Coastal and Estuarine Environment Department

Beach protection system with gravity drainage

An abrupt beach erosion in a storm is caused by the groundwater seepage from around the top of the berm in the foreshore. In a storm, the groundwater level near the shoreline gets high owing to the penetration of run-up long period waves developed in the surf zone, and the groundwater seeps out from around the top of the berm with transporting sediments seaward.

Because in the gravity drainage system, schematically shown in Figure 1, the groundwater is naturally drained seaward through a permeable layer installed below the beach surface and the groundwater level is lowered, the system is expected to prevent an abrupt beach erosion in a storm and protect the beach.

A beach protection system similar to the drainage system is the sub-sand filter system, which lowers the groundwater level by pumping groundwater, and its effect was confirmed with laboratory tests and small-scale field experiments. Compared to the sub-sand filter system, an advantage of the drainage system is not to require electricity, which is needed for pumping in the sub-sand filter system. In the drainage system, the groundwater level is naturally lowered.

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Field experiments for the drainage system have been conducted near HORS since 1994. A permeable layer of 8 m in width and 88 m in length was installed about 3 m below the surface in the foreshore (Figure 2). The layer consists of units shown in Figure 3. The dimension of the unit, which is framed with two sheets of corrosion-proof expanded metal and covered with geotextile fabric, is 1 m in width, 2 m in length and 20 cm in thickness. The units are connected with coupler as shown in Figure 3. Groundwater in the permeable layer is drained into the sea at y = 144 m through a pipe connected to the seaward edge of the permeable layer.

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Figure 4 shows the beach profile changes caused by the typhoon #9424 with and without the permeable layer. Although the beach with the permeable layer experienced erosion, the amount of the erosion was less than that of the beach without the permeable layer probably owing to the permeable layer.

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