[HISTORY: Adopted 11-20-2006 by Ord. No. 366; amended 10-9-2007 by Ord. No. 420; amended 6-14-2010 by Ord. No. 556; 12-20-2010 by Ord. No. 574; 4-18-2011 by Ord. No. 585. Subsequent amendments noted where applicable.]
A.
Stormwater sewers or channels provide the facility for removing and
transporting surface runoff produced from rainfall. Design requirements
differ from those for either sanitary or combined sewers.
B.
This section gives the minimum technical design requirements of the
City of Moscow Mills drainage facilities. In general, the formulae
presented herein for hydraulic design represent "acceptable" procedures
not necessarily to the exclusion of other sound and technically supportive
formulae. Any departure from these design requirements should be discussed
before submission of plans for approval and should be justified. All
construction details pertaining to storm sewer improvements shall
be prepared in accordance with the Metropolitan St. Louis Sewer District
(MSD) Standard Construction Specifications Latest edition unless otherwise
noted.
C.
Bioretention for both storm sewers and detention facilities for the City of Moscow Mills. Bioretention is an upland water quantity and quality control practice that uses the chemical, biological and physical properties of plants and soils for stormwater filtration, absorption (retention) and storage (detention). The Maryland Stormwater Design Manual shall be used as a basis for all Bioretention designs in conjuncture with the requirements of this chapter (Chapter 48) of the City Code. Each Bioretention design shall be reviewed and approved by both the City Engineer and the City of Moscow Mills Planning and Zoning Commission.
All storm sewers shall meet the following general requirements:
A.
Size and shape. The minimum diameters of pipe for storm water sewers
and combined sewers shall be 12 inches. Sewers shall not decrease
in size in the direction of the flow unless approved by the City.
Circular pipe shall be used for storm water sewers unless the City's
engineer approves otherwise in writing.
B.
Materials. All materials shall conform to MSD Standard Construction
Specifications, latest edition, except as specified herein. Reinforced
Concrete Pipe (RCP), Class III, or High Density Poly Ethylene (HDPE)
Corrugated Pipe may be used for storm water sewers and combined sewers,
except that HDPE Corrugated Pipe cannot be used under or through existing
street right of ways or through an area where potential future right
of ways may occur. HDPE Corrugated Pipe must be installed in accordance
with the manufacturer's specifications and the City's specifications
as set forth herein.
C.
Specifications for HDPE corrugated pipe. High Density Polyethylene
(HDPE) Corrugated Pipe shall be specified and inspected in accordance
with MoDOT Standard Construction Specification Section 730, latest
revision. Installation shall be in accordance with MoDOT Standard
Plan drawing 730.00, latest revision. As directed in MoDOT's
specifications, this shall include, but not be limited to the following:
1.
Twelve inches to 60 inches nominal metric inside diameters.
2.
Visual inspection of all pipe prior to final acceptance. If the City
believes a section is deflected greater than 5%, no less than 10%
of the pipe runs will be dimensional inspected.
3.
Minimum cover as follows:
a.
Twelve inches to top of rigid pavement or bottom of flexible pavement
for pipe sizes 12 inches to 48 inches.
b.
Twenty-four inches to top of rigid pavement or bottom of flexible
pavement for 60 inches pipe.
c.
Twelve inches of temporary additional cover is required where heavy
construction load traffic is anticipated.
4.
Flared-end sections are required when outfall erosion is a concern.
Flared-end sections shall be metal or precast concrete in accordance
with MoDOT Standard Drawing 732.00, latest revision.
5.
In areas of a high groundwater table, the burial depth and resulting
cover must be sufficient to balance the hydrostatic uplift force.
D.
Bedding.
1.
The project plans and Specification shall indicate the specific type
or types of bedding, cradling, or encasement required in the various
parts of the storm sewer construction if different than current MSD
Standard Construction Specifications. Special provisions shall be
made for pipes laid under or over fills or embankments in shallow
or partial trenches, either by specifying extra strength pipe for
the additional loads due to differential settlement, or by special
construction methods to prevent or to minimize such additional loads.
Pipes having a cover of less than three feet shall be encased in concrete.
2.
If the storm and sanitary sewers are parallel and in the same trench,
the upper pipe shall be placed on a shelf and the lower pipe shall
be bedded in compacted granular fill to the flow line of the upper
pipe.
E.
Pipe or conduit under streets and pavements.
1.
Reinforced concrete pipe shall be Class III, Minimum.
2.
Any pipe or conduit material beneath a highway, road, street, or
pavement, or with reasonable probability of being so located, shall
have ample strength for all vertical loads, including the live load
required by the highway authority having jurisdiction, and in no case
shall provide for less than an AASHTO HS-20 loading. For other locations,
the minimum live load shall be the HS-10 loading. Special considerations
may be required for adverse conditions. Compacted granular backfill
shall be utilized to the base of the pavement.
3.
Monolithic reinforced concrete structures shall be designed structurally
as continuous rigid units. Las much concrete as is practical shall
be poured in one single operation with the reinforcing steel not terminated
at the ends of a member but carried over at the joints into adjacent
members.
F.
Alignment. Sewer alignments are normally limited by the available
easements which in turn should reflect proper alignment requirements.
Since changes in alignment affect certain hydraulic losses, care in
selecting possible alignments can minimize such losses and use available
had to the best advantage.
Sewers shall be aligned:
1.
To be in a straight line between structures, such as manholes, inlets,
inlet manholes and junction chambers, for all pipe sewers 30 inches
in diameter and smaller.
2.
To be parallel with or perpendicular to the centerlines of straight
streets unless otherwise unavoidable. Deviations may be made only
with approval of the City.
3.
To avoid meandering, off-setting and unnecessary angular changes.
4.
To make angular changes in alignment for sewers 30 inches in diameter
or smaller in a manhole located at the angle point, and for sewers
33 inches in diameter or larger, by a uniform curve between two tangents.
Curves shall have a minimum radius of 10 times the pipe diameter.
5.
To angular changes in direction greater than necessary and any exceeding
90°.
G.
Location. Storm sewer locations are determined primarily by the requirements
of service and purpose. It is also necessary to consider accessibility
for construction and maintenance, site availability and competing
uses, and effects of easements on private property.
Storm sewers shall be located:
1.
To serve all property conveniently and to best advantage.
2.
In public streets, roads, alleys, rights-of-way, or in sewer easements
dedicated to the City of Moscow Mills.
3.
In easements on private property and streets only when unavoidable.
4.
On private property along property lines or immediately adjacent
to public streets, avoiding diagonal crossings through the central
areas of the property.
5.
At a sufficient distance from existing and proposed buildings including
footings, and underground utilities or other sewers to avoid encroachments
and reduce construction hazards.
