Research on Evaluation and Countermeasures of Waterlogging Prevention and Cure in Urban Built-up Area—Taking Nankai District in Tianjin as an Example
赵越 张斌* 王新亮
ZHAO Yue, ZHANG Bin*, WANG Xinliang
赵越 / 女 / 1982年生 / 天津大学建筑学院城市规划与设计在读博士生 / 主要研究方向：应对洪涝灾害的城市空间规划
ZHAO Yue, female, was born in 1982,Ph.D. students majoring in urban planning and designing, School of Architecture in Tianjin University. Major research direction: urban spatial planning for flood disasters.
张斌* / 男 / 1981年生 / 天津大学城市规划设计研究院中级工程师/主要研究方向：景观规划与设计
通信作者邮箱（Corresponding author Email）：firstname.lastname@example.org
ZHANG Bin, male, was born in 1981. He is the intermediate engineer of Urban Planning and Design Institute in Tianjin University. Major research direction: landscape planning and design.
王新亮 / 男 / 1983年生 / 天津市城市规划设计研究院高级规划师/主要研究方向：市政工程规划
WANG Xinliang, male, was born in 1983. He is the senior unban planner of Tianjin Urban Planning and Design Institute. Major research direction: municipal engineering planning.
Abstract: For better solving waterlogging issue in urban built-up area renewal, the paper takes Nankai District of Tianjin as the example, considering the vulnerability and exposure, indicator system evaluation as the research method to assess waterlogging risks. And considering current situation and the difficulty level of the built-up area renewal, the paper incorporates urban construction executable evaluation factors to divide urban waterlogging prevention and control sections. While building up an entire green waterlogging prevention system, according to section conditions, the paper proposes feasible planning measures, and offers reference to waterlogging prevention and control work in urban built-up area.
Key words: urban built-up area; waterlogging prevention and cure evaluation; the guidance zoning
天津市近年因暴雨频发造成内涝， 城市安全和人民生活受到了极大的影响。2016年7月20日，天津市遭遇50年一遇的特大暴雨，据天津市排管处数据及暴雨期间各区上报情况，中心城区共有69处积水片（积水深度20 cm以上），其中南开区积水片区共14个，占总数的20.3 %，是中心城区6个行政区中内涝片区较多的行政区（表1）。
南开区的高程值大多分布在1 m～5 m（大沽高程，下同）区间内（图2）。其中，高程值小于1 m的地块多为河道水系，内涝风险小；高程值大于4 m以上的地块主要分布在中部（鞍山西道至天拖北道以南）、南运河河堤两侧和鼓楼街区，内涝风险较小；而高程值介于2 m～3 m之间的大面积区域主要分布在南开区北部（鞍山西道至天拖北道以北），其中长江道与咸阳路交口周边地区地势最为低洼，内涝风险大。
通过对区内一、二级河道两侧堤顶高程分析，发现北部南运河堤顶高程为3.5 m～5.6 m，津河堤顶高程为2.6 m～4.4 m，均高于两侧用地高程，高差在0.5 m～2 m。其中，津河北段堤顶高程仅为2.6 m～2.7 m，暴雨时期河道水位升高，极易发生河道水漫堤现象（图3）。加之南开区北部地区地势低洼，河道两侧地块的内涝风险相当高。
管渠覆盖率（m/m2）是区域管渠长度（m）与区域面积（m2）的比值。因南开区目前管道标准各不相同，以1 km2收水面积为例，南开区三年一遇设计标准下排水流量约为6 m3/s，一年一遇设计标准下排水流量约为4 m3/s，以二者比值1.5为修正系数，三年一遇排水区域乘以该系数进行修正。计算得到管渠覆盖率在0.001 m/m2～0.004 m/m2区间范围内，其值越低，内涝风险越高。
以上六项风险评估因子具有不同的计量单位和数量级，相互之间不具有可比性，所以要采用百分制评分法对各因子进行无量纲化处理，得到各因子的评分Si ，并通过专家打分法确定各风险评估因子的权重Wi （表4）。
Figure 1 Planning Map of Drainage System In Nankai District
Figure 2 Analysis Map of Top Height In Nankai District
Figure 3 Analysis Map of Top Height of Riverbanks In Nankai District
式中：Rj ——地块j 的内涝风险评估结果；
Si ——风险评估因子Bi 的评分值；
Wi ——风险评估因子Bi 的权重值；
Pj ——地块j 近期暴雨积水赋值。
Figure 4 Population Distribution in Nankai District
Figure 5 Distribution of Importance of Urban Function In Nankai District
Figure 6 Distrbution of Water Area Caused by Recent Rainstorms In Nankai District
构建城市、社区、邻里三个尺度下的线性绿地空间作为城市海绵体系的廊道，连接绿地斑块，并结合城市慢行系统形成绿道体系（图13）。一级绿道规划宽度120 m～150 m，由现状市级环城绿道、南运河绿道和规划津河绿道构成，形成绿道主要骨架；二级绿道规划宽度60 m～90 m，由在建天拖绿道和规划西营门绿道、水上西路绿道构成；三级绿道规划宽度20 m～30 m，由邻里范围内带状绿地构成。上述绿道体系不仅可以创造市民休闲游憩的活力场所，同时结合雨水花园、下凹绿地、植草沟等具体技术措施可削减雨水径流，减少城市内涝，实现绿色基础设施的作用。
Figure 7 Assessment Image of Waterlogging Risk In Nankai District
Figure 8 Distribution of Renewable Plots
Figure 9 Distributions of Recent Reconstructed Roads
In recent years, frequent waterlogging strongly impacts the development of cities in our country, causing "ocean in the city", a challenge many big cities face . Because built-up area is regions where environment and live qualities are in urgent need of improvement, it's necessary to put city waterlogging risk prevention and cure evaluation as a precondition of update planning. Meanwhile, given the impact of various current factors and the physical truth in built-up area, the waterlogging risk evaluation result should incorporate with the complexity, feasibility and sequence of practical renewal. And operable planning strategy should also be proposed based on that.
