toward the back of the bank and riprap 20 is piled over the unrolled portion of the roll mat 10 so that the roll mat 10 and the riprap 20 will be embedded together in the ground fig3 is a schematic perspective view showing a trench formed at a top of a bank to install the roll mat 10 and the riprap 20 fig4 is a schematic cross sectional view taken along the line b b of fig3 as shown in fig3 and 4 the top surface of the bank 100 is excavated to form a trench 110 at which the roll mat 10 is to be disposed in the present disclosure the bank is not entirely dug or damaged but just the top surface of the bank is excavated to a certain depth to form the trench 110 while remaining the bank slope as it is in particular in the present disclosure the trench 110 may be formed at the bank only in a region where bank erosion is expected for example in case of a river based on a direction along which the river flows or a longitudinal direction the trench 110 may be excavated at the top of the bank in the longitudinal direction only for a region where bank collapse is expected as shown in fig3 and 4 fig5 is a schematic perspective view showing that the roll mat 10 is disposed at the trench 110 as depicted in fig1 fig6 is a schematic cross sectional view taken along the line c c of fig5 as shown in fig5 and 6 after the trench 110 is completely formed the roll mat 10 is disposed in the.