By Jurek Janota-Bzowski
It is not often in one’s career that a project so obviously demands a high level of creative freedom in designing a large river outfall for disinfected effluent from a municipal treatment plant.
Such was the case with the Northern Rockies Regional Municipality (NRRM) who contracted Kerr Wood Leidal Associates to design a 25 ML/d river outfall in the Muskwa River for the Town of Fort Nelson, British Columbia. Initially, this seemed like a dream opportunity to build something special, design some unique features, and incorporate some innovative ideas that had never been designed before.
However, once the initial euphoria had passed, the magnitude of the task ahead became readily apparent. Challenges and constraints for operation, maintenance and construction started to seem overwhelming. These included:
- Designing, operating, and monitoring an outfall in a very active river that ranges in depth from 0.75 m (half of which is ice) in winter, to 8 m in summer.
- Mitigating the risk for an outfall in a fast-flowing river, where the key objectives are to: protect the manifold and diffusers from extensive river scour; withstand impact forces from debris, logs, and ice breakup; and ensure diffusers are flexible, self-cleaning, and operating within a 0.3 m water zone beneath the ice in winter.
- Making provisions for an outfall to survive the loss of any diffuser without blocking the manifold through ingress of sands and gravel.
- Building an outfall with its manifold 3 m below the existing riverbed level.
- Incorporating operation and maintenance features that allow operators to: inspect and monitor performance of each diffuser and replace as necessary; clean out the entire length of the manifold using suction, air, or water pressure; and re-excavate the manifold in case of catastrophic failure at minimum cost.
The challenge was further complicated by the fact that the manifold had to extend landward under a relatively high, unstable and erodible riverbank. It also required sufficient cover to avoid damage from any resulting slip circle in case of bank failure. Once past this obstacle, it was necessary to connect to a deep outfall chamber set back some 50 m from top of bank, and from there, just 3 km of outfall back to the treatment plant.
Research and planning
A literature search failed to uncover any design guidelines for outfalls, any comparable designs, or any similar challenges. References only pointed to extraordinarily expensive failures, where the sight of a clogged outfall pipe was a clear indicator of what went wrong. It seemed that no one had dealt with the issues we were facing and that we would need to draw on all our previous experiences with outfalls and get innovative.
For a year, we studied river cross-sections, ice thicknesses, timing of river ice breakup, effluent dispersion models, environmental impacts, scour mechanics, and undertook geotechnical investigations along the outfall route, and completed stability analyses of adjacent river banks. This information allowed us to identify the optimum stable reach of the river, select the best site for an outfall chamber, and determine the exact coordinates for the 12 diffusers required to satisfy dilution ratios.
The next hurdle was to develop a plan for construction. It became abundantly clear that the in-river works would be a winter construction project with a definite construction window. Ice breakup was a great unknown variable, occurring at the earliest in mid-April and at the latest in early May. Delay was not an option.
Construction would require a cofferdam no less than 35 m in length, 6 m wide, and a working depth of 5 m. The design philosophy for the cofferdam was for it to become part of the post-construction structural protection for the outfall, with sheet piles driven down to riverbed level on completion. This approach would provide the required flexibility to re-raise the cofferdam should future major repairs be required.
The next decision was to install a carrier pipe to house the outfall pipe. This would provide protection against riverbank failure and river scour. It would also provide an opportunity to extract and replace the entire outfall if it was ever required. After a review of several horizontal drilling options, we settled on horizontal augering as the least disruptive technique to install a 1,067 mm diameter steel casing pipe through the cofferdam wall seal, underneath the riverbank, and through to the outfall chamber.
Diffuser challenges and innovation
The first issue to be resolved was how to prevent manifold clogging with river sands and gravels in the event of a diffuser(s) loss. We visualized some innovative options but opted for a specially modified stainless-steel Tideflex backcheck wafer valve that opened normally under 50 mm of differential water pressure between the river level and the driving head of effluent in the outfall chamber. The valve was located in the upper part of the vertical riser, and its closing action would prevent migration of materials downward into the manifold in the event of diffuser breakaway.
The second issue was to provide individual sensors for each of the 12 diffusers to determine which (if any) had broken off. We examined several electronic sensors, in-line flow meters and pressure gauges; none could provide the flexibility, reliability or accuracy we required. In the end, we resorted to basic physics by installing 50 mm diameter HDPE sensing lines on each riser between the backcheck valve and the diffuser.
