Improved ecological development model for lower Yellow River floodplain,China

Jin-ling Zhng ,Yi-zi Shng ,b,*,Ji-xing Liu ,Jin Fu ,Meng Cui

a Yellow River Engineering Consulting Co., Ltd., Zhengzhou 450003, Chinab State Key Laboratory of Simulation and Regulation of Water Cycle in River Basins, China Institute of Water Resources and Hydropower Research,Beijing 100038, China

Received 15 October 2019;accepted 22 July 2020

Available online 13 December 2020

Abstract In this study,a model for the development of the wide floodplain in the lower Yellow River Basin,in China was developed.This model includes flood control schemes using grading criteria,enables sediment deposition in partitioned zones,and allows free exchange between channel runoff and sediment.The wide floodplain located between the main channel and levees is divided into three typical regions:the tender,low,and high floodplains.Different ecological models should be applied when these floodplains are constructed.This paper describes the associated research ideas and methodology,and clarifies several key issues,including sediment prediction and regulation,land planning,land use,and a multi-dimensional framework of safeguard measures for industries on the lower Yellow River floodplain.A refined ecological development model is proposed for the lower Yellow River floodplain,and future work on ecological and sustainable development of the lower floodplain is suggested.To establish a comprehensive system integrating runoff and sediment resource regulation in the Yellow River Basin,future work should focus on runoff and sediment exchange mechanisms in the wandering lower reaches.Furthermore,it is necessary to improve theories on floodplain planning and ecological construction,and these theories should be integrated with the research findings on land development across the lower Yellow River floodplain.© 2020 Hohai University.Production and hosting by Elsevier B.V.This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords:Ecological development;Lower Yellow River;Floodplain;Sediment;Eco-friendly construction model

1.Introduction

The Yellow River is the most sediment-laden river in the world,with a maximum annual sediment load of 3.91×109t(in 1933)and a maximum sediment concentration of 972 kg/m3(in 1977)(Fig.1).The middle reaches of the Yellow River flow through the Loess Plateau with unconsolidated soil and sparse vegetation,resulting in large amounts of sediment being brought into the river through its tributaries(Li et al.,2019).The river channel gradually becomes wider with low slopes as it enters the plain area.This leads to severe sediment deposition,which has created suspended rivers and levees.Throughout history,China has aimed to stop or slow down the continuous deposition of sediment in the lower reaches of the Yellow River,because this has caused a remarkable rise in the water level(Zhang et al.,2018a).Many years of practical experience have proven that the most effective strategy is to allow natural flood discharge to deposit sediment on the floodplains along the lower Yellow River(Xi,2013).In this context,short-duration and high-intensity floods can be artificially created at appropriate times through coordinated scheduling between reservoirs in the upper and middle reaches of the river.Hence,sediment can be flushed into the sea(Li and Sheng,2011;Wang et al.,2018).Currently,1.6×109t of annual total sediment are transported into the lower Yellow River from its middle and upper reaches.Of this amount,1.2×109t of sediment flow into the sea,and 4.0×108t of sediment are indefinitely deposited on the riverbed of the lower Yellow River.This indicates that,during the annual flood season,cities along the lower Yellow River are at risk of flooding,particularly the regions located in sedimentdeposition zones beside levees.Moreover,because of serious land resource shortages in this region,most of the local people live on the floodplains of the lower reaches,even though it is dangerous to do so.Currently,the Yellow River floodplain region has a population of 1 895 200 and croplands of 226 733.3 hm2(Zhang et al.,2018a,2018b).The current floodplain management strategies have effectively guaranteed the safety of inhabitants and their properties beside levees.However,these strategies are sometimes controversial because they hinder flood discharge and sediment deposition on floodplains.

Fig.1.Location of lower reaches of Yellow River.

Suspended rivers in the Yellow River Basin are formed by large amounts of sediment from the midstream that are carried downstream and continually deposited in the lower watercourses.Meanwhile,dikes on both river sides restrict the lateral space for sediment deposition(Qi et al.,2011).Consequently,riverbeds in the lower reaches continually rise.This means that the elevation of beach lands is higher than that of the regions outside of the levees.As a result,a Level I suspended river is formed(Hu et al.,2006;Zhang,2005).Owing to the uneven lateral distribution of sediment in the watercourse,a new beach near the watercourse has a high depth of siltation,and both the dike foot and the beach far from the main channel have low siltation depths.This raises the beach lips and channel bottom,thereby forming a Level II suspended river,which is characterized by a high channel,a low beach,and a low-lying dike foot(Hu,2003;Zhang et al.,2018d)(Fig.2).

