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Design For Fish Passage at Roadway - Stream Crossings: Synthesis Report
6 Design Considerations, Methods and Tools
How to use this chapter
A culvert designed for fish passage must also meet applicable flood conveyance requirements, such as allowable headwater elevations during the design flood and local Federal Emergency Management Agency (FEMA) regulations.
Many factors will determine the suitability of any particular set of fish passage design criteria for a culvert installation, replacement or retrofit. With fish passage as the overall goal, economics, site logistics, regulatory requirements and roadway characteristics may dictate a particular design procedure. The following categories have been developed to aid in the classification and selection of design approaches based on project goals. These goals are based on biological, geomorphic and hydraulic considerations.
Category 1: Geomorphic Simulation, also known as stream simulation, approaches recreate or maintain natural stream reach geomorphic elements including slope, channel-bed width, bed materials and bedform by using the reference reach. The approaches are based on the assumption that crossings approximately matching natural conditions will readily pass fish that are moving in the natural channel. This approach has five benefits: (1) it will provide passage for fish more readily than for much narrower spans; (2) it may provide passage for other aquatic and some terrestrial organisms; (3) for discharges less than approximate bankfull values, it will not increase downstream channel velocities; (4) for discharges exceeding bankfull values, it will increase downstream velocity less than for culverts with narrower spans, and (5) maintenance requirements (debris clearing, streambed manipulation) should be less than for narrower spans.
Category 2: Hydraulic Simulation techniques utilize embedded culverts, natural or synthetic bed mixes and natural roughness elements such as oversized rock to provide hydraulic conditions conducive to fish passage. These techniques operate on the assumption that providing hydraulic diversity similar, but not identical, to that found in natural channels will create a fish passable structure without checks for excessive velocity or turbulence. Hydraulic Simulation will generally have the benefit of creating smaller spanning structures that have a reduced cost when compared to Geomorphic Simulation.
Category 3: Hydraulic Design techniques create water depths and velocities that meet the swimming abilities of target fish populations and life stages during specific periods of fish movement. Hydraulic Design is most often used in retrofit projects. General considerations include the effect of culvert slope, size, material and length. Flow control structures such as baffles, weirs, or oversized substrate are commonly utilized to create acceptable hydraulic conditions. This technique generates a smaller diameter culvert that keeps cost of materials to a minimum. Installation costs, however, are highly variable due to unique designs of baffles, weirs, steps or other controls. Hydraulic Design produces a less conservative design for fish passage than Geomorphic or Hydraulic Simulation.
All of the fish passage design methods seek to allow passage when fish are believed to be moving in the natural stream system. Note that none of the methods provide for unaltered flow hydraulics during flood events used for design by state DOTs.
Infrastructure Safety and Service Life
6.1.1 Biological Considerations
18.104.22.168 Fish Passage Requirements
Crossing designs create different levels of stream reach connectivity. In general, Geomorphic Simulation creates the greatest connectivity, followed by Hydraulic Simulation and Hydraulic Design. In all cases, passage is presumed not to occur for discharges exceeding approximately bankfull conditions. The emphasis for fish passage design, therefore, is for discharges much smaller than those used in flood conveyance checks. A few pertinent questions can significantly narrow design option selection based on project goals.
What are the species of fish and life stages for which passage should be provided?
This question requires consultation with the fisheries biologist team member and likely consultation with natural resource agencies. The answer may depend upon, for example, regulations (e.g., Endangered Species Act), the desire to exclude invasive species, or economics (sport fishing considerations).
Hydraulic Designs can be completed to cater to a particular fish species and life stage; however, such a structure may provide a barrier to weaker swimming fishes at some or all flows.
What is the weakest swimming fish species and life stage for which passage is required?
Example: Adult Salmon; Juvenile Salmon; resident trout; benthic fish; all species and life stages present.
All techniques are designed to ensure fish passage; however, Geomorphic and Hydraulic Simulation approaches will allow passage for a wider variety of fish species and other aquatic organisms.
Do we know the swimming abilities, behaviors and timing of these species?
See Section 2.2.
At what flows, and time periods are these fish migrating? What is the allowable delay?
Design may depend on the timing of fish migration and relative flows. Delay impacts may be less crucial for resident fish than a spawning salmon. This problem can be compounded, for example, by several culverts in series or culverts that provide passage only after short delays.
22.214.171.124 Ecological Significance
Further consideration should be paid to the ecological significance of the roadway-stream crossing. The only way to truly preserve habitat at a crossing is to use a bridge or open bottom structure. Figure 6.1 shows a representation of the range of ecological solutions available at a roadway-stream crossing. Extreme ends of the spectrum include traditional design for flood capacity, and bridges or road removals that will permit valley and floodplain processes.
It is recommended that new culvert designs incorporate auxiliary barrels on the floodplains at the roadway crossing. Adding barrels at an elevation higher than bankfull will provide a flowpath for waters when discharge exceeds bankfull values. Such relief will decrease the main channel velocity and scour potential. This procedure is described in more detail in sections 7.2.4, 7.3.7 and 8.2.1.
6.1.2 Geomorphic Considerations
Site geomorphology is another important consideration in design for fish passage. Slope, channel location, channel stability and bed material are all examples of geomorphic elements that affect design selection. For example, installations located at slope breaks or in sediment sensitive areas may have a high propensity to degrade, aggrade or elicit a change in channel conditions, eventually creating another barrier or destroying valuable habitat (Bates et al. 2006).
126.96.36.199 Form and Key Features
Channel form and key features can aid in understanding channel processes including sediment transport, channel stability, and channel migration (Bunte and Abt 2001). Key features describe stream elements such as large woody debris (LWD), rock, vegetation, or channel confinement, all of which can play a large part in channel form and stability (Montgomery and Buffington 1998). While features such as LWD may be prominent in some channels, exact placement and development of such influences, and associated features, may be fairly unpredictable (Montgomery and Buffington 1993), and an understanding of overall influence and importance will be essential.
Channel stability refers to the likelihood that a channel will retain its current placement, gradation, shape and form over time. Channels in highly entrenched mountain streams will be less likely to show lateral or vertical changes over time, while meandering valley streams may show great variation both laterally and vertically in response to minimal inputs (Montgomery and Buffington 1993; Rosgen 1994).
