9.02 PERMITTED FLOWS:
A. No sanitary sewers, sanitary sewer services or septic tanks shall be discharged into storm sewers.

B. No flows from commercial car washes shall be discharged into storm sewers.

C. Sump pump discharge of ground and surface water is permitted.

D. Only ground water is permitted in sump pump discharge tiles less than 12 inches in diameter. No surface water or roof drains are permitted in these systems.

9.03 DESIGN FLOWS AND CAPACITY:
A. Storm sewer systems consist of pipe and overland flow routes to convey stormwater. All storm sewer designs shall contain both components.

B. Storm sewer pipes shall be designed to convey the five-year return frequency flow except for pipe used to discharge water from stormwater management facilities (see 9.12).

If the five-year return frequency flow exceeds the capacity of a 48-inch pipe, open channel flow may be used in lieu of an enclosed system upon approval of the City Engineer. Generally, this will be approved only where a stream or creek exists.

C. The overland flow route shall be designed to convey the 100-year return frequency flow assuming the pipe is inoperative. Such routes shall be designed to convey the flows without damage to property and shall be clearly designated on the construction drawings and final plat or site plan.

D. All design flows shall be calculated using commonly accepted engineering practices appropriate for the size of the drainage area under consideration. All designs shall consider existing and fully developed conditions and use the larger of the two flows.

E. The minimum size for any storm sewer receiving surface water is 12 inches in diameter, excluding stormwater management control structures. The minimum size for subsurface drainage pipes receiving only ground water and stormwater management control structures, is 6 inches in diameter.

F. All storm sewer pipes shall have a slope which will give a mean velocity when flowing full of not less than 2.0 feet per second based on Manning's formula using a minimum "n" value of 0.013.

G. Where velocities in a pipe are greater than 15 feet per second are calculated, special provisions shall be made to protect against displacement, abrasion or shock.

H. All area intakes (excluding street intakes) shall include trash racks and be designed assuming 25% blockage by debris.

I. The maximum slope for storm sewer outlet lines shall be 10%.

• If RCP is utilized for outlet lines having a slope greater than or equal to 8%, all joints must be tied together.

• The flared end section and last two pipe joints shall be tied together in all instances.

9.04 OPEN CHANNEL FLOW
A. The erosion potential of the soil shall be evaluated with regard to anticipated velocities. Appropriate measures shall be taken to protect the soil and/or reduce velocities to prevent erosion.

B. The channel capacity in conjunction with possible over-bank flow shall be designed to safely convey the 100 year return frequency flow. If over-bank flow is considered, its impact must be evaluated and area of conveyance protected by easement.

9.05 SUBSURFACE DRAINAGE:
A. In predominantly wet areas, areas containing hydric soils and other such areas designated by the City Engineer, a subsurface drainage system shall be provided to receive the discharge from sump pumps.

B. A 2" service connection manufactured specifically for the pipe being used and approved by the City Engineer shall be provided for each lot abutting the subsurface drainage system.

C. The end of all sump pump services shall be marked with a metal post or #4 reinforcing steel at least 24" in length buried to within one foot of the finished grade.

9.06 INTAKES AND DRAINAGE OF STREETS:
A. All intakes on public streets will be designed to the IDOT Type RA standards.

B. DOT RA-16 intakes are allowed on storm sewer laterals.

C. Intakes shall be spaced so that for a five (5) year storm arterial streets shall have two traffic lanes free of excess water at all times. Local and collector streets shall be designed to have one twelve (12) foot traffic lane free of water. Storm water shall not cross an intersection but can follow a curb around its radius.

D. For a one hundred (100) year storm, the ponding of surface water on local, collector, and arterial streets must not exceed a depth of one (1) foot at the gutter. Water exceeding one (1) foot shall be designed to overflow into an overland flow route. This applies to temporary ponding from local drainage only.

E. Streets in flood plains designated on the FEMA Flood Insurance Maps shall be elevated at their lowest point to the 100-year flood elevation.

F. Minimum drop through intake structures shall be the same as required for manholes (see 9.08 B).

9.07 PIPE STANDARDS AND STRENGTH DESIGN:
A. Reinforced concrete pipe meeting the following standards may be used for pipe 12 inches in diameter or larger.

1. Joints

a. Circular pipe: All joints will be confined O-ring gasket meeting ASTM C443. All pipe 36-inch diameter and smaller will have bell and spigot joints. Pipe larger than 36-inch diameter may have tongue and groove joints. If the hydraulic grade line is less than 6 inches above the top of the pipe during the 100-year event, O ring gaskets may be omitted, provided each joint is wrapped with 24 inches of approved filter fabric.

b. Low head pipe: All joints will be sealed using an approved mastic material and each joint shall be wrapped with 24 inches of approved filter fabric. If the Hydraulic grade line is less than 6 inches above the top of the pipe during the 100-year event, mastic joint material may be omitted.

