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Land Use Planning Information>Floodplains


Flooding happens when water bodies receive a greater volume of water than they can handle at one time. Floods are a natural part of the water cycle and can even be beneficial, however humans have affected the landscape drastically. By building on floodplains, draining wetlands, and controlling stormwater, we have increased the likelihood of flooding and the extent of damage done by floodwaters such as erosion, loss of property, loss of frontage, loss of habitat, and loss of life.
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What is a Floodplain?
Why Flooding Occurs
Preventing Flooding
Federal Emergency Management Agency (FEMA)

What is a Floodplain?

Click here to view full size picture A floodplain is an area next to a river, stream, or creek that may be covered with water following heavy rainstorms. This plain holds the excess water allowing it to be slowly released into the river system and seep into groundwater aquifers. Floodplains also give time for sediment to settle out of floodwaters, thereby keeping it out of water bodies. Floodplains often support important wildlife habitat and are frequently used by humans as recreation areas.

The 100-year floodplain is the land that will be covered with water during a 100-year storm, and is the accepted limit for protection. Replacing highly developed land with a landuse that is functionally connected to the river, a "hydrologically-active area", would be an improvement. The 100-year storm is an amount of rain so heavy that the chances of experiencing such a storm are one in 100. (Note that those are the chances every year. It is possible to have this unusual storm two years in a row or even more frequently.)

Why Flooding Occurs

Click here to view full size picture In the past, we have developed on the floodplains and then tried to control stormwater, keeping it out of the floodplains. This practice causes water to overflow riverbanks in other locations - often creating floods of a greater magnitude and danger. Building on floodplains increases the risk of property damage and life threatening situations. Diverting stormwater into channels and easing its path to bodies of surface water forces water to flow faster. This greater velocity destroys habitats and causes greater erosion including the loss of topsoil often creating a need for increased fertilizer use.

There are also other factors that increase flooding:
* The removal of stabilizing vegetation around stream banks and rivers.
* Erecting structures that deflect or inhibit the flow of floodwaters. This modifies flow paths and can spread flooding problems and increase erosion.
* Constructing bridges, culverts, buildings, and other structures that encroach on the floodplain. These developments reduce the storage area available for floodwaters and cause an increase in flood elevations.
* Building drainage systems that feed stormwater quickly into the receiving body.
* Straightening meandering watercourses to hasten drainage. This transfers flooding problems downstream and also alters habitat.
* Filling and dumping in floodplains. Floodwaters can transport this debris, which may interfere with the movement of the floodwater causing increased flood elevations.

With increasing development in the floodplain, open spaces, and wetlands, our land has lost the ability to soak up rain. Areas that were once effective sponges storing precipitation are now being replaced by buildings and pavement that have made the land increasingly impervious. As a result, floods have become far larger and more frequent. Local governments can prohibit building in the floodplain, but only a few municipalities in the Dowagiac River watershed have taken this important step.

Preventing Flooding

Click here to view full size picture Taking the time to word your community’s zoning ordinances so that they reflect the importance of preserving water quality is an important task. There are many angles to do this from including:
* Stormwater Control Ordinances
* Floodplain regulations
* Impervious Surface Restrictions
* Shoreline Vegetation Cover Restrictions
* Building and Septic field setbacks

following are things to think about adding to your zoning ordinances:
* Prohibit construction of buildings and facilities subject to water damage in the 100-yr floodplain
* Require flood-proofing measures on buildings presently in floodplain
* Remove flood-prone structures from floodway portion of floodplain
* Establish construction standards for development in the floodplain
* Adopt provisions to protect natural vegetative cover in floodplains
* Require tree and shrub planting in floodplains to prevent erosion
* Restrict dredging, filling, dumping or backfilling of floodplain areas
* Avoid land divisions within floodplain areas that would create parcels or lots that cannot be used
* Require that flood insurance be obtained for all facilities in floodplain under the National Flood Insurance program
* Require that proposed new structures or modifications to existing structures be subject to approval
* Create an overlay zone within a prescribed setback from the river, stream, or creek that regulates development and use of the floodplain based on the severity of the flooding hazard
* Require that before local approval can be granted for activities in a floodplain, a permit must be secured from the MDEQ
* Building Officials & Code Administrators, and the Uniform Building Code contain regulations concerning construction in floodplains that are commonly used.