1 Selection and Correction of Evaluation Methods
There are three main methods of waterlogging evaluation in domestic and abroad. The first method is based on the mathematical statics of historical disasters. This method is simple on calculation but obtaining the enormous observation data from long history is not easy. Thus, the method has great limitations in practical applications. The second method is based on hydrological and hydraulic model and simulation method. It has great visuality but the data amount is huge and the model is complex. The third is evaluation method of indicator system based on the feathers of disasters. Comparing with previous two methods, it has advantages of simple model, small data requirement and strong versatility . We take universality and construction guidance as main principle, considering the construction system from vulnerability and exposure, and combining with the urban construction executable evaluation factors, to strengthen the guidance effect of evaluation result to renewal implement.
2 Scope of the Research Example
Located in the southwest of downtown Tianjin, Nankai district has a population of 1136 thousand until the year of 2015, with a total area of 40.64 square kilometers. It's a higher civilized region among central urban area with an urban construction average plot ratio of 1.28.Nankai district was an open sinkage area lying in the south of old town of Tianjin, and there are still large scale of low-lying land there.
3 The Situation and Reason of Waterlogging In Nankai District
3.1 The Situation of Waterlogging In Nankai District
Waterlogging resulting from rainstorms in Tianjin brought extreme impact to urban security and citizens in recent years. On 20th July 2016, Tianjin suffered from the worst torrential rain in 50 years. According to the data from Tianjin Drainage Office and report from all districts during the raining, there were totally 69 pieces of water area in downtown (20 cm or deeper), including 14 pieces in Nankai District, accounting for 20.3% of the total. It's one of administration districts with more water pieces in six districts of downtown (table 1).
3.2 The Reason of Waterlogging In Nankai District
Severe waterlogging in Nankai District is a comprehensive result from multiple factors such as specific climate, hydrology, topography, drainage system and urbanization.
(1) Climate Factor: Climate in Tianjin is warm. Precipitation amount from June to August accounts for 80% of whole year, which tends to form summer flood; and global-warming has increasing the frequency and intensity of extreme rainstorms. (2) Hydrology Factor: Tianjin downtown is located at the end of nice rivers. Due to the tidal topping, the water in first-class rivers cannot smoothly access to the ocean and thus water level is raised. As a result, water in coastal drainage gate and second-class rivers cannot run into the first-class rivers, further exacerbating the risk of urban waterlogging.
(3) Topography Factor:Most area in Nankai District are at low elevation, in which the north part is a large bottomland, where tends to form water areas after rainstorms; plus the low river embankment top, it's easy to cause river overtopping situation.
(4) Drainage Facility Factors: Most built-up area in Nankai District built earlier with old pipe nets were designed for worst rainstorms in one year. And there are still some confluence areas which can hardly meet the drainage demand of downtown. Now the per capita possession of drainage mains pipe length is 0.07 m, which is only 1/60-1/80 of developed countries . Through the investigation of each water area in the district, we found the diameters of main rainwater pipes in roads such as Xianyang Rd are too small; the second rivers like Jinhe River and Weihe River have problems such as small cross section, scare lift ability of pump station lack of pump station (figure 1).
