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Potential of tipping flush gate for sedimentationmanagement in open stormwater sewerC.H.J. Bonga, T.L. Laub & A. Ab. Ghanica Department of Civil Engineering, Faculty of Engineering, Universiti Malaysia Sarawak,Sarawak, Malaysiab School of Civil Engineering, Universiti Sains Malaysia, Engineering Campus, Seri Ampangan,Penang, Malaysiac River Engineering and Urban Drainage Research Centre (REDAC), Higher Institution Centreof Excellence (HICoE) for Service, Universiti Sains Malaysia, Engineering Campus, SeriAmpangan, Penang, MalaysiaPublished online: 12 Jan 2015.

To cite this article: C.H.J. Bong, T.L. Lau & A. Ab. Ghani (2015): Potential of tipping flush gate for sedimentationmanagement in open stormwater sewer, Urban Water Journal, DOI: 10.1080/1573062X.2014.994002

To link to this article: http://dx.doi.org/10.1080/1573062X.2014.994002

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RESEARCH ARTICLE

Potential of tipping flush gate for sedimentation management in open stormwater sewer

C.H.J. Bonga*, T.L. Laub and A. Ab. Ghanic

aDepartment of Civil Engineering, Faculty of Engineering, Universiti Malaysia Sarawak, Sarawak, Malaysia; bSchool of CivilEngineering, Universiti Sains Malaysia, Engineering Campus, Seri Ampangan, Penang, Malaysia; cRiver Engineering and UrbanDrainage Research Centre (REDAC), Higher Institution Centre of Excellence (HICoE) for Service, Universiti Sains Malaysia,

Engineering Campus, Seri Ampangan, Penang, Malaysia

(Received 10 March 2014; accepted 29 September 2014)

This paper presents the design of a tipping flush gate and its potential use in servicing open storm sewers in terms ofsedimentation management. The tipping flush gate was installed in a section of open concrete storm sewer located in acommercial area in Taman Pekaka, Nibong Tebal, Penang, Malaysia. Monitoring of the gate operation and performance wasdone from 14th November 2012 to 15th March 2013 covering the beginning and end of the wet season. The sediment profileinside the sewer was measured after each operation of the gate or rainfall event. Results showed that the gate was effective inreducing naturally accumulated sediment in the chosen sewer section. However, proper litter management is needed if thegate is to be implemented in open storm sewer systems. A design guideline for the usage of a tipping flush gate for openstorm sewers is also presented in this paper.

Keywords: open storm sewer; sedimentation management; tipping flush gate

1. Introduction

Open channels are frequently used in developing and less

developed countries to convey storm water runoff (Geiger,

1990). However, without proper maintenance; sediment

tends to build up in open storm sewers. Losses of hydraulic

capacity of storm sewers due to sedimentation have been

identified as one of the factors of flash flooding in urban

areas (Ab. Ghani et al., 2008; Liew et al., 2012; Rodrıguez

et al., 2012). Sediments in developing countries that use

open storm sewers, like India (Kolsky, 1998) and Malaysia

(Ab. Ghani et al., 2000; Bong et al., 2014), tend to be

coarser than those found in European countries (Ashley

et al., 2004). Clearly open sewers allow easier ingress of

larger sediments (Ashley et al., 2004) as compared to

closed conduit sewers.

Conventionally, removal of sediment from open storm

sewers often involves manual handling which is costly due

to periodic maintenance (Bong et al., 2013). On the other

hand, various techniques have been developed, especially

in European countries, to clean sediment in closed conduit

sewers. These techniques may be manual or automated

and include the use of rodding, balling, flushing, poly pigs

and bucket machines (Dinkelacker, 1991). With the

exception of flushing, these techniques are generally used

in ‘reactive’ mode to clear blockages once they have

formed and as preventative maintenance (Ashley et al.,

2004). Hydraulic flushing was probably the most widely

applied (Bertrand-Krajewski, 2008) and a preferred

technique as a control concept and is a means both to

reduce hydraulic restriction problems and to prevent

pollution. The flushing effect can be created by storing

water in upstream chambers and then being discharged

through a gate/tipping bucket located above water level or

mobile tipping plates like the Hydrass gate (Chebbo et al.,

1996; Lorenzen et al., 1996). Though there exists various

flushing methods, flushing gate technology appeared to be

the most cost-effective means of flushing sediments in

large diameter flat sewers (Pisano et al., 2003).

