RELATED APPLICATIONS

This application derives priority from New Zealand patent application number 794333 filed on 10 November 2022 with WIPO DAS code F94D incorporated herein by reference.

TECHNICAL FIELD

Described herein is a self-cleaning stormwater apparatus and method for operation of same. More specifically, a stormwater management system for attenuation/detention of stormwater is described comprising an overflow system configured to self-clean debris from a flow restrictor orifice.

BACKGROUND ART

Stormwater management as referred to herein refers to the process of detaining stormwater in a tank and slowly releasing the detained stormwater back into the stormwater system. Stormwater is held (detained) for long enough to prevent overflow into rivers and streams, preventing bank erosion and harm to aquatic life. Additionally, stormwater detaining may act to protect properties, roads, bridges and other infrastructure from flooding.

The need for stormwater management is growing, particularly in urban areas where more undeveloped land is covered by hardscapes such as bitumen, concrete and steel. Grass and plants on undeveloped land is permeable and, in most circumstances, capable of absorbing stormwater and rainfall events. Hardscapes tend to be impermeable or poorly permeable and as such, stormwater is directed away from the area of falling and concentrated as it runs off the hardscape.

Stormwater management via a tank is commonplace and can be an effective means of stormwater flow management. A purpose of stormwater management via a tank, may be to achieve 'stormwater mitigation' or 'stormwater neutrality', equivalent to returning the stormwater run-off from a site to the same or a lower level than prior to site development, as if the rain was still falling on undeveloped 'green' areas, even after homes, patios and driveways have been built. A civil engineer may calculate the flow of stormwater egress from these impermeable surfaces and then know how much attenuation/detention is required to account for the difference.

One drawback of existing stormwater tank designs is that the tanks require regular maintenance as the talk outlets may be become blocked by debris. Debris such as sticks, leaves, soil particles and the like may enter the tank and then be drawn to the tank outlet following the natural flow of stormwater as it drains from the tank. Since the tank outlet is typically a small orifice opening, out flow may easily be blocked by even small leaves or other smaller sized debris. Councils may require regular maintenance checks of stormwater tanks but compliance with such requirements may be poor.

In addition, when property ownership changes, new owners may be unaware of where the stormwater system is, or how to maintain it. As a result, maintenance of such stormwater systems may be poor, even if the owner is dutiful in wanting to meet Council requirements.

The problem of poor stormwater tank management was highlighted by a report conducted by an independent engineer on behalf of the Auckland City Council of fifty stormwater management devices. The report found that only one of the fifty devices was functioning normally. Further investigation found that this one device was the only one which had regular user-invoked maintenance. The remaining 49 stormwater management devices were effectively rendered non-operational relative to their initial design and performance parameters due to debris to blocking and hence, no useful detention occurring.

NZ 511142 discloses a rainwater tank cleaning system. However, the purpose of this system is to collect rainwater for re-use and comprises a device configured to remove the build-up of sediment from rainwater tanks. In particular, the device sucks debris out of the tank en-masse whenever overflowing from the outside of the overflow pipe and is intended to regularly overflow to maintain regular cleaning of the tank base. This device is not configured to clean debris that may occur from inside the overflow Pipe.

Figure 1 shows an exist commercial tank outflow arrangement. When the orifice plate is blocked by debris, the water volume rises in the tank until the overflow is reached at which point, stormwater flows from the tank continuously via the overflow path. There is no means to clean the orifice other than the manually access the tank and orifice opening to move away debris. Figure 2 shows an alternative overflow system. Like in Figure 1, if the orifice is blocked by debris, the water volume in the tank rises until over the overflow line at which point, water then flows via the overflow line.

From the above background, it may be appreciated that it may useful to provide a stormwater management apparatus and method that is configured self-clean in the event of an outflow blockage. Further, it may be useful to return the tank to normal detention functionality post debris removal or after a high rainfall event with minimal or no user input or, at least provide the public with a choice.

Further aspects and advantages of the self-cleaning stormwater apparatus and method for operation will become apparent from the ensuing description that is given by way of example only.

