Chapter 28.36


28.36.010    Introduction.

28.36.020    Channel drop structures.

28.36.030    Grouted sloping boulder (GSB) drop structure.

28.36.040    Vertical riprap drop structure (vertical hard drop basin).

28.36.050    Straight drop spillway.

28.36.060    Baffled aprons (USBR Type IX, baffle chute drop).

28.36.070    Energy dissipation structures.

28.36.080    Increased roughness basins.

28.36.090    Induced hydraulic jump basins.

28.36.100    Impact basins.

28.36.110    Overbank prevention structures and wasteways.

28.36.120    Side-channel spillways.

28.36.130    Gated turnouts.

28.36.140    Pipe appurtenances.

28.36.150    Pipe collars.

28.36.160    Thrust blocks at pipe bends.

28.36.170    Valves.

28.36.010 Introduction.

This chapter presents criteria regulating the design and construction of certain hydraulic structures commonly encountered in Mesa County at storm drain and culvert outlets as well as in open channels that are intended to convey storm runoff. Many of the structures discussed herein vary widely in physical and hydraulic parameters, and are thus presented with general design criteria only. The designer is referred to the applicable references for further detail on the hydraulic theory and design processes for these structures.

Many of the structures discussed in this chapter are highly visible and easily accessible to the general public. In some cases, these structures may attract onlookers who are unaware of the inherent dangers associated with their operation. Therefore, it is imperative that the design of such structures incorporate posted warnings, pedestrian barriers, fences, and/or other safety apparatuses. It is recommended that the designer consult with Mesa County and/or other local jurisdictions to coordinate the planning and design of the structures in this chapter.

(Res. 40-08 (§ 901), 3-19-08)

28.36.020 Channel drop structures.

The design of open channels for conveyance of stormwater runoff is governed by the maximum permissible velocities for a given channel type. These velocities, presented in Chapter 28.32 GJMC, Open Channels, are primarily related to the erosive potential of nonclear stormwater flow. In some locations, such as those adjacent to schools or parks, it may be advisable to further reduce design velocities to diminish safety risks to children. Drop structures are used at locations where the use of channel lining materials is undesirable or does not sufficiently reduce design velocity in the channel (see Chapter 28.32 GJMC for the design of lined channels). A properly designed drop structure will effectively reduce the design slope in a channel segment and dissipate the energy produced by the drop without adverse erosive effects to the channel bed.

Control sill grade control structures, or low-flow check structures, are used for velocity and grade control in wide, relatively stable floodplains and wetland areas. These structures are addressed in GJMC 28.32.340(d)(3).

The first three structures addressed in this section are typically selected for drops of 5.0 feet or less, but may be used in series (stepped drops). The vertical riprap drop structure (hard drop basin) is limited to 3.0 feet per drop for safety reasons. For larger single drops, the straight drop spillway, the baffled apron, or one of the structures found in GJMC 28.36.070 through 28.36.100 may be employed.

(Res. 40-08 (§ 902), 3-19-08)

28.36.030 Grouted sloping boulder (GSB) drop structure.

The GSB drop structure has recently become one of the most commonly installed drop structures in both new construction and channel retrofit situations. Relatively good hydraulic effectiveness and generally pleasing aesthetics likely contribute to this trend. However, the local availability of rock that meets the size and quality requirements for this structure weighs heavily on the economic viability of GSB drops. Much of the design data presented herein is attributed to the Urban Drainage Criteria Manual for the Denver-area UDFCD.

The excess energy created by the invert drop is dissipated in two ways in a GSB structure. The additional channel roughness of the grouted boulders themselves is secondary in energy dissipation to the hydraulic jump formed in the drop basin downstream. However, improper design or construction of the grouted boulders, including faulty rock or grout selection and placement, may result in sweepout of the hydraulic jump due to excessive velocity in the drop basin.

The GSB structure is intended for use only in grass-lined channels with upstream velocities within the limits set forth in Chapter 28.32 GJMC. With some variation in design (as outlined in this chapter), GSB structures may be used with channels containing or not containing a trickle channel or a low-flow channel.

Plans, profiles, sections, and details of typical GSB structures are found in Figures 28.36.030(a), 28.36.030(b), 28.36.030(c), 28.36.030(d), 28.36.030(e), and 28.36.030(f). Design of these structures involves five components: Rock and grout, upstream channel and approach apron, the drop face, the drop basin, and the exit apron. Seepage concerns are addressed in the design of the approach apron and associated cutoff wall.

(a)    Rock and Grout. Grouted boulders must be placed upstream of, and along, the crest of the drop, in the drop face and basin, and along the sill at the end of the basin.

Boulder sizing is based upon the critical velocity, Vc1, in the channel upstream from the drop structure. If a trickle channel or low-flow channel is present, the maximum critical velocity of that channel and the main channel is used to find the rock-sizing parameter:





Rock-Sizing Parameter



Maximum Upstream Critical Velocity (fps)



Longitudinal Slope (ft./ft.)



Specific Gravity of Rock = 2.55 unless otherwise certified by quarry

This parameter is used in Table 28.36.030(a) to find minimum boulder dimension, Dr.

Table 28.36.030(a): Grouted Sloping Boulder (GSB) Drop Structure Rock Sizing

Rock Sizing Parameter, Rp

Minimum Boulder Dimension, Dr

Less than 4.50

18 inches

4.50 to 4.99

24 inches

5.00 to 5.59

30 inches

5.60 to 6.39

36 inches

6.40 to 6.99

42 inches

7.00 to 7.50

48 inches

Adapted from USDCM (UDFCD) Table HS-5

Note that standard riprap rock gradation is not utilized in GSB structures. Instead, the boulders are placed in one layer directly on the graded and compacted subgrade (compaction per GJMC 28.32.310(g)(6)), as close together as is possible, and in such a manner so as not to adversely disturb the subgrade. The flattest surface of each boulder shall be oriented upward and shall be as horizontal as possible. The boulders shall be cleaned with water before grouting to improve grout-rock adherence. The boulders shall be placed by such methods that are less likely to cause breakage or significant blemishes and shall be checked for significant cracking before grouting. Damaged boulders shall be replaced before grouting.

