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International Journal of Sediment Research 34 (2019) 531e536

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International Journal of Sediment Research

journal homepage: www.elsevier .com/locate/ i jsrc

Original Research

From fluvial dynamics to eco-fluvial dynamics

Guojian He, Hongwei Fang*, Jianyu Wang, Tao ZhangState Key Laboratory of Hydro-science and Engineering, Department of Hydraulic Engineering, Tsinghua University, Beijing, 100084, China

a r t i c l e i n f o

Article history:Received 4 May 2019Received in revised form21 May 2019Accepted 22 May 2019Available online 7 June 2019

Keywords:Fluvial dynamicsEco-fluvial dynamicsWater and sediment transportAquatic ecosystemsInteractive response

* Corresponding author.E-mail address: fanghw@tsinghua.edu.cn (H. Fang

https://doi.org/10.1016/j.ijsrc.2019.05.0021001-6279/© 2019 Published by Elsevier B.V. on behaland Erosion Research.

a b s t r a c t

Sediment plays a very important role in the functioning of river ecosystems. It is the basic substance forthe survival of benthic animals and aquatic plants. On the other hand, the growth of biofilms and bio-disturbance of benthos affect the sediment transport characteristics. With the increasing attention toprotect aquatic ecosystems, the importance of habitats has become increasingly researched. The need tostudy the interactions among sediment, flow, riverbed deformation, and aquatic ecosystems naturallyleads to the proposed discipline of eco-fluvial dynamics. In this paper, the basic concept and mainresearch content of eco-fluvial dynamics is introduced with the Yarlung Tsangpo River as the researchexample. This case study is an example of an aquatic ecosystem in an ever-changing environmentbecause of the effects of climate change. The results of analysis of eco-fluvial dynamics will provide ascientific basis for decision support for the government.© 2019 Published by Elsevier B.V. on behalf of International Research and Training Centre on Erosion and

Sedimentation/the World Association for Sedimentation and Erosion Research.

1. Introduction

The Tibet Plateau has one of the highest biodiversities in theworld, and is known as the “rare wildlife natural park and plateauspecies gene pool”. The Yarlung Tsangpo River, originates from theHimalayas, and is one of the highest rivers in the world. The terrainis complex with high mountains and deep valleys. It is high in thewest and low in the east, and water flows out of China throughPasighat as shown in Fig. 1. The upstream reaches flow throughwide valleys on the plateau. The middle reaches basically flow fromwest to east, and the valleys are wide and narrow and braided. Thelower reaches of the river are alpine valleys. Fig. 2 shows theelevation changes along the lower reaches of the Yarlung TsangpoRiver. From Nuxia to Motuo, the distance is less than 100 km, whilethe elevation changes about 2,000 m. Such complex and variabletopography and flow conditions make the river ecosystem andbiodiversity extremely rich. The flora and fauna in the ecosystemalso affect sediment transport, channel geometry, and the riverbedmorphology.

Sediment in the riverbed is known as the substrate in biology. Itis the basic substance for the survival of the benthic animals andaquatic plants, and is an important part of the river ecosystem. Itnot only plays a key role for the breeding and spawning of many

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aquatic animals, but also can be used as a refuge for aquatic animalsand habitats.

Fig. 3 shows the mutual relations among water chemistry,habitat, flora and faunal in a river system. The sediment particlesboth in the water and on the bed will absorb amounts of inorganicmatter, organic matter, and bacteria; and biofilms may develop onthe sediment. Sediment movement will affect thewater quality andthe eco-system. Particle size and surface properties (Fang et al.,2014), riverbed morphology (Han et al., 2018), and permeabilityof the sediment to water (Fang et al., 2018) also affect the transportof dissolved oxygen and nutrients across the sediment-waterinterface. All of these factors affect the growth of biofilms,benthos, and aquatic plants.

Richards et al. (1997) pointed out that the levels of water flowand sediment on the river reach scale are good indicators of thebenthic community characteristics. Their work fully demon-strated the strong correlation between the structure of thebenthic community and the flow rate and sediment levels. Theresearch has gradually evolved from traditional fluvial dynamicsto eco-fluvial dynamics as researchers pay more attention to riverecosystems.

2. The definition of eco-fluvial dynamics

Traditional fluvial dynamics studies the sediment transport,erosion, deposition, and riverbed deformation under the action ofgravity, water flow, waves, and wind. It mainly focuses on the

ning Centre on Erosion and Sedimentation/the World Association for Sedimentation

Fig. 1. Plan of the middle and lower reaches of Yarlung Tsangpo River.

Fig. 2. Elevation changes of lower reaches of Yarlung Tsangpo River.

G. He et al. / International Journal of Sediment Research 34 (2019) 531e536532

physical processes (Fang et al., 2015). However, in addition to thesephysical processes, there are many other interactions related tosediment transport in the river system. For example, the migrationand transformation of nutrients, pollutants, and toxic organiccompounds are affected by the sediment concentration; and thesediment transport characteristics are affected by the growth ofbiofilms and bio-disturbance of the benthos.

