Design For Fish Passage at Roadway - Stream Crossings: Synthesis Report

7 Design Methods

How to use this chapter

• See specific design procedures in each Design Category as currently used by State DOTs and State and Federal natural resource agencies
• Learn what data are needed for each of the selected methods
• Compare and contrast methods and procedures
• Learn about auxiliary components that can improve design: fishways, floodplain relief culverts and two-cell installations
• Learn about the use of tide gates in coastal areas for fish passage

The design methods summarized in this chapter represent the spectrum of techniques that are currently available to meet fish passage. Variability is in part due to the conditions under which criteria were developed, and in part due to specific agency policies and regulatory thresholds. It is not clear from the source materials which guidelines are based on physically tested field conditions or different mandates, conservatisms and local practice. More research is required to provide a better basis for the methods described in this chapter (see Chapter 11, Future Research Needs). Applicability may be limited to specific geomorphic and hydraulic conditions. Careful attention should be paid to applicability and limitations, and engineering judgment is required. It is recommended that the featured agencies be contacted directly with questions as these methods evolve.

Equations provided are based on the recommendations of design manuals for States or local areas. Designers should be familiar with the source, derivation, and limitations of these equations before using them. A review of method applicability was not conducted as part of the development of HEC-26, and engineering judgment must be used when applying state-of-practice technologies, remembering the importance of monitoring in the future refinement of these methods.

All of these methods require careful attention to the project alignment and profile. To ensure that the project layout is properly aligned with the eventual channel profile, a two-dimensional plan view, connecting the upstream and downstream channels, must be combined with a streambed profile, connecting vertically stable points upstream and downstream of the crossing. This will provide insight into channel degradation and eventual channel elevation.

7.1 Geomorphic Simulation (Category 1)

As defined in Chapter 6, Geomorphic Simulation approaches are based on recreating or maintaining existing channel geometry for approximately bankfull conditions. These design techniques attempt to mimic (or maintain) natural stream reach characteristics including slope, channel-bed width, bedform, and bed materials. The basis of these methods is the presumption that crossings matching natural conditions will readily pass fish that are moving in the natural channel.

The three examples of Geomorphic Simulation included in Section 7.1 represent the spectrum of design techniques available. The USFS takes a stream reference reach approach, while Washington State utilizes a specific set of general culvert-span criteria. Both methods are very similar and describe different procedures to achieve the same objective. Massachusetts (River and Stream Continuity Partnership 2006) has recently established similar procedures that have proven helpful in obtaining general permits for fish and aquatic-organism passage culverts. Washington's No Slope and NMFS's Active Channel technique, combined due to their similarity, provide simple and conservative design approaches that are applicable in very limited situations. A summary of design approaches is included in Table 7.2 at the conclusion of Section 7.1.

Although maintaining stream continuity through the structure is the goal, these techniques are subject to the constraints of existing channel conditions, including slope, available bed material and others identified in Section 6.2. The USFS and WDFW criteria for stream simulation provide equations that allow for adjustment of bed mobility and stability. If substantially larger substrate is required, the design becomes a Hydraulic Simulation approach (Section 7.2).

7.1.1 U.S.F.S. Stream Simulation - DRAFT Manual

Source

• Bates et al 2006

Applicability

• New and replacement installations
• Passage required for all fish and aquatic organisms

Limitations

• Slope of crossing resembles slope of natural channel or representative reach
• Limited examples for cohesive soils

The United States Forest Service recently produced a draft manual of their "Stream Simulation" design technique. This methodology utilizes a reference reach approach to understand bed material, channel morphology and structures found within the natural channel. A crossing structure is then designed to match reference reach characteristics. This ideally creates a crossing that is self-sustaining and free to adjust similarly to the natural channel.

This approach is simplest for new installations, where open bottom structures can be placed to span the stream channel, leaving natural bed material and bedforms in place. In replacement installations, past channel degradation may require a culvert to be steeper than the natural channel.

Although the following discussion summarizes design procedures, adequate understanding of channel processes and site characteristics is necessary to complete a viable fish passage culvert. The draft manual is quite comprehensive, but appropriate designs will require a skilled group of design professionals with breadth of knowledge covering engineering, hydrology, biology, and geomorphology. For further details refer to Bates et al 2006. Note - many criteria, such as slope, width and applicability are largely left to the discretion of design professionals who work as a team to find the appropriate combination of variables to meet project objectives.

7.1.1.1 Biological Characteristics

This design allows for passage of all fish and aquatic organisms.

7.1.1.2 Geomorphic Characteristics

The slope of the crossing resembles the slope of the natural channel or reference reach, ideally creating a crossing that is self-sustaining and free to adjust to the natural channel.

In new installations, where open bottom structures can be placed to span the stream channel, natural bed material and bedforms will be left in place, although significant disturbance may occur during construction. Replacement installations may require a culvert to be steeper than the natural channel due to past channel degradation.

Ideally natural bed material will be used. If not, a well-graded mix of materials should be created to closely approximate the natural streambed, especially with respect to mobility and particle size distribution. Angular rock may be necessary to simulate large wood structures of the reference reach. A basic V-shaped low flow channel should be constructed within the culvert barrel to provide a continuous channel thalweg. The culvert should accommodate anticipated widening or narrowing of incising channels.

7.1.1.3 Hydraulic Characteristics

Analysis of flows for aquatic organism passage is not required. Hydraulic capacity must be checked to meet required headwater-flood policy for the responsible agency.

7.1.1.4 Data Requirements

Channel Type:

Table 7.1 depicts a number of design recommendations based on channel type. Channel types are based on Montgomery and Buffington, described in Section 6.5.2.1 (1997).

Bed Material:

When natural bed material cannot be used, a well-graded mix of materials should be created to closely approximate the particle size distribution of the reference reach. The most important elements of a constructed bed are large particles to provide bed structure, and fines to limit bed permeability and bind the bed mix together. Analysis of bed material can be done through a sieve analysis, but is most commonly done through a pebble count. When distribution is calculated by a pebble count, D₁₀₀, D₈₄, D₅₀ of the reference reach are taken directly from the surface pebble count, and smaller grain sizes are determined through use of the Fuller-Thompson equation (Section 6.5.5.1) or some other method.

Channel Width:

Considerations of channel width will affect the culvert sizing and material selection. Channel width should consider channel entrenchment, key features and incision. In general, it is recommended that channel width be greater than or equal to:

1. Bankfull width of the reference reach, or
2. Four times the diameter of the largest particle in the simulated bed.

In situations where the channel is incising, culverts should be designed to accommodate anticipated widening or narrowing.

Bed Structure:

At a minimum, a basic V-shaped low flow channel should be constructed within the culvert barrel (Figure 7.1), providing a continuous channel thalweg until the channel is reshaped by higher flows. Temporary bed structures can also be used in low gradient channels to provide channel form until natural processes can shape the channel. Recommended structures include rock bands and clusters (to replicate the shape of dune-ripple and pool-riffle channels), marginal features to simulate the reference reach banklines and edge diversity, and key features to simulate specific structural features in the reference channel. Specific design of these features is included in the Stream Simulation Manual (Bates et al. 2006).

Figure 7.1 Low flow channel in an open bottom structure

Culvert Span:

Culvert span is determined through a combination of bankfull width calculations and provisions for banklines and overbank surfaces. This should also incorporate channel width considerations. If banklines are desirable, an initial estimate of culvert width could be bankfull width plus 2 to 4 times the diameter of the largest mobile particle in the bed is suggested. A minimum barrel width of 1524-1830 mm (5-6 ft) is recommended to allow placement of sediment within the barrel.

Bed Mobility and Stability:

Checks can be made to ensure that bed material is mobile when channel material is mobile, and that banklines and key features remain stable at high design flows. Typically, analysis is conducted on the particle size that provides structure (D₈₄). Bathurst's unit discharge equation, the modified Shield's equation and U.S. Army Corps of Engineers riprap-sizing equation (see Section 6.5.5.5) are recommended for this analysis. Designers should have a thorough understanding of the source, derivations, and limitations of these equations before use. Further discussion of these methods is included in the Stream Simulation Manual Appendix E (Bates et al. 2006).

7.1.1.5 Design Procedure

1. Perform site assessment to determine the reference reach.
2. Determine project alignment and profile (see Introduction to Chapter 7).
3. Design bed material and arrangement.
4. Select structure size and elevation.
5. Verify mobility / stability of simulated streambed.

7.1.2 WDFW Stream Simulation

Source

• Bates et al 2003

Applicability

• New and replacement installations
• Passage allowed for all species

Limitations

• Culvert slope does not exceed 125% of channel slope

In new installations, it is desirable to use open bottom structures placed at stream grade to allow natural bed material and form to remain undisturbed if possible. In replacement installations, culvert slope should be within 125% of the upstream channel slope. In the case that natural bed material must be disturbed during construction, Washington's manual considers two design scenarios - outlined in Section 7.1.2.4.

7.1.2.1 Biological Characteristics

This design allows for passage of all aquatic organisms.

7.1.2.2 Geomorphic Characteristics

In new installations, the use of open bottom structures placed at stream grade allows natural bed material and form to remain undisturbed, if channel widths exceed about 5 m (16.4 ft). In replacement installations, the culvert slope should be within 125% of the upstream channel slope. One design scenario applies to streams of grades lower than 4%. Natural bed material is interspersed with bands of coarse material to control initial grade and cross-sectional shape, providing a low flow channel for fish passage and addressing slow channel formation in low-gradient streams with much fine sediment. The channel thalweg forms toward the culvert center, and in wide lower-gradient culverts, the low flow channel should meander.

In the second scenario, for slopes greater than 4%, coarser sediment found in streams is assumed adequate to control bed stability and create paths for fish passage.

When culvert bed slope matches natural channel slope, sediment supplied to the structure will rebuild the culvert bed after extreme flooding. When the slope ratio approaches upper limits, coarse bed materials will not be recruited and finer materials lost over time. Bed stability must be addressed. If greatly oversized bed material is needed, the culvert will not resemble the natural channel.

