Ruwenbos is a residential location on the south side of Enschede with over four hundred houses. They are mainly owner-occupied houses: from premium sales houses to houses in the free sector. The price range varies from € 144,000 to € 476,000 (2023 price level). There are also seventy luxury rental flats. Construction of the first homes started in autumn 1994.
It is a neighbourhood with a garden-city character in which almost all existing landscape elements have been retained. For instance, the preserved wooded banks still determine the structure in the neighbourhood and all property fences are hedges. And water drainage in the form of the new bioswales follows the original site profile.
In the early 1990s, the Enschede municipal authorities introduced a new water system in the form of bioswales in the Ruwenbos district, and later also in the Oikos district. The goals were to reduce peak drainage and overflows, to manage groundwater flooding and shortages and to improve the quality of surface water and groundwater. The most striking element in this neighbourhood is the bioswale system, called wadi in Enschede, where rainwater infiltrates into the ground through ditches.
Most of Ruwenbos has no rainwater sewer. The downspouts of houses discharge above ground into gutters that drain the water through the garden to the street or back path. The paving of streets and paths is hollow, without gutter or pavement gullies, and drains accurately into bioswales. Rainwater from neighbourhood access roads and car parks does not go to the bioswales but is collected in street gullies connected to an improved separate sewer system. A separate system carries household wastewater from the neighbourhood to the treatment plant.
The bioswales infiltrate rainwater through trenches into the bioswale soil. Below the bioswale bottom is an infiltration device containing a drainage pipe. In the bioswale walls are gullies (also known as ‘slobber heads’) at about 0.25 m above the bottom. The design assumption is that the bioswales are filled to this level a maximum of once every two years, to prevent muddy situations and to avoid attracting mosquitoes.. These slobber heads pass through the bioswale bottom and the infiltration facility and lead the water directly into the drainage pipe. After the water has filled the bioswale up to and including the slobber heads, the water in the bioswale can still rise to a level that occurs once every twenty-five years: at that time, there is about 0.35 m of water in the bioswale. If the water rises above this water level, the bioswale acts as an open ditch and the water flows above ground to the next bioswale compartment or to surface water.
Drainage is installed in the infiltration facility under the bioswale bottom. The drainage spreads the water in the facility and can also help dewater the area when groundwater levels are high. In this way, a bioswale can combine several functions in one facility: infiltration at low groundwater levels (in summer) and drainage at higher groundwater levels (in winter).
Walking through the Ruwenbos neighbourhood, a bioswale looks like nothing more than a wide ditch overgrown with grass, a lowering of the ground level. The bioswales run right through the residential area, following natural depressions in the terrain. At its lowest point, the bioswale is about 3 m wide and 40 cm deep. Usually, a bioswale is dry, only in the period after a rain shower does it contain water. From the bioswale, the water sinks into the soil. Bioswales feed the groundwater by infiltrating rainwater from roofs and streets into the soil. Bioswales ensure that rainwater drains more naturally than via a sewer system. Moreover, in periods of high groundwater levels, the bioswale has a draining function. Finally, the bioswale can serve as a connecting zone for animals and can attract flora and fauna to the urban area.
The top layer of the bioswale consists of improved soil whose composition has been chosen to make it both a good soil for vegetation and optimal infiltration for the situation. The humus in the soil helps the grass develop better and makes the bioswale more resistant to treading; a proportion of sand in the soil ensures good infiltration.
Below the top layer is the drainage trench. The drainage trench consists of a geotextile-wrapped package of granules; it stores water until it infiltrates into the subsoil through the walls and soil. When the water level rises above the level of the slobber heads, these drain the water directly to the drainage pipe laid in the package. This drainage pipe follows the course of the bioswale from compartment to compartment and eventually acts as an overflow of the system. Along the way, the water may possibly infiltrate into other compartments of the bioswale before reaching the surface water.
In the implementation of the bioswales in Ruwenbos, rainwater from the roofs in the neighbourhood is collected above ground and directed to the bioswale via gutters in the streets. The bioswales meander through the neighbourhood; they are perpendicular to the roads. The intersections of the streets with the bioswales are called fords. These fords are higher than the bioswale soil, but lower than the street. This allows water to flow easily from the streets into the bioswale. Road users experience the fords as reverse speed bumps. The bioswales eventually lead to surface water in the form of large ponds at the edge of the neighbourhood.
The maximum storage height in the bioswale is adjustable and intended to set a summer or a winter level; above that maximum, the control wells at the end of a bioswale compartment transfer water directly into the drain pipe of the next bioswale compartment. Alternatively, the storage can be turned off completely. And when the groundwater level reaches the drain pipes, these begin to drain the infiltrated water and the excess groundwater or drain the area until the groundwater level is acceptable again.
In terms of costs, [Boogaard et al, 2006] shows that a bioswale is more expensive per linear metre than a sewer leading to a treatment plant but that a bioswale is cheaper than a sewer leading to a treatment plant when converted to costs per m2 of connected paved surface. Maintenance of a bioswale is more expensive, but it should be borne in mind that the bioswales also form green areas, which would (partially) have to be present in case of a conventional sewer; in that case, part of the costs would be borne by green management. Maintenance is important to keep the bioswale is good condition and requires coordination between different municipal services.
By combining soil infiltration with green space in the neighbourhood, the bioswales need not take up more space than in traditionally designed neighbourhoods.
Over time, the bioswales in Ruwenbos continued to perform well: the emptying time of the bioswales does not seem to increase; clogging of the top layer, silting up of grass to geotextile, there are many conditions that could lead to this and thus require vigilance and awareness on the part of residents. Micropollutants did show up in the groundwater and the top layer of the bioswale; these can be traced to construction materials from the paving but more long-term research is needed to determine whether they are a problem.
Residents actually only seemed to become more positive about living in a bioswale district. A stumbling block for some was the design of the level junctions of street and bioswale, the fords. Also, a large proportion of them would like more information on the rules to follow and would appreciate additional ways of water management such as rainwater use in the home. In 1999, as many as 94% of the residents were pleased with the bioswale system. By 2005 that number had grown to 98%. However, the detailing of the gutters requires attention to prevent residents from falling. Of the residents, 50% would like to see additional measures, such as using rainwater to flush toilets, rainwater tanks and ponds. The residents would prefer the bioswales to have more natural vegetation and perceive the bioswale system as a qualitative improvement to the district’s appearance. [Boogaard et al, 2006]