The complex topography and flow conditions in the YarlungTsangpo River, as well as the vulnerability of cherished species,requires complete consideration of fluvial dynamics, and utilizationof multi-disciplinary knowledge to fully study the complex

Fig. 3. Mutual relations among water chemistry

interactions of the physical and biological processes. The studies ofthe river water environment and aquatic ecology are merged intoone research field and a new discipline, eco-fluvial dynamics, isformed built upon traditional fluvial dynamics. This new field willimprove the understandings of the micro-geomorphology, hydro-dynamics, water environment, and indicators of biological habitatand support the protection of aquatic ecosystems.

Eco-fluvial dynamics is a science that studies the coupled re-lations among sediment transport, flow, riverbed deformation, andbiochemical processes of aquatic ecosystems. Surface characteris-tics of sediment particles, sediment transport, and its ecological

, habitat, flora, and fauna in a river system.

Fig. 4. The main research components of eco-fluvial dynamics (Fang et al., 2019).

G. He et al. / International Journal of Sediment Research 34 (2019) 531e536 533

influence mechanisms are the key points of eco-fluvial dynamics.By studying the interaction of these physical, chemical, andecological processes, the interactions will be quantified, and thencomputer models can be developed. The ultimate goal of studyingthe eco-fluvial dynamics is to restore and protect the structures andfunctions of river ecosystems, maintain the health of river ecosys-tems, and eventually to mitigate the impact of human activities ornatural disasters on these ecosystems (Fang et al., 2019).

3. The components of eco-fluvial dynamics

According to the definition of eco-fluvial dynamics, the mainresearch components as shown in Fig. 4 include: surface charac-teristics of sediment particles, sediment transport variation withbiofilms, interactions between sediment transport and aquaticvegetation, interactions between micro-geomorphology andbenthic animals, and the impacts of vegetation on sedimenttransport and bed deformation.

Through high-resolution microscopy, it is found that the surfacemorphology of sediment is undulating and there are pores ofvarious sizes, which determine the surface charge characteristicsand adsorption characteristics of the sediment particles (Fang et al.,2009). Due to this phenomenon, the interactions between sedi-ment and pollutants are complex and variable (Chen & Fang, 2013;Fang He et al., 2015; Fang Zhao et al., 2015). Based on a surfacecomplexation model, the adsorption distribution coefficient of thesediment particles can be simplified (Cui et al., 2017). Under-standing this coefficient will help to obtain the distribution relationbetween pollutants in water and suspended sediment.

At the same time, the surface of sediment particles adsorbs alarge number of bacteria. These bacteria formbiofilms bygeneratingextracellular polymers, which constitute the “microbial skin of theriver” (Battin et al., 2016; Fang, Cheng, Fazeli, & Dey, 2017; Fang,Chen, Huang, & He, 2017; Fang, Lai, Cheng, Huang, & He, 2017).They affect the sediment movement and bed deformation (FangCheng et al., 2017; Fang Chen et al., 2017; Fang Lai et al., 2017; Lai

et al., 2018). The sediment particles and the biofilm coated on thesurface adsorb various types nutrients and pollutants. The attachednutrients, pollutants, and biofilms move along with water andsediment, together affecting the ecological processes of the river.

Environmental and ecological problems caused by water andsediment transport usually occur within micro-topography in asmall scale range from centimeters to a meter. The microstructureof a riverbed can greatly affect the local mean and instantaneousflow fields, and generate non-uniform flow fields such as backflowand cross-section secondary flow (Liu et al., 2017). The heteroge-neity and instantaneous nature of these flows not only change thesediment deposition and scouring pattern near the bed surface, butalso change the concentration distribution of dissolved oxygen orpollutants, affecting the living environment of microorganisms(Han et al., 2018). The channel geometry and the ecosystem areinter-related. A large number of benthic animals digging andfeeding, and causing biological disturbances, will change themicro-topography. In 2010, the American National Academy of Sciencespublished a report calling on researchers to investigate how or-ganisms affect the surface topography of the earth because modelsincluding only physical processes often are insufficient to predictgeophysical processes (National Research Council, 2010).

The influence of sediment laden flow on aquatic vegetation ismanifested in many aspects. First, the flow velocity can affect thegrowth rate and concentration of phytoplankton directly (Gaoet al., 2018). Second, the existence of suspended particles willreduce the transparency of water, thereby reducing the photo-synthetic rate of phytoplankton and other aquatic plants. More-over, sediment particles can absorb nutrients, especiallyphosphate (Huang Fazeli et al., 2015; Huang, Fang, & Reible,2015), which will limit the growth of phytoplankton.

The influence of aquatic plants on hydrodynamic processes hasbeen studied for a long time (Nepf, 2012). Under the influence ofvegetation, the change of the flow structure also has a strong in-fluence on the sediment transport and riverbed deformationprocesses.

Fig. 5. The schematic gird map of the study area.

