[Ord. No. 777 §26-142, 3-23-1994]
A.
Drainage Area Plan. A plan of the drainage area at a scale
of one (1) inch equals one hundred (100) feet with two (2) foot contour
intervals using U.S.G.S. datum for areas less than one hundred (100)
acres or a plan of the drainage area at a scale one (1) inch equals
three hundred (300) feet with five (5) foot contour intervals for
larger areas. This plan shall include all proposed streets, drainage
and grading improvements with flow quantities and direction of flow
at all critical points. All areas and subareas for drainage calculations
shall be clearly distinguished.
B.
Hydraulic Data. Complete hydraulic data showing all calculations
shall be submitted. A copy of all nomographs and charts used in the
calculations shall be submitted if other than those included in this
Article.
C.
Plan And Profile. A plan and profile of all proposed improvements
at a scale of one (1) inch equals fifty (50) feet horizontal and one
(1) inch equals five (5) feet vertical shall be submitted. This plan
shall include the following:
1.
Location, sizes, flow line elevations and grades, type of pipe, channels,
boxes, manholes and other structures drawn on standard plan profile
sheets.
2.
Existing and proposed ground line profiles along centerline of the
drainage improvement.
3.
A list of the kind and quantities of materials.
4.
Typical sections and reinforcement of all boxes and channels.
5.
Location of property lines, street paving, sanitary sewers and other
utilities.
D.
Field Study. A field study of the downstream capacity of
all drainage facilities and the effect of additional flow from the
area to be improved shall be submitted. If the effect is the endangerment
of property or life, the problem must be solved before the plan will
be given approval.
E.
Stormwater Flow Quantities. Stormwater flow quantities in the street shall be shown at all street intersections, all inlet openings and at locations where flow is removed from the streets. This shall include the hydraulic calculations for all inlet openings and street capacities. Street flow shall be limited according to the requirements of Section 400.1230(C).
[Ord. No. 777 §26-143, 3-23-1994]
A.
General Design Requirements.
1.
All bridges shall be designed to accommodate a 100-year frequency
rain. Box culverts, pipe culverts, channels and ditches shall be designed
to accommodate a 100-year frequency rain at all locations having a
drainage area in excess of one (1.0) square mile. Locations having
a drainage area of less than or equal to one (1.0) square mile shall
be designed to accommodate a 25-year frequency rain.
2.
Channel improvement shall be:
Such to prevent erosion while maintaining natural channels where
possible. Velocities shall be low enough (less than five (5) fps)
to prevent scouring. A series of detention structures, restrictions,
etc., are recommended. Concrete lined channels and pipes shall be
used when scouring velocities cannot be controlled.
B.
Specific Requirement For Various Improvements.
1.
Bridges and culverts. Bridges, box culverts or concrete
pipe culverts shall be provided where continuous streets or alleys
cross watercourses. The structures shall be designed to carry HS-20
loadings. Culverts and stormwater drainage pipes under public streets
shall be Class 3 or 4 reinforced concrete pipe (RCP) with a minimum
diameter of fifteen (15) inches or sized based upon a 25-year stormwater
flow from the watershed, whichever is greater. Box culverts shall
be designed for 100-year stormwater flow.
2.
Closed storm sewers. Closed storm sewers shall either
be reinforced concrete box or pipe of approved type designed for HS-20
loadings. Reinforced concrete pipe or reinforced concrete boxes must
be used within two (2) feet of the back of the street curb and under
paved areas. All storm sewers having trench walls within two (2) feet
of the back of the street curb shall be backfilled with granular material.
The use of corrugated steel, zinc-coated pipe and extra strength clay
pipe will not be permitted within two (2) feet of the curb or under
pavement areas.
Grades for closed storm sewers shall be designed so that the
velocity shall not be less than three (3) feet per second nor exceed
twelve (12) feet per second.
3.
Open paved concrete channels. Grades for open paved
channels shall be designed so that the velocity shall not be less
than three (3) feet per second nor exceed twelve (12) feet per second.
Such concrete channels may be of different shapes according to existing
conditions; however, a channel with a flat bottom and 4:1 to 5:1 side
slopes is the most desirable type and shall be used whenever possible.
The thickness of channel paving shall depend on conditions at site
and size of channel; however, a minimum thickness of six (6) inches
is required. A six (6) inch free board must be provided. An eighteen
(18) inch toe wall is required at both the outlet and inlet ends of
the channel.
4.
Open ditches (earth channels). Ditches shall have
a gradient that limits the velocity within one and one-half (1.5)
to five (5.0) feet per second depending on existing soil conditions.
Such ditches shall have a minimum side slope ration of 3:1. Encroachment
of buildings and improvements on natural or designated drainage channels
for the channel's flood plains is prohibited. Such flood plains are
areas of land adjacent to an open paved channel or an open sodded
ditch that may receive a flood condition from a 100-year frequency
rain. The limits of such flood plains shall be indicated on drainage
improvements plans.
5.
Riprap shall not be used for erosion control on public rights-of-way.
Concrete or a geo-fabric shall be used for erosion control at stormwater
or culvert pipe outlets and at such other locations as determined
or directed by the City Engineer or City Public Works Director.
