Image Analysis-Based Flow Observation Software, Part 1
Background

Data about flow rates obtained from flow observation is critical, forming the core of river management
and road planning. Since around 1955, high-water (flood) flow observations in Japan have been
performed using the floater method, which involves the use of rod-shaped floaters (rod floaters).
However, as flooding has intensified over the past several years, surveyors have been unable to takemeasurements because they cannot safely access rivers in an increasing number of cases. Securingpersonnel is also an issue due to avoidance of long hours and the decreasing birth rate and aging
population in rural areas.

There are many issues with the floater method

Measurement section≧50m

Approach section≧30m

Measurement posts are
red and black・Difficulty securing personnel
・Difficulty setting up by the time flooding peaks
・Accuracy of observations depends on personnel

Floater

・Working during storms is dangerous
・Floaters cannot flow downstream smoothly because of trees,
Floater input section

No. 1 measurement section

wake from bridge piers, etc.

No. 2 measurement section

Source: E de miru suimon kansoku (Hydrological observation illustrated)

A new method of observation is needed to replace thefloater method

Image analysis: Space-time image velocimetry (STIV)
A method of measuring river surface flow velocity based on video images,
developed by Kobe University Professor Emeritus Ichiro Fujita
Space-time image (STI) 1

Time (10 seconds in this case)

Image at 1/30 sec

STI
Image at 2/30 sec

Image at 3/30 sec

Measurement line (Length: 15 m)

Video speed: 30 frames/second

Space-time image (STI) 2

Length of measurement line
(15 m in this case)

Measurement line

Space-time image (verticallyalong measurement line)
TimeBrightness data along measurement line
Captured image (at time T+◿T)

Image at 10 sec

Measurement line (Length: 15 m)

Time

Captured image (at time T_END)
Illustration of STI

The striped pattern of STI
represents the buildup ofdebris, foam, ripples, and thelike over time along themeasurement line

Striped pattern

Image filter used to emphasize the stripedpattern

Path of blue ball alongmeasurement line (the gradient isthe speed in this case)Captured image (at time T)

Flow velocity V is determined by finding the gradientof the striped pattern (tanφ)Gradient of the striped pattern determined using the brightness
gradient tensor method

Divide the STI by the specificarea size to determine theangle from the brightness
gradient

Heavy weighting when thepattern is clear, lightweighting when the patternis blurryVideos of river surfaces captured with a video camera are displayed on a computer screen, and measurement lines are drawn on the river surface. The measurement lines are roughly 1 pixel thick, and thelength in this case is the actual scale: 15 m (actual length depends on the filming conditions). The video is played with the lines drawn, and brightness data along the measurement lines are extracted andlined up for each frame as the video is played. The two-dimensional image resulting from the alignment of this brightness data is known as the space-time image (STI), and it has a horizontal axis the samelength as the measurement lines (15 m) with time on the vertical axis. The shooting speed of videos in Japan is 30 frames per second, so if a video is 10 seconds long, there will be 300 measurement lines(300 pixels) lined up vertically. Ripples, debris, or the like observed flowing along the measurement lines in videos appear as diagonal stripes in STI. The slopes of these diagonal stripe patterns represent
the flow velocity (Length (distance) ÷ Time); therefore, in STIV, lines are set automatically, and their slopes are determined and used to calculate flow velocity.

Image Analysis-Based Flow Observation Software, Part 2
Software for simple, high-precision calculations of flow velocity and flow rate from images and water level data
Screenshot of the system in operation

Main Screen

Geometric conversion image

Cross-section flow velocity
display

STIV operator

Procedure
Screenshot: Loading

Screenshot: Cross-section/measurement line setting

Conclusion

Screenshot: Geometric conversion

Screenshot: Flow velocity/flow rate calculation

・Equipment costs less than other methods (monitoring cameras can be used instead of video cameras)
・Using stationary cameras in the field is safer during floods because personnel do not need to work in the field・Accuracy of observation is at least as good as the floater method
・Continuous observation is possible
・Images are saved, making it possible to verify the causes of abnormal values at a later time・Easy to operate (operations are intuitive)*The software (Hydro-STIV) is sold by Hydro Technology Institute Co., Ltd.