Grout is used to fill the voids between the boulders from the subgrade to one-half of the boulder height from the subgrade. In the drop basin only, this is increased to three-quarters of the boulder height from the subgrade to promote draining. Excessive grouting leads to a reduction in hydraulic capacity and energy dissipation, and may endanger the structural stability of the drop. Selection, mixing, placement, and finishing of grout shall comply with the specifications set forth in Table 28.36.030.

(b)    Upstream Channel and Approach Apron. Grouted boulders shall be placed on grade with the upstream channel for a minimum of 8.0 feet upstream from the drop crest (this shall be referenced as the “approach apron”). Buried riprap shall be installed from the upstream end of the approach apron to a point at least 8.0 feet upstream along the channel flowline. The riprap shall be D50 = 12 inches (UDFCD Type M) and shall be installed per the criteria described in GJMC 28.32.300. The grouted boulder approach apron shall be continuous across the width of the channel (except as described in the following paragraph) and up each bank to the elevation of the normal depth for the design flow at that location. The buried riprap shall be installed across the channel bottom and up each bank to the elevation of one-half the normal depth for the design flow at that location.

For grass-lined channels with a concrete or rock-lined trickle channel (see GJMC 28.32.280(c)(1)), the approach apron and upstream riprap protection are discontinuous across the channel cross-section to allow the trickle channel flowline to continue unimpeded to the drop crest (see Figure 28.36.030(a)). While this is necessary to retain the effectiveness of the trickle channel in conveying base or nuisance flows, it tends to create a concentrated jet at the location of the trickle channel during higher flow periods. The additional energy introduced to the basin in these cases may be partially dissipated by the installation of large boulders or baffles in the trickle channel and/or a meandering trickle channel through the drop basin itself. These options are not shown in the GSB details (Figures 28.36.030(a), 28.36.030(b), 28.36.030(c), 28.36.030(d), 28.36.030(e) and 28.36.030(f)), but are similar to the trickle channel/drop basin controls used for the vertical riprap drop structure (see Figure 28.36.030(g)).

Grouted rock is particularly susceptible to failure from undermining and the subsequent loss of the supporting bank material. (HEC-11) This refers to the high potential for seepage and piping under and around the drop structure. Since the GSB structure is rigid and essentially monolithic, seepage under the grouted boulders and the resultant transport of subgrade particles will eventually lead to structural failure. Therefore, a seepage cutoff section is required as shown in Figures 28.36.030(a), 28.36.030(b), 28.36.030(c), 28.36.030(d), 28.36.030(e) and 28.36.030(f). As noted in the details, the dimensions of the vertical cutoff shall be determined based on geotechnical investigations and seepage analysis or shall comply with the minimum cutoff criteria set forth in the appropriate figures. The seepage cutoff shall be installed prior to the placement of the grouted boulders at the drop crest, and shall include a keyway for the grout/cutoff interface as shown in the details.

(c)    Drop Face. The drop face shall consist of grouted boulder “steps” of vertical dimension no greater than one-half of the minimum boulder dimension, Dr, from Table 28.36.030(a). The overall drop face slope must not exceed 4H:1V; flatter slopes are permissible and encouraged due to improved aesthetics and energy dissipation. Slopes steeper than 4H:1V may reduce structural stability.

The grouted boulders are continuous across the entire bottom width on the drop face – the trickle channel flowline equals the main channel flowline in the drop section. The grouted boulders also continue up each bank to the elevation equivalent to the downstream channel normal depth (sequent subcritical depth) plus freeboard or the channel critical depth plus 1.0 foot, whichever is greater.

A weep drain system shall be installed behind the drop face to relieve hydrostatic pressure in drops exceeding 5.0 vertical feet. See details in Figures 28.36.030(a), 28.36.030(b), 28.36.030(c), 28.36.030(d), 28.36.030(e) and 28.36.030(f).

(d)    Drop Basin. The basin area shall be constructed of continuous grouted boulders of the same dimensions as the drop face section (boulder size, crest and basin width, height of bank protection). However, the grout level is increased to three-quarters of the boulder height in the basin, and shall be sloped to drain to the centerline of the channel (or trickle channel if applicable).

The basin is depressed below the downstream channel invert by 2.0 feet for drops of 5.0 feet or less. This helps to stabilize the hydraulic jump. For drops exceeding 5.0 feet, a sequential depth analysis is necessary to determine basin depression depth (a minimum of 2.0 feet applies). Sequential depth analysis is not presented in this title; refer to a hydraulics text such as Open-Channel Hydraulics (Chow, 1959) for explanation.

Basin length shall be a minimum of 15 feet for nonflexible downstream channel lining (concrete, grouted riprap, geosynthetic linings) and a minimum of 20 feet for downstream channels with flexible linings. A row of 36-inch or larger grouted boulders shall be placed at the downstream end of the basin. The top of this sill shall be equal to the invert of the downstream channel. For channels with a concrete or rock-lined trickle channel, there shall be a break in the end sill of width equal to that of the trickle channel. The trickle channel shall continue downstream through the sill and exit apron with scour protection as specified in Chapter 28.32 GJMC.

(e)    Exit Apron and Downstream Channel. The exit apron shall consist of buried riprap of size D50 = 12 inches (UDFCD Type M) and shall be installed per the criteria described in GJMC 28.32.300. The riprap shall extend across the channel (except in the trickle channel as applicable) and up the banks to an elevation equal to the top of the adjacent grouted boulders. This riprap protection shall extend downstream from the end sill a minimum distance of twice the drop height or 10 feet, whichever is greater.

