Extended Abstract 11 th ISE 2016, Melbourne, Australia HYDRODYNAMIC EFFECTS AND ROLE OF RIVER MORPHOLOGY IN CONSERVATION OF COBBLE BAR VEGETATION
KAYO ASAMI University of Hyogo, Yayoigaoka 6, Sanda, Hyogo, 669-1546, Japan
AKIHIKO NAKAYAMA Department of Environmental Engineering, Universiti Tunku Abdul Rahman, Kampar, Perak 31900, Malaysia
TAKESHI KAWATANI Construction Engineering Res. Inst., Tsurukabuto 1-3-10, Nada Kobe, 657-0011, Japan
In order to conserve cobble bar vegetation, it is necessary to understand he hydrodynamic effects and river morphology surrounding such vegetation. We conducted two-dimensional flow analysis and large-eddy simulation for a middle reach of the Ibogawa River, Western Japan. The prominent feature of the river reach studied herein is a cobble bar, in which a refugium exists during large floods. This cobble bar has certain morphological characteristics: a strong bend, ridge at the center of the bar, shelving slope at the upstream end, and steeply sloping bar front line. These characteristics prevent the fast main flow and strong secondary flow running over the downstream tip. Therefore, the establishment of refugia, where a particular endemic species survives during large floods, depends on these three-dimensional features. As these morphological characteristics are identical to the bed forms of bars, the conservation of cobble bar vegetation might require the continuous existence of bars with typical bed forms.
1 INTRODUCTION On the dry and low-nutrient areas of cobble and/or gravel bars, sparse perennial communities are often established creating habitats for various endemic flora and fauna. Considering conservation measures is difficult, specifically assessing the disturbance regime. The vegetation is maintained by moderate disturbances from floods. Hence, the occurrence of floods is a necessary condition; conversely, floods that are too large and strong can wipe out the entire population. Previous studies of conservation of sparse perennial vegetation have indicated that a decrease in the frequency and strength of flooding disturbances leads to a decline in the community owing to the progress of vegetation succession [1]. However, none of those studies addressed how plant species on bars can survive large floods. To address this problem, we hypothesized the existence of “large flood event refugia” where the hydrodynamic forces do not exceed a critical value in large floods. This study defines the habitat where Anaphalis margaritacea sub-species yedoensis (Franch. et Savat.) survives even strong disturbances from a high-severity, low-probability flood event as “large flood event refugia.” The plant species is one of the main component species of the cobbled bar vegetation. It is distributed throughout Japan, but designated as an endangered species in one quarter of prefectures [2]. Therefore, it can be regarded as a sensitive indicator of the health and condition of the cobble-bar ecosystem. In our previous study [2], we showed that “large flood event refugia” for the sparse perennial community, called cobble bar vegetation in this study, exist where the shear stress does not exceed a threshold level, though discharge increased using one-dimensional hydraulic analysis. Moreover, we demonstrated how “large flood event refugia” were formed at the downstream tip of the cobble bar, where the shear stress did not exceed the critical value. However, the survey site for that study was in a place where the river channel has large bends. The ability of one- or two-dimensional analysis to deal with three-dimensional flow features and the shear stress distribution around a strong bend is limited. In this study, the changes in bed shear stress distribution with increasing discharge and flow velocity distribution for a large discharge are investigated using two-dimensional analysis. Subsequently, the three- dimensional structure of flows on the cobble bars is simulated using three-dimensional analysis. Finally, we describe the hydrodynamic and morphologic effects on the conservation of “large flood event refugia”. 2 MATERIALS AND METHODS
2.1 Study area The study area is located along the Ibogawa River in Hyogo Prefecture, Japan. The river has a river basin area of 810 km 2, and the length of the main region is approximately 70 km. The study area is in a river section located 24–27 km from the river mouth and includes two cobbled bars (Figure 1). The bed slope in this area is 1/200– 1/400. The average annual maximum discharge over the past 40 years recorded at Yamazaki Second Gauging Station, located upstream of the study area at 29.5 km, is approximately 750 m 3/s. A discharge of 1515 m 3/s was recorded in 2004, the fourth largest in history, and 1595 m 3/s in 2009. The probability of the occurrence of the 2004 and 2009 floods was once per fifty or sixty years. When the discharge rate surpasses 1500 m 3/s, the river channel becomes almost full. The 2009 flood is referred to as the large flood in this study. Investigation sites have been surveyed at two bars in the study area for ten years (Figure 1). The upper bar is named site IK and the lower site IH. Site IH has the prominent features of bed bathymetry, a riffle in the center of the bar, a bar front line with a steep slope, and a shortcut channel during medium to large floods. Site IK does not have these features. The cobble bar around the cross-section IK-d (Figure 1, center) had been removed by river bed excavation but subsequently formed gradually over 30 years.
2.2 Field conditions To evaluate the vegetation dynamics, the main component species of cobble bar vegetation were counted and a vegetation map was created over a ten-year period. The conditions before and after the large flood were reported in a previous study [2]. After the large flood, the number of species significantly decreased. For a particular perennial plant that is regarded as a sensitive indicator of the health of the cobble bar vegetation, Anaphalis margaritacea (L.) Benth. et Hook. fil. subsp. yedoensis (Franch. et Savat.) Kitam, a few thousand individuals existed before the large flood. After the large flood, more than 100 individuals survived at site IH, but only 15 individuals at site IK. Figure 1 shows the potential habitat, i.e., the distribution region of the cobble bar vegetation, before the large flood and location of the individuals of A. margaritacea that survived the large flood.
