Hydrodynamic Modelling of Flood Inundation on Welang River,

Indonesia

Ilham Cahya and Suntoyo

Department of Ocean Engineering, Faculty of Marine Technology, Institut Teknologi Sepuluh Nopember,

Surabaya 60111, Indonesia

Keywords: A 1D-2D Coupled Hydrodynamic Model, Flood Inundation, Welang River, MIKE FLOOD.

Abstract: A flood inundation in a river can be caused by an increase in river water level due to the addition of water

discharge from both the rainfall and the tides from the sea. Flood inundation simulation can be done by using

a numerical flood model flood model. In the Welang basin, floods occur every year and cause many problems.

Floods that occurred on April 29, 2019, caused the closure of national and regional roads and many houses

that were flooded. In the present study, flood model was carried out by using MIKE FLOOD software. Semare

Village and Tambakrejo Village are critical points where floods occur. The depth of the water from the

simulation model occurs 0.3 m - 2.2 m compared to the flood map report by the Indonesian National Disaster

Management Agency which estimates it at 0.1 m - 1.5 m.

1 INTRODUCTION

The Welang Basin is threatened with flooding every

year. It happened during the rainy season from

December to June. The higher the surface of the river

water from the increasing intensity of rainfall and

rising sea level due to the tide cycle, this results in a

higher risk of flooding. The Welang River has many

tributaries (about 21) with a total watershed area of

509.5 km

and a length of about 53 km, so that the

river's water level becomes high when the intensity of

rainfall increases.

(Wahyudi et al., 2018) investigated a method to

apply eco-drainage to control flooding that often

occurs in the Welang river, however, the flood can’t

be predicted how far it can spread of eventhough the

flood affects Pasuruan districts so that flood control

using the method of harvesting rainwater, by using

infiltration ponds requires a large number of wells to

be applied. (Wahyudi and Sumirman, 2019), showed

how the flood management in the Welang basin can

be done by injecting flood water into ground water

using Artificial Storage Recharge (ASR). However,

there have been no studies on two-dimensional flood

inundation models in the Welang River Basin.

MIKE FLOOD 1D-2D coupled model is a

powerful software for integrated hydraulic (1D) –

hydrodynamic (2D) modelling purpose. A lot of

research has been done using MIKE FLOOD 1D-2D

coupled model (e.g., Dat, et al., 2019, CTCN, 2017,

Timbadiya et al., 2014, Kadam and Sen, 2012, Patro

et al., 2008). In the present paper, the existing

condition of flood in the Welang river is simulated by

using the MIKE FLOOD 1D-2D coupled model. The

expected final result is a flooding map.

2 STUDY AREA

The modelling area is located at the downstream of

Welang River. The desired model area is located at

Kraton sub-district, Pohjentrek sub-district (Pasuruan

district), and Gadingrejo sub-district (Pasuruan City)

from Dhompo village to river mouth on the north,

where river levels data is measured at Dhompo

village in the coordinate system (-7.665908 latitude,

112.856342 longitude). The model area showed in

Figure 1 within the red area.

Cahya, I. and Suntoyo, .

Hydrodynamic Modelling of Flood Inundation on Welang River, Indonesia.

DOI: 10.5220/0010218602450250

In Proceedings of the 7th International Seminar on Ocean and Coastal Engineering, Environmental and Natural Disaster Management (ISOCEEN 2019), pages 245-250

ISBN: 978-989-758-516-6

245

Figure 1: Modelling area of Welang River (Google Earth Pro).

Figure 2: Flow chart of MIKE FLOOD 1D-2D coupled model.

3 METHOD

The methodology applied for solving this problem

consists of the data collection, the model settings on

MIKE 11 1D and MIKE 21 2D, the models added

MIKE FLOOD 1D-2D, and the model validation can

be seen in Figure 2. The MIKE FLOOD 1D-2D

coupled model consists of upstream river surface and

sea level at the river mouth (Figure 3). Hourly river

level data collected for 1 year in 2019 and sea level

forecast data were collected from measurements for

15 days in 2014 in Grati, Pasuruan.

Bathymetry and topography data were collected

from the Indonesian Geospatial Information Agency.

Surface height and cross-section data collected from

the Water Resources Public Works Department of

East Java Province. The model runs from 27 April

2019 11:00 PM and finishes on 29 April 2019 6:00

AM.

ISOCEEN 2019 - The 7th International Seminar on Ocean and Coastal Engineering, Environmental and Natural Disaster Management

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Figure 3: Modelling setup of MIKE FLOOD coupled model.

