Application of fibrous streambank protection against groundwater seepage erosion

Abstract

Groundwater flow is one of the main driving factors in the 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, and when acting in concert with fluvial forces. Previous research has mainly focused on 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. Reduced-scale sandy (14 kN/m(3) unit weight) streambank models (45 degrees bank slope) with dimensions of 195 cm long, 100 cm wide and 110 cm high were constructed in an erosion flume. 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 measured pore-water pressures near the wall section. Erosion of sediment from the streambank initiated concurrently as seepage flow emerged on the bare streambank surface. Erosion volumes were computed by three-dimensional laser scanning. Triaxial compression tests on sand samples (fiber gravimetric content ranged from 0% to 1.0%) 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 boundary condition (BC), respectively. Seepage erosion rate and seepage discharge demonstrated a power-law relationship. Due to the increased cohesion, fibrous streambank protection with 1.0% fiber content effectively prevented seepage erosion during streambank experiments under the same BCs.