Various experimental studies are available in the

literature on the effect of flushes for flushing devices such

as sluice gate/lifting gate (Campisano et al., 2004, 2008),

vacuum flushing (Guo et al., 2004) and the Hydrass gate

(Bertrand-Krajewski et al., 2005). A long term study on

flushing in a man-entry sewer in Lyon, France had shown

that the Hydrass gate was efficient and themass of sediments

appeared to move downstream linearly with the number of

flushes (Bertrand-Krajewski et al., 2006). Bong et al. (2013)

conducted a preliminary laboratory study on the hydraulic

characteristics of a tipping flush gate and the efficiency of

flushing in removing uniform sediment with different sizes.

Numerical simulation on the erosive effects of sewer

flushing revealed that the modelling of sediment bed erosion

q 2015 Taylor & Francis

*Corresponding author. Formerly with River Engineering and Urban Drainage Research Centre (REDAC), Universiti Sains Malaysia.Email: bhjcharles@feng.unimas.my

Urban Water Journal, 2015

http://dx.doi.org/10.1080/1573062X.2014.994002

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required the use of sediment transport formulae specifically

developed for high stress conditions (Campisano et al.,

2007; Shirazi et al., 2014).What is lacking in the literature is

the on-site use of flushing devices for sediment management

in open storm sewers.

Since the existing flushing gates in the literature, such as

the Hydrass gate, are designed to function in closed conduit

sewers; some modifications are needed if the gate is to be

used in open storm sewers. The Hydrass gate has a lower

part (below the gate hinge/axis) with a smaller area but

heavier than the upper part (above the gate hinge/axis)

(Schaffner, 2008). This design is suitable for closed conduit

sewerswhich can allow a highwater level for gate operation

without overflow. This condition is different with open

storm sewers which could not allow a high water level for

gate operation in order to avoid overflow and nuisance.

Thus a modification in the design for the Hydrass gate is

needed before it could be used in an open storm sewer.

This paper highlights the design of a tipping flush gate

(which was modified from the Hydrass gate) and its

potential to be used for sedimentation management in an

open storm sewer. From on-site installation and monitor-

ing of the tipping flush gate, the potential of the gate for

usage in open storm sewer could be evaluated in terms of

the performance of sediment removal and the problems

faced in using the gate. A design guideline for the usage of

a tipping flush gate for open storm sewers was also

presented. It is hoped that this study will fill the gap in the

literature on the usage of flushing devices in open storm

sewers as a technique for sedimentation and sewer asset

management, which is still lacking.

2. Methodology

2.1. Site description

To test the potential of the tipping flush gate for

sedimentation management, a section of the open concrete

storm sewer located in a commercial area in Taman

Pekaka, Nibong Tebal, Penang, Malaysia was chosen as

shown in Figure 1. The chosen storm sewer section was

straight with no discontinuity. The total length of the

chosen section for monitoring purposes was 46m (the

sewer section with the straightest alignment) with the

cross-section of the rectangular sewer being 1.20m (W)

£ 0.80m (D). The catchment that drained into the chosen

Figure 1. Site map of study area in Taman Pekaka, Nibong Tebal, Penang, Malaysia with the insert photo showing the chosen opensewer section for this study.

C.H.J. Bong et al.2

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sewer section has an area of about 0.11 km2 with a roughly

90% impervious area. From site survey work, the average

slope for the sewer was 0.00021m/m.

The last sewer clean out was done on 1st August 2012

by the local council, Majlis Perbandaran Seberang Perai

(MPSP), prior to the study. Monitoring of water level and

sediment level were done daily using a steel ruler with the

flat end of a measuring tape with an accuracy of^0.5mm

from 2nd August 2012 prior to the installation of the

tipping sediment flush gate. Sediment samples were

scooped randomly from the surface to the bottom of the

sediment across the sewer’s cross-section at a few

randomly selected points in the chosen sewer section.

Results from the sediment sampling have shown that

the median grain size of the sediment sample was

d50 ¼ 1.00mm and geometric standard deviation sg ¼ 3.8

(non-uniform) with a specific gravity of 2.52. The

sediment sample was non-cohesive with predominantly

sand at 68.4%, gravel at 30.1%, and silt and clay at 1.5%.

2.2. Site installation

For site monitoring purposes, two closed-circuit televi-

sions (CCTVs) were installed on top of a tree by the

chosen sewer section; about 40m downstream from the

tipping flush gate. One CCTV was to capture the video

clips of the gate operation while another CCTV was to

monitor the water level at 40m downstream of the gate

(the end of the monitoring section). Stick gauges were also

installed at 10m interval along the chosen sewer section.

