Flood Prevention Basics:
What Can You Control?

Floods are natural events. However, humans can improve flood prevention efforts 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), and vegetation. Humans can control those last two factors.

1. Impervious surface reduction

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 (speed) is nearly as vicious as volume (quantity) in causing damage.

The value of plant materials in processing stormwater runoff is three-fold: (a) they slow the rate 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 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.

Types of Floods

Floods may be characterized as flash floods, which usually occur on smaller streams and rivers, or the more slow-moving riverine floods. Flash floods are quite dangerous because people sometimes try to walk or drive through rising water and underestimate the difficulty of the task. The riverine floods cause extensive property damage, and if people insist on returning to flood-damaged homes, mold and other health-related issues are additional hazards to be considered.

Current climate change patterns seem to indicate that humid areas will receive even more moisture in the future, while dryer climates receive even less. This increase in rainfall in humid areas is very likely to increase the frequency of flooding. So flood prevention is a timely topic.

Understanding Flooding Terms

The terminology of flooding is important to master if you are a community leader or activist in a flood-prone area. These are the important concepts:

• Base flood. The base flood level has at least a 1 percent chance of being reached each year. Note that this does not mean you can't have the base flood this year because it happened last year or last month. It's just a probability statement. For practical purposes, consider this the same as the 100-year flood, in which the probability is that the land would flood once every 100 years.

• Floodway. The floodway is the actual stream or river plus the adjacent area that is required to discharge the base flood without raising the water elevation more than one foot. So our editorial comment is that clearly the floodway needs to be kept clear of buildings, debris, and obstruction of any type.

• Floodplain. The term floodplain is often used to mean the area outside of the floodway, but within the limits of the 100-year flood. A floodplain thus is wider than the floodway in most circumstances, although a bluff or other absolute barrier could mean that the boundary of the floodplain and floodway would be the same.

• 100-year flood, 500-year flood, 2-year flood, and so forth. This does not mean that this area probably would flood only once every 100 years, 500 years, or 2 years. The 100-year flood does mean that this is the flood level that statistically could be expected once in 100 years. You might well have two 100-year floods only a year apart. This is a measure of probability, but there's no guarantee that nature will be random in its distribution of flooding.

• FIRM map. This is the U.S. Flood Insurance Rate Map, maintained by FEMA, which became part of the Department of Homeland Security. The map shows the flood hazard area, called Zone A, where flooding danger is highest. It also shows areas of shallow flooding, called the AO or VO zones. As the name suggests, federal flood insurance rates are based on this map, but informally the map may play an important role in real estate financing and other local regulatory matters as well. It is possible to have this map changed, but specific engineering studies must be conducted and then reviewed by the government. So it isn't a fast or sure process, and my advice is don't count on it happening.

• Floodplain administrator. Each municipality that elects to participate in the federal flood insurance program has a floodplain administrator, and it is his or her job to rule on whether flood prevention measures proposed as part of new development are adequate.

Municipal Flood Ordinances

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

• Limits on the construction of man-made flood prevention structures that would unnaturally divert flood waters or increase flood danger for others.

Value of Watershed Planning in Flood Prevention

While localized measures may help with flood prevention in small geographic areas, the basic approach to flood prevention needs to be planned and executed on a watershed basis. The U.S. Environmental Protection Agency (EPA) operates this fun webpage that allows you to determine what your watershed is and to make contact with any groups that are active in caring for the watershed.

The U.S. Geological Service monitors stream levels and reports them on the basis of an eight-digit watershed classification system. The USGS provides an on-line map of current flooding or high water locations, as well as additional on-line information you might find useful.

The best flood prevention measures preserve or enhance the watershed's capacity to handle stormwater on a gradual basis, and restore the flood storage capacity of natural floodplains wherever possible.

Forming and maintaining watershed groups or councils is frustrating, because the watersheds are often a much different shape and geography than the political subdivisions. However, it's very interesting and rewarding to bond with the people who share a watershed with you. Give it a try.

Then you can formulate a simple or elaborate watershed plan, which would describe measures that need to be taken to improve the functioning, or perhaps even the cleanliness and appearance, of the watershed. Volunteer groups may be able to produce such a plan without the use of baseline measurements, but of course if you can figure out how to afford it, some scientific help would be very useful. Your state conservation department or even an Army Corps of Engineers unit nearby might be very willing to help you.

Building Structures for Flood Prevention

As a practical matter, defensive structural measures against floods, including construction of floodwalls and levees, must be maintained in some situations. Sometimes farmers have built small agricultural levees that merely limit the nuisance of constantly having fields flooded in the spring. At the maximum end, levees are protecting large sections of cities, as we all know from the Hurricane Katrina experience.

Building a levee should be the last resort when the local decision makers consider present development to be too valuable and too expensive to move. And of course sometimes the culprits are upstream of you, and your community suffers because they have permitted too much development too close to the stream.

If a new levee is constructed, that's no excuse to permit even more of the urban development that led to the dangerous flooding situation in the first place. As we should all know now, levees can and do break, fail, and are "overtopped," as people say. So if a levee is the only practical compromise among interests, at least make sure it doesn't continually take on a greater burden of property to protect.

And regardless of the "economic development" impact that some developers may claim for erecting a new levee and then building a giant development behind it on the floodplain, the environmental and social costs are just too great. Encourage greater density and intensity of uses in areas outside the floodplain instead. A metropolitan area needs to work together through regional planning to keep its floodplains clear.

Where levees are in place and have been for many years, make sure they are inspected and maintained regularly if you expect them to hold when tested.

Practical Flood Prevention Steps for Individual Communities

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 width of streets required 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. Vegetate 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. 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. Be mindful of 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.

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.

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