(Res. 40-08 (§ 902.1), 3-19-08)

28.36.040 Vertical riprap drop structure (vertical hard drop basin).

This type of drop structure consists of an approach apron (grouted rock), a vertical concrete crest wall, a jump basin with end sill (grouted rock or concrete), and downstream channel scour protection. While an effective method for drop design, these structures shall be avoided if possible in areas of significant public use or in highly visible locations due to safety concerns and low aesthetic appeal. Vertical drop structures shall be avoided in channel reaches which may be utilized for boating or other recreation activities in or adjacent to the water. The maximum allowable drop for a vertical drop structure of this type shall be 3.0 feet.

(a)    Rock and Grout. Rock used upstream of the crest wall shall have a minimum dimension of 12 inches in any direction. Rock used downstream of the drop shall have a minimum dimension of 18 inches in any direction. Grouting requirements are identical to those presented in GJMC 28.36.030 for the GSB drop structure.

(b)    Approach Apron. A grouted rock apron shall be installed across the entire bottom width (including trickle and low-flow channels) and up each bank to the elevation equal to upstream channel normal depth plus 1.0 foot. The rock shall be buried to a depth such that the top of the grout is equal to the invert of the upstream channel at every point across the channel. This approach apron shall extend upstream from the crest wall a minimum of 10 feet.

(c)    Vertical Crest Wall. The concrete crest wall conforms to the upstream inverts for the trickle or low-flow channel and the main channel across the bottom width. The wall shall extend a minimum of 5.0 feet into the undisturbed banks. However, all design dimensions including minimum structural width, wall thickness, footer size and geometry, and reinforcement shall be determined using accepted structural analysis methods and determination of potential creep, heave, buoyancy and uplift due to seepage pressures, and all other considerations associated with the design of a retaining wall.

An impervious backfill material is recommended both upstream and downstream adjacent to the crest wall and footers to act as a horizontal seepage cutoff. In lieu of this material, other appliances may be employed to ensure minimized seepage around/under the crest wall. Piping, the transport of structural supporting material away from its intended location, is a common cause of structure instability and failure.

(d)    Basin. The basin is a depressed, hard-surface area which redirects the plunging flow from the crest horizontally. At lower flows, the energy dissipated by this redirection may be sufficient to return the flow to a subcritical state. However, the primary energy dissipation method for this structure is a hydraulic jump formed in the basin. When the upstream channel is composite (utilizing a trickle or low-flow channel), the approach velocity tends to be higher in the smaller sub-channel zone than in the main channel zone. Therefore, for the design flow, the basin length and downstream protection requirements may differ for the two zones. By placing large boulders (60 percent to 80 percent of critical depth in height) between the location of nappe impingement on the basin floor and a point at least 10 feet from the end sill, the required basin length for the sub-channel zone may be reduced to that of the main channel zone. Otherwise, the following calculations must be applied to both zones independently.

The drop will be treated hydraulically as a straight-drop spillway and analyzed per Chow’s (1959) method:

A “drop number,” DN, must first be calculated in order to relate other associated lengths and depths:





Discharge per Unit Width for the Subject Zone (cfs/ft.)



Gravitational Constant = 32.2 ft.2/s



Effective Height of Drop (ft.)

Note that the effective drop height must include the basin depression depth. Using the drop number, the following relationships can be solved:








Drop Length (ft.)



Pool Depth Under Nappe (ft.)



Depth Upstream of the Hydraulic Jump (ft.)



Subcritical Sequent Depth (ft.)

See Figure 28.36.030(g) for illustration of these variables. These values assume that atmospheric pressure is maintained under the nappe, thus the designer is responsible for incorporation of aeration devices as necessary. Drop length, Ld, refers to the horizontal distance from the crest wall to the location of depth y1, upstream of the hydraulic jump.

The basin design length, for the subject zone, is given by Equation 28.36-7:





Basin Design Length (ft.)



Distance from Location of Depth y1 to Jump (ft.)



Length of Jump ≅ 6 · y2

The distance from the point of nappe impingement on the basin floor to the upstream end of the hydraulic jump is determined by a water surface profile analysis as presented in most hydraulic design texts.

Basin depression depth below the downstream channel invert is determined by comparing the subcritical sequent depth, y2, with the tailwater depth in the downstream channel, yTW. If y2 exceeds yTW, the jump will be swept downstream and possibly out of the basin. This situation is to be avoided since significant erosion may take place if the jump occurs in an unarmored location in the channel. If yTW exceeds y2, the jump is pushed upstream toward the wall, potentially submerging jump. Hydraulically, this is not problematic, but the structural design of the crest wall may be affected by the additional forces. Basin depression effectively adds to the tailwater depth in the downstream channel, controlling the location of jump formation. Therefore, the minimum basin depression depth, B, is the maximum of the following:


This is the height of the end sill and downstream invert above the downstream end of the depressed basin. The end sill shall be constructed of reinforced concrete or grouted boulders of a minimum 36-inch dimension. This acts as a protected transition back to the channel invert.

(e)    Downstream Channel Protection. The channel directly downstream from the end sill shall be protected for a minimum of 10 feet in the direction of flow with buried riprap of size D50 = 12 inches (UDFCD Type M) or grouted rock with a minimum dimension of 12 inches.

In cases where the sub-channel zone basin length is longer than the main channel zone (no additional boulders or baffles placed in the basin to dissipate the center jet), the additional protection shall extend a lateral distance equal to the bottom width of the trickle channel from each edge of the trickle channel. This results in an extended protection zone with a width equal to three times the trickle channel bottom width.

(Res. 40-08 (§ 902.2), 3-19-08)

28.36.050 Straight drop spillway.

The straight drop spillway is very similar hydraulically to the vertical hard drop basin presented in GJMC 28.36.040. The primary difference exists in the shaping of the spillway downstream from the crest to closely resemble the shape of the lower nappe, i.e., the bottom of the jet formed by the flow suddenly departing the crest. This results in a “classic” spillway shape, as used for major reservoir spillways and channel drops alike. The straight drop spillway itself is not a significant energy dissipation structure, and must be paired with an induced hydraulic jump basin as presented in GJMC 28.36.090.

