Where Does All that Heavy Rain Go?
By Heidi Sonen & Roscoe Shaw
We sure had a lot of rain last month. Over eight inches fell at our house and some of it came down really hard. In fact, on June 1, over two inches of rain fell in just 20 minutes. Most people don’t think about what happens after it falls, but the runoff from downpours has been a major environmental topic and source of change in the last couple of decades.
In a natural setting, rain falls onto the grasslands and forests. Much of it is absorbed into the ground and the excess runs into creeks and streams, eventually heading to the ocean. The water that is absorbed into the ground is filtered, cleansing both the water and the earth. This absorption also dramatically slows the speed of the water runoff.
The invention of modern cities has dramatically changed the environmental equation when it comes to runoff. Imagine a giant parking lot at the mall. When a thunderstorm hits, all the water and trash and road grime almost instantly race to the nearest creeks and rivers with no absorption, no filtering, and no cleansing. Also, many cities originally mixed storm water runoff with sewage so it all just went straight into the river.
The result was an increase in flash flood risk plus greater pollution of waterways. Another unforeseen problem was the creation of urban heat islands (UHI). A giant parking lot bakes in the sun and has no evapotranspiration to aid in cooling. Even a place the size of Crozet can create an urban heat island that makes it measureably warmer than the surrounding countryside.
In the last 30 years, better and better environmental practices have evolved and been regulated to require storm runoff to simulate a more natural course. Crozet has doubled in size in the last 10 years and all the new development has come with provisions to slow runoff and prevent stream pollution.
The most common environmental change has been the creation of stormwater dry ponds. If you look for them, they are everywhere these days. When new construction takes place, generally a dry pond must be built to capture the runoff. This slows and filters the water so that it makes it to the rivers at roughly the same speed and water quality as before the new construction. “Dry ponds” are dry most of the time and only fill during rainy periods. “Wet ponds” are similar detention ponds but stay at least partially filled all year. Both are often planted with water-loving vegatation that makes the pond a better filter and more visually attractive.
A significant purpose of the recent streetscape project in downtown Crozet was to improve the storm drainage. Water from the buildings and pavement downtown is now captured in dry detention ponds in several locations and finally into a giant wet detention pond next to Animal Wellness on Crozet Ave. During a heavy rain, the water can collect here and slowly filter through the plants and ground before easing into Powell’s Creek, then Lickinghole Creek to Mechums River then to the Rivanna River near Ivy Creek Natural Area before joining the James River south of Palmyra and then to the Atlantic Ocean at Norfolk.
June was wet. Very wet. Our rain gauge picked up 8.36” of rain which is more than double the normal but short of the record of 12 inches. On a quiet afternoon, if you listened carefully, you could hear the grass growing. The grass grew an average of 12” in June and 37” so far this year. Neither is a record but enough to keep you busy trying to keep up.
- Greenwood 6.65
- Mint Springs 8.36
- Beaver Creek 5.23
- Ivy 4.94”
- Wintergreen 7.17”
- White Hall 8.77”
- Waynesboro 4.58”
- Nellysford 4.53”
- CHO Airport 7.85”