(A) 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.
(B) Computation of runoff shall be based on a fully developed drainage area, or watershed, in accordance with the minimum provisions of the City of Austin drainage criteria. The drainage system shall be designed to convey the theoretical two, ten and 25-year storm as predicted in the Drainage Criteria Manual, as amended. 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) Critical environmental features shall have a standard setback of 150 feet around said feature. An administrative variance may be granted by the City Engineer for a 50 feet setback on the downstream side only of said feature.
(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 two, ten and 25-year storm peak flow using the City of Austin Drainage Criteria Manual, as amended.
(3) 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:
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 control entries).