There are many types of best management practices (BMPs) that the County has determined are acceptable methods of stormwater pollutant removal. However, no single BMP is suited for every situation and each type has limitations, advantages and disadvantages relating to efficiencies, space, maintenance requirements, etc. In addition, County policy and adopted design criteria may preclude certain BMPs from being acceptable options for specific projects.
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GENERAL DESIGN GUIDELINES
3.6.3 BMP LOCATION
3.6.4 BMP ACCESS
3.6.5 BMP MAINTENANCE
3.6.6 EXISTING WET PONDS
3.6.7 SAFETY DESIGN
3.6.8 BMP INSPECTION
3.6.9 FIELD CONDITION CHANGES
3.9 PLAN SUBMITTAL REQUIREMENTS
BMP POLLUTANT REMOVAL EFFICIENCIES
The specific design criteria for the various types of BMPs are included in the Minimum Design Standards of Chapter 9 of the Henrico County Environmental Program Manual. However, below are listed the various types of BMPs, design variations and their respective phosphorus removal efficiency. BMPs must be designed based on these efficiencies and shall be located and maintained to effectively reduce the pollutant load to the required level.
Average Total Phosphorus Removal Efficiency
With 10% Additional Volume In Forebay
Extended Detention Basin
Design 1 - 1 x WQV detained 12 hours 20%
Design 2 - 2 x WQV detained 30 hours 30%
Design 3 - 2 x WQV detained 40 hours 35%
Extended Detention with Shallow Marsh
2 x WQV detained 30 hours
Retention Basin (Wet Pond)
Design 1 - 1 x WQV 35%
Design 2 - 2.5 x WQV 40%
Design 3 - 4 x WQV 50%
Design 4 - 10 x WQV 65%
Design 5 - 4 x WQV with Wetland System 65%
Design 1 - 1 x WQV 50%
Design 2 - 2 x WQV 65%
Delaware Sand Filter
All Other Sand Filters
WQV = ½" of runoff per impervious acre
Although all extended detention and retention basins require forebays, the basin may achieve an additional 5% removal efficiency if the required forebays provide 10% additional volume.
Where a method or facility without an efficiency rating is proposed, the designer must adequately substantiate the rating before the design will be considered for approval.
BMPs may be constructed in series; however, the maximum combined pollutant removal efficiency recognized by the County of Henrico is 65%. Henrico County uses a maximum phosphorus removal efficiency of 65% for both individual BMPs and BMPs in series. Phosphorus in urban runoff is approximately 60% particulate form and 40% soluble form. Settling is the principal mechanism used by BMPs to remove pollutants and therefore, the soluble phosphorus remains in runoff after treatment by a BMP.
All proposed projects required to comply with the stormwater quality criteria must address the following general design guidelines:
1. Stormwater management (SWM) addresses the quantity of runoff, while Best Management Practices (BMPs) address the quality of runoff. While there are differences between the performance criteria, the two management strategies will often be employed within the same structure or facility. Standards which apply to one set of performance criteria shall not lessen the performance criteria of the other.
2. If both quantity and quality management are provided within the same facility, the final design shall ensure that the performance criteria and maintenance are compatible.
3. Pollutant loadings shall be determined by the calculation methods set forth in this manual.
4. For development incorporating an existing BMP, basin, or pond, the engineer shall certify that the facility is in good working order and performing at the necessary level of service and provide supporting calculations/information as necessary.
5. Underground structures detaining flows and not providing infiltration, or not documented to reduce pollutant loads, shall not be allowed as BMPs.
6. Volume control BMPs, such as porous pavement and modular pavement, are recognized as difficult to maintain, are subject to high rates of failure, and are not accepted as BMPs unless approved by the Director of Public Works.
7. Gravel parking areas which are subject to vehicular traffic and subsequent compaction shall be considered as impervious.
8. Maintenance activities such as repaving a previously approved or legally "nonconforming" impervious area shall not be considered as development when calculating the pollutant removal requirement.
