A large number of agricultural irrigation facilities exist in Mesa County, and many have historically intercepted runoff from rural and agricultural areas with little consequence. However, the development (urbanization) of these areas results in storm runoff of much higher peak flows and larger total volumes. In addition, water quality of the runoff is often adversely impacted by this urbanization. As a result, the traditional practice of utilizing irrigation ditches, drains, and reservoirs for stormwater control must be reexamined on a case-by-case basis.
It is recommended that the designer/engineer, when faced with a specific irrigation/drainage structures interface, to review in detail GJMC § 28.16.200 through 28.16.230. Only after a thorough review and understanding of this chapter, and with coordination with the parties involved, shall the user proceed with the specific tasks that need to be performed. Further, the designer/engineer is cautioned to verify that damage to downstream properties will not occur by bypassing of storm runoff.
(Res. 40-08 (§ 1301), 3-19-08)
It is common for a storm drainage system to encounter irrigation ditches, canals, or even conduits, especially in agricultural areas. Mesa County contains a large percentage of agricultural lands, thus the interaction of storm runoff systems and agricultural irrigation structures is common, especially for new developments. Storm drains are often buried with sufficient cover to completely avoid an interaction with existing irrigation structures. However, it may occasionally be necessary to install storm drain pipe by boring or jacking to avoid disruption of the irrigation flow. Where the invert of a drainage channel is low enough in relation to the irrigation structure, it may be possible to utilize a standard culvert design for the crossing (see Chapter 28.48 GJMC).
In locations where stormwater flow is in an open channel or relatively shallow pipe at an intersection with an irrigation structure, other options must be considered. In certain (rare) cases, it is allowable for stormwater flow to enter an irrigation canal and then be removed (see side-channel spillways in GJMC § 28.36.120) at another location. Otherwise, stormwater flow shall be kept separate from irrigation conveyances as reasonably possible. Two methods for completing this task are presented in GJMC § 28.52.030 and 28.52.040: inverted siphons and overchutes. Overchutes include both flumes and pipe overchutes for the conveyance of stormwater over another channel.
(Res. 40-08 (§ 1302), 3-19-08)
(a) 
An inverted siphon consists of a closed conduit used to convey water under an obstruction such as an irrigation structure or roadway where the use of a continuous-slope conduit would interfere with said obstruction. Sometimes called “sag pipes,” the conduit drops to an invert low enough to pass under the obstruction then rises to the channel invert at the downstream end. This infers pressure pipe flow, with successful operation being dependent on sufficient head at the upstream end to overcome the rise as well as pipe losses in the siphon section.
(b) 
Transitions are recommended for the upstream and downstream ends of all siphons to reduce head losses and to prevent excessive erosion. Entrance head losses reduce the effective head on the inverted siphon, thereby requiring larger upstream depths to achieve the same flow through the conduit. Concrete inlet and outlet transitions are required for siphons which:
(1) 
Cross railroads or State/federal highways.
(2) 
Are 36 inches in diameter or larger and cross a road.
(3) 
Are used with an unlined channel and the pipe velocity exceeds 3.5 feet per second.
(c) 
In locations where the siphon may be affected by groundwater flow, it may be necessary to include pipe collars to reduce piping effects. Cutoff walls may also be necessary depending on site conditions.
(d) 
It is recommended that the design of long inverted siphons include a blowoff structure at the low point of the alignment to allow for draining of the system (see GJMC § 28.36.170(a)). These can be designed for operation by pumping or gravity draining. Shorter siphons can usually be easily drained by pumping from either end of the structure.
(e) 
Pipe used for inverted siphons shall be pressure-rated as required per design, shall utilize rubber gaskets (may use other joint connection devices in addition), and shall comply with the applicable pipe selection criteria set forth in GJMC § 28.40.120.
(f) 
It is good design practice to include features in the design of an inverted siphon to minimize the risk of flooding due to the failure of the siphon to properly convey the channel flow. These features may include, but are not limited to:
(1) 
Increased freeboard in the upstream channel in the vicinity of the siphon.
(2) 
The use of multiple barrels to allow for at least partial operation if one barrel fails.
(3) 
The installation of a wasteway (and associated side-channel spillway) to limit the depth of water in the upstream channel.
