Culverts and Fish Passage

The Problem that some Culverts Pose!

    Small tributaries are often overlooked regarding their importance in our fisheries resource! Furthermore, because of their proximity to the valley bottoms of our major water courses, they are crossed by a network of highway and roadways that require bridges and culverts. Too often in past years, has the importance’s of fish passage fit into the plans of transportation engineers and road construction supervisors.

    It has only been in recent years that our fisheries managers have grasped this dilemma and in many cases, measures to remediate the problems caused by improperly constructed culverts are taking place! In many cases, existing culverts can be modified to allow fish passage upstream. In other scenarios, older culverts are being replaced with more modern designs.

The most common problem with road culverts is the hydraulic jump created by the drop in flow on the bottom end where the water plunges down from the lip of the culvert. For small trout, this obstacle is too high to pass over. Often, when the trout negotiate the smaller elevated culverts, the velocity of the flow will immediately push the trout back down into the pool. For the high elevation drops, the height is just too much.

    The pool depth below a culvert is also important to fish passage. Trout require a pool depth of at least 1.25 times the height of the hydraulic jump, so that they can gain enough swimming speed to get a good launch.

    The photo to the right shows a small trout unable to clear the lip of a culvert on a small stream crossing. Despite numerous attempts, the small fish could not clear the jump.

    A simple fix on many culvert crossings that block passage, is to elevate the pool depth below the culvert, by installing flow constriction structures at the outflow of the pool. This creates a damming effect and raises the pool elevation to a more acceptable height. The other solution is to replace the old culvert with a larger embedded culvert with a natural streambed substrate that will withstand high flow events.

    BVHD completed the design and supervised the construction of the streambed substrate for this new culvert on a stream crossing near Hinton, Alberta. The designed substrate has withstood a few high flow events, which is evident on the markings of the culvert wall in the photo. The photo was taken three years after the project was completed!

    Under high flow events, the large boulders are submerged and woody debris is flushed down the system! The key to success using this method of culvert bed design and material is the blanket anchor around the boulders.

Blanket Anchor

When the larger boulders are seated into the new bed material of a new culvert, large cobblestones are placed around the larger boulders, creating a snug fit. This is called a blanket anchor. As long as the cobblestones are touching each other, it creates a stable blanket that holds the larger boulders in place, during high flow events.

The bed of 3/4 inch crushed gravel is placed on the bed of the new culvert and compacted. Next, the large boulders are placed into position at the right depth and then the cobble is added, surrounding the large boulders. The photo to the left shows a culvert with bed material that is just about completed. The creek is diverted past the construction site, it is on the left-hand side of the culvert.

After the construction is completed, materials will continue to move down the system annually, but the large boulders and the round stone cobble will stay in place.

The creek’s flow is diverted past the construction site using a corrugated poly pipe. The roughness factor of the pipe slows the flow down before it enters the stream channel below the site. This keeps the water flowing clean during the entire process. Bow Valley Habitat Development built and used this by-pass pipe on a number of stream habitat enhancement programs. It was also used to build pool habitats both upstream and downstream of the culvert site.

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