[Ord. No. 1620 §1, 9-25-2006]
A.
Proposed
street improvements within the City shall conform to the pattern established
in the major street plan as adopted by the City of Hermann.
B.
Street
improvements shall be designed to conform to applicable codes, regulations,
ordinances and the provisions set forth in these criteria as established
by the City of Hermann. Plans for said improvements shall be submitted
to the City's consulting engineer for approval and shall include all
information as may be required or described hereinafter.
C.
Deviations
and/or alternatives from the established design criteria set forth
herein shall be authorized upon final approval of the City's consulting
engineer and only after a review of the submitted plans and written
justification for said design alternative is provided from the project's
design engineer.
[Ord. No. 1620 §1, 9-25-2006]
A.
The
classification of streets shall be generally defined as follows:
1.
Local streets. A street designed to provide access
to abutting property from collector and arterial streets.
2.
Collector frontage streets. Streets which, in addition
to serving abutting properties, intercept local streets, connect with
community facilities and carry neighborhood traffic to the arterial
street systems. Commercial streets serve areas predominately zoned
for commercial or industrial uses.
3.
Arterial streets. A street or road of considerable
continuity which serves or is intended to serve as a principal traffic
way between separated areas or districts and which is the main means
of access to the collector street system, highways or expressways.
[Ord. No. 1620 §1, 9-25-2006]
A.
Design Table.
|
Collector
|
Local
| ||
|---|---|---|---|
|
Minimum right-of-way width
|
60 feet
|
50 feet
| |
|
Minimum number of lanes
|
2
|
2
| |
|
Minimum lane width (feet)
|
12 feet
|
12 feet
| |
|
Street width (BOC to BOC) (feet)
|
36 feet
|
28 feet
| |
|
Minimum street light spacing (feet)
|
300 feet
|
400 feet
| |
|
Minimum compacted subgrade depth (inches)
|
6 inches
|
6 inches
| |
|
Minimum aggregate base course depth Type 1 or Type 2 (inches)
|
10 inches
|
8 inches
| |
|
Minimum asphaltic concrete pavement depth (inches)
|
6 inches
|
4 inches
| |
|
Volume range (trips per day)
|
1,000 — 4,500
|
Less than 1,000
| |
|
Design speed (mph)
|
35 mph
|
35 mph
| |
|
Maximum road grade
|
8%
|
10%
| |
|
Minimum road grade
|
0.5%
|
0.5%
| |
|
Minimum interior corner radius of pavement (feet)
|
50 feet
|
25 feet
| |
|
Minimum stopping sight distance
|
300 feet
|
250 feet
| |
|
Minimum intersection spacing
|
500 feet
|
300 feet
| |
|
Maximum intersection spacing
|
N/A
|
1,400 feet
| |
|
Minimum horizontal curve radiiu
|
300 feet
|
280 feet
| |
|
Minimum length of vertical curve
|
100 feet K=40
|
100 K=30
| |
|
Minimum driveway entrance spacing (feet)
|
200 feet
|
One per property
| |
|
Minimum sidewalk width (feet)
|
5 feet (one side)
|
5 feet (one side)
| |
|
Parking permitted
|
No
|
One side only
| |
|
Storm sewers
|
Yes
|
Yes
| |
|
Curb and gutter type
|
Barrier
|
Roll back
|
B.
Off-Center Street Intersections. A minimum centerline-to-centerline
dimension of one hundred fifty (150) feet shall separate off-center
street intersections.
C.
Intersection Vertical Alignment. In all cases where a higher
functional street intersects with a lower functional street, normal
street crown shall be maintained on the higher functional street.
Where streets of equal function intersect, street grades shall coincide
in the center of the intersection with reduced ride ability for both
streets or a warping of the cross slope for both streets.
D.
Minimum Angle Of Intersection. All intersections shall meet
at approximately a ninety degree (90°) angle. Skewed intersections
should be avoided and in no case will the angle be less than sixty
degrees (60°).
E.
Maximum Gradient. The maximum gradient for streets as noted in Subsection (A) may be exceeded only upon written approval of the City's consulting engineer. Such approval will only be granted in unusual cases where grades within the acceptable limits cannot be obtained.
