DESIGN OF STORM SEWER Drainage SYSTEMS

SECTION 403 DESIGN OF STORM SEWER Drainage SYSTEMS

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SECTION 403

DESIGN OF STORM SEWER DRAINAGE SYSTEMS 403.00 GENERAL All storm drainage improvements within the City of Gillette (City) shall be designed and constructed according to the City of Gillette Storm Drainage Design Manual, these design standards, the City of Gillette Standard Construction Specifications and other City regulations and ordinances as they apply to storm drainage. Depending on specific site conditions storm drainage systems may need to exceed these minimum standards in order to meet the goals of City policies and provide adequate protection from flooding. Should these design standards conflict with the requirements of other agencies the designer shall seek direction from the City Engineer to determine the appropriate standard to apply. The storm sewer system is to be used for transmission of natural surface water and uncontaminated groundwater only. No pollutants, trash or other deleterious substances shall be placed in the storm sewer system or area tributary to the system. No discharge of industrial waste or process waste of any type shall be made into the storm sewer system. Storm drainage design must be compatible with comprehensive regional plans and should not be based on the premise that problems can be transferred from one location to another. Natural historic drainage ways should be used for storm runoff wherever feasible. Major consideration must be given to the floodplain, park and open space requirements of the area. The City of Gillette encourages multi-use of drainage facilities with parks and open spaces. 403.01 STORMWATER MANAGEMENT To comprehensively address storm water management multiple local, state and federal laws and regulations may apply. The designer of storm drain systems must consider all laws and regulations that affect the design of each project. These include, but are not limited to the City of Gillette subdivision regulations, floodplain ordinance, WYDOT standards, Wyoming DEQ regulations or USACE Permits and other sections of these Design Standards. In addition, agencies other than the City may require permits to complete proposed improvements. The designer shall consider the requirements of other permitting agencies and incorporate them into the design as necessary. Storm water management involves more than conveying rainfall runoff from the land. Storm drainage designs must consider the impacts to vegetation, public safety, facility maintenance access and costs and disturbances during construction.

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403.02 DESIGN STORM FREQUENCIES Storm drainage improvements shall be designed to convey “Minor” and “Major” storm frequencies as defined in Table 1 below.

Table 1 Design Storm Frequency by Zoning District/Land Use

Zoning District/Land Use1 Design Storm Frequency (Return Period, Year)

Minor Major Residential 2 100 Commercial/Office/Business 5 100 Industrial/Central Business 10 100 Parks, Cemeteries, Open Public Land 2 100

1 Where multiple zoning designations and land uses apply within the same drainage basin, the greater design storm shall govern.

All drainage improvements shall be designed to convey the Minor storm, as a minimum. Minor facility improvements may need to be designed to carry some part or all of the Major storm flows if Major storm flows cannot be conveyed safely to a suitable receiving system or if allowable flow depths in streets cannot be maintained. In addition, consideration should be given to the impact of storms greater than the Major storm to anticipate and reduce the potential for significant flood damage. 403.03 RAINFALL AND RAINFALL LOSSES Rainfall depths, intensities and storm distributions as described and summarized herein shall be applied to all projects within the City to estimate rates and volumes of rainfall runoff. Rainfall depths for the City are summarized in Table 2.

Table 2 Rainfall Depths (inches), Durations and Frequencies

Storm Frequency (Return Period, year)

Storm Duration (hours)

1 6 24 2 0.83 1.20 1.60 5 1.25 1.70 2.00 10 1.50 2.00 2.60 25 1.80 2.50 3.20 50 2.05 2.80 3.60

100 2.35 3.10 4.00 Source: NOAA Atlas 2, Precipitation-Frequency Atlas of the Western United State, Volume II, Wyoming, 1973

Rainfall intensities for durations to be used for the Rational Method are shown in Table 3. When hydrology models requiring a design storm to be applied, the 2-hour distribution of 1 hour rainfall depths described in Table 4 shall be used for basins up to 10 square miles.

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Table 3 Rainfall Intensity-Duration-Frequency Data (in/hr) Storm Frequency 2-Year 5-Year 10-Year 25-Year 50-Year 100-Year Duration/Tc (min)

5 2.82 4.24 5.09 6.11 6.95 7.976 2.68 4.03 4.84 5.80 6.61 7.587 2.55 3.84 4.61 5.53 6.30 7.228 2.44 3.67 4.41 5.29 6.02 6.919 2.34 3.52 4.23 5.07 5.77 6.62