The shape of a straight drop spillway is dependent on the shape of the nappe, which varies with head over the crest and the shape of the approach to the spillway. The reader is referred to Open-Channel Hydraulics (Chow, 1959), Hydraulic Design of Spillways (USACOE, 1992), or other texts for design of these structures.

Figure 28.36.050 shows a typical straight drop spillway configuration.

(Res. 40-08 (§ 902.3), 3-19-08)

28.36.060 Baffled aprons (USBR Type IX, baffle chute drop).

The fixed costs associated with the construction of a baffled apron structure (hereafter referred to as a baffle chute drop) typically limit their use to larger drops from an economic standpoint, although the actual minimum size is limited to that length required to incorporate the minimum number of baffle rows. These drop structures are most effective at unit discharge rates between 35 and 60 cfs per foot. However, a value in this range can often be attained by altering the width of the chute. Most often, transition walls are employed to direct wider upstream channel flow to a narrower chute, decreasing the cost of the drop structure. When designed and built correctly, these structures are effective and last for many years with minimal maintenance requirements.

While the baffle chute drop can pass most sediment and debris, larger debris may become caught behind the baffles or in the narrowed chute, disabling the structure’s ability to dissipate energy. This can lead to an effectively higher invert in the chute and overtopping, and can also allow the nearly unimpeded flow in the chute to exit the structure at erosive velocities. Therefore, debris-control structures are recommended upstream of the drop, and regular inspection and maintenance may be necessary.

The baffle chute drop structure does not rely on the formation of a hydraulic jump as its primary energy dissipation process. Instead, excess energy in the chute flow is dissipated by redirection over and around baffle blocks, which are arranged in offset rows to avoid the passing of high-velocity jets between the blocks. Since a hydraulic jump is not part of the design, there are no tailwater requirements for this structure. However, potential scour due to relatively high velocities at the end of the chute and in the downstream transition section necessitate a protected exit apron and/or scour hole.

Figure 28.36.060 presents an isometric view of a baffle chute drop with typical dimensional requirements. Note that this figure does not indicate structural requirements such as concrete thickness or reinforcement, footer depths and dimensions, or seepage control. These factors shall be assessed and approved by qualified professionals.

(a)    Upstream Channel Transition. Typically, the design width of the baffle chute drop is less than the upstream channel width for economic and sizing reasons as well as to attain unit discharge rates in the desired range. The headwalls and/or wingwalls associated with this transition are subject to design constraints set forth in this manual and shall be designed using proper structural analysis techniques. The designer should note that the effective width of a conduit or channel is often considerably smaller than the physical width due to the separation of flow from the abutment/conduit interface.

The approach section downstream from the transition is designed to maintain an approach velocity of less than the critical velocity at the crest. Recommended approach velocities are presented in Figure 28.36.060. The concrete flow alignment apron, reaching from the abutment/conduit interface to the chute crest, shall be a minimum of 5.0 feet in length and shall be equal to the chute in width along its entire length. In certain cases, the transition section may not sufficiently reduce the specific energy of the flow to achieve the proper approach (alignment) apron velocity. In these situations, the crest may be raised by up to 12 inches above the approach apron invert.

If a trickle channel is present in the upstream channel, it shall continue through the transition section and apron, and shall maintain a continuous flowline through any raised crest.

Transition and apron wall heights are determined by backwater analysis at peak flow, with a freeboard equal to or greater than that of the upstream channel.

(b)    Baffled Chute. The chute floor, walls, and baffles shall be constructed of reinforced concrete and shall be structurally designed to withstand all geotechnical, hydrostatic, and hydrodynamic (impact and frictional) forces imposed by the specific site conditions, including a reasonably conservative factor of safety for all loading. The chute floor shall have a slope no steeper than 2H:1V (Z:1, ZMAX = 2). The chute walls shall be vertical and shall be tied to the floor, upstream wall or abutments, and downstream abutments with properly sized and installed steel reinforcement.

The baffle blocks shall be reinforced concrete of the dimensions shown in Figure 28.36.060. Baffle blocks shall be adequately reinforced and tied with steel reinforcement to the chute floor. A key-in interface is recommended to stabilize the blocks on the chute floor. The block height normal to the chute floor is defined in Equation 28.36-9:





Block Height Normal to Chute Floor



Critical Depth at Peak Flow

There shall be at least four rows of baffle blocks. Baffle block rows shall be spaced at Z · H along the direction of flow, and shall be staggered such that jets of water not directly impinging on a baffle block within a two-row distance are minimized. The blocks and the spaces between the blocks shall be equal to 1.5H except where the width is limited by the chute wall. All baffle rows shall be symmetrical along the centerline of the chute. When a trickle channel exists, the top row of baffles shall be aligned such that the maximum percentage of the trickle channel flow width is not impingent upon any baffles in the first row.

Chute walls shall be at least 3H in height normal to the chute floor. Other dimensional requirements may be found in Figure 28.36.030.

Where a hard-surface exit apron is not employed, at least 1.5 rows of baffles shall be buried in riprap. This allows for the exposure of additional baffle blocks as loose rock is displaced to form a scour hole or to adapt to a lower downstream channel invert.

Downstream transition walls (headwalls and/or wingwalls) shall be of a height equal to the design normal depth in the downstream channel plus 1.0 foot of freeboard. They shall extend from the chute walls at an angle of 45 to 90 degrees for a distance necessary to contain any eddies that may form in this area.

(c)    Basin/Exit Apron. There exist two primary design options for the basin downstream from the baffle chute. The first, a hard-surface basin, is used if the invert of the downstream channel is expected to remain approximately constant over the life of the drop structure. This basin is constructed of either reinforced concrete tied to the downstream end of the chute floor or grouted rock, the latter of which further dissipates energy in the flow and protects the downstream channel from excessive degradation.

