Flow rate of leakage Flow rate of
drainage pump
Change in flooding Dischage of
diversion channel
Rainfall Inflow of landslide dam Overflow channel
Landslide dam
Groundwater recharge
LANDSLIDE DAM HYDROLOGICAL OBSERVATION AND HYDROLOGICAL BALANCE CALCULATION PROCEDURES
Soichi KAIHARA
1*
, Noriko TADAKUMA
1
, Yasumasa FUJIWARA
1
,Wataru SAKURAI
2
, Makoto OYAMA
3
and Ryo SAKAI
2
1Eight-Japan Engineering Consultants Inc., Japan
2 Kii Mountain District Sabo Office, Kinki Regional Bureau, Ministry of Land,Inflastructure,Transport and Tourism, Japan
3Kinki Regional Bureau, River Division, Ministry of Land,Inflastructure,Transport and Tourism, Japan
*Corresponding author. E-mail:kaihara-so@ej-hds.co.jp
INTRODUCTION
Numerous landslide dams formed in southern Nara Prefecture following the over 1,000 mm of rainfall caused by typhoon no. 12 in 2011, and some of these remain. When the water level rises, these dams pose a risk of overflow and washout. It is necessary to understand the hydrological properties of the landslide dam catchment area in order to predict the water level during flooding. However, in many cases, landslide dams that form in valleys in mountainous land do not have the structure of fixed beds in river channels. Here, we present a calculation of the hydrological balance of the landslide dam circumference for a rain event based on hydrological observations the runoff characteristics of the catchment area.
INVESTIGATION AREA AND METHOD
The investigation areas were two landslide dams, whose locations are shown in Fig. 1. The specifications of the two landslide dams are shown in Table 1, and a diagram of the
hydrological balance of the landslide dam is shown in Fig. 2. To understand the hydrological balance in terms of inflow, rate of flooding
increase/decrease, and other variables, the observations shown in Fig. 3 were performed. The hydrological balance of the landslide dam circumference was calculated by the following method.
Fig. 1 Scope of study area
Fig.2 The concept of the hydrological landslide dam circumference
・The depth–volume curve of the landslide dam was calculated using a laser scanner and acoustic doppler current profiler.
・The amount of change was calculated using the flooding water gauge.
・The groundwater level was measured in the boreholes of two or more landslide dams to determine the quantity of water that was leaking downstream from the flooding. This allowed calculation of the percolation area and hydraulic gradient by Darcy's rule using the inverse operation coefficients of permeability.
CONCLUSIONS
The hydrological balances of the rain event associated with typhoons no. 16 and 17 in 2012 in Nagatono and Kuridaira are shown in Fig. 4. In the Nagatono area, the rate of rain loss and water leakage amount was large compared with those in the Kuridaira area. There were few diversion channel flows at the time of typhoon no. 16 in the Kuridaira area. The opposite occurred at the time of typhoon no. 17, when the situation favored easy erosion in the Kuridaira area. By investigating the hydrological balance using this method, runoff
characteristics such as the amount of rainfall loss could be calculated to water level prediction during flooding.
Keywords: landslide dam, hydrological observation, hydrological analysis, hydrological balance
Tab.1 The summary of landslide dam
8.95 km2 5.1×106m3
100 m Kuridaira
4.63 km2 1.9×106m3
80 m Nagatono
Catchment area Reservoir
capacity The height
of the dam Area
8.95 km2 5.1×106m3
100 m Kuridaira
4.63 km2 1.9×106m3
80 m Nagatono
Catchment area Reservoir
capacity The height
of the dam Area
0 50 100 150 200 250 300 350
2012/9/16 6:00～2012/9/19 17:00 2012/9/30 10:00～2012/9/30 20:00 Event period (Kuridaira)
Height (mm)
Height of rainfall loss Height of leakage Height of change in flooding Height of diversion chanel dischage Height of drainage pump
0 20 40 60 80 100 120 140 160 180
2012/9/15 15:00～2012/9/19 12:00
2012/9/29 19:00～2012/9/30 20:00 Event period (Nagatono)
Height (mm)
Height of rainfall loss Height of reakage Height of change in flooding
no.16
no.17 no.17
no.16
Fig. 4 Hydrological balance of the two landslide dam circumferences on for typhoons 16 and 17 in 2012 Fig. 3 The hydrological balance observation method for
the landslide dam circumference Rainfall gauging Water-level
gauging
Discharge measurement (monthly)
Grandwater level
Water-level gauge Rain gauge
Using magnetic current meter Borehole
Rainfall
A laser scanner and acoustic Doppler current profiler surveys for depth-volume curve
Rainfall gauging Water-level gauging
Discharge measurement (monthly)
Grandwater level
Water-level gauge Rain gauge
Using magnetic current meter Borehole
Rainfall
A laser scanner and acoustic Doppler current profiler surveys for depth-volume curve
