9:45 AM - 10:00 AM
[ACG43-04] Investigation of Longshore Drift Sediment Movement from the Mouth of the Tedori River to Chirihama Beach Using the IRSL Method, Japan
Keywords:drifting sand, coastal erosion, IRSL, tracer
1. Introduction
Chirihama, located in Ishikawa Prefecture, is a unique sandy beach where cars can drive. However, beach erosion has accelerated. To initiate effectively beach conservation measures, it is important to hold comprehensive understandings of the sediment transport from the mouth of the Tedori River to Chirihama Beach. In this study, a preliminary investigation was conducted on longshore drift sediment movement using the IRSL method.
2. Materials and Methods
The luminescence method has been widely used in studies of sediment transport in fluvial or coastal environments. This is due to the attenuation of luminescence signals by exposure to sunlight (Rink et al., 2003; McGuire et al., 2014). Sato et al. (2020) also revealed that the IRSL intensity of feldspar decreases from the upper Tedori River to the mouth of the river and beach along the western coast of Ishikawa Prefecture.
To understand widespread longshore drift sediment movement, this study was conducted in the coastal zone along the western coast of Ishikawa Prefecture in water depths up to 10m. Surface sediments were collected between September and November 2022 (Figure. 1)
a. Grain Size Analyses
A sieve analysis was conducted on each sample in accordance with JIS standards.
b. IRSL Analyses
The collected samples underwent a series of procedures, including washing with water, drying, and sieving to extract sand with a grain size of 150-212µm. K-feldspar (2.60<ρ<2.62) was then extracted using heavy liquid separation and magnetic separation, and the IRSL intensity was measured.
An automatic IRSL measuring machine (co-developed with Medec) was used for IRSL measurements. Every sample was heated to 50℃ while irradiated with infrared rays, and the number of photons of blue light (450 nm) emitted from K-feldspar was counted. Continuously, same measurements were carried out at 225℃. The number of photons at a test dose (42.6 Gy) for the same sample was also counted and normalized. In this study, the results of 225℃ measurement (pIRIR225) were analyzed.
c. IRSL Intensity Distribution
The IRSL measurement results and ArcGIS (ESRI) were used to create an IRSL intensity distribution map, which was overlaid on the aerial photograph.
3. Results and discussion
a. Grain Size Analyses
Figure. 2 shows the results of grain size analysis. The shoreline of Chirihama (46km) predominates, with approximately 60% of the sand with a grain size of 150-212µm. The sand with this grain size is present throughout almost the entire coastal area, suggesting that it travels from the mouth of the Tedori River to Chirihama along the coastal area up to a water depth of 10m.
b. IRSL Analyses
Figure. 3 shows the IRSL intensity distribution of the feldspar with a grain size of 150-212 µm. The trend IRSL intensity decrease appears to be uncertain from the mouth of the Tedori River to Kanazawa Port (IRSL intensity range: 0.07-0.19). However, it is observed to decrease rapidly in the northeastern part of Kanazawa Port (IRSL intensity range: 0.03-0.15). In addition, in the more northeastern region, similar low IRSL intensities are distributed regardless of water depth (IRSL intensity range: 0.01-0.04). According to the results, it appears that the IRSL intensity decreases as one moves from the mouth of the Tedori River towards the northeast. This suggests drifting sediment moves in that direction. Besides, sand with low IRSL intensity accumulates on the northeast side of Kanazawa Port, including Chirihama. This suggests that a considerable amount of the 150-212 µm sand that is newly released from the Tedori River does not reach the northeast side of Kanazawa Port.
4. Conclusion
The study investigated the longshore drift sediment movement from the mouth of the Tedori River to Chirihama using the IRSL method. The results suggest that Kanazawa Port impedes the transport of coastal sand drift towards the northeast, which in turn affects the supply of sand to Chirihama and contributes to coastal erosion.
Chirihama, located in Ishikawa Prefecture, is a unique sandy beach where cars can drive. However, beach erosion has accelerated. To initiate effectively beach conservation measures, it is important to hold comprehensive understandings of the sediment transport from the mouth of the Tedori River to Chirihama Beach. In this study, a preliminary investigation was conducted on longshore drift sediment movement using the IRSL method.
2. Materials and Methods
The luminescence method has been widely used in studies of sediment transport in fluvial or coastal environments. This is due to the attenuation of luminescence signals by exposure to sunlight (Rink et al., 2003; McGuire et al., 2014). Sato et al. (2020) also revealed that the IRSL intensity of feldspar decreases from the upper Tedori River to the mouth of the river and beach along the western coast of Ishikawa Prefecture.
To understand widespread longshore drift sediment movement, this study was conducted in the coastal zone along the western coast of Ishikawa Prefecture in water depths up to 10m. Surface sediments were collected between September and November 2022 (Figure. 1)
a. Grain Size Analyses
A sieve analysis was conducted on each sample in accordance with JIS standards.
b. IRSL Analyses
The collected samples underwent a series of procedures, including washing with water, drying, and sieving to extract sand with a grain size of 150-212µm. K-feldspar (2.60<ρ<2.62) was then extracted using heavy liquid separation and magnetic separation, and the IRSL intensity was measured.
An automatic IRSL measuring machine (co-developed with Medec) was used for IRSL measurements. Every sample was heated to 50℃ while irradiated with infrared rays, and the number of photons of blue light (450 nm) emitted from K-feldspar was counted. Continuously, same measurements were carried out at 225℃. The number of photons at a test dose (42.6 Gy) for the same sample was also counted and normalized. In this study, the results of 225℃ measurement (pIRIR225) were analyzed.
c. IRSL Intensity Distribution
The IRSL measurement results and ArcGIS (ESRI) were used to create an IRSL intensity distribution map, which was overlaid on the aerial photograph.
3. Results and discussion
a. Grain Size Analyses
Figure. 2 shows the results of grain size analysis. The shoreline of Chirihama (46km) predominates, with approximately 60% of the sand with a grain size of 150-212µm. The sand with this grain size is present throughout almost the entire coastal area, suggesting that it travels from the mouth of the Tedori River to Chirihama along the coastal area up to a water depth of 10m.
b. IRSL Analyses
Figure. 3 shows the IRSL intensity distribution of the feldspar with a grain size of 150-212 µm. The trend IRSL intensity decrease appears to be uncertain from the mouth of the Tedori River to Kanazawa Port (IRSL intensity range: 0.07-0.19). However, it is observed to decrease rapidly in the northeastern part of Kanazawa Port (IRSL intensity range: 0.03-0.15). In addition, in the more northeastern region, similar low IRSL intensities are distributed regardless of water depth (IRSL intensity range: 0.01-0.04). According to the results, it appears that the IRSL intensity decreases as one moves from the mouth of the Tedori River towards the northeast. This suggests drifting sediment moves in that direction. Besides, sand with low IRSL intensity accumulates on the northeast side of Kanazawa Port, including Chirihama. This suggests that a considerable amount of the 150-212 µm sand that is newly released from the Tedori River does not reach the northeast side of Kanazawa Port.
4. Conclusion
The study investigated the longshore drift sediment movement from the mouth of the Tedori River to Chirihama using the IRSL method. The results suggest that Kanazawa Port impedes the transport of coastal sand drift towards the northeast, which in turn affects the supply of sand to Chirihama and contributes to coastal erosion.