Magnolia Grove

What happens to stormwater when natural land is developed?

Land development comes with great responsibility to the environment and to neighbors. Clearing natural land (land-disturbing activity) is regulated to protect the environment. Two factors impact stormwater run-off when land is developed; the clearing of forest, prairies and woodlands and channelizing of water. Several entities are responsible for the careful management of development; developers, engineers, contractors and municipalities.

The Forest is a Solar Pump

Leaves and branches of trees capture some rain as it falls. The rain that reaches the ground can be absorbed by the vegetation, both living and dead, on the ground. More rain soaks into the earth and much of that is captured by the roots of the trees. Thus, the trees work like a solar pump to control much of the rain water. Trees are about 50% water. In a year a 100-foot tree will turn 11,000 gallons of water from the soil and release it as oxygen and water vapor through the leaves. The roots anchor the tree and hold the soil around the tree while filtering harmful substances from the water. Trees help control erosion, sediment and substances in the water.

You can observe a similar effect when your garden hose is running on the grass and the water disappears. However, the same amount of hose water on the driveway remains on the concrete.

Excess rainwater gathers and finds a path to waterways. The natural vegetation helps to keep the water entering streams and rivers clean. When land is cleared special care must be taken to keep the water leaving that land clean (free from sediment and debris) and under control (volume and flow rates similar to pre-development).

Channelizing Water

Developing land for subdivisions, business or industry typically takes natural land (forest, woodland, prairie) and disturbs the native materials (trees, grasses, shrubs) and covers the land with roads, sidewalks parking, roofs and other materials that do not absorb water. The rain (stormwater) is controlled by channelizing the water with curbs, watercourses (streams, ditches, culverts), storm-sewers and detention ponds. After development, water that ran in all possible directions is forced into specific channels (channelized). As land is cleared the natural vegetation is removed and no longer absorbs water, so more rain or stormwater must be handled on bare land or the water will run off because it can’t be absorbed as it once was.

Think of this; hold a ball under a faucet with the water turned on. The water runs off the ball in all directions (360 degrees). If you held a straw under the same faucet the water would be forced into the straw. If the water leaving the faucet is turned up so more water comes out, at some point the water would not fit into the straw. The straw would handle part of the volume of water and the rest would find another route around the straw. Thus, it is very important that the plan for handling stormwater is accurate when natural land is developed and the water is channelized.

Generally, natural watercourses (tributaries, streams, rivers etc.) are not to be disturbed by development. The Army Corps of Engineers can give special permission to developers to enter and alter waterways. Waterways are public and belong to the people of Alabama.[1] Equipment is not supposed to enter streams and the path, volume and rate of the natural watercourse should not change as a result of development.

Land developers are facing two challenges. First, there will be more water because trees and vegetation are no longer present to absorb water. Second, channelized water must be controlled. This means keeping water away from structures (houses, buildings, sidewalks, etc.) and directed or stored in a safe and equitable manner. The City of Hoover[2] has required drainage facilities be designed for twenty-five-year rainfall event, except for major drainageways which shall be designed for a one-hundred-year rainfall event, and construction site run-off[3], since 1990.

Engineers, Contractors and City of Hoover

In 1990 Hoover obtained a Stormwater Permit as required by the EPA (Environmental Protection Agency). This plan included Construction and Post-construction stormwater runoff regulations including annual inspections post-construction. Success was to be measured by clean streams in 10-20 years (so by 2010). Sediment is the most common pollutant in rivers, streams, lakes and reservoirs according to the EPA.

Since 1990 regulations have been updated. Developers must employ registered professional engineer (hydrologists) to plan stormwater management in new developments. The engineer can either test or model the impact of stormwater; most often modeling is used with the assumption the model will work. If it does not work flooding, is a significant risk. The municipality approves the plan. In Hoover, the City Engineer approves the plan, however, the caveat is that the city is not responsible for outcomes of that plan[4]. This appears to be the responsibility of the developer and their engineer. Contractors are expected to carry-out the stormwater plan, including using best management practices during construction. This could be observed as silt fences and straw bales as well as temporary detention ponds. The goal is to keep sediment in the construction area and away from streams, tributaries and rivers.

The City of Hoover has regulated subdivision development since 1982 with specific concern for the impact of new developments on adjacent property and updated verbiage since; The City of Hoover will not allow development of one site to cause an adverse effect on adjacent property (Ord. No. 89-857, § 1, 1-3-90). The aim of protecting neighbors is admirable; the challenge is implementation and monitoring[5].

Summary

Nature is reasonably efficient at controlling stormwater and keeping water clean. Development, particularly land-disturbing activities, can be a threat to clean water and may threaten downstream existing homes and roads. For more than three decades the City has acknowledged their role in keeping residents and property safe and healthy through regulations for subdivisions (1982) and stormwater (1990).