Doboku Gakkai Ronbunshuu B Vol.62 No.3, 238-250, 2006. 8
EXPANSION MECHANISM OF SALICACEOUS SPECIES FOREST AND ITS MANAGEMENT TECHNIQUE IN THE ASAHI RIVER
Satoshi WATANABE1, Shiro MAENO2, Hironao SHIRAI3 and Masaki FUJIWARA4
1 Member of JSCE, Dept .Environmental Planning, WESCO Co., Ltd (2-5-35, Shimadahonmachi, Okayama, 700-0033, JAPAN) [email protected] 2 Member of JSCE, Assoc. Professor, Dept. Environmental and Civil Engineering, Okayama University (3-1-1, Tsushima-Naka, Okayama 700-8530, JAPAN) [email protected] 3 Member of JSCE, Dept .Environmental Planning, WESCO Co., Ltd (2-5-35, Shimadahonmachi, Okayama, 700-0033, JAPAN) h-shirai @wesco.co.jp 4 Member of JSCE, Dept .Environmental Planning, WESCO Co., Ltd (2-5-35, Shimadahonmachi, Okayama, 700-0033, JAPAN) masaki.fujiwara @wesco.co.jp
The expansion mechanism of Salicaceous species forest in the middle reaches of the Asahi River was investigated by ecological field surveys and experiments. The results of the study show that Salicaceous species take root on a damp riverbed, wither if the riverbed immediately dries. A bank protection effect by Salicaceous species contributes to the stability and the expansion of the bar, and the direction of flood flow and the existence of a dammed up area influence the habitat of Salicaceous species. The control technology and the spatial layout of Salicaceous species forest were proposed based on the obtained in- clusive scenario for the expansion of Salicaceous species
Key Words : thick growth of trees, Salicaceous species, sandbar, shoot establishment
1. INTRODUCTION long-term viewpoint is not established yet. Since the River law amendment in 1997, pres- Many research works with relation to the devel- ervation of the river environment is emphasized and opment of natural environment in the river were car- its essential preservation method is discussed ac- ried out considering the dynamic behavior of bed tively. An original purpose of the amendment is a loads and sandbars under flood flow. (for example, harmony of the flood control, water-utilization and 1㧕 2㧕 3㧕 Lee et al. , Fujita et al. , Fukuoka et al. , Hattori et environment, then finding a planning and control 4㧕 5㧕 al. , Teramoto and Tsujimoto 㧕. These works method as the optimum solution based on this back- clarified the required environmental conditions of ground. In order to solve the real problem of the river workings of nature on the development in river control mentioned above, it is required to grasp the inherent nature of the river. Even though such the river system and vegetation’s life history of each a river system can be understood to some extent, river. Furthermore, it is necessary to develop a some questions and confusion arise in an actual river planning and control technique to a practical level. management. For example, although we under- This study focused on the thick growth of trees on stand that free rampageous river flow is the most the sandbar in the river, which is one of the most important factor for growth in the river vegetation, it important environmental problems for many rivers. is not realistic to abandon the flood control methods The middle reach of the Asahi River, chosen as the such as banks, revetments and dams. With respect subject of the study is no exception, that is, Salica- to the problem of forest in the river, an effective ceous species grew rapidly there in the last 30 years. method to eliminate or control them from a Unlike such an introduced spieces as Robinia pseu-
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Kurare weir Goudou weir Otoide weir Mino station Santei weir Kagoide weir Asahi River A
Shimizu weir Myoujou weir Nakaide weir Hyakken River(diversion channel) Restored rubble bar(Fig.14) Fig. 1 Research area of the Asahi river (A : shooting location of Fig. 14)
㪌㪃㪇㪇㪇 㪋㪃㪌㪇㪇 㪋㪃㪇㪇㪇 CPPWCNUECNGHNQQF U 㪊㪃㪌㪇㪇 OU 㪊㪃㪇㪇㪇 㪉㪃㪌㪇㪇 㪉㪃㪇㪇㪇 㪈㪃㪌㪇㪇 FKUEJCTIG O 㪈㪃㪇㪇㪇 㪌㪇㪇 㪇
