Multi-row sand fences are commonly employed to control wind-blown sand on beaches. In this study, the accumulation characteristics of wind-blown sand around multi-row sand fences are identified and examined based on a literature survey. An attempt is made to describe the two-dimensional accumulation processes and resulting morphological forms until the front fence is buried. These processes and forms are complicated and depend on the fence properties (e.g., porosity, shape of openings, height, separation distance between fences, and number of rows), sand characteristics, and wind conditions. Important results for two-row fences obtained from field and wind tunnel studies are: (1) sand accumulation speed at the upwind side of the front fence varies with the porosity of the front fence, if the rear fence is located downwind at a distance of more than twice (2H) the front fence height (H); (2) the accumulated form at the upwind side of the front fence, when the front fence is just buried, is the same regardless of the separation distance between the front and rear fences and the porosity of the rear fence; and (3) accumulated forms at the downwind side of the front fence vary with the separation distance between the fences and the porosity of the rear fence. In addition, selected results for two-row fences obtained from field studies are: (1) with increase in fence porosity, the back-slope gradient of the formed dune becomes gentle when the separation distance of a two-row fence is 2.86H; (2) fences with a separation distance of less than 2.86H function as if they were single structures and sand is intensively deposited between the two fences; (3) sand volume accumulated around fences becomes large, if the fence separation distance becomes large. Furthermore, selected results for two-row fences obtained from wind tunnel studies are: (1) the two-row fences with 4H separation distance trapped as much as 8 to 30% more of the sand than a single row fence for wind speeds between 8.3 and 16.0 m/sec, (2) the two-row fences trapped as much as 30% of the total amount of sand in motion when the wind speed was higher than 16.0 m/sec; however, a single row fence did not trap any sand, (3) a single fence and the two-row fences did not function well and trapped little sand if the wind speed exceeded 21.5 m/sec; (4) two-row fences where the separation distance is less than 4H function as if they were single structures and sand is intensively deposited between the two fences. The sand accumulates around the front fence in a similar manner to a single fence, and accumulation around the rear fence progresses after the front fence is buried, if the separation distance is larger than 10H; (5) in the early stage of the accumulation process around the two-row fences, lowering of the sand surface (erosion) takes place on the downwind side of the rear fence. The point that the lowering generates progresses up-windward with time elapsed, but the point then recedes down-windward after it has reached a certain location. The distance between the rear fence and the most up-windward point that is reached becomes larger with increasing fence porosity; (6) for two-row fences with different porosities between the front fence and the rear fence, the accumulated sand volume around the two-row fences for the case where the front and the rear fence porosities are 50% and 20%, respectively, is more than that of the two-row fences where the corresponding porosities are 20% and 50% at a time when either the front or rear fence is just buried.
In 2009, numerous shallow landslides occurred due to heavy rainstorms in a cretaceous granite region near Hofu city, Yamaguchi Prefecture, in western Japan. We examined the relationship between the degree of denudation of the mountains and several geographic features of the landslides, including the soil profile, information from geological and topographic surveys, geographical properties, and detailed field surveys. We classified mountain slopes into three categories: (1) gentle slopes on the summit (Gen-S), (2) upper dissected slopes (Up-S), and (3) lower dissected slopes (Low-S). The number of landslides in the Gen-S, Up-S, and Low-S regions was 23, 54, and 21, respectively. The density of landslides was 76.9/km2, 112.2/km2, and 41.6/km2 in the Gen-S, Up-S, and Low-S regions, respectively, with average volumes of 432 m3, 410 m3, and 173 m3 in the Gen-S, Up-S, and Low-S regions. Landslides in the Gen-S and Up-S region were “saturated soil slides” while “soil fall” landslides occurred in the Low-S region. The landslide areas in Gen-S were covered by thick weathered residual soils, and the landslide areas in Up-S and Low-S were covered by colluvial soils. Landslides in the Up-S area had a thick soil layer, while the landslides in the Low-S area contained many rocks, and soil depth was, therefore, low. These differences in soil layer structures were regulated by the type and properties (e.g., density, volume) of the landslides.