Swell-dominated sediment re-suspension in a silty coastal seabed

https://doi.org/10.1016/j.ecss.2020.106845Get rights and content

Highlights

  • The height of swell strongly affected sediment resuspension in coastal seabed.

  • Large swell coming from offshore (NE in our study area) dominated sediment resuspension events in offshore zone (water depth >10 m).

  • During and after swell, residual currents are the major dynamics that transport sediment from coastal areas.

  • Abstract.

Abstract

Waves are fundamentally important for sediment re-suspension in estuary and coastal areas, especially for silty sediments, which can be easily suspended by waves, but the differential effects of swell and wind waves are still unclear. Integrated field observations were made from November 2012 to March 2013 including waves, currents, and suspending sediments on the offshore seabed of the Huanghe Delta to explore the mechanism of sediment re-suspension in silty coastal zones. During the five months of observation, there were more than 30 winter wind events that affected the study area and induced sediment re-suspension with varying suspended sediment concentration. The observed wave composition was separated into swell and wind waves using a bandpass filter. Results show that large swell (with significant height > 1.0 m) coming from the offshore direction (NE in our study area) dominated sediment resuspension in the coastal seabed due to the fact that this wind direction had the longest average fetch. Winds from the onshore direction usually had smaller swell due to their short fetches and caused limited sediment re-suspension. The residual currents caused by NE winds also transport larger sediment. An individual NE wind event could transport sediment 8–13.6 t/m2 and 5.1–8.2 t/m2 in directions parallel and perpendicular, respectively, to the isobaths, which is much higher than the sediment transportation during an individual NW wind event, which could transport 1.5–4 t/m2 and 0.6–5 t/m2 parallel and perpendicular, respectively, to the isobaths. Our research shows that large swell and the accompanying residual currents caused by NE winds (from offshore direction) are a vital driving force for sediment resuspension and transportation in the offshore zone.

Introduction

Waves and currents play important roles in sediment re-suspension in shallow coastal waters. Waves are locally generated by wind or propagate from the open sea, while currents are partly driven by tidal, wind, and density differences (Green and Coco, 2014). The waves in nature are composed of various components (e.g., wind waves, swell, infragravity waves). Swell is a series of waves that break in the surf zone (Thornton and Guza, 1982), while infragravity waves (frequency: 0.005–0.04 Hz) may be a few centimeters high in deep water (Aucan and Ardhuin, 2013; Crawford et al., 2015; Inch et al., 2017) or over 1 m high in nearshore environment (Guza and Thornton, 1982; Senechal et al., 2011; Fiedler et al., 2015). Diverse wave components possibly make distinct contributions to sediment movement as well as seabed variations, so it is necessary to discriminate the differing contributions of swell and wind waves on sediment resuspension and transportation in the coastal seabed.

Many studies have investigated the roles of wind wave and swell playing in suspending sediment with different environments. Based on field observations in various bays and estuaries, the distinct mechanisms of sediment re-suspension under two types of waves have been identified. Wind waves can resuspend sediment from the bed with 2–12 m water depth (You and Yin, 2007), while swell is possibly more important for sediment re-suspension in the inner shelf zone (Jing and Ridd, 1996). Early in 1986, Oradiwe (1986) investigated the hydrodynamic conditions and sediment budget in Monterey Bay, California and found that the swell produced by an open ocean are the most important source of wave energy in the bay, with larger sizes and longer periods than local wind waves resulting in much more damage in the form of shoreline erosion. Jing and Ridd (1996) investigated two high sediment re-suspension events in Cleveland Bay, California, and found that swell was the most important factor in sediment resuspension during high suspended sediment concentration (SSC) events, while wind waves seemed to have a limited effect on SSC levels. In a comprehensive field study in Moreton Bay, Australia, You and Yin (2007) compared sediment resuspension dynamics between deep water (the main entrance and the central bay) and shallow water (small bays) and found that the dominant driving force in deep water is a combination of ocean swell and tidal currents, whereas in shallow water, wind waves predominate. Jia et al. (2014a) calculated the bottom shear stresses (BSSs) and analyzed their effect on sediment re-suspension based on observations in the Modaomen Estuary, China. Their results showed that the swell-induced BSS are the most important part of the total BSS. In addition to these field studies, Dalyander et al. (2017) used numerical modeling to analyze the bottom shear stress in the entire Mid-Atlantic Bight (MAB) and determined that the contribution of swell to bottom shear stress in the middle MAB is much higher than that of local wind waves. All of these studies considered the mechanism of sediment re-suspension in the deep water within bays and estuaries, and suggested that swell is the dominant physical force. These investigations are limited by their duration and have not investigated the resuspension process under different types of swell. Hence, this research examines the components of natural random waves and their different action on sediment re-suspension based on long time observation to understand the mechanisms of sediment re-suspension and transportation during different stormy winter winds.

