(A) Although a certain percentage of impervious cover is discussed and designated within this section, nothing in this section shall release a person from meeting the requirements of the zoning, landscape, and other provisions of this Code.
(B) (1) Impervious cover and calculations thereof shall include, but shall not be limited the following:
(a) Alleys, streets, and any kind of pavement driveways, and parking areas used for vehicular purposes. For roadways or streets located within the boundaries of a proposed subdivision, the area to be included in an impervious cover calculation will be based on roadways widths of up to and including 24 feet. Requirements for sizing water quality controls or detention facilities for runoff from such roadways are not affected by this provision.
(b) Buildings, concrete, and other impermeable construction covering the natural land surface.
(c) Roadways or streets located adjacent to a development area shall be included in the calculation of impervious cover as described in the City of Austin Environmental Criteria Manual, as amended, except that this provision does not apply to those roadways that either already have water quality controls in place or water quality controls have been previously approved for construction by the city council and construction of such controls timely occurs as approved before the earlier of expiration of the permit or one year after the issuance of the subject watershed development permit.
(d) Site area used for the storage of scrap, auto, and metal salvage.
(2) Impervious cover and calculations thereof shall not include the following when located on property zoned and used for single family residential purposes:
(a) Water surface area of a swimming pool, hot tub, wading pond, fountain, bird bath, fish pond, and other similar city approved structure.
(b) Uncovered decks meeting the following standards:
1. The deck must be constructed on piers or posts to allow for the unabated flow or passage of water underneath the deck;
2. The deck floor must be constructed of slatted material that provides for a minimum of quarter-inch drainage spaces between the slats; and
3. The deck must be constructed over pervious cover.
(c) Up to 350 square feet of city approved rainwater harvesting systems whose sole purpose is for the collection, treatment, and/or storage of rainwater for on-site use.
(d) Walkways and uncovered patios used for landscaping and pedestrian use only that are placed on pervious material and allow for the unobstructed passage of water between or through the material used in the construction thereof, as permitted pursuant to regulations adopted by the Director of Public Works for pervious structures.
(e) Pads for air conditioning units provided they are less than ten square feet in area per unit.
(C) In calculating projected impervious cover, all existing impervious surfaces shall be included in the calculations and charged against impervious cover allowances. This provision does not include any “natural” imperviousness, such as rock outcrops, or any surfaces excluded from impervious cover pursuant to the terms of divisions (D) or (E) below, provided there is full compliance with any pollution reduction measures required pursuant to such divisions. Pedestrian sidewalks located within the public right-of- way and approved water quality controls/treatment devices and other drainage facilities are not impervious cover for the purposes of impervious cover calculations. In addition, trails in green spaces and conservation areas no more than 11 feet wide constructed of natural soil, granite gravel, and stabilized soil aggregate, with concrete or stone edging no more than six inches wide on each side, are exempted from impervious cover calculations. Pervious concrete may also be exempted from impervious cover calculations in trail areas prone to high erosion.
(D) Roads, parking area, buildings and other construction are to be assumed as 100% impermeable unless specific proposed alternate surfaces are authorized as being less than such by the City Council.
(E) Properties in Sunset Valley are limited to 18% impervious cover or less.
(F) Residential impervious cover exemption allowance.
(1) To assist property owners with small residential improvements the city has considered certain exemptions on residential properties. Residential properties zoned and used for single family residential purposes will be allowed the following exemptions from impervious cover totals:
Impervious Cover Exemptions | |
Lot Size | Total Exemption in Square Feet |
Up to 1/4 acre | 200 |
Between 1/4 acre and 1/2 acre | 400 |
Between 1/2 acre and 3/4 acre | 600 |
Between 3/4 acre and 1 acre | 800 |
Greater than 1 acre | 800 + an additional 50 feet per 1/4 acre |
(2) Exempted items shall fall within the following categories:
(a) Pedestrian sidewalk or walkway that is:
1. Not greater than three and one-half feet in width;
2. Connects to the primary single-family residential structure; and
3. Has a minimum of one foot of pervious cover on each side.
(b) Covered decks or gazebos meeting the following specifications:
1. The deck or gazebo must be constructed on piers or posts to allow for the substantially unobstructed flow or passage of water underneath the deck or gazebo;
2. The deck or gazebo floor must be constructed of slatted material that provides for a minimum of one-fourth inch drainage spaces between the slats;
3. The deck or gazebo must be constructed over pervious cover; and
4. Each wall of the deck or gazebo, excluding those sides of the structure adjoining a primary single-family residential structure, does not exceed 20% of the area measured from the top of the overhead cover to the top of the floor.
