Building and maintaining culverts

What is a culvert?

A culvert is a structure that allows water to flow under an obstruction (i.e. road or track) from one side to the other. In either case, they need to be properly sized and installed, and protected from erosion and scour.

Choosing your culvert

Landholders have a right to access property and the construction of culverts should be allowed provided the proposal satisfies the following criteria:

Small (<1m height) dry weather access culverts (low environment impact culverts)

Should meet standard design/specification outlined in this fact sheet. Design plans need to be lodged with a water affecting activity application to the landscape board for approval.

Larger all-weather culverts (significant environmental impact culverts)

Should be designed by a qualified engineer. Design plans need to be lodged with a water affecting activity application to the landscape board for approval.

What a culvert shouldn’t do

The construction of any culvert should not:

Increase erosion up or downstream,
Increase flooding up or downstream,
Cause alteration to natural flow regimes
Cause change to the natural timing or duration of ponding upstream
Cause loss of upstream/downstream connectivity of ecosystems
Cause detrimental impacts on water dependent ecosystems, for example habitat destruction, alteration of flows or structures affecting fauna migration.

Site selection

Aim to site a culvert across a straight section of the watercourse where the flow line and stream power is concentrated in the center of the watercourse (figure 1).

Avoid locating culverts:

At sharp angles to the stream
Near sharp bends
Near sections of unstable bank
Across sinuous, meandering waterways (as their migration rates may be increased due to changes in flow dynamics)
At sites that will require the removal of vegetation (important for bank stabilisation)

Culvert sizing

Culvert pipes / boxes should be designed to replicate the same area provided by the stream channel up to the flow depth of 0.5m.

Culvert pipes / boxes must not be perched above the streambed as this causes erosion downstream (higher stream velocities) and sedimentation upstream. This reduces access for normal fish passage and can cause ponding, upstream stagnation and water quality problems (refer figure 2).

Fish passage consideration

It is important to give consideration to fish passage requirements when selecting the type of culvert box or pipe, concrete or corrugated metal, single cell or multi-cell.

Requirements are as follows:

Where practical, align the culvert with the downstream channel to minimise bank erosion.
In urban areas, a multi-cell culvert usually requires a combination of elevated “dry” cells to encourage terrestrial movement, and recessed “wet” cells to facilitate fish passage.
Avoid using culverts on a waterway that has a gradient of more than 2 per cent (1:50). The gradient immediately downstream of the culvert should be less than 5 per cent (1:20) so fish can approach the culvert outlet. Unless specific advice from a fish ecologist indicates that a steeper slope will be satisfactory.
The culvert invert should be buried so a minimum of 20 per cent of the diameter (round culvert) or 20 per cent of the height (box culvert) lies below the channel bed.
The culvert should have at least 600 millimetres of space above the typical base flows so it is light enough inside that fish are not discouraged from entering and swimming through.
If possible, the culvert should be designed so its hydraulics are similar to that of the stream and the weakest fish species can swim through. The water depth should also allow the largest fish species to remain submerged.
Wet cells should have a minimum water depth of 50 to 200 millimetres to encourage fish passage.
As a first priority, effective flow area under the waterway crossing should be at least equal to the natural or existing flow area of the channel below the deck / crest level of the crossing (high flow design). Where this is not feasible, the second priority would be to design the culvert such that the effective flow area is at least equal to the natural or existing channel flow area below the roof of the culvert (medium flow design). A medium flow design also requires the depth of the deck slab to be minimised.
In all cases, the culvert should be designed to maximise the geometric similarities of the natural channel profile from the bed of the culvert up to a flow depth of 0.5 metres (low flow design).
Lining the base of the culvert with a rough concrete finish and / or natural substrate will increase turbulence and make it easier for fish to swim through. Velocities of less than 0.3 metres per second will allow most native fish to swim through a 5 metre long culvert. Placing small rocks along the base may also help other species (e.g. platypuses and water rats) pass through the culvert.
If a smooth bed culvert must be designed, then flow velocities for water depth up to 0.5 metres ideally should be no more than 0.3 m/s. Baffles or large angular rocks typical of the area can be cemented along the base of longer concrete culverts to reduce flow velocities and allow fish and invertebrates to pass through.
In sand and gravel-based streams, natural bed material should be placed along the bed of the wet cells or allowed to deposit in these cells. The hydraulic design of these culverts should allow for this added bed roughness which facilitates upstream fish movement.
In clay-based streams that do not experience significant movement of bed load sediment, artificial roughness units such as rounded stone, can be grouted across the bed of the wet cells to provide the desirable bed roughness and fish resting areas.
If fish passage is desirable during bankfull flows, then consideration should be given to the placement of artificial sidewall roughness units along the cell wall immediately adjacent to the channel banks.
Where conditions allow, construct pools at the inlet and outlet of the culvert to assist in the dissipation of flow energy and to act as resting areas for migrating fish. Fish resting pools should be at least two metres long along the direction of flow, be deep enough for fish to remain submerged, and contain rocks and vegetation to provide cover. Aquatic and riparian plants can provide shading.
If a low-flow channel is constructed within the base slab of the culvert, then ensure that the channel extends across the inlet and outlet aprons.
Debris deflector walls can be used to reduce the impact of debris blockages on fish passage while also reducing maintenance costs.
Maximise light penetration within the wet cells by maximising the height or diameter of the cells, and possibly by introducing skylights or grated stormwater inlets into the median strip of divided roads. These skylights are only required within the nominated wet cells.
To avoid the formation of a perched culvert and damage to the stream’s bed and banks, erosion at the outlet should be controlled with the use of rock protection and / or the formation of a stabilised energy dissipation pool.

