Expansion Mechanism of Salicaceous Species Forest and Its Management Technique in the Asahi River

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- 238 U 㪌㪃㪇㪇㪇㪋㪃㪌㪇㪇㪋㪃㪇㪇㪇 FKUEJCTIG O 㪊㪃㪌㪇㪇㪊㪃㪇㪇㪇 station Mino 㪉㪃㪌㪇㪇㪉㪃㪇㪇㪇㪈㪃㪌㪇㪇 Kurare weir doacacia 㪈㪃㪇㪇㪇 necessary factors of the typical feature of the river. However, too much growth of causes a reduction of the safety degree of flood, de- 㪌㪇㪇 terioration of ecological diversity and typical land- scape of the river. Therefore, it is urgently required to take appropriate measures to prevent the growth 㪇 of forest. Salicaceous Santei weir Otoide weir conditions and how they proliferate there. Consid- ering flood control, ecology and landscapeIn this study,preserva- we investigated the area where tion points, we examined the management tech- , niques of anti-development of thick growth of caceous Salicaceous countermeasures are if undertaken and from wha kind of place and how to start. Fig. 1 2. OUTLINE OF INVESTIGATION FIELD (1) Outline of the Asahi River in Okayama Prefecture located in the western part o Hyakken River(diversion channel) Research area of the Asahi river (A : shooting location of JAPAN, originates from Mt. Asanabe Washigasen (1,081 m) in th The Ashahi River, one of the three largest rivers species grows in such kind of hydraulic 142 km long and has a catchment of 1,800 km species and how appropriate and effective The river basin extends from north to south, with the upper and middle reaches of the river cutting through the Ch Myoujou weir species can be one of the Fig. 2 Salicaceous Doboku Gakkai Ronbunshuu B Vol.62 No.3, 238-250, 2006. 8 e Chugoku Mountain The flood history after 1957 (more than 500 m ugoku Moutains and Ki Shimizu weir Asahi River CPPWCNUECNGHNQQF O species U A Nakaide weir FCVG Range. Itis Sali- ranking Goudou weir 1 1934. 9.211 1934. 6,000 typhoon b t 9.182 1945. 4,800 i Highland and the lower reaches forming Okayama Plain, be- 3 1998.10.18 4,405 Kagoide weir fore emptying out into Kojima Bay. The river ba- 7.124 1972. 3,700 Fig. 14 sin is mostly mountainous. Only 14.7 % of the 5 1971. 7. 1 3,180 b The average rainfall in the river basin is about 1,450 asin is plains, most of it being on Okayama Plain. mm/year. Table 1 Asahi River (10.8-17.4 km from the river mouth). Restored rubble bar(Fig.14) f Mean bed slope is 1/670, annual mean maximum discharge is 1,400 m date 2 representative grain-diameter of the bed is 40 ) . mm. The existence of a large meandering part and many crossing works such as weirs controls the 3 flood flow and ordinary water flow. The overall length of dammed pools by weirs is 3.3 km, corre- /s) Main floods of Asahi River sponding to half of the research site. (2) Flood history 239 Big floods are due to typhoons or seasonal rain fronts. Table 1 Fig. 1 discharge (m shows the lower reaches of the shows the main floods of the Asahi River. Fig. 2 3 3 /s) /s, about 300 m in width and shows the flood (more than 500 seasonal rain cause typhoon seasonal rain typhoon front front 㨪 70 Doboku Gakkai Ronbunshuu B Vol.62 No.3, 238-250, 2006. 8 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- 240 Doboku Gakkai Ronbunshuu B Vol.62 No.3, 238-250, 2006. 8 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.

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