ABSTRACT
Bed erosions take place around the sabo facilities such as sabo dam, weir and so on. Several patterns of bed erosion are in downstream reach of the sabo dam, one is the local scouring due to drop of overflow in between main and counter dams and in downstream of the counter dam, and the other is bed erosion due to imbalance of sediment transportation along downstream reach of the counter dam.
Longitudinal bed profile has typical formation either rotational or parallel degradation due to hydraulic conditions. Those bed erosions are well-known phenomena, and countermeasures against those erosion problems are classic study in sediment hydraulics. Maximum erosion depth in equilibrium condition was evaluated by ASCE (1962), and there are many researches concerning to local scouring due to the drop flow. One research (e.g., Martin et al., 2006) will discuss the bed slope of between sills in comparison with bed slope without sills, and the bed slope becomes milder due to the local head loss for sediment transportation even if several sills are set properly. However, the main mechanism related to decrease of bed slope does not seem to be discussed, and the research is a little different from our study because present study could discuss the preferable interval of main dam and counter dam, overlapping height, bed variation in downstream reach of the counter dam.
More effective countermeasure in downstream reach is preferable for bed erosion in between a main and counter sabo dam, because a horizontal distance and an overlapping height between dams, which are shown in Fig. 1, are determined empirically in Japanese standard of sabo dam design.
In present study, influences of sabo dams on the bed variations in downstream reach of the dam are discussed experimentally for combinations of overlapping height, horizontal distance between dams and apron. Bed erosion depth is not noticeable around the counter dam if the counter dam is set behind main dam such that the drop water of over flow from main dam could attack directly on the top of the counter dam in case of large magnitude of floods. Differences of overlapping height affect bed erosions in long downstream reach of the counter dam, and does not affect the bedload runoff at downstream end. In addition, smaller overlapping height of dams is effective to protect bed erosions around the counter dam.
