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February 18, 2004 Accurate estimation of the flow roughness coefficient, often expressed as Manning's n, is critical for predicting water surface elevations or stream discharge. Flow roughness generally cannot be measured directly, and common methods for estimating Manning's n may not be applicable to Alaska streams with steep gradients and coarse bed material. For this study, Manning's n was computed from field measurements and methods for estimating Manning's n in similar streams were explored. Manning's n for 13 small Alaska streams in various physiographic settings was verified from field measurements of discharge, water surface elevation, and two to four channel cross-sections. Width-to-depth ratios at high flow ranged from 10 to 40, drainage areas from 1.5 to 140 square miles, and water surface slopes from 0.4 to 7 percent. Gravel and cobble bed material provided grain roughness. Large boulders, poorly organized boulder steps, bedrock protrusions, mild contractions and expansions, and riffle-pool sequences provided form and spill roughness. Measurements at both flood conditions and at moderate to low discharge quantified the corresponding change in flow roughness. Preliminary results suggest that field estimates
by experienced hydrologists underpredict Manning's n for this type of
stream. A common equation using friction slope and hydraulic radius overpredicted
Manning's n for these streams. Spatial variability in Manning's n appears
to be inherent to all the streams but particularly prevalent in streams
with high values for form and spill roughness. Reach-average Manning's
n was correlated to slope, suggesting that slope may provide a means for
estimating flow roughness coefficients. Reach-average Manning's n was
not inversely correlated to hydraulic radius, as was originally expected,
but hydraulic radius was useful for assessing the discharge-dependency
of Manning's n. |