Proceedings of 2013 IAHR World Congress

ABSTRACT: This paper demonstrates the application of InfoWorks CS model for the calibration of hydraulic performance of Stage 1 and also for the hydraulic design of Stage 2 HATS system. The HATS Stage 1 system is calibrated and validated to determine the roughness so as to use for the hydraulic design of Stage 2. A similar model for HATS Stage 2 system also developed to facilitate the hydraulic design and option selection. Finally, the hydraulic performance of the HATS system under ultimate development scenario is also simulated. Challenges in calibration, limitations of InfoWorks CS and results of the modelling are discussed. KEY WORDS: HATS, flows, roughness, InfoWorks CS, Real Time Control, calibration, model 1 INTRODUCTION

To improve the water quality of the Victoria Harbour, the Harbour Area Treatment Scheme (HATS), was committed in late eighties to be implemented in stages to provide treatment for the sewage collected from the urban areas on both sides of the Harbour. The HATS Stage 1 system as shown in Figure 1 comprises the Stonecutters Island Sewage Treatment Works (SCISTW) and 23.6 km of deep tunnels for collecting and treating sewage from Tseung Kwan O (TKO), Kwun Tong (KT), To Kwa Wan (TKW), Kwai Chung (KC), Tsing Yi (TY) and north-eastern Hong Kong Island Preliminary Treatment Works (PTWs) before discharge into the western harbour through a submarine outfall. Since its commissioning in December 2001, Stage 1 system has resulted in substantial and widespread improvements to the general water quality of the Harbour.

The Government of the Hong Kong Special Administrative Region (SAR) is now implementing the next phase of HATS, namely HATS Stage 2A also shown in Figure 1, which comprises, among other elements, a 21 km long deep tunnel system to convey sewage from eight PTWs on the northern and southwestern shores of Hong Kong Island to a new Main Pumping Station (MPS) to be constructed in SCISTW. The new MPS includes eight number of duty pumps. Construction of HATS Stage 2A system is now underway for commissioning in 2014.

Application of InfoWorks CS Software for the Hydraulic Design of HATS Stage 2 Sewage Conveyance System in Hong Kong, China

Anil Kumar Senior Engineer, ARUP, Level 5, Festival Walk, 80 Tat Chee Avenue, Kowloon Tong, Kowloon, Hong Kong, China

Chan Pak Keung Assistant Director, Drainage Services Department, the Government of the Hong Kong Special Administrative Region, 43/F Revenue Tower, 5 Gloucester Road, Wan Chai, Hong Kong, China

David Pickles Associate Director, ARUP, Level 5, Festival Walk, 80 Tat Chee Avenue, Kowloon Tong, Kowloon, Hong Kong, China

Fergal Whyte Director, ARUP, Level 5, Festival Walk, 80 Tat Chee Avenue, Kowloon Tong, Kowloon, Hong Kong, China

Figure 1 HATS Sewage Conveyance System 2 OBJECTIVE

A HATS hydraulic model is developed to serve as a design tool for the following purposes:- (a) calibration of HATS Stage 1 system to determine roughness; (b) to facilitate the hydraulic design of the HATS Stage 2 system and evaluation of options to

optimise the system hydraulic and to relieve any capacity constraints identified; (c) to investigate the likely overflow events (including the overflow locations, frequency, duration,

overflow volume, etc.) of the HATS system in future years under different development scenarios during dry and wet seasons; and

(d) to serve as a planning tool for the HATS system for investigating options to modify and improve the system performance.

