A field experiment on streambed stabilization by gravel structures

We report an experiment to study particle restraint imposed by bed surface aggregate structures in a small headwater stream. Extant structures (clusters, stone lines, and partial stone cells) were manually destroyed in one reach of the channel (treated reach) whilst leaving them intact in an adjacent reach (reference reach), establishing two reaches of similar sediment texture and gradient that differed primarily in degree of structure development. After the first flood, the sediment yield from the treated reach was 32% greater than from the reference reach. For material > D84, the treated reach yielded 24× that from the reference reach. The critical Shields number was reduced by more than 18% in the treated reach to the value generally accepted for loose, heterogeneous sediment. The effect of structural development on particle mobility was subsequently studied in the treated reach over the course of three flow events that did not significantly exceed the Shields threshold, so that bed material transport remained low. After these events, mobility of the larger clasts was substantially reduced, permitting the inference that particle restraint in the treated reach approached that of the reference reach. This corresponds with direct evidence of structure development in the treated reach as indicated by the movement and emplacement of marked grains. A major flood, however, caused a reversion toward unstructured conditions in the treated reach, indicating that structures had not developed the resilience exhibited in the reference reach.