Are woody debris jams suitable to prevent the Noor brook channel bed erosion? Groundwater and bank erosion impacts.
The seasonal variations of the chemical composition of shallow groundwater in the Noor brook (Limburg, The Netherlands) can be affected by drastic changes of seepage through this incisive and erodible brook. Channel bed erosion can lead to increased seepage directly to the brook system and decreased seepage to the adjacent wetland. The groundwater in the top soil root zone can be modified from groundwater type to rainwater type. To maintain or increase the Noor brook sediment accretion and prevent the erosion, dead woody debris jams were installed.
To study the woody debris jams effects on groundwater, sediment transport and banks erosion the research method combines field survey and modelling. Piezometric groundwater level and electrical conductivity in 13 piezometers and 209 pins lengths distributed in channel bed and banks were gathered from October 2016 to July 2017. The model HEC-RAS was used to compute vertical bed changes in response to dynamic sediment mass balance and bed processes, including lateral bank erosion by using the coupled model Bank Stability and Toe Erosion Model (BSTEM).
The research area has a wide variety of characteristics, as banks morphologies, channel bed gradients, sediment grain sizes, distribution of riparian forest, groundwater influences and the woody debris jams. All these factors intervene in a complex geomorphology interrelation. The high number of pins installed provides hydraulic effect evidences of woody debris jams influencing the sediment transport and bank erosion in the Noor brook research area. The woody debris jams installed promote internal preferential flows, increasing the erosion inside of the jams by a rate of 5 cm per year and decreasing sediment supply derived from bank erosion processes.