(c) Coping with a maximum width of 18 inches on: a swimming pool, hot tub, wading pond, fountain, bird bath, fishpond, and other similar structures.
(d) Solar or wind power structures and facilities used for the primary purpose of providing on-site energy.
(e) An accessory building meeting the following specifications:
1. Structure is elevated to allow for the substantially unobstructed flow or passage of water underneath the structure.
2. Structure is constructed over pervious cover.
(3) Nothing in this division shall exempt a lot or accessory structure from the requirements of Chapter 2 of the Land Development Code, including but not limited to the limitations on the number of structures allowed per lot or setback requirements
(G) Variance allowing additional development in the water quality transition zone. The provisions of this division apply only to development of property not described in § 150.327. The City Council may grant a variance allowing development of commercial, multi-family, and developments of single family residential lots comprised of more than two lots in the water quality transition zone as provided in this division, and as permitted pursuant to § 150.323(A).
(1) A person desiring to develop in the water quality transition zone may submit an application for a variance at the time of submission of the preliminary site plan, and the application shall be acted upon by the City Council at the same time that the preliminary site plan is acted upon.
(2) The granting of a variance under this division shall not be construed as relieving the grantee from obtaining a watershed development permit under applicable ordinances or this Code, nor shall the granting of a variance be construed as committing the Council to approving the grantee's application for a watershed development permit.
(3) A variance may be granted only if the following standards are met:
(a) Impervious cover in the water quality transition zone shall be no more than 8% of the total portion of the lot that is in the transition zone. The impervious cover within the water quality transition zone shall be part of, and not in addition to, the total amount of impervious cover allowed on the lot as a whole.
(b) A minimum setback of at least 75 feet shall be maintained between the critical water quality zone and developed, impervious, or pollutant source areas, or areas with disturbed vegetation or soil in the water quality transition zone. Within the aquifer recharge zone, a 100-foot minimum setback shall also be preserved between developed, impervious, or pollutant source areas, or areas with disturbed vegetation or soil in the water quality transition zone and any identified recharge features.
(c) Impervious cover, water quality controls, and drainage shall be designed to allow maximum infiltration of clean rainfall runoff. The applicant shall provide an increased average annual infiltration equal to 125% of the infiltration volume lost due to development within the transition zone. The increase must be shown compared to the average annual infiltration volume without the proposed water quality transition zone encroachment. Increased infiltration can be achieved using retention/re-irrigation of stormwater runoff, infiltration basins, disconnected impervious cover, and/or engineered vegetative buffers. Infiltration estimates must be based either on soil data for the site published by the Soil Conservation Service (U.S. Department of Agriculture, 1974), or on field measurements of the infiltrative capacity of the surface soil, using such devices as a ring infiltrometer. Subsurface infiltration testing methods for septic systems are not acceptable.
(d) Water quality controls shall be designed to be at least 25% more efficient at reducing the average annual pollutant load for total suspended solids, total nitrogen, total phosphorous and total organic carbon or chemical oxygen demand than water quality controls required for development in the Upland Zone.
(e) Where development encroaches into the water quality transition zone, associated turf and landscaped areas requiring fertilizer, pesticides, herbicides, insecticides, or fungicides for maintenance shall be prohibited. Disturbances of the natural vegetation and tree cover shall be prohibited except within the building footprint and the surrounding construction disturbance area. The surrounding construction disturbance area within the water quality transition zone shall be limited to a maximum radius of 20 feet from the building footprint, unless the developer can demonstrate that a greater, specified radius is necessary and would not produce greater adverse effects than a 20-foot radius.
(f) A variance under this division may be granted only if the property to be developed in the water quality transition zone has not already been used for transferring development intensity credits.
(4) The conditions of a variance permitted under this division shall be imposed on the property as a restrictive covenant running with the land, in a form approved by the City Attorney, and recorded in the real property records of the Travis County, Texas once a final site plan is approved by the City Council.
(Ord. 211109-B, passed 11-9-2021)