Reservoir Sedimentation and Water Supply Reliability By: Aubrey Mescher, MESM Ms. Mescher is a water resources specialist at Aspen’s Agoura Hills headquarters office. Our reservoirs are filling with sediment. That’s the same as dirt. Mud, muck, silt. Rocks, gravel, even boulders. All of this stuff where there’s supposed to be just water. Water for drinking, irrigation, recreation, commercial and industrial uses, flood control, and groundwater recharge. This isn’t a new issue. Actually, reservoirs are assumed to have a finite lifetime, limited by loss of function due to sedimentation. But reservoirs are filling with sediment far more quickly than anticipated, and important water supply and flood control facilities are not only being rendered useless before their time, but in doing so they are simultaneously introducing new issues with public safety and water supply reliability. What is sedimentation? Sedimentation is a natural process that occurs when soil particles suspended in water settle out of the main water column to the bottom. Sediment content in a waterway is higher during and after storm events, when rates of flow and erosion are higher, and lower during dry months, when these rates tend to be lower. Under natural conditions, unconstrained by a dam, the quantity of water and sediment in a waterway is generally balanced, as the ground surface and riverbed erode into the waterway, and sediment is deposited in downstream areas, where it provides habitat and replenishes riverbanks and beaches. But when a dam is constructed in a waterway, it traps the flow of both water and sediment. The sediment gradually accumulates behind the dam, larger particles such as rocks and gravel settling to the reservoir floor while the spaces in between fill with finer material such as silt and mud. Over time, accumulated materials reduce reservoir storage capacity, and the dam loses value as a flood control and water supply facility. Hydroelectric dams also lose value, as decreased reservoir capacity results in decreased power generation. Perhaps it is time to consider that operation and maintenance of a dam require as much focus on sediment management as on water management. What causes increased sedimentation? As a natural process, the rate of sedimentation is affected by factors such as topography, geology, flow volume and velocity, and climate conditions. Therefore, reservoir sedimentation will occur at different rates, depending on local conditions. Hetch Hetchy Reservoir, which is formed by the 90-year-old O’Shaughnessy Dam on the Tuolumne River, is estimated to contain only two inches of sediment because the Tuolumne riverbed is mostly granite and erodes very slowly (Biba, 2012). In comparison, Matilija Reservoir, located on a tributary of the Ventura River, was only 17 years old when it lost 27 percent of its capacity due to sedimentation from a 100-year storm event in 1969; today the 65- Hetch Hetchy Valley, prior to construction of O’Shaughnessy Dam. year-old reservoir is virtually Source: PBS, 2013 1 useless, with more than 90 percent of its capacity lost to the six million tons (two million cubic yards [mcy]) of silt and sediment trapped by Matilija Dam (Matilija Coalition, 2002a). It’s not that any greater consideration to upstream conditions was provided when O’Shaughnessy was constructed in 1923 than when Matilija was constructed in 1947; conditions on the Tuolumne River are just so different than on Matilija Creek and the Ventura River. Two of the greatest factors influencing increased rates of sedimentation include development that replaces natural ground cover, and changes to soil composition that make it more susceptible to erosion. These factors are not always mutually exclusive. Development tends to replace natural or vegetated ground cover with impervious or less permeable surfaces; in response, the rate of surface water runoff increases, and rates of erosion and sedimentation increase. A 2004 study of sedimentation in Lake Elsinore, located approximately 75 miles southeast of Los Angeles, supported this connection by concluding that average 20th- century sedimentation rates are several times higher than 18th- and 19th-century rates (Byrne, R. and Reidy, L., 2004). The 20th century also marked a time of substantial urban growth in southern California, Lake Elsinore, CA. Source: grandfathersmc, 2013 compared to the 18th and 19th centuries. Regarding soil composition changes which facilitate erosion and sedimentation, the introduction of hydrophobic conditions is the most dramatic. Hydrophobic soil is water-repellent and prevents water from infiltrating the soil surface. Wildfires commonly result in hydrophobic conditions, because the burning of organic materials creates hydrocarbon residue which settles