Evaluation of the ecosystem functionsof gravel bars in rivers using drone
View from 100‐above the ground, Tenryu River

1Sohei Kobayashi (Disaster Prevention Research Institute, Kyoto University)

My study interests
Riverbed morphology, invertebrate community,
their relationships
Forewater

Backwater
Pool

Bar
RiffleRiffleRiffle

Bar

Pool

Pool

Backwater

Sediment conditions and river ecosystem
Natural rivers vs degraded rivers downstream of dam
Increase in sediment supply by dam removal, sediment bypassing, sluicing

2

My field study

4‐5 years ago

Drone, batteries, sd cards
Sampling net, white pan, sieve, forceps
ethanol, pH/EC meter, current meter, etc.

Sampling net, white pan, sieve, forceps
ethanol, pH/EC meter, current meter, etc.

3Gravel bars: basis of habitat diversity within reaches
Gravel bar formed at downstream of dam
after sediment sluicing

Gravel bar formed
after upstream dam removal

Without GB(before dam removal)

With GB(after dam removal)

Heterogeneous
flow, bed↓
Diverse habitats

Pool
Uniformflow

With Gravel BarNo GBRiffle

Homogeneous flow,bedPool

Wando
Pond

RiffleStagnant water,muddy
Fast flow, shallow
coarse4

Gravel bars: places of water filtration
Exchanges of water between surface and subsurface (hyporheic) at bars
Surface flow
Subsurface flow(hyporheic)

PoolBarDown‐welling
Up‐welling

RifflePoolBarBy the filtering process bars
contribute to reduce turbidity inthe downstream direction.
5

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.
hyporheic
hyporheic
surfacesurface

20℃
18Hyporheic outflow
1624, Oct. 26

Fish seek and stay cool places during
hot periods (daytime of summer).

28

30

1, Nov.

3

A large number of juvenile fish in
hyporheic outflow
6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 walkaround and determine places for survey.

Single flight (10‐20 min) is enough tooverview the bar

In addition, many physical data of bars can be obtained from drone photos

7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.

Photo alignment and detecting tie points

Building 3D polygonal mesh model

Building dense point cloud

Generation of orthomosaic and DEM

8Vegetation coverIf there is a sufficient contrast in color or brightness, area of different cover types
can be evaluated by a free image software

Vegetation cover (%)
1009080706050403020100GRB1 GRB2 GRB6 GRB5 GRB3 GRB4ImageJ (color threshold, count pixel)

9Bed particle size
If you are interested in bed materials size larger than a few centimeter, a free
software can also automatically define particle and measure size.
1008060
40200100
80
60
40
200Cumulative percentage

Using Basegrain

Normal digital camera

Using ImageJ

ダム下流2ダム下流1ダム上流

Drone

110100Grain size (mm)Average grain size (mm)140
120
1008060
40200GRB1 GRB2 GRB6 GRB5 GRB3 GRB4Upper Lower
10

Drift wood volume on the surface of bed
You can sometimes estimate volume of LWD from the image.

Before segmentation

Color thresholdBinary

After segmentation
Long axis

Original image

VolumeVolumeShort axis

11

Velocity of flow surface from video

STIV (space for time velocimetry)

Search lineTime

A commercial software

12

Velocity of flow surface from video
LSPIV (large‐scale particle image velocimetry)
Free (Fudaa‐LSPIV) and commercial
software are available

2015

Frequency (%)

40

2017

Riffle

30

2015
2017

20
100‐0.1 ‐0.2 ‐0.3 ‐0.4 ‐0.5 ‐0.6 ‐0.7 ‐0.8 ‐0.9 ‐1 >140Run

2015
2017

3020Riffle

Riffle100‐0.1 ‐0.2 ‐0.3 ‐0.4 ‐0.5 ‐0.6 ‐0.7 ‐0.8 ‐0.9 ‐1 >1
Flow velocity (m/s)

13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‐jp
Okada et al., 2018
River Engineering Symposium

14Detection of hyporheic outflow by thermography
Because of the temperature difference, hyporheic outflow can be detected by using
drone with infrared camera, thermography.
hyporheic
20
hyporheic
surface℃
surface18

Hyporheic outflow
1624, Oct. 26

Surface flow

Infrared camera is usually used for detecting
animals or electric problems of facilities.

28

30

1, Nov.

3

Hyporheic
outflow

Gravel bar
15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)Hyporheicoutflow

Surface flow

• The difference in temperature between surface and subsurface changes by timeof 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 verticallydown and get good resolution images.
16Can you detect hyporheic outflow?
Main‐flow (surface flow)

Hyporheic outflow (subsurface flow)

100m

100m

17

Can you detect hyporheic outflow?
Hyporheic outflow (subsurface flow)

Main‐flow (surface flow)

100m

100m
18

The purpose is to show what kind of gravel bar
has high filtering potential
2nd year: Jan. 2017

1st year: Oct. 2015
38 m3/s

Hyporheic exchange
potential

Hyporheic outflow

43 m3/s

1.2 m3/s

Hyporheic outflow

0.12 m3/s

湧出場
（伸長的）

湧出場
（パッチ状）

湧出場
なし
0102030
代表粒径D50 (cm)

00.2

0.4

0.6

0.8

1

水面標高差（m）

Bar characteristics

012345標高差勾配（%）

Kobayashi et al. 2017
River engineering symposium
19Various thermography drones are available,Marvic 2 pro
Phantom 4

Phantom 3

but are probably still very expensive.
20

What I tried at first
DJI Phantom3 Professional

Android
Flir One

Drone camera

Flir one &Android21

Summary• Spatial scale of gravel bars (100m‐1km, reachscale) is suited to use drones.
• Powerful software and free software for analyzing
river environment are increasing.
• Gravel bars are the basis of habitat diversity andhyporheic exchange of water.
• Hyporheic outflow around bars can be detectedby drones with infrared camera.

Thank you22