Research Trends and Results

Protect the sand dune (regarded as a natural dike) using the“Sand pack” -Beach scarp recession preventive worksSUWA Yoshio, HeadNOGUCHI Kenji, Senior researcher
WATANABE Kunihiro (Ph. D. Agriculture), Researcher
Coast Division, River Department(Key words) Sand pack, Beach scarp recession preventive works, GeoTextile, Beach nourishment
1. What is the “Sand pack”?
“Sand pack” is a large GeoTextile bag filled with beachmaterial collected on site or shared with the beachnourishment. Although it is applied in foreign countries,
problem has been whether it is applicable or not to theJapanese beach composed of coarse-sand-containing beachmaterial formed by hard waves grown by the steeply sloped
sea floor. If the sand pack was proved applicable, it isexpected to see earlier accomplishments of the beach
prevention effect taking advantage of its merit of shortperiod construction work and economic efficiency.
2. Collaborative research and editing of the “Guide line to
the Beach scarp recession preventive works (draft)”
Coast Division has performed the collaborative researchcalled “Research on the performance evaluation technologyof the sand back fill work in the beach conservation” fromthe fiscal year 2010 to 2012. In that research, we havecarried out the hydraulic model test of the sand bag,
durability test for sand bag material, construction
experiments on site, follow-up monitoring, and so on andorganized performance evaluation methods.
As a result, it was proved that the lifetime of the bagmaterial becomes shorter near the shore line on the sanderoding beach, since those beaches are always attacked bytireless waves propagating over steep topography. On theother hand, we also found that the abrasion external force issmaller if there was wide sand beach on the front even in
the coast facing the outer sea, which performed the bag
lifetime longer to approximately ten years or more.
Accordingly we have decided to make a proposition of thepractical use of the sand pack to the Beach bank scarp
retreatrecession preventive construction works of the sanddune as its application based on such characteristics. As wehave carried out the site test on Sumiyoshi area of Miyazaki
coast shown in photo 1, we have made a summary of theresult of that collaborative research as the Guideline to theBeach bank retreat preventive construction work (draft).

Photo 1 Preventive effects to the earth fill on the back3. Beach scarp recession preventive works
Fig. 1 shows the cross section of the Beach scarp
recession preventive works. The construction work consistsof the standalone sand pack laminated body and beachnourishment earth fill located on the front of the beach fill,in which sand covering beach nourishment is implemented
on the laminated sand pack if necessary. The laminatedsand pack and beach nourishment fill on the back have thefunction to decrease the denudation of the lower part of thebeach due to waves and prevent the beach bank fromretreating. The Guideline (draft) shows how to set the crosssection of the Beach scarp recession preventive works, thesand pack weight assessment method, evaluation method of
the tension reacting on the bag material, how to set theabrasion external force and climatic aging deterioration
external force, calculation method of the required force forthe bag material, performance evaluation test method and soon.
In the proximity of Oidanohama, Miyazaki coast shownin photo 2, it was decided to adopt the Beach scarp
recession preventive works taking advantage of theexperience of the site test on the Sumiyoshi proximity andconcept of the Guideline (draft)【Reference】1)

Coast Division HP http://www.nilim.go.jp/lab/fcg/

Sand pack Sand covering ofbeach nourishment

Beach bank
Beach nourishment
earth fill

Fig, 1 Cross-section of the Beach scarp recession

preventive works

Photo 2 Proximity of Oidanohama that the first
Sand pack project is being carried out