11:00 AM - 1:00 PM
[AOS22-P01] Estimation of freshwater inflow into a semi-enclosed bay during heavy rainfall: A case study in Nanao Bay, Ishikawa Prefecture
Keywords:small river, coastal sea, Nanao Bay, Climate Change, extreme precipitation
Many studies have suggested that extreme precipitation will increase under climate change. For example, the Global Warming Projection Information, Volume 9 (Japan Meteorological Agency, 2017) reports an increase in heavy rainfall with daily precipitation of 100 mm or more and short-term intense rainfall with hourly precipitation of 50 mm or more under the RCP8.5 scenario.
These large amounts of precipitation supply large amounts of freshwater in a short period of time, resulting in rapid increases in river flow and lower salinity of the marine environment. Extreme low-salinity environments have been reported to have growth-inhibiting and lethal effects on many marine organisms in coastal areas, including bivalves. Therefore, it is necessary to quantify the impacts of extreme precipitation on fishing ground with high biodiversity in coastal seas.
In order to assess the impact of increasing extreme precipitation on the coastal area, it is necessary to estimate the amount of freshwater flowing into the sea. However, in many cases, data on river discharge are not measured or published for small rivers such as Class-B rivers and Class-C rivers. This makes it difficult to estimate freshwater inflow to coastal areas where the main sources of freshwater are small rivers.
Therefore, the purpose of this study is to quantify the freshwater inflow from rivers by field observations and to determine the relationship with precipitation at the catchment area, in the coastal sea, Nanao Bay in Ishikawa Prefecture, where the major source of freshwater are small rivers of Class B or smaller.
Nanao Bay is a semi-enclosed bay located in Noto Peninsula, Ishikawa Prefecture, and is divided into three areas by Noto Island in the center: North Bay, South Bay, and West Bay. In this study, we focused on the West Bay, which has the shallowest average depth of 6.3 m and has many small rivers.
First, the following observations were conducted for 6 major small rivers that flow into West Bay: Kumaki River, Hiyo River, Kasashi River, Shiotsu River, Mibiki River, and Ninomiya River. For the Kumaki River, which is one of the largest rivers in West Bay, water level and flow velocity were continuously observed from July to August 2021 to calculate hourly continuous flow rate. In addition, for all 6 rivers, water levels and flow velocities were observed 12 times during the two-month period from July to August 2021 to calculate the instantaneous flow rate.
Next, for the Kumaki River, the amount of precipitation in the catchment area were extracted from the 1km mesh precipitation analyzed by radar-AMeDAS, and modeled the relationship between the continuous flow rate and the amount of precipitation. For the rivers other than the Kumaki River, the model created for the Kumaki River was applied, and the parameters of the model were fine-tuned using results of flow rate observation.
In early July and mid-August in 2021, heavy rains of more than 15 mm per hour and heavy rains of more than 50 mm per day were observed, and the flow rates of the 6 rivers fluctuated correspondingly. The flow fluctuations during the precipitation period of the 6 rivers flowing into the West Bay were highly different each other. Even for the adjacent rivers, the contribution rate and time series variation of freshwater inflow to coastal seas differed greatly, due to extreme locality of precipitation.
These large amounts of precipitation supply large amounts of freshwater in a short period of time, resulting in rapid increases in river flow and lower salinity of the marine environment. Extreme low-salinity environments have been reported to have growth-inhibiting and lethal effects on many marine organisms in coastal areas, including bivalves. Therefore, it is necessary to quantify the impacts of extreme precipitation on fishing ground with high biodiversity in coastal seas.
In order to assess the impact of increasing extreme precipitation on the coastal area, it is necessary to estimate the amount of freshwater flowing into the sea. However, in many cases, data on river discharge are not measured or published for small rivers such as Class-B rivers and Class-C rivers. This makes it difficult to estimate freshwater inflow to coastal areas where the main sources of freshwater are small rivers.
Therefore, the purpose of this study is to quantify the freshwater inflow from rivers by field observations and to determine the relationship with precipitation at the catchment area, in the coastal sea, Nanao Bay in Ishikawa Prefecture, where the major source of freshwater are small rivers of Class B or smaller.
Nanao Bay is a semi-enclosed bay located in Noto Peninsula, Ishikawa Prefecture, and is divided into three areas by Noto Island in the center: North Bay, South Bay, and West Bay. In this study, we focused on the West Bay, which has the shallowest average depth of 6.3 m and has many small rivers.
First, the following observations were conducted for 6 major small rivers that flow into West Bay: Kumaki River, Hiyo River, Kasashi River, Shiotsu River, Mibiki River, and Ninomiya River. For the Kumaki River, which is one of the largest rivers in West Bay, water level and flow velocity were continuously observed from July to August 2021 to calculate hourly continuous flow rate. In addition, for all 6 rivers, water levels and flow velocities were observed 12 times during the two-month period from July to August 2021 to calculate the instantaneous flow rate.
Next, for the Kumaki River, the amount of precipitation in the catchment area were extracted from the 1km mesh precipitation analyzed by radar-AMeDAS, and modeled the relationship between the continuous flow rate and the amount of precipitation. For the rivers other than the Kumaki River, the model created for the Kumaki River was applied, and the parameters of the model were fine-tuned using results of flow rate observation.
In early July and mid-August in 2021, heavy rains of more than 15 mm per hour and heavy rains of more than 50 mm per day were observed, and the flow rates of the 6 rivers fluctuated correspondingly. The flow fluctuations during the precipitation period of the 6 rivers flowing into the West Bay were highly different each other. Even for the adjacent rivers, the contribution rate and time series variation of freshwater inflow to coastal seas differed greatly, due to extreme locality of precipitation.