A Long Term Study of the Response of a Piedmont Headwater Stream to Rapid Development: An Evaluation of Relationships Between Trends in Bankfull Width/Depth Ratios, Bankfull Quantity, Bankfull Area, and Shear Stress
Bourne, Robert L.
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This continued assessment of a long-term study of the response of a Piedmont headwater stream to rapid development addresses trends and behavior of selected physical attributes of stream morphology including bank-full width/depth ratio, bankfull area, bankfull quantity, and shear strength. For the five study reach cross-sections reported, annual mean bankfull area and mean percent silt and sand in pebble counts were highly correlated (n=5) with and followed a similar trend to annual rainfall lagged two years, while mean bankfull width/depth ratio, and to a lesser, extent mean bed elevation are negatively correlated, suggesting an approximately two-year lagged response to annual rainfall. On the other hand mean calculated quantity, mean bankfull width, and mean bankfull depth were highly inter-correlated and followed the same gradually increasing trend as the increase in the percent of the impervious portion of the watershed. The mean width depth ratio for the cross-sections has decreased over time indicating that the bankfull cross-sections are deepening more rapidly than widening in response to the development of the watershed. Initial estimates of the bankfull area at the study cross-section ranged from 23.3 to 44.2 ft2 and enlarged at widely varied rates (9.4- 121.4% over the ten-year study period. The bankfull areas were higher and increased at a greater rate in the lower sinuous and pooled portion of the study reach as compared to the upper and straight, riffle-run crosssections. Proffered explanations for these widely varied estimates were that varied bankfull areas could satisfy the continuity equation (Qbf = Abf * V) for open channels, varied physical conditions at cross-sections could affect flow dynamics, particularly velocity, rapid and pronounced changes in channel configurations could mask or obscure the key visual determination of bankfull elevation, and other influences. Mean cross-sectional calculations of bankfull quantity with the Manning equation increased 14.1% from 103.3 CFS in 1999 to 117.9 CFS in 2006, with a substantial range in the mean quantities for individual cross-sections (68.7 to 92.1 and 179.5 CFS for lower Cross-sections #1 and 2). These variations in calculated bankfull flow suggest that the continuity equation does not completely explain crosssection variations in bankfull area. Closer examination of the pronounced upstream changes in channel configuration, widely varied velocities, and pronounced changes at Cross-section #2 suggest that e masking or distortion of key visual indicators of bankfull elevation may have contributed to overestimating bankfull area and consequently quantity calculations. Backwater effects, downstream flow impediments, and substantial degradation of the mean channel bed elevation in advance of adjustments in visual indications of bankfull elevation may have contributed to relatively greater estimates of bankfull area and estimates of Qbf at Crosssection #1.