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The Evolving “100-year” Floodplain of the San Jacinto River, Montgomery County, Texas

Authors: Ross Guida*, SHSU, Adam Camp, SHSU, Matthew Hilburn, SHSU, Hanna Hoffman, SHSU, Brooke Jennings, SHSU
Topics: Hazards, Risks, and Disasters, Hazards and Vulnerability, Human-Environment Geography
Keywords: flooding, flood risk, flood exposure, flood damage, record precipitation, land use/land cover change
Session Type: Paper
Presentation File: No File Uploaded

In Montgomery County, directly north of Harris County (Houston, TX), population growth has been explosive, doubling from 294,000 residents in 2000 to an estimated 590,000 in 2018. This growth has been fueled largely by converting forested land to new developments. The San Jacinto River Basin (SJRB) is one of the river basins in Southeast Texas that has experienced major changes in land use/land cover with development encroaching into floodplains. Further exacerbating these changes, record precipitation events have been recorded within different subbasins of the SJRB during each of the years from 2015-2019. As a result, regulatory floodplain boundaries will continue to expand, as will risk and expected damages. This study assesses the dynamic risk from incorporating one record discharge event, Hurricane Harvey, into the statistical record to determine how the floodplain boundary has evolved along the rapidly-developing West Fork of the San Jacinto River. Using FEMA flood insurance study baseline data, updated flood frequency analyses and rating curves, and county assessor’s data, this study quantifies the increase in property exposure and expected damage pre-Harvey and post-Harvey. Initial results demonstrate that expected 100-year flood discharges have increased as much as 30% post-Hurricane Harvey with corresponding flood heights increasing several feet. As a result, over $100 million worth of additional property can be expected to flood during large events. Despite proposed buyouts in some high-risk SJRB areas, new building continues in the expanding floodplain, likely offsetting or exceeding damages avoided during future floods of record.

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