• Pump Station Risk: Laying Low as Sea Levels Rise

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Pump Station Risk: Laying Low as Sea Levels Rise
Watermark Magazine: Winter 2019

KWL's Robin Hawker and Amir Taleghani's article, Pump Station Risk: Laying Low as Sea Levels Rise, was published in page 28 of the Watermark Magazine: Winter 2019.

Often located, “as low as they can go,” wastewater pump stations in coastal areas are particularly vulnerable to coastal flood events, which are expected to become more frequent and severe with climate change in the future.

Public discourse on sea level rise tends to focus on protecting buildings in waterfront areas. However, damage to low-lying critical infrastructure such as wastewater pump stations can cause widespread disruption of services and affect people and structures, including those located in upland areas outside of flood inundation zones.  In addition, there are significant public and environmental health risks associated with potential wastewater overflows.

Preliminary results from a recent risk assessment of sea level rise impacts on Vancouver’s North Shore suggest that under a 2 m sea level rise scenario, up to 65% of North Shore wastewater pump stations could be flooded during an extreme flood event (0.5% annual probability / 200-year return period) if no adaptation measures are implemented. This study also found that a 1 m sea level rise scenario paired with a less severe flood event (10% annual probability / 10-year return period flood) could affect 45% of all North Shore wastewater pump stations (Kerr Wood Leidal, 2019 draft) if no adaptation measures are implemented.

Coastal communities in BC recognize these risks and are beginning to build wastewater pump station resilient to coastal flood events through a range of approaches. While some communities have focused on retrofitting existing pump stations to protect against flooding, other communities have taken an adaptive management approach that incorporates at-station floodproofing with coastal floodplain management (e.g. land use planning, dikes, etc.) and long-term plans for pump station relocation out of flood zones.

This article summarizes how sea level rise can impact wastewater pump stations and shares case studies of pump station adaptation measures being implemented in BC communities.

Coastal Flooding & Sea Level Rise

Coastal flooding refers to a temporary overland flooding event caused by the combination of tides, storm surge (a rise in water level due to low atmospheric pressures and winds from nearby or distant storms) and wind-generated waves. 

According to current BC government guidelines, sea levels in BC are expected to gradually rise by 1 m by the year 2100 and 2 m by the year 2200 due to climate change, causing flood waters to potentially reach higher elevations and further inland (Ausenco Sandwell, 2011).

Critical Risks to Public & Environmental Health

The most direct impact of sea level rise on wastewater pump stations is inundation. Inundation of sea water can impact wastewater pump stations in two primary ways:

  1. Inundation of electrical equipment and backup generators halting pump operation.
  2. Inflow of sea water into the wastewater system through pump station facilities or linear infrastructure.

If water enters the system faster than the pumps can discharge it from the station, it can result in wastewater overflows, backups into buildings, damage to structures and service disruptions in coastal and inland areas. These situations can have significant impacts on public and environmental health as contaminated floodwaters overflow into receiving environments.

Rare, severe flood events could potentially damage pump station facilities via direct water, wave and debris impact, whereas more frequent, less severe events could impact maintenance programs and result in re-occurring service disruptions.

Sea level rise can also cause longer term impacts, including pump failure over time as the pumps turn on more frequently or for longer durations, resulting in shorter life spans and increased maintenance costs. Gradual raising of the water table from sea level rise can also lead to structural issues associated with soil loading causing the wet well structure to fail.

Pump Station Vulnerability

Pump station vulnerability to flooding varies according to several factors, including:

  • the elevation of electrical and mechanical control equipment
  • the vulnerability of power sources to flooding and whether backup power is provided
  • equipment construction material and vulnerability to corrosion from seawater exposure
  • the distance between pump station and shoreline as related to wave impacts and coastal erosion due to waves
  • gravity sewer connections and potential for sea water inflow and infiltration through manholes and service connection interfaces

While some stations are built above ground, others are buried and more vulnerable to inundation and damage during flood events.  Many older pump stations were built without backup power, or in a “wet well & dry well” configuration in which both the mechanical and electrical equipment are underground and are therefore more vulnerable to flooding. 

Building Pump Station Resilience

While there are a range of solutions available to increase pump station resilience to sea level rise, approaches should be selected carefully to best suit local needs and context. 

Strategies can be grouped into to three main approaches, including:

  1. Station Floodproofing to seal or raise existing pump station components within flood inundation zones
  2. Floodplain Management Measures in the surrounding area, including structural and non-structural approaches to minimize the likelihood and/or impacts of flooding
  3. Gradually Retreating by planning to relocate pump stations out of inundation zones over time

1. Station Floodproofing

Floodproofing of existing pump station components often involves a combination of measures to minimize the amount of sea water ingress into the station and wastewater system overall.  

