IMPROVEMENT OF FLOOD REDUCTION FUNCTION OF FORESTED BASIN BY REINFORCEMENT OF EVAPORATION RATE AND GOUUND ROUGHNESS

流域治水における活用を念頭に，遮断蒸発率と地表面抵抗の増強による森林の洪水低減機能の向上について洪水ピーク流量の視点から検討した．徳島県内に位置する２つの林地（一斉植林・皆伐型施業のスギ人工林，弱間伐・択伐型施業のスギ・ヒノキを中心とする針広混交複層林）を対象に水文観測を実施し，地表面流分離直列二段タンクモデルと将来予測降雨を用いた流出シミュレーションを実施した．遮断蒸発率と地表面抵抗の洪水ピークの流量低減と遅延効果について検討した結果，対象としたスギ人工林を針広混交複層林・弱間伐に変更すると20％程度の洪水ピーク流量低減効果を得られる可能性が示唆された．Two forest operation techniques with an flood peak reduction effect were discussed using a tank-model and two small artificial forested basins where forest physiognomy was different. One operation was the way to strengthen the insulation evaporative rate by changing its physiognomy, other was the way to make forest floor vegetation and ground roughness increased. The effect when artificial cedar forest was replaced with a mixed conifer-hardwood forest was discussed. As a result, it was estimated that the flood peak flow decreased around 20% before the replacement

FLOOD CONTROL FUNCTION OF FARMLAND AS GREEN INFRASTRUCTURE USING INUNDATION ANALYSIS MODEL

本研究では,徳島県にある海部川支川善蔵川流域を対象とし,農地や湿地をグリーンインフラとし,洪水調節機能を定量的に評価する内水氾濫解析モデルを構築し,人口減少が進む当該流域における将来の土地利用の在り方を考察した.平成26年台風12号を元に,Hazen法によって確率雨量を評価し,降雨外力の違いと土地利用の違いによる洪水調節機能に与える影響を評価した.前者では,床上浸水の被害が20年確率降雨以上で顕著に増加したことから,水害リスクの高い場所と農地による洪水調節機能の発揮限界について検討した.後者では,宅地転換が進む前の過去の土地利用では,湛水量の増減の応答が早く,とりわけ集中的に浸水する領域を特定した.以上から,浸水リスクがある場所での開発を控え,社会状況に連動した優先的な退避の施策の成立が今後重要視される.In this paper, flood control function of agricultural land and wetlands were considered using an inundation analysis model, expecting to work as green infrastructure against flood. The study area is located in Zenzo River basin, a tributary of the Kaifu River, Tokushima Prefecture, where population decline is severe in recent years. Based on the rainfall by Typhoon 1412 several probable rainfalls were evaluated, and the flood control function was examined with the different rainfalls, as well as the difference of land use. The results show that, since the flood damage due to inundation above floor level remarkably increase at and over 20-year period probable rainfall, the flood control function has a limit at some stage against extreme rainfall events. On the other hand, in the past land use where more farmlands existed, temporal variation of inundation volume reduces more rapidly compared with the current land use. The results indicate it will be necessary to suppress development in high risk areas, and the past land use will be a good reference for a wise land use

Self-managerial activity by local people for keeping coastal pine forest and complemental policy of Fukutsu City, Fukuoka Prefecture, Japan

海岸マツ林は重要なグリーンインフラの一つであるが，現在，その多くは放置に伴う遷移や松枯れ病によって劣化してきている．そのような中，福岡県福津市の海岸マツ林は，地域住民を主体とする自律的活動で荒れた状態から再生され，継続的な管理作業及びマツ林を活用したイベントが行われている．当地を対象に，地域住民が海岸マツ林の管理に主体的に取り組むようになったプロセスと，それを誘導してきた福津市の地域自治の促進に関する政策・施策を明らかにした．そして，福津市の政策・施策過程を，ガバナンス論をベースに評価した．海岸マツ林の再生・保全に係る活動のエネルギーは「子供の頃に見ていた白砂青松の景観を取り戻し，再び人々が憩う場にしたい」という，体験に基づいた地域で共有されている価値であった．そして，福津市の後押しによって創出された「郷づくり推進協議会」が活動のエンジンとなっていた．海岸マツ林を誰がどのように継続的に管理してゆくのかとの課題に対して，福津市は，権限と財源を「郷づくり推進協議会」に移譲しながら地域自治を強化することで，ガバナンス型問題解決を導いてきていた．Coastal pine forest is one of the important green infrastructures, which provides several ecosystem services such as disaster prevention and beautiful scenery. Most of the coastal pine forests in Japan, however, has been unmanaged and changing to different types of forests through natural succession. In addition, the structure has been declining due to pine wilt disease. In this situation, coastal pine forest in Fukutsu City, Fukuoka Prefecture, has been restored by autonomic activities of local people. Enthusiasm come from people's memory of the past such as beautiful landscape of the pine forest and thought of recovering it. Local council, which was established under the policy of Fukutsu City, took a roll to share people's memories, recognize challenges and establish a goal in the local area. The council has act as the platform for public involvement and the motivation for the activities. Fukutsu City has transferred authority and fiscal resources to the local council according to the policy on promoting local governance. Thus it has successfully brought the governance-based solution to ecosystem management for keeping the functions of green infrastructure