6.
To avoid interference between other stormwater sewers and house connections
to foul water or sanitary sewers.
7.
In unpaved or unimproved areas whenever possible.
8.
To avoid, whenever possible, any locations known to be or probably
to be beneath curbs, paving or other improvements particularly when
laid parallel to centerlines.
9.
To avoid sinkhole areas if possible. However, if sinkhole areas cannot
be avoided, see sub-section 48.020.08 for requirements.
10.
Crossing perpendicular to street, unless otherwise unavoidable.
H.
Sinkhole areas.
1.
Sinkhole report.
a.
Where improvements are proposed in any area identified as sinkhole
areas, a sinkhole report will be required. This report is to be prepared
by a Professional Engineer, registered in the State of Missouri, with
demonstrated expertise in geotechnical engineering, and shall bear
his or her seal.
b.
The sinkhole report shall verify the adaptability of grading and
improvements with the soil and geologic conditions available in the
sinkhole areas. Sinkhole(s) shall be inspected to determine its functional
capabilities with regard to handling drainage.
c.
The report shall contain provisions for the sinkholes to be utilized
as follows:
(1)
All sinkhole crevices shall be located on the plan. Functioning
sinkholes may be utilized as a point of drainage discharge by a standard
drainage structure with a properly sized outfall pipe provided to
an adequate natural discharge point, such as a ditch, creek, river,
etc.
(2)
Non-functioning sinkholes and sinkholes under a proposed building
may be capped.
(3)
If development affects sinkholes, they may be left in their
natural state; however they will still require a properly sized outfall
pipe to an adequate natural discharge point.
(4)
Special siltation measures shall be installed during the excavation
of sinkholes and during the grading operations to prevent siltation
of the sinkhole crevice.
2.
Procedure for utilization of sinkholes.
a.
Excavation. Prior to filling operations I the vicinity of a sinkhole,
the earth in the bottom of the depression will be excavated to expose
the fissure(s) in the bedrock. The length of fissure exposed will
vary, but must include all unfilled voids or fissure widths greater
than 1/2 inch maximum dimensions which are not filled with plastic
clay.
b.
Closing fissures. The fissure or void will be exposed until bedrock
in its natural attitude is encountered. The rock will be cleaned of
loose material and the fissures will be hand-packed with quarry-run
rock of sufficient size to prevent entry of this rock into the fissures,
and all the voids between this hand-packed quarry-run rock filled
with smaller rock so as to prevent the overlying material's entry
into the fissures. For a large opening, a structural (concrete) dome
will be constructed with vents to permit the flow of groundwater.
c.
Placing filter material.
(1)
Material of various gradations, as approved, will be placed
on top of the hand-packed rock with careful attention paid to the
minimum thicknesses. The filter material must permit either upward
or downward flow without loss of the overlying material.
(2)
The fill placed over the granular filter may include may include
granular material consisting of clean (no screenings) crushed limestone
with 10 inch maximum size and one inch minimum size or an earth fill
compacted to a minimum density of 90% modified Proctor as determined
by ASTM D-1557.
d.
Supervision.
(1)
Periodic supervision of the cleaning of the rock fissures must
be furnished by the Engineer who prepared the Soil Report. Closing
of the rock fissures will not begin until the cleaning has been inspected
and approved by that Engineer.
(2)
During the placement and compaction of earth fill over the filter,
supervision by the Engineer shall be continuous. Earth fill densities
will be determined during the placement and compaction of the fill
in sufficient number to insure compliance with the specification.
The Engineer is responsible for the quality of the work and to verify
that the specifications are met.
I.
Flow line. The flow line of storm sewers shall meet the following
requirements;
1.
The flow line shall be straight or without gradient change between
the inner walls of connected structures; that is, from manhole to
manhole, manhole to junction chamber, inlet to manhole, or inlet to
inlet.
2.
Gradient changes in successive reaches normally shall be consistent
and regular. Gradient designations less than the nearest 0.001 foot
per foot, except under special circumstances and for large sewers,
shall be avoided.
3.
Sewer depths shall be determined primarily by the requirements of
pipe or conduit size, utility obstructions, required connections,
future extensions and adequate cover.
4.
Stormwater pipes discharging into lakes shall have the discharge
flow line a minimum of three feet above the lake bottom at the discharge
point or no higher than the normal water line.
5.
A concrete cradle is required when the grade of a sewer is 20% or
greater. A special design and specification is required for grades
exceeding 50%.
J.
Manholes. Manholes provide access to sewers for purposes of inspection,
maintenance and repair. They also serve as junction structures for
lines and as entry points for flow. Requirements of sewer maintenance
determine the main characteristics of manholes.
1.
For sewers 30 inches in diameter or smaller, manholes shall be located
at changes in direction; changes in size of pipe; changes in flow
line gradient of pipes, and at junction points with sewers and inlet
lines.
For sewers 33 inches in diameter and larger, manholes shall
be located on special structures at junction points with other sewers
and at changes of size, alignment change and gradient. A manhole shall
be located at one end of a short curve and at each end of a long curve.
2.
Spacing of manholes shall not exceed 400 feet for sewer pipes 36
inches in diameter and smaller; 500 feet for sewer pipes 42 inches
in diameter and larger, except under special approved conditions.
Spacing shall be approximately equal, whenever possible.
3.
When large volumes of stormwater are permitted to drop into a manhole
from lines 21 inches or larger, the manhole bottom and walls below
the top of such lines shall be of reinforced concrete.
4.
Manholes shall be avoided in driveways or sidewalks.
5.
Connections to existing structures may require rehabilitation or
reconstruction of the structure being utilized. This work will be
considered part of the project being proposed.
K.
Overland flow system.
1.
The design components of the drainage system include the inlets,
pipe, storm sewers, and improved and unimproved channels that function
during typical rainfall events. The overland flow system comprises
the major overland flow routes such as swales, streets, floodplains,
detention basins, and natural overflow and ponding areas. The purpose
of the overland flow system is to provide a drainage path to safely
pass flows that cannot be accommodated by the design system without
causing flooding of adjacent structures.
2.
The criteria for the overland flow systems shall be as follows:
a.
The overland flow system shall be designed for the 100-year, a twenty-four-hour
event, assuming the design system is blocked. The Natural Resources
Conservation Service (NRCS) Unit Hydrograph method shall be used to
calculate the overland flow peak flow rate. (100-year, twenty-four-hour
rainfall 7.21 inches).
b.
The capacity of the overland flow system shall be verified with hydraulic
calculations at critical cross-sections. The overland flow system
shall be directed to the detention facility, or as approved by the
City.
c.
The low sill of all structures adjacent to the overland flow system
swales shall be above the 100-year high-water elevation.
d.