10 2.25 3.38 4.06 4.87 5.55 6.3611 2.16 3.25 3.91 4.69 5.34 6.1212 2.08 3.14 3.77 4.52 5.15 5.9013 2.01 3.03 3.64 4.36 4.97 5.7014 1.95 2.93 3.52 4.22 4.81 5.5115 1.88 2.84 3.41 4.09 4.65 5.3316 1.83 2.75 3.30 3.96 4.51 5.1717 1.77 2.67 3.21 3.85 4.38 5.0218 1.72 2.60 3.12 3.74 4.26 4.8819 1.68 2.53 3.03 3.64 4.14 4.7520 1.63 2.46 2.95 3.54 4.03 4.6221 1.59 2.40 2.88 3.45 3.93 4.5122 1.55 2.34 2.80 3.37 3.83 4.3923 1.51 2.28 2.74 3.29 3.74 4.2924 1.48 2.23 2.67 3.21 3.65 4.1925 1.45 2.18 2.61 3.14 3.57 4.1026 1.41 2.13 2.56 3.07 3.49 4.0127 1.38 2.09 2.50 3.00 3.42 3.9228 1.36 2.04 2.45 2.94 3.35 3.8429 1.33 2.00 2.40 2.88 3.28 3.7630 1.30 1.96 2.35 2.82 3.22 3.6931 1.28 1.92 2.31 2.77 3.15 3.6232 1.25 1.89 2.26 2.72 3.10 3.5533 1.23 1.85 2.22 2.67 3.04 3.4834 1.21 1.82 2.18 2.62 2.98 3.4235 1.19 1.79 2.15 2.57 2.93 3.3636 1.17 1.76 2.11 2.53 2.88 3.3037 1.15 1.73 2.07 2.49 2.83 3.2538 1.13 1.70 2.04 2.45 2.79 3.1939 1.11 1.67 2.01 2.41 2.74 3.1440 1.09 1.65 1.97 2.37 2.70 3.0941 1.08 1.62 1.94 2.33 2.66 3.0542 1.06 1.60 1.91 2.30 2.62 3.0043 1.04 1.57 1.89 2.26 2.58 2.9644 1.03 1.55 1.86 2.23 2.54 2.9145 1.01 1.53 1.83 2.20 2.50 2.8746 1.00 1.51 1.81 2.17 2.47 2.8347 0.99 1.48 1.78 2.14 2.43 2.7948 0.97 1.46 1.76 2.11 2.40 2.7549 0.96 1.44 1.73 2.08 2.37 2.7250 0.95 1.43 1.71 2.05 2.34 2.6851 0.93 1.41 1.69 2.03 2.31 2.6552 0.92 1.39 1.67 2.00 2.28 2.6153 0.91 1.37 1.65 1.98 2.25 2.5854 0.90 1.36 1.63 1.95 2.22 2.5555 0.89 1.34 1.61 1.93 2.20 2.5256 0.88 1.32 1.59 1.91 2.17 2.4957 0.87 1.31 1.57 1.88 2.14 2.4658 0.86 1.29 1.55 1.86 2.12 2.43

Formatted: Right: 0.9", Bottom: 0.8",Section start: New page

Formatted: Tab stops: Not at 3" + 6"




59 0.85 1.28 1.53 1.84 2.10 2.4060 0.83 1.25 1.50 1.82 2.07 2.35

Source: NOAA Atlas 2, Precipitation-Frequency Atlas of the Western United States, Volume II, Wyoming, 1973.

Table 4 2-Hour Design Storm Distribution of 1-Hour NOAA Atlas Depths

Storm Frequency 2-Year 5-Year 10-Year 25-Year 50-Year 100-Year 1-Hour Depth (in) 0.83 1.25 1.5 1.80 2.05 2.35

Time (min) Rainfall (inches)

0 0 0 0 0 0 0

5 0.017 0.025 0.030 0.023 0.027 0.024

10 0.033 0.046 0.056 0.063 0.072 0.071

15 0.070 0.109 0.123 0.090 0.103 0.108

20 0.133 0.191 0.225 0.144 0.164 0.188

25 0.208 0.313 0.375 0.270 0.308 0.329

30 0.116 0.163 0.180 0.450 0.513 0.588

35 0.052 0.073 0.084 0.216 0.246 0.329

40 0.042 0.055 0.065 0.144 0.164 0.188

45 0.025 0.045 0.057 0.090 0.103 0.146

50 0.025 0.045 0.048 0.090 0.103 0.118

55 0.025 0.038 0.048 0.058 0.066 0.094

60 0.025 0.038 0.048 0.058 0.066 0.094

65 0.025 0.038 0.048 0.058 0.066 0.094

70 0.017 0.038 0.048 0.043 0.049 0.047

75 0.017 0.031 0.048 0.043 0.049 0.047

80 0.017 0.028 0.038 0.032 0.037 0.028

85 0.017 0.028 0.029 0.032 0.037 0.028

90 0.017 0.028 0.029 0.025 0.029 0.028

95 0.017 0.028 0.029 0.025 0.029 0.028

100 0.017 0.019 0.029 0.025 0.029 0.028

105 0.017 0.019 0.029 0.025 0.029 0.028

110 0.017 0.019 0.029 0.025 0.029 0.028

115 0.008 0.019 0.026 0.025 0.029 0.028

120 0.008 0.016 0.020 0.025 0.029 0.028

Totals 0.960 1.446 1.736 2.081 2.370 2.717 Rainfall losses shall be based on land uses. Soil types according to the National Resource Conservation Service hydrologic soil group designations (A, B, C and D) shall be used to determine infiltration rates. 403.04 RAINFALL RUNOFF METHODS AND COMPUTER MODELS The Rational Method, the Colorado Urban Hydrograph Procedure (CUHP), the Urban Drainage Storm Water Model (UDSWM), the Environmental Protection Agency’s Storm Water Management Model (EPASWMM), or other acceptable methods (to be pre-approved prior to design) shall be applied to all projects within the City to develop peak rates of runoff



