This article was first published in the CHALT newsletter, October 2014
Last week on September 29th, the Town Council voted unanimously to adopt the Stormwater Management Master Plan. What does this mean, and why does it matter? At its simplest, stormwater management is directing rainfall after it falls in a watershed, that is, the land area drained by a creek or stream. Under natural conditions, a large percentage of rainfall will sink into woodland soils. But as Chapel Hill has grown, and structures and roads are added on top of the soil, the amount of rain that can be absorbed in a watershed decreases, often quite dramatically, causing sudden and high velocity flows downstream into creeks and lakes. Under those conditions, water quality is diminished as the rainwater carries heavy metals, soil and other pollutants into our waterways. It’s well-known that our creeks flow into Jordan Lake, a drinking-water supply for upwards of a million people.
In an effort to mimic natural conditions, the Town’s ordinances seek to control the rate and volume of rainwater that leaves each construction site. However, Chapel Hill only controls for storms of the severity we are likely to see every two years on average. In more severe storms, such as we might experience only once every 5 or 10 years, most of the rainwater runs off-site downhill to lower parts of the watershed. The diagram below illustrates the dramatic role trees and forest cover play in regulating the rate and flow of rainfall across the landscape.