G. He et al. / International Journal of Sediment Research 34 (2019) 531e536534

4. The application of eco-fluvial dynamics in the YarlungTsangpo River

Eco-fluvial dynamics was applied to the Yarlung Tsangpo River.A two-dimensional numerical model for the reach from Nang toPasighat with a length of about 500 km was developed using thefinite volume method. The computational domain was divided intoa total of 41,370 unstructured elements, with the minimum sidelength of 20 m and the maximum side length of 200 m, as shown inFig. 5. Three scenarios were simulated to evaluate the impact ofglobal climate change. The first scenario uses the current annualaverage flow rate which corresponds to the current greenhouse gasconcentrations. The next two scenarios are based on the green-house gas concentration predictions using the RepresentativeConcentration Pathway (RCP) value. RCP 4.5 and RCP 8.5 were usedto predict the annual average flow rates in 2,035 for scenarios 2 and3, respectively.

Micro-geomorphology has an important impact on benthichabitat and is a key factor in the study of eco-fluvial dynamics. It is

Fig. 6. Criterion (Q versus Fr) for prediction of bedforms.

also affected by the biofilm growing on the sediments (Fang Chenget al., 2017; Fang Chen et al., 2017; Fang Lai et al., 2017). The Shieldsparameter, Q, can be used to evaluate the sediment transport rate,and, thus, the bed configurations. Hence, the Shields parameter, Q,usually is chosen to predict the bedform types. Fig. 6 shows theprediction of bedforms for the three scenarios with the variation offlow Froude number, Fr, in 2,035. The increase of the possible rangeof radiative forcing values will melt the glaciers and permafrost,resulting in an increase in the discharge. This increase will causethe Fr to be a little higher than the current Froude number. Q will,however, increase rapidly, which will make the bedforms moreactive.

In addition to using the Shields parameter, the bedformdimensions can be calculated with the steepness, D/l, where D isthe bedform height and l is the bedform length. An empiricalrelation between the bedform dimensions and a flow parameter, h,

Fig. 7. Bedform steepness, D/l, as a function of h

Fig. 8. Concentration variations for different scenarios.

G. He et al. / International Journal of Sediment Research 34 (2019) 531e536 535

is directly established. Where h ¼ Q/Qc, and Qc is the thresholdShields parameter for sediment motion. The variation of bedformsteepness D/l with h is shown in Fig. 7. h is water depth and d50 isthe median diameter of the sediment. It is evident that the steep-ness initially increases with h until a maximum value is reachedand then decreases as h increases. The range of the steepness of thebedforms with the scenario RCP 8.5 is the highest among the threescenarios as shown in Fig. 7.

Based on hydrodynamic and water quality conditions in theNiyang River (Gong et al., 2012; Liu et al., 2013), the daily changeseries (2020e2030) of the plankton and benthos biomass (dryweight) are calculated. The average concentrations are shown inFig. 8. The units are g/m2 for peri-diatom and carp, and mg/L fortotal nitrogen (TN), total phosphorus (TP), plankton-diatom, andthe rotifer. In the two scenarios with RCP 4.5 and RCP 8.5, themelting glaciers and permafrost caused the water level of theNiyang River to increase, the bed-forming force to intensify, and thecomplexity of the river bed configuration to accordingly increase.These changes will result in more diverse habitat conditions foraquatic organisms.

As a result, the biomass of primary producers such as diatomsincreases. Also, the increasing biomass of the low tropic levelcreatures provides a greater food source for the higher tropic levelcreatures such as the carp. As shown in Fig. 8, when pH � 8, theconcentration of TN slightly increased from 4.12 to 4.21 mg/L forRCP8.5, while the concentration of TP slightly decreased because ofthe uptake of the aquatic organisms. For the increasing water areaand water discharge, the concentration of peri-diatom and carpincreased more dramatically than plankton-diatom and rotifer. Thebiomass of the carp increases from 2.053g/m2 to 2.827 g/m2 forRCP4.5 and 3.341 g/m2 for RCP8.5.

5. Conclusions

In the current paper, the concept andmain research componentsof eco-fluvial dynamics are presented, and the interaction betweenflow, sediment transport, and river biochemical processes wasanalyzed. This newly developed discipline covers many researchfields, such as flow and sediment transport, sediment surfacecharacteristics, biofilms, benthos, plankton, and aquatic plants. Eco-fluvial dynamics faces challenges of multi-disciplinary, interdisci-plinary, and multi-scale coupling, which requires researchers withdifferent backgrounds to work in depth cooperatively. In recentyears, with the increasingly intensified global warming, the TibetPlateau, as a climate sensitive area, has undergone changes in itsglacial permafrost and vegetation, which have a direct impact on theamount of water flow, sediment transport, and riverbed deforma-tion of the Yarlung Tsangpo River. Protecting the aquatic ecology

system of Yarlung Tsangpo River and other rivers under conditionsrequires an understanding of the complex interactive Eco-fluvialprocesses, which requires a deeper understanding of the eco-fluvial dynamics to provide a scientific basis for decision supportin response to the adverse effects of climate change.

Acknowledgments

This investigation was supported by the 111 Project (Grant No.B18031) and the National Natural Science Foundation of China(Grant No. 91647210), China.

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