Where the topography will not allow for an overland flow path:
(1)
The storm sewer shall be designed for the 100-year, twenty-four-hour
storm; and
(2)
If this storm pipe is smaller than 36 inches in diameter, a
designated ponding area shall be identified, assuming the pipe is
blocked; and
(3)
The ponding area shall be based on the 100-year, twenty-four-hour
storm; and
(4)
The low sill of all structures adjacent to the ponding area
shall be above the 100-year high-water elevation.
e.
The overland flow system shall be clearly designated on the drainage
area map and on the grading plan.
f.
All overland flow systems will be considered on a site-specific basis.
A.
Flow quantities. Flow quantities are to be calculated by the "Rational
Method" in which:
Q = API
|
Where:
| ||
Q
|
=
|
Runoff in cubic feet per second
|
A
|
=
|
Tributary area in acres.
|
I
|
=
|
Average intensity of rainfall (inches per hour) for a given
period and a given frequency.
|
P
|
=
|
Runoff factor based on runoff from pervious and impervious surfaces.
|
P (runoff Factors) for various impervious conditions are shown
in Table 4-1.
| ||
P.I. Values for various impervious conditions are shown in Table
4-2.
|
1.
Rainfall frequency. A fifteen-year, twenty-minute duration rainfall
shall be used for all stormwater sewer design in the City of Moscow
Mills unless otherwise directed by the City's Engineer Figure
4-1 gives rainfall curves for two-, five-, ten-, fifteen-, twenty-
and 100-year frequencies.
2.
Impervious percentages and land use.
a.
Minimum impervious percentages to be used are as follows:
(1)
For manufacturing and industrial areas: 100%.*
(2)
For business and commercial areas: 100%.*
(3)
For residential areas, including all areas for roofs of dwellings
and garages; for driveways, streets, and paved areas; for public and
private sidewalks; with adequate allowance in area for expected or
contingent increases in imperviousness:
In apartment, condominium and multiple dwelling areas
|
75%*
|
In single family areas:
| |
1/4 Acre or less
|
50%
|
1/4 Acre to 2 Acre
|
40%
|
One acre or larger
|
30%
|
Playgrounds (Non-Paved)
|
20 - 35%*
|
*NOTE:
|
Drainage areas may be broken into component areas with the appropriate
run-off factor applied to each component, i.e.; a proposed development
may show 100% impervious for paved areas and 5% impervious for grassed
areas.
|
(4)
For small non-perpetual cemeteries.
(5)
For parks and large perpetual charter cemeteries: 5%.
b.
The planning engineer shall provide adequate detailed computations
for any proposed, expected or contingent increases in imperviousness
and shall make adequate allowances for changes in zoning use. If consideration
is to be given to any other value than the above for such development,
the request must be made at the beginning of the project, must be
approved in writing before its use is permitted.
c.
Although areas generally will be developed in accordance with current
Zoning requirements, recognition must be given to the fact that zoning
ordinances can be amended to change the currently proposed types of
development, and any existing use. Under these circumstances the possibility
and the probability of residential areas having lot sizes changed
or re-zoned to business, commercial, or light manufacturing uses should
be given careful consideration.
d.
Average twenty-minute values of P.I. (Cfs per acre) to be used are
as follows:
Percent Imperviousness
|
15-year Interval, 20-Minute Duration
|
---|---|
5%
|
1.7
|
10%
|
1.8
|
20%
|
2.0
|
30%
|
2.2
|
40%
|
2.6
|
50%
|
2.6
|
90%
|
3.4
|
100%
|
3.5
|
Roofs
|
4.2
|
3.
Reduction in P.I. with time and area. Reduction in P.I. Values for
the total time of concentration Exceeding 20 minutes and for tributary
areas exceeding 300 acres will be allowed only in trunk sewers and
main channels. The reduced average P. I. value for the tributary area
shall not be less than the value determined as follows on the basis
of:
a.
Time. As the time of concentration increases beyond 20 minutes, select
the appropriate P. I. Value from Table 4-1. The travel time through
a drainage channel should be based on an improved concrete section.
These reduced values shall be used unless a further reduction is allowed
for area.
b.
Area. As the total tributary area at any given location in a channel
increases in excess of 300 acres, the P. I. Value may be further reduced
by multiplying it by an area coefficient "Ka". The average rainfall
rate, for a given storm, for a given period for the tributary area,
is less than the corresponding point value as determined from recording
rainfall gauges. The curve data is as follows:
P. I. Coefficients Ka
| |
---|---|
Area (Asbscissass)
(acres)
|
"Ka" (Ordinates)
|
300 to 449
|
1.00
|
450 to 549
|
0.99
|
550 to 549
|
0.98
|
750 to 999
|
0.97
|
1,000 to 1,280
|
0.96
|
1,281 to 1,600
|
0.95
|
1,601 to 1,920
|
0.92
|
1,921 to 2,240
|
0.91
|
B.
Hydraulic grade line for closed conduits.
1.
Computation methods.
a.
The hydraulic grade line is a line coinciding with: the level of
flowing water at any given point along an open channel, or the level
to which water would rise in a vertical tube connected to any point
along a pipe or closed conduit flowing under pressure.
b.
The hydraulic grade line shall be computed to show its elevation
at all structures and junction points of flow in pipes, conduits and
open channels, and shall provide for the losses and the differences
in elevations as required below. Since it is based on design flow
in a given size of pipe or conduit or channel, it is of importance
in determining minimum sizes of pipes within narrow limits. Sizes
larger than the required minimum generally provide extra capacity,
however consideration must still be given to the respective pipe system
losses.
c.
There are several methods of calculating "losses" in storm sewer
design. The following procedures are presented for the engineer's
information and consideration. It is expected that the design will
recognize the reality of such "losses" occurring and make such allowances
as good engineering judgment requires.
(1)
Friction loss. The hydraulic grade line is affected by friction
loss and by velocity head transformations and losses. Friction loss
is the head required to maintain the required flow in a straight alignment
against frictional resistance because of pipe or channel roughness.