Even more energy dissipation is often achieved with the installation of a preformed or nonpreformed scour hole at the chute exit. The former is a riprap-lined depressed basin that approximately imitates the dimensions of the scour hole that would form if loose rock was placed as backfill. The riprap and basin sizing requirements are found in GJMC 28.32.340(c) and Figure 28.32.340(a). The designer may substitute the downstream design flow normal depth for Do in the relevant equations and figures. Wo is equal to the width of the chute for the purpose of this design.

A nonpreformed scour hole is constructed by backfilling up to the existing downstream channel invert with loose rock. The loose rock shall be at least 2.0 feet deep and shall extend a minimum of 4H feet horizontally parallel to the chute. The rock backfill area shall be of such a width to reach the ends of the downstream abutment walls. Rock size is based on the riprap selection criteria set forth in Chapter 28.32 GJMC. Placement of the rock must not damage the buried baffle blocks. With sufficient operation time, the force of the flow from the baffle chute will displace the loose rock in such a way so as to form a stable scour hole.

The scour hole options, especially the latter, tend to adapt somewhat automatically to changing conditions in the downstream channel, including a gradually lowered invert elevation. However, it is still recommended that a protective channel lining be installed in the downstream channel for an appropriate distance to allow flow to return to a nearly steady state.

(Res. 40-08 (§ 902.4), 3-19-08)

28.36.070 Energy dissipation structures.

(a)    The structures described in this section are similar in many ways to the channel drop structures of GJMC 28.36.020 through 28.36.060. However, while the drop structures’ primary purpose is to allow a channel to quickly change elevation without excessively increasing the specific energy of the flow in the downstream channel, these structures are designed to dissipate excess energy already present in the upstream channel. These energy dissipation structures are often employed at transitions from nonflexible channels or conduits to channels with flexible linings or other velocity restrictions. This includes culvert and storm drain outlets to open channels. They are also occasionally used at locations where the energy produced by a channel drop exceeds the limitations of the channel drop structures. As mentioned in GJMC 28.36.050, straight-drop spillways must be paired with one of the structures in this section to dissipate the energy associated with the high-velocity flows.

(b)    The structures in this section are divided into three categories:

(1)    Increased roughness basins.

(2)    Induced hydraulic jump basins.

(3)    Impact basins.

(Res. 40-08 (§ 903), 3-19-08)

28.36.080 Increased roughness basins.

Increased roughness basins are designated for use in locations where the upstream Froude number does not exceed 3.0. Further restrictions apply to each type, including maximum velocities and maximum cross-sectional flow areas. These basins include the riprap basin (preformed scour hole) and the array of drop structures introduced in GJMC 28.36.020 through 28.36.060.

The FHWA’s Hydraulic Design of Energy Dissipators for Culverts and Channels (HEC-14) also presents methods for the design of increased resistance devices for pipes, box culverts, and channels. These devices are intended to create a tumbling flow pattern along steep reaches of conduits and channels, thereby maintaining an allowable average velocity. However, due to the economic advantages of other options and to the relatively flat terrain in developed portions of Mesa County, these structures are not included in this title.

(a)    Riprap Basin. The riprap basin/preformed scour hole is effective for the dissipation of excess energy from upstream conduits and channels complying with the following:

(1)    Maximum allowable upstream flow area must be equal to or less than the equivalent full-flow area of a 36-inch pipe.

(2)    Maximum upstream flow velocity must be equal to or less than 15 feet per second at any flow depth.

The design procedure for this structure is presented in GJMC 28.32.340(c) and Figure 28.32.340(a).

(b)    Drop Structure as an Energy Dissipator. While the first two drop structures listed in GJMC 28.36.020 through 28.36.060 are intended to dissipate only energy produced by the drop itself, they can in certain cases be used as dissipators of upstream energy. The most significant restriction is that flow in the upstream channel must be in a subcritical state before reaching the structure. The combination of a small channel drop and local energy dissipation can effectively reduce the velocity in the downstream channel.

(Res. 40-08 (§ 903.1), 3-19-08)

28.36.090 Induced hydraulic jump basins.

Induced hydraulic jump basins are commonly used for large and small projects alike. They are highly effective at utilizing the hydraulic jump phenomenon to dissipate excess energy and return the flow to a subcritical depth. While the space required for these structures is relatively large, they are typically less expensive than impact-type basins on a unit-discharge basis. Five distinct induced hydraulic jump basin designs are presented herein. The designer shall incorporate adequate seepage controls as part of the design of all structures. Riprap protection shall be provided for an appropriate distance downstream of all structures in this section where the receiving channel has a flexible lining.

(a)    CSU Rigid Boundary Basin. The Colorado State University Rigid Boundary Basin (CSU RBB) utilizes offset rows of baffles (roughness elements) to force supercritical flow from a conduit into a hydraulic jump. The only basin in this category to be designed as entirely on-grade, the CSU RBB is useful for locations with restrictive vertical alignment criteria. However, the upstream Froude number is restricted to a value of 3.0. Figures 28.36.090(a) and 28.36.090(b) present sketches and data for the design of this structure.

The design procedure for the CSU RBB is presented in HEC-14, Chapter VII-A.

(b)    USBR Type II Basin. This basin utilizes chute blocks and a dentated sill to induce a hydraulic jump in the basin. An isometric sketch of this basin is presented in Figure 28.36.090(c). Unlike the CSU rigid boundary basin, the USBR Type II basin does not allow for subcritical upstream flow by forcing the flow through critical depth prior to the jump basin. Therefore, this basin requires an upstream Froude number between 4.0 and 14.0. The required tailwater depth for this basin varies with the Froude number per Figure 28.36.090(d), in which the solid “design curves” incorporate the required 10 percent factor of safety. This basin is intended for rectangular sections only, thus transitions may be required upstream of the structure. The design procedure herein is intended for unit discharge rates of up to 500 cfs per foot width. The incoming chute to the basin can be of any slope, but slopes greater than 2:1 shall incorporate a radius curve to allow for a smooth transition to the basin floor. Sequent depths for a free hydraulic jump, USBR Type II basin, and USBR Type III basin are shown in Figure 28.36.090(e).

