Use of basin outlet velocity to determine the basin concentration time and storage coefficient

Most empirical formulae for the basin concentration time (T_c) and storage coefficient (K) focus on estimating the representative values under the ordinary condition, with their return period being a maximum of 100–200 years. Under more extreme conditions, those parameters should be modified to consider faster velocity conditions. The main objective of this study is to examine the possibility of determining these parameters corresponding to the given peak velocity (v_p) at the basin outlet. Two issues are involved in this problem; one is whether T_c can be fully expressed by v_p, while the other is whether K is still linearly proportional to T_c under extreme conditions. In this study, these two issues are resolved by the theoretical review of these parameters, as well as an analysis of the rainfall–runoff events collected at the Chungju Dam basin, Korea. It is observed that as v_p increases, T_c and K decrease. Their relationship is close to inverse but in linear proportion. That is, strong linear relationships are found among T_c, K, and v_p. As a result, the ratio of K to T_c is found to be almost identical, regardless of v_p. This ratio at a basin can be assumed as a basin characteristic that is unchanged, regardless of the size of rainfall events.