188.8.131.52 Morphological Adjustment Potential/Ability
A crossing can be built to buffer for slight lateral and vertical channel adjustments. Although this can increase the size and initial cost of a structure, benefits can include decreased maintenance requirements and increased design life.
184.108.40.206 Rigid Structure in Dynamic Environment
Bates et al. provide a detailed discussion about managing the stream profile at crossing. All culverts act as constraints in a dynamic environment, remaining at a specific location and elevation, and prevent channels from maintaining their natural processes (2006). By attempting to understand the possible impacts of a crossing on the channel, it is possible to select design options that provide optimum fish passage while ensuring acceptable design life and maintenance requirements.
6.1.3 Hydraulic Considerations
220.127.116.11 Flood Flow Conveyance
Flood flow capacity must be considered at all roadway-stream crossings. Each state has established flood flow requirements for culverts as a function of roadway category. Typical values of required flood capacity range from the 4% chance flood (25-yr) to the 1% chance flood (100-yr). If designing for fish passage, culvert size is often larger than that required for flood conveyance; however, hydraulic capacity must still be checked to ensure adequate flood flow conveyance.
18.104.22.168 Culvert Flow Characteristics
Slope and span will have a large impact on culvert flow characteristics. Crossings that are designed to create passage for specific fish and lifestages may require additional hydraulic considerations such as low and high fish passage flows and induced turbulence.
22.214.171.124 Targeted Fish Passage at Design Flows
Hydraulic Design options require detailed hydrologic information in order to ensure fish passage at specific periods of fish migration, while Geomorphic and Hydraulic Simulation methods attempt to match (or closely mimic) natural stream reach characteristics, and require little to no additional hydrologic information.
126.96.36.199 Passage for All Fish
Geomorphic and Hydraulic Simulation techniques are intended to provide passage for all fish species within the reach through any period during which they are moving. It may be difficult, or very costly, to provide passage for all fish by designing for specific hydraulic conditions.
188.8.131.52 Sediment Transport
Culverts that maintain a natural bed will be sized to retain natural reach sediment transport properties (Bates et al. 2003; Bates et al. 2006; National Marine Fisheries Service Southwest Region 2001). If crossings constrict flow, there will likely be associated impacts on sediment transport including aggradation upstream and increased velocities, scour and degradation downstream from the structure (Castro 2003).
184.108.40.206 Outlet Control
For fish passage velocities and depths to be met, it is recommended that flow remain subcritical through the culvert and at the outlet, requiring that culverts be designed to maintain outlet control (Alaska Department of Fish and Game and Alaska Department of Transportation 2001; Behlke et al. 1991; Bates et al. 2003). Characteristics governing outlet control include culvert inlet area and shape, barrel area and shape, barrel slope, barrel length, barrel roughness, and water surface elevation at the culvert outlet (Norman et al. 2005). Depressed inverts, or artificial roughness created by weir baffles, and deep corrugations can also be used to slow velocities within the culvert barrel (Behlke et al. 1991). Figure 6.2 from Hydraulic Design Series 5 depicts a culvert under outlet control with an unsubmerged entrance. Outlet control may also exhibit a submerged entrance with part of the barrel or the entire barrel flowing full.
Other than biological, geomorphic and hydraulic considerations, a number of project and site constraints will help determine the appropriateness of a particular design technique. These include, but are not limited to, funding, cost, right-of-way, and physical, environmental, and regulatory issues. Most of these issues apply to all roadway-stream crossings but have unique aspects when applied to fish passage.
Culvert barrel cost increases with span. If this were the only parameter used to compare the cost of design methods, Geomorphic Simulation techniques would be more costly than Hydraulic Simulation. Span cost is even less applicable to Hydraulic Design, where baffle, weir and other appurtenances can cost more than the culvert barrel.
Total roadway-stream crossing cost includes several other capital and recurring items such as installation and long-term maintenance. To date there are insufficient case histories to conclusively state that Geomorphic Simulation techniques incur greater life cycle costs than Hydraulic Simulation or vice versa. Limited experience in Alaskan rivers indicates that there is about a 20-30% capital cost reduction for Hydraulic Simulation installations when stream slopes exceed 3%, and little difference for stream slopes less than 3% (Gubernick, Personal Communication). These costs do not include long-term maintenance, posited to be inversely related to culvert span.
The cost of replacement may be prohibitively high due to deep fill or location (Interstate highway, for example). These cases may favor Hydraulic Design techniques.
Total life cycle cost for fish passage culverts is therefore difficult to compare for different design methods. It should not be assumed, for example, that one method is more costly than another based on culvert span. A complete cost analysis must be made for each crossing.
Many design techniques are still considered experimental, and long-term monitoring is still required to understand the true impacts and implications of a selected method (Chapter 10). Careful consideration of goals and requirements should be taken before selecting design criteria.
6.2.2 Right of Way
Right of way will affect the ability of designers to modify the channel outside of the culvert structure. Some design situations will require hydraulic control structures to ensure adequate backwatering, or to control channel slope, scour, and incision. Right of way costs may limit options on small retrofit projects with limited budgets. Clear communication with local landowners will provide an understanding of right of way, and innovative agreements and easements may extend access beyond existing rights of way.
6.2.3 Physical Constraints
In addition to right of way, a number of physical barriers or obstacles could force the designer to consider the costs of moving those obstacles vs. a change in design direction. Examples include utility crossings, extreme gradient changes, and incised or degrading channels. A roughened channel (Hydraulic Design, Section 7.3.4), for example, may be required instead of a Geomorphic Simulation procedure when protecting a utility. Correcting a perched culvert may also dictate a method that protects the streambed from incision.
6.2.4 Environmental Constraints
Environmentally sensitive areas will require a high degree of design consideration. For example, at a new crossing in a salmon spawning area, it may be pertinent to design an open bottom structure that allows natural substrate to remain relatively undisturbed through the crossing. To illustrate, a culvert barrier replacement in northern California utilized natural substrate and experienced salmon spawning within the structure only two years after installation (Furniss, Personal Communication).
6.2.5 Regulatory Constraints
Regulatory requirements, like those discussed in Chapter 1 may reduce design options. For example, the presence of endangered or threatened fish species will require specific and immediate consideration, and if passage for weak swimming fish is required, Hydraulic or Geomorphic Simulation may be the best option.
