Fish Passage in Large Culverts With Low Flow

CHAPTER 3. METHODOLOGY AND ANALYTICAL FORMULATION

At the flood flows for which culverts are typically designed, knowing the flow depth and average velocity within the culvert and in the stream channel are generally sufficient for design at these higher discharges.⁽¹⁸⁾ However, at lower flows of interest when considering fish passage, the velocity distribution within the culvert may become relevant for design. The objective of this fish passage research is to assist in the design of culverts by identifying zones of lower velocity during low flows that are conducive to fish passage and to develop practical design methods based on quantifying these lower-velocity zones. The focus of the study is corrugated metal pipes.

To accurately evaluate the ability of specific fish with specific swimming capabilities to traverse corrugated metal culverts, it is desirable to examine the velocity distribution within the culvert flow field to identify zones of lower velocity adjacent to the culvert wall under low flow conditions. Other studies have documented the tendency of fish to seek out a swimming location with the lowest velocity.^(19,20)

This study addresses low flows in culverts as illustrated in figure 1. For low flows, a fraction of the total barrel opening is carrying water, and the flow depths are shallow compared with the culvert diameter. As with high flows, the low flow condition may be characterized by its discharge Q, average velocity V_a, and maximum depth y_max.

Figure 1. Illustration. Culvert water surface at low flow

Figure 2 illustrates the variation in depth-averaged velocity and depth in a circular cross-section. Typically, the local depth-averaged velocity (V₁, V₂, V₃, etc.) will approach zero at the culvert wall and will be at a maximum near the center of the culvert cross-section. The depths y₁, y₂, y₃, etc. will also vary as shown in the figure.

Figure 2. Illustration. Variation of velocity and depth in a circular cross-section

Recommendations for a required minimum depth (y_f) and maximum allowable velocity (V_f) are expected to vary by location, species, and life stage. Other important considerations may include the sustained and prolonged speeds of the various species for which the fish passage is being designed. The minimum depth and maximum allowable velocity will generally be provided by a team that includes an aquatic biologist familiar with fish passage needs.

The design hypothesis is that even though the average velocity in the culvert exceeds the maximum allowable velocity for fish passage, a fish path may exist where the local velocities do not exceed the allowable velocity and provide sufficient depth (and width) for passage. Figure 3 illustrates this concept by showing a path on both the left and right sides of the symmetrical cross-section where hydraulic conditions are favorable for passage. As illustrated, the depth-averaged velocity in the eighth strip, V₈, is less than or equal to V_f, while the depth-averaged velocity in the ninth strip is greater. In addition, depth in the fish path must exceed the minimum fish depth, y_f, over a sufficient width to allow passage.

Figure 3. Illustration. Fish path concept

The necessary fish path must exist during the periods at which the subject fish species/life stage is moving up and down stream for a successful design. Culvert Design for Aquatic Organism Passage recommends the use of the high and low passage flows to define the discharge range within which fish passage movement is anticipated.⁽¹⁾ The document also provides recommendations on estimating the high and low passage flow rates.⁽¹⁾

The high passage flow, Q_H, represents the upper bound of discharge at which fish are believed to be moving within the stream. At this discharge, a fish path must exist if fish passage is to be facilitated; achieving a sufficiently low velocity is typically the challenge. The culvert discharge that includes an acceptable fish path is compared with the high passage discharge. If the culvert discharge is equal to or greater than the high passage discharge, then this criterion is satisfied. If not, then the design of the crossing must be revised until the criterion is satisfied.

Similarly, the low passage flow, Q_L, represents the lower bound of discharge at which fish are believed to be moving within the stream. At this discharge, a fish path must also exist if fish passage is to be facilitated; achieving the minimum depth is typically the challenge. The culvert discharge that includes an acceptable fish path is compared with the low passage discharge. If the culvert discharge is equal to or less than the low passage discharge, then this criterion is satisfied. If not, then the design of the crossing must be revised until the criterion is satisfied.

For the foregoing design approach to succeed, the following conditions must be met:

• The necessary design criteria for the target fish species, (i.e., maximum velocity, minimum depth, and minimum width) must be available.
• The channel width and depth at the culvert outlet should provide a flow path and a flow depth sufficiently deep to facilitate the entry of fish into the culvert at the low passage flow. The downstream and upstream culvert inverts should be at or below the elevation of the bed of the outlet and inlet channel, respectively.
• The channel width and depth upstream of the culvert should transition to the width of the culvert entrance in a manner that avoids any significant contraction, drawdown of the flow, or critical flow at the culvert entrance at both the low and high passage flows.