What is an overland flowpath?
This term describes the path followed by rainwater during a storm event.
An overland flowpath is a section of land that is reserved from development, and must be kept clear so that during storms water can flow over it until it reaches a stream. Overland flowpaths are generally the lowest point in a property and houses and buildings must be kept clear of the flowpath so as to not restrict its capacity.
Streams are similar to overland flowpaths, in that they are nature’s natural path for rain to flow during a storm. The difference between a stream and an overland flowpath is that a stream runs permanently.
Overland flowpaths, just like natural streams, collect and convey rainwater but only flow for the duration of a rain event. They are critical to the effective drainage of an area. It is common urban myth that streams are no longer necessary when development occurs and rainwater is drained by pipe systems.
Underground pipes are limited in their capacity and ability to deal with large rain storms. Pipes rely on surface inlets such as grated covers in streets, and roof gutters and downpipes to collect rainwater and convey it into the pipe network. However these systems are easily blocked by storm debris and are only big enough to cope with rain events which occur relatively frequently.
What is a one in one hundred year storm?
When describing the intensity of storms, it is usual to use the expression “a one in ten or one in twenty or one in one hundred year storm event”. The larger the number, the more intense is the rain, the greater the volume of rainwater generated, and the more severe the effects.
For example, a one in one hundred year event will generate about twice the amount of water which might flow from a one in two year storm. Urbanisation will increase the amount of rainwater generated in a storm event by a factor of two to three over that which existed when the land was in bush or pastoral cover. It can be readily seen that an urban area in a severe storm event will have to cope with four to six times the flow that might have been generated by the same storm when the land was “undeveloped”.
Depending on the size of the catchment area, that can translate into water flows which can amount in a developed urban area to tonnes of water a second.
Because underground pipes can only cope with relatively small rain events, we need areas where overflow from these pipes can run, that won’t flood houses or damage property. It is important during a storm that rain can get to streams and wetlands as quickly as possible so as not to flood houses. Overland flowpaths are designed to convey the rainwater generated in an extreme storm event. For that reason it is essential that they remain clear and are not blocked with fences or other structures. Fences can be readily smashed by water pressure and the danger is that while the water is building up against the fence or other obstruction it may be diverted away from the predicted path and cause un-wanted damage elsewhere.
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Overland flowpath – the route followed by a stream of water flowing down to a river or harbour outlet. Usually along a continuous depression similar to a natural stream
Catchment area – the area of land that slopes towards the low point that will constitute a stream whether natural or man-made. Usually measured in hectares for larger areas and square metres for smaller areas.
Run-off – any rainfall generated by a rainstorm and conveyed away in pipes and overland . Usually measured in litres per second or for larger flows in cubic metres per second.
Impermeability – the surface of the land within a catchment area is divided in natural soil – permeable and that covered in roof, roadway, and other concrete or asphaltic covered areas. The extent of impermeable surface will affect the run-off generated by a particular storm event.
GIS layer – Geographic Information System. A system of maps which can be overlaid providing a range of information on land related issues. For stormwater, the layers are overland flowpath showing the route and magnitude of the flowpath. Flood plains are a companion layer which in conjunction with the overland flowpath layer can indicate the potential extent of flooding in a one in one hundred year event. The two layers need to be applied simultaneously to determine if the issue to be resolved is merely overland flow or whether a flood plain analysis is required.