Chapter 6 Street, Inlets, and Storm Drains

Chapter 6 Street, Inlets, and Storm Drains

Table of Contents

6-1 Introduction ...... 1 6-1-1 Urban Conveyance System Components ...... 1 6-1-2 Minor and Major Storms ...... 1

6-2 Street Drainage ...... 2 6-2-1 Allowable Spread and Depth ...... 2 6-2-2 Inlet Location and Spacing ...... 3 6-2-3 Cross-Street Flow Conditions...... 3 6-2-4 Hydraulic Evaluation ...... 3

6-3 Inlets ...... 10 6-3-1 Inlet Function and Selection ...... 10 6-3-2 Types of Inlets ...... 11 6-3-3 General Design Guidelines ...... 11 6-3-4 Nuisance Flows ...... 12 6-3-5 Hydraulic Evaluation of Inlets ...... 12

6-4 Storm Drain Systems ...... 15 6-4-1 Introduction ...... 15 6-4-2 Design Storms ...... 16 6-4-3 Pipe Material, Size and Service Life ...... 16 6-4-4 Other Storm Drain Design Considerations ...... 17 6-4-5 Vertical Alignment ...... 18 6-4-6 Horizontal Alignment ...... 18 6-4-7 Easements ...... 18 6-4-8 Manholes ...... 19 6-4-9 Hydraulic Design ...... 20 6-4-10 Hydraulic Calculations ...... 21

6-5 Examples ...... 22

Tables Table 6-1. Allowable Street Encroachment and Depth of Flow ...... 2 Table 6-2. Inlet selection considerations ...... 11

Figures Figure-6-1. Gutter Section with Uniform Cross Slope ...... 4 Figure 6-2. Typical Gutter Section—Composite Cross Slope ...... 6 Figure 6-3. Calculation of Composite Street Section Capacity: Major Storm ...... 8 Figure 6-4. Reduction Factor for Gutter Flow (Guo 2000b) ...... 9 Figure 6-5. Typical V-Shaped Swale Section ...... 10

October 2018 City of Durango 6-i Storm Drainage Design Criteria Manual

Chapter 6 Street, Inlets, and Storm Drains

6-1 Introduction

The purpose of this chapter is to provide design guidance for stormwater collection and conveyance utilizing streets and storm drains. Procedures and equations are presented for the hydraulic design of street drainage, locating inlets and determining capture capacity, and sizing storm drains. This chapter also includes discussion on placing inlets to minimize the potential for icing.

6-1-1 Urban Conveyance System Components

Urban stormwater collection and conveyance systems are comprised of three primary components:

1. Street gutters and roadside swales,

2. Storm drain inlets, and

3. Storm drain pipes (with appurtenances such as manholes, junctions, etc.).

Street gutters and roadside swales collect runoff from the street (and adjacent areas) and convey the runoff to a storm drain inlet while maintaining the street’s level of service.

Inlets collect stormwater from streets and other land surfaces, transition the flow into storm drains, and provide maintenance access to the storm drain system. Storm drains convey stormwater in excess of street or swale capacity along the right-of-way and discharge into a stormwater management facility or directly into a receiving water body. All of these components must be designed properly to achieve the objectives of the stormwater collection and conveyance system.

6-1-2 Minor and Major Storms

Rainfall events vary greatly in magnitude and frequency of occurrence. Major storms produce large flow rates but rarely occur. Minor storms produce smaller flow rates but occur more frequently. For economic reasons, stormwater collection and conveyance systems are not normally designed to pass the peak discharge during major storm events without some street flooding.

Stormwater collection and conveyance systems are designed to pass the peak discharge of the minor storm event (and smaller events) with minimal disruption to street traffic. To accomplish this, the spread and depth of water on the street is limited to 6 inches during the minor storm event. Inlets must be strategically placed to pick up excess gutter or swale flow once the limiting allowable spread or depth of water is reached. The inlets collect and convey stormwater into storm drains, which are typically sized to pass the peak flow rate (minus the allowable street flow rate) from the minor storm without any surcharge.

For the City of Durango, storm sewer inlets are to be sized and spaced to meet the 10-year minor storm event and the 100-yr major storm event allowable street encroachment and depth of flow requirements as shown in Table 6-1. In addition, storm sewer inlets should be sized to accept all flow generate by the 2-yr event with no bypass.

October 2018 City of Durango 6-1 Storm Drainage Design Criteria Manual

Street, Inlets, and Storm Drains Chapter 6

During the major storm event (100-yr storm), runoff exceeds the minor storm allowable spread and depth in the street and the capacity of storm drains and storm drains may surcharge. Street flooding occurs, and traffic is disrupted as the street functions as an open channel. The designer must evaluate and design for the major event with regard to maintaining public safety and minimizing flood damages. Guidance on major and minor storm design specifications can be found in the “City of Durango Development Standards for Public Improvements and Construction Specifications,” otherwise referred to as the Construction Specifications Manual hereafter. 6-2 Street Drainage

6-2-1 Allowable Spread and Depth

Streets are typically given classifications such as local, collector, arterial. Street design standards for the City of Durango can be found in the Construction Specifications Manual. Other street cross-sections shall be reviewed and approved by the City Engineer. Allowable spread and depth for each street classification are presented in Table 6-1. In general, storm drain or surface drainage swales and culverts shall be installed when the carrying capacity of the street is exceeded.

