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A Study on Channel Bed Scour and Debris Flow Control by Bottom Infiltration Screen
The frequency of disasters increased by debris flow after 921 earthquake happened in Taiwan. The scale of debris flow enlarged also. In order to prevent such disasters, many new works have been created. Among those works, dams are usual ways to be used. The main types of dams are closed-type and opened-type. The most problem is that both of them easy to be hit by large stones carried by debris flow. Besides, for blocking all silt behind its back, closed-type dam will lose solid contents after silt deposited at the upstream. For that reason, closed-type dam could only prevent the scale of debris flow from increasing, but could not decrease to . Viewing amount of 1420 rivers which have potency to occur debris flow in Taiwan . Where have been built so many closed-type dams, and they can't prevent debris flow actively. In order to activate ability of those dams for reducing scale of debris flow, we study on combining Bottom infiltration screen with it.
Flume experiment is the method for this study. We simulated where silt has deposited at upstream, the slope became 2/3 in original slope at upstream. And closed-type dams combined with Bottom infiltration screen crossed it. According to mechanism of scour by over-fall water theorem and Dimensional analysis theorem, we researched the mechanism of scour by over-fall water passing through co-construction. After flume experiment, we got the regression equations which could express the interaction between distance from scour center to dam, length of scour, depth of scour and slope in upstream, wide of rails gap in Bottom infiltration screen, length of rails in Bottom infiltration screen, average diameters of channel bed, wide of over-flow in top of dam. We proved that co-construction can reduce threaten in scour at downstream. Besides, we simulated debris flow passing through co-construction and observed the variation in ratio of concentration of sand and amount of sand. According to Transport equation and Dimensional analysis, we resolve the interaction between ratio of concentration of sand and wide of rails gap in Bottom infiltration screen, length of rails in Bottom infiltration screen. And got regression equation with those parameter and factors also. Finally, we proved that Bottom infiltration screen can promote the ability for preventing debris flow passing through co-construction. After 1134 times flume experiment, we proved that the closed-type dam could only reduce the concentration of debris flow with average less than 5.7%. By combining with Bottom infiltration screen, the closed-type dam could increase the average of concentration of debris flow to 30.3% ~ 56.8%. With combination, the average of reducing concentration of debris flow would be 570% to 890% than by Check-Dam only.
After analysis, we knew the capability of reducing concentration of debris flow could be over 25% when the value of b/d95 is less than 0.99 after debris flow passing through co-construction.
Proof from flume experiment, the depth of scour center after water passing through co-construction was less than closed-type dam only. The rates of relative reducing depth of scour center were more than 0 in general and the length of scour and distance between scour center and closed-typed dam was longer under same comparison after combining both. The rates of displacement between scour center and closed-type dam were less than 0 in general. The factor of shape of scour was 2.57 after water passing through Check-Dam in Sample A. It increased to be 3.89 after passing through co-construction. The Sample B increased from 2.50 to 3.01. Those proved that co-construction could make Check-Dam more secure and safer than before.
In a word, when closed-type dams have been built only, the capacity of reducing scale of debris flow is limited due to silt deposited at its upstream. If dams could combine Bottom infiltration screen. Not only can save budget in works for preventing debris flow, easy to choose address for constructing works, but also can be complementary to each other. We hope the result of this originality can achieve in reality. Therefore, the margin value in closed-dam can be promoted.
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