Chapter 28.40


28.40.010    Introduction.

28.40.020    Storm drain design criteria.

28.40.030    Storm drain design criteria – Allowable capacity.

28.40.040    Storm drain design criteria – Allowable velocity.

28.40.050    Storm drain design criteria – Pipe roughness.

28.40.060    Storm drain design criteria – System layout.

28.40.070    Storm drain hydraulics.

28.40.080    Gravity-flow analysis.

28.40.090    Pressure-flow analysis.

28.40.100    Computer hydraulic modeling.

28.40.110    Construction standards.

28.40.120    Storm drain pipe.

28.40.130    Manholes.

28.40.140    Inlets.

28.40.150    Outlets.

28.40.160    Storm drain system design.

28.40.170    Initial storm drain design.

28.40.180    Preliminary/final storm drain design.

28.40.190    Example design application.

28.40.010 Introduction.

(a)    Storm drains are used to convey runoff in locations where streets or other drainage facilities exceed their designated capacity or are otherwise unable to drain. The most common method for the introduction of water into a storm drain is the street inlet, discussed in Chapter 28.44 GJMC. However, water may also enter the system via grated area inlets, culvert-type inlets (typically for the conveyance of drainage-channel flow into the drain), pump stations, or other entry points. The design of a storm drain system is dependant on topography, street rights-of-way and drainage easements, the need to convey flows from multiple locations, existing and proposed structures and utilities, outfall locations, local hydrology, and regional and local design criteria.

(b)    Typically, storm drains are sized to convey the peak runoff from the minor storm in excess of the contributing street flow capacity. This means the upper end of a storm drain branch will usually be located at the first inlet encountered by runoff in a given sub-watershed. As discussed in Chapter 28.44 GJMC, the first inlet will either be located at the point where street flow from the design storm exceeds street capacity for that storm (on-grade inlet) or where there is a vertical sag in the street (sump inlet). In some cases, however, street inlets discharge their intercepted flow to drainage facilities other than a storm drain (e.g., a drainage channel). Storm drains shall be sized to carry the maximum difference between street capacity and peak runoff for any given design storm. This could be the difference between the major storm peak runoff and the allowable street capacity for the major storm, or it could be the difference between the minor storm runoff and allowable street capacity for the minor storm. This is discussed further in GJMC 28.40.160 through 28.40.190.

(c)    Occasionally, inlets and storm drains must be sized to convey the entire major storm event flow. Two examples of this situation follow:

(1)    Locations where street flow is not in the desired direction and there is no other feasible drainage solution (such as closed basins – natural ponding areas).

(2)    Locations where the standard allowable major storm street capacities do not apply, such as negative slopes outside the curb but within the right-of-way.

(d)    Peak runoff values are found using the methods set forth in Chapters 28.24 and 28.28 GJMC.

(Res. 40-08 (§ 1001), 3-19-08)

28.40.020 Storm drain design criteria.

GJMC 28.40.020 through 28.40.060 present certain parameters relating to the design and construction of storm drain systems in Mesa County.

(Res. 40-08 (§ 1002), 3-19-08)

28.40.030 Storm drain design criteria – Allowable capacity.

As described in GJMC 28.40.010 and 28.40.160 through 28.40.190, a storm drain is designed to convey up to the entire design storm for all sub-watersheds tributary to it. The design of pressure-flow, or surcharged, storm pipes is allowed under certain restrictions as specified in this chapter. These include the calculation of energy grade lines (EGLs) and hydraulic grade lines (HGLs) indicating all hydraulic losses due to friction, junctions, and other structures and phenomena. The EGL for the storm drain design flow must at no time or location exceed the manhole rim or inlet throat elevation. More restrictive local criteria may apply; it is the responsibility of the designer to select the most restrictive of all applicable design criteria. Note that calculation of EGL and HGL is mandatory on all projects for the drainage plan submittal.

For the purpose of completing a conceptual storm drain system design, calculation of an EGL and HGL is not required. In these cases, the initial design methods presented in GJMC 28.40.170 (using open-channel hydraulics as presented in GJMC 28.40.080) are considered sufficient. The specific requirements for a conceptual drainage report are detailed in GJMC 28.12.030 through 28.12.050.

(Res. 40-08 (§ 1002.1), 3-19-08)

28.40.040 Storm drain design criteria – Allowable velocity.

Minimum velocities are required in storm drains to reduce sedimentation and promote positive drainage through the pipe at all depths. A minimum design-flow velocity of 2.5 feet per second is required for standard (positive-slope) storm drains. Table 28.40.040 provides required slope values needed to maintain this minimum velocity for different pipe sizes and roughness factors.

While concrete pipe itself “can carry clear water of extremely high velocities without eroding” (ACPA, 1996), there exist numerous other factors that indicate the need for a maximum velocity in storm drains. Among these are the use of other pipe materials and shapes, expected flow conditions, and the “type and quality of construction of joints, manholes, and junctions” (Washoe County, 1996). Therefore, storm drains shall have a maximum design-flow velocity of 15 feet per second. Note that maximum outfall velocities are more restrictive to protect those areas from extensive erosion. See Chapter 28.32 GJMC, Open Channels; Chapter 28.36 GJMC, Additional Hydraulic Structures; and Chapter 28.48 GJMC, Culverts and Bridges, for details.

