Pilebroen in Allinge. Diversion of roof run-off in existing area

Pilebroen. Diversions in Allinge – utility company establishes soakaways, citizens take over

The community sewer in a catchment area 8.26 ha in size in Allinge has more than 30 overflows per year to Knuds Næs, which is therefore under threat as a beach. The municipality is now demanding that the overflows are reduced to no more than 2 occasions per year. Bornholms Forsyning has assessed that separation is not a solution as rocks will make things disproportionately expensive. The present community sewer is located in a natural watercourse, but separation would require work to be done on the rock. Thus expansion of the community system to include additional basins has been rejected as a solution due to a lack of even ground. Therefore, a solution has been selected in which roof water and road runoff from approx. 250 households (all single-family residences) will handle the rainwater locally in sokaways. This ought to mean that the number of overflows is reduced to below 2 per year.

This work will start off with diversion of a partial area containing 46 households and associated roads. Bornholms Forsyning has entered into a contract with an authorised sewer contractor who is responsible for the construction work in both the public and the private areas. It is anticipated that 36 households will have soakaways with overflows to an existing clay pit, watercourse or ditch along a disused railway, while the last 10 households will have larger soakaways constructed with no overflow to the off-site recipient. All water which is currently connected to the sewer will be diverted, i.e. roof water, any water from car ports, terraces, etc.

When the construction work is complete, the landowners will take over responsibility for the systems and will be unable subsequently to re-join the community sewer for rainwater as the Waste Water Plan states that the sewer must carry only waste water in future. Any surplus amount (the connection fee for rainwater less capital costs) will be paid to the landowners, who are each guaranteed a minimum of DKK 1,000. Rainwater from the public roads will be fed from each individual road drain to soakaways constructed beneath the pavement. There is overflow to the sewer from the soakaway. 

Besides expansion of the drainage capacity (as described above), it is necessary to implement measures to prevent backwater from upstream agricultural areas behind Allinge. These contain clay and cover an area of 8.9 ha. The field drains are unable to keep up during long periods of precipitation or in heavy rain, and the rainwater runs off the surface and down through the town, exacerbating the problem with overflow in the sewer. To prevent this, ditches and channels will be constructed outside the town where the water will be delayed and fed more slowly to a watercourse. An elevation model generated on the basis of a digital terrain model will be used to assess where the water comes from, where it flows to, and where best to position ditches and channels as a result.

This process has begun with Bornholms Forsyning preparing a supplement to the Waste Water Plan in February 2008, in which the above solution is described. This proposal was submitted for consultation on 26 June 2008, and a public meeting was held at Borholms Højskole on 2 October 2008 during the consultation period. At this meeting, the utility company (Forsyning) ran through the background and proposals. The municipal council adopted the proposal on 22 January 2009. An outline project has been prepared for each property.

Jakob Post, Bornholms Region Municipality: "We need to think innovatively, and without doubt diversions are the best solution here. With the selected model, we will go in and help citizens overcome the technical difficulties with regard to how the soakaway is to be established and provide guidance on how citizens have to maintain the soakaway once it has been handed over."

Establishment costs:
For each individual site, it is envisaged that resources equivalent to DKK 14,000-15,000, totalling DKK 700,000, will be used on average.
A budget of DKK 1,560,000 has been set for reestablishment of watercourses all the way down to the coast (partly in pipes).  
The results will mean a smaller reduction in pumping charges (total head 135 m) and treatment at Tejn Waste water treatment plant. Estimated at approx. DKK 200,000 per year.

Operation and maintenance
Responsibility for operation and maintenance of the systems on private land will be transferred to the landowners. Bornholms Forsyning will retain responsibility for soakaways in public roads and is responsible for emptying of gullies with sludge traps once a year. All soakaways for road runoff must be designed in a manner which facilitates cleaning.

Catchment (the first 46 households)
Residential area of 8.26 ha (2.07 red. ha.)
Average annual precipitation is unknown, estimated at 650 mm. (Cf. DMI's precipitation map + 100 mm.)
Surface runoff in the order of 150 m3 has been seen for several years.
The total volume of extraneous water is estimated to amount to approx. 30,000 m3 of drainage and surface water annually.

Service life
The service life of systems on both private and public land is expected to be in the order of at least 50 years. There are already soakaways more than 30 years old in the area.

Hydraulic capacity
Roof water and road runoff from 46 properties must be handled by means of a total of approx. 300 m3 of soakaway units, with overflow to a total of 2 existing recipients (marlpit, ditch), a re-established historical watercourse, or two private gardens. Soakaways with overflow are designed for a 2-year event, while soakaways without overflow are designed for a 10-year event.

The solution significantly reduces the discharge of untreated waste water to Knuds Næs. Citizens have been notified that they must not use algaecides and pesticides. Road runoff is fed untreated to soakaways, from which it can seep out. The risk of groundwater contamination is deemed to be insignificant as the traffic load is very low. There are no groundwater interests. Groundwater formed in the area flows to the Baltic Sea.

Pilebroen 2, DK-3770 Allinge

Name: Jakob Post
Organisation: Bornholms Region Municipality
E-mail: jakob.post@bornholmsforsyning.dk
Telephone: +45 56 92 24 50

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