For reference purpose, rainfall data was obtained from the

Department of Irrigation and Drainage (DID), Malaysia

for Kolam Takongan Bkt. Panchor (station no: 5105051)

which is about 5.45 km from the site.

2.3. Tipping flush gate design

The Hydrass gate, as mentioned in the existing literature

(Chebbo et al., 1996; Lorenzen et al., 1996), has a lower

part (below the gate hinge/axis) with a smaller but heavier

area than the upper part (Schaffner, 2008). This results in a

high water level to provide more force to open the gate

with a heavier lower part; a condition which closed-

conduit sewers could allow without overflowing. This

condition however is different with open storm sewer. The

tipping flush gate for the current study in open storm sewer

was designed with the bottom part (below the gate hinge)

lighter than the upper part of the gate by positioning the

gate hinge so that the bottom part of the gate has smaller

area as compared to the upper part of the gate. This allows

easier tipping of the gate during an opening phase which

requires a lower water level as compared to a gate with

heavier lower part so as to prevent overflow from the open

storm sewer.

2.3.1. Forces on tipping flush gate

The tipping flush gate was designed based on the

difference of moment on the upper portion (above the

hinge) and lower portion (below the hinge) of the gate

(see Figure 2). The water force above the gate hinge F1[N]

is given as:

F1 ¼ 1

2gwbh

21 ð1Þ

where gw is the specific weight of water [N/m2]; b is the

width of the gate [m] and h1 is the distance of the water

surface to the centre of the hinge [m]. The force below the

gate hinge F2[N] is made up of two components of force

namely F2a[N] (the triangle shape pressure prism) and

F2b[N] (the rectangle shape pressure prism) as shown in

Figure 2 and given by:

F2a ¼ 1

2gwðh2 2 h1Þðh2 2 h1Þb ð2Þ

F2b ¼ gwh1ðh2 2 h1Þb ð3Þ

where h2 is the distance of the water surface to the bottom

of pressure prism [m].

Taking the moment at the hinge which is off centre, the

gate will open if the sum of the moment above the gate

hinge is larger than the moment below the gate hinge.

Thus:

F1 £ h1

3

� �. F2a £ 2

3ðh2 2 h1Þ

� �

þ F2b £ 1

2ðh2 2 h1Þ

� �ð4Þ

Figure 2. Forces acting on the tipping flush gate beforeopening.

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or

1

2gwbh

21

� �h1

3

� �.

1

2gwðh2 2 h1Þðh2 2 h1Þ

b2

3ðh2 2 h1Þ þ gwh1ðh2 2 h1Þb 1

2ðh2 2 h1Þ

ð5Þ

Equation (5) could be simplified by removing gw, band 1/2 from both sides of the equation and obtain:

h313. ðh2 2 h1Þðh2 2 h1Þ 2

3ðh2 2 h1Þ

þ h1ðh2 2 h1Þðh2 2 h1Þð6Þ

or

h1 . ½2ðh2 2 h1Þ3 þ 3h1ðh2 2 h1Þ2�1=3 ð7ÞBy using trial and error, the position of the hinge for the

tipping flush gate could be calculated by fulfilling Equation

(7) and the allowable water level upstream of the gate just

before the gate tips open and the gate height hg were

predetermined from the condition on-site. The height of the

hinge hhng from the bottom of the pressure prism is given by:

hhng ¼ h2 2 h1 ð8ÞAfter the gate has tipped to open, it will close if the

moment below the hinge is larger than the moment above

the gate hinge (see Figure 3). This happens when the water

starts to recede after opening.

In Figure 3, the water force/thrust above the gate hinge

F01½N� when the gate tilts is given as:

F01 ¼

1

2gwbh

021 ð9Þ

where gw is the specific weight of water [N/m2]; b is the

width of the gate [m] and h01 is the vertical distance of thewater surface to the centre of the hinge [m]. The force

below the gate hinge F02½N� is made up of two components

of force, namely F02a½N� (the triangle shape pressure prism)

and F02b½N� (the rectangle shape pressure prism) as shown

in Figure 3 and given by:

F02a ¼

1

2gwðh02 2 h01Þðh02 2 h01Þb ð10Þ

F02b ¼ gwh

01ðh02 2 h01Þb ð11Þ

where h02 is the vertical distance of the water surface to thebottom of pressure prism [m]. The weight of the gateWg is

given as:

