Bathymetric Survey for Estimating the Local Scour at Suramadu

Bridge

Octavia Prihanesti Kartika Rini

, Suntoyo

and Aditya Kusumadinata

Department of Ocean Engineering, Institut Teknologi Sepuluh Nopember, Surabaya, Indonesia

Ministry of Public Work and Housing, Directorate General of Highway, Jakarta, Indonesia

Keywords: Bathymetri, SBES, MBES, Scouring.

Abstract: The Suramadu Bridge is the longest bridge in Indonesia, located at Madura Strait causes an increase in the

speed of ocean currents as well as obstacles to seawater flow acceleration of ocean currents due to the reduce

of the wet cross-sectional area of seawater flow. This influence cause changes in the seabed bathymetry along

the Suramadu Bridge and the potential for local scouring on the bridge foundation. In dangerous stage of

scouring, can affect the stability and carrying capacity of the foundation structure under the bridge. The

Suramadu Bridge has operated since 2009, has been almost 10 years, and bathymetric surveys is needed to

ensure the actual conditions on the seabed under the Bridge. The purpose of the bathymetric survey activity

is to obtain the latest bathymetric data and the scouring patterns arround the main pillar (P46 and P47) of the

Suramadu Bridge. The Bathymetric Survey method used are SBES and MBES. The first Bathymetric survey

was obtained at the year of 2016 since the bridge had operated, shows that the scouring was indicated at the

main pillar the increase is about 7 – 8 m. Based on the bathimetric survey data of the 2017, shows the scouring

is increased is about 1 meter below the result of bathymetric survey of 2016 at the seabed in the main bridge.

1 BACKGROUND

Many bridges around the world failed or damaged

because of extreme scour around piers and abutments.

Any constructed bridges must be continuously

monitored for any changes in the structure of the

bridges that usually focus on the changes of the soil

in the area of the bridges constructed. The typical

changes can be seen is from the bridge scouring or in

this case (Akib, 2011).

Suramadu Bridge is located in the northern part of

East Java Province, Indonesia. With an overall length

of 5.4 km, it spans Madura Straits and connects

Surabaya and Madura Island (Jatnika et al, 2006).

Suramadu Bridge in Madura Strait reduces the wet

cross sectional area of sea water, increase the ocean

current speed and acceleration. The seabed along

Suramadu Bridge can be change after more than 10

years after the last study of bathymetri in 2005.

Scour is a natural phenomenon caused by the

erosive action of flowing water on the bed and banks

of alluvial channels. The local flow around a

hydraulic structure such as a bridge pier or abutment

is associated with an enhanced sediment-carrying

capacity, such that scour may occur near a structure

even when there is no transport of sediment away

from the structure. At a bridge site, scour around

bridge piers or abutments may lead to reduced

support and hence constitute a potential catastrophic

hazard (Masjedi, 2010).

Scour is local lowering of streambed elevation

that takes place around structures that are constructed

in flowing water (Akib et al, 2014). Local scour is the

removal of sediment from around bridge piers due to

flowing of water. A large amount of local scour is

dangerous to the bridge piers and causes the structure

tend to collapse and loss of life without any warning

(Ahmad et al, 2016).

Local scour around a bridge pier is largely

depending on the shape of the bridge pier and how the

design is fashionable from the view of construction.

Local scour is a complex phenomenon which depends

on the discharge, depth of flow, geometry of the pier

and type of sediment particle (Roy, 2017).

Scouring depth assessment for Approach Bridge

and Main Bridge of Suramadu Bridge for a 20-year

return period carried out in 2005 for pier foundation

P37 - P56 by Yu (2005) from the Department of

Hydraulic Engineering of Tsinghua University Figure

1 and Table 1. In 2016 was the first study of

Bathymetri on Suramadu Bridge by Bathymetry

Consultans was hired by Ministry Of Public Work

Kartika Rini, O., Suntoyo, . and Kusumadinata, A.

Bathymetric Survey for Estimating the Local Scour at Suramadu Bridge.

