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Alaska Section, American Water Resources Association

Jeff Conaway, Alaska Section AWRA Southcentral-Region Director

This is a tentative list of winter speakers for AWRA, South-Central Section. The Southcentral AWRA generally meets 12:00 - 13:00 on the 3rd Wednesday of each month in the EROS conference room, 2nd floor, Grace Hall, 4230 University Drive, APU campus, Anchorage. Presentations are informal, and designed to be informative, timely, but not necessarily polished. If you would like to present a work in progress, chair a panel discussion of a current water range of government and private hydrologists, please call or email Jeff Conaway at - (907) 786-7041.

February 18, 2004

Verification of flow roughness coefficients in small Alaska streams
Janet Curran, USGS

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.

Southcentral Region Brown Bag Agendas and available abstracts

Please visit our previous years Southcentral Region Brown-Bag abstracts and talks. Information is kept on-line to better serve our speakers, members, and the public.

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