(Res. 40-08 (§ 1002.2), 3-19-08)

28.40.050 Storm drain design criteria – Pipe roughness.

Roughness effects tend to vary with changes in flow depth and installation inconsistencies. To simplify design and ensure consistency, this manual specifies roughness values and does not permit the use of pipe manufacturers’ values. Table 28.40.050 provides a range of Manning’s n values for many pipe materials and configurations as developed by Chow in 1959 and Normann in 1985 (adapted from tables found in HDS-4 and HEC-22). For purposes of storm drain design, hydraulic roughness shall be specified by the largest Manning’s n value in the provided range.

The designer may choose to use a higher Manning’s n value if conditions warrant.

(Res. 40-08 (§ 1002.3), 3-19-08)

28.40.060 Storm drain design criteria – System layout.

The layout of a storm drain system is dependent on topography, hydrology, surface hydraulics, easements and rights-of-way, existing structures and utilities, outfall locations, and other factors. General criteria for the design of a storm drain layout follow.

(a)    Vertical Alignment.

(1)    Minimum and maximum cover are determined by the size, material, and class of pipe, as well as the characteristics of the cover material and the expected surface loading. The designer shall consult the appropriate data sources, to include:

(i)    Colorado Department of Transportation Standard Specifications for Road and Bridge Construction, Section 700 (Materials Details).

(ii)    Concrete Pipe Design Manual (ACPA).

(iii)    Handbook of Steel Drainage and Highway Construction Products (AISI).

(iv)    Pipe manufacturer specifications.

(v)    Other applicable references.

Storm drains crossing under railroads and highways must comply with any cover requirements specified for culverts (Chapter 28.48 GJMC).

(2)    Pipes installed under any driving or parking area shall be designed for H-20 minimum live load. (City of Grand Junction Standard Specifications for Construction of Underground Utilities – Waterlines, Sanitary Drain, Storm Drains, Underdrains and Irrigation Systems).

(3)    Storm drain mains (any storm drain to which laterals connect) shall have a minimum cover of 36 inches over the top of the pipe. This minimum includes any pavement thickness, but does not replace the minimum cover and compaction requirements designated by local standards and the application of valid structural loading computations.

(b)    Horizontal Alignment.

(1)    Storm drain bends, whether completed using the pulled-joint method, bend pipe, or radius (curved) pipe, shall be avoided where possible. Bends are not allowed for storm drain pipe of less than 48-inch diameter. Table 28.40.060(a) shows the maximum allowed deflection for pulled-joint construction.

Table 28.40.060(a): Maximum Allowed Deflection for Pulled-Joint Construction

Pipe Diameter or Span (inches)

Allowed Deflection (Pull) Per Joint

48 – 54


60 – 78


84 – 102


108 – 144


(2)    Per the City of Grand Junction General Utility Details, storm drain manholes shall be located at the centerline of a traffic lane. Storm drain mains are to be located on the south or west side of a roadway, and must have a minimum horizontal clearance of 6.0 feet from the roadway centerline to the storm drain centerline. In cases where the sanitary drain main is not located at the street centerline, the designer shall consult with the appropriate local jurisdiction to determine the required horizontal and vertical clearances.

Maximum allowable spacing between manholes is presented in subsection (d) of this section and Table 28.40.060(b).

(c)    Utility Clearances. The designer shall consult with the most recent versions of the following documents to ensure compliance with the most restrictive (largest) utility clearance values applicable to the subject location:

(1)    City of Grand Junction Standard Specifications for Construction of Underground Utilities – Waterlines, Sanitary Drains, Storm Drains, Underdrains and Irrigation Systems.

(2)    City of Grand Junction Standard Details for Construction of Streets, Storm Drains, and Utilities.

(3)    City of Grand Junction Transportation Engineering Design Standards (TEDS) Manual, GJMC Title 29.

(4)    Any utility clearance requirements set forth by a local jurisdiction or special district.

Pipe encasement may be required in some locations where minimum utility clearances are unable to be met. Standards for the design and installation of casing pipe and concrete encasement can be found in the General Utility Details of the City of Grand Junction Standard Details for Construction of Streets, Storm Drains, and Utilities.

(d)    Manholes. Manholes are necessary to provide maintenance and inspection access to the storm drain. When designed correctly, they also provide more hydraulically efficient pipe junctions and other transitions. All manhole lids must bear the words “storm water” for identification purpose.

(1)    For storm drain pipes of less than 48-inch diameter, a manhole must be located at all changes in main-line pipe size or grade, junctions where a lateral joins the main-line alignment at a higher elevation (vertical drops), main-line vertical drops (drop manhole), and main-line direction changes or bends. Manholes located at storm drain bends shall be located at either tangent intersection or within the bend itself.

(2)    Pipes of 48 inches or larger diameter do not necessarily require manholes at all locations specified above. However, manholes in addition to those required by standard maximum spacing may be stipulated by local jurisdiction.

(3)    Table 28.40.060(b) indicates maximum spacing for manholes. Noncircular pipes shall be converted to equivalent diameters based on pipe area.