Wg ¼ ggbhgtg ð12Þ

where gg is the specific weight of the gate [N/m2]; hg is the

total height of the gate [m] and tg is the thickness of the

gate [m]. Taking the moment at the hinge, which is off

centre, the gate will close if the moment below the gate

hinge is larger than the sum of the moment above the gate

hinge with the moment due to the gate weight. Hence:

F01 £

h013

� �þW 0

g

ðhg 2 hhngÞ sin u

2

, F02a £

2

3ðh02 2 h01Þ þ F0

2b

h02 2 h012

� �ð13Þ

or

1

2gwbh

021

� �h013

� �þ gg £ tg £ b £ hg

cos u

ðhg 2 hhngÞ sin u

2

,1

2gwðh02 2 h01Þðh02 2 h01Þb

2

3ðh02 2 h01Þ

þgwh01 h02 2 h01� �

bh02 2 h01

2

� �

ð14Þwhere W 0

g½N� is the vertical component of the weight of

the gate ðW 0g ¼ Wg= cos uÞ; ðhg 2 hhngÞ sin u=2 ½m� is the

Figure 3. Forces acting on the tipping flush gate when open.

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vertical distance between the gate hinge and the centre of

gravity of the gate where the weight of the gate acted; and

u is the angle of gate opening from the horizontal axis.

Equation (14) could be simplified by taking the

thickness of the gate as 0.012m (thickness of plasboard in

this study) and the specific weight of gate (material is

plasboard and homogeneous) gg ¼ 10712.5 N/m2 and

gw ¼ 9810N/m2. Hence:

1635h031 þ64:275hgðhg 2 hhngÞ sin u

cos u

, 3250ðh02 2 h01Þ2 þ 4905h01ðh02 2 h01Þ2 ð15Þ

or

h031 þ 0:0393hgðhg 2 hhngÞ tan u

, 2ðh02 2 h01Þ3 þ 3h01ðh02 2 h01Þ2 ð16Þ

Using trial and error to solve Equation (16), the water

level h02 when the gate closes could be determined

provided that the height of the hinge, the angle of gate

opening and gate height have been predetermined.

2.3.2. Tipping flush gate sizing

To determine the sizing of the tipping flush gate, the on-

site conditions such as the storm sewer dimensions and the

invert level of the inlet pipes to the sewer needed to be

determined. As mentioned earlier, the chosen storm sewer

had a dimension of 1.20m (W) £ 0.80m (D). It was also

observed that there were inlet pipes of 0.15m diameter

spaced at an interval of every 6m along the right wall

(looking downstream) of the sewer to convey stormwater

from the main road next to the sewer; and at every 14m

along the left wall of the sewer to convey stormwater from

the parking lots next to the sewer. The lower invert level of

the pipe was about 0.61m from the bottom of the sewer

(see Figure 4).

In considering the width, the gate opening width must

be as close as possible to the width of the sewer (so as not to

have much contraction effect on the flow during the storm).

As for the gate opening height, there must be an allowable

freeboard between the height of the gate with the total depth

of the sewer (for emergency overflow). The gate opening

height must be below the lower invert level of the inlet

pipes to prevent backwater through the inlet pipes.

Based on the observation on-site and also taking the

consideration for sizing as mentioned in the previous

paragraph, it was decided that the gate has an opening

width of 1.00m (to allow for the gate frame on both side of

the gate and also less contraction effect) with the gate

opening height of 0.50m (so that the water level behind

the gate just before the gate open will not be higher than

the lower invert level of the inlet pipes, thus preventing

backwater through the inlet pipe and water ponding on the

road and parking lots). The dimension for the gate

(including the frame) was 1.20m (W) £ 0.60m (D).

Though the total depth of the drain was 0.80m, the total

height of the gate including the frame was 0.60m which

has an allowable freeboard of 0.20m. This is for the

purpose of emergency cases, for example if the gate stuck

and cannot open; stormwater could still flow over the gate

installation.

To determine the level of the hinge hhng for the gate,

trial and error was performed using Equation (7) to find h1.

To perform the trial and error, it was first determined that

the allowable water level h2 behind the gate was 0.56m

from the bottom of the sewer so that there was a difference

of 0.05m between the allowable water level and the lower

invert of the inlet pipes. From the trial and error, the right

hand side (RHS) of Equation (7) must be smaller than the

value of h1 (see Table 1). This is achieves when h1 is

0.40m and the location of the hinge hhng as calculated

using Equation (8) is 0.16m from the bottom of the

pressure prism (see Figure 2).