(5) Urbanization Factors: Rapid urbanization exacerbates the urban heat island effect and improves the integrated runoff coefficient ; urban expansion leads to a reduction in natural waters, and weakens the drainage ability of the surface.
Figure 10 Distributions of Key Development Plots
Figure 11 Executable Evaluation Image of Urban Construction In Nankai District
Figure 12 Guidance Zoning Image of Waterlogging Prevention and Cure Construction in Nankai District
Figure 13 Planning Image of GreenwaySystem In Nankai District
Figure 14 Planning Image of Green Plagues In Nankai District
4 Risk Assessment of Water logging in Nankai District
Facing the severe situation of water logging prevention and cure in Nankai District, for the waterlogging reasons, a waterlogging risk assessment system based on the data of topography, network of rivers, drainage and location is created. A guidance zoning of waterlogging prevention and cure is proposed combining with executable factors such as current construction and future planning direction in Nankai District. And prevention and cure strategy is proposed from the aspect of planning formulation and policy guidance.
4.1 Waterlogging Risk Assessment in Nankai District
Under the same precipitation intensity, the waterlogging risk in Nankai District mainly depends on the difference on vulnerability and exposure of affected objects. In the research area, vulnerability is reflected as terrain elevation, risk of river levee, municipal pipe coverage and current pump stations; while exposure is reflected as factors like population density and importance of urban function. The six risk assessment factors and their quantization parameters based on the universality and versatility of the data are determined as below in table 2.
(1) Terrain Elevation
The elevation values of Nankai District are mostly distributed at the range of 1-5 m (estimated value, the same below) (figure 2). In the map, blocks below 1m are river water system who has less risk of waterlogging; areas above 4 m are mainly distributed in the middle part (south of Anshanxidao - Tiantuobeidao), banks of Nanyunhe Canal and Gulou district with low waterlogging risk; and areas between 2-3 m are on the north of Nankai District (north of Anshanxidao - Tiantuobeidao) including region around the cross of Changjiangdao Street and Xianyanglu Road which has most waterlogging risk.
(2) River Levee Risk
By analyzing the bank elevation value of first and second class rivers, we found on the north part, top height of Nanyunhe Canal bank is 3.5-5.6 m; top height of Jinhe River is 2.6-4.4 m, both of which are 0.5-2 m higher than nearby area. Elevation of Jinhe River north section is only 2.6-2.7 m, and water level would rise in rainstorms tending to cause flooding over banks (figure 3). In addition to the low terrain in the north of Nankai District, there is extreme high risk of waterlogging on bank zone.
(3) Pipeline Coverage
Pipeline coverage ratio (m/m2) is the ration of the length (m) of the zone pipe to the area of the zone (m2). The pipeline standards are various in Nankai District. Take area of receiving water in 1 square meter as an example, drainage flow under the design for worst in 3 years is about 6 m3/s, while for worst in 1 year is about 4 m3/s, take 1.5, the ration between them as correction factor, correct drainage area for worst in 3 years by timing the factor. The result of pipeline coverage is between 0.001-0.004 m/m2. The lower the result, the higher the waterlogging risk.
(4) Current Pump Stations
Current pump stations include rainwater pump stations and river pump stations. The less the pump stations, the higher the waterlogging risk, vice versa. According statistics data, there are 0-3 pump stations in Nankai District.
(5) Population Density
The impact of population on the waterlogging risk is mainly reflected in safety of residence, travel convenience and efficiency. Densely populated areas in the district is mainly concentrated in the middle and north region and along eastern side of Hongqidonglu Road,a main road. Population density distribution could be acquired according to population date in figure 4 (table 3).
(6) Importance of urban function
The economic and social losses caused by waterlogging are positively related to the importance of urban function. According to the relevancy between different urban function blocks and the product and life as well as visitors flow, the importance is divided into level 0-4, with increased importance. Large culture facilities, medical facilities, sport facilities, business facilities, tourist attractions are in the highest level (figure 5). See details in table 3.
The six factors above are not comparable with different unit of measurement and magnitude. Thus, the factors should be dimensionless processed using percentage rating, to obtain score Si, and then determine the weight Wi of each factors through score by experts (table 4). rea caused by recent rainstorms (figure 6), based on all risk assessment factors.
