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<br />
<br />'Vith specific regard to buildable area requirements,
<br />Hovvard County, Maryland, requires that 3D percent of
<br />each lot be outside the floodplain. Sacramento County,
<br />California, has requirements for buildable area that vary
<br />according to the zoning district. The AR-5 zone
<br />(agricultural use that allows one d\velling unit per five
<br />acres) must have at least one acre of buildable land
<br />above the 1 DO-year floodplain_ In the RD-5 to RD-7
<br />zones (residential use that allows from five to seven
<br />units per acre), the minimum buildable area
<br />requirement per lot is 5,200 square feet or the entire lot,
<br />whichever is less (Sacramento County Department of
<br />Public Works 1993).
<br />Limiting buildings to natural high ground is not
<br />ahvays feasible. The third level of our hierarchy
<br />suggests a policy that can be used in such cases; namely,
<br />the community would require developers to make
<br />maximum use of the natural high ground and al1mv
<br />them to use fill only in limited areas ,vhere it is
<br />necessary to provide road access and to establish
<br />building sites above BFE.
<br />Finally, the lowest level in the hierarchy would be
<br />governed by a policy that applies to cases in "vhich the
<br />land to be subdivided is located entirely or mostly in the
<br />floodplain. In these situations, the community would
<br />require that the amount of fill or other disturbance of the
<br />land be kept to a minimum by requiring clustering of
<br />building sites in areas subject to the shallowest flooding
<br />and as far from the flooding source as possible.
<br />It should be noted that AP A and FEMA do not
<br />encourage the use of the policies in levels three and four
<br />of this suggested hierarchy_ They are included here as
<br />alternatives only inasmuch as those polices promote
<br />standards and requirements that are better than none.
<br />They also promote development practices that are
<br />preferable to what has been practiced in many communi-
<br />ties. The shortcomings of those practices have been borne
<br />out in an unfortunate history of lost lives, family suffer-
<br />ing, property loss, and flood insurance payouts.
<br />
<br />ADDITIONAL CONSIDERATIONS TO MINIMIZE
<br />ON- AND OFF-SITE DISRUPTIONS
<br />Regardless of the policy adopted by a community,
<br />there are design principles for subdivisions in flood
<br />hazard areas that must be followed to minimize on- and
<br />off-site disruptions. First, the site must be designed in a
<br />manner that will prevent an increase in downstream
<br />flooding, and, second, the homes and accessory
<br />structures must be configured in a ,vay that precludes
<br />them from becoming obstructions to flood ,vater.
<br />Furthermore, there are a range of site design issues
<br />aimed at minimizing damage to structures, allowing
<br />emergency access and reoccupation of flooded areas,
<br />and minimizing damage to utilities and roads.
<br />The methods described belmv to prevent do\vnstream
<br />flooding are part of a broad range of best management
<br />practices (BMPs) for storm,vater management. Local
<br />governments apply BMPs through land development
<br />regulations (storm"water management and drainage
<br />ordinances, floodplain ordinances) ,vith the dual goals
<br />of reducing the dO\vnstream impacts of stormwater
<br />runoff and minimizing water-quality degradation from
<br />nonpoint pollution sources. Bl\11Ps are used in planning
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<br />for all subdivisions, not just those that are all or
<br />partially in a floodplain. Those BMPs described below,
<br />hmvever, have direct applicability to the goal of
<br />minimizing damages from floods. For general
<br />information on storm water management, please refer to
<br />the list of references in Appendix A of this report.
<br />
<br />Preventing an Increase in Downstream Flooding
<br />To protect an individual site from flooding ,,,,'ithout
<br />increasing flood levels elsewhere, the developer must
<br />ensure that the storm"vater runoff rate after development
<br />does not exceed the rate that existed prior to the site
<br />being developed. BIvlPs can be used in varying
<br />combinations to meet the runoff-rate goal. Common
<br />BMPs include impervious surfaces limits, retention and
<br />detention ponds, infiltration devices, and s\vales.
<br />Impervious surfaces and landscaping. :Minimizing the
<br />amount of impervious surface (pavement, roofing, etc.)
<br />in a development is an elemental requirement for
<br />controlling runoff and reducing flood damage.
<br />Impervious surfaces that do not absorb rain include all
<br />buildings, roads, sidewalks, parking areas, and other
<br />areas covered ,vith concrete or asphalt. Impervious
<br />surfaces add to the volume and velocity of ,vater runoff;
<br />they also add pollutants (e.g., oil leaked from cars) to
<br />runoff and, consequently, contribute to groundwater
<br />pollution. To minimize these effects, all development
<br />site;--but particularly those in floodplains-should
<br />retain as much vegetation and natural ground cover as
<br />possible.
<br />Driveways and parking areas can create runoff
<br />problems. Parking areas can be configured to minimize
<br />the amount of impervious surface by requiring that a
<br />certain percentage of the lot area be devoted to
<br />landscaping. There are also a number of options for
<br />porous paving material, such as gravel, permeable
<br />brick, and turf/brick combinations that can be used to
<br />limit runoff. A number of works (Smith 1988; Schueler
<br />1995; Schueler 1987) offers illustrations, ordinance
<br />language, and examples of parking lot designs and
<br />standards that use porous materials and control runoff.
<br />Floodplain, aquifer protection, and other sensitive
<br />lands ordinances commonly stipulate the maximum
<br />amount of impervious surface (this figure generally
<br />ranges from 30 to 50 percent). Many performance-based
<br />zoning ordinances and environmental controls apply
<br />maximum impervious surface ratios (number of acres of
<br />impervious surface divided by the gross site area) for
<br />each type of land use. The maximum allowable
<br />impervious surface stipulated in these ordinances
<br />generally ranges from 20 to 50 percent.
<br />Altering community street standards to allow for
<br />narrmver roads can also help minimize the amount of
<br />surface area that is paved. By and large, conventional
<br />subdivision road \vidths (as required by ordinances) in
<br />the U.S. far exceed what is needed to accommodate
<br />typical residential traffic volumes. Many existing
<br />subdivisions use the Institute for Transportation
<br />Engineers 1965 standard of 34 feet from curb to curb.
<br />In a performance-based approach to subdivision
<br />design, street ,,,ridths vary according to intended traffic volumes and speeds, and lot sizes and widths. The
<br />larger the lot (e.g., four d\velling units per acre or
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