9. Subdivisions which are to be submitted in phases (Section A, Section B, etc.) must address stormwater quality and quantity (where applicable) for the entire subdivision as part of the first phase submittal. BMP calculations, location(s), and design option(s) must be identified on an overall subdivision plan and approved by the Department of Public Works prior to any phase plan approval. The BMP must be constructed during the phase that increases the impervious cover for the subdivision to 16% or greater.
10. The Water Quality Volume may not be stored in any County maintained easements or rights-of-way. It is recommended that privately maintained systems should consider using O-ring pipe in situations where the storm sewer will be temporarily inundated. O-ring pipe must be installed on non-County maintained storm sewer systems that will permanently hold the Water Quality Volume.
Proposed BMPs must be located to adhere to the following siting constraints:
1. A BMP must meet minimum front yard setback requirements from the public right of way. The setback for the BMP is measured as the distance from the highest continuous contour elevation inside the BMP to the ultimate right of way line. These setbacks are identified below:
a. Residential Zoned Districts (all "R" Districts including A-1) - The BMP must honor the setback specified for the primary dwelling. b. Non-residential Zoned Districts - The BMP must honor a minimum setback of 25 feet from the ultimate right of way.
2. A distance equivalent to the rear yard setback in residential development must be provided from the rear of the buildable area to the highest continuous contour elevation inside the BMP.
3. The BMP must be located outside all transitional or proffered buffer areas or planting strip easements.
4. If highly visible from the public right of way, the BMP shall be effectively screened from the public right of way or less intense uses of adjacent properties. Examples of acceptable screening include fencing, landscape hedging or a combination of these features.
5. Infiltration trenches, sand filters, and wet ponds are not permitted in any single family residential development. Regional wet ponds and existing ponds are allowed in residential development under certain conditions. BMPs can not be located within the limits of the Resource Protection Area (or the 100' buffer) or the Stream Protection Area unless:
a. There are no other reasonable alternatives, and
b. The encroachment is the absolute minimum necessary to site the BMP. BMPs can not be located in the lower (seaward) 50 feet of the RPA buffer or in the lower (seaward) 35 feet of the SPA , and
c. Buffer equivalency has been provided in accordance with the Chesapeake Bay Local Assistance Manual for impacts to the RPA buffer.
6. BMPs may be located within the 100-year floodplain, however, they must be located outside the 25-year floodplain.
7. In general, BMP facilities for subdivisions shall be extended detention ponds (with or without a shallow marsh). Draw down time for extended detention ponds shall not exceed 30 hours in subdivisions. These BMPs shall be located in the rear yard of all residential units or in a common area controlled and maintained by a homeowners association. The BMP must be a minimum of 20 feet from the principal structure.
In general, all BMPs shall be designed to provide adequate access for maintenance activities. However, the following specific provisions must be met for all County maintained BMPs:
1. An access easement around the BMP facility is required. In addition, BMP facility easements shall be provided to adequately contain the 10-year ponding level (plus six inches of freeboard), embankment, outlet structures, and an appropriate width (minimum 12 feet) of maintenance area around the 10-year ponding area that permits access to the dam, outlet structures, and embankments.
2. Access to a BMP facility shall be contained within an easement of not less than twenty (20) feet in width and shall not exceed a grade or cross-slope of 12:1. The easement must either be in a common area or located completely on a lot (not split by a property line).
3. The access easement shall not have any obstacles, vegetation, or cross-slopes that would prevent ease of access for maintenance equipment.
4. A 12' minimum access should be provided for maintenance capabilities for all dams and inlet and outlet structures.
5. A 12' easement is required from the downstream toe of slope or from the limits of cut on an excavated basin.
Periodic maintenance of BMPs is required and inspections will be conducted to determine whether the facilities are being maintained. All BMPs constructed in the County must have a maintenance agreement on file with the Department of Public Works and recorded in the Circuit Court Clerk's Office, regardless of whether long term maintenance is provided by the County. A copy of the required maintenance agreement is included in the Appendix of this Manual.
