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Water level and height of flood line.* The points where the flood line is lower than the other points may underestimate the actual high-water levels because the values were determined based on evidence from plants, such as damage to the grass. Using ArcGIS, we calculated based on the topography, water depth, and area of the retarding basin that 378,203.0 m 3 of floodwater was stored on the flood plain in Okawano at the time of the maximum water level of 19.05 m, demonstrating that the open levee reduced the downstream flood volume. Rice had been planted in the paddy fields in the retarding basin, but except for a very small area near the inlet, the plants survived. There was little debris influx, driftwood, or mud deposition, and there was no significant damage to the paddy fields (Figure 6).
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![Figure 7. Plan view of Azame-no-se (Drawing by Hironori HAYASHI) [43].](publication/349264903/figure/fig4/AS:990595468259328@1613187772050/Plan-view-of-Azame-no-se-Drawing-by-Hironori-HAYASHI-43_Q320.jpg)
Large-scale disasters, such as hurricanes, cyclones, tsunamis, and forest fires, have caused considerable damage in recent years. This study investigated two case studies of discontinuous open levees (kasumi-tei), which are a traditional Japanese river technology, on the Matsuura River at the sites of Okawano and Azame-no-se, and evaluated the adva...
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Context 1
... status at the time of flood event [27,34]. It is concluded by examining the flood lines in Figure 5 that the lines are concentrated near an elevation of 19 m at each location within the retarding basin of the Oka-wano open levee system. In other words, within the retarding basin, the height of the water surface was almost constant, regardless of the distance from the inlet. ...
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One of the most important parts of disaster management and disaster risk reduction strategies is the special vulnerability of children and the need to increase disaster awareness among them. It is very effective to design and develop awareness-raising tools suitable for children’s learning needs in this process. The main purpose of this study is to...
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... Source: 13 September 2022, Asahi Shinbun Digital (Takatoki river, Shiga prefecture) Flood control based on such retarding functions of agricultural land has been shown to have significant disaster mitigation effects by Terayama and Shimatani (2021) [3] and Osawa et al. (2022) [4]. However, since there is no institutional mechanism established to compensate for the damage caused by the deliberate flooding of agricultural land, this flood control measure is premised on the sacrifice on the part of the farmers (Yamada et al., 2022) [5]. ...
Japan’s susceptibility to and severity of floods have necessitated flood control policies by the government. “Overflowing flood control”, in which the floods due to torrential rains are systematically diverted to agricultural lands in the upper to middle reaches, is one of them. More information is needed on the public assessment of the overflowing flood control policy, and this research seeks to bridge this gap. Data evaluating rice affected by the policy were collected from a random nth-price auction using a developed online system. The sample consisted of 47 consumers living in the downstream areas of the Edogawa River, one of Japan’s first-class, or prime, rivers. Data on their attitudes toward the policy were collected with a questionnaire. Multiple ordered probit models are used for regression analysis. The results show that the sample respondents were willing to pay an average of JPY 1578 for 5 kg of rice, slightly higher than the national average rice production cost, and that 36% of the sample agreed with the flood control policy, which is positively associated with large families or owning many assets.
... However, Eco-DRR evaluations still face shortcomings and do not sufficiently acknowledge the impacts of Eco-DRR on all three components of disaster risk [3,7,13]. Eco-DRR evaluation is not currently comprehensive, as it oftentimes focuses on solely hazard mitigation (e.g., [14,15,16]), overlooking benefits for vulnerability reduction. ...
... These studies have enabled us to understand the significance of the levee arrangements and their effects on flood control. Some case studies addressing its Eco-DRR functions have shown the balance between flood control, environmental and agricultural benefits, and risks of allowing floodwaters to flow from open levees [28,29]. All of these demonstrate the advantages of this open levee system over flood control by continuous levees. ...
... Based on the results, the implementation of open levees is discussed. Open levees work as flood drainers or peak flow reducers [28] which are strongly related to the structure and location. In this study, we regard it as a drainer in sites B and C, and a peak flow reducer in site A. The open levee closing in sites B and C had a significant impact on the severe damage. ...
... Moreover, by comparing ensembles by levee location, it is possible to quantify the risk-benefit relationship of introducing (or eliminating) open levees. Not only for the flood risk, we also have to pay attention for an environmental co-benefit of an open levee system, such as environmental connectivity, aquatic habitat creation, or refuges [28,29]. However, this multi-perspective assessment method has not been common in practice. ...
An open levee system is a traditional flood mitigation system for reducing the expansion of inland flooding and decreasing the peak flow. However, there have been few quantitative studies on its feasibility. Furthermore, the differences in applicability depending on the topography and the construction of continuous levees have not been fully examined. We studied its feasibility based on simulations in the Kuji River area, where the vast Typhoon Hagibis occurred. Morphological models representing the past (the 1940s) and the present (2019), obtained by modifying the highly accurate digital elevation models (DEM) via the tracing of aerial photos, were applied to a 2D unsteady flow simulation model to reveal the effects of the levee system on river hydrography and overland flood behavior. The results indicated that inundation flow through an open area decreased both inundation duration and depth, while the reduction of peak discharge is relatively insignificant at approximately 10%. The sub levees are not adequate under the current conditions and floodwater volume, and their effectiveness depends on the surrounding conditions, such as the development of continuous levees.
