[HGG01-02] Sediment discharge from a small catchment affected by earthquake-induced shallow landslides and its environmental impacts
Keywords:The Hokkaido Eastern Iburi Earthquake, shallow landslides, sediment deposition, sediment discharge, ecological environment
The M6.7 Hokkaido Eastern Iburi Earthquake on 6 September 2018 induced numerous shallow landslides. The induced shallow landslides are characterized by that (1) occurrence was quite high in density, (2) slid materials were mostly volcanic ash and pumice (tephra) and buried humus between tephras, and (3) the large amount of slid materials filled the floodplains of the tributary valleys of the Atsuma River. The valley-filling materials are to be transported downstream for coming years during discharge events. However, it is less understood how long a large amount of sediment will be transported from an affected/disturbed catchment and how sediment discharge and change in catchment landforms are related. Also, it is not necessarily clear how the large amount of transported sediment will have downstream and coastal impacts environmentally. As a first step of investigation for these questions, we report the first-year monitoring data of sediment discharge from a small catchment affected by earthquake-induced shallow landslides and discuss its environmental impacts in the downstream and coastal areas.
A river having been monitored is the Towa River, a tributary of the Atsuma River. Its catchment has an area of 4.9 km2 and the relief of 209 m. Bedrocks are Neogene sandstones, mudstones, and conglomerates. The Towa catchment was disturbed by the highest rates of landslide occurrence (0.23 in area) in the 2018 event and the total number of shallow landslide was 248. The floodplains were filled with slid materials continuously from the upstream to downstream reaches.
The monitored data of sediment and water discharge, together with rainfall records of the AMeDAS Atsuma (about 4 km away from the Towa River), from April to December 2019 were summarized as follows:
(1) Recognized rainfall and/or discharge events were recorded 22 times during the period. While rainfall and water discharge events were almost coupled, water and sediment discharge did not always simultaneously occur.
(2) First weeks of June were the dry period, and the three discharge events during this dry period accompanied remarkably high turbidity, exceeding 40000 degree (mg/L Kaorin).
(3) Very high turbidity, exceeding 25000 degree, was recorded in two discharge events in August. These events were coupled with good amounts of rainfall.
(4) Very high turbidity, exceeding 25000 degree, was recorded once in October. The total rainfall of this event was only 6 mm.
(5) A high turbidity event, exceeding 5000 degree, occurred once in December. The total rainfall of this event was 16.5 mm.
(6) A peak of turbidity mostly appears within one hour after a rainfall peak.
(7) High turbidity exceeding 1200 FTU (Formazin turbidity unit) continued more than a few hours when the total rainfall exceeded 50 mm.
(8) It took a few days for a smaller event, more than one week for a lager event that discharge (i.e. water level) decreased down to the level before the event.
Water of the Towa River is turbid not only during discharge events but also between events when water level is steady. Continuous supply of turbid water from the upstream should result in alteration of light environment in the downstream and the coastal areas and also should change in river morphology and particle-size composition of river-bed materials, both of which are expected to have negative impacts on ecological environments.
Acknowledgement
Takeshi Fujinami, Yasuhiro Murakami, Yu Inami, Hiromi Akita of the Civil Engineering Research Institute for Cold Region provided field assistance. This work was supported by JSPS KAKENHI Grant Numbers JP18H03819, and a research grant of the River Center of Hokkaido ‘Inter-disciplinary study on the sediment disaster in the Atsuma River catchment by the Hokkaido Eastern Iburi Earthquake’ (PI: Hideto Kon, Hokkaido University).
A river having been monitored is the Towa River, a tributary of the Atsuma River. Its catchment has an area of 4.9 km2 and the relief of 209 m. Bedrocks are Neogene sandstones, mudstones, and conglomerates. The Towa catchment was disturbed by the highest rates of landslide occurrence (0.23 in area) in the 2018 event and the total number of shallow landslide was 248. The floodplains were filled with slid materials continuously from the upstream to downstream reaches.
The monitored data of sediment and water discharge, together with rainfall records of the AMeDAS Atsuma (about 4 km away from the Towa River), from April to December 2019 were summarized as follows:
(1) Recognized rainfall and/or discharge events were recorded 22 times during the period. While rainfall and water discharge events were almost coupled, water and sediment discharge did not always simultaneously occur.
(2) First weeks of June were the dry period, and the three discharge events during this dry period accompanied remarkably high turbidity, exceeding 40000 degree (mg/L Kaorin).
(3) Very high turbidity, exceeding 25000 degree, was recorded in two discharge events in August. These events were coupled with good amounts of rainfall.
(4) Very high turbidity, exceeding 25000 degree, was recorded once in October. The total rainfall of this event was only 6 mm.
(5) A high turbidity event, exceeding 5000 degree, occurred once in December. The total rainfall of this event was 16.5 mm.
(6) A peak of turbidity mostly appears within one hour after a rainfall peak.
(7) High turbidity exceeding 1200 FTU (Formazin turbidity unit) continued more than a few hours when the total rainfall exceeded 50 mm.
(8) It took a few days for a smaller event, more than one week for a lager event that discharge (i.e. water level) decreased down to the level before the event.
Water of the Towa River is turbid not only during discharge events but also between events when water level is steady. Continuous supply of turbid water from the upstream should result in alteration of light environment in the downstream and the coastal areas and also should change in river morphology and particle-size composition of river-bed materials, both of which are expected to have negative impacts on ecological environments.
Acknowledgement
Takeshi Fujinami, Yasuhiro Murakami, Yu Inami, Hiromi Akita of the Civil Engineering Research Institute for Cold Region provided field assistance. This work was supported by JSPS KAKENHI Grant Numbers JP18H03819, and a research grant of the River Center of Hokkaido ‘Inter-disciplinary study on the sediment disaster in the Atsuma River catchment by the Hokkaido Eastern Iburi Earthquake’ (PI: Hideto Kon, Hokkaido University).