FCVG Fig. 2 The flood history after 1957 (more than 500 m3/s) doacacia, Salicaceous species can be one of the Table 1 Main floods of Asahi River necessary factors of the typical feature of the river. ranking date discharge (m3/s) cause However, too much growth of Salicaceous species 1 1934. 9.21 6,000 typhoon causes a reduction of the safety degree of flood, de- 2 1945. 9.18 4,800 typhoon terioration of ecological diversity and typical land- 3 1998.10.18 4,405 typhoon scape of the river. Therefore, it is urgently required seasonal rain to take appropriate measures to prevent the growth 4 1972. 7.12 3,700 front seasonal rain of forest. 5 1971. 7. 1 3,180 front In this study, we investigated the area where Salicaceous species grows in such kind of hydraulic conditions and how they proliferate there. Consid- and the lower reaches forming Okayama Plain, be- ering flood control, ecology and landscape preserva- fore emptying out into Kojima Bay. The river ba- tion points, we examined the management tech- sin is mostly mountainous. Only 14.7 % of the niques of anti-development of thick growth of Sali- basin is plains, most of it being on Okayama Plain. caceous species and how appropriate and effective The average rainfall in the river basin is about 1,450 countermeasures are if undertaken and from what mm/year. Fig. 1 shows the lower reaches of the kind of place and how to start. Asahi River (10.8-17.4 km from the river mouth). Mean bed slope is 1/670, annual mean maximum discharge is 1,400 m3/s, about 300 m in width and 2. OUTLINE OF INVESTIGATION FIELD representative grain-diameter of the bed is 40㨪70 mm. The existence of a large meandering part and (1) Outline of the Asahi River many crossing works such as weirs controls the The Ashahi River, one of the three largest rivers flood flow and ordinary water flow. The overall in Okayama Prefecture located in the western part of length of dammed pools by weirs is 3.3 km, corre- JAPAN, originates from Mt. Asanabe Washigasen sponding to half of the research site. (1,081 m) in the Chugoku Mountain Range. It is 142 km long and has a catchment of 1,800 km2. (2) Flood history The river basin extends from north to south, with the Table 1 shows the main floods of the Asahi River. upper and middle reaches of the river cutting Big floods are due to typhoons or seasonal rain through the Chugoku Moutains and Kibi Highland fronts. Fig. 2 shows the flood (more than 500
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1947
1974
1991 Fig. 3 Transition of Salicaceous species in the Asahi river
㪈㪋㪇 㪚㫆㫍㪼㫉㪼㪻㩷㪹㫐㩷㫋㫉㪼㪼㫊 㪈㪉㪇 㪪㪸㫅㪻㩷㪹㪸㫉 ) 㪈㪇㪇 ha The other forest 㪏㪇 ( 㪍㪇 Salicaceous
Area species forest
㪘㫉㪼㪸㩷㩿㪿㪸㪀 㪋㪇 㪉㪇 㪇 2001 year 㪈㪐㪋㪇 㪈㪐㪌㪇 㪈㪐㪍㪇 㪈㪐㪎㪇 㪈㪐㪏㪇 㪈㪐㪐㪇 㪉㪇㪇㪇 㫐㪼㪸㫉 1991 1996 Fig. 4 Transition of the area covered by trees and sand bar Fig. 5 Transition of the area covered by trees since Showa era
ceous species and most of the other area was cov- m3/s) history after 1957. Floods whose discharge ered by a bamboo forest. Since then, Celtis sinen- was more than 2,000 m3/s occurred frequently from sis-Aphananthe aspera forest and Ulmus parvifolia 1957 to 1981, whereas no such floods occurred from forest and so on have grown there. The present 1981 to 1994. The third largest flood since the state is that more than 50% of the sandbar in the Showa period occurred in 1998. river is covered by forest. Most of Salicaceous species in the river was composed of two different (3) Transition of growth of forest species, Salix chaenomeloides and Salix eriocarpa. As shown in Fig. 3 thick growth of trees rapidly These two Salicaceous species are the dominant progressed after 1980’s. Although the area of sand species there. bar has not changed so much since 1970, the area of forest in the river was increased until now (see Fig. 4). These areas were measured by GIS technique 3. EXPANSION MECHANISM OF SALI- using aerial photographs in 1949 and 1974. Vege- CACEOUS SPECIES tation maps of river front census were used for the data in 1991, 1996 and 2001. Despite the third (1) Ecesis of Salicaceous species largest flood in 1998 washing out some trees, the There are two types of the initial ecesis of Salica- area of trees slightly increased from 1996 to 2001. ceous species, one is seed propagation and the other Discharge of the flood was 4,405 m3/s and its occur- is vegetative propagation. In case of seed propaga- rence probability is once per 50 years. tion, light seeds with a small pappus are distributed Fig. 5 shows the occupied area of Salicaceous by wind and water flow. These seeds can grow species in total area of forest since 1991. In 1991, under a damp bed condition with a stable water level more than 80% of the area was abundant in Salica- state because seeds and shoots of Salicaceous spe-
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Table 2 Experimental conditions Case type of planter installation condition 1 floating 2 submerged and buried 3 submerged and inserted water supplied planter 4 driven to the shoreline 5 inserted in damp bed 6 put on the damp bed surface 7 buried in the dry bed 8 dry planter inserted in the dry bed 9 put on the dry bed surface
