Effect of Flow Regime on Accelerated Recruitment and Establishment of Vegetation in Unregulated Sandy Rivers – A Case Study at Naeseong-cheon Stream in Korea

This study investigates possible causes for accelerated vegetation recruitment and growth in an unregulated sandy stream in Korea. Flow and sediment regime, represented here by critical bed shear stress, is considered a dominant physical factor affecting the stability of sediment bars with vegetation at germination and seedling periods. A good inverse correlation is obtained between the area of vegetation expansion on sediment bars and the annual peak flow during early summer (June and July) where other minor effects seem not to be important. Three comparative reaches are selected in a stream; one strongly meandering reach, one moderate, and one mildly meandering reach. Hydraulic calculations are made for the whole stream reaches using a verified numerical model to evaluate two-dimensional distribution of the critical bed shear stresses at those reaches. Results are compared with past aerial photos of the study stream. Other critical physical factor of soil moisture is considered not to be limiting to the present case of sandy stream. The results are relatively in good agreement between the calculated distribution of bed shear stress and actual distribution of vegetation on the riparian sediment bars, indicating that stability of riparian sediment bars would be critical for vegetation recruitment and growth.