Recently, coastal environmental engineers have made attempts to understand the role of sediment (particulate organic matter adsorbed on fine soil particles, diameter range 1???100 ??m) in subsurface environment which is considered to be related to the biodiversity of estuaries. Since the retention amount of sediment is one of important factors in considering the biodiversity, understanding sediment retention is vital to good management of the estuarine environment. In this study, laboratory experiments were conducted to propose a model for investigating the sediment retention based on variations of water head in a sand bed. Field observations were also conducted to verify the validity of the proposed model. From laboratory experiments, variations of water head in the sand bed could be represented by our proposed model with a maximum relative error of 3%. As the proposed model takes the porosity and the hydraulic conductivity of the sand bed into account, sediment retention in the sand bed can be evaluated on the basis of variances in the porosity and the hydraulic conductivity when variations of water heads at the boundary and in the sand bed are known. A method was proposed to measure variations of river water head and water head in a tidal flat. Furthermore, sand material of the tidal flat was sampled in order to determine the porosity and the hydraulic conductivity of the tidal flat. Based on the observation results, the variation of water head in the tidal flat could be reproduced by the proposed model when the porosity and the hydraulic conductivity of the tidal flat were applied into the model. In other words, the porosity and the hydraulic conductivity of the tidal flat can be predicted by the proposed model when variations of river water head and water head in the tidal flat are measured, leading to the understanding of sediment retention in the tidal flat from temporal changes in the porosity and the hydraulic conductivity