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and runoff hydrographs for the design of storm drainage improvements. These methods shall be applied as described in the program user’s manuals. Estimates of runoff shall be made for Minor and Major storm events as needed for the type of development and drainage facility being designed. Using more than one method is recommended to verify results. The CUHP may be applied to individual basins between 90 acres and 5 square miles in area. Larger basins may be divided into sub basins of 5 square miles or less. The Rational Method may be applied to basins smaller than 160 acres. The CUHP program provides an option using the Rational Method time of concentration to adjust CUHP hydrographs for basins between 5 and 90 acres in area. This option allows the development of runoff hydrographs for small basins, but has been found to generate significantly higher peak flows when compared to the Rational Method peak flows. The UDSWM or EPASWMM may be applied to basins of any size, but care should be used in applying them to basins larger than 1 square mile. The Rational Method is generally used for smaller catchments when only the peak flow rate or the total volume of runoff is needed (e.g., storm sewer sizing or simple detention basin sizing). The CUHP, UDSWM or EPASWMM are used for larger catchments and when a hydrograph of the storm event is needed (e.g., sizing large detention facilities). However, UDSWM and EPASWMM can be used for smaller basins to determine peak flow rates and when a hydrograph is needed. Runoff from upstream undeveloped land, not part of the proposed project, shall at a minimum be based on historic conditions assuming that detention storage will be provided if the upstream land is developed. Runoff from developed upstream land must be determined based on existing conditions or approved drainage plans. 403.05 STREET CAPACITIES The primary function of streets is to safely convey traffic. Drainage systems must be designed to preserve this primary function during a range of storm events and avoid excessive flooding. Storm flows must be conveyed within public right-of-way or easements. Allowable street encroachments shall be as described in Table 5.

Table 5 Allowable Street Flow Depths

Street Classification Minor Storm Major Storm

Local/Local Through No curb overtopping with spread to crown and 6” at cross streets

18” over gutter flow line and at cross streets

Collector No curb overtopping with spread not covering one center lane and 6” at cross streets with cross pans

18” over gutter flow line, 12” at cross streets

Arterial No curb overtopping with spread not covering one lane in each direction and no cross street flow

12” over gutter flow line, no cross street flow


403.06 STORM SEWERS Storm sewer improvements shall be designed so that Minor storm peak flows do not exceed curb-full street capacities, are contained within the storm sewer without surcharging and so that Major storm flows do not exceed allowable flow depths in streets. To maintain flow depths less than those allowed during the Major storm event, it may be necessary to convey flows greater than the Minor storm. Storm sewers shall also be located to drain low points. Minimum design standards for storm sewer are as follows:

A. Alignment – storm sewers shall be designed in straight segments with manholes at each horizontal and vertical change in alignment. Storm sewers shall be placed within street sections and easements to maintain ten feet of separation from water mains and provide maintenance access. Curved horizontal alignments may be allowed as long as the pipe radius does not exceed the pipe manufacturer maximum joint deflection requirements. Horizontal bends may be allowed as long as the bend does not exceed 22¼˚. If a curved horizontal alignment or bends are used, tracer wire and test stations shall be provided for locating purposes. Test stations shall be provided at all pipe junctions. Vertical bends shall not be allowed. When pipe sizes increase in a downstream direction, pipe crowns shall be set at the same elevation.

B. Sizes – the minimum diameter storm sewer trunk lines shall be 18 inches. The

minimum diameter for lateral pipes connected to storm inlets shall be 15 inches. For non-circular pipe sections the minimum dimension shall be 12 inches. Pipe sizes shall normally increase in a downstream direction to avoid trapping debris and maintain design capacities.

C. Flow conditions – storm sewer pipes shall be designed to convey their flow-full capacity during the Minor storm event. Storm sewers may be designed to surcharge during Major storm events, however, surcharging may not exceed one-half the pipe diameter or cause the hydraulic grade line elevation to be higher than one foot below the adjacent gutter flow line elevation. The hydraulic grade line at inlets, including lateral losses, shall be at least one foot below the gutter flow line elevation adjacent to the inlet.

D. Velocities – mean pipe velocities shall be between 2 fps and 15 fps for Minor

storm events. Mean velocities may be as high as 20 fps during the Major storm event if high strength concrete (6,000 psi) is used for pipes and manholes. The pipe manufacturer shall provide the 6,000 psi concrete mix design to the City Engineer for review prior to casting of pipe or structures.

E. Materials – storm sewer pipes may be of any material specified in the City of

Gillette’s Standard Construction Specifications, except as specified herein. Pipe wall strengths and coatings shall be suitable for soil conditions, design depths and trench details. Minimum depth of bury shall be two feet below finished grade for all pipe materials unless a structural design analysis is approved by the City Engineer. All pipes with a depth of cover greater than eight feet shall be analyzed for structural strength. Allowable maximum depths of bury shall be

Formatted: Tab stops: Not at 0.5"





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determined based on the type of pipe material, pipe diameter, wall thickness, trench details and soil conditions. Depth of bury shall be based on the amount cover for each storm sewer segment from finished grade to the top of the pipe’s exterior wall.

Corrugated metal pipe (CMP) is not permitted for storm sewer construction except for under driveways and in slotted drain installations. Where minimum buried depth cannot be met, the pipe design must be approved by the City Engineer. Supporting documentation shall include pipe strength calculations, loading conditions, soil conditions, trench cross sections, bedding materials and any other information necessary to determine the suitability of the proposed design. Corrosion, abrasion and other appropriate observations of field conditions shall also be considered in determining appropriate pipe materials. Corrosion resistance shall be evaluated based on minimum resistivity, pH, sulfate content and chlorine content of the soil and groundwater. Tests shall be conducted along the proposed alignment of the drainage system.

F. Manhole spacing – manholes shall be placed at each vertical bend in the storm sewer alignment, at each change in pipe size, at lateral connections, at the upstream terminus of each trunk line and shall have maximum spacing as follows:

Table 6 Maximum Allowable Manhole Spacing

Storm Sewer Diameter Maximum Distance 18” to 36” 400’ 42” to 60” 500’

66” and Larger 600’

G. Manhole sizes – Manhole diameters shall be as follows:

Table 7 Minimum Allowable Manhole Sizes

Storm Sewer Diameter Manhole Diameter 18”to 24” 4’ 27” to 36” 5’

42” 6’ 48” and larger Appropriate Sized

Manhole, Junction Box or Tee

Larger manhole diameters or junction boxes may be required when storm sewer alignments are not straight through or when lateral pipes also enter manholes. Manhole bases and junctions boxes may be cast-in-place or pre-cast.