Table 28.40.060(b): Maximum Manhole/Accessible Junction Spacing

Equivalent Pipe Diameter

Maximum Allowable Manhole Spacing

Less than 48 inches

400 feet

48 inches or larger

600 feet

(Res. 40-08 (§ 1002.4), 3-19-08)

28.40.070 Storm drain hydraulics.

GJMC 28.40.080 through 28.40.100 present the hydraulic methods used to calculate storm drain capacities and thereby to design a storm drain system. The actual design process is presented in GJMC 28.40.160 through 28.40.190. The majority of the methods in this section are adapted from those presented in HEC-22 (Urban Drainage Design Manual) and HDS-4 (Introduction to Highway Hydraulics).

(Res. 40-08 (§ 1003), 3-19-08)

28.40.080 Gravity-flow analysis.

Initial storm drain design is completed by selecting pipe sizes based on “just full” capacity. This means that the drain capacity is calculated using open-channel (nonpressurized) flow computations. Starting at the uppermost reach of the storm drain (at the first inlet), the designer applies Manning’s equation (Equation 28.40-1) for each segment of drain. A segment is a reach of pipe with a junction, transition, grade change, horizontal bend, or pipe size change at each end.






Full Flow Discharge (cfs)



Manning’s Roughness Coefficient (see GJMC 28.40.050)



Full Flow Area



for circular pipes




Full Flow Hydraulic Radius = D/4 for circular pipes (ft.)



Pipe Slope (So = Sf for full flow) (ft./ft.)



Pipe Diameter (feet)

Equation 28.40-2 is a form of Manning’s that can be used to directly solve for the minimum required pipe diameter for circular pipes. The designer shall always round up to the nearest available pipe size, keeping in mind that minor losses in the pipe may decrease available capacity. Initial pipe size, Di (ft.), is based on the peak design flow for that pipe segment, QP (cfs).


For noncircular pipes, Equation 28.40-2 provides the equivalent diameter based on flow area.

To better account for energy losses that will occur in the system, the designer may choose to calculate preliminary head losses through inlet and manhole junctions. Application of these approximate losses will allow for better estimation of required pipe sizes during the initial design process, expediting the preliminary and final design phases. HEC-22 presents the following equation and table for the calculation of approximate junction head loss:





Preliminary Junction Head Loss Estimate (ft.)



Head Loss Coefficient from Table 28.40.080



Flow Velocity = QP/Af (fps)



Gravitational Constant = 32.2 ft.2/sec.

Table 28.40.080: Preliminary Head Loss Coefficients (for Conceptual/Initial Design ONLY)

Structure Configuration

Coefficient, Kah

Inlet – Straight Run


Inlet – Angled Through (θ)










Manhole – Straight Run


Manhole – Angled Through (θ)










From HEC-22, Table 7-5a and Figure 7-4.

Figures 28.40.080(a) through 28.40.080(d) present relative velocities and flows for circular, elliptical (horizontal and vertical), and arch pipe under gravity-flow conditions. Similar charts for box sections can be found in the Concrete Pipe Design Manual and other design aids.

(Res. 40-08 (§ 1003.1), 3-19-08)

28.40.090 Pressure-flow analysis.

Following the initial “just-full” storm drain design, the system is analyzed using energy-momentum theory to account for specific energy losses. This method allows for the calculation of hydraulic and energy grade lines (HGL and EGL) for a given storm drain line by starting with the water surface elevation of the outfall and working upstream, accounting for all losses due to pipe friction, manholes, transitions, bends, junctions, and pipe entrances and exits. In cases where pressure flows exist, certain limitations exist on the maximum elevation of the EGL in relation to the ground surface (finished grade). Compliance with minimum and maximum flow velocities is based on peak design flow in the final selected pipe size for each segment. See GJMC 28.40.020 through 28.40.060 for specific design criteria.

Energy-momentum theory is based upon the concept that energy, typically expressed in hydraulics as “head” in a linear dimension such as feet, is conserved along a given conduit segment. For a segment where A is the upstream end and B is downstream, the steady-flow energy equation can be expressed as:





Invert Elevation above any Horizontal Datum (ft.)



Fluid Pressure lbf/ft.2



Specific Weight of Water ≅ 62.4 lbf/ft.3



Flow Velocity (fps)



Head Added by a Pump (if applicable) (ft.)



Sum of Head Losses in Segment A – B as calculated per the methods prescribed in this section

Each term in Equation 28.40-4, and thus the sum of the formula, has a linear dimension (e.g., feet). Each term represents the hydraulic head contributed to the total energy head by that term. For instance, the third term, V2/2g, is the velocity head. The EGL elevation at a given point is equal to:


and the HGL elevation is simply the EGL minus the velocity head:


In cases where outfall water surface is equal to or higher than the outlet flow elevation, the EGL and HGL are assumed to be equal, i.e., velocity is zero at the downstream point where calculations start. However, if the outfall water surface is lower than the outlet pipe flow elevation, the latter value is used as the outlet HGL. Note that the outfall water surface elevation used must be determined coincident with the time of peak flow from the storm drain.

The HGL at the next structure (e.g., manhole) is determined by the equations presented in Table 28.40.090(a). The equations are separated by HGL at the pipe inlet downstream of the manhole and the pipe outlet at the inlet to the manhole. For nonsurcharged flow (less than 80 percent pipe depth), the free water surface at the pipe inlet (downstream end of the manhole) is added to head loss across the manhole to find the pipe outlet HGL (upstream end of the manhole).

