Numerical modeling of hyperpycnal flows in an idealized river mouth Yan Wang a , Houjie Wang a, b, c, * , Naishuang Bi a, b , Zuosheng Yang a, b a College of Marine Geosciences, Ocean University of China, 238 Songling Rd., Qingdao 266100, China b Key Laboratory of Submarine Sciences and Prospecting Technique, Ministry of Education, 238 Songling Rd., Qingdao 266100, China c State Key Laboratory of Estuarine and Coastal Research (SKLEC), East China Normal University, 3663 Zhongshan N. Rd., Shanghai 200062, China article info Article history: Received 5 February 2010 Accepted 16 February 2011 Available online 2 March 2011 Keywords: hyperpycnal flows suspended sediment transport settling velocity tidal mixing numerical modeling abstract Numerical experiments in an idealized river mouth are conducted using a three-dimensional hydrody- namics model (EFDC model) to examine the impacts of suspended sediment concentration (SSC), settling velocity of sediment and tidal mixing on the formation and maintenance of estuarine hyperpycnal flows. The standard experiment presents an illustrative view of hyperpycnal flows that carry high-concentrated sediment and low-salinity water in the bottom layer (>1.0 m in thickness) along the subaqueous slope. The structure and intra-tidal variation of the simulated hyperpycnal flows are quite similar to those previously observed off the Huanghe (Yellow River) mouth. Results from the three control experiments show that SSC of river effluents is the most important parameter to the formation of hyperpycnal flows. High SSC will increase the bulk density of river effluents and thus offset the density difference between freshwater and seawater. Low SSC of river effluents will produce a surface river plume, as commonly observed in most large estuaries. Both the settling velocity of sediment particles and the tidal mixing play an important role in maintaining the hyperpycnal flows. Increasing settling velocity enhances the deposition of sediment from the hyperpycnal layer and thus accelerates the attenuation of hyperpycnal flows, whereas increasing tidal mixing destroys the stratification of water column and therefore makes the hyperpycnal flows less evident. Our results from numerical experiments are of importance to understand the initiation and maintenance of hyperpycnal flows in estuaries and provide a reference to the rapidly decaying hyperpycnal flows off the Huanghe river mouth due to climatic and anthropogenic forcing over the past several decades. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction When a river with extremely high suspended sediment concentration (SSC) flows into an estuary, the river effluents will be probably much denser than that of the ambient ocean water. As a result, the river water carrying suspended sediment will plunge beneath the oceanic water and generate hyperpycnal flows (Bates, 1953; Wright et al.,1988; Mulder and Syvitski, 1995; Wright and Nittrouer, 1995). In the estuaries of many small to medium size rivers, the hyperpycnal flows generate mostly during flood seasons or during some episodic events including jÖkulhaups, dam breaking and draining, and lahars (Wright et al., 1988; Mulder et al., 2003), when the SSC of the river water are at least 35e45 kg/m 3 depending upon the density of the ambient water (Mulder and Syvitski, 1995). The main hydrodynamic factor for the generation of hyperpycnal flow is the negative buoyant effect caused by the density contrast between the river effluents and the ambient ocean water (Wright and Nittrouer, 1995). Hyperpycnal flows can travel along the slope for hours to weeks until its higher density disappears when most of the sediment deposits on the seabed from the hyperpycnal layer (Mulder and Syvitski, 1995). By transporting and depositing a large amount of sediments that are closely tied terrestrial elements as well as metal contamination in particulate form, hyperpycnal flows in estuaries play an important role in estuarine morphology and coastal biogeochemical cycle in marine environment. Most direct observations of hyperpycnal flows were from fresh- water of lakes (Gould, 1951; Lambert et al., 1976; Nelson et al., 1999) and reservoirs (Wiebe, 1939; Ford and Johnson, 1983). In marine environment, it has long been postulated that hyperpycnal flows exist in the estuaries of high turbid rivers and small mountainous rivers (Mulder and Syvitski,1995; Milliman and Kao, 2005; Milliman et al., 2007); however, most of our knowledge about hyperpycnal flows in marine environment comes either from the inference of the sedimentary records on the continental shelves (e.g., Foster and Carter, 1997; Mulder et al., 1997; Normark et al., 1998) or from the observations of hyperpycnal discharges during extreme events (e.g., Milliman and Kao, 2005; Milliman et al., 2007). Thus, there is a lack of knowledge about the process and mechanism of hyperpycnal flows in estuaries, because it is hard to monitor such process on site * Corresponding author. College of Marine Geosciences, Ocean University of China, 238 Songling Rd., Qingdao 266100, China. E-mail address: hjwang@mail.ouc.edu.cn (H. Wang). Contents lists available at ScienceDirect Estuarine, Coastal and Shelf Science journal homepage: www.elsevier.com/locate/ecss 0272-7714/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2011.02.011 Estuarine, Coastal and Shelf Science 93 (2011) 228e238