(g) 
Inverted siphons pose a significant risk to human and animal safety. Specific features must be included in the design of these structures to help alleviate these risks. It is recommended that the location and safety features of any proposed inverted siphon be discussed with Mesa County and any local jurisdictions early in the design process. At some locations, a jurisdiction may disallow the use of these structures where they pose excessive or unwarranted risk to the public.
(h) 
The design procedure for an inverted siphon is as follows (USBR 1974):
(1) 
Determine an initial system layout with all known elevations and lengths. Pipe slopes between the inlet/outlet transitions and the main section of the siphon are limited to a maximum slope of 2:1. All siphon pipes shall have a slope of at least 0.005.
(2) 
Determine the type of inlet and outlet structures required (transitions, headwalls, etc.).
(3) 
Determine the type of pipe to be used. This is typically pressure-rated reinforced concrete pipe.
(4) 
Select initial pipe size based on the table in Figure 28.52.030(a). This is based on design flow, transitions used, and the subjective length of the siphon. Presented with the table are maximum permissible pipe velocities for different siphon lengths and transition types. Siphons are considered to be relatively short if they are crossing under a road or a canal. Only flows of up to 99 cfs are included in Figure 28.52.030(a) since it is typically more economical to consider a bridge at flows of 100 cfs or higher. However, multiple barrels may be utilized to convey larger flows.
(5) 
Using the design flow rate and the properties of the initially selected pipe, determine the velocity head in the pipe (Hvp) and the friction slope (Sf). Using the normal depth in the upstream channel, find the velocity head (Hv1).
(6) 
Determine the additional freeboard required (FBadd) for the 50 feet of channel upstream of the structure:
(28.52-1)
(7) 
Invert elevations of the transitions are set to allow for an adequate hydraulic seal at the inlet (to minimize hydraulic loss) and to avoid submergence at the outlet. Due to the sloping pipe inlet, the effective diameter is larger than the pipe diameter:
(28.52-2)
Where:
D1
=
Effective Inlet Diameter (feet)
D
=
Siphon Pipe Diameter (feet)
α
=
Slope of Inlet (α1) or Outlet (α2) Pipe (degrees)
Required hydraulic seal is based on the difference in velocity heads between the upstream channel and the pipe:
(28.52-3)
Where:
Hseal
=
Hydraulic Seal Required at Inlet, Min. 0.25' (feet)
Hvp
=
Velocity Head in the Pipe (V2/2g) (feet)
Hv1
=
Velocity Head in the Upstream Channel (feet)
Throughout the remainder of this process, the designer is referred to Figure 28.52.030(a) for the locations of Stations A through H and J. Note that the siphon in Figure 28.52.030(a) is crossing under a roadway. In this section, the focus is on irrigation canal crossings, so the cover requirements may differ from those in the figure. Siphons crossing under a channel with flexible lining shall have a minimum of 2.0 feet of cover, and those crossing under a canal with concrete or other nonflexible lining shall have a minimum cover of six inches.
Table 28.52.030 presents equations for finding invert elevations at Stations A through H:
Table 28.52.030: Inverted Siphon Invert Elevation Equations
Station (see Figure 28.52.030(a))
Invert Elevation
A
Channel IE @ 10' U/S from the U/S transition
C
NWS Elev. @ Sta. A – Hseal + Di
B
Minimum: Channel IE @ U/S end of transition
Maximum: IE Sta. C + pinlet
G
Channel IE @ 10' D/S from the D/S transition
H
Channel IE @ 10' D/S from the D/S transition
F
IE @ Sta. G – poutlet
Where:
IE
=
Invert Elevation (feet)
NWS
=
Normal Water Surface (Design Flow) (feet)
p
=
Difference in invert elevations between the ends of the transitions (Sta. B and C or F and G) (feet)
pinlet
≤
3/4D (feet)
poutlet
≤
1/2D (feet)
The invert elevations of Stations D, J, and E are determined by cover requirements and pipe slope.
