Evaluation of FLO-2D runoff and sediment loading models in tropical humid to semi-arid catchments of Hawaii
Abstract
The FLO-2D flood routing model with topography derived from 10 m DEMs was used to estimate runoff and suspended sediment fluxes of the Hanalei and Kawela catchments located on the Hawaiian islands of Kauai and Molokai, respectively. These catchments differ in average annual rainfall (~17.0 mm/day in the Hanalei vs. ~3.81 mm/day in the Kawela), discharge (~540 x 106 L/day in the Hanalei vs. 7.80 x 106 L/day in the Kawela), suspended sediment content (~13.3 mg/L in the Hanalei vs. ~39.8 mg/L in the Kawela) and vegetation cover (evergreen forests in the Hanalei vs. grassland in the Kawela). However, they are similar in catchment area (~80 km2), bedrock lithology (basalt and marine carbonates) and relief (3-70% slopes normal to river channel). Models were calibrated and validated using 4 years (2005-2008) of daily observed discharge and suspended sediment content data. Alternative scenarios of spatial distribution of rainfall, evaporation, infiltration and surface roughness were evaluated. Once surface flow begins, slope becomes the principal parameter controlling flow direction in both catchments and velocity and a convex topography promote a rapid spreading of the flow towards the estuaries, especially considering the smoothing effect on surface roughness of the gently dipping lowlands. Monthly runoff fluxes predicted by FLO-2D compared well with the measured data in the Hanalei; however, a significant difference between the actual and simulated runoff was observed in the Kawela River in the dry season. Predicted and observed monthly sediment yield correlated reasonably well in both catchments; however for the events of small rainfall magnitude, the model generally overestimated sediment yield, while the opposite was true for larger rainfall events. Our modeling results showed that sediment loading is strongly influenced by infiltration which, in turn, is controlled by the soil, land use and vegetation properties of the catchments. Lower proportions of riparian vegetation and soil cover in the semi-arid Kawela Catchment inhibit water infiltration and promote more contact time and surface area at the water-rock interface. This enhances physical and chemical erosion of bedrock material and promotes higher suspended sediment yields. Overall, FLO-2D performs well in small tropical river systems where climate and soil properties are well constrained. The model allows identification of areas prone to sediment loading and is proposed as a valuable tool in long-term catchment management.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFMEP53B0626H
- Keywords:
-
- 1807 HYDROLOGY / Climate impacts;
- 1815 HYDROLOGY / Erosion;
- 1862 HYDROLOGY / Sediment transport;
- 1928 INFORMATICS / GIS science