The design procedure for the USBR Type II Basin is presented in HEC-14, Chapter VII-D.

(c)    USBR Type III Basin. This basin utilizes chute blocks, baffle piers, and a solid end sill (no dentates) to induce a hydraulic jump in the basin. An isometric sketch of this basin is presented in Figure 28.36.090(f). This basin requires an upstream Froude number between 4.5 and 17.0. This can in part be controlled by the height and slope of the upstream chute, but the designer shall be aware that lower basin elevations can cause the jump to move upstream and submerge the chute, negating its ability to increase the influent Froude number. The required tailwater depth for this basin is at least full conjugate depth as indicated in Figure 28.36.090(d). This basin is intended for rectangular sections only, thus transitions may be required upstream of the structure.

The USBR Type III basin is limited to a unit discharge rate of 200 cfs per foot width, but can handle velocities up to 50 or 60 feet per second. The design is intended to effectively initiate and shorten the hydraulic jump, thereby reducing the space requirements for the structure. However, the baffle piers, which are essential for controlling the jump, must be carefully designed to comply with the procedure outlined below. The incoming chute to the basin can be of any slope, but slopes greater than 2:1 shall incorporate a radius curve to allow for a smooth transition to the basin floor.

The design procedure for the USBR Type III Basin is presented in HEC-14, Chapter VII-E.

(d)    USBR Type IV Basin. At locations where the upstream flow is supercritical but still in the relatively low range of Froude numbers, the USBR Type IV basin can be employed. Designated for Froude numbers between 2.5 and 4.5, the jump is defined by Chow (1959) as an “oscillating jump.” This type of hydraulic jump can produce potentially destructive downstream wave action, so the recommended tailwater depth for this structure is higher than that for the Type III basin.

Like the Type II basin, this structure utilizes chute blocks and an end sill. However, the end sill in this case is solid, not dentated. This basin is intended for rectangular sections only, thus transitions may be required upstream of the structure. An isometric sketch with general dimensions is presented in Figure 28.36.090(g).

The design procedure for the USBR Type IV basin is presented in HEC-14, Chapter VII-F.

(e)    SAF Stilling Basin. The Saint Anthony Falls (SAF) stilling basin is similar to the USBR Type III Basin in that it utilizes chute blocks, baffle piers (floor blocks), and an end sill to induce and maintain a steady hydraulic jump in the basin. Also similar to the Type III, it produces a jump that is significantly shorter than a natural hydraulic jump (approximately 80 percent of the length), thereby reducing the required length of the basin and downstream protection.

Plan and profile views of the SAF basin are provided in Figure 28.36.090(h). Note that the basin itself may be laterally flared to better fit the downstream channel. This flare is labeled as z (longitudinal):1 (lateral), wherein the variable z is limited to values equal to or greater than 2.0. However, all side walls, headwalls, and wingwalls shall be vertical.

The SAF Basin may be used at the base of straight-drop spillways, at culvert and storm drain outlets, and in canals. It is required that flow entering the basin be supercritical, but this can usually be achieved by proper upstream chute design. The allowable range of Froude numbers for this structure is 1.7 to 17.0.

The design procedure for the SAF stilling basin is presented in HEC-14, Chapter VII-G.

(Res. 40-08 (§ 903.2), 3-19-08)

28.36.100 Impact basins.

Impact basins dissipate energy by causing the high-velocity flow to encounter an obstruction, redirecting the flow in directions other than the influent path. This action effectively negates a large percentage of the velocity head that would otherwise potentially cause damage to the downstream channel. While these structures tend to be costly on a unit-discharge basis, they require far less space than many other dissipation options. Three types of impact basins are presented in this section.

The designer of the energy dissipators discussed herein is responsible for ensuring adequate structural design, including the analysis of all forces incident on the structure, calculation of creep and overturning potential, and design and installation of seepage controls. Necessary seepage controls may include cutoff walls, liners, weep drains, and/or other devices. The designer is referenced to applicable texts concerning subgrade compaction, concrete mixing, steel reinforcement, calculation of external forces, and retaining wall design.

(a)    Contra Costa Energy Dissipator. This structure is intended for use with small to medium culverts with medium to high velocity flows. It is also designed to operate with minimal tailwater, although some tailwater improves the dissipator’s performance. Tailwater depth is limited to one-half of the culvert height. The Contra Costa dissipator is best for locations where the design flow depth at the culvert outlet is less than the culvert height. Therefore, culvert effluent depth is limited to one-half of the culvert height. The Froude number of the culvert outlet flow is limited to a maximum of 3.0.

The Contra Costa energy dissipator is a concrete structure designed to be placed in a trapezoidal channel with side slopes of 1:1 and a bottom width between one and three times the culvert height (D ≤ W ≤ 3D). If a natural channel exists at the structure location, the structure width shall conform to that channel, with a maximum width of 3D. The structure consists of two continuous baffles of different heights across the basin floor as well as a vertical end sill. All parts of the structure shall be reinforced concrete and shall be tied to the downstream end of the culvert with steel reinforcement bars if possible. Profile and section views with dimensional definitions are provided in Figure 28.36.100(a).

The design procedure for the Contra Costa energy dissipator is presented in HEC-14, Chapter VIII-A.

(b)    Hook-Type Energy Dissipator. The hook-type dissipator, also called aero-type, is used at culvert outlets with Froude numbers in the range of 1.8 to 3.0. Each dissipator utilizes three hook structures in the basin that redirect a portion of the high-velocity flow up and back into the basin flow. This action creates a large amount of turbulence, thereby dissipating some of the excess energy in the flow. At Froude numbers exceeding about 3.0, the dissipation effects are greatly diminished.

This energy dissipation structure is designed to use either of two basin configurations. The first type contains vertical wingwalls at the culvert exit which are warped smoothly to side slopes of 1.5:1 at the end sill (see Figure 28.36.100(b)). The second configuration is a trapezoidal channel with a constant cross-section throughout the basin (see Figure 28.36.100(d)). Hook details for the two configurations are found in Figures 28.36.100(c) and 28.36.100(e), respectively.