Table 6-1. Allowable Street Encroachment and Depth of Flow Minor Storm (10-year) Major Storm (100-year) Allowable Street Allowable Cross- Classification Allowable Encroachment Allowable Encroachment Cross-Street Street Flow Flow No curb overtopping; where no curbing exists, Depth of water in gutter 6 inch encroachment shall not shall not exceed the Local depth in be over property lines. adjacent ground line, crosspan. Flow may spread to unless buildings are flood 12 inch depth crown of street. proofed, or the depth of above gutter No curb overtopping water over the gutter flow line. (same as above). Flow flowline shall not exceed spread shall leave the 12 inches. The smaller of Collector None1 equivalent of one 10 foot these two depths shall driving lane clear of control. water. No curb overtopping No cross- (same as above). Flow street flow. spread shall leave the Depth of water in gutter Maximum equivalent of two 10 foot shall not exceed 12 depth at Arterial driving lanes clear of None inches, nor extend beyond upstream water – one lane in each ROW. gutter on road direction. No more than edge of 12 two lanes in each inches. direction shall be flooded. 1. If use of cross pan is determined necessary/allowed, depth of flow should not exceed 6 inches

6-2 City of Durango October 2018 Storm Drainage Design Criteria Manual Chapter 6 Street, Inlets, and Storm Drains

6-2-2 Inlet Location and Spacing

On a long continuous grade, stormwater flow increases as it moves down the gutter and picks up more drainage area. As the flow increases, so does the spread and depth. Since the spread (encroachment) and depth (inundation) are not allowed to exceed the values provided in Table 6-1, inlets must be strategically placed to remove some of the stormwater from the street. Locating these inlets requires design computations by the engineer. Inlet Location Requirements

The City of Durango will require inlets at the following locations:  Immediately upstream of intersections to reduce stormwater flow impacts for both at pedestrians and traffic.  At all sump locations.  Minimum of every 400 feet along continuous grades.

6-2-3 Cross-Street Flow Conditions

Cross-street flow can occur in an urban drainage system under three conditions:

1. For local and collector streets, runoff in a gutter spreading across the street crown to the opposite gutter is allowed during the major event – this is prohibited for all streets for the minor event. During the minor event at least one lane must be available to pass traffic.

2. When cross-pans are used – Cross-pans, which are only to be used for local streets, shall be designed to convey the minor and major storm event within the criteria presented in this chapter. The design engineer shall evaluate the carrying capacity (with calculations provided) considering water on the roadway, as well as the side street.

3. When the flow in a drainageway exceeds the capacity of a road culvert and/or bridge and subsequently overtops the crown of the street – Culverts design shall adhere to the CDOT design manual for roadway overtopping. Additional information is referenced in the Culverts and Bridges Chapter of this Manual.

6-2-4 Hydraulic Evaluation

Hydraulic computations are performed to determine the capacity of roadside swales and street gutters and the encroachment of stormwater onto the street. The design discharge is based on the peak flow rate and usually is determined using the rational method (covered in the Hydrology Chapter). Although gutter, swale/ditch and street flows are unsteady and non-uniform, steady, uniform flow is assumed for the short time period of peak flow conditions.

October 2018 City of Durango 6-3 Storm Drainage Design Criteria Manual

Street, Inlets, and Storm Drains Chapter 6

Curb and Gutter

Both the longitudinal and cross (transverse) slope of a street are important in calculating hydraulic capacity. The capacity of the street increases as the longitudinal slope increases. The City of Durango requires a minimum longitudinal slope of 0.5% for positive drainage. Longitudinal slopes of 4% or greater require an analysis of drainage inlet bypass and soil erosion in ditches or swales along with mitigation facilities as required by the City Engineer (refer to the Construction Specifications Manual). The cross slope represents the slope from the street crown to the interface of the street and gutter, measured perpendicular to the direction of traffic. For all streets, the City of Durango recommends a minimum cross slope of 3% for positive drainage. The gradient within one hundred (100) feet of any four-way street intersection shall not exceed 5%, for driver comfort and safety, considerations shall limit the maximum cross slope. Use of standard curb and gutter sections typically produces a composite section with milder cross slopes for drive lanes and steeper cross slopes within the gutter width for increased flow capacity.

Each side of the street is evaluated independently. The hydraulic evaluation of street capacity includes the following steps:

1. Calculate the street capacity based upon the allowable spread (T) for the minor storm as defined in Table 6-1.

2. Calculate the street capacity based upon the allowable depth (y) for the minor storm as defined in Table 6-1.

3. Calculate the allowable street capacity by multiplying the value calculated in step two (limited by depth) by the reduction factor provided in Figure 6-4. The lesser value (limited by allowable spread or by depth with a safety factor applied) is the allowable street capacity.

4. Repeat steps one through three for the major storm using criteria in Table 6-1.

Capacity When Gutter Cross Slope Equals Street Cross Slope (Not Typical)

Streets with uniform cross slopes like that shown in Figure 6-1 are sometimes found in older urban areas. Since gutter flow is assumed to be uniform for design purposes, Manning’s equation is appropriate with a slight modification to account for the effects of a small hydraulic depth (A/T).

Figure-6-1. Gutter Section with Uniform Cross Slope

6-4 City of Durango October 2018 Storm Drainage Design Criteria Manual Chapter 6 Street, Inlets, and Storm Drains

For a triangular cross section as shown in Figure 6-1, Manning’s equation for gutter flow is written as:

1.8 0.56 5 / 3 1/ 2 Q  AR 2 / 3 S 1/ 2  S S T 8 / 3 Equation 6-1  o  x o Where:

Q = calculated flow rate for the half-street (cfs)

η = Manning’s roughness coefficient (0.016 for asphalt street with concrete gutter, 0.013 for concrete street and gutter)

R = hydraulic radius of wetted cross section = A/P (ft)

A = cross-sectional area (ft2)

P = wetted perimeter of cross section (ft)

Sx = street cross slope (ft/ft)

So = longitudinal slope (ft/ft)

T = top width of flow spread (ft).

The flow depth can be found using:

y  TSx Equation 6-2

Where:

y = flow depth at the gutter flowline (ft).

Note that the flow depth generally should not exceed the curb height during the minor storm based on Table 6-1. In order to solve for the anticipated gutter flow capacity of a street section as a function of the flow depth, Manning’s equation can be written in terms of the flow depth, as:

0.56 1 2 8 3 Q  SO y Equation 6-3 S x

The cross-sectional flow area, A, can be expressed as:

2 S x T A  Equation 6-4 2

The gutter velocity at peak capacity may be found from continuity (V = Q/A).