Measures typically include:

  • Raising electrical equipment and backup power
  • Sealing pump station access points and upstream manholes

These approaches are ideal for protecting existing pump stations that have few alternative options for relocation or that would be too costly to move out of the flood inundation zone.

However, if infrastructure within the flood zone is not entirely sealed, saltwater ingress can overload pump station capacity and impact the operation of the station over time, downstream trunk sewers and forcemains, and ultimately the wastewater treatment plant. For this reason, sealing of wastewater infrastructure may be required over a large area to avoid service impacts.

There are a number of strengths and challenges associated with this approach: 

Strengths
  • lower up-front costs than moving the station.
  • minimized service disruption during upgrade.
  • floodproofing retrofits can be paired with other initiatives to extend station service life. 

Challenges

  • often applied to select components only (partial coverage).
  • temporary measures that can require ongoing maintenance and replacement of floodproofing components to ensure their reliability (higher long-term O&M costs).
  • access to the station during flooding may still be difficult or dangerous.

 

The Wiggins Pump Station in the City of Burnaby was retrofitted by raising electrical components and adding standby power to extend the station service life in the face of increasing coastal flood risk.   This approach provided a much lower cost solution than moving the entire pump station and enabled the station to continue serving the developing area nearby.

Figure 1 Wiggins Pump Station, City of Burnaby

2. Floodplain Management Measures 

In addition to strategies that retrofit existing pump station components, risk to the station and broader wastewater system should also be addressed through floodplain management measures.

These measures can include:

  • structural flood protection measures (e.g. dikes or other flood barriers to reduce the likelihood of flooding of the station)
  • non-structural measures (e.g. land use and building control such as flood construction levels and sewer backflow prevention valves)
  • cumulative risk reduction measures (e.g. flood protection of at-risk interconnected infrastructure to minimize potential cumulative impacts to pump stations.  An example would be protecting electrical substations in inundation zones to minimize the risk of wide-scale power outage leading to pump station failure)

For pump stations located near the coastal shoreline, nature-based adaptation solutions (e.g. wetlands, vegetated shorelines) may also help reduce risk by attenuating wave energy and infiltrating runoff while strengthening local habitats.  Over time, these nature-based solutions may require less maintenance and management than traditional “grey” infrastructure (e.g. concrete sea walls, riprap, etc.) once the engineered natural environments become established and thrive.

Strengths and challenges of incorporating flood management into pump station planning may include: 

Strengths

  • can reduce the likelihood of flooding to reduce risk overall. 

Challenges

  • higher cost associated with longer term planning and flood management measures.
  • ongoing maintenance requirements. 

3. Station Retreat

Moving a pump station entirely out of the flood inundation zone is the most effective way to reduce or eliminate the risk of coastal flooding from sea level rise.

This approach is ideal for siting new pump stations or moving existing pump stations that are relatively small, are critical assets serving a large area or are at a very high risk of failure from inundation.

In most cases, however, this approach is limited by high costs or the availability of suitable land for relocation. Another consideration is that when a pump station can be moved outside of the flood zone, some low-lying or waterfront properties can be left out of the new service area if underdeveloped or may need to install individual sewage pumps for their wastewater service. 

For these reasons, pump station retreat is often framed as a phased approach to occur at the end of the station’s expected useful life, with floodproofing strategies established in the interim along with new requirements for redeveloping existing properties to include individual sewage pumps (or provisions for them in the future). 

Station retreat can also be considered as part of a larger, gradual land use adaptation strategy that involves all development being gradually relocated out of high flood risk areas. This can also factor into risk-based decision making for pump station retrofitting: if a community has a long-term managed retreat and land use change strategy in place, it may find that it can tolerate the interim risk of pump station flood impacts knowing that the pump station will eventually not be required due to land use change.

Strengths and challenges associated with this approach include: 

Strengths

  • most effective approach (lowest residual risk)

Challenges

  • high cost and limited siting options.
  • increased costs to low-lying and waterfront properties that will need to install individual sewage pumps and force mains.

