Ecological Planning as a Tool for Urban Flood Resilience: Lessons from Community Developments in the Houston Region

This study probes the differences observed in urban flood resilience and presents empirical findings from two watershed-scale projects (89.4 km² and 55.7 km²) located in Houston, Texas. Houston has seen several significant, regional storms of historic proportion (e.g., 1979 flood, 1994 Hurricane Rosa, 2001 Tropical Storm Allison, 2008 Hurricane Ike, 2015 Memorial Day flood, 2016 Tax Day flood, and 2016 Memorial Day flood). In August 2017, catastrophic Hurricane Harvey released an unprecedented 9 trillion gallons of water in two days, paralyzing a large portion of the Greater Houston area. Harris County Flood Control District officials reported that Harvey actually made the third “500-year” flood in three years in Houston (2015, 2016, and 2017, respectively), and produced enough rain to supply the US’s water usage for 102 days. Despite the flood-prone situation in the Texas coastal region, The Woodlands, a community lying 43 km north of Houston, sustained relatively minor impact during the above events, including Hurricane Harvey. The adjacent suburbs of Oak Ridge North and Timber Ridge, like the greater Houston area, however, were awash during these events. We find the sharp contrast of flood resilience is a result of The Woodlands’ comprehensive ecological plan, which is lacking in Houston’s conventional community development model. The Woodlands’ ecologically-based approach to community design features green infrastructure (GI) strategies throughout, including an integrated suite of on-site, infiltration-based stormwater management techniques. The Woodlands is compared to an adjacent community that follows the conventional design approach to stormwater management (i.e. a network of underground pipes and detention areas) used in most US communities. The study uses parcel data to estimate each site’s impervious cover area, which is then correlated with observed streamflow and water quality data (i.e., NO₃^-N, NH₃^-N, and TP). Results show that the impervious cover percentage in The Woodlands (GI) site (32.3%) is more than twice that of the conventional site (13.7%). However, the GI site’s precipitation-streamflow ratio maintains a steady, low range, whereas this ratio fluctuates substantially in the conventional site, suggesting a “flashy” stream condition. Furthermore, in the conventional site, annual nutrient loadings are significantly correlated with its impervious cover percentage (p<0.01), whereas in the GI site there is little correlation. We conclude that integrated GI design is an effective strategy to reduce stormwater runoff and improve water quality, thus enhancing urban flood resilience in watershed-scale community development.