SUMMARY

Described herein is a self-cleaning stormwater apparatus and method for operation of same. More specifically, a stormwater management system for attenuation/detention of stormwater is described comprising an overflow system configured to self-clean debris from a flow restrictor orifice.

In a first aspect there is provided a stormwater management system configured to self-clean in an event of an outlet blockage, the stormwater management system comprising: a tank comprising a tank volume, a tank inlet and a tank outlet, the tank configured to receive and detain a stormwater volume therein; the tank outlet comprising: a first flow path for stormwater exiting the tank volume via an orifice with an opening to the tank outlet, wherein the orifice is configured to restrict flow of stormwater through the opening from the tank outlet via the first flow path; a second flow path configured to detour stormwater from a point upstream of the orifice and directing stormwater to the tank outlet at a point downstream of the orifice; wherein: the orifice is located at least partly within a protrusion and the protrusion is positioned to extend at least partly upstream of the orifice during first flow path flow and, also to extend at least partly into the second flow path; and in the event of stormwater flow via the second flow path, stormwater flow across the protrusion results in a venturi effect across the protrusion and orifice that urges any trapped debris from the orifice and protrusion and into the flow of stormwater via the second flow path.

In a second aspect there is provided a method of operation of a stormwater management system configured to self-clean in an event of an outlet blockage, comprising: selecting the stormwater management system substantially as described above; and, in a first mode of operation for a range of stormwater flows from nil to a predetermined maximum, having stormwater enter the tank via the tank inlet and, as a stormwater volume increases in the tank, water flows via the first flow path, through the orifice and to the tank outlet; in a second mode of operation where high stormwater flow occurs above the predetermined maximum, having stormwater enter the tank via the tank inlet and, as a stormwater volume increases in the tank, water flows via the second flow path, avoiding the orifice and to the tank outlet; and in a third mode of operation where debris blocks or limits flow via the first flow path, stormwater enters the tank via the tank inlet and, as a stormwater volume increases in the tank, water flows via the second flow path and to the tank outlet and in doing so causes debris to move from the orifice at which point, stormwater flow commences again via the first flow path.

Advantages of the above include active cleaning and reduced user-required maintenance of the stormwater management system. For example, in an event the orifice is obstructed or blocked with debris, the stormwater management system allows overflow to the tank outlet and in doing so, optimises flow over the orifice to cause removal of debris from the orifice and return the stormwater management system to normal detention functionality with minimal or no user input.

BRIEF DESCRIPTION OF THE DRAWINGS

Further aspects of the self-cleaning stormwater apparatus and method for operation will become apparent from the following description that is given by way of example only and with reference to the accompanying drawings in which:

Figure 1 illustrates a cross-section front elevation view of a prior art stormwater management system;

Figure 2 illustrates a cross-section front elevation view of an alternative prior art stormwater management system;

Figure 3 illustrates an example of the assembled system in a perspective view;

Figure 4 illustrates an end elevation of the system as seen from the tank outlet opposing end;

Figure 5 illustrates a front elevation view of the system about section A noted in Figure 4;

Figure 6 illustrates a detail front section view of detail C shown in Figure 5;

Figures 7 to 10 illustrates schematic diagrams of the stormwater management system filling up with stormwater and the first flow path and second flow path along with a visual scenario of flow when the orifice is blocked by debris;

Figure 11 illustrates a close up view of the venturi effect and optimised water flow that occurs at the junction or interface between the orifice and protrusion i.e. stormwater orifice self-cleaning component;

Figure 12 illustrates a series of schematic diagrams of one example of a vortex generator;

Figures 13a, 13b and 13c illustrate a series of schematic diagrams of examples of a vortex generator in the used to test for improved siphon activation requiring lower flow rate of water in the pipe connection;

Figures 14a, 14b and 14c illustrate a table and graph of various shapes and dimensions of vortex generators tested for optimal siphon activation;

Figure 15 illustrates a table of results and graph of various shapes and dimensions of reducers tested for optimal siphon activation;

Figures 16a, 16b and 16c illustrate a table of results and graph of various dimensions of lip design vortex generators tested for optimal siphon activation; and

Figure 17 illustrates a cross sectional view of an exemplary 3D printed self-cleaning stormwater management system and installation to a stormwater tank with stormwater flow.

