Results
Total Inundation Area
​
The total inundation area is the area below the flood stage (everything that is underwater!). Total inundation areas are determined by analysis of area under the flood water levels (elevations) in the Digital Elevation Model (DEM) for the watershed. Sixteen (16) inundation area cases are calculated for the 10-year, 50-year, 100-year, and 500-year events for the downstream water level and peak discharges. As shown below, the largest flooding area is 4417 acres with 0.2% discharge and downstream water level. In contrast, the flooding area becomes the smallest with 1662 acres when both discharge and water level are 10-year events. The average flooding area of the 16 cases is 3059 acres. An increase in return periods for either discharge or water level will lead to growth in flooding areas. However, the rise in discharge will cause a much larger increment in the inundation region than the rise of downstream water level, indicating that precipitation-induced discharge changing is the main factor for flooding hazard at the East River watershed.
The areal extent of the flooding is illustrated below for four cases: a 10-year event, a 50-year event, a 100-year event, and a 500-year event.
Flooding area of different land use​
Flooding areas are categorized by nine general land use based on the land use map at Brown County in 2014. As shown below, the largest land use region that suffers from flooding issues is natural areas, which have an average of 1252 acres and account for about 41% of total flooding areas. 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. For the upstream of the East River, flooding lands are mainly natural and agricultural areas, but for the downstream, the flooded regions are mostly used for residence, transportation, and outdoor recreation.
Flooding areas with different land use types show various changing patterns as the return period of discharge and water level increases, as shown below. We further compare the flooding area of each land use under 16 cases with the smallest area from the case with 10-year discharge and 10-year water level.
The most sensitive land use to flooding issues is institutional/governmental area, where an increment of 2300% happened in the flooding area as discharge grows from 10-year event to 500-year event. Also, there are huge increases of 1100% and 1300% for commercial and communication areas, respectively. For residential and industrial land use, the growth is smaller but also significant, with a value of around 900%. On the contrary, natural and recreation areas are relatively insensitive to discharge or water level rise with the maximum increment of 48% and 67%, respectively. Overall, extreme flooding events are likely to cause more urbanized, densely-populated, high-valued areas, such as institutional and commercial regions, to be inundated than small events, while natural and agricultural areas are insensitive to an increase in discharge or water levels.
