‘Flow Over’ structures 

These structures are designed to slow the passage of water within a stream channel. Flow Over structures are constructed from natural materials such as rock and logs, and they are permeable. They work by causing water to pool behind the structure, so that a pond is formed. The pond allows mobilised sediment to settle. Over time, this deposited material will rebuild the stream bed. By slowing the velocity of stream flow, this kind of structure also reduces the risk of stream bank erosion and enables aquatic and riparian plants to re-establish.  

The sequence of backwater ponds produced behind each structure causes water to be retained in that part of the watercourse for longer, which allows shallow groundwater systems beneath the stream bed to be recharged. As water builds up behind the structure, the water level of the channel is also raised. This allows water to more frequently spill onto adjacent floodplains. The structures are designed so that when the water reaches a certain point it ‘flows over’ the structure itself and carries on its downstream journey.  

The establishment of vegetation in and around the structure is essential to stabilise, strengthen and eventually ‘naturalise’ 'flow over’ structures over the long term. 

‘Flow Through’ Structures 

These are semi-permeable structures that are designed to increase surface roughness, slow the flow of water and capture mobilised sediment in overland flow down gullies and in-stream flows.   

'Flow Through’ structures are typically made from branches, logs and brush which are strategically placed and secured to logs, woven through wooden pins or braced against the upstream side of stakes. To ensure the structure is robust, it is important that heavier woody material is placed on the ground in contact with the soil surface across the full length of the branch and is heavy enough to withstand heavy overland flows. 

These are low-cost and low-risk structures that are quick to install. They are useful for raising the bed of minor incised gullies and providing immediate ground cover on scalded and/or eroded sites. ‘Flow Through’ structures are very suitable for sites that have limited ground cover and exposed soil, where sediment and organic matter is on the move. Pin weirs, brushpacks and brush mattresses are examples of ‘Flow Through’ structures. 

Flow Around Structures 

Flow Around’ structures are appropriate in alluvial systems where an intermittent stream once existed but is no longer active due to erosion upstream, or where overland flows are the dominant flow pattern. These structures are primarily designed to lift water from incised stream channels, and re-direct flows using a spillway so that it can again spread across the alluvial landscape. These structures can be constructed from materials found on site, most commonly earth, making them cost-effective and convenient. 

Water will collect behind the structure, forming a pond in which mobilised sediment and material will be deposited. Plants can then capture and utilise the mineral particles and organic matter that is suspended in that water.  

These types of structures are made of earth and not armoured with rock or log material, so any water that breaches the top of the structure will compromise its structural integrity. This means that ‘Flow Around’ structures must be built higher than the channel banks and higher than the expected peak flood level on the floodplain. As water builds up behind the structure, the flow will not overtop the structure but will rather flow via the spillway onto the floodplain. Earth bunds, contours and gully plugs are examples of ‘Flow Around’ structures. 

Protection structures 

Structures that protect landscape integrity are designed to stabilise actively degrading features and prevent them from compromising functional processes in the surrounding landscape. Such features include areas of exposed sodic soils, gully head cuts and unstable stream banks where overland flows re-enter the main watercourse. 

‘Protection Structures’ are made from rock or brush and are used to amour exposed soil and safely transport water to the gully floor or stream bed. They are usually located in areas where plants will never be able to establish themselves and designed to manage all the water the flows across that section of the landscape. When planning and designing this kind of structure, it is important to mitigate the risk that water may flow around a structure, sodic soils may be mobilised, or that the structure can deteriorate if it is poorly maintained.