Short-term variations and influencing factors of suspended sediment concentrations at the Heisha Beach, Guangdong, China
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Abstract: Knowledge of sediment variation processes is essential to understand the evolution mechanism of beach morphology changes. Thus, a field measurement was conducted at the Heisha Beach, located on the west coast of the Zhujiang River (Pearl River) Estuary, to investigate the short-term variation in suspended sediment concentrations (SSCs) and the relationship between the SSC and turbulent kinetic energy, bottom shear stress (BSS), and relative wave height. Based on extreme event analysis results, extreme events have a greater influence on turbulent kinetic energy than SSC. Although a portion of the turbulent kinetic energy dissipates directly into the water column, it plays an important role in suspended sediment motion. Most of the time, the wave-current interaction is strong enough to drive sediment incipience and resuspension. When combined, the wave-current interaction and wave-induced BSSs have a greater influence on suspended sediment transport and SSC variation than current-induced BSS alone. The relative wave height also has a strong correlation with SSC, indicating that the combined effect of water depth and wave height significantly impacts SSC variation. Water depth is mainly controlled by the tide on the beaches; thus, the effects of tides and waves should be conjunctively considered when analyzing the factors influencing SSC.
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Figure 2. Linear least-squares line of best fit between turbidity and SSC of the water samples.
Figure 3. Power spectral density of cross-shore velocity (v) vs. frequency (blue line) for Burst 19. The red and black dashed lines indicate different spectral slopes.
Figure 6. Hydrodynamic conditions during the observation period of tide level (TL) (a), water depth (h) (b), mean wave height (Hm0) (c), mean wave period (Tm0) (d), mean cross-shore current velocity (v) (e), and mean alongshore current velocity (u) (f).
Figure 8. Fractions of extreme SSC events to the total data points (n1/n) and the SSC percentage contained in the extreme events in the entire time series (n1m1/nm).
Figure 10. Fractions of extreme TKE events to the total data points (n1/n) and the TKE percentage contained in the extreme events in the entire data set (n1m1/nm).
Figure 11. The fractions of correlated extreme events to the SSC (a) / TKE (b) extreme events (n0/n1) and the percent of the SSC (a) / TKE (b) contained in the correlated extreme events in relation to that in the entire time series (n0m0/nm).
Figure 12. Wave-induced BSS (τw), current-induced BSS (τc), combined wave-current BSS (τcw), and mean SSC value of each burst. The horizontal magenta line indicates the critical BSS (τcr=0.16 Pa).
Figure 13. Linear regression results between SSC and wave-induced BSS (a), current-induced BSS (b), and combined wave-current BSS (c).
Figure 15. Linear regression results between SSC and water depth (h) (a), mean wave height (Hm0) (b), and relative wave height (Hr) (c).
Table 1. Sediment parameters of the samples
Sampling point Dme($ {\text{ϕ}}$) D50($ {\text{ϕ}}$) Sc Sk Ku HSW01 2.40 2.48 0.84 −0.22 1.19 HSW02 2.65 2.65 0.67 −0.11 1.11 HSW03 2.70 2.68 0.76 −0.17 1.32 HSW04 2.62 2.64 0.78 −0.22 1.24 HSW05 2.62 2.64 0.72 −0.17 1.17 HSW06 2.74 2.73 0.63 −0.11 1.06 HSW07 1.82 2.08 1.14 −0.21 0.81 HSW08 2.59 2.61 0.58 −0.06 0.99 HSW09 2.73 2.61 1.15 0.27 1.99 HSW10 2.95 2.82 1.03 0.36 2.47 HSW11 2.90 2.88 0.47 0.02 0.95 HSW12 2.86 2.83 0.49 −0.02 0.95 
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Table 2. Statistical average of extreme events of overall bursts (332)
Parameter (n1/n)/% (n0/n1)/% (n1m1/nm)/% (n0m0/nm)/% SSC 10.88 (23.64) 11.31 (20.06) 12.57 (24.62) 4.02 (10.73) TKE 14.79 (14.90) 10.93 (24.17) 41.80 (32.69) 8.81 (20.58) Note: Data are shown as the mean value (standard deviation).
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Table 3. Statistical averages of extreme events from Bursts 235–318
Parameter (n1/n)/% (n0/n1 ) /% (n1m1/nm)/% (n0m0/nm)/% (n0/n)/% SSC 40.82
(31.57)30.72
(19.52)45.27
(30.44)15.36
(16.76)14.35
(16.78)TKE 29.37
(16.62)41.60
(32.18)79.77
(11.60)34.25
(28.39)14.35
(16.78)Note: Data are shown as the mean value (standard deviation).
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Table 4. Extreme event analysis results utilizing the comparison parameters of wave-induced BBS (τw), current-induced BBS (τc), and combined wave-current BSS (τcw)
Parameter (n0/n)/% (n0m0/nm(SSC))/% ${\tau _{\rm{w} } }$ 18.71 31.35 ${\tau _{\rm{c} } }$ 13.80 25.04 ${\tau _{ {\rm{cw} } } }$ 20.86 35.61 Notes: n0 is the number of correlated extreme events where the extreme SSC events and comparison parameters occur synchronously, n is the number of total data points (332), m0 is the mean SSC value of the correlated extreme event data points, and m is the mean SSC value of the total data points.
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Table 5. Evaluation of the relationship between SSC and wave-induced BSS (τw), current-induced BSS (τc), and combined wave-current BSS (τcw) via the Pearson (rp) and Spearman rank (rs) correlation coefficients
Parameter rp rs ${\tau _{\rm{w} } }$ 0.78 0.84 ${\tau _{\rm{c} } }$ 0.77 0.79 ${\tau _{ {\rm{cw} } } }$ 0.84 0.89
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Table 6. Extreme event analysis results utilizing the comparison parameters of water depth (h), mean wave height (Hm0), and relative wave height (Hr)
Parameter (n0/n)/% (n0m0/nm(SSC))/% h 5.83 9.44 Hm0 11.66 18.63 Hr 18.10 30.30 Notes: n0 is the number of correlated extreme events where the extreme SSC events and comparison parameters occur synchronously, n is the number of total data points (332), m0 is the mean SSC of the correlated extreme event data points, and m is the mean SSC of the total data points.
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Table 7. Evaluation of the relationship between SSC and water depth (h), mean wave height (Hm0), relative wave height (Hr) via the Pearson correlation coefficient (rp) and Spearman rank correlation coefficients (rs)
Parameter rp rs h −0.37 −0.44 Hm0 0.27 0.25 Hr 0.82 0.84
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