Dynamic State of River-Mouth Bar in the Yuragawa River and Its Control Under Flood Flow Conditions

Dynamic state of river-mouth bar in the Yuragawa River and its control under flood flow conditions

H. Miwa National Institute of Technology, Maizuru College, Kyoto, Japan K. Kanda National Institute of Technology, Akashi College, Hyogo, Japan T. Ochi Kyoto University, Kyoto, Japan H. Kawaguchi NTT Infrastructure Network Corporation, Osaka, Japan

ABSTRACT: The topography of a sand bar is continuously affected by river runoff, ocean currents and sea wave action. Disruptions in any of these factors can alter the geometry of the sand bar, causing sedimentation problems and increased risk of flooding. In order to mitigate the risks associated with such changes, it is important to understand the characteristics of the topographic changes of river-mouth bars, and to develop a method for controlling the bar geometry. This study examined the effect of variations in the geometric properties of a river-mouth bar using hydrological data (river water discharge and sea wave height). The application of spur dikes to promote erosion of the sand bar by altering the direction of river flow was evaluated by means of flume experiments. A two-dimensional numerical model (iRIC Project) was also applied to examine the effectiveness of spur dikes on river-mouth bar control. The results of this study clarified the efficiency of measures for managing sand bar formation at the mouth of the Yuragawa River in Kyoto Prefecture, Japan.

1 INTRODUCTION strong three-dimensional flow field in the periphery of a river-mouth bar with a complex geometry, The topology of river mouths, which are defined as Tateyama et al. (1995) recently developed a new an- regions where river runoff enters an ocean, is char- alytical method (referred to as the bottom velocity acterized by the combined influence of sediment computation (BVC) method) to assess bottom flow transported by the river and near shore currents as- velocities without any assumptions of shallow flow, sociated with tides and sea waves. River mouths and applied the BVC to analyze and reproduce the along the coast of the Sea of Japan are particularly changes in bar morphology that occurred during the affected by the action of winter sea waves, and tidal flooding that occurred in the Aganogawa River flood prisms are small. Under these conditions, river- in 2011 (flow discharge approximately 11,000 m3/s). mouth bars tend to develop under conditions of On the other hand, in their study on river-mouth longshore sediment transport, and blocking of river- channel width of rivers with river-mouth bars, Sato mouths is frequent. Floods tend to breach the river- et al. (2004) modeled sediment transport by near mouth bars that develop during dry periods, and shore currents associated with sea waves and tides, complex changes occur in bar behavior during as well as by river runoff, and derived predictions of floods and in post-flushing river-mouth channel the channel width for different equilibrium states. width. These changes vary depending on flood hy- However, as shown in the results of this study, bar drodynamics, fluvial morphology near the river dynamics in times of flooding are affected by the mouth, and the presence or absence of harbor facili- pre-flood fluvial morphology, coastal sea level, and ties and other man-made structures along the coast. flood discharge. In many cases, the bar is not com- Hosoyamada et al. (2006) examined the changes pletely flushed out, even in design-scale floods. in river-mouth bar morphology during flash flooding In the Yuragawa River located in the north of by numerical analysis using nonlinear shallow-wave Kyoto Prefecture, the mid-west in Japan, as shown modeling, and quantitatively assessed the stage- in Figure 1, topographic changes of its river-mouth lowering effect of sediment flushing, in their study bar are continuously activated by sediment transport on the river-mouth bars of the Aganogawa River, due to river flow and sea wave. In October 2004, a which were not effectively breached by flooding on large part of the river-mouth bar was eroded by the a scale of approximately 50% of its design discharge huge flood flow due to typhoon. The river-mouth bar (i.e. 6,000 m3/s). Kuwahara et al. (1996) also per- has developed along the right bank only, and the riv- formed a two-dimensional analysis of riverbed er-mouth channel has been fixed along the left bank changes in the Natori River. In response to the since then. This situation may cause some problems such as bank erosion, washout of bank protection 2 FIELD MEASUREMENT ON RIVER MOUTH works and harmful effects on other coastal struc- TOPOGRAPY tures. Even effects of water discharge during flood periods on responses of the river-mouth bar are not 2.1 Outline of the Yuragawa River clarified. Therefore, the risk of high water level The origin of the Yuragawa River is Mikunidake caused by a river-mouth clogging is formidable. In situated on the borders of three prefectures of Kyoto, order to avoid these problems and risk, it is im- Fukui and Shiga. The length of the river is 146 km, portant to understand the characteristics of the topo- and the size of its basin is 1,880 km2. The Yuragawa graphic change of the river-mouth bar and its cause, river system is one of the 109 Class A river systems and to propose a control method of the bar geome- in Japan. The Yuragawa River can be classified into try. Miwa et al. (2014) investigated the effects of the three reaches. In the upper reaches, V-shaped ra- flood discharge on the area, height and volume of vines and river terraces have developed. The width the river-mouth bar through the numerical simula- of river increases in the middle reaches and pools tion. Then, they showed that the bar area and height and riffles are developed in the main channel of the of the river-mouth bar considerably decrease in the river. The lower reaches consist of the valley plain, early stage of the flood period, and that the decrease the stream flows along the long and narrow bottom rate after that is relatively small. Over 3,000 m3/s of of the mountains. Figure 2 shows the longitudinal flood discharge accelerates the erosion of the bar, bed profile of the lower and middle reaches of the and it also decreases its area remarkably. Ochi et al. Yuragawa River. The average bed gradient around (2015) also investigated the relationship between the Fukuchiyama (37 km away from the river-mouth) in sea water level and channel width at the river- the middle reaches is about 1/1,500, whereas the bed mouth, and they clarified that the channel width gradient in the lower reaches is approximately tended to increase with decreasing sea water level 1/6,000 to 1/8,000. Therefore, the length of tidal sec- and increasing water discharge, which was in action of the river reaches over 20 km. cordance with the analytical results obtained by Altitude (m) Kuwahara (1996) in their investigation of the effect of changes in tidal level during flooding on the phe- Fukuchiyama Ohkawa-bashi nomenon of river-mouth bar collapse. Bridge

