10th Asian Regional Conference of IAEG (2015)

Rainfall-recharge-runoff processes through bedrock groundwater: implications fortriggering of deep-seated catastrophic landslides
Yuki MATSUSHI(1), (2), Yosuke YAMAKAWA(2), (3), Yusuke TAKAMI(4),Naoya MASAOKA(4), Ken’ichiro KOSUGI(2) (4)(1) Disaster Prevention Research Institute, Kyoto University, Japan
E-mail: matsushi@slope.dpri.kyoto-u.ac.jp(2) JST CREST, Japan, (3) Agricultural and Forestry Research Center, University of Tsukuba, Japan,(4) Graduate School of Agriculture, Kyoto University, Japan
Abstract
We observed discharge from bedrock springs and fluctuations in deep groundwaterlevel in bore holes in a high-relief steep hillslope along a high-angle strike-slip active fault
underlain by accretionary sedimentary rock, western Hira Range, central Japan. Bedrock inthis area is extensively fractured through active faulting and gravitational hillslope
deformation, resulting in a characteristic hydro-geological structure. Rainwater percolated
through thin soil cover recharges the deep groundwater body expanding in a fissure networkin bedrock. This bedrock aquifer is separated by subsurface walls of impermeable fault
gouges, and the groundwater overflow maintains the discharge from persistent springs
aligned on a similar altitude. Quick raise in deep groundwater level were observed in
response to large rainfall events in summer seasons, demonstrating high connectivity ofshallow soil-water to the deep bedrock aquifer through fractures. Discharge from thesprings correlates well with the upslope groundwater level with a cubic or quadratic
function, suggesting constraints of bedrock aquifer by subsurface geological structure suchas beddings and foliations. The functional relationship between them may provide a
potential way to infer bedrock groundwater level based on spring runoff monitoring inhillslopes with high susceptibility to deep-seated catastrophic landsliding.
Keywords: bedrock groundwater, deep-seated landslide, spring, hydrological monitoring
1.

Introduction

bedrock for better understanding of deep groundwater
system, which should depend on subsurfacehydro-geological structure.
Bedrock groundwater in the specific geologicalcondition causes deep-seated rock mass movements,
when water storage exceeds a critical level for slope
stability by intense rainwater infiltration. Mechanicaland hydraulic discontinuity in bedrock often
functions as a potential sliding surface for acatastrophic landslide, especially in areas underlain
by accretionary sedimentary rocks and volcanic rocksin active orogens. In fact, dip-structures by beddings
or faulting appeared at slip scars in several cases ofrecent bedrock landslides in SW Japan (Chigira et al.,2013). Almost no information is available for how
these subsurface discontinuities affect bedrock
groundwater behavior and storage capacity that limits
the rainwater amount to be safely drained from thehillslope. A proper hillslope conservation and disastermitigation need knowledge of bedrock groundwater

Groundwater storage in bedrock buffers rainwater
infiltration and drainage in hillslopes, maintainingbase flow in mountain streams. Many hydrologicalstudies demonstrated significant contribution ofgroundwater in bedrock to rainfall-runoff system inmountainous watersheds (e.g., Onda et al, 2001,
Katsura et al., 2008, Kosugi et al., 2006, 2008, 2011).
However, infiltrated water flow paths andmechanisms of quick/slow responses of such deep
groundwater remain controversial. Fast fissure flowor pressure propagation through rock fractures may
bear the prompt raise of groundwater level in bedrock
during a rainstorm (Gabrielli et al., 2012; Salve et al.,2012; Fujimoto et al., 2014), while slow water
convergence for a long traveling distance may
contribute to the delayed or seasonal groundwaterfluctuation (Abbott and Stanley, 1999). We needdirect observation of infiltrated water behavior into1

