In this paper the flow characteristics of stormwater are analyzed as it travels from a roof gutter down-pipe and the turbulent flow generated on entering an individual lot on-site stormwater detention (OSD) unit beneath a residential carport. Comparison was made between a full-scale model and computational fluid dynamic (CFD) simulations to determine the flow characteristics. These modular tanks with multi-unit chambers can capture the roof run-off from a 15-minute, 10-year return period storm. The results from the physical and CFD models matched well, suggesting that turbulent flow occurs when stormwater is directed to an individual lot stormwater detention tank. However, turbulence in the OSD was concentrated around the inlet, after which the pattern changed from turbulent to laminar flow. This work implies that the use of modular underground storage tanks is practical for managing stormwater from a roof.

INTRODUCTION

Impervious areas such as parking lots, paved streets, and building roofs produce stormwater discharges during rainfall events. Uncontrolled runoff can cause significant environmental problems (Mah et al. 2016). One control method for stormwater discharges is by on-site stormwater detention (OSD). Stormwater runoff can be slowed and the peak discharge reduced using OSD, which is expected to solve flash flood problems (Haberland et al. 2012; Mah et al. 2014). Such a system has been widely applied in Sydney, Australia, since 1991; detention storage is provided for stormwater on project development sites to limit rates of runoff (O'Loughlin et al. 1995). In previous OSD studies, centralized, large-scale control structures were used, such as wet ponds (Chang et al. 2013; Ahmad et al. 2014) and detention basins (Vorogushyn et al. 2012). In spite of that, the availability of empty land is decreasing in urban areas.

Because of the above, locally distributed control structures are gaining in importance as means of catering for physiographic and urban development characteristics (Hamel et al. 2013). There is extensive literature on best management practice covering various measures (Baker & Doneux 2012; Reynolds et al. 2012; Ritchie 2014) but there is no mention of the use of carports. This arises from different living styles.

In Malaysia, in general, a carport has become a necessity of house ownership. A large carport is a common strategy used by developers to attract buyers. Making use of the area provided by a carport, enables stormwater from the roof to be held in a detention tank beneath it. At the lot – i.e., single dwelling – scale, stormwater control measures have been reported as effective in reducing stormwater runoff (Loperfido et al. 2014). The cumulative effect is great when combining the small effects of individual lots.

One issue that needs to be dealt with before using OSD in such a way is the flow characteristics of stormwater directed to an OSD beneath a carport. High runoff rates can be generated from roofs during intense rainfall events and the flow is expected to be supercritical. Such thrusting, high speed water flows could cause congestion at the OSD inlet, so the system hydraulics need to be studied carefully.

In this paper, a semi-detached house is analyzed to gauge the effectiveness of OSD under the carport. Only stormwater generated on the front portion of the roof is considered. It drains to the OSD (Figure 1). As the roof's catchment area is 95 m2, it is within the limit of the Rational Method for small catchment below 800,000 m2 to calculate the runoff.
Figure 1

Typical carport space in Malaysian detached houses.

Figure 1

Typical carport space in Malaysian detached houses.

The type of housing design shown in Figure 1 is common in Malaysia. Column 1 – see Figure – would usually have a drainage down-pipe built into or attached to it. Using this, stormwater from the roof can be directed to an underground tank, as indicated. The OSD could be expanded as far as Column 2, in which case two inlets would be needed. The outflow could be directed to the house perimeter drain.

Design of OSD

To design a storage tank under such a carport, a reference point was taken based on the 4.33 m length of a typical car; and 4.625 m width – which would allow for two cars with room to open their doors and get between them. These dimensions provide the optimum surface area for the OSD, giving a useful area of 20.02 m2. As the span is over 4 m, several supports are needed for the beam, but this creates a hollow in the middle and the carport floor could collapse. So, instead of a single tank, modular tanks with multi-unit chambers were designed, to support the vehicles, etc.

A single modular unit consists of a top cover and bottom plate, with a hollow cylinder between them. The dimensions of these three pieces are shown in Figure 2. Both the top and bottom plates can withstand loads of up to 10 tonnes, and are hexagonal for stability.
Figure 2

Dimensions of the OSD as designed.

Figure 2

Dimensions of the OSD as designed.

METHODS

OSD screening model

The Storm Water Management Model (SWMM) can show the efficiency of the proposed OSD in terms of total inflow (runoff). SWMM can also be used to model special elements like the storage units, which were used to represent an OSD in this study. Using the orifice outlet in SWMM enables it to show the runoff volume and peak flow, so the model was chosen to provide insights into the proposed OSD.

