Research Trends and Results

Recover the Seashore vegetation utilizing the waves
WATANABE Kunihiro, Researcher (Ph. D., Agriculture)
SUWA Yoshio, HeadRiver Department, Coast Division（Key words）

Sand beach, Vegetation, Site survey

1. Background and purpose
Though Sea coast projects aiming at maintaining thebeach sand has been implemented throughout thenation, it was proved that some cases have failed todevelop natural seashore vegetation even when the sea
sand was recovered. The cause being that too muchtranquilized sea surface due to the off-shore facility,
although a method to cultivate periodically the beaches
sand has been executed, in the future we think we needto introduce the seashore management that enables theself-sustaining recovery on recognizing theestablishment and maintenance mechanism of theseashore vegetation.
As the essential research for that purpose, we havebeen carrying out the site survey on the formation
process of the seashore vegetation on Kitafujihara
work area in the Isewan south-west coast (Fig. 1).
This beach is the one that has implemented the “Setback” to change the construction site onto the inland
when reconstructing the old beach dike in 2009,
which has created a new sand space at the front of
the beach dike.

Fig. 1 Old and new dike location at Isewan
south-west beach
2. Formation process of the seashore vegetation
After we have carried out the vegetation survey once tothree times a year from just after the completion of thedike reconstruction work, though almost no seashore
plant has appeared in the earlier stage, it was confirmedthat grass land plants such as Mehishiba (Digitariaciliaris) and Himemukashiyomogi (Conyzacanadensis) have swiftly grown as far as to coveralmost all the beach. After such a state has remained
unchanged for two and half years, it has immediately

moved into the seashore vegetation mainly based on
the Hamahirugao (Calystegia soldanella) during theperiod as short as two months and half after September
2011 (Fig. 2). As this transition has occurred in avery short period and it has never appeared in thesurvey line shielded from the wave, it was estimatedthat it was caused by the disturbance made by the wave
of significant wave height 2.5m observed in the same
period at site. Afterward, beach lower trees such as
Hamagou (Vitex rotundifolia) have appeared in thatplace, types of the seashore plants are increasing
gradually.

Fig. 2 Change of the vegetation distribution before
and after disturbance due to waves
On the other hand, as it has been observed also theincrease of the land hardness and ignition loss in thesand in the survey line shielded from the wave (L20 inFig. 2), the place is changing into a hard land including
a lot of organic substances, not appropriate for theseashore plants. It is being the plant compositions closeto the waste inland, proved result that appearance ofthe sand space could not create the seashore vegetation
by itself.
3. Future deployment and utilization of the results
We are to make clear the environmental factor
controlled to the condition better for the seashore
plants by the wave disturbance and degree of the wave
disturbance required for the maintenance of theseashore vegetation to use those results to help thenationwide beach management. Results of this research
are expected to become the reference to see how muchperiod is required to recover naturally the seashore
vegetation destroyed by The Great East Japan
Earthquake and what kind of means are effective topromote the recovery, if possible.