Quantitative relationship of runoff and tide to saltwater spilling over from the North Branch in the Changjiang Estuary: A numerical study

This study focuses on the quantitative relationship of runoff and tidal range to the saltwater spilling over (SSO) from the North Branch in the Changjiang Estuary. In order to determine the intensity of SSO, which is directly influenced by the upstream river runoff and downstream tidal range simultaneously, the net transectional water mass flux (NTWF) is numerically calculated on diurnal and semimonthly basis, respectively. Since a series of experiments were conducted, including river runoff ranging from 7000 to 40,000 m3 s−1 and each experiencing throughout a spring–neap tidal cycle, the corresponding NTWFs were obtained. In the diurnal case, the NTWF can be determined by 533.1 × exp(0.361 × Isso)−533.1 (Isso ≥ 0) and 226.0 × Isso (Isso < 0), where Isso (index of SSO) is defined as TR−1.58 × exp(0.2457 × RF/10,000), TR is the tidal range (m) at the Qinglong Harbor and RF is the runoff (m3 s−1). The Isso has the same sign as the diurnal NTWF. In the semimonthly case, the NTWF is determined by 701.89 × ln(−RF/10,000+6.2)−553.54, which decreases with the increasing of runoff and declines to zero when the runoff reaches 40,000 m3 s−1. The tidal range and runoff are measured at Qinglong Harbor Station and Datong Station everyday, respectively. Thus, the NTWF can be easily obtained by using these formulae.