Why Worry about Your City’s Stale Stormwater Design Plans?

As climate changes in the 21^st century, we expect rainfall events over the United States to intensify. Adaptation to these changes requires us to reassess existing floodplains. However, local municipalities rely on floodplain plans using intensity-duration-frequency (IDF) curves derived from NOAA Technical Paper 40 (TP40) or other sources that are outdated for today’s existing conditions, let alone those developed for future climates. Such divergence due to stateliness is especially the case for basins situated in complex terrain, such as the South Dakota Black Hills whose stormwater plans’ rainfall criteria predate the 1972 Rapid City flood, where the TP40 and more contemporary Atlas 14 IDF curves can show a significant disagreement between the two products.

Here, we demonstrate the streamflow and floodplain sensitivity to older design storms (TP40), “contemporary” design storms using the NOAA Atlas 14, and projected future storms over test creek basins for the front range of the SD Black Hills. Our future design storms use the IDFs derived from daily CMIP6 LOCA2 downscaled analyses adjusted to shorter event durations using the existing TP40 and Atlas 14 analyses. These products are used to generate streamflow results for both short-and-long range models with SWAT, HEC-RAS, and HEC-HMS.