(8) 
Determine the total amount of hydraulic head available (Hprofile) across the siphon profile:
(28.52-4)
(9) 
Determine the approximate head loss across the preliminary profile, as well as a 10 percent factor of safety as included in Equation 28.52-5:
(28.52-5)
Where:
HL
=
Head loss across siphon profile (with 10% F.S.) (feet)
hi
=
Inlet transition head loss = 0.4ΔHv (feet)
hf
=
Pipe friction losses = Pipe Length × Sf (feet)
hb
=
Bend losses (See Equation 28.40-26) (feet)
ho
=
Outlet transition head loss = 0.7ΔHv (feet)
ΔHv
=
(Hvp – Hv1)
For the siphon to function properly, the total head loss H L must not exceed the available head across the profile, Hprofile.
(10) 
Headwall height above the transition invert is dependent on backfill height and required freeboard. In many cases, the top elevation of the headwall is equal to the top of wall elevation at the cutoff, Station B. (The wall height there is determined by adding standard channel freeboard to the additional freeboard calculated in Equation 28.52-1.)
(11) 
Transition dimensions C, B, and cutoff depth e, and wall thickness tw are defined by Figure 28.52.030(b) (USBR 1974). Transition length L shall be at least three times the pipe diameter D.
(12) 
For most siphons, forces exerted on the pipe bends are not great enough to warrant additional structural considerations. However, for large pipes, high heads, poor foundation conditions, and large deflection angles, thrust blocks and other appurtenances shall be considered. (USBR 1974)
(13) 
The locations of Stations C and F are determined by the width of the obstruction and the siphon pipe slope requirements. Using the new locations for Stations C and F, determine new locations for Stations A, B, G, and H, and recompute invert elevations as applicable.
(14) 
Recompute total head loss (Equation 28.52-5) and verify that the final siphon profile is viable.
(15) 
Erosion protection may be required upstream and/or downstream from the siphon transitions. See Chapter 28.32 GJMC. Pipe collars may be necessary to reduce the effects of piping due to percolation along the outer wall of the pipe. A blowoff (drain) valve may be warranted for longer pipes. See Section 906 for criteria regarding these appurtenances.
(16) 
Air vents, pressure-release valves, or air jumper pipes may be necessary to relieve air pressure in the siphon pipe, especially under less-than-capacity flows. A hydraulic jump can occur in the pipe, causing blowback and significantly reducing the capacity of the siphon. The aforementioned appurtenances allow for the release of trapped air from the pipe. See GJMC § 28.36.170 for criteria regarding these appurtenances.
FIGURE 28.52.030(a)
FIGURE 28.52.030(b)
(Res. 40-08 (§ 1302.1), 3-19-08)
The term “overchutes” refers to cross-drainage structures that pass over the normal water surface elevation of the drainage being crossed. While typically used to convey stormwater (or other) across an irrigation canal or natural drainageway, other obstructions such as roads and railroad tracks occasionally require these structures. Where the obstruction being crossed conveys anything other than water (such as vehicles), clearance to the overchute must be considered. The designer shall ensure that an overchute installed at a waterway will not adversely impact the design capacity of that waterway. This includes a minimum clearance of 1.0 foot over the normal water surface of the waterway.
Overchutes can be open-channel structures (flumes) or pipes, depending on site conditions. Rectangular concrete flume sections are typically used for larger cross-drainage (e.g., stormwater) flows and for areas where debris is expected to prohibit the use of pipes. Figure 28.52.040(a) presents a typical plan and profile for a rectangular concrete flume overchute. For smaller cross-drainage flows, pipe overchutes are typically employed for economic reasons. See Figure 28.52.040(b) for typical plans and profiles for two pipe overchute types. Note that welded steel pipe is specified for the suspended section of each due to the additional forces and exposure to which those sections are subjected. Both Figures assume that the obstruction being crossed is an existing trapezoidal canal.
(a) 
Flume Overchutes.
The design of a flume overchute (“chute”) is as follows (USBR 1974):
(1) 
The chute invert slope is dependent upon the selected method of energy dissipation at the chute outlet. Some, such as a stilling basin, best function with supercritical influent. Others, such as a baffled apron drop (GJMC § 28.36.060), require subcritical flow at the entrance. Also, a wider, shallower flume section will require a smaller depth at the inlet pool and will more easily satisfy freeboard requirements. Therefore, the most economical flume section may not be the most efficient overall.
Where supercritical flow is desired, the following equations are used to determine critical slope:
(28.52-6)
(28.52-7)
(28.52-8)
(28.52-9)
(28.52-10)
(28.52-11)
Where:
dc
=
Critical depth (feet)
q
=
Flow per unit width (cfs/ft.)
