[Ord. No. 1620 §1, 9-25-2006]
A.
Design Factors. Sanitary sewers should be designed for the
ultimate tributary population. Due consideration should be given to
current zoning regulations and approved planning and zoning reports
where applicable. Sewer capacities should be adequate to handle the
anticipated maximum hourly quantities of sewerage and industrial waste,
together with reasonable consideration given to infiltration/inflow.
B.
Sewer Design. Sewers shall be designed for the total tributary
areas using the following minimum criteria: (Note: Using this criteria
all pipes are to be sized flowing full.)
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Interceptors and trunk lines
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0.01 CFS/Acre
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Laterals and submains
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0.02 CFS/Acre
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C.
Maximum Size. The diameter of sewers proposed shall not
exceed the diameter of the existing or proposed outlet, whichever
is applicable, unless otherwise approved by the City's consulting
engineer.
D.
Minimum Size. No public sewer shall be less than eight (8)
inches in diameter. Stub lines for service connections shall not be
less than four (4) inches in diameter and shall be extended at a ninety
degree (90°) angle to the main sewer line.
E.
Materials Of Construction.
1.
Sanitary sewers shall be constructed of pipe material resistant to
or protected from bacterial degradation, acid and alkaline solutions,
normal sewer temperature variation, abrasion and industrial wastes
or other material that may be transmitted by the collection system.
2.
The following types of commercial pipe are approved for gravity sanitary
sewer systems constructed in the City of Hermann. HDPE and fiberglass
reinforced pipe may be used for special applications only after review
and approval of the design drawings and specifications by the City's
consulting engineer.
PVC Pipe — ASTM D3034, Type PSM Polyvinyl (Chloride),
SDR 35; PVC material shall conform to ASTM D and shall have a cell
classification of 12454-B, 12454-C or 13364-B. The minimum pipe stiffness
for pipe used for stub lines shall be SDR 35.
F.
Minimum Slope. All sewers shall be designed to give mean
velocities when flowing full of not less than two (2.0) feet per second.
1.
All velocity and flow calculations shall be based on the Manning
Formula using an "n" value of thirteen-thousandths (0.013). The following
slopes shall be minimum for the size indicated.
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Minimum Slope in Percent
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Sewer Size
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Full and Half Full Flow
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8 inches
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0.40
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10 inches
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0.28
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12 inches
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0.22
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15 inches
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0.15
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18 inches
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0.12
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21 inches
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0.10
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24 inches
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0.08
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Exceptions to these minimum slopes shall be made at the upper
end of the lateral sewers serving fewer than thirty (30) houses. Said
sewers shall have a minimum slope of 0.76%.
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Where lateral sewers serve less than ten (10) houses, the minimum
slope shall not be less than 1% unless otherwise approved by the City's
consulting engineer.
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G.
Increasing Pipe Size. When a sewer joins a larger one, the
invert of the larger sewer should be lowered sufficiently to maintain
a continuous energy gradient.
H.
High Velocity Protection. In situations where flow is continuous
and grit is a problem and where velocities greater than ten (10) feet
per second are possible, special provisions shall be made to protect
against abrasion damage to the pipe. Such protection may be attained
utilizing ductile iron pipe.
I.
Alignment. All sewers shall be laid with straight alignment
between manholes.
J.
Manhole Construction. Manholes shall be installed at the
end of each line; at all changes in grade, size or alignment; at all
intersections; and at a distance not greater than four hundred (400)
feet for sewers fifteen (15) inches or less in diameter and not greater
than five hundred (500) feet for larger sewers.
K.
Manholes. The construction of all manholes shall conform
to the details shown on the standard drawings.
1.
The minimum horizontal clear distance within the barrel of standard
manholes shall not be less than four (4) feet. Manholes with connecting
pipe diameters greater than twenty-four (24) inches shall have a minimum
inside clear dimension of five (5) feet.
2.
Drop manholes should be avoided as much as possible. However, an
outside or inside drop pipe shall be provided for a sewer entering
a manhole at an elevation of twenty-four (24) inches or more above
the manhole invert. The outside drop pipe shall be protected against
breaking or settling by the use of concrete encasement. The drop pipe
shall have the same nominal diameter as that of the incoming sewer.
The minimum diameter of an inside drop type manhole must be increased
to five (5) feet.
3.
Without utilizing drop manholes, the difference in elevation between
the invert of any incoming sewer and the invert of the outgoing sewer
should not exceed twenty-four (24) inches except where required to
match crowns. When a sewer joins a larger one, the crown of the smaller
sewer shall not be lower than the crown of the larger one. The minimum
drop through manholes shall be two tenths (0.2) feet.
5.
All other sanitary sewer lines (sewer lines across unplatted land,
etc.) shall have the tops of manholes set flush with the existing
ground elevation. The top of all manholes shall be located above the
100-year floodplain.
6.
Any variation from the above top of manhole criteria will require
a letter of explanation to be submitted with the drawings and be subject
to approval by the City's consulting engineer.
L.
Sewer Locations. Sanitary sewers shall be located within
street or alley rights-of-way (outside pavement limits) unless topography
dictates otherwise. When located in easements on private property,
access shall be provided to all manholes. A manhole shall be provided
at each street or alley crossing.
1.
End lines shall be extended to provide access from street or alley
rights-of-way where possible. Imposed loading shall be considered
in all locations.
2.
Not less than four (4) feet of cover shall be provided over the top
of pipe in street and alley rights-of-way and five (5) feet in all
other areas unless otherwise approved by the City's consulting engineer.
M.
Cleanouts And Lampholes. Cleanouts and lampholes will not
be permitted.
N.
Protection Of Water Supplies. There shall be no physical
connection between a public or private potable water supply system
and a sewer or appurtenance thereto which would permit the passage
of any wastewater or polluted water into the potable water supply.
1.
Horizontal separation. Sewer mains shall be laid
at least ten (10) feet horizontally from any existing or proposed
water main. The distances shall be measured edge to edge. In cases
where it is not practical to maintain a ten (10) foot separation,
the engineer may allow deviation on a case-by-case sewer closer to
a water main, provided that the water main is in a separate trench
or on an undisturbed earth shelf located on one (1) side of the sewer
at such an elevation that the bottom of the water main is at least
eighteen (18) inches above the top of the sewer.
2.
Crossings. Sewers crossing water mains shall be
laid to provide a minimum vertical distance of eighteen (18) inches
between the outside of the water main and the outside of the sewer.
This shall be the case where the water main is either above or below
the sewer. The crossing shall be arranged so that the sewer joints
will be equidistant and as far as possible from the water main joints.
Where a water main crosses under a sewer, adequate structural support
shall be provided for the sewer to prevent damage to the water main.
3.
Special conditions. When it is impossible to obtain
proper horizontal and vertical separation as stipulated above, the
sewer shall be designed and constructed equal to water pipe and shall
be pressure tested to assure water-tightness prior to backfilling.
4.
No water pipe shall pass through or come in contact with any part
of a sewer or a sewer manhole.
O.
Aerial Crossings. Adequate support shall be provided at
all joints in pipes utilized for aerial crossings. Only ductile-iron
pipe with restrained joints shall be used unless otherwise approved
by the City's consulting engineer.