F.
Grading Gradients. The finished grade within the limits
of the right-of-way shall slope from one-quarter (¼) inch vertical
to one (1) foot horizontal minimum, to one-half (½) inch vertical
to one (1) foot horizontal maximum measured above the back of the
curb. The grading gradients may be varied only upon written approval
of the City's consulting engineer.
G.
Tangent Length. The minimum tangent length between reverse
curves shall be fifty (50) feet for local streets and one hundred
(100) feet for collector and arterial streets, except that no tangent
will be required for radii longer than five hundred (500) feet.
H.
Connections To Existing Pavements. Where a new street is
to connect to an existing street, all deteriorated or cracked asphalt
within five (5) feet of the connection point shall be removed to a
point where sound material is found. If full-depth pavement removal
is required, the subgrade will be recompacted to ninety-five percent
(95%) to standard density.
I.
Storm Drainage. All storm drainage works constructed in
connection with street improvements shall be designed in accordance
with the design criteria for storm sewers and appurtenances.
J.
Cul-de-sacs. At locations where streets are to be terminated
and a vehicular connection between adjacent streets is not required,
a cul-de-sac may be required. Such cul-de-sac shall be constructed
with a minimum radius of forty-five (45) feet to the back of the curb.
K.
Temporary Turnarounds. At locations where streets are to
be temporarily terminated which will be extended at a later date and
said street extends beyond the intersection of an adjacent street
more than five (5) lots, a temporary cul-de-sac shall be constructed
with a minimum radius of forty-five (45) feet. The temporary cul-de-sac
shall be constructed of asphaltic concrete with a minimum depth of
six (6) inches. Curb and gutter will not be required. The cul-de-sac
shall be constructed within the limits of a permanent construction
right-of-way.
L.
Monument Boxes. Monument boxes conforming to the standard
drawings shall be installed at all quarter section corners as involved
in the street construction.
M.
Other Design Criteria. The American Association of State
Highway and Transportation Officials (AASHTO) or other applicable
AASHTO design guides shall in accordance with the most current edition
of "A Policy on Geometric Design of Highways and Streets" author all
other street design elements not contained within this criteria.
N.
Driveway Elevations. Driveways shall attain top of curb
elevation within the right-of-way. Driveway grades within right-of-way
shall be eight percent (8%) maximum until curb height is reached.
Break-over grades for crest drives shall be eight percent (8%) maximum
and sag drives shall be twelve percent (12%) maximum. Driveway elevation
shall be not more than six (6) inches above or below the normal shoulder
elevation at the right-of-way line to allow for a smooth sidewalk
profile.
[Ord. No. 1620 §1, 9-25-2006]
A.
This
Section sets forth the minimum technical criteria for the analysis
and design of drainage systems. All development plans submitted for
approval to the City of Hermann must be accompanied by an adequate
storm drainage system analysis and design in accordance with the criteria
as hereinafter described and shall be performed by a licensed professional
engineer in the State of Missouri.
B.
The
criteria set forth in this Section shall apply primarily to that element
of the drainage system outside the limits of the 100-year regulatory
floodplain unless otherwise noted. Improvements within the limits
of the 100-year floodplain shall conform to requirements set forth
in applicable City Codes and ordinances and the regulations of any
other agency having jurisdiction over such area.
C.
Minimum Standards Of Analysis. Unless otherwise approved
by the City's consulting engineer, the following criteria will be
utilized to determine the adequacy of any storm drainage facility
design submitted for approval.
1.
Methodology of analysis. In determining the amount
of stormwater runoff resulting from a development and the amount of
flow at various points throughout the drainage system, it is important
for the designer to relate the methodology to be utilized in his calculations
to the proportionate size of the tributary watershed areas. In developments
where the area contributing runoff is one hundred (100) acres or less,
the Rational Method of calculating the quantity of runoff shall be
utilized. Developments where the area contributing runoff exceeds
one hundred (100) acres shall be designed using the unit hydrography
method (SCS) or other methodologies approved by the City's consulting
engineer.
2.
