Improvement of riemann solver for hydrodynamic simulation of natural river channel sections
[Objective]Natural rivers have the characteristics of complex cross-sections and variable flow patterns,which affect the convergence and stability of hydrodynamic simulations.Based on existing research,improving the Riemann solver for complex cross-sections to solve the numerical instability problem caused by discontinuous cross-sections at low water levels is of great sig-nificance for enhancing the applicability of one-dimensional hydrodynamic simulations of natural rivers.[Methods]For complex natural river sections with"conical depression"morphology characteristics at low water levels,the traditional irregular section HLL Riemann solver may encounter flux evolution obstruction at the dry wet interface of the river.To address this issue,the in-terface flow value is directly used at the dry wet interface of the river to replace the flux calculated by the traditional irregular sec-tion HLL Riemann solver.The traditional Riemann solver is improved to make it suitable for simulating the dry wet interface of complex natural river sections with"conical depression"morphology characteristics.Based on the improved Riemann solver,a one-dimensional hydrodynamic model of the irregular river section is constructed,and the applicability of this method under dif-ferent conditions is verified using five universal examples.[Results]The result showed that in a one-dimensional dam failure case,for complex river sections with"conical depression"characteristics,numerical instability and obstructed flux evolution oc-curred at the dry wet interface of the river when simulated using the traditional irregular section HLL Riemann solver.The appli-cation of the improved HLL solver can effectively solve the above problems.The correlation coefficient between the numerical so-lution and the theoretical solution is higher than 0.99,and the result are accurate and reliable;In the dam failure test of triangu-lar water retaining structures,the simulation result of the flood arrival time and flood process at four measuring stations by the model are consistent with the measured data,and the correlation coefficients are all above 0.95;In the calculation of the water surface profile of a steep slope river using one-dimensional constant flow over a hump and alternating simulation of rapid and slow flow,the model performs similarly to traditional models and can accurately predict the changes in water jump position and depth before water jump in different river section shapes;In the tidal simulation calculation of the Chengtong section of the Yangtze Riv-er Estuary,the correlation coefficients between the simulated and measured tidal data of the four observation stations predicted by the model are all above 0.99,and the root mean square error is within 0.08.[Conclusion]The result show that the improved Riemann solver can effectively solve the problem of obstructed flux evolution in the hydrodynamic model at the dry wet junction due to the existence of"conical depression"shape characteristics in natural rivers.The constructed one-dimensional hydrody-namic model of irregular river sections can accurately simulate the interaction between flood wave processes and tide levels in complex terrains.
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