Flow Resistance in Broad Shallow Grassed Channels

Resistance to sheet flows over natural turf surface is investigated experimentally. The formulation of a functional relationship between the resistance coefficient and controlling parameters for shallow flows over various turf surfaces is essential to the mathematical modeling of the rainfall-runoff process and sediment yield from a watershed which may consist of various vegetated surfaces. An analysis of results obtained from laboratory experiments for laminar flow on Kentucky Blue grass and Bermuda grass surfaces reveals that a relationship exists between the Darcy-Weisbach friction coefficient, Reynolds number, and bed slope. A further analysis of previous investigators' experimental results for flow on various smooth and rough surfaces under or no rainfall leads to the qualitative determination of a general trend of the resistance relationship for shallow flows over such dense turf surfaces as affected by bed slope, raindrop impact, and roughness size.