The impact of urbanization on the streamflows and the 100-year floodplain extent of the Sims Bayou in Houston, Texas

Abstract

ABSTRACTThe Houston Metropolitan Statistical Area has experienced rapid population growth in the past decades. The impact of this growth on the streamflows and 100-year floodplain extent in the Sims Bayou over the 1980–2000 period has been studied and is reported. Development and imperviousness over time was determined using parcel data provided by the Harris County Appraisal District and Fort Bend Central Appraisal District, and aerial photographs. The change in the annual streamflow volume over time was studied, and found to have a statistically significant increasing trend. A similar analysis of the annual instantaneous peak flows showed that, although an increasing trend in their values is not apparent, there is a statistically discernible increase in their variability. Strong relationships between the annual streamflow volume and annual instantaneous peak flow, and the imperviousness were found. The imperviousness increased from 15% in 1980 to 18% in 2000. For the 100-year 24-h storm event, hydrologic analysis with HEC-HMS resulted in peak flows of 41,000 cfs under the 1980 development conditions, and 43,000 cfs under the 2000 conditions. A HEC-RAS hydraulic analysis of the flood inundation extents, resulting from these peak flows, estimated 6500 acres flooded in 1980, and 7500 acres in 2000. Assuming that residential parcels have an area between 0.12 and 0.30 acres, it was concluded that, because of development in the 1980s, around an additional 2100 households were subject to flooding in a 100-year event, while, because of development in the 1990s, a further additional 1400 households. Although development is needed to accommodate population and employment growth, the negative consequences to people and the built environment can be devastating. Local government implementation of low-impact development policies is needed to reduce the effect of urbanization on the watershed’s hydrology and hydraulics, and also to limit adverse flooding impacts.