BACK TO THE FUTURE: A NOVEL METHOD OF DOWNSCALING CESM FUTURE PRECIPITATION PROJECTIONS TO SMALL WATERSHEDS TO ESTIMATE INCREASES THE EXTENT OF CURRENT FLOODPLAINS

In the mid-Atlantic region of the USA, most climate models, including the Community Earth System Model (CESM) project that global warming will drive an increase in the mean annual amount of precipitation and cause more intense and frequent storm events (IPCC, 2013). This increased intensity and frequency of storms that forecasters are worried about is exemplified by the large amounts of rainfall dumped by hurricanes Irene and Sandy in the past several years. Rainfall leads to an increase in stream discharge, thus more frequent and increased rainfall would cause flood stage discharge levels to be reached more frequently, which translates to more urban flooding. However the CESM makes general regional projections, while the relevant information for urban planning is at the smaller, county scale. The methods for adjusting projections to smaller areas, called downscaling, are computationally intensive and complex, so are impractical for local planners.

This project aims to illustrate a simpler and thus more widely useful technique to correct the CESM projected future 100-year, 24-hour rainfall depth based on its divergence from the 100-year, 24-hour rainfall depth calculated from historical precipitation. The rainfall depths are then converted to water surface elevations at various river cross sections using several software packages produced by the US Army Corps of Engineer’s Hydraulic Engineering Center (HEC-HMS and RAS). A future 100-year floodplain will be delineated and compared to the current, historically-based 100-year floodplain zone. The Monocacy and Saucon Creeks in Bethlehem, PA are used in this study as a good proxy for the scale of watershed that are relevant to many small cities.

To evaluate additional risk that climate change can pose, delineation of a future 100-year floodplain based on adjusted CESM future rainfall may also be developed for a watershed in which unconventional gas (fracking) wells have been drilled. Geographic analysis and remote sensing will be used to help determine how this type of floodplain development could exacerbate potential pollution under new climate norms.

Policymakers, insurance providers, and home buyers would all benefit from considering the impact that a warming climate will have on the severity of flooding in the next century.