For BMPs in subdivisions for which the County collected $100.00 per lot prior to recordation, the County will provide long term maintenance (i.e. dredging) of extended detention basins and shallow marsh basins. Short term maintenance of basins (grass cutting, trash pick-up) shall be provided by the developer or Homeowners' Association. This also applies to zero lot line developments.
For BMPs on POD and certain subdivisions that did not contribute $100.00 per lot prior to recordation, maintenance of the BMPs is the responsibility of the owner or Homeowners' Association. This includes short term (i.e. grass cutting, trash pick-up) as well as long term (i.e. dredging) maintenance activities. All designs must address potential maintenance problems as part of the submittal.
Existing wet ponds in single-family residential subdivisions are permitted to remain in place; however, the subdivision must be designed in such a manner that the existing pond meets each of the following criteria:
1. The pond must be located in a common area, and
2. There must be a minimum of 50 feet between any buildable area and the normal pool elevation of the pond, and
3. All access and easement requirements must be met, and
4. The pond must be an amenity for the entire subdivision, and
5. In order to use an existing wet pond as a BMP:
a. The pond must meet the design criteria included in Minimum Design Standard 9.05, and
b. A letter from the U.S. Army Corps of Engineers must be submitted stating the pond can be maintained as a BMP and that either a permit would not be required or a permit would be issued for such maintenance activity.
The design of the BMP shall contain any features necessary to eliminate safety concerns for the public. For example, guardrail may be required due to the proximity of the BMP to vehicular traffic. The Director of Public Works reserves the right to deny approval of any BMP facility in a single family residential development which is deemed to be a potential nuisance or hazard.
BMPs will be inspected at two phases of construction - "rough grading" and "final conformance". The developer, the developer's engineer, and County staff should be present at these inspections. Prior to final acceptance by the County, a letter of conformance may be required from the developer's engineer. These inspection requirements are incorporated into the County's standard Erosion and Sediment Control plan sheet.
Prior to final acceptance, the County will determine whether the BMP has been built in accordance with the approved plans. If discrepancies between the designed and "as-built" BMP exist, they must be addressed. If these discrepancies cannot be corrected in accordance with the approved plan, a revised plan may be required.
Proposed BMP revisions that alter the type of BMP may need to be reviewed by the Planning Commission. For example, if during construction springs were encountered and the previously approved dry basin design needs to be changed to a shallow marsh, approval must first be obtained from the Planning Commission.
The following activities are exempt from the requirements of this Chapter:
1. Minor land disturbing activities involving less than 2,500 square feet of land disturbance.
2. Agricultural and silvicultural operations as outlined in Chapter 10 and Chapter 24 of the Code of the County of Henrico Virginia.
3. Surface mining, exploration or drilling operations as exempted in Chapter 10 of the Code of the County of Henrico Virginia.
4. Individual utility connections and/or the maintenance of utility lines.
5. Other applicable exemptions as identified in Chapter 10 of the Code of the County of Henrico Virginia.
The Director of Public Works may consider granting a waiver of the stormwater quality requirements for the following:
1. Single family developments which involve three lots or less, with an aggregate impervious area less than 25 percent.
2. Applicable waivers for Chesapeake Bay Preservation Areas identified in Chapter 24 of the Zoning Ordinance (§ 24-106.3).
3. Inadequate site availability for a suitable BMP structure, as determined by the Director of Public Works. The waiver request shall include technical documentation and computations necessary to support the request.
4. If full compliance with the pollutant removal requirement is demonstrated to be unachievable, the portion that cannot be achieved by the proposed BMP(s) shall be accomplished through a contribution to the Environmental Fund. However, the pollutant removal that is not achieved must be less than the pollutant removal requirement associated with 25% impervious coverage (L25% - L16%) on the site.
All requests for waivers must be submitted in writing to the Director of Public Works and sent to the attention of the County's Review Engineer.
