There’s more to managing stormwater than ‘pipe to pond’
Looks all too familiar, doesn’t it? Far too often we are seeing images across our news channels on localised flooding. According to the Environment Agency, over 5 million homes are at risk of flooding in England alone. The average cost of flooding to a home is £30k and the mental health-impacts are long-lasting.
Even if you’ve never been flooded, you can sympathise with people whose homes you see on TV, with furniture floating in filthy water. But consider this: we are responsible. We all contribute to these floods. From the roofs over our heads to the roads we drive on; from pavements to shopping centres and their car parks; from schools to business parks – all the impermeable surfaces that make way for our everyday lives prevent rainwater from being absorbed where it falls.
Drainage systems designed to take rainwater away may have been up to the job when they were first designed, but they have become overwhelmed by the challenges of larger populations, increased urbanisation and a changing climate. That’s why people are looking increasingly to sustainable solutions for the answer: solutions that can accommodate changes without compromising the future.
Susdrain’s 2020 SuDS champion, George Warren, makes the case that “We need sustainable urban drainage systems (SuDS) – solutions that replace hard engineering works with nature-based designs – to tackle the looming challenges of poor water quality, climate change and tight budgets.” The Environment SuDS in 2030 – cleaner, greener towns and cities.
This doesn’t mean replicating a hard engineered system with something ‘softer’, although this is how some designers and developers have interpreted the legislation that underpins SuDS. Often referred to as ‘pipe to pond’ systems, a great many SuDS solutionsuse hard engineering to transport rainwater to an attenuation area which may be a natural solution; a swale or maybe a pond.
Of course, a solution that’s designed to alleviate flooding must perform its primary task. For that, the designer has to have confidence that comes from the numbers. Calculating the flow and volume required to handle runoff can lead to an obvious ‘pipe to pond’ solution and can result in the ‘pond’ becoming a dominant feature, attracting negative publicity along the way. Even ‘successful’ SuDS schemes – i.e. those that reduce the risk of flooding and meet building regulations – have received criticism for being unsightly, polluted and sometimes hazardous by posing a drowning risk to children (frequently used as an objection to planning applications). What’s more, they aren’t even the most efficient way for developers to manage runoff as the size of the pond or swale could take up more valuable development land than necessary to handle exceedance flows.
These unimaginative ways of meeting regulatory demands don’t take full advantage of the opportunities that SuDS offer, especially with regards to the four pillars of sustainable drainage outlined by Susdrain: quantity, quality, amenity and biodiversity. By looking more holistically at the problem and its solution to incorporate other, natural structures in a drainage design, designers and engineers can manage resources better and create more sustainable solutions. Slowing the flow by intercepting runoff closer to its source naturally reduces the volume of water reaching an attenuation point – reducing the amount of land take and increasing amenity along the way.
For planning purposes, local authorities naturally need key information in the planning submission. They expect to see a flood risk assessment with a drainage strategy. They want to know the overall drainage plan as well as specific drainage arrangements. They want to know:
When there is an exceedance
When it surcharges
Where the excess water is going and how long it takes to empty
As mentioned earlier, one barrier to designing with additional structures lies in the calculations. It’s just easier to calculate the ‘pipe to pond’ numbers – after all, that’s what drainage engineers have been doing for years by following the ‘pipe to tank’ model. With the GreenBlue Urban stormwater management solutions, the growing medium is used both as the treatment stage for pollutant removal and also as the flow control. This latter feature may confuse some people who are more used to flow control being at the end of the system – like orifice control at the end of a tank – rather than at the beginning. By using the soil as a flow control we can calculate how fast the stormwater will travel through the system, and therefore reduce the amount of storage that is required. This presents a huge benefit, and also an entirely different way of calculating drainage requirements.
At GreenBlue Urban, we’ve recognised this as a stumbling block and have worked to overcome it. We have developed ways to help you determine the degree of interception, ground infiltration and saturation within a linked system of engineered tree pits, and the impact this has on your overall scheme. Our specially engineered products have been used to very positive effect in schemes around the world – particularly close to home in the UK in Grangetown in Cardiff, Keighley in West Yorkshire and Kenmont Gardens in London to reduce volume, remove pollutants and even alleviate traffic problems.
So, some new thinking is required: a way of bringing engineering and landscape design closer by incorporating natural engineering into practical, sustainable drainage design. The good news is that when drainage engineers embrace this new way of thinking, they create innovative nature-based solutions that encourage amenity and biodiversity while achieving the primary goal of reducing flood risk, solving multiple problems along the way.