Flood Risk
The Delta is affected by riverine flows from the Sacramento, San Joaquin, Cosumnes, and Mokelumne Rivers, along with tidal conditions from the San Francisco Bay. The Delta covers more than 700 square miles in five counties and includes nearly 60 islands surrounded by natural and man-made channels and sloughs. Because about two-thirds of the islands are below sea level, the Delta relies on a maze of levees to protect land and key infrastructure from floods and daily high tides. There are more than 1,100 miles of levees in the Delta, including many levees that were built more than a century ago to protect farmland. The entire city of Isleton lies within a 100-year flood zone, involving different standards for floor level elevation for residential, rehabilitated residential, and industrial construction. The most recent housing construction typically has involved two stories, with garages at ground level and living quarters on the second story above the 100-year intensity flood level (City of Isleton, 2000).
The city of Isleton is protected by a system of leveed channels, multipurpose reservoirs, and other structures, which comprise the Sacramento River Flood Control Project (SRFCP). The goal of the SRFCP is to reduce the chance of flooding for communities like Isleton. Even with the realization of major physical improvements to the flood management system, the risk of flooding will continue to exist within the city. The Brannan-Andrus Levee Maintenance District (BALMD) and Reclamation District 556 (RD 556) system of levees provide flood protection for the city of Isleton. Isleton is located along the left bank of the Sacramento River approximately 3.12 miles upstream of its confluence with Steamboat Slough. Isleton sits at an approximate elevation of +5 feet. BALMD staff monitors and maintains the levees on the island and maintains and controls the operations of the seven pumping stations to keep the island dry.
Brannan-Andrus Island has a population of approximately 1,837, which is made up of a major recreation contingent and the city of Isleton. The maintenance of the levee system is critical to maintain the economy supported by over 10,856 acres of prime agricultural land and 1,500 residents. District levees protect about 1,500 residents, 613 residential structures, 33 gas wells, and approximately 9,088 acres of gas fields (Sacramento County and GEI Consulting Engineers, 2017).
Along RD 556, the flood protection facilities include the Sacramento River Unit 2, Segment 390 and Georgiana Slough Unit 1, and Segment 129 (see Figure 10). BALMD SPFC facilities include Sacramento River Unit 2, Segment 378, Georgiana Slough Unit 1, Segment 40, Mokelumne River Unit 3, Segment 1050, San Joaquin River Unit 4, Segment 1049, and Sevenmile Slough Unit 5, 1048 (see Figure 11). The Sacramento River and Georgiana Slough are project levees, and the Mokelumne and San Joaquin Rivers and Sevenmile Slough are non-project levees. These levee systems primarily protect the small communities of Isleton and the Delta Loop recreational area, in addition to other rural agricultural areas. Oxbow Marina is accessed by the District’s levee system. In the event of a flood, the area would be disconnected from services and the area’s population would be forced to evacuate.
Isleton and the Delta Loop Recreational Area are protected by a levee system that is bordered by the Sacramento River, Georgiana Slough, Mokelumne River, San Joaquin River, and Sevenmile Slough. Oxbow Marina is outside of the flood protection system, but its access is protected by the system.
There are approximately 26.2 miles of levee that surround Brannan-Andrus Levee Maintenance District with 16.2 miles of project levees and 10 miles of non-project levees. In addition, BALMD is separated from Upper Andrus Island (RD 556) by a 0.46- mile cross levee that is owned by RD 556. RD 556 is surrounded by 11.2 miles of project levees. In the event of a levee failure and flood on RD 556, the cross levee could likely overtop and flood BALMD and the small communities of Isleton, Oxbow Marina, and the Delta Loop Recreational Area. Therefore, an evaluation is needed of the entire levee system surrounding the two districts as well as the cross levee (Sacramento County and GEI Consulting Engineers, 2017).
Most of the levees surrounding the districts do not meet the FEMA 100-year levee standard. In addition, there are several areas with Bulletin 192-82 (agricultural levee design standards) geometry deficiencies. As part of the Non-Urban Levee Evaluation (NULE) investigations, DWR analyzed the deficiencies currently (2017) associated with passing the 1957 design flow. The investigations concluded that some segments of the levees protecting Isleton, Oxbow Marina, and the Delta Loop Recreation Area suffer from under-seepage, through-seepage, landside stability, and geometry deficiencies (see Figure 12).
The County, BALMD, and RD 556 have been active participants in DWR’s Delta Flood Emergency Preparedness, Response, and Recovery Program, including activities associated with updating the Reclamation District’s Flood Emergency and Safety Plans (ESPs), and Multi-Hazard Mitigation Plan (MHMP) for each of the County’s RDs/ Levee Maintaining Agencies (LMAs) located in the Delta. The ESPs and MHMPs for each of the RDs/LMAs in the Sacramento-San Joaquin Delta have been prepared and updated as non-structural measures to improve flood emergency preparedness, response, and recovery actions for each of the communities and adjoining agricultural areas. Simulated levee breaches of the State Plan of Flood Control (SPFC) levee sections indicating timing and potential deep flooding conditions have been developed for each of the RDs/LMAs inclusive of all the small communities and adjoining agricultural areas in the Delta, including the Isleton region.
The BALMD ESP provides mapping to show the projected flood depth and flood timing if a levee protecting BALMD were to fail. There were three hypothetical levee failure “breach” model scenarios provided for BALMD. The first scenario model was on the left bank of the Sacramento River across from Rio Vista. Tidal conditions as well as flows from the Yolo Bypass greatly impact the results of the levee failure in this location. The second scenario model was on the right bank of the Mokelumne River downstream of the confluence with Georgiana Slough. The third scenario model was on the right bank of the Sacramento River upstream of the city of Isleton and downstream of the cross levee. The criteria to approximate the hypothetical levee failure opening in the hydraulic model was developed by the DWR and USACE (Sacramento County and GEI Consulting Engineers, 2017).
Maps in Figure 13, Figure 14, and Figure 15 provide the estimated depth of flooding (in feet) in assuming a levee fails and no relief cuts were placed in the levee. The results indicate that a levee failure located on the Sacramento River could produce enough volume to fill the basin close to the effective Federal Emergency Management Agency (FEMA) elevation. The lowest ground elevation in the basin is approximately El. -22.0 feet and the lowest top of levee along the San Joaquin is approximately El. 11.0 feet (Sacramento County and GEI Consulting Engineers, 2017).