(a) Generally.
(1) Drainage systems, including all conveyances, inlets, conduits, structures, basins, or outlets used to drain storm water, must be designed and constructed to promote the health, safety, and welfare of the property owner and the public. Adequate provision must be made for the acceptance, collection, conveyance, detention, and discharge of storm water runoff drainage onto, through, and originating within the subdivision. No final plat release may be issued until proper provision has been made for drainage.
(2) Private drainage systems are those which serve one lot or tract, or any open system that serves more than one lot or tract for which a private entity has maintenance obligations. Private systems are owned and maintained by a private entity. Easements must be provided to allow access by the city to any open system in the event that private system failure or diminished function jeopardizes the public's health, safety or welfare. Private storm water drainage systems must be designed in general conformance with the design standards of the department of water utilities as set forth in the Drainage Design Manual of the city of Dallas. Private enclosed systems are not required to be constructed according to the Standard Construction Details, File 251D-1.
(3) Public drainage systems are those systems which serve more than one lot or tract, excluding open systems maintained by a private entity. The portion of a drainage system located downstream from a lot or tract boundary, and the portion of any drainage system within the lot or tract boundary which conveys storm drainage from outside the lot or tract boundary are public systems. Public storm water drainage systems must be designed and constructed in strict conformance with department of water utilities requirements.
(4) The city owns and maintains public systems that have been constructed and accepted pursuant to Section 51A-8.612.
(5) All storm drainage facilities must be designed and constructed to safely drain a one-percent annual chance storm event as outlined in the Drainage Design Manual of the city of Dallas. Paved streets and alleys, ditches, and swales may be used for emergency overflow capacity in parallel with enclosed systems provided the requirements of the Drainage Design Manual of the city of Dallas are met.
(6) Storm water must be discharged in an acceptable form and at a controlled rate so as not to endanger human life or public or private property.
(7) The owner shall fund and construct all storm drainage outfalls necessary to safely and adequately drain the subdivision.
(8) The city may provide new public drainage outfalls and public drainage system upgrades to serve existing and future subdivisions through specific items in the capital improvement bond programs.
(b) Erosion and sedimentation control.
(1) The owner shall provide erosion control plans for review and subsequent approval by the department of development services for any development requiring grading or clearing where sediments can be carried to natural or manmade drainageways. Erosion and sedimentation control plans are required in the following instances:
(A) When the property to be platted is located in the escarpment or in a geologically similar area (See Division 51A-5.200).
(B) Where ground cover is disturbed over an area larger than three acres.
(C) If required as a condition of approval of the preliminary plat.
(2) Erosion control plans must include the following:
(A) A timing schedule indicating the starting and completion dates of the development activities sequence and the time of exposure of each area. Written approval of the director of development services is necessary to authorize any time of exposure exceeding six months.
(B) A complete description of all control measures designed to control erosion and sedimentation of soils during and after construction. The owner is responsible for maintenance of erosion and sedimentation control measures during development and shall remove sediment from city right-of-way or storm drainage systems that occurs during the construction phase. Revegetation of the disturbed area is required as a part of the approved erosion control plan.
(c) Detention.
(1) Detention facilities required in this subsection must be designed to provide detention for the one-percent, two-percent, 10 percent, and 50 percent annual chance storm events. Detention must be provided in the following instances:
(A) The property to be platted is in or drains through the escarpment zone or a geologically similar area as defined in Division 51A-5.200 of this chapter.
(B) The development of the platted area results in an increase to the existing rate of runoff due to a rezoning of the platted area that allows higher density. Detention will not be required if:
(i) the rezoned area is in the redeveloped area and there is no increase in impermeable surface;
(ii) the change in zoning results in less than a 20 percent increase in the runoff, and the area rezoned is less than 3 acres, or an adequate outfall exists to handle the developed discharge; or
(iii) the rezoned area is less than one acre in size and adds less than 5,000 square feet of additional impervious surface relative to existing conditions.
(C) The proposed development does not have adequate outfall to carry the one-percent annual chance storm event without damaging property downstream, or the owner of downstream property refuses to provide the needed easements to the city. Detention will not be required under this subparagraph if the owner funds and constructs the storm drainage system to provide a one-percent annual chance storm event runoff carrying capacity.
(D) The property to be platted contributes to the storm drainage of a neighboring municipality having detention requirements, provided there are written agreements with the neighboring municipalities.
(2) Detention facilities must be designed and constructed in conformance with the Drainage Design Manual of the city of Dallas.
(3) Detention area easements must be dedicated on the plat when detention facilities are on-site, and dedicated by a separate instrument when detention facilities are off-site.
(4) Each adjoining property owner and his successors and assigns shall be responsible for simple, routine maintenance of the detention area easement. The city of Dallas is responsible for any major maintenance and repair work necessary for the public safety and welfare.
(5) The constructed detention facilities and pond area must remain to line and grade and must not be altered without the approval of the director of water utilities.