H. Manhole materials – manholes shall be pre-cast with cast-in-place or pre-cast bases and eccentric cones and iron rings and covers. Flat top lids may be used if design depths are too shallow to allow eccentric cones. Steps shall be installed at appropriate spacing in line with the eccentric cone to provide safe access to the storm sewer. Adjustments to rings and covers to match design grades shall be made with pre-cast concrete or HDPE rings and shall not exceed 8 inches in height.

I. Maximum manhole depth – manhole depths shall not exceed 20 feet without

special safety provisions such as intermediate platforms and minimum diameter risers of 48 inches.

403.07 STORM INLETS

Storm inlets shall be placed at low points in flow lines and sumps and where necessary to maintain allowable street flow depths for Minor and Major storm events. Minimum design standards for storm inlets are as follows:

A. Inlet types – allowed storm inlet types include: curb opening, grated and combination (grated with curb opening). Typical inlet details are provided in the City’s Standard Drawings. Other inlets may be allowed with City Engineer approval.

B. Capacity – single inlet capacities shall be adjusted according to the efficiency

factors listed in the table below. Multiple inlet capacities shall be determined by applying the clogging coefficient to single inlet efficiencies. The minimum inlet length shall be three feet and the maximum length shall be fifteen feet. If the required length exceeds fifteen feet, multiple inlets shall be used. Table 8 summarizes efficiency factors for single storm inlets.

Table 8

Efficiency Factors for Single Storm Inlets

Condition Inlet Type Percent of Theoretical Capacity Allowed

Sump Curb Opening 80 Sump Grated 50 Sump Combination 65

Continuous Grade Curb Opening 80 Continuous Grade Deflector 75 Continuous Grade Longitudinal Grate 60 Continuous Grade Transverse Bar Grate 50 Continuous Grade Combination Same as percentage for type of

grate times 1.10

C. Lateral pipe connections – no more than two inlets shall be connected by each lateral pipe entering trunk line manholes or junction boxes.

403.08 OPEN CHANNELS






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Open channels shall be designed to maintain stable cross sections and slopes based upon the design flows to be conveyed, soil types and channel cover. Wherever practical, open channels shall have slow flow characteristics, be wide and shallow, be compatible with surrounding land uses and be “natural” in appearance. Open channels should be designed to integrate recreation and aesthetic needs, protect wildlife and support plant populations. Minimum design standards for open channels are summarized as follows:

A. Alignment – open channels may have a curvilinear alignment with a centerline radius, at least, equal to two times the top width of the design flow or 100 feet, whichever is greater.

B. Maximum flow depth – the main channel depth of flow shall be no greater than 5 feet, except on major drainage channels.

C. Freeboard – freeboard shall be 1.0 foot minimum for grass-lined channels, 2.0 foot minimum for riprap or concrete and as otherwise required due to channel design and flow conditions.

D. Side Slopes – Maximum side slopes for grass-lined channels shall be 4:1 and 2.5:1 for riprap-lined channels.

E. Maximum Froude Number – open channels shall be designed for a subcritical

flow condition with a Froude Number no greater than 0.8.

F. Velocities – Maximum flow velocities for grass-lined open channels during the Major storm shall be 5.0 fps for sandy soils and 7.0 fps for erosion resistant soils. Riprap-lined channels shall have a maximum velocity of 12.0 fps and concrete-lined channels shall have a maximum velocity of 18.0 fps.

G. Roughness coefficient – values for Manning’s “n” shall be as specified in Table

9. The design should be evaluated using a range of values to address various design parameters, higher values for depth of flow calculations and lower values for velocity calculations.








Table 9 Roughness Coefficients (“n”) for Channel Design

Channel Type Roughness Coefficient (n) Minimum Typical Maximum

A. Excavated or Dredged 1. Earth, straight and uniform a. Gravel, uniform section, clean 0.022 0.025 0.030 b. With short grass, few weeds 0.022 0.027 0.033 2. Earth, winding and sluggish a. Grass, some weeds 0.025 0.030 0.033 b. Dense weeds or aquatic plants 0.030 0.035 0.040 c. Earthy bottom and rubble/riprap sides 0.028 0.030 0.035 3. Channels not maintained, weeds, brush uncut a. Dense weeds, high as flow depth 0.050 0.080 0.120 b. Clean bottom, brush on sides 0.040 0.050 0.080 B. Lined or Built-Up Channels 1. Concrete a. Trowel/float finish 0.011 0.015 0.016 b. Shot Crete 0.016 0.020 0.025 2. Gravel bottom with sides of: a. Formed concrete 0.017 0.020 0.025 b. Random stone in mortar 0.020 0.023 0.026 c. Dry rubble or riprap 0.023 0.033 0.036 3. Grassed Flow Depth Flow Depth a. Buffalo grass, Kentucky bluegrass = 0.1-1.5 ft > 3.0 ft i. Mowed to 2 in 0.035 0.030 ii. Length = 4-6 in 0.040 0.030 b. Good stand, any grass i. Length 12 in 0.070 0.035 ii. Length 24 in 0.100 0.035 c. Fair stand, any grass i. Length = 12 in 0.060 0.035 ii. Length = 24 in 0.070 0.035

H. Low flow and trickle channels - Low flow and trickle channels may be required in the channel cross section to improve channel conditions during sustained flow conditions and frequent storm events when a vegetative lining alone would not be appropriate to handle the flows.

I. Grade control – grass-lined channels will generally have longitudinal slopes

between 0.2% and 0.6%. Grade control structures should be used to provide channel stability and be limited to a height of three feet. Sloping grouted boulder drop structures are preferred, however, limited availability of materials may require concrete vertical drops with riprap basins or other materials to be used.



