Systems for infiltration, buffering and drainage
In bioswale systems, the water running off from roofs and roads does not flow into the sewers but instead is led into the bioswale via above-ground gutters and/or ditches. Bioswales can be incorporated into the green infrastructure and can help enhance biodiversity and quality of life.
A bioswale is a ditch with vegetation and a porous bottom. The top layer consists of enhanced soil with plants. Below that layer is a layer of gravel, scoria or baked clay pellets packed in geotextile. These materials have large empty spaces, allowing the rainwater to drain off. The layer is packed in geotextile to prevent the layer from becoming clogged by sludge or roots. An infiltration pipe/drainpipe is situated below the second layer. To prevent the bioswale from overflowing its banks during heavy rainfall, overflows are added that are connected directly to the infiltration pipe/drainpipe. Rainfall infiltrates into the ground via the ditch and the packed layer. If the water rises above the level of the overflow, the water runs through it to the drainpipe. The bioswale’s dimensions should be sufficient to ensure that this occurs no more than once every two years. If the drain and the overflow both fill up, the bioswale acts as an above-ground drainage system and leads the water directly to surface water.
The dimensions of most bioswales are designed in such a way that water from heavy rainfalls infiltrates into the ground within 24 hours. In most cases, the bioswale will only serve as an above-ground drainage system once every 25 years. This means that bioswale systems must always be connected to surface water. The drainage system also serves to transport water from areas of ground with a low infiltration capacity to areas with a more favourable infiltration capacity.
In the winter, when water levels are high, the infiltration pipe/drainpipe also serves as a drainage system. To utilise the drainage function and to regulate groundwater levels, pumping/inspection drains can be installed in the bioswale, from which the water can be drained off.
The permeability and structure of the ground differ greatly from one location to the next, and even in different parts of individual locations. The structure of the ground and the groundwater levels must be determined before a bioswale system can be chosen for draining rainwater.
Today there are more than 500 residential areas with bioswales spread all over the Netherlands, especially cauliflower districts, renovation districts and villa districts. Research also shows that bioswales continue to function well even in extreme weather conditions such as drought and in low-lying areas of the Netherlands with average high groundwater levels and low soil permeability. Instead of the layer of gravel, also crates and the like can be used. Boogaard en Roest, 2020
Almost every bioswale system created requires above-ground drainage, as most parts of the Netherlands are relatively level and groundwater levels are relatively high. Underground drainage systems for precipitation are situated too low to connect properly to bioswale systems.
The highest possible groundwater level at which a bioswale system can be introduced is -1.5 metres, and preferably even -2 metres (see cross-section).
Roads that carry more than 1000 vehicles per 24 hours must always have a connection to the sewer system.
The area required to build a bioswale system is approximately 10% of the total connected and paved area of a new district. This percentage includes the use of gentle nature-friendly banks. Past experience in Enschede shows that the banks of a bioswale should be no steeper than 1:3 to allow mowing.
The bioswale system can be combined with green facilities in the district. When combined, bioswales and green facilities do not take up more space than the usual landscaping in a district. Boogaard et al., 2006
Planting more specifically suited vegetation in bioswales can allow bioswales to play a greater part in enhancing the town or city’s biodiversity and affords them a more diverse and pleasing appearance.
Naturally designed bioswales can play a part as ecological connection zones in districts and in towns and cities. What is particularly important is that the vegetation rise high enough to shelter and camouflage the movements of small mammals, amphibians and insects such as butterflies and grasshoppers. Deliberately designed and linked green-blue veins running through the town or city are of great importance to the town or city’s biodiversity and quality of life.
Unlike what is now common practice, more use could be made in bioswales of plants that are suited to fluctuating water levels as occur naturally in stream valleys and along banks. The most common choice now is for strong grass mixtures that can survive being walked on and extended dry spells. These grass mixtures are less capable of surviving extended wet spells. More varied vegetation, with grass for playing on and with higher plants for the diversity of flora and fauna, will add to the aesthetic appeal of bioswales. Bioswales can then fulfil more functions than water management alone. More diverse vegetation also improves the root systems in the ground, which will then remain more permeable in the long term. However, the increased plant volume will reduce storage capacity by 1% (a negligible amount).
Primarily natural bioswales require different kinds of maintenance. Whereas bioswales sown with grass mixtures need mowing at least once every week in the growing season (26 times a year), more natural bioswales demand less routine maintenance but more attention in terms of litter. Boogaard et al., 2003
SPECIES THAT CAN BE APPLIED IN A NATURAL BIOSWALE
Woody plants (trees and shrubs)
Scirpus lacustris lacustris
Higher growing plant species (no helophytes)
Other plant species (staying low)
Bioswales are an excellent instrument for improving urban water systems. A properly designed bioswale system buffers rainwater and allows it to infiltrate; this minimises overflow, improves the quality of surface water and prevents the ground from drying out. In periods of groundwater flooding, bioswales can serve as drainage systems.
