5:15 PM - 6:30 PM
[HCG28-P01] Preliminary study on the relationships between landform of coastal to alluvial plain and sediments supply from watershed
Keywords:Geomorphology, Denudation, Sedimentation, Watershd, On-going earth processes
1. Introduction
Sedimentation data in water reservoirs became to be accumulated and annually published (e.g. Water Management and Land Conservation Bureau, MLIT, 2012), we are able to concretely discuss on the relationships between sediment production in upper course of watershed and sedimentation in the lower course of it. The presenter tried to make roughly clear some relationships among volume of the Holocene alluvium deposits, estimated duration of its sedimentation (temporary fill-up duration: estimated from sediment supply and volume of the Holocene alluvium) and geomorphological features especially river-channel landform, taking the case of seven coastal to alluvial plain which have different rate of sediment supply and geomorphological shape of sedimentary basin of the Holocene deposits.
2. Subject
ATST have continued a research project named “Investigation on Geology and Active Faults in the Coastal Zone of Japan” since 2007, its purpose is to make clear continuity of sedimentary bodies and geological structures. Subjects of this study are 7 areas (Table 1) of this AIST project except for Noto area (which do not accompany with alluvial plain).
3. Data
The author used deposition rates measured in water reservoir after Hasegawa et al. (2005) as basic data and Water Management and Land Conservation Bureau MLIT (2012) as supplementary data for estimating denudation rates in the upper course of the watershed (D), and roughly estimated sediment supply rates (S) by multiplying the area of watershed except for depositional area (Aw) and denudation rate (D). Volume of the Holocene alluvium and coastal deposits (Aa) is roughly estimated by multiplying mean thickness of the Holocene deposits (Ta) and total area of the Holocene alluvial and coastal plain (Aa). However, denudation rate (D) is short periods’ one (during only several ten years) when artificial action has done, so it may be larger than natural and long-term rate. And volume of the Holocene deposits (Aa) includes large errors (The author did not use GIS calculation).
4. Results
Va, S, temporary fill-up duration (F = Va / S), distance between recent river mouth and outer edge of the continental shelf (breadth of continental shelf), and so on in each district are shown in Table 1. This table indicates as follows. 1) The meandering channel and branching channel zones do not develop and only braided channel zone is dominant in the Sagami and Shizuoka Plains which F is less than several thousand years and less than several kilo-meters in breadth of continental shelf. 2) The braided, meandering and branching channel zones develop in the Niigata, Nobi and Tokusima Plains where F is several thousand years, breadth of continental shelf is several tens kilo meters. 3) The braided and meandering channel zone do not develop in the Yufutsu and Fukuoka Plains where F is larger than ten thousand years.
5. Discussion
F values of the Sagami and Shizuoka Plains are significantly shorter than actual duration of the Holocene deposits (several thousand to ten thousand years) and both plains directly face to deep sea. Much of sediments supplied to these plains should removed to deep sea bottom in short period. The other hand, F values of the Yufutsu and Fukuoka Plains are longer than ten thousand years, and these plains’ deposits include large amount of organic materials and they accompanies beach ridges. This investigation is no doubtfully loose one, however the author think it also have some values for discussion on sediment linkage from mountain, plain and sea.
Refference
Hasegawa, K., Wakamastu, K. and Matsuoka, M. (2005) Mapping of potential erosion-rate evaluated from reserboir sedimentation in Japan. Jouranal of Japan Society of Natural Disaster Science, 24, 287-301.
Water Management and Land Conservation Bureau, MLIT (2012) Situation of the reservoir sedimentation in whole Japan (at end of the 2008 fiscal year) (http://suigenren.jp/wp-content/uploads/2012/09/H20zenkokutaisade-ta.pdf)
Sedimentation data in water reservoirs became to be accumulated and annually published (e.g. Water Management and Land Conservation Bureau, MLIT, 2012), we are able to concretely discuss on the relationships between sediment production in upper course of watershed and sedimentation in the lower course of it. The presenter tried to make roughly clear some relationships among volume of the Holocene alluvium deposits, estimated duration of its sedimentation (temporary fill-up duration: estimated from sediment supply and volume of the Holocene alluvium) and geomorphological features especially river-channel landform, taking the case of seven coastal to alluvial plain which have different rate of sediment supply and geomorphological shape of sedimentary basin of the Holocene deposits.
2. Subject
ATST have continued a research project named “Investigation on Geology and Active Faults in the Coastal Zone of Japan” since 2007, its purpose is to make clear continuity of sedimentary bodies and geological structures. Subjects of this study are 7 areas (Table 1) of this AIST project except for Noto area (which do not accompany with alluvial plain).
3. Data
The author used deposition rates measured in water reservoir after Hasegawa et al. (2005) as basic data and Water Management and Land Conservation Bureau MLIT (2012) as supplementary data for estimating denudation rates in the upper course of the watershed (D), and roughly estimated sediment supply rates (S) by multiplying the area of watershed except for depositional area (Aw) and denudation rate (D). Volume of the Holocene alluvium and coastal deposits (Aa) is roughly estimated by multiplying mean thickness of the Holocene deposits (Ta) and total area of the Holocene alluvial and coastal plain (Aa). However, denudation rate (D) is short periods’ one (during only several ten years) when artificial action has done, so it may be larger than natural and long-term rate. And volume of the Holocene deposits (Aa) includes large errors (The author did not use GIS calculation).
4. Results
Va, S, temporary fill-up duration (F = Va / S), distance between recent river mouth and outer edge of the continental shelf (breadth of continental shelf), and so on in each district are shown in Table 1. This table indicates as follows. 1) The meandering channel and branching channel zones do not develop and only braided channel zone is dominant in the Sagami and Shizuoka Plains which F is less than several thousand years and less than several kilo-meters in breadth of continental shelf. 2) The braided, meandering and branching channel zones develop in the Niigata, Nobi and Tokusima Plains where F is several thousand years, breadth of continental shelf is several tens kilo meters. 3) The braided and meandering channel zone do not develop in the Yufutsu and Fukuoka Plains where F is larger than ten thousand years.
5. Discussion
F values of the Sagami and Shizuoka Plains are significantly shorter than actual duration of the Holocene deposits (several thousand to ten thousand years) and both plains directly face to deep sea. Much of sediments supplied to these plains should removed to deep sea bottom in short period. The other hand, F values of the Yufutsu and Fukuoka Plains are longer than ten thousand years, and these plains’ deposits include large amount of organic materials and they accompanies beach ridges. This investigation is no doubtfully loose one, however the author think it also have some values for discussion on sediment linkage from mountain, plain and sea.
Refference
Hasegawa, K., Wakamastu, K. and Matsuoka, M. (2005) Mapping of potential erosion-rate evaluated from reserboir sedimentation in Japan. Jouranal of Japan Society of Natural Disaster Science, 24, 287-301.
Water Management and Land Conservation Bureau, MLIT (2012) Situation of the reservoir sedimentation in whole Japan (at end of the 2008 fiscal year) (http://suigenren.jp/wp-content/uploads/2012/09/H20zenkokutaisade-ta.pdf)