1:45 PM - 3:15 PM
[O06-P06] Proposal for installing a sedimentation fence on the Samejima beach in Iwata City 2
Keywords:coastal erosion, sediment fence, contribute to the local community
Our school has conducted various studies on the Samejima Coast in Iwata City. Since the erosion of the coast is progressing year by year, we considered the suppression of coast erosion by installing sand fences and depositing blown sand. In this study, we examine the effective installation method and structure of sediment fences at Samejima Coast.
First, four 90cm x 180cm cross sectional sand fences made of round piles, bamboo screens, and bamboo were made and placed on the coast with the round piles facing east-west, north-south, northeast-southwest, and northwest-southeast, respectively, in order to examine the difference in the amount of sand deposited depending on the direction of the wind.The sandy sediments around the sand fences were collected for about five months.
We measured the height difference of sandy sediments around the sand fences at 30 points within a 3m x 3m area surrounding the fences using an auto-level.
The largest difference in height was 21 cm in the north-south direction. The second largest difference was 17 cm in the northwest-southeast direction. In both directions, high dunes were formed on both sides of the sand fences. However, the sediments below the barriers were reduced due to scouring. According to AMeDAS Iwata, the maximum wind direction during the study period was west. Therefore, it was found that more sand would be deposited if the fence was placed in an orthogonal direction to the wind. This is thought to be due to the decrease in wind velocity when passing through the sand fences. However, there was a problem that the wind hitting the sand fences eroded the sediments in the lower part of the fences. Therefore, we thought that the erosion could be suppressed by burying roots and leaves in the lower part of the sediment fences.
We therefore installed four types of compost fences with or without buried roots and leaves and with different porosity conditions, and investigated them for approximately four months in the same manner as the aforementioned method.
As a result, the amount of sand deposited increased in the sediment hedge with buried roots and leaves in the lower part of the hedge. This is thought to be due to the fixation of sand in the lower part of the sand fence by the roots and leaves. On the other hand, the sandy hedges without buried roots and leaves were subjected to wind erosion at the bottom, resulting in a decrease in the amount of sand. The average value of the amount of sand increase on the leeward side of the compost hedge for three types of compost hedges with different porosity showed that the amount of sand increase also increased as the porosity increased. A model of a composting sand fence with split branches was made and tested in a wind tunnel. The gaps between the splittings were 3 mm and 5 mm apart, and the porosity was 37.5% and 50%, respectively. The wind speed at the reference point in front of the model was set to 6.0 m/s, and the average wind speed at 30 points was measured at the front and back of the model, respectively. As a result, it was found that the wind velocity at the back of the model was also higher when the porosity was larger.
Therefore, the relationship between the porosity and the amount of sand deposited is considered to be that when the porosity is small, the sand is impacted by the sand fences and does not move to the back of the model. On the other hand, when the porosity is large, the wind velocity passing through the sand fence is also large, and more sand is transported by the passing wind, and sand is considered to be deposited on the back side, which is less subject to erosion at the bottom.
Therefore, it is concluded that a sandbank hedge with a porosity of about 50% can efficiently deposit sand and reduce coast erosion by installing it in an orthogonal direction to the westerly to southwesterly winds and burying roots and leaves in the lower part of the hedge.
First, four 90cm x 180cm cross sectional sand fences made of round piles, bamboo screens, and bamboo were made and placed on the coast with the round piles facing east-west, north-south, northeast-southwest, and northwest-southeast, respectively, in order to examine the difference in the amount of sand deposited depending on the direction of the wind.The sandy sediments around the sand fences were collected for about five months.
We measured the height difference of sandy sediments around the sand fences at 30 points within a 3m x 3m area surrounding the fences using an auto-level.
The largest difference in height was 21 cm in the north-south direction. The second largest difference was 17 cm in the northwest-southeast direction. In both directions, high dunes were formed on both sides of the sand fences. However, the sediments below the barriers were reduced due to scouring. According to AMeDAS Iwata, the maximum wind direction during the study period was west. Therefore, it was found that more sand would be deposited if the fence was placed in an orthogonal direction to the wind. This is thought to be due to the decrease in wind velocity when passing through the sand fences. However, there was a problem that the wind hitting the sand fences eroded the sediments in the lower part of the fences. Therefore, we thought that the erosion could be suppressed by burying roots and leaves in the lower part of the sediment fences.
We therefore installed four types of compost fences with or without buried roots and leaves and with different porosity conditions, and investigated them for approximately four months in the same manner as the aforementioned method.
As a result, the amount of sand deposited increased in the sediment hedge with buried roots and leaves in the lower part of the hedge. This is thought to be due to the fixation of sand in the lower part of the sand fence by the roots and leaves. On the other hand, the sandy hedges without buried roots and leaves were subjected to wind erosion at the bottom, resulting in a decrease in the amount of sand. The average value of the amount of sand increase on the leeward side of the compost hedge for three types of compost hedges with different porosity showed that the amount of sand increase also increased as the porosity increased. A model of a composting sand fence with split branches was made and tested in a wind tunnel. The gaps between the splittings were 3 mm and 5 mm apart, and the porosity was 37.5% and 50%, respectively. The wind speed at the reference point in front of the model was set to 6.0 m/s, and the average wind speed at 30 points was measured at the front and back of the model, respectively. As a result, it was found that the wind velocity at the back of the model was also higher when the porosity was larger.
Therefore, the relationship between the porosity and the amount of sand deposited is considered to be that when the porosity is small, the sand is impacted by the sand fences and does not move to the back of the model. On the other hand, when the porosity is large, the wind velocity passing through the sand fence is also large, and more sand is transported by the passing wind, and sand is considered to be deposited on the back side, which is less subject to erosion at the bottom.
Therefore, it is concluded that a sandbank hedge with a porosity of about 50% can efficiently deposit sand and reduce coast erosion by installing it in an orthogonal direction to the westerly to southwesterly winds and burying roots and leaves in the lower part of the hedge.