Hydraulic Geometry in subtropical headwater catchments, Florianópolis, Southern Brazil

Código

I-EBHE0146

Autores

EWERTHON CEZAR SCHIAVO BERNARDI, Luiz Felipe Pereira de Brito, Pedro Luiz Borges Chaffe

Tema

WG 1.05: Comparative understanding of runoff generation processes

Resumo

Headwater streams expansion and contraction modify hydraulic properties, cross-section stability, and nutrients, sediment and organic matter exchange rates. While At-a-Station Hydraulic Geometry is commonly used to describe how width, depth and velocity vary with discharge at a given cross-section, its applicability in headwater streams remains limited. Here, we test if these hydraulic geometry relations, commonly applied to alluvial rivers, are valid in coastal headwater streams in Southern Brazil. These streams are representative of the coastal landscape of Brazil comprising highly steep and heterogeneous morphology with step-pools, plane bed and riffle channel type. We analyzed discharges controls and stream stability by hydraulic geometry relations based on field measurements at eight cross-sections. We surveyed discharge, width, velocity, depth at each cross-section across four small tributaries (areas ranging from 0.03 km² to 1.12 km²) of the Peri Lake Catchment (20 km²), Southern Brazil. The catchment is in a transition between tropical and subtropical climate (27°S), mainly covered by subtropical Atlantic Forest. Twenty eight measurements were made from October 2023 to May 2024 with a handheld Acoustic Doppler Velocimeter. Top width was measured directly as the water surface between banks using survey tape. Mean flow depth was calculates dividing total area by top width. Discharge varies from 0.0002 m³/s to 0.0765 m³/s. At-a-station Hydraulic Geometry relations were generated and their exponents were analyzed by Rhodes Diagram. Range value for exponents of width (0.01 - 0.27), depth (0.06 - 0.94) and velocity (0.05 - 0.88) power law function were within the range found by other studies in steep gradient channels. Our findings indicates that banks are more stable (i.e., less erodible) than bed at six cross-sections, since width to depth ratio decreases when discharge increases. Velocity rises faster than depth, increasing sediment transport rates with higher discharges at most sites. Discharge increases are predominantly controlled by velocity in plane bed and riffle reaches. Laterally expansion and contraction is higher in step-pool reaches. Hydraulic Geometry relations presented are valid for in coastal headwater streams and can be useful in morphodynamic models, water management and stream stability studies.