Fig.2.Schematic diagram of Level I and Level II suspended rivers.

The most severe Level II suspended river is located within the 235-km section of the lower Yellow River between Dongbatou and Taochengpu.The floodplain lip is between about 3 and 5 m above the ground surface of the dikes on riversides,and the transverse gradient of the floodplain is approximately 0.1%,which is much steeper than the slope of the watercourse(0.014%)(Ma et al.,2010).When water flows over the floodplain lip,it aggressively inundates the floodplain along the dikes,thereby increasing the risk of the dike breaching.Since 1960,many dikes have been constructed in the floodplain region(Liu and Zhao,1998).However,since the 1980s,water inflow from the Yellow River has decreased,and the dikes have been encroaching channels(Cao et al.,2011).Although dikes function to ensure the safety of residents on the floodplain,they affect the normal exchange of water and sediment within the channel.This exacerbates sediment deposition in the channel,further reducing the watercourse flow capacity and causing the deterioration of Level II suspended rivers(Zhang et al.,2018a).

On a global scale,many different strategies for river floodplain management have been employed(Neill and Galay,1967;Leahy et al.,2005;Schindler et al.,2013).According to national and local requirements for river ecological restoration,different types of waterfront landscape construction have been implemented.Rational waterfront landscape planning and construction can enhance the social benefits of river ecological landscape functions and maintain the main river functions,such as flood control,drainage,sewage discharge,and water storage(Wohl,2018).Although these strategies and types of construction have been employed in many regions,they are not particularly relevant to the lower Yellow River,where the river segment with most severe sediment deposition is 22 m higher than riverine cities with a continuous riverbed rise of 10 cm/year on average.Therefore,it is difficult to use the common strategies to transform the lower Yellow River riverbed into a traditional riverside landscape.

Flood detention and sediment deposition are the main functions of the lower Yellow River floodplain,and these processes differ depending on the category of the floodplain.The current local strategy is to consolidate dikes by increasing the distance of dikes on both sides of the river(Zhou,1996).However,this is only effective to maintain the flood discharge capacity of the main channel of the lower Yellow River.When floods occur in the lower Yellow River Basin,flood flow slows down after entering the floodplain,and large amounts of entrained sediment are deposited on the floodplain.Afterward,clear water from the floodplain flows into the main channel,thereby reducing siltation there.Implementation of these strategies will result in silted and elevated floodplains.This indicates that the current strategy fundamentally conflicts with China's long-term goal to restrict the riverbed rise of the lower Yellow River.Since 2019,the Chinese government has actively promoted the national strategy of high-quality development in the Yellow River Basin.People on the floodplain have gradually increasing sense of risk reduction.They have requested that the distance between dikes on wide channel segments be reduced,to free the existing floodplains,which means floods would no longer completely inundate the plains.However,with no floodplains to detain flooding runoff and receive deposited sediment,narrowing the distance between dikes would further deteriorate the formation of Level II suspended rivers.Therefore,the current management strategies are not sufficiently rational.Zhang(2017b)proposed a new governance strategy to overcome the inherent disadvantages of the current management strategies,suggesting that dikes be consolidated by broadening the distance of dikes on both river sides.With this strategy,floodplains from the Yellow River dikes to the main channel would be categorized into three types according to the ascending rate of siltation and river dredging:high,low,and tender floodplains.From high to tender floodplains,flood control requirements should be gradually reduced,with gradually increased probability of sediment deposition and more frequent water-sediment exchange in plain channels.Practices have shown that this strategy is effective in changing existing suspended rivers and can enhance the flood prevention capability of floodplains due to no restrictions of the flood detention and sediment deposition functions of floodplains.Notably,the model proposed by Zhang(2017b)has potential to reduce the risks for people who live and work on floodplains.

In this study,a comprehensive technical framework system was established to support the implementation of the model proposed by Zhang(2017b).The requirements for the techniques are presented.The aim of this study is to promote discussion and associated research on the governance of the lower Yellow River,as well as to contribute to a refined scientific basis for the new technical framework.