It is determined by the equation:
hf = L x Sh
|
Where:
| ||
hf
|
=
|
Differences in water surface elevation, or head in feet in length
L
|
L
|
=
|
Length l feet of pipe or channel
|
Sh
|
=
|
Hydraulic slope required for a pipe of given diameter or channel
of given cross-section and for a given roughness "n", expressed as
feet of slope per foot of length.
|
From Manning's formula: Sh = [V
n/(1.486 R0.667)]2
|
Where:
| ||
R
|
=
|
Hydraulic radius of pipe, conduit or channel (feet) (Ratio of
low area/wetted perimeter)
|
V
|
=
|
Velocity of flow in feet per second (fps)
|
n
|
=
|
Manning's value for coefficient of roughness
|
Use:
| ||
n
|
=
|
.013 for pipes of concrete, vitrified clay, and PVC pipe
|
n
|
=
|
.0112 for formed monolithic concrete, i.e., vertical wall channels,
box culverts and for R.C.P. over 48" in diameter.
|
n
|
=
|
.015 for concrete lining in ditch or channel inverts and trapezoidal
channels
|
n
|
=
|
.020 for grouted riprap lining on ditch or channel side slopes
|
n
|
=
|
.033 for gabion walled channels
|
Note:
|
---|
"n" will have a weighted value for composite lined channels.
|
"n" values for unlined channels to be determined on an individual
basis
|
(2)
Curve loss. Curve loss in pipe flow is the additional head required
to maintain the required flow because of curved alignment, and is
in addition to the friction loss of an equal length of straight alignment.
It should be determined from Figure 4-2 which includes an example.
(3)
Entrance loss at terminal inlets. Entrance loss is the additional
head required to maintain the required flow because of resistance
at the entrance. The entrance loss at a terminal inlet is calculated
by the formula:
Hti = (V2/2g)
|
Where:
| ||
V
|
=
|
Velocity in flow of outgoing pipe
|
g
|
=
|
Acceleration of gravity (32.2 Ft./Sec/Sec)
|
(4)
Turn loss. Head losses in structures due to change in direction
of flow (turns) in a structure, will be determined in accordance with
the following:
Change in Direction of Flow (A)
|
Multiplier of Velocity Head of Water Being Turned (K)
|
---|---|
90°
|
0.7
|
60°
|
0.55
|
45°
|
0.47
|
30°
|
0.35
|
0°
|
0.0
|
DIAGRAM:
MANHOLE OR INLET LATERAL OR MAIN
Formula: HL = K(VL)2/2g
|
Where:
| ||
HL
|
=
|
Feet of head lost in manhole due to change in direction of lateral
flow
|
VL
|
=
|
Velocity of flow in lateral, (Ft./Sec)
|
g
|
=
|
Acceleration of gravity, (32.2 Ft./Sec/Sec)
|
K
|
=
|
Multiplier of Velocity Head of water being turned
|
(5)
Junction chamber loss.
(a)
A sewer junction occurs for large pipes or conduits too large
to be brought together in the usual forty two inch diameter manhole
or inlet where one or more branch sewers enter a main sewer. Allowances
should be made for head loss due to curvature of the paths and due
to impact at the converging streams.
(b)
Losses in a junction chamber for combining large flows shall
be minimized by setting flow line elevations so that pipe centerlines
(spring lines), will be approximately in the same planes.
(c)
At junction points for combining for combining large storm flows,
a manhole with a slotted cover shall be provided.
(d)
A computation method for determining junction chamber losses
is presented below:
Hj = Δy + Vh1 -
Vh2
|
Where:
| ||
Hj
|
=
|
Junction chamber loss (ft.).
|
Vh1
|
=
|
Upstream velocity head.
|
Vh2
|
=
|
Downstream velocity head.
|
Δy
|
=
|
Change in hydraulic grade line through the junction in feet.
|
Where:
| ||
Δy
|
=
|
□(Q2V2) -
{(Q3V3) - (QnVnCos en)}]\0.5(A1 + A2)g
|
Where:
| ||
Q2
|
=
|
Discharge in cubic feet per second (cfs) at the exiting conduit.
|
V2
|
=
|
Velocity in feet per second (fps) at the exiting conduit.
|
A2
|
=
|
Cross sectional area of flow in square feet for the exiting
conduit.
|
Q1
|
=
|
Discharge in cfs for the incoming pipe (main flow).
|
V1
|
=
|
Velocity in fps for the incoming pipe (main flow).
|
A1
|
=
|
Cross sectional area of flow in Square Feet. For the incoming
pipe (main flow).
|
Q3Qn
|
=
|
Discharge (s) in cfs for the branch lateral (s).
|
V3Vn
|
=
|
Velocity (ies) in fps for the branch lateral (s).
|
e3en
|
=
|
The angle between the axes of the exiting pipe and the branch
laterals(s).
|
g
|
=
|
Acceleration of gravity (32.2 ft./sec/sec)
|
Where:
| ||
e
|
=
|
The angle between the axes of the outfall and the incoming laterals.
|
The terms within [ ] indicate the summation of all incoming
laterals (not on main sewer).
|
(6)
Losses at junctions of several flows in manholes and/or inlets.
(a)
The computation of losses in a manhole, inlet or inlet manhole
with several flows entering the structure should utilize the principle
of the conservation of energy. This involves both the elevation of
water surface and momentum (mass times the velocity head). Thus, at
a structure (manhole, inlet or inlet manhole). Thus, at a structure
(manhole, inlet or inlet manhole) with laterals, the sum of the energy
content for inflows is equal to the sum of the energy content of the
outflows plus the additional energy required by the turbulence of
the flows passing through the structure.
(b)
The upstream hydraulic grade line may be calculated as follows:
Hu = [VD2/2g] - [Qu/QD) (1-K) Vu2/2g)]
- [QL1/QD) (1-K) VL12/2g] +[(QLN/QD) (1-K) (VLN2/2g)] + HD
|
Where:
| ||
HU
|
=
|
Upstream hydraulic grade line in feet.
|
QU
|
=
|
upstream main line discharge in cubic feet per second.
|
QD
|
=
|
Downstream main line discharge in cubic feet per second.
|
QL1-QN
|
=
|
Lateral discharges in cubic feet per second.
|
VU
|
=
|
Upstream main line velocity in feet per second.
|
VD
|
=
|
Downstream main line velocity in feet per second.
|
VL1-VLN
|
=
|
Lateral velocities in feet per second.
|
HD
|
=
|
Downstream hydraulic grade line in feet.
|
K
|
=
|
Multiplier of Velocity of Water being turned.
|
g
|
=
|
Acceleration of gravity, 32.2 ft./sec/sec.
|
(c)
The above equation does not apply when two almost equal and
opposing flows, each perpendicular to the downstream pipe, meet and
no other flows exist in the structure. In this case the head loss
is considered as the total velocity head of the downstream discharge.
(7)
Transition loss.
(a)
The relative importance of the transition loss is dependent
on the velocity head of the flow. If the velocity and velocity head
of the flow are quite low, the transition losses cannot be very great.
However, even small loses may be significant in flat terrain. The
sewer design shall provide for the consideration of the necessary
transitions and resulting energy losses. The possibility of objectionable
deposits is to be considered in the design of transitions.
