Agondian Reservoir, located in Kaohsiung, south part of Taiwan (Fig.1), has been operated since 1953 for the flood control. 71% of the total storage volume was filled with deposited sediment in 1991 because of the mud rock in the upstream of the reservoir. To recover flood control ability, a reservoir renewal project was started in 1997. The idea of empty flushing was applied in Agondian Reservoir. The intake was re-construct as the spillway to release the water and sediment during rainfall season. All the re-construct engineer were done in 2004, and the empty flushing operating was started from this year. To understand the effect of the empty flushing, a monitoring system were setup from 2009. There are three station set at the inflow and outflow of the reservoir, which included rainfall gage, water level meter, flow velocity meter, water sampler (Fig.2). There are 9 events were recorded from 2009–2015, included the rainfall, inflow and outflow discharge, the water concentration, the reservoir water level. The deposition in the reservoir were measure every year at same period. Figure 1 Location of Agondian reservoir. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315623207/4fbc492d-6678-4a12-aaf6-5c2b8ea38e5f/content/fig185_1.jpg"/> Figure 2 Location of monitoring system and equipment. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315623207/4fbc492d-6678-4a12-aaf6-5c2b8ea38e5f/content/fig185_2.jpg"/>

The results show some characteristic of empty flushing operating listed follow.

First, with the comparison of the rainfall intense and inflow concentration, the occurrence of the high concentration flow needs 2-3 hour continued rainfall.

Second, the reservoir water level is the key factor during operating. The transportation of the high concentration flow would be cut with the high reservoir water level. The suitable value is suggesting as EL30M.

From these characteristic, some suggestion could feedback to improve the effect of the empty flushing operating.