Federal Emergency Management Agency (FEMA)

In 1968, Congress created the National Flood Insurance Program (NFIP) in response to the rising cost of taxpayer funded disaster relief for flood victims and the increasing amount of damage caused by floods.
The NFIP makes Federally-backed flood insurance available in communities that agree to adopt and enforce floodplain management ordinances to reduce future flood damage. National Flood Insurance is available in more than 19,000 communities across the United States and it's territories.
The NFIP's Community Rating System (CRS) was implemented in 1990 as a program for recognizing and encouraging community floodplain management activities that exceed the minimum NFIP standards. Under the CRS, flood insurance premium rates are adjusted to reflect the reduced flood risk resulting from community activities that meet the three goals of the CRS: (1) reduce flood losses; (2) facilitate accurate insurance rating; and (3) promote the awareness of flood insurance.
This series credits programs that provide increased protection to new development. These activities include mapping areas not shown on the FIRM, preserving open space, enforcing higher regulatory standards, and managing stormwater. The credit is increased for growing communities. These activities work toward the first and second goals of the CRS, damage reduction and accurate insurance rating.

Stormwater Management Practices That Are Credited

1. The most common approach to reduce the impact of stormwater from new development is to require each developer to construct facilities to restrict the rate at which the increased runoff leaves the property. A volume of stormwater runoff is required to be stored on the developer's site. It is released at a restricted rate after the runoff subsides (stormwater detention). A developer may store stormwater runoff for irrigation or groundwater recharge or to reduce pollution (stormwater retention). This approach is recognized as element SMR, which stands for stormwater management regulations.

2. As an alternative to using a uniform standard for all areas, many communities regulate development according to a master plan that analyzes the combined effects of existing and expected development on stormwater and flood flows in the watershed. Such watershed-specific regulations may allow different amounts of runoff for different areas in order to control the timing of increased flows into the receiving streams. A watershed master plan may also be used to preserve wetlands, riparian areas, or other important habitats. Sometimes instead of requiring developers to build stormwater facilities on site, a plan may require them to contribute funds for a regional facility. Regulating new development through such a master regulatory plan is recognized as element SMP, which stands for stormwater management plan. By planning the runoff from entire watersheds, this approach can be more effective in reducing increases in downstream flooding. Watershed master planning usually uses more sophisticated modeling techniques and may consider alternative storm intensities and durations.

3. A third approach is to regulate new construction throughout the watershed so that new buildings will be less likely to be affected by local drainage problems. Much of the nation's flood damage (including one-third of all flood insurance losses) occurs in B, C, and X Zones. There are two regulatory approaches to preventing local drainage problems in new construction. The first approach is to require the lowest floors or basement openings to be elevated above street level. This allows the street system to carry excess surface flows to the streams without damaging buildings. The other approach is to require that developers show that they have accounted for local drainage in their site plans. This element is recognized by Activity 450 as FRX, for freeboard in X Zones (i.e., those zones not in the floodplain).

4. The fourth element credited under Activity 450 is regulation of construction (and other projects that disturb vegetation and soil) to minimize sediment runoff. When an area is stripped of its ground cover, much soil can be eroded during a storm. The stormwater runoff carries the sediment to downstream channels where it is dropped. The result of this process is that the channels become filled with sediment and lose their capacity to carry larger flows. The acronym ESC is used to recognize erosion and sediment control regulations. These regulations usually govern only construction projects but sometimes they include all modifications to the land, including farming.

5. Regulating new development for water quality purposes is the fifth stormwater management element the CRS credits. Water quality or WQ credit is based on one or more requirements for new developments to include "best management practices" that clean, filter, or otherwise improve stormwater runoff in the design of stormwater management facilities. Unlike ESC, which credits measures taken during construction, WQ credit is for measures that are permanently incorporated into a development's drainage facilities.
This page last updated on 9/3/2003.
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