10th Asian Regional Conference of IAEG (2015)

dynamics and pore-pressure fluctuations in bedrockwith such hydro-geological conditions leading to acatastrophic landslide.
A clue to infer groundwater system in a hillslope
is springs that drains bedrock groundwater directlyfrom the deep aquifer. It is well known that
distribution of bedrock springs reflects geologicalstructures. These springs may align on a contact
between land surface and groundwater table, or may
arise at an outlet of preferential flow paths such as
rock fissures or permeable fault bands. In any cases,
spatial distribution and temporal runoff fluctuation ofsuch bedrock springs should reflect the subsurfacehydro-geological structure and groundwater dynamics
in it.
Monitoring of spring runoff may provide useful
information for predicting pore-water pressures in
bedrock, if a functional relationship exists between
runoff amount and groundwater level. This
relationship must be controlled by the local
hydro-geological structure, hence we are also capableto estimate subsurface water flow system by thesimultaneous observation of spring discharge andbedrock groundwater level. Once a functional
relationship is established between them, thisapproach enables us to evaluate hillslope stability
based on spring discharge that is easily observablethan bedrock groundwater fluctuation.
Here we show how spring distribution anddischarge correlates with groundwater in bedrock fora case in a high-relief steep hillslope underlain byaccretionary sedimentary rocks. Observations werecarried out for precipitation, runoff from springs, andgroundwater level in boreholes. These data
demonstrated geo-structural constraints on expansionof bedrock aquifer, which seems to control
pore-pressure raise that affects occurrence of bedrock
landslides.
2.

Fig. 1 Topographic map of the study site (A) andhillslope profile surveyed (B). Three borings were
drilled on the profile and used as observation wellwith a screen for depths represented by broken lines.
Groundwater tables are indicated by max, mean andmin levels during 2013-2014.

Study site

The study site is a hillslope adjacent to an active
strike-slip fault, Hanaore Fault, in south western Hira
Range, SW Japan (Fig. 1A). The eastern area of the
Hanaore Fault is thought to have actively uplifted
during the late Quaternary to form the Hira Range
with remaining low relief surfaces on its ridge crest.
A major valley, upper Ado River, along the fault
deeply incises the terrains to form gorges walled bysteep lower side-slopes. Topography in these area aredominated by high-relief (>300 m), steep (>30°)
hillslopes.
Bedrock in this area is Jurassic accretionary
sedimentary rocks, Tamba-Mino Belt, consisting ofsand–mud alternating beds, chert, and muddy chaotic
rocks. The strata show beddings and foliations with ageneral attitude of north–south strike with west
dipping. The substrates beneath the eastern part of thestudy area is thermally metamorphosed by the contact
with Cretaceous granitic intrusion that is presently
exposed in the eastern part of the Hira Range.
High-angle sub-faulting of the Hanaore Fault systemand porphyritic dykes often observed along the Ado
River with both north–south directions.
Climate in this area is characterized by humid
temperate condition with mean temperature of 11 °C
and average annual precipitation of 1800 mm. About
40% of rainfall is supplied by frontal activities andtyphoons in summer and fall seasons. The area
receives snowfall equivalent to ~500 mm in winter.
Natural vegetation under this climate would be an
evergreen broadleaf forest, but in the present day,planted coniferous forest occupies the majority ofhillslopes.

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10th Asian Regional Conference of IAEG (2015)

In the Hira Range, we have poor records ofdeep-seated catastrophic landslides. An earthquake in
AD1662 (Kanbun Earthquake) in the northern part of
Hanaore Fault caused a deep-seated catastrophic
landslide, so called Machii-Kuzure, which shuttered
the Ado River until its outburst at few weeks later(Imamura and Inoue, 2002). The most recent
rainfall-triggered deep-seated catastrophic landslides
occurred in 18-20 Aug 1992 by rainfall about 600
mm per 2 days; the event caused landslides with avolume of 105 m3 at two locations near the field site.
In the airborne LiDAR (Light Detection And
Ranging) based topographic image, many landslide
scars and gravitational deformation scarplets arevisible on the hillslopes (Fig. 1A, Matsushi et al.,2014).

