Flow characteristics of individual lot stormwater detention

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.

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.

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.

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.

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.

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.

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.

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.

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).