(b)
For design purposes it shall be assumed that the energy loss
and changes in depth, velocity and invert elevation, if any, occur
at the center of the transition. These changes shall be distributed
throughout the length of the transition in actual detailing. The designer
shall carry the energy head, piezometric head (depth in an open channel),
and invert as elevations, and work from the energy grade line. Because
of inherent differences in the flow, transitions for closed conduits
will be considered separately from those for open channels.
[1]
Closed conduits.
[a]
Transitions in small sewers may be confined within
a manhole. Special structures may be required for larger sewers. If
a sewer is flowing surcharged, the form and friction losses are independent
of the invert slope; therefore, the transition may vary at the slopes
of the adjacent conduits. The energy loss in a transition shall be
expressed as a coefficient multiplied by the change in velocity head
(ΔV2/2g) in which ΔV is the change in Velocity
before and after the transition. The coefficient may vary from zero
to one, depending on the design of the transition.
[b]
If the areas before and after a transition are
known, it is often convenient to express the transition loss in terms
of the area ratios and either the velocity upstream or downstream.
[c]
For an expansion:
HL = K(V1-V2)2/2g = [K(V1)2/2g] [1-(A1/A2]2
|
In which HL is the energy loss;
K is a coefficient equal to 1.0 for a sudden expansion and approximately
0.2 for a well-designed transition and the subscripts 1 and 2 denote
the upstream and downstream sections, respectively, i.e., A1 = Area Before Transition and A2 = Area After Transition.
[d]
For a contraction:
HL = [K(V2)2/2g] [(1/Cc)-1]2 = [K(V2)2/2g] [1-(A2/A1)]2
|
In which K is a coefficient equal to 0.5 for a well-designed
transition, Cc is a coefficient of contraction,
and the other terms and subscripts are similar to the previous equation.
Losses in closed conduits of constant area are expressed in terms
of (V2/2g).
[e]
The above equations may be applied to approximate
the energy loss through a manhole for a circular pipe flowing full.
If the invert is fully developed, that is, semi-circular on the bottom
and vertical on the sides from 1/2 depth up to the top of the pipe,
for the expansion (A1/A2) = 0.88, and for the contraction (A2A1) =
0.88. The expansion is sudden; therefore, K = 1. The contraction may
be rounded if the downstream pipe has a bell or socket. In this case,
K may be assumed to be 0.2.
[f]
The expansion energy loss is 0.014 [(V1)2/2g] and the contraction energy loss is 0.010 [(V2)2/2g]. If the invert is
fully developed, the manhole loss is small, but if the invert is only
developed for 1/2 of the depth, or not at all, the losses will be
of considerable magnitude.
[2]
Open channel transitions. The hydraulics of open
channel transitions are further complicated by possible changes in
depth. As a first approximation to the energy loss, unless a jump
occurs, the equations given above may be used with a trial-and-error
solution for the unknown area and velocity. The K value for a well-designed
expansion should probably be increased to 0.3 or 0.4 Whether the properties
of the upstream or downstream section will be known will depend on
the characteristics of the flow and the channel, but can be determined
by a profile analysis. In transitions for supercritical flow, additional
factors shall be considered. Standing waves of considerable magnitude
will be produced in transitions. The height of these waves must be
estimated to provide a proper channel depth. In addition, in long
transitions, air entrainment will cause bulking of the flow with resultant
greater depths of the air-water mixture.
C.
Hydraulic grade line limits.
1.
The hydraulic grade line shall not rise above the following limits as determined by flow quantities calculated per § 48.030A.
a.
The hydraulic grade line at any inlet shall not be higher than six
inches below the inlet sill. The hydraulic grade line at any storm
manhole shall be no higher than one foot below the top.
b.
Storm sewers shall not flow with greater than three feet of head
without special pipe joint requirements.
2.
Inlets function entirely as entry points for stormwater flow. They
also may be constructed to serve as a manhole on separate stormwater
sewers, and are then termed inlet-manholes. Steep gradients may give
such low inlet capacities that additional inlets should be located
at more favorable grade locations or special inlets designed for steep
gradients must be used. Provisions must be made to control by-pass
flow and to provide additional capacity in the inlet and line affected
by such increased flow. Six inch open throat inlets should be used
at all times.
3.
Grated inlets, without an open throat or other provision for overflow
shall be avoided except under exceptional conditions, and are prohibited
in grade pockets. Any exceptions shall be used only with the City's
approval.
4.
Curb inlets shall be placed at street intersections or driveways
such that no part of the inlet structure or sump is within the curb
rounding.
a.
Inlets are shown in the MSD Standard Details of Sewer Construction,
latest edition. The minimum depth of a terminal inlet is four feet
from the top of the inlet to the flow line of the outlet pipe. Greater
depth shall be used for intermediate inlets if necessary for the required
depth of the hydraulic grade line.
b.
Inlet capacity should not be less than the quantity flow tributary
to the inlet and by-pass flow shall be avoided whenever possible.
c.
Inlets at low points or grade pockets should have extra capacity
to compensate for possible flow by-pass of upstream inlets.
d.
Figure 4-3 shows inlet capacity/maximum gutter capacity with a given
gutter line grade and flow.
e.
Connections to existing structures may require rehabilitation or
reconstruction of the structure being utilized. This work will be
considered part of the project being proposed.
D.
Open channels. All open channels shall meet the following requirements:
1.
Size and shape. Open channels shall not decrease in size in the direction
of flow. Open channels shall be vertical and constructed of reinforced
concrete or other materials approved by the City Engineer. Where possible
the bottom of open channels should be constructed of pervious materials
and protected with non-armored erosion protection. All open channel
designs shall be approved by the City Engineer.
2.
Materials. All materials used in construction of open channels shall
be approved by the City Engineer.
3.
Bedding. Special provisions shall be made for channels or paved swales
laid over fill on non-supportive soils to support the channel on paved
swales. Pipes extended to the channel in a fill area shall have compacted
crushed limestone bedding for support.
4.
Structural considerations. Provisions must be made for all loads
on the channel.
5.
Alignments. Open channel alignments may be limited by available easements,
physical topography, existing utilities, buildings, residential development,
maintenance access and roadways.
6.
Locations.
a.
Storm channel locations are determined primarily by natural drainage
conditions. It is also necessary to consider accessibility for construction
and maintenance, site availability and competing uses, and evaluating
effects of easements on private property.
b.
Storm channels shall be located:
(1)
To serve all adjacent property conveniently and to best advantage.
(2)
In easements or rights-of-way dedicated to the City.
(3)
In easements on common ground when feasible.
(4)
On private property along property lines or immediately adjacent
to public streets, avoiding crossings through the property.
(5)
At a sufficient distance from existing and proposed buildings
and underground utilities or sewers to avoid future problems of flooding
or erosion.
(6)
To avoid interference between stormwater sewers and house connections
to foul water or sanitary sewers.
(7)
In unpaved or unimproved areas whenever possible.