(S&DLmini, OYO Corp.). Groundwater temperatureand EC were also measured at the bottom of the bore
holes. At the uppermost bore hole, subsurface profile
of temperature were recorded at depths of 1.50, 2.50,
3.95, 5.85, 10.85, 17.80 and 30.60 m below surface.
Meteorological observations were carried out at
narrow flat space nearby the surveyed slope (Fig. 1A).
Total precipitation were measured by a heatabledipping bucket rain gauge with 0.2 mm resolution. In
winter season, snowfall were received by a 1.00 m2
lysimeter, and snow melting were recorded by adipping bucket flow meter with 0.125 mm resolution.
Air pressure was monitored by a barometer tocalibrate the pressure sensor in bore holes.

3.

4.1 Spring locations and groundwater distribution
in bedrock
The spatial distribution of major springs shows
two series of north-south alignments with two distinct
ranges of altitudes (Fig. 1A). The higher group
appears from 450 to 500 m a.s.l (above sea level),while lower springs are located below 400 m. Several
springs have irregularly large amount of discharge(>5 L/s) even at the higher altitude, regardless of theirrelatively small topographic contributing area.
Almost all these springs accompany outcropping of
clayey black fault gouges containing mudstone andquartz fragments. Indeed the alignment of the twogroups of springs matches to the lines of high-angle
sub-faults of the Hanaore Fault. In addition, thesprings were often located within scars of past
bedrock landslides or at the end of scarplets formedby gravitational slope deformation (Fig. 1A, Matsushi
et al., 2014). These facts indicate that the distributionof bedrock aquifer may be constrained by faults
accompanying impermeable gouges as subsurface
water-damming structure, and groundwater may gushout to form springs at the insection of the fault gouge.
The hypothesis of subsurface water dam-up by
faults was verified by large difference in water levels
across spring observed at the bore holes on thesurveyed slope profile (Fig. 1B). Average water levelat the upper bore hole is 463 m a.s.l., while it isaround 400 m in the lower hole, indicating a large
drop of groundwater table larger than 60 m. As theboring cores show highly fractured appearance evenin a deeper part, the bedrock seems to retain
unconfined aquifer with free groundwater table. The
aquifer should be separated by the high-angle fault,
and the groundwater overflows the top of its
impermeable clayey gouge (Fig. 1B).

4. Results and discussion

Method

3.1 Field surveys
Hydro-geological field surveys were conducted in
the selected hillslope. Location of major springs was
mapped using portable GNSS tools with LiDAR
images, with measurement of rough amount ofdischarges. Distribution of lithology was investigatedwith description of attitudes of beddings, joints, andfoliations. Outcrops of thick band of fault gouges
were also located at head hollows at springs andalong gullies surveyed.
A long-sectional survey line was set to carry out
detailed hydro-geological observations (Fig. 1B). Soil
thickness was surveyed by cone penetration
soundings for few-meter intervals at the upper part ofthe slope profile. A thin piezo-pipe was installed ateach sounding site to check water seepage into soil
layer from bedrock. Geological structures beneath thesoil layer was investigated with boring cores
extracted by drilling into bedrock at three sites along
the slope profile (Fig. 1B).
3.2 Hydrological observations
Discharges were monitored at the spring named
K_C1 on the profile (Fig. 1B). We set a V-notch weirat the spring outlet, and measured the height ofoverflow by a water-pressure sensor (PA-830-101G,
Copal Electronics Corp.). The overflow height (inmm) were converted to runoff amount (in L/s) basedon in-situ calibration. Temperature and electricalconductivity (EC) of the inlet water also monitored(by HOBO UA-001-64 and U24-001, Onset
Computer Corp., respectively). All these data wererecorded at 5 or 10 min intervals by data loggers.
Groundwater fluctuation in bedrock wasmonitored using the bore holes named K_a1 (Fig. 1B).
The bore holes were screened at depths below normal
groundwater tables, so they function like wells into
bedrock aquifer. Temporal changes in water level inthe bore hole were monitored by a pressure sensor