Criteria for drainage system. All calculations
relating to runoff analysis shall be based upon the proposed land
use and shall take into consideration any contributing runoff from
areas adjacent to the development site. Stormwater runoff analysis
from adjacent existing developed areas shall be based upon current
land usage and topographical features. Property adjacent to the study
area that is undeveloped shall be considered as fully developed in
accordance with the most probable anticipated future land use. Such
land use shall be determined from the City Comprehensive Plan and
the City Zoning Map. In the event that the future land use of a specific
undeveloped property cannot be adequately projected from available
information, the average runoff coefficient © to be used shall
not be less than sixty-five hundredths (0.65) for use in the Rational
Method or an appropriate equivalent value as approved by the City's
consulting engineer for any other method. The most likely flow pattern
to be utilized for an undeveloped area shall be based upon existing
natural topographical features.
3.
Average land slopes in both developed and undeveloped areas may be
utilized to calculate runoff rates. The exception to this shall be
in areas with existing well-defined drainage patterns and slopes;
in which case the actual slope shall be used.
4.
Existing runoff flow rates and velocities at locations of discharge
from adjacent upstream developments shall be utilized in the drainage
system design. Drainage facilities shall be designed to minimize the
velocity of overland flow so as not to cause erosion damage. In areas
where excessive velocities exist, adequate dissipating structures
shall be provided as required to result in velocities appropriate
for the type of erosion control to be utilized or as specified in
this criteria.
5.
The primary function of roadways within a development shall be reserved
for the conveyance of traffic. The use of these facilities as a storm
runoff facility shall be restricted to the requirements established
and set forth in these design criteria.
6.
The utilization of on-site or on-stream detention and natural drainage
ways is recommended and encouraged where feasible. Relocation of existing
natural drainage ways will not be approved unless such relocation
has been substantiated as a result of a thorough and complete analysis
of the resultant consequences.
7.
The designer shall consider all problem areas of his design and analysis
to prevent the transfer of these problems from one location to another.
All points of drainage outfall shall be designed to preclude creation
of downstream flooding problems and hazards to the public. Approval
will not be given to any project that involves the construction of
any structure or the placement of fill material that will hinder or
impair surface or subsurface drainage from surrounding areas.
D.
Minimum Acceptable Standards Of Design. Storm water runoff
shall be carried by enclosed systems or open channels on the basis
of criteria established in this Section and subject to the final approval
of the City's consulting engineer.
1.
Open channels. Open channels, natural or improved,
may be placed to the rear or side of properties upon approval of the
City's consulting engineer where the design provides adequate protection
to the adjacent property and structures. Such protection shall be
through the provision of a 50-year floodplain setback and a minimum
clearance from the top of bank to any building of thirty (30) feet.
2.
Enclosed systems. Runoff from drainage areas outside
of established or proposed right-of-ways greater than three (3) acres
in size shall be collected and transported in an enclosed system.
Enclosed systems shall also be utilized within the limits of established
or proposed right-of-way for roadways and streets. The drainage system
shall remain enclosed until the flow rate is such that the runoff
from the design storm for a development can no longer be contained
within a seventy-two (72) inch R.C.P. equivalent conduit and an open
channel can be entered without negative impact.
3.
At the point of intersection and discharge with the receiving open
channel, an energy-dissipating structure acceptable to the City's
consulting engineer shall be provided to limit the discharge velocity
from the enclosed system to not more than five (5) fps.
4.
Where storm drainage facilities are located along side property lines,
such systems shall be enclosed to a point at least thirty (30) feet
beyond the rear corner of adjacent buildings unless otherwise directed
or approved by the City's consulting engineer. A surface swale shall
be designed over this area to contain additional runoff from a 50-year
storm.
5.
Complete side and rear drainage systems meeting the criteria established
previously shall be provided along the boundaries of new subdivisions
or developments by the developer or property owner.
6.
Design storm frequencies. The minimum rainfall
event to be utilized in determining the intensity of rainfall for
storm flow calculations shall be based on the following land uses:
|
Table of Storm Return Frequency
| ||
|---|---|---|
|
Storm Return
|
Land Use/Zoning Frequency
| |
|
Residential
|
10 year
| |
|
Commercial
|
25 year
| |
|
Industrial
|
25 year
| |
|
Parks, open space, etc.
|
10 year
| |
|
Open channels
|
25 year
| |
7.
