Site drainage with an Eco Twist – Féidhlim Harty

Féidhlim Harty explains how to maximise the potential for biodiversity in your next stormwater management project

Flood control is very much on the agenda in this period of climate consciousness; fuelling an increase in planning requirements for Sustainable Drainage Systems (SuDS) across Ireland. But the ecological benefits of SuDS go far beyond flood mitigation; they engage in carbon sequestration and biodiversity habitat creation as well. How can we use SuDS as a springboard to maximise the ecological benefits of the developments we work on?

WHAT TO AVOID

Concrete has a high embodied energy for the fuel needed to manufacture cement. For every m2 of paving that can remain as green space, or every planned concrete pipe that can be redesigned as an open swale, the lower the overall carbon footprint of your project. Plastic has a high climate and biodiversity impact. Increasingly so, as more production is based on fracked gas. Fracking (hydraulic fracturing for shale gas extraction) leaks a significant percentage of gas directly into the atmosphere at the fracking site, with a carbon equivalent of up to 86 times the potency of CO2. Every metre of plastic we can avoid using means we have more time and resources to invest in making all the other societal changes needed to avert runaway climate breakdown and mass species extinctions.

WHAT TO ADD

An easy way to increase ecological value is to introduce plants. Each one is a small solar-powered carbon capture unit in itself. Planted SuDS components include green roofs, swales, filter strips, Bioretention filters or rain gardens, infiltration areas, detention basins, wetlands and ponds. These not only sequester atmospheric carbon, but also provide food and shelter for countless other species of animals and microscopic flora and fauna that form the foundations of the food web. Following is a summary of these green SuDS components.

SuDS TYPES

Green Roofs are designed, built and planted to allow drought-resilient species to thrive, consequently holding rainfall for slow-release into the drainage network and providing habitat where there would otherwise be none. Care is needed to select the correct roof pitch (typically between 1:80 and 1:3), to ensure the roof is structurally strong enough and that it includes a sufficient number of drainage exits to minimise the risk of blockage. By varying the size and placement of the lightweight growing media and by diversifying the plants used, the biodiversity potential can be significantly increased.

On the subject of roofs, it’s worth noting that while water butts and rainwater harvesting are not planted systems, they are SuDS components in their own right. They have a climate benefit in reducing the energy needed for the treatment and delivery of mains water.

Swales are open vegetated channels used for water conveyance, filtration and/or storage. Depending on the design and soil substrate, they may be dry during low flows or flooded permanently and planted with wetland plant species for added wildlife benefits. Dry swales may be planted with a wildflower mix to add value for pollinators. Design so that water depth is typically ≤100mm, with a base width of ≥0.5m and top flow velocities not greater than 1-2m/s. Bank slopes are typically 1:3 or shallower.

Filter strips are often built between the runoff yard or road and the swale. These are typically grassed strips for water filtration and are usually mown to maintain a low dense vegetated layer for water filtration. However, where the pollution load is likely to be modest, native wildflower mixes may be used to promote biodiversity. Allow ≥6m width and an even inlet distribution at the upper edge of the strip to achieve optimum filtration. Gradients are much shallower than for swales, at 1:20 to 1:50.

Bioretention areas, bioretention filters or rain gardens are relatively highly engineered with different components within their overall design. These can include grass filter strips or channels, ponding areas for temporary water storage, mulch areas for bacteria proliferation and a variety of planted areas and sand filtration. They are sized to store the design runoff volume at <150mm depth of standing water. Unless they are lined, the system base should be ≥1m above groundwater to avoid causing pollution. Include a bypass system to avoid inundating the system during extreme storm events.

Infiltration basins and detention basins both store incoming stormwater for slow-release into the receiving environment. Infiltration basins do this through percolation to groundwater; and detention basins by using a restricted outlet weir. They are both generally dry under normal weather conditions. By planting with wildflower species or wetland plants you can maximise the benefits for biodiversity while still serving the main function of flood control. Infiltration basins are unsuitable for siting over vulnerable aquifers or for polluted runoff areas. Detention basins should be set out at least twice as long as the width to increase pollution reduction. Allow for full storage of the design runoff volume within the infiltration or detention basin. Side slopes of 1:4 are recommended for safety.

Stormwater wetlands and ponds can double as rich wildlife habitats, particularly where the incoming water is relatively free of hydrocarbons or other contaminants. Conversely, where stormwater contamination is high (it can be up to 15% as polluting as raw sewage!) wetlands can be very effective filter systems, cleaning the water before it enters nearby rivers or streams. They should be sized to cater for the full treatment volume and allow variable depth during storm events for attenuation of high flows. Include a sediment forebay to settle debris and silt early in the process, (10% of the overall basin volume). Allow water to remain in the wetland even in dry conditions and provide varying water depths for habitat enhancement. Ponds can have a permanent pool of up to 2m depth.

A length:width ratio of >3:1 is recommended, to maximise pollution uptake. Gently sloping banks are to be used in either wetlands or ponds for safety reasons. Suitable native species include, amongst others: Typha latifolia (bulrush), Iris pseudacorus (yellow flag), Sparganium erectum (branched burr reed), and Mentha aquatica (water mint).

FINAL THOUGHTS

Next time your project brief includes stormwater management of any sort consider how you can provide benefits for biodiversity. The mass extinction event that we are beginning to see around us is the sum total of making lots and lots of decisions every day that erode the base of biodiversity in Ireland and all around the world. By consciously making decisions that support wildlife, we can begin to reverse this trend and have a world that can support and nurture all species, including us. ✽

FÉIDHLIM HARTY is the director of FH Wetland Systems environmental consultancy and writer. His most recent book Towards Zero Waste is now available online and in selected bookshops nationwide. For more SuDS information and links visit www.wetlandsystems.ie