Riparian Zone Structural Characteristics And Water Environment Response Of Suzhou Creek Under The Background Of Different Drainage System

As the important mother river in Shanghai,Suzhou Creek has experienced serious water environment pollution issues.After three rounds of comprehensive environmental improvement projects,point source pollution from the riparian zone of Suzhou Creek has been basically controlled,the problem of black and odor in the river have been generally solved,and the water environment quality have been significantly improved.However,the pollution caused by overflow events is becoming increasingly prominent.The government could only build underground storage tanks with limited capacity in the limited riparian zone to cope with overflow events issue,which has become an extremely important factor affecting the stability of water quality in the urban central area of Shanghai.Correspondingly,the integrity of the spatial structure of Suzhou riparian zone is generally degraded.At the same time,the land use in the riparian buffer is characterized by highly impervious surface cover.A lack of applicable planning and management measures for non-point source pollution,such as the low impact development(LID)and best practice management(BMP),has aggravated pollution load from urban rainfall runoff.In addition,drainage system is one of the artificial intervention factors that have a profound impact on the hydrological cycle in the riparian zone in the process of urbanization.It also brings about regional differences in water environment.Therefore,this study applied Suzhou Creek as a typical case of Chinese urban rivers to explore the characteristics of the water quality response under the combined effect of different sewage system modes and riparian land use,aiming at solving the environmental issue of different regions caused by sewage system.The study is not only of great theoretical exploration significance,but also an urgent practical need for Shanghai and similar areas in China to further stabilize and improve the quality of water environment.This study systematically analyzed the spatial differences of Suzhou Creek riparian structure under different sewage system modes,based on various analysis methods and three cases,then proposed the classification of physical structure types of urban riparian based on an integrated drainage system mode and land use.The main research work and conclusions can be divided into the following aspects:1? This chapter systematically analyzes the physical structural characteristics and spatial variances of urban river riparian zone under different drainage mode zones,from a dual perspective of drainage system and land use,using Suzhou Creek as a case.This chapter attempts to discuss the classification of urban riparian structure types,and specifically analyzes the water environment issues which caused by different drainage systems,to provide relevant theoretical research reference for further discussing the water environmental response characteristics of Suzhou Creek under the combined impact of drainage system and riparian land use.Suzhou Creek is located in the lower reaches of the river basin,which characterized by intensively land use in the riparian zone,and the embankment is made of hardened material after artificial rebuilding.The classification of riparian zone type has important meaning on the urban river restoration planning,because it related to the selection of the expression of riparian function.However,no consensus has been reached on the classification method of urban river riparian zones.It is difficult to characterize the spatial differences of riparian zones caused by artificial intervention damage,because mainstream methods mostly refer to vegetation communities,soil properties and land use development on both sides of the river.After reviewing and analyzing the connotation of traditional riparian buffer width and ecological function,an riparian classification method was proposed based on the natural structure and a concern of anthropic disturbance,which provides a theoretical basis for the restoration and management practice of urban riparian zone.Based on different drainage system areas,this paper discusses the phyzical structural characteristics of riparian zone,which is a new perspective for Suzhou Creek with different intensity of waterfront space development and utilization.Because the regional difference of drainage system has reflect the changes of non-natural factors such as urban construction,and also had a fully consideration on the variation of natural interaction mode between riparian and water environment.This determines that under the current situation of high-intensity construction of urban river in the region,a study on the structure of the riparian zone can integrate the influence of the artificial intervention factors of the urbanization and the changes of the ecological function of the riparian zone.Based on the division of the "Three zones" of the traditional riparian classification in natural area,this paper discusses the physical structure and ecological characteristics of Suzhou Creek riparian zone.On the basis of a comprehensive consideration on intertidal zone structure,buffer width,vegetation community,land use,drainage mode and other factors,the edge positions of various parts of riparian structure(intertidal zone,revetment zone and nearshore zone)are defined,according to management baselines in standard specifications such as river water conservancy in China.The riparian zone along the Suzhou Creek is classified into three basic types: the "Four lines and three belts" which has a natural riparian structure,the "Three lines and two belts" which characterized by artificial greening,and the "Two lines and one belt" with missing of ecological space.The investigation results showed that the "Four lines and three belts" and the "Three lines and two belts" types are only distribute in the natural drainage area(NDA)of Suzhou Creek,which characterized by a relatively complete physical structure and vegetation community,but fragmented in the vertical direction.It is noteworthy that the "Two lines and one belt" type structure is distributed in both natural drainage area to urban drainage area(UDA),the integrity and