WRRCTR No.127 Urbanization-Induced Impacts on Infiltration Capacity and on Rainfall-Runoff Relation in an Hawaiian Urban Area

Abstract

The effect of urbanization on infiltration on rainfall-runoff relations was investigated at Mililani Town, O'ahu, Hawai'i, a residential community. The double-ring method determined the mean constant infiltration rate to be 0.298 cm/min (7.03 in. /hr) under preurban, long-abandoned, and overgrown pineapple field conditions. Following the transitional grubbing and lot-shaping operations, the rate slowed respectively to 0.126 cm/min (2.98 in. /hr) and 0.022 cm/min (0.51 in. /hr). The rate rose slightly to 0.050 cm/min (1.18 in. /hr) under lawn, the new permanent cover. There was no significant difference between recreational and nonrecreational lawns. Incorporated in these results are unmeasurable quantities of lateral flow through the porous soil under original and grubbed conditions. Based on the data, however, the constant infiltration rate is reduced 83% after urbanization. For single-family residential areas the impermeable area overlaid by paving and buildings was 27% of the total surface area; for town houses it was 64%. Based on the experimental data for single-family residential areas, the combined effect of reduced soil infiltration and accrued impermeable surface area is estimated to reduce infiltration opportunity 10 to 1 after urbanization. Or, there may be as much as 93% reduction in infiltration after urbanization, depending on whether rainfall frequency, intensity, and duration exceed infiltrative capacity. The impact of urbanization was evaluated with a digital computer using the St. Louis Heights Watershed Model (modified ILLUDAS) with field measured rainfall-runoff relations as the basis for verification. When pervious lawn surface is substituted for impervious areas as inputs, the peak runoffs are much less than those simulated with existing impervious lands. The reduction in peak runoff from one selected storm is estimated at 79% of the observed peak.