Abstract
In this article, we proposed an indirect method of measuring seepage of natural dam and considered the effect of seepage on prediction of water level in natural dam. It is one of the most important subjects to predict overtopping time of natural dam. It has not been obvious whether pondage depends on seepage in natural dam reservoir, because monitoring of inflow and seepage is difficult in natural river bed. First, we estimated seepage volume of natural dam with hydrological data. Second, we back-calculated inflow volume. The data was observed around a natural dam in Nagatono Valley in Kii Peninsula, from Sep. 2012 to March 2014. In our proposed method, it was assumed that seepage was followed Darcy's law. We compared observed inflow with back-calculated inflow by the saturated hydraulic conductivity, and obtained the same results. Using the back-calculated inflow, it clarified that 30-90% of inflow volume leaked from a natural dam, and 30-70% of seepage is observed as spring water from lower of the natural dam during rainfall period and 24 hours after stopping rainfall. The existence of substantial seepage in this natural dam shows necessity of consider in prediction of water level in natural dam.