Scour pool stabilisation

Preparation of the area downstream of pipes is needed to prevent the scour pool and bank erosion. This involves excavating to lower the creek bed, then using rock armour to fill the bed back to the existing level. This prevents scouring and erosion downstream of crossing when floodwaters fall over the top of the crossing. Downstream banks are also stabilised with armour rock to prevent bank erosion as floodwaters move over the top of the crossing and back into the watercourse.

To protect the streambed, rock armour should be placed directly downstream of the culvert and at the end of the scour pool. The end of the scour pool can be defined as the distance of 10 x the height of the culvert pipe or box, or if more than 1 pipe 13 x the height (refer to figure 3).

Height of the culvert

Small, dry weather access crossings are designed such that high flows and floodwaters will pass over and around the crossing and back into the watercourse downstream.

The middle section of the crossing should always be lower than the left and right bank to enable higher flows to remain within the channel and not back up around the structure. It must also enable the flood plains to “flood out” naturally and re-enter the stream without causing erosion.

The top of the culvert pipes must also be as close as practical to the top of the crossing to allow overtopping without causing the backing up of flow, which can cause erosion around the structure (refer to figure 4).

Construction of the culvert

Construction material must be clean graded fill using large rock (50cm), small rock (10-20cm) and a clean gravel for surfaces. Building site rubble is poor quality material and will erode.

To stablise the position of the pipes, use large armour rock around and between the pipes. Smaller rock can then used to provide fill over the width of the crossing.

Culverts are ideally constructed early during the summer period to allow the maximum amount of time for stabilising prior to winter. Crossings stabilise through compaction from stock activity and rainfall.

Use a small 2% cement mix in the gravel surface for the crossing. This assists in stabilising surface material and resists erosion of surfaces during periods when floodwaters are passing over crossings. The rubble surface / raceway over the crossing should stretch further than the top of the creek banks as this area may be boggy during winter months and with heavy stock activity (e.g. dairy) may become unstable.

Disclaimer

All landholders using this information do so at their own risk and are encouraged to seek engineering advice. While every reasonable effort has been made to verify the information in this guide, use of the information contained is at your sole risk. The Murraylands and Riverland Landscape Board recommends that you independently verify the information before taking any action.

This webpage is based on information developed by Landscapes Hills and Fleurieu and republished with permission.