2. Wall Thickness: Minimum wall thickness will be B-wall as defined in ASTM C 76.

3. The required pipe strength and bedding requirements shall be calculated on a case-by-case basis. The minimum pipe strength shall be Class III as defined in ASTM C 76.

4. Pipe Markings: All pipe shall be marked with the date of manufacture and ASTM class. If quadrant reinforcement is used, the top shall be marked on the outside of the pipe.

5. No lift holes.

6. All RCP shall be manufactured by an IDOT certified facility.

C. Ductile iron pipe meeting the following standards may be used.

1. Joints: Use push on gasketed joints.

2. Thickness: Use Pressure Class 350 standard unless a thicker wall is required because of depth.

3. Lining and Coating:

a. Inside of pipe and fittings: Double thickness cement lining and bituminous seal coat conforming to ANS A21.4.

b. Outside of other pipe and fittings: Standard bituminous coating conforming to appropriate AN Standard.

D. Corrugated PVC with a smooth interior wall meeting the following standards may be used only under the conditions specified below:

1. The pipe shall meet the standards of ASTM F949-90 constructed of resins meeting the requirements of ASTM D1784, cell class 12454B.

2. Size: 12" through 18".

3. Joints: Gasketed.

4. Perforated PVC pipe meeting the standards of paragraph 1 and 2 above may be used provided the pipe is surrounded by a freely draining aggregate or fabric sock.

5. Manhole connections shall be approved by the City Engineer.

6. No corrugated PVC pipe shall be used within public right-of-way or with easements shared with other utilities except for subsurface drainage pipes.

E. Subsurface Drainage Pipes meeting the following specifications may be used for subsurface drainage systems:

1. Corrugated PVC with a smooth interior wall meeting the standards of ASTM F949-90 constructed of resins meeting the requirements of ASTM D1784, cell class 12454B may be used for 6", 8" and 10" diameters.

2. Corrugated HDPE with a smooth interior wall from a manufacturer approved by the City Engineer may be used for 6" diameter.

3. All subsurface drainage pipes shall be perforated and shall be protected by a fabric sock or freely draining aggregate placed around the pipe. Subsurface drainage pipes serving as pavement subdrains shall be surrounded by a freely draining aggregate that extends up to and contiguous with the aggregate base of the pavement.

F. The designer shall specify pipe material, bedding and trench width to withstand anticipated dead and live loads. Minimum pipe bedding shall be Class B.

G. See pipe bedding diagrams, Figure 8.4

H. HDPE (ADS N-12) 4”-36”. Gasketed joints. Crushed stone encasement. Allowed for culverts within right-of-way. Not allowed for storm sewer under or within 5 feet of streets.

9.08 MANHOLE AND INTAKE STANDARDS:
A. Manholes or intakes shall be located as follows:

1. At the end of each line.

2. At all changes in grade, size, or alignment.

3. At all intersections of pipes.

4. At distances not greater than 400 feet for sewers 15 inches or less in diameter and 500 feet for sewers 18 inches to 30 inches in diameter. Greater spacing may be permitted in larger sewers.

5. Flared end sections are required at all storm sewer line outlets.

B. Minimum Drop Access Manholes

1. For the same size pipe with a change in alignment of 45º or less, no drop is required.

2. For the same size pipe with a change in alignment of greater than 45º, or junction of two pipes, a 0.2' drop is required.

3. For the same size pipe with a junction of three or more pipes, a 0.3' drop is required.

4. When a smaller sewer joins a larger one, the invert of the larger sewer shall be lowered sufficiently to match the 0.8 depth point of both sewers at the same elevation.

C. Bedding: All precast manhole bases shall be placed on a minimum of 6 inches of crushed stone bedding.

D. Materials

1. Joints: All joints will be confined O-ring gasket meeting ASTM C443.

2. No lift holes through the entire wall.

3. Mark Class and date of manufacture.

4. Inverts: Precast and cast-in-place inverts must provide a channel at least one-half the depth of the pipe and match the full cross-sectional area of the pipe. All junctions and changes in directions of inverts shall be smooth and rounded to the maximum extent possible to supplement flow through the manholes.

