Retention of Sediment and Nutrient Loads with Peak Runoff Control

The purpose of this research was to evaluate peak runoff control as a water protection method to reduce sediment and nutrient loads. Increased eutrophication of surface waters and risk of floods demands cost effective methods to reduce pollutant input and risks of flooding. With the peak runoff control it is possible to cut the main peaks and store the runoff water temporarily in ditches. The method decreases the suspended solids (SS) and nutrient loads by reducing flow velocities, and improving the settling of sediment particles. The method was tested in two heavily drained adjacent peat harvesting areas suffering considerable erosion. The peak flows were cut by 27–87%, the SS load by 61–94%, the total nitrogen $\left({\mathrm{N}}_{\mathrm{tot}}\right)$ by 45–91%, and the total phosphorus $\left({\mathrm{P}}_{\mathrm{tot}}\right)$ load by 47–88%. The peak runoff control method operated most effectively during extreme events when most of the SS load is transported. A detailed particle analysis of runoff water showed that water detention reduced the median particle size of SS load as the largest particles settle. The results clearly indicate that the peak runoff control is an effective method to control the sediment loads and peak flows from peatland drainage.