| Flows are much stable and sediment transport is much limited in the downstream than upstream reaches of dams.The downstream of dams is often in a sediment deficiency, which cause riverbed degradation (Erskine, 1985). In such conditions, attached algae on the riverbeds tends to overgrow due to a reduced opportunity of its detachment associated with movement of sediment particles (Minagawa et al, 2007). Thick mats of algae are not edible for grazing animals (e.g., Heptageniid mayflies, Japanese Ayufish) for the most part, because the mats consist of older algae with much silt deposition and low nutritional quality. Bedload and sediment management has been conducted in many Japanese reservoirs to improve the sediment deficiency of the downstream reaches. Sediment replenishment is one of the effective methods to prevent the deficiency. The method is excavating and transporting part of the sediment in reservoirs to the downstream channels, and flushing them by flood (Kantoush et al, 2010). It is discussed that the sediment replenishment also has an effect on detaching algae.There are several principal factors controlling gain and loss of plant biomass in streams (Figure 1) (Biggs,1996). The factors about gain are light, water quality (carbon, nitrogen, and phosphorus), and temperature (Biggs,1996). Toda et al (2001) tried to predict the growing rate by equations consisted of the resources. The factors about loss are hydraulics, bed and suspended load sediment by hydraulics, grazing (Biggs, 1996), and selfgenerated detachment (Bouletreau et al, 2006). The sediment replenishment can promote disturbance of certain frequencies through bedload transport that induce algal detachment. An increased quantity of detached algae after sediment plenishment has been reported in the downstream reach of Managawa Dam, which lacked in sediment for transport previously, Fukui Prefecture, Japan. (Sakamoto et al, 2005). But volume of a sedimenttransport is dependent on the volume of sediment replenishment, and the distance from the sedimentreplenishment point. Flows in the downstream are also change when sediment transport to the downstream reach causes. Even if flows are the same, tractive force to riverbed is different by riverbed material distribution.Further, some reports say that algae recover in a month after detachment. Those complex relationships prevent predicting sediment replenishment effect on attached algae biomass. So, for evaluating the effect of algae detachment by sediment replenishment, analysing several patterns of flow regimes and riverbed material distribution is needed.This study shows the effect of sediment replenishment to bed material distribution and algae biomass. For achieving it, years of data which was measured at a downstream of dam are analysed. |