Proper culvert alignment requires consideration of channel shape, morphology, and culvert length. Installations that run perpendicular to the road will allow the shortest installations. In addition, flow exiting a culvert at an angle is likely to induce scour (Baker and Votapka 1990; White 1997), requiring wider culverts or channel treatments to protect against stream movement (Bates et al. 2006). Highway alignment should avoid sharp stream bends, severe meanders, confluences or other areas of converging and diverging flow (Maryland State Highway Administration 2005). When situations require installation at a skew, Figure 6.3 depicts a series of alignment options. Following the current channel form will require a longer culvert. Straightening the channel will shorten the crossing but require channel protection. Creating a wider crossing will provide a slight buffer for channel migration but may also significantly increase material and construction costs (Bates et al. 2006). Also, the aligned culvert might result in the greatest direct habitat loss, and the perpendicular culvert might result in the greatest overall disturbance and the greatest risk due to the skew of the culvert to the stream. For locations at skews or bends, all three options should be considered, and the final design is often a combination of the three.
Treatments recommended for minimizing culvert length include adding headwalls, steepening embankments, and narrowing and lowering the road (Bates et al. 2003; Maryland State Highway Administration 2005). Specifications for such options are included in HDS-5 (Norman et al. 2005).
6.3 Design Approaches
6.3.1 No Impedance
DEFINED - No Impedance - Crossing design produces no impedance to aquatic organism passage by spanning both the channel and floodplain.
Aside from road removal or relocation, bridges provide optimum biological, geomorphic and hydraulic connectivity (Robison et al. 1999). Often bridges will be more expensive to install and have shorter effective lives than culverts (Venner Consulting and Parsons Brinkerhoff 2004). The No Impedance procedure will not be described further
6.3.2 Geomorphic Simulation (Category 1)
DEFINED - Geomorphic Simulation approaches are based on recreating or maintaining natural stream reach geomorphic elements including slope, channel-bed width, bed materials, and bedform.
The basis of these methods is the presumption that crossings matching natural conditions will readily pass fish that are moving in the natural channel. For this reason, analysis of fish passage flows is not required. Design methods are based on a reference reach (see Glossary). Geomorphic Simulation is also known as Stream Simulation (Bates 2006; WDFW 2000). This method has expedited regional permitting in some regions of the country (Bates, Personal Communication).
220.127.116.11 Biological Characteristics
Successful installations should pass fish, debris, and sediment at rates very closely resembling the natural stream reach. Geomorphic Simulation assumes passage is provided for all fish species and life stages moving through the natural channel for all flows at which they are moving. Culverts spans wider than the bankfull width can provide dry bank margins that can serve to provide passage for aquatic and terrestrial organisms.
18.104.22.168 Geomorphic Characteristics
To allow natural processes to occur within the culvert, the crossing slope must remain close to that of the natural channel. A review of such culverts in Washington State found that installations remaining within 25% of natural channel slope successfully replicated natural channel conditions (Barnard 2003). New open bottomed and embedded installations can be placed to minimize disturbance of bed material, or laid below grade and backfilled with natural material to maintain natural channel grade.
Geomorphic Simulation creates wide spanning culverts that exceed channel bed width. For example, in Washington, Barnard found that these structures should be 1.3 times the channel bankfull width in order to replicate stream processes (2003). In new installations, wide spanning culverts allow crossings to maintain natural bed material.
The wide-spanning culverts and open bottom structures needed to meet such requirements will allow a slight buffer against lateral and vertical stream adjustments (Bates et al. 2006). Although success has been achieved in high gradient situations, methods simulating the natural stream have been limited to gravel and cobble beds with only a few applications in sand bed streams (Bates et al. 2006).
22.214.171.124 Hydraulic Characteristics
Geomorphic Simulation avoids the need for consideration of target species/life-stage, timing of fish migration, or fish passage hydrology. Since crossings are generally much larger than culverts designed for hydraulic capacity alone, Geomorphic Simulation will typically control design (hydraulic capacity must still be checked to meet the required headwater-flood policy).
126.96.36.199 Data Requirements
Geomorphic Simulation is illustrated in Section 7.1.
6.3.3 Hydraulic Simulation (Category 2)
DEFINED - Hydraulic Simulation techniques utilize embedded culverts, natural or synthetic bed mixes, and natural roughness elements such as oversized rock, to provide hydraulic conditions conducive to fish passage. These techniques operate on the assumption that providing hydraulic diversity similar, but not identical, to that found in natural channels will create a fish passable structure without checks for excessive velocity or turbulence. Many techniques are based on regional design experience.
Regardless of specific criteria, Hydraulic Simulation will generally create smaller spanning structures that have a reduced capital cost but higher maintenance requirements (debris removal) when compared to Geomorphic Simulation.
188.8.131.52 Biological Characteristics
By creating a crossing that resembles natural stream slope and substrate, passage is assumed adequate for fish in the stream reach. This assumption is often based on regional experience and project monitoring (Alaska Department of Fish and Game and Alaska Department of Transportation 2001; Maryland State Highway Administration 2005; Robison et al. 1999; Miles, Personal Communication; Browning 1990). In Alaska, experience has found that culverts following Hydraulic Simulation, they call it "Stream Simulation," criteria adequately pass fish, and permitting has been expedited (Alaska Department of Fish and Game and Alaska Department of Transportation 2001). Techniques developed by Maryland State Highway Administration (2005) and Browning (1990) check channel velocities for compliance with local stream flows. Although structures aren't specifically oversized to provide stream bank margins, low flows may provide dry bank areas that will allow terrestrial organisms to pass (Miles, Personal Communication).
184.108.40.206 Geomorphic Characteristics
Hydraulic Simulation creates hydraulic roughness, low flow paths, and resting areas conducive to fish passage by utilizing natural or artificial bed material (Robison et al. 1999; Browning 1990), or oversized substrate that remains stable during design floods (Alaska Department of Fish and Game and Alaska Department of Transportation 2001). Bed structures and key pieces are used to create flow diversity and resting areas, ideally matching bed characteristics of the natural channel.
Culvert span is generally close to or slightly less than bankfull (Alaska Department of Fish and Game and Alaska Department of Transportation 2001; Browning 1990; Robison et al. 1999; Maryland State Highway Administration 2005), allowing sediment and debris flow to continue through the crossing at flows up to bankfull. Substrate does not necessarily mimic stream reach substrate and form as in Geomorphic Simulation.