A plot with water area is 100, while without water area is 0. R, assessment value of waterlogging risk in Nankai District, could be obtained according to formula (1), and they are responding to different levels (table 7). Assessment Results of waterlogging risk in Nankai District see figure 10 bellow.
4.2 Guidance Zoning for Waterlogging Prevention and Cure Construction In Nankai District
Executable assessment of waterlogging prevention and cure in Nankai District is mainly based on executable factors such as current land stocks, recent construction plan and future planning direction, including practicable factors like renewable plots, recent reconstructed roads, and key development plots (figure 8, 9, 10).
(1) Renewable Plots
Renewable plots have features of small construction amount, low renewal cost and high waterlogging prevention and cure maneuverability. This kind of plots is distributed around Xiyingmen area and along Hongqilu Road, and most of them are industry zones and Science Parks which already moved away or waiting for reconstruction and wasteland to be renewal. The design for their future construction is rational, and has addressed the drainage problem in advance.
(2) Recent Reconstructed Roads
Drainage pipe nets could be updated combining with road reconstructions to improve flood storage capacity. Recently, plots like Tiantuo are been considered to be reconstructed, and roads like Xianyanglu to be improved, in order to change the combined system to separate system and fit the high-strength and highdensity construction environment in downtown.
(3) Key Development Plots
The key development plots have higher requirement for waterlogging prevention and cure standards, and will probably implement in recent future. These plots concentrate around Xiyingmen and Nanjing University. They are used to be business area, industry zone or universities with all types of operation like business, retail and residence. As a key point of future urban development in Nankai district, its waterlogging problem has to be settled urgently.
By combining waterlogging risk assessment and executable factors, we can obtain waterlogging prevention and cure practicable assessment (figure11).the dark grey area in the map has highest risk of waterlogging, the red is area for the future construction and prevention of waterlogging. And where the two overlapped is area in urgency and could be constructed for improvement. Further divide Nankai District according to waterlogging prevention and cure practicability to create guidance zoning map. The district can be divided into advance zone, key zone and guide zone (figure 12). Advance zone has the highest waterlogging risk, the most prevention and cure necessary and most renewability including Xiyingmen, Tiantuo, Science Square; key zone has high waterlogging risk and urgent necessary of prevention and cure, and has certain constructability including south of Xianyang Road, Xihu Road and Wanxing Street area; besides are guidance zone.
5 Waterlogging Prevention and Cure Strategy in Nankai District
Start from the holistic perspective to improve greenways and plaques on each level and construct a whole green waterlogging prevention and cure system in the district. Meanwhile, divide the guidance zone and consider the specific strategy of "grey-green" combining current renewal practicability.
5.1 Construct Green Waterlogging Prevention and Cure System in Nankai District
Urban green space can not only create a city view and provide a place to relax but also could provide function of water storage and flood reduction in waterlogging disaster. As a result, we should properly take advantage of current green space in Nankai District; improve the green space system from aspect of urban green corridor and plaques green space, to construct urban sponge system for rain water storage and usage .
5.1.1 Urban Green Way System
Establish linear green space under city, community and neighborhood as corridor of urban sponge system, to connect green space plaques to form greenway system by combining urban low speed system (figure13). Planned width of first level greenway is 120-150m. It consists of current city level ring road greenway and planned Jinhe Road, forming the main greenway frame; planned width of second level greenway is 60-90 m, it consists Tiantuo greenway in-constructing, planned Xiyingmen greenway and Shuishangxilu Road green way; width of third level greenway is 20-30 m, and it consists banded greenway in neighborhood.
Above greenways can not only create activity places for citizen to relax and visit, when combining with specific technical measures like rainwater garden, subsided green space and grass gutter, it can also reduce rainwater cutoff, remit waterlogging and provide the function of green facilities.
5.1.2 Urban Plaques System
Green space plaques are effective in enriching urban green space and controlling storm runoff. Renewal current plaques and increase amount of plaques on each level in Nankai District to reduce waterlogging (figure 14). First level plaques are large scale urban green space, which are mainly current urban parks like waterpark, Changhong Park, Nancuipin Park, Lvshui Park. Improve elevation of drainage storage area in the park, to make sure the holistic elevation is below watershed partition nearby, and rainwater can converge. The second level plagues are small scale green space like community public green space and street garden, which can be flexibly set in renewal of built-up area, and mainly increased in the plan. The third plagues can be combined with neighbor public area to plan subsided green space, rainwater garden and green roofs to reduce internal cutoff. It can improve drainage ability without large scale renovating existing pipeline nets.
