Summary

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1. Flood hazards at the East River are assessed by re-building the HEC-RAS model and steady flow conditions. Floodplain is delineated based on 16 cases with different combinations of discharge and downstream water levels, where inundation areas classified by general land use are compared.

2. An average of 3059 acres area is exposed to flooding hazards at the East River watershed, and the inundation area is more likely to be affected by changes in discharge rather than water levels.

3. Natural area is the largest region that suffers from flooding, accounting for an average of about 41% of total inundation areas for 16 cases. The agricultural/silvicultural and residential regions have the second and third largest flooding-affected areas, constituting an average of 656 (21%) and 391 (13%) acres. Also, about 11% of the inundation area is for recreation and 9% for transportation.

4. Extreme flooding events are likely to cause more urbanized, densely-populated, high-valued areas, such as institutional and commercial regions, to be inundated, while natural and agricultural areas are insensitive to an increase in discharge or water levels.