Classification, subsurface and surface sediment physical properties, and bank stability of non-perennial and perennial headwater streams of a tropical climate

Over 100 reaches of perennial streams (PS) and non-perennial streams (NPS) were classified based on the Rosgen stream classification. NPS were mainly type B (39%), characterized by moderate entrenchment and low sinuosity. The remainder were almost equally split between three different classes, highlighting the morphological diversity of NPS. Fifty-nine percent of PS belonged to type C; such streams are slightly entrenched, less sinuous and have a sequential riffle-pool configuration. Surface particles were significantly coarser than the subsurface in both stream types in thalweg and low flow areas, whereas in NPS, this was prominent, showing 4-5 times more armoring than PS. Even though the NPS had a significantly coarser surface sediment layer than PS in thalweg and low flow areas, its subsurface sediment showed similar particle sizes to PS; this is an indication of surface armoring and provision of more infiltration of fine particles in NPS. A two-year return period flow event did not result in a change of the cross-section profiles. In both stream types, the horizontal force required to uproot herbaceous vegetation with unexposed roots under moist conditions manually was higher than the tractive force at high flows; however, at bankfull flows, it was lower. Also, the uprooting force in PS was lower than that of NPS, indicating that NPS banks are more stable, and numerical analyses showed they are stable under the self-weight.