To understand the relationship between turbid water and coral damage caused by the heavy rain disaster at the end of October 2010 in Amami-Oshima, Kagoshima Prefecture, Japan, turbid water and coral damage distribution monitoring was attempted using satellite imagery and a diving survey immediately after the disaster. ALOS AVNIR-2 images (spatial resolution: 10 m) on October 6 (before the disaster), October 24, October 30, and October 31 (after the disaster) were obtained as satellite data in 2010. The red-silt deposition index (RSI) map based on the method by Nadaoka and Tamura (1992) was also created. Moreover, a diving survey was conducted via the spot check method on December 18, 2010. As a result, comparison between the high turbidity (RSI) areas estimated using AVNIR-2 data and the coral damage areas judging from the field survey was considered relatively light. It is shown that satellite data such as AVNIR-2 can be a powerful tool to monitor damage distribution of coral reefs after heavy rain.
Catastrophic natural disasters in recent years, such as the volcanic ash due to the explosion of Eyjafjallajökull, Iceland in April 2010 [
In Japan, “red soil pollution” is notorious for causing the same problem as that caused by the sediment discharge in the coral reef region around Okinawa and the other Nansei Islands. When there is heavy rainfall on the island of Okinawa, coral reefs near river mouths often develop a “cloudy red” appearance where once there was clear water. Most of the red soil carried to the sea by rivers is deposited on semienclosed reef moats adjacent to the coast, and the red soil is often repeatedly stirred up by turbulence, including typhoons and monsoons [
In the interior of the Amami-Oshima Island, Kagoshima Prefecture, Japan, a subtropical evergreen broad- leaved forest that is the largest in the country spreads across the study site. Many unusual and rare animals and plants live in this forest. Moreover, the coastal region has various environments such as coral reefs, mangroves, and tidal flats. Therefore, the land area and ocean space are highly valued for their biodiversity in an area of very high preservation importance, internationally as well as in the country. In such an important area, due to the record heavy rains late in October 2010, turbid water streams flowed out of the river into the sea, seriously damaging the coral reef. The circumference of the Sumiyo Bay, which has mangrove forests on a leading national scale, especially tidal flats, was most critically damaged. Because a field survey of the water area itself was conducted, measures against damage to land areas have been prioritized. In many cases, the area following such a catastrophic disaster is dangerous, and investigation of damage to corals immediately after a disaster is very difficult. In contrast, urgent observation by aircraft or satellites can be rapidly performed as one of the monitoring tools for large-scale disasters that have frequently occurred in recent years [
From this background, satellite imagery immediately after the heavy rain disaster that occurred in Amami- Oshima and subsequent diving survey data of October 2010 are used, and the turbid water distribution of the coral reefs and understanding of the coral damage distribution situation are discussed in this study.
The region of this study is the Amami-Oshima Island located in Kagoshima about 400 km southwest of Kagoshima City. The area in which the coral reef in the Amami Archipelago is located is 27˚ - 29˚ north latitude and accounts for a coral community of about 6000 ha calculated from the moat area. The diversity of hermatypic coral as compared with the coral reef community in the same latitude is also rich. Approximately 220 species of hermatypic corals have been identified in the Amami sea area [
A diving survey was conducted at four points at a depth of 3 - 4 m around Tobira Island on December 18, 2010, two months after the heavy rain disaster, as shown in
Stn. | Name | Latitude | Longitude | Depth |
---|---|---|---|---|
1 | Takahama-Higashi | 28˚13'50.4" | 129˚27'27.5" | 4 m |
2 | Takahama | 28˚14'00.7" | 129˚27'11.8" | 3 m |
3 | Hatonosaki | 28˚13'56.9" | 129˚27'02.5" | 3 m |
4 | Tobira | 28˚13'53.2" | 129˚26'46.1" | 4 m |
To determine the turbid water distribution on the coral reef before and after the heavy rain disaster, ALOS AVNIR-2 data (level 1B product) were used over four days, i.e., October 6, 2010, October 24, October 30, and October 31, as shown in
Data were acquired under observation conditions (different sun elevation and camera angle) from which AVNIR-2 target data differ each day. That is, in addition to the change in spectral characteristics reflecting the change in muddiness of the offshore water, there are also spectral changes due to atmospheric absorption and dispersion. Thus, atmospheric correction was performed to reduce the influence of spectral change on interpreting the color or brightness under turbid water conditions. There is a method assuming the near-infrared reflectance of open sea to first be 0 as the atmospheric correction of ocean space [
To estimate the deposition of sediment from the land, the red-silt deposition index (RSI) by Nadaoka and Tamura [
Date | Time | Sun Elevation | Pointing Angle | Sea Level (MSL) |
---|---|---|---|---|
(˚) | (˚) | (cm) | ||
Oct. 6 | 10:50 | 53 | −30 | 43 |
Oct. 24 | 11:32 | 49 | 39 | 98 |
Oct. 30 | 10:41 | 44 | −41.5 | 151 |
Oct. 31 | 11:22 | 47 | 24 | 143 |
Band | Band 1: 420 - 500 nm |
---|---|
Band 2: 520 - 600 nm | |
Band 3: 610 - 690 nm | |
Band 4: 760 - 890 nm | |
Spatial resolution | 10 m (at nadir) |
Swath | 70 km |
Quantization | 8 bit |
Date | Band | A | b | R2 |
---|---|---|---|---|
Oct. 24 | 1 | 1.21 | 8 | 0.92 |
2 | 1.12 | 8.7 | 0.99 | |
3 | 1.17 | 6.6 | 0.97 | |
4 | 1.18 | 4.4 | 0.90 | |
Oct. 30 | 1 | 1.51 | −84.5 | 0.91 |
2 | 1.48 | −48.1 | 0.96 | |
3 | 1.31 | −10.2 | 0.92 | |
4 | 0.81 | −1.5 | 0.96 | |
Oct. 31 | 1 | 1.26 | 7.6 | 1.00 |
2 | 1.15 | 13.5 | 0.99 | |
3 | 1.26 | 2.2 | 1.00 | |
4 | 0.75 | 7.1 | 1.00 |
Coefficient of relative atmospheric correction = a + b ´ (DN), R2 = Coefficient of deter- mination.