The design procedure for the Hook-Type Energy Dissipator is presented in HEC-14, Chapter VIII-B.

(c)    Impact-Type Energy Dissipator (USBR Type VI). Also called the baffle-wall energy dissipator or baffled outlet, this structure is compact and highly effective for the control of high-energy flows exiting a conduit or rectangular channel section. Consisting of a vertical-walled basin with a single large vertical hanging baffle, energy is dissipated by impact with the baffle and secondarily by eddies formed in the basin. At the design flow, this structure dissipates energy more effectively than a hydraulic jump (See Figure 28.36.100(g)), and has no minimum tailwater depth. However, its debris-handling capability and maximum tailwater depth limit (discussed later) limit the locations at which the structure can be used. Further limitations include a maximum discharge of 400 cfs per structure and a maximum upstream velocity of 50 feet per second. This latter value is intended to minimize damage to the baffle due to cavitation. Where these limits are exceeded, two or more structures may be built adjacent to one another to accommodate the excess flow.

For upstream conduits with a slope greater than 15 percent and for all open channels, it is recommended that there be a horizontal section from the outlet brink to a point at least four conduit widths upstream. Rectangular upstream channels shall have sidewalls of a height equal to or greater than the walls of the dissipator basin and shall always have a zero longitudinal slope for a minimum of three channel widths upstream from the entrance to the basin.

Figure 28.36.100(f) presents the configuration and necessary dimensions for the design of the USBR Type VI structure. Note that the optional notches near the edges of the basin are included to create concentrated jets for self-cleaning purposes.

One of the most important design features of this structure is its ability to pass the entire design discharge over the top of the baffle. This is important to prevent upstream flooding in the case of complete clogging of the area under the baffle. However, this flow configuration is not nearly as effective and shall not be relied upon as an alternative energy dissipation method. Therefore, the debris and ice buildup potential at a given location shall be analyzed prior to selection of this structure as the energy dissipator for that outlet.

While some tailwater (up to h3+h2/2) improves the performance of the dissipator, depths over this height shall be avoided. Significant degradation of performance occurs with tailwater depths greater than h3+h2, thus the USBR Type VI structure shall not be installed in these conditions.

The design procedure for the impact-type energy dissipator is presented in HEC-14, Chapter VIII-C.

(Res. 40-08 (§ 903.3), 3-19-08)

28.36.110 Overbank prevention structures and wasteways.

Every channel has a maximum allowable flow depth which, when exceeded, may cause damage to the banks and eventually failure of the channel. Occasionally, overflow from a storm drainage system enters an irrigation canal (this shall be avoided unless specific consent is granted by the owner/operator of the canal). In these situations, it is typically necessary to remove the overflow from the canal at some downstream location. The structures in this section are intended to remove excess water from a channel to maintain a specified water surface elevation or to allow the water in a channel section to be drained. The latter may be necessary to inspect, maintain, or repair the channel, or in the event of an embankment failure, to redirect some of the escaping flow to an acceptable location.

Wasteway is the term commonly applied to the channel to which the main channel excess flow is diverted. A wasteway shall have the capacity to convey the maximum flow that can be diverted through all diversion structures located upstream, and shall deliver the excess flow to an acceptable disposal point.

Two types of diversion structures which can act as overbank prevention structures are presented herein: the side-channel spillway and the gated turnout.

(Res. 40-08 (§ 904), 3-19-08)

28.36.120 Side-channel spillways.

A side-channel spillway is the most effective structure for automatic removal of excess flow in a channel since its capacity increases with depth over its crest. The spillway crest is usually parallel to the channel alignment except at terminal wasteways (at the end of a canal). Typically, the spillway crest is set approximately 0.2 feet above the normal design depth for the channel to allow for normal wave action. The length of the spillway is then controlled by the required overflow discharge capacity and the maximum allowable water surface elevation in the channel. A standard rule of thumb is to ensure no more than 50 percent encroachment on the freeboard of the channel banks in the vicinity of the spillway. A detailed procedure for the design of a side-channel spillway turnout and wasteway is not presented in this manual due to the infrequent application of such a structure in stormwater runoff designs. However, Equation 28.36-10 is the basic design equation for the side-channel spillway (suppressed rectangular weir):





Design Flow over the Spillway (cfs)



Crest Length (ft.)



Height of Channel Water Surface over Crest (ft.)

(Res. 40-08 (§ 904.1), 3-19-08)

28.36.130 Gated turnouts.

To allow for manual release of water from a channel for the purpose of water level control, maintenance access, et cetera, gated turnouts are often installed at wasteways. It is common practice to include at least one gated turnout at any side-channel spillway location for flushing and additional water level control. Again, specific design procedures are not presented, but the general orifice equation is given:





Design Flow through Gate (cfs)



Orifice Coefficient ≅ 0.6



Height of Water Surface Over Gate Centerline (ft.)



Area of Orifice (ft.2)



Gravitational Constant (32.2 ft./s2)

(Res. 40-08 (§ 904.2), 3-19-08)

28.36.140 Pipe appurtenances.

GJMC 28.36.150 through 28.36.170 present appurtenances for use in conjunction with pipe systems, specifically those designed for the transport of stormwater.

(Res. 40-08 (§ 905), 3-19-08)

28.36.150 Pipe collars.

Pipe collars are transverse fins that extend from the pipe into the surrounding earth and function as barriers to percolating water and burrowing rodents (USBR 1974). Due to the relative smoothness and impermeability of pipe, percolated water tends to collect and move along the soil adjacent to a pipe’s outer wall. This action, typically called piping, tends to transport soil particles away from the pipe, potentially causing the pipe to experience structural problems. Failure of the backfill and ultimately the pipe itself can lead to hydraulic failure of the pipe system as well as the failure of surface structures such as roadways and buildings.

