§ 1.2.2. General

A.

Stormwater runoff peak flow rates for the two (2), ten (10), 25 and 100-year frequency storms shall not cause increased inundation of any building or roadway surface or create any additional adverse flooding impacts.

B.

Street curbs, gutters, inlets and storm drains shall be designed to intercept, contain and transport all runoff from the 25-year frequency storm.

C.

In addition to B. above, the public drainage system shall be designed to convey those flows from greater than 25-year frequency storm up to and including the 100-year frequency storm within defined public rights of way or drainage easements.

D.

Stormwater runoff peak flow rates shall not be increased at any point of discharge from a site for the two (2), ten (10), 25 and 100-year storm frequency events.

E.

Regulation of peak flows to allowable levels, as determined by the provisions of this policy, shall be achieved by storage on-site or off-site or by participation in an approved Regional Stormwater Management Program. The Stormwater Management Section of this manual provides a guide to acceptable methods, but does not limit the designer to the methods presented therein. Guidelines for participation in the Regional Stormwater Management Program are contained in the Stormwater Management Section of this manual.

F.

For those developments which are immediately adjacent and discharge directly into Lake Travis, Lake Austin, Lady Bird Lake and the Colorado River, on-site detention is not required.

G.

Development activity is not subject to D. or E. above if the following conditions are satisfied:

1.

The activity is located in the Waller Creek, Shoal Creek (downstream from the West 38th St. bridge), Boggy Creek, East Bouldin Creek, West Bouldin Creek, Blunn Creek, Johnson Creek, Harper's Branch or Lady Bird Lake watersheds; and

2.

The activity involves an existing legal lot that is limited to single-family (SF-1, 2, or 3) land use by zoning, restrictive covenant or plat note; and

3.

The proposed impervious cover does not exceed 45%; and

4.

A site plan is not required; and

5.

The existing lot size is equal to or less than one (1.00) acre; and

6.

The existing lot does not receive significant off-site drainage.

7.

An engineer submits a signed, sealed and dated letter, addressed to the Director of the Planning and Development Review Department, stating the following without qualification: "I certify that I have personally conducted a topographic review and field investigation of the existing and proposed flow patterns for stormwater runoff from the subject development to the main stem of (name of creek). At build-out conditions allowable by zoning, restrictive covenant or plat note, the stormwater flows from the subject development will not cause any additional adverse flooding impacts for storms of magnitude up through the 100-year event."

8.

Calculations supporting the certification statement in Part 7 must be submitted if requested.

Cross reference

Supplemental requirements for development applications in certain planning areas, § 25-7-66.