In addition to distinctiveness, condition, and strategic significance, watercourses have two additional parameters that determine the biodiversity units achieved: watercourse encroachment and riparian encroachment.
Watercourse encroachment refers to any features that negatively affect the natural function of a watercourse. It may be features that engineer the banks of a watercourse or features that enter or cross the watercourse. Criteria and examples for selecting the appropriate watercourse encroachment value are provided in the table below:
Watercourse encroachment category | Multiplier | Description | Examples |
No encroachment | 1 | Less than 5% of the bank length is an engineered bank revetment and there is no encroachment into the channel | N/A |
Minor | 0.8 | Between 5% and 20% of the bank length is an engineered bank revetment or there is encroachment across up to 10% of the watercourse width at any one point | Headwalls, jetties, pontoons |
Major | 0.5 | More than 20% of the bank length is an engineered bank revetment or there is encroachment of more than 10% of the watercourse width at any one point | Weirs, headwalls, bank revetment |
N/A - Culvert | 0.68 | Only applies to culverts | Culverts |
Riparian encroachment refers to any feature that negatively affects the quantity, quality, or function of riparian habitats. The riparian zone is defined as the area of habitat that occurs after the top of the watercourse bank (where the bank slope ends). Habitats that may otherwise be terrestrial can be present in the riparian zone and may provide different ecological, hydrological, and geomorphological functions. Riparian zones are a set width depending on the watercourse type, as shown in the table below:
Habitat | Riparian zone width |
Priority rivers | 10 metres from the top of each bank |
Other rivers and streams | 10 metres from the top of each bank |
Canals | 10 metres from the top of each bank |
Ditches | 5 metres from the top of each bank |
Culverts | Not applicable |
Criteria and examples for selecting the appropriate riparian encroachment value are provided in the table below:
Riparian encroachment category | Definition for rivers and canals | Definition for ditches |
No encroachment | No encroachment within 10 metres of the bank top | No encroachment within 5 metres of the bank top |
Minor | Any encroachment between 8 and 10 metres from the bank top or where the footprint of encroachment occupies between 0 and 10% of the riparian zone between 4 and 10 metres from the bank top | Any encroachment between 4 and 5 metres from the bank top or where the footprint of encroachment occupies between 0 and 10% of the riparian zone between 2 and 5 metres from the bank top |
Moderate | Where the footprint of encroachment occupies between 10 and 25% of the riparian zone between 4 and 10 metres from the bank top | Where the footprint of encroachment occupies between 10 and 25% of the riparian zone between 2 and 5 metres from the bank top |
Major | Any encroachment between 0 and 4 metres from the bank top or where the footprint of encroachment occupies more than 25% of the total riparian zone area | Any encroachment between 0 and 2 metres from the bank top or where the footprint of encroachment occupies more than 25% of the total riparian zone area |
In addition to enhancing by distinctiveness or condition, watercourses can also be enhanced by reducing the severity of watercourse or riparian encroachment.