Experimental Study on Local Scour in the Downstream Area of Low Drop Structure Types ()
1. Introduction
The stability of a natural river depends on the bed slope, silt movement, state of natural ground, depth of low water channel, structure of channel and riverside and riverside vegetation. Among these, bed slope is the most important factor; thus, a method of properly maintaining bed stability should be sought (H. J. Kim et al. [1]). For the natural river’s stability, installing mainly riverbed maintaining facilities is a universal practice. The drop structure as a typical structure of riverbed maintaining facilities is a type of river cross-cutting structure installed to prevent bed scour and erosion at the steep slope of the channel. The drop structure is generally defined as a structure with sharp fall of 0.5 m or more. In Korea, installing scour protection structure in the apron and downstream area is a universal practice to prevent scouring in the downstream area, whereas most bodies and apron are installed as single concrete structure (River Design Standard [2]).
The installation of such drop structure is designed and constructed in line with the installation purpose of securing bed stability based on the river design standard; although effective for channel stability, it is known to cause various ecological and environmental damages such as interruption of the channel environment’s continuity (H. J. Kim et al. [1]). As a typical example, a river cross-cutting structure at high altitude disrupts the migratory fish’s movement between the upstream and downstream areas of a river and reduces the number of fish and diversity of fish species. Moreover, some fish can become extinct (Ministry of Environment, MOE [3]). To overcome such demerit, continued efforts have been made toward using natural materials for the body and auxiliary structures of a drop structure or reducing ecological and environmental damages by installing fishway at the body of the drop structure, with some achievements. Note, however, that the supplementary methods proposed to date have not presented a fundamental solution. To solve the ecological and environmental problems caused by the installation of cross-cutting structures including drop structures, understanding of the living things inhabiting the river in question is essential. Based on such understanding, a most effective method may be installing cross-cutting structures through which fish living in the river in question can easily move.
Although ecologically disadvantageous since a river is interrupted because of drop structure installation, such structure installation is also deemed ecologically advantageous in terms of scour and complex flows in the direct downstream area. Cooper and Knight [4] as well as the US Washington State Department of Fish and Wildlife [5] suggested that the scour generated in the downstream area of a structure for artificial slope adjustment, i.e., drop structure, functions as a fish habitat. Note, however, that they did not present specific habitat evaluation and standard on the size of scour hole (scope, depth, etc.) according to the drop structure. Even though some study achievements have been presented in Korea including the suggestion of low drop structures using huge stones to secure an effective fish habitat environment, they simply presented the standard on the structure’s stability. Those studies merely mentioned that the effect of fish habitat improvement is in the early stages, i.e., citing only water quality improvement (MOE [3]).
The evaluation on the habitat improvement effect in the downstream area of drop structures can be performed through habitat simulation. The study on habitat began in developed countries including the US and Europe in the 1970s. In Korea, the concept of river maintaining flow considering the fish was adopted only in the latter part of the 1990s, and various studies have been conducted since then (D. G. Lim et al. [6]). Since the 2000s, various types of natural-type drop structures have been suggested to improve the habitat of fish as part of river restoration efforts and have been applied to actual problems. Note, however, that the studies to date merely present a comprehensive, conceptual result i.e., the physical habitat can be improved by complex flow through the installation of boulder or embankment within the channel. Actually, systematic and specific studies are insufficient.
This study started with the premise that a scour hole has value as habitat and sought to analyze local scour adjustment possibility in the downstream area through drop structure type change and to offer habitat through scour in the future. Various types of scour holes including straight and V shapes were used in this study, and quantitative comparison for a tentative physical habitat was carried out by evaluating the scour effect on the downstream area of the channel in question. As for the structure in question, a low drop structure with drop height where fish for simulation can ascend was selected, and a wood drop structure that was environment-friendly and easily acquired was the object of this study. This study conducted hydraulic experiments to check the scour effect downstream according to the drop structure type.