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J. Compliance with Environmental Regulations – All Federal and State regulations, such as Section 404 Permits must be complied with and all applicable permits must be received.

K. 1. Rear Lot Drainage – This section shall only apply to residential areas where the

average lot size is less than 15,000 square feet. Dedicated rear lot drainage tracts shall not be required where 2 lots or less drain across a downstream lot. Where more than 2 lots drain across a downstream lot, the drainage area shall be dedicated to the City of Gillette as a tract or other approved dedication. The conveyance system through a drainage tract shall be in accordance with this section 403 Design of Storm Sewer Systems.

2. Rear Lot Drainage – This section shall only apply to residential areas where the average lot size is 15,000 square feet or greater. Drainage areas shall be dedicated as a tract or other approved dedication where flows are concentrated or otherwise deemed necessary.

403.09 ROADSIDE DITCHES AND DRIVEWAY CULVERTS Roadside ditches shall only be allowed where approved with appropriate zoning. Ditches shall be designed to convey at a minimum the Minor design storm. Where design velocities will create unstable ditch conditions, ditch protection or grade control structures shall be provided. Roadside ditches shall also be designed with the following limits:

A. Minimum depth - 24 inches B. Maximum side slopes – 3 H to 1 V C. Maximum bottom width – 4 feet D. Driveway culverts – driveway culverts shall be sized at a minimum to pass the

Minor storm peak flow for the adjacent street classification without overtopping the driveway and shall have a minimum diameter of 15 inches. Driveway culverts shall be of a suitable material and strength for driveway traffic loads and shall be placed with adequate cover to protect culvert. Minimum depth of bury shall be 9 inches.

403.10 CULVERTS Culverts shall be placed wherever drainage ways cross public roadway alignments. Minimum design standards for culverts are as follows:

A. Street overtopping - culverts shall be sized so that street overtopping is limited

to depths shown in Table 10.









Table 10 Allowable Street Overtopping Depths at Culvert Crossings

Street Classification Minor Storm Major Storm

Local/Local Through None

12 inches at street crown

Collector None 12 inches at street crown

Arterial None No overtopping allowed

B. Minimum size – culverts shall have a minimum diameter or height of 18 inches. C. Culverts must also be sized so that channel capacities and design water levels

are maintained. D. Velocities – culverts shall be designed to maintain a minimum velocity of 2 fps

during the Minor storm event and to maintain maximum velocities to limit erosion potential during Major storm events unless adequate erosion control or energy dissipation is provided.

E. Minimum slope – culverts shall be designed to carry storm flows without

accumulating sediment. Generally, the minimum slope shall be 0.5%. F. Blockage – culverts crossing streets shall be sized for Major storm events

assuming 20% of its flow capacity is lost to blockage to determine overtopping depths.

G. Materials –Culverts shall be constructed of HDPE pipe, pre-cast concrete or

cast-in-place concrete, except under driveways where corrugated metal pipe may be used.

H. Culverts that are used to span a drainage more than 20 feet wide are considered

bridge structures and require the design to be reviewed and approved by Wyoming Department of Transportation (WYDOT).

403.11 DETENTION PONDS Detention storage shall be provided for all new development and redevelopment projects so that peak rates of runoff are reduced to rates less than or equal to their historic (predevelopment) rates. Peak release rates shall be controlled for both the Minor and Major design events consistent with the capacity of the downstream receiving system. Underground detention facilities shall not be allowed when the facilities shall be maintained by the City. For basins larger than 90 acres or when upstream detention ponds are present, detention ponds should be designed using hydrographs. Detention ponds for basins smaller than 90 acres can be sized using the Rational Formula-based FAA procedure or other acceptable methods. Minimum design standards for detention ponds are as follows:








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A. Minimum bottom slopes – pond bottoms shall be designed with a recommended 0.5% minimum longitudinal and a 2.0% minimum cross slope.

B. Side slopes – pond side slopes shall be 4H to 1V, preferred, and 3H to 1V, maximum. Side slopes greater than 3H to 1V must be approved by City Engineer and, if approved, protected from erosion and instability and not require routine maintenance.

C. Trickle channels – ponds shall have a trickle channel with a recommended

minimum capacity of 1% to 3% of the peak flow of the major design storm inflow to the pond and a recommended minimum slope of 0.5%.

D. Freeboard – detention ponds shall be designed with a minimum freeboard of

one foot above the emergency spillway water surface elevation or 6 inches above the water surface elevation during Major storm spills over the emergency spillway, which ever is greater.

E. Inlets – points of inflow to the pond shall be protected to prevent erosion. The

design of protection measures shall be based on no storage in the pond. F. Outlets – outlet structures shall control release rates to no more than the historic

Minor and Major flows. The minimum diameter for outlet pipes shall be 12 inches.

G. Emergency spillway – an emergency spillway shall be designed to safely pass the

peak discharge for the Major storm event assuming that the outlet structure is ineffective and the upstream basin is fully developed to current zoning or current development levels, whichever produces the greatest runoff. No buildings or structures shall be constructed in the path of flows passing over the emergency spillway.

H. Parking lot detention – detention storage in parking lots shall have a maximum

depth of 15 inches with 6 inches of freeboard above emergency spillway elevation or 3 inches above the water surface elevation during Major storm spills of the emergency spillway, which ever is greater.

I. Roof top detention – detention storage shall be provided in dedicated areas of

the site such as, parking lots, landscaped areas or along drainage channels. Rooftop detention shall not be allowed.

