computational hydraulics aspects of the drainage …€¦ · page 1 computational hydraulics...

Computational Hydraulics Aspects of the Drainage System

Planning for Happy Valley Flood Protection Scheme, Hong Kong

K.W. Mak*, Jimmy Poon**, Kelvin Lau***, Eric Lo****, Kevin Chan*****

* Assistant Director, Drainage Services Department, the Government of the Hong Kong Special Administrative Region (E-mail: [email protected]) **Senior Engineer, Drainage Services Department, the Government of the Hong Kong Special Administrative Region (E-mail: [email protected]) ** Associate Vice President & Project Director, Black & Veatch, 25/F, Millennium City 6, 392 Kwun Tong Road, Kowloon, Hong Kong

(E-mail: [email protected]) **** Technical Director, Black & Veatch, 25/F, Millennium City 6, 392 Kwun Tong Road, Kowloon, Hong Kong (E-mail: [email protected]) ***** Engineer, Black & Veatch, 25/F, Millennium City 6, 392 Kwun Tong Road, Kowloon, Hong Kong (Email: [email protected])

Abstract: Happy Valley is a low-lying urban area located in the hinterland of the Wan Chai District and is surrounded by a hilly terrain. Serious flooding occurred in the Happy Valley catchment, in particular the Happy Valley Recreation Ground (HVRG), Hong Kong Jockey Club (HKJC) racecourse and nearby streets, during heavy rainstorms in 2000, 2006 and 2008. To address the flooding problem, the Drainage Services Department of the Government of the Hong Kong Special Administrative Region proposed a Happy Valley Underground Stormwater Storage Scheme (HVUSSS) project at the HVRG. The project would raise the flood protection level of the drainage system in Happy Valley to cater for rainstorms with a return period of 1 in 50 years, and in turn significantly reduce the risk of flooding in Happy Valley and areas in the vicinity. 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. In the planning and design stages, 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, automatic movable crest weirs with real-time monitoring of upstream and downstream water levels have been designed. The adjustable crest weirs use real-time water levels as the basis for controlling the timing and amount of stormwater flowing from the twin cell diversion box culverts to the storage tank so as to avoid early filling of the storage tank which in turn resulted in reducing the required volume of the storage tank by 30%. To further enhance sustainability by saving energy consumption and recurrent costs of discharging water from the storage tank, a shallow tank has been designed 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 after rainstorm events. Another advantage of minimising the depth of the storage tank is to facilitate the adoption of lower pressure pumps, with a lower capacity, to pump the remaining stored water out of the tank thus helping to save energy. This paper discusses the design and computational hydraulics aspects of the planning for 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.

Keywords: 3-dimensional Computational Fluid Dynamics; inlet structure; underground storage tank; overflow side weir

Introduction

Happy Valley is one of the urban areas with a long development history in Hong Kong. Over the past few decades, Happy Valley has undergone significant urbanization and changes in land use from green land to developed areas. The low-lying and hilly-surrounded topographical characteristics, together with the city development, has induced major increases in overland runoff. The existing drainage system is no longer fully capable of protecting the region from flooding in which Lap Tak Lane,

Morrison Hill Road and Wong Nai Chung Road in Happy Valley have all experienced serious flooding during severe rainstorms and high tides in years 2000, 2006 and 2008. The maximum flood depth exceeded 1m and the flood extent was as much as 30 hectares during the rainstorm in 2008 (Figure 1 refers).

The Government of the Hong Kong Special Administrative Region (HKSARG) has commissioned two major flood relief projects in the northern part of Hong Kong Island, namely the Hong Kong West Drainage Tunnel (HKWDT) and Lower Catchment Drainage Improvement Works (LCDI), to relieve the flooding hazards of the Happy Valley catchment. The HKWDT serves to intercept stormwater runoff from an upstream catchment area of approximately 140 hectares, while the LCDI at the downstream catchment involves conventional drainage improvement methods to mitigate the flooding problem in the low-lying areas including Wan Chai and Causeway Bay (Figure 2 refers). 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 3 refers). Therefore, further improvement works are required in the mid-stream catchment of Happy Valley to enhance flood protection to an acceptable level in order to safeguard the continuous and sustainable development of the area.

