Hydraulic connectivity of impervious surfaces as a key indicator of urban flood control

Abstract:

The total impervious area (TIA) has been the most common indicator of predicting the hydrologic impact of urbanization. Directly connected impervious area (DCIA) is a subset of TIA that particularly contributes to direct discharge of stormwater to downstream channels through connected routes of sewer systems. Which measure better predicts hydrologic response of urban watersheds has long been debated. The purpose of this study is to empirically assess the performance of TIA and DCIA in controlling urban stormwater runoff for changing storm patterns. The study area includes ninety-two watersheds located in three metropolitan statistical areas in Texas (including Greater Houston, Greater San Antonio, and Greater Austin). Using streamflow monitored from USGS gauge stations, runoff depth and peak flow at each watershed outlet were computed at a monthly basis from 2010 to 2017. Multiple ordinal regression models were then developed with a set of imperviousness, land use configuration, climate, and biophysical variables to predict the probability of runoff yields. The results reveal the outperformance of DCIA over TIA in runoff depth prediction and vice versa in peak flow prediction. Yet, DCIA better predicted far-above-average runoff depth, while its contribution to far-above-average peak flow was comparable to that of TIA. Most importantly, the impacts of TIA and DCIA were subject to changes in total and 24-hour peak depths of monthly precipitation. After their performances reached the peak, the impacts of TIA and DCIA no longer depended upon rainfall patterns for large, infrequent storms. The findings demonstrate the importance of controlling the hydraulic connectivity of impervious surfaces over the total amount under certain thresholds of rainfall depth and return period to mitigate high-risk runoff. This study contributes to preventing overestimation of TIA impacts and encourages the use of suitable indicators for effective flood control.