DOI: 10.5220/0010287002630270

In Proceedings of the 7th International Seminar on Ocean and Coastal Engineering, Environmental and Natural Disaster Management (ISOCEEN 2019), pages 263-270

ISBN: 978-989-758-516-6

263

and Housing, and the result was the local scouring 7

– 8 meters at the main pilars (P.46 and P.47). So, that

to ensure that the scheme of local scouring not

damaged the bridge, in 2017 the survey of

Bathymetry was continued by Bathymetry

Consultans was hired by Ministry of Public Work and

Housing.

The location of activities was along the Suramadu

Bridge corridor and around the main pillar (P46 -

P47) with a corridor width of 500 m to the left and

right of the bridge. Conduct a detailed bathymetry

survey along the Suramadu Bridge corridor and

around the main pillar (P46 - P47) with corridor width

of 1 km.

Figure 1: Maximum Scouring Depth on Pier 37 - Pier 56 of

Suramadu Bridge for a 20-Year Period Based on 2005

Study Results.

Table 1: Scour depth at major piers under tide with 100-

year return period.

Pier

Number

Scour

depth

(m)

Pier

Number

Scour

depth

(m)

Pier

Number

Scour

depth

(m)

Pier

Number

Scour

depth

(m)

P37 8,35 P42 8,94 P47 11,36 P52 6,16

P38 8,79 P43 7,41 P48 8,9 P53 5,28

P39 9,27 P44 6,7 P49 5,34 P54 5,19

P40 8,88 P45 10,2 P50 5,35 P55 5,09

P41 9,42 P46 11,5 P51 6,17 P56 4,99

The purpose of the bathymetry survey activity is

to obtain data on the latest bathymetry conditions and

scouring patterns in the main pillar of the Suramadu

Bridge. In addition, this paper gives recommendation

and mitigation if needed. Protecting the bridge piers

against scour is a crucial step to preventing bridge

failure because there is a close relationship between

bridge failure and scour at bridge foundations (Tang

et al, 2009).

2 METHOD

2.1 Data Collection

Bathymetry is a method or technique in determining

sea-depth or seabed profile from the result of sea-

depth analysis (Anugroho et al, 2017). From late 2017

the survey using Single Beam Echo Sounder (SBES)

and Multi Beam Echo Sounder (MBES). Survey

location along Suramadu Bridge with corridor width

of 1 km (500 meters to the right and left side of the

bridge). Tide measurements carried out at these

locations used for correction when carrying out

bathymetry measurements.

Data processing of multibeam depth survey

results was carried out using Qinsy software and

single beam echo sounder was processed using Hydro

pro navEdit and Terramodel software. The License of

the software was licensed to Bathymetry Consultant

that hired by Ministry of Public Works and Housing.

And the Author was a part of Bathymetry Consultant

Members.

2.2 Determination of Bench Mark

(BM)

This survey started with data collection. The first data

in this study is determine of Bench-Mark (BM) points

as measurement reference points that represent the

bridge spans and survey corridors. Mapping changes

in seabed elevation along the Suramadu Bridge,

especially in the area around the bridge foundation.

The navigation system used to determine the

position of moving objects such as ships when

conducting bathymetry surveys known as DGPS

(Differential GPS). This differential correction can

either be a pseudo range correction (such as RTCM

SC-104) or coordinate correction. With differential

correction, the coordinates obtained have relatively

higher horizontal position accuracy compared to the

absolute method (Abidin, 2000).

2.3 Bathymetry Survey

According to IHO (International Hidrographic

Organization), the bathymetry survey is "measured or

charted depth of water or the measurement of such

depth". Tides also affect the survey due to sea level

variations, so tidal observations needed to reduce the

results of the survey on the dynamics of the seawater

(Rinaldy et al, 2014).

Sounding is one of the methods of determining

depth using the principle of the reflection of an

aquatic wave (Qhomariyah and Yuwono, 2016). The

device used for this activity is echo sounder. The

using of this device is an indirect measurement of

depth by measuring the travel time of acoustic wave

pulses emitted by Transducer (Figure 2).

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264

Figure 2: Bathymetri Measurement.

Bathymetry surveys use a combination of Multi-

Beam Echo sounder (MBES), is a survey equipment

for areas > 3 m deep and Single-Beam Echo sounders

(SBES) for areas < 3 m deep with 100% coverage for

locations near bridge foundations. Using Real Time

Kinematic (RTK) Global Positioning System (GPS)

equipment.

