Temporal and spatial evolution of surface sediments characteristics in the Dagu River estuary and their dynamic response mechanism

Xiao-ying Chen, Da-hai Liu, Ping Yin, Jin-qing Liu, Ke Cao, Fei Gao

a Qingdao Institute of Marine Geology, China Geological Survery, Ministry of Natural Resources, Qingdao 266071, China

b Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266071, China

c The First Institute of Oceanography, Ministry of Natural Resources, Qingdao 266061, China

Keywords:

Dagu River estuary

Surface sediments

Global climate and environmental change

Water dynamics

Coastal zone geological survey engineering

Qingdao

Shandong Province

China

A B S T R A C T

Based on the 39 surface sediment samples collected in the flood season and the dry season in 2012 respectively and the measured hydrological data in October 2012, the sediment grain size characteristics has been analyzed and the response mechanism of surface sediments to estuarine hydrodynamics was revealed by calculating the range of waves and tidal currents.The results show that: (1) The grain size of the surface sediment samples decreased gradually from land to sea in the flood season.The fine sediment was redistributed under marine hydrodynamics in the dry season and the sediments showed coarser tendency ingeneral; (2) tidal current stirring sediment was very obvious in Dagu River estuary area, and wave stirring sediments mainly occurred in the tidal flat area and estuary sand bar area; (3) in the flood season, surface sediment sat the estuary were transported towards south and southeast.In the dry season,surface sediments were transported towards southwest at the north area of Jiaozhou Bay Bridge, and sediments were transported towards northeast area at the south of Jiaozhou Bay Bridge.

1.Introduction

The study of sedimentary process from small river systems has become a focus in recent years (Smith SV et al.,2003; Dadson SJ et al., 2003; Milliman JD and Kao SJ., 2005;Kao SJ and Milliman JD., 2008; Hilton RG et al., 2011).When extreme events (such as flood events induced by typhoons) occur, river flux increases rapidly and can have great impact on sedimentary environments of river estuary and coastal zones (Gaston TF et al., 2006; Milliman J and Farnsworth KL, 2011; Sirdari ZZ et al., 2014; Eidam EF et al., 2016; Lu WW et al., 2018; Deng K et al., 2019).Dagu River is a typical small mountain river in the north of China.It is 179.9 km in length and originates in Fushan mountain in Zhaoyuan of Shandong province.The drainage area is 6131.3 km2and the gradient of the slope decreases gradually.Dagu River is the largest river flowing into Jiaozhou Bay and supplies over 85% of the fresh water to Jiaozhou Bay (Han SZ et al., 2007; Li NS et al., 2006).In this research, the highresolution sedimentary process at Dagu River estuary has been analyzed (Fig.1).

Fig.1.The location map of surface sediment sampling and the in-situ monitoring of Dagu River estuary in July and October, 2012.

2.Data and methods

A platform with autonomous instruments was developed to investigate synchronized waves, currents, water level,seabed changes and water turbidity at Dagu River estuary from September 24th to October 4th, 2012.The related data has been seized and recorded effectively during the strong wind weather process.

39 surficial sediment samples were collected within 2 cm below the bed surface at cross-shore sections of the coastal zones in July 2012 (flood season) and October 2012 (dry season) respectively.The positions of sampling sites were determined with GPS.All the samples were treated by the following procedure: (1) 2 g dry sample of sediment, 15 mL distilled water and 5 mL H2O2(30%) were mixed and slightly heated, then left for 24 hours and organic matter has been removed; (2) 1-2 drops of (NaPO3)6solutions were added as dispersant and the mixture was placed in an ultrasonic bath to agitate and disperse for 2 hours.A Malvern 2000 was used for measuring the grain size distribution of the sediments.According to their gravel+sand/silt/clay ratios, the sediments were classified following the classification of Folk RL et al.(1970).The sediment parameters were calculated according to the methods of Folk RL and Ward WC (1957).From cumulative grain-size distribution curves, the grain diameters at 5%, 16%, 50%, 84% and 95% are taken as the values ofφ5,φ16,φ50,φ84,φ95respectively.Then sediment parameters were calculated based on equations (1), (2), (3) and (4).

WhereMzis mean grain-size, σ1is sorting coefficient, andSK1,KGshow skewness and kurtosis respectively.

A ternary diagram for a textural classification of hydrodynamic subdivisions on the basis of sand/silt/clay ratios as proposed in this paper (Flemming BW, 2000).In this scheme, 25 sediment classes are distinguished,each defined by a generic name and a letter-number code (Fig.2).The sediment transportation tendency was based on the method of Gao S and Collins M (1992, 1994).

Fig.2.Sediment ternary diagram in Dagu River estuary.