October 2018 City of Durango 6-5 Storm Drainage Design Criteria Manual

Street, Inlets, and Storm Drains Chapter 6

Capacity When Gutter Cross Slope is Not Equal to Street Cross Slope (Typical)

Streets with composite cross slopes like that shown in Figure 6-2 are often used to increase the gutter capacity and keep nuisance flows out of the traffic lanes.

Figure 6-2. Typical Gutter Section—Composite Cross Slope

For a composite street section:

Q  Qw  Qx Equation 6-5

Where:

Qw = flow rate in the depressed gutter section (flow within gutter width, W, in Figure 6-2 [cfs])

Qx = flow rate in the section that is outside the depressed gutter section and within the street width, TX, in Figure 6-2 (cfs).

In Hydraulic Engineering Circular No. 22, Third Edition, the Federal Highway Administration (FHWA 2009) provides the following equations for obtaining the flow rate in streets with composite cross slopes. The theoretical flow rate, Q, is:

Q Q  x Equation 6-6 1 Eo

Where:

1 EO  Equation 6-7 Sw / Sx 1 8/3  Sw / Sx  1  1  (T /W ) 1

6-6 City of Durango October 2018 Storm Drainage Design Criteria Manual Chapter 6 Street, Inlets, and Storm Drains

and, a S  S  Equation 6-8 w x W

Where:

EO = QW/Q, the ratio of gutter flow, QW, to total flow Q

W = width of the gutter (typical value = 1.5 ft)

SW = the gutter cross slope (typical value = 0.5/12 or 0.0.0417 [ft/ft])

a = gutter depression = WSW - WSX (typical value for WSW for a 1.5-ft gutter section is 0.0625 ft).

Figure 6-2 depicts all geometric variables. From the geometry, it can be shown that:

d  TSx  a Equation 6-9 and,

S T 2  aW A  x Equation 6-10 2

Where:

y = flow depth above depressed gutter section (ft). Note that the depth of flow at the gutter line is defined as d, where d = y + a (see Figure 6-2) and y = TSx (see Equation 6-2)

A = flow area (ft2)

Due to the complexity of Equation 6-7, care should be taken when calculating EO. Additionally, EO cannot be correctly calculated using Equation 6-7 when T-Tx < W or when ponding depth exists at the street crown. For these special cases, the principle of similar triangles may be applied in conjunction with Equation 6-5 (see Figure 6-3).

October 2018 City of Durango 6-7 Storm Drainage Design Criteria Manual

Street, Inlets, and Storm Drains Chapter 6

Figure 6-3. Calculation of Composite Street Section Capacity: Major Storm

Allowable Capacity

Stormwater flows along streets exert momentum forces on cars, pavement, and pedestrians. To limit the hazardous nature of large street flows, it is necessary to set limits on flow velocities and depths. As a result, the allowable half-street hydraulic capacity is determined as the lesser of:

QA  QT Equation 6-11 or

QA  R Qd Equation 6-12

Where:

QA = allowable street hydraulic capacity (cfs)

QT = street hydraulic capacity where flow spread equals allowable spread (cfs)

R = reduction factor (allowable street and gutter flow for safety)

Qd = street hydraulic capacity where flow depth equals allowable depth (cfs).

6-8 City of Durango October 2018 Storm Drainage Design Criteria Manual Chapter 6 Street, Inlets, and Storm Drains

There are two sets of safety reduction factors developed for the UDFCD region (Guo 2000b) that are also applicable in Durango. One is for the minor event, and another is for the major event. Figure 6-4 shows that the safety reduction factor does not apply unless the street longitudinal slope is more than 1.5% for the 100- yr event and 2% for the 10-year event. The safety reduction factor, representing the fraction of calculated gutter flow at maximum depth that is used for the allowable design flow, decreases as longitudinal slope increases.

It is important for street drainage designs that the allowable street hydraulic capacity be used instead of the calculated gutter-full capacity. Where the accumulated stormwater amount on the street approaches the allowable capacity, a street inlet should be installed.

Figure 6-4. Reduction Factor for Gutter Flow (Guo 2000b)

Swale Capacity

Where curb and gutter are not used to contain flow, swales are frequently used to convey runoff and disconnect impervious areas. It is very important that swale depths and side slopes be shallow for safety and maintenance reasons. Street-side drainage swales are not the same as roadside ditches. Street-side drainage swales provide mild side slopes and are frequently designed to provide water quality enhancement. For purposes of disconnecting impervious area and reducing the overall volume of runoff, swales should be considered as collectors of initial runoff for transport to other larger means of conveyance. To be effective, they need to be limited to the velocity, depth, and cross-slope geometries considered acceptable.

October 2018 City of Durango 6-9 Storm Drainage Design Criteria Manual

Street, Inlets, and Storm Drains Chapter 6

Equation 6-13 can be used to calculate the adjusted cross slope in a V-section swale (using the appropriate roughness value for the swale lining) for use in geometric equations such as Equation 6-3:

Sx1Sx2 Sx  Equation 6-13 Sx1  Sx2

Where:

Sx = adjusted side slope (ft/ft)

Sx1 = right side slope (ft/ft)

Sx2 = left side slope (ft/ft).

Figure 6-5 shows the geometric variables for a swale with a v-shaped cross-section.

For safety reasons, paved swales should be designed such that the product of velocity and depth is no more than six for the minor storm and eight for the major storm.

For grass swales, refer to the Open Channels Chapter. During the 2-year event, grass swales designed for water quality should have a Froude number of no more than 0.5, a velocity that does not exceed 1.0 ft/s, and a depth that does not exceed 1.0 foot. Grass swales designed for conveyance and not water quality shall be capable of conveying the minor event (10-year), while complying the inundation criteria from Table 6- 1.

Note that the slope of a roadside ditch or swale can be different than the adjacent street. The hydraulic characteristics of the swale can therefore change from one location to another.

