Modified Models of Wash-Off from Roofs and Roads for Non-Point Pollution Analysis during First Flush Phenomena

Continuous monitoring for runoff pollution was conducted at an urban drainage. Stormwater sampling was extensively done for suspended solids (SS) measurement during initial 8 hours of rainfall focussing on first flush phenomena as well as monitoring rainfall and water level in conduit. Reasonable simulation of runoff quantity was obtained using a commercially available distributed model, considering distribution of roof and road as impervious surface. There were good agreements of water level between observation and simulation after adjustment of values on initial loss and single linear reservoir constant for roof and road surfaces. The Sartor and Boyd model was applied to investigate SS wash-off process from impervious surface. Although simulations with the calibrated model parameter values captured general trends of pollutographs, they did not work well to express dynamic pattern. The disagreement seemed to be caused by different behavior of pollutant wash-off from roofs and roads. It was necessary to allocate different wash-off, coefficient for each surface. In addition, the concept of critical runoff rate was also introduced to initiate delayed wash-off from road surface. The modified model for wash-off process explained well dynamic SS runoff pattern in several events with different rainfall characteristics. It was found that distinction of roof and road surfaces for runoff pollution was essential to explain first flush phenomena in cases of weak rainfall.