堤防险情及其处置

长江中下游堤防多位于由表层弱透水层及下卧透水层组成的双层地基上，堤防及其地基、边滩在汛期出现的险情有散浸，管涌，流土、接触冲刷引起的渗水流砂、漏洞，穿堤建筑物渗漏，滑坡，崩岸，溢溢等。

Dangerous situations in levees and treatment measures

The levees in the middle and lower reaches of Yangtze River are mostly on two-layer foundation consisting of the weakly pervious surfae layer and the pervious underlying layer.Dangerous situations occurring in levees and their foundations,side beaches in flood periods include infiltration,piping,soil flow,quicksand caused by direct scouring of seepage,leakage chink,leakage due to structures passing through levees,landslide,bank caving,overtopping of levee,etc.In plain regions of the middle and lower reaches of Yangtze River,primitive weakly-pervious clay layers possess perfect mechanical characteristics and seepage resistance,so levee breaches are mostly the results of not finding dangerous situations,not taking measures for them in time,or inappropriate rush repairs,leading to continuously aggravating the dangerous situations.This paper expounds and summarizes in details the judgment of dangerous situations,the principle for rush repairs and blocking chinks,and experiences for handling infiltration,piping and soil flow,chink and collapse sink,cracking of levee,slope failure and bank caving,etc.,in consideration of specific conditions for flood control of Yangtze River.