Healthy Country   Leaky Weirs

Leaky Weirs

Leaky_Weirs_Article_2010

The Healthy Country Waterways Restoration Project focuses on defined areas within the Bremer and Logan Rivers and Lockyer Creek. These regions have a long history of agricultural use and today boast a wide variety of enterprises that rely on healthy waterways for productivity.

SEQ Catchment project officers have been working with landholders within their focal areas to implement innovative methods to reduce sediment and improve water quality. The project aims to trial a variety of designs to monitor their effectiveness and suitability to South East Queensland landscapes.

One such trial is the implementation of a series of leaky weirs to slow flood waters down by spreading them over floodplains and trapping sediments before they enter the river system. Leaky weirs aim to slow down runoff and spread it over the floodplain alluvial areas. By doing so, more of the sediment and nutrient is deposited on the flood plain, not passed to the major river systems and into Morton Bay.

*Image: Flows greater than the pipe capacity diverted to floodplain


Sediment_Leaky_Weirs_2010Two locations have implemented leaky weirs. In the Upper Bremer 3 weirs located at 100 m intervals pass flood waters over 10 ha of flood plain and out of the stream line which is starting to erode and destabilize the high fertility flood plain. The design has safely passed two storm events – one of 75 mm in 45 minutes and one of 80 mm in 20 minutes and fundamentally aims at re-instating flood plain flows. These weirs leak because they have 100 mm PVC pipe with slotted drainage pipe installed through the walls. 

The weirs are built with minimum keyways (keyways are the main design tool to stop leakage through the wall) and the walls are constructed from compacted clay. The leaky pipe ensures normal stream flow continues to pass down the stream bed and thus maintain the downstream stream bed aquatic environment. Most of the sediment is silt and clay fractions and deposition is likely to be greatest in the flood plain and velocity of flow lessens. Another advantage of the design is that recharge to the local alluvial groundwater system is increased.
 
*Image:
Flows greater than the pipe capacity diverted to floodplain

In Knapp Creek, two weirs have been built on a gully leading into the creek. The junction of the gully and creek is highly unstable and eroding. The upper weir is of a similar design to the Bremer Weirs but it is strongly keyed in to prevent tunnelling in the sodic materials. The lower weir does not have a pipe installed as the bed of this weir is sandy loam and therefore leaky.

Both weirs pass storm flows into what are termed Z (or zee) runoff control systems – a system of short contour banks linked to small dams that passes runoff to an alluvial plain some 300m away. At this site, there is a significant amount of coarse sediment and this will be deposited in the weirs with the finer sediment deposited in the Z system.

 

Leaky_Weirs_2010

*Image: One of three leaky weirs installed in the Upper Bremer Healthy Country Focal area to spread flood waters across floodplain and deposit sediment.

Article Bill Thompson
Photographs Amanda Bland & Bill Thompson