The particle size distribution is similar at both sites. The largest particle diameter (d 100) is 30–40 cm, and the 2 mean diameter (d mean ) is 7–10 cm. The critical shear stress at each size is estimated to be 130-140 N/m for d 100 2 and 55–85 N/m for d mean [2].
2.3 Hydraulic analysis We carried out two numerical simulations: horizontal two-dimensional flow analysis and Large Eddy simulation (LES). In the two-dimensional flow analysis, steady-state calculations were performed for relatively stable discharge rates of 300, 600, 900, 1100, 1300, and 1600 m 3/s. In either case, the riverbed degree of roughness was homogeneous and a constant value of 0.035 was used for the Manning's roughness coefficient. In the two- dimensional flow analysis, bed shear stress was calculated as an index of disturbance intensity. The computation is done using the LES code KULES, which is explained in detail in [3, 4]. It is based on the finite difference
Figure 1. The study area (left), site IK (middle), and site IH (right). method constructed on a rectangular grid and can be used to calculate flows over arbitrary topography with rapidly changing free surface and wetted area. The sub-grid turbulence is modeled by the Smagorinsky eddy viscosity but the stress on the solid surface is determined by the wall-law for rough surfaces from the velocity near the bed. Arbitrary distributions of bed roughness and resistances due to obstacles within the flow such as riparian plants and vegetation on the bars can be represented. The computation is done in time advancing manner removed for clarity. In the present computation, the initial low flow conditions of the flow rate 20m 3/s are first constructed and the flow rate is increased gradually to 1300m 3/s. The resolution of the grid used is 2m horizontal and 0.15m vertical, so any obstacles, including sudden drop and large boulders of sizes larger than these, are represented by the boundary geometry remove for clarity.
3 RESULTS AND DISCUSSION
3.1 Change of shear stress as discharge increases Figure 3 shows the relationship between the discharge and the bed shear stress using two-dimensional analysis . At site IK, the shear stress obviously increases with discharge in every plot. Even in the downstream plot, where the rate of increase is the lowest of the three plots at site IK, the shear stress for a discharge of 1600 m 3/s exceeds 2 the critical shear stress of d mean (56 N/m ). At site IH, the shear stress also increases with discharge; however, in the downstream plot, the shear stress remains low after the discharge exceeds 1100 m 3/s. The shear stress for a 3 2 discharge of 1600 m /s is less than half the intensity of the critical shear stress of d mean (84 N/m ).
3.2 Mean velocity distribution Figure 3 shows the distributions of the depth-averaged velocity in the case of 1600 m 3/s using two-dimensional analysis. In site IK, every distribution regardless of cross section is comparatively similar in shape. In contrast, in site IH, the flow on the bar separates to the right and left of the ridge creating a flow directed into the main flow and a flow toward the secondary channel. Thus, the flow discharge above the downstream tip of the cobble bar becomes lower than the discharge above the other area of the bar, and the velocity is smaller than that of the main flow. These results suggest that the shear stress, which is proportional to the square of the velocity, maintains a low value at the downstream tip of the bar. In the area, approximately 90% individuals of A. margaritacea survived the large flood, that is 109, was confirmed. That is because the shear stress is low.
Figure 2. Shear stress at different discharges on the potential habitats above cross sections at site IK (left) and site IH (right).
Figure 3. Velocity distribution obtained using two-dimensional analysis around site IK (left) and site IH (right). Figure 4. Velocity distribution obtained using two-dimensional analysis around site IK (left) and site IH (right). Black arrows show the potential habitat of the cobble-bar vegetation.
3.3 Velocity distribution of the vertical direction component Figure 4 shows the LES results of the velocity magnitude and secondary flow in five representative cross sections for a discharge of 1300 m 3/s. In site IK, the three results regardless of cross-section comparatively show the same tendency: the main flow runs straight on the bar, including the potential habitat of cobble bar vegetation, with a weak secondary flow. In contrast, in the main flow, the flow field in site IH differs from that in site IK as it is very fast and the secondary flow is clear. However, only above the flat ground of the downstream tip, where many individuals of A. margaritacea survived, the flow velocity is much smaller.
4 CONCLUSION The cobble bar where many individuals of A. margaritacea survived during the large floods possesses some morphological characteristics: a ridge at the center of the bar, a shelving slope upstream, and a bar front with a slope as steep as the angle repose. These characteristics are common bed forms of all cobble and/or gravel bars [5]. Furthermore, the bar is established at a point bar with a strong bend. The results show that these characteristics prevent the fast main flow and strong secondary flow running over the downstream tip and that the establishment of “large flood event refugia” during large flood is dependent on these three-dimensional features. “Large flood event refugia” might be established on fully developed bars. However, the spatial and temporal scales of the succession of perennial plant communities, such as cobble bar vegetation, are far smaller than those of the processes of bar formations. This finding suggests that the conservation of cobble bar vegetation might require the continuous existence of bars with typical bed forms. This study has qualitatively revealed the relationship between the bed forms of bars and “large flood event refugia” of cobble bar vegetation. Further studies are required to find the index based on the mechanisms of maintaining cobble bars.
REFERENCES
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