Land development reference 5.3.1
Stormwater connections can have many and diverse impacts on waterway health, including:
Direct impacts associated with construction. These include disturbance to bank material, vegetation, the stream bed and the permanent alteration of the bed and banks through the provision of rock beaching.
Direct impacts of high velocity outflows. Stormwater outlet designs which allow high velocity outflows can cause the scour of bed and bank material. Depending on the nature of the connection and on the robustness of the receiving stream, ongoing disturbance can result during successive storm events.
Indirect and broader scale impacts. Suspended load sediment generated at poorly designed connections can impact on water quality. The coarser sediments generated by such connections will contribute to potential adverse changes in stream bed and bar form.
Open waterways and channels
Drainage connections to our managed waterways are owned and maintained by a diverse range of individuals and organisations, as is the land adjoining our managed watercourse. These arrangements make for complex management. However, we do have responsibility for the health of waterways into which the subject drains discharge. These guidelines seek to ensure drainage connections are designed to mitigate impacts.
Objectives and principles
This guideline is based on the principle drainage connections shouldn't adversely impact on waterway health, assisting us to meet our river health objectives. This means designers shouldn't rely on the receiving watercourse to absorb or assimilate the impacts of urbanisation.
The energy of drainage flows should be adequately dissipated prior to the flow entering the receiving watercourse, not expended eroding the watercourse. Drainage connections shouldn't require associated rock lining of the bed and opposite bank of the stream to prevent erosion.
Drainage flow of an energy that's incompatible with the receiving water may also carry a sediment load that's detrimental to the stream. The dissipation of the flow energy and hence deposition of the sediment upstream of the drainage connection is preferred.
Outlet flow velocity
For the purposes of this discussion we define the drainage connection as where the flow enters the receiving water, as shown below.
Figure 1: drainage connection
Outlet flow velocities from drainage connections should be limited to those in the following table, depending on the stream type and vegetation criteria described.
Table 1: outlet flow velocities
Silt and sand bed and bank streams, prone to ongoing erosion.
|1.0m/s||Existing high velocity flow associated with steep streams containing gravel and cobble bed and intact riparian vegetation.|
|1.5m/s||Robust streams with basal t and or stiff clay in the bed and banks.|
|2.0m/s||Robust systems with exposed bedrock in bed and banks with low likelihood of scour.|
In the absence of an assessment of stream type and vegetation condition, the default maximum permissible outlet velocity is 1.5 m/s. This may be below the self-cleaning velocity for the pipe. In this case an energy dissipation structure may be required to connect the pipe to the waterway.
However, in accordance with water sensitive urban design, it's preferable the stormwater system be designed not to capture such sediment, rather than being designed to transport such sediment to the receiving water.
Overall, the governing principle is, if the stormwater system is supplying high energy flow and sediment that are incompatible with the receiving water, such impacts shouldn't be imposed on the stream but resolved upstream.
The distance from the closest stream bank of the waterway to the pipe outlet/endwall should be a minimum of 3x the pipe diameter.
Drainage connections will regularly be sited on overland flow paths. The detailed design of the connection should ensure the structure is stable with overland flows spilling over it.
All energy dissipation necessary to achieve the target discharge velocities is to be provided within the drainage system, upstream of the connection to the waterway. Thus, the distance between the pipe outlet and the waterway may need to exceed 3x the pipe diameter to accommodate the energy dissipation structure.
It's recommended energy dissipation structures be of the rock chute style. Vegetation (grasses, sedges, shrubs) can be established alongside rock chutes to improve visual amenity.
If sediment supply to the stormwater system isn't adequately managed at the source, sedimentation will occur within the energy dissipation structure and maintenance may be required.
Open and closed connection
Open watercourses provide better river health outcomes than piped systems and should be adopted wherever possible. An open watercourse treatment should be adopted across parkland adjacent to our managed open waterway where:
the stormwater drainage systems services an area greater than 30 hectares
urban development has resulted in an increase in low flow within the sub catchment exceeding the low flow produced by a 30 Hectares rural or forested catchment
Multiple and single connections
The number of stormwater connections to waterways are to be minimised by:
limiting connections to natural overland flow entry sites for new development areas
where practicable, using and upgrading existing stormwater connections within areas of existing developments
To underground pipes
Where the proposed inlet pipe has a diameter d, and our drain has a diameter D, such that d is greater than D/3, the follow these guidelines.
The stormwater connection necessitates the construction of a junction pit in accordance with either of our following drawings (depending on pipe size).
If a connection to our existing Pit is required, an assessment of required pit modifications (including the relocation of step irons) shall be undertaken prior to construction.
In both of the above cases, the stub shall be constructed so the inlet connection(s) are angled 45o or as close as practicable downstream to our Main Drain, and the inlet connection shall be made to the base of the junction pit. It may be necessary to re-shape the chase to satisfy this requirement.
Where d is less than D/3, the inlet connection(s) must be made as a pipe to pipe connection. In this case, the connection(s) must be angled 45o or as close as practicable downstream to our drain and be set in the middle third of the drain.
No inlet connections are permitted to enter into the crown of the drain. All domestic junction/side entry pits and gross pollutant traps are to be located a minimum of two metres from the outside edge of our Main Drain. Side entry pits, and other structures such as energy dissipaters and gross pollutant traps shall not be constructed on our drainage system without prior approval.
Where the drainage system consists of a high flow channel with a low flow underground drain, the stormwater connection shall be made to the low flow pipe. The above requirements are applicable.