Hydraulic characteristics of groundwater-fed open ditches in a peatland

Abstract

The Environmental Ministry of Schleswig-Holstein has implemented a novel peatland management strategy in order to utilize the high nutrient retention potential of degenerated peatlands, and to improve the habitat at the same time. The effect of raised water levels and passive land use management on hydraulic properties of heavily vegetated and groundwater-fed open ditches was investigated at the ‘Eider River Valley’. The flow properties of selected representative open ditches were regularly assessed at different discharge levels during different vegetation cover periods. The hydraulic residence time is predominantly a function of the open ditch geometry and the overall obstruction cover including the presence of macrophytes. Typical mean discharges measured ranged between 0.2 and 2.5 l/s. Within-ditch vegetation and other hydraulic obstructions, like accumulated silt and organic debris, slow down the hydraulic residence time. The experimental results were evaluated to acquire more realistic values for total bed-roughness under different hydraulic flow regimes. Further findings show that the lower parts of the open ditches were flooded by the Eider River due to the absence of a mowing scheme. Typical water level increases were 0.5 m. This led to a reduction of the velocity and an increase of the dispersion within the ditch.