Company:
Santa Margarita Water District (SMWD)
Status:
Awarded
Awarded:
Geotechnical Project of the Year
Additional Files







Additional Information

Project description on owner’s website:
https://www.smwd.com/313/Trampas-Reservoir

TRAMPAS CANYON DAM AND RESERVOIR ORANGE COUNTY, CALIFORNIA

Project Location:

The Trampas Canyon Dam and Reservoir site is located within Rancho Mission Viejo, south of Ortega Highway and east of San Juan Capistrano.

Project Description:

Santa Margarita Water District (SWMD) has constructed south Orange County’s largest reservoir and the first reservoir built in decades. The Trampas Canyon Reservoir is a recycled water reservoir that provides seasonal and operational storage for the District’s existing and proposed recycled water system. The proposed reservoir supplies irrigation demands for Talega Ranch, Ladera Ranch, Village of Sendero, Village of Escencia, the proposed Ranch Plan development, City of San Juan Capistrano and other existing irrigation demands within south Orange County. Recycled water is supplied by the Chiquita Water Reclamation Plant (CWRP) and will be supplemented by other non-domestic water supply sources and reclamation plants. In addition, the proposed reservoir will provide storage for other recycled water purveyors in south Orange County.

SMWD has reconfigured the existing Trampas Canyon Dam and Reservoir to convert it from a tailings retention facility to a recycled water supply reservoir. The dam and reservoir were reconstructed on 177-acres of property to create a 1.6-billion-gallon recycled water reservoir. The proposed Trampas dam reconfiguration project includes raising the existing earthfill embankment (Main Dam), constructing two saddle dams (East and West Saddle Dams) and appurtenant facilities (i.e. an inlet/outlet structure, inlet/outlet tunnel and pipeline, spillway, roadways, and a multi-zone booster pump station). The reconfiguration for the proposed Trampas Canyon Reservoir includes the following features:
• Raising the existing Main Dam to increase reservoir capacity and provide the minimum freeboard required by the California Department of Water Resources Division of Safety of Dams (DSOD);
• Removing a portion of the existing Main Dam crest to allow the proposed raised dam core to tie into the existing core;
• Adding a downstream buttress and filter/drain layers to mitigate historical deficiencies associated with the existing Main Dam (e.g., seepage, stability);
• Constructing an inlet/outlet structure and associated pipeline with gravity outfall for reservoir emergency drawdown;
• Constructing a new, higher capacity, spillway;
• Constructing two saddle dams; one at the existing Saddle Dam (East Saddle Dam) and the other would be required due to recent road cuts at the west end of the reservoir (West Saddle Dam). 

Project Justification:

The project is worthy of an award because it is a central component of a sustainable, future water supply for Southern California and the 165,000 people currently served by the Santa Margarita Water District.  It is OC’s largest and the first reservoir built in decades. “Trampas Reservoir allows the district to maximize recycled use and bring us closer to zero wastewater discharge into the ocean,” said Betty Olson, president of the water district.
While the water district currently imports 100% of its drinking water from the Colorado River and northern California, the new Trampas Canyon Reservoir is part of a plan to generate 30% of potable water supplies locally and to recycle more wastewater. That in, in turn, will provide a sustainable buffer from droughts and other circumstances that could reduce imported flows.
In the early 1970s, a reservoir was built as part of the sand mining operation in the hills south of Ortega Highway in Rancho Mission Viejo. Nearly 40 years later, in 2014, Santa Margarita Water District (SMWD) identified the opportunity to repurpose and reconstruct the reservoir to provide additional recycled water seasonal storage for its use during dry periods.
SMWD worked with Rancho Mission Viejo and, in 2017, acquired the land and took the steps toward a new era of water sustainability in south Orange County. The new reservoir has a capacity of 1.6 billion gallons of water – enough water to fill 2,500 Olympic size swimming pools. The reservoir is the largest surface water reservoir in south Orange County, with the capacity to hold more than one-half of the recycled water SMWD will generate from its Chiquita Water Reclamation Plant. The reservoir may also one day provide SMWD and its partners with the opportunity to reuse the recycled water for direct or indirect potable purposes.
This $123 million investment, which will enhance water reliability and storage capacity in south Orange County, is paid through a State Water Resources Control Board low-interest loan, a Proposition 1 grant, Rancho Mission Viejo, designated District reserves, and other funds.

