Flooding in coastal areas can occur as a result of several factors, including low-lying areas, land subsidence causing adverse slopes in storm sewer pipes, high groundwater, inadequate hydraulic carrying capacity, and the age of the stormwater system, which may have been laid in the ground before current design standards were implemented. In recent years, another factor reared its head: sea level rise (SLR). Engineers started factoring sea level rise in their designs. More recently in some geographic areas, climate change has resulted in an increase in storm intensity and frequency, which is another contributing factor to the nuisance flooding issues we face today.

To address the increase in rainfall depth over the years, Naval Facilities Mid-Atlantic (NAVFAC) issued a task order to revise and update the 95^th percentile rainfall depth published in the Unified Facilities Criteria (UFC) for Low-Impact Development (LID). At over 50 US and 12 overseas installations, GKY analyzed rainfall data and calculated the updated 95^th percentile rainfall depth.

Local, state, and federal government agencies are considering changes that must be made to build resilient stormwater infrastructure for the future. Updated rainfall analysis is a must for resiliency planning and design efforts. For this reason, we did further research on the available data to answer the questions, “Is the rainfall depth increasing across the board, and what rainfall depth could we expect 10-20 years from now?” In this presentation, we discuss rainfall analyses and current trends in ten major US cities and what those trends will be. Below are bar graphs developed using rainfall data collected at the Norfolk International Airport.