Final answer:
Floodways are marked on a new-format Flood Insurance Rate Map with distinct shading or patterns to indicate areas reserved for passing floodwaters. The 21,200 cfs discharge measured during the 9/23/2009 flood event can be plotted on a graph to help understand the flood's recurrence interval.
Step-by-step explanation:
The question relates to how a floodway is shown on a Flood Insurance Rate Map (FIRM), which is used to identify different flood risk areas. FIRMs are used by the National Flood Insurance Program (NFIP) to dictate the requirements for flood insurance and are vital for urban planning and building codes in flood-prone areas. The new-format FIRMs display floodways typically with some form of shading or hatch patterns along the rivers or coastal areas. This shading usually differs from the surrounding floodplain areas to indicate the designated area that would be preserved to allow for floodwaters to pass without increasing flood heights.
During a flood event, such as the one on September 23, 2009, measuring the discharge in cubic feet per second (cfs) is critical for understanding the extent and impact of the flood. The discharge you've noted, 21,200 cfs, would be plotted on a hydrograph or similar graphing tool provided by the FIRM. By plotting this data, professionals can calculate the recurrence interval, which indicates the estimated average time between floods of a specific size.