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Design For Fish Passage at Roadway - Stream Crossings: Synthesis Report
3 Culverts as Passage Barriers
How to use this chapter
3.1 Stream Fragmentation
Culvert installations can significantly decrease the probability of fish movement between habitat patches (Schaefer et al. 2003). Figure 3.1 depicts the possible results of ineffective roadway-stream culverts on fish populations. In the undisturbed case, fish are free to use the entire stream system as habitat. After a road interrupts stream continuity, fragmented populations are forced to survive independently. Over a short time, smaller populations are more likely to die of chance events (Farhig and Merriam 1985), but over the long-term, genetic homogeneity and natural disturbances are also likely to extirpate larger populations (Jackson 2003). Figure 3.1 shows this process sequentially from top left to bottom right.
3.2 How a Culvert can be a Barrier
A culvert becomes a barrier to fish passage when it demonstrates conditions exceeding fishes' biological ability. Common obstructions to fish passage include excessive water velocities, drops at culvert inlets or outlets, physical barriers such as weirs, baffles, or debris caught in the culvert barrel, excessive turbulence caused by inlet contraction, and low flows that provide too little depth for fish to swim. Figure 3.2, from Natalie Cabrera and the FishXing Team, depicts five common barrier types.
The severity of obstacles to fish passage compounds when a series of obstacles cause fish to reach exhaustion before successfully navigating the structure. For example, fish have been observed successfully passing an outlet drop, but having insufficient white muscle capacity to traverse a drop upon reaching the culvert inlet (Behlke et al. 1989). As noted in Chapter 2, fish swimming abilities are not cumulative, and a fish that reaches exhaustion in any category of muscle use will require a period of rest before continued movement (Bell 1986).
3.2.1 Drop at Culvert Outlet
Drops in water surface will create passage barriers when they exceed fish jumping ability. Drops can occur at any contiguous surface within the culvert, but they are most commonly seen at the culvert outlet (see Figure 3.3), where scour and downstream erosion leads to culvert perching (Forest Practices Advisory Committee on Salmon in Watersheds 2001). At existing sites, drops will need to be addressed through culvert replacement, retrofit, or channel modification, such as backwatering the culvert outlet. See Chapter 2 for examples of species-specific jumping abilities.
3.2.2 Outlet Pool Depth
Fish require a jump-pool to gain the momentum necessary to jump into the structure. Early field observations suggested that successful fish passage at falls occurs when the ratio of drop height to pool depth is greater than or equal to 1:1.25 (Stuart 1962). Aaserude and Orsborn later correlated fish passage to fish length and the depth that water from the falls penetrates the pool (1985). For practical application, jump pool requirements are generally specified based on a ratio of drop height to pool depth. Oregon, for example, uses 1.5 times jump height, or a minimum of 0.6 m (2 ft) depth (Robison et al. 1999). An adequate jump-pool neither guarantees that a fish has the ability to make the required leap, or once in the culvert, has the energy to overcome the water velocity in the culvert barrel.
3.2.3 Excessive Velocity in Barrel
Figure 3.4 depicts a culvert outlet presenting a drop and velocity barrier to fish passage. There are many categories of velocity that impact fish passage within a culvert crossing. These include boundary layer velocity, maximum point velocity, average cross-sectional velocity, and inlet transition velocity. The importance of each is discussed below.
220.127.116.11 Boundary Layer Velocity
Due to the no-slip condition in fluid mechanics, water velocity at all points of contact with the culvert is zero. The velocity increases away from the boundary, forming a so-called boundary layer. Boundary roughness increases the depth of reduced velocity. Fish have been observed to use this area to hold and rest, or swim upstream through culverts (Behlke et al. 1989; Powers et al. 1997). Investigation of the development of low velocity zones has quantified velocity reduction in round culverts for use in fish passage design (Barber and Downs 1996). However, variability in flow patterns and fish utilization are likely too great for this phenomenon to be consistently accounted for in design standards (Lang et al. 2004). To ensure passage, Powers recommended that design be based on average cross-sectional velocity - without direct considerations of roughness (1997). Although the impacts of roughness have not been directly correlated to fish passage success in the field, using corrugated pipe and large corrugations is still common practice to increase roughness and decrease boundary layer velocity (e.g. Maine Department of Transportation 2004; Bates et al. 2003; Robison et al. 1999).
18.104.22.168 Average Velocity
Average cross-sectional velocity is the most common velocity parameter used in culvert design. Although the characteristics of a fish's chosen path may not be well represented by average velocity (Powers et al. 1997; Barber and Downs 1996), little is understood about the utilization and development of boundary layers within a culvert, and average velocity represents a conservative design parameter (Lang et al. 2004).