While percolation is expected around many storm drains and culverts, especially near pipe inlets, those with higher (5H:1V or greater) percolation gradients are often candidates for pipe collars. The percolation gradient is the slope of a line from an inlet water surface to a point of relief for the percolated water. The difference in water surface elevations between the upstream end of the percolation path and the point of relief is ΔHperc. Lane’s weighted creep method is used to determine a percolation factor (weighted-creep ratio), which is compared to the allowable ratio for the soil type at a given site. First, determine the weighted-creep length:





Weighted creep length (ft.)



Vertical path distance along the structure (steeper than 45°) (ft.)



Horizontal path distance along the structure (flatter than 45°) (ft.



Percolation path distance that shortcuts the soil (ft.)

Then determine the percolation factor, Rwc to 1:


Table 28.36.150 presents minimum recommended weighted-creep ratios for a range of soil types:

Table 28.36.150: Lane’s Minimum Recommended Weighted-Creep Ratios


Minimum Ratio

Very fine sand or silt


Fine sand


Medium sand


Course sand


Fine gravel


Medium gravel


Course gravel w/cobbles


Boulders w/some cobbles and gravel


Soft clay


Medium clay


Hard clay


Very hard clay/Hardpan


Adapted from USBR 1974, Untitled Table, Page 364

Where the weighted-creep ratio calculated in Equation 28.36-13 does not exceed the applicable recommended ratio from Table 28.36.150, or does not exceed 2.5:1, pipe collars shall be installed.

Figure 28.36.150 presents basic dimensions for pipe collar fittings on reinforced concrete pipe (RCP) and corrugated metal pipe (CMP).

(Res. 40-08 (§ 905.1), 3-19-08)

28.36.160 Thrust blocks at pipe bends.

Every horizontal or vertical pipe bend in a storm drain, culvert, inverted siphon or other pipe structure shall be analyzed for stability. As the momentum of flow changes around a bend, forces are exerted on the bend that must be countered by the pipe walls, soil pressure, pipe joints, and friction. When the dynamic thrust exceeds the allowable force on any of these resistance devices, a thrust block is installed at the bend. A thrust block typically consists of a rough block of concrete poured around the outside of a pipe bend in direct contact with the outer wall of the pipe.

The thrust force on a pipe bend is calculated by vector components (x, y, and z) to simplify the process. In the equations below, “x” represents the horizontal direction of flow upstream of the bend, “y” represents the horizontal direction of flow normal to “x,” and “z” represents the vertical direction along which gravity acts. Equations 28.36-14 through 28.36-16 are adapted from Roberson et al., 1998, using conservation of momentum to find reaction forces. Pipe cross-sectional area and internal pressure are assumed to be constant through the bend, with pressure assumed to equal the surcharge depth above the pipe crown, if applicable.







Reaction force required to hold bend in place, lbf



Density of water 62.4 lbs/ft3



Flow rate in pipe, cfs



Average pipe velocity upstream of the bend, fps



V1x cos θ, V2y = V1x sin θ, V2z = V1x sin θ



Internal pipe pressure, psf



Gravitational constant, 32.17 ft./s2



Cross-sectional flow area of pipe, sf



Total bend angle (vertical or horizontal)



Weight of the pipe in the bend, lbs.



Weight of the water in the bend, lbs.

Subscripts 1 and 2 indicate conditions just upstream and just downstream of the bend.

In addition to soil bearing pressure, force on a bend is resisted by friction between the pipe and the soil. A sliding coefficient of 0.35 is recommended for purposes of calculating the friction force (USBR 1974).

Where calculations indicate that sliding or displacement of a horizontal bend may occur, a thrust block is installed to increase the effective bearing area on the soil such that the load is adequately dispersed. Vertical bends may require an anchor block to provide additional weight to resist the resultant vertical force. Calculation of resisting forces for a vertical bend may include full pipe weight and anchor block weight, but shall not include the weight of earth cover on the bend. This allows for safe operation of the pipe even with reduction or removal of cover material (USBR 1974).

(Res. 40-08 (§ 905.2), 3-19-08)

28.36.170 Valves.

References for this section include USBR, 1974 and Linsley and Franzini, 1964.

(a)    Drain (Blow-Off) Valves. A blow-off valve is intended to allow for the draining of a structure that typically will otherwise not fully drain. Most commonly used in long inverted siphons, blow-off valves may be gravity-fed, pumped, or a combination of both, depending on the invert of the discharge pipe. The design and installation of blow-off valves and related pipes shall incorporate pressure-rated joints and provisions for operation and maintenance access.

(b)    Pressure Relief Valves. Pressure relief valves are used to exhaust excess air pressure from a pipeline to protect the pipe from bursting and to remove large volumes of entrapped air that may significantly impact the hydraulic capacity of the pipe. The valves are set to open at a predetermined pressure so as to allow for a sealed pipeline under normal operating pressures.

These valves are commonly utilized in smaller pressure pipelines such as water supply lines to limit the effects of hydraulic transients (water hammers), but are occasionally used in stormwater systems. Inverted siphons (GJMC 28.52.030) often require a venting system to prevent blowback of air entrained in the water, although an open air vent (no valve) is usually an acceptable solution given an exhaust point that is well above the hydraulic grade line.

An air venting system of some type is required at all locations where the crown of a pipe is higher than the crown elevations upstream and downstream from that point.

(c)    Air Inlet Valves. Air inlet valves operate in a similar fashion to pressure relief valves, but instead allow air into a pipeline to avoid internal pressure to drop too far below atmospheric pressure. As water drains from a sealed pipeline, a partial vacuum is created that can collapse or severely damage the pipe. Air inlet valves operate either by a float (water level) control or by opening at a set pressure difference like a pressure relief valve.

High points in a pipeline shall always be designed with an air venting system to avoid extreme positive or negative internal pressures as compared to atmospheric pressure.

(Res. 40-08 (§ 905.3), 3-19-08)