Measurement and Simulation of Flow on Surfaces with Extreme Low Slope for Determination of Time of Concentration

Field laboratory experiments and numerical simulations were used to explore the question of estimating time of concentration for regions with negligible slope. Current methods for estimating time of concentration generally have the slope of the land surface in the denominator of the functional relationship. This means that as slope decreases to zero, the time of concentration becomes infinite, which is contrary to observations. Researchers performed a set of experiments with a rainfall simulator over beds of very low slope. The resulting hydrographs were used to determine the time of concentration as a function of the slope and other pertinent variables. A numerical model was also developed to simulate the flow of water over a low-slope surface with random microtopography. While the field measurements provided some successful insight, the results from the two approaches were not able to determine definitively a relationship for time of concentration that can be used in all cases. The field laboratory study may have suffered from being too small scale compared to the area of the watersheds (l 00 Ac) for which time of concentratio is sought. The numerical study suffered from instabilities brought about by the nature of the equations modeled and, possibly, from lack of knowledge about the boundary conditions. Based on the experience and knowledge gained from this work, it is recommended that future work to determine the time of concentration for regions of low slope would benefit from one or more field observation facilities, where rainfall and flow records could be measured. Additionally, because different times of concentrations were derived in this study depending on the flow conditions or time of concentration determination technique, a standard definition (or standard definitions) of time of concentration is needed.