Distance (km) Figure 2. Longitudinal bed profile of lower and middle reaches.

2.2 Changes of river-mouth topography for 60 years Figure 3 shows the changes of river-mouth topography from 1947 to 2009. In 1947 and 1963, the river- mouth bar on the right bank was developed considerably and the width of the river-mouth channel was approximately 80-100 m. The river-mouth bar on the Figure 1. Location and basin of the Yuragawa River. right bank was disappeared and it on the left bank was developed in 1972. The river-mouths on the In this study, the temporal variations in geomet- both bank were developed in 1975 and 1982. The rical properties (e.g., bar area and shape) of the riv- widths of the river-mouth channel were approxi- er-mouth bar were analyzed on the basis of the hy- mately 80 m for both years. After that, a large part of dro-logical and topographical data in the Yuragawa the river-mouth bar was flushed out by the flood 3 River estuary. The effects of the river water dis- flow (peak discharge Qp=3,600 m /s) of the typhoon charge and the sea wave height on the geometrical No.10 in 1982 and 1983, the width of the river proper-ties of the bar were also investigated. As for mouth channel increased to about 300 m. However, the bar control, the effectiveness of a trench excava- the river-mouth bars were developed as shown in tion and a spur dike, which can increase the erosion photographs of 1986 to 2001. A large part of the riv- area and change the flow direction respectively, for er-mouth bar was flushed out again by the flood 3 erosion of the bar was evaluated by means of flume flow (peak discharge Qp=5,400 m /s) of the typhoon experiments. The two-dimensional numerical model No.23 in 2004. The river-mouth bar has developed was also applied to further investigate the effects of on the right bank only after then, and the river- the trench excavation and spur dike on the bar con- mouth channel has been fixed along the left bank trol. The simulation results were verified against the (2009). The construction works of detached break- experimental results. waters have been carried out from 1967 and there Figure 3. Temporal changes in river-mouth topography in Yuragawa River (1947-2009). are 8 detached breakwaters for each bank. The for- height at Kyoga-Misaki (30 km far from the river- mation of tombolo is confirmed after 1982. Effects mouth) and (c) the water discharge at Fukuchiyama of the detached breakwaters on river-mouth bar for- for 5 years and 8 months (from May 2010 to De- mation are not clarified. cember 2015). The datum level was the sea surface at each measurement time, and there was no correc- tion of the datum level because the tidal range was at 2.3 Results of field measurements most 30 cm. The river-mouth bar on the left bank In order to clarify the effects of river flow dis- has developed since December 2012 (The survey charge and sea wave height on changes in river- was started in March 2013). In Figure 4(a), the area mouth topography, survey works were conducted by of the bar on the right bank shows increases and de- using GPS periodically. A surveyor walked along creases in the short term, whereas it gradually de- the shore line receiving the satellite signals, and reg- creases as a long term tendency. The decreasing rate istered the signal to the controller at intervals of ap- of the area for four years is approximately 40%. proximately 10 to 20 m. The number of stations for From the short term tendency, it can be seen that the each survey was about 70 to 100. The area of the bar area increased in the winter seasons of 2010, 2012 and its shape were calculated by means of identify- and 2013, and it decreased in the rainy and summer ing the coordinates of the stations. Basically, the seasons of 2011 and 2013. As shown in Figure 4(b), survey was conducted once a month. Kanda et al. (2012) clarified that the significant Figure 4 shows (a) the temporal variations in the wave height over 2.55 m influences the development area of the river-mouth bar, (b) the significant wave of the river-mouth bar. Therefore, it can be consid-

