Responses of estuarine circulation to the morphological evolution in a 1 convergent, microtidal estuary 2

: 12 The Huangmaohai Estuary (HE) is a funnel-shaped microtidal estuary in the west 13 of the Pearl River Delta (PRD) in southern China. Since China's reform and opening up 14 in 1978, extensive human activities have occurred and greatly changed the estuary's 15 topography, and modified its hydrodynamics. In this study, we examined the 16 morphological evolution by analyzing remote sensing data with ArcGIS tools and 17 studied the responses of hydrodynamics to the changes in topography from 1977 to 18 2010 by using the Delft3d model. We took the changes in estuarine circulation during 19 neap tides in dry seasons as an example. The results show that human reclamation 20 caused a narrowing of the estuary, and channel dredging deepened the estuary. These 21 human activities changed both the longitudinal and lateral estuarine circulations. The 22 longitudinal circulation was observed to increase with the deepening and narrowing of 23 the estuary. The lateral circulation experienced changes in both the magnitude and 24 pattern. The momentum balance analysis shows that when the depth and width changed 25 simultaneously, the longitudinal estuarine circulation was modulated by both the channel deepening and width reduction, in which the friction, pressure gradient force, 27 and advection terms were altered. The analysis of the longitudinal vortex dynamics 28 indicates that the changes in the vertical shear of the longitudinal flow, lateral salinity 29 gradient, and vertical mixing were responsible for the change in the lateral circulation. 30 The changes in water depth are the dominant factor affecting lateral circulation intensity. 31 This study has implications for sediment transport and morphological evolution in 32 estuaries heavily impacted by human interventions. 33


Introduction
, river discharge, tides (Pritchard, 1952), and  (Fischer, 1976;Dyer, 1977). 47 Human activities may change the estuarine topography, leading to changes in the 48 estuarine circulation and associated material transport. Therefore, a study of the 49 estuarine circulation and its response to human activities is essential for integrated  In this study, we used a state-of-the-art three-dimensional baroclinic model (Delft  The HE is located in the west of the PRD in southern China and exhibits a 136 distinctly convergent geometry, with a latitude ranging from 21°50′ to 22°13′ N and 137 a longitude ranging from 113°00′ to 113°51′ E (Fig. 1). The estuary is composed of a 138 bay (Huangmao Bay) and a tidal river. The bay is trumpet-shaped with an area of 409 139 km 2 . It has a complex bathymetry comprising of two channels and three shoals, namely 140 the West Channel and East Channel, the West Shoal, Middle Shoal, and East Shoal. In 6 / 39 recent decades, the West Channel is observed to shrink and almost disappear now (Jia 142 et al., 2012). The width of the bay is 30 km at the estuary mouth and decreases to 1.8 143 km at the head. The mean water depth of the bay is 4.5 m (Gong et al., 2014). The bay 144 is connected to the upstream river catchment by two constrictions (Yamen and 145 Hutiaomen Outlets). Several islands, namely Dajin Island, Hebao Island, and Gaolan 146 Island, are scattered at the estuary's mouth (shown in Fig. 1b). 2003, a total of 142.29 km 2 tidal flat was reclaimed, with an average reclamation rate 173 of 3.74 km 2 /a, and the reclamation rate continuously but gradually increased during that 174 period. After 2003, the reclamation rate slowed down. In terms of channel dredging, 175 the Yamen Waterway Project was conducted in 1997 to deepen the channel between 176 S0 and S3 in Fig. 1b (Luo, 2010 (1977, 1994, 2003, and 2010), published by the Navigation Safety Guarantee Bureau.

191
The filling and excavation toolbox of ArcGIS was used to calculate the difference  provide ocean boundary conditions for the MD2 model.

219
As mentioned above, the hydrodynamics in the HE experiences distinct seasonal  In this study, the Willmott skill score (SK) was used to evaluate whether the model 241 result is consistent with the observed data (Willmott, 1981). The SK is defined as:  (Table   252 3). The simulation of the current speed is worse than that of the current direction, but  As a whole, the simulation of surface currents is worse than that in other layers,       can be ignored, therefore, formula (2) can be simplified as: when is positive, the lateral circulation is an anticlockwise vortex, conversely, 432 when is negative, the lateral circulation is a clockwise vortex.

433
The results of the averaged intensity of estuarine circulation along Sec.A and the 434 averaged intensity of vorticity at the cross-sections are listed in Table 4.  Table 4 indicates that the longitudinal estuarine circulation intensity increased with   To further identify the changes in different terms, the advection term was divided 519 into lateral (X-direction), longitudinal (Y-direction), and vertical (Z-direction) 520 advection terms (Fig. 8). It is worth noting that the sum of the advection terms in X 521 and Z directions represents the effect of the lateral circulation.  In the right side of Eq. 5, the first term represents the tilting of the planetary 552 vorticity by vertical shear in the along-channel flow, the second term is the baroclinicity 553 caused by the lateral salinity gradient, the third is the vertical diffusion, and the fourth 554 is the horizontal diffusion, which is typically two orders of magnitude smaller than the 555 vertical diffusion term. Therefore, we only show the first four terms in Fig. 9.  The vertical diffusion of the vorticity was overall negative, indicating its effect in 592 dissipating the vorticity. The vertical diffusion term was larger than the baroclinicity 593 term, especially in the middle water, which was inconsistent with the conclusion that The reason may be that in our study site, the vertical mixing was strong as the estuary 596 became shallow. However, the existence of a pycnocline greatly weakened the 597 momentum exchange between the upper and lower layers: above the pycnocline, the Ralston and Geyer (2019). In our study site, the salinity gradient at the upstream part 649 of the longitudinal section was increased owing to an enhanced salt intrusion where 650 water depth increased, which led to an increased gravitational circulation in the 651 upstream of the HE (Fig. 4).

652
The tidal straining-induced estuarine circulation is another important component of  barotropic process, i.e., the water elevation gradient, and thus by the intensity of the 703 ebb jet. Different from the wet season when the river discharge was higher, the lateral 704 circulation in the dry season was more affected by the baroclinic effect. We speculate 705 that with the narrowing and deepening of the estuary, the lateral circulation even in the 706 wet season will be enhanced with the ebb jet in the deep channel strengthened.

707
In the HE, the channel underwent siltation, and sediment was carried from the 708 channels to side banks by the lateral circulation, making the estuary overall shallower  Table 4, the width-to-depth ratio has been decreasing from 1977 to 2010, but 720 the estuarine circulation has been increasing. The difference would be caused by the 721 fact that in our study site, the tidal mixing is not strong enough to generate an effective 722 tidal straining-induced circulation.

745
The changes in the longitudinal estuarine circulation were dominated by the 746 changes in the baroclinic pressure gradient force and advection. As the estuary was 747 narrowing and deepening, the pressure gradient force and advection term (especially 748 the horizontal advection term) increased, which increased the longitudinal circulation.

749
The change in lateral circulation intensity was mainly caused by the change of the