Bathymetry surveys carried out using a

combination of Single beam Echo sounder (SBES)

and Multi beam Echo sounder (MBES) equipment.

Furthermore, SBES and MBES work methods and

work principles explained in the following sub-

chapters:

Figure 3: Echo Sounder Working Principle.

Acoustic waves with a frequency of 5 kHz or 100

Hz will maintain their intensity loss to less than 10%

at a depth of 10 km, while acoustic waves with a

frequency of 500 kHz will lose their intensity at

depths of less than 100 m. The principle of this

method is distance measurement by utilizing acoustic

waves emitted from the transducer (transmitter). A

transducer is a part of an echo technology device that

converts electrical energy into mechanics (to generate

sound waves) and vice versa. Acoustic waves travel

through the water to the seabed and reflected back to

the transducer (received by the receiver).

Bathymetry measurements influenced by the

dynamics of seawater media in the form of tides,

making it very difficult to determine the same object

at different times. Thus on the measurement of the

depth of the seabed it is necessary to do three

measurements at the same time, namely the

measurement of the depth, the measurement of the

position of the depth measuring instrument and the

measurement of tides.

2.4 Determine of Point Control

The control points used in the bathymetry survey

activities along the Suramadu Bridge were

determined using BM VKS2. The use of this control

point is intended as a verification point for

Differential GPS that is used as a horizontal

positioning system or navigation during the survey.

Here are the results obtained.

Point Name : BM VKS2

Stipulated by : BPPT

Location Point : Tambak Wedi Surabaya

Geodetic coordinates

WGS 84

Latitude : 7 ° 12 '29,201 "S

Longitude : 112 ° 46 '40,130 "E

UTM Zone (Zone 49S)

East : 696303,803

North : 9202862,814

Point Condition : Good

Verification of the coordinates obtained from the

VSPS Hemisphere VS330 positioning system is

carried out against the BM VKS2 Benchmark

coordinates located on the west side of the Suramadu

Bridge in Tambak Wedi. Verification was carried out

on September 13, 2017 before the start of water depth

measurements.

This verification is intended to ensure that the

position system used, including the geodetic

parameter system entered in the navigation software

is in accordance with the coordinate system and the

position value of the coordinates of the control points.

If the difference between the coordinates is more than

the tolerance required, then the equipment used must

be replaced or calibration must be repeated while

ensuring that there are no wrong parameters in the

software used.

The DGPS antenna is placed above the existing

control point, then the coordinates obtained from the

position of this antenna are recorded in the navigation

software that is used for approximately one hour with

an observation interval every 5 seconds (Figure 3 and

4). The coordinate data obtained are then averaged

Bathymetric Survey for Estimating the Local Scour at Suramadu Bridge

265

and the results are compared with the coordinates of

the existing control points.

Figure 4: Determine Point Control.

Figure 5: Output of Determine Point Control.

2.5 Singlebeam Echosounder

Single beam echo sounder is a measurement device

that uses a single beam as the sender and receiver of

sound wave signals. The working principle of SBES

is to use the principle of measuring the phase

difference in pulses, which is calculating the time

difference from the time of the emission and

reception of the acoustic pulses. SBES is also quite

accurate, where SBES is able to provide accuracy to

0.1 meters at depths of less than 100 meters

(Lekkerkerk et al, 2006).

Bathymetry systems using a single beam generally

have an arrangement: a transducer (transmitter/

receiver) mounted on the hull or the bearing side of the

ship. This system measures the depth of water directly

from the inquiry vessel. Transmitter mounted on the

hull sends acoustic pulses with high frequency

contained in the beam (sound waves) directly down the

water column. The receiver will capture acoustic

energy emitted from the transmitter.

The data obtained from the process is the time

interval the waves begin to emit and the waves

received again, so that the depth data obtained by the

recording device is a function of the time interval. The

process described in the equation (Poerbandono et al,

2005):

𝑑  ½ 𝑣𝛥𝑡 (1)

Where :

d : see water level (m)

v(t) : sound wave velocity (m/s)

Δt : interval (s)

Transceiver consists of a transmitter that has a

function as a control of the wavelength of the emitted

pulse and provides electrical power for a given

frequency and the receiver receives repeatedly

reflected waves at high speeds, up to the order of the

speed of the millisecond. Continuous water depth

recording from under the ship results in a high-

resolution depth measurement along the surveyed lane.