(a)

(b)

(c)

Figure 4. Temporal variations in (a) river-mouth bar area, (b) significant wave height at Kyoga-Misaki, and (c) water discharge at Fukuchiyama in Yuragawa River. ered that an increase in the onshore sediment gravity s of 1.47. transport may contribute to the increase of the area The grain size of the river-bed material in the ex- of the river-mouth bar. A possible reason why the perimental model was larger than that for a scaled- area did not increase in the winter season of 2011 is grain replica of the field-site sediment, as its reduc- that the onshore transported sediment from the fore- tion to the scale of 1/150 would have required a shore may be accumulated on the seabed, where it grain size of 0.2 mm or less, which would have re- was eroded by the flood flow in the fall season of sulted in suspended-sediment and sand-wave pre- 2011. On the other hand, as shown in Figure 4(c), dominance. In this large-grained, and consequently, the water discharge over 1,500 m3/s was recorded a distorted scale model, similitude (Shields similitude) total of four times in the summer seasons of 2011 in the sediment discharge rate was not completely and 2013. In particular, the flood with discharge of satisfied. In view of the important influence of the about 5,500 m3/s occurred due to the typhoon No.18 critical tractive force of the river-bed material on in September 2013. The area of the bar was reduced sediment transport and changes in the river bed by 17% in 2011 and by 40% in 2013 due to these around the bar during overflow, we used uniform large flood discharges. Moreover, it is found that the coal grains with a density of 1.47 g/cm3 and a mean area was gradually reduced in the spring to fall sea- grain size of dm=1.3 mm as the river-bed material, to sons of 2012. The water discharge of 300 m3/s clas- obtain a correspondence between the river site and ses had frequently occurred during these seasons, the model in the ratio of the shear velocity of the these flows might have eroded the river-mouth bar. flow to the critical shear velocity of the river-bed material. The critical shear velocity in the model, as calculated by Iwagaki’s equation (Iwagaki, 1956), 3 FLUME EXPERIMENT AND NUMERICAL was u*c=1.44 cm/s. Experimental measurement of SIMULATION FOR RIVER-MOUTH BAR the shear velocity u* of the flow in the region up- CONTROL BY TRENCH AND SPUR DIKE stream of the bar yielded values in the range u*= 2.01-2.83 cm/s and thus a u /u ratio of approxi- 3.1 Experimental set-up and procedure * *c mately 1.4-2.0, indicating the occurrence of a dy- Flume experiments using the large channel, which namic state throughout that range. Application of was modeled based on the Yuragawa River, were these values to river-site flood conditions yielded a conducted in order to clarify the effects of the spur grain size d of approximately 20 mm, which corre- dikes on the control of the river-mouth bar topogra- sponded closely to the maximum grain size of the phy. The experiments were conducted in a horizon- river-bed material at the river site (mean grain size tal rectangular open channel, which was 8.75 m long of 1.24 mm, maximum grain size of 18 mm). and 2.87 m wide as shown in Figure 5. The width of The movable bed was flattened with a scraper and this channel corresponds with the 1/150 scale of the the river mouth-bar model, which was modeled Yuragawa River. The channel has a water level ad- based on the results of the topographical survey, was justing weir in the returning flume in order to con- formed on the channel bed. The river mouth-bar is trol the water elevation at the downstream end of the located on the right bank in the Yuragawa River, but channel. Nearly uniform coal dust was used in the the river-mouth model was set to the left bank in or- experiments as bed sediment. The coal dust had a der to avoid the effect of the lateral flow toward the mean grain diameter dm of 1.3 mm and a specific returning flume. The height of the river-mouth bar was 0.02 m, the sea bed behind the river-mouth bar was set to a slope Ib of 0.05 (1/20). The other part of the bed was made horizontally. The experimental conditions are listed in Table 1. In the table, Q = water discharge, Qc = corresponding water discharge, T = experiment duration, hd = water surface elevation at the downstream end of the channel, B= width of trench excavation, L = length of the spur dike. The discharges in the experiments correspond with the flood discharge of Q = 3,300 to 4,900 m3/s at the river-site channel. The experiment duration (T=20min) corresponds with the peak discharge duration of 3.7 hours. Runs 13B and 13C are the experiments using trench excavation which set at center of the river-mouth bar as shown in Figure 5. Runs 14A and 14B are the experiments using spur dike, which was made of plywood, and its thickness Figure 5. Experimental set-up (Trench and spur dikes were and height were 0.01 m and 0.12 m, respectively. set on the corresponding Run). Three pieces of spur dike were set on the right bank with intervals of 0.2 cm and placed at an angle of 60 degrees to the wall as shown in Figure 5.