Table 28.40.090(a): Equations for Determining HGL

Surcharge Conditions

Outlet Submergence

HGL in Manhole/Junction


Equation Number

dn/D > 0.80


= HGLPipe Outlet + hf + hminor

Pipe Inlet (D/S from MH)


dn/D > 0.80


= HGLPipe Inlet + hmh

Pipe Outlet (U/S from MH)


dn/D ≤ 0.80


= WSEPipe Inlet

Pipe Inlet (D/S from MH)


dn/D ≤ 0.80


= WSEPipe Inlet + hmh

Pipe Outlet (U/S from MH)


dn/D ≤ 0.80


= Larger of Equations 28.40-7 and 28.40-9 OR

= Larger of Equations 28.40-8 and 28.40-10




Normal Flow Depth in Pipe (feet)

HGLPipe Outlet


Larger of Tailwater Elevation, Flow Depth Elevation at Pipe Outlet, and HGL at Next Downstream Pipe Inlet

WSEPipe Inlet


Free Water Surface Elevation at Pipe Inlet

hf, hmh, hminor


Head Losses as Described in This Section

Occasionally, design flow through a pipe may be not only gravity-flow (nonsurcharged) but also supercritical. Pipe losses (hf and hminor) in a supercritical pipe section are not carried upstream. (HEC-22)

In locations where two adjoining pipe segments flow in supercritical conditions, manhole losses are also ignored for that line. The designer shall be careful to include these losses where only one of the pipes on the line under investigation contains supercritical flow.

Inlet pipes to a manhole must occasionally have an invert significantly above that of the outlet pipe. In locations where the outlet pipe water surface elevation (or HGL if pressure flow) is below the invert of an inlet pipe, that inlet pipe is treated as an outfall pipe. In this case, the outfall water surface elevation is always lower than the pipe outlet water level, so the latter elevation is used for the initial HGL of the new upstream reach. The outflow pipe from the manhole in such a situation acts as a culvert under either inlet or outlet control. See Chapter 28.48 GJMC and/or FHWA Hydraulic Design of Highway Culverts (HDS-5) for information regarding the computation of an HGL at the manhole and calculation of head loss due to a culvert inlet.

The following subsections prescribe methods for determining the energy losses induced by pipe friction, manholes, and other structures (minor pipe losses) that may be encountered by storm drain flows.

(a)    Pipe Friction Losses. Pipe friction is a significant source of energy dissipation in storm drains, whether in gravity-flow or pressure-flow conditions. For the former, friction slope (Sf) can be assumed to be equal to the slope of the pipe invert (So). For pipes with a surcharge flow condition (dn/D > 0/80), Equations 28.40-11 and 28.40-12 define friction slope (units for variables are the same as in Equation 28.40-1 when using English units).





2.21 (English Units)



1.0 (S.I. Units)






0.46 (English Units)



0.312 (S.I. Units)

Equation 28.40-11 is a form of the Chezy-Manning formula, and is based on average velocity in the pipe segment. Since flow rate and cross-sectional area typically remain constant through one segment of pipe, average velocity can be assumed to equal flow rate divided by flow area. Where flow rate and/or pipe size changes within one segment (such as at a pipe transition without a manhole or a no-access junction), this velocity is the average of those calculated at the ends of the pipe segment (Linsley, 1992). Equation 28.40-12 is based on the average flow rate in the pipe segment.

Once the friction slope is known, pipe friction head loss is calculated by multiplying the friction slope by the pipe segment length:


(b)    Manhole Junction Losses. This subsection details the energy-loss method used by the HYDRAIN program (FHWA) as presented in HDS-4 for calculation of approximate head loss through a manhole. This method applies to any junction of two or more pipes accessible by a manhole. The approximate head loss coefficient values presented in Table 28.40.080 are replaced by the values computed herein.

For each manhole, the designer must first calculate the initial head loss coefficient (Ko) and all applicable coefficient correction factors (Cx). The adjusted head loss coefficient (K) and head loss in the manhole (hmh) are then computed.







Angle Between Inflow and Outflow Pipes (≤ 180°)



Manhole of Junction Diameter (at water level)



Outlet Pipe Diameter

The coefficient correction factors are calculated using the equations presented below and are applied to the initial head loss coefficient per Equation 28.40-15. Note that some correction factors do not apply to all manhole configurations. These nonapplicable factors are set to unity.

(1)    CD – Correction Factor for Pipe Diameter. This applies to pressure flow when the ratio of water depth in the manhole above the outlet pipe invert to outlet pipe diameter is greater than 3.2. dmho/Do > 3.2.





Outlet Pipe Diameter



Inlet Pipe Diameter

(2)    Cd – Correction Factor for Flow Depth. This applies to gravity flow and low-pressure flow when the ratio of water depth in the manhole above the outlet pipe invert to outlet pipe diameter is less than 3.2. dmho/Do < 3.2.





Water Depth in Manhole Above Outlet Pipe Invert



Outlet Pipe Diameter

For purposes of this calculation, water depth in the manhole is approximated as the vertical distance from the outlet pipe invert to the HGL at the upstream end of the outlet pipe.

(3)    CQ – Correction Factor for Relative Flow. This applies to manholes with three or more pipes entering the structure at similar elevations (one of these pipes will be the outlet pipe). This correction factor does not apply to the effects of inflow pipes with flowlines far enough above the outlet pipe to qualify as plunging flow (see Equation 28.40-20 and explanation, this section).





