Fieldcase Venner Bruchkanal by LWK Niedersachsen
The Lower Saxony Chamber of Agriculture is involved in field demonstrations at Dümmer lake , the second largest lake (13 km²) in Lower Saxony. The lake suffers from eutrophication which is probably the result of intensive farming in the catchment area (324 km2) and already a matter of public concern since 1972. The large variety of beneficiary interests on lake Dümmer (tourism, water sports, commercial fishing, natural reserve, residents...) predisposes its catchment area as a good study region for NuReDrain and other simular projects.P concentrations in the drainage water are too high (up to 4,0 mg/l) to meet the EU standard in the receiving surface water. The basic concept to reduce these P losses in drainages is to install an experimental inline P filter with a P-sorbing material (PSM) just before the end of the drainage pipes. In order to remove mineral and organic substances from the drain water, a pre-filter is installed upstream of the P filter. The water flows through the pre-filter into the P filter material that allows the removal of P from it before entering the receiving surface water (Figure 2). There is only a low hydraulic pressure head between the level of the tile drain and the surface water in the creek. The working principle of drainage at that (specific) site is suction drainage by streaming (creek) water. Therefore, a horizontal percolation of the P filter had to be facilitated.
Specific challenges at the field site
The measuring operation at the site is significantly influenced by two factors. On the one hand, by the input of amorphous organic matter (AOM) from the drainage pipes into the individual filter stages. The consequences of AOM discharge are clogging, piling up of the water, preferential flow or overflowing of the P filter and thus a reduction in the effectiveness of the P filter. Secondly, the flow velocity in the Venner Bruchkanal plays an important role: only with sufficient flow velocity is there sufficient suction on the drainages so that they can discharge their water. If the flow velocity is not sufficient, water flows back from the Venner Bruch canal into the P filter. The consequences are free standing water on the P filter without outflow. No measurements and values can be collected causing a gap in the data table. Under such conditions, the measuring operation must be stopped. The main cause of insufficient flow velocity is a high vegetation cover in the ditch. After mowing and dredging by the watercourse maintenance association in autumn/winter, the flow velocity increases and the drainage discharge is ensured.