7.1.2.3 Hydraulic Characteristics

This design avoids the need for analysis of flows for aquatic organism passage. The culvert must be checked for adequate flood capacity. If channel slope is greater than 4%, the 100-yr flood is used as design flow in determining bed stability.

7.1.2.4 Data Requirements

Site Suitability:

• The ratio of culvert bed slope to channel slope (slope ratio) must be less than 1.25. Channel slope is generally taken as the upstream channel slope, but downstream slope can be used if it is representative of channel slope.
• The culvert itself should be placed as flat as possible to reduce shear stress between the culvert bottom and the bed material. Long installations will likely require the culvert to be placed with slope.
• Channel susceptibility to vertical changes should be assessed, and taken into account with culvert size and countersink elevation. Larger culverts will be required if material is likely to aggrade, and a lower countersink will be required in situations where channel degradation could undermine culvert stability.

Adjacent Stream Reach:

For most new and replacement installations, a representative reach will be used to determine the proper bed sizing and culvert span. This reach is typically found upstream, with considerations of slope ratio mentioned above (S_culvert/S_channel ≤ 1.25).

Two design scenarios are considered for these structures. The first scenario is applicable in low-gradient alluvial channels matching pool-riffle channel forms, or exhibiting the characteristics of Rosgen C, E or F-type channels. A second scenario applies in higher gradient streams with step-pool or cascade-type channels that are likely to be more stable - corresponding to Rosgen's stream classifications of A, B, F or G. In Washington, a somewhat arbitrary 4% threshold is used to divide these two methods.

Culvert Type and Size:

Minimum bed width in any culvert should be determined by:

W_{culvert bed}= 1.2W_ch + K

where:

K = 0.6 m (2 ft)
W_ch = the width of the bankfull channel, m (ft)

Equation 9.1 applies to confined and moderately confined coarse-bedded channels. Future channel widening (of an incised channel) should be taken into account. A full discussion of reasoning for these width criteria is included in the WDFW manual, and should be addressed before deviating from Equation 9.1.

Culvert Bed Configuration:

The decision to use a particular slope scenario (Figure 7.2 and 7.3) is based on channel assessment. Channel-bed composition should be described by a sample of the bed material or by a surface pebble count. In situations where large wood or roots dominate the reach, a representative reference reach (exhibiting similar slope and width) should be used as a design template.

The first design scenario, depicted in Figure 7.2, is utilized when slopes are less than 4% in the natural reach. Natural bed material is interspersed with bands of coarse material (1 to 2 times D₁₀₀) to control initial grade and cross-sectional shape. This provides a low flow channel desirable for fish passage, and addresses the likelihood of excessively slow channel formation in low-gradient streams with a large proportion of fine sediments. This also ensures that the channel thalweg forms towards the culvert center, reducing the probability of channel formation along culvert boundaries. In wider, low-gradient culverts, the low flow channel should meander to match natural conditions.

Figure 7.2 Washington Department of Fish and Wildlife Stream Simulation approach for low slope situations, where bed slope < 4.0% (Bates et al. 2003)

(Structure is filled with native streambed material and bands of well-graded rock to control initial shape of the culvert bed)

Spacing and Sizing of Rock Bands:

The distance between rock bands should be the lesser of five times channel width or the distance necessary to provide a drop between bands of less than or equal to 0.24 m (0.8 ft). The first and last rock bands in the structure should be distanced from the culvert inlet and outlet by more than 2 channel widths or 7.62 m (25 ft), whichever is less.

For slopes greater than 4%, native or engineered bed material is used without bed-control structures. Coarser sediment found in streams is assumed adequate to control bed stability and create paths for fish passage. Figure 7.3 depicts this culvert configuration.

Figure 7.3 Washington Department of Fish and Wildlife High-Slope Stream Simulation Approach (Bates et al. 2003)

(Span and embedment criteria remain the same, but bed material consists of native sediment mix)

Culvert-Bed Design:

Bed mix requirements vary with slope considerations:

• When culvert bed slope matches natural channel slope, sediment supplied to the structure will allow the bed in the culvert to rebuild after large flood events. Appropriately sized culverts will have bed material matching that found in the natural channel.
• When the slope ratio approaches the limits of Geomorphic Simulation (1.25), coarse bed material required to maintain the slope will not be recruited, and finer materials will be lost over time. In this situation, a number of approaches aid in bed stability design: Reference Reach Approach, Unit-Discharge Bed Design and Paleohydraulic Analysis (see sections 6.5.5.6, 6.5.5.7 and 6.5.5.8, respectively). The method producing the coarsest D₈₄ should be used. When stability requires bed material to be greatly oversized, it will no longer look or respond like the natural channel, and the resulting design may be more appropriately classified as Hydraulic Simulation.

Bed Material:

Once the largest material (D₈₄) has been sized, the rest of the bed mixture should be well graded to minimize permeability. If material is imported, a synthetic streambed mix should be used. Relations for gradation are given as a starting point, and may be refined according to the availability of materials. Typical relations for gradation include:

D₈₄/D₁₀₀ = 0.4
D₈₄/D₅₀ = 2.5
D₈₄/D₁₆ = 8.0

Note - When ratios indicate impractical sizing, the adjacent channel should be looked at for guidance. For example, a D₈₄of 1.8 ft requires a D₁₀₀ of 4.5 ft that is likely not represented in the natural channel (Bates et al. 2003).

Gradations are not overly restrictive so as to be practical and economical.

Bed material comprised entirely of fractured rock is inappropriate for stream simulation, as jagged edges will interlock and dissuade appropriate migration of channel bed material.

Sediment finer than fine sand should account for 5-10% of the mix to prevent low flows from traveling through coarse voids.

Bed Retention Sills:

Although WDFW does not consider this a desirable option, the application of bed retention sills can be considered (as a last resort) to hold bed material within the culvert when slopes approach 1.25 times the reach slope. These sills can be steel or concrete placed at the bottom of the culvert to hold bed material within the barrel.

If desired, the crest of bed-retention sills should be V-shaped with a 10:1 slope laterally. These are placed at 20% of the culvert diameter below the streambed as constructed in the culvert. The maximum drop between sills should not exceed 0.24 m (0.8 ft), ensuring that each sill backwaters the next in the case that the bed material is scoured out.

7.1.2.5 Design Procedure

Washington State has developed a preliminary design process for stream simulation design based on local experience. Because of the relatively small amount of field experience, after consulting WDFW personnel for suggested updates, this procedure should be applied conservatively.

7.1.3 No-Slope and Active Channel Design

Source

• Bates et al 2003
• National Marine Fisheries Service Southwest Region 2001

Limitations

• Stream reach slope <3%
• Culvert length <30. 5 m (100 ft), or product of slope times length < 0.2D subject to requirements in "Culvert Length" section below
• Embedment requirements can be met

Applicability

• New and replacement installations
• Passage required for all species
• Low risk crossings

The No-Slope design specifies a culvert that is installed flat, and sized sufficiently large to allow natural movement of bed material and the formation of a stable bed within the barrel. This method avoids the need for detailed survey information or fish passage hydrology.

Aside from span requirements, NMFS's Active Channel design is almost identical to WDFW's No-Slope design. In Washington's guidelines, the culvert span must exceed 1.25 times channel bankfull width, while NMFS recommends 1.5 times active channel width. California guidelines suggest that the active channel is generally less than bankfull width (Taylor and Love 2003). Entrenched streams in Washington may show little variation between active channel and bankfull widths (Bates et al. 2003). Discrepancies in regional manifestation of bankfull and active channel indicators likely lead to a similarly sized structure, although it would be conservative to take the larger of bankfull and active channel width.

7.1.3.1 Biological Characteristics

This design allows passage for all aquatic organisms.

7.1.3.2 Geomorphic Characteristics

The No-Slope and Active Channel designs are appropriate for streams with less than 3% slope. Culvert size is sufficient to allow the natural movement of bed material and the formation of a stable bed within the barrel.

7.1.3.3 Hydraulic Characteristics

This design is intended for use where detailed survey information on high and low fish passage flow is not available. Hydraulic capacity must meet the required headwater-flood policy.

7.1.3.4 Data Requirements

Channel Slope:

Natural stream channel slope should not exceed 3%. For all installations, future channel elevation and slope should be predicted using unaffected stream reaches both up and downstream. This should be projected for the project lifetime.

Culvert Span:

Structure span is 1.25 times channel-bed width^† (minimum 1830 mm (6 ft)). WDFW recommends that this be taken as the average of at least 3 typical cross sections Bates et al. 2003). Pipe, pipe-arch, and elliptical culverts are applicable for this design. Round culverts have the advantage of providing additional vertical clearance for a given span.

^†Washington uses bankfull width as a design standard.

Embedment:

The bottom of the culvert is buried no less than 20% and no greater than 40% of the culvert height. If bottomless structures are used, footings are designed for the largest anticipated scour depth, and the culvert should be placed so as to minimize the disturbance of the natural bed.

Culvert Length:

Due to embedment requirements, the product of slope times length must be less than or equal to 20% of the culvert diameter. In general, installations should not exceed 30.5 m (100 ft) in length.

Culvert Slope:

Culvert is laid flat within the stream reach.

Upstream Headcut Potential and Impacts:

Evaluation of upstream headcut potential and impacts should be completed. This is necessary because if the substrate is washed out of the barrel as a result of degradation from downstream, the headcut can continue upstream past the culvert and endanger the crossing.

7.1.3.5 Design Procedure

No detailed design procedure is provided by the guidelines, but this method is intended for simple design situations, avoiding detailed survey information or high and low fish passage flow data. The No-Slope design option is depicted in Figure 7.4.

Figure 7.4 WDFW No-Slope option (Bates et al. 2003)

7.1.4 Summary of Geomorphic Simulation Procedures

Table 7.2 provides a summary of Geomorphic Simulation techniques.