Storm drainage systems having more than one (1) land use or zoning
classification tributary to the system shall be designed on the basis
of the highest runoff producing land use comprising thirty percent
(30%) or more of the total tributary area.
8.
Runoff computation. The Rational Method of calculating
stormwater quantities, Q = KCiA shall be used with the following definitions
of terms and arbitrary values:
a.
Where "Q" is the quantity of runoff in cubic feet per second and
is the basis for design of the storm drainage system.
b.
Where "K" is a dimensionless coefficient to account for antecedent
precipitation.
c.
Where "C" is the weighted coefficient of runoff from the tributary
area. Coefficient of runoffs are specified in the "Table of Coefficient
of Runoff".
d.
Where "i" is intensity of rainfall in inches per hour and shall be
determined for the yearly frequency or as derived from intensity duration
curves to support this criteria.
e.
Where "A" is the area in acres contributing to the drainage system.
All upstream tributary areas are to be considered as fully developed
as zoned or planned at the time of design.
|
Coefficient of Runoff Table
| |||
|---|---|---|---|
|
Land Use/Zoning Classification Runoff Coefficient ("C")
|
Average
|
Impervious
| |
|
Commercial:
| |||
|
Business areas
|
0.90
|
95%
| |
|
Neighborhood areas
|
0.80
|
85%
| |
|
Residential:
| |||
|
Single-family areas
|
0.50
|
35%
| |
|
Multi-family areas and mobile home parks
|
0.65
|
60%
| |
|
Churches and schools
|
0.75
|
75%
| |
|
Industrial:
| |||
|
Light areas
|
0.65
|
60%
| |
|
Moderate areas
|
0.80
|
80%
| |
|
Public use:
| |||
|
School areas
|
0.75
|
75%
| |
|
Church areas
|
0.75
|
75%
| |
|
Parks and cemeteries
|
0.35
|
10%
| |
|
Playgrounds
|
0.35
|
10%
| |
|
Agricultural:
| |||
|
Open space areas
|
0.30
|
0%
| |
|
Permanent unimproved areas
|
0.30
|
0%
| |
9.
As an alternate to the above coefficients or for areas not specifically
listed above, a composite runoff coefficient based on the percentage
of the different types of surfaces involved shall be used.
10.
Coefficients with respect to surface type shall not be less than
those listed in the following table:
|
Surface Coefficient Table
| ||
|---|---|---|
|
Surface Type
|
Coefficient ("C")
| |
|
Asphalt
|
0.90
| |
|
Concrete
|
0.90
| |
|
Roof areas
|
0.90
| |
|
Turf
|
0.30
| |
11.
Time of concentration ("TC") equals the overland flow time to the
most upstream inlet or other point of entry to the system plus the
time of flow in the system upstream from the point under construction.
(TC = Ti + Tt)
a.
Inlet time "Ti" shall be calculated utilizing the following formula
but shall not be less than five (5.0) minutes or greater than fifteen
(15.0) minutes:
Ti = 1.8 x (1.1 — C) x D ½ minutes S ⅓
b.
Travel time ("Tt") shall be calculated as the length of travel in
the channelized system divided by the velocity of flow. Velocity shall
be calculated by Manning's equation assuming all system elements are
flowing full without surcharge.
c.
When the upstream channel is unimproved, it shall be assumed that
future construction of drainage system improvements will increase
the velocity of flow. Velocity used for calculating "Tt" shall be:
|
Velocity Table
| ||
|---|---|---|
|
Average Channel Slope (percent)
|
Velocity (fps)
| |
|
Less than 2%
|
7 fps
| |
|
2% to 5%
|
10 fps
| |
|
Greater than 5%
|
15 fps
| |
12.
Antecedent precipitation. "K" represents the frequency
factor used to account for antecedent precipitation and shall have
the following values. The product of K x C shall not exceed one (1.0).
|
Antecedent Precipitation Factors
| ||
|---|---|---|
|
Storm Return Period (years)
|
Frequency ("K")
| |
|
10 years
|
1.00
| |
|
25 years
|
1.10
| |
|
50 years
|
1.20
| |
|
100 years
|
1.25
| |
13.
