California American Water initiated the first phase of its three-year, $84 million Carmel River Reroute & San Clemente Dam Removal (CRRDR) project that will remove the 106-foot-tall San Clemente Dam and begin a long-term watershed restoration process.
Owned and operated by California American Water, the San Clemente Dam, located in California’s Central Coast in Monterey County, was originally built in 1921 to supply water to the Monterey Peninsula. Engineering assessments of the aging dam determined that the structure potentially will not withstand a probable maximum flood (PMF) or a maximum credible earthquake (MCE). After a comprehensive study and coordination with multiple state, federal, local, and environmental groups, California American Water in cooperation with other stakeholders opted to remove the dam and reroute the Carmel River, returning the river to its original flow.
Engineers developed a conceptual plan to reroute the river, remove the dam, and manage 2.5 million cubic yards of sediment trapped behind the dam.
While significant analysis and site assessments have been done to develop the reroute and removal plan, California American Water and its stakeholders, and the design-build team of Granite Construction and Kleinfelder understand that the truth is in the details. During the next six to eight months, Kleinfelder’s engineering and geotechnical teams must dig deep to gain a clear and comprehensive understanding of the subsurface conditions throughout the 100-acre area and develop an appropriate design to ensure the feasible construction during the next three years of a long-term successful solution.
The upper portion of the Carmel River watershed lies within the largely undeveloped Santa Lucia Mountains. Constructed at the confluence of San Clemente Creek and the Carmel River, the San Clemente Dam and Reservoir is currently more than 90 percent filled with 2.5 million cubic yards of sediment. A key part of the dam removal plan is to deal with the sediment.
A considerable part of the early analysis was to determine the best way to manage the sediment to ensure public safety and environmental care. The team evaluated four sediment management strategies: notch release, rapid release, sediment hauling/removal, and in-place stabilization.
Assessments indicated that notch release and rapid release were not feasible options because of the potential dangers to downstream structures and impacts to the environment and private property. Portions of the lower Carmel River floodplain have been developed with a variety of residential, commercial, and recreational (including golf courses) uses, some of which are subject to periodic inundation from the Carmel River. If handled incorrectly, periodic inundation from the Carmel River could be exacerbated by the release of sediments and changes to the river channel from aggrading to a higher baseline elevation. Additionally, sediments (particularly finer materials) could severely impact steelhead spawning and rearing habitat within the lower Carmel River below San Clemente Dam.
Hauling the sediment out of reach also was discarded as an option for environmental and economic reasons. Because of the dam’s remote location, a large number of trucks would have been required to remove impounded sediments, which would create significant traffic impacts in established communities, as well as additional costs for handling and disposal.
After extensive analysis, the project sponsors opted to reroute a half-mile portion of the Carmel River into San Clemente Creek, use the abandoned reach of the former reservoir as a sediment storage area, and then remove the dam. The new stream will be bio-engineered and stair-stepped for fish passage, using boulders and woody debris harvested during construction.
The option for the reroute and removal project received a great deal of support from resource protection agencies, as well as environmental groups such as the Carmel River Watershed Conservancy, Carmel River Steelhead Association, the California Planning and Conservation League, and The Nature Conservancy (which supported removing the dam because it serves as a barrier to threatened Central Coast Steelhead Trout). By removing the dam, steelhead will have unimpaired access to more than 25 miles of natural spawning and rearing habitat. Perhaps most unusual is that two state and federal regulatory agencies, the California Coastal Conservancy and National Marine Fisheries Service, are stakeholders in the project, not just regulators.
While fundamentally a very large earthwork project, the CRRDR project is made up of several interdependent, complimentary elements including the diversion dike (to block the Carmel River), diversion dike abutment, sediment stockpile, stabilized sediment slope and buttress (to hold the sediment), reroute channel (cuts through Santa Lucia ridgeline), and combined flow reach – all of which must be designed to last a minimum of 100 years. Every element integrates, influences, and compliments the other.
The diversion dike will be about 260 feet wide and cross the Carmel River floodplain. The foundation will be dug 60 feet below the surface and its crest rise more than 40 feet high and be 22 feet wide at the top. The reroute channel connecting the Carmel River to San Clemente Creek will have cuts 70 feet deep and be approximately 500 feet long. The stabilized sediment slope on the downstream side, where the former reservoir sediments and stockpiled construction rubble will be located, will have slope heights approaching 70 feet and have a length of about 500 feet.
The Granite and Kleinfelder engineering team developed a temporary river diversion system to bypass the Carmel River around the construction site until the reroute and combined flow stretch are completed in 2015. The contractor has begun construction of the temporary river diversion system even as the geotechnical investigation proceeds in the summer and fall of 2013. The geotechnical investigation will provide the necessary information to complete detailed design of the permanent diversion dike, reroute channel, stabilized sediment slope, and sediment stockpile. Testing includes in-depth subsurface testing, geophysical evaluations, deep rock coring, and extensive laboratory testing.
The borings will define the width and depth of the bedrock. The subsurface explorations, currently ongoing, will evaluate the competency of the rock and alluvial materials, the depth of groundwater, and gather enough data to model conditions and assess liquefaction potential in the event of seismic activity. Twenty-seven borings and seven cone penetrometer test holes (CPTs) will be drilled as deep as 230 feet below the ground surface. HQ rock coring will be conducted into bedrock in all borings with as much as 220 feet of core collected from the bedrock ridge where the reroute channel will be cut. Split spoon and Shelby Tube samples will be collected, downhole seismic and packer testing conducted, and televiewer imagery collected. Some borings will be instrumented with vibrating wire piezometers and inclinometers.
Another challenging aspect that will require significant geotechnical investigation is sediment dewatering. The 2.5 million cubic yards of sediment is made up of a wide range of material from gravel to fine-grained silts and organic peat. Fine grained silts and peat do not drain easily. Piezometers and dewatering wells will be installed and pump tests conducted. The geotechnical team will summarize anticipated dewatering requirements that will help the contractor determine the means and methods to control the water during construction of the diversion dike foundation and stabilized sediment slope.
The hydraulics and hydrology of the river will also influence the construction plan. Engineering studies will evaluate probable maximum flood scenarios that range from average precipitation and flow characteristics of the Carmel River to the influence of a 100-year storm to water elevations. These factors will dictate the top elevations of the diversion dike and buttress for the sediment stockpile and the flow capacity of the combined flow reach.
Ready for renewal
Most of the design work will be completed during the winter of 2013 and spring of 2014. Even as the geotechnical studies get underway, the contractor is fine-tuning the construction schedule to ensure that all pieces – the diversion dike, stockpile, slopes, and channels – are constructed in a timely manner to meet the aggressive three-year schedule. In the 2014 construction season, crews will build the permanent diversion dike, excavate the cut between the Carmel River and San Clemente Creek, and construct a sediment stockpile and the buttressing structure that will hold the sediment behind the dam.
All excavated materials, including rock from the reroute channel, will be used onsite for construction or disposed with the concrete rubble in the sediment stockpile area. No construction wastes will need to be trucked off site.
The estimated cost for dam removal, river reroute, and in-place sediment management is $84 million. Of the $84 million cost, California American Water is putting up $49 million, the state of California is contributing $25 million from previously authorized bonds and private donations, and the federal government is providing $2.4 million.
Peter L. Stroud, CEG, is a principal engineering geologist and senior associate with Kleinfelder. He can be reached at firstname.lastname@example.org.