Flow Analysis for Navigation Safety by Using iRIC Model Nays2DH

Abstract

For safer navigation, it is important to reduce the shallow water effects and reduce the possibility of sedimentation. The shallow water effect is a vital factor in confluence areas where the depth of the navigation routes is subject to frequent changes during high flow and low flow times. This study summarizes the flow analysis during the process of improving the navigation channel around a confluence area by excavating shallow areas up to two possible bed elevation levels (3.3 OPm and 2.8 OPm). The analysis results, demonstrated that the depth has increased and the velocity has decreased in the excavated areas showing an improvement. Velocity has reduced from 1.1 m/s in the existing condition to 1.0 m/s for 3.3 OPm at excavation level and 0.8 m/s for 2.8 OPm at excavation level. Water depth is improved for 2.8 OPm at excavation level more than 3.3 OPm level. However, for both excavation levels water depth has been improved than the existing condition. Overall, the shallow water effect would be lessened after at least some of the proposed excavation plans.

1 Introduction

Inland navigation is a famous transport method since ancient times due to its cost-effectiveness and environmental friendliness. The main factors affecting navigation safety are navigable flow, water surface slope, water depth, and velocity (Zhang et al. 2019). For safer navigation in rivers, it is very important to reduce the shallow water effect which creates high velocities and turbulence under the hull increasing hull resistance. This increased resistance can cause instability of the vessels and more fuel consumption. Increased turbulence will result instability which might affect safety. Further, when the shallow areas are present, the reduced depth creates high velocities under the boat making low-pressure areas under the hull making boats squat. 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. In inland navigation on rivers, these kinds of problems are very common especially around the confluence areas as it is easier to form shallow areas due to the effect of flow separation zones at confluences. Therefore, it is vital to study possible dredging methods to improve the shallow points in navigation routes around confluences and their resulting flow conditions in the vicinity.

Yodo river in Osaka prefecture Japan, is a famous inland navigation area since 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 the confluence of three rivers is present. Figure 1 shows the location of Yodo river, Yodo river basin, and navigation route in the area considered. This location is in a center of the tourism boat route from Hirakata to Yamasaki. On the navigation route, several shallow points exist near the confluence area and they have sometimes caused disturbances to the safety of the navigation in the area. Especially when two vessels cross their paths, the area may become critical. Figure 2(a) shows the bathymetry of the area and the navigation route with shallow points obtained from river bathymetric survey. Figure 2(b) shows the shallow points identified by interviewing the boat operators of the area (marked in purple and green marks according to the size) and bathymetric survey data. Figure 2(c) shows an aerial photograph taken during the green laser trial survey and the circled area shows high elevation rocky bottom. To improve navigation safety, several measures to improve the bed have been identified such as excavating the rocky bottom down to 3.3 OPm level (OP is with respect to a datum in Osaka called Osaka pale). However, when the rocky bottom is excavated to 3.3 OPm level, there are places that become deeper than the existing bed level. Therefore, the new flow field is required to analyze for velocity, depth and water surface elevations. This study summarizes the analysis of the existing condition and the conditions after the possible excavation options, for depth, velocity, and water surface elevations.

Fig. 1.

The location of Yodo river, Yodo river basin and navigation route in the area considered

Fig. 2.

(a) Elevation of the area and the boat route with shallow points (b) Shallow areas as per the interviews with boat operators (c) Arial photograph taken during green laser trial survey showing the rocky areas

2 Methodology

The basic test of improved navigation safety in rivers by excavation is to check the reduction of the shallow water effect. The reduction of the shallow water effect can be checked by the change in depth, velocity, and water surface elevation before and after the excavation along the channel in the vicinity of excavated area. Numerical simulations were carried out to identify the above effects for various possible excavation options proposed, considering the existing condition at the study location. For the numerical simulations, high-resolution bathymetry data were obtained with a high-density bathymetric survey using green laser about 1 m mesh size, and a calculation mesh of the area was then created with 5 m mesh in both x and y directions. Figure 3 shows the area modeled and river bed elevation plots of the area. Lines A, B and C are the lines along the navigation route. There are shallow points in the middle of the route as shown in the figure. The area inside the rectangle is the proposed excavation area which will be explained in Sect. 4.2.

Fig. 3.

Simulated area with proposed excavation area and the river bed elevation along the three lines and associated water surface elevation

3 Numerical Model

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 the calculation of unsteady flows accompanied by turbulence and laminar flow, and it is capable of dynamically showing the realistic motions of unsteady eddies.

