Ponds, streams and lakes can attract wildlife, flora, fauna and bring a real sense of calm and beauty to any place. They are fundamental to the landscape and crucial to the way we design new places.
And yet water is an increasingly precious resource in the UK that is difficult to manage. Extreme weather events are multiplying in frequency and intensity, yet they are ever more localised, causing extreme flooding.
Drainage matters more than ever. Tom Hydes, a Chartered Civil Engineer, explains our approach.
The Challenge
Building in any area can change the way rainwater drains into sewers, streams and rivers. You may have heard of places where the new runoff of water has caused more flooding.
Hard surfaces (like tarmac and paving) accelerate water flow. As it washes off roofs, drives, roads and commercial areas, it can also collect pollutants. When this is from a single home it will not tend to have a noticeable effect, but a new town can create a different scale of impact.
The danger doesn’t only come from built-on land. Rainwater runoff from farmland, forestry activities, community and amenity green spaces can contain fertiliser. And without management and maintenance, sewers, streams and rivers can also create flood risks.
Our Approach
- Understanding Site Conditions Prior to Development
We start by working with specialist engineers to determine how water flows around and off an undeveloped site. They apply detailed calculations that include:
- Local rainfall data collected over many decades
- Soil conditions (clay soils cause rainwater to run off faster than, say, limestone)
- Water levels and the contours of the land (water will typically flow faster down steeper gradients)
- Groundwater monitored over several months
- Existing drainage and land uses.
This baseline data is used to set parameters. They are adopted into our designs, ensuring that the new drainage system maintains current conditions at the very least.
These parameters and calculations will be included in the drainage strategy within the planning application, with important parameters stipulated in the planning conditions (as in South Canterbury).
- Understanding the Impact of the Proposed Scheme
As we develop our masterplan, our specialist engineers assess the potential impact on the speed and quantity of rainwater flowing across the new place. They use statistics to assess the effect of particularly intense or adverse weather. We add allowances based on predicted increases in rainfall intensity due to climate change (there’s fascinating guidance here: www.gov.uk/guidance/flood-risk-assessments-climate-change-allowances). Our calculations at this stage include the full range of factors examined in Stage 1.
The new designs will include man-made devices to control the rate, amount and quality of rainwater around the new place. These are called Sustainable Urban Drainage Systems or SUDS.
- Engineered Water Management
Our landscape-led approach to master planning means we always try to drain the land naturally (using nature’s preferred tool, gravity!). However, there are areas that require additional measures. These water management practices and facilities are designed to drain surface water in a sustainable way (rather than the more traditional pipe running into a watercourse).
They slow the flow of water by passing it through special filter material, or by holding water temporarily in a pond. These allow water to leave the pond at a controllable rate (like a bath plug at home).
Conclusion
Water and SUDS are typically explained in complex and technical reports. We always try to ‘translate’ these to make them accessible to all. Our approach is always to start planning a place around its topography, so we look at where water is most likely to settle. We then look at the probability of inconvenience or flooding occurring across the place. That risk can be alleviated through sustainable engineering.
We are also determined to explain this in ways that can be understood without industry or engineering knowledge.
For example, when others describe greenfield runoff rate, we talk about the rate at which rainwater flows off undeveloped land. Other examples are:
Detention Basin – a pond used to temporarily hold water (as per the bathtub)
Bioretention system / Raingarden – a shallow trench or hole filled with sand or gravel to collect water and filter it into the soil (or an underground pipe).
Source Control – placing SUDS as close as possible to where rainwater accumulates
Treatment Train – a way of managing the run-off from storm water.
We run a number of events to discuss engineering challenges in detail across all the places we create. It would be great to meet you there!