Pipe sizing. Pipe sizes in integrated underground
systems shall be determined in accordance with the Manning Formula
Q = A (1.49) AR 0.67 S 0.5n
a.
Manning's roughness coefficients shall be used for the values of
"n" in the Manning Formula.
b.
The minimum size storm sewer size shall be twelve (12) inches in
diameter (fifteen (15) inches for CMP).
c.
Storm sewers and inlets shall be of sufficient capacity to adequately
carry the anticipated runoff from the design storm. Capacity shall
be rated at either inlet or outlet control, whichever condition indicates
the least capacity. The drainage system and appurtenant storm inlets
shall commence at all locations where the allowable street capacity
for the conveyance of stormwater runoff is exceeded or where there
is a possibility of ponding.
14.
All storm drainage systems shall be designed so as to maintain a
minimum velocity of flow of two (2) FPS feet per second and a maximum
velocity of fifteen (15) feet per second when flowing full. All systems
discharging at a velocity in excess of five (5) feet per second shall
be designed with an acceptable energy-dissipating structure.
a.
Depth. All storm drainage lines shall have a minimum
cover of eighteen (18) inches where practical. Cover may be decreased
to avoid conflicts or on short laterals as approved by the City's
consulting engineer. Special bedding and backfill may be required
where cover is less than eighteen (18) inches.
b.
Curb inlet, junction boxes and other points of entry. In general, curb inlets shall be installed at intersections and
as required at intermediate points to limit gutter flow width during
runoff occurring from the design peak discharge from the tributary
watershed area to that which will not encroach on the following center
width of streets:
|
|
Arterial streets
|
24 feet
|
|
|
Collector/commercial streets
|
14 feet
|
|
|
Local streets
|
10 feet
|
15.
Because of the potential for street debris to clog inlets and to
reflect potential cross section changes due to resurfacing, inlet
capacity shall be rated at eighty percent (80%) of the theoretical
inlet capacity unless otherwise approved by the City's consulting
engineer.
16.
Design shall provide that the hydraulic gradient at any opening through
which surface water may enter (or backflow from) the system is five-tenths
(0.5) foot or greater below the opening elevation.
a.
The hydraulic gradient elevation shall be calculated at the entrance
to the outlet line of each structure.
b.
The crown(s) of pipe(s) entering a structure shall be at or above
the crown of the pipe exiting from the structure to provide a minimum
fall of the invert in the structure of two-tenths (0.2) feet for straight
flow through the structure or five- tenths (0.5) feet fall for all
other types of flow (bends more than twenty-two and one-half degrees
(22.5°) deflection angle, multiple lines entering, enlargement
transition, etc.) through the structure.
17.
Open channels. Unless in a 100-year designated
floodplain or a critical area as determined by the City's consulting
engineer, open channels shall be designed for the 25-year frequency
storm. Open channels shall be sized to adequately carry the design
rate of flow without damage. Whenever practical, the channel shall
be characterized as slow flowing, be wide and shallow and be natural
in its appearance and functioning.
a.
Channel capacities shall be computed using the Manning Formula for
uniform flow.
b.
Design flow rates shall be carried within the confines of the open
channel with a minimum allowable freeboard of one (1.0) foot measured
from the water surface to the top of bank.
c.
Pipe culverts, box culverts and other structures entering channels
shall not project into the normal waterway area.
d.
Channel design shall include lining or treatment of the invert and
sides as required to minimize erosion. Minimum treatment shall including
seeding.
e.
The following table lists the minimum lining for channel inverts
and sides:
|
Channel Lining Table
| ||
|---|---|---|
|
Mean Flow Velocity (fps)
|
Type of Channel Lining
| |
|
Less than 3 fps
|
Seeded
| |
|
3 to 5 fps
|
Staked sod
| |
|
5 to 10 fps
|
Stone rip-rap (15" minimum depth)
| |
|
10 to 15 fps
|
Grouted stone rip-rap, gabion revetment or concrete
| |
|
Over 15 fps
|
Concrete or bedrock
| |
f.
Lining materials having equivalent erosion control properties to
those shown in the foregoing table may be used in lieu thereof with
the approval of the City's consulting engineer.
g.