into small In support of a Supplemental EIS/EIR for the spaces between soil particles, effectively Tehachapi Renewable Transmission Project creating a layer of waterproof soils. These (TRTP), which traverses the Angeles National soils may be located on the ground surface Forest (ANF) and Station Fire burn area, Aspen or at shallow depths. After the Station Fire of hydrology and computer resources specialists prepared a GIS-based model of erosion and 2009 burned 161,188 acres of watershed in sedimentation on the ANF resulting from the the Angeles National Forest, the USDA Station Fire, in order to develop project-specific Forest Service conducted hydrologic mitigation measures for potential erosion and analyses of the burn area, and determined water quality impacts of the TRTP. that extremely hydrophobic soils were located one-half inch to two inches below the ground surface (USDA, 2009). Due to the hydrophobic soils being buried, it was anticipated that the first precipitation events after the fire would saturate and wash away the soils resting on top of the hydrophobic layer(s). The Forest Service estimated that the amount of sediment and debris that could be delivered to downstream areas as surface runoff would increase by 100 to more than 400 percent in the first few years following the Station Fire (USDA, 2009). 2 Downstream and adjacent to the Station Fire burn area are the cities of Los Angles, Glendale, La Crescenta, La Cañada Flintridge, Pasadena, Altadena, and Acton; flood control facilities in these jurisdictions received dramatically higher sediment loads during the wet seasons immediately following the Station Fire. The County of Los Angeles Department of Public Works (LADPW) is currently working to clear flood control facilities in these areas. As part of this effort, the LADPW has proposed and/or is currently executing sediment removal projects on the following major flood control facilities: Big Tujunga Dam, Cogswell Dam, Devil’s Gate Dam, Morris Dam, and Pacoima Dam. As one example, in the Devil’s Gate Reservoir, sediment inflow after the Station Fire has significantly reduced the reservoir's capacity and in its current condition, the reservoir’s outlet works are at risk of becoming clogged and inoperable (LADPW, 2013a). As with other flood control facilities in the area, the Devil’s Gate Reservoir no longer has the capacity to safely contain another major debris event, and removal of sedimentation from the reservoir is necessary to restore its functional capacity. As noted, the effects of development and altered soil conditions on sedimentation rates are Devil’s Gate Reservoir in the Los Angeles Basin. not mutually exclusive. For instance, development Source: LADPW, 2013a in forested areas has limited natural burn cycles due to fire prevention and suppression efforts, and our forests therefore tend to accumulate far more density and fuel than would occur under natural conditions. As a result, when fire is eventually introduced to a forested area, it tends to burn much hotter and faster than it would naturally, increasing both the potential for hydrophobic conditions and the removal of soil-stabilizing vegetation. Why should reservoir sedimentation be addressed? There are three primary objectives to managing water supply facilities, including reservoirs affected by sedimentation: water storage, environmental protection, and public safety. Public safety concerns include both the provision of flood hazard protection, and the removal of hazards associated with potential dam failure. As mentioned above, multiple flood control facilities within the jurisdiction of the LADPW are currently compromised due to sedimentation associated with runoff from the Station Fire burn area, and efforts are underway to rehabilitate these facilities and provide essential flood hazard protection to residents of the Los Angeles basin. In addition, aging dams which trap volumes of sediment risk structural failure and release of constrained materials to downstream areas, potentially burying property and habitat. The San Clemente Dam located on the Carmel River in Monterey County currently has only 70 acre- feet of its planned 1,425 acre-feet in storage capacity, due to more than 2.5 mcy of sediment entrained in the reservoir (CCC, 2010). In the early 1990s, the California Department of Water Resources (DWR) Division of the Safety of Dams determined that the dam could fail in the event of either the maximum credible earthquake or probable maximum flood (PMF), and a safety order for the dam was issued (CCC, 2010). Rather than simply stabilizing the structure, which would meet the requirements for public 3 safety, a plan is underway to temporarily divert the river and remove the dam structure in order to restore habitat along