How do you build a more durable river or creek crossing? Here are some principles to consider when building a crossing:
- Streams have active and less active erosion and deposition points. These are a part of their natural development. A crossing at a naturally stable erosion site is likely to be more durable that one at a less active site.
- The crossing should not be constructed on a bend as the main flow typically accelerates around the outside of the meander and can cause bank scouring. By constructing a crossing downstream of a bend, the energy of flow is less concentrated. The crossing also reduces the flow velocity by creating a pool upstream that back floods and dampens the power of these flows.
- The selection of a crossing location should also consider the existing bed profile. The siting should take advantage of naturally high points along the channel, thus reducing the height to which the crossing must be constructed.
- Naturally erosion and deposition points of a stream can be destabilised through removal of vegetation, uncontrolled stock access, solid structures placed in the stream, removal of logs, straightening of the channel, poorly sited vegetation and other bank earthworks. Crossings in these areas are at greater risk.
- The bed material and conditions at the site should also be taken into consideration. Boggy areas should be avoided. Existing river sills (in-stream channel blockages) or where the bed is naturally hard can be enhanced to form a crossing.
- The siting should also consider where existing livestock trails are located which could be stabilised to formalise a crossing point. At the crossing site, the banks of the channel should not be too steep. Ideally the grade should be less than 1:4 for the banks to be stable.
- Crossings should be aligned perpendicular to flow.
- The design should reflect the natural cross-sectional area and flow capacity requirements of the channel.
- Downstream scour protection should be provided. This may take the form of a rocky riffle, vegetation on the flanks, rock paving or gabions at culvert outfalls.
- Bank stabilisation works may need to be carried out at both the inlet and outlet of the crossing.
- Revegetation of the area will provide long term stability. Sedges and rushes can be strategically planted to armour the 'toe' of the banks and the abutments of crossings. Trees and shrubs provide reinforcement to the banks.
- The Department of Environment recommends attempting to construct crossings as low as practicable. As the height of the crossing is increased, so is the associated risk during flooding.
- The placement of culverts allows a dry crossing, if desired, at low flows. It is recommended that numerous smaller diameter pipes or culverts are installed beneath a crossing, rather than a larger structure that requires the level of the crossing to be raised.
- The crossing should not be unnecessarily high or create a large difference between the maximum water level able to pass through the culverts and the water level as it overtops the crossing.
- Crossings need to be shaped and armoured, especially at the ends and flanks, so that flood flows overtop the crossings with a minimum risk of damage.
- A downstream scour apron or rock gabion can be constructed to have a riffling effect to that dissipates the energy of flow rather than allowing it to concentrate and jet through the outlet of the culverts or pipes.
Proposals to construct larger stream crossings of a significant height should be referred to the Department of Environment for specific advice on a case by case basis.
Construction of Crossings
The Department of Environment's general advisory notes for land managers on the design and construction of low level waterways crossings on farms are outlined below. The objective of the guidelines is to protect waterways from bank erosion and degradation of the riverine environment by providing formalised crossing and watering points and controlling livestock access to the riparian zone. The preferred management strategy is to keep livestock out of the river channel by providing off-stream livestock watering points.
Unrestricted livestock access also results in poor water quality and the loss of productivity. Correctly designed crossings can also provide environmental benefits during normal and low flow events such as:
- Having a riffling effect which aerates the water and provides a variety of riverine habitats,
- Allowing for fish and crustacean passage,
- Maintaining river pools that are important in providing summer refuges and
breeding areas for certain species such as marron.
- Contributing to channel stability by controlling the velocity of flow and reducing the downstream movement of sediments into the river.
Engineering design guidelines
The following are guidelines for designing and constructing stable livestock/vehicle crossings that do not adversely affect the flood conveyance or stability of the channel.
Design guidelines will vary according to the condition of the waterway. Where a channel is actively eroding, further protection works may need to be conducted.
- Rock floodway crossings should be constructed with a maximum upstream slope of 1:4.
- The rock scour apron of the crossing should have a maximum downstream slope of 1:10 (1:20 to allow passage for fish).
- The height of the crossing can be slightly higher than the existing bed level, but should not exceed 300 millimetres.
- Design advice should be sought for installations that raise the height of the channel by more than 300 millimetres above the existing natural level or significantly inhibit downstream flow of the waterway.
- The crossing should ideally be constructed as bed hardening and should not significantly change the cross sectional shape or the horizontal natural shape of the floodway.
- On soft bed material, the bed should be dug out to a depth of at least the height of the crossing and the rock laid to below bed level to provide resistance to flow passing beneath the crossing and causing destabilisation. Alternatively, a geo-textile can be used to stabilise the bed by burying the upstream edge about one metre into the bed and banks and overlaying with rock.
- The crossing should be dug into the banks of the channel to avoid outflanking, but use as little disturbance to the consolidated bank material as possible.
- The rock banks should extend to the full height of the embankment (above high water level) and downstream for at least one channel width. This will stabilise the access points to the crossing and prevent scouring of the banks. The banks of the crossing should also be graded to a maximum 1:4 slope. The lowest point of the crossing should be in the centre of the channel.
- Both sides of the crossing should be fenced across the full width of the waterway, connecting to riparian fencing so that livestock cannot have access along the channel to the riverbanks.
- Hanging fences or electrified fences can be used across waterways to allow flood flows, limit accumulation of debris and reduce potential damage to the fences. Gates should be installed at the entry and exit to control livestock access through the crossing.
Timing of the works
Ideally construction should be undertaken earlier in summer and disturbed vegetation around the crossing should be reinstated prior to the following winter. The site should be monitored through winter and modified if required.
Where to build a crossing
The site selection for construction of the crossing is very important. Incorrect siting or alignment of crossings can initiate or accelerate channel erosion. Crossings should always be built along a straight section of the river or at the crossover point in the middle of a meander. The crossing should be built perpendicular to the main flow channel to direct flows to the centre of the channel. Particular sites will of course have other factors that may demand a compromise of some sort, but if the above principles are applied as far as is practicable then damage risk will be reduced.
Crossings constructed of rock tend to be most successful. Rock gabions, logs or other material could alternatively be used for construction. These materials would need to be sufficiently anchored into the bed and banks, for example by securing to posts driven into the bed, and the banks stabilised using rock.
If the crossing is being used as a watering point, a rough, variable surface should be created so that livestock do not remain in the river channel for prolonged periods of time. Well-graded quarry rock should be used to construct the crossing to form an interlocking rock matrix. Suitable rock sizing should be selected to resist the forces of the flow on the bed material.
If the crossing is used only to provide access across the river, then the surface can be smoother by overlaying with smaller gravel and compacting.
A PDF document covering stock crossing construction is available from the DoE website.