Angle Between the Inflow Pipe of Interest and the Outflow Pipe



Flow in the Inflow Pipe of Interest



Flow in the Outflow Pipe

The “pipe of interest” is the inlet pipe to the manhole on the line being investigated. This factor accounts for streamline interference by flow from other pipes entering the manhole. See Figure 28.40.090(a) for an illustration of the relative flow effect.

(4)    Cp – Correction Factor for Plunging Flow. This applies to manholes with an inflow pipe of interest that is affected by plunging flow from another inflow pipe with a higher flowline. The factor does not apply to the line with the pipe that is discharging the plunging flow, and only applies when the height of the plunging-flow pipe flowline above the outlet pipe center exceeds the manhole water depth above the outlet pipe invert: h > dmho





Vertical Distance of Plunging Flow (height of plunging flow pipe flowline above center of outlet pipe)



Water Depth in Manhole Above Outlet Pipe Invert



Outlet Pipe Diameter

A common application of this correction factor occurs at locations where inlets convey intercepted flow directly (vertically) to the storm drain main line (drop inlets) or where laterals enter a manhole well above the main line invert.

(5)    CB – Correction Factor for Benching. This applies to all flow conditions. See Figure 28.40.090(b) and Table 28.40.090(b) for proper correction factor selection.

Table 28.40.090(b): Benching Correction Factors

Bench Type (see Figure 28.40.090(b))

Outlet Pipe Conditions

Fully Submerged, Pressure Flow*

Unsubmerged, Free Surface Flow**

Flat or Depressed



Benched: 1/2 Pipe Diameter



Benched: 1 Pipe Diameter



Improved Bench



Adapted from Mesa County SWMM 1996, Figure “H-4”

*Applies for dmho/Do ≥ 3.2

**Applies for dmho/Do ≤ 1.0

As can be seen in Table 28.40.090(b), benching in manholes significantly reduces head loss due to outlet inefficiency, especially in unsubmerged conditions. Note that in this case, the submerged pressure-flow factors do not apply until flow depth in the manhole has exceeded 3.2 times the outlet pipe diameter. Therefore, for depths between free surface (gravity) flow and full pressure-flow conditions (1.0 > dmho/Do < 3.2), the designer shall use a linear interpolation to compute the benching correction factor.

(c)    Minor Pipe Losses. This subsection describes the methods used in Mesa County for the calculation of head losses caused by pipe transitions (expansions or contractions), bends (curved drains), no-access junctions, on-grade inlets, and exits (outlets). The minor losses are added together for a given pipe segment per Equation 28.40-21:


(1)    he and hc – Transition Losses. Transition losses occur when pipe size is changed at a location other than a manhole. Expansions may be necessary due to changes in flow rate or slope. Contractions are locations where pipe size is decreased, and are allowed only through a variance. Methods for head loss calculation through a pipe contraction are included in this title.

The calculation of head loss through a transition differs for nonpressure flow and pressure flow.

(i)    Nonpressure Flow Transitions.






Expansion Coefficient (see Table 28.40.090(a)(1))



Contraction Coefficient (see Table 28.40.090(a)(2))



0.5 · Ke for Gradual Contractions



Velocity Upstream of the Transition



Velocity Downstream of the Transition

(ii)    Pressure-Flow Transitions.






Expansion Coefficient (see Table 28.40.090(b)(1), (2))



Contraction Coefficient (see Table 28.40.090(b)(3))



Velocity Upstream of the Transition



Velocity Downstream of the Transition

See Figure 28.40.090(c) for illustration of the “Angle of Cone” variable used in Tables 28.40.090(a) and 28.40.090(b).

(2)    hb – Bend Losses (Curved Drains). The minor loss that accompanies a storm drain bend can be approximated by:





Angle of Curvature (degrees)

This equation does not apply to bends located at manholes. Head losses due to manhole bends and deflections are addressed in subsection (b) of this section.

(3)    hj – No-Access Junctions. This term applies to head loss associated with locations where a lateral pipe connects to a larger trunk pipe without the use of a manhole structure. While these junctions are not recommended for trunk pipes of less than 48 inches in diameter, it is sometimes physically or economically inefficient to place a manhole at every junction location. At locations where more than one lateral joins the main line (trunk), a manhole is required. The head loss at no-access junctions is related to the relative flows and velocities of all three pipes, the angle between the lateral and trunk pipes, and the cross-sectional area of the trunk pipe.





Outlet, Inlet and Lateral Flow Rates



Outlet, Inlet and Lateral Velocities



Outlet and Inlet Velocity Heads = V2/2g



Outlet and Inlet Cross-Sectional Areas



Angle of Lateral with Respect to Outflow Pipe

(4)    hi – On-Grade Inlets (Culvert-Type Inlets). In some locations, water may enter a storm drain system from a drainage channel, overflowing pond, or other conveyance with a flowline approximately equal to that of the storm drain inlet. These storm drain entrances are hydraulically equivalent to culvert inlets, thus the coefficient Ki in Equation 28.40-28 is equal to the culvert entrance loss coefficient Ke provided in Chapter 28.48 GJMC, Table 28.48.110. (Note that Ke represents the expansion loss coefficient in this chapter.)





