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Geomorphic evaluation of the Thompson Creek Watershed in Santa Clara County, CA, indicated that several stream segments are suffering from excessive erosion and bed incision leading to headcuts and bank failure. Comparison of stream segments upstream of development in the watershed, which continue to be stable and do not show excessive erosion, with stream segments downstream of outfalls that drain impervious areas indicated that the increase in runoff as a result of urbanization (hydromodification) was the most plausible cause of erosion. The magnitude of change in flows (hydromodification) was computed by modeling the runoff from 50 years of continuous rainfall record and calibrating the model to last winter's runoff measurement at the downstream end of the watershed. The results showed that the pre-urban (defined as end of agricultural period in early 1960s) 10 year event runoff occurs once every 2.5 years today; the pre-urban 2 year event occurred 3 to 4 times each year (every 0.25 year). The density of development is fairly high in eastern Santa Clara County, with average impervious areas exceeding 25 percent for the watershed. Analysis of bed material showed that the critical shear stress that mobilizes the bed was exceeded at 10 percent of the 2-year event for the Thompson Creek, whose riverbed is principally composed of fine gravel and sand with relatively steep slopes, typical of creeks draining the Diablo Mountain Range in Santa Clara County. Based on an analysis of flow duration control, it is recommended that solutions be designed for a range of flows from 10 percent of the 2-year event to the 10-year event. The matching is done based on an acceptable value of an erosion potential index (Ep), which is the ratio of cumulative excess shear stress under post development conditions to the pre-urban condition. The recommended regional solution to stabilize the creek for hydromodification from existing development includes grade control structures to prevent head cut migration and bioengineering techniques (brush mats, rock with vegetated cover, etc.) to increase the resistance of the stream channel to erosion. For new developments, the solutions can include basins to match post development runoff volume and duration to pre-urban conditions, in addition to upgrading the channel downstream to accept modified flows. The regional solution is expected to reduce erosion to pre-urban levels and improve water quality.
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Document Type: Research Article

Publication date: 2003-01-01

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