WSP04601 - Laserfiche WebLink

Home Browse Search

. . ..-f 00 N N c.- O DRAFT LC-732 February 28, 1984 LAS VEGAS WASH UNIT CRBSCP, NEVADA GROUND-WATER FLOW REDUCTION GENERAL DESCRIPTION OF PROJECT FEATURES Ground-water flow reduction is to be accomplished by the placement of a system of underground walls extending from bedrock to a few feet above the ground surface. These walls are typically formed with a slurry mixture which fills an excavated trench. The relatively impermeable underground slurry walls would be placed along the south side of the Wash to block tributary ground-water inflow to the Wash. Although the slurry walls would reduce subsurface ground water flow, triubutary inflow would continue as less dense brackish surface water. Detention basins would help retard the tributary brackish surface water as it flows parallel to, but separated from, the sewage effluent flow on the north side of the Wash. High flows of fresh storm runoff filling any basin would spill across to the north side of the Wash. Thus, the detention basins would not be managed like evaporation ponds to completely contain all inflow, but only serve to help attentuate fluctuations in tributary inflow. The evapotranspiration capacity of the existing vegetated areas within the proposed detention basins is expected to exceed the annual average tributary inflow. However, the flushing action of intermittant storm runoff is expected to continue to maintain near surface salinity levels tolerable to the vegetation. In effect, minimal changes in existing vegetated or inundated land area is expected within the detention basin system. In some areas more salt tolerant plants may replace existing cattails. The system would be conducive to management (if desired) for a diversity of vegetative communities. Existing wetlands on the north side of the Wash would be unprotected by the detention basin system and subject to drainage by natural erosive processes. A set of two surge ponds would be formed by floodplain excavation, dikes, and slurry walls to provide surge capacity for attenuation of peak storm runoff. The normal water surface of 14.5 acres for both ponds is expected to be maintained by effluent from the city and county sewage treatment plants. Above Pabco Road, approximately 55 acres of floodplain currently inundated by sewage treatment plant effluent would be seperated from the effluent flow by the slurry wall system and utilized for ground-water detention. Approximately 10 miles of slurry wall may be built in the next 3 years. The salinity control effectiveness of the system would be monitered as construction proceeds to help estimate the potential effectiveness of further construction.