1 12.0 Hydrology - Flood Management 2 This chapter describes the environmental and regulatory settings for flood management 3 and environmental consequences and mitigation, which could potentially be affected by 4 implementation of Project alternatives. 5 12.1 Environmental Setting 6 The environmental setting for flood management includes a discussion of flood 7 protection history in the San Joaquin River basin, flood management structures, and flood 8 management operations and conditions. Much of the information presented in this section 9 was obtained from the Upper San Joaquin River Basin Storage Investigation Initial 10 Alternatives Report Information Report, Flood Damage Reduction Technical Appendix 11 (U.S. Department of the Interior, Bureau of Reclamation [Reclamation] and California 12 Department of Water Resources [DWR] 2005) and is summarized below. 13 12.1.1 Historical Perspective of Flood Protection in the San Joaquin 14 River Basin 15 Historically, the San Joaquin River had insufficient capacity to carry heavy winter and 16 spring flows generated by precipitation and/or snowmelt within its channel banks. Once 17 flows exceeded channel capacities, the channels overflowed onto the surrounding 18 countryside, forming vast floodplains. Velocities in overbank areas were greatly reduced 19 from velocities in the channels reducing the sediment-carrying capacity of the water 20 allowing material naturally eroded from mountain and foothill areas to drop out of 21 suspension. In this way, over many years, the San Joaquin River built up its bed and 22 formed natural levees composed of heavier, coarser material carried by flood flows. Finer 23 material stayed in suspension much longer and dropped out when overflow water ponded 24 in basins that developed east and west of the river. The higher elevation land formed by 25 the natural levees attracted the first settlements in the Central Valley. In the early 1800s, 26 settlers and Native Americans described the Sacramento and San Joaquin rivers as “miles 27 wide” during flooding. 28 Early Flood Protection 29 Initial flood protection in the Central Valley developed in a piecemeal fashion with the 30 construction of levees to protect local areas from flooding. Levees were typically 31 constructed in response to a past flood, with little or no coordination between different 32 localities. As the private levee system developed, the protection afforded by individual 33 levees decreased because of the increased heights of floodwaters constrained between the 34 levees. The increased flood danger led to competition between landowners to continually 35 raise and strengthen levees by stages to protect local areas and direct floodwaters 36 elsewhere. Mendota Pool Bypass and Reach 2B Improvements Project Draft Draft Environmental Impact Statement/Report 12-1 – June 2015 San Joaquin River Restoration Program 1 By the early 1900s, it was evident that local efforts would not be adequate to provide 2 flood protection to agricultural lands in the Sacramento River and San Joaquin River 3 basins. In 1920, Colonel Robert Marshall, chief geographer for the U.S. Geological 4 Survey (USGS), proposed a major water storage and conveyance plan to transfer water 5 from Northern California to meet urban and agricultural needs of central and Southern 6 California. This plan ultimately provided the framework for development of the Central 7 Valley Project (CVP). Under the Marshall Plan, a dam would be constructed on the San 8 Joaquin River near Friant to divert water north and south to areas in the eastern portion of 9 the San Joaquin Valley, and provide flood protection to downstream areas. The diverted 10 water would be a supplemental supply to relieve some of the dependency on groundwater 11 that had led to overdraft in areas of the eastern San Joaquin Valley. Water in the 12 Sacramento Valley would be collected, stored, and transferred to the San Joaquin Valley 13 by a series of reservoirs, pumps, and canals. 14 In 1933, the California State Legislature approved the Central Valley Project Act, which 15 authorized construction of initial features of the CVP, including Shasta Dam; Friant Dam; 16 power transmission facilities from Shasta to Tracy; and the Contra Costa, Delta-Mendota, 17 Madera, and Friant-Kern canals. However, the Great Depression prevented the State from 18 financing the project so the State appealed to the Federal Government for assistance in 19 constructing the CVP. 20 Congress appropriated funds and authorized construction of the CVP and construction 21 began on October 19, 1937, with the Contra Costa Canal. Construction of Shasta Dam 22 began in 1938 and was completed for full operation in 1949. Friant Dam, on the San 23 Joaquin River, was also completed in 1949. 