Does sedimentation or erosion trigger river forestation? A numerical modelling approach

Transcription

1 5 th International Multidisciplinary Conference on Hydrology and Ecology April 215 Vienna Does sedimentation or erosion trigger river forestation? A numerical modelling approach Takashi Asaeda, Kelum Sanjaya, and Md Harun Or Rashid Saitama University asaeda@mail.saitama-u.ac.jp

2 Background of the study

3 Okotsk Hokkaido Tohoku Japan Sea Coast Sanriku/Joban Northern Kanto Tokyo Metropolitan Area Tokai Hokuriku Kii Peninsula Kansai Metropolitan Area Sanin Seto Inland Sea Coast Southern Shikoku Northern Kyushu Southern Kyushu Fraction of vegetation coverage Intensive forestation widely occurs in East Asian rivers. deteriorates the ecosystem of gravelly and sandy bars. affects flood protection. changes the landscape of rivers vegetation Vegetation coverage in Vegetation cover in Vegetation cover in Fraction of vegetation coverage increased in Japanese rivers Tama River (Tokyo) Ota River (Hiroshima) Asaeda et al. (213) Proc. IAHR

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5 6 yrs transition of the river channel An example of Sagami River near Tokyo 1947 October 1977 December present Channel is filled with stony sediment Low in vegetation Reduced sediment + Channel incision + Vegetation invasion

6 Sediment is deficit in the river channel Fukashiro Dam 5,m 3 Numamoto Dam Sagami Dam 7,,m 3 Sediment inflow 11,,m 3 3,,m 3 5,m 3 Doshi Dam 4,m 3 Sediment budget (>.2mm) between 1944 and 21 1,m 3 Shiroyama Dam 2,,m 3 Weirs 18,m 3 Sediment harvesting before ,,m 3 Miyagase Dam 1,,m 3 Remarkable amount of sediment was harvested before 1964 and trapped by dams Total inflow 14.5 milli m 3 << Total amount of harvesting 24 milli m 3 Total amount of trapped 1.55 milli m 3 Flow into sea??? Deficiency of sediment in the river channel

7 Vegetation colonization at deposited and eroded areas Flooded at Sep 27 View of 21 (3 years later) Eroded area Example of Arakawa River North of Tokyo Eroded area: exposure of underlying nutrient rich sediment Deposited area Deposited area: covered by cleansed sediment low in nutrient and seed bank Deficiency of sediment decreases the deposition at flood time Reduction of deposited areas introduces earlier colonization of vegetation The vegetation coverage increases even with the same flood frequency

8 1993 Example of Kurobe River Before flood Deposited location Eroded location 1996 After flood Deposited eroded Deposited yrs later eroded

9 Simulation with Dynamic Riparian Vegetation Model (DRIPVEM) Asaeda et al. River Res. Appl. (214) Online

10 Development processes of vegetation communities on the sediment bars Herbs Accumulation of nutrient Increase in biomass Repeated process of vegetation flushing, and the deposition of cleansed sediment (low nutrient) or exposure of surface sediment (same as previous nutrient) with floods Flushing of vegetation and accumulation of cleansed sediment For herb development, edaphic (nutrient and sediment size) condition must be processed after floods Growth with self-thinning Tree seeds are dispersed at floods at floods, then grow afterwards, decreasing by self-thinning. Trees Colonization of trees hydrochory

11 Model structure hydrology flooding Cleansed sediment deposition or erosion Recruitment of seeds Flushing of vegetation flooding herbaceous plants loss of herbs trees Recruitment of trees shading seeds mortality & decomposition loss of trees defoliation remaining trees soil nutrients denitrification Atmospheric fallout N-fixation

12 Tree density and biomass for organs (module TREE) Model scheme for each mesh d dt Tree density = Recruitment (flood characteristics) * Self-thinning function - Flush(inundation, age) Tree morphology, biomass = F(age) Herb biomass (module HERB) Herb biomass = F(soil N, particle size, shading by trees) Soil N concentration (module SOIL) d dt Soil N = - Uptake by plants (biomass, N content) + N fixation (biomass, fixed N content) + decomposition (dead biomass, fixed N content) + Atmospheric fallout - denitrification Geomorphology of the river channel, hourly flood level, sediment size : observed data used; The ground surface was divided into 1mx1m mesh size Initial condition : no vegetation at 5 years before Simulation time step: 1 month (with recorded hourly highest flood level in the month)

13 Composition of tree ages Number of mesh Number of mesh Distribution of Salix Observed Validation of the model Example of simulated results Hii River (after 5 years simulation) Distribution of herbs 21 Observed herb 21 Simulated herb distribution Arakawa River (after 2years simulation) Observation Salix Robinia Simulation Salix Robinia Simulated Age comparison Tree age (yrs) Observed Observed Simulated Simulated Simulated results Compatibility of vegetation coverage simulation to the observed data Hii River Total simulated mesh Bare soil 64.7% Herbs 8.4% Trees 75.4% Compatible mesh Asaeda et al. Riv Res Appli 214 Kuzuryu River Total simulated mesh Bare soil Herbs Trees 53.7% 79.3% Compatible 71.6% mesh

14 Avg.biomass(gdw/m 2 ) Simulated delay time of herb colonization (years) Avg.Biomass(gdw/m 2 ) Avg. Avg. iomass(gdw/m 2 ) Delay time of herb colonization Kuzuryu River Time(years) biomass(gdw/m 2 ) Hii River Time (years) Sagami River 5 Time(years) Kurobe River Time (years) Kuzuryu River Hii River Sagami River Kurobe River non-deposited Observed delay time of herb colonization (years)

15 Deposited area fraction Deposited area fraction Effects of deposition on the vegetation coverage - Long period simulation results- Conditions of simulation Geomorphology: Kuzuryu River Flood condition: 1/year, 1/1years Deposited area fraction: ~1. Simulation period: 5 years (until periodical steady condition is achieved) Vegetation coverage of periodical steady condition Every year flood Flood level (m) Observed condition Every 1 year flood Flood level (m) Vegetation cover increases with decreasing fraction of deposited area, as well as the flood intensity and flood frequency

16 Possible mechanism of the increased vegetation coverage in the river channel Harvesting of sediment / termination of sediment inflows by dams Reduction of movable sediment in the river channel Decrease in sediment deposition area at flood time Fast recovery of vegetation after the flood Increase in vegetation coverage in the river channel

17 Summary 1. Vegetation coverage is increasing according to the present observations. 2. Vegetation colonization after a flood is delayed by the deposition of cleansed sediment. 3. The sediment stock in the river channel has been reduced due to harvesting and termination of sediment inflows by upstream dams, consequently sediment deposition during floods was substantially decreased. 4. Decreasing sediment deposition area during floods has enhanced the vegetation coverage of the river channel. 5. A dynamic model was developed to describe the processes. The model consists of four interacting modules; hydrological processes, trees, herbaceous plant biomass, and nutrient concentration of the soil. It describes the recruitment and the later growth of trees, herbaceous plant biomass, the nitrogen concentration of the soil, and flushing of vegetation. Simulation results indicates the vegetation coverage decreases with the fraction of deposited area as well as flood intensity and frequency.

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