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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 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.
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.
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.
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
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