Posted on Friday, April 27, 2018
Intake structures in high-deposition and cold-regions environments require a designer to consider the entire life-cycle of the system. Over the years, we learned many lessons from our experience with structures on the Peace, Elbow, North Platte and North Saskatchewan Rivers in particular. Here’s our top seven for now.
1. Maintenance and operations dominate geometry. Intake designs in deep-water systems with turbid water require careful design of screen geometry, electrical and instrumentation terminations, materials compatibility and replacement logistics. At greater water depths, particularly in silt-laden waters, divers must overcome have both limited visibility, reduced mobility due to cold water and restrictions on dive durations that change even routine operations into an “extreme” experience. As a result, we’ve had to design our intake systems to allow for rapid recovery, servicing and re-installation with safety as the primary goal.
2. Sedimentation drives system hydraulics. Intake screen types/sizes, inflow geometry and pump hydraulics must consider a variety of sedimentation rates and operating conditions to prevent premature plugging and avoiding biomass fouling. Intake velocities must be low enough to prevent sand incursion that can damage pump internals, but high enough to prevent silt deposition during low-flow, storage and shutdown conditions.
3. Materials selection is paramount. Both concrete and steel structures require careful selection of compatible materials and appropriate coatings to avoid both galvanic and stray current corrosion.
4. Frazil ice can be a deal-breaker. Frazil ice is collection of randomly-oriented needle-shaped ice crystals that forms when air temperatures fall below 6 deg C (21 deg F) or lower, particularly during ice-in conditions on northern rivers. Frazil ice is neutrally buoyant and can therefore appear throughout the water column at the intake location and plug off the intake. Owners have tried mechanical and thermal systems to mitigate the problem with limited success: suspended operations can often result.
5. Sparging may not solve these problems. Systems that include sparging or back-flushing can have limited success due to the variability in hydraulics of intake plugging. It is important to remember that the intake geometry and hydraulics are a dynamic system with variations in flow velocities, pressure head and composition of suspended solids. Sparging and backflush systems can short-circuit when heterogeneity is extreme: design should focus on preventing the problem rather than correcting it operationally.
6. Infiltration galleries loom large. A fully-functional infiltration gallery requires careful consideration of all-of-the-above. Infiltration rates should prevent sand incursion while maintaining adequate velocities to prevent siltation. Accessibility rapid repair, replacement and cleaning should be factored into system geometry.
7. Innovative technologies reduce cost of service. Remote control bathymetry and sonar tools improve the assessment of conditions, support better planning and reduce maintenance frequency plus cycle times. By choosing the right suppliers of technology and systems, we’ve been able to improve safety and reduce the cost of long-term structures.
We’ve worked with top river hydraulics engineers to make sure our intake systems stand the test of time and deliver on our commitment to lowest life-cycle cost. We don’t pretend to do it all – but you can be certain that we deliver a team that can.