Experimental study of the water depth and rainfall intensity effects on the bed roughness coefficient used in distributed urban drainage models

• Variability of roughness coefficients with water depth and rainfall is studied.
• Experimental measurements and numerical calibration are performed.
• Results show bed friction variations, not well captured by any standard formulation.

SummaryThe work presented in this paper analyses the effect of water depth and rainfall intensity on the surface roughness coefficients used in overland flow models based on the shallow water equations. The relation between the Manning coefficient and the water depth and rainfall intensity has been quantified using different methodologies based on the analysis of two sets of experimental data. In the first set uniform overland flow conditions were generated, and the bed roughness coefficient was computed from direct measurements of the water depth and discharge. In the second set of experiments, unsteady rainfall–runoff transformations with different rainfall intensities were generated in a flume and computed with a shallow water model in which different bed friction formulations were implemented and calibrated. Results show that for very low water depth values there is a significant increase in the surface resistance, which is not captured by any standard bed friction formulation. Rainfall intensity also increases surface resistance especially as the water depth diminishes below a critical threshold. Using a Reynolds dependent formulation for the Manning coefficient improves model predictions.