PIANC Smart Rivers 2022

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Abstract : *(250~1000 words)

INTRODUCTION
Inland navigation is a famous transport method from ancient times due to its cost effectiveness and environmental friendliness. The main factors affecting navigation safety are navigable flow, water surface slope, backflow, water depth, velocity and wave height, (Zhang et al., 2019). The most important factors for rivers are navigable flow, water depth, water surface slope and velocity.  For safer navigation in rivers, it is very important to reduce the shallow water effect which creates high velocities and turbulences under the hull increasing hull resistance. This increased resistance can cause for instability of the vessels, more fuel consumption and environmental pollution. Increased turbulences will result in problems in visibility which might affect the safety.  Therefore, the shallow water effects have to be reduced for safer navigation. This shallow water effect occurs mainly due to the presence of shallow areas on the navigation route.
Yodo river in Osaka prefecture Japan is a famous inland navigation area from ancient times for tourism and other purposes. Even though the most famous navigation area is the downstream areas of the river around Osaka bay, this study focuses on upstream areas of the river where confluence of three rivers are present. On the navigation route, several shallow points exist near the confluence area and they have sometimes caused disturbances to the safety of the navigation system. Especially when two vessels cross their paths, the area becomes very critical. To improve the navigation safety, several measures to reform the bed has identified and the proposed dredging areas and their effects to the flow field along the river are needed to be analyzed. Therefore, a two dimensional hydrodynamic model was created for the area and existing and expected flow analysis were carried out.

MODEL DESCRIPTION
The flow analysis has been carried out with Nays2DH in iRIC GUI. Nays2DH, developed by Shimizu et al., 2014 is a computational model for simulating unsteady horizontal two-dimensional (2D) flow, sediment transport and morphological changes of bed and banks in rivers using boundary-fitted coordinates within general curvilinear coordinates. This model has an established reputation for calculation of unsteady flows accompanied with turbulence and laminar flow, and it is capable of dynamically showing the realistic motions of unsteady eddies.

ANALYSIS OF THE FLOW CONDITIONS UNDER PRESENT AND PROPOSED CONDITIONS 
In this study, effects of the various dredging options were analyzed by checking the variations in depth, velocity, water surface elevation and shear stresses along the river. The area near the confluence of three rivers is used for the analysis as it was identified as the critical area specially due to the confluence. Three possible dredging areas with two bottom levels were identified considering the navigable flow and the low flow situations. Six different proposed cases and three cases of their existing conditions were simulated. Very fine bathymetry data of 1m mesh size was used and 5m and 4m grid sizes were used for simulation considering the simulation costs. Comparing the elevation and velocity profiles along the navigation route for nine cases of existing and with proposed dredging options, it is clearly visible that the water depth of the dredged area has increased and the velocity of the dredged area has decreased. During the dredging processes, it is important to confirm there is no change in water level and the velocities of other areas. Otherwise, that will cause for changes in sediment transport and the dredged areas will be easily filled up or some other areas might result in erosion. In this study for all the dredging cases no specific change in velocity expanded by the dredging effect in the downstream area was confirmed clearly. Each case has variations in change in velocity and water depths. Therefore, from all the cases, the optimum condition has to be identified in terms of costs and benefits.

APPLICABILITY OF THE METHOD 
This study calculated current velocity patterns with respect to the morphological patterns of dredging by using Nays2DH model in iRIC-GUI. From the simulations, the complex velocity pattern near the shallow point and the improved flow pattern by the proposed dredging scenarios have been observed. Changes in water depth and velocities were found appropriate for safer navigation.  
The full paper will discuss detail model description, data used, analysis methods and the results under each dredging scenario and applicability of the method for safer navigation systems around confluences.

REFERENCE
Shimizu Y., Takebayashi H., Inoue T., Hamaki M., Iwasaki T., Nabi M. (2014) iRIC-Software: Nays2DH Solver Manual. 
Zhang S., Jing Z., Li W.,Wang L., Liu D.,Wang T. (2019) Navigation risk assessment method based on flow conditions: A case study or the three river reach between Georges dam and the Gezhouba dam, Ocean Engineering, 175 pp 71-79.

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