5. Diameter: The minimum diameter for manholes is 48 inches for pipe 24 inches in diameter and smaller, and 60 inches for pipe greater than 24 inches in diameter. 72" diameter manholes may be required for pipes greater than 36" in diameter. Reducers may be used above the chamber section of 60" and 72" manholes. Precast T's may be used on pipes 48" and larger in diameter.

6. Castings shall be Neenah R-1670 non-rocking self-sealing or approved equal and shall have “storm sewer” cast in the cover.

E. Standard manholes, step details, and risers are the same as for sanitary sewers as shown in Figures 8.1, 8.2 and 8.3, except cretex chimney seals are not required.

F. All manholes shall be marked with two metal fence posts to remain in place until landscaping is complete. In subdivisions, the posts shall remain in place until landscaping has been completed by the lot owner.

G. Manholes in paving shall have an interior Cretex manhole chimney seal (or approved equal).

H. Concrete spacer rings, metal shims and bricks shall be the only materials utilized to adjust manhole frame elevations. Mastic or grout must be used to bond and seal materials used for adjustment.

9.09 PROTECTION OF WATER SUPPLIES:
A. There shall be no physical connection between a public or private potable water supply system and a sewer appurtenance which would permit the passage of any sewage or polluted water in the potable supply.

B. Under normal conditions, water mains parallel to sewers shall be placed at least 10 feet horizontally from any sanitary sewer, storm sewer or manhole. Where local conditions prevent this separation, the water main may be laid closer provided the bottom of the water main is at least 18 inches above the top of the sewer and the water main is placed in a separate trench or in the same trench on a bench of undisturbed earth at a minimum horizontal separation of 3 feet from the sewer.

C. Water mains crossing sewer services, storm sewers or sanitary sewers shall be laid to provide a separation of at least 18 inches between the bottom of the water main and the top of the sewer. Where local conditions prevent this vertical separation, the water main shall not be placed closer than 6 inches above a sewer or 18 inches below a sewer under any circumstances. Additionally, one full length of water pipe crossing the sewer shall be centered at the point of crossing so that the water pipe joints will be equal distance as far as possible from the sewer. The water and sewer pipes must be adequately supported and have pressure tight joints. A low permeability soil shall be used for backfill material within 10 feet of the point of crossing.

D. No water pipe shall pass through or come in contact with any part of a sewer manhole. A minimum horizontal separation of 3 feet shall be maintained.

9.10 MINIMUM COVER AND BACKFILL AND BEDDING:
A. The minimum cover for storm sewers shall be 1' below the bottom of pavements and 2' below the surface or non-paved areas. All shallow pipe shall be designed to withstand anticipated live loads.

B. Within public right-of-way, backfill shall consist of crushed stone placed in one foot lifts and compacted to 90% modified proctor density or suitable job excavated material placed in one foot lifts and compacted to 90% Modified Proctor Density. If crushed stone is used, the top 12 inches of backfill shall consist of suitable job excavated materials. Flowable mortar may be used upon approval of mix design by the City Engineer. Sand backfill is not permitted.

C. All other areas - backfill shall consist of suitable job excavated material placed in one foot lifts and compacted to 85% Modified Proctor Density.

9.11 PERFORMANCE & TESTING:
A. All sewers and manholes will be lamped and visually checked prior to acceptance.

B. All tests will be completed after backfill is complete.

C. All flexible pipe 12" in diameter and larger shall have a deflection test conducted after the final backfill has been in place for at least 30 days. No pipe shall exceed a deflection of 5%. The deflection test is to be run using a rigid ball or mandrel having a diameter equal to 95% of the inside diameter of the pipe and the tests shall be performed without mechanical pulling devices.

9.12 SEWER LOCATION AND EASEMENTS:
A. Manholes in street right-of-way must be located in areas which allow direct access by maintenance vehicles. Manholes in areas outside the street right-of-way shall be subject to the approval of the City Engineer in which case access to the manhole shall be along a route in which the transverse slope does not exceed 4% and longitudinal slope does not exceed 12%.

B. Sewers shall be placed a minimum horizontal distance of 1.5 times the depth from potential or existing building sites. Greater separations are desirable.

C. All storm sewers outside public right-of-way shall be placed in an easement for operation and maintenance. Easement width from the center of the pipe shall generally be 1.5 times the sewer depth rounded up to the nearest 5 feet.

D. The minimum easement width is 10 feet.

E. Drainageway easements for overland flow shall be of sufficient width to contain the 100-year flow and as a minimum shall include the bottom width and side slopes of the drainageway and any necessary overbank areas.

