SEDIMENT TRANSPORT BY RIVER AND SEDIMENT INTERACTION WITH RIPARIAN VEGETATION GROWTH

概要

Sediment yield from a catchment is a complex phenomenon of weathering, land sliding, glacial and fluvial erosion which depends on geological as well as fluvial characteristics. The bed shear stress (τb), specific stream power (ω), and flow velocity (v) associated with maximum boulder size transport were determined throughout years 2003 to 2011 by using derived lower boundary equation from the available data for Kali Gandaki (KG) River at Setibeni, Syangja located about 5 kms upstream from a hydropower dam. The river transported an average of 40.904±12.453 Mega ton (Mt) of suspended sediment (SS) during the period 2006-2011 and the artificial neural networks (ANNs) predicted the daily SS rate and annual sediment load as 35.190±7.018 Mega tons (Mt) satisfactorily compared to multiple linear regression, nonlinear multiple regression, general power model, log transform models including sediment rating curve (SRC).
Fluvial discharge is a principal driver of SS transport in Himalaya mountain. The study basin has three hydrometric stations in main river, four hydrometric stations in its tributaries, and a hydropower reservoir site where suspended sediment transport is measured. This study also analysed relationship of specific discharge of main and tributaries rivers with rainfalls of the main basin and its sub basins together with SS transport. Annual anticlockwise hysteresis loops were developed between specific discharge and rainfall at three hydrometric stations of main KG River, whereas its tributaries namely Aandhi Khola, Seti Khola and Modi Khola followed anticlockwise, and Myagdi Khola showed an eight-shaped hysteresis loop. The clockwise hysteresis loop developed between SS concentration and fluvial discharge was eliminated and changed to linear relationship with the direct runoff discharge. The result showed that about 97% of SS was transported during monsoon season (June-September), comprising an annual weathering rate of KG basin estimated to be 4390 tons/km2/yr which is equivalent to 1.66 mm/yr between the period 2006-2017. Particularly, specific direct runoff discharge and monsoonal daily erosion rate of major rainfall events (>30 mm/day) showed linear increasing trend with rainfall in KG basin.
Stream flow alteration is one of the most noticeable effects of rainfall change patterns in this catchment. The study also depicted fluvial flow changing patterns of three hydrometric stations of KG River, four hydrometric stations of tributaries due to changing seasonal and annual rainfall trends in twenty-seven rain gauge stations located between the elevation ranges of 700 to 2744 m MSL of this basin over the period of 1957-2018. Monthly rainfall data were used to examine the rainfall and discharge trends. Mann- Kendall trend test (MKT) test along with Sen’s slope and sequential Mann- Kendall trend (SQMKT) analysis on homogenized time series data were used to evaluate the existence of monotonic trends, magnitude of trend and identify shifting of rainfall trend.
Finally, the ecological dynamics of riparian area interact with sediment transport in river system, which plays an active role in riparian vegetation growth in the floodplain. Frequently flood disturbance, extreme floods, sediment transport with nutrients and seeds by fluvial discharge, sediment deposition and erosion phenomena which frequently occurred in the floodplain, changed without vegetation land area to vegetative area and vice versa. The sediment grain size (D50) plays important role in riparian vegetation area coverage in the floodplain. Mathematical models describing vegetation growth in flood plain during short period of time available in literature. However, long-term prediction by modelling and validations of riparian vegetation are still lacking. The long-term prediction of riparian vegetation is important in perspective of floodplain management. In order to cover long-term vegetation growth modelling, a Dynamic Riparian Vegetation Model (DRIPVEM) was coupled with Dynamic Herbaceous Model used to establish the interactive relationship of sediment grain sizes (D50) and riparian vegetation in the two rivers: Narayani River floodplain, Nepal and Kuzuryu River floodplain, Japan. In context of Nepal, the calibration of coupled DRIPVEM, observation of species wise herbs and tree biomass, sediment particle sizes with nutrients availability in the floodplain should be conducted. The riparian vegetation observation and calibration of coupled DRIPVEM and prediction of spatial distribution of riparian vegetation in Narayani floodplain, Nepal would be future perspective of this study.

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