4.2 Characteristics of rainfall-runoff hydrographs
and groundwater level fluctuation
Hydrographs observed at the K_C1 spring show
seasonal fluctuations ranging in two orders of3

10th Asian Regional Conference of IAEG (2015)

Fig. 2 Hydrograph observed at a bedrock spring (blue) and groundwater response in bedrock (black).
magnitude (Fig. 2). It shows drastic increase by large
events of summer rainfall to the highest discharge of~10 L/s and declines gradually within few months.
The spring runoff has almost no response to the small
rainfall events. In winter, the discharge exhibits
gradual increase in response to snowfall and melting,forming rounded runoff summits up to ~5 L/s incontrast to the prominent peaks in summer seasons.
Groundwater levels observed in the K_a1 bore
holes indicate a similar pattern to the spring discharge(Fig. 2). They changed in response to large events ofrainfall and snowmelt over a range of few meters. Of
note is that the groundwater level show a fast raise bya large rainfall supply even the water table isobserved in the deep wells of 20 to 30 m below
surface. Although the mechanism of this quick
response of the bedrock groundwater is unknown atpresent, the lag-time of the peak of groundwater levelfrom rainfall peak becomes shorter with increasing
preceding effective rainfall (or snowmelt) (Fig. 2).
This implies that wetness of soil or shallow part ofbedrock plays an important role in percolating wateror propagating pressure into deep bedrock aquifer, as
pointed out also in recent studies (Gabrielli et al.,2012; Salve et al., 2012).
Focusing into individual runoff event, we caninfer significant contribution of deep groundwaterflow through bedrock to the discharge from spring.
Fig. 3 represents a typical hydrograph in response tosummer intense rainfall. The first small runoff peak
appears promptly after the rainfall peak, and the large
secondary peak follows with a lag time of few hours
to days from the rainfall peak only for the event
started 13 Oct 2014 (Fig. 3A). Temperature of therunoff water increases simultaneously with the firstrunoff peak then down back almost to the initial value

Fig. 3. Typical responses in spring runoff andgroundwater level for intense summer rainfall
events. (A) Rainfall and discharge at K_C1 spring.
(B) Responses in temperature and electricalconductivity (EC) of the runoff water from thespring. (C) Responses in groundwater level and EC
at the K_a1 bore hole. EC of spring runoff (grey) is
indicated for reference. (D) Changes intemperature in the depth profile at the K_a1 bore
hole. Temperature of spring runoff (grey) is
indicated for reference.

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10th Asian Regional Conference of IAEG (2015)

even the discharge increases to form the secondary
runoff peak (Fig. 3B). Electrical conductivity (EC)of the runoff water from spring drops during the firstpeak.
The groundwater level in bedrock show a similar
response to the slow component of discharge (Fig.
3C). The timings of initiation of groundwater raise
match to the initial runoff peak, whereas the change
in EC of the deep groundwater delays to the responseof the groundwater table. This strongly supports theidea that prompt pressure propagation due to rain
infiltration into surface soil layer generates apiston-like flow through fractures in weathered upper
bedrock, leading to a quick raise in deep groundwaterlevel. This mechanism to connect surface rainwater todeep groundwater may play a significant role intriggering bedrock landslides.
The profile of subsurface temperatures wasbasically stable during these events (Fig. 3D),
supporting the hypothesis of pressure propagation.
The change in temperature of spring discharge can be
explained by mixing of water components withdifferent temperatures, most simply, of two
endmembers of deep bedrock water of 11.5 °C andshallow soil water of 17 °C. Since the outlet of
bedrock drainage for the K_C1 spring is covered bysoil mantle, the runoff-water temperature may bebuffered by warm soil overburden.