 

As part of a long-term sea level rise adaptation on flood risk management strategy, the City of Colwood adopted a combined approach for the Ocean Boulevard Pump Station that involved:

  • floodproofing pump station outdoor equipment (e.g. raising one electrical kiosk, sealing wet well doors and hatches)
  • raising an existing sea wall to reduce the risk of flooding due to waves
  • planning to move the station out of the flood zone in the future

Figure 2 Floodplain Management at Ocean Boulevard Pump Station, City of Colwood

Adaptive Management 

Ideally, communities will plan for a combination of the strategies discussed above to best address flood risk over the entire design life of an existing or proposed pump station and the associated linear infrastructure. This combined and adaptive approach has been demonstrated by the City of Colwood at the Ocean Boulevard Pump Station described above. The consideration of design life is important because sea level rise is happening gradually and some pump station components may need to be replaced before sea level rise increases the coastal flood hazard to a level that may impact them.

However, despite advances in atmospheric and oceanic research, there continues to be uncertainty about the magnitude and timing of sea level rise. For example, a recent study found that sea levels could rise by as much as 2 m by 2100 (Bamber et. al, 2019). In response to this uncertainty, some communities are now preparing for more extreme impacts and are trying to take more adaptive approaches that are more flexible to changing conditions.

An adaptive management approach for pump station flood risk could involve careful planning and sequencing of potential options under various sea level rise scenarios. This approach is made more powerful by drawing on other perspectives that affect pump stations, including land use and development. Taking an adaptive management approach could be used by coastal communities to determine when and how to apply each of the resilience strategies discussed above.

Multi-Disciplinary Perspectives

Developing adaptive management strategies to address pump station flood risks is extremely site-dependent and should be approached carefully. Drawing on input from a multidisciplinary team (e.g. planners, coastal engineers, pump station design engineers, biologists, etc.) can provide a wider system-level perspective and address trade-offs between design and community priorities to support safe and long-lasting wastewater infrastructure.

At KWL, we work closely with communities to develop adaptive flood management strategies and infrastructure designs that stand up to sea level rise, making both our natural and built environments more resilient to climate change.

To view the published article in PDF format, click on the download button below.

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Authors:

Robin Hawker, MScPl, MCIP, RPP is an Environmental Planner and Climate Change Adaptation Planning Lead with Kerr Wood Leidal Associates Ltd. She works closely with municipal and indigenous communities across BC to assess risks and develop integrated solutions for infrastructure resilience in the face of flooding and climate change.
Contact: rhawker [at] kwl [dot] ca  1-604-293-3107

Amir Taleghani, M.Eng, P.Eng is a Water Resources Engineer at Kerr Wood Leidal Associates Ltd. with expertise in sea level rise adaptation and floodplain management.
Contact: ataleghani [at] kwl [dot] ca  1-604-293-3212

Alina Torres, P.Eng is a former Project Engineer at Kerr Wood Leidal Associates Ltd. with expertise in pump station design and construction.  Her design experience includes pump selection, ventilation systems, and detailed design of facilities systems. She has also been the field reviewer for pump stations, sewer, and water infrastructure projects.

Photo Credits:

Kerr Wood Leidal (2017) Wiggins Pump Station.

Kerr Wood Leidal (2018) Ocean Boulevard Pump Station. 

References:

Ausenco Sandwell (2011). Climate Change Adaption Guidelines for Sea Dikes and Coastal Flood Hazard Land Use.  Prepared for BC Ministry of Environment.
http://www.env.gov.bc.ca/wsd/public_safety/flood/pdfs_word/coastal_flooded_land_guidelines.pdf

Fraser Basin Council (2016). Lower Mainland Flood Management Strategy.
https://www.fraserbasin.bc.ca/_Library/Water_Flood_Strategy/FBC_LMFMS_Phase_1_Report_Web_May_2016.pdf

J.L Bamber, M. Oppenheimer, R.E. Kopp, W.P. Aspinall, and R.M. Cooke (2019). Ice sheet contributions to future sea-level rise from structured expert judgement. Proceedings of the National Academy of Sciences PNAS. 116 (23), 11195 - 11200.
https://www.pnas.org/content/116/23/11195

Kerr Wood Leidal Associates Ltd. (2019). North Shore Sea Level Rise Risk Assessment and Adaptive Management Strategy (Draft).  Prepared for the District of North Vancouver, City of North Vancouver, District of West Vancouver, Squamish Nation, Port of Vancouver, and North Shore Emergency Management.

Ministry of Environment and Climate Change Strategy (2019). BC Preliminary Risk Assessment. Summary of Results.
https://www2.gov.bc.ca/assets/gov/environment/climate-change/adaptation/climate-risk-summary.pdf

Resources: 
Article - Pump Station Risk: Laying Low as Sea Levels Rise (application/pdf, 0.794Mb)