Multiple areas of open space can detain a large cumulative quantity of stormwater runoff across a catchment.
Existing ecosystems such as gullies and floodplains can also provide natural detention areas. Detention functions can be enhanced through minor grading or construction of a berm or wall to provide temporary ponding. Dam regulations should be considered to ensure all appropriate regulations are complied with.
Property owners and the wider community must understand that ponding is intended to occur in the area and should be made aware of the extent and frequency of flooding of a ponding area. Because of the multipurpose nature of areas being used, safety is an important consideration and the period of inundation should be less than 24 hours depending on access requirements.
In terms of hydrological function, above-ground detention (AGD) should have a controlled discharge outlet and an overland flow or large capacity pipe for larger storm events. These overflows also provide a safety valve, should an outlet from an AGD become blocked. Plants within ponding areas can provide some water quality benefit but must have tolerance to both temporary inundation and extended dry periods.
Challenges and solutions
The table below describes some of the common issues and constraints relating to AGD. In all circumstances, there is a potential design solution, which must be balanced against other objectives for the project.
Standing water may be unsightly, have smells, and be a vector for insects
AGD are temporary storage practices, where surface ponding is designed to occur for a maximum 24 hour period between storms to prevent any stagnation. Any ponded water can be screened by vegetation and/or rocks.
AGD on hard surfaces should be regularly maintained by sweepers to prevent ‘sediment rings’ from occurring.
Access may be restricted by AGD
Consult with stakeholders to manage expectations for public facilities.
Alternative access options should be provided for any areas that will be inundated. Structures or facilities should remain above ponding areas to prevent water damage and allow continual use.
Velocities and water depth may cause a hazard
Where overland flow paths are accessible to the public (roads and pedestrian routes), ensure that there is adequate signage to warn the public of the potential hazard.
Flooding may extend to other properties
Make sure there is sufficient freeboard (see PAUP rules) to buildings in the 100 year ARI event floodplain and allow for a dedicated overland flow path.