evaluation of the ecosystem functions of gravel...
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Evaluation of the ecosystem functions of gravel bars in rivers using drone
Sohei Kobayashi (Disaster Prevention Research Institute, Kyoto University)
View from 100‐above the ground, Tenryu River
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Pool
RiffleBar
Backwater
BackwaterForewater
Pool
Riffle
Pool
BarRiffle
Riverbed morphology, invertebrate community,their relationships
My study interests
Sediment conditions and river ecosystem
Natural rivers vs degraded rivers downstream of damIncrease in sediment supply by dam removal, sediment bypassing, sluicing
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My field study
Sampling net, white pan, sieve, forcepsethanol, pH/EC meter, current meter, etc.
Drone, batteries, sd cards
4‐5 years ago
Sampling net, white pan, sieve, forcepsethanol, pH/EC meter, current meter, etc.
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Gravel bars: basis of habitat diversity within reaches
Riffle
Pool
Wando
Fast flow, shallowcoarse
Homogeneous flow,
bed
With Gravel BarNo GB
Stagnant water, muddy
Pond
Heterogeneous flow, bed
↓Diverse habitats
Gravel bar formed at downstream of dam after sediment sluicing
Gravel bar formedafter upstream dam removal
Without GB(before dam removal)
With GB(after dam removal)
Riffle
Pool
Uniform flow
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Gravel bars: places of water filtration
Pool
Riffle
PoolSurface flow
Down‐wellingUp‐welling
Subsurface flow(hyporheic)
Bar
Bar
By the filtering process bars contribute to reduce turbidity in the downstream direction.
Exchanges of water between surface and subsurface (hyporheic) at bars
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Gravel bars: creation of thermal heterogeneity
Temperature change is usually smaller for hyporheic than surface water,which cause local difference in temperature within reaches.Hyporheic outflow act as nursery for fish juvenile and refugia for insect/fish during hot summer/cold winter.
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18
20
24, Oct. 26 28 30 1, Nov. 3
hyporheichyporheicsurfacesurface℃
Hyporheic outflow
Fish seek and stay cool places during hot periods (daytime of summer).
A large number of juvenile fish in hyporheic outflow
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Drone as a helpful tool to monitor gravel bars
The spatial scale of bars are 100 – 1000 m in many rivers.
It takes an hour or a whole day to walk around and determine places for survey.
Single flight (10‐20 min) is enough to overview the bar
In addition, many physical data of bars can be obtained from drone photos
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Photo alignment and detecting tie points Building dense point cloud
Building 3D polygonal mesh model Generation of orthomosaic and DEM
Orthoimage map and DEM
Take drone photos with camera facing vertically down and a high overlap between two consecutive images.A commercial software automatically creates orthomosaic image and DEM.
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0102030405060708090100
GRB1 GRB2 GRB6 GRB5 GRB3 GRB4
Vegetation cover (%)
ImageJ (color threshold, count pixel)
Vegetation cover
If there is a sufficient contrast in color or brightness, area of different cover types can be evaluated by a free image software
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Bed particle sizeIf you are interested in bed materials size larger than a few centimeter, a free software can also automatically define particle and measure size.
020406080
100
ダム上流ダム下流1ダム下流2
020406080
100
1 10 100Grain size (mm)
Cumulative pe
rcen
tage
Normal digital camera
Drone
020406080100120140
GRB1 GRB2 GRB6 GRB5 GRB3 GRB4
Average grain size (mm)
Upper Lower
Using ImageJ
Using Basegrain
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Original image Color threshold Binary
Volume
Short axis
Long
axis
Volume
Before segmentation After segmentation
Drift wood volume on the surface of bedYou can sometimes estimate volume of LWD from the image.
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Velocity of flow surface from video
STIV (space for time velocimetry)
A commercial software
Search line
Time
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Velocity of flow surface from video
0
10
20
30
40
‐0.1 ‐0.2 ‐0.3 ‐0.4 ‐0.5 ‐0.6 ‐0.7 ‐0.8 ‐0.9 ‐1 >1
20152017
0
10
20
30
40
‐0.1 ‐0.2 ‐0.3 ‐0.4 ‐0.5 ‐0.6 ‐0.7 ‐0.8 ‐0.9 ‐1 >1
20152017
Flow velocity (m/s)Freq
uency(%
)
2015 2017
Riffle Riffle
Riffle
Run
LSPIV (large‐scale particle image velocimetry)
Free (Fudaa‐LSPIV) and commercialsoftware are available
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Flow discharge
Flow velocity and water depth are needed to calculate discharge (m3/s).Water depth is still difficult to obtain from drone photo.
Non‐expensive fishfinder sonar can be used for depth measurement.
Deeper: record gps and depth (from 0.3‐0.6 to 80 m)
https://deepersonar.com/jp/ja_jp/home‐jpOkada et al., 2018River Engineering Symposium
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Detection of hyporheic outflow by thermography
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18
20
24, Oct. 26 28 30 1, Nov. 3
hyporheichyporheicsurfacesurface℃
Hyporheic outflow
Because of the temperature difference, hyporheic outflow can be detected by using drone with infrared camera, thermography.
Surface flowHyporheicoutflow
Gravel bar
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Infrared camera is usually used for detecting animals or electric problems of facilities.
Surface flow
Hyporheicoutflow
Temperature difference of >0.3‐0.5℃ can be detected visually.
Source of temperature variation: surface/subsurface/groundwater andmidflow/edge (heated by sunlight, or cooled by air)
• The difference in temperature between surface and subsurface changes by time of the day .
• You cannot detect hyporheic outflow if it flows under surface flow.• You need good camera stabilizer and tilt (angle control) to keep camera vertically
down and get good resolution images.
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100m
100m
Main‐flow (surface flow)
Hyporheic outflow (subsurface flow)
Can you detect hyporheic outflow?
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Can you detect hyporheic outflow?
100m
100m
Main‐flow (surface flow)
Hyporheic outflow (subsurface flow)
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The purpose is to show what kind of gravel bar has high filtering potential
0 10 20 30 0 0.2 0.4 0.6 0.8 1 0 1 2 3 4 5代表粒径D50 (cm) 水面標高差(m) 標高差勾配(%)
湧出場なし
湧出場(パッチ状)
湧出場(伸長的)
Bar characteristics
Hypo
rheicexchan
ge
potential
1st year: Oct. 2015 2nd year: Jan. 2017
Hyporheic outflow 1.2 m3/sHyporheic outflow 0.12 m3/s
43 m3/s38 m3/s
Kobayashi et al. 2017River engineering symposium
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Various thermography drones are available,
but are probably still very expensive.
Marvic 2 pro
Phantom 4 Phantom 3
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What I tried at first
Flir One
DJI Phantom3 Professional
Android
Drone camera Flir one &Android
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Summary
• Spatial scale of gravel bars (100m‐1km, reach scale) is suited to use drones.
• Powerful software and free software for analyzing river environment are increasing.
• Gravel bars are the basis of habitat diversity and hyporheic exchange of water.
• Hyporheic outflow around bars can be detected by drones with infrared camera.
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Thank you