J. Groundwater – the presence of groundwater within detention ponds must be

addressed in design so that standing water is avoided and drainage structures are adequately protected.

K. State Permit – Detention pond designs shall comply with the current Wyoming

State Engineer’s Office requirements. State regulations may be updated or revised periodically. It is the responsibility of the owner and the designer to comply with all State requirements for design and permitting.

403.12 END TREATMENTS












All storm sewers and culverts shall be designed with appropriate end treatments at their inlets and outlets such as flared end sections, headwalls, wing walls, erosion protection or energy dissipaters to provide smooth transitions to embankment slopes and limit erosion due to turbulent flow and high velocities.

A. Minimum riprap size – where riprap is used for protection at inlet and outlets the minimum size shall meet the requirements of the City of Gillette Standard Construction Specifications, Section 02190.

B. Trash racks – Generally, trash racks are needed on culverts if one cannot see

clearly through the culvert, if the culvert is more than 150 feet long, if a 48 inch diameter object can not pass through or if the outlet may trap or injure a person being carried through. The open area of trash racks shall be, at least, a minimum of 4 times the open area of the culvert. Trash racks shall be constructed of smooth steel pipe, with a corrosion protection finish and capable of withstanding the full hydraulic load when completely blocked under maximum submergence. Bar clear spacing shall not exceed six inches and bars shall be generally perpendicular to flow. The longitudinal slope of the trash rack shall not exceed 3.0 H to 1.0 V. Trash racks shall be attached by removable devices such as bolts or hinges to allow access for maintenance, prevent undesirable access and prevent vandalism. Trash racks shall only be installed at inlets to drainage conduits. Trash racks at outlets to drainage conduits can pose a safety issues and trap debris reducing capacity and causing maintenance problems. The rack must not cause water to rise higher than the maximum allowable flood elevation.

403.13 WATER MAIN CROSSINGS AND ALIGNMENTS The alignment of storm drainage pipes relative to water mains shall comply with Standard Construction Specifications, Section 02725, Paragraph 3.01.E. 403.14 BRIDGES Bridges shall be designed to pass flows in accordance with the State of Wyoming Department of Transportation (WYDOT) design requirements. Bridge designs must be submitted to WYDOT for review and approval. 403.15 RIGHT OF WAY AND MAINTENANCE ACCESS To provide maintenance access to drainage facilities to be maintained by the City outside of already established public right-of-way, the following minimum right-of-way or easement widths shall be conveyed to the City:

A. Storm sewers - twice the pipe invert depth (at its deepest point) or 20 feet, whichever is greater.

B. Culverts – culverts extending beyond available public right-of-way or easement

shall be within easements with a width of twice the depth to the culvert invert at



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the right-of-way or easement line (at the deepest point) or 20 feet wider than the culvert width, whichever is greater. The culvert easement shall extend 10 feet beyond any end treatments or protection materials or 20 feet beyond the end of the culvert, whichever is greater.

C. Open channels – The greater of the channel top width, including freeboard, or

20 feet. All channels to be maintained by the City shall be contained within a tract of land dedicated to the City of Gillette or other approved dedication and be provided with maintenance access by adjacent public right-of-way or easements as required herein.

D. Detention ponds – Detention pond right-of-way shall be as required to contain

the required storage volume, including freeboard, outlet works and embankments and maintenance access. All detention ponds to be maintained by the City shall be contained within a tract of land dedicated to the City of Gillette or other approved dedication and be provided with maintenance access by adjacent public right-of-way or easements as required herein. Detention ponds will normally remain in private ownership unless upstream basins from outside of the subject property contribute to the pond or runoff from more than one property contributes to the pond.

E. Maintenance access – Minimum of one point of access shall be designed with a

minimum of 10 foot wide driving surface. Maintenance access may be combined with other public access improvements such as sidewalks and trails. Maintenance access to detention ponds shall be provided to the pond bottom and have a maximum slope of 15%.

F. Rear/Side Lot Lines – In local drainage areas where more than 2 lots drain across

a downstream lot, and the lots are 15,000 SF or less, the drainage area shall be dedicated to the City of Gillette as a tract of land or other approved dedication. In local drainage areas where the average lot sizes are 15,000 SF or greater, drainage areas where flows are concentrated, the drainage area shall be dedicated to the City of Gillette as a tract of land or other approved dedication.

Right-of-way or easement conveyed to the City shall provide the legal right of access to maintain drainage facilities so that they function as intended and they shall be kept clear of obstructions to flow and obstructions to access. Public right-of-way adjacent to drainage improvements may be used to provide maintenance access.

403.16 STRUCTURAL DESIGN

All storm sewer improvements placed under traffic carrying routes shall be designed to withstand static and live loads in accordance with the pipe manufacture’s recommendations. As a minimum, all improvements shall be designed to withstand an AASHTO HS-20 loading in accordance with the design procedures of AASHTO, “Standard Specifications for Highway Bridges”. 403.17 GRADING, EROSION AND SEDIMENT CONTROL


Erosion and sediment control practices shall be required on all developing and redeveloping sites. All projects shall be designed and managed to minimize the loss of soils from disturbed areas and the deposition of eroded materials downstream during and after construction. Designs shall also include the restoration and stabilization of disturbed areas to minimize the loss of soils after construction. Methods for controlling erosion and sedimentation during construction are listed in paragraph C of this section. Additional or alternative measures will be considered on an individual basis.

A. State Permits (DEQ) – sites shall comply with State regulations and permitting. B. Seed mixtures – where necessary the selection of seed mixtures shall be based

on recommendations from an expert knowledgeable in plant materials. As an alternate, the seed mixture described below shall be used for temporary and long-term vegetation of disturbed lands.

