Application of fibrous streambank protection against groundwater seepage erosion

Authors: AKAY, ONUR - Okan Universitesi; OZER TOLGA - Okan Universitesi; FOX, GAREY - North Carolina State University; Wilson, Glenn

Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/6/2018
Publication Date: 8/31/2018
Citation: Akay, O., Ozer Tolga, Fox, G.A., Wilson, G.V. 2018. Application of fibrous streambank protection against groundwater seepage erosion. Journal of Hydrology. 565:27-38. https://doi.org/10.1016/j.jhydrol.2018.08.010.
DOI: https://doi.org/10.1016/j.jhydrol.2018.08.010

Interpretive Summary: Groundwater flow is one of the main factors causing erosion of streambanks particularly during conditions of flow back into the stream of water stored in the streambank. Sediment carried by the seepage flows back into the stream causing bank failure. Previous research mainly focused on the seepage erosion processes, whereas in this study, a practical solution using 6-mm-long polypropylene fibers that mimic the behavior of plant roots was evenly mixed with bank material for investigation in laboratory streambank experiments. For this purpose, sandy streambanks were modeled with dimensions of 195 cm long, 100 cm wide and 110 cm high constructed in an erosion flume. The surface angle of the model was 45o. Two different seepage conditions were tested within the streambanks by maintaining water levels of 50 cm-H2O, and 100 cm-H2O in the upstream section of the erosion flume during experiments. Models were equipped with instruments to measure the water pressures within the streambank and near the side-walls. Erosion of sediment from the streambank started at the same time as seepage flow emerged from the bare streambank surface under 50 cm-H2O level. As the water level was raised to 100 cm and the groundwater seepage flow rate increased, the erosion increased. Erosion volumes were computed by laser scanning. Triaxial compression tests on sand cores indicated an increase in streambank cohesion by adding fiber. Fibrous streambank protection with 0.3% fiber content reduced the total amount of seepage erosion by 35%, under 50- and by and 47% under 100 cm-water level. Due to the increased cohesion, no seepage erosion was observed during streambank experiments with fibrous protection of 1.0% fiber content. Lastly, the sediment discharge and seepage flow discharge data were used to determine a mathematical expression that described the seepage erosion sediment transport.

Technical Abstract: Groundwater flow is counted as one of the main driving factors causing erosion of streambanks particularly during return flow of bank storage as sediment particles on the bank face may be entrapped or liquefied by seepage flow into the stream. Previous research mainly focused on the seepage erosion mechanisms, whereas in this study, a remedial solution using randomly distributed 6-mm-long polypropylene fibers mimicking the behavior of plant roots in slopes was investigated by laboratory physical streambank model experiments. For this purpose, reduced-scale sandy streambank models with dimensions of 195 cm long, 100 cm wide and 110 cm high were constructed in an erosion flume. The surface angle of the model was 45o and dry unit weight of the sand was 14 kN/m3. Two different seepage gradients were generated within the streambanks by maintaining piezometric heads of 50 cm-H2O, and 100 cm-H2O in the upstream section of the erosion flume during experiments. Models were equipped with vibrating wire piezometers to measure the pore-water pressures within the streambank. In addition, tensiometers were installed on one side of the erosion flume to measure pore-water pressures developed near the wall section. Erosion of sediment on the streambank initiated concurrently as seepage flow emerged on the bare streambank surface under 50 cm-H2O hydraulic head boundary condition (BC). As the seepage gradient and hence the groundwater seepage flow discharge increased under 100 cm-H2O piezometric head BC, the extent of the erosion increased. Erosion volumes were computed by three-dimensional laser scanning. Under the light of the triaxial compression tests on sand cores (fiber gravimetric content ranged from 0% to 1.0%) that indicated an increase in cohesion by fiber content, fibrous streambank protection with 0.3% fiber content inclusion reduced the total amount of seepage erosion by 35%, and 47% under 50-, and 100 cm-H2O piezometric head BC, respectively. Due to the increased cohesion, no seepage erosion was documented during streambank model experiments with fibrous protection of 1.0% fiber content inclusion under the same BCs. Lastly, the sediment discharge and seepage flow discharge data was used to determine the empirical coefficients of a dimensionless seepage erosion sediment transport model.