To determine when the gate will close, trial and error

was performed using Equation (16). Trial and error was

performed by taking the angle of opening for the gate as

308 from the horizontal axis and h2 2 h1 is 0.16m (based

on hinge location calculated previously). Preliminary

Figure 4. Distance between lower invert of inlet to sewerbottom.

Table 1. Trial and error to determine the hinge level of gate.

h1(m) h2(m) RHS hhng(m) Gate Condition

0.20 0.56 0.56 0.36 Close0.30 0.56 0.51 0.31 Close0.31 0.56 0.46 0.26 Close0.35 0.56 0.40 0.21 Close0.40 0.56 0.34 0.16 Open

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testing for the gate before the on-site installation had found

that 308 from the horizontal axis was the suitable opening

angle due to the ease of self-automated closing of the gate

when the water level recedes. From trial and error as

shown in Table 2, the gate starts to close when the water

level h02 is 0.40m.

Figure 5 shows the front and isometric view of the

tipping flush gate. The material used for the gate in this

study was plasboard with a specific gravity of 1.092.

The two pieces of wooden bar frame installed on the gate

was to prevent the gate from buckling due to the pressure

of water since the gate has a rather long continuous width

of 1.0m (see Figure 5a). The iron stopper bar behind the

gate (see Figure 5b) was installed to prevent the gate from

tipping further from the desired angle of opening (during

opening) and also to avoid the gate from tipping further

back when closing. Springs were added to the gate to

ensure proper closing. This was due to findings from

preliminary testing that the gate sometimes could not close

tightly due to litter caught at the gate, especially around the

hinge area.

Table 2. Trial and error to determine the water level when thegate closes.

h01(m) h02(m) hg(m) hhng(m) LHS RHS Gate Condition

0.34 0.50 0.50 0.16 0.043 0.034 Open0.29 0.45 0.50 0.16 0.028 0.030 Open0.24 0.40 0.50 0.16 0.018 0.027 Close0.19 0.35 0.50 0.16 0.011 0.023 Close

Figure 5. Views of the tipping flush gate: (a) front view; and (b) isometric view (not to scale).

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2.4. Sediment profiles monitoring

Sediment was left to accumulate naturally in the chosen

sewer section since 1st August 2012 until 14th November

2012. After installation of the tipping flush gate on 14th

November 2012, sediment profile was measured after each

operation of the gate or after each rainfall event. The

sediment profile was measured manually using a steel ruler

together with the flat end of a measuring tape with an

accuracy of^0.5mm (see Figure 6a). Sediment profilewas

measured with an interval of 2m along the 40m

downstream of the gate and at every 0.15m for each cross

section. The sediment profile was also measured at every

2m interval and at 0.15m for each cross section along the

6m upstream of the gate. The mean height of sediment at

each cross-section was calculated. To further study the

performance of the gate, non-cohesive sand with d50 size of

1.11mm with specific gravity of 2.55 was added into the

sewer as sediment on 26th December 2012 starting from2m

downstream of the gate with the sediment bed extended for

4m (see Figure 6b). The average sediment height for the

sediment bed ranged from 5.2 cm to 10.9 cm. Figure 7

shows the sediment size distribution for the sand bed added

into the sewer. Themonitoring periodwas fourmonths from

14th November 2012 to 15th March 2013 covering the

beginning and end of the wet season.

3. Results and discussion

3.1. Tipping gate operation

Between 14th November 2012 (gate installation date) to

15th March 2013 (end of monitoring period), there were a

total of 121 rainfall events recorded by the rainfall station

at Kolam Takongan Bkt. Panchor (station no. 5105051)

with rainfall duration from 5 minutes to 245 minutes

and rainfall intensity from 1.1mm/hr to 61.5mm/hr. The

tipping flush gate was observed to only operate during or

just after the rainfall event. The tipping flush gate operated

18 times during the monitoring period with the duration of

flushing (gate opening) ranging from 22.8 minutes to

176.5 minutes before the gate automatically closed

(see Table 3). The rainfall duration and rainfall intensity

was calculated based on the total rainfall from the start

of the rain event until the time the gate opened. The

equivalent ARI (annual recurrence interval) was also

calculated from the rainfall intensity and duration prior to

the gate opening and was compared with the intensity-

duration-frequency (IDF) curve for Sg. Simpang Ampat

Tangki (station no: 5204048), the closest rainfall station

(17.07 km from the site) with available IDF curve from the

Urban Stormwater Management Manual for Malaysia 2nd

Edition (2012) (DID, 2012). From Table 3, the gate was

observed to operate at minimum rainfall duration of

10 minutes with rainfall intensity of 8.98mm/hr and less

than 0.5month for the ARI equivalent.