Salix Water supplied chaenomeloid planter
Salix eriocarpa Dry planter Fig. 6 Initial conditions for each branch (Numbers in the figure correspond to each case in Table 1)
FGCF
㧔5CNKZEJCGPQOGNQKFGU㧕㧔5CNKZGTKQECTRC㧕 Fig. 7 Experimental results(6 months later) cies are extremely weak in dry conditions6). research works focusing on an initial ecesis process Some research works7), 8), 9) were carried out fo- of Salicaceous species by vegetative propagation cusisng on a relative elevation of sand bars above were not found. the water level with respect to the places where Therefore, a propagation experiment was carried Salicaceous species took root. It was clarified that out by using branches of Salicaceous species. the ececis area strongly depends on the relative ele- Considering branches may be driven to various vation above the water level and Salicaceous species places, nine different experimental conditions were developed mainly in the area where the relative ele- introduced as shown in Table 2 and Fig. 6. Two vation is small. different annual, biennial and triennial branches of We have studied for an initial ecesis of Salica- Salix chaenomeloides and Salix eriocarpa cut into ceous species at the research site for the last three 10 cm lengths, which are the dominant species in the years, all reproduced Salicaceous species were field, were used in each experiment. Planters were vegetative reproductions from branches that had installed out of doors and experiments started at the drifted there. The reproduced areas were also end of April. Water level was held constant in the damped bars at the edge of the water. However, case of the planter filled with water. In the case of
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Salicaceous species over 25 years old 5㨪25 years old under 5 years old grey zone : present sand bar Edge of sandbar formed by flood in 1972 (Salicaceous species over 25 years old)
Edge of sandbar formed by flood in 1998 Edge of sandbar formed by flood in 1980 (Salicaceous species under 5 years old) (Salicaceous species 5 to 25 years old) Fig. 8 Formation of sand bar by flood and expansion process of Salicaceous species
15 Km from the river mouth sand bar ) 3 water-course sand bar (in 2000) (in 2000) (in 1968) (in 1968)
water level (numerical result) sediment (m 10.68m (in 1968) 1978 1989 2000 10.21m (in 2000) 1968 1988 1996
water-course ) Bed level (m) 3
aggradation shear stress decreased
degradation 2000 sediment (m shear stress increased 1968 1978 1989 2000 Distance (m) 1968 1988 1996 Fig. 9 Change of Cross section between 1968 and 2000 (15 km from the river mouth)
the dry planter, water supply depended only on rain- shown in ten days from the start of the experiments. fall. Obtained results are: Fig.7 shows the experimental results in autumn a) Salix chaenomeloides which are washed ashore which is at the end of growing period. In the case can grow only on the edge of the sandbar. of the water supplied planter, Case 4 and 5, branches b) Salix eriocarpa can grow not only on the edge of established and grew, but in Case 2 and 6 they did the sandbar but also in the shallow water region not establish. In Case 1 and 3, only Salix erio- where the shoot can grow to reach the water sur- carpa grew. In the case of the dry planter, Case 7, face. 8 and 9, all shoots could not grow. With respect to c) Branches driven to the dry bed can not grow. Salix eriocarpa, which grew favorably, leaves did d) In a damp river bed caused by submergence or not appear unless the grown shoot had reached the rain fall, a shoot can grow temporarily. How- water surface in Case 1 and 3. In Case 7, although ever, if the bed gets dry later, the shoot withers. Salix eriocarpa germinated and grew temporarily The above mentioned experimental results ex- during the rainy season, it withered away under the plain well that distributed on the edge of the sandbar following dry weather conditions. Except for Case in the Asahi River. 7, tendency of the above mentioned results were
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1947 ̪ Shrubby Salicaceous species in 1947
1961
1974
1995
2000
Distribution of Salicaceous species The area that Salicaceous species washed out or destroyed by flood in 1998 Fig. 10 Transition of Salicaceous species
