Change order approved for Moosa Canyon Erosion Control project

Work on the San Diego County Water Authority’s Moosa Canyon Erosion Control project began in June, but groundwater was encountered at a higher level than expected and the SDCWA approved a change in its contract with Granite Construction to allow the installation of a dewatering system.

The CWA’s unanimous board vote, July 26, approves the change order covering the additional scope of work along with an additional $200,000 funding authorization. The Moosa Canyon erosion control project will protect the CWA’s Second Aqueduct by preventing stream bank erosion, preventing head cuts from moving upstream and installing half-ton riprap. The construction will place engineered grouted and non-grouted riprap in the creek channel.

The CWA’s Second Aqueduct includes pipelines 3, 4 and 5. The pipelines transport water through steep terrain and cross under Camino Del Rey in Bonsall. Moosa Creek is a riparian environment with dense vegetation, and in 2013 CWA staff observed that a portion of pipeline 4 was exposed in Moosa Creek due to creek bed erosion. The location coincides with a horizontal bend in the pipeline where the pipeline crown is visible.

The exposed bend in pipeline 4 is subject to internal water force, or thrust, as water changes direction. A structural analysis of the internal water forces conducted in 2015 indicated that those forces were not a concern at the time, but the CWA was concerned that future storms could cause additional erosion and threaten the suitability of the pipelines at the creek crossing.

During 2016 CWA staff installed interim erosion control measures to restore cover over the pipeline and stabilize the area before the next wet season. In June 2016, the CWA board authorized a $279,620 professional services contract with Michael Baker International for the design of pipeline protection and for construction phase design support services. The design incorporates the interim repair improvements into a long-term erosion control solution. The CWA board approved the project, along with the environmental Mitigated Negative Declaration and the Mitigation Monitoring and Reporting Program, in April 2017.

Temporary right of way agreements, a U.S. Army Corps of Engineers letter allowing the project, and a streambed alteration agreement with the California Department of Fish and Wildlife were prerequisites to advertising the project for bids. The project was advertised for bids, Feb. 14. Six firms responded by the March 13 bid deadline, and the Granite Construction bid of $1,235,889.40 was the lowest.

The Granite Construction proposal included all work specified in the bid package, but the geotechnical borings conducted during the design phase showed groundwater at a depth of 9 feet below the surface and when Granite Construction began excavation activities in June groundwater was encountered approximately 1 foot below the surface. A dewatering system will be necessary to maintain the water level below the bottom of the excavation.

The change order allows an increase of the contract amount to up to $1,435,889.40 without further CWA board authorization, although if the actual costs are less than $200,000 a credit to the CWA will be processed. The additional authorized amount covers the anticipated costs of groundwater discharge permits, installing and operating a dewatering system and a treatment system if needed, and installing a disposal system.

Once the dewatering system is installed water quality samples will be taken. The sample results and the anticipated volume of groundwater requiring disposal will determine the most cost-effective means of treatment and disposal. The treatment system will only be needed if the groundwater has constituent levels greater than the limits required for the Regional Water Quality Control Board to issue a conditional waiver for the discharge of construction and nuisance water.