§ 36-288. Drainage plat review.

The following are the minimum items required for drainage plan review:

(1)

Site plan showing existing and proposed contours; existing and proposed previous and impervious cover areas.

(2)

Predevelopment and post-development runoff calculations using the rational, state department of transportation (TxDOT) or other widely accepted methods.

(3)

Detention pond required volume calculations using City of Austin or other widely used methods. The proposed pond shall have a volume greater than or equal to the required volume with one-foot of freeboard. The flow line of the detention pond shall be concrete lined.

(4)

Design analysis and details of outlet structure. Outlet structures shall be designed to restrict the 2-, 10-, 25- and 100-year storm rate of discharge to predevelopment rate of runoff. For small facilities it is recommended that a concrete outlet structure with a vertical slot be used instead of small pipes for maintenance reasons. Outlet structure to include a spillway for storms greater than the 100-year storm.

(5)

Depth/storage/discharge table. Discharge leaving the pond and discharge leaving the entire site (in the event there is off-site drainage that does not flow into the pond, but must be accounted for in the pond calculations).

(6)

Seal of a state registered professional engineer on all drawings and calculations.

(7)

Finish floor slab is one foot above top of curb (TOC) or the CL (centerline) of the street.

(8)

Approved TxDOT permit if required.

(9)

Two copies of construction drawings.

(10)

Show right-of-way line on all streets.

(11)

Manholes. Manholes (inlets or junction boxes) shall be provided at all changes in grade or alignment, sewer intersections, and at a maximum of 1,000 feet.

(12)

Inlets. Design of inlets shall conform to the city standard specifications.

(13)

Pipe. Pipe for storm drains shall be concrete pipe in sizes as shown on the approved plans. Pipe 18 inches or larger in diameter shall be reinforced concrete pipe (RCP), ASTM C76, class 3. Where, in the opinion of the city engineer, added strength of pipe is needed for traffic loads over minimum cover or for excessive height of backfill, concrete pipe. Monolithic, reinforced concrete sewers may be used for storm sewers 36 inches and larger.

(14)

Major drainage ways. Design of major drainage ways through a subdivision and major structures, such as box culverts or bridges, across a major drainage channel, shall be coordinated with the floodplain coordinator.

(15)

Drainage collection facilities (all on site).

a.

Drainage collection facilities (all on site) shall be constructed with underground storm sewers. If it can be established by certified engineering data to the satisfaction of the city engineer that storm sewers are not physically feasible, open ditches may be used, provided that such on-site channels are lined with concrete. These structures shall be of sufficient cross section and slope as to fully contain design flows and facilitate self cleaning. Outfalls shall enter major collector drainage ways and major streams at grade or be designed and constructed with adequate concrete aprons, energy dissipaters or similar features to prevent erosion;

b.

Detention ponds and related structures may utilize either existing natural open sections which may be modified, or newly constructed facilities. If modified or newly constructed facilities are utilized, they shall be lined with permanent materials including, but not limited to: concrete or vegetation (see subsection (15)c of this section for special conditions and exceptions for vegetation);

c.

Vegetated channels be allowed only to convey off-site water through a subdivision shall have sufficient grade to provide velocities that will allow self-cleaning but will not be so great as to create erosion. Side slopes shall not be a steeper ratio than three to one to allow for future growth and to promote slope stability. All slopes shall be hydromulched, sodded or seeded with approved grass, grass mixtures or ground cover suitable to the area and season in which they are applied;

d.

Sodding shall be St. Augustine or Bermuda with sufficient soil attached to sustain growth and must be alive at the time of application;

e.

Hydromulch shall be applied as follows:

1.

Between April 15 and October 1, for each 1,000 square feet, two pounds of hulled Bermuda seed, and 20 pounds of fertilizer (16-20-0 with magnesium, sand and sulfur). On slopes, add 40 pounds of cellulose fiber mulch and one-half-pound Hydro-Tack soil binder or acceptable substitute;

2.

Between October 1 and April 15, for each 1,000 square feet, six pounds Rye grass seed, 20 pounds of fertilizer (15-10-10 with magnesium and sulfur). On slopes add 40 pounds of cellulose fiber mulch and one-half pound of Hydro-Tack soil binder or acceptable substitute. As soon as practical after April 15, the April 15 to October 1 application described above must also be made; provided, however, surfaces must be reshaped to original configuration prior to the second application.

Hydromulch growth must be established over 85 percent of applied areas prior to acceptance of subdivision improvements by the city, with no exposed area exceeding ten square feet. The term "established growth" means the vegetation has reached a height of 1½ inches and is of a density such that it can be reasonably expected to be self-sustaining;

f.

Major streams shall not be modified without consent of applicable state and federal agencies and authorization for the director of public works and city engineer;

g.

If, in the opinion of the city engineer, either conditions such as drought, excess precipitation or extreme heat or cold are unsuitable for hydromulching or sodding, such applications shall be deferred by the developer. Under these circumstances, subdivision improvements may be accepted upon the provision of a letter or credit in a form acceptable to the city attorney, in an amount of twice the city engineer's estimated cost of the sod or hydromulch application and where appropriate, surface reshaping, maintenance requirements and reapplication. If the developer is unable to meet the requirements of subsection (15)e of this section within nine months of subdivision acceptance, the letter of credit will be drawn on and the proceeds used to obtain the required vegetation cover;

h.

The developer shall be required to use concrete or similar permanent cover in lieu of vegetation if the city engineer determines that future maintenance is materially impaired or where channel bends and intersections, flow dissipation or similar circumstances so warrant.

(Code 1990, ch. 8, § 6(I)(8))