Water system connectivity evaluation in plain urban river network based on AHP-entropy weight method:A case study of Langfang City
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Abstract:
Under water conservancy projects control such as gates and pumps,the complex river network in plain cities has a complicated water flow and a blocked water system,which affects its self-purification capacity.Instead,there are diverse factors that affect water system connectivity,makes more uncertain due to the regulation of dam gates.Therefore,the system pattern,structural connectivity,and hydraulic connectivity should be integrally considered to evaluate the connectivity of river networks in plain cities. Taking the optimal solution of water system connectivity as the target layer and water system pattern,structural connectivity,and hydraulic connectivity as the criterion layer,considering both static and dynamic factors,four indicators of water surface rate,river network density,river frequency,and river channel storage were selected to evaluate the morphological characteristics of urban water system more comprehensively.Water system ring degree,node connection rate,and water system connectivity were selected to evaluate structural connectivity.Based on structural connectivity,the influence of water flow capacity on the connectivity of the water system was fully considered,and three indicators of flow velocity,water flow potential,and water level are used for hydraulic connectivity evaluation.The above 10 evaluation indicators are used as the index layer to build the water system connectivity evaluation system.To comprehensively evaluate the water system connectivity of different schemes,a comprehensive evaluation method is used to assign weights to the 10 indicators,and then to comprehensively evaluate different water system connectivity schemes. Taking Langfang urban water system as an example,the water system connectivity evaluation system of the plain city river network was constructed to quantitatively evaluate the water system connectivity status of eight water system connectivity schemes.The results showed that there were differences in the three criterion levels of water system pattern,structural connectivity,and hydraulic connectivity for different working conditions.Combining the comprehensive evaluation scores and the grade cut-off values,the optimal water system connectivity solution can be obtained to improve the water system connectivity of the whole river network for Langfang city and could suggest that the river network should open the Shengli branch canal,Shengfeng branch canal,and Xiaoliuzhuang branch canal in the water transmission process,and dredge and desilt of the three branch canals with serious pollution according to the local actual situation. The system can evaluate the water system connectivity of the plain city river network in a more comprehensive way,and reflect the water system connectivity status of different water system connectivity schemes.The hydraulic connectivity index can reflect the mobility of water bodies in water systems.The calculation method of the hydraulic connectivity index in the hydraulic connectivity criterion layer was improved,so that it can be applied to the whole river network,making up for the limitation that the index was only applicable to the calculation of a single river in the past.To address the limitations,the evaluation of water system connectivity by index values can not take into account the influence of multiple factors,based on the water system connectivity evaluation system.The AHP-entropy integrated evaluation method was used to evaluate different water system connectivity schemes in terms of water system pattern,structural connectivity,and hydraulic connectivity to obtain the final evaluation scores of different water system connectivity schemes,to determine the optimal solution for water system connectivity and to make reasonable planning suggestions for the whole river network.The overall connectivity of the river network is optimal after connecting the three heavily polluted rivers with other rivers.
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