The Urban Water System Design

Although, the ancient cities had a complex design to adapt to the local water environment especially the rivers, but the tracks of regular pattern of urban water system can still be followed. Usually, the model of water system in ancient Chinese city was usually combined with（1）Water supply river,（2）Urban rivers,（3）Internal draining system,（4）Ponds and lakes,（5）City moats（fosse）, and（6）Draining river（Figure 1）. However, these parts formed an urban water system in a city. Usually, each part performs its function, but they are connected to each other to perform the integrated functions in a city.

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

The water supply system

The water supply system is a canal or river that directs water into the city for water supply. According to the regulation of city site, a major city is usually located close to the river. However, if the river was not suitable to be used for water supply directly due to unexpected food risk by the river drop, the city founders have to build a canal to lead water into the city. But the fact is that，many cities were built very close or crossed the river to make the river as the urban water supply source. For instance, the East Capital of North Song Dynasty（960—1127 A. D.）, today's Kaifeng City, was built near the Yellow River. Hence, there were four rivers: Bian river（汴河）、Jingshui river（金水河）、Cai river（蔡河）、and Wuzhang river（五丈河）which crossed flow the city, and performs the functions of water supply and transportation. Also，many cities were built close to the Grand Canal Jinghan. The canal and its branch rivers usually cross the urban areas of the city, and became the water supply rivers directly due to lower flood risk. Also, if the flood risk was not high, the city built close to the river in the urban area would always be in a favourable situation.

The urban river

The urban river is the rivers or canals that flow inside the city as a water cycling system. During the ancient ages, the urban rivers were very important to perform the functions of water use and drainage. Sometimes, they include the natural rivers inside the city when the city was built. Other times, canals were built as the urban river. For example, in Shuzhou city, there are many rivers inside the city. Consequently, there are 7 major rivers flow in the city, but at same time, many other smaller rivers flow inside the city. For example, in every one street in some district, one river flows through. In the Han Chang' an city, not just the natural rivers in the city were present, but some canals were also built; for example, a very famous canal“The Open Ditch”was built to introduce water into the palaces. Urban river performs the functions of water supply, drainage and transportation, waterscape, etc. Furthermore, the residents especially, use water from the rivers（Chinese usually do not drink from river，but from wells especially in major cities）according to the Chinese tradition. Thus, urban rivers were convenient for residents to get daily water，wash foods and clothes, etc. Urban river is also a function of the urban drainage, and most rainwater and wastewater were drained by the urban rivers. In another way, urban river also plays an important role in transportation, especially in many cities in Yangtze River basin. Along with the Grand Canal Jinghan, river transportation is not just busy inside the city, but is also important for the trade outside, for example, the cities of Jaingsu province like Suzhou, Yangzhou, Huaiyin, etc.

Ponds and lakes

Ponds and lakes are usually natural or are built in a city. Thus, it is very popular to find ponds and lakes inside the old cities. Ponds and lakes are very important part of the city's natural water system, but many of them were built for multi-purpose use e. g. for storage and sluice of water, and also as the city waterscapes. For example, in the ancient Chang' an city, 10 big ponds were built for rainwater sluice and water storage in summer, but water supply in winter. At same time, many cities also included the natural lakes inside the city. The ponds and lakes were usually linked with the urban rivers during the dry season, and were used for water storage. Also, this ponds and lakes were im portant for sluice of rainstorm water during summer. At same time, the waterscape made the city more beautiful.

The draining system

The drainage system for rainwater and wastewater were strongly considered during the construction of a city. Usually, it was designed and built with a city's original construction plan. The archeological discovery showed that early sewer in China was dated back to 4000 years ago（Zheng, 2013）. In major cities, the network of draining channel was built in palace and residential areas. However，some were underground channel, while some were surface channel for draining waste water and rainwater into the urban rivers, and later, into the drainage rivers. The draining system usually constitutes of four parts. They include: 1）the small sewers from the houses; 2）the street sewers and channels receives water from the house sewers. In many cities, there were open ditches or underground sewer along the street, and is a very popular way of building sewer along the streets, which is then covered with flagstones;3）the urban rivers receives water from the street sewers;4）the draining river to drain water to the outside the city. In addition, an urban draining system also includes other parts of the water system such as ponds, lakes, and moat.

Moat（Fosse）

The ancient city was usually built with the town walls. Also, a canal was built outside the walls which surrounded the town. Basically, the moats were built for the urban defense, but it was also linked with the city's water system for drainage. When the city meets rainstorm, it will perform the function of sluice from the town to reduce the risk of flood. Accordingly, the function of moat was often used for drainage purpose than its original purpose, which is for defense.

Draining river

Draining river is the river or canal that passed the city which leads water into the lower basin of the river or to other places. However, it is important that all cities should have or build a draining river for urban drainage. If a river was included inside the city, the rainwater and wastewater will be drained out of the city by it. Also, if the city does not have a river, a draining canal has to be built to drain water from the city.

Through the above systemic design and construction of a water system, water was introduced from the nearby river to supply the city. Water cycling in the urban water system includes the urban rivers, channels, ponds and moats，which then flows into the lower basin of the rivers or the farmlands outside the city. If rainstorm comes, the urban rivers, ponds and towns surrounding moats would serve as sluice for the storm water to reduce the risk of flooding. Therefore, if a city was built with such a logical water system, a water cycling system from the nearby river then returns the water to the river again. This puts the city in a superposition with its natural environment especially the water sources. In such cases, not just the citizen can use and enjoy the water conveniently, but would also make the city flourish. Consequently, some cities even faced high flood risk, but the towns were still built close to the river or include the river as an urban river. For example, the Kaifeng City in Song Dynasty was built near the Yellow River. Also, the Bian river（汴河）inside the town, before the city was destroyed by flood was from the Yellow River. The Bian riversides had become a very flourishing area of the city. However, Figure 2 is a most famous painting which shows the flourishing view of the riversides of Kaifeng city at that time.