During flood disaster

the water head at the upstream of the embankment changes rapidly

and the water pressure acting on the embankment soil is not stable.In order to study the water head lag effect in cohesionless soil under unsteady seepage conditions

water permeability test was conducted through a self-designed test device.Three kinds of test materials including fine pebble

fine sand and middle sand were chosen.The impact of seepage path length

void ratio

permeability coefficient and particle size of cohsionless soils on water head lag effect under unsteady seepage conditions was studied

considering the factors of start-up of seepage

stability of downstream water pressure and recovery of hysteresis.The results show that:the larger the particle size is

the weaker the water head lag effect is.The water head lag effect can be ignored in the fine pebbles with grain size larger than 2 mm;the faster the hydraulic gradient of the upstream head rises

the shorter the downstream water pressure start-up and stabilization lag time is.But the downstream hydraulic pressure needs a longer time to recover;the relationships between the void ratio and permeability coefficient of the cohesive-free soil and the downstream water pressure transmission delay time are both inversely proportional;the permeability coefficient is in a logarithmic relationship with the start-up and stabilization lag time of the downstream water pressure.The results of the study reveal the law of water head lag effect in the cohesive-free soil

which is of great significance in studying the most dangerous moments of flood peaks during flood season and arranging flood protection and emergency rescue.