Culvert renovation improves fish migration in Washington
The Washington State Department of Transportation (WSDOT) spearheaded a culvert replacement project as part of a long-range federally mandated “fish passage program” to restore impeded fish migration that has been an unintended consequence of outmoded state roadway stream-crossing and culvert design.
Before the culvert renovation, migrating fish traversed a constricted and fast-flowing stretch of creek caused by a 12-ft box culvert under State Route 92 in Lake Stevens, Wash. The culvert was removed and replaced with a 66-ft-wide arched bridge, which then was backfilled so that SR-92 could be reconstructed over it. The new wider creek bed with less steep sides has slowed the speed of the water flowing through it, making it easier for migratory species such as trout and salmon to pass under SR 92.
The strategy of rebuilding the creek from the ground up allowed the project design and construction team to restore Little Pilchuck Creek to its natural state. As part of the fish passage program, the project was one of many in the effort to help the state comply with a district court ruling regarding historical tribal fishing rights. As many as 1,513 fish passage barriers have been identified across the state.
Out of the three precast structure options shown on the bid drawings, the winning bid, submitted by Graham Construction, used a 66-ft-span-by-24-ft-rise Contech BEBO Bridge Concrete Arch System, which is 72-ft in length, as well as 4,800 sq ft of the Keystone Compac II Retaining Wall System for the headwalls and wingwalls. The BEBO system has precast concrete footings and arch elements. This package met WSDOT criteria by providing product quality and cost savings. It also met the criteria for accelerated schedule capability; the BEBO structure was installed using two cranes in 24 hours and the road itself was closed for only 13 days.
For the headwall design, structural segmental retaining walls (SRW) were chosen. SRWs are an economical and effective system when used in conjunction with many types of multi-plate arches, precast concrete panel arches, and various types of culverts. The effectiveness of the system comes from the size of the block unit, which is 18-in. width by 8-in. height by 12-in. depth. This size has several benefits, including ease of block unit placement, which allows units to be placed by a single construction worker without equipment. The units also provide flexibility for customizing to many different wall layout and alignment scenarios.
Rate of placement also is a major benefit. The precast units are ready to use, allowing the wall units to be placed and backfilled immediately. Speed of placement was a key factor for this project due to a mandated two-week construction window.
Arch headwalls and wingwalls were constructed with the Keystone Compac structural wall system, which has been approved by WSDOT for use on state and federal highways. Total area for the two walls is 4,800 sq ft. The wall length is 160 ft each with a maximum wall height of 29.67 ft. Design considerations included the tall wall height, 2H:1V backslope, and high seismic acceleration requirements.
Seismic coefficients are an integral part of all design evaluations. Concrete segmental retaining wall structures have shown considerable resistance to seismic forces due to the system’s inherent flexibility, permitting minor yielding during a major seismic event, particularly when compared to other more rigid wall systems.
The wall block used on this project has a 1-sq-ft face area and an easy-to-handle 1-ft depth. It is made of high-strength, low-absorption concrete. For interconnection and alignment, fiberglass pins achieve strong shear connection and provide a secure connection with soil reinforcement materials. Available face textures include tri-plane or straight split options having a weathered symmetry, coloring and appeal, while the Compac Hewnstone’s face has the look of naturally chiseled stone.
Restored Fish Migration
In 2013, the U.S. District Court for Western Washington ruled the state must restore historical tribal treaty fishing rights by removing approximately 1,000 culverts that were blocking fish passage under many state roads. This type of stream restoration work, which aims to return streambeds to a more natural state and includes the planting of native shrubs and trees in the area surrounding replaced culverts, provides better habitat for salmon and trout and is more inviting to other local wildlife.
The ability of salmon and steelhead to swim upstream to their traditional spawning grounds is vital to the recovery of their populations across the state of Washington. Deteriorating culverts, outdated bridges, and other barriers have been found to block fish passage and undermine the state’s recovery efforts. When fish are not able to spawn upstream or reach traditional rearing areas, their numbers decrease and they may not survive locally.
While progress has been made over the last few decades and numerous fish passage barriers have been remediated, they still are a problem on more than one front. In addition to harming fish migration, the state’s environmental services office determined that undersized culverts and bridges also contribute to flood damage, threaten public safety, and drain funds for emergency repairs. Because Washington is among the most flood-prone states in the west, well-designed culvert and bridge replacement projects have enormous importance, benefiting fish and wildlife habitats, and reducing flood risks and the associated emergency repair costs. The Washington Department of Fish & Wildlife, which administers the program, works with other public agencies, private landowners, local governments and nonprofit community groups to locate fish passage barriers and identify the highest priority projects to ensure limited funds are allocated appropriately.
For future fish passage program projects, a precast structure approach-or a variation of it-may play a significant role. With Little Pilchuck Creek, a bridge arch system in combination with a concrete SRW wall system for the headwalls and wingwalls met the needs of WSDOT for all the major construction goals: quality, cost-efficiency, efficacy, and the ability to provide a streamlined construction schedule.
In this and similar projects across the 7,056 miles of Washington State highways, an environmental issue has been eliminated. Miles of rearing and spawning habitat, which had become inaccessible when roadways in the state were constructed over these environmentally sensitive waterways, have been restored, which in turn protects and restores fish populations. The new, wider passage on Little Pilchuck Creek allows it to flow more naturally under State Route 92, improving prospects for fish over the course of 30 miles of waterway.