Some Hydraulic Simulation approaches create a stable channel within the culvert (i.e. Alaska Department of Fish and Game and Alaska Department of Transportation 2001; Bates et al. 2003). In such a case, bed load and suspended load still move through the culvert, but foundation bed material is not scoured out at high flows (i.e. a 50-yr event). This requires less flow area within the culvert barrel, as higher flows can pass through the culvert without scouring the bed material (Miles, Personal Communication).
In situations where a mobile bed is created, or allowed to develop within the crossing, sediment and debris movement is similar up to bankfull flows. Bed material can be washed out during a flood event, leaving a bare culvert, and, without upstream grade control, lead to upstream progressing channel incision. Recruitment may replace material that is scoured out, but it cannot be relied upon to do so. Regardless of bed stability, fines must be part of the bed material mixture to seal voids and avoid flows going subsurface, which would create a low flow barrier.
220.127.116.11 Hydraulic Characteristics
Culvert spans designed for Hydraulic Simulation are generally very close to, or slightly less than, bankfull width. Methods that call for increased bed sizing and roughness will decrease flow velocity but increase turbulence.
Hydraulic capacity must be checked to ensure adequacy.
18.104.22.168 Data Requirements
Channel and valley type (Section 6.5.2)
Hydraulic Simulation is illustrated in Section 7.2.
6.3.4 Hydraulic Design (Category 3)
DEFINED - Hydraulic Design techniques create water depths and velocities that meet the swimming abilities of target fish populations during specific periods of fish movement. General considerations include the effect of culvert slope, size, material, and length. Flow control structures such as baffles, weirs, formal fishways or oversized substrate are commonly utilized to create adequate hydraulic conditions.
Hydraulic Design is most applicable to retrofits, but can be used for new and replacement culverts. This technique generates a smaller diameter culvert, while still meeting fish passage criteria including leap height, average cross-sectional velocity, flow depth, and drop height. Hydraulic Design is specifically tailored to meet target fish species requirements, but produces a less connected design than Geomorphic or Hydraulic Simulation. These designs are applicable for slopes up to 5% (Robison et al. 1999; Bates et al. 2003; Katopodis 1992).
22.214.171.124 Biological Characteristics
Hydraulic Designs have been shown to aid in upstream migration by providing resting pools, low velocities, and deep flow (Gregory et al. 2004). These techniques utilize the swimming abilities of target fish populations in order to develop hydraulic criteria necessary to ensure fish passage. The target fish species and lifestage should be determined through consultation with fisheries biologists, and will generally focus on the weakest swimming fish known to require passage during specific periods of fish movement. Designs to meet specific hydraulic criteria are likely to constrict flow, disrupt ecosystem connectivity, and require a more rigorous design and permitting process than geomorphic or Hydraulic Simulation (i.e. Alaska Department of Fish and Game and Alaska Department of Transportation 2001; Bates et al. 2003). Hydraulic Design does not account for ecosystem requirements or the movement of non-target species.
126.96.36.199 Geomorphic Characteristics
Hydraulic Design is applicable over a range of slopes. Installations on mild slopes may create fish passable conditions without grade control structures, while moderately sloped (1-3.5%) installations and retrofits may require weirs or baffles to attain fish passable conditions (Bates et al. 2003; Alaska Department of Fish and Game and Alaska Department of Transportation 2001).
The structures created by Hydraulic Design are more likely to affect flow through and around the structure than those designed by Geomorphic or Hydraulic Simulation. Localized aggradation and degradation due to channel constriction may have to be addressed (Castro 2003), and regular debris maintenance is generally required for Hydraulic Design culverts. This can be especially important in retrofit situations where structure modifications, such as baffles or weirs, have the propensity to catch and hold debris, increasing the risk of debris clogging (Bates et al. 2003).
188.8.131.52 Hydraulic Characteristics
Low and high fish-passage flows must be determined to ensure that hydraulic criteria are met during periods of fish movement (Chapter 5). This requires knowledge of the times of the year and flow regimes at which fish move within the natural channel. In new installations, fish passage considerations will generally control structure size, but flood conveyance must still be checked. Smaller diameters, especially when combined with the effects of baffles, or other roughness elements, can restrict passage of water and debris through the culvert, decreasing the flood flow capacity while increasing the likelihood of plugging and culvert failure.
184.108.40.206 Data Requirements
Hydraulic Design is illustrated in Section 7.3.
220.127.116.11 Further Considerations
This design approach is often recommended as a last alternative, when other possibilities are found to be unfavorable (Alaska Department of Fish and Game and Alaska Department of Transportation 2001; Bates et al. 2003; Flosi et al. 1998; Robison et al 1999; Maine Department of Transportation 2004). In Washington for example, design guidelines recommend that use of Hydraulic Design be limited to culvert retrofits, producing inexpensive, short-term, benefits until the crossing can be replaced (Bates et al. 2003).
Baffles have a much larger failure rate than other techniques. They are prone to clogging, and are difficult to prefabricate as settling may cause the baffles to pop out leading to damage to the culvert itself and to culvert failure (Robison et al. 1999; Gardner 2006). Hydraulically designed structures will have a shorter design life, increased maintenance needs, and a more intensive permitting process than Geomorphic or Hydraulic Simulation culverts.
6.4 Design Selection
The selection of an appropriate design technique will be the result of project goals and the design techniques applicable to a particular situation or region of the country. In Chapter 7, design techniques from across the country are explained within the context of the design categories listed above. Design examples are included in the Chapter 8 to further clarify the design process.
A first step in the decision process is to verify the necessity of a road crossing. Abandonment or removal of a crossing may be a plausible and desirable solution for fish passage problems, especially on forest land where road use is intermittent or logging and fire traffic can be rerouted with little consequence (Robison et al. 1999). Temporary structures and fords might also be considered.
It is recommended that State DOTs meet with State natural resource agencies and appropriate federal agencies (for example, U.S. Army Corps of Engineers) to discuss these methods for general applicability for a region.
Agreements between State Departments of Transportation and Resource agencies can greatly expedite the design and permitting process, ensuring that the requirements of all parties are met satisfactorily through a common vision. For example, Alaska and Oregon currently have agreements between their respective resource agencies aimed at more timely approval of permit applications for culvert installations, and recognizing the priority of replacement/repair of current fish passage barriers (Venner Consulting and Parsons Brinkerhoff 2004).
A comparison table of the design categories is presented in Table 6.1.