Special Circumstances:

The project encountered challenges during design and construction. Some of the challenges for the site during design included the geologic investigation of a potential seismic fault running through the area.  Various remnant stockpiles and undocumented fills were found on the site which impacted not only the design of the proposed dam but the need for a larger quantity of materials than anticipated during the preliminary phase.
During the preliminary geologic/geotechnical investigations, it was recognized that a splay (branch) of the Mission Viejo Fault (MVF) projects towards the Trampas Canyon Dam Site.  AECOM performed a non-invasive investigation of the fluvial terraces to evaluate the displacement history and activity of the MVF.  The investigation included extensive geologic literature review, independent geologic mapping and profiling, and pedogenic age dating of key terraces.  The investigation concluded that the MVF is not considered an active fault (a fault that has had at least one displacement event within the last 35,000 years).  Based on the study the potential for future displacement of the Trampas Canyon Dam from faults in the project area was considered negligible.
Various remnant stockpiles and other fills from the sand mining operations were found to be present within the project area.  A large non-engineered fill stockpile near the right abutment of the Main Dam was identified during the design geotechnical investigation.  This area was assumed to be bedrock during the preliminary phase of the design.  The available onsite materials for the dam were limited by the property and environmental boundaries.  The discovery of the large non-engineered fill impacted the design by increasing the size of the new dam, and the need for more embankment material.  AECOM was committed to minimize the need for material import.  Through detail soil characterization and testing, AECOM revised the design to be able to use most of the materials found onsite.  This not only made it possible to complete a design with minimal import, but to create minimal waste which was able to be kept onsite without the environmental and cost impacts of disposal.
The project team was also able to minimize impacts to the existing habitats and natural drainages surrounding the construction site. The reservoir and related pumping and conveyance facilities were designed to blend in with the native habitat to minimize viewshed impacts.

Project Attachments:

Articles in local papers:
https://www.ocregister.com/2020/10/09/vast-new-reservoir-in-south-orange-county-gets-its-first-drops-of-water/

https://www.thecapistranodispatch.com/big-dig-trampas-canyon-reservoir-serve-south-countys-largest-water-project/

FAST FACTS
PROJECT: Trampas Canyon Dam & Reservoir
• PURPOSE: Seasonal storage of recycled water to maximize a local resource
• CAPACITY: 1.6 billion gallons (5,000 acre-feet) = 2,500 Olympic size swimming pools
• LOCATION: San Juan Capistrano, CA. Southwest of Ortega Highway and Christianitos Road
• FACILITY OWNER/OPERATOR: Santa Margarita Water District
• COST: $123 million
• FUNDING SOURCES: State grants, Low-interest loans, Rancho Mission Viejo, SMWD designated reserve funds
• CONSTRUCTION PERIOD: 30 months
• ANTICIPATED COMPLETION: January 2021
• ADDITIONAL USES: Potential for future indirect/direct potable reuse
• ENGINEERING: AECOM
• CONSTRUCTION: Sukut Construction
• GEOTECHNICAL TESTING AND FIELD SUPPORT: GMU Geotechnical, Inc.
• CONSTRUCTION MANAGEMENT: Butier Engineering
• Largest surface water reservoir in south Orange County. Spanning 3,000 feet wide and reaching over 80 feet in depth at its lowest point
• Helps us use 100% of our community’s wastewater, building regional sustainability and resiliency.
• Creates an additional 1.6 billion gallons of recycled water storage – enough water to fill 2,500 Olympic-size swimming pools.
• Saves drinking water supplies by allowing increased availability to use recycled water to irrigate parks, medians, and common area landscapes.
• Features three dams. The main dam face reaches 216 feet high.

Award Citation::

Santa Margarita Water District’s new Trampas Canyon Dam and Reservoir has a capacity of 1.6 billion gallons of water – enough water to fill 2,500 Olympic-size swimming pools – and is the largest surface water reservoir in south Orange County.

Suggested Award Summary:

Santa Margarita Water District’s new Trampas Canyon Dam and Reservoir has a capacity of 1.6 billion gallons of water – enough water to fill 2,500 Olympic-size swimming pools – and is the largest surface water reservoir in south Orange County and the first reservoir built in decades. The project features three dams that reach 216 in height and a reservoir that is 3,000 feet across and 80 feet deep. The project helps the District to use 100% of our community’s wastewater, building regional sustainability and resiliency. The project will allow the region to save drinking water supplies by using recycled water to irrigate parks, medians, and common area landscape. The reservoir may also one day provide the District and its partners with the opportunity to reuse the recycled water for direct or indirect potable purposes.

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