Impact of Summer Storms on Rainwater Collecting Lakes in Gansu, China

Abstract: The rapid population growth in China during the last decades has put an enormous pressure on cultivation production and water supply. Many regions in China are therefore facing water scarcity and shortage of arable land. One of such regions is Gansu and it is considered as the driest and poorest province in China. This arid and semi-arid region is signifying a very uneven annual precipitation pattern and the majority of the rainfalls occur during summer. Rainwater harvesting is therefore necessary in order to make maximum utilization of the rainfall. Rainwater harvesting (RWH) is a technology used for collecting and storing stormwater for domestic use, irrigation and flood control. The local government of Qingyang in Gansu has sponsored a pilot RWH project and it involves the man-made lake Tian Hu, which consists of three reservoirs: the Sun, Moon and Star. The man-made lake serves as storage where stormwater is harvested from a highway and a square. From heavy rainstorm events in the past, the storage of the Sun reservoir of Tian Hu was filled to its maximum during less than one hour. With experience of this a great volume of the harvested rainwater is wasted during these heavy rainfalls due to an insufficient storage capacity. In this study the main focus is put on calculating the quantity of this flood water generated from heavy rainfall events. The focus is also put on identifying the response time and location of floods in the existing conveyance system of Tian Hu. The numerical computer model SWMM (Storm Water Management Model) has been chosen to analyse the impact of different rainfall events on the conveyance system and the reservoirs of Tian Hu. In order to represent the rainfall pattern of Qingyang, simulations of daily and 3-hours rainfall pattern is applied in SWMM. Two major catchments areas are created in SWMM; the Century Avenue Catchment and the Tian Hu Catchment. The establishment and properties of the catchment areas are determined with different field measurements; infiltration, soil moisture content and field capacity. The field measurements show expected soil types around Tian Hu which are silt and clay. The measured infiltration rate show similar trend as typical infiltration rates of loess soil. It is necessary to implement more sampling tests and use modern instruments in order to obtain more accurate results for future research. The simulations of different rain events in SWMM show different impacts on the two catchments of Century Avenue and Tian Hu. The simulations with 3-hours rainfall confirm that the maximum rain intensity the Century Avenue Catchment can handle is below 61 mm/h and the maximum generated runoff is 20 cubic meters per second. The flow generated from the Century Avenue Catchment carries on down to Tian Hu and provides as the major input to the artificial lake. Simulations of daily rainfall pattern resulted in more floods in the Sun reservoir than the other two (Moon and Star). The floods in the Sun reservoir usually appear during June to September. The excess water simulated and gathered in an imaginary storage proves a volume which is five times larger than the actual lake. This strengthens the evidence that the existing construction of Tian Hu is under dimensioned. The impact of overland flow generated from the surrounding subcatchments is significantly contributing to 20-40 % of the reservoir capacity, thus overland runoff should not be neglected and be included when planning future artificial lakes in Qingyang. Furthermore, the annual evaporation loss is estimated to 1000 mm or 44 000 m3 in Tian Hu. Finally when simulating with extreme rain scenarios the Tian Hu is reaching maximum water levels after 6 hours.