Application and evaluation of the HEC-RAS - riparian vegetation simulation module to the Sacramento River

Life cycles of riparian vegetation are substantially impacted by river flow regime, groundwater and morphodynamics. The polygon-based riparian vegetation simulation module (RVSM) was developed and integrated into HEC-RAS one-dimensional flow model to predict spatially-explicit seed germination, seedling establishment, plant growth and mortality in response to fluvial processes. The HEC-RAS - RVSM system was applied to the Sacramento River reach to evaluate its capability in modeling temporal and spatial changes of riparian vegetation and the interactions between flow and riparian vegetation dynamics. River hydraulics, groundwater level and five vegetation types of the study reach were simulated for the eight-year (1999-2007) period. Model results demonstrate that the HEC-RAS - RVSM system reproduced the coverage increase of cottonwood, riparian shrub, invasive species and grass as well as the coverage decrease in mixed forest over the eight-year. The RVSM was able to capture sites for cottonwood establishment observed on certain point bars. The modeled variations of cottonwood coverage in response to dynamic flow regime facilitated determining and managing environmental flow for riparian vegetation restoration. The height of capillary fringe and root growth rate were two key parameters influencing riparian vegetation distribution.