Engineering and the Environment
The engineering work for the purposes of this report was done to determine feasibility and an order of magnitude costs. The key issues are how to construct a Land Bridge that could carry development, and what steps would be required to create and maintain the Lake as a stable, usable and clean body of water.
Land Bridge
There are two possible approaches for construction of the Land Bridge – wet or dry construction. The latter, although a more controllable method of fill, would require extremely expensive cofferdams (cost could be approx. 2x greater than wet construction) to build. Therefore, the wet approach has been deemed the more feasible. Construction would have to begin in low water (probably late August) with the creation of rip-rap containment dams at either end of the Land Bridge. Hydraulic fill would be pumped into the annular space until the grade level reached the top of the dams. A stabilization mat and granular drainage/work course would be installed and a sheet pile cut-off wall would be driven at the center of the bridge to minimize seepage. This procedure would be repeated in approximate 10’ lifts (rip-rap at ends, hydraulic fill, drainage/work course, extension of sheet pile cut-off wall) until the Land Bridge was brought to final grade. Construction would probably take at least one year, and it could take anywhere from six months to three years before the fill material consolidates to the extent where development could begin.
Creation of a Lake
The Lake is essentially the result of the formation of the Land Bridge to Mud Island. In creating the Lake, two major design factors create the need for new or modified infrastructure: 1) maintaining a stable Lake level, and 2) elimination of the existing floodwall. By eliminating the floodwall and creating the Lake, the Gayoso Bayou detention basin could become obsolete, as the Lake would serve as a much larger detention basin. The basins could become permanent water features as a part of the Lake. The permanent water surface elevation of the Lake would be approximately 216’ (32.1 on the gage).
Stability of the Lake Due to Exfiltration / Evaporation
Most (96%) of the time, the river will be below the permanent Lake level. Water will be lost through Mud Island and the Land Bridge due to the head caused by the difference in elevations. Based on soil borings from Mud Island, it is estimated that approximately 225 gpm will be lost through exfiltration to the river. In addition approximately 275 gpm will be lost to evaporation. The total loss for the Lake would be approximately 864,000 gallons per day. This could be controlled with a relatively small pump station with two 1000 gpm pumps.
Due to Stormwater Outfall
There are two major storm water outfalls that would contribute runoff to the Lake – those currently handled by the Gayoso and Marble pumping stations. In addition, there are several smaller outfalls. When the river is below Lake level, the outfall could be handled with a sluice gate on the north end that empties into the river. When the river is above Lake level, the sluice gate would be closed, and the Lake would have to be pumped to control the level. In order to prevent flood potential in the area around the Gayoso basins, the water level would have to be maintained within an approximately 4’ range. This would require a 300,000 gpm pump station, which would handle the worst case 100-year flood. The Gayoso Pumping Station (which has a 500,000 gpm capacity) could probably be converted for this use, but it would require a very long force main, which would also be of considerable expense. The Gayoso Pumping Station, which is eligible for the historic register, could be converted to a museum if a new pump station is built on the north end of the Lake, and the existing detention basins could be converted to permanent water features.