Dryland Pasture Seed Mix

Nardan Crested Wheatgrass 30% Russian Wild Rye 20% Y.B. Sweet Clover 15% Slender Wheatgrass 10% Oahe Intermediate Wheatgrass 10% Fairway Crested Wheatgrass 5% Western Wheatgrass 4% Other 6%

C. Erosion and sediment control criteria – Criteria for implementing erosion and

sediment control measures are summarized below. The list below is a summary of erosion and sediment control activities.

a. Erosion Control. Permanent or temporary soil surface stabilization must be

applied to disturbed areas and soil stockpiles as soon as possible but no later than 14 days after final grade is reached. Soil surface stabilization should also be applied within 14 days to disturbed areas that may not be at final grade but will remain dormant (undisturbed) for longer than an additional 30 days.

b. Surface Roughening. Surface roughening should be performed after final grading to provide a rough soil surface with horizontal depressions created by operating a tillage or other suitable implement on the contour, or by leaving slopes in a roughened condition by not fine-grading them.

c. Mulching. All disturbed areas must be properly mulched, seeded, or seeded and mulched, within 14 days after final grade is reached if not otherwise permanently stabilized. A properly anchored mulch shall be provided for areas reaching final grade in a non-growing season.

d. Revegetation. A viable vegetative cover should be established within one year on all disturbed areas and soil stockpiles not otherwise permanently stabilized. Vegetation is not considered established until a ground cover is achieved which is sufficiently mature to control soil erosion and can survive severe weather conditions.

e. Temporary Revegetation. Temporary revegetation is required on all disturbed areas having a period of exposure prior to final stabilization of one year to two




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years. Temporary seeding may be required to be mulched at the direction of the City Engineer.

f. Permanent Revegetation. Permanent revegetation is required on all disturbed areas having a period of exposure greater than two years, or for an indeterminate length of time. A perennial grass mix should be planted and may be required to be mulched at the direction of the City Engineer.

g. If stockpiles are located within 100 feet of a drainage way, additional sediment controls, such as a diversion dike or silt fence, should be provided.

h. Sediment Control. Properties and roadways adjacent to a construction site should be protected from eroded sediment being transported to them.

i. Vehicle Tracking. Wherever construction vehicles enter onto paved public roads, provisions must be made to prevent the transport of sediment (mud and dirt) by vehicles tracking onto the paved surface. It is recommended that coarse-aggregate rock surfacing be provided to keep most construction traffic from coming into contact with mud and dirt. All accesses, parking areas, staging areas, and loading/unloading areas should be stabilized. For sites greater than two (2) acres, a stabilized vehicle tracking control must be constructed. Whenever sediment is transported onto a public road, regardless of the size of the site, the roads shall be cleaned at the end of each day.

j. Slope Diversion Dikes. Temporary diversion dikes shall be provided as required by the provisions of Section 4.2. Diversion dikes located above disturbed areas may be discharged to a permanent or temporary channel. Diversion dikes located midslope on a disturbed area must discharge to temporary slope drain. Diversion dikes located at the base of a disturbed area must discharge to a sediment trap or basin.

k. Roads and Roadside Swales. For road areas that are not paved within 30 days of final grading, and have not received early application of road base, rough-cut street controls should be provided.

l. Sediment Entrapment Facilities. Sediment entrapment facilities include straw bale barriers, silt fences, and sediment basins. The limitations for selection and use of sediment entrapment facilities are given in Table 11.

Table 11

Sediment Entrapment Facility Limitations Sediment Control Facility

Allowable Maximum Limit Tributary

Drainage Area (ac)

Tributary Slope Length

(ft)

Tributary Slope Gradient

Straw Bale Barrier or Silt Fence

0.5-1.0 per 100 lineal ft

150 2:1 (50%)

m. All runoff leaving a disturbed area shall pass through at least one sediment

entrapment facility before it exits the site. n. Physical Barriers. Physical barriers to prevent wind erosion from development

sites include snow fence, silt fence, construction fence, or similar devices. These measures should be strategically located throughout the site to allow airborne particles to settle out of the air and be contained within the development site. Physical barriers shall be used in conjunction with surface roughening for all development sites disturbed more than five (5) acres.

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Physical barriers may be required for development sites smaller than five (5) acres determined on a case by case basis.

o. Watering. The application of water to disturbed areas within the site shall be used as necessary to suppress dust.

p. Site Traffic Control/Earthmoving Management. A plan to minimize construction dust by properly locating haul roads should be implemented. Regular watering and blading of haul roads should occur as necessary to minimize the amount of dust generated on-site. During periods of high winds, suspension of earthmoving activities may be necessary.

q. Limiting Cleared Areas. Proper construction planning shall be used to minimize areas without topsoil or vegetation.

r. Physical Stabilization. Physical stabilization measures to suppress dust include straw mulch, surface roughening, matting, and erosion control blankets.

s. Chemical Stabilization. The application of chemicals to suppress dust generated on-site may be used.

t. Steep Slopes. Slopes steeper than 4 to 1 will require vegetative matting to eliminate erosion concerns on the slope.

u. Working Within or Crossing a Waterway. Construction vehicles shall be kept out of waterways to the maximum extent practicable. Where an actively flowing watercourse must be crossed regularly by construction vehicles, a temporary stream crossing or channel diversion must be provided.

v. Outlet Protection. The outlets of temporary slope drains, culverts, sediment traps, and sediment basins must be protected from erosion and scour.

w. Inlet Protection. All storm sewer inlets made operable during construction must have sediment entrapment facilities installed to prevent sediment-laden runoff from entering the inlet.

x. Chemicals, Oils and Material Storage. Areas used for storage of chemicals, petroleum-based products and waste materials, including solid and liquid waste, shall be designed to prevent discharge of these materials in the runoff from a construction site.

y. Underground Utility Construction. Construction of underground utility lines shall be subject to the following criteria:

1. No more than 200 feet of trench are to be opened at one time. 2. Where consistent with safety and space considerations, excavated

material is to be placed on the uphill side of the trenches. 3. Trench dewatering devices must discharge in a manner that will not

adversely affect wetlands, drainage systems, or off-site property. Site dewatering permit requirements should be discussed with the appropriate State agency.