2.Novel ideas for development and regulation of wide lower Yellow River floodplain

The lower Yellow River has wandering channels and changing river regimes,and overflow can potentially occur during the flood season,thereby threatening the safety of floodplain residents and their property(Zhang et al.,2018b).Dikes help store the overflow on the floodplain and improve the flood control capacity in residential areas.However,once an extreme flood event occurs in the basin,dikes are likely to significantly contribute to the risk of levee breaches(Zhang et al.,2018c).Zhang(2017a,2017b)proposed a model to plan the development and utilization of the wide lower Yellow River floodplain,which includes flood control through graded sediment deposition within partitioned zones and free exchange of channel runoff and sediment.As shown in Fig.3,this eco-friendly and systematic planning adopts a comprehensive approach to flood risk control and management,and can resolve the conflicts of disaster prevention,socioeconomic development,and ecological rehabilitation in the lower Yellow River Basin.It can also promote harmony between inhabitants and the river environment.

Fig.3.New model for development and ecological restoration of wide lower Yellow River floodplain(the blue dashed line indicates the channel morphology prior to modification,and the gray area indicates channel morphology after modification).

To enable model implementation,studies in four technical aspects are required.First,to develop the techniques for sediment prediction and control and to identify the sediment deposition mechanism in partitioned zones,it is necessary to investigate the mechanisms associated with channel runoff and sediment exchange in wandering suspended rivers.Second,to assist the development of techniques for dynamical partition of floodplains and adaptive flood control within the wide floodplain of wandering suspended rivers,the sedimentation characteristics in different regions should be identified,and the processes associated with the dynamic floodplain evolution should be investigated.Third,factors currently causing the wide floodplains of the wandering suspended rivers should be examined.This means that principal component analysis(PCA)should be employed in such investigation:a pressurestate-response(PSR)model that considers regional economic,social,and resource factors should be established;and techniques that enable ecological planning,construction,and restoration should be defined.Finally,it is necessary to identify methods that would enable the integrated regulation of flood and sediment resources in the river basin,such as project operation,runoff and sediment regulation,and flood and drought control,which are critical to facilitating the coordination of various services and ultimately to safeguarding all regions of the lower Yellow River floodplain.

2.1.Runoff and sedimentexchange features and associated prediction and regulation within lower Yellow River in a changing environment

2.1.1.Variations of runoff and sediment exchange characteristics in lower Yellow River floodplain

The environment of the Yellow River Basin has undergone substantial changes,and the free exchange of channel runoff and sediment has varied over time(Zhang et al.,2018b).In the lower Yellow River floodplain,variations in runoff and sediment exchange are driven by intensified human activities in the upper and middle reaches of the river,combined with simultaneous changes in hydrological and meteorological conditions(Zhang,2017a).Factors affecting the channel's ecological responses to the changes in runoff and sediment conditions should be investigated.Such investigation helps to illuminate the variations of runoff and sediment exchange in the lower Yellow River floodplain and to clarify the variations that have occurred since the construction of large water conservancy projects such as the Dongzhuang and Guxian projects.In this respect,four types of studies should be conducted:(1)assessment of the floodplain-channel ecological response to runoff and sediment in the river basin;(2)analysis of the floodplain-channel response features in the context of upstream and midstream runoff-and sediment-regulation systems and downstream flood control systems;(3)development of methodology and criteria for floodplain ecosystem health assessment;and(4)establishment of a model for ecological floodplain development.

2.1.2.Flow-carried sediment prediction in lower Yellow River based on altered runoff and sediment exchange mechanisms

Accurate predictions are necessary in developing management processes(Yang et al.,2010;Gianikellis and Dávila,2010).Sediment prediction in this paper refers to flowcarried sediment prediction in the lower reaches of the Yellow River in a changing environment.As flow-carried sediment prediction is closely related to the occurrence of natural floods and regulated floods regulated by upstream and midstream water conservancy systems(Wang et al.,2018;Ochoa-Rodríguez et al.,2015),accurate predictions are required to develop an integrated runoff regulation strategy for the Yellow River.It is necessary to focus on the trends of sediment discharge frequency,if sediment deposition can be accurately forecasted on monthly,annual,and intra-annual scales(Zhang,2008a).Therefore,to enable effective downstream governance,accurate sediment prediction should include multiple temporal scales,such as real-time,short-term,medium-term,and long-term scales.

