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ITEM: |
CONSENT
CALENDAR |
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4. |
Consider Expenditure for Carmel River THALWEG
PROFILE |
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Meeting
Date: |
August 17, 2015 |
Budgeted: |
Yes |
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From: |
David J. Stoldt
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Program/ |
Water
Supply |
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General
Manager |
Line Item
No.: |
1-8-1-B |
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Prepared
By: |
Larry Hampson |
Cost
Estimate: |
$25,000 |
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General Counsel Approval: N/A |
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Committee Recommendation: N/A |
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CEQA Compliance:
N/A |
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SUMMARY: Staff proposes to amend an existing contract with Central Coast Surveyors for survey work along five miles of the lower Carmel River channel between Schulte Road Bridge and Boronda Road Bridge. Detailed ground elevation data would be gathered along the profile of the channel at its lowest point (thalweg). Data will be used to maintain a long-term record and to compare with past and future monitoring data. Comparisons of repeated surveys carried out over long periods yields information about the long-term rate of aggradation (sediment build-up in the channel) or degradation (loss of sediment from the channel), and changes in bank storage that affect the capacity of the Carmel Valley Alluvial Aquifer to store water. A portion of the river proposed for survey work between Schulte Road Bridge and the Narrows at River Mile 9.8 was partially surveyed in 2007 and showed that several feet of degradation had occurred in some reaches in the previous 10 years. Anecdotal evidence suggests additional degradation has occurred since 2007. The portion of the river from the Narrows to Boronda Road Bridge was last surveyed in 1984.
RECOMMENDATION: Authorize the General Manager to amend an existing agreement with Central Coast Surveyors for an additional amount not-to-exceed $25,000 to conduct a topographic survey in the Carmel River channel.
DISCUSSION: Most of the riverbed and streambanks along
the lower 15.5 miles of the
Gravel mining, main stem reservoirs, and streambank armoring have contributed to a sediment-starved condition in the river channel downstream of Los Padres Dam, located at approximately River Mile (RM, measured from the ocean) 25. A chronic lack of sediment from the watershed area above San Clemente Dam for 95 years has been a factor in aquatic habitat degradation, channel incision, streambank instability, infrastructure damage, loss of property, and episodes of bank erosion along the river. In addition, incision and removal of alluvial valley deposits reduces the volume of water that can be retained in storage in the Carmel Valley Alluvial Aquifer.
Survey data will be used by MPWMD staff to adjust Carmel Valley Alluvial Aquifer parameters and to monitor effects to downstream areas from sediment retention at San Clemente and Los Padres Reservoirs.
Due to the difficult nature of obtaining access to and data from the
river channel between Schulte Road Bridge and Boronda Road Bridge, the cost may
be in excess of $3,500 per mile. Central
Coast Surveyors (CCS) is a local firm with previous experience in the Carmel
River, including completion of eight miles of survey work in 2014. Completion of survey work in proposed for
2015 would provide the District with a recent full profile of the Carmel River
from the lagoon to Carmel Valley Village and set a baseline for comparison in
future years after San Clemente Dam is removed in 2015.
IMPACT TO STAFF/RESOURCES: Funds for this project are included in the FY 2015-16 budget under Project Expenditures for item 1-8-1-B Carmel Riverbed Topographic Data, Account 35-03-7895.90, $25,000. Staff time will be required to administer the contract.
U:\staff\Boardpacket\2015\20150817\ConsentClndr\04\Item4.docx