Suitable LiDAR Platform for Measuring the 3D Structure of Mangrove Forests

Investigating the three-dimensional structure of mangrove forests is critical for their conservation and restoration. However, mangrove forests are difficult to survey in the field, and their 3D structure is poorly understood. Light detection and ranging (LiDAR) is considered an accurate and dependable method of measuring the 3D structure of mangrove forests. This study aimed to find a suitable LiDAR platform for obtaining attributes such as breast height diameter and canopy area, as well as for measuring a digital terrain model (DTM), the base data for hydrological analysis. A mangrove forest near the mouth of the Oura River in Aza-Oura, Nago City, Okinawa Prefecture, Japan, was studied. We used data from terrestrial LiDAR scanning “TLS” and unmanned aerial vehicle (UAV) LiDAR scanning “ULS” as well as data merged from TLS and ULS “Merge”. By interpolating point clouds of the ground surface, DTMs of 5 cm × 5 cm were created. DTMs obtained from ULS could not reproduce the heaps of Thalassina anomala or forest floor microtopography compared with those obtained from TLS. Considering that ULS had a few point clouds in the forest, automatic trunk identification could not be used to segment trees. TLS could segment trees by automatically identifying trunks, but the number of trees identified roughly doubled that of the visual identification results. The number of tree crowns identified using TLS and ULS was approximately one quarter of those identified visually, and many of them were larger in area than the visually traced crowns. The accuracy of tree segmentation using the canopy height model (CHM) was low. The number of canopy trees identified using Merge produced the best results, accounting for 61% of the visual identification results. Results of tree segmentation by CHM suggest that combining TLS and ULS measurements may improve tree canopy identification. Although ULS is a promising new technology, its applications are clearly limited, at least in mangrove forests such as the Oura River, where Bruguiera gymnorhiza is dominant. Depending on the application, using different LiDAR platforms, such as airborne LiDAR scanning, UAV LiDAR scanning, and TLS, is important. Merging 3D point clouds acquired by different platforms, as proposed in this study, is an important option in this case

Development of a Monitoring Method Using UAVs That Can Detect the Occurrence of Bark Stripping by Deer

The occurrence of bark stripping associated with increased deer densities can severely damage forests. Identifying trends in bark stripping is crucial for forest management, but such data are often difficult to obtain through field surveys. Therefore, this study aimed to develop an efficient monitoring method using unmanned aerial vehicles (UAVs) that can detect the occurrence of bark stripping and enable long-term monitoring. The area around the Ochiai Pass in Higashi-Iya Ochiai, Miyoshi City, Tokushima Prefecture, Japan, was selected as the study area for the survey of Abies homolepis, which was found to be significantly bark-stripped by deer in the field. The location and root diameter of A. homolepis were measured, and the percentages of bark stripping and tree growth were visually determined. Simultaneously, normalized difference vegetation index (NDVI) and visible light orthomosaic images were produced using a UAV. A canopy polygon of A. homolepis was created, and the average value of the NDVI within the polygon was calculated. Where the bark stripping rate at the root edge was greater than 75%, the number of “partially dead” and “dead” trees increased significantly, indicating that bark stripping by deer was the primary cause of the death of A. homolepis in Ochiai Pass. In addition, the mean value of the NDVI was significantly lower, with a bark stripping rate of 75% or higher, indicating that the NDVI of the canopy of A. homolepis can be used to estimate individuals with a high bark stripping rate at the root tips, that is, those with a high probability of mortality. Furthermore, by extrapolating the results of the tree-by-tree survey to the nontarget A. homolepis, we detected 46 (8%) A. homolepis with an average NDVI value of 0.8 or less (i.e., those with a bark stripping ratio of 75% or higher and a high probability of mortality). Therefore, the utilization of remote sensing technology via UAVs, as demonstrated in this study, proves to be a potent means for monitoring the incidence of bark stripping