DETAILED DESCRIPTION

As noted above, described herein is a self-cleaning stormwater apparatus and method for operation of same. More specifically, a stormwater management system for attenuation/detention of stormwater is described comprising an overflow system configured to self-clean debris from a flow restrictor orifice.

For the purposes of this specification, the term 'about' or 'approximately' or 'substantially' and grammatical variations thereof mean a quantity, level, degree, value, number, frequency, percentage, dimension, size, amount, weight or length that varies by as much as 30, 25, 20, 15, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1% to a reference quantity, level, degree, value, number, frequency, percentage, dimension, size, amount, weight or length.

The term 'comprise ¹ and grammatical variations thereof shall have an inclusive meaning - i.e. that it will be taken to mean an inclusion of not only the listed components it directly references, but also other non-specified components or elements.

Stormwater Management System

In a first aspect there is provided a stormwater management system configured to self-clean in an event of an outlet blockage, the stormwater management system comprising: a tank comprising a tank volume, a tank inlet and a tank outlet, the tank configured to receive and detain a stormwater volume therein; the tank outlet comprising: a first flow path for stormwater exiting the tank volume via an orifice with an opening to the tank outlet, wherein the orifice is configured to restrict flow of stormwater through the opening from the tank outlet via the first flow path; a second flow path configured to detour stormwater from a point upstream of the orifice and directing stormwater to the tank outlet at a point downstream of the orifice; wherein: the orifice is located at least partly within a protrusion and the protrusion is positioned to extend at least partly upstream of the orifice during first flow path flow and, also to extend at least partly into the second flow path; and in the event of stormwater flow via the second flow path, stormwater flow across the protrusion results in a venturi effect across the protrusion and orifice that urges any trapped debris from the orifice and protrusion and into the flow of stormwater via the second flow path. Tank

As noted, the tank comprises a tank inlet and a tank outlet. The tank is configured to receive and detain a volume of stormwater therein.

The tank may be configured to be buried underground. The tank may be located below a hardscape.

The tank may have a tank top closest the hardscape and an opposing tank base. The tank may be elongated with opposing ends. The tank may have a volume that stormwater may be detained in.

The tank may be generally elliptical cross-section or ovoid in shape.

The tank inlet may be located at one end of the tank. The tank inlet may be located at a tank top.

The tank outlet may be located at an opposing end of the tank to the tank inlet.

The tank outlet may be located below the tank inlet. The tank outlet may be located towards the tank base.

Tank Outlet

As noted the tank outlet discharges stormwater from the tank. Discharge may be via the first flow path or the second flow path. The first flow path and the second flow path may be located intermediate the talk volume and a tank outlet.

The first and second flow paths may be configured to provide both stormwater detention, slowing of flow of stormwater to the outlet and self-cleaning of the orifice.

First Flow Path

The first flow path may be a pipe. The first flow path pipe may be generally elongated. The pipe may be straight along a longitudinal length of the pipe. The first flow path may exit the tank about a low point in the tank volume. The pipe may have a first end and a second end, the first end linked to the tank and the second end linked to the tank outlet.

Orifice

As noted above, stormwater flow exits the tank volume via an orifice to the tank outlet. The orifice is configured to restrict flow of stormwater from the tank via the first flow path.

The orifice may restrict flow of stormwater through the first flow path via a reduced opening size relative to an opening size of a wider flow path of the first flow path. For example, the first flow path maybe a pipe with a selected diameter and the orifice may have an opening with a smaller diameter than the pipe diameter. The smaller diameter may interfere with first flow path stormwater flow thereby reducing a flow rate of stormwater from the tank and to the tank outlet.

Other flow restriction designs may be used that impede or slow stormwater flow e.g. an orifice having a tortious path. Reduced opening size is however a common and often regulatory specified approach.

The orifice may have a 1-20, or 5-15, or approximately 10mm diameter opening. This size may be a regulatory requirement.

The orifice opening may be generally centrally located within the first flow path. The opening of the orifice may be generally centrally located within the first flow path when stormwater flows via the first flow path. The orifice opening, if circular, may in cross-section, be aligned with a centre of the first flow path.