(2) Relationship between Ecesis of Salicaceous bility of sandbar and the growth of Salicaceous spe- species and stability of sandbar cies was recognized on the site observation. In the previous section, it is clarified that Salica- Fig. 8 shows the developing process of Salica- ceous species easily grew on the edge of the sandbar. ceous species. The age of trees in the figure were But a driven branch will be washed away if the established by the annual growth rings of a stump in sandbar is scoured under flood flow. Therefore, a 2004. This figure proves that flood flow washed necessary condition for the growth of the Salicaceous out seeds or branches to the edge of sandbar and the species is the stability of the sandbar until the driven developed Salicaceous species expands the sandbar branches develop enough root against scouring. in one direction without retreating from the shore On the other hand, the driven branches at the edge line. And then again, the driven branches devel- of the sandbar develop rootlets like a mat-shape, oped on the expanded edge of the sandbar. which considerably prevents the sandbar from The stabilization of the sand bar by vegetation scouring. That is, the symbiosis between the sta- enhanced progress of the scouring at river course
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(a) with the effect of vegetation
Shear stress (b) without the effect of vegetation
Fig.11 Shear stress distribution with or without vegetation
2m/s 2m/s
(a) with the effect of vegetation (b) without the effect of vegetation
Fig.12 velocity distribution on the bar with or without vegetation
and deposition at sand bar in the Asahi River. Fig. (3) Destruction of Salicaceous species by flood 9 shows the cross sections in 1968 and 2000. The flow water levels in the figure show the water level ob- Fig. 10 shows the change of the Salicaceous spe- tained by the normal flow analysis under an annual cies ditribution in the Asahi River. In the subject scale flood 1,400m3/s. Manning’s roughness used area, not only Salicaceous species but also the sand- in the the numerical analysis was 0.035. This fig- bar in the river disappeared temporarily because of ure shows that the bed level has decreased by about large-scale gravel digging in the 1960’s. After that, 2 m at the water-course in the last 30 years, whereas the occupied area of Salicaceous species expands it has increased by about 1 m at the sand bar. Ac- together with the expansion of the sandbar. As cording to the bed level variation, non-dimensional mentioned before, the third rank huge flood occurred shear stress at the water-course decreased by in 1998 destroying some of Salicaceous species. 20-40 % under the mean annual scale flood dis- The destroyed area corresponds to the river bed cov- charge, whereas it increased by 20-40 % at the sand ered by gravel in the photo of 1947, where develop- bar. The decrease of sediment supply from the up- ment of vegetation is restricted due to the dynamic stream due to the Asahi Dam built in 1954 and the behavior of the river bed under flood flow. The gravel digging in the 1960’s also enhanced a lower- undestroyed area corresponds to the area where ing of the bed of the river course. Above men- Salicaceous species developed in 1947. These re- tioned tendency of the bed profile is almost the same sults show that the area where Salicaceous species as in other sections in the research site. grow well is considerably controlled by the direction
244 Doboku Gakkai Ronbunshuu B Vol.62 No.3, 238-250, 2006. 8
C $GPFGF D +PENKPGF E 7RRGTRCTV F 5EQWTCTQWPFTQQV G 7RTQQVGF FGUVTWEVKQP CPFKPENKPGF FGUVTWEVKQP
Fig. 13 Destruction forms of trees by the heavy flood in 199811)
&COCIGQH /CKPVGPCPEGYQTM %QRRKEG WRRGTRCTV TGIGPGTCVKQP Fig. 15 Sprouts after an upper part destruction
Fig. 14 State of Salicaceous trees during flood in 1998 struction and disturbance caused by flood flow. (From point A in Fig. 1) Salicaceous species in the Asahi River was not uprooted during the heavy flood in 1998 (see Fig. 13, (a)-(d)). That is, as shown in Fig. 14, we may of the meandering feature of the river and crossing say that uprooted destruction of Salicaceous species structures such as weirs. rarely occurs under flood attack. In most cases, On the other hand, the existence of grown Sali- part of the trees remains even if the upper part of caceous species has much influence on the flood them is destroyed. And then, as shown in Fig. 15 flow. Fig. 11 shows shear stress distribution ob- the remaining part rapidly sprouts. Therefore, it tained by the numerical simulation of the flood flow can be stated that total destruction of Salicaceous in 