O-82-951 - 4/8/1982ORDINANCE NO. 95/
AN ORDINANCE ADOPTING A STORMWATER DRAINAGE POLICY.
BE IT ORDAINED BY THE COUNCIL OF THE CITY OF ROUND ROCK,
TEXAS, THAT
I.
The City Council hereby adopts and establishes the
following policies for stormwater drainage planning and
management.
A. General Policies
1. Storm drainage facilities shall be designed in
compliance with the provisions of the Drainage
Criteria Manual of the City of Austin, sections 2
through 9 and errata sheets dated July 21, 1977. In
case of conflict between Round Rock City Ordinance
and Manual content, Round Rock City Ordinance shall
prevail. No Manual amendments by the City of Austin
shall become effective without first being adopted
by the Round Rock City Council.
2. All facilities shall be designed to intercept and
transport the projected runoff from a 25 -year
frequency storm. Overflow and/or transport provi-
sions shall be provided for the 100 -year storm.
3. Projected runoff rates for the design of drainage
facilities shall be based on the expected ultimate
developed state of the upstream contributing area.
4. Development involving an increase of 0.5 acre or
more of impervious cover shall provide facilities
to prevent any increase in the peak rate of runoff
from a 25 -year frequency storm. Impervious cover is
defined as that man made or constructed coverage of
the natural ground surface with roofs, parking
lots, streets, drives, etc. This coverage shall be
calculated as accurately as possible but in no case
shall it be less than 2500 square feet for a single
family residence or duplex lot. The City Engineer
shall have authority to permit exemptions to the
above requirement upon proper application. Applica-
tions for exemption shall be accompanied by
certified engineering data and calculations justify-
ing the request and demonstrating no adverse impact
on all conveyances and property between the
applicant's property and the receiving major stream.
B. Drainage Channels
1. The 25-year and 100-year flood plains shall be
determined for water courses draining 50 or more
acres. Calculations for flood plains shall utilize
generally recognized backwater computational
methods and actual field channel and overbank con-
figuration.
2. No importation of fill material or channel modifica-
tions shall be undertaken within the area of the
100-year flood plain without permission of the City
Engineer. Such permission shall be based upon certi-
fied engineering data and calculations furnished by
the proposing permitee.
3. All constructed or modified earthen channels shall
be designed utilizing a side slope of 3 to 1, or
flatter, to allow for future maintenance and
promote adequate slope stability.
C. Streets and Storm Sewer
1. All street sections shall be in accordance with
City Standards with an allowable design drainage
capacity for gutter flow no deeper than the top of
the curb.
2. Depth of flow in streets is to be controlled to
allowable levels by modification of crossfall,
gradient changes, or the use of curb inlets and
storm sewers.
3. Curb inlets shall be spaced as required to control
flow in streets to allowable levels and placed to
minimize interference of runoff to traffic flow,
particularly at intersections.
4. All storm sewer pipe shall be reinforced concrete,
minimum size 18" diameter, and installed in compli-
ance with the City's Standard Specifications for
Public Works Construction of the City of Round Rock.
D. Bridges and Culverts
1. All bridge or culvert structures shall be designed
to carry and/or store the upstream runoff from a
25-year storm.
2. Runoff from the 100-year storm may, however, over-
top the road surface at bridge or culvert crossings
a maximum of 6 inches for a major or collector
street crossing, or a maximum of 12 inches on a
minor or residential street crossing.
E. Computations and Plans
1. Plans for proposed drainage facilities shall be
submitted to the City Engineer for acceptance prior
to construction.
2. Computations for all drainage related design shall
be submitted with the plans for review. Data sub-
mitted shall include a drainage area map, a summary
of methodology employed and resulting data, land
use and runoff coefficient assumptions and other
pertinent hydrologic and hydraulic data.
3. Certification shall be submitted by the design
engineer that the plans and computations are in
compliance with the requirements of this policy and
the Drainage Criteria Manual, referred to above.
4. Following construction, but prior to acceptance of
improvements by the City, the design engineer shall
furnish certification that based upon his periodic
inspection of the work all improvements, including
those covered by this Ordinance, have been
constructed in compliance with the City's require-
ments.
F. Subdivision Plats
1. Preliminary plans for subdivisions shall show the
limits of the 25 and 100 -year flood plain for all
waterways draining 50 or more acres.
2. The final plat of any proposed subdivision shall
provide adequate drainage easements to cover
25 -year flood plain areas, drainage channels, pipe
systems and any other related drainage facilities.
3. The final plat shall establish specific minimum
floor slab elevations for construction of any struc-
tures on a lot adjacent to or within and adjacent
to any 100 -year flood plain area to assure future
structures are constructed a minimum of one foot
above the 100 -year storm levels.
4. The final plat shall also contain a certification
by an engineer that the easements, slab elevations,
and any other drainage related notes are in compli-
ance with the policies and criteria of the City of
Round Rock.
G. Building Permits
1. Plans submitted for building permits other than
single-family residential
or duplex construction
shall include the necessary drainage related facili-
ties designed and provided for
this policy and the City of Round Rock Drainage
Criteria Manual.
2. Plans and design calculations for all drainage
in compliance with
facilities shall be submitted to the City
for acceptance prior
permit.
3. The design engineer
and calculations for
compliance with the
City of Round Rock.
Engineer
to issuance of the building
shall certify that the plans
all drainage facilities are in
policies and criteria of the
4
4. Following construction, but prior to issuance of a
Certificate of Occupancy by the City, the design
engineer shall certify that the public works
improvements, including those covered by this
Ordinance have been constructed in compliance with
the City's requirements based on his inspection of
the completed work.
(Mark through the following alternative that is not appli-
cable)
Alternative 1.
By motion duly made, seconded and passed with an affirma-
tive vote of all the Council members present, the requirement
for reading this ordinance on two separate days was dispensed
with.
READ, PASSED, and ADOPTED on first reading this
day of , 1982.
Alternative 2.
READ and APPROVED on first reading this the �/ TN day
, 1982.
READ, APPROVED and ADOPTED on second reading this
the TN day of
ATTEST:
, 19
ANNE LAND, 'City Secretary
RRY L. ONN, Mayor
City of Round Rock, Texas