Thirty-one percent of land in the watershed was developed with no stormwater regulation, county officials previously said. Presently, development must meet standards that would withstand a 100-year storm — a storm with a rainfall total that has a 1% probability of occurring in Ellicott City in that year. The storm that dropped more than 6 inches of rain — much of it in less than 90 minutes — on July 30, 2016, and the storms that dumped a total of 15 inches of rain on May 27, 2018, were considered 1,000-year storms.