Figure 1111: Serious flooding on 7 June 2008: (a) a Happy Valley Recreation Ground; (b) at Junction between Morrison Hill Road and

Queen’s Road East; (c) at Wong Nai Chung Road.

(a) (b) (c)

Figure 3: Flood Extent under 50-yr Return Period Rainfall Event after completion of HKWDT and LCDI

After Completion of the HKWDT and

LCDI

Figure 2: Happy Valley Sub-catchment Plan and Locations of HKWDT and LCDI

Mid-level area of

Happy Valley

Catchment

LCDI at Wan Chai and

Causeway Bay

HKWDT HVRG

HKSARG therefore proposed to implement a stormwater storage scheme to temporarily store the discharges from the major stormwater box culverts/drains so as to alleviate the loading on the existing downstream drainage system during heavy rainstorms. When the rainstorm is over, the stored water in the underground storage tank would then be discharged to the sea via the existing drainage system. The main components of this Happy Valley Underground Stormwater Storage Scheme comprise an inlet structure with a stilling basin for intercepting the discharges from the major box culverts/drain, a diversion twin-cell box culvert conveying intercepted discharges to the underground storage tank and downstream existing drainage system, an automatic moveable overflow side weir penstock system, and an underground storage tank and associated pump house. Happy Valley Underground Stormwater Storage Scheme (HVUSSS)

To meet the flood protection standard of a 1 in 50-year return period, an underground storage tank with a capacity of 60,000m3 would be needed. Of the limited land available in the Happy Valley area, HVRG was found to be the most suitable site for locating the underground stormwater storage tank (Figure 2 and 5 refer). HVRG is an open area with 11 sports pitches currently managed by the Leisure and Cultural Services Department of HKSARG and situated close to the existing trunk drainage network of this catchment. It is surrounded by the race track of Happy Valley Racecourse of the Hong Kong Jockey Club (HKJC).

Runoff from the upstream catchment will be converged to the inlet structure at Crescent Garden before discharging further downstream. The existing box culvert will be modified and the runoff will be conveyed through the newly-constructed approximately 630m long diversion twin-cell box culvert crossing underneath the HKJC Race Course and HVRG. The inlet structure will also function as a stilling basin to stabilize the flow before entering the diversion twin-cell box culvert (Figure 4 refers). A stabilized flow is crucial for avoiding excessive runoff flowing into the storage tank via the overflow weir system. During the days of low flow, stormwater collected at the inlet structure will be conveyed along the diversion twin-cell box culvert to the downstream drainage network without entering the underground

Figure 4: Major Components of the HVUSSS

Inlet Structure

Diversion Twin-cell

Box Culvert

Underground Storage Tank

Overflow Weir Pump House

HKJC Race

Course

Crescent Garden

Figure 5: Photomontage of the HVUSSS

storage tank. During heavy rainstorm events, the excess runoff in the diversion box culvert will enter the storage tank via the “movable” overflow weir system. The crest levels will be adjusted automatically based on the real-time monitoring data of flow depths in the drainage network at the upstream and downstream of the storage tank, water levels in the storage tank, and tide levels in Victoria Harbour which thus allow filling of the storage tank at the most optimal time. Subsequently, emptying the tank by means of gravity draining firstly and then pumping will be adopted to discharge the stored water from the tank after a rainstorm event. The underground storage tank beneath HVRG will have a capacity of 60,000m3 with a plan area of about 20,000m2. The associated pumping station with design pumping rate of 1.5m3/s will be constructed for pumping the remaining stored water from the underground storage tank. After completion of the HVUSSS, it is expected that the risk of flooding in Happy Valley catchment will be substantially lowered. The amenity facilities on the HVRG will be restored for public use after completion of the HVUSSS. Hydraulic Modelling Approach

In recent years, numerical hydraulic modelling analysis has been widely adopted to simulate the flow conditions and the hydraulic performance of a drainage system under heavy rainfall for decision making. Three types of hydraulic modelling technique which can simulate unsteady and gradually varying flow are commonly used in urban drainage system analysis and planning.

i) One Dimensional (1-D) Model – A 1-D Model provides fast simulation results as long as no overflow is predicted as the overland flow path cannot be simulated by the 1-D Model.

ii) One Dimensional (1-D) with Two Dimensional (2-D) Ground Model – This type of model can be regarded as an enhancement of the 1-D Model as the overflow from the 1-D Model could be simulated flowing on the ground profile of the catchment.

iii) 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. This type of model requires accurate modelling data and a longer simulation time.