Ac
=
Critical area (sf)
Vc
=
Critical velocity (fps)
Hvc
=
Velocity head (critical flow) (feet)
Esc
=
Specific energy (critical flow) (feet)
Sc
=
Critical slope (ft./ft.)
n
=
Manning’s roughness = 0.015
R
=
Hydraulic radius (feet)
The chute invert slope shall be at least 20 percent greater than critical slope (Sc) for the selected cross-section to avoid unstable flow that occurs around critical flow.
Where subcritical flow is desired, the slope shall be set appreciably less than critical slope as defined by Equation 28.52-11.
Normal depth at design flow in the flume shall be a minimum of 1.0 foot below the top of the flume wall for all nonpiped overchutes. Normal depth shall be based upon a Manning’s roughness coefficient of 0.015 for the purpose of this criterion.
(2) 
Determine the required pool depth at the inlet transition, do. Equation 28.52-12 assumes that velocity in the inlet pool is zero:
(28.52-12)
Provide 2.0 feet of freeboard above the pool depth in the channel upstream from the overchute, and 1.0 foot of freeboard above the pool depth in the flume from the inlet pool to the suspended portion.
(3) 
The chute wall height across the suspended portion of the flume shall be equal to the maximum depth in the chute plus 1.0 foot of freeboard.
(4) 
Determine the flow depth at the downstream end of the chute using the Bernoulli Equation iteratively on d2:
(28.52-13)
Where:
Es2
=
D/S Specific Energy = d2 + Hv2 (feet)
hf
=
Friction Loss (feet)
Es1
=
U/S Specific Energy = d1 + Hv1 (feet)
SoL
=
Vertical Drop across Chute (feet)
(5) 
A stilling basin or other energy dissipation structure shall be included as necessary per GJMC § 28.36.020 through 28.36.060 and 28.36.070 through 28.36.100.
(b) 
Pipe Overchutes.
The design of a pipe overchute is as follows (USBR 1974):
(1) 
The invert of the inlet transition is limited by the following:
(i) 
At least 1.0 foot of clearance shall be provided between the irrigation canal water surface and the pipe, where applicable. Larger clearances may be required for other types of obstructions.
(ii) 
2.0 feet of bank freeboard shall be provided above the maximum water surface in the upstream channel.
(iii) 
Inlet losses reduce the effective capacity of the pipe. Therefore, it is recommended that the inlet opening be submerged by a minimum of 1.5Hv(pipe) to offset this loss and maintain one full velocity head in the pipe.
(2) 
Determine inlet/outlet control:
For inlet control, the required depth at the inlet (di) for a discharge of Q through the pipe is determined using Equation 28.52-14:
(28.52-14)
Where:
C
=
Orifice discharge coefficient = 0.6
di
=
Depth (feet)
Q
=
Flow rate (cfs)
A
=
Cross-sectional area of pipe (sf)
q
=
Gravitational constant, 32.2 ft./s2
Under outlet control, the required head for discharge Q is equal to the head losses through the pipe:
(28.52-15)
Where:
HL
=
Head loss across pipe (feet)
hi
=
Inlet head loss (feet)
hf
=
Pipe friction losses = Pipe length × Sf (feet)
ho
=
Outlet transition head loss (feet)
(3) 
Pipe overchutes shall be designed with a maximum full pipe velocity of 10 feet per second for concrete transition outlets or 12 feet per second for baffled outlets.
(4) 
Pipe diameter is determined by:
(28.52-16)
The minimum recommended diameter for an overchute pipe for the purpose of conveying stormwater is 24 inches. Depending on expected sediment and debris loads, some sites may necessitate larger minimum diameters.
(5) 
Structural and reinforcement requirements are not specified herein as they are outside the scope of this manual. Support piers shall be located so as to minimize potential adverse effects on the operation of the obstruction being crossed.
(6) 
An energy dissipation structure and/or other erosion protection shall be installed downstream of the overchute as applicable per Chapter 28.32 GJMC, GJMC § 28.36.020 through 28.36.060 and 28.36.070 through 28.36.100, and Chapter 28.48 GJMC.
FIGURE 28.52.040(a)
FIGURE 28.52.040(b)
(Res. 40-08 (§ 1302.2), 3-19-08)