3.Characteristics of sediment distribution

3.1.Sub-environments classification

Based on the ternary diagram of Flemming BW (2000),the closer sediment samples are located to the silt end, the higher is the energy level; the closer samples are to the clay end, the lower is the energy.As shown in Fig.2, it can be seen that all sediment samples are distributed at Band I and Band II, indicating that the hydrodynamic conditions in the study area were generally strong.On the other hand, the sample points are located in Zone A (slightly muddy sand area), Zone B (muddy sand area), Zone C (sandy mud area)and Zone D (slightly sandy mud area), indicating sand contents of the sediment samples varied greatly.According to the location of sediment samples and the geomorphological background of the study area, the study area can be divided into four sub-environments (Fig.3): Estuary sand bar, tidal flat, inter-distributary bay and subtidal zone.

Fig.3.Sedimentary environment sub-divisions of the Dagu River estuary.

Estuary sand bar is located at the river estuary, with the highest deposition rate.The sediment is generally composed of fine sand and silt.The samples in estuary sand bar are mainly distributed in Zone A and Zone B in the ternary diagram.

Tidal flat area is located between the average low tidal line and the average high tidal line.Due to long-term submergence and exposure of periodic sea water, the surface sediments composition in tidal flatwas relatively complex,and the samples are mainly distributed in Zone C of the ternary diagram.

Yanghe River is located to the Southwest of Dagu River.The inter-distributary bay is the relatively low area of the underwater branching channel deposits between Dagu River and Yanghe River.The hydrodynamics of the interdistributary bay is very weak, the sediments are dominated bymud.The samples are mainly located at the back of Zone C and Zone D of the ternary diagram.

Subtidal zone is slightly concave in topography.Waves and tidal currents action are very weak, and the sediment was relatively finer.The samples are mainly located in the back of Zone C and Zone D of the ternary diagram.

3.2.Distribution characteristics of sediment types

It can be seen from Fig.4 that there were 7 types for the surface sediments near the Dagu River estuary, such as sandy silt, silty sand, silt, sandy clay, sandy mud, mud and clay.In the flood season, the sandy mud in the estuary area was widely distributed.Clay and sandy silt was mainly distributed at the tidal flat area.Sandy mud, clay, and sandy silt were widely distributed at the river estuary.However, due to the sharply decreased sediment supply from Dagu River in the dry season, the fine sediment in the surface of the estuary area was redistributed under marine hydrodynamics.The types of surface sediments were changed significantly.

Fig.4.Diagram of the surface sediment types in the Dagu River estuary.a-Flood season; b-dry season.

3.3.Distribution characteristics of sediment grain size parameters

According to the statistics and calculation of the grain size parameters of Dagu River estuary, grain size parameters diagrams of the sediments were drawn (Fig.5-Fig.8), and the typical samples of the different zones were selected to plot the grain size frequency of the sediment samples (Fig.9).

Fig.5.Seasonal changes of average grain size of surface sediments in Dagu River estuary.a-Flood season; b-dry season.

Fig.6.Seasonal changes of sorting factor (σ 1) of surface sediments in Dagu River estuary.a-Flood season; b-dry season.

Fig.7.Seasonal changes of skewness (Sk1) of surface sediments in Dagu River estuary.a-Flood season; b-dry season.

Fig.8.Seasonal changes of kurtosis(KG) of surface sediments in Dagu River estuary.a-Flood season; b-dry season.

Fig.9.Comparison of frequency distribution of surface sediments of the different sub-environments.a-Estuary sand bar; b-inter-distributary bay; c-tidal flat area; d-subtidal zone.

3.3.1.Sediment characteristics in flood season

From land to sea, the average grain size of surface sediments decreased gradually, the sorting became worse, the skewness was gradually negative, and the kurtosis was narrow.The kurtosis showed a tendency from narrow peak towards a medium-narrow peak.

In detailed, the average sediment grain size of the estuary sand bar area and tidal flat area were relatively coarse,generally less than 4φ.The average grain size of the subtidal zone and inter-distributary bay were relatively finer, generally greater than 5.6φ.

The sorting was generally poor, and the surface sediment sorting coefficients were all greater than 1.3.The sorting was best at estuary sand bar among the other three subenvironments.

The particle distribution curves of sediment at all the study area were relatively symmetrical, and the skewness coefficient was between -0.45 and 0.45.For the subtidal zone an dinter-distributary bay, the skewness coefficient was between -0.05 and 0.05, showing very symmetrical curves.For estuary sand bar, the skewness coefficients were larger than 0.25, indicating there dominated the coarser sediment contents in this area.