Figure 6-5. Typical V-Shaped Swale Section 6-3 Inlets

6-3-1 Inlet Function and Selection

Inlets collect excess stormwater from the street, transition the flow into storm drains, and can provide maintenance access Photograph 6-1. City approved CDOT Type 13 Inlet. to the storm drain system. There are three major types of inlets: grate, curb opening, and combination. Table 6-2 provides considerations for proper selection of an inlet. Roadway geometry often dictates the location of street inlets along the curb and gutter. In general, inlets are placed at all low points (sumps), along continuous grade curb and gutter, at median breaks, intersections and crosswalks. The spacing of inlets along a continuous grade segment of roadway is

6-10 City of Durango October 2018 Storm Drainage Design Criteria Manual Chapter 6 Street, Inlets, and Storm Drains

governed by the allowable spread of flow as discussed in Section 6-2-1 of this chapter.

Photograph 6-2. City approved CDOT Type R Inlet. Photograph 6-3. City approved Combination Inlet.

Table 6-2. Inlet selection considerations Inlet Type Applicable Setting Advantages Disadvantages Road Class Grate Sumps and Perform well over Can become clogged Sump/Area continuous grades wide range of Lose some capacity Drains/Alley V- (should be made grades with increasing grade pans/Inverse bicycle safe) Crowns Curb- Sumps and Do not clog Lose capacity with Collector and opening continuous grades easily increasing grade Arterial (but not steep Bicycle safe grades) Combination Sumps and High capacity More expensive than Local continuous grades Do not clog grate or curb-opening (should be made easily acting alone bicycle safe)

6-3-2 Types of Inlets

Inlets approved for use within the right-of-way are specified in the City of Durango Construction Specifications Manual (other types of inlets may be acceptable with review and approval by the City Engineer).

6-3-3 General Design Guidelines

The following guidelines shall be used when designing inlets along a street section:

 Design and location of inlets shall take into consideration pedestrian and bicycle traffic. All inlet grates shall be pedestrian and bicycle-safe.

 Design and location of inlets shall be in accordance with the allowable spread and depth criteria provided in Section 6-2-1 of this Chapter.

 Maintenance of inlets shall be considered when determining inlet locations. The slope of the street, the potential for debris and ice accumulation, the distance between inlets and/or manholes, and

October 2018 City of Durango 6-11 Storm Drainage Design Criteria Manual

Street, Inlets, and Storm Drains Chapter 6

other factors shall be considered. Maintenance access shall be provided for all inlets.

 To avoid potential damage from large vehicles driving over the curb return and interference with pedestrian traffic, inlets shall not be placed in the curb return radii.

 Selection of the appropriate inlet grate shall be based on a number of factors, including, but not limited to, the adjacent land use and potential for pedestrian or bicycle traffic, the potential for debris and ice accumulation, visibility, expected loading from vehicles, and hydraulic capacity.

 In many cases, inlets are necessary at grade breaks, where street or ditch grades change from steep to relatively flat, because of the reduced conveyance capacities at those locations. Additionally, it

Additional Information on Nuisance Flows

For more information on nuisance flows, multiple Colorado-based publications are available to provide guidance related to landscape management practices and snow and ice removal. Representative resources include:  USDCM Volume 3, Source Control BMPs (http://udfcd.org/)  GreenCO BMP Manual (http://www.greenco.org/current-bmps.html)  Colorado State University Extension Yard and Garden Fact Sheets (http://extension.colostate.edu/topic-areas/)

is common for icing or sediment deposition to occur with nuisance flows in reaches where grades are relatively mild.

 Inlets, whether precast or cast-in-place shall be constructed so that the cap or top of the inlet matches the slope of the street. This is particularly important on streets with steep slopes, as the elevation difference between the beginning and end of the inlet can be significant, particularly when double or triple inlets are used.

6-3-4 Nuisance Flows

The location of inlets is important to address the effects of nuisance flows and avoid icing. Nuisance flows are urban runoff flows that are typically most notable during dry weather and come from sources such as over-irrigation and snow melt. Nuisance flows cause problems such as algae in warm weather months and ice in cold weather months. While it is possible to minimize nuisance conditions through design, irrigation practices in the summer and snow and ice removal in the winter make it very difficult to eliminate nuisance flows entirely. Because these practices are largely controlled by residents and businesses, designers should consider the need for maintenance access to address nuisance flow conditions, particularly in the winter when ice accumulation can impede the ability of the drainage system to serve its purpose.

6-3-5 Hydraulic Evaluation of Inlets

Inlet Capacity Factors and Calculations

The capacity of an inlet located on a continuous grade is dependent upon a variety of factors including gutter slope, depth and velocity of flow in the gutter, height and length of the curb opening, street cross slope, and the amount of depression at the inlet. Inlets placed on continuous grades rarely intercept all of the flow in the gutter during the minor storm. This results in flow continuing downstream of the inlet and is typically referred to as “bypass.” The amount of bypass must be accounted for in the drainage system

6-12 City of Durango October 2018 Storm Drainage Design Criteria Manual Chapter 6 Street, Inlets, and Storm Drains

evaluation, as well as in the design of the downstream inlet. Inlets are most efficient in a sump condition or along mild continuous street grades. The capture efficiency of a curb-opening inlet depends on the length of the opening, the depth of flow at the curb, the street cross slope, and the longitudinal gutter slope. If the curb opening is long, the flow rate is low, and the longitudinal gutter slope is small, all of the flow may be captured by the inlet. However, a portion of the stormwater often bypasses the inlet as indicated by the inlet efficiency. See the Streets/Inlets/Storm Drains Chapter in Volume 1 of the UDFCD Manual for additional information on the efficiency and design of curb opening inlets on continuous grades.

The City of Durango Construction Specifications Manual provides capacity charts for acceptable curb opening inlets on continuous grades along standard street sections for the minor and major storm events, based on the maximum allowable flow in the street section, incorporating appropriate clogging factors. Sump condition figures are also provided. These charts may be used to evaluate the street if it is at the maximum allowable flow. The UDFCD Manual also includes charts for other types of inlets, which may be considered and approved on a case-by-case basis if standard inlets are not sufficient.