The Huanghe Delta is frequently hit by strong winds during the winter, which results in various wave components that possibly have different effects on sediment suspension. Our research focuses on the area outside of the Huanghe Delta (the inner shelf of Bohai Sea) where both swell and wind waves are strong during the winter. The different roles of various wave components play in sediment re-suspension within the inner shelf sea are identified in this paper.

Section snippets

Study area

Our study area was located on the east side of the modern Huanghe Delta, where the surface sediments consist mainly of silty sand and clay (Yang et al., 2011). Tidal current in this region is irregular and shows semi-diurnal characteristics. There is an amphidromic point of the semi-diurnal M2 tide in the northeast portion of the delta (Yang et al., 2011).

In winter, strong winds (usually >11 m/s) primarily come from the NE and NW. Yang et al. (2011) pointed out that strong winds exceeding grade

Site setup

The SSC and accompanying hydrodynamics were simultaneously measured with an instrumented tripod that was equipped with an up-looking Acoustic Doppler Current Profiler (ADCP) at z = 70 cm, and an Optical Backscatter Sensor (OBS) at z = 40 cm (z is the height from the seabed). The current profile and OBS measurements were configured to simultaneously sample every 10 min from November 1 to December 26, 2012 and every 30 min from December 27, 2012 to March 30, 2013. The wave measurement was set to

Hydrodynamic conditions

The observation lasted for five months throughout the long windy winter. The strong winds were mainly from the NE and NW. The observed maximum wind speed was 17.25 m/s, on March 22, 2013. Only 20 percent of the wind speed observations exceeded 10 m/s, and those exceeding 15 m/s were only from the NE and NW directions.

Generally, the height and period of local wind waves depend on fetch length, wind speed, wind event duration, and water depth (Green and Coco, 2014). Fig. 4 shows the observed wind

Discussion of sediment re-suspension mechanism under winter winds

Waves and currents are important dynamic forces in sediment re-suspension in shallow coastal waters (Warrick, 2013), but the composition of bottom shear stresses varies with water depth. It can be inferred from the research results that the bottom shear stress in the inner shelf area near the Huanghe Delta is swell dominated during strong winter wind events, although the development potential of swell in this area is strongly affected by the local terrain. The orientation of the isobaths is

Conclusions

  • (1)

    Swell controls the incipient motion of sediment in the inner shelf zone, and swell that is larger than 1 m is responsible for all the strong resuspension events. The differences in swell heights strongly affect the process of sediment resuspension.

  • (2)

    Due to topographic effects, the development potential of swell from different directions tends to show significant variations, which further affect the sediment motion and transport. In this study, large swell from the NE direction was responsible for

CRediT authorship contribution statement

Jianwei Niu: Formal analysis, Visualization, Methodology, Writing - original draft. Jishang Xu: Conceptualization, Funding acquisition, Investigation, Data curation, Formal analysis, Writing - review & editing. Guangxue Li: Supervision, Funding acquisition, Resources, Writing - review & editing. Ping Dong: Conceptualization, Validation, Writing - review & editing. Jinghao Shi: Formal analysis, Methodology, Validation. Lulu Qiao: Resources, Writing - review & editing.

Declaration of competing interest

The author(s) declare no competing interests.

Acknowledgments

This study was supported by the Natural Science Foundation of China [Grant Nos. 41976198, 51479182], the National Key Research and Development Program of China [Grant Nos. SQ2017YFGH001475, 2016YFC0802301], and the Taishan Scholar Project granted to Guangxue Li. We thank LetPub (www.letpub.com) for its linguistic assistance during the preparation of this manuscript.

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