7.2 Hydraulic Simulation (Category 2)

As defined in Chapter 6, Hydraulic Simulation techniques utilize embedded structures, natural or synthetic bed mixes, and roughness elements to create hydraulic conditions conducive to fish passage. Structure design is optimized to provide and sustain existing substrate. These techniques represent the middle-ground between Geomorphic Simulation, which closely matches natural channel geomorphology, and Hydraulic Design, which sizes a culvert for specific fish species. Table 7.5 (end of Section 7.2) provides a summary of Hydraulic Simulation techniques.

More research is required to better substantiate the methods used to determine streambed stability (Section 6.5.5). Users are encouraged to use best engineering judgment when employing Hydraulic Simulation procedures.

7.2.1 Oregon Department of Fish and Wildlife Stream Simulation

Source

• Oregon Department of Fish and Wildlife (Robison et al. 1999)

Limitations

• Moderate gradients: 1.5-8%
• Stream width ≤ 4.6 m (15 ft)
• Valley fill must be adequate to allow adequate countersinking

Applicability

• New and replacement installation
• Fish passage required for all species

7.2.1.1 Biological Characteristics

This design reportedly allows for passage of all fish species, even with a channel velocity and turbulence regime (see 9.2.1.3).

7.2.1.2 Geomorphic Characteristics

This design is appropriate for streams with channel slopes from 1.5-8% and stream widths of less than about 4.6 m (15 ft). Natural and oversized bed material is used to create hydraulic roughness, low flow paths and resting areas conducive to fish passage. Sediment and debris flow is allowed to continue through the crossing at flows up to bankfull.

Channel impacts should be considered, hydraulic controls may be required to improve structure entrance and exit conditions (beveled inlet configuration; providing resting pools at entrance and exit, etc); concentrate low flows; prevent erosion of the streambed and banks; and allow passage of bedload material (this provision is designed with ODFW consultation).

A recommended bed mix consists of 30% fines (to seal voids to avoid sub‑surface flow), 30% small rock, 30% large rock and 10% shadow rock (to simulate undercut banks, large wood and boulders, and to remain stable during flood events).

7.2.1.3 Hydraulic Characteristics

In this design, the culvert span should match the channel bed width (defined as the active channel width).

The use of increased bed sizing or roughness will decrease flow velocity but increase turbulence.

Hydraulic capacity must be checked to ensure adequate culvert size. Culvert capacity must pass the 50-yr flood.

Channel velocities must be checked to ensure compliance with local stream velocities.

7.2.1.4 Data Requirements

Watershed Information:

Channel slopes must be between 1.5-8%. Bridges are suggested if stream width is greater than about 4.6 m (15 ft). Valley fill should be adequate for sinking the culvert into the streambed. The barrel should be sunk more than 20% of the culvert rise, or 0.46 m (18 in), for pipe arches and box culverts, and a minimum of 40% of the diameter, or 0.6 m (2 ft), for round culverts.

Culvert Size:

The culvert span should match that of the channel bed width (defined as active channel width). Multiple width measurements should be made above and below the culvert, as well as areas outside the influence of the culvert installation. This width should represent stream reach conditions prior to the impacts of the existing structure. Table 7.3, in customary units, aids in the selection of appropriate structure so that the span or diameter matches that of the stream channel. Data from the columns labeled, "MAX FLOW in Culvert," is not to be used in fish passage designs. It was computed assuming a thin edge projecting pipe under inlet control with HW/D equal to 1.0. For pipe-arch culverts not covered in Table 7.3, approximations of culvert area can be found using Equation 7.2, in customary units.

Area (ft²) = Rise (inches) * Span (inches) * 0.005472

* Data from the columns labeled, "MAX FLOW in Culvert," is not to be used in fish passage designs. It was computed assuming a thin edge projecting pipe under inlet control with HW/D equal to 1.0.

^† If culvert embedment is considered, oversizing is completed as detailed below.

Countersink:

Appropriate countersink depth should be created according to the following criteria, assuming a minimum span of 1524-1830 mm (5-6 ft):

1. Circular culverts: 0.4 times diameter or 600 mm (2 ft), whichever is greater
2. Pipe-arch culverts: 0.2 times rise or 460 mm (18 in), whichever is greater
3. culverts: 0.2 times width, or 460 mm (18 in), whichever is greater

For channel slopes 0-4%: The outlet and inlet inverts are sunk at the same depth.

For channel slopes 4-8%: Use circular and pipe-arches only. Countersink the outlet according to the above criteria, (a) and (b). Determine the outlet invert elevation relative to some datum, and determine the depth to countersink the inlet using Equation 7.3.

Elevation inlet invert = (culvert length)*[(channel slope-1.5%)/100] + elevation outlet invert.
Note - use the inlet countersunk values in calculating the effective cross-sectional area.

Effective Cross-Sectional Area (ECSA):

Calculate "effective cross-sectional area" and the flow capacity of the culvert using Equation 7.4 and Table 7.4.

ECSA = (Culvert cross-sectional area for chosen culvert)*(% loss in cross-sectional area/100)

Flow capacity is determined by comparing the cross-sectional area to the corresponding maximum flow in the culvert on Table 7.3. It may be necessary to interpolate to find cross-sectional areas for odd-sized culverts.

Culvert Capacity:

Culvert capacity must also be checked to ensure that it passes the 50-yr flood in order meet Oregon Department of Transportation standards for culverts.

Bed Material Specification:

A bed mix is recommended based on local experience. Shadow rocks, 50-100% larger than the natural D₁₀₀, are placed to protrude 30-50% above the final streambed elevation. Large rocks, small rocks and fines should be mixed before placing, and the final surface should be washed into interstitial spaces to ensure a good seal.

7.2.2 Alaska DF&G and DOT Stream Simulation

Source

• Alaska Department of Fish and Game and Alaska Department of Transportation 2001

Limitations

• Natural channel slope ≤ 6%
• Culvert slope is within 1% of the natural channel slope (natural slope +/- 1%)
• Stable channels

Applicability

• New and replacement installations
• Passage required for all fish species present

When the following criteria have been met, fish passage is assumed to be adequate without further hydraulic calculations. This design methodology has worked well in Alaska, and fish have been observed successfully passing structures that have been in place (Miles, Personal Communication). A memorandum of agreement between ADOT and ADF&G ensures that permitting goes quickly, and structures are designed to be smaller than Geomorphic Simulation, resulting in smaller initial cost.

7.2.2.1 Biological Characteristics

This design allows for the passage of all present fish species.

7.2.2.2 Geomorphic Characteristics

This design uses oversized substrate, sized to be stable up to and including the 50-yr flood, to create hydraulic roughness, low flow paths and resting areas conducive to fish passage by matching the bed characteristics of the natural channel. This creates a stable channel within the culvert, where bed load and suspended load still move through, but bed material is not scoured out at 50-yr flood.

The design applies to streams with gradient less than or equal to 6%. The culvert slope will be within 1% of the natural channel slope.

7.2.2.3 Hydraulic Characteristics

Hydraulic capacity of the culvert must be checked to ensure adequate size.

When the design procedure is followed, analysis of fish passage flow levels is not required.

Culvert span is greater than or equal to 90% of the channel width at Ordinary High Water stage, unless the channel slope is less than 1%, and the culvert is installed with a slope of less than 0.5%, in which case the span may be greater than or equal to 75% of the channel width at Ordinary High Water stage.

7.2.2.4 Data Requirements

Stream Gradient:

Stream gradient is less than or equal to 6%.

Culvert Span:

Culvert span is greater than or equal to 90% of the channel width at Ordinary High Water stage.

Where the channel slope is less than 1%, culverts may be installed at slopes less than 0.5%, with a span of at least 75% of the channel width at Ordinary High Water stage.

Culvert Slope:

Culvert slope is within 1% of the natural channel slope (i.e., 4% channel slope, 3‑5% culvert slope).

Bed Material:

Bed material is sized to be stable up to and including the 50-yr flood (possibly requiring sediment retention baffles).

Embedment:

Circular culverts should be buried at least 40% of the culvert diameter, while pipe arches must be buried 20% of the culvert rise.

7.2.2.5 Design Procedure

Although no specific design procedure is provided, fish passage is assumed when the above data requirements are met.

7.2.3 Browning et al. 1990

Limitations

• Slope ≤2-5% (see discussion below)
• Stable Stream Systems

Applicability

• New and Replacement
• Passage required for all species

A 1990 survey of culverts in Oregon had the primary goal of determining which type of culvert provided the best fish passage, and if current design practices would have produced that type of culvert. It was also hoped that results would resolve current disagreements surrounding fish passage requirements. This study included collection of field data and hydrologic and hydraulic analysis of each of the selected sites. A comparison was made between culvert velocities and velocities present in the natural channel during the 2-yr and 50-yr flood events.

Adult salmon passage was a main concern at many sites, although trout were included as important species in many cases (Browning 1990). Study sites were largely located in stable stream systems that had reached dynamic equilibrium (Browning, Personal Communication). Based on the results of this survey, Browning recommends a design procedure that utilizes Hydraulic Simulation to create a fish passable structure.

This method is unique in that it does not require determination of channel bed width. Channel bed width is difficult to measure consistently, and boundary roughness in slightly constricted culvert installations may actually increase flow depth and slow velocities during fish movement (Browning, Personal Communication).

7.2.3.1 Biological Characteristics

This design allows for passage of all species, though it was derived from sites where passage of adult salmon was primarily of concern. Trout were also considered important in many cases.

7.2.3.2 Geomorphic Characteristics

This design uses natural bed material to create hydraulic roughness, low flow paths and resting areas conducive to fish passage. Cohesive soils should be replaced with fine gravels in the likelihood that cohesion will be disrupted when installation occurs. To keep flows from going subsurface, placement of a non‑permeable barrier between the culvert bed materials and foundation materials can be considered. Small boulders can be included in the bed mix to increase roughness and reduce downstream scour, where the installer cannot match velocity and scour conditions. If structures for reducing scour are improperly placed, they could pose a barrier to fish passage (see Section 6.5.6.1 on riprap).