When flow in the gutter is less than the maximum flow, the UD-Inlet spreadsheets shall be used to determine the interception by the proposed inlet. See the Streets/Inlets/Storm Drains Chapter of the UDFCD Manual for further discussion on the use of UD-Inlet for less than maximum allowable flow.

The Streets/Inlets/Storm Drains Chapter of the UDFCD Manual provides detailed instruction on the appropriate analysis of inlet capacities including equations, coefficients, and examples. The worksheets are the most accurate means of determining inlet capture rates and capacity calculations. The spreadsheets analyze both street capacities and inlet interception rates for both the minor and major storm events simultaneously. The UD-Inlet Spreadsheets may be downloaded from the UDFCD web site at www.udfcd.org.

Design of Inlets on Continuous Grade

The primary design considerations for the location and spacing of inlets on continuous grades are the encroachment and inundation limitations. This was addressed in Section 6-2-1 of this chapter. Table 6-1 lists pavement encroachment and inundation standards for minor storms.

Proper design of stormwater collection and conveyance systems makes optimum use of the conveyance capabilities of street gutters, such that an inlet is not needed until the spread (encroachment) and depth (inundation) reach allowable limits outlined in Section 6-2. To place an inlet prior to that point on the street is not economically efficient. To place an inlet after that point would violate the encroachment and inundation standards. Therefore, the primary design objective is to position inlets along a continuous grade at the locations where the allowable spread and/or depth is about to be exceeded for the design storm.

Based on the encroachment and inundation standards and the given street geometry, the allowable street hydraulic capacity can be determined using Equation 6-1 and Equation 6-5. This flow rate is then equated to some hydrologic technique (equation) that contains drainage area. In this way, the inlet is positioned on the street so that it will service the requisite drainage area. The process of locating the inlet is accomplished by trial-and-error. If the inlet is moved downstream (or down gutter), the drainage area increases. If the inlet is moved upstream, the drainage area decreases.

October 2018 City of Durango 6-13 Storm Drainage Design Criteria Manual

Street, Inlets, and Storm Drains Chapter 6

The hydrologic technique most often used in urban drainage design is the rational method. The rational method was discussed in the Hydrology Chapter. The Rational Method, repeated here for convenience, is:

Q  CIA Equation 6-13

Where:

Q = peak discharge (cfs) C = runoff coefficient described in the Hydrology Chapter I = design storm rainfall intensity (in/hr) described in the Hydrology Chapter A = drainage area (acres).

The design process starts with the selection of the proposed first inlet in the system. The peak discharge for the half-street at this point is calculated by the rational method, using runoff coefficients and rainfall intensities as described in the Hydrology Chapter. Next, the allowable peak discharge is found using the allowable spread and depth calculated as functions of the street geometry at the design point. If the allowable peak discharge is less than the watershed peak discharge, the proposed design point is too far downstream in the watershed and must be moved upstream. If the allowable peak discharge is much greater than the calculated peak discharge, no inlet is required at the proposed design point and a new location for the proposed first inlet in the system is selected somewhere downstream of this location. Inlets shall be placed upstream of the point where the allowable spread with reduction factor and/or depth criteria would otherwise be exceeded.

Once the first inlet location is identified along a continuous grade, an inlet type and size can be specified. The first inlet’s hydraulic capacity is then assessed. Generally, it is uneconomical to size an inlet (on continuous grades) large enough to capture all of the gutter flow. Instead, some bypass flow is expected. This practice reduces the amount of new flow that can be picked up at the next inlet. However, each inlet should be positioned at the location where the spread or depth of flow is about to reach its allowable limit.

The gutter discharge used for sizing inlets (except for the most upstream inlet), consists of the bypass flow from the upstream inlet plus the stormwater runoff generated from the intervening local drainage area. The carryover flow from the upstream inlet is added to the peak flow rate obtained from the rational method for the intervening local drainage area. The resulting peak flow is conservatively approximate since the carryover flow peak and the local runoff peak do not necessarily coincide.

Design of Inlets in Sump Condition

Inlets located in sumps (low points) must be sized to intercept all of the design storm flows at an allowable depth of ponding. The capacity of an inlet in a sump condition is dependent upon the depth of ponding above the inlet and the amount of debris clogging the inlet. Ponded water is a nuisance and can be a hazard to the public; therefore curb opening and combination inlets (where approved for use) are highly recommended for sump conditions due to their reduced clogging potential versus grate inlets acting alone. Capacity charts for curb opening and combination inlets in a sump condition are provided in The City of Durango Construction Specifications Manual. The depth of ponded water shall not exceed the maximum allowable water depth for the given street classification as summarized in Section 6-2-1 of this chapter.

6-14 City of Durango October 2018 Storm Drainage Design Criteria Manual Chapter 6 Street, Inlets, and Storm Drains

6-4 Storm Drain Systems

6-4-1 Introduction Runoff Reduction and Stormwater Quality Considerations

Traditionally, urban development has relied on storm drain systems in the upper portions of watersheds to prevent local flooding and to carry flows away quickly. As storm drains pick up more drainage area, they increase in size and convey urban runoff quickly downstream with little to no reduction in rate or volume or improvement in water quality.

Today, with the emphasis on runoff reduction and water quality enhancement, stormwater management practices are being revised to promote infiltration, attenuation and water quality enhancement. Properly designed sites with grass swales and other conveyances that provide opportunity for infiltration can serve in place of storm drains and/or allow smaller and less extensive storm drains to be constructed downstream. Disconnecting impervious area through the use of downspout routing to pervious areas, grass buffers and/or porous landscape detention (bioretention) can reduce demands on the downstream conveyance system. These types of practices, often termed “minimizing directly connected impervious areas” or “Low Impact Development” can also improve the quality of stormwater runoff and reduce the amount of dedicated water quality features required.

