(A) Minimum soil depths in the vegetative filter zone must be 18 inches or more. Soil depths shall be determined from hand or mechanical borings at representative locations at a density of at least one per two acres in the vegetative filter zone.
(B) The vegetative filter will be constructed along the entire length of the contributing drainage area and shall receive runoff as sheet flow.
(C) Flows from impervious surfaces must be delivered to the vegetative filter in an even, diffuse, shallow, overland flow manner. If necessary, a flow spreader meeting the requirements of the City of Austin Environmental Criteria Manual will be provided to distribute flow evenly across the top of the strip.
(D) The runoff flow path across the vegetative filter shall be as long as possible, within topographic site constraints.
(E) Native, existing vegetation in the vegetative filter shall be preserved to the greatest extent possible.
(F) Existing vegetation shall be supplemented with grasses and ground cover as necessary to prevent erosion. Juniper trees must be trimmed and native grasses established beneath. Supplemental vegetation should be selected to be drought resistant, sturdy under short periods of inundation and shall require minimum fertilization for vigorous growth.
(G) Fertilizer application to the vegetative filter at a rate greater than 75 pounds/acre per year must be preceded by an assessment of the nitrogen requirements of the vegetation and the measurement of nitrogen concentration in the soil.
(H) The maximum slope of the vegetative filter must be 5% or less. Alternatively, a demonstration, sealed by a registered professional engineer, may be presented to show that flow velocities through the filter during the 10-year design storm will not be erosive.
(I) Flow velocities through the vegetative filter shall not exceed one foot/second for the one-year design storm.
(J) The average annual hydraulic load to the vegetative filter shall not exceed 20 inches per year.
(K) Vegetative filters shall not be used for construction-phase erosion and sedimentation controls.
(L) Vegetative filter strips for commercial or multifamily development will be operated and maintained either by the City of Sunset Valley or by the owner. If maintained by the owner, an annual operating permit to ensure the continued effectiveness of the flow spreader and that there is no erosion or gullying, and the presence of adequate vegetation to prevent erosion shall be required.
(M) Vegetative filter strips for residential development will be maintained by the City of Sunset Valley.
(N) The vegetative filter strips shall be inspected visually by the owner at least once each year and following every rainfall event of eight inches or more within 24 hours. Inspection will include, at a minimum:
(1) Examination of vegetation for indications of distress.
(2) Examination of the soil and topography for indications of erosion or flow channeling.
(3) Examination of the entire length of all diversion berms and level spreaders.
(O) Any changes in vegetation which reduce the effectiveness of the vegetative buffers strip will be repaired. If repeated channeling and erosion of the vegetative filter occur, it may be necessary to supplement or replace the filter with structural controls.