Hydraulic and Sediment Continuity Modeling of the San Joaquin River from Friant Dam to Mendota Dam, California

Mussetter Engineering, Inc. (MEI) conducted a geomorphic analysis, and hydraulic and sediment transport modeling, of an approximately 60-mile long reach of the San Joaquin River between Friant Dam and Mendota Dam near Fresno California. The objectives of the study were to explore opportunities to modify the physical system to benefit riparian habitat along the reach in support of the San Joaquin River Riparian Habitat Restoration Program (SJRRHRP). The overall objectives of the SJRRHRP are to develop and implement a plan for restoration of a continuous riparian corridor along the reach, and to construct specific riparian habitat restoration projects in a reach of the river that has been significantly impacted by sand and gravel mining, altered flow regimes, elimination of upstream sediment supplies, groundwater pumping-induced subsidence and implementation of a flood-control project.

MEI analyzed the existing flood-flow operation release and distribution criteria, as well as the low-flow hydrology, and developed two alternative flood release scenarios that may provide additional benefit to riparian habitat along the reach. A detailed hydraulic and sediment continuity model of the reach was then developed to assess the effects of the modified flood release criteria. The hydraulic model is very complex, due to the presence of numerous split-flow reaches and instream sand-and-gravel ponds. Due to the necessity to accurately predict conditions over a wide range of flows, including very low discharges, the model contained over 800 cross sections, and analyzed 34 separate split-flow reaches. MEI developed a special batch file routine and a series of computer programs that are linked to the HEC-2 program to automatically balance the energy at each of the flow splits.

The study results showed that sufficient water is available in about 1 in 4 years to make higher than normal flow releases from Friant Dam during the spring when cottonwoods and other riparian seeds are being dispersed. The potential benefits of such releases included increased duration at which the gravel- and cobble-bed material in the upstream portion of the reach would be mobilized, development of improved habitat for colonization of riparian tree species on the resulting bare substrate, and increases in the amount of area within the riparian zone that would be inundated and thereby irrigated during the flood releases. Potential negative impacts of the releases include increased frequency of flooding in low-lying areas, increased stress on the flood-control levees due to seepage, increased sediment deposition in the backwater areas upstream from the Chowchilla Bypass Control Structure and Mendota Dam, and increased tendency for bank erosion in portions of the reach.

Major Project Features

• Flood Frequency Analysis (HECWRC)
• Geomorphic and Channel Stability Analysis
• Environmental Restoration
• Hydraulic Analysis (HEC-2)
• Flow Duration Analysis
• Sediment Continuity Analysis
• GIS; ARCINFO
• Flood Routing Analysis (HEC-1)