Here, Vλ1 and Vλ2 are TM band 1 (420 - 520 nm) and TM band 2 (520 - 600 nm), respectively, and V∞ shows the darkest pixel value (clear open ocean water) at each band. In this study, Vλ1 and Vλ2 were transposed to AVNIR-2 band 1 (420 to 500 nm) and band 2 (520 to 600 nm), respectively. In addition, for V∞, the radiance value (average value of 11 × 11 window pixels) of a water area (N28˚14'26", E129˚27'26"), which had sufficient depth in Sumiyo Bay on October 6 (before the disaster), was used.
To investigate the relation between satellite imagery and geographical features, the water depth expected for the diving survey and elevation data used are from the “Bathymetric Chart” of the Japan Coast Guard and “Digital Map 50 m Grid (Elevation)” from the Geospatial Information Authority of Japan (GSI), respectively. A Bathymetric Chart was made from overlays to obtain true color images of AVNIR-2. Digital Map 50 m grid. It was changed to a 3-D image using the AVNIR-2 image with Digital Map 50 m grid interpolated at 10 m.
An outline of the diving survey results on December 18, 2010, two months after the disaster, in an approximately 2-km2 area of Tobira Island of Sumiyo Bay is shown in
AVNIR-2 natural color composite images of before and after the heavy rain disaster are shown in
Stn. | Coral | Death/ | ||
---|---|---|---|---|
Coverage | Dominant | Bleaching | Transparency | |
(%) | Coral Community | Rate (%) | (m) | |
1 | 60 | Tabular Acropora | 0 | <10 |
2 | 40 | Tabular Acropora | 0 | <10 |
3 | <5 | Massive Porites | <5/<5 | <3 |
4 | 10 | Massive Porites & Branching Acropora | 30/30 | <3 |
the heavy rain on October 20, the sea surface of eastern Amami-Oshima became remarkably turbid, as seen in the image for October 24 (upper part of
As mentioned above, from the result of the diving investigation, chlorosis of Acropora and Porites was observed at Stn. 3 and Stn. 4, but healthy coral remained at Stn. 1 and Stn. 2. A massive amount of sediment brought about from the small river due to the heavy rain (photograph on the leftmost side of
confirmed once more through this study using high-resolution satellite data such as AVNIR-2 that turbidity and geographical features are also very important parameters to growing coral.
In this study, to understand the relation between turbid water and coral damage, turbid water and coral damage distributions were estimated in some areas of the circumference of Tobira Island whose coral damage was serious using a diving survey and satellite imagery of the heavy rain disaster that occurred at the end of October 2010. As a result, comparison between the high turbidity (RSI) areas estimated using AVNIR-2 data and the coral damage area judging from field survey was considered relatively light.
These observations can continue for a long period of time in this water area from now on as a method of elucidating the mud deposition mechanism on the coral reefs of Amami-Oshima. I would like to estimate the amount of mud deposition by combining satellite data and local observational data in the future.
This study was supported in part by the River Fund under the Foundation of River and Watershed Environment Management (FOREM), Japan and JSPS KAKENHI (24560623, 80313295). The ALOS AVNIR-2 data used in this study were offered by JAXA under the agreement of the JAXA Research Announcement entitled “Monitoring of water pollution and aquatic plants in coastal lagoon environments using ALOS data” (JAXA-PI 324).
YujiSakuno,KatsukiOki, (2015) Relationship between Turbid Water and Coral Damage Distribution Using ALOS AVNIR-2 Images and Diving Survey Data Immediately after the Heavy Rain Disaster of the Amami-Oshima Island, Japan. Advances in Remote Sensing,04,25-34. doi: 10.4236/ars.2015.41003