Sec. 16.2-20. Technical criteria.
(a) An evaluation shall be performed for each proposed land development project not exempted from this chapter as provided in 16.2-5(b) to assure compliance with state law and the city ordinance concerning stormwater management.
(b) Ordinance applicability thresholds
(i) Neither stormwater quality nor quantity control shall be required for projects with new impervious cover of less than one thousand (1,000) square feet.
(ii) For projects with new impervious cover between one thousand (1,000) and five thousand (5,000) square feet, stormwater management shall be achieved through new landscaping at the rate of one (1) large shrub for every three and one half (3.5) feet of new building exterior perimeter and one (1) ornamental tree for every twenty (20) feet of new building exterior perimeter.
(iii) Both water quality and quantity control shall be required for projects with new impervious cover greater than or equal to five thousand (5,000) square feet.
Stormwater plan not
required if … Single-family dwelling < 1,000 ft2 of new impervious area
Residential subdivisions < 5,000 ft2 of new impervious area if
containing less than 5 lots landscaping is planted and the site
and no new street construction has an adequate receiving channel
required if …
Single-family subdivision Construction of commercial or
with 5 or more lots industrial building or amenities
exceeds 5,000 ft2 of new
Single-family subdivision with
new street construction
Construction of multi-family units
or amenities exceed 5,000 ft2 of
new impervious area
(c) All stormwater facility design, including hydrologic, hydraulic, water quality, and velocity components, must be verified with submitted engineering calculations, in accordance with the procedures outlined in the latest editions of the Virginia stormwater management handbook, the Virginia erosion and sediment control handbook, and the City of Lynchburg manual of specifications and standard details, including all appendices, or other methods approved by the city’s plan approving authority.
(d) All design calculations must be submitted in formats as specified in the City of Lynchburg manual of specifications and standard details, and by the central Virginia erosion and sediment control committee.
(e) For proposed residential, commercial, and/or industrial subdivisions, these stormwater management criteria shall be applied to the land development as a whole, excluding any previously developed parcel(s) which have no new impervious area; individual lots in new subdivisions shall not be considered separate land development projects, but rather the entire subdivision shall be considered a single land development project. Hydrologic parameters which reflect the ultimate land development shall be used in all engineering calculations.
(f) Stormwater management facilities implemented prior to the date of the adoption of this ordinance shall be maintained in accordance with the stormwater management regulations that were in effect at the time the property on which the facilities are located was developed. Except where specifically required by federal, state or local laws or regulations, it is not the intent of this ordinance to require existing stormwater management facilities that were constructed under prior regulations to be upgraded until the property for which such facilities were constructed is developed or redeveloped to such an extent that it triggers the applicability thresholds set forth in section 16.2-20(b).
(g) Construction of stormwater management facilities or modifications to channels shall comply with all applicable laws and regulations. Evidence of approval of all necessary permits shall be presented.
(h) The design of impounding structures shall be checked by a design professional certified to practice in the Commonwealth of Virginia for structural integrity and floodplain impacts for the one hundred (100)-year storm event.
(i) Outflows from storm water management facilities shall be discharged to an adequate channel or an adequate stormwater conveyance system. Complete calculations and a narrative shall be included on the plan. Verification of channel adequacy should include the following:
(1) Channel adequacy shall be checked at all critical locations, as determined by the plan approving authority, downstream of the discharge as far as the point where the disturbed area represents less than one percent of the contributing watershed.
(2) The channel lining material shall be evaluated to determine its ability to withstand the design velocity as found in Table 5-22 of the Virginia erosion and sediment control handbook, 1992 edition and other published materials as approved by the plan approving authority.
(3) For natural channels, relative elevations should be taken along the channel length at the channel cross-sections in order to determine the average longitudinal slope of the channel.
(4) For enclosed conduits, a submitted hydraulic grade line calculation should verify that the flow is contained within the system during the design event.
(5) The necessary construction easements to accomplish the proposed design shall be required.
(6) Permanent drainage easements shall be dedicated to the city to permit reasonable access for maintenance of all stormwater facilities which are to become public property. A stormwater maintenance agreement shall be required for all stormwater management facilities which shall remain private property. Private drainage easements shall be provided for maintenance access to all stormwater facilities that will be located on private property but will not be owned by the entity directly responsible for maintenance.
(7) Determination of the size and capacity of the drainage system shall be based on the planned development and existing development within the watershed.
(8) The drainage system shall be adequate for both off-site and on-site surface waters and properties over which the surface waters are conveyed shall not be adversely affected.
(9) Concentrated surface waters shall not be discharged on adjoining property, unless an easement expressly authorizing such discharge has been granted by the owner of the affected land or unless the discharge is into a natural watercourse or other appropriate discharge point as set forth above.
(i) Velocity dissipaters shall be placed at the outfall of all stormwater discharge points as necessary to provide a non-erosive velocity of flow from the outfall into an adequate channel. Conveyance channels shall be designed to provide a non-erosive surface for designed flows. Riprap shall only be used when protection of the channel geometry and lining cannot otherwise be achieved.
(j) Suitable safety measures should be incorporated into the design of all stormwater management facilities. These may include but are not limited to, safety ledges, fencing, warning signs, anti-vortex devices, stadia rod indicating depth at the lowest point, and outlet structures designed to limit public access. The plan approving authority reserves the right to require safety measures consistent with local policies for public safety.
(k) If stormwater management facilities are provided through which water passes at times other than following rainfall, documented provisions shall be made to pass existing flows. It is necessary for detention requirements to be met, despite the necessity of passing certain low flows. This applies to all onstream or online stormwater management facilities.
(l) All stormwater management facilities shall be designed to function without manual, electrical or mechanical controls, unless authorized in advance in writing by the plan approving authority.
(m) Detention facilities shall be designed to minimize propagation of insects, particularly mosquitoes.
(n) Outlets for all stormwater management facilities shall be designed to impede the downstream migration of trash and debris into natural and man-made channels. (Ord. No. O-01-079, 4-24-01, eff. 5-1-01; Ord. No. O-07-047, 4-10-07)
Last updated date: 8/27/2007 10:07:41 AM