Study On Channel Leakage And Its Effects On Flood Routing Of Dagu River

With the development of computer technology, flood routing model for flood control security system has become an important component of the theory and powerful analysis tools. In the event of flooding, the right decisions on flood control coulde be made with proper flood routing calculations,and flood disasters could be reduced to a minimum. However, the present model of flood routing has little considering the effect of channel leakage. In the arid or semi-arid areas the rivers are mostly ephemeral rivers.Before the flood season, river-beds are also mostly dry and floods have the characteristics of rising and then landing sharply. There is a large amount of leakage in flood period. Therefore, it is very important for studying on channel leakage and flood routing incorporating its effects. Research results not only would have important practical significance for the estimated amount of groundwater recharge, rational use and protection of groundwater resources, but also have important application value for seasonal river flood forecasting and flood mitigation.In this paper, the leakage law of the Dagu River is studied with determination of unsaturated infiltration characteristics, indoor field infiltration tests and field infiltration experiments. The infiltration parameters are accessed in different sections of the channel. Then, based on the basic principles of fluid mechanics, flood mathematical model considering leakage is set up.And, the influencing factors on the evolution of flood are analysed through the numerical calculations. Finally, based on the MIKE SHE\MIKE11 model and incorporating the channel leakage effects, flood routing model system of the Dagu River is researched and developed. The reliability and validity of the model is verified. On the basis of a series of tests, theoretical analysis and research and development of flood routing model system, some new understanding and conclusions drawn from this paper can be summarized as follows:(1)The characteristics of the unsaturated seepage of different sand are experimentally studied, including the water characteristic curve and hydraulic conductivity. The moisture content curves of coarse and fine sand are closer to the basic overlap within the suction of 0-30cm water column.As the suction increases, the curves gradually separate,and the water holding capacity of coarse sand is larger than fine sand. In the desorption-absorption processes, hydraulic conductivity of coarse sand is greater than that of fine sand with the same moisture content; when water content less than 20%, the hydraulic conductivity is less than 10cm/d.(2) The results from indoor infiltration tests show that the initial moisture content or sand texture of sand has demonstrable affection on infiltration. Vertical or lateral infiltration rate increases as the initial moisture content decreases, the particle stem increases and the bulk density decreases. Initially, infiltration rate is great.Then, infiltration rate decreases rapidly. Infiltration process can be divided into non-stable infiltration stage and stable infiltration stage. The vertical infiltration rate of coarse sand is about 0.0022cm/s and 0.002cm/s when the corresponding initial moisture content is 0.44% and 9.30%. The vertical infiltration rate of fine sand is about 0.002cm/s and 0.0018cm/s when the corresponding initial moisture content is 2.50% and 9.65%. The lateral infiltration rate of coarse sand is about 0.002cm/s, 0.0011cm/s and 0.0008cm/s when the initial moisture content is 2.6%,7.35% and 13.98%, respectively. The lateral infiltration rate of fine sand is about 0.0026cm/s, 0.002cm/s and 0.0016cm/s when the initial moisture content of fine sand is 3.20%, 7.61% and 16.20%, respectively.(3) The results from field infiltration tests show that hydraulic properties of vadose zone affect the infiltration capacity of the riverbed; ephemeral channel infiltration process can be divided into non-stable infiltration stage and stable infiltration stage; the more coarse the vadose zone sand, the more permeable as possible and shorter duration of non-stable infiltration stage. Infiltration rate is less when the particles fine, the permeability coefficient smaller. Infiltration rate is higher when particles rough, loose, the permeability coefficient greater. The duration of transient infiltration was about 1.5～2.0 hours, and the ratio of total leakage of the channel gradually reduced over time to extend. Non-steady infiltration ratio of total leakage is only 2.75%～4.36% when three days later. (4) Based on the basic principles of fluid mechanics, flood routing equations containing leakage terms are derived and established,being solved with Preissmann a four-point eccentric implicit scheme. The numerical results show that the channel leakage influences on the flood routing, expressed as that flow rate decreases, the water level lower, the time of peak value delays. The main affecting factor is infiltration rate f. As the infiltration rate increases, the effect is more obvious. The channel slope has little influence on the calculation results.(5) Based on MIKE SHE\MIKE11 model, the Dagu River flood routing model system is researched and developed ,being carried out for flood routing simulation incorporating leakage effects. Software which demonstrates results is user-friendly, simple in operation and convenient for scientific research and management departments.Model validation shows that the calculated results is in good agreement with the measured data, model parameters selection are about right.