GrokSurf's San Diego

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Posts Tagged ‘Reservoirs’

San Diego-Sweetwater intertie could boost local reservoir storage by 100,000 acre-feet of water

Posted by George J Janczyn on April 18, 2011

Since the end of California’s severe 1987-92 drought it has taken a 1993 Army Corps of Engineers reconnaissance study, U.S. Senate and House legislative support from Rep. Duncan Hunter (Sr.), Rep. Susan Davis, and Sen. Dianne Feinstein, a California Proposition 50 grant via the Integrated Regional Water Management Plan, and negotiations between Sweetwater Authority, the City of San Diego, and the U.S. Bureau of Reclamation — plus another major drought — just to reach this stage.

This stage is a proposal for a pre-feasibility study of a four-reservoir intertie between the City of San Diego and Sweetwater Authority systems.

The proposal will be presented today (Apr 18) at the Independent Rates Oversight Committee (IROC) [agenda] and Wednesday (April 20) at the Natural Resources and Culture Committee [agenda] where support will be sought in bringing it to the San Diego City Council. If full Council approval is obtained an agreement will then be executed by the City of San Diego, Sweetwater Authority, and the Bureau of Reclamation.

As described in a San Diego Integrated Regional Water Management document:

“Connecting the San Vicente, El Capitan, Loveland, and Murray Reservoirs would create an enhanced and integrated reservoir system to more efficiently use the reservoirs and increase accessibility to approximately 100,000 acre-feet of surface storage without creating new reservoirs or new storage capacity. The environmental effects of the future conveyance system would be minimal because each reservoir has been in place since the 1940s or earlier, and reservoir footprints would not increase. The Sweetwater Authority is serving as the lead agency for the regional intertie project.”

[Update: at the IROC presentation it was announced that the lead agency has been changed from Sweetwater to San Diego]

Since that report was written, of course, one reservoir footprint is increasing due to the San Vicente Dam Raise Project currently underway that will give it a more significant role within the potential intertie system.

As things are now, overall reservoir capacity is underutilized. To give one example, San Diego’s El Capitan reservoir (110,120 AF) is rarely full because its small imported water supply connection has very limited ability to supplement local runoff. An intertie connection would facilitate more optimal reservoir levels.

[I’ve since learned that another reason El Capitan water levels often appear to be low is that its catchment area produces the greatest amount of runoff of all the reservoirs in the San Diego area, so the city intentionally keeps water levels low there in order to capture the maximum amount of water during wet years and not lose any to spillway overflow].

Further south, Sweetwater’s Loveland Reservoir (25,225 AF) relies solely on watershed runoff which is usually not enough to keep it filled. An intertie would allow adding more water during drier times. On the rare occasions that very wet seasons such as we recently experienced result in spillway overflow, excess runoff at Loveland could be diverted to the larger El Capitan or San Vicente reservoirs instead of being lost.

A bay in the San Vicente Reservoir. Water cascades down the fill chute from the San Diego Aqueduct tunnel portal. Water level has been drawn down while the dam raise project is underway.

San Diego’s much smaller Murray reservoir (4684 AF) was included because of its Alvarado Water Treatment Plant supplying a large part of the city.

The shared connections would permit more water overall to be stored in local reservoirs. That translates into increased reliability in the event of a cutoff of imported supplies due to disaster or other problem. The 1993 Army Corps of Engineers reconnaissance study of San Diego’s water supply (not available online*) also estimates that 23% of the total benefits from an intertie system would be from increased flood control.

Because an intertie system would be an immensely expensive and complicated undertaking, the pre-feasibility study would carry out “an appraisal investigation to determine the prudence of a full-scale feasibility study for the proposed intertie system” according to the Army Corps study.

(A subsequent Army Corps Executive Summary of Lessons Learned from the California Drought (1987-92) says one confirmed lesson is that “local and regional interconnections between water supply systems are effective and flexible options against severe water shortages.”)

If the pre-feasibility study recommends it, then a full feasibility study could be performed as a phase 2 project.

Funding for the pre-feasibility study will be shared between the U.S. Bureau of Reclamation ($344,332), state grant funding from Proposition 50 ($112,832), the City of San Diego ($171,500), and the Sweetwater Authority ($60,000) for a total of $688,664.


* Thanks to Marguerite S. Strand, Assistant General Manager of Sweetwater Authority for allowing me to examine the Army Corps reconnaissance study and answering my questions.


Posted in Environment, Sweetwater Authority, Water | Tagged: , | 1 Comment »

California’s water storage dilemma

Posted by George J Janczyn on August 9, 2010

Many large reservoirs in California need to store and release water in a way that balances flood control needs against water supply needs (San Diego’s reservoirs have limited flood control capability but were mainly designed for storage). Those state reservoir levels need to be lowered in late summer and fall in order to have enough capacity to capture and hold floodwaters that will come during the wet season, but they also need to retain enough to supply needed water in the dry months.

Shasta Dam and reservoir. Photo: UC Davis.

“Reservoir rule curves” help manage when water should be released or held back.

Mountain snow is a factor in the timing and quantity of reservoir releases; the frozen snowpack serves as water storage that gradually melts and provides a predictable flow of water into the reservoirs.

Over recent years we have seen a gradual trend where more precipitation falls as rainfall instead of snow, and the snow that does fall tends to melt earlier in the season. This means that larger water flows into the reservoirs in a short period may threaten to exceed storage capacity, which in turn means more water needs to be released than would be desirable. The unwanted outcome can be that later, during the dry season, with insufficient snowmelt to help restore water levels, reservoirs could be unable to meet demand. The report Climate Change and Reservoir Rule Curves discusses that business in some detail.

In considering the collective needs of environmental, agricultural, and urban/rural interests over the long term (say 50 years), one viewpoint is that we need to increase storage capacity throughout the state in order to have more flexibility in managing the alternating flow requirements. In other words, build more dams or raise the height of existing dams.

San Vicente Reservoir August 2010. Water level is currently drawn down to allow dam raise construction. Most water in the reservoir is piped in through the San Diego Aqueduct, although it does receive some local runoff. Photo taken from a trail on the Oak Oasis Open Space Preserve.

In San Diego, we’re only indirectly affected by changing precipitation patterns elsewhere but we’re still raising the San Vicente Dam in order to more than double the capacity of its reservoir. That’s being done because of San Diego’s meager local water resources and very limited emergency reserves. In this we’re fortunate to have a reservoir that was suitable for enlargement. Keeping it filled will partly depend on an increasingly variable amount of imported water. Eventually a small amount of IPR water may also be a source for the reservoir.

For the state as a whole, in the face of changing precipitation patterns and growing demand, should we or could we increase reservoir storage on a large scale? Or can we adequately meet our needs by further refining reservoir rule curves and trying to make better use of existing resources? Funding for additional infrastructure proposed in the California Water Bond suggests increased storage is a priority although it’s looking like the bond may not make it to the ballot this year. One outcome seems easy to predict: whatever we end up doing, there are going to be a lot of unhappy people.

[April 22, 2011: The Sacramento Bee raises this issue [should we increase storage capacity] now that California had a wet winter filling up its reservoirs.]

More background: Managing an uncertain future: climate change adaptation strategies for California’s water / California Department of Water Resources.


Posted in Environment, Water | Tagged: , , | 2 Comments »