Medium-and long-term sediment predictions are essential to developing and implementing upstream and midstream runoff and sediment regulation systems,and they help the river basin administration authority to develop annual runoff and sediment regulation rules.Medium-and long-term sediment predictions involve sediment deposition predictions in the river basin for the near future on annual and intra-annual temporal scales.Regarded as the basis for designing and scheduling coordinated reservoir activities,short-term sediment prediction should focus on channel sediment erosion and the development of river regimes.Real-time sediment prediction provides an important basis for flood risk assessment and decision-making support for emergency dispatch and commands.

With respect to short-term sediment prediction,a twodimensional(2D)water-sediment model for the lower Yellow River has been developed(Shang et al.,2014,2016).This model can analyze the effect of water-sediment regulation when different river channel management models are employed.The numerical simulation system comprises a set of sub-systems,in which one-dimensitional(1D),2D,and threedimensional(3D)models are integrated.The 2D floodplain water-sediment mathematical model can be used to solve orthogonal and nonorthogonal curvilinear coordinate grids,unstructured triangular meshes,hybrid meshes,and other computational gridding systems.This system has been applied to river channel management and river basin planning for the Yellow,Yangtze,and Huaihe rivers in China and has been proven to be adaptable in complex regions(Zhang et al.,2020;Shang et al.,2017,2019).

Real-time sediment prediction should adaptively simulate sediment transport processes with moving boundaries and reflect the characteristics of downstream riverbed sediment.In addition,a 2D runoff-sediment simulation should be performed using a practical model that provides prompt and accurate results of sediment dispatch.A comprehensive hydrological and sediment monitoring system for the Yellow River is currently installed in multiple locations along the river,facilitating the technical development of smart runoff and sediment prediction schemes.In the future,data mining,self-learning,and artificial intelligence technologies will be integrated into the existing knowledge system to provide realtime sediment predictions for critical river sections.

2.2.Classification and utilization of floodplains and adaptive grading flood control criteria

2.2.1.Classification and utilization of floodplains under applicable flood risks

Different flood control specifications can be applied to different floodplain regions.In this respect,the functions for the classified floodplains are illustrated in Fig.4.

Regarding the current floodplain-channel pattern,it is necessary to construct high,low,and tender floodplains in association with the criteria shown in Fig.3.The high floodplain region is situated adjacent to the levee and is elevated by deposited sediment.Thus,it can sustain floods with a 20-year return period with no threats to residents.Tender floodplains are located adjacent to watercourse,which are utilized for water discharge and sediment transport in conjunction with watercourse.Tender floodplains are designed to sustain floods with a five-year return period(Ren and Lin,2005).Low floodplains are located between high and tender floodplains and they can be efficiently utilized for sightseeing and agriculture.They control flood detention and sedimentation to some extent and can be used as inundated area during nonstandard floods as well.

Fig.4.Planning and development goals for classified floodplains in lower Yellow River Basin.

In addition to flood control,it is necessary to coordinate the development and utilization of flood-prone lands within the levees with economic-development planning for the regions beyond the levees.In some cases,the range of inundation and land uses of floodplains should be changed.Thus,in-depth studies are required to identify land use types of floodplains,to conduct scientific planning,and to propose floodplain development and utilization guidelines,including flood risk criteria and medium-and long-term planning for land uses in floodplains with wandering suspended rivers.Such changes allow for the rational exploitation and ecologically sustainable development of floodplains.They help control the risk of overbank floods as well,although it is difficult to achieve flood risk control alongside the assigned floodplain utilization based on national and local regulations for land resource management.

2.2.2.Adaptive flood control in floodplains

When flood control measures are implemented in grading criteria with respect to floodplain utilization,high safety levels can be achieved,and the maximum flood prevention for local industries can be guaranteed.Adaptive flood control includes static and dynamic flood control,which collectively ensure the security of floodplain development and utilization.