... Many of the river embankments in Japan in the early modern period (until the mid-19th century) had openings at key points, and when river flow exceeded the channel capacity, river overflow was intentionally generated. While embankment technology was immature, such river planning was effective in predicting and mitigating flood damage, as well as reducing flood peaks [9][10][11][12][13]. In the latter half of the 19th century, the modern hydraulic control method of promptly discharging floods to the sea by continuous embankments was introduced by European engineers, and a modern river plan was made in 1911 for 13 nationally controlled rivers, as stipulated by the River Law at the time. ...
Due to the recent increase in the intensity of rainstorms, the Japanese government has announced a new policy of flexible flood mitigation measures that presupposes the release of water volumes exceeding the river channel capacity onto floodplains. However, due to the limited amount of quantitative measurement data on excess runoff, it will take time to formulate planning standards for remodeling and newly constructing flood control facilities reasonable enough under current budgetary constraints. In this study, the capacity shortage of a flood detention pond was evaluated against the excess runoff from a severe 2019 flood event by combining the fragmentary measurement data with a numerical flow simulation. Although the numerical model was a rather simple one commonly used for rough estimation of inundation areas in Japan, the results were overall consistent with the observations. Next, in accordance with the new policy, an inexpensive remodeling of the detention basin, which was designed according to conventional standards, was simulated; the upstream side of the surrounding embankment was removed so that excess water flowed up onto the floodplain gradually. Numerical experiments using the simple model indicated that the proposed remodeling increased the effectiveness of flood control remarkably, even for floods greater than the 2019 flood, without much inundation damage to upstream villages.
... Mitigation of landslide damages is often optimized according to local ecological knowledge, where people for instance allocate grassland on slopes of thin-layer volcanic soil to keep the biomass or potential mass flows small [92]. In some parts of Japan, the locals utilize agricultural land as a retarding basin by the use of traditional open levees and mitigate flood damages at the site and downstream [93]. For ecosystem services optimized at the local level, suitable proxies were not available for all municipalities in Japan, and regional scales are suited to studies that investigate where and to what extent services are maintained by agricultural management. ...
Agriculture provides a wide range of ecosystem services and has the potential to contribute to biodiversity conservation. In Japan, many of the resources associated with agroecosystems are threatened by farmland abandonment. Identifying where and to what extent agricultural ecosystem services and farmland biodiversity are affected by farmland abandonment is essential for developing effective strategies to counter the potential loss of these services and the biological communities that support them. Our study aimed to examine how a set of indicators for ecosystem services and biodiversity linked to agroecosystems (proportions of land dedicated to rice production and other agricultural production, proportion of agricultural land on slopes potentially providing landscape aesthetics, proportion of villages promoting rural tourism, and densities of forest
edges and irrigation ponds in agricultural land) are distributed at the municipal level across the Japanese Archipelago, and to analyze their spatial patterns in relation to the distribution of farmland abandonment. It was hypothesized that hotspots of agricultural ecosystem services and farmland biodiversity occur in areas at risk of farmland abandonment owing to shared drivers. The cluster analysis identified four distinct ecosystem service bundle types, two of them representing areas specializing in agricultural production, while the other two provided high levels of cultural services
and habitats for diverse biological communities. The latter two bundles were located in hilly and mountainous areas and accounted for 58% of rice production, 27% of other agricultural production, 77% of landscape aesthetics, 77% of rural tourism, 64% of forest edges, and 87% of irrigation ponds in Japan. In support of the hypothesis, farmland abandonment was pronounced in these areas, with 64% of recently abandoned fields located where 44% of agricultural land was found. This spatial overlap suggests that substantial losses of ecosystem services and biodiversity may occur if current
patterns of farmland abandonment continue. In order to prevent large‐scale losses of agricultural ecosystem services and farmland biodiversity, measures to counteract the ongoing abandonment trends should prioritize hilly and mountainous areas, and future studies should further evaluate the multiple functions of agricultural areas to improve policies that aim to ensure sustainable development of rural areas in Japan.
Community assembly can change over time following variations in connectivity among habitats. Previous studies have demonstrated the influence of seasonal pulse floods on community structures by facilitating species dispersal, but the effects of short‐term flash floods have been poorly investigated. We investigated fish community structures before and after a short‐term flood in drainage ditches of paddy fields in the Kita River basin, Japan. The study sites were classified into three connectivity types, “Transient” (connected only during the flood, with a temporally disappearing vertical gap between drainage ditches and downstream rivers), “Connected” (always connected, without the vertical gap), and “Disconnected” (always not connected, with the vertical gap). The abundance of Misgurnus anguillicaudatus , the only species that dispersed from paddy fields to drainage ditches, increased after the flood irrespective of the connectivity types, although the total abundance of the other species significantly increased after the flood only in the “Transient” sites. The dissimilarity of species composition between the ( n − 1)‐th and n ‐th surveys at each site was consistently low in the “Connected” and “Disconnected” sites, whereas it rose just after the flood and gradually declined in the “Transient” sites. Species composition was significantly different among the connectivity types, indicating that some fish species need stable connectivity even during the non‐flood period. The results showed that the short‐term flood facilitated fish dispersal from paddy fields by the increased water discharge and that from downstream rivers by the temporary removal of vertical gap, emphasizing the importance of hydrological connectivity for biodiversity conservation in an agricultural landscape.