With the adoption of an integrated modelling approach in HVUSSS, 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 module 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 module. In this 2-D module, the ground surfaces act 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 Models 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, inlet structures and 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. The application of CFD modelling techniques in designing the complex hydraulic structures under HVUSSS will be further discussed in the following paragraphs. Inlet structure of HVUSSS The inlet structure will be located inside Crescent Garden. It is designed to intercept four existing box culverts/drains, with invert levels at about +4.61 mPD, and discharge to the proposed downstream diversion box culvert at an invert level of +2.2 mPD. Three out of the four existing box culverts/drains will be grouped into an approach box culvert and connected to the stilling basin through 14 openings, each 1.5m wide x 2m high, while the remaining box culvert will connect directly to the stilling basin. Inside the stilling basin, a chute leading the discharges from the openings to the stilling basin and a 1m deep silt trap, having retention capacity of 800m3 at the bottom, are being proposed (Figure 6 refers). The longitudinal profile of the existing box culvert, inlet structure and the diversion box culvert is shown in Figure 7.

Shing Woo Road 3850mm (W) x

2200mm (H) Box Culvert

(Inet A)

Chute

Wong Nai Chung Road 2500mm

(W) x 2500mm (H) Box Culvert

(Inlet C)

14 nos. of 1.5m

x 2m high

openings

Diversion Box Culvert

Stilling basin

with 1m deep

Silt Trap

Existing 900mm dia.

Drain (Inlet D)

Downstream

Upstream

Inlet structure

Shan Kwong Road 1450mm (W) x

2775mm (H) Box Culvert

(Inet B)

Location of Inlet structure

Figure 6: Major Components of the HVUSSS

The inlet structure is designed to fulfil the following functions:

� Achieve equal flow split at the diversion box culvert;

� Reduce flow velocity in order to enhance the intercepting performance by the overflow side weirs of

the storage tank; and

� Trap silt

Equal flow spilt at diversion box culvert

During the design rainstorm of 1 in 50-year return period, the flow rate inside the diversion box culvert will be as high as 32.4m3/s. As high velocity flows might not be balanced in the two cells of the diversion box culvert through the balancing holes along the internal wall, it is important to maintain uniform flow as much as possible inside the inlet structure before entering the diversion box culvert. Reduction in flow velocity

The efficiency of intercepting the flow from the diversion box culvert to the underground storage tank through the overflow side weirs would be reduced if the flow velocity is high. By introducing the inlet structure and retarding the flow velocity, the intercepting performance through the overflow side weirs of the storage tank could be enhanced.

Silt trapping

To minimise future maintenance inside the diversion box culvert underneath the race track and at HVRG, a 1 m deep silt trap will be provided at the bottom of the stilling basin at an invert level of +1.2mPD. It is purposely planned that the desilting works will be carried out mainly inside the silt trap with an access opening at the footpath outside Crescent Garden. With such an arrangement for desilting works, the disturbance to the public at Crescent Garden, carriageway, race track and HVRG will be minimized. With a difference in invert levels between the existing box culverts at Wong Nai Chung Road and the inlet structure, the flow is designed to drop into the stilling basin via a chute which has a profile that avoids negative pressures from developing on its glacis. The shape of the chute is based on the trajectory

+4.61mPD +2.2mPD

+5.2mPD

+1.2mPD

Figure 7: Longitudinal profile of the existing box culvert, inlet structure and diversion box culvert

of a falling jet: Y = (g � X2) / (2 � V2)

where: Y is vertical distance below the crest of chute (m)

g is gravitational acceleration (= 9.81m/s/s)

X is horizontal distance from the crest of chute (m)

V is upstream velocity of the flow (m/s)

An angular transition at the intersection between the chute and the stilling basin will be provided to facilitate energy dissipation. The calculation for deriving the profile of the chute is shown in Figure 8.

