Comparing methods for estimating Manning's roughness coefficient on a portion of the South Fork of Chester Creek

Abstract

This report compares several different methods used for estimating the Manning roughness coefficient of a reach using observed conditions during spring runoff. A survey was taken of the South Fork of Chester Creek in the small valley between the Engineering and Computation Building and the North-West Parking Lot at UAA. The survey included 16 cross sections, a center line profile, water surface elevations (WSE’s), and flow velocity readings at each cross section. Based on observed conditions, a model was created in HEC-RAS to calculate the hydraulic characteristics at each cross section. These hydraulic characteristics, along with downstream cross section lengths and head losses, were used to calculate the composite roughness of the reach. This value was then compared to a visual method that accounts for flow-retarding factors, and an empirical method developed in a laboratory. The roughness values computed for the analytical, empirical, and visual methods were 0.071, 0.045, and 0.077 respectively. Additionally, the three computed coefficients were compared with two methods that involve matching table values. The two approximate table values were 0.11 and 0.10. Supplementary methods not used in the analysis are also discussed.