EFFECTIVE DISCHARGE IN URBANIZING BEDROCK CHANNELS, NORTH TEXAS

Four gaged channels with bedrock bottoms and alluvial banks in Ft. Worth, Texas were surveyed in order to calculate effective discharge relationships for use in stream channel restoration efforts. The survey included channel dimensions, slope, and bedload grain size distribution. All four streams were former USGS gage sites with record lengths of 10 to 25 years. Gage data was analyzed to construct flow duration curves. The survey and gage data were combined with two bedload transport equations (Schoklitsch and Yang sand and gravel) to model the effective discharge for each stream. The D₅₀ of the bedload ranged from pebble to cobble size.

Problems inherent to establishing effective discharge in urban channels relate to increasing discharges due to addition of impervious surfaces and storm sewer systems, channel alterations at gage sites, upstream and downstream channelization, and limited length of of gage records. Despite these problems, correlations between channel slope, grain size distribution, active channel width, effective discharge and drainage area were established. Effective discharges of drainage areas ranging from 2 to 53 square miles were strongly correlated to basin size (R²=0.9437) and had recurrence intervals from 1 to 1.85 years. These trends allow a comparison of effective discharge estimated with historic gage and land use data to current channel and basin characteristics; ratios of discharge allow prescription of new urbanized effective discharge for use in channel design. Present research infers a lag in channel adjustment may be greater than 25 years.