§ 1.2.4. Drainage System

A.

All drainage system components that are within public right of way or public drainage easements shall be manufactured and installed in compliance with the City of Austin Standard Specifications and Standards and shall comply with all applicable portions of the City of Austin's Transportation Criteria Manual published by the Department of Transportation, unless:

1.

Those components receive stormwater runoff solely from private property,

2.

That property consists solely of the property being developed, and

3.

Those components are privately maintained.

B.

Construction plans for proposed reinforced concrete box culverts, bridges and related structures may be adaptations of the Texas Department of Transportation (TxDOT) Standards.

C.

For bridges and culverts crossing local streets, runoff from the fully developed 100-year frequency storm shall not produce a headwater elevation at the roadway greater than either twelve (12) inches above the roadway crown elevation or any top of upstream curb elevation, whichever is lower.

D.

For bridges and culverts crossing streets other than a local street, runoff from the fully developed 100-year frequency storm shall not produce a headwater elevation at the roadway greater than six (6) inches above the roadway crown elevation or six (6) inches above any top of upstream curb elevation, whichever is lower.

E.

All drainage facilities (including but not limited to headwalls, open channels, storm drains, area inlets, and detention, retention and water quality controls and their appurtenances) shall comply with the following requirements, unless otherwise noted in this section or the Environmental Criteria Manual (ECM). See Figure 1-1 in DCM Appendix D for approximate representation. For the purposes of this section (1.2.4.E), the term residential structure means a building used for living accommodations.

1.

(a)

Where a portion of the stormwater facility either has an interior slope or wall steeper than three (3) feet horizontal to one (1) foot vertical with a height exceeding one (1) foot, or, an exterior slope or wall steeper than three (3) feet horizontal to one (1) foot vertical with a height exceeding three (3) feet above adjacent ground, barrier-type fences at least six (6) feet high, and/or steel grating are required for all single-family or duplex residential development, City maintained stormwater facilities, and/or for any privately maintained stormwater facilities located within 500 feet of a residential structure. Barrier type fences include, but are not limited to chain link, solid wood, masonry, stone or wrought iron.

(b)

Metal components of the fence shall be corrosion resistant and wood components of the fence shall be weather resistant. Placement of the fence, shall be 20 feet past the toe of the embankment or to the edge of the property line. Any fence posts used shall be galvanized steel with a concrete footing of at least twelve (12) inches in diameter and at least eighteen (18) inches in depth (refer to City of Austin Standard Specification No. 701). All fences shall have at least one gate, which shall open fully inward and outward and shall be at least twelve (12) feet in width. The first gate shall provide access from either the easement or right of way. Access to the outfall structures is required for inspection and maintenance. If the fencing prohibits access to the outfall structure, then a second gate shall be provided allowing access to the outfall structure. Corrosion resistant aluminum or hot-dipped galvanized steel grates of load bearing strength are an acceptable means of vertical access control. Grates shall be securely bolted to the structure but shall allow for easy removal and replacement during maintenance operations.

2.

The landscaping requirements of Section 2.9.1, part B. and C. only, of the ECM apply for all detention, retention and water quality facilities for single-family or duplex residential development and/or for such facilities that will be maintained by the City. Landscaping and other vegetation shall not encroach into or impede use of any access drive or access strip, based upon the nominal size of the landscaping or vegetation at maturity.

3.

(a)

A 12-foot maintenance access strip is required around the perimeter of detention, retention, and water quality facilities for single-family or duplex residential development and/or for such facilities that will be maintained by the City. This 12 foot maintenance access strip shall be outside the toe of any fill slope and the top of any cut slope and shall not have a post construction longitudinal slope greater than 15% nor a post construction transverse slope greater than 5%, shall not have a vertical grade break of greater than 12%, shall have an inside turning radius of no less than thirty-six (36) feet, shall be cleared and graded, and shall have vertical clearance from existing and proposed vegetation and all other objects of no less than 14 feet.

(b)

A setback shall be provided for all detention, retention, and water quality facilities for single-family or duplex residential development and/or for such facilities that will be maintained by the City so that no such facility shall be located within 50 feet of a residential structure. All detention, retention and water quality facilities that will be for single-family or duplex residential development and/or for such facilities maintained by the City shall have a permanent maintenance equipment access ramp into each basin of the facilities, with a longitudinal slope not to exceed 4:1, a transverse slope not to exceed 5%, no vertical grade break greater than 12%, no vertical curve with a grade change greater than 1% per horizontal foot, an inside turning radius of no less than 36 feet, a minimum width of 12 feet and a clear distance from the bottom of the ramp to any facility interior slope of at least 15 feet.

(c)

Access/drainage easements and access drives are required for those detention, retention and water quality facilities for single-family or duplex residential development and/or for such facilities that will be maintained by the City. Access drives shall have a longitudinal slope not to exceed 15%, a transverse slope not to exceed 5%, no vertical grade break greater than 12%, no vertical curve with grade change greater than 1% per horizontal foot, a centerline turning radius of no less than 50 feet, and a minimum width of 12 feet. The access drive shall include a means for equipment to turn around when located more than 200 feet from a paved public roadway. Access drives shall be cleared, graded and stabilized with stones in accordance with Standard Detail 662S-2, Pond Maintenance Road Typical Cross Section.

4.

(a)

For drainage facilities within a drainage easement, but not for single-family or duplex residential development or such facilities maintained by the City, the set back rule discussed in section 3(b) above is not applicable for the proposed internal building(s), if they have a restrictive covenant for maintenance and 4-foot high barrier-type fences with accessible maintenance ramp.

