Abstract
Flooding in California’s Central Valley is very common and expected to occur every year anywhere throughout the region. The climate and geography of the Central Valley, together, are responsible for over flow streams, rivers, and lakes causing water flooding, particularly at the lower elevation areas. San Joaquin Basin is located between the Sierra Nevada (on the east) and the Coast Ranges (on the west) where flooding is typically characterized by infrequent severe storms during winter season due to heavy rain and snow-melt runoff coming from the foothills, east of Merced County (Jesse Patchett 2012). There are many tributaries that flow from the Sierra Nevada into the San Joaquin Valley. The Merced River, located in the southern portion of California’s Central Valley, is one of the most major tributaries that is flowing into the San Joaquin Valley. Historical floods records show that whenever heavy rain and snow-melt runoff combine together and the discharge from Exchequer Dam (located under Lake McClure reservoir, at the lower part of Merced River) exceeds 4,000 cfs, a backwater occurs at the lower part of the Merced River at the confluence with San Joaquin River as a result of San Joaquin River high stages. According to J.C. Blodgett study, the backwater effect may extend as far as 8 miles upstream from the Merced River mouth causing some over-bank flooding onto the surrounding countryside. A study was conducted by Mr. J.C. Blodgett and Mr. G.L. Bertoldi on October 1968 (prepared in cooperation with the California Reclamation Board) to determine the channel capacity of the Merced River, lower part, downstream from Merced Falls Dam (Blodgett and Bertoldi, 1968). This report is prepared to investigate the water surface elevations and peak flows at the lower part of the Merced River near the confluence with San Joaquin River. The study area of this report begins from Merced Falls (near the New Exchequer Dam) to the confluence with the San Joaquin River, with a river reach length of about 55 miles. The objective of this study is to find a practical solution to increase conveyance capacity of the Merced River at the confluence with the San Joaquin River in order to prevent flooding and prevent or reduce properties damages. To achieve this goal, two engineering alternatives were investigated: 1)A flood control structure, adding a lateral structure (Weir-1 with length of 100 ft., or Weir-2 with length of 500 ft.). 2)Applying a channel modification (widening the bottom width of the last seven cross-sections by setting the channel bottom width to 100 ft., 200 ft., or different values using new cross-sections geometries). The Merced Irrigation District Hydrologic and Hydraulic Optimization Model was used for this study area (55-mile reach) simulations to investigate the effect of the above engineering alternatives (lateral structure and channel modification) on water surface elevations and peak flows at the lower part of the Merced River at the confluence with San Joaquin River. The model was created by Dewberry Company prepared for Merced Irrigation District MID (Dewberry). Three different scenarios were used for adding a lateral structure alternative (Alternative 1): Steady flow (Feb 2017 storm event), Unsteady flow-hourly data (May 2006 storm event), and Unsteady flow-daily data (June 1983). A scenario of steady flow of 6,000 cfs was used for the channel modification alternative (Alternative 2). The model simulations results at station 766.656 ft. (766.656 ft. upstream of the two rivers junction) are described below: 1) Alternative 1 (Lateral structure): A. Steady Flow (Feb 2017 storm event) The reductions of Water Surface Elevation WSE was 1.22 ft. for Weir-1 (100 ft.), 2.21 ft. for Weir-2 (500 ft.). The reduction of river flow was 839 cfs for Weir-1 (100 ft.), 1,733 cfs for Weir-2 (500 ft.). B. Unsteady Flow-Hourly (May 2006 storm event) The reductions of Water Surface Elevation WSE was 0.83 ft. for Weir-1 (100 ft.), 1.87 ft. for Weir-2 (500 ft.). The reduction of river flow was 1,060 cfs for Weir-1 (100 ft.), 1,775 cfs for Weir2 (500 ft.). C. Unsteady Flow-Daily (June 1983) The reductions of Water Surface Elevation WSE was 0.62 ft. for Weir-1 (100 ft.), 1.13 ft. for Weir-2 (500 ft.). The reduction of river flow was 951 cfs for Weir-1 (100 ft.), 1,689 cfs for Weir-2 (500 ft.). 2) Alternative 2 (Channel Modification): A. Steady Flow (6,000 cfs) A steady flow of 6,000 cfs was used in all the channel modification scenarios (channel bottom width of 100 ft. and 200 ft., and the template scenario). By using a modified channel (bottom width of 100 ft.) the reduction in Water Surface Elevation WSE was 2.87 ft., using a modified channel (bottom width for 200 ft.) the reduction in Water Surface Elevation was 4.54 ft., finally, using the template method had resulted in 5.64 ft. reduction in Water Surface Elevation WSE. In conclusion, the modifying channel scenario resulted in more reduction of WSE than the weir scenario (two to three times), and was more effective in achieving the project goal (to increase conveyance capacity of the Merced River at the confluence with the San Joaquin River). However, channel modification activities can have a variety of impacts on riverine processes, associated riparian ecology, and terrestrial environment. Channel modification can lead to draining wetlands, increasing erosion, sedimentation, and turbidity due to bed and bank instability. Widening, deepening, or relocating existing stream channels can cause reduction in aquatic habitat diversity, and degradation in water quality due to increasing water temperature. Although the channel modification alternative leads to more reductions in WSE, but because of its great adverse impact on the riverine processing and the surrounding environment, the lateral structure alternative is highly recommended.