(A) The interpretation of the requirements set forth in this section shall be made by the City's Engineer or designate, unless specified otherwise by the City Council. The subdivider shall be responsible for the conveyance of all storm drainage flowing through or abutting the subject property, including drainage directed to the property by prior development as well as that naturally flowing by reason of topography.
(B) Where new drainage improvements are required along the boundary of a subdivision, the owner proposing development shall be responsible for designing and constructing all the required improvements at or before the time of development, including the dedication of all necessary rights-of-way or easements necessary to accommodate the improvements. Where the subdivider proposes to subdivide only a portion of the property, only the drainage improvements for the portion being subdivided shall be required to be installed, except as drainage improvements outside the portion being subdivided are deemed necessary by the city for proper drainage of the portion being subdivided.
(C) The responsibility of the subdivider shall extend to the provision of adequate off-site drainage facilities and improvements to accommodate the full effects of the development of said property.
(D) When the subdivider certifies by affidavit that a bona fide attempt to acquire property rights to meet off-site drainage requirements was not successful, the city may assist at its discretion in the acquisition of necessary property rights to provide for the construction of off-site drainage improvements. In such cases, the subdivider shall make adequate guarantees that he will stand the full cost of acquiring said property rights and constructing the off-site improvements and facilities.
(E) Unless otherwise specified herein, the design of all storm drainage facilities shall at least meet the requirements of the City of Austin Drainage Criteria Manual.
(F) Computation of runoff shall be based on a fully developed drainage area, or watershed, in accordance with City of Austin drainage criteria. The drainage system shall be designed to convey the theoretical 25-year storm as predicted in the Drainage Criteria Manual. The design shall further provide for system overflows from larger storms up to the intensity of the 100-year storm without increasing the risk of flood damages to development.
(1) Natural drainage channels shall be preserved whenever possible. Open surface drainage through grass-lined swales shall be preferred over the use of enclosed sewers, streets and street right-of-way as the central drainage network. The provisions of § 150.063 shall apply in regard to street drainage. Drainage into or across sinkholes, faults and other areas of rapid groundwater recharge shall be avoided whenever practicable.
(2) The rate of runoff after construction shall not exceed the site's runoff rate prior to construction. Rate of runoff shall be computed on a 25-year storm peak flow using the City of Austin Drainage Criteria Manual as of the effective date of this Code.
(3) In the event that stormwater drainage systems and/or culverts are necessary, such systems shall be designed to mitigate their impact on water quality through the use of approved control strategies to control sediment and dissipate energy and through the use of multiple smaller outlets whenever practical and by locating discharges to maximize overland flow.
(4) Surface drainage channels shall be designed to reduce velocity, minimize potential erosion and to maximize the bottom width to flow depth ratio, in accordance with the following criteria:
(a) Channel cross-sections shall be trapezoidal in configuration.
(b) Side slopes of channels shall be no steeper than four horizontal to one vertical.
(c) For a six-month design storm assuming wet antecedent conditions, channel bottom flow depth shall not exceed four inches and design flow velocity shall be two and one-half feet per second.
(d) All constructed and altered drainage channels shall be stabilized and vegetated immediately after final grading.
(e) The City Council may allow exceptions to the design flow velocities or depths in the following situations in conformance with the purpose of § 150.307(F)(3) hereof.
1. On lands with greater than 15% slope, or less than 2% slope; provided that the design flow velocity shall never be greater than three feet per second or design depth greater than six inches.
2. In limited transitional channel sections (such as culverts, culvert entries and exits, drop sections, sharp bends, and water quality basin entries).
(G) Public easements or rights-of-way shall include all drainage at least to the limits of the 25-year flood as indicated on the floodplain maps or as determined on the basis of the Drainage Criteria Manual. The minimum drainage easement or right-of-way width shall be 25 feet. All drainage easements across private property shall contain the necessary language to permit the required unobstructed water flow, require maintenance of vegetation by the property owner(s), and permit the necessary access by city officials for inspection and repairs. All easements, 25 and 100-year floodplain boundaries and water quality zones shall be clearly shown on drainage plans and the final plat.
(H) As a general rule, drainage carried in roadside channels shall be minimized and off-roadway locations shall be used as the primary drainage network whenever practicable.
(I) When roadside channels are required, they shall be contained within a dedicated right-of-way or right-of-way easement. Channel side slopes shall be no steeper than four to one, except for curves and transitions where slope stabilization acceptable to the City's Engineer may be allowed. Roadways shall be designed for fordable driveway approaches whenever practical. All driveways shall be designed such that drainage flow from a 100-year storm shall not exceed a depth of 12 inches on any portion of the driveway. Should driveway culverts be required, the culvert design, capacity, and general location shall be shown on the construction plans. Minimum driveway culvert diameter shall be 12 inches. In no case shall driveway approaches constitute a blockage of roadway drainage.
(J) All drainage facilities located in the street rights-of-way shall be maintained by the appropriate jurisdiction. All drainage facilities located on private property shall be maintained by the property owner.
(K) Duly authorized inspectors of the city shall have the right of entry on the land or premises where property owners are required to maintain drainage facilities or detention facilities, at reasonable times, for the purpose of inspection of writing that the property owner comply. This notice shall describe the measures required to be taken. If, within three months of the notice the maintenance required is not accomplished, the city shall either:
(1) Cause the necessary restoration to be accomplished and assess the property owner for the city's actual cost; or
(2) Bring an action for mandatory injunction to require the property owner to accomplish the necessary maintenance.
(L) The required maintenance by private landowners and the power of the city shall be noted on the plat.