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access.
(b) Maintenance Access: Maintenance access shall be provided along the entire
length of all major drainageways and shall connect with a public street to allow
access by medium and large scale construction and maintenance equipment. An
access road shall be at least 12 feet wide and designed to adequately support the
loads of expected maintenance equipment. The maintenance road may be shared
as a greenway trail, subject to approval by the City.
(c) Water Surface Profiles: Water surface profiles shall be determined for all
drainageway designs using standard backwater methods, taking into
consideration losses due to velocity changes produced by changing channel
sections, drops, waterway openings, or obstructions. The water surface and
energy gradient profiles shall be shown on the construction plans.
(4) Roadside Ditches and Drainage Swales: The design of roadside ditches and drainage
swales is similar to the standards for unlined channels with modifications for application
to minor storm drainage. The standards are as follows:
(a) Capacity: Roadside ditches and drainage swales shall have a minimum capacity
for the 10-year design storm.
(b) Flow Velocity: The maximum velocity for the design storm runoff peak is not to
exceed 5 feet per second. The Froude number shall be less than 0.8.
(c) Longitudinal Slope: The slope shall be limited by flow velocity of the design
storm. Swale widening or check drops may be required to control velocities.
Effective: November 16, 2000 DESIGN AND CONSTRUCTION STANDARDS 7-25
(d) Freeboard: Freeboard above the design flow depth shall be at least 6 inches.
(e) Curvature: The minimum radius of curvature shall be 25 feet.
(f) Roughness Coefficient: Manning s n, as adjusted by channel bottom
conditions outlined in the UDFCD Drainage Criteria Manual, shall be applied.
(g) Depth: A drainage swale shall be at least 1 foot deep. A maximum depth for
drainage swales shall not exceed 5 feet, and shall be dictated by the design flow
and cross-sectional standards.
(h) Side Slopes: Side slopes shall be no greater than 3:1; however, 4:1 side slopes or
flatter are recommended for landscaped areas and to enhance water quality.
(i) Driveway Culverts: Along roadside ditches, driveway culverts shall be sized to
pass the design storm flow without overtopping the driveway, having a minimum
culvert diameter size of 12 inches in height with at least 6 inches of cover.
Flared end sections or headwalls with appropriate erosion protection shall be
provided. Given the depth constraints along roadside ditches, more than one
culvert may be required to pass the design flow. Maintenance of all driveway
culverts shall be the responsibility of the property owner served by the driveway.
(j) Major Drainage Capacity:
(i) The major drainage (100-year storm) capacity of roadside ditches is
restricted by the maximum flow depth allowed at the street crown or by
the ground surface at the edge of the street right-of-way.
(ii) The major drainage capacity of drainage swales is restricted to the
maximum flow that can be passed without inundation to and damage of
downstream properties.
(E) Hydraulic Structures
(1) Where Required: Hydraulic structures are used in open storm water systems to control
the flow of the runoff. The energy associated with flowing water has the potential to
create damage to the drainage system, especially in the form of erosion. Hydraulic
structures are intended to control the energy of storm water flow and minimize the
damage potential of storm water runoff. Typical hydraulic structures may include
without limitation the following:
(a) Channel drop and check structures,
(b) Rip rap and rock linings,
(c) Energy dissipaters and stilling basins,
(d) Channel rundowns,
(e) Bridges and culverts,
(f) Irrigation ditch crossings,
(2) Design Standards: The standards to be used in the design of hydraulic structures shall be
in accordance with these Standards and the UDFCD Drainage Criteria Manual.
7-26 DESIGN AND CONSTRUCTION STANDARDS Effective: November 16, 2000
7.08 Storm Sewers
(A) System Design
(1) Where Required: Storm sewers shall be required when the other parts of the minor
storm water system, primarily streets, curbs, gutters, and roadside ditches, no longer have
the capacity for additional runoff in the initial storm event.
(2) Gravity Flow Conditions: Storm sewers shall be designed for gravity (open) flow
conditions, using a Manning s roughness coefficient from Table 7-3, Manning s n
for Storm Sewers.
Table 7-3: Manning s n for Storm Sewers
Sewer Type Manning s n
Concrete 0.015
Plastic 0.013
Corrugated Metal 0.013
(3) Flow Depth: Storm sewers are to be designed to carry peak flows at full pipe depth.
(4) Pressure Flow Prohibited: Pressurized surcharged or depressed (inverted siphon) storm
water mains are prohibited in the City s storm water system.
(B) Location
All storm sewer mains shall be installed in public rights-of-way or easements, in conformance with
Section 4.03, Utilities Easements, of these Standards.
(C) Depth
The cover for all storm sewer mains shall be at least 18 inches deep, measured from the top of pipe to the
final surface grade, and shall be capable of withstanding AASHTO HS-20 highway traffic loadings.
(D) Size
Storm sewer mains shall be at least 18 inches in diameter, and storm sewer laterals shall be at least 12
inches in diameter.
(E) Slope
(1) Minimum and Maximum: Minimum allowable slope shall provide flow velocities of at
least 2-feet per second and maximum allowable slope shall provide flow velocities no
greater than 10feet per second during peak flow conditions.
(2) Constant Slope: All storm sewer mains shall be laid at a constant slope between
manholes.
(F) Alignment
(1) Straight Alignment: All storm sewer mains shall be laid in a straight alignment between
manholes.
Effective: November 16, 2000 DESIGN AND CONSTRUCTION STANDARDS 7-27
(2) Curvilinear Mains Prohibited: Curvilinear storm sewer mains shall not be allowed.
(G) Separations and Crossings
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