The orifice may have a slightly elongated tubular length. The longitudinal axis of this slightly elongated tubular length may be coaxial with at least part of a longitudinal axis of the first flow path.

The orifice opening may be located in a plate, wall, pipe or partition.

Second Flow Path

As noted above, the second flow path is configured to detour stormwater from a point upstream of the orifice and directing stormwater to the tank outlet to a point downstream of the orifice.

The second flow path may be a pipe, the pipe being U-shaped with a first end coupled to the first flow path at a point upstream of the orifice and a second end coupled to one end of the first flow path about the tank outlet.

Stormwater flowing via the second flow path may not pass through the orifice.

The second flow path pipe size may be generally the same as the first flow path pipe size.

The second flow path may intersect the first flow path via an intersection located immediately upstream of the orifice. In cross-section, the intersection may form an inverted T-shape junction with the second flow path intersecting the first flow path orthogonally to a flow direction of stormwater via the first flow path.

Flow of stormwater in this example may be about a generally horizontal plane when flow occurs via the first flow path. Flow via the second flow path may be in three or more sections from a first generally vertical plane initially from upstream of the orifice, to a second generally horizontal plane and, a third generally vertical plane back to the tank outlet. Other shapes and configurations of second flow path may be used.

An inlet of the T-shape junction may be located below the surface of the stormwater where the debris may typically be caught. In this way, the T-shape inlet may initially block any debris, whereby any debris that reaches the T-shape inlet may be forced downwards and then upwards to reach the orifice i.e. the T-shape inlet provides a physical barrier to the orifice.

Protrusion

As noted, the orifice comprises a protrusion. The protrusion is positioned to extend at least partly upstream of the orifice during flow of stormwater along the first flow path. The protrusion may also be positioned to extend at least partly into the second flow path of stormwater when stormwater flows via the second flow path. In the event of detouring stormwater flow via the second flow path, the protrusion is configured to impose a directional flow or a venturi effect across the protrusion and orifice.

Multiple protrusions may be used, although in the inventor's experience, a single protrusion may be sufficient to generate the described directional flow or venturi effect.

As noted, the protrusion shape and position in the second flow path may create a directional flow or venturi effect. This directional flow or venturi effect may be in a direction away from the orifice location. This directional flow or venturi effect may be in an orthogonal and/or a reverse direction to the first flow path of stormwater through the orifice. Owing to the directional flow or the venturi effect, debris obstructing the orifice may be urged away from the orifice and entrained into the second flow path. Once removed from the orifice, flow of stormwater along the first flow path may again commence at a normal rate and flow of stormwater via the second flow path may reduce and halt as stormwater flows lower and can be catered for by flow via the first flow path.

Protrusion Shape

The protrusion may be contoured, shaped and dimensioned to create or enhance the directional flow or venturi effect.

In one example, the protrusion may have a shape that causes a change in water flow about the protrusion when stormwater flows via the second flow path. This shape may mimic that of a wing or sail to create and pressure change across the protrusion. A point of maximum pressure difference over the protrusion may coincide with the location of the orifice on the protrusion. A design of this nature may be useful to maximise the directional flows or venturi effects of the protrusion and hence maximise any forces acting on debris blocking the orifice.

Example shapes of the protrusion may be semi-circular or semi-elliptical in cross-section. In 3- dimensional form, the protrusion may have a part ovoid or part spherical shape although, other shapes may be used such as a wing cross-section shape. Polygonal cross-section forms such as hexagonal shapes may also be used. A key feature of the protrusion shape and form is that the protrusion acts to position the orifice directly in the path of stormwater when stormwater flows via the second flow path. Initial tests have found that a part ovoid shape, or part elliptical cross-section shape, may be optimal and may mimic the flow dynamics a wing of an airplane. The smooth exterior surface of the part ovoid shape may also be useful to minimise debris such as a leaf adhering to the protrusion or orifice. Without being bound by theory, the part ovoid shape may create an uneven surface that water must pass over causing a change in pressure. Furthermore, the protrusion moves the orifice into the line of water flow and away from a flow surface. This may reduce the likelihood of debris adhering to the wall around the orifice. This design may also reduce turbulence for debris to adhere to or get caught in. This is because the part ovoid shape may provide a smooth radius around the orifice where water may pass without creating turbulence.