1998. Numerical procedure is the same as species hardly occurs. Maeno et al10). In the numerical analysis, not only the effect of vegetation but also its inclined degree (4) Regeneration of Salicaceous species was included. Details of vegetation’s treatment are Two investigations were carried out on the re- described in the paper10). Fig. 11(a) shows the re- generation speed from the underground part of Sali- sults including the effect of vegetation in the river, caceous species which remained after destruction of whereas Fig. 11(b) shows the results disregarding the above-ground under attack of flood or tree cut- the effect of vegetation. Velocity distributions ting management for the improvement of the flood surrounded by the dotted line in Fig. 11 are shown control. One is based on the annual ring analysis in Fig. 12(a) and (b). Generally speaking, shear using the cut trees. The other one is based on the stress becomes large due to a strong current on the measurement of the growth rate of the germination sandbar if the effect of vegetation in the river is branch which regenerated from the felled stock. eliminated. For example, as shown in Fig. 12, the Fig. 16 shows a growth rate of Salicaceous species area surrounded by a white solid line in Fig. 11 by annual ring analysis. In the figure, trees with a (sand bar of downstream area of the Shimizu weir), height of more than 10 m were considered. Gener- flood flow is deflected to the right bank side in the ally, Salicaceous species grew about 1.0 m per year case of assuming an existence of vegetation on the in the first ten years. After that they grew about sand bar, whereas strong shear stress is acting there 0.5 m per year. Gregarious trees generally grew in the case of assuming no vegetation because the faster than solitary trees. The struggle for survival flood flows across on the sand bar. At present, is more intense for gregarious trees because the ad- high trees of Salicaceous species developed on the jacent distance of each stump is shorter than for sandbar just downstream of the Shimizu weir. On solitary trees. All Salicaceous species investigated the other hand, on the right bank side where the here were Salix chaenomeloides and their critical sandbar has been controlled as a grass sampling height in the research field was 12 to 14 m. ground, the development of the vegetation is sup- Fig. 17 shows the growth speed of shoot just after pressed there. It means that Salicaceous species the tree cutting. Germination branches just after developed in groups has an effect to weaken de- the trimming in March grew about 97 cm and the
245 Doboku Gakkai Ronbunshuu B Vol.62 No.3, 238-250, 2006. 8
Gregarious trees
Hight of trees (m) Solitary trees (Age) Fig. 16 Growth rate of Salicaceous species by annual ring analysis
Average length of shoot 㪤㪸㫏㫀㫄㫌㫄㩷㪣㪼㫅㪾㫋㪿㩷㫆㪽㩷㫊㪿㫆㫆㫋㩷
Cut in March Cut in March
Cut in August Cut in August Length of shoot (cm) (cm) shoot of Length (cm) shoot of Length
Mar. Aug. Dec. Mar. Aug. Dec. Fig. 17 Growth speed of shoot just after tree cutting
Water㪮㪸㫋㪼㫉㩷㫃㪼㫍㪼㫃㩷䋨㫄䋩 level (m) Flood in 㪍㪅㪇 Flood in May September 㪌㪅㪌 And October 㪌㪅㪇 Water level after flood 㪋㪅㪌 㪋㪅㪇 㪊㪅㪌 㪊㪅㪇 㪉㪅㪌 㪉㪅㪇 㪉㪆㪏 㪊㪆㪋 㪊㪆㪉㪐 㪋㪆㪉㪊 㪌㪆㪈㪏 㪍㪆㪈㪉 㪎㪆㪎 㪏㪆㪈 㪏㪆㪉㪍 㪐㪆㪉㪇 㪈㪇㪆㪈㪌 㪈㪈㪆㪐 㪈㪉㪆㪋(Date) 㪈㪉㪆㪉㪐 Fig. 18 Water level of nearest observatory station “Mino” (year:2004) maximum length was 365 cm. A germination typhoons on 30th of September and 20th of October branch just after the trimming in August grew 59 cm respectively (see Fig. 18). As shown in Fig. 1, on average and its maximum length reached 123 cm. many weirs exist in the research site to irrigate the If the germination branches grow more than 150 cm, rice fields of the Okayama plain. Therefore, the they can grow without withering by being covered water level of the river is usually kept higher for rice by herbaceous plants. cultivation from the middle of May to the middle of June. At the same time, much rain fell during this (5) Ecesis place of Salicaceous species time in 2004, which also contributed to keeping the Generally, branches of Salicaceous species higher water level after the flood. Contrary to this, snapped by strong wind or flood flow were washed the water level after the flood in September and Oc- out and driven to the edge of the sandbar. Three tober was kept lower because it was the end of the flood flows which occured in 2004 washed out rice cultivation season. many branches of Salicaceous species. On the The lower figure of Fig. 19 shows a restored gravel bed at the downstream of the Kagoide weir gravel bar where the developed trees were cut and restored in March 2004, the branches were driven the sand bar surface was cut 1.5 m at the deepest there and established. part (the area surrounded by slid lines). It is ex- The first flood occurred due to heavy rain on 17th pected that the creation of the gravel bar has an ef- of May, the second and third one occurred due to fect to enhance the dynamic action of the bar under