This indicates that the groundwater drainage frombedrock is controlled by seepage through fractureswith high connectivity, forming an unconfined
aquifer in bedrock with a free water table. The
relationship indicates a cubic function for the upperbound while it accords with a quadratic function forlower envelope. This suggests effect of geologicalstructure for the cross sectional shape of the aquifer,that is capable to be approximated to a rectangular in
the lower discharge, but changes to triangular in acondition of higher discharge (Fig. 4). Thishydro-geological structure may be formed by a
combination of high-angle faulting and low-angle
beddings/foliations, but not certainly confirmed themechanism at present that the aquifer drains different
amount of spring discharge even in a similar
condition of groundwater level.
The formulation may be applicable to estimate
groundwater level (and might be pressure head) in
bedrock from spring discharge. More data are needed
to check spatial variability of this functional
relationship between them, and specify which
structure affect the groundwater recharge anddrainage. Even though, we can find out a clue to infer
groundwater states in deep bedrock from the spring
runoff, by an easily accessible monitoring approach.

4.3 Connection between spring discharge andbedrock aquifer
The spring discharge shows a clear correlation to
groundwater level at the upslope bore hole (Fig. 4).

This paper reveals bedrock groundwater dynamics
and its relation to spring discharge based onlong-term field monitoring in a hillslope underlain byaccretionary sedimentary rocks near active strike-slip
fault system. Spatial distribution of springs anddifference in groundwater level across a high-angle
sub-fault indicate subsurface water damming by the5.

Conclusions

Fig. 5. Schematic illustrating bedrock aquifer
system separated by high-angle impermeable
fault gouge. Quick piston flow due to rainfall
infiltration through fissures recharges the
bedrock groundwater. The bedrock aquifer
drains groundwater by overflowing of thesubsurface dam as persistent spring discharge.

Fig. 4. Correlation between upslope groundwaterlevel in bedrock and discharge from spring (blackdots). Blue and red curves indicates a cubic and aquadratic function to explain the overflow
drainage from bedrock aquifer.

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10th Asian Regional Conference of IAEG (2015)

fault with impermeable clayey gouge. This bedrock
aquifer system can be drawn schematically as Fig. 5.
Rainfall infiltration recharges the bedrock aquifer bya piston-like flow through fractured upper bedrock,
resulting in quick raise of the deep bedrock
groundwater table. The bedrock groundwater drainsby overflowing to form persistent spring discharge. A
clear correlation between spring discharge andupslope groundwater level supports the formation of
unconfined aquifer within the highly fractured
bedrock. The functional relationship between them
implies the constraint of cross sectional shape ofaquifer by low-angle beddings or foliations. This typeof groundwater retention in bedrock may cause adeep-seated deformation and/or landslide across thefault. Our results demonstrated the potential way toestimate pore-water pressure in bedrock bymonitoring of spring discharge in order to assess
slope stability.

scales, Journal of Hydrology, Vol. pp. 450-451,
117–133.
Imamura, T. and Inoue, K. (2002): Biwako-seigan
Earthquake(1662) and Machii Landslide Dam,
Historical Earthquakes, Vol. 18, pp. 52-58 (inJapanese).
Katsura, S., Kosugi, K., Mizutani, T., Okunaka, S.
and Mizuyama, T. (2008): Effects of bedrock
groundwater on spatial and temporal variations insoil mantle groundwater in a steep granitic
headwater catchment, Water Resources. Research,
Vol. 44, W09430.
Kosugi, K., Katsura, S., Katsuyama, M. andMizuyama, T. (2006): Water flow processes inweathered granitic bedrock and their effects on
runoff generation in a small headwater catchment,
Water Resources. Research, Vol. 42, W02414.

Acknowledgements
Kosugi, K., S. Katsura, T. Mizuyama, S. Okunaka,
and T. Mizutani (2008), Anomalous behavior of soil
mantle groundwater demonstrates the major effectsof bedrock groundwater on surface hydrologicalprocesses, Water Resources. Research, Vol. 44,
W01407.

This research is conducted with the support by
JST CREST, KAKENHI (23221009, 24300321,
25247082, 26702010), and General Collaborative
Research in DPRI-KU (26A-04).
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