The Metropolitan Water District of Southern California will be undertaking comprehensive assessments of the spillways and dam structures at Lake Mathews and Lake Skinner along with preliminary design of upgrades to the dam monitoring systems at both lakes.
The MWD board vote, Dec. 12, also approved an agreement with AECOM, an international design firm, for technical support of those activities. The $2.86 million appropriation for the project covers $2,000,000 for the study and $860,000 to upgrade the monitoring systems and includes $1.53 million for AECOM’s consultant work.
MWD’s Colorado River Aqueduct runs from the Parker dam to the Lake Mathews reservoir, and the Lake Mathews dam was constructed in 1938. The dam at Lake Skinner was completed in 1973, and Lake Skinner receives both Colorado River supply from Lake Mathews and State Water Project supply which routes water into the California Aqueduct for transport to Southern California. Oroville Dam and Lake Oroville comprise the start of the State Water Project, and the winter 2017 overflowing at Oroville Dam led to measures by the state’s Division of Safety of Dams to prevent a similar situation in the future. DSOD now requires that agencies which own and operate dams assess the condition of spillways to confirm that they meet minimum safety standards, and in July 2017, DSOD issued an initial list of 93 dams which will require comprehensive spillway assessments to evaluate hydraulic capacity, geotechnical stability, structural integrity and potential erosion from dam release. A total of 20 MWD facilities are permitted by DSOD although only Lake Mathews and Lake Skinner have been directed to have the comprehensive assessments. DSOD has a deadline of December 2018 to submit the draft reports.
The DSOD classification of Lake Mathews and Lake Skinner as high hazard dams is not based on the dams’ conditions but on the size of the dams’ reservoirs and the population which could be impacted if a dam fails. During the most recent DSOD annual inspections, the Lake Mathews and Lake Skinner dams along with their spillways were declared to be in satisfactory condition.
The original Lake Mathews facilities included the main dam embankment, a 250-foot-long concrete-lined spillway, the lake’s initial outlet tower and a forebay with its own outlet tower. The main dam embankment was raised in 1961, and two dams were constructed to increase the lake volume to its current capacity of 182,000 acre-feet. A second outlet tower was added in 2004. The area of Lake Mathews’ upstream watershed is 40 square miles, and the watershed drains into the lake through Cajalco Creek. Lake Mathews conveys water to MWD member agencies through the upper feeder to MWD’s F.E. Weymouth Treatment Plant in Laverne, and through the lower feeder to MWD’s Robert B. Diemer Treatment Plant in Yorba Linda.
The maximum storage capacity of Lake Skinner is 44,000 acre-feet. Lake Skinner has two adjacent spillway structures which release flows into Tucalota Creek: a concrete-lined primary spillway which is designed to handle normal flood flows from the 52-square mile upstream watershed and a concrete-lined emergency spillway designed to handle excess flood flows. Water from Lake Skinner is delivered to the Robert A. Skinner Water Treatment Plant and to the San Diego Canal.
MWD previously submitted work plans to re-evaluate the Lake Mathews and Lake Skinner spillways, and DSOD approved those plans in September. Phase 1 will be the research and review of available information; Phase 2 will be the structural and geotechnical investigations; Phase 3 will be the analyses and evaluations and Phase 4 will develop measures to remediate any deficiencies.
The research and review phase will identify and catalogue historical reports, review design reports, evaluate construction documents and quality control data and assess post-construction modifications, inspection reports and maintenance records.
The structural and geotechnical investigations phase will focus on investigating the condition of structural elements of the existing spillway structures and the condition of the soil and rock adjacent to and beneath those structures. Visual inspections of the spillway will identify signs of concrete degradation, cracking or spalling and will examine the condition of waterstops, joints, subdrain systems and weep holes. The investigations will include scanning with ground-penetrating radar and concrete coring to determine slab thickness and the condition of reinforcing steel and will also characterize the foundation bedrock weathering and hardness. Video pipe inspection will determine the condition of weep holes and drain pipes. Visual inspections and aerial mapping of the surrounding soil and rock conditions will identify areas of potential erosion of downstream earthen features and landslides which might constrain spillway releases. Detailed geologic mapping will identify rock discontinuities such as joints, shears, faults and bedding in the adjacent rock formations. Field investigations such as trenching, drilling and coring will determine the soil and rock characteristics such as the degree of weathering and rock hardness. Laboratory testing will establish physical properties to be used in subsequent technical analyses.
The analyses and evaluations phase will include technical analysis to confirm the hydraulic capacity, geotechnical stability and structural integrity of the spillways. The evaluations will compare the spillways’ current condition with current design practices and performance guidelines. A comprehensive spillway assessment report will be prepared and submitted to DSOD for each dam, and DSOD will review the reports before making a decision whether to approve those. The outlet tower and conduit at Lake Skinner will be re-evaluated to identify the potential risks and vulnerabilities of lowering the reservoir pool after a major seismic event.
The development of measures to remediate any deficiencies will be implemented if necessary. If such activities are needed MWD staff will return to the board for authorization of that work.
The study expenses include $1.13 million to AECOM, which is headquartered in Los Angeles, for specialized investigations. MWD staff will conduct the technical review, the preparation of environmental documentation, consultation with DSOD and project management and $540,000 is budgeted for those tasks. The $126,000 for aerial surveys and mapping and the $25,000 for peer review by an independent specialty firm are below the threshold which would require MWD board approval, so those contracts can be approved by MWD General Manager Jeff Kightlinger. The other $179,000 which was budgeted will cover contingencies.
Instrumentation along with regular inspections is a component of MWD’s dam monitoring program. The instrumentation provides warning signs of dam distress and provides real-time monitoring of the embankments and foundations. The equipment at Lake Mathews includes 80 piezometers, which measure pore water pressure, 13 weirs, which measure seepage flow and five strong motion accelerographs. The equipment at Lake Skinner includes 71 piezometers, seven weirs and six strong motion accelerographs.
Recent inspections have shown that several piezometers and weirs no longer function reliably and require rehabilitation or replacement. Improved dam instrumentation technology since the existing instrumentation was manufactured has enhanced the accuracy, timeliness and communication of data, and the current industry practice is for extensive data to be collected on a frequent schedule with minimal if any delay in transmitting the data to the reviewing office.
The monitoring system upgrade project will begin with MWD staff identifying potential upgrades to the Lake Mathews and Lake Skinner monitoring systems so that the dams will employ state-of-the-art equipment similar to that at Diamond Valley Lake. Condition assessments will be conducted at both dams to develop a staged replacement schedule, and the results of those assessments will allow MWD staff to return to the board to authorize the installation of automated monitoring systems for each dam.
The preliminary design phase activities will include conducting field surveys and condition assessments of the existing dam monitoring instruments, developing final design criteria, value engineering, DSOD permitting and development of cost estimates. AECOM will provide specialized support for the condition assessments, a specialty firm will perform the third-party value engineering under an agreement Kightlinger will execute, and MWD staff will perform the other activities. The $860,000 budget covers $400,000 for the AECOM field investigations and technical activities, $385,000 for consultations with DSOD, technical review, value engineering, development of a cost estimate, preparation of environmental documentation, project management and $75,000 for contingency. MWD staff will return to the board at a later date to authorize the final design.
AECOM’s scope of work includes geotechnical field investigation, hydraulic and structural analyses for the spillway evaluations, seismic assessments of the outlet works and condition assessments of the existing dam monitoring systems. AECOM will utilize five subcontractors; all of whom are based in Southern California.