HYDRODYNAMIC CONTROL OF AN EMERGENT AQUATIC PLANT (SCIRPUS-ACUTUS) INOPEN CHANNELS

Control of emergent aquatic plants such as tule (Scirpus acutus Muhl.; Bigel.) is of direct interest to managers of surface waters in Wester n North America. Where conditions of water velocity and depth occur th at permit this and similar species to colonize and grow, their clonal habit may restrict, or even block, open channels within several season s after their establishment. Fortunately, sufficient flow depth and ve locity naturally prevent these plants from growing into and blocking c hannels. We investigated physical constraints for tule stem growth wit h the ultimate intent to apply this knowledge in rehabilitating 60 mil es of the diverted Owens River in Eastern California, presently choked with emergent growth. Bending stress resulting from hydrodynamic drag on tule stems was found to induce lodging; permanent deformation and consequent loss of function. The depth-velocity envelope describing th is process (at 95 percent confidence) is uD/d = 12.8 where u = average velocity acting upon the stem (m/s), D = local depth of flow (m), and d = tule stem diameter at the point of attach ment (m). Maintaining a discharge or reconfiguring a channel so this critical depth-velocity- stem diameter envelope is exceeded (predictable using flow models) thr ough the summer growing period should prevent encroachment into an act ive channel.