Relationship between the surface water system structure and the flood peak in the Xiangxi river karst basin

The Xiangxi river basin is located in the mountainous area of western Hubei Province with an area of about 3，211 km2. The basin hosts a tectonic erosion and dissolution landform, with gentle elevation variations. The terrain is generally high in the north and low in the south. The Xiangxi river is a large tributary close to the Three Gorges Dam, with a length of 97 km and an average annual flow 40.6 m3·s-1. The Nanyang river, Gufu river and Gaolan river are the major tributaries of the Xiangxi river, covering an area of 678 km2, 1,183 km2 and 928 km2, respectively. The strata are well exposed in the basin, except for the absence of carboniferous and Cretaceous strata, from the metamorphic rocks of Shuiyuesi group of Archean to the loose rocks of Quaternary system. The study area is basically an exposed karst area, where carbonate rocks are widely distributed, which generally reflects the characteristics of the transition zone between north and south karst, and groundwater flows from north to south on the whole.The purpose of this work was to study the structural characteristics of the surface water system and the attenuation process of flood peak flow in the karst area. The drainage structure of typical sub-basins was analyzed by a statistical method, and the drainage attenuation equation was used to examine the typical dewatering process. The results show that topographic slope affects the development of the water system by affecting the erosion ability of slope runoff. The greater the slope is, the stronger the scouring force of surface runoff is, and the more developed the water system. The stratigraphic lithology is an important factor affecting the development of the water system. Karst development, water system density, river frequency and other water systems in the carbonate river basin are small in number, but the water system length is large. The attenuation rate of surface water flux is variable, and the dewatering coefficient decreases with time. There are several sub-dynamic state in the flow attenuation process of surface water. The water distribution ratio of each sub-dynamic state is taken as the characteristic value to describe the characteristics of flood peak, and the number ratio of the water system and length ratio of water system are taken as the characteristic values to describe the structure of the water system for correlation analysis. The results show that the ratio of water length in the karst area has a high correlation with the ratio of the water volume in the first and third sub-dynamic states, with a correlation coefficient as high as 0.84. The ratio of water length in the karst area can better characterize the structure of the water system.