On-Grade Inlet Coefficient (see Table 28.48.110)

(5)    ho – Outlets (Pipe Exits). This term applies to pipe outlets other than those which exit to a manhole. Outlet losses are always associated with a storm drain system outfall to an open channel, detention/retention basin, or other receiving waters. Outlets that discharge into a body of water with essentially zero velocity in the direction of the storm drain exit lose all velocity (one velocity head). This includes outlets perpendicular to an open channel and all submerged outlets. The storm drain flow is also assumed to lose all velocity when it discharges to open air and plunges to the receiving waters.





Flow Velocity at Storm Drain Exit



Flow Velocity (in the direction of storm drain flow) in Receiving Waters

Allowable storm drain velocities often differ from those for open channels. Chapters 28.32, 28.36 and 28.48 GJMC present criteria for the proper design of outlets to open channels, including the design of riprap and other energy dissipation structures to reduce channel scour potential.

(Res. 40-08 (§ 1003.2), 3-19-08)

28.40.100 Computer hydraulic modeling.

Since the storm drain system design process tends to be somewhat iterative, computer programs are now commonly used to develop and/or model proposed and existing storm drainage networks. Numerous hydrologic modeling programs now exist that can often achieve more accurate results due to hydrograph routing capabilities. Many of these hydrologic programs also include hydraulic simulation modules based on hydrologic computations and system parameters. The tediousness of creating hydrographs for every convergence and divergence point in the system is avoided by using these programs, and hydrograph timing consistency is vastly improved. Other more simple programs are stand-alone hydraulic calculators, and are useful if peak flows have been previously determined.

HGL and EGL calculations may be prepared using computer software subject to review by the local jurisdiction. At this time, a list of approved or disapproved public or proprietary computer programs for hydrologic and hydraulic modeling is not maintained. However, the designer is urged to use sound professional judgment to select the program(s) that are most applicable to local design standards and the requirements of the given project. It is recommended that the designer consult with the local development review engineer before using any software that is newly released or has not already been broadly accepted by the engineering community.

(Res. 40-08 (§ 1003.3), 3-19-08)

28.40.110 Construction standards.

GJMC 28.40.120 through 28.40.150 outline standards for the construction of storm drain systems as based on the most recent versions of all reference publications. The designer is responsible for procuring and complying with the most current version of each applicable reference document. For hydraulic design, the most restrictive criteria among said references and this manual are to be used.

(Res. 40-08 (§ 1004), 3-19-08)

28.40.120 Storm drain pipe.

(a)    Minimum Size. Minimum pipe sizes are required in order to allow for maintenance and inspection and to reduce the effects of expected sedimentation and debris build-up. All storm drain pipes within the public right-of-way shall have a minimum diameter of 18 inches. For noncircular pipes, these minimum diameters represent equivalent diameters based on cross-sectional areas.

(b)    Maximum Size. There is no maximum pipe size specified. However, the designer shall consider the possibility of utilizing multiple barrels (pipes) where physically and economically advisable.

(c)    Pipe Material and Shape. All storm drain pipes shall comply with the Grand Junction Standard Specifications for construction of underground utilities, as well as the most recent revision of the Colorado Department of Transportation Standard Specifications for Road and Bridge Construction (CDOT).

Public storm drain mains (to which laterals are connected) may be circular, elliptical, arch, or box (rectangular – concrete only) pipe of reinforced concrete pipe, corrugated aluminized steel, corrugated aluminum, corrugated polymer coated galvanized steel, corrugated or profile wall polyethylene, or polyvinyl chloride. However, pipe material and shape shall be selected based on not only hydraulic capacity, but also “the ability of a pipeline to maintain full cross-sectional area and function without [excessive] cracking, breaking, or undergoing excessive deflection” (Mesa County SWMM, 1996). The designer should be aware that local jurisdictions may have varied regulations for allowable pipe materials.

(d)    Joint Fillers, Sealants, and Gaskets. All pipe joint fillers, sealing compounds, and gaskets, and the installation thereof, shall be governed by the specifications set forth in Section 705 of the CDOT Standard Specifications. Rubber gaskets shall be used at pipe section joints where greater than 5.0 feet of pressure head is expected in the design storm. This is equivalent to locations where the HGL elevation is 5.0 feet higher than the pipe crown.

(e)    Backfill Loading. Backfill and cover requirements for storm drain pipe are discussed in GJMC 28.40.060.

(f)    Pipe Bedding. Specifications for pipe trenching, bedding, and backfill may be obtained from the City of Grand Junction General Utility Details and Standard Storm Drain Details.

(Res. 40-08 (§ 1004.1), 3-19-08)

28.40.130 Manholes.

Construction details for a standard storm drain manhole are provided in the City of Grand Junction Standard Storm Drain Details. Nonstandard manhole designs shall meet the design and construction criteria set forth in Section 604 of the CDOT Standard Specifications. The EGL for all design flows must be at or below the manhole rim. Locking manhole covers are not permitted.

(Res. 40-08 (§ 1004.2), 3-19-08)

28.40.140 Inlets.

Chapter 28.44 GJMC describes the selection and placement criteria for storm drain inlets in Mesa County. Construction details for street inlets can be found in the City of Grand Junction Standard Storm Drain Details.

