Analysis of impacts of polders on flood processes in Qinhuai River Basin, China, using the HEC-RAS model

Flood control with polders is prevalent in East China. Their impact on flood processes is critically important for flood control, but has not been well documented. The Qinhuai River Basin was selected as the study area. A Hydrologic Engineering Center – River Analysis System (HEC-RAS) hydraulic model was developed to simulate and predict storm flood processes and the associated impact of polders. The study shows that the HEC-RAS model is capable of simulating the impact of polders on flood processes in the Qinhuai River Basin. The polders increased the water level outside of the polders. The polders in upstream watersheds have a greater impact on the water level than polders close to basin outlets when individually distributed. The maximum water level at Dongshan section shows an increasing trend for different sized flood with the increasing number of polders in the basin, and a linear increasing trend associated with urbanization. The smaller the flood scale is, the greater the maximum water level changes. This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY 4.0), which permits copying, adaptation and redistribution, provided the original work is properly cited (http://creativecommons.org/licenses/by/4.0/). doi: 10.2166/ws.2018.008 om https://iwaponline.com/ws/article-pdf/18/5/1852/251683/ws018051852.pdf 2020 Yuqin Gao (corresponding author) Yu Yuan Huaizhi Wang Liu Ye College of Water Conservancy and Hydropower Engineering, Hohai University, No. 1, Xi Kang Rd, Gu Lou District, Nanjing City, Jiangsu Province 210098, China E-mail: 178164576@qq.com Zhenxing Zhang Illinois State Water Survey, Prairie Research Institute, University of Illinois at Urbana-Champaign, Champaign, IL 61820, USA


INTRODUCTION
The hydrological effects of urbanization have been receiving increasing attention. The increasing impervious area due to the land use and land cover change caused by urbanization has been found to have a profound impact on urban hydrological process (Liu et al. ; Yerramilli ; Zhang & Song ). The impact of urbanization on the hydrological cycle in urban areas includes changes to rainfall, evaporation, and runoff (Song & Zhu ; Zhang ). found that hydrological processes would be greatly affected when the impervious percentage of a drainage reaches 10% or more (Hammer ). Klein () studied 23 small watersheds in Maryland and concluded that baseflow was negatively correlated with watershed imperviousness (Klein ). Ng & Marsalek () found that peak flows and flood volumes would increase by 20% when the impervious rate of a basin increases two times (Ng & Marsalek ). Jennings & Jarnagin () found that urban expansion would increase river runoff ( Jennings & Jarnagin ). Zhou et al. () analyzed the influence of urbanization on the hydrological processes in the Yangtze River Delta region, and pointed out that the surface runoff and baseflow respond strongly to urbanization (Zhou et al. ).
Flood control is critical to a city's long-term sustainable development. Flood-related hydrological parameters of an area change during the urbanization process. Flood control policies must adapt to these changes (Stevens ).
Polders are the areas enclosed by embankments in coordination with advantageous terrain, natural rivers, or artificial rivers (Gao & Mao ). In flood control practices, flood protection zones, also known as urban polders, are built with existing dikes. Runoff in the polders has no direct contact with the external rivers. Polders are connected with rivers through gates and pumping stations. and HEC-RAS model (Yerramilli ).
This study is mainly aimed to simulate flood processes, examine the impact of urban agglomeration polders, and explore the relationship between the flood control and flood level, using the HEC-RAS model.

Study area and data
The Qinhuai River Basin, with an area of 2631 km 2 , is located in the lower Yangtze River Basin (Figure 1). The

HEC-RAS model in Qinhuai River Basin
The HEC-RAS model is commonly used to simulate one- The influence of the Yangtze River tide is not taken into account and the water level is subject only to flood processes in the river. The basin outlet is set to be free outflow and the lower boundary conditions are set to be normal water depth (outlet river slope) to analyze the influence of polders on mainstream river water levels.
The upper boundary discharge is based on the HEC-HMS model, which was first devoloped without considering polders, and then with polders added. Since the proportion of urban land in the polder is bigger than that without polder, within the hydrological model considering polders, The direct progressive method is used to calculate the water surface curve section by section. The formula is: (1) where: where: ρ is the density of water, kg/m 3 ; u is the velocity, m/s; t is time, s; and x is distance, m.
The momentum equation is as: where: f is the mass force, kg·m/s 2 ; p is pressure, kg·m/s 2 ; v is the kinematic viscosity coefficient of water, m 2 /s.        Fourth, urbanization appears to have a linear impact on water level. Minor floods tend to be more sensitive to urbanization than major floods.