24 The Flood Control Act of 1944 authorized the Lower San Joaquin River and Tributaries 25 Project. The project included constructing levees on the San Joaquin River below the 26 Merced River, Stanislaus River, Old River, Paradise Cut, and Camp Slough. Construction 27 was initiated on the Lower San Joaquin River and Tributaries Project in 1956. The 28 Chowchilla and Eastside bypasses were constructed by the State as part of the Lower San 29 Joaquin River Project. 30 12.1.2 Flood Management Structures 31 Friant Dam 32 Friant Dam is the principal flood damage reduction facility on the San Joaquin River and 33 is operated to maintain combined releases to the San Joaquin River at or below a flow 34 objective of 8,000 cubic feet per second (cfs). Several flood events, as described below, 35 in the past few decades have resulted in flows greater than 8,000 cfs downstream from 36 Friant Dam and, in some cases, flood damages resulted. 37 The existing Friant Dam is a 319-foot-tall concrete gravity dam with a crest length of 38 3,488 feet and a crest width of 20 feet. Millerton Lake, formed by Friant Dam, has a 39 volume of 524 thousand acre-feet (TAF). The dam serves the dual purposes of storage for 40 irrigation and flood management. The minimum operating storage of Millerton Lake is 41 130 TAF, resulting in active available conservation storage of about 390 TAF. The Draft Mendota Pool Bypass and Reach 2B Improvements Project 12-2 – June 2015 Draft Environmental Impact Statement/Report 12.0 Hydrology - Flood Management 1 minimum operating storage allows for diversion from dam outlets to the Friant-Kern 2 Canal, Madera Canal and the San Joaquin River. During the rainy season of October 3 through March up to 170 TAF of available storage space must be maintained for 4 management of rain floods. 5 San Joaquin River 6 Except for a small area to the west and south of Fresno Slough, the Project area is located 7 in a Federal Emergency Management Agency (FEMA) Special Flood Hazard Zone A (no 8 base flood elevations have been determined). The area adjacent to Fresno Slough is 9 designated as Zone AO (1-3 feet of flood depth). 10 Chowchilla Bypass and Chowchilla Bifurcation Structure 11 The flood control structure most relevant to Reach 2B is the Chowchilla Bypass and 12 Chowchilla Bifurcation Structure, owned by DWR and the Central Valley Flood 13 Protection Board (CVFPB) for the State of California. The Chowchilla Bypass begins at 14 the Chowchilla Bifurcation Structure in the San Joaquin River and runs northwest, 15 parallel to the San Joaquin River, to the confluence of the Fresno River, where the 16 Chowchilla Bypass ends and essentially becomes the Eastside Bypass. The design 17 channel capacity of the Chowchilla Bypass is 5,500 cfs. The bypass is constructed in 18 highly permeable soils, and much of the initial flood flows infiltrate and recharge 19 groundwater. The Chowchilla Bifurcation Structure is a gated structure that controls the 20 proportion of flood flows between the Chowchilla Bypass and the San Joaquin River 21 Reach 2B. The bifurcation structure has a drop (plunge pool) on the downstream side in 22 both the San Joaquin River and Chowchilla Bypass, and has no fish passage facilities. 23 The Chowchilla Bifurcation Structure is operated to keep flows in Reach 2B at a level 24 less than 2,500 cfs because of channel design capacity limitations. Therefore, operating 25 rules for the Chowchilla Bifurcation Structure are based on initial flow to the San Joaquin 26 River and initial flow to the Chowchilla Bypass (McBain and Trush 2002). The intended 27 design capacities for the various sections of the San Joaquin River reaches in the Project 28 area are described in Table 12-1. 29 Mendota Dam 30 Mendota Dam is located at the confluence of the San Joaquin River and Fresno Slough. 31 Mendota Pool is a small reservoir, with approximately 8,000 acre-feet of storage, created 32 by Mendota Dam. The Mendota Pool does not provide any appreciable flood storage. The 33 water surface elevation in the pool is maintained by a set of gates and flashboards that are 34 manually opened/removed in advance of high-flow conditions. This process lowers the 35 water level in the pool for passing high flows to reduce seepage impacts to adjacent 36 lands, but hinders distribution of flows into the canals. 37 Over time, the Mendota Pool has partially filled with sediment during infrequent 38 high-flow releases from Friant Dam. During times of high flows, some unknown portion 39 of this sediment is able to flush and route downstream when flashboards have been 40 removed, restoring much of the Mendota Pool storage capacity. If the flashboards are not 41 removed before a high-flow event from either the San Joaquin River or Kings River via 42 Fresno Slough, the increased water surface elevations cause seepage problems on 43 upstream and adjacent properties. Additionally, there have been recurring problems with Mendota Pool Bypass and Reach 2B Improvements Project Draft Draft Environmental Impact Statement/Report 12-3 – June 2015 San Joaquin River Restoration Program 1 water seeping under Mendota Dam, threatening the structural integrity of the dam.