3 DEVELOPMENT, CALIBRATION AND VALIDATION OF STAGE 1 MODEL

This section focuses on simulation of the hydraulic performance of the Stage 1 sewage conveyance system (SCS). The model comprises major components of the Stage 1 system including the Stage 1 PTWs, the transfer pumping stations, the sewage conveyance system including drop shafts and riser shafts, and the main pumping stations at SCISTW. Compared with other equivalent software, InfoWorks CS model is capable of simulating unsteady as well as gradually varied flow for a network with flat or negative gradients along with ancillary structures and was therefore selected. 3.1 Development of Model

The model was developed in accordance with EPD’s Sewerage Infra-structure Planning Group Guidelines for Sewer Network Hydraulic Model Build and Verification. As-built details with existing settings, and flow data were collected for the development of an accurate and reliable model. Information was also collected by conducting a survey to verify and confirm the as-built details and levels of the facilities at SCISTW and the Stage 1 system before inputting them into the model. The SCS was modelled as an inverted siphon except the section between Tseung Kwan O and Kwun Tong which is modelled as a rising main. 3.2 Operational Data

The following data at 1-second interval corresponding to 3-5 March 2006, 29-31 August 2009 and 29-31 August 2010 was obtained from the plant operator for the calibration and validation of the model:

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• Inflow to drop shaft at PTW’s; • Flow data of Pumping Stations – Stage 1 Stonecutter Island Main Pumping Station (SCIMPS),

Kwun Tong Transfer Pumping Station (KT TPS) and Tseung Kwan O Pumping Station; • Water level at PTWs drop shaft; • Water levels within inlet chamber of Stage 1 SCIMPS and KT TPS. One of the main reasons for selecting the above periods in 2006, 2009 and 2010 is to, determine if

there has been any increase in the roughness of conveyance system with time by calibrating and validating the model. At the same time, the months March and August were selected with an aim to investigating if there is any difference in hydraulic performance between dry and wet seasons in the same year. 3.3 Calibration of Model

The model parameters play a key role in simulating the hydraulic performance of conveyance system. Calibration is the estimation of model parameters by comparing simulated results against measured to ensure the same response over time. In this process, model parameters varied until recorded water level patterns are reasonably simulated. The head loss along the SCS are the key parameter which determine the hydraulic performance of the system, as well as the water level in the drop shaft at various PTWs. The entrances, alignment changes, junctions and gradient would cause minor head loss whereas tunnel wall roughness would cause major head losses. Therefore, the majority of the head losses are due to tunnel wall roughness which has been calibrated against Stage 1 operation data.

For calibration of model, recorded Initial Control Target Water Level (ICTWL) in MPS, the inflow and water level recorded at PTW’s drop shaft along the Stage 1 system are used. The operation data of HATS Stage 1, including sewage flows and water level at various PTWs drop shaft and also ICTWL at Stage 1 SCIMPS, of the above periods in 2006 and 2009 was used in the calibration. One of the main purposes of calibration of Stage 1 system is to find out representative roughness. Figure 2 below shows hydraulic operation of inverted siphon and Figure 3 shows water level at various drop shaft and MPS.

Figure 2 Hydraulic Operation of Inverted Siphon

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Figure 3 Drop Shaft & SCIMPS Inlet Chamber Water Levels of Stage 1 System 3.4 Validation of Model

Validation is comparison of the model outputs with an independent dataset without further adjustments of the values of the parameters. Operation data of 29-31 August 2010 was used for the validation and results are presented in Section 3.7 below. The verified model was used to simulate the hydraulic performance in the ultimate development scenario. The ultimate development scenario refers to scenario with completion of all planned developments within Sewage Catchment Area (SCA) of HATS. 3.5 Challenges in Calibration

The setting up of the model and simulation poses a number of challenges throughout the modelling process. Some of the major challenges include: defining invert level lower than -99.0mPD (InfoWorks would not accept data greater than 99.0), modelling of vertical drop shaft, setting initial boundary conditions for variable speed pumps and their sequential operations, time step, and mathematical modelling of pump operation in response to the signal from flow meters. There is no direct way to define the majority of these aspects in the model but to write a Real Time Control (RTC) algorithm, as indicated in Figure 4.

To facilitate the modelling works for this project, the software developer, HR Wallingford, has updated the software (InfoWorks CS model version 10.0) to allow the users to define invert level lower than -99.0mPD. As the pumps come into operation in sequence and therefore initial boundary conditions should be variable which is not possible with the existing InfoWorks CS model. Initial water level was therefore defined after performing sensitivity tests. In real time operations, variable speed pumps change flow rate even at a fraction of second. However, in InfoWorks CS model the pumps can only change flow rate at a time step which will also be constant throughout the modelling process. This was significantly overcome by using 1-sec time step which is similar to time interval used to record the data along the Stage 1 system.

Another important aspect is controlling of water level at the drop shafts and within the wetwell of MPS. InfoWorks CS model allows controlling of water level at drop shaft and within the wetwell of MPS. Sensitivity tests were performed and concluded that controlling of water level within the wetwell of MPS provides the desired simulation which is due to minimisation of error between calculated and simulated water level within wetwell of MPS.

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Figure 4 RTC for HATS Stage 1 System Model

3.6 Limitations of InfoWorks CS

InfoWorks CS model is a powerful tool to simulate the hydraulic performance of network system. However, extensive calibration is required to use the model for design or to simulate the hydraulic performance of a treatment works under various flow scenarios. Apart from this, the model could not simulate vertical drop shaft. This was overcome by modelling the drop shaft as tunnels with length of tunnels determined from equivalent head losses. Apart from this, pump on/off only happen at a time step which is different from actual operation. In addition, the operation of pumps could only be described through RTC mentioned above which is complicated and not user friendly.

In real time operations, variable speed pumps can change flow rate even at a fraction of second whereas in model this only happen with a constant rate and only at a time step. 3.7 Results and Analysis

For Stage 1 operation, ICTWL in the wetwell of Stage 1 MPS were calculated using an empirical equation which depends upon the head losses from various PTWs drop shaft to Stage 1 MPS and control water level (-2.00mPD) at drop shaft. The calculated ICTWL varies with time and is being maintained by sequential operation of pumps to provide hydraulic gradient to conveyance system. The controller instructs the pumps to maintain the minimum water level in the wetwell to avoid any overflow from PTWs drop shaft through the overflow weir when inflow to the system increases. Regarding operation of Stage 1 pumps, there are 8 nos. of pumps (6 duty and 2 standby) each with maximum discharge capacity of 5.21m3/sec. Among the 6 duty pumps, 2 are variable speed and 4 are fixed speed pumps.

As mentioned above the hydraulic model can be used to assess the hydraulic performance of HATS Stage 1 SCS, in particular to check if there is any deterioration in hydraulic performance of the tunnel system since its operation in 2001. Therefore, recorded and simulated water levels were compared at various drop shafts along the Stage 1 system using 3mm, 6mm and 9mm roughness for 3-5 March 2006 and 29-31 August 2009 recorded data to estimate the representative roughness. Subsequently, operation data from 29-31 August 2011 were used for validation of the model.

The figures below show the simulated and recorded water levels at Stage 1 MPS and Kwun Tong Transfer Pumping Station drop shaft using 3mm, 6mm and 9mm roughness for 3-5 March 2006 event. It can be seen from Figure 5 that calculated and simulated water levels in inlet chamber of pumping station match with each other, indicating that the model is performing in the correct way. It can also be seen that

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there is a minor difference between recorded and simulated/calculated water levels at MPS which is due to limitation of model which uses discharges in conduit to calculate head losses in RTC whereas empirical equations using discharge (inflows) recorded at manhole in estimating the head losses. In addition, there is a poor agreement in the beginning of simulation between simulated and recorded water level which is due to initial boundary condition in model and will not be used for comparison.

Figure 5 Calculated/Simulated/Recorded Water Level at Stage 1 MPS

The accuracy of the model depends upon simulated and recorded water level at inlet of MPS. Once

there is good agreement between simulated and recorded water level at ICTWL, the water level at PTWs drop shaft would be influenced by head losses only. Figure 6 shows the simulated and recorded water level at KTTPS drop shaft for 3mm, 6mm and 9mm roughness. It can be seen from Figures 6 that, the simulated and recorded water levels at various drop shafts are in good agreement except at a few points which are due to errors in measurement.

Figure 6 Simulated/Recorded Water Level at KT TPS Drop Shaft with 3/6/9mm Roughness

Statistical analysis was carried out to determine the optimum roughness by analysing the simulated

and recorded water levels at various drop shafts. The best estimate of roughness would provide minimum variance. It can be seen from Table 1 that 6mm roughness would provide least variance between simulated and recorded water levels at various drop shafts, which were used subsequently for validation of model corresponding to the 29-31 August event. Validation of the model confirmed the 6mm

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roughness which has been recommended for the hydraulic design of the Stage 2 system, as well as assessing the hydraulic performance in the ultimate development scenario of HATS system.

Table 1 Variance with 3mm, 6mm and 9mm Roughness along the Stage 1 System

Drop Shaft 3-5 March 2006 29-31 August 2009 29-31 August 2010 3mm 6mm 9mm 3mm 6mm 9mm 6mm

KT PS 0.67 0.19 0.52 0.46 0.20 0.43 0.21 KT PTW 0.44 0.15 0.41 0.38 0.18 0.40 0.19 TKW PTW 0.30 0.14 0.32 0.35 0.19 0.36 0.18 TY PTW 0.48 0.29 0.45 0.18 0.17 0.20 0.18 KC PTW 0.51 0.29 0.49 0.25 0.18 0.28 0.19

4 HYDRAULIC DESIGN OF HATS STAGE 2A SYSTEM 4.1 HATS Stage 2 System

HATS Stage 2A comprises a 21 km long deep tunnel system to convey sewage from eight PTWs on the northern and southwestern shores of Hong Kong Island to a Stage 2 Main Pumping Station (Stage 2MPS) in SCISTW. Stage 2 MPS will include eight number of duty pumps. Construction of HATS Stage 2A system is now underway for commissioning in 2014. Figure 7 below shows a pictorial view of overall HATS Stage 2A system.

Figure 7 HATS Stage 2A System

4.2 Sewage Flows

The build up of sewage is one of the important considerations for operation and maintenance of the conveyance tunnel. Peak sewage flows will determine the depth of main pumping station and also frequency of overflows at the PTW drop shafts. In view of the significant capital cost involvement for the construction of a deep MPS, operation data for Stage 1 was analysed to estimate the peak flows and hence to determine optimum depth of MPS.

Figure 8 below shows the percentage of time sewage flow at Stage 1 MPS not exceeding the peak flow vs peaking factor based upon 2003 to 2006 operation data. It can be seen a peaking factor 1.7 could intercept 99.7% of sewage flows in wet season and 99.98% of sewage flows in dry season. Therefore, a peaking factor 1.7 has been used in Stage 2 as compared to peaking factor 2.0 used for Stage 1 design. This relatively minor change has realized considerable savings in capital and recurrent costs with an almost indiscernible reduction in the water quality of the Victoria Harbour.

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Figure 8 Relationship of Peaking Factor against % of Time not exceeding Peak Flow in SCISTW

4.3 Option Evaluation and Recommended Options Calibration of Stage 1 system calculated the tunnel roughness to be 6mm and the same has been

used for the hydraulic design of Stage 2A conveyance system. In order to optimise the hydraulic design, three options namely (i) deep Stage 2 MPS without any transfer pumping station (ii) shallow Stage 2 MPS and transfer pumping station at Wan Chai and Sandy Bay (iii) shallow Stage 2 MPS and transfer pumping station at Wan Chai and Cyberport were investigated. These three options were evaluated by considering life cycle cost, construction programme, constructability, environment, health & safety, planning, land and operation flexibility, and any impacts on the implementation programme for Stage 2 to arrive at preferred option. Table 2 below shows weightage percentage to various factors used to analysed the three options.

Table 2 Weighting Percentage of Criteria Primary Level Secondary Level Weighting %

Life Cycle Cost Civil Construction, Equipment Cost and O&M Costs

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Construction Programme Duration 15

Constructability Construction & Machine (Technology) Risks

20

Availability of Additional Land (Works Area) Required during Construction

3

Interfacing with SCS and PTW Operational Requirements

7

Environment, Health & Safety (Construction Safety Risks) 15

Planning and Permanent Land Requirement Consideration Effect on Land Planning Intention, 10 Effect on Future Development Potential of the Surrounding Area (within 100m).

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TOTAL 100

Graph of Peaking Factor vs. % of Time NOT Exceeding Peak Flow in HATS Overall in Year 2003 to Year 2006

1

1.05

1.1

1.15

1.2

1.25

1.3

1.35

1.4

1.45

1.5

1.55

1.6

1.65

1.7

1.75

1.8

1.85

1.9

1.95

2

90.0% 90.5% 91.0% 91.5% 92.0% 92.5% 93.0% 93.5% 94.0% 94.5% 95.0% 95.5% 96.0% 96.5% 97.0% 97.5% 98.0% 98.5% 99.0% 99.5% 100.0%

% of Time NOT exceeding Peak FLow

Peak

Fac

tor

% of Flow Less than Peak Flow in HATS Overall in Year 2003 to Year 2006 (dry days only)% of Flow Less than Peak Flow in HATS Overall in Year 2003 to Year 2006

PF = 1.7

99.7%

99.98%

Time Time (dry and wet days)

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The ratings were assigned on a 5-point scale to each hydraulic option under each of the criteria, with higher scores for the better options. The relative importance of the criteria is taken into consideration by applying weights to these criteria. The scoring of each criteria is multiplied with the applied weight to give the weighted score. The overall score of each hydraulic option is then obtained by summing the weighted scores of all the criteria. Figure 9 below shows scores for three options with highest for option 1 which is recommended to be taken forward for detailed design and construction.

Figure 9 Summary of Option Evaluation

4.4 Methodology for Hydraulic Design

One of the key objectives of the hydraulic modelling is to optimise the pumping requirement and therefore the water levels at drop shafts have been designed to be kept as high as possible. At the same time, the depth of Stage 2 MPS is designed in such a way that no overflow will occur at drop shafts during peak flow (1.7 x average dry weather flow (ADWF)) condition in ultimate development scenario. Regarding tunnel shape, self cleansing velocity, maximum head losses and construction cost are some of the important considerations for the design of conveyance system for HATS Stage 2. The major headlosses through HATS system are due to friction in conveyance system. Based on the flow projections, there will be gradual build up of flows from the commissioning year (2014) of HATS Stage 2A to the ultimate development scenario. Therefore, various tunnel shapes such as circular, elliptical, egg shaped were analysed so as to optimize the headlosses. Finally, oval shape tunnel were adopted from self cleansing velocity, head losses and cost considerations. The oval shape is defined in the model using a symmetrical shape with width at different heights from the invert of the tunnel.

The hydraulic profile under (ADWF) and peak flow (PF) condition with ultimate development scenario is presented below under the preferred option which is currently under construction.

3.0

3.1

3.2

3.3

3.4

3.5

3.6

3.7

3.8

3.9

4.0

Mar

ks

Base Weightings EqualWeightings for

All Criteria

Weighting forLife Cycle Cost

+50%

Weighting forProgram +50%

Weighting forConstructability

+50%

Evaluation Scenario

Summary of Option Evaluation

Option 1Option 2Option 3

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Figure 10 Hydraulic Profile for Stage 2 HATS System

5 HYDRAULIC PERFORMANCE OF HATS SYSTEM UNDER ULTIMATE DEVELOPMENT SCENARIO

The hydraulic performance of HATS Stage 1 and Stage 2 systems were also assessed against the ultimate development scenario. Figure 11 and Figure 12 show the variation in water level in the inlet chamber of Stage 1 MPS and at Kwun Tong Preliminary Treatment Works (KTPTW) drop shaft respectively. It can be seen that the minimum water level in the inlet chamber will be -16.50mPD (which is above -19.30mPD i.e., design lowest water level of Stage 1 MPS) to avoid any overflow at KTPTW drop shaft.

Figure 11 Simulated Water Level at Stage 1 MPS with Peak Flow under Ultimate Development Scenario

Figure 12 Simulated Water Level at KTPTW Drop Shaft with Peak Flow under Ultimate Development Scenario

Similarly, Figure 13 and Figure 14 show variation in water level of inlet chamber of Stage 2 MPS

and at Aberdeen drop shaft respectively. The minimum water level in inlet chamber will be -21.15mPD (which is above -22.75mPD i.e., design lowest water level of Stage 2 MPS) to avoid any overflow at Aberdeen drop shaft.

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Figure 13 Simulated Water Level at Stage 2 MPS with Peak Flow under Ultimate Development Scenario

Figure 14 Simulated Water Level at Aberdeen PTW Drop Shaft with Peak Flow under Ultimate Development Scenario

In summary, it can be concluded that the existing Stage 1 MPS arrangement and the proposed Stage

2 MPS design can maintain the targeted water level at respective drop shafts. 6 CONCLUSIONS

A model for the HATS Stage 1 system has been successfully developed, calibrated and validated by InfoWorks CS. One of the major difficulties is modelling a set of variable/fixed speed pumps with continuously changing operational properties, has been resolved by composing a complicated real time control for the operation of these pumps. Simulated and recorded water levels are compared for a number of flow events at various drop shafts. However, there is a need to further enhance the model by improving upon RTC, initial boundary condition and time step for simulation.

Three options were developed for the hydraulic design of Stage 2 with the option of no transfer pumping stations taken forward and this is currently in construction stage. The model will continued to be reviewed and updated in relation to any changes in hydraulic design of the system during construction. ACKNOWLEDGEMENT This paper is based upon findings undertaken as part of the consultancy under Agreement No. CE 8/2006 (DS) Harbour Area Treatment Scheme Stage 2A Upgrading of the Stonecutters Island sewage treatment works and the preliminary treatment works – Investigation, Design and Construction, commissioned by the Drainage Services Department (DSD), the Government of the Hong Kong Special Administrative Region, China. References Agreement No. CE 34/2005 (DS). Harbour Area Treatment Scheme Stage 2A Sewage Conveyance System -

Investigation, Design and Construction (SCS). DSD’s Research & Development No. RD1006 (2002). A review of three Existing Sewage Tunnels and

Recommendations on the Guidelines for Tunnel Design with respect to Future Tunnel Maintenance and Inspection. DSD’s Research & Development No. RD1022 (2002). Desk-top Study on Sewage Flow Pattern of Preliminary

Treatment Works Discharging to the SSDS Stage 1 Tunnels. EPD Technical Paper No. EPD/TP1/05. Guidelines for Estimating Sewage Flows for Sewage Infrastructure Planning. EPD’s Sewerage Infra-structure Planning Group Guidelines for Sewer Network Hydraulic Model Build and

Verification. HATS Stage 1 System (2002) – Commissioning Completion Report. Kumar A., Pickles D., Whyte F. and Lawrence K M Ho., 2012. Development and Calibration of a Hydraulic Model

for HATS Conveyance System Using InfoWorks CS Model. HKIE Civil Division International Conference 2012. iCiD: innovation and Creativity of infrastructure Developments for Quality Cities

Sewerage Manual Part 1 (1995). “Key Planning Issues and Gravity Collection System”, Drainage Services Department.

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