(8)
Crossing perpendicular to streets, unless unavoidable.
7.
Flow line. The flow line of open channels shall meet the following
requirements.
a.
Gradient changes shall be kept to a minimum and be consistent and
regular.
b.
Gradient designations less than the nearest 0.001 foot per foot shall
be avoided.
c.
Channel and swale depths shall be determined primarily by the requirements
of the channel size, utility obstructions and any required connections.
8.
Other open channel considerations and requirements.
a.
All natural channels and ditches shall be improved unless otherwise
authorized by the City.
b.
Drainage within private property should be controlled to prevent
damage to the property crossed. Swales, or broad shallow grass lined
ditches with non-erosive slopes, are generally located at or near
rear lots and along common property lines. If erosion protection is
necessary, a non-armored type of erosion protection is preferred by
the City (where feasible). All erosion protection shall be approved
by the City Engineer.
c.
Drainage channels and water courses draining through a subdivision
shall be enclosed if the required pipe size does not exceed 60 inches.
When it is undesirable or impractical to enclose a channel with a
pipe across a road or street, a suitable bridge or culvert shall be
required.
d.
For flows greater than four cfs, area inlets or inlet manholes are
required to intercept the gutter or swale flow.
e.
All improved concrete channels shall have a forty eight inch chain
link fence on each side of the channel, or other protective measures
as directed by the City.
f.
Channels and water courses draining large areas shall be located
in rights-of-way or easements previously approved by the City as a
part of an adequate overall plan for drainage.
9.
Design limitations.
b.
If the channel is within an area designated in a community's
flood insurance study, then the channel shall also meet all of the
City at Moscow Mills floodplain requirements.
c.
Other agencies of jurisdiction may have requirements which must be
met. An Army corps of Engineers permit may be required for any construction
affecting a watercourse.
10.
Hydraulic grade line.
a.
Computation methods.
(1)
In open channels the water surface is identical with the hydraulic
grade line. The hydraulic grade line shall be computed throughout
the channel each to show its elevation at junctions with incoming
pipes or channels and at the ends of the channel reach under consideration.
It shall also provide for the losses and differences in elevations
as required below. Since it is based on design flow in a given channel,
it is of importance in determining minimum sizes within narrow limits.
The depth at which the actual flows will occur is controlled by the
two end conditions of the reach considered, and by the relationship
between the energy available and by the energy required to overcome
the losses that are encountered along the channel.
(2)
There are several methods of calculating "losses" in channel
design. The following procedures are presented for the engineer's
information and consideration.
(3)
It is required that the design recognize the reality of such
"losses" occurring and make such allowances as good engineering judgment
indicates.
(a)
Control sections. The engineer should locate all
possible control sections for the reach in question. A control section
refers to any section at which the depth of flow is known or can be
controlled to a required stage. At the control section, flow must
pass, through a control depth which may be the critical depth, the
normal depth or any other known depth. Three types of control sections
include (a) Upstream Control Section; (b) Downstream Control Section;
Artificial Control Section, which occurs at a control structure, such
as a weir, dam, sluice gate, roadway embankment, culvert, bridges
or at the confluence with a major river or stream.
(b)
Friction loss. The friction loss may be calculated by the same procedure as is presented in § 48.030B of this chapter.
(c)
Flow in curved channels.
[1]
The centrifugal force caused by flow around a curve
reduces a rise in the water surface on the outside wall and a lowering
of the inner wall. This phenomenon is called superelevation. The flows
tend to behave differently according to the state of flow.
[2]
In sub critical flow, fiction effects are of importance,
whereby in supercritical flow, the formation of cross-waves is of
major concern.
[a]
Curve losses. Curve losses may be estimated from
Figure 4-2 by replacing D, diameter, with b, width of channel.
[b]
Superelevations. In addition to curve losses, an
evaluation of superelevations should be considered and, if required,
an allowance made in the top elevation of outside wall. Equations
are presented below which may be used to determine the superelevation
at channel bends.
{1}
|
Trapezoidal Channels:
|
Subcritical Flow: ΔHW = 1.15 (V2/2grc) [b+(D(ZL+Zr))]
| |
Supercritical Flow: ΔHW=2.6 (V2/2grc) [b+(D(ZL+ZR))]
| |
{2}
|
Rectangular Channels:
|
Sub critical Flow: ΔHW = (V2b/2grc)
| |
Supercritical Flow: ΔHW = (V2b/2grc)
|
Where:
| ||
ΔHW
|
=
|
Change in water height above the centerline water surface elevation.
|
V
|
=
|
Average velocity of design flow in Fps
|
g
|
=
|
Acceleration of gravity (32.2 Ft./Sec/Sec)
|
rc
|
=
|
Radius of curve on horizontal alignment in feet
|
b
|
=
|
Base width of channel in feet
|
D
|
=
|
Depth of flow in straight channel
|
ZL
|
=
|
Left side slope (ft./ft.)
|
ZR
|
=
|
right side slope (ft./ft.)
|
(d)
Transitions.
[1]
Transitions should be designed to accomplish the
required change in cross section with as little flow disturbance as
possible.
[2]
The following features are to be considered in
design of transition structures.
[a]
Proportioning.
{1}
|
The optimum maximum angle between the channel axis and a line
connecting the channel sides between the entrance and exit sections
is 12.5°.
|
{2}
|
Sharp angles in the structure should be avoided.
|
[b]
Losses. The energy loss in a transition consists
of the friction loss and the conversion loss. The friction loss may
be estimated by the Manning Formula. The conversion loss is generally
expressed in terms of the change in velocity head between the entrance
and exit sections of the structure.
Ht = Kt ΔVH
|
Where:
| ||
Ht
|
=
|
Conversion Loss
|
Kt
|
=
|
Coefficient of head loss in transition
|
ΔVH
|
=
|
Absolute change in velocity head
|
Average design values for Kt are
presented in the table Below:
Type of Transition
|
Contraction Section
|
Expanding Section
|
---|---|---|
Warped
|
0.10
|
0.20
|
Wedge
|
0.20
|
0.50
|
Cylinder-Quadrant
|
0.15
|
0.25
|
Straight Line
|
0.30
|
0.50
|
Square End
|
0.40
|
0.75
|
See Figure 4-4 for sketches of each type of transition.
|
[c]
Freeboard. A transition shall have a minimum of
one foot of freeboard above the hydraulic grade line.
[d]
Hydraulic jump. The existence of a hydraulic jump
in a transition may become objectionable, and the design of the transition
should be checked for such.