9.13 STORMWATER MANAGEMENT FACILITIES:
A. Thresholds:

1. Stormwater management will be required for all new subdivisions and resubdivisions of residential developments larger than 3 acres in size and for commercial and industrial developments larger than 3 acres in size.

2. In developments where the natural drainage is divided into more than one watershed, the individual watershed drainage areas must meet the criteria mentioned in A.1 above before stormwater management is required. However, the designer has to account for the stormwater runoff, which may include easements from downgrade property owners, to ensure public safety and not create property damage.

3. No stormwater management facilities are required if stormwater runoff from a development, up to and including the 100-year storm, can be piped or conveyed in its entirety directly to the Iowa River without significant adverse impact to intervening properties.

4. No stormwater management facilities are required if stormwater runoff from a development, up to and including the 100-year storm, can be piped or conveyed in its entirety directly to Muddy Creek, Clear Creek or Ralston Creek provided:

1. It can be shown by hydrograph analysis that the runoff from the subdivision will arrive at Clear Creek or Ralston Creek sufficiently ahead of the peak flow on Clear Creek or Ralston Creek so as not to increase the peak flow on said creeks, and

2. Flows from the subdivision can be safely conveyed to Clear Creek or Ralston Creek without significant adverse impact to intervening properties.

Similar waivers of stormwater management requirements may be considered on other watersheds on a case by case basis.

5. No stormwater management facilities are required for developments upstream from the north and south branch detention facilities on Ralston Creek. All developments within these waterheads shall be designed to safely convey the 100-year flow from upstream areas assuming no stormwater management.

6. A stormwater management fee may be assessed to developments excluded from the stormwater management requirements by Section 9.13 A; if established by Council.

B. Detention Requirements:

1. New developments which require stormwater management shall be required to detain the difference in the volume between the five-year undeveloped storm and the 100-year developed storm events for their development site. For redevelopment of a site that does not presently have stormwater management. The undeveloped condition shall be calculated assuming pasture conditions.

2. The maximum release rate for detention calculations shall be the five-year undeveloped storm.

C. Methodology:

1. The SCS TR-20 computerized runoff volume program or other technically proven method, shall be used to determine the volume of runoff which must be detained.

2. The results of these calculations shall be submitted on a standardized form which shall include all of the individual parameters that the designer inputs into the program.

D. Locational Criteria:

1. Regional stormwater management facilities are encouraged.

2. Regional stormwater management facilities which are of sufficient size may be deeded to and maintained by the City. The conditions for City ownership will be reviewed on a case by case basis.

3. Stormwater detention is not encouraged within any front or side yard setbacks required by building code, or within 25 feet from the estimated back building line. Location is subject to approval by the City.

E. Design Requirements, Dry-bottomed Detention Facilities:

1. Dry-bottomed detention facilities shall be oversized by 10% to help offset anticipated sedimentation prior to total watershed development.

2. Maximum side slopes of dry bottom facilities shall not exceed 4:1.

3. Low flow pipes are required to convey low flows from storm sewer outlets to the detention facility outlet structure. Low flow pipes shall be a minimum of 12 inches in diameter unless it can be shown that the bottom of the detention facility will remain dry. Low flow pipes shall be slotted and bedded in freely draining aggregate. Alternate methods of subsurface drainage may be proposed.

F. Design Requirements, Wet Detention Facilities:


1. Maximum side slopes from the point of normal water level to a point 20 feet land side and 20 feet water side shall be 8:1. If riprap is provided around the pond normal water level perimeter, the maximum riprap slope may be increased to 3:1.


2. The pond bottom shall be contoured to encourage fish habitat. Minimum depth shall be 10 feet for at least 25% of the pond normal water level area. Ponds shall be stocked with appropriate bass, bluegill, catfish combination. Pond owner may contact the Iowa Department of Natural Resources Fisheries or City of North Liberty for fish stocking.

3. Short (3 to 4 foot height) urban mix native and / or wetland plantings shall be provided for at least 25% of the pond normal water level perimeter. Plantings shall be on average at least 10 feet wide and specified from list of North Liberty approved mixes or approved equivalent. Preferred location of plantings is at or around the outlet device.


4. Pond outlet shall be three stage.

• Stage one outlet shall consist of orifice, weir, or pipe to retain the volume of ½ inch times the pond drainage basin area. Outlet device shall be designed to drawdown this volume in not less than 24 or more than 48 hours. Provisions shall be made for protection of the outlet device from debris buildup and clogging.

• Stage two outlet shall control the 100-year developed maximum release rate to no more than the 5-year undeveloped maximum rate.

• Stage three shall be an overflow spillway to the designated 100-year overflow route.