The single beam echo sounder used to make 3D

combined with the physical surface of the base

location, the physical surface of the seabed location,

which used to conduct preliminary surveys before

using multiband sonar. Single beam echo sounder

consists of two types, namely:

a. Single frequency is a single echo sounder that

uses only one frequency, namely high

frequency.

b. Dual frequency is a single echo sounder that

uses two frequencies, namely high frequency

and low frequency. High frequency provides

more accurate depth in relation to shipping

safety, while low frequency is able to penetrate

into the seabed mud deep) so it is not safe for

shipping.

The frequency range used in this system

according to the WHSC (Woods Hole Science

Centre) Sea-floor Mapping Group operates the

frequency range from 3.5 Hz to 200 kHz. Single beam

echo sounder is relatively easy to use, but this device

only provides depth information along the track line

traversed by the ship. This activity the use of Single

beam Echo sounder (SBES) for areas that covered by

Multiband Echo sounder (MBES) because, the ship is

less than the minimum MBES operation.

Figure 6: Bathymetry SBES Method.

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Correction for bathymetry measurements by the

SBES method with the following equation:

D = Du + s – h + BC (2)

Where :

D : definitive of hight

Du : echosounder water level measuring

s : transducer

h : water level hight

BC : barcheck correction

Figure 7: Echo sounder SBES Filed Survey.

2.6 Multibeam Echosounder

Multi beam Echo Sounder (MBES) is a used in

seafloor mapping alng with a Single Beam Echo

Sounder (SBES). MBES can measure a wider range

of water depths and can acquire high resolution

images simultaneousley through the beamforming

process. (Jung et al, 2017).

Multi beam sonar (MBS) technology developed

to examine in detail large stretches of the seafloor

surface, providing accurately positioned and

excellent 2-D and 3-D images of features as small as

a few centimetres or covering areas as large as

hundreds of square meters (Stanley et al, 2011).

The principle of operation of this device based on

the sound beam (beam) that emitted and reflected

directly toward the seabed, and captured again

through sensors that are on the MBES device. The

detection algorithm on the seabed, to determine the

depth and distance of the transversal, calculates the

two-way propagation time between sending and

receiving. Multi beam Echo sounder can produce

bathymetry data with high resolution.

Unlike the side, scan sonar beam pattern that

owned by MBES it widens and crosses the hull. Each

beam emits one sound pulse and has its own

reception. When the ship moves, the MBES sweep

results in an area of the seabed surface area (Moustier,

2005).

The transducer contained in the multi beam sonar

consists of a series of elements that emit sound pulses

at different angles. Usually only one beam is

transmitted but it produces a lot of reflected energy

from each of the transmitted sound pulses. The ability

of each transducer element to reclaim the reflected

sound pulse depends on the calibration method of the

ship's motion applied.

This MBES has excellent accuracy in measuring

depth. In addition to having excellent accuracy, the

advantage of using this MBES is a wide range of

measurement areas (Moustier, 2005). Coverage of the

seabed area that surveyed by ships using multi beam

in one sweep is called swath. The swath width for

each type of multi beam echo sounder can vary.

The principle of MBES is roughly the same as

measurement devices that use acoustic waves, such as

SBES and sonar side scans, which is simply by

emitting acoustic waves through its reflected wave

transmitter.

Figure 8: Bathymetry MBES Method.

Then measure the time difference between the

waves when the initial emitted until the reflection of

the wave is captured again by the sensor. With the

previously determined speed of sound propagation in

the study area, so that it can be calculated the depth

of the seabed.

Transducer configuration is a combination of

several projectors arranged like an array (matrix). The

projector is part of the MBES transducer which

functions as a channel to emit acoustic pulses towards

the seabed. All acoustic signals from all directions

reflecting objects on the seabed will be received again

by the hydrophone array. All acoustic signals will be

Bathymetric Survey for Estimating the Local Scour at Suramadu Bridge

267

received by each hydrophone simultaneously with a

difference in time of reception of 4/3 to 8/3 mill

seconds depending on the length and duration of the

acoustic signal.