Districts with bioswale systems have green-blue veins running through them that can help reduce heat stress. That effect can be enhanced by planting carefully selected vegetation along the bioswales.
Water surfaces and areas of vegetation have a small positive impact on air quality, in particular by binding particulate matter.
If bioswale systems are used in residential environments, the residents must be informed beforehand and be kept informed. The assessment of the bioswale system in Enschede showed that proper education about the system leads to a high level of acceptance and greater awareness of how cleaning products and other environmental pollutants are used. Citizens need to be involved in the planning and construction of bioswale systems in new development projects and restructuring situations. The bioswale system itself should preferably be realised in a public area. Some of the inflow gutters leading the water from the above-ground precipitation drains to the bioswale run across privately owned land. The design, the connections from private to public land and the upkeep of private land need to be agreed upon with the residents. Past experiences with the bioswale system in Enschede’s Oikos district, part of which has been realised on privately owned land, are less positive. Management by private individuals and on privately owned land is vulnerable.
In most cases, residents regard vegetation and water as positive elements. The same is true of bioswale systems. Bioswales with more diverse vegetation are perceived as even more positive. Bioswales that vary grass with more diverse vegetation can be perceived as flowery veins running through the district. However, bioswales require proper maintenance: otherwise they can quickly become polluted by litter and dog faeces. It is advisable to delineate special dog-walking areas.
Because a wadi combines a green area with a rainwater storage and drainage facility, several municipal disciplines are involved in the wadi from design to realization and during the entire lifespan of a wadi. And in addition there are also water boards, developers, home owners and residents. This requires coordination not only in budgetary terms and in terms of liabilities, but also during maintenance. While the greatest potential for success of a wadi lies in the combination of functions, this is also where the greatest chance of failure lies; Chapter 10 of Boogaard et al., 2006 lists the possible bottlenecks during the planning process from initiation phase to management phase.
In Enschede, to prevent the bioswales from becoming clogged or polluted, leaf waste and litter are removed and the overflow drains pumped clean twice yearly, and the drainage system is hosed clean once per year.
Road salting policies needs to take the bioswale system into account. In most cases this will not present any problems since most main roads are connected to the sewer system.
Bioswales where grass is sown need to be mowed once every fortnight on average, while natural bioswales need to be mowed twice a year. The grass need not be removed: organic matter from natural bioswales does have to be removed. Over a whole year, bioswales with grass are four times as expensive as more natural bioswales in terms of mowing. Boogaard et al., 2006
In general, installing a rainwater sewer is 50% more expensive than a bioswale system per connected m2 . The bioswale system is 40% more expensive in terms of upkeep. It should be noted that the costs of maintaining bioswales in part are costs for maintenance and management of green that are also to be made on a conventional sewage system. When applying nature-friendly bioswales, the costs are reduced because of a lower mowing frequency (2x instead of 26 times per year). Boogaard et al., 2006
Costs comparison bioswales / nature friendly bioswales / conventional sewage system
Construction costs per connected m2
Bioswale: € 5,17/m2 (incl. taxes, prices 2005)
Sewage: € 8,97/m2 (incl. taxes, prices 2005)
Maintenance and management costs per connected m2
Bioswale: € 0,37/m2 (incl. taxes, prices 2005)
Sewage: € 0,26/m2 (incl. taxes, prices 2005)
The construction of a bioswale is custom work. Nevertheless, design guidelines have been drawn up that are based on the experience gained with the first bioswales in Enschede and that can serve as a starting point and benchmark for the design of bioswale systems. The research report Wadi’s – Aanbevelingen voor ontwerp, aanleg en beheer Boogaard et al., 2006 contains in its summary an overview of the design guidelines that apply to wadi’s in the Netherlands. The report also contains tables that compare the guidelines in the Netherlands, Germany, England and Belgium.
More information on wadi’s can be found in the research report Boogaard et al., 2006 that can be found in de Kennisbank Groenblauwe Netwerken.
- Boogaard F., Roest A.; Leren van twintig jaar wadi’s in Nederland; Land+Water, 2020
- Boogaard F., Jeurink N. & Gels J.; Vooronderzoek natuurvriendelijke wadi’s, inrichting, functioneren en beheer, RIONED/Stowa, Utrecht/Ede, 2003
- Boogaard F., Bruins G., Wentink R.; Wadi’s - Aanbevelingen voor ontwerp, aanleg en beheer; Stichting RIONED, Ede, 2006