The latest revision (as of the adoption of this ordinance) of the Montgomery County Drainage Criteria Manual shall be used as a base standard for open channels and detention/retention facilities within a subdivision, unless otherwise approved by the City Engineer.
(A) Open channels. Open channels may be used for outfalls and major drainage ways. The following criteria shall be used in determining the nature of the open channel:
(1) Channels may be left in their natural state, provided that the channel has capacity for the developed flows with one foot of freeboard and velocities are 6.0 feet per second or less.
(2) If the natural channel is to be replaced by an improved channel, the flow from the 100-year design flood must be contained within the improved channel while allowing for one foot of freeboard.
(3) Improved channels shall include a lined section if the design velocity is greater than four feet per second. Lining types, such as concrete, rock walls, erosion control mats and gabions, may be used upon the approval of the City Engineer. Improved channels with design velocities less than the permissible velocities given by the City Engineer may be earthen if the channels are revegetated properly.
(4) Lined channels shall have the entire channel bottom and at least the first three feet (vertical height) of the side slopes up from the channel bottom lined, unless approved by the City Engineer.
(5) Earthen sides above the lined section or totally earthen channels shall be on at least a three feet horizontal to one foot vertical slope, and shall have approved ground cover to prevent erosion.
(6) Improved channels shall have the following minimum side slopes, unless shown to be feasible in a soils report sealed by a registered professional engineer in the state, and approved by the City Engineer:
(a) Three feet horizontal to one foot vertical slope for earthen grassed side slopes.
(b) One and one-half feet horizontal to one foot vertical slope for side slopes in rock.
(7) The developer or owner shall use low maintenance vegetation for vegetative cover, as approved by the City Engineer prior to planting. The selection of planting materials shall comply with locally accepted industry standards.
(8) The developer or owner shall provide a drainage easement and required maintenance easement which shall be dedicated to the city as a permanent drainage right-of-way and open space corridor.
(B) Detention/retention facilities. Runoff rates shall be limited to rates from undeveloped areas. Such requirement shall also apply to the development of sites such as churches, schools and other institutional uses. Detention/retention facilities to reduce runoff rates shall be provided to offset the impacts of developments to runoff rates equal to undeveloped areas. Developments that include 15,000 square feet of impervious area or less shall be exempt from the detention/retention requirements. Detention/retention facilities shall be designed for the 100-year design flood in accordance with the following:
(1) The minimum amount of storage volume of the detention basin shall be the volume required to reduce runoff rates to an undeveloped rate. Dedicated detention/retention basins shall also include an additional one foot of freeboard.
(2) Detention areas in parking lots shall not be:
(a) In required parking spaces, but shall be in extra spaces.
(b) Behind speed bumps, unless the speed bumps are made with reinforced concrete.
(c) Deeper than six inches, unless warning signs are posted.
(3) Drainage easements shall be provided for all regional detention/retention facilities and for other detention/retention facilities where two or more owners are involved.
(4) Detention/retention facilities shall be designed to empty all detention storage in less than 24 hours.
(5) Detention/retention facilities shall not be counted as an erosion control technique, unless:
(a) The basin is designed to empty a minimum of 24 hours after the storm event; and
(b) Adequate sediment storage areas in the basin have been set aside and are maintained.
(6) Detention/retention facilities shall be maintained by the owner, unless such facilities are dedicated to and accepted in writing by the city.
(7) The engineer shall certify that the detention/retention facilities were constructed in accordance with the plans and specifications. The engineer shall provide a signed as-built drawing showing dimensions and elevations of the detention/retention facility.
(8) Any existing subdivision/development that increases the impervious (or semi-impervious) area by more than 15,000 square feet must provide detention/retention/mitigation for the increase in impervious or semi-impervious development.
(Ord. 09-0721, passed 7-21-2009; Am. Ord. 10-0803, passed 8-3-2010; Ord. 19-1022B, passed 10-22-2019) Penalty, see § 155.999