Although grass swales are compatible with many land uses, such as residential, parks, institutional, and others with relatively low densities, grass swales may not be practical in areas where there are many access points across the planned drainage path. Therefore, storm drains will continue to be an integral part of many drainage systems. Once stormwater is collected from the street by an inlet, it is directed into the storm drain system. The storm drain system is comprised of inlets, pipes, manholes, bends, outlets, and other appurtenances.

Apart from inlets, manholes are the most common appurtenance in storm drain systems. Their primary functions include:

 Providing maintenance access.

 Serving as junctions when two or more pipes merge.

 Providing flow transitions for changes in pipe size, slope, and alignment.

 Providing ventilation.

Manholes are generally made of pre-cast or cast-in-place reinforced concrete. They are typically four to five feet in diameter and are required every 400’ per Section 14-218 of the City’s Municipal Code, even in straight sections, for maintenance reasons. Manholes are also required at the location of any bends. Standard size manholes cannot accommodate large pipes, so special junction vaults are constructed for that application.

Outlet structures, covered in the Hydraulic Structures Chapter, are transitions from pipe flow into open channel flow or still water (e.g., ponds, lakes, etc.). Their primary function is to provide a transition that minimizes erosion in the receiving water body. Occasionally, flap gates or other types of check valves are placed on outlet structures to prevent backflow from high tailwater or flood-prone receiving waters.

October 2018 City of Durango 6-15 Storm Drainage Design Criteria Manual

Street, Inlets, and Storm Drains Chapter 6

6-4-2 Design Storms

Both the “minor” and “major” storm events must be considered for properly designing storm drains. In each case, storm drains are to be designed to carry the portion of runoff that cannot be conveyed on the surface, as dictated by the available capacity in streets and swales.

Minor Event

At a minimum, storm drain systems are to be designed to convey storm runoff for the minor (10-year) event. Inlets shall be located to intercept runoff before street capacities are exceeded. See Section 6-2 for a more detailed discussion on inlet size and spacing requirements. Additional information on storm drain design is provided in the Streets, Inlets and Storm Drains Chapter of the UDFCD Manual.

Major Event

Under certain conditions, the storm drain system must be designed to convey flows greater than the minor storm runoff, in some cases up to the major (100-year) storm runoff. These conditions include the following types of locations and configurations:

 Where the street capacity for the major storm is exceeded.

 Where street crown overtopping would otherwise exceed criteria.

 Where major storm flows can split off in undesirable directions (i.e., flow splits at intersections).

 Where the storm drain system is accepting flow from an upstream storm drain system or branch that is designed for the major storm.

 Where regional storm drains are designed for the major storm.

 Where storm drains must convey undetained flows to a detention pond.

 Situations where surface flow of the major storm event would unduly interfere with use of the property or cause unwarranted damage.

If a storm drain is designed to carry major storm flows, the inlets to the storm drain shall be sized accordingly. The major storm event hydraulic grade line may be allowed to rise above the top of the storm drain pipe and surcharge the system. The ability of the storm drain to convey the major storm event shall be based on its capacity when the hydraulic grade line elevation is at least 1 foot below the flowline of gutter. In no case shall the surcharge create system velocities in excess of the maximum defined in Section 6-4-9 of this chapter.

6-4-3 Pipe Material, Size and Service Life

Pipe Material

All storm drains located within public rights-of-way, public easements or tracts shall be constructed with reinforced concrete pipe (RCP), SDR-35 PVC, or Non-Corrugated High Density Polyethylene Pipe (HDPE) up to 30 inches in diameter. Circular pipe is the most cost-effective option for reinforced concrete, but elliptical pipe or box conduits may be a more appropriate option in areas where available cover is limited or to avoid utility conflicts. 6-16 City of Durango October 2018 Storm Drainage Design Criteria Manual Chapter 6 Street, Inlets, and Storm Drains

Alternate pipe materials may be considered with approval prior to submittal of drainage reports for review. Trench details, bedding material, installation specifications, minimum cover or fill height limits, service life and construction testing requirements for alternate pipe materials shall be consistent with those recommended by the manufacturer/supplier or as determined appropriate.

Minimum Pipe Size

The minimum pipe diameter for storm drains located within rights-of-way, public easement or tracts shall be 12 inches for laterals and 18 inches for trunk lines that collect flow from laterals or from upstream trunk lines.

Service Life

The service life for storm drain systems shall be 50 years. An extended service life of 100 years shall be required under these conditions:

 The depth of cover exceeds 15 feet.

 The system is located within the travel lanes of 4-lane or major and minor arterial roadways.

 The centerline of the storm drain pipe is located 15 feet or less horizontally from any building/permanent structure.

6-4-4 Other Storm Drain Design Considerations

RCP Pipe Class

Reinforced concrete pipe (RCP) shall conform to the requirements of AASHTO M 170. The wall thickness and strength class of RCP and non-RCP shall be determined in accordance with Colorado Division of Highways Standard M-603 RC unless otherwise specified. For storm drainage only, concrete pipe with tongue and groove joints may be used.

The minimum class of reinforced concrete pipe shall be Class III, however, the depth of cover, live load, and field conditions may require structurally stronger pipe. Trench installation requirements, trench installation details, and allowable fill heights are shown in City of Durango Construction Specifications Manual. It is the responsibility of the design engineer to develop and submit alternate trench and installation details when project specific conditions or loadings require modification to the standard installation. Alternate designs shall follow ASTM C1479.

Joints

All storm drains shall have gasketed and/or water-tight joints, regardless of material type, and installed in accordance with the manufacturers recommendations. Properly installed tongue-and-groove concrete pipe should have an interior joint gap of no more than 3/8-inch for pipe less than 36-inch diameter installed in a straight run. ASTM Standard C 443 covers flexible watertight joints for circular concrete sewer pipe and precast manhole sections, using rubber gaskets for sealing the joints.