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before
after
Fig. 19 Work of gravel bar restoration
- Distribution of Salicaceous species - ٨㧦Established after the flood in May ٤㧦Established after the flood in September and October
Area of restored gravel bar
㧦direction of flood flow Fig. 20 Ecesis of Salixcaceous species on the restored gravel bar
bed loads deposited area Change of river bed level (m) r Scour osition p 5EQWT Scou
osition : Area of restored rubbl e bar De &GRQUKVKQP
p Fig. 21 Area of scour and deposion accoding to flood in September and October attack of the annual scale flood flow. The bed level of September and October (established positions are of the edge of the water becomes lower because the shown in white circles). The characteristics of the cross slope of the restored area was transversely cut established positions of each flood proves that the by 2 percent. water level after the flood had a great influence on After the main floods, field observations were the development of shoots from a driven branch. carried out in August and November. Fig. 20 Although the effect of flood flow increased on the shows the established area of Salicaceous species on created gravel bar by cutting the sandbar, the gentle the restored gravel bar. The investigation in Au- flow area appeared at the downstream part of the gust shows the ecesis area where Salicaceous spe- created gravel bar adjacent to the dense forest of cies established after the flood occured in May (es- Salicaceous species. And then, not only the bed tablished positions are shown in black-painted cir- loads but also the branches of Salicaceous species cles). And the investigation in November shows and the seeds of vegetation deposited there (see Fig. the driven branches which established after the flood 20, Fig. 21). That is, the area where the branches
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Main flood flow Flood flow across the sandbar Fig. 22 Gravel bar where the flood flows across
2m/s
Fig. 23 Numerically obtained velocity distribution on the gravel bar where the flood flows across (Q=4405m3/s)
Salicaceous species forest The other forest Fig. 24 Thick growth of trees except for Salixcaceous species of Salicaceous species drift ashore is restricted to across the sand bar. And then, Salicaceous species the calm flow area of the flood to some extent. is rarely seen on well established bars located a cir- However, whether branches of Salicaceous species tain distance from the shoreline where there is thick can establish or not depends on the damp condition growth of forest such as Celtis sinensis, Aphananthe of the bed. That is, as shown in Fig. 20, the water aspera, bamboo and Ailanthus altissima (see level near the shore line has to be higher to keep the Fig. 24). Above mentioned results show that the damp bed condition. That is the reason why growth of Salicaceous species depends on not only branches of Salicaceous species could establish at a the stabilization but also the dynamic factor of the higher place of the created gravel bar after the flood river. In other wors, the growth of Salicaceous of May, whereas they could establish only at a lower species is restricted due to the reduction of the place after the flood of September and October. shoreline where the effect of flooding frequency is In the research field, there are two sandbars where large as well as the reduction of a damp bed where mean annual scale flood flows across them. Sali- the relative elevation is small from the water sur- caceous species can not establish in such places face. And the growth is also restricted if the river where the sandbar suffers frequent disturbance by bed fluctuates as deep as the root during flooding or flood activity (see dotted arrows in Fig. 22). Veloc- if the water level decreases due to lack of rain. The ity distribution across the bar is shown in Fig. 23. growth of Salicaceous species depends on the scale Numerical results prove the existence of the flow of these dynamic factors.