As shown in Fig.5,the adjustment of floodplain land uses serves as the static flood control during design floods.In this respect,the potential flood storage capacities of reservoirs in the upper and middle Yellow River reaches should be estimated first.Meanwhile,the flood control capacities of upstream water-retention structures should be calculated with consideration of the coordinated reservoir operation.Given that potential extreme weather events might occur in the basin,the probability and severity of floods with different return periods(e.g.,five-,ten-,and 20-year return periods)or extreme rainfall should be estimated,and the likelihood and severity of human-induced mistakes should be analyzed.Afterward,floodplain design and verification methods should be reviewed,with consideration of natural and anthropogenic changes.Finally,the land use types and inundation ranges of identified regions,which are below flood control standards,should be adjusted.

Fig.5.Static and dynamic flood control using grading criteria.

Dynamic flood control is defined as combined usage of water engineering works within the river basin for floodplain safety management(Zhang and Wei,2007).As dynamic flood control mathematically adopts a complex kinetic model that is high-dimensional,non-linear,and dynamic,it is a great challenge to derive the analytical or optimal solutions(Zhang and Suo,2005).To address this issue,we re-designed the mathematical method using the variable fuzzy optimization modeling method to select the best protective measures.This updated method allow us to refine or adjust flood control emergency planning in response to the predicted flood timing,location,and features,including such characteristic parameters as flood volume and peak.The model also helps to refine or adjust flood control emergency planning,with regard to food production and inhabitants on the downstream floodplains.Hence,targeted measures in different regions in response to various flood types can be implemented.

As shown in Fig.5,libraries containing data on flood features and flood control emergency plans are currently under development,with the aim of enhancing the adaptive flood control capacity and managing floods of different magnitudes.Meanwhile,future work will focus on coordinated regulation of upstream and midstream runoff and sediment regulation systems for the downstream flood control systems in the Yellow River Basin.Based on this research,a dynamic flood control technique will be developed.This technique will integrate flood warning and forecasting,coordinated reservoir operations,and real-time control of water retention structures,and it will provide program evaluation and feedback as well.

2.3.Ecological planning,construction,and restoration of floodplains

2.3.1.Ecological planning and orderly development across lower Yellow River floodplain

Ecological planning,construction,and restoration of floodplains involves rational distribution and exploitation of sediment resources in order to facilitate safe utilization and ecological development of floodplains(Ouyang,2005;Zhang et al.,2006).To achieve sediment deposition in partitioned zones,sedimentation must be in line with economic,ecological,and environmental progresses.In other words,downstream floodplain development should agree with national and local land use planning goals and meet the environmental requirements in the river basin.Therefore,a comprehensive approach should be adopted to develop a land planning model and methods for ecological construction and restoration of the lower Yellow River floodplain should be studied.

Fig.6 displays the methodology for scientific floodplain planning in the lower Yellow River Basin.The PCA and PSR models are key techniques in the methodology.To simplify the evaluation system,PCA is used to evaluate the selected indices and their estimated weights and to convert several indicators into one or several key composite indicators.The PSR model is used to determine the iterative interaction chain between river basin planning for floodplain use and regional development planning.The combination of these two techniques facilitates cyclic model optimization,which includes model construction,model solution,analysis,evaluation,feedback,and refinement.

At first,an ecological evaluation system should be established,and its indicators should be identified.Afterward,indicator-specific criteria should be defined according to national and regional planning standards and desired values,and the indicators should be weighted via expert assessments and social surveys.After an error analysis is conducted,the swarm intelligence and planning algorithms are used to establish an evaluation model.These algorithms are also used to refine and solve the model.The findings from the evaluation model should be adopted to facilitate continuous plan improvement.

The PSR model-based indicator system was established using the selected ecosystem health evaluation indicator characteristics.The utility and vegetation status of floodplains,spatial patterns of land use,service functions of the ecosystem,and status of regional socioeconomic development were investigated in detail.Table 1 presents the associated ecological impact assessment(EIA)indicator system.

Fig.6.Flowchart for land use planning and development in lower Yellow River floodplain.

2.3.2.Ecological construction and restoration of floodplains

A large amount of sediment is deposited in the Yellow River channels during floods.The cross-sections of the lower Yellow River channels have a significantly higher gradient than the vertical sections.When the watercourse is silted,the mainstream flow moves laterally and collides with dikes.Consequently,overflows occur over the dikes.This is unfavorable for flood control and seriously undermines the ecosystem stability of the lower Yellow River floodplain.Ecological construction of floodplains aims to maintain stable cross-sectional morphology on a long-term basis.Pilot studies have been conducted to develop measures to regulate sediment deposition in partitioned zones and to develop techniques for cross-section maintenance(Lu et al.,2018a).

