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.
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.
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.
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:
A. 404 permit from Corp. of Engineers-(land disturbance).
B. 401 permit from MDONR-(land disturbance).
C. Sewer line extension permit from MDONR.
D. Water line extension permit from MDONR.