Channel sections shall be compatible with the type of lining and
maintenance practical to be used. Side slopes shall be as flat as
practical. Side slopes of 3:1 shall be considered a normal maximum.
Under special circumstances where acceptable lining material is to
be utilized, slopes of 2:1 may be considered. Such use in the channel
design shall be only where approved by the City's consulting engineer.
Friction factors used in the design shall consider the type of lining.
h.
Alignment changes shall be achieved by curves having a minimum radius
of:
|
|
R
|
|---|---|
|
=
| |
|
V
| |
|
W
|
|
8
| |
|
D
|
|
|
R = Minimum radius of centerline in feet.
|
|
|
V = Average velocity of flow in feet/sec.
|
|
|
D = Depth of flow in feet.
|
i.
Lining height on the outside (concave) side of curves shall be increased
by:
|
|
y
|
|
=
| |
|
D
| |
|
4
|
|
|
D = Normal depth.
|
|
|
y = Increased vertical height of lining in feet.
|
j.
Increased lining height shall be transitioned from y to zero (0)
feet over a minimum distance of thirty (30) (y) feet downstream from
the point of tangency (p.t.) ten (10) (y) feet upstream from the point
of curvature (p.c.)
19.
Culvert. Culverts under major and minor arterials
shall have sufficient capacity to pass the runoff from the appropriate
design storm considering twenty percent (20%) of the inlet opening
plugged.
a.
The culvert, including inlet and outlet structures, shall properly
take care of water, bed-load and debris at all stages of flow.
b.
Inlet. Culvert inlets shall be designed to minimize
entrance and friction losses. Inlets shall be provide with either
flared-end sections or headwalls with wingwalls. Projecting ends will
not be acceptable. For large structures, provisions shall be made
to resist possible structural failure due to hydrostatic uplift forces.
c.
Outlets. Culvert outlets shall be designed to avoid
sedimentation, undermining of the culvert and erosion of the downstream
channel. Outlets shall be provided with either flared-end sections
or headwalls with wingwalls. Projecting outlets will not be acceptable.
Additional outlet control in the form of riprap, channel shaping,
etc., may be required where excessively high discharge velocities
occur.
d.
Slopes. Culvert slopes should be such that neither
silting nor excessive velocities and scour occur. Generally, the minimum
slope of culverts shall be limited to 0.005 FT/FT.
e.
Headwater. Generally, the headwater to diameter
ratio (HW/D) should not exceed those recommended as follows:
|
Storm Frequency
|
HW/
| |
|---|---|---|
|
10 year
|
< 1.0
| |
|
25 year
|
< 1.2
| |
|
50 year
|
< 1.5
| |
|
100 year
|
< 1.5
|
f.
Tailwater. The depth of tailwater at the outlet
shall be subject to the criteria set forth in this criteria and in
the technical specifications of the City of Hermann and shall be as
approved by the City's consulting engineer.
E.
Permanent
drainage easements are required to provide adequate access for construction,
inspection and maintenance of all storm drainage system components.
All easements shall be dedicated to the City. For new subdivisions,
all required easements and setbacks shall be shown on the final plat
recorded with the Register of Deeds.
1.
Drainage easements shall have minimum widths as described below.
A wider easement width may be required at structures or if the easement
is shared with other utilities or as determined by the City's consulting
engineer.
a.
Storm sewer. Easements for storm sewers shall be
either twenty (20) feet wide or the outside dimension of the conduit
plus ten (10) feet (centered on the conduit), which ever is greater.
A wider easement may be required if the depth of cover exceeds four
(4) feet.
b.
Improved open channel. Easements for improved open
channels shall be as wide as the top bank width plus ten (10) feet
on each side and shall be continuous to the end of the channel.
c.
Natural open channel. Easements for natural open
channels shall be the areas between the high bank lines of the channel,
plus additional width on each side of the channel as deemed necessary
by the City to allow access for maintenance equipment. The minimum
width for a natural open channel easement is thirty (30) feet.
F.
The
engineer must submit the following for review of a stormwater collection
system:
1.
Basic design criteria including the rainfall intensity, percentage
of imperviousness, runoff coefficients for each tributary basin area
in the drainage area, time of concentration, peak flow rates and any
other pertinent design criteria.