Study sites were largely located in stable stream systems that had reached dynamic equilibrium. Recommendations were based on installations on grades of 1-2%, with very few sites exceeding 3%.

Where system wide degradation is possible, the installation may require lowering to match the anticipated stream surface lowering.

7.2.3.3 Hydraulic Characteristics

Culvert span is determined by keeping the headwater to depth ratio at the 50-yr flood less than or equal to 1.0.

Average barrel velocity remains within 25% of the natural stream velocity during discharges less than the 2-yr flood. When stream gauge data is not available, barrel velocity calculations should be done for a number of flows, ideally covering the range at which fish are moving, including analysis of depth and velocity in the culvert and natural channel at each discharge.

Outlet scour depth must be less than 150 mm (0.5 ft) during the 2-yr flood event. It is recommended that outlet scour potential be computed at each of the discharges used for velocity analyses.

7.2.3.4 Data Requirements

Culvert Span:

Headwater to depth ratio at the 50-yr flood should not exceed 1.0. This is intended to ensure that the culvert does not excessively constrict the stream reach.

Bed Slope:

Although no specific limitations are given for slope applicability, recommendations were based on installations on grades of 1-2%, with limited sites approaching 5%.

Embedment:

Culverts less than 3.2 m (10 ft) diameter are buried a minimum of 150-300 mm (0.5-1.0 ft) below the natural stream slope. Culverts with diameters greater than 3.2 m (10 ft) are buried a minimum of 1/5^th the culvert rise. In situations where system wide degradation is possible, the installation may require lowering to match the anticipated stream surface lowering.

Barrel Velocity and Depth:

Barrel velocity remains within 25% of the natural stream velocity during discharges less than the 2-yr flood.

When stream gauges were not available at sites, U.S.G.S. regression equations were used to determine 2- and 50-yr flows for hydraulic analysis. Manning's equation was used to compute velocities in a typical section of the stream and compared to culvert cross section. Stream channels were approximated by using topographic data of the stream site to create a representative trapezoidal cross section. Slope was based on typical slopes in the vicinity of the culvert and a roughness value (n) is based on local streambed materials. For the study, Manning's "n" values were taken from:

Chow, V. T., 1959. "Open-Channel Hydraulics." McGraw-Hill Book Company, Inc, New York, NY.

Barrel velocity calculations should be done for a number of flows, ideally covering the range of flows at which fish are moving. This includes analysis of depth and velocity in the culvert and natural channel at each discharge.

Outlet Scour:

Outlet scour should be limited to 150 mm (0.5 ft) during the 2-yr event. Analysis was conducted based on the method in "Hydraulic Engineering Circular No. 14" (Thompson and Kilgore 2006), with specific methods depending on the bed-material present. It is recommended that outlet scour potential be computed at each of the discharges used for velocity analyses.

If it is determined that outlet scour is likely to be a problem, boulders can be placed just downstream of the culvert outlet to reduce stream energy and potential scour depth.

Bed Material:

Bed material should be similar to the natural stream reach placed to match stream reach conditions. At the time of writing (1990), Browning said cohesive soils should be replaced with fine gravels since cohesion will likely be disrupted during installation. He also said to keep flows from going subsurface, placement of a non-permeable barrier between the culvert bed materials and foundation materials can be considered. More recent procedures recommend washing fines and silts into the streambed to seal voids instead of using a barrier blanket (Bates et al. 2003).

Culvert Slope:

The culvert barrel should be placed on as flat a slope as possible (in general less than 2%). Culverts placed on a slope greater than 2% may require consideration of bed retention baffles.

Roughness:

In situations where the installer cannot match velocity and scour conditions, small boulders can be included in the bed mix to increase roughness, and reduce downstream scour. These should be embedded, and not protrude more than 0.30 m (12 in).

Culvert Capacity:

Culvert headwater-to-rise ratio is not to exceed 1.0 (i.e. during 50-yr event).

7.2.4 Maryland

Source

• Maryland 2005

Maryland culvert design incorporates the use of a main-channel culvert to maintain stream characteristics during bankfull flow, with floodplain culverts to handle overbank flows when practicable. Rather than creating "standard" design methods, Maryland addresses considerations surrounding the culvert design process.

7.2.4.1 Biological Characteristics

This design allows for passage of all species.

7.2.4.2 Geomorphic Characteristics

The main-channel culvert span matches stable bankfull flow of the upstream approach channel in order to maintain natural stream characteristics, allowing sediment and debris flow to continue through the crossing.

The main-channel culvert outlet is designed to minimize impacts to the downstream channel and stabilize flow conditions for fish passage. Modifications may be considered, such as baffles or downstream grade control structures, such as cross vanes or w-weirs designed to match the stable bankfull geometry.

The upstream transition section should be designed to achieve continuity of flow and maintain sediment transport characteristics of velocity and shear, avoiding deposition and scouring. Cross vanes and w-weirs may be necessary upstream.

7.2.4.3 Hydraulic Characteristics

Bankfull flows are determined based on field investigation and stream morphology surveys. For a stable riffle within the reach, the bankfull flow is computed either by using a Manning's equation (Equation 6.1) or by setting up a gradually varied flow model. Bankfull geometry is measured and then verified using a sediment mobility analysis based on the largest mobile particle size and the critical boundary stress for that particle.

Hydraulic capacity must be checked to ensure adequacy.

7.2.4.4 Data Requirements

Main-Channel Culvert Size:

A main-channel culvert should accommodate bankfull flow with minimum change in the hydraulic characteristics of unit discharge, width, depth and velocity. When applicable, bankfull flow should be accommodated in a single pipe, up to 4.9 m span (16 ft), or a single box culvert cell, up to 6.1 m (20 ft).

Sizing should be done by a trial and error solution using HEC-RAS and HY-8 to aid in the iterative design process. HY-8 is used to select efficient culvert sizes, with downstream tailwater elevations taken from the water surface (HEC-RAS) hydraulic model. Results of culvert selections should be reviewed to ensure that they are reasonable.

Two-Cell Installations:

When two culverts are required, box culverts are suggested to minimize the distance between spans. W-weirs may be included upstream of a multiple cell installation to reduce bar deposition and scour, increase competence of bed material transport and reduce debris accumulation at the center wall.

Embedment:

Culverts should be depressed a minimum of 20% below the existing channel bed, and allowed to fill naturally with bed material. In two culvert installations, the stream is expected to form a natural thalweg in one of the cells to accommodate low flows - minimizing fish passage problems.

Slope, Type, Roughness, and Dimensions:

Determine a composite "n" value based on bankfull flow, streambed materials, and culvert material above the streambed. Use HEC-RAS to run water surface profiles while attempting to match continuity of bankfull flow widths, depths, and velocities through the culvert. Plot bankfull depths in channel and adjust culvert invert elevations to maintain selected depression.

Main-Channel Culvert Outlet:

The main-channel culvert outlet should be designed to minimize impacts to the downstream channel and stabilize flow conditions for fish passage. When bankfull flow velocities are significantly higher in the culvert than in the channel, or the channel bed may be swept out, modifications such as baffles or downstream grade control structures may be considered.

Upstream Transition:

The upstream transition section should be designed to achieve continuity of flow and maintain sediment transport characteristics of velocity and shear - avoiding deposition and scouring.

This transition is likely less important for stable stream systems such Rosgen type B, C, and E, but may be very important for A, D, DA, F or G channels. Special considerations are covered in more detail in Maryland's design manual.

Culvert Silting:

Maryland addresses culvert silting but design guidelines were not available for this document.

7.2.4.5 Design Procedure

Floodplain Culverts - Floodplain culverts can be added in situations where a single culvert would overly constrict flow for discharges exceeding bankfull, and lead to effects on downstream morphology. Floodplain culverts can be installed to collect and convey flood plain flows, reducing the impact of the main channel culvert. This may exclude situations where the culvert is on a small ephemeral stream, short culvert installations, locations where fish passage is not required, crossings on streams with small floodplains that convey little flow and crossings where a larger main channel crossing is desirable for debris passage.

Floodplain culverts should be positioned on the floodplain, well beyond the influence of the main culvert. This will avoid channel undermining, degradation or migration into the area of the floodplain culvert. It will also avoid clogging due to debris carried in the main channel.

7.2.5 Summary of Hydraulic Simulation Procedures

Table 7.5 provides a summary of Hydraulic Simulation techniques.

7.3 Hydraulic Design (Category 3)

Hydraulic Design creates water depths and velocities that meet the swimming abilities of target fish populations during period of fish movement. This design option is most often used in retrofit projects, but can be used in new or replacement projects if Geomorphic Simulation and Hydraulic Simulation are not appropriate. General considerations include the effect of culvert slope, size, material and length. Hydraulic Design can include adding baffles to a culvert, adding sediment or sediment catching devices inside the culvert, backwater through crossing by installing downstream weirs, or modification of the culvert inlet or inlet approach to remove a constriction (Robison et al. 1999). Figure 7.5, from Robison et al., depicts the general flow of hydraulically designed structures (1999).

Figure 7.5 Steps in Hydraulic Design (Robison et al. 1999)

Note -Weirs vs. Baffles

Weirs act as small dams which control water depth within a culvert, while still passing the necessary design flow. Multiple weirs can create a series of drops and pools, allowing fish passage through a steeper structure (Zrinji and Bender 1995). A series of baffles work together to increase the hydraulic roughness of a culvert, thereby reducing the cross-sectional velocity (Bates et al. 2003). Baffles provide flow diversity, adding both ineffective flow areas for fish to hide/rest in, and areas of increased flow velocity (and turbulence) in the constricted flow section.

7.3.1 General Hydraulic Design

Source

• The Washington Department of Fish and Wildlife (WDFW) provides a general design procedure for Hydraulic Design that will be described below (Bates et al. 2003). Additional weir/baffle configurations and culvert methodologies are included to expand upon the WDFW method.

Applicability

• New and replacement installations (when other options are precluded)
• Retrofit
• Fish passage required for target species

Limitations

• Requires knowledge of fish movement timing and swim speeds.
• May require additional monitoring due to propensity for roughness elements to catch debris.
• The addition of baffles will decrease culvert capacity (especially important in retrofit situations).

