In this research, a numerical hydrodynamic and sediment transport model is developed for the analysis of reach-scale morphological evolution of the large sandy Red – Thai Binh river. This model can nowadays provide valuable information about the river evolution to river system managers.
Tuyển tập Hội nghị Khoa học thường niên năm 2018 ISBN: 978-604-82-2548-3 MOBILE BED NUMERICAL MODELING OF THE RED RIVER Dinh Nhat Quang, Vu Van Kien Thuyloi University, email: quang.dinh@tlu.edu.vn INTRODUCTION The big reservoirs in the upstream catchments constructed for hydroelectricity production, also act as flood control structures, and significantly alter sediment supply downstream and consequently river equilibrium [1, 2] Additionally, instream sand mining is also deeply affecting river morphology enhancing the bed incision process The noticeable acceleration in river bed degradation is a danger for agriculture water supply as well as for infrastructure stability In this research, a numerical hydrodynamic and sediment transport model is developed for the analysis of reach-scale morphological evolution of the large sandy Red – Thai Binh river This model can nowadays provide valuable information about the river evolution to river system managers supply to the large and populated agricultural districts of the low-lying delta area, as well as infrastructure stability Figure Model scheme Collected data include: i) 75 surveyed cross section along the reach of interest in 2000 and 2009; ii) daily discharge and water level from 1960-2015 at Son Tay, Ha Noi, Thuong Cat and Hung Yen gauging stations; and iii) daily suspended solids concentration METHODOLOGY The choice of a one-dimensional model permits to represent the main geomorphic The study is focused on the 82 km lower processes of interest in our case study (i.e course of Red River, downstream the river bed aggradation-degradation for river confluence of the three main tributaries (Da, reaches 100 km long) Thao, Lo) at Viet Tri and upstream the Governing equations: bifurcation located at Hung Yen (see Figure 1) This reach includes the important The model is based on a set of three bifurcation with Duong river, few km differential equations, stating mass and upstream Hanoi In this stretch from the early momentum conservation for the liquid phase 2000s the river is suffering from a bed and mass conservation of the solid degradation process, whose natural and phase along the main stream direction [3] anthropogenic drivers (instream sediment In the equations t is the time, x is the mining, dams, climatic and land use longitudinal stream coordinate, A is the changes) Continuous decrease of the water cross-section wetted area, Q is the liquid level during dry season endangers water discharge, g is the gravity acceleration, CASE STUDY AND DATA 600 Tuyển tập Hội nghị Khoa học thường niên năm 2018 ISBN: 978-604-82-2548-3 I1 is the static moment of the wetted area A Finally, the local bed elevation variation with respect to the water surface, Sf is due to erosion or deposition is calculated the friction slope, A b is the sediment volume directly by Δs = k.h per unit length of the stream subject to erosion or deposition (“sediment area”), Qs is the solid discharge, q and qs are the liquid and solid lateral inflows (or outflows) per unit length, respectively The sediment discharge can be computed with different formulae and is calculated as sediment transport capacity The choice fell on Engelund-Hansen total load formula (Equation 4, [4]) In this equation, B is the channel width, ρ and ρs are the densities of water and sediments, respectively; s is the relative density ρs /ρ; ds is the sediment representative diameter; U is the water average velocity; τ0 is the bed shear stress /2 Qs 0.05 s gBU ds 0 g( s 1) g( s ) (4) The bed evolution model The solution of the balance eqn (3) updates the value of sediment area A b at every time step This value has to be converted into a bed elevation variation, Δs, for every wetted point of the cross section It is assumed herein that this variation Δs is proportional to bed shear stress, which in turn is related to the local water depth h through a proportionality constant k (Δs = k.h) The variation of sediment area ΔAb , at every time step, is given by integrating Δs along the wetted perimeter P: (5) sdp Ab P Integration of k.h along P gives the wetted area A, and it follows that: k Ab A Figure Example of cross section change (start – end of a simulation) RESULTS The validation of the model is performed taking as initial conditions the cross section survey of 2000, and running 10-year long simulation runs with the recorded discharge series in Son Tay (2000-2009) as upstream boundary condition Two alternative erosion trends of Duong river initial section are considered, which drive the conveyance ratio and the flow distribution The trends are inferred from recorded minimum water levels in the cross section of Thuong Cat on Duong River, just downstream the bifurcation Bed roughness in the reach is assumed as invariant The results of these runs (codes VD1 and VD2) are compared with the recorded time series of discharge of the Duong river at the bifurcation, and of water level in Ha Noi The agreement between simulated and recorded data series in 2000-2009 is analyzed Concerning bifurcation, flow in Duong distributary is slightly overestimated during dry season (Figure 3) In VD1, the Duong bed lowering follows a power-law decrease, with faster lowering at the beginning of the run, while in VD2 the lowering follows a S-shape curve, with faster lowering in the middle years The total 601 Tuyển tập Hội nghị Khoa học thường niên năm 2018 ISBN: 978-604-82-2548-3 decrease is m for both simulation runs Moreover, we tried to calibrate a distinct friction slope ratio for dry and wet seasons, following once again the observed data On the other hand, peak flow in the Duong is slightly underestimated by the model The overestimation of flow in the distributary leads to an underestimation of water level in Hanoi (Figure 4) during the first three - four years Maximum levels provided by the model are in excellent agreement with recorded data CONCLUSIONS A finite-volume mobile bed hydrodynamic model was developed, which suitable for the analysis of the morphological evolution of the Red River A first validation in the period 2000-2009 was carried on which shows the good agreement between simulated and recorded data series This model will be further validated using observed data and sand mining rate and then is adopted to forecast river bed incision in the future under different simulated scenarios REFERENCES Figure Discharge in Duong distributary, simulation runs VD1 and VD2 Figure Water level in Hanoi, simulation runs VD1 and VD2 [1] Kondolf G M 1997 Hungry water: effects of dams and gravel mining on river channels Environmental management, 21(4), 533-551 [2] Dang T H., Coynel A., Orange D., Blanc G., Etcheber H., Anh Le L 2010 Long term monitoring of the river sediment trans port in the Red River watershed (Vietnam): temporal variability and dam-reservoir impact Science of the Total Environment, 408, 4654-4664 [3] Schippa, L and Pavan, S 2009 Bed evolution numerical model for rapidly varying flow in natural streams Computers & Geosciences, 35, pp 390–402 [4] Engelund F., Hansen E 1967 A monograph on sediment transport in alluvial streams, Teknisk Forlag, Copenhagen 602 ... invariant The results of these runs (codes VD1 and VD2) are compared with the recorded time series of discharge of the Duong river at the bifurcation, and of water level in Ha Noi The agreement... ratio and the flow distribution The trends are inferred from recorded minimum water levels in the cross section of Thuong Cat on Duong River, just downstream the bifurcation Bed roughness in the reach... velocity; τ0 is the bed shear stress /2 Qs 0.05 s gBU ds 0 g( s 1) g( s ) (4) The bed evolution model The solution of the balance eqn (3) updates the value of sediment area