3.1 Governing Equations

Equation of continuity and equations of motion of water are used as the governing equations. In order to consider the river shape accurately, a general coordinate system is used. The continuity and momentum equations in Cartesian coordinate system can be converted to the general coordinate system. The basic equations used in Cartesian coordinate system are shown in Eq. (1), (2) and (3).

$$\frac{\partial h}{{\partial t}} + \frac{\partial (hu)}{{\partial x}} + \frac{\partial (hv)}{{\partial y}} = 0$$

(1)

$$\frac{\partial (uh)}{{\partial t}} + \frac{{\partial \left( {hu^{2} } \right)}}{\partial x} + \frac{\partial (huv)}{{\partial y}} = - hg\frac{\partial H}{{\partial x}} - \frac{{\tau_{x} }}{\rho } + D^{x}$$

(2)

$$\frac{\partial (uh)}{{\partial t}} + \frac{\partial (huv)}{{\partial x}} + \frac{{\partial \left( {hv^{2} } \right)}}{\partial y} = - hg\frac{\partial H}{{\partial y}} - \frac{{\tau_{y} }}{\rho } + D^{y}$$

(3)

Where, h is water depth, t is time, u is velocity in the x direction, v is velocity in the y direction, g is gravitational acceleration, and H is the total water depth. Here, the bed shear stresses in x and y directions ($\tau_{x}$ and $\tau_{y}$) are expressed by using the friction coefficient of river bed C_f which can be given as in Eq. (4) and (5).

$$\frac{{\tau_{x} }}{\rho } = C_{f} u\sqrt {u^{2} + v^{2} }$$

(4)

$$\frac{{\tau_{y} }}{\rho } = C_{f} v\sqrt {u^{2} + v^{2} }$$

(5)

The friction coefficient of river bed C_f is estimated by Manning’s roughness parameter n_m as in Eq. (6)

$$C_{f} = \frac{{gn_{m}^{2} }}{{\sqrt[3]{h}}}$$

(6)

The diffusion terms in x and y directions are expressed as in Eq. (7) and (8) respectively.

$$D^{x} = \frac{\partial }{\partial x}\left[ {v_{t} \frac{\partial (uh)}{{\partial x}}} \right] + \frac{\partial }{\partial y}\left[ {v_{t} \frac{\partial (uh)}{{\partial y}}} \right]$$

(7)

$$D^{y} = \frac{\partial }{\partial x}\left[ {v_{t} \frac{\partial (vh)}{{\partial x}}} \right] + \frac{\partial }{\partial y}\left[ {v_{t} \frac{\partial (vh)}{{\partial y}}} \right]$$

(8)

Here, v_t is the eddy viscosity coefficient. For the calculation of v_t zero equation model was used and v_t can be calculated by the Eq. (9).

$$\nu_{t} = \frac{ \, \kappa }{6}Au_{*} h$$

(9)

Here κ is von Karman constant (=0.4), $u_{*}$ is the shear velocity.

4 Analysis of Flow Conditions Under Present and Proposed Conditions

4.1 Analysis of Existing Conditions

Navigable flow condition of 125 m³/s and low flow condition of 50 m³/s were considered for the initial analysis. Figure 4 shows the spatial variation of depth, velocity, and water surface elevation of the two cases. The area marked with a rectangle shows the excavation extent while the triangular marks show the shallow points. From Fig. 4, it is clearly visible that there are low depth areas along the navigation route near the excavation extent area and high velocities in the middle. Further, the velocity vectors show the concentrated flow occurring from the left bank side has increased the velocity on the navigation route. These low depth areas can cause the shallow water effect where high velocities and low pressure zones developed under the hull can cause the boats to squat depending on the speed of the boat and its specifications such as draft. Therefore, it is necessary to remove these kinds of shallow areas by excavation.

As shown on the right side of Fig. 4, the low flow condition has much shallower depths and high velocities in the middle. In this condition, it seems the depth is not sufficient for boat navigation in some locations.

Fig. 4.

Existing condition around the confluence area under navigable and low flow conditions (a) water depth, (b) Velocity and (c) Water surface elevation (Δ - shallow points)

4.2 Analysis of Proposed Excavation Options

Three possible excavation areas with two bottom levels were identified considering the navigable flow and the low flow situations. The excavation level is decided based on 200 day water level at Takahama which is 4.195 OPm. Assuming the water surface gradient between Takahama and the excavation area is the same as the average river bed gradient, the water surface elevation should be at around 4.6 OPm at the excavation area. Water depth at the location is targeted to be maintained as 0.8 m and a minimum safety margin of 0.5 m is allowed for the ship/boat passage. Therefore, the bed level should be, 3.3 OPm (=4.6 − 0.8 − 0.5 OPm). Therefore, in this study, one case is considered as 3.3 OPm bed elevation. Furthermore, according to harbor standards, a margin of another 0.5 m is required during low flow conditions. Accordingly, a second case with bed elevation of 2.8 OPm (=4.6 − 0.8 − 0.5 − 0.5 OPm) is also considered. Two main cases considering the two bed levels and extending excavation areas 6 possible scenarios are identified. Figure 5 shows the proposed excavation options and a cross-section which schematizes the excavation area in 2 different bed elevations (2.8 OPm and 3.3 OPm). Six different cases with proposed excavation options as shown in Fig. 5 were simulated.

