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Interaction bedrock groundwater – Runoff in a catchment affected by creep deformation.
Implications in the generation of deep seated landslides.
University of Tsukuba, Graduate School of Life and Environmental Sciences.

żCristóbal PADILLA
Yuichi ONDA
Tomoyuki IIDA
Shinya TAKAHASHI

Public Work Research, Department of Volcano and Debris Flows.

Taro UCHIDA

1. Introduction
The analysis of the interaction between the bedrock groundwater and surface-subsurface discharge has been
gain importance in the Japanese territory in the last years. Progressively more studies show the relevance ofbedrock groundwater in the hydrological behavior and response of mountainous and forested hillslopes andcatchments. However, the analysis of this interaction with highly fractured bedrock has been poorly studied.
The characteristics of the bedrock groundwater are important in the analysis of the generation of landslides,
especially deep seated landslides, a disaster that repeatedly happens in Japan after heavy rainfall events.
This study presents the analysis of the interaction bedrock groundwater – runoff in catchments of Mt
Wanitsuka (1,118 m asl), Miyazaki prefecture. In this area, the pass of the typhoon No 14 in 2005 induced thegeneration of several deep seated landslides (DSLs). The objective of this study is to characterise theinteraction bedrock groundwater – runoff in catchments of Mt Wanitsuka defining water pathflows in thehillslopes affected by creep deformation. This characterization can allow to understand the mechanism of thegeneration of deep-seated landslides in this area.
2. Study area and instruments
The study area is located in Mt. Wanitsuka (Fig 1). The
bedrock geology corresponds to shale interbeded withsandstone. In some slopes it was identified creep deformation
which affect also to the bedrock increasing the fracture
conditions on it. In the area selected for this analysis it wasidentified creep deformation. It were installed 2 boreholes(Bh10: 10m depth, deposit cover groundwater; Bh40: 40 m
depth, bedrock groundwater) to measure the groundwaterlevels (Gw) and two parshall flumes to measure the dischargeof two catchment (D: 1.44 ha; U: 5.42 ha). The instruments
are situated in the vicinities of a DSLs scar that took place in2005. Form the measured point in were collected water
samples to analyse the electrical conductivity, EC,concentration of stable isotopes (18O and 2H, deuterium) andmajor elements (Ca2+ and Si). The collection of data andsamples was carried out from 2008 to 2012.

Fig 1. Study area and instruments location.

3. Results and discussion
3.1 Groundwater responses: The analysis ofgroundwater response shows a very quick response of
Bh40 which represent the response of the bedrock
groundwater after rainfall events (Fig 2). These quickchanges in the bedrock groundwater levels are also
expressed in a quick decrease in the EC properties. In
the other hand, the Bh10 borehole presents small
variations of groundwater levels and EC. The quickchanges in the bedrock groundwater can be interpreted
as fast infiltration of rainfall water into the bedrock aquifer.
The detailed analysis of the response of groundwatershows also differences depending on the precipitationlevel (Fig 3). Two types of responses were identified in
Bh40. For precipitations lower than 100mm of API6
Fig 2. Rainfall - Gw level - Gw EC - Discharge catchment U and D.
(Antecedent precipitation index with 6 hour half life) theresponse in bedrock groundwater shows a double peak as it shown in Fig 3a, Lp. Rainfall over 50 mm of API6
can generate another type of response, Hp, where the double peak is replaced by a single peak with a very㪄㩷㪙㪄㪊㪊㪏㩷㪄steep rising limb and a stable maximum at approximately 11.5 m bgs. Between 50 and 100 mm of API6 therainfalls could generate both types of responses, Lp and Hp. Observations in the borehole core samples
reveal the existence of sets of fractures that can be involved in
the different response of Gw. Those sets are associated to thecreep deformation that affects the deposit cover and the bedrock.
The analysis of Gw during Hp peaks indicates a high variabilityof concentration of 18O and 2H during the peak, but more stablevalues during the base level before and after a response. Thiscan be interpreted as that most of the water that rapidly infiltrate
into the bedrock can also rapidly be discharged from thebedrock aquifer.
3.2 Catchment responses and evidences of interaction withgroundwater: Even though the catchment U and D present
different areas and therefore different discharges, thecomparison of the specific discharge shows similar quickflow
discharges, represented by the first peaks in the catchment
response (fig 3). After the similar quickflow, it is observed anFig 3. Detailed hydrographs of Gw and discharge inincrease on the specific discharge in catchment D over catchment
catchment U and D During Lp and Hp responses.
U. That increment in the specific discharge in D generates adelayed peak; similar situation observed in the Lp peaks in Bh40. During the base level, it is observed acontinue discharge in catchment D even in dry season whereas in U there is no discharge after extended
periods of no rain. Form the water balance in both catchments it is possible to observe that catchment D
present a very small storage; periods of positive storage are followed by negatives storages not observed incatchment U (Fig 4). In terms of water quality, it was measured relatively high concentration of ion Ca2+ incatchment D (7 ~15 ppm) especially during the base level that can becorrelatedtoHighconcentrations of Ca2+
measured in Bh40 (morethan 20 ppm) but no in
Bh10 (less than 5 ppm).
Thehydrographseparation based on theconcentrationofdeuterium (rainfall waterandGwasend
Fig 4. 'Storage of catchment D and U .
members)
showagroundwater contribution over 50% of the specific discharge (fig 5). The
slightly greater contribution of Gw in U can be reflecting the greater
contribution of new water (rainfall water) in the discharge of D coming fromthe discharge from the bedrock aquifer. The analysis of the concentration ofCa2+ and Si in the catchments show the greater decrease of Ca2+ values in
D from the base level compared with U. The lower concentration of Ca2+ iscoincident in both catchments with the maximum discharge. However, thelowest concentrations in Si values are correlated with the peak of intensityof rainfalls, during the quickflow. This also can be related with relatively newwater that is discharged from the bedrock groundwater.

4. Conclusions
The characteristics of groundwater response reveal a strong control of its
levels by structures in the bedrock. Those structures can be linked to theFig 5. Chemical analysis of discharged
creep deformation observed in the hillslope. The analysis of the discharge inwater in D and U.
two catchments (catchments U and D) in the area shows how thegroundwater levels affect more strongly the one who present evidences of creep deformation (Catchment D).
The water balance show low storage for catchment D (short residence time).The rapid response of bedrock
groundwater and its relatively rapid expression in the discharge seems to be related to the relationshipbetween the timing of peak rainfall intensity and the occurrence of landslides in 2005 almost simultaneously.
The proportion between the infiltrated water that remains in the catchment D and the groundwater discharged
by the catchment is proposed as a parameter in order to define conditions of generation of DSLs in the area.

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