Imagine the following situation. A developer buys a 100-acre tract of land near you, with the goal of building a shopping mall. The tract is undeveloped. It is a forest -- a mix of 40-year-old pines and hardwoods. The developer cuts down all the trees, levels the land, builds the mall and paves a parking lot to hold the cars of all the shoppers. Eighteen months later the mall opens for business.
Now here is the question: What has changed⢠Many things actually. The economic value of the land, and all of the land around it, has grown immensely. Traffic patterns around the new mall will increase. The area will be a little warmer because the cool green trees have been replaced with hot black asphalt.
But one thing that has changed dramatically is the way water flows.
Imagine that 2 inches of rain fall on the new mall over the course of a couple of hours. When the forest was there, chances are that there would be no runoff at all from the downpour. The forest floor is soft and absorbent, with a bed of decaying leaves and topsoil to catch the falling rain so it percolates down into the subsoil.
But, when rain falls on 100 acres of parking lot and mall roof, there is nothing to absorb. Two inches of rain on 100 acres of land is an immense amount of water -- more than 5 million gallons. It is hard to visualize 5 million gallons, but imagine a swimming pool 10 feet deep and 250 feet square and you have the general idea. All 5 million gallons head straight for the nearest creek or river.
Before the mall, this 100-acre tract produced no runoff in most rainstorms. Now, it produces 5 million gallons of water after a 2-inch squall. In any suburban area or city, this same thing happens across hundreds of malls, shopping centers, apartment complexes, subdivisions and schools. And don't forget all the roads, which tend to be relatively narrow but very long, and therefore cover thousands of acres of land with asphalt. A rainy day dumps billions of gallons into local waterways.
What you end up with is a situation where every creek and river now rises much higher than it ever did before development started. The chance of flooding is significantly greater in low-lying areas because of all the runoff.
Is there any way to control the flooding⢠The solution is called a retention pond. Any new development is required to have one, and if you look closely, you will start to see these ponds everywhere.
The basic idea is simple. Instead of letting runoff from the mall flow straight into a creek, it first flows into the retention pond. This pond can be either wet or dry. A wet pond always has some water in it, and therefore looks like a pond all the time. A dry retention structure may be as simple as a grassy basin that fills when it rains and then dries out.
In both cases, the point is for water to enter the retention structure and sit there for a while instead of flowing straight into a waterway. A wet pond will let the water trickle out slowly until the pond returns to its normal level. A dry pond will let the water soak into the ground, evaporate, or trickle slowly.
By holding the storm water for a period of time, the retention structure helps to limit downstream flooding, and also improves water clarity. Any crud in the water will sink to the bottom while the water sits in the pond.
The next time you go to a recently built shopping center, school or apartment complex, look for the retention pond. The easiest way to find it is to notice the direction the land is sloping, and head toward the lowest point. Sometimes you will find a pretty pond complete with ducks and grass. Sometimes you will find a fairly ugly hole lined with rip rap and a chain-link fence. But in all cases, it is serving an important function. It is keeping the runoff from the development from flooding the folks downstream.
