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東北地域災害科学研究 第56巻（2020）

Morphology recovery and prevailing sediment transport along sand spit at theKitakami River mouth after the 2011 Tsunami
Nguyen Trong Hiep1, Hitoshi Tanaka2, Nguyen Xuan Tinh3
Abstract
The Kitakami River mouth was one of many estuaries that experienced and got despoiled by the2011 Tohoku Tsunami. The tsunami was highly energetic that the entire area of the river mouth wasdestructed severely and the remainder was a small area of sediment on the right bank. Afterwards, thephenomenon of intrusion pushed this deposited sediment towards upstream to a certain distance. Since
that, the right sand spit has not been able to recover to its former shape due to special topography and theparticular wave conditions near the river mouth. This study provides a thorough understanding of themorphology at the Kitakami River mouth by analysing aerial photographs obtained from many sources.
Furthermore, unstable long shore sediment transport related to the seasonal variation is also discussed tointerpret the change of the sand spit at the Kitakami River mouth.
Keywords: the Kitakami River mouth, the 2011 Tsunami, sand spit, recovery process, seasonal effect.
1. INTRODUCTION
The 2011 Tohoku Earthquake triggered a giant
Tsunami waves with a record of 40m wave heightin Iwate. With regard to the 2011 Tohoku
Earthquake and Tsunami, many studies wereconducted to immidiately assess the destruction
caused by the tsunami onto the coastal areas andalso give possible prediction of the recoveryprocesses aftermath. Namely, Tanaka et al. (2012),
Tappin et al. (2012), Supasri et al. (2012) and Udoet al. (2012). In detail, many remarkable changes
in coastal and estuarine areas in Miyagi Prefecturesuch as beach erosion and breaching, sand barriers
and sandspits’ loss were reported in Tanaka et al.
(2012).
The river mouths in Miyagi Prefecture sufferedalmost similar devastation that the sand spits
located in each area were disappeared by thetsunami. Roh et al. (2016) and Hiep et al. (2019a)
both discussed the disappearance for 7 years of thesandspit at the Naruse River mouth till the artificial
construction to reform the sandspit in 2018.
Recently, Hiep et al. (2019b) has brought adetailed understanding of the Natori River mouth

recovery process with the assistance of humanintervention to recreate the situation before
Tsunami. In contrast, some areas have recovered
fully due to the existence of sufficient sedimentsources; for instance, in Abukuma River mouth as
discussed thoroughly in Tanaka et al. (2017) and
Hiep et al (2019c). The Kitakami River Mouth wasalso among the river mouths could not uphold theintensity of the tsunami that its sandspit waseroded into small area and the coast on its rightside was destroyed entirely. The topography andwave conditions in the Kitakami River mouth are
special resulting in the slow recovery process tothe pre-tsunami morphology.
There are some former studies discussing about
the morphology of the Kitakami River Mouth suchas Tinh et al. (2011) discussing the morphology
changes of the Yokosuka Coast and Purwanto et al.
(2006) revealing the flow regime in the
Nagatsura-Ura Lagoon nearby the Kitakami Rivermouth. The understanding of the mechanism at theKitakami River, in this study, is examined moredeliberately by considering the longshore
sediment transport along the sand spit.

(1) Graduate Student, Department of Civil and Environmental Engineering, Tohoku University,
6-6-06 Aoba, Sendai 980-8579, Japan(2) Professor, Department of Civil and Environmental Engineering, Tohoku University, 6-6-06 Aoba,Sendai 980-8579, Japan(3) Assistant Professor, Department of Civil and Environmental Engineering, Tohoku University,
6-6-06 Aoba, Sendai, 980-8579, Japan

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Figure 1: The Kitakami River mouth
2. STUDY AREAThe Kitakami River is ranked as the fourthlargest river in Japan and the largest in the Tohoku
region. The catchment area is approximately
10,150 km2 flowing through the southern part ofIwate Prefecture and pouring into the Oppa Bay.
The upstream river separates into 2 channels: TheOld-Kitakami River and the currently so-called
Kitakami-River (Fig. 1). Near the entrance area, asandspit is located roughly 750m from theYokosuka Coast at the right side.
The offshore wave condition in this area isdriven by two main directions which are East andEast South East. (Figure 1)

3. DATA COLLECTION
For the goal of understanding the morphologyof river mouth, this study aimed to collect many
aerial photos before and after the Tsunami taken by
Tohoku Regional Bureau of Ministry of Land,
Infrastructure, Transport and Tourism (MLIT) as
well as satellite images from Google Earth andSentinel II. All of the photos were geo-referenced
and detected shoreline in order to monitor
consistently the recovery process of the rivermouth.
Moreover, transverse data of the several
locations at the sandspit are also combined toemphasize the special conditions and thecontribution of depth change around the sand spiton the development of the spit at the KitakamiRiver Mouth after the tsunami.