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(6) Summary of expansion mechanism of Salica- just cutting the above-ground part of Salicaceous ceous species species may not be useful to prevent a germination. The obtained knowledge of the expansion mecha- Therefore, an uprooted removal or other germina- nism of Salicaceous species is as follows tion regeneration countermeasures are required. a) Seeds and branches, which develop as a vegeta- tive reproduction, easily drift ashore where flood (2) Measures using Salicaceous species flow becomes gentle. Once developed Salicaceous trees are hard to b) Branches of Salicaceous species can take root remove even after an attack of a large-scale flood. only in a damp bed condition. Even if a shoot Considering 3. (6). e), f), management measures grows, if the bed gets dry later, the shoot withers. which positively utilize the development of Salica- Therefore, a damp bed condition maintained by a ceous spieces are also proposed. stable water level condition is an essential condition a) Allow moderate development of Salicaceous for ecesis of Salicaceous species. trees for the flood control, by which a temporary c) Salicaceous species cannot establish in such storage function of the flood can be expected. places where the sandbar suffers frequent distur- b) Use Salicaceous trees as a flow control structure. bance by flood activity. Thick growth of Salica- The diversion system from the Asahi River to the ceous trees contributes to the stability of the sand Hyakken River will be promoted if Salicaceous bar. trees are properly arranged. d) Salicaceous trees easily reproduce from the re- c) Use Salicaceous trees as a habitat protection mained part even if upper part is destroyed unless it structure. A habitat, a stable river space where en- is uprooted by flood impact. dangered species prefer to live, trees arranged e) Only a part of the Salicaceous trees is destroyed around it has an effect to prevent it from destruction under attack of a large-scale flood. The destroyed even under flood impact. area depends on a curvature degree of the river d) With respect to 4.(1). c), use Salicaceous trees as course and crossing hydraulic structres such as a groin which facilitates a dynamic bed13). weirs. a) and b) aims for the flood control aspect, and c) f) On the other hand, the developed Salicaceous and d) for the environmental aspect. Another impor- trees become one of the factors to regulate the place tant viewpoint of the application method is that which is likely to bear flood action. Salicaceous species can be used as a landscape im- provement or for the creation of an amenity space.
4. SUGGESTION OF MANAGEMENT MEASURES 5. CONCLUSIONS
The purpose of this study is to propose effective Considering the various findings obtained in this measures to manage the developed riparian forest of study, the expansion mechanism of Salicaceous spe- Salicaceous species. Considering the obtained ex- cies was clarified and several measures to control pansion mechanism of Salicaceous species, the fol- development of Salicaceous species were proposed. lowing measures are suggested. Considering the flood control aspect, because Sali- caceous species reduces flood capacity of the river, (1) Measures to restrict the development development of Salicaceous species should be re- a) With respect to 3.(6). a) ; Landform improve- stricted. On the other hand, with respect to the ment such as cutting sand bars is useful to eliminate restoration of an amenity space in the river, if Sali- the dead water region, which has an effect to reduce caceous species did not exist there, it is a loss of part the driven seeds and branches at the shore of the bar. of the river environment in a sense. Therefore, b) With respect to 3.(6). b) ; An artificial water level positive and negative influences of the development control is introduced to make a dry bed using dis- of Salicaceous species should be evaluated in detail charge operation of the dam during a flood recession in view of the flood control aspect as well as the period or water level control by movable weirs after environmental aspect in the future study. flood. c) With respect to 3.(6). c) ; To make a dynamic river bed, a discharge control giving a dynamic bed REFERENCES variation using dam12) or flood control hydraulic 1) Lee, S., Watanabe, S., Mochizuki, T., Kouichi, H. and Tu- kahara, T. : The disturbance of plant community and transfer structures such as spur dikes can be introduced as a of gravels, Proceedings of the 52th Annual Conference of destabilization system of the sand bar. the Japan Society of Civil Engineers, pp.286-287, 1997. (in d) With respect to 3.(6). d) ; Trimming management Japanese)
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