[e]
Sudden enlargement and contraction. A sudden enlargement
results when an intense shearing action occurs between incoming high-velocity
jet and the surrounding water. As a result, much of the Kinetic energy
of the jet is dissipated by eddy action. The head loss at a sudden
enlargement, HLe, is:
HLe = Ke(ΔV2/2g)
|
Where:
| ||
Ke
|
=
|
Coefficient of had loss for enlargements = 1.
|
ΔV
|
=
|
Change in velocities between incoming and outgoing sections.
|
G
|
=
|
Acceleration of gravity (32.2 Ft./Sec/Sec).
|
The flow in a sudden contraction is first contracted
and then expanded resulting I high losses as compared to a sudden
enlargement. Thus the head loss at a sudden contraction, HLe, is:
HLc = Kc(ΔV2/2g)
|
Where:
| ||
Kc
|
=
|
Coefficient of head loss for contractions - 0.5.
|
ΔV
|
=
|
Change in velocities between incoming and outgoing sections.
|
g
|
=
|
Acceleration of gravity, Ft./Sec/Sec.
|
(e)
Constrictions. A constriction results in a sudden
reduction in channel cross section. The effect of the constriction
on the flow depends mainly on the boundary geometry, the discharge
and the state of flow. When the flow is sub critical, the constriction
will induce a backwater affect that extends a long distance upstream.
If the flow is supercritical, the disturbance is usually local and
will only affect the water adjacent to the upstream side of the constriction.
A control section may or may not exist at a constriction. The control
section, when it exists, may be at either side of the constriction
upstream or downstream), depending on whether the slope of the constricted
channel is steep or mild. The entrance and outlet of the constriction
then acts as a contraction and an expansion, respectfully.
(f)
Obstructions. An obstruction is open-channel flow
creates at least two paths of flow in the channel. Typical obstructions
include bridge piers, pile trestles, and trash racks. The flow through
an obstruction may be sub critical or supercritical.
11.
Hydraulic jump. When flow changes from the supercritical to sub critical
state, a hydraulic jump may occur. A study should be made on the height
and location of the jump, and for discharges less than the design
discharge, to endure adequate wall heights extend over the full ranges
of discharge.
12.
Open channel junctions.
a.
General.
(1)
Consideration shall be given in the design of open channel junctions
to the geometry of the confluence of flows in order to minimize undesirable
hydraulic effects due to supercritical velocities.
b.
Confluence design criteria.
(1)
The momentum equation can be applied to the confluence design
if the below stated criteria is used.
(2)
The design water-surface elevations in the two joining channels
should be approximately equal at the upstream end of the confluence.
(3)
The angle of the junction intersection can vary from 0°
to 12°.
(4)
The width of the main channel shall be expanded below the junction
to maintain approximate flow depths throughout the junction.
(5)
Flow depths should not exceed 90% of the critical depth.
13.
Erosion protection. A Rock blanket, minimum one-foot thick, shall
be required at each end of the improved channel. The minimum length
of the rock blanket shall be 25 feet. A rock toe wall, minimum two-foot
deep, shall be constructed at the free end of each blanket.
14.
Sanitary sewer crossings. The characteristics of any sanitary sewer
crossing shall be given consideration in the design of the channel
floor.
E.
Culverts.
1.
The design of culverts shall include consideration of many factors
relating to requirements of hydrology, hydraulics, physical environment,
imposed exterior loads, construction and maintenance.
2.
With the design discharge and general layout requirements determined,
the design requires detailed consideration of such hydraulic factors
as shape and slope of approach and exit channels, allowable head at
entrance (and ponding capacity, if appreciable), tail water levels,
hydraulic and energy grade lines, and erosion potential.
a.
Hydraulic design. The hydraulic design of a culvert for a specified
design discharge involves (1) selection of a type and size, (2) determination
of the position of hydraulic control, and (3) hydraulic computations
to determine whether acceptable headwater depths and outfall conditions
will result. Hydraulic computations will be carried out by standard
methods based on pressure, energy, momentum and loss considerations.
b.
Entrances and headwalls - outlets and end walls. Where an existing
culvert is to be extended, the possibility for maintaining or improving
existing capacity should be investigated. Marked improvement may be
obtainable by proper entrance design. All culverts shall be designed
for possible extension unless there are extenuating circumstances.
Bridges shall be designed to meet the current criteria of the
governing agencies.
A.
Waterway capacity and backwater effects. Sufficient capacity will
be provided to pass the runoff from the design storm determined in
accordance with principles given elsewhere in this manual.
B.
Clearance. The lowest point of the bridge superstructure shall have
a (freeboard) clearance of two feet above design water surface elevation
for the fifteen-year frequency and one foot for the 100-year frequency.
C.
Waterway alignment. The bridged waterway will be aligned to result
in the least obstruction to stream-flow, except that for natural streams
consideration will be given to future realignment and improvement
of the channel.
D.
Erosion protection. To preclude failure by scouring, abutment and
pier footings will usually be laced either to a depth of not less
than five feet below the anticipated depth of scour, or on firm rock
if such is encountered at a higher elevation. Large multispan structures
crossing alluvial streams may require extensive pile foundations.
To protect the channel, revetment on channel sides and/or bottom,
consisting of concrete or rock blanket should be placed as required.
The governing authority should be contacted regarding their design
requirements.
A.
If outlet velocities exceed 5 f.p.s., an appropriate erosion protection
must be provided. Erosion protection may be required at outlets where
velocities are less than 5 f.p.s. if soil conditions warrant.
B.
All erosion protection shall be approved by the City Engineer. Non-armored
erosion protection is preferred.
Area inlets shall be required to intercept overland flows greater
than one cfs to prevent that flow from crossing sidewalks of curbs.
Any area which is to be paved, repaved, expanded or otherwise
improved, that is over 3,000 square feet in area, whether presently
paved or not, shall at such time as it is to be paved, repaved, expanded
or be otherwise improved, be provided with storm water drainage facilities
constructed in accordance with plans and specifications submitted
to and approved by the City.
A.
Requirements and submittals.
1.
The requirement of stormwater detention shall be evaluated for all
projects submitted to the City for plat approval. The Planning and
Zoning Commission may request review by the City's engineer prior
to plat approval. The applicant for plat approval shall reimburse
the City for any engineer fees incurred as a result of said review.
Detention facilities shall be designed and provided as required by
the City's engineer.
2.
A Storm Water Drainage and Detention Design Report, with three extra
copies, shall be submitted with all applications for plat approval.
Said report shall contain a written summary and all hydraulic calculations.
The Report shall be signed, dated and sealed by the Missouri Professional
Engineer who is responsible for its preparation.
3.
The City's Superintendent of Public Works may issue a written
stop work order to any person who is found violating the provisions
of this chapter or otherwise failing to conform to their submitted
plans or the City's specifications for storm drainage facilities.