Table 1 EIA indicator system used in land development and utilization model for lower Yellow River floodplain.

Since 2003,runoff and sediment regulations has been conducted 19 times in the Yellow River through the coordinated operation of midstream and upstream reservoirs.This helps to establish a harmonious relationship between runoff and sediment and to stabilize river regimes with respect to the watercourse.Multiple sediment treatment techniques have been developed as well,including pipeline sediment transport,sediment control,river dredging,and impinging jets.The joint application of these techniques has the potential to maintain stable cross-section morphologies over long-term periods(Zhang,2017b).

The supply chain of sediment transport management will be explored in the next section.To achieve cost-effective crosssection maintenance,the costs of sediment deposition,arrangements of personnel and material resources,rational sediment distribution,and balanced resource allocation should be considered.The development and cultivation of flood-and salt-tolerant crops is also important,as it helps to improve soil quality to enable ecological floodplain construction.Research associated with these aspects is a crucial factor in reducing the negative impacts of floods on industries and associated economies in floodplains.

2.4.Multi-dimensional frameworkforfloodpreventionand integrated regulation of flood and sediment resources in river basin

Currently,three major conflicts exist in the development and utilization of the lower Yellow River floodplain.The first is that facilitating water discharge,flood detention,and sediment deposition functions on floodplains conflicts with the protection of human life,physical safety,and property.The second is the tension between increasing land resource demand for economic development and land resource shortage in the Yellow River Basin.The third occurs when the ecological deterioration caused by uncontrolled land development conflicts with the need for environmentally sustainable development and ecological progress.To address these conflicts and the requirements for both basin and local management,combined governmental and market-based instruments should be adopted to promote the implementation of the ecological development model for the lower Yellow River floodplain.

With respect to the government,migration,the economy,the ecology,and the power supply,the following relationships should be analyzed.First,the political relationship between the central and local governments and between different river basins and regions,as well as the competitive relationship between regions on the left and right riverbanks and between upstream and downstream regions should be clarified.Afterward,it is necessary to study and propose decision-making strategies that involve the central government,river basin authorities,and the local government,as well as management entities,users,and stakeholders.To refine and develop a security-based floodplain ecological utilization model,the control and authority balance between the central government,river basin authorities,and local government should be determined or re-distributed if necessary.

The systems for runoff and sediment regulation,water resources management,and water quality management in the lower Yellow River Basin should reflect the natural attributes and features of the water environment and aquatic resources(Zhang,2008b).These systems should also consider the functional interactions,which include the political relationships between the central and local governments and between river basins and regions,as well as competition between upstream and downstream regions.In addition,these aspects should be investigated under specified conditions,and the economic,political,and social impacts of the associated functions should be evaluated as well(Lu et al.,2018b).

Zhang et al.(2017a,2017b)proposed a multi-dimensional safeguard system that promoted the implementation of the ecological development model within the lower Yellow River floodplain.In this study,an integrated system for runoff and sediment resource regulation in the river basin was established.In this system,several stations have been set up to monitor water quantity,quality,and sediment and to observe the aquatic ecology of rivers,canals,lakes,and reservoirs.This system comprehensively utilizes advanced techniques,such as satellitebased remote sensing,unmanned aerial vehicles,and video surveillance,to detect the changes in rivers,canals,lakes,and reservoirs,thereby facilitating the dynamic monitoring of runoff and sediment resources(Zhang et al.,2003).In addition,a refined reservoir regulation model was developed,which enables ecological restoration flood and sediment management.This model focuses on the application of muddy water-based power generation technology that can be employed in sandy rivers and reservoirs.The model achieves the multi-purpose synergy of single reservoirs,including flood control,power generation,sediment transport,water supply,shipping,and ecology,and it has the potential to regulate cascaded reservoirs(Zhang,2005;Zhao,2004).