22.214.171.124 Maximum Point Velocity
Points of maximum velocity will also occur within the culvert as water flows over or around constrictions such as weirs or baffles. While average design velocity will likely be based on a fish's prolonged swimming ability, fish may be required to use their white muscle tissue to burst through zones of maximum velocity (Rajaratnam et al. 1991).
126.96.36.199 Inlet Transition Velocity
The culvert inlet requires special consideration, as it is the last barrier for a fish traversing a culvert. Velocity at the inlet may be higher than in the barrel if bedload deposits upstream from the entrance increase the local slope. Inlet conditions are especially important in long installations, or when successful navigation through a series of other obstacles has required significant use of fishes' white muscle tissue. The addition of tapered wingwalls may significantly reduce the severity of an inlet transition (Behlke et al. 1991). Finally, a skewed entrance will produce higher entrance velocities than a non-skewed entrance.
3.2.4 Insufficient Depth
Insufficient depth can be a barrier within the culvert or on any continuous flow area before or after the culvert installation. Insufficient depth will impair fishes' ability to generate maximum thrust, increase fishes' contact with the channel bottom, and reduce the fishes' ability to gather oxygen from the water (Dane 1978). Combined, these effects reduce a fish's swimming potential and increase the risk of bodily injury and predation. Criteria for sufficient depth vary from state to state, and although species specific depth requirements can be found, it may also be desirable to provide a "fish factor of safety" (Gebhards and Fisher 1972). State criteria for fish passage depth are included in Table 3.1, and comparison with literature values will show that most criteria are conservative.
3.2.5 Excessive Turbulence
Treatments used to reduce culvert velocity or increase depth may also increase turbulence, and dissuade fish from entering or traversing the structure or confuse their sense of direction. Although little is understood about the effects of turbulence on fish passage, recent studies at University of Idaho have found that fish prefer to hold in zones of low turbulence (Smith and Brannon 2006). Washington and Maine design guidelines suggest fish turbulence thresholds, quantifying turbulence with an Energy Dissipation Factor (EDF) (Bates et al. 2003; Maine Department of Transportation 2004):Equation 3.1 (Bates et al. 2003)
EDF = γQS/A
EDF = Energy Dissipation Factor, m-N/m3/s (ft-lb/ft3/s)
Washington State suggests the EDF be less than 335 (7.0) for roughened channels, 191 (4.0) for fishways, and 144-239 (3.0-5.0) for baffled culvert installations. These criteria are based on experience in Washington, and will be modified with future research and evaluations (Bates et al. 2003). Maine DOT has similar guidelines (Maine Department of Transportation 2004).
3.2.6 Behavioral Barriers
Certain conditions at or within a culvert may dissuade fish from entering or attempting passage, even when passage is possible. These conditions include long culverts, darkness, confined culverts and shallow depths. Longer culvert installations require fish to maintain speed for extended periods of time, leading to increased energy expenditure. For this reason, maximum allowable velocity thresholds decrease with increasing culvert length (Bates et al. 2003; Robison et al. 1999). Longer culverts with natural substrate may not represent a barrier if fish can rest in reduced velocity zones.
Extreme length can also cause a culvert to be dark. Research has noted behavioral differences in light vs. dark passage of fish species (Welton et al. 2002; Kemp et al. 2006; Stuart 1962), suggesting that darkness may dissuade certain fish from entering a structure (Weaver et al. 1976). This theory has yet to be accepted as common knowledge (Gregory et al. 2004), but deserves consideration when installations require long structures. The NMFS Southwest Region, for example, requires consideration of lighting in culverts exceeding 150 ft in length (2001). It is important to consult with the appropriate natural resource agency before considering the addition of lighting to a culvert installation.
Some culverts have reduced flow areas due to excessive sediment accumulation or damage at the entrance or exit. These confined conditions may dissuade fish from attempting passage.
3.2.7 Debris Accumulation
Culverts with baffles, large roughness elements, or small diameters may have a high propensity to collect debris. This debris can include natural materials such as Large Woody Debris (LWD) and warrants specific consideration in areas where anthropogenic or natural debris accumulation is likely. The designer cannot assume that debris at culverts will be removed on a regular or timely basis. A monitoring and maintenance program can identify culverts that require more attention than others (Forest Practices Advisory Committee on Salmon in Watersheds, 2001).
Culverts can act as barriers to fish passage by presenting any number or combination of impassable obstacles. Treatments designed to treat one barrier must ensure that another is not created in the process. For example, localized treatments, such as moderately sloped aprons, may eliminate a drop, but can present a low flow or velocity barrier (Whitman, Personal Communication). Rock weirs designed to backwater a culvert may create a drop or debris barrier if not properly installed. Successful installations will consider all possible obstacles in terms of local fish requirements and crossing context.