Manning coefficient is used for determining the

bed roughness parameters. Applied bed roughness

coefficients are divided into river, landuse and sea

bed areas, which use a uniform distribution. The bed

roughness coefficient values are estimated based on

(Chow, 1959) and the MIKE 21 recommendation

summarized in Table 1.

Table 1: Applied Manning number.

Natural stream Manning’s n

River upstream 0.040

River downstream 0.045

Landuse Manning’s M

Vegetation,

natural areas

Sea Manning’s M

Seabed 32

Table 2: Average Bed Slope of Welang River.

Area Stations (m) Slope

Dhompo 0 – 1000 0.002408

Sungi Wetan 1000 – 3000 0.001601

Sukorejo 3000 – 5200 0.001890

Tambakrejo 5200 - 6000 0.002657

Semare 6000 – 11400 0.000870

According to the Manning equation, the slope of

the river bed also contributes to the increase in river

water flow and flow discharge. Areas that have a low

slope are at risk of water deposition, causing flooding

to occur. Semare village has a low slope shown in

Table 2. Because the location of the Sumare village

close to the river mouth causes a very high tidal effect

as seen in Figure 4.

Model validation was performed using the flood

map data from National Disaster Management

Authority of Indonesia (BNPB, 2019) shown in

Figure 5.

4 RESULTS AND DISCUSSION

Comparison of flood maps from flood models and

flood map reports from BNPB is shown in Figure 6.

Green polygons represent inundation in Tambakrejo

Village and yellow polygons represent inundation in

Sukorejo Village. At 01:00 28 April 2019, this model

showed good results compared to the flood map

report from BNPB, 2019. The validation process is

carried out through water depth data where the results

of this validation are shown in Table 3.

The time of the flood occurs when the highest tide

is 0.47 m MSL at 20:00 28 April 2019. At 20:00, 28

Hydrodynamic Modelling of Flood Inundation on Welang River, Indonesia

247

Figure 4: Modelling setup of MIKE FLOOD coupled model.

Figure 5: Flood map report from National Disaster Management Authority of Indonesia, (BNPB, 2019).

April 2019, the surface of the river shows 5 m as

the initial flood time. At 21:00 April 28, 2019, the

highest river water level reached 6.27 m which

occurred at a tidal height of 0.39 m MSL until 0:00

April 29, 2019 with a river height of 5.91 m at tidal

height -0 , 24 m MSL (Figure 7). At 3 am on April 29,

2019, the height of the river did not exceed the height

of the riverbank so that the flood spill subsided.

Figure 7 shows the relationship between river water

level and tidal change when flooding occurs in the

Welang river from the results of a modeling

simulation. However, the temporal variation of flood

map on Welang River has not been examined in

detail, yet.

Table 3: Water depth validation 4/28/19 1:00 AM.

Type Water depth (m)

Model 0.3 – 2.2

BNPB 0.1 – 1.5

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Figure 6: Flood model validation at 1:00 AM 28 April 2019.

Figure 7: The relation of the river levels and tide levels when flood occured on Welang River.

5 CONCLUSION

Based on the modeling results and analysis in the

paper, a few conclusions can be drawn as follows:

1. From the results of the hydrodynamic model it

has been shown the critical point at which floods

begin to overflow. Semare Village and

Tambakrejo Village are critical areas where

floods begin to overflow.

2. This Kali Welang flood modeling simulation is

validated using water level data from the flood

map report of the Indonesian National Disaster

Management Agency (2019), which shows quite

valid results, the simulation model results show

0.3 m - 2.2 m while the data used for this tie tie

is 0.1 m - 1.5 m.

3. River water level 6.27 m occurs during tidal

height 0.39 m MSL until 0:00 April 29, 2019

with river height 5.91 m at tidal height -0.24 m

MSL. At 3 am on April 29, 2019, the height of

the river did not exceed the height of the

riverbank so that the flood spill subsided.

ACKNOWLEDGEMENTS

Authors are grateful for the supported to the Centre of

Research and Development of Marine and Coastal

Resources, the Ministry of Marine Affairs and

Fisheries, Republic of Indonesia for providing the

facilities of DHI’s Mike 21/3 model. And authors also

thank to the Water Resources Public Works

Critical moment

Flood spill eased

Flood start

Hydrodynamic Modelling of Flood Inundation on Welang River, Indonesia

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Department of East Java Province for providing some

informations and data related the Welang River.

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