Severe consequences of flash floods: Ellicott City

What’s the worst that can happen if you urbanize an area without thinking carefully about stormwater? Ellicott city in Maryland, US is a good example, with terrible consequences.    

Ellicott city was urbanized before stormwater standards were required, but it doesn’t mean stormwater didn’t exist before the requirements.  

Wherever it rains, there will be water draining of the surface. The outcome of a rainfall event is characterised by the areas climate, topography and land use. With urbanization, the use of impervious surfaces increases, and stormwater will run above the surface instead of infiltrating to the ground.  

In addition to urbanization, the topography around Ellicott city isn’t favourable, as the city is places at the bottom of a topographical funnel with steep slopes, at the confluence of several streams.  

When the hydrological characteristics in cities are changing due to urbanization and climate change, the stormwater volumes increase.  Hence the consequences of developing a city without thinking about stormwater can be severe.  

In two years, Ellicott city experience two extreme rainfall events with a return period of 1000-years. A rainfall event with a return period of a 1000-year is not only to occur every 1000 year but has a 1:1000 probability of occurring in any given year.  

In May 2018, 167 mm of rain fell in about 3 hours, which is equal to almost 17 10-liter water buckets per square meter. 

And in July 2016, 152 mm of rain fell in 90 minutes, which is equal to 15 10-liter water buckets per square meter.  

These intense rainfall events, combined with the land use and topography in Ellicott city’s caused extreme flooding and significant damage.  

The floods had deadly consequences in addition to significant damage on buildings, roads, and infrastructure - also on the constructions rebuilt after 2016. Combined the floods have had more than 30 million USD worth of damage, and just the following month after the event in 2016, 42 million USD was lost in economic activity.  

Now Howard County is leading the state to develop new, more rigorous stormwater management regulations.  

Ellicott city is only one of countless examples of a city that hasn’t been prepared for future climate. We don’t have to look far to see local examples on the consequences of urbanization and climate change, just think about it the next time there is heavy rain.

Written by Ida Haveraaen, stormwater engineer and enthusiast

Photocredits: The Baltimore Sun via AP | Shannon Baranoski | David Kidd