(A) Construction plans for proposed reinforced concrete box culverts, bridges and related structures may be adaptations of the current Texas Department of Transportation (TxDOT) Standards.
(B) For bridges and culverts in residential streets, runoff from the 100-year frequency flow shall not produce a headwater elevation at the roadway greater than either six inches above the roadway crown elevation or any top of upstream curb elevation, whichever is lower.
(C) For bridges and culverts in streets other than a residential street, runoff from the 100-year frequency storm shall not produce a headwater elevation at the roadway greater than three inches above the roadway crown elevation or three inches above any top of upstream curb elevation, whichever is lower.
(D) All drainage facilities (including but not limited to headwalls, open channels, storm sewers, area inlets, and detention, retention and water quality controls and their appurtenances) shall comply with the following requirements, unless otherwise noted in this section.
(1) Storm sewer inlets and gutter transitions shall be designed to avoid future driveways and to avoid conflicts with standard water and wastewater service locations. No utilities shall be allowed to cross under a storm sewer inlet unless approved by the City Director of Public Works or City Engineer.
(2) Drainage channels and detention ponds that are to be maintained by the public (city) shall be contained within drainage easements. Adequate room for access shall be provided for drainage channels and detention ponds. Ramps no steeper than five feet horizontal to one foot vertical shall be provided at appropriate locations to allow access to drainage channels and detention ponds. The minimum bottom width for any channel with vegetative side slopes shall be four feet.
(3) Detention ponds shall be designed with adequate area around the perimeter for access and maintenance. The said area shall be a minimum of seven feet wide for ponds with depths of five feet or less (back slopes included) and a minimum of 15 feet wide for ponds over five feet deep or with back slopes in excess of five feet high. The said area shall not slope more than 5%.
(4) Rip-rap for slope protection or velocity dissipation shall be formed by concrete dissipaters or designed rock rip-rap. Designed rock rip-rap shall include a D50 sizing and a gradation for the rock. Mortared rock or stone shall not be allowed.
(5) Storm drains between lots (crossing blocks) shall be avoided as much as possible. When unavoidable, such drains shall be underground storm drains, unless underground storm drainage is not practical. Flumes will be allowed only if approved for use by the City Engineer or Director of Public Works. In either case, the improvements shall be contained within the dedicated drainage easement. Storm drains along rear of residential lots (through back yards) shall be avoided. Easements shall be a minimum of 15 feet in width.
(6) All bends, wyes and pipe size changes in storm sewers shall be prefabricated or shall occur at manholes/junction boxes.
(7) Storm drains shall be reinforced concrete pipe (RCP), ASTM C76, minimum 18 inches diameter. The Engineer shall provide load analysis to the Planning and Development Department as appropriate to demonstrate that class of pipe used is sufficient for the loading conditions as set forth in TxDOT Item 464. Higher strength pipes shall be used where loadings warrant such. Storm drains shall have a minimum of two feet of cover in unpaved areas and a minimum of one and five-tenths feet of cover from sub-grade in paved areas. Embedment of RCP shall be to the spring line of the conduit.
(8) The use of High Density Polyethylene (HDPE) shall be allowed up to 36 inches in diameter. Pipe sizes larger than 36 inches in diameter may be used if approved by the City Engineer or Director of Pubic Works. The use of HDPE pipe shall be limited to unpaved areas outside of city streets. All cross street storm drainage conduit shall be reinforced concrete pipe (RCP). All outfall structures shall be constructed of reinforced concrete and the connection with the outfall structure shall be accomplished using RCP. A transition fitting from HDPE to RCP shall be made upstream of the outfall structure. Embedment of HDPE shall be to six inches above the top of the pipe.
(9) Junction boxes and manholes shall be reinforced concrete. Junction boxes in lieu of manholes shall be provided where any pipe opening exceeds 37 inches and where the distance from the outside surfaces of any two pipes entering a manhole is less than one foot, measured along the inside of the manhole.
(10) Prefabricated wyes, mitered angle fittings and pipe size reducers shall be allowed in lieu of junction boxes and manholes.
(11) Channels.
(a) Concrete channels. Concrete channels shall be of sufficient cross section and slope as to fully contain design flows and facilitate self- cleaning. Outfalls shall enter major collector drainage ways and major streams at grade or be designed and constructed with adequate concrete aprons, energy dissipaters or similar features to prevent erosion.
(b) Vegetated channels. Vegetated channels shall have sufficient grade but with velocities that will not be so great as to create erosion. Side slopes shall not be steeper than three feet horizontal to one foot vertical for channels four feet or less in depth and no steeper than four feet horizontal to one foot vertical in all other channels to allow for future growth and to promote slope stability. All slopes shall be hydro-mulched, sodded or seeded with approved grass, grass mixtures or ground cover as indicated in TxDOT Item 164 for use in the Waco District and season in which they are applied. All earthen channels must have vegetation established prior to acceptance by the City of Harker Heights. If vegetation has not been established at the time applicant/developer requests approval of the final plat, the channel side slopes and bottom shall be lined with erosion control matting prior to being seeded and approval of the final plat shall be conditional to the establishment of the vegetation.
(c) Major streams shall not be modified without consent of applicable state and federal agencies and authorization from the City Engineer or the Director of Public Works.
(12) Discharge from storm sewer outfalls shall not cause channel, bluff, or stream bank erosion. If the storm drain discharges to an open drainage facility (as determined by the city), the applicant must show acceptable non-erosive conveyance to that drainage facility, appropriate energy dissipation at the outfall and a stable headwall. No outfalls shall be allowed to discharge on the slope of the receiving channel without adequate erosion control measures.
(13) If the development is located such that there is considerable drainage from potentially developable upstream areas, the developer may request participation by the city for the cost of over- sizing of elements of the overall drainage system. The city shall consider these requests on a case-by-case basis.
(Ord. 2007-18, passed 6-26-07)