Predicting the existence of stormwater first flush from the time of concentration

Simulations were performed using a one-dimensional kinematic wave model to examine the relationships between the time of concentration (T_c) and the first flush (FF) for impervious landuses. The model used chemical oxygen demand parameters from a highway runoff site in west Los Angeles for eight storm events. FF indices such as the mass first flush ratio (MFF_n) and the partial event mean concentration to event mean concentration ratio (PEMC_t/EMC) were calculated from simulations using various rainfall intensities, rainfall durations, bed slopes, and watershed lengths (L), and matched measured values with an RMS error of 25% for six events. Resulting values of MFF₂₀ and PEMC₆₀/EMC were correlated to T_c calculated using ASCE's kinematic wave formula, and showed a non-monotone relationship, with very short or long T_c having low MFF₂₀ with a maximum for mid-range T_c. Contours of the MFF₂₀ and PEMC₆₀/EMC were created to predict and quantify FF from known T_c, L, and rainfall duration (T).