ecological function of the riparian structural has already been damaged.This study found that the physical structure and ecological function of Suzhou Creek riparian zone tend to be simplified from NDA to UDA.If land use development and drainage system constructions are ignored in the river restoration plan,the riparian structure may possibly be degrade from "Four lines and three zones" to "Two lines and one zone" type.The result have a important meaning on the protection of ecological riparian condition.2? This chapter analyzes the variation of riparian land use and landscape pattern from NDA to UDA,and explored the relationship between water quality variation and land use on different widths of riparian buffer of Suzhou Creek by using redundancy analysis model.The main land use factors and the critical buffer width on which affecting the water quality are identified.Based on the results,this chapter puts forward suggestions on low-impact development and renovation of land use in the riparian zone,which could provide scientific references for the management and land use planning of Suzhou Creek restoration.The analysis result shows that the water quality differentiation of the main stream of Suzhou Creek and the tributaries in the NDA have significant relationships to the spatial variation of land use and landscape pattern in the riparian zone respectively,but the response relationships vary from different riparian buffer widths.On a relatively smaller buffer width(100 m-600 m),the proportion of construction land and the level of impervious rate in the mainstream riparian zone are negatively correlated with water quality,while the landscape patch density in the NDA is positively correlated with water quality.On a larger buffer width(800 m-1200 m),the density of agricultural landscape patches in the riparian zone is positively correlated with water quality,while the irregular shape and arrangement of industrial landscape patches in the NDA riparian zone is negatively correlated with water quality.The explanatory ability of the model shows that,with a decrease of the riparian buffer width,the impact from land use and landscape pattern to water quality variation has gradually increased.The strongest impact on the variation of water quality were detected on the 100 m width(the maximum cumulative explanatory ability reaches to 90.5%).The impact of land use and other factors on water quality variation has been significantly enhanced from NDA to UDA.(81.7% of the main stream explanatory ability and 41.1% of the NDA).In conclusion,1)for areas with different drainage systems in the mainstream,a high proportion of construction land and a high impervious rate in the riparian zone are the main factors leading to the deterioration of regional water quality.For NDA,the protection of river system with complex branch structures and wetland in the riparian zone,and enhancement of river connectivity can improve the quality of regional water environment.2)As the main stream passes through different drainage system areas,the impact from land use and landscape pattern in the riparian zone on water quality variation is stronger than that in the NDA.The urban drainage system increases the impact of land use and landscape pattern characteristics in the riparian zone on water quality variation and expands the influence range of the riparian zone may be a possible explanation.3)The impact of land use structure and landscape pattern on water quality in the riparian zone became stronger when distance to the edge of the river decreases.3.This chapter focused on the overflow events in UDA as prominent water quality issue of Suzhou Creek.Using a redundancy analysis and linear regression model based on a set of high frequency on-line monitoring data,the characteristics of water quality degradation response after the occurrence of overflow events of drainage facilities(pumping stations)along the riparian zone were analyzed.This paper attempts to reveal the dynamic response characteristics and influencing factors of the received water quality under typical overflow events in the UDA.Based on the evaluation method of urban water black and odor issue by the Ministry of Housing and Urban-Rural Development in China,the response mechanism of water quality deteriorate after overflow events is discussed,which can provide scientific insights for the control of overflow pollution of drainage facilities along the riparian area of UDA.The water variation trend during total 22 typical events were evaluated applying WQI as an indicator.It is found that the water quality response characteristics can be mainly described as "significant decreasing","slight decreasing" and "decreasing first and then increasing" after the overflow event.The RDA model is applied to analyze the impact factors of water quality variation after overflow events.Results showed that factors including total rainfall,total overflow flow,rainfall intensity and drought days in antecedent dry weather period can significantly affect the variation trend of water quality response after overflow within 46 hours(total explanatory ability > 50%).The relationship between water quality variables and overflow variables has a dynamic change after the occurrence,which also provide an explanation on the dynamic change trend of water quality " decreasing first and then increasing" after overflow events.Taking the overflow events of Kaixuan and Wuning flood pumping stations during 2014-2015 as a case,a linear regression model was formulated and combined with the black and odors water assessment method from the guidance by the Ministry of Housing and Construction,to explore the correlation between response time of a black odor condition of the water and overflow volume,overflow frequency.The results showed a goodness of fit of the linear model,and demonstrated the monthly duration of black odor condition is positively and significantly affected by monthly overflow volume and overflow frequency.Therefore it is suggested that the overflow volume and frequency should be taken into account when controlling the black and odor condition hours in UDA,but the overflow frequency should be given priority in control measures as low overflow volumes may also lead to serious water quality degradation.