(6) If detention is provided due to inadequate outfall pursuant to Section 51A-8.611(c)(1), then upstream storm drainage systems must be designed for a one-percent annual chance storm event, up to the outfall into the detention basin. Drainage systems constructed downstream must be designed for a one-percent annual chance storm event of the drainage basin without taking into consideration the reduction in flow provided by the detention facility upstream, unless a lesser criteria is approved by the director of water utilities when the proposed development does not increase the stormwater drainage from the property and the director determines that the drainage system is not necessary to preserve public health or safety.
(7) Storm water runoff from any plat into a contiguous city may be required to comply with the criteria of the contiguous city as directed by the director of development services provided there is a written agreement in effect at the time.
(8) When development of the property downstream results in the construction of facilities designed to accommodate the one-percent annual chance storm event, and the detention facilities upstream are no longer necessary, the detention facilities may be abandoned and the land reclaimed for other purposes.
(d) Floodways.
(1) Generally. Floodways must be provided in accordance with the recommendation of the director of water utilities and the requirements of the commission to accommodate the one-percent annual chance storm event drainage flows. Floodway dedications must be identified on the plat and monumented on the ground. Floodway conditions must be satisfied before submitting a final plat for a certificate of approval. Division 51A-5.100 applies to all floodways.
(2) Floodway easements.
(A) Floodway easements are drainage areas dedicated to the city as an easement to prevent obstructions of floodway capacity in a flood plain. Except as provided in Paragraph (3), a floodway easement is required for any portion of a property that is within a flood plain.
(B) A 15-foot wide floodway access easement from a publicly dedicated right-of-way may be required and may extend along a creek, parallel to the top of the bank to inspect or maintain a floodway easement.
(C) The owner of a lot that includes a floodway easement is liable for floodway easement maintenance in compliance with this subsection, taxes, and all other standard property owner liabilities.
(D) Unless approved by the directors of development services and water utilities in an instrument filed in the county deed records or by a city council approved tree mitigation plan, structures, fencing, trees, shrubs, or any other improvement or growth may not be placed in or across any floodway easement.
(E) Common areas, such as in a CUD, may be located within floodway easements. Before the release of a final plat, access to the common area must be shown on the plat and a permanent maintenance plan must be approved as to form by the city for a common area within a floodway easement. Owners of a common area within a floodway easement are jointly and severally liable for the floodway easement common area maintenance in compliance with this subsection, taxes, and all other standard property owner liabilities.
(F) For purposes of this subsection, "maintenance" means removing any object or condition that, as determined by the director of water utilities, impedes the free flow of water. Maintenance includes:
(i) keeping the floodway easement free from any structures;
(ii) removing debris;
(iii) desilting lakes, ponds, and detention areas; and
(iv) controlling the growth of vegetation.
(G) The city retains the right, but not the obligation, to enter onto the floodway easement to inspect or maintain the easement. If the floodway easement is not maintained in compliance with this subsection, the city has the right, but not the obligation, to put the floodway easement in compliance with this subsection. The owner of the floodway easement must pay the city for maintenance work performed within 180 days after the date of presentation of the bill. If a bill is not paid by the owners when due, the city shall file a lien statement that includes expenses assessed, the name of the owner, if known, and the legal description of the lot with the county clerk of the county where the lot is located.
(3) Floodway management areas.
(A) Floodway management areas are drainage areas dedicated to the city in fee simple to prevent obstructions of floodway capacity in a flood plain. All floodways not dedicated as a floodway management area must be dedicated as a floodway easement.
(B) If any portion of the subject property is (1) within a flood plain and (2) abuts a public park, green belt, open space, trail system, or the Trinity River that has been recommended for improvement in a flood plain management plan, the Trinity River corridor plan, the park and recreation long range development plan, the park and recreation master plan, the trail network plan, or any other master plan adopted by the park and recreation board or city council, the directors of water utilities, development services, and parks and recreation must be notified and given an opportunity to negotiate for the acquisition of the property for a floodway management area before a final plat is approved. The property owner is encouraged, but not required, to donate the floodway management areas to the city.
(C) If the applicant donates or commits to sell that portion of the property to the city, the applicant must designate that portion as a dedication or reservation on the plat. Upon acquisition of the property, the city maintains the floodway management area.
(D) The area for each floodway management area must be identified on the plat in square feet or in acres.
(E) No lot may extend into a floodway management area.
(e) Lot to lot drainage. Each lot must be drained to an abutting street or alley unless the director of development services determines that drainage to a street or alley is infeasible. If the director of development services determines that street or alley drainage is not feasible, drainage may be provided as follows:
(1) If no more than the rear 15 feet of a lot drains toward the rear lot line, a well-pronounced swale must be provided as approved by the director of development services.
(2) If more than the rear 15 feet of a lot drains toward the rear lot line, a paved invert in a common area or a drainage easement is required. In order to accommodate the one-percent annual chance storm event, an enclosed drainage system with inlets may be designed. Each portion of the system that drains one lot must be a private system. Each portion of the system that drains more than one lot must be a public system within an easement. (Ord. Nos. 20092; 23384; 25047; 27333; 27572; 27697; 30994; 31314; 32002)