[Ord. No. 777 §26-144, 3-23-1994]
A.
The
rate of runoff concentrated at any point shall be determined by the
Rational Formula:
|
Q
|
=
|
CIA, in which
| |
|
Q
|
=
|
Runoff in cubic feet per second
| |
|
C
|
=
|
The runoff coefficient for the area
| |
|
I
|
=
|
Design rainfall intensity in inches per hour over the area based
or time of concentration and rainfall intensity curves included as
a part of this document. A five (5) minute time of concentration is
the minimum permitted.
| |
|
A
|
=
|
Drainage area, in acres.
|
1.
Runoff coefficient. The runoff coefficient "C" is
the variable in the Rational Formula least susceptible to precise
determination and the one which requires the greatest exercise of
engineering judgment because of the many area characteristics which
affect the runoff coefficient. Among the factors to be considered
in influencing the runoff coefficients are the following: present
and future zoning; terrain; local ponding or depressions; the amount
of pavement; roofs, turf and other areas having different degrees
of imperviousness.
The selection of a coefficient should take into consideration
the probable ultimate development of presently undeveloped area. Suggested
values of runoff coefficients are included in the following table:
|
Suggested Runoff Coefficients "C"
| |
|---|---|
|
"C" Value
|
Surface Conditions
|
|
.10 — .15
|
Tall grass, brush
|
|
.15 — .20
|
Parks, golf courses, farms and 1 acre single-family residences
|
|
.35
|
Single-family residences on lots of not less than 15,000 square
feet
|
|
.45
|
Single-family residences on lots of not less than 10,000 square
feet
|
|
.47
|
Single-family residences on lots of not less than 7,500 square
feet
|
|
.51
|
Single-family residences on lots of not less than 6,000 square
feet
|
|
.90
|
Gravel surfaces
|
|
.95
|
Asphalt concrete surfaces
|
|
1.00
|
Buildings and other structures
|
2.
Rainfall intensity. The average frequency of rainfall
occurrence used for design determines the degree of protection afforded
by a drainage system. (See Appendix C, Drawing No. 1 to this Chapter)
Maximum intensity of rainfall of a given expectancy is greater
for a short period of time than for longer periods. Therefore, it
is assumed that the maximum runoff will occur as soon as all parts
of the drainage area under consideration are contributing. The length
of time from the beginning of rainfall until runoff from the most
remote point in the drainage area reaches the point under consideration
is called the time of concentration. This may include overland flow
time and channel or gutter flow time. Nomographs which may be used
for determining time of concentration are reproduced in this document.
Once the time of concentration is known, the design intensity rainfall
may be determined from the rainfall intensity curves included in this
document.
[1]
Cross Reference — As to flow time chart, see Appendix
C Drawing No. 2.
[Ord. No. 777 §26-145, 3-23-1994]
A.
The
size of closed storm sewers, open channels, culverts and bridges shall
be designed so that their capacity will not be less than the runoff
computed by using the Manning Formula (see Appendix C Drawing No.
3 Manning's Formula):
|
Q
|
=
|
1.486
n
|
ar2/3 s1/2
| |||
|
Q
|
=
|
Capacity = discharge in cubic feet/second
| ||||
|
a
|
=
|
Cross-sectional area of water in conduit or channel in square
feet.
| ||||
|
r
|
=
|
Hydraulic radius of water in conduit or channel = area/wetted
perimeter.
| ||||
|
s
|
=
|
Mean slope of hydraulic gradient in feet per foot.
| ||||
|
n
|
=
|
Roughness coefficient, based on condition and type of material
of conduit or channel lining.
| ||||
|
Values of "n" for various kinds of pipe for use in Manning Formula
(see Appendix C Drawing No. 3 Manning's Formula):
| ||
|
Concrete pipe — 0.013
| ||
|
Corrugated metal pipe — 0.024
| ||
|
Concrete lined channel — 0.015
| ||
|
Earth channels — 0.030 to 0.050
| ||
B.
Design Tabulations. For systems of storm sewers with inlets
in various locations, the time of concentration at any point will
be time of concentration at the most remote inlet upstream, plus the
flow time in the storm sewer to the point under consideration. Computations
for systems lend themselves readily to tabulation showing the drainage
area, time of concentration runoff and capacity of each inlet and
section of sewer under consideration. This data is to accompany the
improvement plans.
C.
Street Flow. Street flow shall be limited by pavement encroachment
and depth of flow as indicated in the following table:
|
Street Flow
| ||
|---|---|---|
|
Street Classification
|
*Maximum Encroachment of a 2-Year Storm
| |
|
Local
|
No curb overtopping. Flow may spread to crown of street.
| |
|
Collector
|
No curb overtopping. Flow spread must leave the equivalent of
one (1) 10-foot driving lane clear of water.
| |
|
Arterial
|
No curb overtopping. Flow spread must leave the equivalent of
two (2) 10-foot driving lanes clear of water. One (1) lane in each
direction.
| |
|
* Where no curbing exists, encroachment shall not extend past
property lines.
| ||
|
The storm sewer system shall commence at the point where the
volume of flow equals five (5) cfs.
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