Our reasoning was that, under normal operating conditions, all water levels in the sensor pipes would be equal, based on the 50 mm water head pressure required to open the diffusers. With breakaway of any diffuser, the water level in that sensor pipe would drop by the same 50 mm and would be readily measured in the sensing array at the outfall chamber. Great care was required to make sure that each sensor line was meticulously labelled, and correctly installed. This simple innovation would not only inform operational staff that one or several diffusers had been dislodged, but also identify which one(s).
With these issues solved, it would be a straightforward installation of the 600 mm diameter HDPE outfall pipe, complete with pipe spacers on which all 12 sensing lines could be strapped to the spacer fins. This arrangement resulted in an annular space between the casing pipe and the outfall. We determined that it would provide an opportune pathway for emergency overflow and operational bypass.
Accordingly, we built a slotted screen diffuser into the cofferdam piping and an emergency overflow in the outfall chamber. A shut-off valve in the outfall chamber could divert flows through the overflow, and allow for operational staff to clean the outfall pipe and manifold through a separate access point.
A key focus for construction was that all design elements would need to be prefabricated and brought to site for storage, and be designed for simple installation like Lego blocks. This would provide the optimum opportunity for the contractor to complete the work on time. The design of the manifold and 12 diffusers provided opportunities to address several of the key project issues.
To simplify installation, the manifold design comprised 12 x 2.25-m-long 600 mm diameter HDPE tees, butt-welded in three sections and bolted together with slip-on flanges terminating in an upward sweep to the river bed elevation.
The terminus was fitted with a camlock coupling for access to clean out debris at the end of the line. Each tee fitting had a bolted vertical riser that included the specially designed Tideflex backcheck valve, the sensing line, and the Tideflex effluent diffuser. The entire assembly was designed to protrude no more than 300 mm above the riverbed level.
Each diffuser was 1,500 mm long to ensure flexibility of horizontal movement. Diffusers were bolted to the vertical riser with specially designed nylon breakaway bolts, and included a stainless-steel chain tether bolted to the adjacent Lock Blocks.
A significant aspect of the structural integrity of the design was the attention paid to details required to mitigate potential damage from impact forces, and to avoid long-term erosion damage from river scour.
Scour protection and horizontal impact forces were readily addressed through use of Lock Blocks wedged between the cofferdam walls. The geometric layout of the Lock Block assembly was based on their dimensions of 1,500 mm x 750 mm x 750 mm. The layout necessitated a great deal of precision, as it determined the horizontal spacing of the manifold tees, and created 1,500 mm wide pockets to house the vertical risers.
For added protection, the vertical risers were installed within a 1,200 mm diameter HDPE pipe sleeve that ran down to the manifold and was filled with gravel. This arrangement ensured that any horizontal forces on the diffuser would not be transferred along the vertical riser to the manifold, but rather to the nylon breakaway flange located at riverbed level. This design feature would also make it a relatively straightforward operation to replace the diffuser assembly if and when required.
Construction preparations commenced in late December 2017 through to January 2018. Prior to cofferdam construction, river ice had to be thickened from 450 mm to approximately 1,200 mm in order to support the weight of construction equipment. This was accomplished by cordoning off a section of the river, and regularly flooding the area with 50 mm layers of river water. This process took over a month before load spreader mats could be laid down and heavy equipment brought on to the ice.
Cofferdam construction went as planned, ice was removed from within its footprint, and marine biologists rescued all fish trapped within. Excavation to more than 5 m below the river level went as planned and horizontal augering started off well. However, as with all construction projects, the best laid plans come with hiccups.
A third of the way into casing installation, the auger jammed, and no amount of manipulation would get it out. This was a critical moment, and put the whole project in jeopardy. After much deliberation, all agreed on a rescue mission to extend the cofferdam almost to the river bank, and remove the entire casing. The casing pipe was re-installed using pipe ramming techniques to drive it to the outfall chamber.
This cost the project precious weeks and it was a race against time to get it finished before the spring thaw. The contractor worked around the clock in -40°C weather and completed the outfall construction phase by April 9, 2018. All that was left was to drive down the sheet piles to riverbed level, flood the construction, and get all equipment and materials off site.
During this final phase, the temperature started to warm up, and had it not been for a brief freezing spell, the outfall would have likely been flooded out. As it turned out, ice breakup started in late April, and the job was done with less than two weeks to spare.
This project allowed us to incorporate many unique and innovative design features into an area of engineering that has little information to draw on. We hope that our efforts will help engineers to use our ideas as a reference and a resource for their own designs.
Jurek Janota-Bzowski, P.Eng., is with Kerr Wood Leidal Associates. This article appears in ES&E Magazine’s August 2019 issue.