Research Trends and Results

Impacts on the riverine estuary area of the KitakamiRiver by the Great east Japan earthquake disaster
NAKAMURA Keigo, Senior Researcher (Ph. D., Engineering)
IWAMI Yoichi, HeadHAGA Masataka, Research Engineer
Environment Department, River Environment Division(Key words) Reedbed, Ground subsidence, Tsunami deposit

2. Overall condition of the vegetation on the rivermouth after disaster
The survey was made on March 2012 in the area fromthe river mouth of the Kitakami River to Kitakami
barrage (17.2kp：Distance from the river mouth (km)) asshown in Fig. 1. Implemented survey items were rivergeomorphology, vegetation and sediment along the rivercorridor.
Before the disaster, almost all the flood channels from
2kp to 9kp were covered by mainly reedbeds, and salt
marsh vegetation and Ogi (Miscanthus sacchariflorus)
have grown in a little higher portions at the periphery of
the flood channels. Survey after the disaster has revealed
that the vegetation area was reduced to the half andalmost all of the observed vegetation were reedbeds with
only a small magnitude of new salt marsh vegetation
observed around 6.5kp.

3. Effects on the reedbeds caused by the Tsunami
deposit and ground subsidence
As probable causes of the effect on the reedbeds in theriverine estuary, it is considered the erosion by theTsunami, changes in the altitude and salinity by groundsubsidence, and burial on land by the Tsunami deposit.
Reedbeds around the Kitakami River mouth werelargely flushed away by the Tsunami, in which survived
dimension was reduced to the half. Further the vegetation
ground level has lowered due to the ground subsidence
and we have found that it was difficult to bring up reeds
due to the effect of the increased salinity in the area from0kp to 4-5kp.
Also, though the effect of the salinity was not critical in
the upper stream over 5kp, still largely affected by thedeposit brought by the Tsunami (mainly sand from thesea), there found no new growth in those places that havethick Tsunami sediments as shown in Fig. 2.
Less deposit &high elevation
1.2

Thick deposit
1.0
0.8

Elevation (m)1. Introduction
Tsunami caused by the Great east Japan earthquake
disaster has given extensive damages on the Tohoku
region mainly on the Pacific maritime area. That Tsunami
has not only deprived people of lives and properties but
also given serious damages on the natural environment of
those areas.
Here, we are to report the effect that the Tsunami has
made on the environment of the riverine estuary mainlyon the reedbeds based on the survey results made on theriver mouth of the Kitakami River that has passed around
one year from the disaster

0.6
0.4

Low elevation &thick deposit

0.2
0.00Survey
Area

1020304050Deposit thickness (cm)Ishinomaki
city
Sendai city

Kitakami riv.

Kitakami Barrage（17.2kp）

Iino-gawa Br.

Fig. 1 Surveyed area in the Kitakami River mouth(downstream of the Kitakami Barrage)

Fig. 2 Impact on Reedbeds by the Elevation Level andThickness of Tsunami deposits (○ : New growthobserved, × : New growth not observed)【Reference】
1) NAKAMURA Keigo (2012), "Impacts of the Tsunami of theGreat East Japan Earthquake on Riverine Estuary, a casestudy: the Kitakami River Mouth," Kasen (in Japanese withEnglish abstract), No. 795, October 2012, pp.41-462.
2) ENDOU Maremi, ONUMA Katsuhiro and AMANO
Kunihiko (2012), "Analysis of the Impact on Estuarine
Vegetation from Sinking Ground by the 2011 Off the Pacific
Coast of Tohoku Earthquake," Journal of River Engineering,
Vol.18, pp. 53-58.