Abstract:
Catchment response time parameters are one of the primary inputs required when design
floods, especially in ungauged catchments, need to be estimated. The time of concentration
(TC) is the most frequently used time parameter in flood hydrology practice, and continues to
find application in both event-based methods and continuous hydrological models. Despite
the widespread use of the TC, a unique working definition and equation(s) are currently lacking
in South Africa. This paper presents the results of the direct and indirect TC estimation for
three sets of catchments, which highlight their inherent variability and inconsistencies. These
case studies demonstrate that estimates of TC, using different equations, may differ from
one another by up to 800%. As a consequence of this high variability and uncertainty, we
recommend that, for design hydrology and calibration purposes, observed TC values should be
estimated using both the average catchment TC value, which is based on the event means, and
a linear catchment response function. This approach is not only practical, but also proved to be
objective and consistent in the study areas investigated in this paper.