Foreign Material Self-Cleaning

As may be appreciated from the above, in the event that the orifice blocks with debris, the second flow path activates and, as a result, cleaning of the orifice opening occurs. Once cleared, the first flow path again becomes the primary flow for stormwater to the tank outlet. No manual cleaning or user maintenance is needed to maintain optimal function. The design also still caters for high stormwater flow events when an overflow arrangement via the second flow path governs tank stormwater flow to the tank outlet. The stormwater system is therefore self-cleaning via the selected design and optimisation of the system fluid dynamics.

Vortex Generator

Optionally, the apparatus may further comprise a vortex generator.

As noted above, the second flow path is configured to detour stormwater from a point upstream of the orifice and direct stormwater to the tank outlet at a point downstream of the orifice. Flow may be directly from the second flow path to the tank outlet. Alternatively, flow via the second flow path may pass through the vortex generator before reaching the tank outlet.

The vortex generator may be configured to interfere with a flow of stormwater along the second flow path and, in doing so, create a siphon effect. Stormwater flow via the second flow path may pass through a vortex generator before reaching the tank outlet. The siphon effect created during the flow of stormwater through the vortex generator may impose a further drawing force on debris located in the second flow path. Expressed another way, the vortex generator causes or enhances a pressure difference between the orifice and second flow path and thereby effectively sucking any debris located in or about the orifice from the orifice.

The vortex generator may be configured to interfere with the flow of stormwater along the second flow path. Interference may be by restricting flow of stormwater along the second flow path to the tank outlet. The vortex generator may comprise a restricted opening size relative to a wider size of the second flow path that stormwater must pass through to reach the tank outlet. This restricted opening size may be larger than the orifice opening size. Assuming the vortex generator has a circular restricted opening size, this may be approximately 20-80, or 30-70, or 30-60, or 30-50, or approximately 40mm in diameter.

The vortex generator may also be conical in shape. The narrowed restricted opening size may be downstream of a wider upstream transition point that the walls of the flow restrictor taper from. Tapering of the walls of the vortex generator may transition smoothly from the wider upstream transition point to the downstream narrowed restricted opening size. The shape of the vortex generator may be termed an oblate spheroid. The vortex generator may have an oblate spheroid shape tapering from a size of the second flow path to the restricted opening size.

The inventor has found that vortex generator may allow the orifice cleaning directional flow or venturi effect to commence with an earlier flow rate of stormwater through the second flow path than if no vortex generator is used. Without being bound by theory, the vortex generator reduces the second flow path size to create tension and an air pocket, thus a vacuum and siphon effect results. Preliminary testing has found that the minimum flow rate required for a vacuum to occur in the system without the vortex generator is HL/s (equivalent to a high rain fall event). By comparison, if the vortex generator is used, the flow rate needed to induce a vacuum was measured as being 5L/s with the vortex generator included representing a marked lowering in flow rate. This has the benefit of urging debris removal at a much lower flow rate of second flow path flow ideally resulting in earlier debris removal and return to normal system operation and avoidance of unwanted continuous over flow via the second flow path.

Apparatus Operation

The stormwater management system may be configured to efficiently operate where stormwater flow and stormwater levels may occur in a one in two-year rain event (0.750 - 0.786L/s). Modelling shows that the stormwater management system may function with a water flow as low as 0.482 L/s and up to 5L/s (with a vortex generator present). These figures are based on a height drop of water level of more than 750mm . This may compare with a flow rate in the range of 11 - 18L/s when a vortex generator is not used.

It may be appreciated that the water flow and water levels for optimal operational conditions may be based on a range of calculated rainfall events for specific locations at which the system is used.

