[Ord. No. 1250, 10-15-2002]
A. Drainage Area Plan. A plan of the drainage area at a scale of one
(1) inch equals one hundred (100) feet with one (1) 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) feet 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
Part.
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. Storm Water Flow Quantities. Storm water 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 to a maximum of five (5) cfs in each
gutter. Flow over this must be removed.
[Ord. No. 1250, 10-15-2002]
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 miles. Locations having
a drainage area of less than or equal to one (1.0) square miles 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 Requirements For Various Improvements.
1.
Bridges And Culverts. Bridges, box culverts or concrete pipe
culverts shall be provided where continuous streets or alleys cross
water courses. The structure shall be designed in accordance with
City specifications for materials and to carry HS-20 loadings in all
cases.
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 with 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 and shall
not exceed twelve (12) feet per second. The use of corrugated polyethylene
pipe is permitted outside the paved areas.
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 and shall not 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 five-tenths (1.5) to five
(5.0) feet per second depending on existing soil conditions. Such
ditches shall have a minimum side slope ratio 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 earthen 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. Short sections, such as those following the outlets of small
culverts, may exceed the velocity of five (5.0) feet per second provided
that:
a.
UV protected geotextile netting (such as North American Green
C-350) is used.
b.
Rip-rap is not acceptable and cannot be used for this purpose.
[Ord. No. 1250, 10-15-2002]
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
on time of concentration and rainfall intensity curves included as
a part of these regulations. 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 areas.
Suggested values of runoff coefficients are included in the following
table:
Suggested Runoff Coefficients
|
---|
"C" Value Surface Conditions
|
---|
0.10 - 0.15
|
Tall grass, brush
|
0.15 - 0.20
|
Parks, golf courses, farms and one (1) acres single-family residences
|
0.35
|
Single-family residences on lots of not less than 15,000 sq.
ft.
|
0.45
|
Single-family residences on lots of not less than 10,000 sq.
ft.
|
0.47
|
Single-family residences on lots of not less than 7,500 sq.
ft.
|
0.51
|
Single-family residences on lots of not less than 6,000 sq.
ft.
|
0.90
|
Gravel surfaces
|
0.95
|
Asphalt and 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. 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 that may be used
for determining time of concentration are included in these regulations.
Once the time of concentration is known, the design intensity rainfall
may be determined from the rainfall intensity curves included in these
regulations.
[Ord. No. 1250, 10-15-2002]
A. The size of closed storm sewers, open channels, culverts and bridges
shall be designed so that capacity will not be less than the computed
runoff using the Manning Formula:
Q
|
=
|
(1.486/n) (r2/3) (s1/2)
|
Q
|
=
|
Capacity = Discharge in cubic feet/sec.
|
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:
|
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 up stream, 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, with five (5)
cfs the maximum flow in each gutter.
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) ten-foot driving lane clear of water.
|
Arterial
|
No curb overtopping. Flow spread must leave the equivalent of
two (2) ten-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 equals five (5) cfs.
|