4. RESULTS AND DISCUSSION
4.1 Morphology change before and after Tsunami(2003-2012)
The Kitakami River mouth from 2003 to 2012encountered an overwhelming change in size andconfiguration. Before the 2011 tsunami, theYokosuka Coast and the sand spit were both
wealthy with slight variations on the head of thesand spit due to the mutual interaction between
long shore sediment transport derived by waves
and the discharge from the river (Figure 2 (a~c)).
The Tsunami came and destroyed almost entire
the Yokosuka Coast and the area behind wasdrowned in water (Figure 2 (d~e)). Afterwards, thewidth of the river mouth certainly became open sincea remarkable area of sand spit was flushed away
during the event. Then, waves propagate further andcarry the sediment to upstream by a certain distance(Figure 2 (f)). This is called the intrusion process
4.2 Morphology change under slow recovery
process (2013-2020)
Because of the intrusion phenomenon, the sandspit was located in a specific position in the river as
seen in Figure 3 (a). The recovery of the sand spit
throughout almost 6 years shows unsignficant
change. The sand spit was not able to deposit intranverse direction and only small amount ofsediment accumulated on the side facing the sea area.
(Figure 3 (b~f)).
However, it is also interesting that on theopposite left bank, sediment started depositing from2017 which had been seen only in more downstream
area.

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Figure 2: Morphology change at the Kitakami River mouth before and after the 2011 Tsunami
4.3 Influences of river mouth’s conditions on
the morphology development
From the aerial photographs at the KitakamiRiver mouth, the sand spit has been solelyaccumulate onto the spit as new layers of sedimentwith a variant at the tip of sand spit. One of thereason for this phenomenon is due to the deep
water depth around the sand spit. It is noticablefrom Figure 4 that the more offshore the measuringlocation was, the less sediment was deposited andonly obsearvable at a small distance from theembankment (Figure 4 (a,b)). Figure 4 (c,d) also
indicates from the tip of the shoreline to nearby
tranverse region, the depth difference is up to 5 m.
Hence, the accumulation can only be seen at thealready seen area
Another main factor contributing the slow
recovery process at the Kitakami is the direction ofwave incident. Waves approach towards the rivermouth in two specific directions which are Eastand East South East since the topography of rocky
cliffs surrounding the entrance. Hiep et al. (2019d)
discussed how the approaching waves refracted

and resust in the breaking waves parrallel to theshoreline of the spit. This certain wave condition
combining with deep water deep has been causing
such a ineffective recovery at the Kitakami Rivermouth.
In the next section, the evaluation of sedimenttransport along the sand spit will emphasize theclaim of the reduction of sediment along the sandspit and examine the effect seasonal varitons on
the Kitakami River
4.4 Longshore sediment transport rate at thesand spit
Based on the notice of sediment movement
which hardly deposit towards the left bank, it isideal to define a fix location where the sediment isassumed to be interrupted or not able to transport
further upstream in order to compute the longshore
sediment transport along the spit. The coordinates
and the direction of sediment transport towards thesand spit are defined as Figure 5 and it is noted thatat the boundary the value of longshore sedimenttransport must be zero.

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Figure 3: Slow recovery process at the Kitakami River mouth
The longshore sediment transport rate (Q) can be
expressed as the equation below (Eq. 1)∆y(1)dxQ(x) = −(D + D )∆t
Where: DC is depth of closure (m), DB is berm∆height (m),is the shoreline change between∆
consecutive times. From transverse section data,
the total depth is assumed to be 6 m and the chosen
shorelines were from 2013 when the newembankment was rebuilt.
The result of the computation is shown inFigure (6). According to the definition, the positivevalues refer to the erosion along the sand spit while
the negative values represent the deposition of thespit. The distances that the sediment transported
during each period did not resemble as a result of
the observation of accretion along theembankment during some periods.
In order to investigate the change of thelongshore sediment transport rate through time,
tranverse sections at several locations along thesand spit are chosen (x = 100 m, 300 m and xmax)(Figure 7). The varitions of longshore sedimenttransport rate at these three locations reveals theinfluence of seasons on the evolution of the spit.
Indeed, the positive values are seen during winterseason in which wave height is not significant
whereas during summer, deposition is certainly
observed.
It is also interesting to compare the maximum
longshore sediment transport at xmax with thevolumetric deposition rate of sand spit. According
to Hiep et al. (2019a), the rate of the area is 9.7x103(m2/y) and from the transverse section data, thethichness of the deposited layer on the sandspit isroughly 4 m. And the value of the volumetricdeposition rate of sand spit is estimated as Eq.
(2).
VDeposition rate = 0.4 x105 (m3)(2)

As indicated in Figure 7(c), the spit deposition
rate is almost similar to the value of longshore
sediment transport rate when the seasonal effect is
not overwhelming indicating the slow recovery of
the sand spit under small sediment supply.

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Figure 5: Coordinates for longshore sedimenttransport’s computation

Figure 6: Longshore sediment transport along thesand spit
5. CONCLUSIONSThe Kitakami River mouth is a typical rivermouth where the recovery process has beenineffective since the 2011 Tohoku Tsunami. Deepwater depth around the sand spit and specific wave
directions are the reasons for the slow restorationof morphology to its former situation. The
longshore sediment transport in this river mouth isaffected by the season effects which explained thedifferent shapes of shoreline during some periods.

Figure 4: Transverse sections at the sand spit

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75(2),
I_773-I_778. (In Japanese)

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Figure 7: Time variation of longshore sediment transport rate along the sand spit
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