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Conservation Subdivision Design:A Brief Overview
<br /> by Randall Arendt,FRTPI,AsLA (Holm.)
<br /> An approach to laying out subdivisions so that a significant percentage of buildable uplands is permanently
<br /> protected in such a manner as to create interconnected networks of conservation lands.The percentage of protected
<br /> land varies according to project density,rising from 30-35%at density levels of three or more dwellings/acre,to 75-
<br /> 80%at four or more acres/dwelling.
<br /> This approach is distinct from earlier"clustering"and"planned unit development"(PUD)in terms of both the higher
<br /> open space ratios and in terms of conscious design to forge community-wide networks of open space.
<br /> This is primarily a design approach for conserving existing natural and cultural resources,although a limited amount
<br /> of active recreation is permissible(such as ballfields and neighborhood greens).Subdivisions where the majority of
<br /> open space is taken up by a golf course do not meet this basic criterion.
<br /> Conservation subdivisions are generally"density-neutral",meaning that the overall number of dwellings built is not
<br /> different from that done in conventional developments.Small density bonuses are sometimes granted in return for
<br /> dedicating some or all of the conservation land for public access or use,for endowing permanent maintenance of the
<br /> open space,or for providing workforce housing.
<br /> Conservation subdivisions are specifically designed around each site's most significant natural and cultural
<br /> resources,with their open space networks being the first element to be"green-lined"in the design process.This
<br /> open space includes all of the"Primary Conservation Areas"(inherently unbuildable wetlands,floodplains,and
<br /> steep slopes),plus 30-80%of the remaining unconstrained land,depending upon zoning densities and infrastructure
<br /> availability.
<br /> The site planning process begins with general mapping of the project site in its context of surrounding properties(up
<br /> to 2000 feet away,typically mapped at 1"=400 feet).On this Location Map(compiled form existing published
<br /> data)are shown vegetative cover,topography,soils,and floodplains.This initial map informs decisions regarding
<br /> the design of the interconnected open space network.
<br /> A more detailed,site-specific Existing Resources and Site Analysis Map is then created,identifying significant
<br /> natural and cultural resource,such as productive cropland,wildlife habitat and travel corridors(meadows and
<br /> forests,stream valleys),and significant trees,with size thresholds related to species:4-6 inches for smaller species
<br /> such as dogwood and redbud,8-10 inches for medium species such as sassafras,cherry and water beech,12-14
<br /> inches for slow growing hardwoods(oak,maple,ash),and 15-18 inches for fast growers(tulip poplar,sycamore,
<br /> conifers).Historic or cultural resources such as farmhouses,barns,cellarholes,wells,stone walls,earthworks,
<br /> trails/traces,and hedgerows are also identified,usually through GPS.(A photo essay illustrating such resources can
<br /> be seen-at b!W://www.terraitLo-r-gt@Mslsstl 8/arendt htm).
<br /> A four-step process then ensues,Step One separating the site's resources into two categories.The first,Primary
<br /> Conservation Areas(PCAs),are limited to inherently unbuildable wetlands,floodplains,and steep slopes(>25%).
<br /> Secondary Conservation Areas are comprised of"the best of the rest".Because the PCAs would be off-limits to
<br /> development in conventional developments in any event,they are not counted toward the minimum required open
<br /> space percentages of conservation subdivisions.Therefore,30-80%of the buildable land is usually designated as
<br /> SCAs,depending on density(as noted above).
<br /> Step Two consists of locating house sites in relation to the protected open space,to add livability,marketability,and
<br /> value to the homes.The third step is to"connect the dots"with streets and trails.Step Four consists simply of
<br /> drawing in the lot lines.
<br /> This process works best when guided by a landscape architect or physical planner,collaborating with a civil
<br /> engineer.The creative skills of a landscape architect or physical planner are essential,balancing the technical
<br /> training of engineers whose expertise lies principally in streets and drains.
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