| Qionghai Lake is in the southeast of Xichang City, about4km far from it. It’s alsolocated in the subtropical plateau mountain area, southwest of China. In this region, thegeological structure is complicated, terrain fluctuation is large, and the erosion is strong.The climate is affected by the alternation influence of the southwest monsoon and thesoutheast inland dry monsoon, and the wet and dry season is distinct. In recent years,due to the extreme climate anomalies, forest vegetation decrease, sloping landreclamation and other reasons, the problem of sediment deposition is serious. Thesediment of Guanba River and Ezhang River promotes to the Qionghai Lake rangingfrom10~100km annually, with the Guanba River is the most serious and typical.Taking the Qionghai Lake as the core, the benefit of the series construction projects andthe tourism development reduce greatly, putting tremendous impact and obstacles onthe prosperity and development of Xichang city. The studies of Qionghai Lakesediment accumulation characteristics, causes and development are of great protectionsignificance to Qionghai Lake.Through field surveys, field tests, laboratory experiments, consulting papers andmonographs and other methods, it describes the sediment deposition characteristics ofQionghai Lake from space and time. It explains that debris flow and flush flood are theforming reasons of sediment deposition in Qionghai Lake in the aspects of sedimentaccumulation characteristics, packing quantity into the lake, and the effect on Qionghaiand so on. It calculates the accelerated metrization earthquake and the SPI droughtindex to reflect the drought level. Combined with the larger debris flow and flush floodevents in the region to analyze the superimposition of earthquakes and drought on thecontrol of debris flow and flush flood. It comes out to the trend of the vegetation coverage by the interpretation of typical TM remote sensing image in different phases.By the superimposition of earthquake, drought and vegetation coverage, and analyzingthe development trend of the debris flow and flush flood in the region, it predicts thetrend of sediment accumulation in Qionghai Lake. The main conclusions are as follows.(1) In space, the more serious siltation region in Qionghai are the inlet of GuanbaRiver in the north, the inlet of Ezhang River in the south and the back silting zone in thenorthwest.(2) In time, in the Holocene, the sediment deposition volume is about2.9billioncubic. Since1950,520thousand cubic silts have sediment there.(3) The manifestation of soil erosion in Qionghai region is debris flow and flushflood. More than half of the soil erosion area is distributed in the basin of the GuanbaRiver.(4) The accumulation characters of silt carried by debris flow and flush flood areaccumulated along the ditch bed by coarse particles (diameter>2mm), while the fineparticles are transported to Qionghai under the action of water. The debris flow andflush flood not only change the coastline morphology and the Qionghai Lake bottomtopography.(5) In the Qionghai basin, the debris flow and flush flood is in closer relation withthe occurrence of earthquake than drought. When the earthquake effective peakacceleration is greater than20gal, the probability of occurrence of debris flow and flushflood in the region is strengthened. When the seismic activity and droughtsuperimposed together, the probability of outbreak of debris flow and flush flood isgreater.(6) Since the1940s, the vegetation coverage in the region has experienced thechanges from less to more and from less to more. The forest coverage is graduallyimproved and shows a proyressive succession way.(7) With the accidental combined effects of strong earthquakes, droughts andheavy rainfall and other factors, it may occur to a large-scale debris flow and flush floodin the Qionghai basin. Under normal circumstances, with the implementation of naturalforest protection project in Xichang City, debris flow and flush flood decrease, so as theamount of silt entering in the Qionghai shows a slow decreasing trend. |
