Proceedings of International Conference on Violent Flows (VF-2007) Organized by RIAM, Kyushu University, Fukuoka, Japan Wave Setup at River and Inlet Entrances Due to an Extreme Event Xuan Tinh Nguyen 1), Hitoshi Tanaka 1), and Hisao Nagabayashi 2) 1) Department of Civil Engineering, Tohoku University, Sendai, Japan 2) Department of Civil Engineering, College of Engineering, Nihon University, Koriyama, Japan Abstract The hydrodynamics of interacted area between ocean and river is very complicated since this area contains a lot of different processes and the full field of data sets is sometime hard to be obtained. Therefore, it needs to be investigated more. The wave setup at a river entrance or tidal inlet due to an extreme event such as typhoon and the low pressure system is a big concerning issue nowadays in terms of river mouth morphology change, saline intrusion, and navigation transportation. The wave setup is the height of Mean Water Level (MWL) above Still Water Level (SWL). In most of cases, the estimation of wave setup at a river mouth have been based on water levels measured some distance upstream from river mouth and then compared with a tidal level that measured in deep water where wave effects can be neglected. In this study, the full data sets including water level in the river, tidal level, wave height, and river discharge as well as water depth at the entrance from 8 rivers and tidal inlets within Japan during the serve typhoon no.18 in October 2006 and the low pressure system in December 2006 were complied. The main purposes are to in-depth investigating the wave set-up phenomena at different morphological river entrances or tidal inlets based on collected data sets, and to examine the relationship between wave setup with wave height, river discharge and water depth. This is a comprehensive study on wave setup due to extreme events at different river entrance morphology. Wave setup height is not only depending on wave height but also depend on river discharge during the high tide. In addition, the wave setup height was attained from 10 to 14 percent of offshore wave height for the cases of narrow and shallow river or inlet mouth, and from 0.2 to 4 percent of offshore wave height for a deep and width river mouth. This percent is dependent on the water depth at river entrance. The deeper it is, the less value of percent will be observed. Keywords: Wave setup height; River mouth and sea interaction; Extreme storms; Hydrodynamics; Wave height; River mouth morphology 1 Introduction Each year, there was several low pressure systems from Pacific Ocean impacted to Japan Coastline. Heavy precipitation and overcast conditions are often associated with low pressure. Due to the Coriolis Effect, low pressure systems often develop typhoon or cyclonic properties. At the beginning of October 2006, a serve low pressure system called typhoon 18 was attracted to Japan Beach. The maximum of wave height and wave period in Enoshima station were obtained up to 8.4 meters and 12.8 seconds, respectively. In addition, a very significant wave setup was observed at many river mouths or tidal inlet along to the coastline. The main purposes of this study are to in-depth investigating the wave set-up at different morphological river entrances or tidal inlets based on collected data sets from 8 river entrances (Figure 1). Because of each river entrance has their own morphology as shown in Table 1, for instance with and without jetties construction, or with and without sand spit at river entrance so the wave setup height needs to be analyzed individually and compare them. There are quite a lot of researchers studying on wave setup at river entrance such as Guza, R.T. and E.B. Thornton (1981), Hanslow et al. (1992, 1996) Tanaka et al. (2000, 2003), Dunn S. L. (2001), and Oshiyama et al. (2001). The wave setup is the height of Mean Water Level (MWL) above Still Water Level (SWL). In most of cases, the estimation of wave setup at a river mouth have been based on water levels measured some distance upstream from river mouth and then compared with a tidal level that measured in deep water where wave effects can be neglected (Tanaka et al. 2000). Figure 2 are shown the definition sketch of wave setup at a river or tidal inlet entrance. The wave setup caused by wave height, however, the water depth at the entrance and river discharge can be affected to its height. 2 Data Collection In order to investigate the wave setup the requirement data sets are wave height in deep water, tidal level and water level in upstream of river, as well as the river discharge at the same time of typhoon. The average water depth at river entrance is also necessary. Tidal level data was ob-
tained from Ayukawa Port, Sendai Port and Onahama Port. Enlarged area Kitakami River Nagatsura Inlet Sendai Port Old Kitakami River Naruse River Ayukawa Port Nanakita River Natori River Abukuma River N Onahama Port Natsui River Fig. 1 Location map of 8 river mouths Q Fig. 2 Definition sketch of wave set-up at a river or tidal inlet entrance River name Table 1 River characteristics River length (km) Catchment area (km 2 ) Average water depth (m) Number of jetties at the entrance Natsui 67 749 1.2 0 Abukuma 239 5,390 5.7 0 Natori 55 984 4.5 2 Nanakita 45 229 1.5 1 Naruse 89 1,130 6.0 2 Old Kitakami 6.5 2 249 10,150 Kitakami 4.0 0 Nagatsura Inlet ---- ---- 1.1 0 2.1 Water level correction Since the T.P datum of five water level stations were unknown excepting Nagastura Inlet, Nanakita River and Natsui River, thus a calibration procedure was carried out to find the correlation parameters. The method is that the water level data in the river is compared with the closest tidal level station which the T.P datum is available. However, the chosen month have to in normal condition, this means there is no special serve events during a month, for example in this study the authors chose data in September 2006. The tidal level at Ayukawa Port was used to compare with the water level in Kitakami River and Nagatsura Inlet. The tidal level in Sendai Port station was used for Old Kitakami, Naruse and Natori River water level data. The results of calibration can be seen in the Figure 3. The results of adjustment constants are illustrated in the
