study and the 1995 Barataria-Terrebonne National Estuarine Program work, apparent subsidence rates from gaged data for the stations Bayou Lafourche at Leeville (1.1 cm/year or 0.036 ft/year) and Bayou Petit Caillou at Cocodrie (1.46 cm/year or 0.048 ft/year) were used for this feasibility study.

1.5 HTRW Concerns
A Phase I HTRW investigation was completed in September 1997. The Phase I included record searches of local state and federal databases for environmental compliance issues, land use records, aerial photographs and limited site visits to accessible areas. Overall no major HTRW concerns were uncovered. However, the Phase I investigation did reveal a number (approximately a dozen) of areas of concern within the project corridor that should be considered during detailed design of project elements. For example the Ashland landfill has been in operation since 1963, while currently it is in compliance with state regulations, it does have a record of being out of compliance in the 1980s and project elements should be designed to avoid this area.