Program with Abstracts
41st Annual Meeting
Association of Engineering Geologists
Seattle, Washington - September 31 - October 3, 1998

ESTIMATING CHANGES IN WATER AND SEDIMENT BUDGETS FOR

DEVELOPING WATERSHEDS: A CASE STUDY

By

Donald W. Tubbs, William R. Clevenger and Charles S. Lindsay

A roughly 1000-acre forested site in western Washington is being developed as an Urban Planned Development. The project is located in an upland area that provides recharge to aquifers and lies within the headwaters of several streams.

Water budgets were used to model existing groundwater recharge at the site and to estimate potential changes in recharge under various development scenarios. Water budgets were estimated for individual sub-basins within the project area using (1) daily rainfall data, (2) monthly air temperature data, (3) soil and vegetation information and (4) existing and proposed land use information. Values of evapotranspiration were estimated using the Thornthwaite/Mather method and values of storm runoff were estimated by the SCS method. The water budgets for existing conditions were calibrated using surface water discharge information from on-site gaging stations. The water budgets were used to estimate the amount of induced infiltration needed to match existing groundwater recharge under developed conditions.

An analysis of geomorphic processes was completed to assess the potential impacts of land use changes on erosion and sedimentation for each development scenario during and after construction. Factors considered in the analysis include development design, construction techniques, effectiveness of erosion control measures, local geologic/soils conditions and the response of geomorphic processes to changes in vegetative cover, runoff and land use. Subareas were designated within the study area based on topography, soil type, surface water, vegetation and geomorphic features. Sediment flux was evaluated for each subarea by estimating the rate of erosion or deposition for each geomorphic process within the subarea based on field observations and published information.

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