A generalized installation without baffles is shown in Figure 7.6.

Figure 7.6 Hydraulic Design option, customary units (Bates et al. 2003)

7.3.1.1 Biological Characteristics

This design allows for passage of target fish species.

7.3.1.2 Geomorphic Characteristics

Products of this design may affect flow through and around the structure, possibly leading to localized aggradation and degradation through channel constriction. Retrofitted roughness elements may have the propensity to catch debris, increasing the risk of clogging.

Backwater elevations of the downstream channel will be greater than or equal to the water surface of the culvert.

Upstream and downstream channel profiles may be adjusted in order to match culvert elevation.

7.3.1.3 Hydraulic Characteristics

Fish-passage flows must be determined in order to provide adequate hydraulics for passage, in accordance with knowledge of fish-passage timing and corresponding flow regimes of the natural channel.

Adequate flood-flow capacity must be verified. Roughness elements may catch debris, decreasing capacity.

7.3.1.4 Data Requirements

Length of Culvert:

Find the length of culvert based on geometry of the road fill.

Fish Passage Requirements:

Determine the target species, swimming capabilities and sizes of fish requiring passage. Use this to determine allowable barrel depth and velocity.

Hydrology:

Determine fish passage design flows at which the fish‑passage criteria must be satisfied.

Velocity and Depth:

Determine size, shape, roughness and slope of the culvert to satisfy velocity criteria, assuming open channel flow and a bare culvert bottom. Verify that the flow is subcritical through the range of fish passage flows.

Velocity and depth requirements can be met through a number of alternatives, including baffles or channel modifications, weirs, sediment catching devices, or roughened channels.

Acceptable velocity and depth are determined through appropriate selection of culvert size, material and slope. Many types of analyses are acceptable, but the simplest is Manning's equation (see Section 6.5.4) with a "n" value appropriate for baffles (if used). Such values may be estimated using techniques in HEC-14 (Thompson and Kilgore 2006).

Channel-Backwater Depth:

Determine backwater elevation at the culvert outlet for fish passage at both low and high fish passage design flows.

Culvert Elevation:

Set the culvert so that channel-backwater elevations are at least as high as the water surface in the culvert.

Channel Backwater:

The downstream culvert invert elevation at the outlet is determined by matching the water-surface profile at the culvert outlet to the backwater elevation of the downstream channel. Downstream water surface elevation can be determined by observation of the water surface at flows near fish passage design flow, or by calculating the water surface profile in a uniform flow condition. This may require several iterations, and modifications may be required to establish the culvert slope and roughness to match the profile to the downstream channel backwater.

Backwatering may also be accomplished by using structures to raise and steepen the channel to an appropriate elevation.

Calculated Backwater:

Channel backwater can be calculated using an open-channel flow calculation such as Manning's equation. WDFW recommends that this be calibrated with at least one high water-surface observation or high water mark (Bates et al. 2003). Selection of the appropriate Manning's n is very significant because it affects calculated water depths. The 'n' value depends on a number of variables including surface roughness, vegetation, channel irregularities, channel alignment, scour and deposition, obstructions, the size and shape of the channel, stage and discharge, suspended material, and bedload. Methods for combining these variables are included in

Chow, V. T. (1959). "Open-Channel Hydraulics." McGraw-Hill Book Company, Inc, New York, NY.

In situations where the project will affect the downstream channel, either as part of the design, or as the channel evolves after installation, the new channel slope, roughness and cross-sectional shape should be used for backwater calculations.

Flood Flow Capacity:

Verify that the flood-flow capacity of the culvert is adequate.

Channel Profile:

If necessary, adjust the upstream and/or downstream channel profiles to match the culvert elevation. Channel modifications (as discussed in Section 6.5.6) may be appropriate to control backwater elevation.

7.3.1.5 Design Procedure

1. Length of Culvert- Find the length of culvert based on geometry of the road fill.
2. Fish Passage Requirements - Determine the target species, sizes and swimming capabilities of fish requiring passage. Use this to determine allowable barrel depth and velocity.
3. Hydrology - Determine fish passage design flows at which the fish-passage criteria must be satisfied.
4. Velocity and Depth - Determine size, shape, roughness and slope of the culvert to satisfy velocity criteria, assuming open channel flow and a bare culvert bottom. Verify that the flow is subcritical through the range of fish passage flows.
   
   Velocity and depth requirements can be met through a number of alternatives including baffles or channel modifications, weirs, sediment catching devices, or roughened channels.
5. Channel-Backwater Depth - Determine backwater elevation at the culvert outlet for fish passage at both low and high fish passage design flows.
6. Culvert Elevation - Set the culvert so that channel backwater elevations are at least as high as the water surface in the culvert.
7. Flood Flow Capacity - Verify that the flood-flow capacity of the culvert is adequate.
8. Channel Profile - If necessary, adjust the upstream and/or downstream channel profiles to match the culvert elevation. Channel modifications (as discussed in Section 6.5.6) may be appropriate to control backwater elevation.

Several iterations of steps 4 through 8 may be required to achieve the optimum design.

Acceptable velocity and depth are determined through appropriate selection of culvert size, material and slope. Many types of analysis are acceptable, but the simplest is Manning's equation (see Section 6.5.4).

7.3.2 Baffle Configurations

Baffles are intended to create allowable velocities during fish passage flows, while not exceeding fish turbulence thresholds. Baffles divide the culvert into a series of cells and bays, creating resting areas between the baffles, and points of high velocity at the baffles (Ead et al. 2002). Fish are assumed to use their prolonged swimming speed along lower velocity areas and in between baffles, and use their burst speed to navigate around baffles (Rajaratnam et al. 1991).

Rajaratnam Et Al.

Some of the most comprehensive baffle information available comes from a number of studies completed at the University of Alberta at Edmonton, Canada. The hydraulics of six fishway baffle configurations were analyzed, resulting in a series of five papers completed by Rajaratnam et al (Rajaratnam et al. 1988; Rajaratnam et al. 1989; Rajaratnam and Katopodis 1990; Rajaratnam et al. 1990; Rajaratnam et al. 1991). Figure 7.7 depicts tested baffles including offset baffles, slotted weir baffles, weir baffles, spoiler baffles, Alberta fishweirs, and Alberta fishbaffles.

Tests were conducted on slopes from 0.5-5% covering baffle heights (h/D), where h is baffle height and D is culvert diameter, of 0.1-0.15, and baffle spacing up to 1.2 culvert diameters. Spacing wider than one culvert diameter was found to decrease velocity while increasing depth (Ead et al. 2002). Culvert material in the majority of these tests was smooth, with the exception of tests conducted on the Alberta fishweir and Alberta fishbaffle, when corrugated pipe was used.

From the baffle systems analyzed, weir and slotted weir baffles are recommended based on effectiveness and simplicity (Ead et al. 2002). Figure 7.8 shows the general layout of these two alternatives.

Figure 7.7 (a) Offset baffle; (b) slotted weir baffle; (c) weir baffle; (d) spoiler baffle; (e) Alberta fishweir; and (f) Alberta fishbaffle
(Ead et al. 2002)

Figure 7. 8 Culvert options (b) slotted weir and (c) weir baffle configurations (adapted from Ead et al. 2002)

(These are recommended for installation in fish passage situations due to simplicity and effectiveness)

Design techniques may be found in the Introduction to Fishway Design (Katopodis 1992).

WDFW Baffles

Washington Department of Fish and Wildlife has three recommended baffle configurations - two for circular culverts, and one for box culverts, see Figure 7.9 (Bates et al. 2003). In each case, drop between baffles should be less than 0.06 m (0.2 ft). Notches are aligned to allow an uninterrupted line of fish passage along one or both sides. The continuously sloped baffle configuration in box culverts is generally used for juvenile fish passage in culverts 1800 mm (6 ft) wide or less. Corner baffles are recommended for use on slopes between 1‑2.5%, with notched baffles being used between 2.5-3.5%. Direct observation of baffle systems have lead to the recommendation that they not be used on slopes greater than 3.5%, with steeper slopes requiring stream simulation or fishway design (Bates et al. 2003).

To avoid inlet contraction that can lead to reduced culvert capacity, the upstream baffle should be placed at least one culvert diameter downstream of the inlet, and be high enough to ensure subcritical flow at the high design flow. It is also recommended that the designer use a mitered end or wing walls to improve hydraulic efficiency.

Figure 7.9 Recommended styles of baffles for round and box culverts in Washington (Bates et al. 2003)

7.3.2.1 Biological Characteristics

Baffle configurations allow passage for target species.

7.3.2.2 Geomorphic Characteristics

Rajaratnam et al. recommend baffle configurations for slopes of 0.5-5%, while WDFW recommends them for slopes less than or equal to 3.5%.

Baffle configurations may affect flow through and around the structure. Localized aggradation and degradation may occur due to channel constriction.

7.3.2.3 Hydraulic Characteristics

Velocities for fish passage must be determined.

Flood capacity must be adequate for the structure. WDFW recommends designing the upstream baffle to avoid inlet contraction and subsequent reduced culvert capacity, and to ensure subcritical flow at high design flow. Debris clogging the culvert may reduce flood-flow capacity.

Baffle spacing greater than one culvert diameter was found to decrease velocity while increasing depth.

An energy-dissipation factor must be calculated to ensure that turbulence and sediment deposition do not impede fish passage.

7.3.2.4 Data Requirements

Velocity for Baffles in Round Culverts:

Velocity is calculated by the flow equations developed by Rajaratnam and Katapodis (Rajaratnam et al. 1989; Rajaratnam and Katopodis 1990). Washington utilizes sloping baffles, and although weir baffles from the studies were horizontal, they provide the most reliable information for predicting roughness of baffles. Data within these papers were simplified to create Equation 7.5 and Table 7.6, aiding in WDFW's baffle design procedure.

