Engineering, 2012, 4, 90-99 http://dx.doi.org/10.4236/eng.2012.42012 Published Online February 2012 (http://www.SciRP.org/journal/eng) Case Study: Hydraulic Model Experiment to Analyze the Hydraulic Features for Installing Floating Islands Sanghwa Jung, Joongu Kang, Il Hong, Hongkoo Yeo Department of Water Resources Research & Environment Research, Korea Institute of Construction Technology, Ilsan, Korea Email: {kikimorah, jgkang02, hongil93, yeo917}@kict.re.kr Received November 13, 2011; revised December 13, 2011; accepted December 20, 2011 ABSTRACT The viewpoint of a river is changing as people regard the river as water-friendly space where they can enjoy and share the space beyond the simple purpose of flood control alongside the improving social level. The floating islands installa- tion was planned featuring three islands. The river’s flow and channel stability could be changed when new structures are built in a river. Hence an analysis of the hydraulic characteristic changes should need. The hydraulic model experi- ment in this study sought to review the impacts of the floating islands installation on the safety of flood control and sta- bility of river channel. This study analyzed the hydraulic features affecting the surrounding stability when installing floating islands and proposed stable floating islands layout in terms of hydraulics based on the experiment results. Keywords: Water-Friendly Space; Floating Islands; Stability of River Channel; Hydraulic Features 1. Introduction plaza and view plaza through riverside improvement. The basic plan is established to shape the ecosystem river The viewpoint of a river is changing as people regard the bank and for nature-friendly or water-friendly river bank river as water-friendly space where they can enjoy and protection in terms of the change of bank protection. share the space beyond the simple purpose of flood con- Moreover, in the Banpo zone, a plan to create cultural trol alongside the improving social level. Floating islands space has been established through the floating islands installation was carried out according to the plan to shape installation. Consisting of 3 islands in Figure 2, the the cultural space along the river, i.e., to provide water planned floating islands shall be installed on the left of culture and leisure space. Floating islands were planned the straight downstream section of Banpo Bridge as wa- featuring three islands. The hydraulic model experiment ter cultural and leisure space in a 9200 m2 area. Here, (HME) in this study aimed at reviewing the impact of two issues are flood control stability and hydraulic move- floating islands installation on the safety of flood control ment. First is topographical change on the left riverside and stability of river channel. The main review items were of Han River in the target area including change in low flow duration, velocity, and shear stress. The experiment target area is the downstream area of Han River located at the center of Seoul. The project sec- tion is an approximately 3.5 km-long straight line down- stream between Hannam Bridge and Dongjak Bridge. The section marked with the red dotted line in Figure 1 is the project target section, and the blue dotted line-marked section, the section of main interest with some changes in the target section. The target area’s flood frequency is high due to the high altitude of the water front of Han River and owing to damage by floods occurring at least 3 times annually. This area was used as grassland and green land-centered area because of the frequent floods. The basic plan of the target area within Han River consists of educational fa- cilities construction including ecosystem natural learning Figure 1. Project target section (red dotted line) and section arena and complex cultural space shaping as an event of main interest (blue dotted line). Copyright © 2012 SciRes. ENG S. JUNG ET AL. 91 consequently be carried out with clear purposes. Like- wise, a variety of plans including experiment size in line with the purposes need to be established. The model and actual-sized prototype in the HME should meet three similitudes: geometric similitude, kinematic similitude, and dynamic similitude [1]. In the geometric similitude, the length ratio needs to be constant at the corresponding point of the actual-sized prototype and model. In the ki- nematic similitude, the ratio of velocity and acceleration needs to be constant at the corresponding point of the actual-sized prototype and model. In the dynamic simili- tude, the ratio of strength needs to be constant at the cor- responding point of the actual-sized prototype and model. The element that becomes a criterion in the law of geo- metric similitude is expressed as Lr = Lp/Lm (L: length, with r, p, and m representing the scale ratio, actual-sized prototype, and model, respectively). The velocity and 2 acceleration ratio is expressed as ar = Ur/Tr = Lr/Tr . Here, time ratio is expressed as Tr, Tr = Tp/Tm. The dynamic similitude is the law matching the ratio of external forces that add to or reduce the flow of fluid and inertia force of fluid. Here, the main external forces are gravity, viscosity, surface tension, and pressure. When carrying out HME, similitude is decided by a main ex- ternal force. In this study, the main external force was gravity because of the open river channel condition; thus, the Froude similitude as the ratio of gravity and inertia force was used. The main physical elements are pre- sented as follows: Inertiaforces LU22 U 2 Fr 2 , Gravity gL3 gL Figure 2. View before and after the installation of floating islands. 