Figure 8 shows the CCTV clips captured during the

gate operation on 27th December 2012. It was observed

from the CCTV clips that the flush wave took about 20 s to

travel for a distance of 40m, thus the average velocity was

2m/s for the flush wave. This velocity was observed for all

Figure 6. Site monitoring: (a) measuring sediment profile; and (b) sediment bed added on 26th December 2012.

Figure 7. Sediment size distribution for the sand bed.

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the flush since the gate was designed to open at the same

water level every time. The gate was observed to open

when the water level reached 0.51m from the sewer

bottom and closed when the water level receded to 0.35m

from the sewer bottom (compare these values with the

design water level of 0.56m and 0.40m for opening and

closing respectively as mentioned in Section 2.3.2).

3.2. Sediment flushing

After the sewer clean out by the city council Majlis

Perbandaran Seberang Perai (MPSP) on 1st August 2012,

sediment was left to accumulate naturally until 14th

November 2012 when the tipping flush gate was installed.

During this period, there were a total of 92 rainfall events

recorded by the rainfall station at Kolam Takongan Bkt.

Panchor (station no. 5105051) with rainfall duration from

5 minutes to 420 minutes and rainfall intensity from

0.9mm/hr to 60.0mm/hr. Though with frequent rains after

the sewer clean out, sediment was observed to accumulate

rather than moving out of the chosen sewer section.

3.2.1. Flushing of naturally accumulated sediment

On 14th November 2012, the tipping flush gate was

installed in the chosen sewer section to flush the sediment

Table 3. Gate operation time with the corresponding rainfall duration, intensity and equivalent ARI prior to the gate opening.

Date

Operationtime(hr)

Operationduration(min)

Operation(during/after)rainfall

Rainfallduration(min)

Rainfallintensity(mm/hr)

Equivalent(ARI)

19/11/2012 13:31:09–14:05:00 33.9 during 10 8.98 , 0.5month2/12/2012 19:19:34–20:00:00 40.4 during 30 37 2month10/12/2012 16:17:14–16:40:00 22.8 during 85 38.81 12month12/12/2012 20:30:06–21:05:00 34.9 during 25 8.39 , 0.5month13/12/2012 03:14:40–04:40:00 85.3 n/a n/a n/a n/a14/12/2012 03:37:28–06:15:00 157.5 n/a n/a n/a n/a15/12/2012 04:18:31–07:15:00 176.5 n/a n/a n/a n/a15/12/2012 18:38:40–19:50:00 71.3 during 30 40 3month17/12/2012 05:22:18–07:07:00 104.7 n/a n/a n/a n/a20/12/2012 18:10:52–18:52:00 41.1 after 15 52 3month22/12/2012 17:19:25–17:55:00 35.6 after 50 31.81 3month27/12/2012 17:58:47–18:50:00 51.2 after 20 61.56 6month2/1/2013 16:48:00–18:04:46 76.8 during 100 22.50 3month25/1/2013 03:23:20–04:29:04 65.7 during 10 8.98 , 0.5month31/1/2013 19:59:00–20:40:22 41.4 during 45 17.33 0.5month9/2/2013 17:16:45–17:46:12 29.5 after 50 32.41 3month12/2/2013 14:31:02–15:37:00 66.0 n/a n/a n/a n/a13/2/2013 19:58:19–20:25:00 26.7 during 60 14.5 0.5monthAverage operation duration (min) 64.5

Figure 8. Tipping flush gate operation: (a) the flush wave created moment after the gate opened; and (b) the flush wave reaching the endof the monitoring section at 40 m downstream of the gate about 20 s after the gate opened.

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which had naturally accumulated since 1st August 2012.

Figure 9 shows the changes of the mean sediment profile in

the chosen sewer section for the period of 14th November

2012 to 26th December 2012. There were 11 flushing

operations by the gate during this period. The flushing gate

was observed to be efficient in reducing the sediment level in

the monitored section as evidenced in the general reduction

of mean sediment level downstream of the gate after 11

flushes as compared to the initialmean sediment level before

flushing. The sediment could be accumulating further

instead of reducing as observed during the period before the

gate installation; if the gate was not installed in the chosen

sewer section. Also observed from Figure 9 is that there

was no significant sediment accumulation up to 6m

upstream of the gate. This confirmed the study by Staufer

et al. (2008) that solids built up during the storage phase

behind a flushing gate will not last due to the sunk waves

generating sufficient shear stress to remobilise these solids.

