Antecedent Bottom Conditions of Reservoirs as Key Factors for High Turbidity in Muddy Water Caused by Storm Rainfall

Abstract

To facilitate administrators’ abilities to handle reservoir operations, factors on how muddy water was formed inside the reservoir should be identified. According to field observation data, during typhoon events, there were big uncertainties caused by only using the reservoir inflowing discharges to evaluate turbidity condition of raw water inside reservoirs. The data also showed that time intervals between typhoon events could be relevant. The state of the bottom silt inside the reservoir and the erosion capacity of the inflowing turbidity current both might play key roles. In view of these facts, a flume experiment was conducted to analyze the threshold condition of bottom-silt entrainment by turbidity currents. Then, to evaluate and validate muddy water phenomena during storm rainfall events, an empirical equation for evaluating the concentration of reservoir bottom silt after a long time was established by a 4-year experiment on flocculent settling with silt-clay consolidation. The results showed that when the ratio of bed shear stress of turbidity currents to critical shear stress was large, peak turbidity formed by the inflowing turbidity current was relatively high; however, the correlation between that ratio and peak turbidity could be further strengthened by considering silt-clay consolidation time on the bottom concentration of reservoirs. This explained the different levels in terms of turbidity for rainfall events of similar scale.