In general, MBES uses interferometric techniques

to detect the direction of reflected waves as a function

of time. By using the accumulation of acoustic signals

received on two separate arrays, an interference

pattern will be formed.

Based on the existing relationship a direction will

be determined. When this information is combined

with distance, depth data will be generated (Sasmita,

2008). The geometrical image of the acoustic waves

emitted by MBES can be seen in Figure 7.

Figure 9: Transducer Mechnisme.

The phase difference of pulses in MBES means

that it is a function of the difference between the

transmitting and receiving time phases. Then the

calculation of the travel time and direction of the

beam angle of each stave is determined from the

measurement of the MBES pulse phase difference.

Figure 10: Echo sounder MBES Field Survey.

3 RESULT

The results of the bathymetry survey obtained around

the seashore can be concluded as follows:

a. The area around Surabaya was surveyed as far

as 1.7 km in front of the beach. The area

surveyed is in accordance with the planned area

in the 500 m corridor to the left and right of the

Suramadu Bridge.

b. According 2016 bathymetery data Table 2, that

the water sea level was 7 – 8 m.

c. In the results of the cross section overlay data

for 2016 with 2017 across the position of Pillar

46 also obtained changes in the form of

sedimentation and scouring at certain spots. The

part that forms the ridge or embankment tends

to occur sedimentation while the lower part

occurs scouring. Sedimentation values that

occur along cross sections 500 west and 500 east

are 0-0.7m per year and the highest

sedimentation rates are seen in the cross section

of STA 575 and 600. While the scouring value

is 0 - 0.9 m per year especially in the STA 700

area up to 825.

Figure 11: Seabed in P46 Pillar.

d. When looking at changes in the results of the

cross section on Pillar 47, the back of the west

and east pillars is very visible in the 3D image

as shown below, where changes occur in

sedimentation at the top of the embankment and

scouring at the bottom of the embankment such

as dark appearance on the underside of the

embankment. The value of this change varies

with the sedimentation value of 0 -1 m per year

as seen in the cross-section and sedimentation 0

- 1.1 m per year.

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Figure 12: Seabed in P47 Pillar.

Besides the seawater depth map, from

measurements with Multi beam echo sounder, you

can also see the profile of the seabed surface around

Pillars P-46 and P47. The condition of the profile seen

in the 3D image below.

Figure 13: 3D Image Sea bed in Main Bridge.

Table 2: Bathymetry Data.

No. Of

Pillars

Preliminary

data

Design 2016 2017

46 -19,995 -31,495 -27,495 -28,395

47 -15,600 -26,960 -23,100 -24,200

From the table above presented in the graph, in

Figure 13. From the data obtained from preliminary

data, bathymetry survey data in 2016 and 2017, which

can explain that scouring, occurred in the main span

until 2016 occurred at 7-8 meters, but in 2017, the

scouring has stabilized result 0 – 1 meters.

Figure 14: Scouring Graphic in Main Bridge.

4 CONCLUSION

The survey results are also presented in the form Point

Cloud with a grid size of 1.0 m. All depth data use the

MSL reference so that it will be commensurate when

compared to 2016 measurement data that use the

same reference. The maximum water depth that

measured in the corridor of the survey area is 22.8 m

and this depth find to the west of the Main Pillar of

P-47.

The water depth of 22.8 m is in the elongated

scouring area that occurs along the southern part of

the P-47 pillar where this scour is parallel to the

sedimentation, which is located side by side in the

northern part of the P-47 pillar. Underneath there is a

scoring direction to the west of Pillar 47.

The depth of water between the main pillars to a

distance of 300 m from the bridge ranges from 15 to

22 m with a composition that is almost evenly

distributed and regularly in the West and East.

Sedimentation occurs forming embankments

longwise to the west and followed on the main pillars

that range between 0 - 1 meter form the embankment

which can be seen clearly on the appearance of 3D

images.

5 RECOMMENDATION

To ensure that the local scouring is not gives damege

to the bridge in the further, bathymetry survey has to

periodically, that level of local scouring can be

estimated as reference to maintenace the bridge.

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