●Research Trends and Results

Attempt to detect seepage in levees from thermalimages
HATTORI Atsushi, HeadNAKAJIMA Haruma, Researcher
River Department, River Division(Key words) Levee inspection, thermal image, seepage, infrared radiation
1. Introduction
River levees are long structures which have,throughout the long history of flood control, been
constructed accompanied by repeated reinforcement
and repair according to the state of past disasters. Soin order to ensure the safety of river levees far into thefuture, it is important to perform inspections closely
coordinated with reinforcement to efficiently andreliably detect locations which impact safety and tomonitor their condition1). An inspection of a riverlevee is done basically by walking on it to visually
examine its condition, and such inspections are
performed over the full length of every levee once ayear. It is necessary for inspections to be done bylimited personnel and restricted budgets, so more
efficient methods are needed. And because the resultsof visual inspections vary in some cases because ofdifferences in the skills of inspectors, it is important todevelop methods of using measuring instruments toaccurately quantitatively clarify and analyze the stateof deformation. So as such a survey method, a methodof combining multiple physical exploration methods
such as electricity and magnetism to clarify soil
quality inside the levee, and a method of measuring
the shape of a levee in detail using an automobile
equipped with laser measuring instruments and GPS
are being studied.
The River Division is conducting research intended
to integrate, rationalize, and increase the efficiency ofinspections and reinforcement by evaluating the extent
that safety is ensured by levee inspections in the same
way as reinforcement work. As part of this effort, theDivision is studying a method of detecting seepage on
the surface of levees using thermal images.
2. Challenges to the application of the infrared
inspection method to river levees
The inspection method using infrared radiation is
now used to inspect and diagnose concrete structuresby measuring the diel variation of the surface
temperature caused by sunlight, air temperaturechange, etc. as thermal images, to detect defects basedon temperature differentials caused by differences inthermal capacity etc.

Table Challenges to the Application of theinfrared inspection method to river levees
Challenge
1

Does a temperature difference great enough to bedistinguished by a thermal image appear at seepage
on the slope of the levee?

Challenge
2

The levee slope is covered with vegetation, but is it
possible to detect a temperature difference on its
surface?

Challenge
3

The soil quality on the levee slope surface isheterogeneous, but to what degree does the soil
quality impact temperature differences?

Challenge4

On a long levee, it is difficult to obtain thermal
images under identical weather conditions, but is it
possible to detect defects regardless of conditions?

Challenge
5Is it possible to perform an efficient inspection basedon long-distance photography in relation to theabove?

The above table shows challenges to its application
to river levees.
3. Results and future challenges
Temperature and soil water content sensors wereburied in the slope surface of an actual levee and used
to measure diel variation at the same time as thermal
images were obtained using an infrared thermo-sensor.
The results confirmed that if the surface is bare groundunaffected by vegetation, parts with seepage and parts
without seepage create temperature differences(Challenge 1). In the future, the results will be further
analyzed to clarify weather conditions which arerepresented by large differences in the temperature.
Deepening studies will be conducted concerning otherchallenges to be resolved to introduce the method: theimpact of the degree of vegetation coverage and plantheight (Challenge 2), differences in soil materialvalues of the levee (Challenge 3), study of times ofday when detection is possible (Challenge 4), andimpact of photography distance limit and angles(Challenge 5). The Division wishes to continue studies
so that the results of the research will contribute tomore efficient and higher quality inspection
technologies to be used to inspect river levees.
[Reference]
1) Manual for River Works in Japan, Management(River), MLIT, 2011