How do you determine hazard? The simple answer is that the purveyor determines it, but every jurisdiction is different. Every city has its own list of named examples for what constitutes the hazard threshold. Here is an example from Washington DC. They also stipulate that if the anticipated use, as you know it, is not named explicitly, then they reserve the right to make the decision during plans review. It’s also important to understand that you cannot, as a designer, over-protect the incoming water service. This means there is no penalty for providing the higher degree of protection.

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Different depths of soil cover (or no cover at all) can be achieved using other types of pipes. The cover specified is measured from the top of the pipe to either the finished ground level or, in the case of paved or concreted areas, to the underside of the paving or concrete.

excavation for drains adjacent to existing footings must be within the area described in Figure 3.1.3.1 as being safe for excavation.

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A designer may specify one of 2 types of backflow prevention devices. First, the Double-Check Valve Assembly. This is often shortened to DC and is typically thought of as the appropriate solution for low hazard conditions. Second, the Reduced Pressure Zone Valve Assembly, often shortened to RP or RPZ, is thought of as the appropriate solution for high hazard conditions.

You’ve heard these phrases and acronyms repeatedly. If you are a designer, you feel sure that you are expected to know what they mean. Here’s a quick rundown on what they are and how they differ.

1Check Valve

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The ground beneath suspended floors must be graded so that the area beneath the building is above the adjacent external finished ground level and surface water is prevented from ponding under the building (see Figure 3.1.3.3).

The Reduced Pressure Zone Valve Assembly (Figure 2) consists of two independently operating check valves just like the Double-Check as well as a hydraulically operated differential relief valve located below the first check valve.

The appropriate slab height above finished ground level and the slope of the external finished surface surrounding the slab may vary depending on:

The Double-Check Assembly (Figure 1) was developed in the 1950s for the fire industry. Any time the pressure on the property (downstream) side exceeds the pressure on the city (public) side, the two redundant check valves close stopping the backwards water flow. There are two problems with the Double-Check backflow preventer. First, no remedy exists in the event of a malfunction of the valve closures or if debris in the water line causes the valves to not close completely. Second, and this is the big problem, there is no way to know when such a failure has occurred without conducting a full test by a qualified professional.  The Double-Check is a closed system. It has no method of revealing whether the internal check valves are functioning properly and no way to detect the presence of debris that is impeding full closure.

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where site conditions exist that create a need for subsoil water to be diverted away from footings, basements, retaining walls etc — sub-soil drainage in accordance with 3.1.3.4; and

the external finished surface surrounding the slab must be drained to move surface water away from the building and graded to give a slope of not less than (see Figure 3.1.3.2)—

3/4backflow preventer

Where a subsoil drainage system is installed to divert subsurface water away from the area beneath a building, the subsoil drain must—

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The stormwater drainage system must be designed so that any overflow during heavy rain periods is prevented from flowing back into the building.

When a flow stop occurs, both check valves close. At that moment, the relief valve opens and evacuates the water between the valves. (Figure 3) Some think that this event defines the limit of how much water can ever flow from the RPZ into a drain. This is not so.

Watch the video below for an example of RPZ flooding and to see just how much water is discharged when a small amount of debris becomes lodged in the device.

where underground drainage from roof areas is required or permitted — underground stormwater drainage in accordance with 3.1.3.5; and

The design and installation of subsoil drainage systems should take into account the nature of the soil and the anticipated water level, quantity and movement. In some cases, detailed investigations involving excavations, field observations and soil tests may be necessary to determine the appropriate solution. Typical subsoil drain configurations are shown in the following diagrams.

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The position and manner of discharge of the stormwater drainage system must be to the satisfaction of the appropriate authority.

Now consider a full failure of the #2 check valve, like one that might occur if the device is knocked out of round or has a mechanical failure. It’s essentially the same event with the exception that due to the larger valve opening, there is actually a higher flood rate. Now you have water flowing through the relief valve at full head pressure. Again, if this is a large or multi-story building, that’s a lot of water very fast.

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Note to Figure 3.1.3.1: Any excavation below the area defined as being safe for excavation will need additional protection measures to be determined by appropriately qualified persons.

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Road drainage design has as its basic objective the reduction and/or elimination of energy generated by flowing water.

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Where an alternative drainage system is proposed as a Performance Solution to that described in Part 3.1.3, that proposal must comply with—

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Consider a flow-stop situation, like one that might naturally occur at the end of the day (Figure 4). If you look closely, you can see that a small pebble has lodged in the #2 check valve. Now imagine some sort of back siphon event over night. Perhaps a nearby building fire leaches the water back with the draw of a fire hydrant; or a pump station fails; or a water main breaks. Because the #2 check valve is not closing, all the water that has been delivered to the building will continue to flow out the relief valve until the private lines are cleared. If this is a four story building, that’s a lot of water.

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The manner of discharge of stormwater drainage systems includes consideration of discharge points. Some examples of discharge points which may be acceptable to the appropriate authority are:

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In clay soil, subsoil drains can alter the long-term moisture content in the soil, adversely affecting the building foundation by removing or, in some cases, introducing water. In such conditions, subsoil drains should only be used where there are no other options for dealing with subsoil water.

The main thing to take away from this is that both backflow prevention device types perform the same function when they are operating properly, but only the RPZ is designed to protect the public water supply by disposing of any backwards-flowing water if any of the check valves or the relief valve fails.

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the level of discharge from the silt pit or sump into an impervious drainage line not less than 50 mm below the invert level of the inlet (see Figure 3.1.3.4); and

Subsoil drainage systems may need to be installed where subsurface water movement could damage buildings or cause loss of amenity through the build up of excessive moisture or lateral water pressure. Typical locations of subsoil drainage systems are on the uphill side of cut and fill sites, adjacent to deep footings, behind retaining walls and adjacent to basement walls.

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In the spirit of reconciliation the Australian Building Codes Board acknowledges the Traditional Custodians of country throughout Australia and their connections to land, sea and community. We pay our respect to their elders past and present and extend that respect to all Aboriginal and Torres Strait Islander peoples today.

This hydraulic valve and its placement makes the RPZ virtually fail-safe but it comes at a cost to the area around the device.

roofs in areas subject to 5 minute duration rainfall intensities of not more than 255 mm per hour over an average recurrence interval of 20 years (as per Table 3.5.3.1a to Table 3.5.3.1h ) where a drainage system is required; and

25 mm over the first 1 m from the building in low rainfall intensity areas for surfaces that are reasonably impermeable (such as concrete or clay paving); or

Performance Requirement P2.2.1 is satisfied for drainage if it is designed and constructed in accordance with AS/NZS3500.3.