The Director of Public Works, in conjunction with the Director of Planning, or on appeal to the County Manager, may consider exceptions to the BMP location guidelines. An exception may be considered only after a conceptual landscaping plan for the BMP and surrounding area is provided in accordance with Minimum Design Standard 9.12 found in Chapter 9 of this Manual. The exception request shall be such that the BMP provides only the minimum encroachment necessary and does not adversely affect:
1. The aesthetic appearance and value of the development.
2. The visibility or value of abutting or adjacent properties.
3. The health, safety and welfare of persons residing on, working on, or visiting the premises.
4. Traffic or pedestrian safety.
5. The improvement or maintenance of the abutting public right of way.
A plan shall not receive final approval from the Department of Public Works unless it includes the manner in which the pollutant requirement resulting from the project will be addressed. The following items and information must be submitted prior to final approval:
1. For projects required to provide a BMP to address the pollutant removal requirement, BMP design shall be submitted as part of the site development plans, including, but not limited to:
a. Calculations, assumptions, and criteria used in the design of the BMP, plans, and profiles.
b. Plans and profiles, including horizontal and vertical controls, elevations, sizes, slopes, and materials.
c. Location, dimensions, and design details required for each BMP facility, as specified in the corresponding sections of this manual.
d. A map of the drainage area served by the BMP.
e. Depth to high water table or seasonal high water table, percolation tests, and calculations for gravel trenches or pits proposed to incorporate infiltration into soil. The Department of Public Works reserves the right to require additional calculations and/or information which may be necessary to evaluate the design of the facility.
f. Additional plan requirements as identified in Chapter 10 (E&S Ordinance) and Chapter 24 (Zoning Ordinance).
g. For each BMP proposed, the BMP Information Summary Chart found in the Appendix of this Manual must be completed.
h. Topsoil, sodding or other measures and techniques should be specified as a part of the BMP design to ensure that a healthy mature stand of vegetation is established immediately after grading for the BMP/Sediment basin.
i. Fencing, landscaping, ground cover and other measures and techniques should be specified as part of the BMP design to provide adequate screening of the BMP structure.
2. All submittals must include the calculations for the pollutant removal requirement on the plans. If the calculations indicate that pollutant removal is required, a compliance worksheet must also be included. If the pollutant removal requirement is met by a BMP, a BMP design worksheet must be shown on the plans as well.
3. In addition to the information listed in items 1 and 2 above, proposals for the construction of extended detention basins without shallow marshes and wet ponds must also include the following:
a. Adequate information to prove all design criteria in Minimum Design Standards 9.03 found in Chapter 9 of this Manual (Extended Detention Basins) and 9.05 found in Chapter 9 of this Manual (Retention Basins) have been met.
b. Calculations of the BMP stage-storage, including elevation, area, incremental volume, and the cumulative volume in cubic feet.
c. Composite discharge curve developed by comparing the inlet control of all the individual structures comprising the basin outfall and the outlet control of the discharge structure. If the BMP discharges to a swale or channel, the outlet control analysis needs to consider the effects of tailwater. If the BMP discharges to a storm sewer system, the outlet control analysis should be based on the hydraulic grade line.
d. Calculations of the 10-year and 100-year post-developed flows.
e. Analysis of the 10-year and 100-year post-developed flows through the basin.
f. Water surface elevations of the water quality volume and the 10-year and 100-year post-developed flows.
g. Detail of BMP outlet structure with all information shown. (Inverts, lengths, widths, slopes, weir elevations, top elevations, etc.)
h. Spillway capacity analysis if a spillway is utilized.
i. Basin length, width, and slopes labeled on the plan (e.g. 3:1, 4:1).
j. Access to and around the BMP identified.
4. In addition to the information listed in items 1, 2, and 3 above, proposals for the construction of extended detention basins with shallow marshes must also include the following:
a. All information needed for extended detention ponds.
b. Soils information.
c. Marsh planting plan (see Minimum Design Standard 9.04).