Introduction

After 1980s, China was involved in rapid urbanization. Consequently, almost all the cities have involved in an enlarging process. Urbanization has effectively promoted the urban development, but has also brought obvious problems simultaneously; for example, incomplete drainage system had brought more and more problems like storm flood and water pollution which does not just occur frequently in common cities but also in major cities. In 21 July, 2011, Beijing，the capital of China experienced a rainstorm flooding with rainfall of about 164mm that was recorded as the highest rainfall in past 61 years. The area of 14000 km2 in the city with 1.9 million people was affected leaving 77 dead in this rainstorm. In recent years, rainstorm has also brought disaster frequently to other major cities like Wuhan, Hangzhou and Nanchang. This situation requires us to rethink on the drainage situation of the city's enlarging process, especially to learn from history for solution. Therefore, it is a fact that China has an extensive history of drainage engineering, as many cities and resident areas formed and developed have successful drainage engineering. Thus, this paper aims to explore the logic and process of the drainage in ancient China and the lessons learnt from history in order to proffer solution to flooding risk in current urbanization.

The history of drainage in China was dated back 4000 years ago. The archeological evidences showed that the drainage facilities had been in existence in the early city of Henan province in 2300 BC. Therefore, it was also evidenced that the construction of drainage system began from the cities early design, and was developed along as the city grows. In the subsequent ages, the drainage system had been developed with the city's development which was incorporated with the urban water system construction. Furthermore, it has also achieved a traditional model of urban drainage before 1980s. Simultaneously, drainage in rural areas has played an important role in agricultural and community development，especially in the river basins and lake countries. The purpose of drainage from the ancient times was for flood control, river management, irrigation and urban waste water management.

In the early age of the cities in China, the major river basins were strongly considered to be the site of the city, especially the Yellow River basin and the Yangtse River basin. However, all the influential ancient cities in China were built near the major rivers, including the 8 most influential ancient capital cities such as Anyang, Changan, Luoyang, Kaifeng, Nanjing, Hangzhou and Beijing. For example, Anyang, the oldest capital city of China, was built across the riversides of the old Heng Shui（洹水）River. Chang'an, today's Xi'an City, was built near 8 rivers such as Wei（渭）River、Jin（泾）River、Lao（涝）River、Feng（沣）River, etc, and it was regarded as the capital city for 1700 years by thirteen dynasties. Kaifeng city is located at a place between the Yellow River and the Huai River. Also, Nanjing city is located at the south riverside of Yangtze River.

Being situated close to rivers, the city was convenient for water supply and irrigation, and this later made transportation convenient. Though a city built close to the river side is at a high risk of flood, the emperors and the governments in any of the dynasty has employed measures to counterattack flood disaster from its early construction to aged times. Accordingly, the history of drainage in China began with the city's early construction. Consequently, when a place has been chosen to be the site for the establishment of a city, drainage was first designed for the water system. Also, when planning a city as the capital, usually the city founder has to construct canals to bring water from the river nearby into the city for water supply. Similarly, the city founder has to construct other canals to direct used and waste water from the city into the lower basin of same river where the water supply canal linked. Therefore, this was the model designed as the system of water supply and drainage in the ancient city of China in general. Inside the city, water supply and drainage system are usually built for palaces preferentially, then for others resident areas. Accordingly, the model of water system in ancient city usually combines（1）Water supply system: It was a canal or river to introduce water into the city.（2）City's rivers: They are rivers or canals flowing inside the city as a water-cycle system with the functions of water supply and drainage.（3）Draining channel: The network of draining channel was built in palace and resident areas. However, some are underground channel and some are upper channel for draining waste water and rainwater into the city's rivers.（4）Ponds: It was built or naturally linked with the city's rivers and functions as a sluice and waterscape.（5）Fosse: A canal commonly built to surround the town with the functions of the defense and sluice, and the canal linked with the city's water system.（6）Draining river: It was the river that passed the city or canal to direct water from the city into the lower basin of the river with the function of draining water from the town. By this systemic design, water was introduced from the nearby river to supply the city, and after cycling in the city through the water system, it then flows into the lower basin of the rivers. Also, the drained waster and rainwater from the town also flows into the lower basin of the river as well. If rainstorm is coming, the city's rivers，ponds and round town-fosse will perform the function of sluice for the storm water to reduce the risk of flooding.（Fig.1）

Fig.1 The urban water system of ancient China in general