5.2 Construction Guidance of Waterlogging Prevention and Cure in Nankai District
5.2.1 Strategy for Advance Zone
Advance zones have possibility of new construction and expansion. At the beginning of construction, these zones are designed with high standard municipal pipeline nets, and large scale sponge systems like large park green space, subside squares are arranged as a whole in the zone. Low-impact development idea is used as much as possible in the engineering and non-engineering measure, in order to reduce emission from source and construct well-built prevention and cure system of waterlogging . New built and expansion plots circumvents the previous construction and deconstruction investment of drainage facilities, thus, have no need to raise funds for waterlogging prevention and cure, and the cost is relative low. Consider from the aspect of regional co-ordination, new built and expansion plots in advance zone should take partial responsibility of the rainstorm cutoff around where have drainage difficult to lower down the difficulty of waterlogging prevention and cure as well as the total cost.
5.2.2 Strategy for Key Zone
Engineering strategy of waterlogging prevention and cure in key zones should be combined with recent reconstruction plan, and flexible use the strategy of "greygreen".
Incorporate with low-impact development idea in proper place, to realize "slow drain and release" as well as "disperse from source". When make strategy of "grey-green" in each key zone, we should respect truth, adjust measures to local conditions and seize the principal problem and contradiction: in plots with well-built grey facilities, construction of green ecological infrastructure should be emphasized; in plots with poor grey infrastructure, "grey-green" should be considered, making up the debt while improve construction level of green ecological facilities.
5.2.3 Strategy for Guidance Zone
Most guidance zones are built-up zones with low risk of waterlogging and less urgency of prevention and cure. The emphasis of these zones is "prevention", not "cure". Thus, partial renewal measures should be taken in guidance zones, together with parks along river or green space to construct dispersive punctate multi-function water storage facilities, such as tanks, rain water sinks, green roof and penetrative pavements. Use these methods to strengthen the ability of absorbing rainwater, reduce pressure on drainage facilities, increase water resources quantity in urban areas, reduce downstream drainage pressure, as well as reduce urban runoff pollution and reduce adverse effects on rivers and lakes.
For the renewal of urban built-up areas, waterlogging assessment with urban practicable assessment takes into the urban disaster prevention need and urban development situation. Comparing to traditional waterlogging risk assessment, it can provide construction guidance zoning to urban construction management, and is convenient to make steady and orderly waterlogging prevention and cure construction plan as well as practical prevention and cure measures, to improve the city's ability to resist waterlogging disasters.
 人民网. 中青报：城市内涝 到底是哪里出了问题[OL]．
http://opinion.people.com.cn/n1/2016/0615/c1003-28445800.html,2016-06-15 People's net. China Youth Daily: What's wrong with urban waterlogging?[OL]．
 张冬冬，严登华，王义成等. 城市内涝灾害风险评估及综合应对研究进展[J]．灾害学，2014，29（1）：144-149
ZHANG Dongdong,YAN Denghua,WANG Chengyi,etc. Research Progress on Risk Assessment and Integrated Strategies for Urban Pluvial Flooding[J]． Journal of Catastrophology，2014，29（1）：144-149
 刘彦涛. 基于海绵城市理念的城市排水防涝综合规划探讨——以天津新八大里地区为例[J]．城市，2016（4）：54-59
LIU Yantao. Discussion on Comprehensive Planning of Urban Drainage and Waterlogging Prevention Based on the Concept of Sponge City: A Case Study of New Badali Area in Tianjin[J]．City，2016（4）：54-59
 陈筱云. 北京7.21 和深圳6.13 暴雨内涝成因对比与分析[J]．水利发展研究，2013，13（1）：39-43
CHEN Xiaoyun. Contrast and Analysis of Causes of Rainstorm Waterlogging in Beijing 7.21 and Shenzhen 6.13[J]．Water Resources Development Research2013，13（1）：39-43
HU Nan, LI Xiong, GE Xiaoyu. Change with Water-The Rational Cognition of Sponge City System from the Perspective of Urban Green Space System[J]．Chinese Landscape Architecture，2015（6）：21-25
CAI Kaizhen. Strategizing Urban Design to Mitigate the DisastrousDamage of Urban Stormwater and Waterlog Based on the Adjustment of Stormwater Runoff on the Block Level[J]．Architectural Journal，2015（10）：73-78