(b)

Any new commercial or multi-family pond(s) shall have a minimum 50-foot setback from any adjacent existing residential structure.

5.

The setback requirement shall be measured from the edge of any portion of the facility such as the top of facility embankment, end of concrete apron/rock rip rap or top of structural wall and shall be dedicated by deed restriction or restrictive covenant, and a note referencing the restriction will be placed on the plat, if a plat is being processed as part of the development. Replatting to add the note is not required.

6.

Discharge from stormdrain outfalls shall not cause channel, bluff, or stream bank erosion. If the storm drain discharges to an open drainage facility (as determined by the City), the applicant must show acceptable nonerosive conveyance to that drainage facility, and appropriately designed outfall including adequate energy dissipation, which may include stream stabilization.

7.

Free standing structural walls/facilities located on or adjacent to a residential structure shall not be greater than six (6) feet in height.

8.

Drainage or drainage access easements that are required along side lot lines shall be located adjacent to a property line and shall not be centered on a property line.

9.

Access drives are required for those area inlets and headwalls for single-family or duplex residential development and/or for such facilities that will be maintained by the City when access is proposed between single family lots or when access from any other location exceeds 20% grade. Access drives shall meet the criteria of section 3(c) above.

10.

Points of access to water quality and detention facilities for single-family or duplex residential development and/or for such facilities that will be maintained by the City shall have a standard City of Austin Type II concrete driveway approach and curb cut on the abutting street. A pipe gate is required at the end of the driveway at the ROW limits in accordance with Standard Detail 662S-1, Pond Pipe Gate At Ramp Detail.

11.

Detention, retention and water quality facilities for facilities that will be maintained by the City shall have a staging area not less than 800 square feet in area (with no dimension less than 20 feet) if the storage volume of the pond exceeds 2,000 cubic feet. No portion of the staging area shall be located further than 100 feet from the detention, retention or water quality facility and the access drive and the staging area shall be within an access easement. No portion of the staging area shall be located within or on any ponding area, interior slope of the facility, or access drive or strip. The staging area shall be cleared, shall have vertical clearance from existing and proposed vegetation and all other objects of no less than 14 feet and shall have no finished slope greater than 10%.

12.

Environmental Criteria Manual Section 1.6.3 shall apply to all Stormwater Control Measures (SCMs), detention, and retention facilities and their appurtenances.

13.

All pond bottoms, side slopes and earthen embankments shall be compacted to 95% of maximum density in accordance with City of Austin Standard Specifications, excluding rain gardens and any water quality features that require infiltration to meet water quality standards. Side slopes for earthen embankments shall not exceed three (3) horizontal to one (1) vertical. Rock slopes may exceed these limits if a geotechnical report warrants a deviation. Actual field conditions may override the geotechnical report. Concrete walls shall be built to City of Austin Standards Specifications. Expansion joints on free-standing walls shall have water tight seals as needed. Ponds for single-family or duplex residential development and/or for City maintained ponds with other than full concrete bottoms must have bottom slopes of no less than 2%. Ponds with full concrete bottoms must have bottom slopes of no less than 0.5%; all grass-lined portions of the pond bottom must have a slope of no less than 2% from that portion to the concrete-lined pilot channel. The pilot channel must be at least four (4) feet wide and have a V-shaped cross-sectional flow area two (2) inches deep. Pilot channels are discouraged in water quality ponds due to the creation of short-circuiting and standing water problems. Where it is deemed necessary due to topographic constraints a pilot channel installed in a water quality pond must be designed to prevent short-circuiting, standing water problems, and shall have a flow spreader installed, refer to Section 1.6.2.D of the ECM. The bottom slope for ponds is defined as the effective slope from any point in the bottom of the pond to the discharge elevation at the point of discharge from the pond, as measured by a straight line between those two points.

14.

Section 8.3.3 of this manual shall apply for all stormwater management facilities, including water quality facilities and stormwater management infrastructure.

15.

All mechanical stormwater management facilities to be maintained by the City shall meet City of Austin Water and Wastewater criteria. OSHA requirements for ingress and egress are also required for any facility that will be located in a confined space above or under ground.

16.

Signs are required on each side of a single-family residential, duplex residential, and/or City-maintained stormwater control measure in accordance with Standard Detail 662S-3, Storm Water Facility Sign.

17.

Maximum retention or "draw-down" time for detention ponds shall not exceed 24 hours from the time of peak storage to the time of complete emptying of the pond, as determined by hydrograph routing or other calculations acceptable to the City. This requirement does not apply to facilities in which retention or "draw-down" time is required to be greater than 24 hours. All volume required for detention shall be available after 24 hours to allow for subsequent storms, including any portion of the water quality volume utilized for detention purposes.

18.

In order to minimize vandalism and deterioration, use of exposed piping and appurtenances and any loose materials (other than access drive rock) shall be avoided or minimized. Use of all such items shall be approved by the City.

F.

Any concentrated flow necessitates the dedication of a drainage easement to the larger of following: the limits of the fully developed 100-year storm water surface elevation, or minimum easements widths as required in this Drainage Criteria Manual.

G.

An easement or right-of-way as required in this Drainage Criteria Manual must be of sufficient width to provide continuous access for the operation, maintenance, or repair of a drainage facility or conveyance of stormwater.

(1)

A minimum of 25 feet in width for an open drainage system; or

(2)

A minimum of 15 feet in width for an enclosed drainage system.

Source: Rule No. 161-15.11, 1-4-2016.