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<p>While a significant body of research has been focused on classical compound channels, limited studies have focused on the multi-stage compound channels. Multi-stage compound channels have more than one floodplain on each side of the main channel. These sections have more complex flow hydraulics than classical channels. So far only a few 1-D and 3-D mathematical models have been used to predict the stage discharge curve in multi-stage compound channels. In this study, considering the limitation of 1-D and 3-D models, the three-dimensional nature of flow has been simplified into a quasi two-dimensional problem by using depth averaged Shiono and Knight Model (SKM). By proposing an iterative procedure for calibrating the three key coefficients involved in SKM, namely bed friction factor (<i>f</i>), eddy viscosity coefficient (<i>λ</i>) and secondary flow term (<i>β</i>), these parameters achieved their best values in the main channel and floodplains for a laboratory three-stage rectangular compound channel. Through this calibration, it was clearly shown that in prediction of lateral velocity distributions, the secondary flow term plays less important role than the friction factor and eddy viscosity coefficients values in this special form of channels. The calibrated SKM model with a mean absolute error of 9.33%, showed a good agreement between the observed and calculated lateral velocity distributions. The mean errors for prediction of total flow discharges as well as the main channel and floodplains discharges are 4.1, 4.9 and 15.7%, respectively. The higher error in floodplains may be due to the strong shear layers and complex momentum exchanges between two adjacent rough floodplains whose beds are covered by grass vegetation. Some degree of experimental error, mainly due to very small values of flow discharges in floodplains, is also expected as another reason for the high error. These errors for the traditional model of vertical divided channel method (DCM) which ignores the momentum transfer between the main channel and the floodplains are obtained as 14.3, 23.2 and 50.5%, respectively. Comparison of boundary shear stresses calculations indicated that the general equation of total bed shear stress which is commonly used in river engineering practices, gives lower bed shear stresses on the floodplains and conversely higher bed shear stresses on the main channel compared to SKM and measured data. Given the importance of proper prediction of flood discharges and also the boundary shear stresses for design of river training schemes and flood management structures, good accuracy of the proposed model shows its potential applicability in these subjects.</p>

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Calibration of a 2D Hydraulic Model for Flow Discharge Prediction in Multi-Stage Compound Channels

  • 期刊论文

  • 发表日期：

    2025年6月16日

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  • Abdolreza Jamali,
  • Babak Aminnejad,
  • Abdolreza Zahiri

摘要

While a significant body of research has been focused on classical compound channels, limited studies have focused on the multi-stage compound channels. Multi-stage compound channels have more than one floodplain on each side of the main channel. These sections have more complex flow hydraulics than classical channels. So far only a few 1-D and 3-D mathematical models have been used to predict the stage discharge curve in multi-stage compound channels. In this study, considering the limitation of 1-D and 3-D models, the three-dimensional nature of flow has been simplified into a quasi two-dimensional problem by using depth averaged Shiono and Knight Model (SKM). By proposing an iterative procedure for calibrating the three key coefficients involved in SKM, namely bed friction factor (f), eddy viscosity coefficient (λ) and secondary flow term (β), these parameters achieved their best values in the main channel and floodplains for a laboratory three-stage rectangular compound channel. Through this calibration, it was clearly shown that in prediction of lateral velocity distributions, the secondary flow term plays less important role than the friction factor and eddy viscosity coefficients values in this special form of channels. The calibrated SKM model with a mean absolute error of 9.33%, showed a good agreement between the observed and calculated lateral velocity distributions. The mean errors for prediction of total flow discharges as well as the main channel and floodplains discharges are 4.1, 4.9 and 15.7%, respectively. The higher error in floodplains may be due to the strong shear layers and complex momentum exchanges between two adjacent rough floodplains whose beds are covered by grass vegetation. Some degree of experimental error, mainly due to very small values of flow discharges in floodplains, is also expected as another reason for the high error. These errors for the traditional model of vertical divided channel method (DCM) which ignores the momentum transfer between the main channel and the floodplains are obtained as 14.3, 23.2 and 50.5%, respectively. Comparison of boundary shear stresses calculations indicated that the general equation of total bed shear stress which is commonly used in river engineering practices, gives lower bed shear stresses on the floodplains and conversely higher bed shear stresses on the main channel compared to SKM and measured data. Given the importance of proper prediction of flood discharges and also the boundary shear stresses for design of river training schemes and flood management structures, good accuracy of the proposed model shows its potential applicability in these subjects.