Young-Jae Yi

Improved Time of Concentration Estimation on Overland Flow Surfaces Including Low-Sloped Planes

AbstractTime of concentration (Tc) is one of the most used time parameters in hydrologic analyses. As topographic slope (So) approaches zero, traditional Tc estimation formulas predict large Tc. Based on numerical modeling and a review of relevant literature, a lower bound for slope (Slb) of 0.1% was identified as a threshold below which traditional Tc estimation formulas become unreliable and alternate methods should be considered. In this study, slopes less than Slb are defined as low slopes. Slopes equal to or exceeding Slb are defined as standard slopes where traditional Tc estimation formulas are appropriate. A field study was conducted on a concrete plot with a topographic slope of 0.25% to collect rainfall and runoff data between April 2009 and March 2010 to support numerical modeling of overland flows on low-sloped planes. A quasi-two-dimensional dynamic wave model (Q2DWM) was developed for overland flow simulation and validated using published and observed data. The validated Q2DWM was used in a ...

Estimating Time of Concentration on Low-Slope Planes using Diffusion Hydrodynamic Model

The time of concentration (Tc) is an important parameter for hydrologic design, analysis, and modeling. Error in the estimation of Tc will cause error in prediction of peak discharge (Qp), resulting in an incorrect design. Overland flow Tc depends on several factors, such as rainfall intensity (i), length (L), topographic slope (S0), and flow resistance (n). Generally, Tc has an inverse relation to S0. For the areas with very small or zero S0, use of such small values for S0 in traditional methods (empirical equations) for estimating Tc can produce very large values of Tc that seem to be unrealistic and incorrect. This study was conducted to identify a lower slope bound (SLB) for the use of traditional equations in determination of Tc and to propose the use of an alternate method for the determination of Tc on low-slope planes. The diffusion hydrodynamic model (DHM) was validated using data collected from published studies and field data collected in this study, including observed rainfall hyetographs and runoff hydrographs for 27 rainfall events for watersheds with relatively low-slope. After model validation, DHM was used to generate parametric relations between Tc and its explanatory variables (rainfall intensity, length, slope and resistance to flow) and extend the regression model to low-slope planes.