Table 2 bellowing. Table 2 Adjusted factors of water level refer to the T.P port datum. River name Water level Adjusted to Station T.P datum, m Kitakami Tsukihama 1.878 Nagatsura Inlet Nagatsura 0 Old Kitakami Kadowaki 2.341 Naruse Nobiru 1.698 Nanakita Nanakita 0 Natori Yuriage 0.315 Abukuma Arahama 0.350 Natsui Mikkakabe 0 2.2 Wave data Wave information is widely available in Japan. However, for this study area there are three suitable wave stations to obtain the offshore wave height during the typhoon 18 namely Enoshima, Sendai and Fukushima stations. The wave data in Enoshima was used to estimate the wave setup height for Kitakami River and Nagatsura Inlet. Sendai wave data was employed for Old Kitakami, Naruse, Nanakita, Natori, and Abukuma Rivers. Fukushima wave data was used for Natsui River. 2.3 River discharge To examine the effects of river discharge during an extreme event to wave setup height at the river entrances, this study have been attempting to collect the monthly fresh water for each rivers. In case of river discharge is not available, the water level at some location in the upstream, which does not have any tidal influence, will be used instead. Fig. 3 Correlation coefficients of 5 water level stations compared with the T.P datum
3 Results and Discussions 3.1 Water level rise at the river entrance Monthly water level rise (WLR) which is equal to water level in the river mouth minus to tidal level, wave height in deep water, Ho, and river discharge in Kitakami River, Q, are plotted in the Figure 4. At the time of WLR occurrence, the wave height and river discharge equal to 5.56 m and 96 m 3 /s, respectively. And then WLR increases together with wave height while river discharge is still very small, this suggests that the WLR in Kitakami River is mainly influenced by wave motion such as wave breaking at the entrance. However, the maximum of WLR attains as the discharge is 1260 m 3 /s and wave height reduces to 4.8 m, it means the river discharge affected to the WLR but smaller in order of magnitude compare to the effect of wave height. This kind of phenomena can also be found in Naruse, Natori and Abukuma River mouths as shown in the Figures 5, 6, 7, respectively. Fig.6 Hydrodynamic conditions and water level rise in Natori River. Fig.4 Hydrodynamic conditions and water level rise in Kitakami River. Fig.7 Hydrodynamic conditions and water level rise in Abukuma River. Fig.5 Hydrodynamic conditions and water level rise in Naruse River. The increase of water level at a river entrance can be attributed not only to wave set up, but also to backwater effect due to constriction of the flow caused by shallow and narrow of rivers or tidal inlets during ebb tide thus saline water that gets into river during high tide might not flow back to the sea. The water level in river is always higher than tidal level even during ebb tide and small wave condition. These effects are clearly seen in Nagatsura Inlet, Nanakita and Natsui River (Figures 8, 9, 10) which have the average water depth at entrance around 1.1m to 1.5m. The river mouth morphology such as water depth at the entrance influences to the height of water level rise. The more water depth is, the less value of water level rise will be obtained. For instance, water level rise in Old Kitakami River is very small compare to others during the event (Figure 11). It is because of a deep and large river, the average of water depth at the entrance is up to 6.5m.
Fig.8 Hydrodynamic conditions and water level rise in Nagatsura Inlet. Fig.11 Hydrodynamic conditions and water level rise in Old Kitakami River. Fig.9 Hydrodynamic conditions and water level rise in Nanakita River. A good case to study these effects is in Natsui River during the second event from 21 st to 28 th in October 2006 as shown in Figure 12. The maximum of wave height and river flood are occurred at different time. By dividing into two periods, which called flood-induced and wave-induced periods, and then plot the water level rise up we can see the effects of wave and flood individually to water level rise at the entrance. The percentage value of water level rise caused by river flood is higher than by wave height as illustrated Figure 13. Figure 14 is shown the relationship between offshore wave heights only with water level rise in Natsui River at two different events in October, 2006. Effect of wave height to water level rise in second event is slightly higher than the first event. Fig.10 Hydrodynamic conditions and water level rise in Natsui River. 3.2 Flood induced and wave induced The phenomenon of a typhoon is normally included a heavy raining, it made a big river discharge or river flood often occurs at the same time with peak of wave height. Therefore, it is difficult to separate the impact of wave height and flood on water level rise individually sometimes. Fig.12 Effects of river flood and wave height to water level rise.