Q = C(y₀/D)^a(gS₀D⁵)^1/2

where:

C =dimensionless coefficient that depends on baffle configuration
D =diameter of the culvert, m (ft)
a =exponent depending on baffle configuration
Q =discharge, m³/s (ft³/s)
y₀ =depth of water, m (ft)
g =gravitational acceleration, m/s² (ft/s²)
S₀ =dimensionless slope
Z₀ =height of the baffle (as depicted in Figure 7.9)

Equation 7.5 should be used to calculate the depth of flow, allowing velocity to be found by dividing the flow by the resulting cross-sectional area.

Velocity for Baffles in Box Culverts:

The hydraulics of baffles in box culverts are described by Shoemaker (1956). This study utilized the Darcy-Weisbach friction equation as a hypothetical model for culverts with baffles (Equation 7.6).

HW = (K_e + C_e + fL_c/D)V²/2g+P-S₀L_c

where:

f = dimensionless friction coefficient
L_c = length of the culvert, m (ft)
D = the diameter of the pipe (four times the hydraulic radius of noncircular pipes), m (ft)
V²/2g = the gross cross section velocity head in the culvert where V is the average velocity, m (ft)
S₀= dimensionless slope of the culvert
K_e = dimensionless culvert entrance head loss coefficient
C_e = dimensionless culvert exit head-loss coefficient
HW = headwater elevation above the invert at the culvert entrance, m (ft)
P = distance from culvert invert to center of flow over a baffle, m (ft)

In Shoemaker's model, baffles were full width and level, with rounded leading edges at a radius equal to one tenth of the culvert height. Baffles heights of 0.10, 0.20 and 0.30 times the culvert rise and spacings of 1.0, 2.0, and 4.0 times the culvert rise were studied. The culvert had inlet and outlet aprons extending 2.5 times the culvert span, and wing walls flared at 34 degrees from the culvert sides, mitered at a 2:1 slope. The baffle furthest downstream from the culvert entrance was placed at the edge of the apron.

Shoemaker's variation of the Darcy-Weisbach friction factor is depicted in Figure 7.10.

Figure 7.10 Variation of Darcy-Weisbach friction factor (Bates et al. 2003)
(L is the baffle spacing; Z is the baffle height, and D is the culvert diameter)

Culvert capacity analysis assumes that entrance, outlet and friction losses are proportional to the velocity head. Equation 7.6 can be used with C₀=K_e+C_e(from Figure 7.11), and other parameters as previously defined. According to Shoemaker, P can be approximated as the distance from the culvert invert to the center of the flow at the opening above a baffle (Shoemaker 1956).

Figure 7.11 Energy coefficients for various baffle arrangements (adapted from Bates et al. 2003)

(K_e and C_e have been combined into a single head loss coefficient C₀, depicted here as a function of baffle spacing and height)

Energy Dissipation:

In order to ensure that turbulence does not prevent fish passage ability, an energy-dissipation factor (EDF) is calculated (see Section 3.2.5). For baffled fishways, WDFW recommends a value of 240 m-N/m³/s (5 ft-lb/ft³/s). It is further specified that the EDF should remain above 144 m-N/m³/s (3 ft-lb/ft³/s) at the high fish passage design flow to ensure that sediment deposition does not make the baffles ineffective or create a direct fish passage barrier.

7.3.3 Maine

Source

• Maine Department of Transportation 2004
• U.S. Army Corps of Engineers 2005

7.3.3.1 Biological Characteristics

This design allows for passage of all species.

7.3.3.2 Geomorphic Characteristics

Hanging outlets are avoided when possible, and the installation is allowed to fill with natural material. Streambed characteristics are maintained as much as practical.

Corrugated elliptical pipe arches with the largest feasible corrugation are used to maximize roughness.

The culvert slope will not exceed the natural gradient, and the culvert should match natural stream depth and width at Q_1.5. Span is equal to 1.2 times bankfull width.

Culverts are to be embedded (Section 7.3.3.4) and are allowed to fill with natural material.

7.3.3.3 Hydraulic Characteristics

Maximizing roughness will decrease flow velocity and increase turbulence.

Culvert capacity is checked using Q₅₀, and must be checked to allow for 100-yr flood. Flow depth during species-specific periods of movement must allow for fish passage.

7.3.3.4 Data Requirements

Bed Material:

Eliminate hanging outlets where practical, and allow installation to fill with natural material. Allow streambed characteristics to be maintained as much as practical.

Culvert Material:

Use corrugated elliptical pipe arches with largest feasible corrugations to maximize roughness.

Culvert Slope:

Culvert slope is not to exceed natural gradient.

Embedment:

1. When culvert diameter is less than 1.22 m (48 in) the culvert should be embedded 150 mm (6 in) into the stream bottom.
2. When culvert diameter is greater than 1.22 m (48 in) the embedment should be embedded 0.30 m (12 in) into stream bottom.

Culvert Capacity:

Culvert capacity is checked using Q₅₀, and the culvert should match natural stream depth and width at Q_1.5.

Fish Passage Flow:

Check flow depth during species-specific periods of movement.

Flood Capacity:

Check 100-year flood.

7.3.3.5 Design Procedure

The Maine Department of Transportation (2004) Design Guide lists four general design steps:

1. Identification of valuable habitat for specific species and need for passage by fisheries biologists in MDOT, resource agencies, and regulatory agencies
2. Determination of calendar periods when passage must be provided
3. Estimation of design flows during passage periods
4. Culvert design
   1. New pipe: size pipe according to natural stream bankfull cross-section; check for extreme flow capacity and passage performance by hydraulic analysis
   2. Rehabilitated pipe: hydraulic analysis to check performance of proposed rehabilitation; design mitigation measures (e.g., weirs, baffles, outlet notch ramps) if fish passage is inadequate

Specific guidelines are given for

• Downstream energy dissipation pools
• End treatments for retrofitted culverts
• Downstream gage control weirs
• Interior weirs
• Fish passage flows

7.3.4 Roughened Channel Design

Source

• Washington Department of Fish and Wildlife (Bates et al. 2003)

Applicability

• Over-steepened channel sections (as in replacement installations with past channel degradation)
• Slopes up to 10% (according to design charts)
• Passage required for target species
• Limited work area or right-of-way

Limitations

• Washington State still considers the "Roughened Channel" an experimental technology requiring more research and monitoring to be a viable design option.
• Velocity and turbulence checks are required to ensure that they do not exceed fish thresholds.
• This technique requires special design expertise, hydrology, and survey information.

Overview

The 2003 Washington Department of Fish and Wildlife fish passage guidelines include criteria for the creation of a roughened channel, either within or upstream of a culvert. Oversized substrate is designed for stability during the 100-year flood, allowing installation on over-steepened channel sections and moderate to high slopes. Roughness elements control depth and velocity, providing passage conditions adequate for the targeted fish species. Average cross-sectional velocity and turbulence are checked against species-specific allowable value.

Culverts designed using this technique are reported to have mixed results in Washington, and are considered experimental at this time, requiring special design expertise, hydrology and survey information (Bates et al. 2003).

7.3.4.1 Biological Characteristics

This design allows for the passage of desired species, although passage requirements of non-target species should be considered in culvert sizing.

7.3.4.2 Geomorphic Characteristics

The Roughened Channel design uses oversized substrate to stabilize over‑steepened channel sections or moderate to high slope, allowing passage conditions for target species. Roughness elements control depth and velocity.

Bed material is designed to be stable for the 100-yr flood, with the largest bed particles being less than one-quarter the culvert span. Bed material is graded to control porosity.

In practice, bed retention sills and engineered substrate have filled culverts to 30% of the rise, the sills keeping bed material in place. Large boulders are added for fish passage. A downstream control structure should be added to protect against the creation of an outlet drop.

7.3.4.3 Hydraulic Characteristics

Velocity and turbulence must be checked to ensure adequacy for fish passage.

The culvert must be checked for adequate extreme-flood capacity (i.e. 100-yr event).

7.3.4.4 Data Requirements

Culvert Span:

Assume a culvert span, beginning with bankfull width. Considerations of debris and sediment transport, habitat, and passage requirements of non-target species should be included. According to WDFW, culvert span should be at least the width of the natural stream channel.

Note - As gradient and unit discharge increase, WDFW recommends an increase in culvert span as the best way to achieve stability and passability, while reducing the risk of scour and extreme hydraulic conditions.

Bed Material Stability:

Size the bed material for stability based on unit discharge for the 100-yr event (Q₁₀₀).

For roughened channel design, bed material should remain in the culvert as placed. Bed material may shift slightly, but should not move an appreciable distance or leave the culvert. For this reason, bed material stability should be calculated before consideration of fish passage velocity. Unlike Stream Simulation design, roughened channels increase hydraulic forces due to increased slope. WDFW considered four methods for sizing bed material for stability (Bates et al 2003). For two of the methods, The U.S. Army Corps of Engineers Riprap Design and the Critical Shear Stress Method, see sections 6.5.5.5 and 6.5.5.9, respectively.

There also exist a number of alternatives for sizing bed material, including those covered in WDFW section on Stream Simulation Design (Section 7.1.2).

Bed Material Size:

Check to see that the largest bed-particle size, as determined by stability, is less than one quarter the culvert span. If not, increase the culvert span, which decreases the unit discharge and, in turn, the particle size.

Bed Material Gradation:

Create a bed-material gradation to control porosity (see WDFW Stream Simulation Design).

Velocity:

Calculate the average velocity at the fish-passage design flow on the basis of culvert span and the bed D₈₄ from gradation. Three equations (see Section 6.5.4), Limerinos equation, Jarrett's equation and Mussetter's equation, are used to find roughness and velocity in order to calculate fish passage velocity. The three equations were derived from data in natural streams and account for roughness characteristics of natural channels. Constructed channels must be designed in such a way to maximize channel roughness and emulate natural channel planform and profile, otherwise the following equations will likely overpredict roughness and lead to an ineffective approximation of constructed channel velocities.

