A MODERN ANALOG FOR RIVER-DELTA SYSTEM STRONGLY AFFECTED BY MONSOON PRECIPITATION IN SEMI-ARID AREA FROM DAIHAI LAKE, NORTH CHINA: IMPLICATIONS FOR MORPHODYNAMICS OF FROUDE SUPERCRITICAL FLOW

This paper focuses on the effects of strongly seasonal and yearly variable precipitation patterns in a semi-arid area. Specifically, it explores the effects on a modern river-delta system, Daihai Lake, north China, what concerns their dynamics, and the resultant sedimentary facies and architectural properties.

Daihai Lake is a hydrological closed basin with a maximum length of 18 km and a maximum width of 10km encompassing an area of 150km².It is located in Inner Mongolia, northern China which a semi-arid zone with an average yearly precipitation in 350mm. Furthermore, most of the annual precipitation falls in a few downpours that last from only hours to days, transform rivers into particularly flashy systems that transmit nearly 100% of their water and sediment discharge during high-intensity floods. These rivers of the drainage built 13 deltas (lobes) along the margin of the lake. In this study, Bantanzi river-delta system, one of these surrounding systems was choose to dissect. To achieve the original purpose, detailed geological information were collected from flied, which include 29 trenches with a depth of 1.5-2.0m, river cut bank section with 1.2km in length and 1.8m in height, and 5 profiles of ground-penetrating radar (GPR) with a total length of 4km.

As a result of high-intensity floods, deposits of this river-delta system are dominated by flashflood deposits. They consist of more than 80-100% of Froude transcritical and supercritical sedimentary structures at a high deposition rate, such as backset cross strata, scour and fill structures, gradational planar laminations, convex-up low-angle bedforms, convolute beddings, and even small scale cyclic steps bedforms, as a result of the high flow velocities and sharp-peaked flood hydrograph. These Froude transcritical and supercritical structures occur consequent and distinct along the cut-bank section from proximal to distal which benefits to analyze the morphodynamics of upper flow regime. Low-angle downstream accretion and aggradational packages instead of well-developed macroforms or bar forms are the recognizable architectural characteristics. These depositional units are commonly capped by mud drapes which commonly show desiccation cracks.