A 1 to 1 slope for the chute profile is adopted prior to reaching the stilling basin. Energy would dissipate within the stilling basin and provide a relatively stable flow condition to the downstream diversion box culvert prior to discharging to the storage tank via the side overflow weir. Because of the complex configuration of the inlet structure and inflow from different directions to the inlet structure, a CFD model was established to assess the hydraulic performance (Figure 9 refers). CFD is a computational modelling tool which simulates three-dimensional (3D) fluid flow. It had been adopted successfully for similar hydraulic structure designs such as “CFD Assisted Spillway and Stilling Basin Design in the mid-west of USA by Flow-3D” and “Computational Modelling of Flow over a Spillway in Vatnsfellsstífla Dam in Iceland”. The method involves setting up of a mesh which divides the water into a number of small elements. The software then calculates the predicted flow by solving iteratively a series of equations for conservation of mass, momentum and energy. The smaller the elements in the mesh adopted, the more accurate and stable outputs will be determined, but the maximum number of elements which can be solved are limited by the hardware used. The software Ansys CFX 12.1 was adopted for analyzing the flow in the inlet structure in this project.

Preliminary profile of the chute of inlet structure Calculation of the chute of inlet structure

Figure 8: Design profile of the chute of inlet structure

The flow rates and water level derived by the 1-D unsteady flow numerical model (InfoWorks CS) were plugged into the CFD model as the inflows and boundary conditions respectively. The objective of the CFD model was to predict the flow regime inside the inlet structure arising from the inflows from different directions into the irregular shape of the stilling basin. The hydraulic performance of the inlet structure could be simulated by modifying the formulation of the CFD model, thus determining an optimal layout of the inlet structure as a result.

The results of the CFD model (Figure 10 refers) indicated that the initial layout of the inlet structure was hydraulically unsatisfactory for the following reasons:

� Supercritical flow and hydraulic jumps in the approach culvert would result;

� Flow with high velocity jetting down on only one side of the stilling basin would lead to

ineffective silt trapping; and

� The asymmetric flow through the stilling basin resulted in an uneven (39% vs 61%) flow

distribution between the two cells of the diversion box culvert.

Flow Split 39%

Flow Split 61%

Hydraulic jump

Inlet A

Inlet D

Inlet C

Inlet B

Boundary Condition (To Storage Tank)

Hydraulic jump

Figure 9: Preliminary CFD model for the inlet structure

Figure 10: Simulation results of the initial CFD model for inlet structure

To minimize the effect of the supercritical flow resulting from hydraulic jumps in the approach box culvert, restraining the rate of inflow to the stilling basin by means of closing five of the 14 openings was introduced. The change caused the upstream flow regime to be subcritical so that the flow could turn gradually at the corner through the openings of the stilling basin. With this amendment, the flow distribution at the two cells diversion box culvert was improved to be 44% vs. 56% (Figure 11 refers).

To further reduce the difference in the flow distribution between the left and right cell of the diversion box culvert, some openings on the internal wall inside the diversion box culvert were maintained. Moreover, three full-height flow guiding vanes were introduced immediately downstream of the openings to guide the flow to the basin. The hydraulic performance of the revised layout of inlet structure was found to have been significantly improved as a better distributed flow in the stilling basin and more even flow distribution between the two cells in the diversion box culverts (49% vs 51%) were noted (Figure

12 refers).

Underground Storage Tank of HVUSSS The excess stormwater would overflow into the storage tank via the overflow weir system when the water level in the diversion box culvert is higher than the top level of the crest weirs. Traditionally, fixed weir overflow systems, with a specific pre-set level to limit the maximum flow depth in the box culvert, are proposed because of their simplicity in operation and maintenance. However, the shortcoming of using a fixed weir is that the crest level cannot be altered to meet the genuine need of

Opening cancelled

Hydraulic jump

Flow Split 44%

Flow Split 56%

Flow Split 49%

Flow Split 51%

Flow Guiding

Vanes

Figure 11: Simulation results of the revised CFD model for inlet structure

Figure 12: Simulation results of the revised CFD model for inlet structure (Final)

Column supporting

top slab

temporary storage of the excessive runoff in a storm event, and there might be premature spilling of stormwater from the box culvert to the storage tank. This premature spilling would take up part of the storage capacity of the tank and hence additional storage capacity would have to be allowed in the design. Adoption of Automatic Movable Crest Weir Overflow Weir System

In the design of HVUSSS, 15 automatic “movable” crest weir penstocks are proposed for the overflow side weir system as these allow for variable weir heights during operation. With the aid of real-time monitoring of runoff and water level by sensors installed 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 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 13.