Culvert-type inlets, such as those directing ditch flows into a storm drain, are required to include a special end section to increase capacity and reduce erosive potential. See Chapter 28.48 GJMC for culvert inlet design criteria.

(Res. 40-08 (§ 1004.3), 3-19-08)

28.40.150 Outlets.

Storm drain outlets typically discharge to a drainage channel, a natural stream or river, or a detention/retention basin. In order to increase storm drain capacity and reduce erosion potential, outlets are required to include a special end section equivalent to those required for culvert outlets per Chapter 28.48 GJMC.

Due to the erosive potential of high-velocity storm drain flow on unlined channels and detention/retention basins, a riprap apron and/or an energy-dissipation structure shall be constructed at all storm drain outlets per requirements set forth in Chapter 28.48 GJMC.

(Res. 40-08 (§ 1004.4), 3-19-08)

28.40.160 Storm drain system design.

Prior to starting storm drain design, the allowable minor and major street capacities must be determined and inlets preliminarily sized and located. In most cases, the storm drain design flow at a given point is equal to the cumulative minor storm runoff exceeding the minor storm street flow capacity to that point. However, since the street and storm drain must cumulatively carry the major storm event flow without exceeding the major storm street capacity, the storm drain must occasionally be sized to carry the runoff exceeding that capacity. Furthermore, in locations where a vertical sag exists in the street (sump inlets) and no overflow path exists for the major storm flow, the storm drain must be sized to accept the entire major storm flow minus street ponding allowances. Note that the latter two cases require that the inlets be resized to accommodate the larger flows.

(Res. 40-08 (§ 1005), 3-19-08)

28.40.170 Initial storm drain design.

The following step-by-step procedure is for the initial layout and sizing of a storm drain. The results of this process must be validated by the procedures set forth in GJMC 28.40.180 before the system can be deemed a viable design. However, this design may be used for conceptual drainage report submittals per GJMC 28.12.030 through 28.12.050.

(a)    Choose a system layout based on street rights-of-way and other drainage easements, developed topography, utility locations, and likely cost and performance. This layout shall include preliminary inlet and manhole locations, if any.

(b)    Complete hydrologic analysis of the project area per Chapters 28.24 and 28.28 GJMC. Compute peak flow in each street (see Chapter 28.44 GJMC) starting at the upper end of the project area and working downstream. Typically, the runoff from multiple streets will converge at a point, so all streets that are tributary to that point must be completed before moving on downstream. An inlet shall be located wherever the minor storm peak street flow exceeds the allowable capacity for that street and at all sump locations.

(c)    Initial storm drain sizing starts at the uppermost inlet for each street, with individual street storm drains combining where appropriate. The design flow for a given storm drain segment is based on the sum of all flow from upstream pipes and the larger of the major or minor street flow exceeding the respective street capacity at the inlet just upstream from that segment.

(d)    Using gravity-flow analysis (Manning’s open channel flow) as presented in GJMC 28.40.080, including approximate junction head losses, compute required pipe size and slope for each pipe segment. In many locations, storm drain slope will be limited by topography or other design criteria including cover and utility clearance requirements, so slopes are often held constant during the initial design phase. It may be prudent to increase pipe size and/or slope at locations where the preliminary energy loss coefficient may not apply and significant energy losses may occur, such as large or complex pipe junctions and major pipe bends. Pipe size shall not be decreased in a downstream direction except in special situations.

(Res. 40-08 (§ 1005.1), 3-19-08)

28.40.180 Preliminary/final storm drain design.

Following the completion of an initial storm drain system design, the preliminary/final design may begin. The level of hydraulic analysis presented in this section shall be met before the design may be included in any final drainage reports (see GJMC 28.12.060 through 28.12.110).

(a)    The hydraulics for each system are recomputed using the energy-momentum theory presented in GJMC 28.40.090, starting at each system’s outfall point. All applicable energy losses must be included in the calculations, including head loss due to manhole/junction chambers, pipe transitions and bends, no-access junctions, and entrances/exits.

(b)    The HGL and EGL shall be calculated and plotted for each end of each pipe segment and each side of all locations of additional energy loss listed in Step 1. The EGL shall be limited to a maximum elevation of manhole rim or inlet throat at all locations along the storm drain.

While many designers may choose to utilize computer software to model storm drain systems, smaller projects are still often completed by hand. Hand calculations are also useful for spot-checking of computer outputs to ensure that software is functioning properly. For this reason, Standard Form 3 in Chapter 28.68 GJMC is provided to assist in tabulation of storm drain hydraulic calculations. Figure 28.40.180 is Standard Form 3 showing input corresponding to the example design application presented herein.

(Res. 40-08 (§ 1005.2), 3-19-08)

28.40.190 Example design application.

This section presents an example of energy and hydraulic grade line computation through a simple storm drain system. It is assumed that initial design has previously been completed, the results of which are shown in Figure 28.40.190.

(a)    Problem: Compute both the energy grade line (EGL) and hydraulic grade line (HGL) at Design Points 1 through 4 for the system shown in Figure 28.40.180 and check for locations where the EGL reaches any manhole rim or inlet throat.

(b)    Solution:

(1)    Step 1. Utilizing Standard Form 3 to organize data and calculations, enter “OUTFALL” in the STATION column for the first row. The “pipe” in this case is just the outlet itself, so calculate ho and enter it in column 19.

