Abstract. In this study, we demonstrate the application of tritium in precipitation and baseflow to estimate groundwater transit times and storage volumes in Hokkaido, Japan. To establish the long-term history of tritium concentration in Japanese precipitation, we used tritium data from the global network of isotopes in precipitation and from local studies in Japan. The record developed for Tokyo area precipitation was scaled for Hokkaido using tritium values for precipitation based on wine grown at Hokkaido. Then, tritium concentrations measured with high accuracy in river water from Hokkaido, Japan, were compared to this scaled precipitation record and used to estimate groundwater mean transit times (MTTs). A total of 16 river water samples in Hokkaido were collected in June, July, and October 2014 at 12 locations with altitudes between 22 and 831 m above sea level and catchment areas between 14 and 377 km 2 . Measured tritium concentrations were between 4.07 (± 0.07) TU and 5.29 (± 0.09) TU in June, 5.06 (± 0.09) TU in July, and between 3.75 (± 0.07) TU and 4.85 (± 0.07) TU in October. We utilised TracerLPM (Jurgens et al., 2012) for MTT estimation and introduced a Visual Basic module to automatically simulate tritium concentrations and relative errors for selected ranges of MTTs, exponential-piston ratios, and scaling factors of tritium input. Using the exponential (70 %) piston flow (30 %) model (E70 %PM), we simulated unique MTTs for seven river samples collected in six Hokkaido headwater catchments because their low tritium concentrations were no longer ambiguous. These river catchments are clustered in similar hydrogeological settings of Quaternary lava as well as Tertiary propylite formations near Sapporo city. However, nine river samples from six other catchments produced up to three possible MTT values with E70 % PM due to the interference by the tritium from the atmospheric hydrogen bomb testing 5-6 decades ago. For these catchments, we show that tritium in Japanese groundwater will reach natural levels in a decade, when one tritium measurement will be sufficient to estimate a unique MTT. Using a series of tritium measurements over the next few years with 3-year intervals will enable us to estimate the correct MTT without ambiguity in this period. These unique MTTs will allow estimation of groundwater storage volumes for water resources management during droughts and improvement of numerical model simulations. For example, the groundwater storage ranges between 0.013 and 5.07 km 3 with saturated water thickness from 0.2 and 24 m. In summary, we emphasise three important points from our findings: (1) one tritium measurement is already sufficient to estimate MTTs for some Japanese catchments, (2) the hydrogeological settings control the tritium transit times of subsurface groundwater storage during baseflow, and (3) in the future, one tritium measurement will be sufficient to estimate MTTs in most Japanese watersheds.

Abstract. In this study, we demonstrate the application of tritium in precipitation and river water to estimate groundwater transit times and storage volumes in Hokkaido, Japan. To establish the long-term history of tritium concentration in Japanese precipitation, we used tritium data from the global network of isotopes in precipitation (GNIP) and from local studies in Japan. The record developed for Tokyo area precipitation was scaled for Hokkaido using tritium values for precipitation based on wine grown at Hokkaido. Then tritium concentrations measured with high accuracy in river water from Hokkaido, Japan, were compared to this scaled precipitation record and used to determine groundwater mean transit times (MTTs). Seventeen river water samples in Hokkaido were collected in June, July and October 2014 at twelve locations with altitudes between 22 and 831 m above mean sea level and catchment areas between 45 and 377 km2. Measured tritium concentrations ranged between 4.07 (±0.07) TU and 5.29 (±0.09) TU in June, 5.09 (±0.09) TU in July, and between 3.75 (±0.07) TU and 5.01 (±0.08) TU in October. We utilized TracerLPM (Jurgens et al., 2012) for MTT estimation and introduced a Visual Basic module to automatically simulate tritium concentrations and relative errors for selected range of MTTs, exponential-piston ratio, and scaling factors of tritium input. Using the exponential(70 %)-piston flow(30 %) model (E70 %PM), we found unique MTTs for seven river samples collected in six Hokkaido headwater catchments because their low tritium concentrations are not ambiguous anymore. These river catchments are clustered in similar hydrogeological settings of Quaternary lava as well as Tertiary propylite formations nearby Sapporo city. However ten river samples of other six catchments produced up to three possible MTT values with E70 %PM due to the interference by the tritium from the atmospheric hydrogen bomb testing 5–6 decades ago. For these catchments, we show that tritium in Japanese groundwater will reach natural levels in a decade, when one tritium measurement will be sufficient to estimate a robust MTT. Using a series of tritium measurements over the next few years with 3 year intervals will enable us to determine the correct MTT without ambiguity in this period. These unique MTTs allow estimation of groundwater storage volumes for water resources management during droughts and improvement of numerical model simulations. In summary, we emphasise three important points from our findings: (1) one tritium measurement is already sufficient to estimate MTT for some Japanese catchments, (2) the hydrogeological settings control transit times with tritium of groundwater watersheds at baseflow, and (3) in future one tritium measurement will be sufficient to estimate MTT in most Japanese watersheds.

