Økologisk byfornyelse og spildevandsrensning, 44 – Lokal afledning af regnvand - effekten af et detaljeret projektforslag på Tingbjerg

Local Stormwater Control - Effects of "Tingbjerg" Porject Proposal

Summary and conclusions

Stormwater source control, i.e. local detention and infiltration, may increase the natural flow in urban watercourses and the groundwater recharge, while at the same time reducing the load on sewer systems and wastewater treatment plants. However, there is a lack of detailed investigations into thesignificance of these effects.

The primary objective of the project is to assess in detail how the water balance changes due to source control of stormwater in a specific housing area, i.e. how the groundwater recharge, the runoff from surfaces and drainage pipes to the combined sewer, and the overflows to receiving waters are affected. The secondary objective is to demonstrate how modern simulation models can be applied in practical planning to assess the functional behaviour of complicated source control systems and their impact on the water balance. Finally, the project aims at demonstrating how stormwater source control can be a positive contribution to spatial planning of green areas between buildings, where other interests than stormwater management are also at stake.

Tingbjerg in Copenhagen municipality was selected because it is topographically separated from the neighbouring areas and because, just like in most areas around Copenhagen, Tingbjerg is situated on glacial moraine deposits. A section of the Tingbjerg area was selected for further investigations involving borings and monitoring of groundwater levels, and calibration of a groundwater model.

A project scenario was developed, where stormwater is made visible on the surface before infiltrating into the ground. Stormwater is collected from roofs, footpaths and traffic areas, passes vegetated surfaces and concentrates in troughs, ponds and infiltration trenches. The different source control elements are connected with internal overflows, and the trenches receive water from the overlying troughs and from pipe systems. An overflow at the downstream end of the system allows water to escape to the external sewer system during heavy rainstorms. The technical solution is illustrated in the report.

A model of the interconnected source control structures that are part of the project scenario was made using the STORM simulation software. Furthermore, infiltration time series computed with STORM was used as input to a MIKE SHE model used for simulating infiltration, building drainage and groundwater recharge in the project area. Finally, the results from the detailed simulations were upscaled to the whole Tingbjerg area, and a MOUSE model was used to simulate the combined sewer overflows from the area.

From the detailed simulations appears that the groundwater table rises during wet periods and limits infiltration while increasing the runoff via building drainage pipes. The effect of this on the sewer system is, however, limited since the runoff via building drainage is relatively slow and therefore does not result in overflows to the receiving water to the same extent as direct surface runoff. The total water balance for the Tingbjerg area shows that:

Rainfall data used as input to the simulations indicate 676 mm of precipitation in 1995, which is distributed between evapotranspiration (32%), runoff via the building drainage and sewer pipes (42%) and groundwater recharge (26%) for the situation without stormwater source control.
Stormwater source control corresponding to the project scenario in the whole Tingbjerg area would increase evapotranspiration to 41% of the annual rainfall, corresponding to an almost 30% increase. The reason for this change is a combination of paved surfaces being changed into open surfaces, free water surfaces being present and generally more moist soil conditions, which increases the potential evaporation.
The runoff from the sewer system would be reduced to 25% of the annual rainfall, corresponding to a 40% decrease. The direct surface runoff would actually be reduced by 58%, because part of the runoff from paved surfaces is infiltrated, but this effect would be limited by overflows from the source control system to the sewer system during heavy rainstorms and temporarily elevated groundwater levels during the winter.
Consequently, the groundwater recharge would be increased to 33% of the annual rainfall, corresponding to a 26% increase compared with the situation without stormwater source control.

The urban drainage system simulations show that stormwater source control in the whole Tingbjerg area corresponding to the project scenario would reduce the frequency of overflows from the two overflow structures by 75-85% and reduce the annual overflow volumes by 80-90%.

The project indicates that it is possible to assess the hydrological effects of urban stormwater source control on a realistically detailed level using modern simulation tools. There are, however, difficulties with integrating simulation models for stormwater source control, groundwater and urban drainage systems, and the modelling uncertainties are difficult to quantify in a planning situation. Realistic projects that utilise the potential for source control fully while respecting the local hydro geological constraints, furthermore requires collaboration and information exchange between several professional disciplines. Integrating the planning basis and simulation models in a common GIS interface will definitely make it more realistic to switch between sketching and simulation while planning ambitious source control projects intended to present more than just a demonstration effect.

It can be concluded that stormwater source control can be an effective way to reinstate the hydrological cycle in urban areas and reduce the environmental impact from urban drainage systems. There are, however, several issues that impede a development in this direction. This project did not include assessments of how soil, groundwater and surface water/sediments are affected by stormwater pollution, how private and public stakeholders are informed and involved in stormwater source control projects, and how economically attractive stormwater source control solutions are.