4. Provide storm sewer inlet protection whenever soil erosion from excavated material has the potential for entering the storm drainage system.

z. Disposition of Temporary Measures. All temporary erosion and sediment

control measures shall be removed within 30 days after final stabilization is achieved, or after the temporary measures are no longer needed, whichever occurs earliest, or as authorized by the City.





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aa. Maintenance. All temporary and permanent erosion and sediment control practices shall be maintained and repaired by the developer during the construction phase and warranty period as needed to assure continued performance of their intended function. All facilities must be inspected and replaced if necessary, following each precipitation or snowmelt event that results in runoff.

403.18 REGULATORY FLOODPLAINS AND FLOODING Regulatory floodplains within the City have been defined and adopted. Proposed improvements that will modify established floodplains must comply with the City’s Floodplain Ordinance and all requirements of the National Flood Insurance Program. In addition, all storm water runoff shall be accommodated so that flows resulting from all storm events, up to and including the Major event, will be safely passed to receiving channels without causing the inundation of private property or any buildings that may sustain damage from inundation. Consideration should also be given to potential damage that may be caused by storms greater than the 100-year event. 403.19 DRAINAGE REPORTS All land use proposals affecting drainage or the potential for erosion and sedimentation shall submit documentation addressing the proposed changes and providing measures intended to mitigate the impacts of the proposed project to drainage facilities and downstream properties. Submittals shall be provided with the appropriate type of land use application including annexation, planning, zoning, platting and site development. Drainage submittals shall be either Phase I or Phase II documents depending on the land use application being submitted. Drainage reports shall be submitted and approved as required by the City regulations. All submitted reports shall follow the outlines provided within this section of the Design Standards and be clearly and cleanly reproduced. Photo static copies of charts, tables, nomographs, calculations, or any other referenced material shall be legible. Washed out, blurred, or unreadable portions of the report are unacceptable and could warrant resubmittal of the report. The submittal shall be reviewed for adequacy based on City guidelines for report content. Incomplete or absent information may result in the report being rejected for review. All reports shall be typed on 8-1/2" x 11" paper and bound. The drawings, figures, plates, and tables shall be bound with the report or included in a pocket attached to the report. The report shall include a cover letter presenting the plan for review and shall be prepared by or supervised by an engineer licensed in the State of Wyoming. A pre-application consultation is suggested for all applicants. The applicant shall consult with City Staff for general information regarding subdivision regulations, required procedures, possible drainage problems and specific submittal requirements.

A. Phase I – Preliminary Drainage Report - The purpose of the Phase I Drainage Report is to identify and/or refine conceptual solutions to the problems which may occur on-site and off-site as a result of the development or any phase of the development. The Phase I Drainage Report shall be submitted during the subdivision process with the application for Preliminary Plat. If the Zoning and



Preliminary Plat applications are submitted concurrently, the submittal requirements of this section shall apply. In addition, those problems that exist on-site prior to development must be addressed during the preliminary phase. The Phase I drainage report shall be in accordance with the outline described in Section 101 of the Design Standards.

B. Phase II – Final Drainage Report - The purpose of the Phase II Drainage Report

is to present the design details for the drainage facilities discussed in the Phase I Drainage Report. Also, any change to the Phase I concept must be presented and fully explained. The report may also address erosion and sediment control issues and the methods to be implemented to control erosion and sedimentation during construction. Several erosion and sediment control mitigation measures are outlined in Section 403.17 (C) of this document. Alternately, a separate Erosion and Sediment Control Report may be submitted in accordance with requirements of these Design Standards. If erosion and sediment control is addressed in the Phase II Drainage Report, the same information required for separate Erosion and Sediment Control Report shall be provided. The Phase II Drainage Report shall be submitted with the Final Plat, Site Plan or Building Permit applications for applicable developments.

The report shall include a cover letter presenting the final design for review and shall be prepared by or under the direction of an engineer licensed in Wyoming.

403.20 EROSION AND SEDIMENT CONTROL REPORT All land use or construction proposals requiring a Phase II – Final Drainage Report shall also require an Erosion and Sediment Control Report. The report shall address the potential for erosion and sedimentation during construction and shall provide documentation addressing the proposed changes and providing measures intended to mitigate the impacts of the proposed project to drainage facilities and downstream properties. The Erosion and Sediment Control Report shall follow the summary outline provided within this section. Alternately, an Erosion and Sediment Control section may be added to the Phase II – Final Drainage Report in-lieu of a separate report. An Erosion and Sediment Control Plan must also be developed and submitted to obtain a construction or site grading permit. Site planning and drainage planning should, whenever possible, occur concurrently with site grading and erosion control planning. When site grading precedes final development, an erosion and sediment control plan for site grading must be submitted. This plan may have to be modified at the time a final site development plan is prepared. This modified plan must be submitted for review and approval prior to final development. Implementation and maintenance of erosion control measures are ultimately the responsibility of the property owner. Should the approved plan not function as intended, and it is determined


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by the City that additional measures are needed, the owner will have to provide additional measures needed to reduce soil erosion and sediment discharged from the construction site.

DESIGN OF STORM SEWER Drainage SYSTEMS

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