6.4.1 Summary Matrix of Design Approaches
6.5 Analysis Tools and Software
Analysis tools and computer software can be useful in the design of fish passable structures. The programs/websites are recommended or specified for use by many design/assessment documents. In addition, reference to Table 1.2 will be helpful for accessing other pertinent FHWA publications.
6.5.1 Channel Geometry
18.104.22.168 Channel Width
The correct determination of channel width is an important prerequisite for many of the design techniques described in this report. Width measurements should describe stable, straight channel conditions between bends and outside the influence of a culvert or an artificial or unique constriction (Bates et al. 2003). Two common design parameters include bankfull width and active channel width. Bankfull width is the result of geomorphic processes, while active channel width is more related to an ordinary discharge. In entrenched and non-adjustable systems bankfull and active channel width may be very similar, while evaluation in other areas, such as meandering valley streams, might show great discrepancies (Bates et al. 2006).
Active Channel Width:
22.214.171.124 Channel Profile and Condition
It is extremely important to understand structure impacts on the channel over time including incision, scour, headcut and regrade (Bates et al. 2006). This requires an accurate survey of the longitudinal profile (River and Stream Continuity Partnership 2004). A longitudinal profile should include the culvert site and 20 channel widths or a minimum of 61-91 m (200-300 ft) up- and downstream of the structure (Castro 2003; Bates et al. 2003). Recent experience shows this distance to be a minimum. Maryland DOT surveys 152 m (500 ft) on each side of the culvert (Kosicki, Personal Communication 2007). This will allow an understanding of the final channel bottom elevation resulting in the vicinity of the replacement structure, ensuring proper invert elevations, embedment, and slope. A good survey is also useful in assessing the potential for downstream flooding, alteration of upstream and downstream habitat, potential for erosion and headcutting, and stream stability in general (River and Stream Continuity Partnership 2004).
Channel Incision, Headcut and Regrade:
In situations where a current culvert installation is acting as a control point, removal, replacement with a larger structure, or lowering may allow channel incision to progress upstream uncontrollably, or until another control point is reached. Regrade will be more immediate and pronounced in sand bed streams (Bates et al. 2003). Stream reaches actively aggrading or incising will cause Geomorphic Simulation culverts to be ineffective, and Hydraulic Design or Simulation incorporating channel grade controls (Section 126.96.36.199) may be more suitable.
6.5.2 Stream Classification
Classifying a stream containing a crossing or the site of a crossing impels the design team to collect meaningful data and discuss the stream dynamics before choosing a design procedure.
Systems for stream classification are useful tools in building awareness of stream form and function. Methods describe the channel in terms of cross-sectional shapes, morphological parts of the stream, and interactions between flow and sedimentation (Bunte and Abt 2001). The following section is intended to introduce the user to popular methods in stream classification and geomorphology, but is not sufficient for structure design. Coordination with a local geotechnical engineer and geomorphologist is necessary for ensuring structure performance. For more information it will be useful to examine references included below and Hydraulic Engineering Circular No. 20 (Lagasse et al. 2001).
188.8.131.52 Montgomery and Buffington
Montgomery and Buffington created a stream classification system based on channel systems in the Pacific Northwest that is applicable to similar regions elsewhere. Their methodology follows changes in channel morphology as steep headwater streams run through steep valleys and hillslopes, gentle valleys, and eventually low gradient valleys (Bunte and Abt 2001). As water flows to the ocean, channel types generally transition from cascade, step-pool, plane bed, pool-riffle and dune-ripple, as shown in Table 6.2. Channel bedform is described by the type and size of sediment, sediment transport capabilities, and hydraulic conditions within a stream reach. Table 6.2 from Bunte and Abt summarizes this classification system with respect to channel geomorphic and hydraulic conditions.
Values in parentheses are the modes of the observed stream gradient distribution; * in terms of channel widths
A reach-scale categorization allows streams to be categorized based on relative positions within the watershed and sediment transport characteristics. This type of analysis is useful in understanding the potential response of a channel reach to a crossing installation. Montgomery and Buffington define reach level morphologies as source, transport and response reaches (Montgomery and Buffington 1993).
Source reaches contain as much or more sediment than the stream can transport. Transport reaches are high gradient supply-limited channels, which are unlikely to respond quickly or severely to disturbance. This includes bedrock, cascade and step-pool channels. Response reaches are lower gradient transport-limited channels with a high potential for morphological adjustment in response to sediment input. This general classification covers plane-bed, pool-riffle and braided channels. The transition from transport to response reach is where the impacts of increased sediment supply will have the largest impact, as sediment supplied by the transport reach will readily settle out at the first reach that cannot maintain sediment transport capacity (Montgomery and Buffington 1993).
A crossing location within a particular reach, as well as the proximity of other reaches will help a designer ascertain the potential impacts and geomorphic response of the stream. Crossings that fall at the intersection of two different channel types, for example, could indicate channel incision, or that the crossing is located at a point of geomorphic transition (Bates et al. 2006). Crossings placed in a response reach typically will require extra consideration of channel processes and morphological impacts (channel aggradation and lateral movement).
184.108.40.206 Rosgen Stream Classification
Rosgen channel classification is based on five morphometric parameters of the channel and its floodplain: entrenchment ratio, width-depth ratio at bankfull flow, sinuosity, stream gradient and mean bed particle size (Rosgen 1996). These characteristics are used to distinguish seven stream types, represented by capital letters A to G. Table 6.3 lists the morphological characteristics of Rosgen's stream types.
Channels can be further distinguished using numbers to represent bed material and particle size, and lowercase letters to represent deviation from expected channel slopes. For example, a stream classified as C4b is a C-type stream with a gravel bed and gradient within the range of 0.02-0.039, which is more typical of a B-type stream (Rosgen 1994). Accurate classification requires a longitudinal and cross-sectional channel survey and sediment sample analysis.
220.127.116.11 Summary of Channel Classification
All stream classification systems can be useful in understanding basic channel reach geometry and dominant geomorphic processes. This can be valuable in predicting channel response to modification or culvert replacement. Certain channel types can carry specific design challenges. For example, risk of floodplain constriction and/or lateral adjustment is associated with Rosgen C, D and E channels (Bates et al. 2006). As mentioned above, plane bed, pool-riffle, and dune-ripple channels are associated with response reaches, and are likely to show the most dramatic response to disturbance (Montgomery and Buffington 1993).
