Quantification of the influencing factors for flood peak discharge increase in the Lower Yellow River

The past decades have witnessed frequent flood peak discharges increase in the Lower Yellow River (LYR). Yet no consensus for its mechanism has been achieved. Here 21 events of the peak discharge increase (PDI) in the period of 1973–2012 are analysed. It is shown that the mean increment of peak discharge increases from 810 m³/s to 1158 m³/s and the frequency also increases from once every 2 years to once every 1.5 years after the completion of Xiaolangdi (XLD) Reservoir. Afterwards, the ordinary differential equations (ODEs) along the characteristics for the discharge are derived, from which seven factors (terms I ∼ VII) that may affect the discharge variation are identified: the effects related to the longitudinal change in flow density (I), and in the product of flow density and flow area (II); the pressure terms due to river width gradient (III) and flow density (IV); the external forces (V); the momentum term due to bed deformation (VI); and the imbalanced advection (VII). Using field data of the 21 events, the bed Manning roughness is back-calculated from the ordinary discharge equation, which agrees with the documented values very well. Quantitative comparisons of the seven factors indicate that the pressure term due to the river width gradient plays a major role in promoting the PDI in most events, whereas the external forces term is the primary cause that attenuates PDI. The rest influencing factors have marginal effects with a much smaller magnitude.