In general the relationship between velocity and roughness is given by:

V/(gRS_f)^1/2 = 1.486R^1/6/ng^1/2 = (8/f)^1/2

where:

V = the average velocity, m/s (ft/s)
g = the acceleration due to gravity, m/s² (ft/s²)
R = the hydraulic radius, m (ft)
n = dimensionless Manning's roughness factor
f = dimensionless Darcy-Weisbach friction factor
S_f = the friction slope of the channel

The use of n or f depends upon convention, but the Darcy-Weisbach equation accounts for the reduction in roughness with increasing depth, whereas Manning's equation does not (Bates et al. 2003).

Turbulence:

Washington State quantifies the impact of turbulence through the calculation of an energy dissipation factor (EDF), see Equation 3.1 for the EDF equation. Calculate the EDF at the fish-passage design flow on the basis of culvert span and the bed D₈₄ from gradation. For roughened channels, the EDF must be less than 7.0. This is based on experience in Washington, and will be modified with future research and evaluations (Bates et al. 2003).

Culvert Capacity:

Check culvert capacity for extreme flood capacity (i.e. 100-yr event).

Fish Rocks and Bed Retention Sills:

In practice, installation of roughened channels has included bed retention sills and engineered substrate filling the culvert to 30% of the rise. Large boulders are then added to provide shadow as a safety factor for fish passage. The sills act to keep bed material in place. Further field experience is expected to eliminate the need for these structures (Bates et al. 2003).

A downstream control structure should be constructed to ensure that the lowest point of the bed elevation at the culvert outlet matches the elevation of a downstream control point. The control structure can be a stable natural feature or a permanent constructed control placed no closer than 6.1 m (20 ft) from the culvert outlet. This protects against the creation of an outlet drop by ensuring that sills do not become exposed.

Figure 7.12 Roughened-channel culverts using fish rocks and bed retention sills (Bates et al. 2003)

Bed Retention Sills - Bed retention sills are typically made of the same material as the culvert, and are attached directly to the culvert.

Bed Material - In low gradient situations, bed material creates the primary source of roughness, and is included to act as a factor of safety. In high gradient situations, the specified bed material may contain elements that will act as boulders.

Fish Rocks/Boulders - Rocks should be no greater than one quarter the culvert span to prevent overly constricting the flow. Boulders are embedded one-third of their diameter (measured along the intermediate axis).

Depth of Flow - The water depth at the fish passage design flow should be less than or equal to two thirds of the exposed height of the boulders. The combination of these constraints should lead to a boulder diameter that is roughly twice the depth of water.

7.3.4.5 Design Procedure

Roughened channel design consists of the following steps:

1. Culvert Span - Assume a culvert span, beginning with bankfull width. Considerations of debris and sediment transport, habitat, and passage requirements of non-target species should be included. According to WDFW, culvert span should be at least the width of the natural stream channel.
2. Bed Material Stability - Size the bed material for stability based on unit discharge for the 100-yr event (Q₁₀₀), as outlined in Step 3.
3. Bed Material Size Check - Check to see that the largest bed-particle size, as determined by stability, is less than one quarter the culvert span. If not, increase the culvert span, which decreases the unit discharge and, in turn, the particle size.
4. Bed Material Gradation - Create a bed-material gradation to control porosity (see WDFW Stream Simulation Design).
5. Check Turbulence and Velocity - Calculate the average velocity and EDF at the fish-passage design flow on the basis of culvert span and the bed D₈₄ from gradation in Step 4 above. If the velocity or EDF exceed the criteria, increase the culvert span.
6. Culvert Capacity - Check culvert capacity for extreme flood capacity (i.e. 100-yr event).

Note - As gradient and unit discharge increase, WDFW recommends an increase in culvert span as the best way to achieve stability and passability, while reducing the risk of scour and extreme hydraulic conditions.

Steps 2-3 can be completed using a variety of recommended methods/equations. Included are the U.S. Army Corps of Engineers Riprap Design and the Critical-Shear Stress Method.

There also exist a number of alternatives for sizing bed material, including those covered in WDFW section on Stream Simulation Design (Section 7.1.2).

7.3.5 Oregon Department of Fish and Wildlife Hydraulic Design

Source

• Oregon Department of Fish and Wildlife (Robison et al. 1999)

7.3.5.1 Biological Characteristics

Fish passage allowed for target species. Velocity requirements are listed for salmon, steelhead, adult trout and juvenile salmonids.

7.3.5.2 Geomorphic Characteristics

This design procedure is appropriate for slopes up to 12%.

Flow through and around the structure may be affected. If the channel is constricted, localized aggradation and degradation may have to be addressed. Retrofitted roughness elements such as weirs and baffles may increase the risk of clogging the structure.

7.3.5.3 Hydraulic Characteristics

Design flows and minimum water depth must be determined for target species.

Flood capacity must be adequate for 100-yr flow. Applied channel width constriction or roughness elements may restrict passage of water and debris, decreasing flood-flow capacity and increasing the likelihood of plugging and culvert failure.

7.3.5.4 Data Requirements

Design Flows:

See Fish Passage Hydrology section, Chapter 5.

Water Velocity:

Note - Hydraulic Design is not allowable in culvert installations longer than 100 ft when juvenile salmonids require passage.

Note – Hydraulic Design is not allowable in culvert installations longer than 100 ft when juvenile salmonids require passage.

Minimum Water Depth:

Minimum water depth is specified by species and lifestage. For example,

• 0.30 m (12 in) for adult steelhead and Chinook
• 0.25 m (10 in) for salmon other than Chinook, sea-run cutthroat trout, or other trout over 0.51 m (20 in)
• 0.20 m (8 in) for trout under 0.51 m (20 in), Kokanee, juvenile steelhead and salmon

Maximum Jump Height:

• 0.30 m (12 in) adult steelhead and salmon
• 0.15 m (6 in) trout, Kokanee, juvenile steelhead and salmon

Jump Pool Depth:

Jump pool depth must be the greater of 1.5 times jump height or 0.61 m (24 in).

Slope of Structure:

• Less than 0.5% if not embedded, baffled, or backwatered
• Up to 5% if baffled.
• 5-12% if installed with a fish ladder or integral weirs

Span of Structure:

The span of the structure is not applicable.

Length of Structure:

The length of the structure must be less than or equal to 30.5 m (100 ft) if juvenile passage is required.

Flood Capacity:

Flood-flow capacity must be adequate to pass the 100-yr flood.

Design Procedure:

Oregon baffle configurations are shown in Figure 7.13-7.15.

Figure. 7.13 Baffle configurations endorsed in Oregon in customary units (Trevis, Personal Communication)

Figure 7.14 8 inch plastic baffle used in Oregon, customary units (Trevis, Personal Communication)

(Flow is from right to left)

Figure 7.15 12 inch plastic baffle used in Oregon, customary units (Trevis, Personal Communication)

(Flow is from right to left)

7.3.6 Maine DOT Culvert Design for Rehabilitation

Source

• Maine Department of Transportation 2004

For culvert rehabilitations, the following objectives are desirable

• Eliminate hanging outlets
• Preserve minimum flow depth during critical periods of species-specific movement.
• Do not exceed maximum flow velocity during periods of species-specific upstream movement.

7.3.6.1 Biological Characteristics

Fish passage provided for target species. Generic design standards are provided when species-specific criteria are not available.

7.3.6.2 Geomorphic Characteristics

Where channel width is constricted, the rehabilitation may affect flow through and around the structure. Localized aggradation and degradation due to such constriction may need to be addressed. Retrofitted roughness elements have the propensity to catch and hold debris, increasing the risk of clogging.

7.3.6.3 Hydraulic Characteristics

Determine design flows adequate for fish passage, preserving minimum flow depth during critical periods of species-specific movement, and not exceeding maximum flow velocity during periods of upstream movement.

When species-specific criteria are not available, the structure must be designed for fish passage during low flow periods, maintaining a minimum depth at design low flows. The average of median September and October flows are to be used as design flows.

Flood conveyance must be checked, with consideration that retrofitted roughness elements may decrease capacity while increasing risk of plugging or failure.

7.3.6.4 Data Requirements

Water Depth:

Maintain at least 0.20 m (8 in) of water depth throughout the length of the culvert at design low flows.

Velocity:

Limit flow velocity to no more than 0.60 m/s (2 ft/s).

Water Surface Elevation Drop at Outlet:

Limit drop in water surface elevation at the outlet to 0.05 m (2 in).

Design Flow:

Use average of median September and October flows as design flow.

Water Level Drop:

Limit water level drop across grade control structures to 0.20 m (8 in).

Weir Dimensions:

When weirs are employed, weir notches should be at least 0.20 m (8 in) wide by 0.20 m (8 in) deep. Calculated dimensions should be rounded to the nearest 0.05 m (2 in) increment.

7.3.6.5 Design Procedure

When species-specific criteria are not available, the structure must be designed for fish passage during the low flow period. Other generic design standards are provided by the Data Requirements section.

7.3.7 Summary of Hydraulic Design Procedures

Table 7.8 provides a summary of Hydraulic Design techniques.

7.3.8 A Detachable Fishway for Steep Culverts

Source

• Clancy 1990

Applicability

• Culverts with spans close to that of the natural channel
• Successfully used in culverts with slopes of 4.4% and lengths of 45 m (148 ft)
• Culvert capacity is adequate to withstand a reduction in cross-sectional area without compromising design flood flow conveyance.

Limitations

• Culvert capacity must be adequate to buffer the impact of added sediment, which was shown to reduce culvert capacity by approximately 15%.

A detachable fishway for culvert retrofits was designed to be inexpensive and easily constructed in the field. Hand-placed rock is held in place by steel crossbars, creating a roughened channel that provides resting areas and low velocity paths within the culvert. The total cost of this retrofit (in 1990 dollars) was $2200 for a culvert that was 45 m (148 ft) long and 1890 mm (6.2 ft) in diameter. Fish passage was observed within the first year. A site visit eight years after culvert installation showed that fish passage remained intact and that bed material had washed between large roughness elements.