The outfall water surface elevation, 4,500.0 feet, exceeds the crown elevation of the outlet pipe, 4,496.0 feet plus 1.5 feet equals 4,497.5 feet, so the pipe is flowing full at this point (outlet control). The outfall pool has no velocity component in the direction of the outlet pipe, so the EGL equals the HGL and water surface elevation (column 23). The U/S EGL (column 24) in this case represents the point just inside the outlet:

(2)    Step 2. Enter stations 1 and 2 in columns 1 and 2 of the next row, as well as all known pipe and flow data. Since the pipe was already shown to be flowing full under outlet control, velocity (column 10) is:

Velocity head (column 11) and friction slope (column 12) are:

Pipe friction head loss is then found and entered in column 13:

The drain schematic (Figure 28.40.190) also indicates a 30-degree bend in this pipe reach. Head loss due to the bend is entered in column 16:

These total to 1.88 feet lost in the pipe reach (column 20), not including losses from the manhole at Design Point 2.

(3)    Step 3. At this point, columns 23, 24, and 25 can be entered. The downstream EGL is in this case simply equal to the upstream EGL (column 24) from the first row. The upstream EGL and HGL are:

(4)    Step 4. The calculation of losses through a manhole is completed per the procedure presented in GJMC 28.40.090(b), and is dependent on the line being investigated. To find the maximum HGL in a manhole, losses for each line must be calculated and compared. The manhole at Design Point 2 has two inlet pipes, and thus two lines. The line to Station 3 is completed first:

Now apply the correction factors to the initial head loss coefficient:

Then apply the corrected head loss coefficient to find the estimated head loss through the manhole (on this line):

Note that the velocity used here is the average velocity in the outlet pipe from the manhole. This value is entered in column 21, and station number “3” in column 22 of the same row.

We now use the same procedure to find the head loss through the same manhole on the other line (2-4). While manhole diameter (b) and outlet pipe diameter (Do) are the same as before, this pipe enters the manhole at a different angle and a different (invert) elevation:


This value is entered in column 21 in the row directly below that containing the 0.12' value. Station “4” is entered in the same row, column 22.

(5)    Step 5. The estimated manhole losses on each line (each row) are then added to the upstream EGL (column 24) and HGL (column 25) to get the EGL and HGL at the upper end of the manhole. The larger pair governs:

The hydraulic and energy grade line elevations for line 2-4 are used for the manhole freeboard check – the design storm maximum EGL is 4503.0 feet, and the manhole rim at Design Point 2 is at 4505.0 feet. The rim is above the EGL, so the design is acceptable to this point.

(6)    Step 6. We now move to analysis of the upper pipe reaches, starting with the pipe between Design Points 2 and 3. As before, fill in known and computed data in columns 1 through 9. Average velocity in the pipe depends on flow conditions, so we must determine outlet conditions. The downstream EGL for this pipe is the larger of the following:

The first value, 4503.0 feet, is entered in column 23. The downstream HGL, then, is 4503.0 feet – Hv = 4503.0 – 0.37 equals 4502.6 feet, which is above the crown of pipe 2-3. Therefore, the pipe will be assumed to flow full under outlet control. Average velocity under full flow is 4.89 fps, resulting in a velocity head of 0.37 feet. Friction slope is:

This results in pipe friction head loss of:

(7)    Step 7. Since there is no known incoming pipe to the manhole at Design Point 3, we will not apply the full energy loss method as before. Instead, we can assume that the outlet pipe from the manhole will act as a culvert, with inlet losses as calculated below:

The value for Ki was taken from Table 28.48.110, assuming a square-edged headwall on concrete pipe. The value of hi is entered in column 18, and the sum of hf and hi is entered in column 20. This is then added to the downstream EGL value in column 23 to find the upstream EGL elevation (column 24):

The manhole rim elevation of 4,505.0 feet is above the EGL of 4,503.6 feet, so this reach is acceptable.

(8)    Step 8. In a new row, enter stations “2” and “4” in columns 1 and 2. Enter data in columns 1 through 9. The full-pipe gravity-flow discharge for this reach is 3.56 cfs per Manning’s equation, so pressure-flow conditions must exist to convey the four cfs design flow. However, there is a no-access junction 40 feet from Design Point 2 to which one cfs of the total four cfs is attributed. Above this junction, the main pipe is carrying three cfs under gravity-flow conditions. The following table organizes the computation of average values for the reach:


Length (ft.)

Flow (cfs)

Flow Area (sf)

Velocity (fps)











Length-weighted averages for flow and velocity are needed for head loss calculations:

These values are entered in columns 9 and 10, respectively. Velocity head and friction slope (columns 11 and 12) are based on these averages:

(9)    Step 9. Determine the friction and minor pipe losses. Friction head loss (column 13) is:

Head loss at the no-access junction (column 17) is calculated as:

Like the manhole at Design Point 3, we will treat the outlet pipe from this manhole as a culvert inlet with a square-edged headwall (column 18):

Total pipe losses (column 20), then, are:

(10)    Step 10. Find the downstream and upstream EGLs and HGLs.

Downstream EGL (column 23) is the larger of:

Upstream EGL (column 24) is:

The manhole rim elevation of 4,507.0 feet is above the EGL of 4,505.7 feet, so this reach is acceptable.

(Res. 40-08 (§ 1005.3), 3-19-08)