Inland Flooding

The past few weeks we’ve seen lots of flooding across the southeast, devasting flooding in North Carolina and Georgia and massive storm surge from the two recent hurricanes-Helena and Milton. Tampa and the western cost of Florida suffered a double hit.

There have always been great storms. The Pittsburgh St Patrick’s day flood in 1936 and the flooding that hit the northeast in 1987, but our Climate is changing, too. Locally (here in Prince William County Virginia), rainfall averages approximately 44 inches per year, but varies from year to year.  Climate forecasts are for our region to get wetter with more intense rainstorms and droughts to get more severe. (ICPRB). The relationship between climate change and flooding is complex. Shoreline flooding is the result of land subsidence, sea level rise, building in the flood plain and storm surge. The most well-established connection between climate change and inland flooding is that more warming leads to more intense rainfall, which in turn increases flood severity in the inland areas. 

This rainfall intensification effect reportedly accounts for more than one-third of all damages from inland flooding in the U.S. in recent decades. Changes in land use are also intensifying inland flooding. Land use changes that increase impervious cover reduces the open area for rain and snow to seep into the ground and percolate into the groundwater and the impervious surfaces cause stormwater velocity to increase preventing water from having enough time to percolate into the earth, increasing storm flooding. 

 Increased stormwater volumes and mud flow are a direct result of clearing the land of trees and vegetation, construction activities for buildings and roads. The devastating flooding we’ve seen in North Carolina’s inland mountain towns that are thousands of feet in elevation and hundreds of miles from any coastline seemed so unexpected (despite major floods and landslides in that area -in 1916, tropical storms that originated in Mississippi and South Carolina caused unprecedented flooding in the Asheville area killing around 80 people and causing massive damage. Back-to-back tropical storms hit the region in 2004. Which resulted in flooding.  Helene inundated parts of southern Appalachia with well over 2 feet of rain. Floodwater carried away entire houses and washed out highways, cutting off access to towns some that are still cut off. 

Prince William County has committed to preparing for the impacts of climate change as part of our goal of becoming a Climate Ready Region by 2030. In line with this commitment, the county staff has concentrated on understanding the susceptibility of our county's people and infrastructure to local climate hazards to meet the community's most pressing needs. The county completed a vulnerability assessment in January 2023. This evaluation was designed to anticipate future conditions that could amplify existing vulnerabilities. From 2018 to 2021 Prince William County has had 60 reported Swift Water Rescue events and 178 reported VDOT road closures due to intense precipitation and flooding. Locations of these events are shown below and are mostly inland events. Though the eastern edge of our county is shoreline. This was unexpected. Prince William County realized that they need detailed stormwater modeling  to better understand how flooding occurs throughout the County during precipitation events and to better understand the limitations of existing stormwater infrastructure.  

from PWC

 In the research paper cited below: “The Northeastern United States (which includes Prince William County in their study) has experienced …increases in extreme precipitation events over the past five decades (Melillo et al., 2014, Walsh and Coauthors, 2014, Parr and Wang, 2015a). While the intensities of the most extreme precipitation events (or the heaviest 1% of all daily events) have increased in every region of the contiguous United States since the 1950s, the maximum change in precipitation intensity of extreme events is occurring in the northeast. “ The models used in the study found that that trend is likely to continue for decades.

The results of this study indicate an increase in mean (20–25%) and extreme (>40%) precipitation in near future over the northeast (including the Potomac River basin). In addition, water depths resulting from extreme inland flooding may increase more than 100% in some regions. Neighborhood scale projections suggest that the major cities of the northeast and the surrounding areas will likely be at higher flood risk. In turn, extreme precipitation and flooding can pose significant risks to their infrastructures and networks. It is time to focus on climate adaptation. Even as close as we are to the federal government, we may still find that we are on our own in a flooding disaster.

 

from Sujan et al

Sujan Pal, Jiali Wang, Jeremy Feinstein, Eugene Yan, Veerabhadra Rao Kotamarthi, Projected changes in extreme streamflow and inland flooding in the mid-21st century over Northeastern United States using ensemble WRF-Hydro simulations, Journal of Hydrology: Regional Studies, Volume 47, 2023, 101371, ISSN 2214-5818, https://doi.org/10.1016/j.ejrh.2023.101371