Figure 10 shows the changes of total sediment volume

for the period of 14th November 2012 to 26th December

2012 where there was a clear reduction in the total sediment

volume from0.1215m3 to 0.0372m3 in themonitored sewer

section.

3.2.2. Flushing of sediment bed

After the sediment bed was added to the chosen sewer

section; for the period of 26th December 2012 to 14th

February 2013, the gate operated seven times and the

changes of the mean sediment profile is as shown in

Figure 11. The scouring effect from the flushes seems to

produce a lowering and lengthening of the initial sediment

bed. There was no more gate operation (gate did not open)

after 14th February 2013 though there were still rains

events. There was no significant change in terms of the

sediment profile up to 6m (the chosen section) upstream of

the gate, which showed that the gate did not cause

significant sediment accumulation upstream of the gate.

The monitoring was stopped on 15th March 2013 when the

wet season ended as evidenced by the rain events

becoming rarer.

Figure 12 shows the changes of total sediment volume

for the period of 26th December 2012 to 14th February

2013 where there was a reduction (from 0.6162m3 to

0.3722m3) but at a diminishing rate in the total sediment

volume in the chosen sewer section. The diminishing rate

in the reduction of the total volume of sediment in the

sewer was probably due to the agglomeration of sand

Figure 9. Changes of mean sediment profile with number of flushes for the period of 14th November 2012 to 26th December 2012.

Figure 10. Changes of total sediment volume in the chosen section for the period of 14th November 2012 to 26th December 2012.

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particles as observed on-site in the sediment bed after

being left too long in the sewer with infrequent flush

operation and rain events. It could also be due to the higher

sediment level near the sediment bed front (around 5m

downstream of the gate) owing to sediment accumulation

or build up near the sediment bed front after being

pushed by the flush waves several times during flushing

operations. This higher level of sediment near the

sediment bed front may cause difficulty for the sediment

bed advancement. For the period of 3rd January 2013 to

25th January 2013, fewer rain events were observed with

only five days with rain events with intensity between

2.3mm/hr to 27.8mm/hr. This caused the storm sewer to

be dry or have almost no flow during this period and no

gate operation; thus causing agglomeration of the sand

particles. Besides that, the amount of the sediment that was

too large and too thick also makes it harder to be removed

and in need of a more frequent flush. It has yet to be seen

whether more flushes happening more frequently could

remove this sediment bed and this could be a subject for

further study. Nevertheless, the tipping flush gate was

proven to be effective in removing naturally accumulated

sediment in the chosen storm sewer section.

3.3. Feasibility to use tipping flush gate on-site

Results from Section 3.2.1 and Section 3.2.2 have shown

that a tipping flush gate has the potential for sediment

removal, especially for naturally accumulated sediment.

However, some problems were observed in the usage of

the tipping flush gate on-site. For an open storm sewer

system, litter such as dry leaves, tree branches, plastic

bottles, plastic bags and papers tend to get into the sewer

system more easily when compared to closed conduit

sewer systems. This litter sometimes tends to get caught at

the gate, thus preventing the gate from closing properly

(see Figure 13a). During the period of the current study,

the tipping flush gate was cleaned weekly of litter that

might catch at the gate. Litter also tends to accumulate

behind the gate between periods of the gate operation (see

Figure 13b). Proper litter management is needed if this

tipping flush gate is to be installed in an open storm sewer

system such as installation of trashrack and grids and also

to educate the community so as not to throw litter into the

sewer system. Another concern is the ponding of water

behind the gate between the periods of gate operation

could be a breeding ground for mosquitoes.

4. Design guideline for tipping flush gate on-site

The purpose of installing a tipping flush gate on-site is to

flush sediment to a section of the open storm sewer where

self-cleansing is possible or where the sediment could be

easily extracted such as a sump rather than cleaning the

whole section of the sewer manually. The tipping flush

gate could also be installed for sewers that are not easy to

access for manual cleaning such as covered open storm

sewers. The following steps were suggested as guidelines

for installation of a tipping flush gate on-site:

Figure 11. Changes of mean sediment profile (after adding sediment bed) with number of flushes for the period of 26th December 2012to 14th February 2013.

Figure 12. Changes of total sediment volume in the chosensection for the period of 26th December 2012 to 14th February2013 (after adding sediment bed).

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(1) Location selection – In selecting the best location

for the tipping flush gate installation along the

sewer, it is recommended to install the gate far

away from sump or after a sump. This is so that the

flush effect could reach further in the sewer

without the disturbance of depression along the

downstream sewer section. Also, avoid using the

tipping flush gate for sewer with a backwater

effect downstream of the gate so that the gate

could operate properly. If possible, install the gate

near to an area where sediment accumulation was

observed or expected.