Normally, as the effective catchment is small, a 15-minute duration, 10-year return period storm is modeled. The OSD is designed for a rainfall intensity of 180 mm/hr – i.e., 124% more than normal. Stormwater runoff generated from the roof drains to the detention tank and is released slowly through the orifice to the discharge outfall. The space excluding the solid concrete in the unit is taken as the unit's storage volume.

Scenario simulations with and without the OSD were ran, and the results recorded. Special attention was paid to cross-checking the computed results with the analytical calculations to ensure that the parameters used in SWMM were appropriate. The runoff generated for the scenario without OSD matched the analytical calculation, with Q for both being 4.74 ℓ/s. Unlike the rational method, which yields a maximum runoff value, SWMM can produce a time-series runoff hydrograph, as shown in Figure 3.
Figure 3

Runoff hydrographs, with and without OSD, for a 15-minute duration, 10-year return period storm.

Figure 3

Runoff hydrographs, with and without OSD, for a 15-minute duration, 10-year return period storm.

As can be seen, the system including an OSD reduces peak flow is by 95% compared to that without. The peak storm discharge was also delayed, occurring after 45 minutes. The model showed clearly that use of an OSD beneath the carport stored the runoff water temporarily, thus reducing congestion in the drainage system.

Laboratory setting

As the modular units are identical, repetitive results are expected so only a portion of the modular tank was used in the lab study – see Figure 4, where the selected section is shown in relation to the full tank. This section, one sixth of the whole (18 full-scale modules), was used to verify the results from the CFD model.
Figure 4

Modular layout as used in the laboratory experiment.

Figure 4

Modular layout as used in the laboratory experiment.

The set up shown should mean that the geometry, kinematics and dynamics of the ‘OSD’ in the experiment are close to those of the real system. With respect to geometry, the materials and structures used were identical. Kinematic similarity is achieved when the streamline patterns are geometrically similar, i.e., on the flow pattern and velocity. Dynamic similarities relate to the forces involved. Since the experiments use the same fluid – water – the density, viscosity, etc., are similar. The experiments were, however, repeated with different flow velocities into the OSD.

Flow velocities were measured using a propeller-type flowmeter, which can record velocity changes over short time periods. The results were compared with those from the CFD for verification. The experiment showed that, although the flows around the OSD inlet are chaotic and fast, the water still spreads evenly through the storage tank. The experiments showed that no congestion was caused by the rapid flows or obstruction in the multi-unit chambers.

CFD

The role of CFD in engineering predictions is so strong that it is now viewed as the third dimension of fluid dynamics. The problems of fluid dynamics are too complex to solve by direct calculation, so numerical methods using computer simulations are used – i.e., CFD. As turbulent flow tends to be nonlinear and chaotic, particular care must be taken when setting up the rules and initial conditions for these simulations. Simulating chaotic flow pattern and velocity changes over short time periods is widely accepted as appropriate.

The set up in the CFD simulation shown in Figure 5 reflects the actual system shown in Figure 4, and concerns internal flow between modular units.
Figure 5

3-Dimensional layout.

Figure 5

3-Dimensional layout.

RESULTS AND DISCUSSION

A 3D layout of the model is depicted in Figure 5. The inlet region is critical, particularly the first 433 mm from the inlet (Figure 6). The calculated maximum velocity, 0.3 m/s, in this region matched that measured with the flowmeter.
Figure 6

Front cut plot of the OSD with inflow of 4.74 ℓ/s.

Figure 6

Front cut plot of the OSD with inflow of 4.74 ℓ/s.

Three scenarios were tested, with inflows of 4.74, 3.69 and 2.77 ℓ/s, respectively, as in Figure 7. The turbulent energy is dissipated quickly 0.1 m from the inlet. As the inlet is submerged, the influent water is slowed when it hits the accumulated water in the flow path. Because of this, turbulent flow only occurs in the vicinity of the inlet, with laminar flow everywhere else.
Figure 7

Distance from inlet vs turbulent dissipation.

Figure 7

Distance from inlet vs turbulent dissipation.

CONCLUSIONS

Turbulent flow occurs when stormwater is directed to an OSD beneath a carport. The investigation found that flow is congested for a short time, although an intense rainfall event produces high runoff rates from roofs. However, turbulence in the OSD is concentrated near the inlet, elsewhere in the tank all flow is laminar. Turbulence in the flow decreases rapidly with distance from the inlet. Because of this, flow congestion is not a problem as any turbulence will be dissipated by the multi-unit OSD chamber design.

ACKNOWLEDGEMENTS

The first author was under the sponsorship of Zamalah Pascasiswazah UNIMAS (ZPU). This project was aided by the Research Acculturation Grant Scheme (RAGS) RAGS/TK01(01)/1315/2015(09) by the Malaysian Ministry of Higher Education (MoHE).

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