For further discussion of stream classification and applicability to channel crossing design, it is useful to review the original documents by Rosgen (1994; 1996), Montgomery and Buffington (1993; 1998), Bunte and Abt (2001), and Bates(2006). It is important to note that these design techniques or classification systems are not well tested outside the regions for which they were created. Installations in low gradient, highly mobile sand bed streams may require special consideration.
6.5.3 Stream Morphology
As a rigid structure in a dynamic environment, culverts must be designed with channel processes in mind. Effective designs consider the channel and watershed context of the crossing location. Channels are continually evolving, and an understanding of stream adjustment potential must be addressed. Without consideration, well intended plans could have detrimental or completely ineffective results/impacts on the stream system and related habitat (Castro 2003; Furniss 2006).
Past channel degradation can require channel modification, or considerations of the impact of increased slope on channel stability, substrate and future conditions (Robison et al. 1999; Bates et al. 2006; Bates et al. 2003). A true Geomorphic Simulation can only be completed when culvert bed slopes very closely match the slopes of the adjacent stream channel. Oversized sediment utilized in Hydraulic Simulations provides more leeway with regards to stream slope, but also require that crossing slopes be close to the adjacent channel.
18.104.22.168 Bed Material and Embedded Culverts
The benefits of natural streambeds and embedded culverts are widely recognized in fish passage applications (e.g. Venner Consulting and Parsons Brinkerhoff 2004; Bates et al. 2003; Taylor and Love 2003; Clarkin et al. 2003). Bed material provides barrel roughness, which provides areas of low velocity that may be conducive to fish passage, mimics natural hydraulics, and is self sustaining when designed properly (White 1997).
22.214.171.124 Key Roughness Elements
In order to provide fish migration paths and resting areas many design techniques utilize key roughness elements to create diversity in flow velocity, depth, and energy dissipation (Robison et al. 1999; Bates et al. 2006; Browning 1990). Key roughness elements describe any number of materials that can be used to provide hydraulic roughness and diversity to a crossing including oversized substrate, constructed channel features including banks, stone sills, boulder clusters, log sills, and baffles. Such features are intended to increase bed stability and provide resting areas and hydraulic diversity conducive to fish passage.
126.96.36.199 Subsurface Flows
Crossings that are filled with a coarse simulated bed mix may allow low flows to seep between rocks - and move solely in the subsurface - until interstitial spaces have been sealed with fine particles. To limit streambed permeability, an appropriate proportion of fine material must be included in the bed mix (5-10%) (United States Forest Service 2006a; Bates et al. 2006). During channel construction, placement of a sediment barrier fabric, mud or straw wattles (Browning 1990; Gubernick, Personal Communication), or washing fines into the streambed during construction can effectively seal the voids (Bates et al. 2006).
6.5.4 Estimating Roughness with the Manning Equation
Estimating roughness with the Manning equation (Equation 6.1, Chow, 1959) is most often used to estimate uniform flow depth given a design discharge:Equation 6.1
K = 1.0 for SI and 1.486 for CU units, respectively
Of primary importance is to determine the Manning's "n" value, or channel roughness coefficient, for low and high values of discharge and for flood discharges. This will govern sediment stability and the hydraulic properties within the culvert barrel. Estimates for Manning's "n" may be found in HEC-20 (Lagasse et al. 2001), Chow (1959) or as determined at U.S. Geological Survey stream gage sites (Barnes 1967). For coarse streambed material, the procedures described in Chapter 7 use the following specialized equations.
188.8.131.52 Limerinos Equation
K = 0.1129 for SI and 0.0926 for CU units, respectively
184.108.40.206 Jarrett's Equation
K = 0.32 for SI and 0.39 for CU units, respectively
220.127.116.11 Mussetter's Equation
1.49R0.17/(n)(g)0.5 = (8/f)1/2 = 1.11(y/D84)0.46 (D84/D50)-0.85 Sf-0.39
y is the mean depth, ft
Note: If Equation 6.4 is used in metric units, the constant 1.49 will equal 1.0.
6.5.5 Bed Mobility
The design engineer must understand the basic concepts of particle sizing and stream stability in order to specify a sediment mixture for the proposed culvert that is appropriate for the selected design approach. For example, Geomorphic Simulation seeks to mimic the natural streambed sediment mixture, while Hydraulic Design usually uses a coarser mix of sediment in the culvert barrel than found in the adjacent stream channel. Nine methods are here presented to assist the engineer in determining the stability of the streambed within the culvert barrel. These are taken from the USFS Stream Simulation design, the WDFW Stream Simulation design and the Roughened Channel design, as illustrated in sections 7.1.1, 7.1.2 and 7.3.4, respectively. Consultation with team members familiar with these procedures will be beneficial. These tools may be used to test the sensitivity of substrate mixes to entrainment.
18.104.22.168 Simulated Streambed Design
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 (6.5).
Fuller-Thompson equation:Equation 6.5
d = particle size of interest, mm (ft)
This equation can be rearranged to find any particle size, for example:
D16 = 0.321/nD50
D5 = 0.101/nD50
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 (6.5). This is based on D50, and creates a simulated bed mix. (This application has not been field tested, and professional judgment is recommended).
22.214.171.124 Modified Shield's Equation
The modified Shield's equation is used to determine particle stability based on critical shear stress. Particle stability is compromised when boundary shear stress in the channel is greater than a critical stress threshold. Boundary shear stress is calculated using Equation 6.6.Equation 6.6
τ = γRS
τ = average boundary shear stress, Pa (lb/ft2)
Once boundary shear stress has been calculated, a critical stress threshold is calculated using Equation 6.7.Equation 6.7
τci = τ*D50 (γs - γ)Di0.3D500.7
τci = critical shear stress at which the sediment particle of interest begins to move, Pa (lb/ft2)
a equation used to determine Shield's parameter for gravel-, cobble-, and boulder-sized particles: τ* = 0.06 tanΦ
126.96.36.199 Critical Unit Discharge Approach
This approach is based on unit discharge, and a value of critical unit discharge will be compared to channel unit discharge to determine particle entrainment (particle lifting into flow).