Figure 6 shows the variation of flow velocity, the water surface, and bed elevation variation along the center line of the channel (line B in Fig. 3). From the 6 cases, it is visible that, flow velocity has reduced at the excavation area (Maximum velocity point) from 1.1 m/s to 1.0 m/s at the 3.3 OPm excavation level and 1.1 m/s to 0.8 m/s at the 2.8 OPm excavation level.

Fig. 5.

Proposed excavation scenarios under navigable flow conditions and the areas to be excavated under each bed elevation along A14 line (This excavation area is different for each line)

Fig. 6.

Variations of (a) velocity and (b) water surface elevation for each excavation option along the center line of the channel

Comparing the elevation and velocity profiles along the navigation route for a total of seven cases of existing and with proposed excavation options, it is clearly visible that the water depth of the excavated area has increased and the velocity within the excavated area has decreased. During the excavation processes, it is important to confirm there is no significant change in water level and the velocities around the vicinity of excavated area. Otherwise, that may cause changes in sediment transport and the excavated areas may be easily filled up or some other areas might result in erosion. In this study, for all the excavation cases, no specific change in velocity expanded by the excavation effect in the downstream area was confirmed. Each case has variations in change in velocity and water depths. Out of all the cases, 2.8 OPm wider excavation extent (case 23) gives the best improved condition in depth and velocity.

Figure 7 is a spatial comparison between the case 0, case 13, and case 23 for depth, velocity, and water surface elevation. Case13 and 23 are the cases with largest excavation extent from 3 cases with 3.3 OPm level and 2.8 OPm level, respectively.

Fig. 7.

Spatial variations of (a) depth, (b) velocity and (c) water surface elevation under existing condition (left), case 13 (middle) and case 23 (right)

According to the Fig. 7, it is clearly visible that the existing condition has been improved with the proposed excavation options. The turbulent area near the left bank has reduced in both cases (in the velocity plots; middle row in the Fig. 7). However, in case 23 where the excavation level is up to 2.8 OPm, the improvement is highly visible in both depth and velocity. The upstream areas and downstream areas away from the excavation extent area has no visible change. Water surface elevation shows no significant change with the existing conditions.

Figure 8 shows the variation of shear stress along the river center line which is related to sedimentation (and erosion) pattern. As seen from the figure, the shear stress has reduced along the excavated area and no specific change outside the excavated area. The shields parameter calculated assuming sandy grains with 0.55 mm–1 mm diameter shows that the sediment movement is bed load dominated. (The sand size was assumed, based on the survey of bed material in the Yodo river a few kilometers away by Azuma 2018). Therefore, the sediment entrainment would be even reduced than the present condition and thus this excavation would not increase the sediment movement which would have led to sedimentation within the study area.

Fig. 8.

Variation of shear stress for each excavation scenario along the center line of the channel

5 Conclusions

An area of 130 m × 40 m near a confluence of the Yodo river was analyzed for possible improvements to enhance navigation safety. From the bathymetric survey and the interviews conducted with local boat operators, existence of shallow areas and high velocity areas were identified. The flow analysis was carried out with 200 days of navigable flow condition of 125 m³/s compared with low flow condition of 50 m³/s to understand the existing condition. In the navigation route, high velocity areas, low depth areas and concentrated flow from the left bank side to the navigation route were identified.

As a measure to improve the safety of the navigation in the area, excavation of shallow rocky areas was identified. Flow analyses were carried out to understand the flow condition before and after the possible excavation options. Flow velocities, depths and water surface elevations were simulated with Nays2DH model (in iRIC GUI) for the 6 cases and compared with the existing condition.

The simulation results demonstrated that, the velocity reduction by increasing water depth and no significant change in water surface elevations were observed in the vicinity of the excavated areas. The current velocity is 1.1 m/s in the existing condition, and it decreases to 1.0 m/s at 3.3 OPm excavation level and 0.9 m/s at 2.8 OPm excavation level. In the case of the 2.8 OPm, the water depth is generally sufficient compared to the 3.3 OPm.

Though both excavation options were found to improve the shallow water condition, the bed level of 2.8 OPm shows more improvement. Since the shallow water effect would be reduced with the increased water depth, as per the analysis the adverse effect should be reduced with the excavation options.

To conclude, it has been demonstrated in this study that, Nays2DH 2D model in iRIC GUI can be used to analyze hydrodynamics of the river flows under various excavation options quite well in order to check the resulting improvements.