Large Projects

Acquisition (Voluntary Buyouts)

Acquisition requires structures to be removed from acquired property, either by demolition or relocation, and property to be forever maintained as open space. Structures that are unsound or otherwise unsafe (e.g., those flooded by contaminated water) probably should be demolished. Certain structures that are sound may be relocated. The least-expensive and most cost-effective structures to relocate are those built on crawl space or basement foundations; those made of wood frame or masonry that are small and compact; and those less than three stories high.

Additional Information:
An Explanation for Homeowners
Property Acquisition Handbook for Local Communities

Detention or Retention Ponds

Detention and retention ponds serve to slow down the runoff from a basin that might otherwise overwhelm the capacity of an existing adjacent waterway. These hydraulic structures can be built out of earth, concrete, or a combination of both. They have some type of release mechanism, possibly a weir or pipe. They normally are empty. However, they are designed to hold the entire quantity of runoff that would be produced in a significant storm event and discharge the water at a predetermined rate. Spreading out the flow of water to the nearby stream over a longer period of time gives the stream a chance to carry the water away without over-topping its banks. Runoff in amounts greater then those for which it was designed would result in water being discharged over a spillway. At that point, the benefit of the pond is eliminated.

Drainage System Maintenance

At most times, a drainage system will do its job and move water to intended areas. However, if a system is not maintained, erosion, material dumping, or deterioration of man-made reinforcement materials may reduce the carrying capacity of a stream. Therefore, regular maintenance, such as sediment and debris clearance, is needed so that the stream may carry out its design function. Also important is detection and prevention/discouragement of discharges into storm-water/sewer systems from home footing drains, downspouts or sump pumps.


Opens up a stream or waterway by changing its shape. Usually this involves clearing out debris, dredging, widening, or straightening a channel. The wider, deeper, and straighter a channel is, the more water it can handle before overflowing its banks. Re-channelization requires a permit from the Army Corps of Engineers, and typically one from a State water regulatory agency. If the stream or river is designated a floodway, then re-channelization also requires a “no rise” certification from a professional engineer, which certifies that the rechannelization will not raise the water elevation in the event of a 100-year flood. Modifications to waterways often have adverse environmental impacts and many produce unintended up- or downstream problems.

Storm Drainage Systems

Flood mitigation can involve installing, re-routing, or increasing the capacity of a storm drainage system that may involve detention and retention ponds, drainage easements, or creeks and streams. It can include separation of storm and sanitary sewerage systems as well as higher engineering standards for drain and sewer capacity.

No Action

Keep in mind that choosing to take no action is a legitimate mitigation alternative that can be used to evaluate the effectiveness of other alternatives by comparing and contrasting them. In some cases, for example where the costs outweigh the benefits regardless of the mitigation option, no action might be the appropriate alternative.


Revised: October 18, 2007