Equation 6.8 is used to calculate channel unit discharge.Equation 6.8
q= Unit discharge, m3/s/m (ft3/s/ft)
Equation 6.9 is used to predict the entrainment of the particle size of interest.Equation 6.9
qc-D50= the critical unit discharge to entrain the D50 particle size, m2/s (ft2/s)
More generally,Equation 6.10
qci= the critical unit discharge to entrain the particle size of interest, m2/s (ft2/s)
D84= the 84th percentile particle size, mm (ft)
188.8.131.52 Boundary Shear Threshold Analysis
τci-u = 0.0814DiEquation 6.13
τci-i = 0.00355Di
τc-u = is the upper critical shear stress value (lb/ft2) for determining particle mobility and immobility for the particle size of interest.
If the average shear stress (τ) is greater than the upper critical shear stress (τc-u), the particle will be mobile at this flow. If the average boundary shear stress (τ) is less than the lower critical shear stress (τc-i), then the particle will be immobile for these flow conditions. If the average boundary shear stress is between the upper and lower critical shear stress values, then the particle has potential to move at these flow conditions.
184.108.40.206 U.S. Army Corps of Engineers Riprap
The U.S. Army Corps of Engineers has developed two riprap models for designing riprap bank protection. These were developed through laboratory and analytical work, and consider angular rock, which is resistant to sliding and rolling. Note that round rock may have to be significantly larger than angular rock to achieve similar levels of stability (Abt, 1988).
Manuals are available at http://www.usace.army.mil/publications/eng-manuals/em1110-2-1601.
For slopes from 2-20%Equation 6.14
D30= dimension of the intermediate axis of the 30th percentile particle, m (ft)
1.25 is a safety factor that may be increased, and designers are cautioned against using this method for rock sizes greater than 0.15 m (6 in).
The U.S. Army Corp of Engineers recommends angular rock with a uniform gradation (D85/D15 = 2). This is not preferred for fish passage situations due to porosity issues. An approximate factor for scaling D30 of a uniform riprap gradation to one that is appropriate for stream channels is 1.5, so that:Equation 6.15
D84 = dimension of the intermediate axis of the 84th percentile particle, m (ft)
220.127.116.11 Reference Reach Approach
Maximum particle size and appropriate sediment size distribution can be determined by examining reaches directly upstream from the culvert, or nearby reaches with similar characteristics to the design channel (e.g. unit discharge, slope, geometry and relative stability).
18.104.22.168 Unit-Discharge Bed Design
J.C. Bathurst developed the following equation to predict the size of D84 particles that would be on the threshold of motion for a given critical discharge in high gradient streams with heterogeneous beds (1987).Equation 6.16
D84 = 3.54S0.747(1.25qc)2/3/g1/3
D84 = intermediate axis of the 84th percentile particle in the sediment distribution, m (ft)
This is recommended as a starting point for development of sediment mixes in high gradient streams. Two design categories are recommended based on slope.
These methods generally agree, but should both be checked. These are mobile or nearly mobile particles at these flows. If it is advisable to create a bed that is more stable, particle sizes should be increased. If bed slope approaches or exceeds 1.25 times the natural reach slope, it may not be possible to simulate stream conditions, and a Hydraulic Simulation approach or a Hydraulic Design approach, such as Roughened Channel may be considered (Section 7.3.4).
22.214.171.124 Paleohydraulic Analysis
Paleohydraulic analysis uses the maximum particle size and flood depth to determine the discharge of flash floods. An equation developed by Costa (1983) to understand velocity based on particle size is useful in substrate sizing for stream channel design. Users should consult Costa (1983) to supplement their understanding of this procedure. This equation and the accompanying table (6.5) are in customary units.
For determining depth, velocity (ft/s) is given by:Equation 6.17
V = 9.57(D84)0.487
D84 = is arrived at by an iterative procedure, ft
At higher slopes, the Costa equation predicts smaller particle sizes than the Bathurst equation, all other conditions being equal (Bates et al. 2003).
126.96.36.199 Critical Shear Stress Method
Critical shear stress methods are used to estimate the initial movement of particles. Particles movement occurs when the maximum shear stress, τ0max, within the channel exceeds a calculated critical shear stress, τc. Critical shear stress is the shear stress required to cause movement of a given particle size (see Sections 188.8.131.52 and 184.108.40.206). The maximum shear stress is 1.5 times γRS, where γ is the unit weight of water, R is the hydraulic radius, and S is the slope. Data used to derive these equations are largely from low-gradient situations, although design charts show slopes up to 10% and particle sizes up to 0.58 m (1.9 ft) (Bates et al. 2003).
6.5.6 Countermeasures for Channel Instability
As a rigid structure in dynamic environment, culverts may require consideration of riprap and channel modification to address scour and channel degradation or incision (Bates et al. 2003; Robison et al. 1999; Maryland State Highway Administration 2005). An undersized culvert will destabilize the adjacent stream reach. A number of alternatives are available to protect the impacted channel. Modification of the channel both up- and downstream of the structure can decrease the slope required at the culvert installation, helping to meet velocity, gradient and embedment requirements.
220.127.116.11 Scour Countermeasures
Energy Dissipation Pool:
18.104.22.168 Channel Modifications
Downstream channel modifications may be necessary to ensure proper culvert backwatering or to control crossing slope. Upstream channel modification can include erosion or grade control structures (detailed below), or a tapering of channel banks to smooth out the impacts of an inlet constriction (Robison et al. 1999). Such grade controls are frequently an element of a cost-effective retrofit; they are also used on replacement projects. A number of techniques for channel modification are included in Table 6.6.
22.214.171.124 Roughened Channel
Roughened channels can be constructed within the natural channel to control channel shape, slope and form. This may be especially pertinent in areas where past degradation causes a culvert installation to be placed at a severe slope. Methods and equations used in the design of roughened channels can be found in Section 7.3.
126.96.36.199 Grade Control Structures
Grade control structures may be necessary upstream or downstream of a culvert to control longitudinal profile and water surface elevations. Downstream of a culvert these installations typically backwater the culvert and stabilize steepened reaches. Figure 6.7 depicts the placement of downstream grade control. Such structures have been shown to cause problems with fish passage (Browning 1990), and a clearance of 20 ft between the culvert outlet and the first downstream control is recommended (Bates et al. 2003; Robison et al. 1999). Upstream of a culvert, grade control is used to stabilize a reach and protect against current or future headcutting.
This type of structure, depicted in Figure 6.8, should end no closer than 35-50 ft from the culvert inlet (Bates et al. 2003).