Hydraulic Design of Overflow Weir System

Based on the results simulated by the 1-D unsteady flow numerical model (InfoWorks CS), 15 3m wide weirs with crest level at +3.3mPD can intercept 30m3/s flow into the storage tank and alleviate the flooding condition in the downstream under a 50-year rainfall event. To have a clear understanding of the intercepting proportion of flow into the storage tank under a range of flow/weir scenarios and examine the flow split in the diversion box culvert, a CFD model was developed accordingly to assess the flow

Figure 13: Stepped Approach for the Operation of Automatic “Movable” Overflow Weir System

behaviours (Figure 14 refers).

Three flow/weir scenarios were simulated for the initial and final designs as summarised in Table 1:

Table 1 Flow Cases for Initial Design

Rainstorm Event

Inflow (m3/s)

Overflow weir level (mPD)

Outlet B boundary condition level (mPD)

Inlet A Inlet B

50-yr 32.4 7.5 +3.3 +3.87

10-yr 19.5 4.5 +3.6 +3.81

200-yr 34.0 16.0 +3.3 +3.94

The CFD model was initially used to simulate a 50-year rainstorm event. The results show that the initial layout could be improved since there were significantly uneven flows at the middle and the end of the box culverts (Table 2 refers). Therefore an improved design was developed and tested afterwards.

Figure 14: The CFD model of the twin box culverts and overflow weir

Table 2 Results for Initial Design

Rainstorm Event

Flow from Inlet A (m

3/s)

Flow Split (m3/s) at Locations L1, L2 & L3 (Locations refer to Figure 14)

(Left vs Right, looking downstream)

To storage

tank

Remain in culverts

L1 L2 L3 Velocity distribution at L3

50-yr 22.0 10.4 13.94 vs 18.46 10.09 vs 0.31 7.28 vs 3.12

The first attempt to improve the design was to add new openings in the internal wall just opposite the weirs so as to evaluate the change of flow behaviour (Figure 15 refers).

The flow rates in the diversion box culvert based on the revised model are listed in Table 3. It indicates that the proposal of adding new openings would allow more flow to be diverted to the storage tank compared with the initial design, but would cause severe back-flow issue (negative flow rates in Table

3) in the diversion box culvert, especially under a 50-year rainfall event.

NO1

NO2

NO5

NO4

NO3

NO6

NO7

O3, O4: Openings proposed in the initial design

NO1, NO2, NO3, NO4, NO5, NO6, NO7: New openings O3 (CH322)

O4 (CH422)

Storage Tank

Figure 15: The CFD model of the twin box culverts and overflow weir (Revised)

Table 3 Results for Initial Design

Rainstorm Event


3/s)

Flow Split (m3/s)


To storage tank Remain in culverts L1 L2 L3

50-yr 33.4 -1.0 18.25 vs 14.15 -0.79 vs -0.21 -0.39 vs -0.61

10-yr 19.2 0.3 12 vs 7.5 -0.56 vs 0.86 0.26 vs 0.04

200-yr 33.5 0.5 17.36 vs 16.64 -5.37 vs 5.87 -1.98 vs 2.48

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 16 refers).

The results as shown in Table 4 indicate that the final design can achieve the design flow intercepting requirement. Moreover, the flow is more balanced in the twin cell diversion box culvert when compared with the initial design.

NO2 (CH350.5)

NO1 (CH342.0)

NO3 (CH359.0)

O3 (CH322)

Storage Tank

O3, O4: Openings proposed in the initial design

NO1, NO2, NO3, NO4, NO5, NO6, NO7: New openings

NO4 (CH388)

NO7 (CH413.5)

NO6 (CH405)

NO5 (CH396.5)

O4 (CH422)

Figure 16: The CFD model of the twin box culverts and overflow weir (Final)

Table 4 Results for Final Design

Rainstorm Event


3/s)

Flow Split (m3/s)


To storage tank Remain in culverts L1 L2 L3

50-yr 30.5 1.9 16.2 vs 16.2 1.49 vs 0.41 0.4 vs 1.5

10-yr 17.8 1.7 9.55 vs 9.95 1.87 vs -0.17 0.56 vs 1.14

200-yr 30.8 3.2 17 vs 17 2.08 vs 1.12 0.48 vs 2.72

To assess the accuracy of the CFD results, a simplified 1-D analysis of stormwater flowing over the side weirs to the storage tank was carried out according to the following equations:

• 1-D momentum equation for flows over side weirs where

so = Initial friction gradient sf = Final friction gradient U = Initial velocity V = Final velocity β= Energy Coefficient Q = Discharge B = width of free surface A = Flow area g = Acceleration due to gravity q = rate of lateral outflow

• The rate of lateral outflow is related to the static head where

CD = Coefficient of discharge g = Acceleration due to gravity p = pressure head

3

2

20

1

2

gA

BQ

gA

qQ

V

Uss

dx

dy

x

xf

β

β

−

−+−

=

2/3)( pygCq D −=

149.0

0

303.0

0

0868.0

0D

p

y

py

L

p

py149.065.0C

−

−−=

−

y = height of water surface

• The flow rate Qx can be obtained by integration

where

Q0 = Initial flow rate q = rate of lateral outflow CD = Coefficient of discharge g = Acceleration due to gravity y = height of water surface p = pressure head

Table 5 Results of 1-D Calculation and CFD Model

Rainstorm Event

Inflow to Storage Tank (m3/s)

CFD Result 1-D Calculation* Difference

50-yr 30.4 29.8 3.5%

10-yr 17.8 19 6.7%

200-yr 30.8 32 4.9%

*Assume the flow split between the two culverts is evenly distributed and the flow is unidirectional

Based on the above counter checking, the rates of stormwater flowing into the storage tank for each case are generally consistent. A wider difference was identified in the 10-year rainstorm event. The reasons behind this might be due to the more uneven flow distribution in the two cells of the diversion box culvert under this rainstorm event and the 1-D calculation cannot simulate the complicated backflow phenomenon. Underground Tank Design

The prime purpose of the underground tank is to store the design interception. The conventional philosophy is that a deeper tank with smaller plan area would be more favourable since it would occupy a smaller site area and hence less disturbance to the public during and after construction. In this project, however, more emphasis has been put on the sustainability of the HVUSSS in determining the configuration of the tank. Since automatic “movable” crest weirs will be adopted for the overflow weir system, the weirs could be fully lowered after the rainstorms, thus providing an opportunity to drain off a large portion of stored water by gravity from the tank to the drainage system if the tank itself is shallow enough. This arrangement enhances sustainability by saving the energy that would be consumed by pumping stored water. To achieve the objective of draining off stored water as much as possible by gravity, a shallower underground storage tank is proposed having due regard to ensuring minimum disturbance to the public facility above the tank and the invert level of the diversion box culvert, which is governed by

∫∫ −+=+=L

xD

L

xx dxpygCQqdxQQ

2/300 )(

the existing upstream and downstream box culverts. Under the latest design configurations, more than two-thirds of the stored water could be discharged by gravity and the remaining volume would be emptied by pumping. An additional advantage of a shallower tank is that pumps with a lower pump head can be chosen.

Conclusions

The challenge of the HVUSSS is to alleviate the flooding problem in a high-density and well-developed low lying urban area. In HVUSSS, an innovative, sustainable and cost-effective solution was developed to relieve the flooding problem in the Happy Valley catchment. It is believed that the adoption of the real-time controlled “movable” weir system in the underground stormwater storage tank has set an example for developing sustainable drainage measures in the future. In addition, the adoption of an integrated advanced modelling technique has greatly enhanced the accuracy of the analysis of the hydraulic performance of the system. Due to the special topographical characteristics and constraints of the site, there was a need to design a special inlet structure and underground storage tank with movable weir penstocks for the satisfactory functioning of this important flood alleviation project. With the special tailored design of the inlet structure, the flow inside the diversion box culvert can be balanced, stabilized and retarded in velocity, and hence the flow intercepting performance of the underground storage tank via the weirs can be assured. The state-of-the-art 3-dimensional CFD hydraulic modelling technique has been applied to allow a better understanding of the behaviour of the flood flows in the hydraulically complex structures in the HVUSSS project. The implementation of the HVUSSS will ensure the future social-economic growth and the safety of the public by improving the level of flood protection to the required standard to cater for rainstorm events of 1 in 50 years in the Happy Valley catchment.

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