21 July 2017
International Centre for Water Hazard and Risk Management (ICHARM)
Public Works Research Institute (PWRI)

Simulation of a flash flood forecast for a mountainous river
We conducted simulation of a flash flood forecast for a mountain stream in a southern part of Japan aspart of flood hazard research on mountainous rivers using a Rainfall Runoff Inundation model (the RRI model1))
with forecasted rainfall data. The figure shows the simulation results on hourly changes in flood water level
resulting from severe rainfall in July 2017 over the Kagetsu River, a tributary of the Chikugo River. In the figure,the forecasted curves of flood water level from 10:00 to 19:00 cross several Warning Water Levels (WWLs),
which are announced for the public to use for safe evacuation. For example, WWL4 denotes that the river waterreaching this level is likely to cause a serious flood disaster.
According to the results, the 16:00 forecast (the dotted line marked with ⑦) shows that the flood waterlevel is anticipated to exceed WWL 4 around 18:40 (with a lead time of 160 minutes).
The results of simulation in the Kagetsu River demonstrated a good reproducibility of the RRI model forflood events in mountainous rivers and the practicality of real-time forecasting of flood water levels, both ofwhich will contribute to ensuring a lead time for safe evacuation.
〔Parameters employed in RRI model〕
Kagetsu River〔Kagetsu River Basin〕
Catchment area : 136.1[km2]River length : 16.6[km]Land use : forest

Subsurface + Surface
Vertical Infiltration

Sd = 0.5[m]Surface soil depthn = 0.030[m-1/3s] Roughness of river channel
N = 0.6[m-1/3s] Roughness of slope
ka = 0.01[m/s]
Saturated hydraulic conductivity
km = 0.0025[m/s] Unsaturated hydraulic conductivity
da = 0.30
Saturated effective porositydm = 0.12
Unsaturated effective porosity

1D Diffusion
in River2D Diffusion on Land

〔Kagetsu gauging station〕
LegendObserved Water Level
Calculated Water Level
Forecasted Water Level①Forecasted W.L. at 10:00②Forecasted W.L. at 11:00③Forecasted W.L. at 12:00④Forecasted W.L. at 13:00⑤Forecasted W.L. at 14:00⑥Forecasted W.L. at 15:00⑦Forecasted W.L. at 16:00⑧Forecasted W.L. at 17:00⑨Forecasted W.L. at 18:00⑩Forecasted W.L. at 19:00⑪Forecasted W.L. at 20:00⑫Forecasted W.L. at 21:00

WWL 4 : water level with high risk of flooding
WWL 2 : pre-cautious water level of inundation

WWL 3 : warning water level to start evacuation
WWL 1 : water level for stand-by of flood fighting corps

Forecasted flood water levels at specified times using the RRI model with analyzed and forecasted rainfall data. Theforecasting used rainfall data forecasted and analyzed by JMA and water stage data from the MLIT water information
system. Flow discharge was converted into water level based on the cross-sectional data collected in 2012.
1) Takahiro SAYAMA, Rainfall-Runoff-Inundation(RRI) Model ver. 1.4.2, ICHARM, PWRI (http://www.icharm.pwri.go.jp/research/rri/rri_top.html)

Chikugo River Basin Map and Kagetsu River

Kagetsu River

Fig. Kagetsu River Basin〔Chikugo river〕
Catchment area : 2,860[km2]〔Kagetsu river〕
Catchment area : 136.1[km2]River length : 16.6[km]Land use : forest

Source：The Fundamental River Improvement Plan of Chikugo River System(July, 2006), Kyushu Regional Development Bureau, MLIT