High groundwater table and impervious layer overlay district
A. Establishment. The High Groundwater Table and Impervious Layer Overlay District (High Groundwater Overlay) is hereby established and may be referred to hereafter as the "High Groundwater Overlay." This overlay district encompasses specific areas of the town as shown on the map entitled "Zoning Map Overlay District Related to Protection of Critical Lands Containing Freshwater Wetlands, High Groundwater Table, and/or Shallow Impervious Layer" dated August 2007 and attached to the zoning ordinance.
B. Findings. The High Groundwater Overlay encompasses areas where nonconforming lots predominate, no public sewer and water are available, and the hydrogeologic conditions provide considerable constraint to development. These conditions require special design approaches and/or infrastructure in order to protect individual homeowners, neighborhoods, and Jamestown's water resources.
C. Applicability. The following activities are subject to the procedures, review, and standards within the High Groundwater Overlay only where the activity is proposed on a lot that is less than 40,000 square feet:
1) Any construction activity that results in the development of new impervious cover.
2) Any construction activity that results in the replacement or expansion of existing impervious cover.
3) Excavation, mining, dredging, filling, or grading that involves the movement of over 200 cubic feet of soil.
D. Sub-districts. The High Groundwater Overlay is divided into two sub-districts as follows:
1) Sub-district "A" consists of those lots where:
a) The seasonal high groundwater table is less than or equal to 18 inches below the original grade; or
b) The impervious layer is less than or equal to 42 inches below the original grade.
2) Sub-district "B" consists of those lots where:
a) The seasonal high groundwater table is greater than 18 inches and equal to or less than 48 inches below original grade; or
b) The impervious layer is greater than 42 inches and up to and including 60 inches below the original grade.
E. Determination of sub-district.
1) Presumption of sub-district A. Absent the evidence required to make a determination of which sub-district applies to a particular lot, the town shall presume sub-district A applies with eight percent maximum impervious surface coverage (see section 82-800 H, Impervious cover, below). An applicant may enter into development permitting procedures voluntarily with this presumption.
2) Evidence. The decision as to whether a particular lot is located in either sub-district A or sub-district B shall be made by the zoning enforcement officer based on the field observation required (subsection E.4). A submission of evidence to the zoning enforcement officer shall include all of the results of examination or testing conducted on the lot and shall be accompanied by a written representation by the owner/applicant or project representative that no such results are being withheld.
3) Previous determination. Where a property owner or the town can produce records of a previous determination made by the town, that determination may be used provided there is a record of the field observations used to make that determination.
4) Field observation required. Soil evaluations verified by a RIDEM Class IV soil evaluator are required to determine the applicable sub-district.
a) Where a new OWTS is proposed, the applicant shall provide record of the two soil evaluations associated with the OWTS, as required by RIDEM.
b) Where an OWTS already exists, and a new OWTS is not proposed, records of the soil evaluations from the original state approval shall be submitted to the town.
c) Where a new structure is proposed on a lot the applicant shall provide a single soil evaluation in the area of each proposed structure's footprint in addition to test hole information associated with the existing or proposed OWTS.
5) Determination. The zoning enforcement officer shall make a determination of which sub-district applies based on the following:
a) If any of the soil evaluations show evidence meeting at least one of the sub-district A criteria, then the lot lies in sub-district A;
b) If none of the criteria for sub-district A are met in any of the soil evaluations, and at least one of the criteria for sub-district B is met in any of the soil evaluations, then the lot lies in sub-district B;
c) If none of the soil evaluations meets either the sub-district A or the sub-district B criteria, then the lot is not subject to the standards promulgated in the High Groundwater Overlay.
F. Permit review.
1) Construction, reconstruction, or renovation of structures or other new impervious surface coverage 200 square feet or greater in sub-district A shall be reviewed first by the Planning Commission through as a recommendation to the Zoning Board of Review. The activities shall subsequently require a special use permit from the zoning board of review. The planning commission recommendation shall be advisory to the special use permit application reviewed by the zoning board of review.
2) A one-time construction, reconstruction, or renovation of structures or other new impervious surface coverage less than 200 square feet in sub-district A shall be reviewed under administrative development plan review. Any additional construction, reconstruction, or renovation to the subject lot shall comply with the requirements of section 82-800 as development over 200 square feet or greater.
3) Activities subject to review in sub-district B shall be reviewed under administrative development plan review.
4) The town may engage one or more professionals as deemed necessary by the town, at the expense of the applicant, to assist with the review of applications at a reasonable cost.
G. Prohibitions.
1) The installation of subsurface drains designed to intercept and lower the groundwater table for the installation of an OWTS.
2) Any proposed new or expanded basement and/or the bottom of any structure's slab, not including pilings/footings that is less than 12 inches above the seasonal high groundwater table is prohibited. In-ground swimming pools are prohibited in sub-district A.
H. Impervious cover. Lots in sub-district A and sub-district B shall have limitations on the amount of impervious cover that may be installed in accordance with the following provisions:
1) A lot in sub-district A shall be limited in accordance with the following chart and associated provisions:
a) Where the examination and/or testing of multiple areas of a lot yield different results as to the water table and/or impervious layer, the percent of maximum impervious cover for the lot shall be determined on the most restrictive results in both categories.
b) Freshwater wetlands shall be subtracted from total lot size prior to calculating maximum impervious cover.
2) In sub-district B, no lot shall have more than 15 percent impervious cover.
3) Impervious surface coverage on a structure shall be calculated at the exterior walls with an ordinary roof projection of 12 inches. Any roof projection beyond 12 inches will count toward the impervious surface coverage.
I. Performance standards. Activities subject to review under the provisions of the High Groundwater Overlay shall comply with the following development standards as applicable. Applicants may request relief from these standards through an application for a special use permit from the zoning board of review.
