THE APPLICATION OF CONCREATE BLOCKS AS SLOPING ENERGY DISSIPATOR WITH LARGE ROUGH ELEMENTSNOBUYOSHI SUGANUMA(AIZAWA Concrete Corporation), YOUICHI YASUDA(Professor, Department of Civil Engineering, College of Science and Technology, Nihon University), KEITO MASUI(Master Student, Department of Civil Engineering, Graduate School of Science and Technology, Nihon University)

INTRODUCTION

ResultsGenerally, a plunging flow is formed below drop hydraulic structure as a stability of jump position. The hydraulic jump isa phenomenon well known as useful method of energy dissipation. If the toe of jump is not plunging, it must be required to keep
jump length and approach length of supercritical flow on concrete apron. In this case, the total length of energy dissipator
might be longer, and the total cost might not be economically adequate. Further, concrete blocks were installed below concreteapron as river-bed protection work, but concrete blocks might be flushed away by the formation of local scour. After floods,
flushed blocksmust be used as recycle resources. In order to stabilize the jump position, a plunging flow is formed. But, a highvelocity near the bottomcontinues far downstream, and the formation of plunging flow might cause local scouring and degradation ofriver bed.
The application of concrete blocks with protrusion type to the sloping energy dissipator. The experimental investigation yields
that the formation of plunging flow can be disappeared by installing the concrete blocks with protrusion type on 1/15 slope for plunging
flow. different discharges. Also, it has beenconfirmed that the surface jet flow is formed without the formation of The velocity
fields at the downstream of concrete blocks were measured, and the velocity measurement supported that the velocity near
the bottom could be reduced by the formation of the surface jet flow.

The mean velocity below concrete blocks was measured, and the velocity profile ateach vertical section was arranged by equation (1).

EXPERIMENTAL SETUP(1)

Figure 4 shows the velocity profiles at downstream of the stacked boulders for F1 = 1.00, hm/h1 = 0.79, and i = 1/15. For the formationof the surface jet flow, as shown in Figure 4, it has been confirmed that the main flow lifts into the water surface. Also, the velocity
near the bottom is smaller than that near the water surface.But,This might be caused by a high velocity flow along the side wall on theslope. As the concrete block was installed as staggered arrangement, a local flow resistance due to the protrusion of concrete block
might be small near the side wall. At y = 0 cm, a reverse flow is formed near the bottom by the formation of stational wave.
Figure 5 shows the velocity profiles at downstream of the stacked boulders for F1 = 2.16, hm/h1 = 1.32, and i = 1/15. At x/h1 = 1.48,
the velocity distribution at y/(B/2) = 0.75 is slightly different from that at y/(B/2) = 0 and 0.5. In this case, the effect of staggered
arrangement on the flow along the side wall might be small.
In order to investigate the maximum mean velocity decay below the installation of concrete blocks for both free overflow type and supercritical flow type, the maximum mean velocity 𝑈max was determined from the mean velocity profile
profile at each vertical section, and the maximum mean velocity was arranged by equation(2).

equation(2)The experimental condition is shown in Table 1. Then, i is slope of installed concrete block defined as hm/L(hm= Height difference,L= horizontal installation length of installed concrete blocks). The inflow condition above the drop was settled by Froude number F1
and relative drop height hm/h1. Here, h1 is the inflow depth defined in the flow passing over the drop (located in 3.5 times of h1
upstream from drop), s is the drop model height, and F1 is the inflow Froude number defined as V1/(gh1)1/2; V1 is average velocity.
If the inflow Froude number is unity, a free overflow type was assumed. The relative drop height hm/h1 (=hm/dc; dc is critical depth)
was settled as 0.79. If the flow above the drop is supercritical(F1 > 1.00), the relative drop height hm/h1 was settled as 1.32.
In addition, the downstream depth is expressed as averaged depth measured in gradually varied flow region (200 cm < x < 300 cm)

Figure 6 shows the maximum mean velocity decay of the surface jet flow under free over flow type. As the flow passing
over the drop transits from critical to supercritical flows, and the velocity on the slope is accelerated. As shown in thisfigure, for the lower limit of surface jet flow, the maximum mean velocity decays within the range of 0 < x/h1 < 8. If thetailwater elevation is increased, the transition flow moves upstream, and the maximum mean velocity decays slightly at the downstreamof the downstream of the installation of concrete blocks.
In the case of supercritical flow type, as shown in Figure 7, the maximum mean velocity is more dissipated comparing
with the case of free over type. Because, a strong turbulent flow is formed.

CONCLUSIONS✓ The application of concrete blocks with protrusion type to the sloping energy dissipator was presented. The installation of the sloping apron with concrete blocks below
low drop structure is effective for the protection of river bed during flood stages if the slope of the installation is settled as 1/15 slope. The concrete block was installed as
staggered arrangement. It might be easy to form surface jet flow by the flow resistance due to protrusion of concrete block for different discharges and tailwater elevations.

REFERENCES
Ministry of Land Infrastructure and Transport. (2012). Ministry of Construction River Erosion Control Technology Standard (Plan),
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Hager, W. H. (1992). Energy dissipators and hydraulic jump. Water Science and Technology Library, Kluwer Academic Publishers.
Kanda, K., Muramoto, Y., and Fujita, Y. (1995). Local Scour and its reduction method in downstream of bed production work,
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✓ The method for the installation of concrete blocks with protrusion type is the most important, because the flow resistance due to the protrusion of concrete block is necessary
to reduce the approaching high velocity. For the formation of the surface jet flow, it has been confirmed that the main flow lifts into the water surface.Also,
the velocity near the bottom is smaller than that near the water surface.The velocity distribution below the installation of concrete block for supercritical flow type differs fromthat for free over type.

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Yasuda, Y., Shinozaki, R. (2018). Flow characteristics of hydraulic jumps below low drop structures.

✓ For the lower limit of surface jet flow, the maximum mean velocity decays in a short distance. In the case of supercritical flow type, the maximum mean velocity is moredissipated comparing with the case of free over type. Because, a strong turbulent flow is formed. If the tailwater elevation is increased, the transition flow move
upstream, and the maximum mean velocity decays slightly at the downstream of the installation of concrete blocks.

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