Capacity and sedimentation of Loch Lomond Reservoir, Santa Cruz County, California

Abstract

A sedimentation study of Loch Lomond Reservoir in Santa Cruz County, California, was begun in 1982 to determine reservoir storage capacity and to establish permanent ranges for future studies. Results of a reservoir survey indicated a total storage capacity of 8,824 acre-feet in 1982. Comparison of thalweg profiles from this survey and a survey done prior to dam construction in 1960 shows that deposition has occurred in the lower reach of the reservoir due to landsliding and in the upper reach due to sediment inflow from Newell Creek.

Analysis of methods to determine storage capacity of, and sedimentation in, Loch Lomond Reservoir, Santa Cruz County, California, 2009

“…Comparison of thalweg transects indicated that deposition had occurred in the lower reach of the reservoir as a result of landsliding and in the upper reach as a result of sediment inflow from Newell Creek (Fogelman and Johnson, 1985). The total storage capacity was computed, by using a method described by Eakin (1936;revised by Brown, 1939), to be 8,824 acre-ft.…”

“…The total storage capacity was computed, by using a method described by Eakin (1936;revised by Brown, 1939), to be 8,824 acre-ft. This data processing method uses the surface area at 10-ft contours to calculate the volume of each prismoid, as described in the 1982 investigation by Fogelman and Johnson (1985), who also describe the inaccuracy of the base maps and initial surveys, which prohibited comparison with the 1971 computations; therefore, an estimate of the volume of sedimentation was not attempted.…”

“…Measurements of sedimentation by means of sequential reservoir capacity surveys were completed in 1971, 1982, and 1998. These surveys provided general sedimentation rates with limited success as a result of the differences in methods and precision among the surveys (Brown, 1973;Fogelman and Johnson, 1985;McPherson and Harmon, 2000).…”

“…Water managers for the City of Santa Cruz periodically measure storage capacity to determine whether sedimentation has occurred, allowing them to take timely and appropriate action to regulate water supply. Sedimentation occurs in the lower reach of the reservoir because of landslides from the surrounding slopes, and in the upstream reach because of the inflow of sediment carried in streamflow from Newell Creek (Fogelman and Johnson, 1985).…”

“…<0.5 0.5 7,940-9,710 As reported by Fogelman (1985), the inaccuracy of the base maps and initial surveys prohibits comparison with the 1982 computations; therefore, an estimate of the volume of sedimentation was not attempted.…”