Key Components of Drainage
Driveway crossing and vegetated swales
A vegetated swale is a vegetation-lined channel used to convey stormwater in lieu of pipes, and remove coarse and medium sediment. Swales may be located within private property, parkland, easements, carparks and along roadway corridors (i.e. within footpaths and center-medians). They can be integrated into the landscape character of an area to enhance its aesthetic value.
Driveway crossings should follow the profile of the swales or they could be raised above the invert of the swale (e.g. like a bridge deck or culvert). A check-dam could be used to spread and slow the flow. See the figure as shown below. The preferred driveway surface should be a porous pavement.
When the existing driveway needs to be replaced or a new access way is required, it is the landowner's responsibility to do so and to get Council’s approval first (working in a utility corridor).
Overland flow paths and flood plains
Overland flow paths are routes stormwater follow during heavy storm events. They are usually illustrated in a Catchment Management Plan or on Council record plans. They are the overland route taken by any concentration of, or significant sheet flow, of stormwater on its way to a flood plain.
A flood plain is an area of land that is adjacent to a river or a stream that is going to be flooded in a severe flood event. Usually, there are restrictions around what can and cannot be done within this area.
When existing, natural or manmade drainage is overloaded, stormwater flows via overland flow paths. These flow paths are a vital component of Hurunui's drainage system, but often they are blocked by solid fences, sheds, unsuitable plantings or even permanent buildings. This can cause secondary flooding around the flow path.
Landowners should not block the flow paths on their property, as they may be held personally responsible for any flooding damage caused by a deliberate blocking of the flow path.
Water course/Urban Stream
A water course is a channel through which water flows or collects – it can be natural, modified or artificial, piped or open in nature.
Any construction works within the water course are tightly controlled by the Resource Management Act and would require discussion with our planning engineers prior to any works commencing.
We don’t know about every water course in Hurunui, as many are not recorded or run through private properties. If you have a water course running through your property, please remember that it is your responsibility to maintain the water course from where it enters your property to where it exits the property.
An urban stream in a garden can be an asset that makes the garden more aesthetically appealing for you and for native wildlife, provided it is managed properly.
Proper stream management reduces flood risk and erosion, both being considerable concerns for many streamside residents. The best way to manage a stream is to let nature do the work for you. The right selection of plants, combined with careful placement of rocks and/or logs, is a more environmentally friendly solution for a stream than hard-engineered concrete channels and/or culverts.
See Manage a Stream for more information.
On-site stormwater management devices
The on-site stormwater management devices are there to provide:
- water quality treatment with final disposal to surface water or to ground soakage and/or infiltration
- peak flow and quantity reduction for sites, where final disposal is to surface water
On-site management devices are an integral part of any water-sensitive designs, that protect and incorporate natural site features into erosion and sediment control and stormwater management plans.
Property owners and developers can manage the stormwater discharge and protect the environment by using water sensitive design concepts and features. Some examples of on-site stormwater management devices are as shown below:
For some new subdivisions and older houses, roof water is discharged to the ground through soak pits (a covered, porous-walled chamber or rock-filled pit that allows water to slowly soak into the ground).
The efficiency of a soak pit depends on the permeability of the surrounding ground and the size of the installed pit. Continued maintenance is required for soakage systems, as silting up of the soakage media may occur over time.
Any design of soakage for rights-of-ways, parking areas or driveway accesses must contain calculations for soak pit sizing and may require specific design of a pre-entry sump.
Flood risk analysis and overland flow designs will be required to be assessed for all soak pit designs. When assessing flood risk and overland flow, no allowance for soakage capacity shall be assumed.
For a copy of guideline for Investigation and design of rock-filled stormwater soak pits in Amberley click here
Retention tanks collect runoff water from impermeable areas, hold it during the storm, and release it back into the stormwater network at a slower rate. This helps reduce the pressure on stormwater drainage systems through peak runoff times.
Rain tanks store rain water for later use, thus reduce reliance upon mains water use. This provides for economic or environmental benefits, and aids in self-sufficiency.
Put soakage trenches along the edges of paving or driveways, in areas with good soakage, to help and aid stormwater soakage into the surrounding ground.
Create rain gardens in areas with good soakage; like volcanic soils. These aid in capturing and detaining stormwater, and slowly let it soak away into the surrounding ground.
Rain gardens also filter out pollutants and contaminants in the stormwater, as the soil particles and plant roots can collect and retain the chemicals, before they reach any streams and coastal environments.
Similar to rain gardens, but they are grown on a roof, thus improving both insulation and stormwater management outcomes.
Using permeable paving for paths and driveways can significantly reduce the amount of stormwater that has to be managed, as they absorb stormwater into the surrounding ground.