2.
Key map of the entire project to scale showing easements, sewer lines
and facilities, both existing and to be constructed.
3.
A drainage area map showing the ridge line of the area tributary
to each inlet, sewer and channel section in the system. The map shall
be accompanied by a table summarizing the basic design criteria.
4.
Plans and profiles of each storm drain showing location, size, design
flow, flow line elevations, gradients and materials; location, depths
and sizes of adjacent or crossing sewer lines and utilities; and special
construction requirements such as concrete encasement, backfill, size
and class of pipe. Typical cross sections of swales, ditches, open
channels.
5.
Details of special structures, channel improvements, culverts, transitions,
headwalls, aprons and junction chambers, all adequately detailed and
dimensioned, including placement of steel in reinforced concrete structures.
[Ord. No. 1620 §1, 9-25-2006]
A.
All
detention basins shall be designed to detain runoff from a 2-year,
10-year or 25-year design storm. Post-development peak discharges
for each design storm shall not exceed the pre-development peak discharge
for each respective design storm. Detention basins shall be designed
to safely pass the 100-year design storm without damage to the facility
or downstream channel.
B.
Stormwater
shall be retained such that the rate of runoff leaving the post-developed
site is no greater than the pre-development runoff rate. The difference
between the two (2) runoff rates is the amount that should be detained.
Any formula may be used — HEC-1, etc. or Rational equation,
etc. Developments over one hundred (100) acres must use the unit hydrography
(SCS) or similar method. Calculations must be submitted to the City's
consulting engineer for review.
Detention may be accomplished in nearly any method viable on
the project site. This can include detention ponds, tanks, below ground
systems or even parking lot or landscape island detention. Discharge
may be by use of V-notch walls or small pipe. Innovative solutions
are usually acceptable depending upon City staff review. Open basins
must be soil lined and have soil placed on interior and exterior side
slopes. Soil must be seeded and mulched. Riprap or rock lined basins
and berms will only be allowable under unusual circumstances and with
special permission from the City.
The maximum depth of water in a dry detention basin shall not
exceed eight (8) feet.
A minimum of one (1) foot of freeboard shall be provided from
the top of the basin to the maximum water surface elevation.
The principal outlet structure shall be designed to function
without requiring operation of any kind or requiring use of equipment
or tools or any mechanical devices.
The engineer must submit the following for review of a stormwater
detention facility:
1.
A drainage area map showing the entire area tributary to the detention
basin areas shall be noted in acres.
2.
Runoff curve numbers, Rational Method coefficients, etc. for the
entire drainage area.
3.
The time of concentration and travel time for entire basin or each
subbasin.
4.
Elevation vs. storage tables and curves for all design storms.
5.
Pre-development and post-development inflow hydrographs for all design
storms.
6.
A tabulation of inflow hydrographs routed through the detention basin
by the storage indication method and a tabulation of subsequent outflow
hydrography, detention pond elevations and storage. Elevation at which
the peak stage occurs should be included.
7.
Site plan showing appropriate design information.
8.
Structural details for the outlet control structures (if required).
Discharge velocities must not be erosive. The following minimum
channel linings are required:
|
Velocity (feet per second)
|
Lining Type
| |
|---|---|---|
|
Less than 3 fps
|
Seeded
| |
|
3 to 5 fps
|
Staked sod
| |
|
5 to 10 fps
|
15" depth of stone rip-rap
| |
|
10 to 15 fps
|
Grouted stone rip-rap, gabion revetment or concrete paved
| |
|
Greater than 15 fps
|
Concrete paved or bedrock
|
|
|
Note: Velocities must be reduced prior to exiting the appropriate
lining.
| |
|
|
All storm sewers under street pavement must be concrete unless
a specific waiver is granted from the City's consulting engineer.
Flared end-sections are required on all CMP and RCP and concrete aprons
and wing walls are required on all concrete box culverts.
| |
|
|
Drainage easements must be dedicated to the City where appropriate.
| |
|
|
An erosion control plan must be developed and implemented prior
to beginning any land disturbance. The plan can include such items
as silt fencing, staked straw bales or silt retention ponds to control
the runoff during the construction.
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