Method of Operation

In a second aspect there is provided a method of operation of a stormwater management system configured to self-clean in an event of an outlet blockage, comprising: selecting the stormwater management system substantially as described above; and, in a first mode of operation for a range of stormwater flows from nil to a predetermined maximum, having stormwater enter the tank via the tank inlet and, as a stormwater volume increases in the tank, water flows via the first flow path, through the orifice and to the tank outlet; in a second mode of operation where high stormwater flow occurs above the predetermined maximum, having stormwater enter the tank via the tank inlet and, as a stormwater volume increases in the tank, water flows via the second flow path, avoiding the orifice and to the tank outlet; and in a third mode of operation where debris blocks or limits flow via the first flow path, stormwater enters the tank via the tank inlet and, as a stormwater volume increases in the tank, water flows via the second flow path and to the tank outlet and in doing so causes debris to move from the orifice at which point, stormwater flow commences again via the first flow path.

Advantages

Advantages of the above include active cleaning and reduced user-required maintenance of the stormwater management system. For example, in an event the orifice is obstructed or blocked with debris, the stormwater management system allows overflow to the tank outlet and in doing so, optimises flow over the orifice to cause removal of debris from the orifice and return the stormwater management system to normal detention functionality with minimal or no user input.

An outcome of the above is that maintenance of the stormwater management system is reduced or even avoided. For example, where the orifice is obstructed or blocked with debris, the stormwater management system allows for overflow, whilst optimising flow over the orifice to remove debris and return the tank to normal tank functionality with minimal or no user input.

The embodiments described above may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, and any or all combinations of any two or more said parts, elements or features.

Further, where specific integers are mentioned herein which have known equivalents in the art to which the embodiments relate, such known equivalents are deemed to be incorporated herein as if individually set forth.

WORKING EXAMPLES

The above described self-cleaning stormwater apparatus and method for operation are now described by reference to specific examples and the following item numbering:

1 Stormwater management system

2 Tank

3 Tank volume

4 Tank inlet

5 Tank outlet

6 Tank top

7 Tank base

8 Opposing tank ends

9 Stormwater volume in the tank

10 First flow path

11 Orifice

12 Second flow path

12a First generally vertical plane initially from upstream of the orifice

12b Second generally horizontal plane

12c Third generally vertical plane

13 Protrusion

14 T-shape junction

15 Vortex generator

16 Vortex generator opening

20 Debris

30 Venturi area

EXAM PLE 1

With reference to Figures 3-6, the overall shape and form of the stormwater management system 1 is shown.

Figure 3 shows the assembled system in a perspective view illustrating the tank 2, tank inlet 4, tank outlet 5, tank top 6, top base 7 and tank opposing ends 8. The first flow path 10 is generally shown being the bottom horizontal plane pipe. The second flow path 12 is generally shown being the U-shape pipework.

Figure 4 shows an end elevation of the system 1 as seen from the tank outlet 5 opposing end 8 of the tank 2. As shown in this view, the tank 2 has a top 6 and base 7. The first flow path 10 shows the tank outlet 5 and the second flow path 12 is partly shown. Section A is noted in Figure 4 which is discussed further below with respect to Figure 5.

Figure 5 shows a front elevation view of the system 1 about section A noted in Figure 4 above. Figure 5 shows the tank volume 3 being the space enclosed within the tank top 6, base 7 and ends 8. The flow paths 10, 12 are more clearly seen in this section view. The first flow path 10 runs through the orifice 11 and in a generally horizontal plane to the tank outlet 5, the first flow path shown in an even dash line. This is the default detention flow path that is preferred in order to slow stormwater flow from the tank 2. Also shown in Figure 5 is the second flow path 12 in an uneven dashed line. The second flow path 12 diverts stormwater flow about the orifice to the tank outlet 5. The second flow path 12 follows a U- shape path, initially upwards in a vertical plane 12a orthogonal to the orifice 11, then about a 90 degree turn to a horizontal plane 12b and about a further 90 degree turn to a vertical plane 12c again linking to the tank outlet 5 and discharging on the other side of the orifice 11. At the exit from the second flow path 12, there may be located a vortex generator 15 described further below.