______Table 1. Experimental Conditions. Run No. ______Q Qc T hd B L 3 ______l/s m/s min m m m 10A 17.8 4,900 20 0.152 - - 10B 17.8 4,900 20 0.139 - - 10C 17.8 4,900 20 0.127 - - 10D 17.8 4,900 20 0.117 - - 13B 17.7 4,900 20 0.122 0.1 - 13C 17.6 4,850 20 0.144 0.1 - 14A 12.1 3,300 20 0.128 - 60 ______14B 17.9 4,900 20 0.135 - 60

In the experiment, the water was stored slowly in the channel in order to avoid erosion of the bed. The prescribed water discharge was fed into the channel after that. The water surface elevation at the downstream end of the channel was controlled by the water level adjusting weir. Water surface elevations were measured with a point gauge, at intervals of 1 m in the longitudinal direction at the location of Y = 0.8 m of the channel before stopping the flow. The motion of PVC particles (size = 0.05 mm) on the water surface was recorded by a video camera for 20 seconds during the experiment. The surface velocity was obtained from PIV analysis (Fujita et al., 1998). Transverse profiles of the bed surface were measured with a laser sensor mounted on a propelled car- riage, at intervals of 4 cm in the longitudinal direction of the channel after stopping the flow. The datum plane for water surface and bed surface elevations was set to the channel-bed surface. Then, initial bed elevation was 0.1 m for the river bed part and was 0.12 m for the river-mouth bar part. Figure 6. Effect of downstream-end water level on river-mouth bar topography.

3.2 Experimental results and discussion lower than the initial bar elevation, the bar was al- most entirely eroded and washed away. This may be Figure 6 shows the river-bed geometry after water caused by the increase of tractive force on the bar passage with a water discharge of Q=17.8 l/s and due to increase of the water surface gradient. The various downstream-end water levels (Runs 10A- deep scouring was shifted downstream as the down- 10D). The water discharge corresponds to a flood 3 stream-end water level decreased. discharge of approximately 4,900 m /s at the river- Figure 7 illustrates the final situations of river- site channel. The area surrounded by the broken line bed geometry in the case of the trench excavation in shows the initial shape of the river-mouth bar. In the river-mouth bar. The surface velocity distribu- Run 10A with the highest downstream-end water tions on the bed are also shown in Figure 8. The hy- level, the flow converged toward the river-mouth on draulic conditions of Q and hd in Run 13B corre- the right bank, and it developed a deeply scoured to- spond between Runs 10C and 10D, those in Run pography upstream of the river-mouth bar tip. Alt- 13C correspond between Runs 10A and 10B. In Run hough the flow also got over the bar, the erosion of 13B, the river-mouth bar was eroded at river-mouth its surface hardly occurs because its velocity was channel side. According to the experimental obser- relatively slow. In Run 10B with relatively lower vation, the erosion of bar area divided by the trench downstream-end water level, the flow into the chan- was progressed. However, the other area was hardly nel was faster and the channel width was wider. In eroded because the primary flow goes through the Run 10C, the water surface gradient increased on the river-mouth channel. As a result, the effect of the bar and its bed was lowered. Then, a depositional trench excavation on the river-mouth bar erosion landform was developed downstream of the bar. In was restrictive. The river-mouth bar was hardly Run 10D with the downstream-end water level being eroded in Run 13C, the trench excavation was not Figure 7. Final river-mouth bar topography (trench excavation). Figure 9. Final river-mouth bar topography (spur dikes).

