In our model pebbles are composed of μm-sized monomer particles. In collisions resulting in partial fragmentation monomers are removed one by one to simulate erosion. The binding energy of the fragment, , is larger (closer to zero) than the binding energy of the original particle, E_bind, since the fragment consists of fewer monomers. The number of monomers released, N_mon, is equal the number of contact surfaces the collision energy can break divided by three contact surfaces per monomer, N_mon = E_coll/⁽3E_roll⁾.