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
Author(s)
Bourne, Robert L.
Mikalsen, Ted
Advisor(s)
Editor(s)
Carroll, G. Denise
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Abstract
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.
Sponsor
Sponsored by:
Georgia Environmental Protection Division
U.S. Geological Survey, Georgia Water Science Center
U.S. Department of Agriculture, Natural Resources Conservation Service
Georgia Institute of Technology, Georgia Water Resources Institute
The University of Georgia, Water Resources Faculty
Date
2009-04
Extent
Resource Type
Text
Resource Subtype
Proceedings