October 2018 City of Durango 6-17 Storm Drainage Design Criteria Manual

Street, Inlets, and Storm Drains Chapter 6

Outfalls

Where storm drains discharge into open channels or detention ponds, outlet protection of the bank and overbank or pond bottom shall be provided to prevent erosion due to flows discharged from the storm drain. Erosion protection shall be designed to convey the storm drain design flow assuming that no flow is in the receiving channel or pond. The stability of the outfall protection must also be evaluated based on the flow conditions in the receiving channel.

Outlets to Junction Creek and the Animas River shall be located above the ten-year flood level and shall be protected with a concrete headwall.

Trash / Safety Racks

Trash racks or grates shall be installed for safety purposes at all storm drain entrances. Typical trash rack details are provided in the City of Durango Construction Specifications Manual. Even relatively small diameter pipes can pose a risk due to the potential for hydrostatic forces to pin a person against the entrance to the storm drain. When trash racks are installed on culvert or storm drain outfalls, frequent inspection is recommended due to potential for accumulation of debris against the interior face of the downstream grate.

6-4-5 Vertical Alignment

When feasible, locating storm sewers below the frost depth (approximately 32 inches in the City of Durango) will aid with freezing and ice buildup; however, in practice, it is not always practical to have storm drains this deep due to conflicts with other utilities and elevations constraints. As specified in the Construction Specifications Manual, the minimum depth of storm sewers from the top of pipe to finished grade should be 2 feet. The maximum cover over storm drains shall also be considered and evaluated according to manufacturer’s specifications.

Please see the City of Durango Construction Specifications Manual for more detailed information on required cover, utility clearances, and other vertical alignment requirements and considerations.

6-4-6 Horizontal Alignment

Alignment

In general, storm drain alignments between drainage structures (inlets or manholes) shall be straight. The angle of confluence where pipe lines intersect shall be 90 degrees or less and where a lateral pipe of 36 inches or greater intersects a trunk line, the angle of confluence shall be 60 degrees or less. Manhole covers shall not be closer than one foot to the edge of the gutter pan. Storm sewer lines shall be placed within the pavement of public streets. Prior City approval is needed for placing storm sewer within the public streets’ tree lawns or medians. Proposed alignments which cannot meet this criteria shall be reviewed and approved by the City Engineer.

6-4-7 Easements

Easement Conveyance

Storm drains shall normally be installed within public right-of-way, easement or tracts, but when it is necessary to route a system through private property, drainage easements are required in order to ensure the proper construction, access and maintenance of storm drains and related facilities. All easements shall be a 20 feet wide or greater depending on pipe size, and conveyed by appropriate legal documents such as plats or grant of easements. Please see the Drainage Principals and Policies Chapter for additional information on easement requirements. 6-18 City of Durango October 2018 Storm Drainage Design Criteria Manual Chapter 6 Street, Inlets, and Storm Drains

Allowable Landscaping and Surface Treatment in Easements

Although storm drain systems are designed to have a significant service life, there are circumstances which may require that the storm drain be accessed for inspection, maintenance, repair, and/or replacement. Storm drain easements should be designed to convey above ground flows in the event the storm drain or inlet becomes clogged or full flows exceed the design flow. It is, therefore, necessary to limit uses within the easement to ensure that surface conveyance redundancy and maintenance access is not impaired. Minor landscaping, including rock, shrubs, etc. may be appropriate where it can be demonstrated that the function of the easement is not compromised by the presence of the materials. Pavement over a storm drain easement may be allowable, providing that the property owner accepts responsibility for replacement in the event it is necessary to remove it to access the system. Improvements that are not allowed on storm drain easements include structures of any kind, retaining walls, permanent fencing, trees and others if determined to be a problem and/or costly to replace. Surface treatments on drainage easements shall be shown on the drainage report plan and final development plan.

6-4-8 Manholes

Please see the City of Durango Construction Specifications Manual for design information associated with various manhole types.

Required Locations

Manholes or inlets are required whenever there is a change in size, direction, material type, or grade of a storm drain pipe to provide a hydraulic transition and maintenance and inspection access. A manhole shall also be constructed when there is a junction of two or more sewer pipes. The maximum spacing between manholes for all pipe sizes shall be 400 feet.

Drop Manholes

The drop within a manhole from the upstream to downstream pipe invert should normally not exceed 1 foot. There are cases when a drop larger than 1 foot may be necessary to avoid a utility conflict, reduce the slope of the downstream pipe, or to account for the energy losses in the manhole. Drop manholes shall be constructed per the City of Durango Construction Specifications Manual.

Other Hydraulic Design Considerations

The following design criteria shall also be met:

 Pipes shall not decrease in diameter from upstream to downstream.

 The invert of a manhole shall be constructed with a drop between the upstream and downstream pipes. The drop shall be 0.2 feet on straight-through alignments, and 0.3 feet for lateral pipes of the same diameter. When different diameter pipes are used, crowns shall be matched.

 All manhole tops shall be eccentric to provide safe access by alignment with manhole steps and with benches in manhole bases.

 It is critical that gutter pans, curb heads, and any other problematic locations be avoided when determining the horizontal placement of manholes.

 In many cases the manhole diameter will need to be increased to account for changes in pipe

October 2018 City of Durango 6-19 Storm Drainage Design Criteria Manual

Street, Inlets, and Storm Drains Chapter 6

alignment or multiple incoming pipes. Manhole bases shall be shaped to match the pipe section below the pipe springline. This shaping significantly reduces manhole losses. The appropriate loss coefficient can be determined using the UDFCD Manual for full shaping. The standard details in the UDFCD Manual provide guidance for shaping in the slab base.

6-4-9 Hydraulic Design

Once the alignment of the storm drain system is determined, the peak flows in the system must be calculated followed by a hydraulic analysis to evaluate system characteristics and determine pipe capacity and size. The pipe size shall not decrease moving downstream (even if the capacity is available due to increased slope, etc.) in order to reduce clogging potential. The City of Durango requires designers to submit pipe capacity and sizing calculations to the City based on results from the UD-Sewer or EPA SWMM programs. Other programs may be used by designer to perform initial design calculations; however, the City will require the use of UD-Sewer or EPA SWMM to present their final analysis of the proposed storm drain system.