2 Lr Ugrr, ULrrL, TLrr Tr water channel cross section according to change of bank The scale of the hydraulic model is decided by com- protection shape. The impacts of such topographical change prehensively reviewing the reproduction ability of the on flood water level and current field should be reviewed. actual-sized prototype, lab’s discharge supply capability, Second is floating islands installation. A review of the experiment model production space, and ease of meas- impacts of the floating islands installation on flood water urement. Generally, the length is much greater compared level and riverbed change should be conducted, including with the depth in a river model. Therefore, a large river a review of the surrounding flow duration when install- tends to have wide river channel compared with depth ing the floating islands to check whether a disadvanta- despite the smaller length. As such, distorted scale is in- geous hydraulic phenomenon occurs. This paper presents evitable when carrying out large river HME. In this ex- a design proposal that can minimize hydraulic change periment, the change of flood water level according to within the river channel based on floating islands instal- the conditions was one of the most important items. For lation. this reason, the scale was determined in the maximum 2. Theory and Scale size allowed by the impact experiment supply discharge and experiment site conditions while securing maximum The hydraulic model experiment (HME) was carried out depth and minimizing distortion considering the accuracy to find hydraulic solutions to phenomena limited to spe- of water level measurement. The model used in this ex- cific areas including the construction of hydraulic struc- periment has 1/120 horizontal scale, 1/50 vertical scale, ture and river improvement. As such, HME has various and 2.4 distortion. The conversion ratios of the hydraulic purposes according to project features, and it should volume of HME are presented in Table 1. Copyright © 2012 SciRes. ENG 92 S. JUNG ET AL. Table 1. Scales and conversion ratios of fixed bed hydraulic model experiment. Conversion ratio of hydraulic Conversion formula Scale volume Horizontal length scale, X r X r 120 Vertical length scale, Yr Yr 50 Area ratio, Ar XYrr 6000 1/2 Velocity ratio, Vr Yr 7.07 3/2 Discharge ratio, Qr XYrr 42426.41 Slope ratio, Sr XYrr 0.42 Coefficient ratio of intensity of 1/2 2/3 XYrr 1.24 illumination, nr Figure 3. Target area model. 3. Model Production and Measurement Method 3.1. Model Production For the model production, the model standard points were set by performing model measurement using a total station after completing the arrangement work on the model installation space. Along the set model standard points, the outer wall was set up using blocks, and the ex- terior was finished with mortar. In this manner, topog- raphical installation work was executed. For the model section, the topographical plate model was manufactured by measurement line by cutting plywood according to the riverbed section type including bank, low water channel, and terrace land of the river after printing out as real Figure 4. Floating islands installation. photo through the conversion of riverbed cross section acquired from actual river measurement into a model periment needed large discharge. Thus, more than the scale. Concerning the produced topographical plates, maximum discharge supply of 0.9 m3/s was guaranteed filling work was done by paving with sand after placing using three pumps. The supplied discharge was measured by measurement line through model measurement. The using the highly reliable weir mode and was calculated fixed bed model was produced by forming paved filling according to a discharge formula and the discharge cali- materials using cement mortar along the reproduced river- bration curve. A weir tank (3 m wide) was used as dis- bed section on the plywood after flushing with water. charge supply tank. The produced topographical plates were installed in the precise locations by measuring with the total station. By 3.2. Measurement Method referring to the ground plan and topographical map in Figure 3, the sections were precisely connected to re- For the measurement of the starting point’s water level produce the same topographical shape as the real one as and depth, a digital water level gauge (PH-355, KENEK) much as possible.