Figure 6 is a detail front section view of detail C shown in Figure 5. This detail view shows the orifice 11 opening and obstruction 13 in more detail. As shown, the obstruction 13 may be part ovoid in shape with a hemispherical cross-section as shown. The orifice 11 opening may be located about an apex of this obstruction 13 shape so as to move the orifice into flow of stormwater when second flow path stormwater flow occurs. Positioning in this manner enables debris blocking the orifice 11 to be forced off the orifice 11 and be entrained into the second flow path 12 of stormwater and discharged from the tank 2. The design of the stormwater management system 1 allows for second flow path 12 overflow, whilst optimising flow over the orifice 11 to remove debris and return the tank 2 functionality to nominal with no user input. The step by step operation of the system 1 is described further below.

Referring to Figure 7, the tank 2 will start filling up with stormwater 9 until it reaches a volume level with the orifice 11. Stormwater in the tank 2 slowly flows from the tank 2 via the outlet 5 via the first flow path 10, this being a normal mode of operation. In a larger stormwater event, the stormwater volume 9 in the tank 2 will rise as shown in Figure 8 with first flow path flow remaining to detain stormwater flow from the tank 2. In the event that the orifice 11 is blocked by debris 20 as shown in Figure 9 and Figure 10, the tank 2 will continue to fill with stormwater volume 9 and flow will commence via the second flow path 12 and divert from the blocked first flow path 10.

Once the stormwater water level reaches the top of the second flow path 12 overflow pipe connection 12a, the stormwater will start to spill over the other side of the second flow path via sections 12b 12c to the tank outlet 5 as shown by the direction of water flow arrows. The stormwater flow and water levels that cause second flow path stormwater flow may be design to occur in a one in two-year rain event (0.75L/s) but has been calculated that function with a water flow as low as 0.48L/s, but only if the height drop of water level is more than 750mm. This second flow path is also activated when the orifice is blocked by debris 20. These water flow and water levels for optimal operational conditions were based on a range of calculated rainfall events for the three major cities of New Zealand including Auckland, Wellington and Christchurch. Other data may be used for other locations to best design the system 1 to suit anticipated conditions at an installation site.

Once the second flow path 12 is activated the stormwater flow creates a venturi effect 30 pulling debris 20 from the orifice 11 and the protrusion 13 increasing the flow of the stormwater and creating an even greater venturi effect 30. This venturi effect as stormwater flows past the protrusion is illustrated by the arrows of flow 30 shown in Figure 11. The changed flow directions change stormwater pressure which in turn generates the directional flow or venturi effect urging debris from the orifice 11.

EXAM PLE 2

With reference to Figures 12 to 17, an overall schematic diagram of the optimal 40mm opening 16 vortex generator 15 is shown with the tests conducted along with the results represented graphically and in table form (Figures 13-16) versus other shapes and configurations to establish an optimal design and show the effects of varying shaped vortex generators 15.

As described above, a vortex generator 15 allows the second flow path to activate with reduced stormwater second flow path flow than if the vortex generator 15 is not present. Without being bound by theory, the vortex generator 15 reduces the second flow path 12 diameter about the vortex generator 15 to create tension and an air pocket, thus a vacuum giving a siphon effect about the second flow path 12.

In this example, preliminary testing found that the minimum flow rate required for a vacuum to occur in the system 1 without the vortex reducer 15 may be approximately HL/s (i.e. high rain fall) compared with 0.482L/s - 51/s which is the minimum flow rate for the second flow path 12 when the vortex generator 15 is present.

Preliminary results show that a vortex generator 15 shaped and dimensioned as shown in Figure 12 being a lip with a 40mm opening 16 allows for a minimum flow rate of rainwater for operation of 0.482L/s. Therefore, testing conducted at this stage has found the lip to be an optimal design. The shape may also be termed an oblate spheroid shape tapering from a size of the second flow path 12 to the restricted opening size prescribed by the vortex generator 15.

Figure 17 shows a visual model of the system 1 in the event of second flow path 12 stormwater flow and a full tank 2 of stormwater indicating stormwater flows (arrows X) and turbulence in the tank 2 volume and flow via the second flow path 12. This system 1 model was based on use of the vortex generator 15 shown in Figure 12.

Aspects of the self-cleaning stormwater apparatus and method for operation have been described by way of example only and it should be appreciated that modifications and additions may be made thereto without departing from the scope of the description herein.