Creating resilience in water infrastructure designs to mitigate flood risks


In light of recent events, we sat down with Woods' GM of 3 Waters, Pranil Wadan, and Senior Associate 3 Waters engineer, Ajay Desai, to discuss flood risks, creating resilience in key water infrastructure solutions for our clients, and the key takeaway lesson for the industry. 

The impact of flooding can be devastating, as recently noticed with severe storms. While the impacts of flooding are well-known, such as damage to property and vehicles and safety risks for residents in flood-affected areas, there are broader environmental concerns too. Designing resilient water infrastructure isn't just for the people in our communities. It is also essential for the receiving environment. 

The broader environmental risks of flood events 

Floodwaters are highly contaminated – and it's not clean water – and all that water eventually ends up in the streams and harbours. This is a significant risk as it can take months for those contaminants to settle and disperse before the water can return to its previous state.  

Household debris, including trampolines, water tanks, trolleys, and sometimes even cars, find their way into the streams and estuaries during flood events. These often become another source of pollution. 

Resilience in water infrastructure design 

The recent floods highlighted the importance of allowing for resilience in water infrastructure design rather than designing for the bare minimum standards specified in various guidelines and codes of practice. Land development and key infrastructure projects are significant investments that will be around for the next 50-plus years, and we want to make sure the foundations or the bones of the design are right. 

When looking at the risk of flooding on a site, we want to understand what it looks like at a more conservative level, i.e. a higher level of rain or with a higher sea level, and then make sure the design can withstand that. Essentially, factoring in climate change so we don't have an issue 100 years from now. 

Identifying the true risk to mitigate effectively 

Before proceeding with the design or development of water infrastructure, we need to understand the true risk for two reasons: 

  1. There will be instances where the risk level might be too high, and 
  2. Once you understand the risk, you can mitigate it more appropriately. 

At Woods, we carry out a lot of technical work upfront to identify the true risk of flooding for our project sites under various scenarios. We ask the ‘What’ questions such as 'What would happen if this downstream culvert we're relying on fails?' or 'What would happen when the primary network or key infrastructure fails during a flood event?' We try to answer these questions upfront and assess all the risks. 

From an engineering perspective, this allows our designs to be much more resilient because we’ve allowed for various scenarios and designed for the ‘worst-case scenario’, or you can call it the ‘best-case scenario’. Often, we are designing for a higher level to avoid possible risks that often go beyond the minimum design standard (1 in 100 years event). 

When a site is in an area with significant flood risk, we might do modelling to understand what the risk looks like for a 1 in a 250-year storm or a 1 in a 500-year storm. Understanding the risk under that much higher level of rainfall, and using that as the baseline for design, allows us to de-risk our clients’ projects and investments as much as we can. 

How Woods' collaborative approach creates better design outcomes  

As a multidisciplinary organisation, we work closely with other specialists in-house from concept to construction. From working with the surveyors to collect high-quality survey data to inform the masterplan and our flood model to liaising with the civil engineering team to ensure the models reflect the right design standards and processes. Most importantly, because we understand development and constructability, we can explain and clarify the limitations (and opportunities) our studies bring to the client regarding design. 

Milldale's water infrastructure tested 

Milldale's water infrastructure was put to the test during the storm. For each stage of the Milldale subdivision that we advance, we develop a flood model as part of a wider model that is continually being refined. We then work with the civil team to understand the flood risks. 

High rainfall events, as devastating as they can be, provide an opportunity to test assumptions and highlight deficiencies in the design. This enables further calibration to create resilience against future events. 

After recent storm events, we are much more confident in the stormwater infrastructure design at Milldale. We are content that this development has shown resilience to extreme flooding, as there was no flooding, and everything that occurred was within design tolerances. The rivers, streams, and all infrastructure operated as intended. 

This validates our approach of creating an overall flood model and refining it on a stage-by-stage basis to incorporate anything and everything that has evolved and changed. 

What can the industry learn from the Auckland floods and Cyclone Gabrielle? 

One of the things that we believe should change in the industry is the focus on primary vs secondary network systems. There needs to be more focus on the secondary design rather than relying on the piped infrastructure networks to mitigate flood risks. 

Even if a property doesn't sit on a floodplain, that doesn't mean there is no risk of flooding. There was a lot of flooding in areas that were predicted to flood, but there was also flooding in areas that were not predicted to flood.  

When the primary network fails, the whole system tends to fail, and we saw this with the recent floods, because there is limited allowance to have flows travel overland. There are other risks here, including pipe blockages which signifies the flood risk. 

We need to take time to understand what failure looks like. We need to understand what happens if there is a blockage within the primary network. Only then can we design appropriately. We would then think along the lines of taking the stormwater alongside housing and then back onto the road, rather than towards the house, to mitigate risk when the infrastructure fails. 

Mitigating risks is crucial, particularly when we're witnessing a rise in severe weather events. It involves more than just relying on an expert in a particular field. To build resilient communities, we need to tap into multiple disciplines, design for worst-case scenarios, and identify potential risks early in planning stages and design appropriately to manage these cost-effectively.  

At Woods, we adopt a highly collaborative approach, involving specialists from various fields to achieve this goal. By working together, we create innovative solutions that can help mitigate risks and improve the safety and wellbeing of communities.