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United States Department of Agriculture

Agricultural Research Service

Title: Determination of Reservoir Sedimentation

Author
item McIntyre, Sherwood

Submitted to: Proceedings 1996 Canberra Users Group Meeting
Publication Type: Proceedings
Publication Acceptance Date: September 26, 1996
Publication Date: N/A

Interpretive Summary: A water depth survey is the conventional way to determine reservoir sedimentation rates, but survey results are dependent upon previous measurements. Previous measurements are often unavailable and those that are may be unreliable which causes uncertainty about sedimentation rates and water storage capacity of many reservoirs. Lake Loiza, a 261 ha water supply reservoir for San Juan, Puerto Rico, was a case where reservoir storage capacity was uncertain because there were doubts about the reliability of previous water depth surveys. To alleviate uncertainty about the existing storage capacity of Lake Loiza, sediment cores were collected in 1990 for Cs-137 dating in conjunction with a water depth survey. Cs-137 techniques which do not rely on previous measurements indicated an average annual sedimentation rate of 1.15 percent of reservoir volume with 43 percent of the reservoir volume lost to sediment by 1990. Findings of the 1990 water depth survey agreed with the Cs-137 findings. Cs-137 findings are significant because they increase confidence in ability to determine available reservoir storage capacity and indicate that remedial action is required to prevent the loss of Lake Loiza in less than fifty years as a water supply.

Technical Abstract: A water depth survey is the conventional way to determine reservoir sedimentation rates, but survey results are dependent upon previous measurements. Previous measurements are often unavailable and those that are may be unreliable which causes uncertainty about sedimentation rates and water storage capacity of many reservoirs. Lake Loiza, a 261 ha water rsupply reservoir for San Juan, Puerto Rico, was a case where reservoir storage capacity was uncertain because there were doubts about the reliability of previous water depth surveys. To alleviate uncertainty about the existing storage capacity of Lake Loiza, sediment cores were collected in 1990 for Cs-137 dating in conjunction with a water depth survey. Cs-137 techniques which do not rely on previous measurements indicated an average annual sedimentation rate of 1.23 percent of reservoir volume from 1953 to 1964 and 1.12 percent from 1964 to 1990 with 42.7 percent of the reservoir volume already filled with sediment by 1990. Results of the 1990 water depth survey were very similar to the Cs-137 results and indicated an average annual sedimentation rate of 1.17 percent from 1953 to 1990 with 43.1 percent of the reservoir volume lost to sediment by 1990. Study findings increased confidence in ability to determine the amount of available water stored in the lake.

Last Modified: 4/21/2014
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