2.0 1.5 1.0 0.5 0.0 3.3 Wave setup and offshore wave height relationship To investigate the wave setup height due to wave we have to neglect the effects of backwater and river flood as mentioned above by only considering the water level rise values during the high tide level. The relationship of wave setup height and offshore wave height can be estimated by using this following equation η = ah (1) 0 where: η is wave setup height at the entrance (m), H the offshore wave height (m), a regression slope. 0 The regression slope, which also means the percentage of wave setup height compare to offshore wave height, is given in the Table 3 by using the linear regression analysis that used the least squares method to fit a line through a set of data. These results illustrate that the wave setup height decreases as water depth increasing. The regression slopes at Nanakita and Natsui River in this study are well in agreement with the results obtained by Oshiyama et al. (2001) see Table 3. 5 Conclusions y = 0.3381x Wave-induced Flood-induced y = 0.1566x 0 2 4 6 8 10 Ho (m) Fig. 13 Wave and flood induced on water level rise 2.0 1.5 1.0 0.5 0.0 1st event in Natsui R. 2nd event in Natsui R. y = 0.1566x y = 0.1351x 0 2 4 6 8 10 Ho (m) Fig. 14 Effect of offshore wave height to wave setup height in Natsui River. A comprehensive study on wave setup due to an extreme event at different river entrance morphologies was carried out. Water level rise at river entrance is not only depending on the offshore wave height but also depend on river discharge and morphology of river mouth. Wave setup height was attained to from 10 to 14 percent of offshore wave height for the cases of shallow and narrow river entrance, and from 0.2 to 4 percent of offshore wave height for deep and width river mouths. Wave setup height is inversely proportional to the average water depth at river entrance. River Name Table 3: Regression slope for each river Aver. water depth at the entrance, m Regression slope Oshiyama Study-2001 Nagatsura Inlet 1.1 12.27 -- Natsui 1.2 13.51 10.07 Nanakita 1.5 10.88 13.3 Kitakami 4.2 3.78 -- Old Kitakami 6.5 1.86 -- Acknowledgments The authors would like very much to express our sincerely thank to Sendai Office of River and National Highway, Ministry of Land, Infrastructure and Transport, Japan, Kitakami River Lower Reach Office, the Ministry of Land, Infrastructure and Transport, as well as Iwaki Construction Office, Fukushima Prefectural Government and Sendai Dam Management Office, Miyagi Prefectural Government for providing us such valuable data sets. This study can not be conducted without the financial support from Grant-in-Aid for Scientific Research from JSPS (No. 17360230) and Grant for "Academic Frontier Project" from the Ministry of Education, Culture, Sports, Science, and Technology. The authors would like also gratefully appreciate their supports. References [1] Dunn S. L. (2001): Wave setup in river entrances. Doctor Thesis in Department of Civil Engineering, University of Queensland. Brisbane, Australia. 191pp. [2] Guza, R.T. and E.B. Thornton (1981): Wave set-up on a natural beach, Journal of Geophysical Research, Vol.86, No.C5, pp. 4133-4137. [3] Hanslow, D.J. and P. Nielsen (1992) : Wave setup on beaches and in river entrances, Proceedings of 23rd International Conference on Coastal Engineering, pp. 240-252. [4] Hanslow, D.J., P. Nielsen and K. Hibbert (1996) : Wave setup at river entrance, Proceedings of 25th International Conference on Coastal Engineering, pp. 2244-2257. [5] Oshiyama S., Lee H., Tanaka H. (2001): Fluctuation
characteristics of water level in medium- and smallscale river mouths. Proceedings of Coastal Engineering, JSCE, Vol. 48, pp. 411-415. (in Japanese) [6] Tanaka, H., Lee, H.S. and Furumichi, K. (2003): Influence of morphological change on water level rise at the Shiribetsu River mouth, Journal of Hydroscience and Hydraulic Engineering, Vol.21, No.1, pp.71-78. [7] Tanaka, H., Nagabayashi, H. and Yamauchi, K. (2000): Observation of wave set-up height in a river mouth, Proceedings of 27th International Conference on Coastal Engineering, pp.3458-3471.