Predicting Manning and Darcy-Weisbach Rouphness Coefficients in a Loess Soil Surface under Different Rock Fragment Covers

Resistance to surface flow such as Manning's and Darcy-Weisbach rouphness coefficients were an important input for estimating soil erosion by soil erosion process models. Also, it is very important for designing and implementing soil and water conservation practices. The objective of present research was predicting Manning's and Darcy-Weisbach rouphness coefficients in surface of a loess soil under different rock fragment covers. For this porpose, a flume was used with 6 m length, 0.5 m width and 3% slope. The treatments included rock fragment cover (0, 10, 20 and 30%) and three levels of flow discharges (3, 6 and 9 lit. min-1). The results showed that Manning's and Darcy-Weisbach rouphness coefficients increased as exponential with increasing rock fragment cover. Darcy-Weisbach coefficient increased 84.3, 83.8 and 85.7% with an increase rock fragment cover from 0 to 30% at 3, 6 and 9 lit min-1 flow discharges, respectively, and Manning's rouphness coefficient increased 96.9, 96.7 and 97.4% at mentioned flow discharges. Rouphness coefficients decreased as exponential (R2=0.99) with increasing flow velocity at a rock fragment cover. Also, (8/f)0.5 increased as logarithmic with increasing relative submergence. Generally, results of this study showed that rouphness coefficients not only were dependent on size and shape rock fragment cover but also, were influenced by factors such as flow rate, depth and velocity and also, rock fragment cover percentage.