Koichi Suzuki¹, Akihiro Kadota¹ and Yoshifumi Fujimori¹

¹Department of Civil and Environmental Eng., Ehime University,

3 Bunkyo-cho, Matsuyama, Ehime,

790-8577, Japan, Tel./Fax: +81 89 927 9831

E-mails: ksuzuki@dpc.ehime-u.ac.jp

akado@dpc.ehime-u.ac.jp

Abstract: Morphological change of large sand bars that was caused by several floods and resulted in the change in vegetation (reed) area in the estuary of a small river named Shigenobu in Matsuyama city in Japan was surveyed. Also, considering with the relation between the vegetation growth and the water qualities near vegetation area, the measurement of salt, nitrogen and phosphorus concentrations around the vegetation area were surveyed by scooping up the ground water near the vegetation area. And grain distribution of bed material on the sand bars were measured. It is proved that even a small morphological change in the estuary causes a large change of vegetation area because of the salt water effect at high tides; The vegetation (reed) area on sand bars can be formed when the salt concentration of the ground water at the reed root is less than about 1.24 percent, which is achieved when the area is submerged only at a tide higher than T.P. 1m and where the mean high and low tides are 1.50m and –0.5m, respectively, and the area is covered by silt and fine sand and it was considered that such fine sand protects the vegetation area from salt water. The vegetation area influences nitrogen and phosphorus concentrations. It was considered that the vegetation has its own ability to remove the pollutant.

 

Keywords: morphological change, large sand bar, vegetation (reed area), salt, nitrogen and phosphorus concentrations

Large sand bars in the estuary of the Shigenobu River in Matsuyama city, Japan, are important relay bases for migratory birds. The watershed and the length of the river are 445km² and 36km, respectively (Fig.1). The vegetation on the bars as well as bare sand field in the tidal marsh should be kept for the migratory birds. Six large floods occurred successively in a short duration from June to September in 1993. The some parts of large sand bars and vegetation were scoured, which had been stable and covered with dense vegetation for more than 20 years. After the floods the large bars in the estuary became braided and some low water ways were formed across the bars. This kind of deformation of the bars with vegetation loss may have a negative influence on their function as relay bases for migratory birds.

In this study, the morphological changes of the large sand bars and the vegetation area in the estuary of the Shigenobu-river were observed, which were caused by the six relatively large floods in 1993. Also the measurement of salt, nitrogen (T-N) and phosphorus (PO₄^3-P) concentrations was conducted by scooping up the ground water around the vegetation area on the large sand bars. Then, the relations between the vegetation area and the salt, and the other water qualities such as nitrogen and phosphorus were discussed. Also, the grain distribution of bed material near the vegetation area on the sand bars was measured in order to discuss the permeation property of salt water.

Riverbed elevation and vegetation area both in 1991 and 1995 are shown in Figs. 2 (a) and (b), respectively (Suzuki et al., 1998). Large vegetation area spreads out in the sand bar from 0.4 to 0.8km in 1991 (before the floods), and relatively small vegetation area exists from 0.15 to 0.3km. In 1995 (after the floods), it is seen that low water way due to the flood in 1993 becomes vivid, and that influence is large especially on the left bank side. Moreover, the formation of low water way that crosses the inside of the sand bar was occurred and also large vegetation area was divided into parts so that the relatively small vegetation area is distributed from 0.2 to 0.5km. As shown in the Figs. 2(a) and (b), it is understood that the vegetation is distributed in the area over the T.P.1m both before and after the floods although large variations of bed elevation and vegetation area can be seen before and after the floods. This relates to a change in river water level, and the river water is affected by the repetition of low and high tides and it gets into the inside of sandbar around T.P.1.0-1.5m at the time of high tide. Also in the field observation, it was found that the invading limit of river water almost corresponded to that of vegetation-growth area.

Temporary change of groundwater (or sea) level, salt, nitrogen and phosphorus concentrations was observed at 4 points scattered along the transverse direction as shown in Fig.3 at the section 600m from the river mouth. Observation point 1 is on the boundary of the vegetation area, observation point 2 is in the vegetation area and point 3 is the bare marsh ithout vegetation. In addition, the observation point 4 is in the low water channel, and used for the observation of river water level. In the left side of point 1, there is a low water way where the river water gets into at the time of high tide. Also, the point 3 is the bare marsh in which the submerged area is variable depending on the tide level.

