Abstract
Accurate modelling of flood flow hydrographs in ungauged catchments is a challenging task due to large errors in the estimation of its response time using existing empirical equations. The time of concentration (Tc) is a key catchment response time parameter needed for forecasting of the peak discharge rate and the timing of the flood event. At least eight different definitions have been presented in the literature for the time of concentration. In this study, a new definition of “Reference Tc” is presented along with a practical procedure for its estimation using readily available basin catchment characteristic parameters with the aim of standardizing this key parameter for practitioners. Nine different empirical models were calibrated and tested on nine catchments of the Credit River watershed, Ontario, Canada to determine which method would provide the most accurate prediction of the Reference Tc. The NRCS velocity method (1986) proved once again to be the most reliable and an accurate method. This study shows that the main reason for the higher accuracy of the NRCS velocity method predictions compared to the empirical equations is attributed to the inclusion of the Manning's roughness coefficient.
Highlights
A fundamental part of the development of a flood risk maps in ungauged catchments is the use of synthetic unit hydrographs (Atieh et al, 2017)
The reference time of concentration (Tc) values for the nine gauging stations along the Credit River were compared against the values for Tc from the “Travel Time Model for the Credit River,” (Environmental Water Resources Group Ltd., 2007)
Manning's roughness is not a constant value for all nine stations along the Credit River; upstream stations have higher roughness coefficients as the river is steeper and faster flowing river with shallower depths and larger bed material sizes while the lower Credit River is deeper and smoother sediments with lower roughness coefficients; the Manning's roughness change from upstream to downstream can explain why all empirical models that have overlooked the effect of Manning's roughness on Reference Tc values for all 9 stations of the Credit tend to under predict Tc values for headwater basins and over predict larger Tc values for the downstream stations
Summary
A fundamental part of the development of a flood risk maps in ungauged catchments is the use of synthetic unit hydrographs (Atieh et al, 2017). Synthetic unit hydrographs have typically been used to develop stream flow hydrographs for a catchment. Examples of synthetic unit hydrographs include SCS (USDA, 1985), Williams and Haan (1973), or Clark (1945) unit hydrographs. All three of these unit hydrograph methods (as well as most others) require the estimation of the time of concentration (Tc). Because of the importance of Tc, different definitions, estimation procedures and empirical equations have been developed over the years to more accurately estimate Tc
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