1) All existing OWTS shall be in good working order. Within sub-district A, all existing OWTS will require a flow test conducted by a licensed maintenance provider. Within sub-district B, for any project involving an addition of 400 square feet or more to a building or structure, all existing OWTS will require a flow test conducted by a licensed maintenance provider. In both sub-Districts A and B, a maintenance report for an existing OWTS certified by a licensed maintenance provider and completed within the past 12 months may be submitted for any flow test required above.
2) All OWTS and wells shall be located on the same lot of the residence to which they are connected. All new OWTS and OWTS requiring major repair shall be approved by RIDEM and shall be designed for advanced reduction of nitrogen. Where necessary for the protection of public health, as defined by RIDEM, the systems shall also be designed for enhanced pathogen removal. OWTS shall use approved technologies listed by the RIDEM and shall be capable of achieving current RIDEM performance standards for denitrification and/or pathogen removal.
3) Applications shall provide a stormwater best management practice capable of storing, on the subject site, the increased volume of runoff resulting from development of the site.
a) The minimum storage volume shall be the difference between the pre-development and post-development runoff volumes using the ten-year, 24-hour, Type III storm event.
b) Stormwater mitigation and water quality treatment practices shall be designed in accordance with the Rhode Island Stormwater Design and Installation Standards Manual, most current edition.
For the purposes of this calculation the following table will be used:
| Assumed Percent of Rainfall Which Becomes Runoff During a Ten-Year, 24-hour Storm Event. | |
| Bare soil | 40% |
| Grassland | 35% |
| Cultivated | 30% |
| Timber/Forest | 15% |
| Lawn 0—5% slope | 15% |
| Lawn >5% slope | 30% |
| Roofs | 95% |
| Paved areas (concrete, asphalt, brick, etc.) | 85% |
| Gravel surfaces (constructed) | 60% |
4) There shall be a 15-foot separation between a leachfield and the edge of any stormwater infiltration system.
5) Proposals must comply with the Jamestown Soil Erosion and Sediment Control Ordinance contained in the Code of Ordinances, chapter 22, article V, division 2.
6) Operation and maintenance requirements of any designed and approved stormwater system shall be recorded with the final approval from the zoning board.
J. Special use permit criteria. The proposal shall incorporate, to the greatest extent possible, the following design criteria and explanation of such shall be provided as a narrative to the proposal where practical. Where a special use permit is required pursuant to the provisions of the High Groundwater Overlay, the zoning board of review may approve, deny, or approve with conditions based on the standards and criteria within this section of the zoning ordinance.
1) General criteria.
a) The proposed use and site design are appropriate to the natural characteristics of the land, including its suitability for use based on soil characteristics, geology, topography and susceptibility to surface and groundwater pollution.
b) The proposal preserves the values of unique or valuable natural resources and features.
c) The proposal is appropriate to the availability and capacity of existing public and/or private services and facilities.
d) The proposal is consistent with the goals and pattern of land use contained in the Jamestown Comprehensive Plan.
e) Site design measures protect the island's vulnerable and limited water supplies by maintaining maximum groundwater recharge of rainfall and treated wastewater to replenish drinking water supplies and avoid salt water intrusion.
f) The site design approach is consistent with the need to prevent further impacts and restore impaired areas where intense development and water use, in combination with limited land development suitability, have resulted in localized flooding, incidents of groundwater contamination, low well yields, and salt water intrusion.
2) Subsurface structures.
a) The design of the subsurface structures shall minimize the problems and hazards created by the seasonal high groundwater table and/or impervious layer and result in the least grading, filling, or other disturbance to the site and to any wetland buffer as possible.
b) The seasonal high groundwater table will not damage, interfere or reduce the potential for the proper functioning of the subsurface structure.
c) The subsurface structure will not pose any threat to public health or safety or to the water resources of the town, including groundwater.
3) On-site wastewater treatment systems.
a) All proposals relating to the installation of an OWTS shall ensure that the system, once in use, will not pose a threat to the public health and safety nor cause unreasonable degradation of ground or surface water quality, including adverse effects due to cumulative impact.
b) All proposals relating to the installation of an OWTS shall demonstrate that the design, siting and selection of technologies for the treatment and dispersal units are the most appropriate for the site.
c) All proposals relating to the installation of an OWTS shall demonstrate that the project has been designed so as to minimize combined impacts related to the OWTS, stormwater runoff, and potential disturbances to wetland buffers.
4) Stormwater management.
a) Stormwater runoff control measures will minimize site disturbance, maximize nonstructural controls, and will not adversely affect subsurface flow of groundwater.
b) The proposed site improvements minimize fill and grading, and maintain, to the greatest extent possible, the existing overland flow of runoff from the site to surrounding areas.
c) All stormwater management measures will maintain the water quality function of wetland buffers and avoid any encroachment that might impair the wetland's pollutant removal capacity. Proposed measures will not direct channelized flow to the wetland, reduce subsurface flow through the buffer, increase sedimentation, reduce shade cover, or include any alteration that would result in fluctuating water levels that negatively impact sensitive habitat.
K. Variances for prohibited uses in sub-district A.
1) Applicants proposing uses prohibited pursuant to [sub]section G hereof shall, after development plan review by the planning commission and recommendation to the zoning board, be required to obtain a use variance pursuant to article 3 hereof. In addition to the standards contained in article 3 hereof, all applicants shall demonstrate that the proposal meets, to the greatest extent possible, all of the performance standards contained in subsections I and all of the criteria in subsection J hereof.
2) Applicants shall file a development plan with the building official containing the information listed on the application available in the office of the environmental scientist, public works department.
(Ord. of 2-3-2025(1), § 1(Exh. A))