Fig.4 shows some results of the ground/river water level in T.P. at each observation point. Observation was conducted in 3 periods, i.e., December 1 (neap tide), December 9 (spring tide) and December 18-19 (between spring and neap tides) in 1999. From the figures of ‘neap tide’ and ‘between spring and neap tides’, it can be seen that the variation of the groundwater level inside sand bar has a time lag of 1 - 2 hours from the variation of river water level. Such time lag may be caused in the process that salt water infiltrates horizontally into the sand bar. The river water level varies largely due to the influence of the tide level, but the variation of groundwater level is small. As for the ‘spring tide’, the rapid rise of the groundwater level is seen with the river water level, and the groundwater level is about equal to the river water level at the time of the high tide. Such phenomenon occurs because the salt water infiltrates vertically from the surface of the earth as well. The range of vegetation growth seems to be restricted depending on the submergence of salt water in such the spring-high tide.

The time variation of salt concentration for each observation point is shown in the Fig. 5. In general, vegetation growth declines with salt concentration of 1.2%Cl, and the limit value of growth is 1.35%Cl, but the limit value becomes 0.76% when submerged in the tidal marsh that repeats invasions and recessions with tide period (Ranvell 1964). In the case of Shigenobu River, effect of salt water from the sea is large in the estuary because the variation of salt concentration is large in the river water and it becomes large with the tide increasing. On the contrary, salt concentration of groundwater is small, especially for point 2, i.e., about 0.85%Cl and always constant. It is considered that surface layer near the vegetation area is covered by silt and sand with about 30cm thick so that the salt water cannot get into the ground. The salt concentration of spring tide shows complicated variations because the points 1 and 3 were completely submerged. The average salt concentrations were 1.24%, 0.87%, 1.30% and 2.06% for points 1,2,3 and 4, respectively, and it becomes larger as much as to be further than point 2. Therefore, it can be concluded that the average salt concentration of 1.24%Cl in the boundary (point 1) between vegetation area and low water way, is the growth criterion of vegetation in Shigenobu River. These results were obtained when it hardly rains until the day before the observation period. However, in the recent observation in Sep. 2000, salt concentrations decrease for every points due to the influence of the river water when it rained on the previous days as shown in Fig.6.

The measurement of the grain distribution was conducted by collecting the sand and silt on the surface around the above observation points 1-4. The accumulation curves of grain size for each point is shown in the Fig. 7. As for the soil at point 2, it is understood more than other points that it is composed of small grain diameter. It is presumed that the sand and silt of small grain diameter deposits around the vegetation because the vegetation restrains the velocity when river water is washed away through the vegetation by the small scale of flood and so on. Furthermore, the environment of vegetation growth becomes better by such the deposition of smaller grain. In point 4, the larger grains deposits because the smaller one has been washed away with the influence of the river flow. The points 3 and 4 are composed of larger grains so that the salt water is easy to get into from the earth, and then the no-good environment is formed for vegetation growth due to high frequency of submergence. Also, the point 1 is hard to grow for the vegetation although the point 1 is composed of smaller grains than points 3 and 4, because it is completely submerged in spring-high tide.

The average phosphorus and nitrogen concentrations for one day in each observation point are shown in Fig.8. The phosphorus concentration in point 2 shows low value in comparison with surrounding it. Especially on December 9 and 18, the amount of decrease is large from points 2 to 3, and from points 1 to 2. The vegetation is considered to be greatly involved in the decrease of phosphorus concentration, and the phenomenon is because vegetation (reed) has phosphorus removal ability. The absorption of phosphorus by the subterranean stem of vegetation is considered as a cause. As for the average nitrogen concentration, it is understood that there are relations that are contrary to the above phosphorus concentration. The nitrogen concentration in point 2 is larger than the other points as well, and it is considered that vegetation influences this phenomenon largely. A factor is considered to be dissolution due to decomposition of blighted vegetation into groundwater within gaps. But, further considerations are necessary about main ingredients and behavior of nitrogen and so on.

In this study, the morphological changes of the large sand bars and the vegetation area in the estuary of the Shigenobu-river in Matsuyama city in Japan were observed. Also the measurement of salt, nitrogen and phosphorus concentrations was conducted. The relations between the vegetation area and the salt, and the other water qualities were discussed. Also, the grain distribution of bed material near the vegetation area on the sand bars was measured and discussed.

Suzuki, K., Kadota, A. and Kok, L. O. (1998), Morphological Change of Tidal Marsh by Several Floods and Change of Vegetation Area after Floods, Proc. of the 11th congress of APD IAHR, Yogyakarta, pp.405-412.

D. S. Ranvell, E. F. C. Bird, J. C. E. Hubbard and R. E. Stebbings, (1964), Journal of Ecology, vol.52, pp.627-642.