In recent flooding, homes were devastated and roads inundated as water roared through our towns. Today, many are asking, why did flooding happen, and what, if anything, can be done to prevent it?
The simple cause: It rained a lot! Immediately prior to the recent flooding, the ground was saturated from record rain. As a result, practically all the rapidly falling rain became runoff. It barreled downhill and filled all the low places on its way into our streams, which swelled precipitously and overflowed their banks. A flood was born.
We know what happened, but what can we do to keep it from happening again?
The answer rests on understanding streams and the geographic nature of where we live. Flooding is a natural part of a stream’s cycle. Each year, a healthy stream floods its banks, dissipating the energy of a storm surge and exchanging nutrients with the land.
Our streams, unfortunately, are not healthy. Our agricultural legacy inundated them with topsoil, and now they are struggling to adjust to our growing urbanism. More impervious surface has led to routine extreme flows – higher and faster during storms, and lower and slower in between. The resulting steeply cut banks create more dangerous flooding conditions, as stormwater builds up greater speed and volume.
How streams flow
Our streams are dynamic systems, which follow certain rules. They speed up on steep, smooth surfaces and slow down in flat areas. They create floodplains in low areas where they spread out and deposit sediment.
Streams don’t naturally stay in one place but continually migrate laterally, particularly in the flood plain. From long experience, we know to oppose a stream’s nature is not only extremely costly but also generally unsuccessful.
Billions of dollars, for example, have been spent on the Mississippi River to control floodwaters. Ditching, channelizing, levees and walls meant to reduce flooding have actually increased peak flows. Nearby residents continue to suffer extensive property damage.
So what can we do? Make room for streams! At every level of municipal planning, we need to take into account water’s natural actions. Each land area has its own water signature, which is created by topography, soil and climate. In Chapel Hill, the signature varies considerably from the steep slate-belt hillsides to the more low-lying Triassic basin. Water is predictable, and by paying attention to its natural characteristics, we can find a better way to live with it.
Our most critical action is to improve our planning for our future. Since Chapel Hill is rewriting its land-use ordinances, as well as planning for more high-density development, it is an opportune time to remember:
• Don’t build in the floodplain. This is a part of the stream.
• Provide enough space as a healthy riparian buffer to contain stream fluctuations.
• Don’t create larger storm flows. Limit impervious surface and include ways to infiltrate stormwater, such as rain gardens. Pay attention to water’s path through the land. If developing a property, maintain or improve this movement.
• Consider improving design standards. Given the current climate outlook, our stormwater volume standard should be set higher than our local two-year frequency, 24-hour storm-duration event.
• Address high flows from existing development. Measures to slow down water from impervious surfaces are critical to protect property and maintain water quality.
A study, a plan
This last point has been considered in our towns’ pilot watershed study of Bolin Creek, one of the most studied streams in our nation. In the recently completed Bolin Creek Restoration Plan, a plan of action is proposed for reducing flooding, addressing stormwater, and restoring the health of Bolin Creek.
Two areas are identified as the largest contributors to Bolin Creek storm surges: the Tanyard Branch and Mill Race tributaries. Due to large amount of impervious surface, steep slopes and extremely poor storm-water control, these areas contribute huge volumes of polluted water to Bolin Creek.
Building regional stormwater control facilities at these two locations will reduce the storm-surge volume significantly.
These projects may be costly but will be an efficient way of addressing problems from existing development.
The facilities will be designed to contain stormwater, to address flooding and protect property owners downstream, including areas that suffered some of the greatest flood damage.
These facilities have the potential to pull Bolin Creek off the list of impaired waters and return it to a healthy water quality.
The choice is ours: live within the context of our geography or impose the consequences of periodic flooding damage on the people of our community.
Betsy Kempter lives in Chapel Hill.