Figure 8. Surface velocity distributions (Trench excavation). Figure 10. Surface velocity distributions (Spur dikes). effective in this case. The technical advantage of redirect flow eroded the river-mouth bar, and the trench excavation was not always found in these ex- erosion area extended into about half of the bar in periments. Further investigations about width, depth spite of relatively low flow discharge in this case. In and number of trench are necessary. Run 14B with high water discharge, the primary and Figure 9 illustrates the final situations of river- redirect flows entirely eroded the river-mouth bar. bed geometry in the case of the spur dike. The sur- The scour depth of the river-mouth channel was face velocity distributions on the bed are shown in small too in this case. The effectiveness of spur dike Figure 10. Although the value of hd in Run 14A cor- is not only bar control but also reduction of the local responds to Run 10C, the value of Q was less than scour in the river-mouth channel. that. On the other hand, Q and hd in Run 14B correspond to Run 10B. As shown in Runs 10A and 10B, 3.3 Simulation model and calculation method the flow converged toward the river-mouth on the right bank, and progressed erosion of the river- Numerical simulations were conducted in order to mouth channel. In Run 14A, the converged flow to- not only follow the flume experiments, but also fur- ward the river-mouth was weakened by setting up ther investigate the control method for the river- the spur dikes. The scour depth of the river-mouth mouth. The two dimensional simulation model, channel became small because the spur dikes strong- Nays2DH (iRIC, 2014), was applied to the investi- ly redirect the flow toward the river-mouth bar. The gation in this study. The continuity equation and the momentum equations for water flow are h uh vh    0 (1) t x y

uh  hu 2 huv H       gh  x  D (2) t x y x  x

2 vh huv  hv H  y      gh   D (3) t x y y  y where x, y = plane Cartesian coordinates; u, v = velocity components in x and y directions; t = time; h = water depth; H = water elevation; g = gravity accel- eration;  = fluid density; x, y = bed shear stress components in x and y directions; and Dx, Dy = dif- fusion terms in x and y directions. The continuity equation of sediment transport is Figure 11. Reproduction calculation results of flume experiments (Trench excavation). z 1  q q   bx by      qsu  w f cb   0 (4) t 1   x y  where z = bed elevation;  = bed porosity; qbx, qby = bed-load sediment transport rates per unit width in x and y directions; qsu = release rate of sediment from bed; wf = settling velocity of sediment; cb = refer- ence concentration of suspended sediment. As for the bed-load sediment transport equation, the Mey- er-Peter & Müller equation (1948) in streamwise direction and Watanabe et al. equation (2001) for calculation of sediment transport vector are employed. The governing equations mentioned above and others are transformed into discrete forms in a gen- eralized coordinate system. In the simulation model, the advection terms are discretized by the CIP method and the zero equation model is applied as a turbu- lence model. The bank collapse and deposition model is also introduced in the simulation model. Figure 12. Reproduction calculation results of flume experi- In the simulation, the research area (5.68 m long, ments (Spur dikes). 2.84 m wide) of the channel was divided into 142 was not always enough. On the other hand, as for the and 71 grids for the longitudinal and lateral direc- spur dike (Figure 12), the flow around the spur dikes tions, respectively. Then, the longitudinal grid size is redirected to the river-mouth bar. Therefore, the (x) and the lateral grid size (y) were the both 0.04 bar of river-mouth channel side eroded both of Runs m. The Manning’s roughness coefficient and the 14A and 14B. The reduction of the local scour in the time increment were taken as n = 0.02 and t = 0.01 river-mouth channel can be reproduced in the simu- second, respectively. Other parameters and hydrau- lation. Moreover, the bar near the left bank of the lic valuables were the same as the experiments. Sus- channel also eroded in Run 14B. These features in pended sediment was not considered in calculation. simulations are same as the experimental results. The simulation result in Run 14 underestimates the 3.4 Simulation results and discussion erosion of the bar near the left bank. The experimental and simulation results suggest- Figures 11 and 12 illustrate the reproduction calcula- ed that the erosion of river-mouth bar depends on tion results of the flume experiments for the trench not only construction works such as the trench exca- excavation and spur dike, respectively. As for the vation and spur dike but also hydraulic conditions of trench excavation (Figure 11), the simulation was water discharge and downstream-end water level. successful in reproducing the erosion features ob- We performed calculations of eroded volume of the served in Run 13B. However, the bar of river-mouth river-mouth bar under some conditions of water dis- channel side was eroded by the flow, the simulation charge and downstream-end water level. Figure 13 river-mouth bar may be mainly activated by an increase of longshore sediment transport rate in winter season; the bar area has a strong correlation with wave height. 2. The trench excavation work was effective to the erosion of the bar area at the river-mouth channel side. However, the effect of the trench excavation on the river-mouth bar erosion was restrictive. On the other hand, the spur dike was effective ade- quately. The effectiveness of spur dike is not only bar control but also reduction of the local scour in the river-mouth channel. 3. The simulation results could reproduced the tendencies of experimental results. Suitable conditions of the construction works such as the trench excavation and spur dike must be investigated by using the numerical simulation model.

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