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In 2003, the Japan Coast Guard (JCG) adopted Airborne Lidar Bathymetry (ALB) to enhance water depth data quality in shallow coastal areas (including updating of nautical charts). These products are used to protect the country’s 30,000 km coastline–6th longest in the world–and to secure navigation safety. In shallow water where surveys from a vessel are difficult, ALB has the advantages of efficiently surveying a large area in a short, and private industry has adopted it. Other government agencies and private enterprises, however, have continued using conventional methods for shoreline surveys, bathymetric surveys, and river surveys because the initial investment for adopting ALB was too high to make it cost-efficient.
The unprecedented great earthquake that struck northeast Japan in March 2011 marked a turning point because it was recognized anew that sand beaches, sand dunes and seaside forests mitigated disaster to a certain extent and that coastal areas should not be managed by simple "lines" of coast but by overall comprehension of "faces" consisting of coast, lands, rivers and sea. After the earthquake, JCG had surveyed seabed topography only in 11 days by means of ALB in the disaster areas of Pacific coast of northeast Japan and the result was made publicly available. Thus ALB demonstrated its remarkable performance of surveying a vast sea area in a short time, in high resolution, efficiently and safely and to acquire topography continuously from land to sea and vice versa.
The majority of Japan’s population is concentrated on flood-prone areas near estuaries and is likely to suffer serious damage when flooded. Thus rivers must be surveyed periodically to check their topography for flood countermeasure planning and maintenance of various infrastructures. A large amount of manpower is consumed for such purposes. Even though it has been developed originally for sea surveying, in light of a looming workforce shortage, ALB is expected to play an important role to make up for the shortage of survey engineers.
Adoption of ALB
In 2012, Asia Air Survey Co., Ltd. (AAS) conducted a feasibility study for using ALB and in 2013 and 2014 it conducted intensive experimental surveys of rivers and sea in Japan with leased ALB equipment. Based on the assessment of these surveys, AAS adopted the Chiroptera II from Leica Geosystems in 2015 and, after completing operational tests, started surveying various locations in Japan in February 2016.
About 70% of the land area of Japan is mountainous and forested. ALB survey aircraft must maintain a low flying altitude above the ground (400 to 600m), which restricts ALB’s performance when measuring winding rivers running from mountainous inland water source. Under these circumstances, AAS had to use fixed-wing aircraft for surveying sea areas and helicopters for rivers. Thanks to the effort of those concerned, AAS received ALB surveying jobs from the Japanese government agencies in 2016 and these work orders are expected to increase.
Case Studies
The followings are 4 case studies: two of ocean measurement and two of river measurement.
Case 1 (Sea): Coastal area (Nanki Shirahama)
The site is a sand beach on a Ria coast located on southwest side of Kii Peninsula. It is wedged between capes on both ends (See Figure 1-1). A river flows in on eastern side and a fishing port is located on the right side of the estuary.
Detached breakwaters and artificial reefs are constructed about 100 meters offshore from the 1.5 km sand beach. This area contains all useable targets on both rivers and sea and is ideal test site for ALB surveying. The first ALB survey revealed that:
Landward side of the breakwaters and the reefs is shallow. Converged water flow between the breakwaters and the reefs has excavated seabed and formed scour which is deeper than the surrounding area by 3 meters. The offshore side has scour as well. Ripple marks are found on the riverbed flowing into the bay. An abyss of 10 meters or more in depth is seen at a hook of the river.
Accuracy verification was made by synchronizing NMB (Narrow Multibeam Echo Sounder of Teledyne Reson’s SeaBat8125) with ALB (Chiroptera II) and the result showed that the difference of water depth acquired by the two system is 10 ~ 20cm, which satisfies "IHO 1b" of bathymetry criteria of ALB for the International Hydrographic Organization.
Although bathymetry limit of ALB is about 20 meters (about 1.3 times Secchi depth), the survey of the entire area was completed in about 2 hours, while it would have taken about a week to survey the same area on boats.
Case 2 (Sea): Dredged navigation route in a port area (Nakagusukuwan port)
This port is located at Nakagusuku Bay in southeast Okinawa Island and is an important international distribution base in Naha Port, where a number of large vessels call. The bay is a shoal with coral reef where vast dry beaches emerge at low tide, hence periodical depth sounding and dredging are needed. Safe and efficient bathymetry on boats is difficult due to shallow water (see Figure 2-1) and frequent passage of vessels. ALB demonstrated its ability; a perfect survey was made to ensure dredging channel in the shallow water area where vessel survey is difficult (see Figure 2-2 and 2-3).
Case 3 (River): Riverbed scouring on downstream of bridge columns (Jinzu River)
Riverbed scouring was surveyed around the columns of a bridge (see Figures 3-1 and 3-2). The target has been scanned in an oblique direction, penetrating under the bridge and revealing topography of the riverbed. Depth of the river is 3 meters and the area surrounding the bridge columns is eroded deeper by 2 meters expanding to about 20 meters square. Odd-shaped streaks are formed on downstream side of the eroded area.
Survey of the area on boats would take time and create a safety risk due to the existence of numerous columns and complicated water flow. River water is sometimes turbid. In such a case, if preliminary ALB survey is conducted to minimize surveys on boats, safety risk can be reduced and consequently, working hours and cost can be reduced significantly.
Case 4: (River): Maintenance of erosion and sediment control (SABO) facilities in a torrential river (Joganji River)
This is the case of using ALB to survey SABO area of Joganji River flowing into Toyama Bay facing Sea of Japan. The river has total length of 56 kilometers but has vertical drop of 3,000 meters and is known as one of the steepest streams in the world. Large volume of sediment is carried from upper stream and trapped in Hongu SABO Dams. (The construction of these dams began in 1935 and was completed in 1937 and they are now Tangible Cultural Assets of Japan), which have the largest sedimentation volume in Japan: 5 million cubic meters.
In order to obtain maximum effect of ALB’s performance, a snow season was selected for the survey, because the water falling from the dams was expected to cause turbidity and bubbles. As a result, we were able to ascertain the shapes of the riverbed, especially the formation of water routes, sedimentation and scouring immediately downstream of the dams, which could not have been obtained by conventional airborne laser or on site survey. (See Figures 4-1 and 4-2)
This is a groundbreaking application of ALB to a river with different water levels. Since the limit of the processing algorithm has been known, the field of application of ALB will expand with cooperation of the supplier of ALB systems to upgrade the algorithm.
Figure 4-2 is a trademarked shaded relief map uniquely developed by AAS and is called "Red Relief Image Map".
About AAS
Since its establishment in 1954, aerial survey has been one of the core businesses of Asia Air Survey Co. (AAS). AAS surveys anything from every angle with the sensors mounted on aircraft, land vehicles, unmanned air vehicles (drones), etc. and analyzes the results to serve society by providing useful technologies in visible forms as a spatial information consultant. Need for consultants like AAS is ever increasing to protect our national land from natural calamities like earthquake and abnormal weather. AAS will keep on challenging the given task by mobilizing its technologies. For more information, please visit: http://www.ajiko.co.jp/en/
As head of the department in charge of maintenance and management of airborne sensors for AAS, Tomowo Ohga is responsible for aerial photography, aerial laser survey, and aerial laser bathymetry. He is also engaged in collecting information to research and select new technologies and equipment.
A 2.547Mb PDF of this article as it appeared in the magazine complete with images is available by clicking HERE