By Jay Lund
Removing sediment from reservoirs is often suggested as a potentially better way to expand storage capacity than raising dam heights or building new reservoirs. This is a natural notion to explore given the cost and likely environmental impacts of traditional expansions.
For perspective, the construction cost of conventional reservoir expansion is about $1,700 to $2,700 an acre-foot (af) of storage capacity. For example:
- Expanding Shasta Reservoir at $1,700/af: $1.1 billion for 634,000 af of new capacity
- Building Sites Reservoir (Colusa County) at $1,800/af: $2.3 billion to $3.2 billion for 1.3 million to 1 .8 million af of new capacity
- Building Temperance Flat Reservoir (upper San Joaquin River) at $1,900/af: $2.5 billion for 1.3 maf of new capacity
- Expanding Los Vaqueros Reservoir at $2,000/af: $120 million for 60,000 acre-ft of new capacity
- Expanding San Luis Reservoir at $2,700/af: $360 million for 130,000 acre-ft of new capacity
The cost of expanding reservoir capacity by removing sediment seems likely to be $5 to $20 a cubic yard or $8,000 to $32,000/af (at 1,600 cubic yards/acre-ft).
At these high costs, removing sediment would be done only rarely, in small quantities, for very valuable purposes when other alternatives are unavailable. For example, sometimes sediment that has accumulated behind flood and debris control dams in Southern California is removed for maintenance. But these are usually small dams built to protect homes and businesses from floods with enormous sediment loads.
Sometimes sediment can be removed by releasing it through low elevation dam outlets when the reservoir is very low. This method, known as sluicing, can sometimes be done for small reservoirs. For larger reservoirs, sluicing is limited by two factors: a) the large amount of water that must be released to remove sediment and b) the tendency for most sediment to deposit at the inflow of the reservoir, far up and away from the dam, making it hard to sluice.
Removing sediment must become more economical to be cost competitive with traditional reservoir expansion. The current cost gap is rather large.
It should be noted that most of the largest dams in California have relatively little sediment because their watersheds yield relatively little of it. One study estimates about 1.7 maf of sediment across 1,200 dams in the state (Minear andKondolf 2009). The scientists estimate that sedimentation could eliminate 15 percent of California’s reservoir capacity within 200 years. Most major reservoirs would see little gain from the high expensive of removing the material.
Of course, a few of the 1,400 reservoirs in California might be worth expanding by sediment removal (and some reservoirs might be worth removing entirely). But these projects are likely to be so small and rare as to not have a major statewide benefit, and can probably be financed locally.
Finally, reservoir capacity is not the same as water deliveries from a reservoir. Reservoirs do not create water; they only deliver water stored from some previous time.
A reservoir expansion in California today is expected to yield on average only 7 percent to 14 percent of its additional capacity annually.
Storage should always be looked at as a component of a larger water management portfolio.
Jay Lund is a professor of civil and environmental engineering and director of the Center for Watershed Sciences at UC Davis.
Lund, J (2011), “Water storage in California,” CaliforniaWaterBlog.com, Posted Sept. 13, 2011
Lund, J (2012), “Expanding water storage capacity in California,” CaliforniaWaterBlog.com, Posted Feb. 22, 2012
Minear, J. T., and G. M. Kondolf (2009), “Estimating reservoir sedimentation rates at large spatial and temporal scales: A case study of California,” Water Resources Research, 45, W12502, doi:10.1029/2007WR006703