Finally,the proposed system involves the joint regulation of water and sediment in the Yellow River Basin,and the scope of the system needs to be expanded,with emphasis on flood control and ecological security of downstream floodplains.It is suggested that smart technologies such as the internet and artificial intelligence be employed in the joint regulation of water and sediment,integrated water resources regulation,and service integration.In this manner,management based on a river-specific approach can be implemented to achieve the integrated and intelligent regulation of floods and sediment resources in the Yellow River Basin.

3.Discussion

The lower Yellow River floodplain,which has a large population,provides the functions of flood detention and sediment deposition.Downstream floodplain management has not achieved satisfying results for years,because of insufficient investment.Residents on floodplains are not adequately protected from floods,and the necessary post-disaster construction is inefficient.Owing to the lack of opportunities for employment and infrastructure development,a poverty belt with low productivity and living standards has developed along the Yellow River.Since 2014,Henan Province has launched two resettlement projects that involve a total population of 56 900.In 2017,the National Development and Reform Commission of China authorized a resettlement plan for the residents in Shandong and Henan provinces.This national plan has ensured flood security for 600 000 residents in Shandong,and 243 200 residents in Henan have migrated from the floodplain.After multiple treatments were conducted in the lower Yellow River floodplain,flood control and security levels have been significantly improved.However,a population of 912 800 remains at the risk of floods with a 20-year return period.Resettlement and reconstruction are encouraged by the Henan and Shandong provincial governments,and many resettlement locations have been expedited.Currently,people who migrated from the floodplains are confronted with high living costs and employment difficulties,which increase the likelihood that they will return.For instance,the migration of residents in flood-prone regions increases the area of agricultural production and expands farming,whereas the migrating farmers find it difficult to adapt to urban life and find jobs.

Since the People's Republic of China was founded,six fundamental water control projects,the Longyangxia,Liujiaxia,Haibowan,Wanjiazhai,Sanmenxia,and Xiaolangdi reservoirs,have been built on the Yellow River main stream,effectively controling flooding and sustaining the water supply for the lower Yellow River region.In 1999,the Xiaolangdi Reservoir started to store water in conjunction with tributary reservoirs such as Luhun,Guxian,and Hekoucun.These reservoirs effectively regulate water and sediment and enhance the dispatch and management capacity of sediment resources in the Yellow River Basin.

The existing runoff and sediment regulation system for the Yellow River provides the prerequisites for implementation of the proposed ecological development model for the lower Yellow River floodplain(Zhang et al.,2003,2008;Zhang,2005;Xu et al.,2005;Shang et al.,2016).Several water control projects are currently under planning or construction,including the Dongzhuang,Guxian,and Malianhe projects,and these projects are expected to enhance coordination between runoff and sediment in the lower Yellow River and to sustain the scale of current downstream channels and floodplains.All these projects will lay the foundation for the sustainable development and exploitation of the lower Yellow River floodplain(Zhang,2016).

4.Conclusions

Houses and farming infrastructure seriously hinder flood discharge on the lower Yellow River floodplain,and the main channel is increasingly elevated by sediment deposition.Therefore,local residents are now in danger from floods with return periods exceeding five years.The Shandong and Henan provincial governments have enacted several resettlement policies that encourage floodplain residents to migrate to urban areas.However,the migrating residents often return to the floodplains due to various reasons,including the increased production radius following mass departures and insufficient land and employment opportunities in urban areas.This seriously undermines the stability of social order.This study redesigned the model proposed by Zhang et al.(2017b)to enable flood control using grading criteria,regulate sediment deposition in partitioned zones,and achieve free exchange of channel runoff and sediment.The wide floodplain between the main channel and levees was divided into three distinct regions:tender,low,and high floodplains.Area-specific ecological construction models were developed for these regions.Furthermore,several key technical studies were carried out to promote the practical application of these models.It is anticipated that the improved model can be smoothly implemented to achieve two objectives:(1)in ten years,the tender,low,and high floodplains should be rationally delineated,the poverty of residents should be effectively alleviated,and a high-quality ecological economy should be developed;and(2)by 2035,an ecologically stable lower Yellow River corridor should be established.This will connect more than 30 large-and mediumsized cities,including Zhengzhou,Xinxiang,Kaifeng,Fuyang,and Jinan,in an economic belt via ecological and green development of the lower Yellow River reaches.

Declaration of competing interest

The authors declare no conflicts of interest.