2. Existing Studies
Scour in the downstream area of drop structures is caused by plunging jet generated by the drop of flow passing through the drop structure. The flow passing through the drop structure shows a type of free over fall and a complex flow type in the riverbed downstream due to plunging jet. The study on scour in the downstream area of drop structures generated by complex flow characteristics was conducted by Schokiltsh [7], Veronese [8], Zimmerman and Maniak [9], Bormann and Julien [10], Fahlbusch [11], Hoffmans [12], Federal Highway Administration (FHWA) [13], and Construction Industry Research and Information Association (CIRIA) [14]. Schokiltsh [7] derived an empirical equation by performing indoor experiments under the condition of similar free fall. This equation has a limitation, i.e., it does not consider the height of the drop structure, and applies to the experiment with height difference of 0.06 m and 0.32 m between the upstream and downstream areas. Veronese [8] carried out an evaluation on the scour depth caused by the low water head’s waterspout in the down-stream area of riverbed protection structures and suggested in FHWA [13]. Note, however, that other equations predict scour depth with only total head loss or water depth in the inlet and downstream area unlike the equation derived as the relation between particle movement and velocity. Bed materials are not reviewed, with the scour depth estimated to be relatively excessive. Zimmerman and Maniak [9] suggested an equation to estimate the scour depth formed in the floor protection structure in the drop structure waterspout, which is the same location in the equation of Veronese [8]. Bormann and Julien [10] proposed an equation of scour depth formed in the downstream area of the slope-type drop structure. As for the equation application scope, from 0.3 m2/s to 2.5 m2/s of flow per width, the limit of maximum scour depth derived through calculation is 1.4 m. The equation of Fahlbusch [11] theoretically approaches shearing stress and drag and lift equilibrium and resistance. This equation suggests that scour depth decreases as water depth and flow decrease, whereas scour depth decreases as downstream depth increases. The limitation of such application is that considering various variables is necessary including sediment size, sediment size distribution, sediment shape, silt density, material’s viscosity, and turbulence level since the equation deals only with the equilibrium of strength, given the fact that various physical variables are associated with silt’s marginal movement. Hoffmans [12] derived an equation with the nondimensional scour variable associated with riverbed sediment size (
) by supplementing the proposed equation of Fahlbusch [11] through the application of the 
-theorem in
. In case of
, 
applies. In case of
, 
applies.
In case of 
and 
and 
given, kinematic viscosity
. This equation was adopted as the scour evaluation equation in the drop structure waterspout in CIRIA [14].
3. Hydraulic Model Experiment
The open channel experiment device used for the naturaltype low drop structure was an experimental channel with length of 30 m, height of 1.2 m, and width of 3.0 m. The experimental channel was installed to supply precise flow through the square weir located upstream; a widthwise laying sluice gate was installed to control the water level in the downstream area of the channel. The drop structure as the experiment object was installed at the center of the channel to check the riverbed change scope downstream. A 3.0 m open channel having the same width as an actual small channel width was used so that physical habitat evaluation could be performed according to fish type in addition to scour analysis. Concerning water level control downstream, as a result of the review of the experiment study carried out by C. S. Kim et al. [15] regarding scour in the downstream area of the vertical drop structure and scour protection structure subsidence, maximum scour depth was noted in the minimum condition of water depth in the downstream area of the vertical drop structure. Therefore, this study also conducted experiments in a state of no artificial control of downstream water level, i.e., in outflow condition. Figure 1 show the structure and flow supply device located at the center of the channel in the experimental channel where scour experiments were performed.
The size of the drop structure used in the experiment targeted a low drop structure with height of 0.5 m or less, which was close to the actual situation. Even though the experiment was carried out in a lab, efforts were made so that the experiment was as free as possible from the issue of the similitude law according to scale. The hydraulic experiment carried out in this study consisted of wood with diameter of 0.1 m, targeting a natural-type low drop structure with height of less than 0.5 m.
This study adopted the straight line type (180˚) and upward V-type as drop structure types, with three angles adopted for the V-type. Figure 2 shows the drop structure types. The straight line type is a drop structure that is easily found in a river, whereas the upward V-type tends to be applied more in foreign countries than in Korea. Various studies have been conducted for the straight line type, and reliable results have been presented. Note, however, that effective study results are insufficient,
Figure 2. Low drop structure types for the experiment.
except studies on the scour of some varied types (labyrinth weir) in the case of the V-type drop structure.
Table 1 shows the experiment conditions. The drop structure in question comes in 4 types whose upward angle (θ) was changed to 180˚, 140˚, 100˚, and 70˚ as shown in Figures 1 and 2. The drop structure conditions are H180, H140, H100, and H70, respectively, in this study, and H180 means a straight line type. Only the upward V-type was considered in this study because erosion can occur at the shore in the case of natural river, since scour can be generated on the left and right sides of the downstream area of the structure owing to overflow of the drop structure in the case of the downward V-type (θ > 180˚). Moreover, the downward-type drop structure was excluded from the experiment object because the scour type could be distorted owing to the wall impact of the experimental channel.