Allowable Velocity and Slope

The allowable storm drain velocity is dependent on many factors, including the type of pipe, the acceptable water level during the pipe design life, proposed flow conditions (open channel versus pressure flows), and the type and quality of construction of joints, manholes, and junctions.

1. Maximum Velocity. In consideration of the above factors, the maximum velocity in all storm drains shall be limited to 10 feet per second (feet/sec) for all design flows.

2. Minimum Velocity. The need to maintain a self-cleaning storm drain system is recognized as a goal to minimize the costs for maintenance of storm drain facilities. Sediment deposits, once established, are generally difficult to remove even with pressure cleaning equipment. Maintaining minimum velocities for frequently occurring flows will reduce the potential for sediment and debris accumulation. Storm drains shall be designed with a minimum velocity of 2.5 feet/sec for a flow equal to 25 percent of the minor storm flow rate

3. Minimum Slope. In general, the minimum allowable pipe slopes (in no case less than 0.5%) ensure that the minimum velocity is achieved, in those cases where the pipe is designed to flow near full.

Minor Storm Event Hydraulic Evaluation

In the minor storm event, inlets are placed along the roadway where the flow in the roadway exceeds the minor event capacity of the street as defined in Section 6-2-1 of this chapter. These inlets intercept flow, as determined by the procedures in Section 6-3 of this chapter, and convey it to a storm drain which must be sized to convey the intercepted flow.

The storm drain system shall be designed to convey the minor design storm without surcharging so that the design flow depth in the pipe is no greater than 80 percent of the pipe height. Hydraulic grade line calculations must be performed to account for energy losses and to ensure that the system is not surcharged during the minor storm event. There may be some special cases where the proposed storm drain pipe is connected to an existing storm pipe (or a detention pond). If this existing pipe is surcharged, then the proposed system will receive backwater from the downstream pipe. In this situation, the minor event hydraulic grade line must be calculated to determine the impacts on the hydraulic grade line through the upstream portions of the system. Where the storm drain outfalls into a detention pond or channel, the tailwater condition will be determined based on the hydraulic grade elevation for the minor design storm event occurring in the receiving facility. Output from the UD-Sewer program for evaluation of the minor 6-20 City of Durango October 2018 Storm Drainage Design Criteria Manual Chapter 6 Street, Inlets, and Storm Drains

storm shall be provided for review by the City.

Major Storm Event Hydraulic Evaluation

The storm drain system layout determined for the minor event analysis must also be evaluated for the major storm event. If necessary, larger or additional inlets must be placed along the roadway when the flow in the roadway exceeds the major storm event capacity of the street as defined in Section 6-3 of this chapter. The interception rates for all of the inlets shall then be calculated for the major storm event, based on the procedures in Section 6-3 of this chapter, and the minor storm pipe sizes must be adjusted to convey the additional flows.

In the major storm event, it is acceptable to have a surcharge in the system. Manning’s equation is not applicable for pipes under pressurized flow conditions. For pressurized flow conditions, use the Bernoulli equation (Darcy-Weisbach Friction Loss) or the Hazen-Williams equation. The UD-Sewer program utilizes the Bernoulli equation for pressurized flow conditions analysis. There may be cases where the major storm event does not result in a surcharge of the system. In these pipes, the capacity can be calculated using Manning’s equation.

Hydraulic grade line (HGL) and energy grade line (EGL) calculations for the storm drain system shall be provided for the major storm event. For the major event, the hydraulic grade line shall be at least 1 foot below the flowline of the gutter. Proposed designs which cannot meet this 1 foot freeboard requirement criteria shall be reviewed by the City Engineer and approved on a case by case basis. Additional information on calculation of the HGL and EGL including loss coefficients through a storm drain system (at bends, junctions, transitions, entrances, and exits) can be found in the UDFCD Manual. The computed HGL shall be plotted on the construction drawings for each design flow, and the design flow and design frequency shall be noted on the drawing. In addition, the computed EGL for the major design flow shall be shown. Where the storm drain outfalls into a detention pond or channel, the tailwater condition will be determined based on the hydraulic grade elevation for the major design storm event occurring in the receiving facility. Output from the UD-Sewer program for evaluation of the major storm shall be provided for review by the City.

6-4-10 Hydraulic Calculations

To show that a proposed design conforms to the design criteria described herein, appropriate hydraulic calculations must be completed and provided in an organized form. The methods and parameters described in the UDFCD Manual for analyzing storm drains are tools that may be applied.

The City requires designers to use the UD-Sewer program or EPA SWMM to present hydraulic calculations. Other computer programs, such as StormCAD, HydroCAD, StormCAD, and others may be used for initial design purposes, however this information must be translated into the UD-Sewer program or EPA SWMM and provided to the City. See the UDFCD Manual for guidance on EPA SWMM, UDFCD methods, and UD-Sewer for analyzing storm drains. In addition to providing a description of the methods used to evaluate the hydraulic design of the storm drain system, adequate documentation of the system characteristics and configuration must be provided in both a detailed and summary format. The summary information for the entire system must show the parameters, coefficients and results for each system element in a tabular format. Documentation must include all input parameters including design flows by location, elevations, sizes, junction losses, coefficients, pipe roughness, alignment deflections, and other relevant information. Documentation must also show the results of the calculations including velocity by location, flow depth, Froude Number, HGL and EGL elevations (profiles), pipe capacities, and other information necessary to confirm that design criteria have been satisfied.

October 2018 City of Durango 6-21 Storm Drainage Design Criteria Manual

Street, Inlets, and Storm Drains Chapter 6

6-5 Examples

See the UDFCD Manual for design examples.

6-22 City of Durango October 2018 Storm Drainage Design Criteria Manual