(2) Site survey – Site surveywork is done to determine

the sewer dimensions and also the location and

height of the bottom invert of the inlets along the

sewer. This is so that the sizing of the gate could be

done based on the results from the survey work.

(3) Gate sizing - From the site survey, the tipping flush

gate size could be determined. The width of the

gate opening must not be too small and cause flow

contraction in the sewer during gate operation

which could cause the sewer to overflow. The

height of the gate opening is dependent on the

bottom invert level of the inlet to the sewer bottom.

This is so that the water level behind the gate just

before the gate opens will not be higher than the

lower invert level of the inlet, thus preventing

backwater through the inlet. Also, the total height

of the gate opening and gate frame must not be

more than the total depth of the sewer. This allows

freeboard for water to overflow over the gate in

case the gate is stuck and could not open.

(4) Gate hinge positioning - After the maximum water

level for the gate to open has been determined,

using Equation (7) and Equation (8), the position of

the hinge can be determined.

(5) Gate opening angle – Preliminary testing of

the gate in the current study has encountered a

problem in closing the gate for large gate

openings (smaller opening angle from the

horizontal axis). Though installation of springs

can improve the closing mechanism it was

decided that an opening angle of 308 from the

horizontal axis was the best angle for the site

monitoring work in the current study. The angle

of opening of 308 from the horizontal axis was

observed to be able to close due to the force of

water alone without the need of springs. The

springs could aid the gate to close properly,

especially when there is litter caught at the gate.

Further study on the gate operation and closing

mechanism could be done so as to allow larger

gate openings during flushing.

(6) Closing water level – To check the water level for

the tipping flush gate to close, Equation (16) can

be used.

(7) Material for the gate - In the current study, the

material used was plasboard. Other materials that

could be used for the gate are stainless steel plate,

fiberglass and Perspex. The material used for the

gate must be durable, water proof and not easily

bent due to the force of water. If other material is

used instead of plasboard then Equation (16) is

subject to checking and revision.

Figure 13. Problem with litter during the tipping flush gate usage: (a) Litter caught at the gate; and (b) accumulation of litter behind thegate during in between period of gate operation.

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(8) Maintenance - Frequent maintenance of the gate,

especially during the wet season, such as weekly

cleaning of litter caught at the gate and checking the

tipping flush gate could operate properly is

recommended. This is to prevent the tipping flush

gate from not working properly such as failure to

open which could cause backflow and overflow for

the section of sewer behind the gate. Less frequent

maintenance with bi-weekly or monthly frequency

could be carried out during the dry season.

(9) Other considerations – Other things that need to be

considered in installing the flush gate on-site are to

have proper litter management like installing a

trash rack and grids so as to prevent litter from

getting caught at the gate. Besides that, if the site

allows, it is recommended to construct a water

storage chamber just behind the gate so that the

water level could build up faster and the flush could

happen more frequently. The gate for the site

monitoring in the current study was observed to be

effective in sediment removal up to 40m down-

stream of the gate which was the limit of the

monitoring. However it was yet to determine the

maximum length the flush waves are still effective

in sediment removal and this could be the subject

for further study.

5. Conclusions

The current study highlights the potential of a tipping

flush gate being used for servicing open storm sewers in

terms of sedimentation management. The design consider-

ations for the tipping flush gate specifically for open storm

sewer were also presented in this paper. Monitoring of the

gate operation and performance in Taman Pekaka, Nibong

Tebal, Penang, Malaysia from 14th November 2012 to 15th

March 2013 had proven that the tipping flush gate was

effective in reducing naturally accumulated sediment.

However, proper litter management is needed if the tipping

flush gate is to be implemented in open storm sewer

systems. Design guidelines that give general rules on

location selection for the gate, initial site surveying work

for the gate design, gate sizing, gate hinge positioning

calculation, gate opening angle determination, water level

for gate closing, material for the gate, maintenance and

other considerations were also presented.

Acknowledgements

Special appreciation goes to the city council; Majlis PerbandaranSeberang Perai (MPSP) for giving the permission to use the sitefor the study.

Funding

The authors would like to thank Universiti Malaysia Sarawak forthe financial support under the SGS grant No. F02

(S149)/1129/2014(14) and Universiti Sains Malaysia for theRU-PRGS grant No. 1001/PREDAC/8044050.

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