Floods are natural events. However, humans can improve flood prevention significantly. Except in unusual cases of dam failure or landslide, a flood is the result of some combination of weather, geology, soils, impervious surfaces (surfaces that water cannot penetrate), failing levees, and vegetation. Communities can control those last three factors.
1. Reducing The Amount Of Impervious Surface
This is a major flood prevention measure. Examples of impervious surfaces are concrete and asphalt paving, as well as buildings. Impervious surface causes water to run off toward a man-made or natural drainage system before the stormwater has an opportunity to percolate into the soil. The runoff also moves faster when there are no plant textures present to slow its progress. This greater velocity is nearly as vicious as volume in causing damage.
2. Minimizing Reliance On Levee Protection
While levees may be appropriate when something already built must be protected, they also are prone to failure, especially when they are earthen levees common in some parts of the world.
A second unpleasant truth about levees is that they simply displace water, forcing it downstream faster than would otherwise be the case. In plain language, that means that you are merely sending your flooding a few miles down the way toward your neighbor. That's not very responsible.
3. Planting And Maintaining Appropriate Vegetation, Often Native Plants
The value of plant materials in processing stormwater runoff is threefold: (a) they slow the rate (speed) of runoff, which allows streams to "catch up" with the amount of rain or snow melt coming their way, (b) their slowing of the flow allows some moisture to percolate into the soil, where plants uptake some water for themselves and let the leftover water seep deeper to that the groundwater recharges, and (c) they capture some sediment and clean up some pollutants as water passes through. The first two points are major flood prevention steps for smaller streams.
Not all vegetation is equal in slowing down and absorbing stormwater. Turf grass, mowed to a typical lawn height, has shallow roots compared with the plants that are to your area. Turf grass does not store as much water, and due to its short height, it also does not slow the velocity as much as a taller native planting plan.
While municipal participation in the federal flood insurance program is voluntary, almost all municipalities do so. As a condition of participating in the program, the municipality must pass an ordinance attempting to limit flood damage. Construction of almost anything is prohibited in the floodway, unless one can demonstrate "no net rise" in flood levels. This could be accomplished perhaps through providing for more flooding capacity in some other fashion.
Most communities adopt the boilerplate language, which includes:
• Restrictions on land uses that result in damaging increases in erosion, flood heights, or flood velocities
• Requirements to raise elevations of the first floors of structures to above the height of the base (100-year) flood
• Limits on altering floodplains, stream channels, wetlands, and other natural forms that hold flood waters
• Control of filling, grading, dredging, or other land development practices that might increase erosion or flood damage
• Regulations about the construction of man-made flood prevention structures that would unnaturally divert flood waters or increase flood danger for others.
Regardless of the fact that your community is interdependent with all other communities in the watershed, there are numerous ways you can do your part to slow the quantity and velocity of runoff.
While we don't recommend building levees, floodwalls, or seawalls unless absolutely necessary, our less dramatic efforts at the community level can have far-reaching effects. We can cooperate with nature on flood prevention in these ways:
1. Reduce the amount of paved parking that is required for new development. Permeable pavement or turf reinforcement mat may be suitable surfaces for overflow parking. Shared parking arrangements should be encouraged when adjacent land uses have opposite peak usage times.
2. Require removal of pavement anywhere that old buildings are demolished or factories cease operation. If the owners or perpetrators have vanished, you may be able to receive some assistance with cleaning up a polluted site, or perceived polluted site, which is called a brownfield.
3. Cancel the requirement that residential driveways be paved, or in many soil conditions, you can encourage driveways with only two paved strips.
4. Change your subdivision regulations to narrow the minimum width of streets when a new subdivision is approved. Most municipalities still require much wider streets than is necessary. The fire departments have a possible legitimate gripe about this, but everyone else will benefit. Severity of pedestrian injury increases exponentially with greater automobile speed, so the decrease in average speeds that occurs when residential streets are skinnier is an added benefit.
5. Investigate permeable pavements for parking lots and other low-usage situations, such as possibly alleys. Permeable versions of both concrete and asphalt are now available. Permeable pavement requires some periodic maintenance that amounts to vacuuming out the silt and debris, but it allows stormwater to percolate slowly into the ground.
6. Require riparian buffers (strips of vegetation) alongside rivers and creeks, with the width of the buffer strip proportional to the size of the water body. At least 25 feet is a minimum flood prevention practice, and an ideal buffer is more in the range of 150 feet for a larger urban stream. No paving or building should be allowed within this buffer.
7. Educate the population on the value of plants that are native to your area (appropriately called native plants). Typically such plants have far longer root systems than plants imported from other geographies. This deeper root system holds soil in place much better and prevents the erosion that can contribute to flash flooding.
8. Vegetated stream banks to hold them in place and to slow down water as it finds its way through the stream. Again, use native plants rather than plants imported from other ecosystems (exotics).
9. Encourage residents to discharge their downspouts away from their foundations but into rain gardens, which is simply a term for a planting bed that includes only plants that will tolerate being partially submerged for some period of time.
10. Permit and encourage the use of grassy swales (or rain gardens) along the sides of streets rather than a curb and gutter system. In conjunction with this flash flood prevention measure, individual homeowners should be encouraged to slow runoff from their roofs, so that stormwater doesn’t immediately course down a driveway, for instance, and pool in the swale.
11. Regularly clean trash and debris from streams to increase their capacity to hold water. If you've never participated in a stream clean-up, you would be shocked as tires, shopping carts, cars, car parts, industrial equipment, building materials, appliances, file cabinets, and thousands of shopping bags, cups, bottles, and wrappers are pulled from the stream.
12. Preserve the role that wetlands play in flood water storage. Wetlands are defined by characteristic soils and plants. They may be man-made or natural. While your state may exempt tiny wetland areas from regulation, preserving smaller wetlands assists with flood prevention, all the same. Your local community can enact a wetland ordinance that is tougher than the state law.
13. Communicate upstream and downstream about the impacts of land development and also of artificial structures such as levees on adjoining communities.
14. Manage any structural flooding protections in a manner that inflicts the least damage possible downstream.