River base level change in mouth channel evolution: The case of the Yellow River delta, China

摘要

The base level is one of the key factors controlling river evolution. In the Yellow River mouth, a base-level rise takes place not only because of a relative sea-level rise, but also as a consequence of the increasing of river channel length associated with the large progradation rate of the delta. A change in river length results in a change in the gradient (slope) of the mouth channel, which further leads to a riverbed aggradation/degradation or a water level change at the deltaic apex. This process is principally associated with the morphodynamic backwater effect. The consequent water level change can be considered as a base-level change of the Yellow River. In this work, a relation was constructed between the water level at the deltaic apex and the riverine water and sediment inputs, as well as with the gradient of the mouth channel in the progradation period of each deltaic lobe. Also, for disclosing the importance of the base-level change due to variations of mouth channel length, it was estimated and compared with the relative sea-level change in the past decades, and for predicting the water level at the deltaic apex, the relation between mouth channel length and cumulative riverine sediment input was investigated. It was found that in the period 1953-2016, the base-level changes associated with variations of the mouth channel length could be larger than 1 m, or even larger than 2 m in one year, with a conservatively estimated annual mean of 34.8 mm/a, which is much larger than the relative sea-level change of 7.0 mm/a In the Yellow River mouth. The mouth channel length is closely and linearly related to the cumulative sediment discharge of the Yellow River. The relation for calculating the water level at the deltaic apex was found to be capable of giving a reliable result with the riverine water and sediment inputs, the relative sea-level, and the mouth channel length estimated from cumulative riverine sediment discharge. This relation was also used to estimate the equilibrium gradient of the mouth channel for sediment transport. The results show that after the year 2000, the equilibrium gradient has remained smaller than the actual gradient of the mouth channel, and both gradients decreased continuously with the decrease of riverine sediment inputs, but their values were becoming closer to each other. It is highly possible that the water level at the deltaic apex will be raised by the increase in sediment discharge of the river in the future.