7.3.8.1 Biological Characteristics

This design allows for the passage of target species, and creates resting areas and low-velocity paths for passage within the culvert.

7.3.8.2 Geomorphic Characteristics

Culvert spans are close to that of the natural channel. The installation has been successfully completed at sites with slopes of 4.4%. A site visit showed that natural bed material washed between roughness elements.

In installations in which the structure constricts the natural channel, localized aggradation and degradation may occur, especially given the fishway's propensity to catch and hold debris and clog the channel.

7.3.8.3 Hydraulic Characteristics

Culvert capacity should adequately buffer impact of added sediment, and should not compromise design flood flow conveyance. Flood conveyance must be checked.

Design flows for target species passage must be determined.

7.3.8.4 Data Requirements

Culvert Span:

Culvert span should be close to the width of the natural channel.

Culvert Capacity:

Capacity must be adequate to buffer the impact of added sediment, which was shown to reduce culvert capacity by approximately 15%.

Culvert Slope:

Successful installation was performed in culverts with slopes of 4.4%.

Culvert Length:

Successful installation was performed in culverts with lengths of 45 m (148 ft).

7.3.8.5 Design Procedure

1. Angle iron and reinforcing bar (as shown in Figure 7.16) is prefabricated in segments and assembled on site.
2. The upstream end of the fishway is anchored to the concrete headwall.
3. Downstream sections are bolted together

   Cross members welded in place every 1.21 m (4 ft)

   Rock holder and hold-downs are angled upstream so that water pressure holds structure in place.

   Large rocks are hand placed on the upstream side of each cross member.

Figure 7.16 Detachable fishway design for culvert retrofit, customary units (Clancy 1990)

7.3.9 Fishways

Fishways are designed on a case-by-case basis. Each design is based on local conditions, policy and custom. Pictured below in Figure 7.17 is a fishway installed on Peacock Creek in California.

Figure 7.17 Fishway installed on Peacock Creek, California (Llanos 2004)

(View is looking downstream towards the culvert entrance)

Applicability

• Excessive drop at outlet cannot be mitigated by downstream grade control.
• Right-of-way is unavailable for developing downstream grade control.
• Steep culvert slope would require numerous closely spaced internal weirs.
• Slopes from 10-25% depending on fish species and life stage requiring passage
• Often built downstream from outlet to avoid debris accumulation or reduced capacity in barrel

Limitations

• Long-term maintenance obligations

Occasionally, weirs and baffles will be either be unfeasible, or will not produce the hydraulics conditions necessary for fish passage (Maine Department of Transportation 2004). Fishways such as the Vertical Slot Fishway, Denil Fishway and Steeppass Fishway are structures consisting of a sloping channel partitioned by flow control devices such as baffles, weirs or vanes with openings to allow fish to swim through. Further discussion of such devices is in Introduction to Fishway Design (Katopodis 1992).

7.3.9.1 Biological Characteristics

Fishways provide hydraulic conditions to allow for target species passage.

7.3.9.2 Geomorphic Characteristics

Fishways are applicable on steep slopes of 10-25%. The fishway may affect flow through and around the structure. When the natural channel is constricted, localized aggradation and degradation may occur. The structure has the propensity to catch and hold debris, and will require long-term maintenance.

7.3.9.3 Hydraulic Characteristics

Design flows for target species passage must be determined.

Flood conveyance must be checked. The potential to catch and hold debris may decrease the structure's flood flow capacity and increase the likelihood of plugging and failure.

7.3.9.4 Data Requirements

Slope:

This design is recommended on slopes from 10-25% depending on fish species and life stage requiring passage.

Design Flow:

Flows must be adequate to provide for species-specific fish passage.

Flood Capacity:

Capacity must be adequate to pass design flood flows.

7.3.10 Floodplain Culverts

As described in Section 7.2, Maryland design guidelines contain specification for floodplain culverts in situations where a single culvert would overly constrict flow (Maryland State Highway Administration 2005). Floodplain culverts can be installed to collect and convey flood plain flows, reducing the impact of the main-channel culvert. Floodplain culverts should be positioned on the floodplain well beyond the influence of the main culvert to avoid channel undermining, degradation or migration into the area of the floodplain culvert. This position also avoids clogging due to debris carried in the main channel.

7.3.10.1 Biological Characteristics

A main-channel culvert allows for target species passage by providing resting pools, low velocities and deep flow.

7.3.10.2 Geomorphic Characteristics

By conveying flood flows, the floodplain culvert reduces the impact of the main-channel culvert on the natural channel. Positioning the floodplain culvert on the floodplain, and away from the influence of the main-channel culvert, also protects local morphology. The main-channel culvert outlet should minimize impacts to the downstream channel and stabilize flow for passage.

7.3.10.3 Hydraulic Characteristics

Design flows for target species passage in the main-channel culvert must be determined. The floodplain culvert increases capacity for flood-flow. The structure must be checked for flood-flow conveyance.

7.3.10.4 Data Requirements

Flood Capacity:

Floodplain culverts must adequately convey design flood flow levels.

7.3.11 Two-Cell Installations

Two-cell fish culverts provide one cell for fish passage and another to ensure flood capacity. Maryland and Maine utilize two-cell installations as described in Section 7.2 (Maryland State Highway Administration 2005; Maine Department of Transportation 2004). For two-cell installations, upstream w-weirs may be included to reduce bar deposition and scour, increase the competence of bed material transport and reduce debris build-up at the center wall. As noted elsewhere, w-weirs can alleviate the accumulation of debris between spans of multi-cell installations. North Carolina has criteria for two-cell culvert installations (Figure 7.18) utilizing a lowered fish passage culvert that creates a sinuous low flow travel path in the lower culvert Twisdale, Personal Communication). Lang et al discourages two-cell installations due to the likelihood of debris collecting on the area between spans (Lang et al. 2004).

Figure 7.18 Example detail of low-flow channel sills, customary units (Twisdale, Personal Communication)

7.3.11.1 Biological Characteristics

One cell allows for passage of target species by creating adequate hydraulic conditions.

7.3.11.2 Geomorphic Characteristics

Generally the cell which allows for fish passage is designed to maintain a low flow channel with natural bed material. Maryland recommends upstream "W" weirs to reduce bar deposition and scour, increase the competence of bed material transport and reduce debris build-up at the center wall, and designs the main-channel culvert outlet to minimize impacts to the downstream channel and stabilize flow for passage.

The propensity for the center wall to catch debris increases the likelihood this structure will influence characteristics of the natural channel.

7.3.11.3 Hydraulic Characteristics

Design flows for target species passage must be met in the corresponding cell.

The two-cell installation provides one cell for flood-flow capacity. Flood-flow conveyance for the structure must be checked. The potential for debris build-up may decrease flood-flow capacity and increase plugging and failure.

7.3.11.4 Data Requirements

Hydrology:

Adequate hydraulic conditions for target species passage must be determined for the fish-passage cell.

Flood Capacity:

Culvert capacity must be sufficient to convey design flood flows.

7.3.12 Tide Gates

In tidal situations, tide gates are used to allow freshwater to flow into estuaries while ensuring that brackish waters are kept from moving upstream. Such structures have been part of a system of dikes used to allow the drainage and development of marshland (Giannico and Souder 2005).

Tide gates (or tide flaps) are attached to culvert outlets as depicted in Figure 7.19, and are controlled by the elevational difference of water levels on either side of the culvert. In a process shown sequentially in Figure 7.20, culverts open as ebbing tides allow fresh water to flow to the estuary side of the culvert, and close as flood tides attempt to bring tidal waters upstream and upland. Fish passage at tide gates is focused on extending the period of time that tide gates remain open, thereby increasing the range of flows over which a fish will be able to pass the structure.

Figure 7.19 Lateral schematic of a culvert with a top-hinged tide gate attached to downstream end of culvert (Giannico and Souder 2005)

Figure 7.20 Tide gate operation cycle: (A) tide gate begins to open when water pressure in culvert overcomes pressure of water on downstream side during ebb tide; (B) tide gate is wide open during ebb tide; (C) tide gate begins to shut when upstream water level drops and tide begins to rise; and (D) tide gate is shut during flood tide (Giannico and Souder 2004

Advances in tide gate technology include gates with permanent holes, aluminum or plastic gates, fiberglass doors, side hinged gates, rubber gates, and fish passage appurtenances such as "pet doors" (Figure 7.21). These technologies are largely unvalidated, and have questionable effects on fish passage and stream ecology (Giannico and Souder 2004).

Figure 7.21 Bottom-hinged pet door (Giannico and Souder 2005)

(The floater allows a small area of the gate to open during periods when water elevations would keep the gate closed; this is intended to allow a longer period of fish movement)

Tide gates impact freshwater/brackish water interaction, and can have a profound effect on channel characteristics including flooding and water flow, channel geometry, water temperature, Ph, salinity, plant communities and fish and fish habitat (Giannico and Souder 2005). The authors warn that there is no such thing as a fish friendly tide gates, only a "fish friendlier" tide gate.

Note on State Guidelines for Design in Tidal Areas

Because of the difficulty in creating fish passage criteria in tidal areas, Washington Department of Fish and Wildlife promotes removal of tidal culverts as the preferred restoration technique (Bates et al. 2003). Maryland culverts are commonly designed for low tide conditions, ensuring that the culvert is accessible in a worst-case scenario (Kosicki, Personal Communication 2006).

7.3.12.1 Biological Characteristics

Designing tide gates for fish passage increases the time the gate remains open, thereby increasing the range of flows over which a fish will be able to pass the structure. Tide gates' impact on the interaction of freshwater and brackish water can profoundly affect fish and fish habitat.

7.3.12.2 Geomorphic Characteristics

Tide gates allow freshwater to flow into estuaries and keep brackish water from moving upstream. Their impact on the interaction of freshwater and brackish water can profoundly affect channel characteristics, including flooding and water flow, channel geometry, temperature, Ph and plant communities.

7.3.12.3 Hydraulic Characteristics

The impact of tide gates on freshwater and brackish water interaction can influence the flood conveyance and water flow of the channel.