The kurtosis values of the sediment samples in the tidal flats and the estuary sand bars were mainly larger than 1.5.The curves showed sharp peaks and the sediments showed a coarser tendency.In contrast, the kurtosis values of the sediment samples were mainly smaller than 1 in the subtidal zone and inter-distributary bay.The curves have the middle and narrow peaks and the sediment in these two areas show a finer tendency.

3.3.2.Sediment characteristics in dry season

The distribution of sediment grain size parameters in the dry season changed significantly compared with the flood season.The average grain size of the whole study area became coarser.The grain size at most of the area was less than 5.6φ.In detailed, the average grain size at estuary sand barwas less than 4.4φand the sorting property became better.The partial skewness value of the whole study area showed an increased tendency and the grain size distribution curve was more positive in the dry season.

4.Hydrodynamics response mechanism

4.1.Contribution of river water and sediment discharge to the estuary

In recent years, due to the temperature increasing, the evaporation of the basin has increased, coupled with the construction of a large number of water conservancy projects in the basin, the runoff has decreased drastically, and the annual runoff of the Dagu River has decreased by 52.38% from 1981 to 2008.About 80% of the precipitation in the Dagu River Basin was concentrated in the flood season (from July to September), especially in July and August, accounting for about 56% of the year (Jiang DJ et al, 2013; Fig.10).During the dry season, there was almost no water and sediment flowing into the sea.Therefore, in the dry season, it could be considered that the Dagu River estuary area was not affected by river affections.

In 2012, the annual runoff and sediment transport of the Dagu River were also concentrated in several flood peaks during the flood season in July and August.The coarsegrained sediment was first deposited at the river estuary, and the finer sediment was transported to the sea under the influence of runoff and tidal current.

4.2.Wave stirring sediment

Wave is an important factor for the resuspension of sediments on the seabed and the coarsening of surface sediments.When waves transported from offshore to near shore, the waves broke down and deformed.The fine sediments on the seabed were stirred up and brought into the upper water, and the surface sediments were coarsened.The effect of wave stirring sediment was mainly related to wave characteristics, local water depth and surface sediment distribution characteristics.

Jiaozhou Bay is a semi-closed bay and the water depth in the bay varies greatly.The depth of central part and bay estuary is more than 30 m.The Dagu River delta is located in the northwest of Jiaozhou Bay, and the water depth was less than 5 m (Chang DF, 1991).In general, Jiaozhou Bay has a small wave height and a short wave period.Even in the case of a cold wave and the gale crossing process, the maximum wave height (H1/10) was only 1.07 m with a wind speed of 14 m/s, and the average wave period was 4.2 s.When the outer sea wave propagated into the bay, to the 5 m depth sea area,wave height was attenuated by more than 90% (Wang YP et al., 2000).Therefore, the Dagu River estuary area was less affected by the outer sea waves, but the spontaneous waves near the estuary played an important role in the sedimentary environment transformation in the estuary area.Based on the measured wave element data near the Dagu River estuary from September 24 to October 4, 2012 (Chen XY et., 2016),the Satou's formula (1962) was used to calculate the critical depth of the surface sediments stirred up at the Dagu River estuary.The details were as follows:

In the formula,H0andL0are wave height and wavelength in the deep water respectively,HandLare the local wave height and wavelength respectively,D50is the local sediment median grain size,his the critical depth of local sediment stirred up by waves,dis the local water depth.When a certain level of wave which can stir up the seabed sediment entirely is greater than or equal to the actual water depth, the level of wave can stir up the seafloor sediment, and vice versa.

According to the statistics of the measured wave data, the waves with a wave height less than 0.1 m in the Dagu estuary accounted for 73%, and the waves with a wave height between 0.1 m and 1.2 m accounted for 17%.According to Sato's formula, the waves with effective wave height (H1/3) of 0.1 m and 1.2 m were calculated respectively.When the effective wave height (H1/3) was 0.1 m, the critical depth of surface sediments stirred up was 0.19 m.When the effective wave height (H1/3) was 1.2 m, the critical depth of surface sediments stirred up was 0.32 m.

4.3.Tidal currents stirring up and transporting sediment

The tidal currents in this area were the direction from northwest to southeast, and the flood current velocity from the northwest was greater than the ebb current velocity (Chen B et al., 2012).Due to the small wave effect in this area, the tidal sediment effect cannot be ignored.The starting current velocity of surface sediments in this area was calculated according to the formula of the starting velocity by Dou GR(1960) in the viscous fine-grained sediment.

