Does sediment shortage cause river forestation? A numerical modeling approach

Takashi Asaeda, Kelum Sanjaya

Monday 29 june 2015

16:00 - 16:15h at Europe 1 & 2 (level 0)

Themes: (T) Hydro-environment, (ST) Ecohydraulics and ecohydrology

Parallel session: 3G. Environment - Ecohydraulic

Intense forestation of stony sediment bars and floodplains at the midstream of a river is often a serious problem. Originally the habitat is stony or sandy and low in nutrient. Nutrient limitation restricts the succession of herb and it greatly depends on nutrients enrichment of soil. Though the soil nutrient is important for tree growth, their recruitment on a site mostly relies on the synchronization of time of seed/propagules dispersal, flooding and the magnitude of floods. After the germination, the growth of trees is governed by self-thinning, hydrological factors and habitat conditions. Based on these processes a numerical model (DRIPVEM) was developed which consists of four modules, viz. ‘hydrological’, ‘tree’, ‘herb’ and ‘nutrient’. The hydrological module provides the hydro-geomorphological information. The tree module simulates the colonization of trees with the flood level in the suitable season. The density and the biomass of individual trees are then simulated age specifically by empirical formulas. The herb module provides the herb biomass based on the habitat condition, and shading by trees. The nutrient module provides the nutrient concentration of the surface sediment by each component of the nutrient budgets. Fair agreements between observed and simulated data sets were found while validating the model with several data sets. Afterward, the model was applied to simulate the effect of sediment accumulation and dissipation in a selected reach of Hii River japan, with assumed magnitude of flood with different frequencies. This showed that, increasing the sediment deposited area fraction substantially reduces the fraction of vegetation coverage. The observed condition by analyzing past aerial photograph in the reach showed a substantial delay of vegetation succession at sediment accumulated areas compared to eroded areas, justifying the simulated results. Therefore it suggests that the reduction of movable sediments in a river channel caused the intensive vegetation invasion.