Infiltration of surface water into the foul water system
Surface water entering the foul water system through gully dishes can cause problems for sewer network utility operators in flood-prone areas. Several parts of the Building Code and its Compliance Documents relate to this situation. Compliance with both E1 Surface Water and G13 Foul Water is required.
Building Code Clause E1 Surface Water
The Building Code Clause E1 Surface Water covers what is commonly called stormwater. There are two relevant parts.
- E1.3.1: Surface water, resulting from an event having a 10% probability of occurring annually and which is collected or concentrated by buildings or sitework, shall be disposed of in a way that avoids the likelihood of damage or nuisance to other property.
- E1.3.2: Surface water, resulting from an event having a 2% probability of occurring annually, shall not enter buildings. Performance E1.3.2 shall apply only to Housing, Communal Residential and Communal Non residential buildings.
For convenience, in this article we will call:
- an event with a 10 percent probability of occurring annually the 1 in 10-year flood
- an event with a 2 percent probability of occurring annually the 1 in 50-year flood.
E1 Acceptable Solution
The E1 Surface Water Acceptable Solution E1/AS1, states:
1.0.1: This Acceptable Solution is limited to buildings and sitework having a catchment area of no more than 0.25 hectares and which are:
- a) Free from a history of flooding
- b) Not adjacent to a watercourse
- c) Not located in a low lying area, and
- d) Not located in a secondary flow path.
Comment: Boundary fences and other site development must not significantly hamper the flow of surface water from the site.
E1/AS1 clearly states what is required to stop water entering buildings, within the scope of the Acceptable Solution that excludes flood-prone land.
2.0.1: Suspended floors and slabs on ground shall be at least 150 mm above the finished level of the surrounding ground immediately adjacent to the building, and:
- a) For sites level with or above the road, no less than 150 mm above the road crown on at least one cross section through the building and roadway.
- b) For sites below the road, no less than 150 mm above the lowest point on the site boundary.
The Acceptable Solution does not contain a solution for protection of other property, such as the sewer network utility operator's system.
G13 Acceptable Solution
G13/AS2 states the following about surface water entering the foul water system.
3.3.1: All gully traps shall be constructed to prevent the ingress of surface water and foreign bodies likely to cause a blockage, shall be located within the legal boundary of the land on which the building is erected, and shall have:
a) The overflow level of the gully dish no less than:
- i) 25 mm above paved surfaces, or
- ii) 100 mm above unpaved surfaces
Comment: It is imperative that the waste pipe connections to the gully trap remain watertight to prevent the ingress of ground/surface water.
The Department is consulting on an amendment to G13/AS2 3.3.1(a) as follows.
3.3.1: (a) The overflow level of the gully dish no less than the height required to prevent the ingress of surface water, resulting from an event having 10% probability of occurring annually, from entering and:
- i) 25 mm above paved surfaces, or
- ii) 100 mm above unpaved surfaces
Comment: 1. It is imperative that the waste pipe connections to the gully trap remain watertight to prevent the ingress of ground/surface water. 2. Gully dish height is required above the 1 in 10 year event to protect the network utility operator's sewer system from surcharge problems.
In summary
All new building work must comply with the Building Code. In this case, both Clauses E1, Surface Water, and G13, Foul Water, apply. To achieve this:
- the floor level must be as given in E1/AS1, providing the location is not within a flood prone area
- if in a flood prone area, E1/VM1 applies. Depending on circumstances, this requires the floor level to be 150 mm or 500 mm above the 1 in 50-year flood level
- the top of the gully dish must be above the 1 in 10-year flood level to meet E1.3.1. In addition, the Network Utility Operator may have further requirements to avoid damage to its network (see E1.3.3(e)).