Takeyoshi Chibana

Physical factors affecting the distribution of leaf litter patches in streams: comparison of green and senescent leaves in a step-pool streambed

We investigated the distribution patterns of senescent and green leaf litter patches on a streambed to evaluate the hypothesis that the different immersion times of senescent leaves in long-term benthic storage and newly retained green leaves affect streambed distribution patterns in summer (June, July, and August). We counted all the leaf litter patches in the streambed of the study reach, comparing the physical condition of patches and classifying the trapping obstacles associated with each patch type. The distribution patterns of senescent and green leaf litter patches differed. Green leaf litter patches were more numerous at every sampling date, with most trapped by cobbles, whereas senescent leaf litter patches were retained by twig obstacles, backwaters, and cobbles. In June and July, senescent leaf litter patches were located in significantly deeper stream areas than were green leaf litter patches. The distribution of senescent leaves would be primarily determined during spring snowmelt-driven floods. We speculate that senescent leaves were originally located at the edges of pools in the main flow pathway of the channel, which overflowed in the floods. We conducted flume experiments to clarify the transport characteristics of senescent and green painted maple and manchurian alder leaves in the water column. Our hypothesis was that the transport characteristics of each leaf type differ when they first enter the water, because of differing leaf properties. The flume experiments showed that duration of surface flotation differed for senescent and green leaves and for the two tree species. These differences in the duration of leaves on the streambed and in the floating time of green leaves of different trees ensure varied food resources for macroinvertebrates in various physical conditions.

CHARACTERISTICS OF RIFFLE-POOL STRUCTURE FORMED BY A DROP STRUCTURE

落差工の存在は周辺地形を変化させ，生物にプラス，マイナス両方の影響を及ぼす．この影響を理解すべく，複数箇所で落差工周辺の地形，流況を観測し，その特徴及び形成機構を解析した．まず，落差工はその周辺に早瀬，淵，平瀬を形成するものの，河床低下区間では淵が形成されないことを示した．また，落差工下流に淵がない場合，その下流の早瀬はアーマー化していた．一方，落差工上流の平瀬では，河道の湾曲に対応し，内岸側が堅い礫，外岸側は軟らかい砂利の河床になるという分級が生じていた．なお，淵では，垂直に落ち込む流れのため，流量が少ない時は表層のみ流れ，多い時は底層に強い流れを生じていた．この様に，落差工周辺の瀬－淵構造は，落差工の構造，河道線形，河道縦断形といった様々なスケールの因子に規定されることを示した．

Hierarchical analysis of physical environment in riffles

When analyzing the physical habitat in a river, it is necessary to understand its structure hierarchically. This paper focuses on a riffle structure in a middle reach of a river. Firstly, as a unit scale analysis, we classified the riffles based on its plane shape. Plane shape of a riffle is determined by the condition of sandbar that is affected by the amount of transported sediment. And plane shape of a riffle determines the distribution of local discharge in a riffle. Next, as a subunit analysis, we clarified the typical distribution pattern of longitudinal and lateral gradient of riverbed and gravel structure in a riffle. And the distribution of longitudinal and lateral gradient of riverbed is determined by the common feature of sandbar shape. Finally, as a point scale analysis, we showed the examples of wave pattern on each subunit. Wave pattern is related to the hydraulic conditions, so it represents the habitat condition for fishes, macro invertebrates and attached algae. Based on this analysis, it is possible to understand the microhabitat condition in relation to the amount of sediment, river shape, and scale of floods that can be controlled in river management.

Physicochemical properties of aquatic habitats from ecology and civil engineering perspectives

Aquatic environments are usually represented by physicochemical properties, including flow regime on riverbed configurations, sediment size and stability, and water quality. This monograph focuses on the habitat of submerged macrophytes. Recent changes to the aforementioned physicochemical properties because of human activities have altered the aquatic habitats for submerged macrophytes, resulting in increases in the number of invasive species and extinction of native species. The expansion of invasive species into new environments often changes the physicochemical properties of these environments through accumulation of fine materials or changes in water quality. This is especially true in stable water areas— such as spring-fed rivers, rivers from lakes, lakes, and lagoons—where the submerged macrophytes are often found. In these locations, sufficient natural discharge and floods sizeable enough to renew the aquatic environment cannot be expected. In this case, increases in underwater invasive species tend to have further negative impacts on the natural environment, as they create conditions that are more suitable to themselves. These areas are considered to be among the most sensitive environments to human influences. In order to overcome this issue, it is necessary to understand the factors that limit the growth of both invasive and native endangered species. With this knowledge, it may be possible to predict future changes to physicochemical properties—focusing on limiting factors with and without the use of some countermeasures—in order to find effective management strategies. Thus, the cooperation of ecologists and civil engineers is essential in order to integrate their knowledge. To that end, all studies introduced here were presented and discussed at the annual meeting of the Ecology and Civil Engineering Society held in Saitama Prefecture, Japan, in 2009. The essential contributions made by several researchers to this issue are summarized here. Shinohara et al. (2013) point out the importance of carbon dioxide (CO2) and light availability to the distribution of submerged macrophytes. Takahashi and Asaeda (2012) clarify that low pH spring water increases the growth rate of Egeria densa by affecting free CO2 concentration in water. It is known that submerged macrophytes play an important role in the ecosystems of shallow lakes and stable streams by changing water quality, creating a refuge for other creatures, reducing bottom shear stress by stabilizing the substrate, and reducing the presence of suspended particles. Submerged macrophytes, however, have been disappearing, and the cause is purported to be a limited supply of photosynthetically active radiation (PAR) because of phytoplankton blooms. Therefore, studies showing the importance of free CO2 are of interest to those who seek to develop the above-mentioned management strategies. Matsui (2012) presents his study on the distribution and habitat of submerged macrophytes in the Seta River as it flows out of Lake Biwa, Shiga Prefecture, Japan. He introduces the clear relationship between sediment type and submerged macrophytes. The study by Gunaratne et al. (2012). addresses changes in lagoon morphology, specifically inlet morphology and flushing properties of Koggala Lagoon, Sri Lanka. According to this study, the flushing half-life, which is the time needed to replace half T. Chibana (&) Department of Civil Engineering, The University of Tokyo, Hongo 7-3-1 Bunkyo-ku, Tokyo 113-8656, Japan e-mail: chibana@hydra.t.u-tokyo.ac.jp

THE RELATIONSHIP BETWEEN ROUGHNESS OF REVETMENT AND CHARACTERISTICS OF GRABEL BAR UNDER FLOOD CONTROLL OF DAM

In the Nakatsu river, after the dam construction in 2001, due to decrease of flood discharge, river course has been fixed, and gravel bar has been covered with vegetation. This study clarified the relationships between roughness of revetment and characteristics of morphological structures from dry riverbed to riffle-pool structures. At first, historical change of Nakatsu river is investigated. River width in the normal flow condition is stable for flow direction due to the concentration of flow, and the mechanism has been clarified through flume experiment. Secondary, through field measurement, in case the roughness of revetment is high, the stream line tend to be steep angle against the flow direction, and the morphology is different due to the difference of revetment roughness. Finally, flume experiment clarified the mechanism of how these difference appears under the controlled discharge.

Urban River Management: Harmonizing River Ecosystem Conservation

In 1997, river environmental conservation was declared one of the important purposes of river management in River Law. Before this year, the main purposes of river management concerned flood control and water use only. During the 1970s, however, ecological conditions in rivers became worse and worse. At this time, government and citizens started to try to restore and preserve river environments. Against such a background, this chapter focuses on how to make urban river ecosystems harmonize with the mission of flood control.