WhereUcis the starting current velocity (average of the vertical line),hais the atmospheric pressure expressed by the height of the water column, which is 10.366 m; δ is the water molecule thickness value of 3×10-8cm, and γ is the water weight, which is 1 N/m3, γsis the sediment gravity value of 2.65 N/m3,Dis the maximum median diameter of all sediments in the dry season, andhis the average water depth.When the actual velocity is greater than the sediment start velocity, the sediment of seabed can be stirred by the current.Based on calculation, the starting current velocity was 32.4 cm/s,and the maximum current velocity of the measured station was 43.5 cm/s.It could be seen that the tidal current velocity was greater than the starting current velocity.Therefore, it could be determined that during the observation period, the tidal current could not only transport sediment but also stir up sediment at Dagu River estuary.

4.4.The hydrodynamic response of the temporal and spatial distribution characteristics of sediments

4.4.1.Estuary sand bar and tidal flat area

In the flood season, the sediments in the river estuary were obviously coarser than those in the deeper water area.Due to the mechanical sorting, the coarse sediments were deposited earlier than the fine sediment.Furthermore, the sea-land interaction was strong at the river estuary and the river runoff,waves and tidal current actions were concentrated here, which was helpful to the flocculation of the sediment, making the bottom sediment coarser.In the dry season, river runoff was cut off and marine dynamics was dominated at the river estuary.The seabed sediments were stirred up and suspended into the water.The coarser sediments were exposed on the surface, and the sorting was also better than the flood season.

4.4.2.Inter-distributary bay and subtidal zone

In the subtidal zone (2-5 m), waves were basically unable to stir up the bottom sediments.Under the dominated influence of the tidal currents, the current velocity of the seabed increased and the fine-grained sediment was resuspended and then the coarse sediments were exposed on the seabed.As shown in the sediment particle size parameter plots, the sediments just showed a coarser tendency, but the sorting, skewness and kurtosis of the particles had no obvious changes.This resulted from the fact that the single tidal current hydrodynamics was relatively weak in this area.

4.5.Tendency analysis of sediment transport

Sediment transportation is affected by various characteristics such as grain size and the shapes of sediment particles.The influence of grain size is the most important factor (Sunamura T and Horikawa K, 1971; McLaren P and Bowles D, 1985; LeRoux JP et al., 2002; Poizot E and Méar Y, 2008).By analyzing the trend of sediment grain size change to determine the net transport of sediment, it has been applied in several bays and achieved good results (Sánchez A and Shumilin E, 2019; Hu G et al., 2018; Rodrigo D et al.,2019; Li T et al., 2019).A two-dimensional grain size trend analysis model by Gao S and Collins M (1992, 1994) was used in this study for calculation and analysis.The comparison distance was set to 0.039° depending on the density of the samples distribution.It can be seen from the calculation results (Fig.11) that in the flood season, surface sediments at river estuary was transported to the south and southeast under the influence of the runoff recharge from the Dagu River and the Yanghe River.In the dry season, marine dynamics became dominated, and sediments transportation mainly was affected by tidal currents.In the north area of Jiaozhou Bay Bridge, sediments showed a transport tendency to the southwest, and in the south area of Jiaozhou Bay Bridge, sediments showed a transport tendency to the northeast.This also confirmed the results of the study that rectilinear cur dominated at Jiaozhou Bay except for a few areas (Kong LS et al., 2004).

Fig.11.Surface sediment transporting tendency in Dagu River estuary.a-Flood season; b-dry season.

5.Conclusions

(i) Dagu River estuary area can be divided into four subenvironments: tidal flat area, estuaries and bar area, subtidal zone and inter-distributary bay.

(ii) In the flood season, the surface sediments of the estuary area were mainly sandy mud, and the average grain size from the land to sea was gradually decreasing; In the dry season, the surface sediments of the estuary area were dominated by sandy silt and the coarse grain range became larger.

(iii) Wave stirring surface sediments of sea bed mainly occurred the water area shallower than 0.19 m.The current velocity was greater than the starting current velocity.So the tidal current not only transported sediment but also stirred up sediment at Dagu River estuary.

(iv) The surface sediments of the sea bed shown a transporting tendency towards south and southeast in the flood season.However, the surface sediments of the sea bed were transported towards southwest at the north of Jiaozhou Bay Bridge and transported towards northeast at the south of Jiaozhou Bay Bridge in the dry season.This is related to the direction of rectilinear current at Jiaozhou Bay.

Acknowledgement

The authors are grateful to Lei Guo , Xiao-lei Liu , Yonggang Jia, Lu-lu Qiao, Bin Chen and others for discussions and help in this study.The authors are grateful to the anonymous reviewers and Dr.Yan Yang for their comments and suggestions, which significantly improved the quality of the manuscript.This study is supported financially by National Natural Science Foundation of China (41506107),Special Program for Basic Research of the Ministry of Science and Technology, China (2013FY112200), "China-ASEAN Marine Geoscience Research and Disaster Reduction and Prevention initiatives Project" Ministry of Foreign Affairs of China.