Numerical Study on Reducing debris flows Impacts in Putih River, Indonesia

Numerical Study on
Reducing debris flows
Impacts in Putih River,
Indonesia
Jazaul Ikhsan, Puji Harsanto, Tria Wulandari
Dept. of Civil Engineering, Faculty of Engineering,
UMY, Indonesia

Outline of Presentation
•
•
•
•
•

Introduction
Research Method
Simulation Lahar (SIMLAR) V.1.1
Discussion
Conclusion

Introduction
U

Mt. Merapi has erupted
regularly and the eruption
has been more active since
1992.
A huge sediment production
threats people live and assets
in the downstream area.
In the rainy season, volcanic
material flows rapidly and
spreads through tributaries
as debris flow.

Merapi’s Eruption in 2010
Merbabu

Woro

Merapi

Opak
Gendol

Boyong
Kuning

( SPOT satellite image © CNES (2010), acquired by CRISP, NUS)

City of Yogyakarta

Runway of Yogyakarta
International Airport

Prambanan Temple (World
Culture Heritage)

The 2010 eruption of Mt. Merapi is one of the
biggest eruption, around 150 million m3.

Progo’s Tributaries: Pabelan, Putih, Blongkeng,
Lamat, Krasak Rivers

Impacts of debris flows on
Putih River, Yogyakarta,
Indonesia (Pebruary, 2011)

Research Method
• The objective of this study is to simulate
impacts of debris flows and effects of sabo
works to reduce the debris flow impacts.
• For simulation, the rain fall data was taken
from Ngepos station. The hydrograph was
generated using Nakayashu method.
• Digital Map Elevation was used from Balai
Sabo (Sabo Center), Yogyakarta

Daily Rainfalls data of Ngepos Station In January 2011

Date

Time
13.00-14.00
14.00-15.00
15.00-16.00
16.00-17.00
17.00-18.00

22

23

24

11.5
2.0
2.5
0.0
0.5

66.5

24.5
1.5
0.5
0.5

0.0
12.5
0.5
0.0
3.6

Simulation Lahar (Simlar) V.1.0.
• Simulation Lahar (Simlar) software is
developed by Sabo Center and UGM, based
on JICA –STC’s 2D Nu erical Si ulatio of
Riverbed Fluctuation and Deposition of Lahar.
• The software is modified on interface and
added some sediment transport formulas, as
Ashida-Takahashi-Mizuyama formula.

Governing Equation
• Momentum equation:

N
 (uN )
 (vN )
H


  gh
  by
t
x
y
y

• Continuity equation:
h M N



0
t  x  y
Where, h is dept of flow, t is time, x and y are the
coordinate along the longitudinal and transversal
direction, g is the gravity force, M and N are velocity in
x and y direction, τby is turbulence stress.

Governing Equation
• Continuity of sediment discharge:
 zb   qBx  qBy


c
t   x
y


0


where is sediment concentration in riverbed; and are
sediment discharge in x and y directions

• Sediment discharge equation: ATM
 B  k*

3/ 2

* c
* c 1 / 2
( 1  0,85
) x( 1  0,92
)
/
1
2
*
*

• Sediment discharge equation: Brown

 B  k( *  * c ) 2.5

Governing Equation
• Sediment discharge equation: MPM

 B  k( *  * c )1.5

where:
B 

qB
3 1/ 2

( sgd )

u*

* 

(    ) gd sgd

2

  ghI c  fU
* c

2

2
c
u* c


 0,05
(    ) gd
sgd

qB is sediment discharge (m3/det), τ฀c is the averaged
alue of critical shear stress, τ*is shield number.

Simulation Scenario

Description

Cases
Additional
of sabo
works

Discharge 1

Type

Existing
of sabo works

Discharge
2

1

No

No

10.26287
m3/s

57.892
m3/s

2

Yes

No

10.26287
m3/s

57.892
m3/s

3

Yes

Yes

10.26287
m3/s

57.892
m3/s

Type 1

Type 2

Type 3

Type

Description

Type 1

There is no sabo work required to simulate with SIMLAR

Type 2

In this type, dimension of existing sabo work are:
- Height : 7.5 m
- Width : 80 m
- Coordinate : 110°17’58.344” E 7°36’15.22” S
- Sabo elevation : +388.339

Type 3

Additional sabo work required in this type besides existing of
sabo work with classifications are:
Existing sabo dimension:
- Height : 7.5 m
- Width : 80 m
- Coordinate : 110°17’58.344” E 7°36’15.22” S
- Sabo elevation : +388.339
Additional sabo work dimension:
- Height : 3.4 m
- Width : 60 m
- Coordinate : 110°18’20.758” E 7°36’15.85” S
- Sabo elevation : +405.144

Results : Discharge 1
• Without
sabo works

• With
existing of
sabo works

•

With existing
and additional
of sabo works

Discussions
• Sabo works could reduce the impacts of debris
flows, especially for reducing velocity.
• In addition, sabo works have function as
reservoir for sediment.
• However, if the sabo works have fulled by
sediment, sediment will flow out sabo works.
• If discharge of debris flow is not so high, river
still could deliver flow from upper to lower
area.

Result : Discharge 2
• Without
sabo works

• With
existing of
sabo works
•

With existing
and additional
of sabo works

Discussions
• If discharge of debris flow is very high, river
could ’t deli er flo fro upper to lo er area,
the area surrounding river is buried by sediment.
• Sabo works could reduce the area buried by
sediment from debris flows.
• Additional sabo works could increase capacity for
sediment, so the impact of debris flows could be
reduced.
.

Conslusions
• Sabo works is effective to reduce the impact
of debris flows.
• However, it is necessary to do some study or
research to obtain the right dimension of Sabo
Works.

Thank you very much
for your attentions