Violation of a stop work order shall be unlawful and shall constitute
a misdemeanor and shall be punished by a fine of not less than $1
and not more than $500 or by imprisonment not exceeding 90 days or
both, recoverable with cost of court. Each day of violation shall
constitute a separate offense.
B.
Design considerations.
1.
The rates (pre-developed and post developed) of runoff are determined
by the Rational Method for the two-, five-, ten-, and fifteen-year
frequencies, with twenty-minute rainfall intensity. The City reserves
the right to require higher rainfall frequencies and intensities as
downstream conditions (problems) would warrant it.
2.
Stormwater shall be detained on site or off site as approved and
released at the rate of an existing pre-development site for two-,
five-, ten- and fifteen-year rainfall event, unless downstream conditions
(problems) warrant otherwise. Note that the stormwater pipes, downstream
from control structures, shall be sized to carry the total tributary
upstream watershed. No reduction in outfall pipe size shall be permitted
because of detention.
3.
The volume of detention may be provided through permanent detention
facilities such as dry basins or ponds, permanent ponds or lakes or
underground storage facilities, parking lot detention will not be
acceptable. Flows from offsite upstream areas should be bypassed around
the detention facility to ensure that the proposed detention facility
will function as designed and will provide effective control of downstream
flows with development in place. If offsite flows are directed into
a detention facility, the allowable release rates shall not be modified
without the City's approval. Modifying the release rate to accommodate
offsite flows may reduce or eliminate the effectiveness of the detention
facility, because it will no longer control the increased volume of
runoff during the critical time period of the watershed.
4.
Detention basin volume will be based on routing each predeveloped
runoff hydro graph through the detention facility while satisfying
the appropriate allowable release rate. The routing computations shall
be based on an application of the continuity principle.
5.
Detention basins shall be located on common ground and have a minimum
fifteen-foot strip maintenance access for vehicles.
6.
Detention basin shall not be located in floodway.
7.
Detention basin shall have an overflow structure capable of passing
a 100-year, twenty-minute design storm. An emergency spillway, capable
of passing a 100-year, twenty-minute storm event, may also be required
by the City to safely route any basin overflow away from developed
areas to a natural drainage channel.
8.
Design of underground Detention system.
a.
Adequate access for basin maintenance and inspection shall be provided.
A means of visual inspection from the ground surface of the low flow
device, overflow weir, and outlet structure is necessary. Access shall
also be provided to allow for cleaning of the low flow device from
the ground surface.
b.
The basin should have sufficient volume and spillway capacity to
pass/contain the 100-year twenty-four-hour event with the low flow
outlet blacked. In some situations it may be desirable to have control
structures with at least two outlet openings, one above the other.
c.
Underground detention facilities shall be acceptable for non-residential
projects only, unless pre-approved by City.
d.
Acceptable materials for underground detention facilities are aluminized
corrugated metal pipe, gauge of pipe approved by City engineer and
polyethylene (PE) pipe.
e.
Provide immediate manhole access from ground surface for both sides
of the low flow device. Also provide a manhole at upstream end of
underground basins, for access, inspection, to facilitate maintenance
and air release.
f.
Adequate flow line spot elevations, sections and profiles including
pipe length and slope shall be labeled to define basin and pipe geometry.
9.
The Engineer must submit the following for review of a detention
facility:
a.
Elevation vs Discharge tables or curves for all frequencies.
b.
Elevation vs Storage tables or curves for all frequencies.
c.
Inflow calculations and data for all frequencies.
d.
Hydraulic grade line computations for pipes entering and leaving
the basin for all frequencies.
e.
If the embankment contains fill material a geotechnical report may
be required.
f.
Site plan showing appropriate design information.
g.
Structural calculations for the outlet control structures (if required).
10.
All pipes discharging into a dry basin or pond shall be reinforced
concrete pipe with a concrete flared-end, concrete toe and head wall
with rip-rap as per MSD standard flared-end detail. PAVED SWALES CONNECTING
THE INLET PIPE WITH THE OUTFLOW CONTROL STRUCTURE WILL NOT BE ALLOWED.
11.
Railroad tie walls cannot be used where water will be in contact
with the railroad tie wall.
12.
Permanent detention ponds or lakes are to be designed to minimize
fluctuating lake levels. Maximum fluctuation from the permanent pool
elevation to the maximum ponding elevation shall be three feet.
13.
The maximum side slopes for basins or ponds, and the fluctuating
area of permanent ponds or lakes shall be 3:1 (three feet horizontal,
one foot vertical) without fencing.
14.
Dry basins or ponds are to be sodded. The slopes are to be kept mowed.
The bottoms are to be allowed to grow (unmowed).
15.
Control structures and overflow structures are to be reinforced concrete.
16.
The outflow pipe shall be sized for the developed flow rate.
17.
In basins with concrete walls or rock blanket covered slopes, the
bottoms shall be left unmowed. This will allow for better infiltration
of water into the ground.
C.
Maximum depths.
1.
The maximum depth of water in a dry detention basin or pond shall
not exceed eight feet. Projects which need a deeper basin to attain
the required detention volume due to physical constraints may be evaluated
on a case-by-case basis. The design and construction of dams greater
than eight feet must be sealed and certified by a Professional Engineer
registered in the State of Missouri with demonstrated expertise in
geotechnical engineering.
D.
Limits of maximum ponding.
1.
The limits of maximum ponding elevation shall be calculated based
on a routing of the design storm assuming the low flow outlet is blocked
with water ponded to the overflow structures sill.
2.
The limits of maximum ponding elevation in dry basins or ponds and
permanent lakes or ponds shall not be closer than 30 feet horizontally
to any building and not less than two feet vertically below the lowest
sill elevation of any building.
3.
A minimum of two feet of freeboard shall be provided from the top
of the basin to the maximum ponding elevation.
E.
Easement required. In subdivisions, the detention basin, access roads
or paths, control structures and outfall pipes are to be located in
easements dedicated to the subdivision trustees.
F.
Maintenance agreement. The owner(s) of the project shall execute
a City Maintenance Agreement for the detention basin; pond or Underground
Facilities to ensure the detention area will be kept in working order,
prior to plan approval. The City will not be responsible for maintenance
of detention basins or Underground Detention Facilities.
G.
Detention basin fencing. A four foot (minimum height) approved fence
shall be provided around the perimeter of any basin where the side
slopes exceed 3:1 (three feet horizontal, one foot vertical).
H.
Detention basin elevation. The low elevation of the detention basin
shall be above the fifteen-year, twenty-minute hydraulic elevation
of the receiving channel or pipe system.
Dams with a height of 35 feet or greater will require approval
from the Missouri Department of Natural Resources.
A copy of all pertinent Federal, State and County permits shall
be submitted to the City before final approval. They shall include
but not be limited to the following: