Proceedings of 2013 IAHR Congress © 2013 Drainage Services Department and Black & Veatch, Hong Kong

ABSTRACT: Combating flooding against urbanization has long been a challenge in many densely populated cities. While drainage improvement by conventional pipe amplification is a direct flood relief alternative, many aspects of disturbance to the public during construction are considered inevitable in view of various site constraints that are intrinsic to most urban areas. As part of an integrated flood alleviation strategy in Happy Valley of Hong Kong, the Happy Valley Underground Stormwater Storage Scheme (HVUSSS) is adopted to minimize the potential vulnerability of the city’s social-economic growth to flood risk.

To ascertain the effectiveness of flood attenuation by the stormwater storage scheme, 1-dimensional and 2-dimensional computational hydraulic modelling of the drainage network and overland flow are adopted to assess the extent of flood risks under various rainstorm events. Combining engineering intelligence with modern technology, the design of the HVUSSS was optimized to obtain a robust and sustainable flood control solution with acclimatization to the effect of climate change. The HVUSSS comprises several major components including an inlet structure, a twin cell diversion box culvert, an automatic movable overflow side weir system, an underground storage tank and a pump house. Particular attention has been given to minimising both the volume and depth of the underground storage tank to achieve a more economical and sustainable design for construction and operation in long-term. Without adopting the traditional fixed crest weirs, the automatic movable crest weirs use real-time water levels measured at the upstream and downstream as the basis to adjust their levels for controlling the timing and amount of stormwater flowing from the twin cell diversion box culverts to the storage tank. Hence, early spillage into the tank could be avoided which could help reducing the required volume of the storage tank by 30%. To enhance the saving of energy consumption and recurrent costs of discharging water from the storage tank after rainstorm events, the tank has been designed shallow enough to allow most of the stored water to be discharged by gravity via the automatic movable crest weirs from the tank to the box culvert.

This paper discusses the design and computational hydraulics aspects of the Happy Valley Flood Protection Scheme in Hong Kong. In particular, the adoption of the latest technique of 3-dimensional Computational Fluid Dynamics in the hydraulic analysis to optimize the design will be elaborated. KEY WORDS: 3-dimensional Computational Fluid Dynamics, flooding, flooding inlet structure, underground storage tank and overflow side weir. 1 INTRODUCTION

Happy Valley is one of the urban areas with a long development history in Hong Kong. Due to its low-lying and hilly-surrounded topographical characteristics together with the increasing urbanization

Numerical Determination and Design Optimization of the Drainage System for Happy Valley Flood Protection Scheme, Hong Kong

Karen K.Y. Chan Engineer, the Drainage Services Department, the Government of the Hong Kong Special Administrative

Region. Email: karenkychan@dsd.gov.hk

Kevin Chan, Senior Engineer, Black & Veatch, Hong Kong Ltd. E-mail: chanks@bv.com

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over the past few decades, a substantial increase in overland runoff has been induced. The existing drainage system is no longer fully capable of protecting the region from flooding in which the low lying area such as Lap Tak Lane, Morrison Hill Road and Wong Nai Chung Road in the Happy Valley area have all experienced serious flooding during severe rainstorms and high tides. During the severe rainstorm in 2008, the maximum flood depth exceeded 1m and the flood extent was as much as 30 hectares.

To address the flooding concern, the Drainage Services Department of Government of the Hong Kong Special Administrative Region (DSD) has implemented a series of flood alleviation projects within the catchment of Wan Chai and Happy Valley, including a drainage tunnel to intercept the runoff collected from the upstream catchment and conventional drainage improvement works at the lower catchment (Figure 1). However, flooding hazards cannot be fully removed by these two projects as the existing pipes in the drainage system in the mid-stream and downstream of Happy Valley catchment are rather flat in gradient and susceptible to tidal influence Hydraulic assessments show that under a 50-year return period storm event, the infield area of the Happy Valley Recreation Ground (HVRG) and the roads in the vicinity are still exposed to a high risk of flooding, which affects about 70,000 residents and 2,000 shops (Figure 2). Therefore, further improvement works are required in the mid-stream catchment of Happy Valley to enhance flood protection to an acceptable level. Hence, DSD proposed to construct an underground storage tank inside the catchment to temporarily store the runoff collected from the major stormwater box culvert / drains during heavy rainstorm. When the rainstorm is over, the stored water would then be discharged to the sea via the existing drainage system.

2 HAPPY VALLEY UNDERGROUND STORMWATER STORAGE SCHEME (HVUSSS)

With the consideration of land and traffic constraints within the Happy Valley area, the HVRG, comprising 11 sports pitches located in the central part of the drainage catchment, was found to be the most appropriate site for implementing the flood detention scheme. In order to retain the existing facilities at HVRG, the flood detention scheme is intended to be constructed as an underground structure in the form of a stormwater storage tank. It would temporarily store the floodwater collected during the peak of storm event to alleviate the flooding situation in the Happy Valley catchment. (Figure 3)

The objective of the HVUSSS is to provide flood attenuation in the Happy Valley catchment during a 1 in 50 year rainstorm event. The main components of the HVUSSS comprise an inlet structure with a stilling basin, a twin -cell diversion box culvert, an automatic moveable overflow weir penstock system, underground storage tank and pump house. (Figure 4)

Mid-level Catchment

Wan Chai

Causeway Bay

Happy Valley

Hong Kong West Drainage

Tunnel (HKWDT)

Lower Catchment

Drainage Improvement

Works (LCDI)

After Completion of HKWDT and LCDI

Figure 1 Happy Valley sub-catchment plan Figure 2 Flood extent under 50-year return period rainfall event

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During heavy rainstorm event, runoff from various streams of the upstream catchment converges to the inlet structure at Crescent Garden prior to discharging further downstream to the diversion box culvert. The existing box culvert will be modified and the runoff will be conveyed through the proposed approximately 630m long diversion box culvert crossing underneath the HKJC Race Course and HVRG. The inlet structure also functions as a stilling basin to stabilize the flows coming from different directions before reaching the overflow weirs. The overflow weir system is a critical component to ensure that only the excess flow would be diverted into the storage tank at the critical timing when the downstream drainage system is approaching its full-flow capacity.

In the design of HVUSSS, 13 automatic “movable” crest weir penstocks are proposed for the overflow side weir system as these allow variable weir heights during operation. With the aid of real-time monitoring of runoff and water level by sensors installed at upstream and downstream of the HVUSSS as well as in the storage tank, the crest weirs would only be lowered as necessary to allow stormwater in the diversion box culvert to be diverted to the storage tank and hence attenuate the peak flow to the downstream drainage systems. This real-time controlled movable weir would ensure that filling of the storage tank occurred at the optimal time to prevent premature or late overspill of stormwater into the storage tank. The design is considered to be a green measure which optimizes the total storage capacity and reduces the design capacity of the storage tank by about 30%. Moreover, the prevention of premature or late overspill of stormwater would minimize the pumping required and thus enhance sustainability during operation. The “movable” crest weir could also allow closure of the opening during normal days to prevent odour from the diversion box culvert entering the storage tank. The mode of operation of this flexible overflow weir system is demonstrated in Figure 5.

Upper catchment

Mid level catchment

HVUSSS

Lower catchment

Figure 3 Location of the HVUSSS

Direction

of Flow

Figure 4 Major Components of the HVUSSS

Figure 5 Stepped approach for the operation of automatic “movable” overflow weir system

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3 HYDRAULIC MODELLING APPROACH

In recent years, numerical hydraulic modelling analysis has been widely adopted in drainage projects. The hydraulic modelling is fast, efficient and can accurately present the results taking into account of time lag due to difference in time of concentration from each contributing sub-catchment. Most of the hydraulic models are able to simulate unsteady, gradually varying flow in looped networks with flat or reverse gradients where the direction of flow may reverse. Three types of hydraulic modelling technique which can simulate unsteady and gradually varying flow are commonly used in urban drainage system analysis and planning.

3.1 One Dimensional (1-D) Model

A 1-D model is adopted in many conventional drainage designs. Once the drainage system is surcharged, the flood water would be temporarily stored in a virtual flood cone on top of the simulation manhole and return to the drainage system when the water levels drop. The maximum flood volume of the flood cone could be used for flood risk analysis. In reality, overflow from the surcharged manhole would possibly follow the topography and discharge into another manhole with surplus hydraulic capacity, which could not be reflected in the 1-D modeling.

3.2 One Dimensional (1-D) with Two Dimensional (2-D) Ground Model

The 1-D with 2-D ground model is adopted in the Happy Valley Flood Storage Scheme project. The 1-D model is adopted to examine the performance and adequacy of the existing underground stormwater drainage system, whereas the flow pattern of flooding water/surface flow is assessed by the 2-D model. The 2-D ground surface acts as the overland flow surface. The results would indicate the behaviour of the overland flows under complex geometries with different flow directions based on the topography of the catchment area. The overland flow, which generates from a flooded manhole, would be able to return to the drainage system, thus providing a more realistic simulation of the flooding situation during a storm event. The flow direction, maximum flow velocity and flood depth of the ground model provide a good reference for the flood risk analysis

3.3 Three Dimensional (3-D) Computational Fluid Dynamics (CFD) Model

A CFD model can simulate the hydraulic response of complex hydraulic structures through the resolution of basic conservation equations. The method involves setting up a mesh which splits the water body into a number of small elements. The software then calculates the predicted flow behaviour by solving iteratively a series of equations for conservation of mass, momentum and energy. This type of model requires accurate modelling data and a longer simulation time. With the adoption of an integrated modelling approach, the hydraulic performance of the drainage system of the entire Happy Valley area could be assessed in a quick and accurate manner. The 1-D with the 2-D ground model was adopted for assessing the drainage performance of the Happy Valley catchment. The 1-D model was used to promptly examine the performance and adequacy of the stormwater drainage system, whereas the flow pattern of the flooded surface flow, if any, was assessed by the 2-D model. In this 2-D model, the ground surface acts as the overland flow surface. The results could indicate the behaviour of the overland flow under complex geometries with different flow directions according to the topography of the catchment area. In addition, the flooded surface flow, which generates from a flooded manhole, could be modelled to return to the drainage system where there is surplus hydraulic capacity, thus providing a more realistic simulation of the flooding situation of a storm event Apart from using the 1-D with the 2-D ground model as a prime modelling tool for HVUSSS, a CFD model has been adopted to simulate and fine tune the hydraulic responses of the utility crossings and other intrusions in the drainage system as well as some complex structure including inlet structures, overflow weirs and diversion box culvert due to their inherent hydraulic complexity. The adverse hydraulic effects determined from the CFD modelling could then be plugged into the 1-D with 2-D ground model for back analysis so that the adverse effects due to the presence of complex hydraulic elements/structures could be reflected in the hydraulic analysis

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4.0 UTILITY CROSSINGS ALONG THE DECKED NULLAH

The decked nullah along Canal Road is intruded at different levels by utilities in the form of pipe clusters and box drains. In order to obtain the headloss value due to the presence of utility crossings within the flow path, a CFD model is developed for the decked nullah under various steady state flow conditions as means to estimate the frictional resistance of the culvert system. The model is of 100m stretch with a series of columns and intruded by utility crossings (Figure 6).

Figure 6 CFD model analyzing flow behaviour with utility intrusion in culvert system

5.0 INLET STRUCTURE

The inlet structure is located underneath Crescent Garden and intended to act as a stilling basin, trapping silt and providing an even flow distribution into the downstream twin cells diversion box culverts. In order to achieve an evenly-distributed flow split at the twin cells diversion box culverts and a low velocity jetting flow, a CFD model is developed to simulate the flow in the steep inlet channels, stilling basin and twin cells box culverts (Figure 7).

Figure 7 The initial layout of the CFD model for the inlet structure

A numbers of iteration were simulated. The optimal layout consists of fewer and narrower openings at the walls of twin cells box culvert. This keeps the upstream flow subcritical so that the flow can be conveyed smoothly around the corner through the openings. In addition, three full-depth vanes were

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proposed to the immediate downstream of the openings to guide the flow into the stilling basin in a predetermined manner (Figures 8a and b).

(a) (b) Figure 8 (a) The Final Layout of the CFD Model for the Inlet Structure; (b) The Velocity Plot of the CFD Model for the Inlet Structure

6.0 OVERFLOW SIDE WEIR AND DIVERSION BOX CULVERT

The third set of component which is analysed by CFD is the twin cells diversion box culverts, overflow weir system and discharge box culverts (part of the existing box culverts) (Figure 9). The flow along the diversion twin cells box culverts discharges into the storage tank via a side overflow weir. During rainstorm event, when water level in the diversion twin cells box culverts rises to a preset water level, excess flow in twin cells box culvert would overflow into the storage tank through the overflow side weir. In order to achieve the required overflow rate for the overflow weir system and improve the flow spilt in the twin cells box culverts, especially in the downstream of the storage tank, a number of scenarios were tested with different weir crest levels and numbers/locations of equalizing holes between the twin cells box culvert.

Figure 9 CFD Model for the Overflow Side Weir and Diversion Box Culver

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The final layout consists of additional balancing holes along the middle wall of the twin cells box culvert. With reference to the results of the CFD modelling, it revealed that the final design of the HVUSSS would have the capability to divert runoff at the desired rate from the upstream catchment through both the inlet structure and the overflow side weir into the storage tank. The final layout with the optimal flow balance is determined by iterating the numbers and locations of the new openings along the internal wall. (Figure 10)

Figure 10 Close-up View of the Final Design of the Overflow Side Weir

7 CONCLUSIONS

The challenge of the HVUSSS is to alleviate the flooding problem in a high-density and well-developed low lying urban area. In the HVUSSS, an innovative, sustainable and cost-effective solution was developed to relieve the flooding problem in the Happy Valley catchment. It is considered that the adoption of the real-time controlled “movable” weir system in the underground stormwater storage tank has established a good precedent for developing sustainable drainage measures in the future. In addition, the adoption of an integrated advanced modelling technique has greatly enhanced the reliability and accuracy of the analysis of the hydraulic performance of the system.

The latest hydraulic modelling techniques in the industry including 1-D and2-D hydraulic models for drainage network and overland flow, as well as the application of 3-D CFD to analyse and understand the behaviour of the complex flood flows such as at the weir intake structures are applied in this project. Numerical modelling of the improved Happy Valley drainage system demonstrated that the implementation of the HVUSSS would uphold the safety by enhancing the flood protection level to the current flood protection standard.

ACKNOWLEDGEMENT

The authors gratefully acknowledge the Director of Drainage Services of the Government of the Hong Kong Special Administrative Region for permission to publish this paper. The authors also thank the colleagues of DSD and Black & Veatch for their assistance on writing the paper.

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