188.8.131.52 Tailwater Control
It may also be necessary to raise the tailwater elevation in order to backwater the culvert and provide minimum flow depths. Sometimes this is all that's required to retrofit a flat, short culvert. Many methods are available including:
Flow over weirs can create velocity and depth barriers, and it may be necessary to design a series of weirs to provide fish passage and backwatering the culvert.
184.108.40.206 Broad Crested Weirs
The Maine Department of Transportationdescribes the following method for the design of a rectangular notch weir-Broad Crested Weir (Maine Department of Transportation 2004). This is a channel-spanning structure at the culvert outlet, which can be used to ensure proper water surface elevation and backwatering. When the drop over a weir will create a barrier to fish passage, it will be necessary to include further control structures to create a series of manageable step pools while maintaining adequate culvert backwater. A series of notch weirs is depicted in Figure 6.9.
Design Procedures are as follows:
At first pass, the weir height can be set at the desired water height (ignores the depth of flow over the weir).Equation 6.18
Q = Cd(2/3)(2g/3)1/2bch13/2
Cd = discharge coefficient (0.9 assumed)
Solving for h1Equation 6.19
h1 = [Q/(Cd(2/3)(2g/3)1/2bc)]2/3
(Note the assumption 0.9 is in view of the uncertainty and variability in the weirs contemplated here.)
Flow over the weir will be critical, and velocity (vc) must be checked for fish swimming ability:Equation 6.20
vc = (gh1)1/2
vc = critical velocity, m/s (ft/s)
This procedure uses constructed materials. Consultation with appropriate State and Federal agencies should occur to determine the acceptability of this design.
Channel regrade promoted by an undersized culvert installation can be a concern with culvert replacement or removal. Grade control structures can be used up and/or downstream of the structure to help protect against catastrophic channel regrade.
6.5.7 Culvert Shapes and Standards
A number of culvert shapes are available to meet the specific needs of a culvert site. Selection will be the result of site conditions including depth of cover, limited allowable headwater elevations, clogging potential, need for natural stream bottom, or structural and hydraulic requirements (Ballinger and Drake 1995). Common shapes for fish passage design include round and elliptical pipes, box culverts, and open-bottom arches. All types of culvert shapes have been used for fish passage, and selection is likely the result of site conditions and personal preference (Bates et al. 2003). Table 6.7 is a collection of noted advantages and disadvantages of culvert shapes and materials.
Corrugated metal culverts are commonly used in fish passage design. These structures provide boundary roughness that may be conducive to fish passage (Powers et al. 1997; Barber and Downs 1996; Behlke et al. 1989), as well as aiding in retention of bed materials (Bates et al. 2003). Culvert embedment is also commonly called for, with some exceptions in hydraulically designed culverts. When new installations utilize natural bed material, bottomless structures have the advantage of allowing natural substrate to remain in place.
Standards for bridges, culverts, foundations and backfill can be found in "Standard Specifications for Highway Bridges, 17th edition" (AASHTO HB-17, AASHTO, 01‑Sept, 2002).
6.5.8 Simulation Software
Several computer programs exist to assist the engineer in the design process. FishXing is most often used to assess culverts for fish passability and is often used in conjunction with FishBase. HY-8 v. 7.0 is used to analyze the detailed hydraulics of culvert flow and for design. HEC-RAS is used for design on larger rivers and for culverts in series where the water surface elevation of one culvert is affected by another. All of these programs predict average cross-sectional velocities within the culvert barrels.
FishXing (pronounced "fish crossing") is a fish passage analysis tool developed by the United States Forest Service. According to product description, FishXing provides the following features (United States Forest Service 2006a):
This software is free and available for download at http://www.stream.fs.fed.us/fishxing/.
Noted limitations include:
This program has been recommended as a first cut analysis tool, but for concrete prioritization, design or analysis site visits and analysis should be completed (Cahoon et al. 2005). Analysis with field assessment and study has found FishXing to match results between 71-100% of the time (Rajput 2003; Cahoon et al. 2005). A powerful use for FishXing is in a culvert assessment of "indeterminate" designated crossings. The software may be able to move a designation to "passable" or "impassable."
FishBase is a searchable relational database catering to different professionals including research scientists, fisheries managers, zoologists and many more. It contains information on over 28,500 fish species, including pictures, data on swimming speeds, distribution, biology, and references. It is available on CD or on the web at http://filaman.ifm-geomar.de/home.htm.
220.127.116.11 HY-8 v. 7.0
The HY-8 v. 7.0 Culvert Analysis program was developed by FHWA in order to automate some of the information contained within HDS-5, "Hydraulic Design of Highway Culverts," HEC-14, "Hydraulic Design of Energy Dissipaters for Culverts and Channels," and HEC-19, "Hydrology." It is intended for hydraulic capacity design, but is useful in evaluating design flood stability, scour potential, and culvert barrel velocity. Maryland suggests the use of other programs for the calculation of tailwater rating curves (Maryland State Highway Administration 2005).
This software is free, and available for download at http://www.fhwa.dot.gov/engineering/hydraulics/software/softwaredetail.cfm.
Noted limitations include:
The Hydrologic Engineering Center River Analysis System (HEC-RAS) is a river modeling program developed by the U.S. Army Corps of Engineers. HEC-RAS can be used to perform hydraulic calculations for a full network of natural and constructed channels. Users have the ability to place culverts within channel context and perform analyses of one-dimensional steady and unsteady flow. The steady flow component is capable of modeling subcritical, supercritical and mixed flow regimes, while the unsteady flow component was developed primarily for subcritical flow calculations.
HEC-RAS is free and available for download at http://www.hec.usace.army.mil/software/hec-ras/hecras-download.html.
Noted limitations include:
18.104.22.168 Commercial Programs
There are many commercial programs available for analysis and design of culverts, but their applicability has not been evaluated for this publication. A short discussion of many of these programs is available in Environmental Stewardship Practices, Procedures, and Policies for Highway Construction and Maintenance. Final Report for NCHRP Project 25-25, Task 4, National Cooperative Highways Research Program Transportation Research Board (Venner Consulting and Parsons Brinkerhoff 2004).
The FishPass Website is a sponsored project of the Bioengineering Section of the American Fisheries Society. FishPass is a forum for professional discussion of the biological and engineering science of upstream and downstream fish passage. Areas of discussion include fish passage technologies, projects, swimming capabilities and behavior and biological and engineering studies and events.
Subscription details are available at http://www.fishpass.org.