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GROWING communities on karst Thursday, September 13, 2007 local government initiatives: Jefferson county, west Virginia. Tony Redman tredman@jeffersoncountywv.org Director of Planning. Karst Area in Jefferson County. General Geology with Caves.
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GROWING communities on karstThursday, September 13, 2007local government initiatives:Jefferson county, west Virginia Tony Redman tredman@jeffersoncountywv.org Director of Planning
Why using GW recharge estimates will do little to curb housing growth in Jefferson County • People have discretion in how much water they use and GW probably supplies less than half of the households in Jefferson. • If we assume each person uses 150 GPD, and four person housholds, and an annual GW recharge rate of 12 inches/year, then: 1 house uses 219,000 gallons/year and requires about 0.68 acres of surface area for recharge. • Jefferson County covers about 135,500 (212 square miles • (Slide courtesy of Bill Jones at Karst Conference, 2006
Management and Protection • Currently Rewriting Subdivision & Zoning Ordinances • Article 4 Environmental Protection • Division 4.200 Resource Protection Standards • Division 4.300 Site Capacity Calculations • Residential and Non-Residential • Division 4.500 Additional Resource Standards • Floodplains, Riparian Buffers, & Water Bodies • Woodlands • Sinkholes & Sinkhole Buffers • Shallow Bedrock • Vulnerable Areas • Wellhead Protection
Site Capacity Requirement • Each property proposed for development shall, at the time of preliminary and final land development plan approval or subdivision, submit a site capacity analysis based on the site's physical conditions, hazards, and natural resources. A preliminary site capacity calculation shall be presented at the time of a proposed rezoning for both the present zoning and the proposed rezoning.
Site Capacity Calculations • Site Capacity Calculation utilizes an 8 step process to ascertain density adjustments based upon the preservation of resources (See Handout).
Sinkholes • The protection of sinkholes is intended to protect the County's groundwater from pollution. The protection takes two forms: preserving the sinkhole as open space and preventing stormwater from being directed to the sinkhole area. The following standards shall apply:
Sinkhole • Sinkhole. The area shall be preserved as open space since any disturbance can result in increasing the rate and quality of water reaching the aquifer. Some areas of sinkholes have been plugged by natural processes and a limited amount of construction could occur (see Mitigation below). • Stormwater.Stormwater should be diverted away or around any sinkhole. If large-scale topography (depressions larger than the sinkhole buffer) prevents this, then all stormwater shall be treated in a wet detention basin or similar and all flows moved through grasses, swales, or constructed wetlands to provide maximum cleaning. If forested, all flows should be overland sheet flow through the forest. • Historic Sinkholes. Historic Sinkholes and their buffers may be permitted to be mitigated. • Mitigation. In no event shall open sinkholes be mitigated. Mitigation shall be permitted only after a geotechnical study indicates that: • The sinkhole or group of sinkholes is fully plugged, meeting the standards of historic sinkhole. The geotechnical study shall prove that the sinkhole has been plugged for a substantial time so that plugging material is compacted and stable and a layer of surface soil to a depth of one foot has been established over the area of the sinkhole and any depression associated with it that slows infiltration. • The geotechnical study shall demonstrate that the proposed development in the sinkhole is not going to lead to increased rates of recharge in both quantity and quality. • Stormwater management shall be such that increased runoff due to buildings, roads, or other impervious surfaces are diverted out of the sinkhole and discharged to stream basins. • In no event shall mitigation result in an increase in the permitted development of the property over that of Division 4.300, Site Capacity Calculations.
Sinkhole Buffer • The protection of sinkholes is intended to protect the County's groundwater from pollution, and the sinkhole buffer furthers that objective. The protection takes two forms: locating development in the best manner to prevent stormwater from being directed to the sinkhole area and managing stormwater. The following standards shall apply:
Sinkhole Buffer • Location. The permitted development in the buffer shall be located as far as possible from the sinkhole in a location where stormwater can most efficiently be drained away from the sinkhole. If draining away is not feasible, then the location should be chosen to optimize the effectiveness of the stormwater management systems. • Open Sinkholes. If the sinkhole is open, then the buffer shall be kept at an open space ratio of 1.00. The density of the site capacity calculation is not changed, but the development must be outside the buffer. • Stormwater Management. That portion of the buffer draining to the sinkhole shall be either left undisturbed and in natural vegetation or the stormwater diverted away from the sinkhole. If that is not possible, stormwater shall meet the requirement that there be no change in quantity or quality of water discharged towards the sinkhole from what it would be in a wooded condition. • Mitigation. Mitigation is prohibited unless the sinkhole is classed as an historic sinkhole by the geotechnical study in Section N D, Mitigation. If the buffer is of an historic sinkhole, the site capacity calculation shall eliminate the area of the sinkhole buffer from the calculation.
Vulnerable Areas • Vulnerable areas are defined as being highly vulnerable or moderately vulnerable. The combination of surface and subsurface conditions makes these areas vulnerable to groundwater pollution or contamination. These areas have been mapped at a large scale in the County, but actual on-site conditions may be more variable.
Vulnerable Areas • Highly Vulnerable Areas. All development in highly vulnerable areas shall require a geotechnical study to determine if there are sinkholes that are not visible on the surface or bedrock conditions that need to be specifically avoided or protected. The study shall recommend corrective and mitigation techniques to be applied to pre-identified problem areas and to areas discovered during the construction process. • Moderately Vulnerable Areas. All developments in moderately vulnerable areas shall require a preliminary geotechnical study to determine if there are areas of the site that are not moderately vulnerable or highly vulnerable. • Geotechnical Study. The geotechnical study shall determine the relative vulnerability of different portions the site. It may also be used confirm or refute the existence of elements that lead to the designation as a highly vulnerable area or moderately vulnerable area. The geotechnical study shall: • Identify the presence of sinkholes or filled sinkholes. • Determine the relative vulnerability of different portions of the site as very highly vulnerable, highly vulnerable, or moderately vulnerable. • Identify areas best suited to development and most in need of protection. • Propose a geotechnical protection plan covering mitigation of existing problem areas and response to conditions that occur during construction. • Propose a stormwater management and facilities plan that will meet the requirements of G below. • Open Space. Areas that are particularly vulnerable on the site shall be placed in the required open space as determined by the site capacity calculation (see Division 4.300). • Open Connections. All open connections to the groundwater shall be sealed or filled.
