Suspended matter elimination in a pre-dam with discharge dependent storage level regulation

Since the phosphorus removal model for pre-dams was developed in the seventies and eighties of the last century, the size of pre-dams was mostly optimised to achieve maximum annual orthophosphate elimination at optimum average retention time of the water flowing through. They usually have been operated entirely filled all the time. At the Forchheim pre-dam (FPD; Saxony, Germany), technical measures allow adapting its filling (i.e. retention time) depending on discharge and turbidity of the inflow. FPD is kept partially filled in periods with low flow rates. In case of high discharge and turbidity, FPD automatically fills up and the residence time of the water on the rising limb of floods carrying the highest loads of nutrients, suspended particulate matter (SPM) and substances associated to them is prolonged accordingly. The influence of the modified operation of FPD on the turbidity elimination was studied from 2003 until 2006. It was shown by means of a model that the SPM retention during floods directly depends on (i) the ratio of the volume difference between complete filling and reduced storage level and the intensity and duration of the floods, (ii) the mode of operation, (iii) the stratification and mixing conditions in the pre-dam, and (iv) the specific settling velocity of the SPM imported. The estimated outflowing total SPM loads of the scenarios assuming partial filling at the start of the calculations were always lower than those of the respective scenarios considering permanent complete filling of the pre-dam. This indicates that adapting the fill level of pre-dams may successfully be applied to reduce SPM load into the main reservoir.