Tomonori SHIMADA, Daisuke TOBITA, Syunichi MAEDA, Kazuhisa KASHIWAYA and Hideaki YOKOHAMA 1 正会員 土木研究所寒地土木研究所寒地河川チーム(〒062-7602 北海道札幌市豊平区平岸1条3丁目)2 正会員 国土交通省北海道開発局帯広河川事務所(〒089-0536 北海道中川郡幕別町札内西町73-6)For the purpose of mitigating flood damage by overflow from a levee breached, the authors conducted an experiment using the full-scale levee at the Chiyoda Experimental Channel on the Tokachi River. The scale of the channel is 1300m long and 30m wide, accompanying a flood area of 80m width. In the experiment works, a group of blocks were installed on the slope of the levee 10 and 20 m downstream from the artificial breach, and water was let flow from upstream. After the water was let flow, when the breach progressed downstream to the concrete armor blocks, the blocks protected the failure levee. The downstream progress of breach was expected to be arrested by the concrete blocks which were expected to weaken the inundation flow from the channel. The control effects by the concrete blocks were examined.

Although damage resulting from levee breaches is particularly serious, there is a lack of technical data on the mechanism behind the phenomenon. Accordingly, the authors have conducted a series of levee breach experiments which were set up in the Chiyoda experimental flume. In this study, the authors focused on the sedimentation within the flood areas attached next to the experimental levees. The experimental conditions were variety of discharge, levee material, and levee shape. Using the results of the experiments, the authors clarified the relationship between the sediment distributions and the experimental conditions. Then, comparing the volume of breached levee and the accumulated sediment, the origin of the accumulated sediment was examined. The authors also conducted a numerical calculation using Nays2D model and identified the relationship of the plenary velocity distributions and the sediment distributions.

：When riparian woods need to be removed for flood control or river patrol, it is often the case that all the trees in the subject area are felled. However, clear cutting of riparian forests has some problems of landscape, ecosystem and nearly simultaneous re-growing of the trees from the roots and stumps. In recent years, with the premise of creating small obstructions in flood control, technological knowledge, which contributes to reducing the load on the riparian landscape and ecosystem and promotes a good forest environment, has been called for. Knowledge about the landscape impacts of various techniques for removing riparian woods from river channels should be modeled so that to evaluate these techniques. However, accumulation of knowledge and insufficient parts are not systematized between wood landscape and river landscape. This study aims at presenting some physical indicators that contribute to the enhancement of the forest landscape and the possibilities for applying qualitative knowledge to cutting riparian forests. To this end, we collected 69 papers on forest landscapes and analyzed them. Through the analysis, we extracted indicators and findings related to the tree height, age and type; the spaces between trees and tree density; the degree of coverage of the forest floor and grass height; and the viewpoint field.