Hydraulic, Vegetation and Water Quality Characteristics of Heavily Vegetated Groundwater-Fed Ditches in a Riparian Peatland in Northern Germany

The Environmental Ministry of Schleswig-Holstein (Northern Germany) has implemented a novel peatland management strategy in order to use the high nutrient retention potential of degenerated wetlands and peatlands, and to improve the habitat conditions at the same time. The effect of raised water levels and passive land use management on hydraulic properties and water quality of heavily vegetated and groundwater-fed ditches at one nationally important wetland case study at the Eider River valley was investigated. Most ditches in the Eider River valley were small and overgrown. The flow properties of selected ditches were regularly assessed at different discharge levels during different vegetation cover periods. The hydraulic residence is predominantly a function of the ditch geometry and overall obstruction cover including the presence of macrophytes. Vegetation cover and other hydraulic obstructions such as accumulated silt and organic debris slow down the hydraulic residence time and lead to a change in the water quality along the ditch. Experimental results were evaluated to get more realistic values for total bed-roughness and pollution loading under different hydraulic flow regimes. The total-roughness K_m values of both vegetated and excavated ditch stretches were about 1 and 10, respectively. However, K_m is actually a crude "fudge factor" depending predominantly on the hydraulic radius rather than on the total-roughness. This is in contrast to the theory presented in the literature. The biochemical oxygen demand and nutrient concentrations of 24 ditches were analyzed. Mean biochemical oxygen demand, ammonia, nitrite, nitrate and ortho-phosphate concentrations were 6.0 (+/-2.74), 0.2 (+/-0.44), 0.0 (+/-0.02), 1.6 (+/-2.04) and 0.0 (+/-0.02) mg/L, respectively. Elevated biochemical oxygen demand and nitrate concentrations for upstream ditch cross-sections were apparent.