Shanrahan Weir Removal
Sweco was commissioned by Inland Fisheries Ireland to undertake a hydrological and hydro-morphological assessment of the potential impacts of removing a weir along the river Duag at Shanrahan, in the county of Tipperary. In addition, Sweco provided a series of recommendations to mitigate the impact of the scheme in terms of flood risk and channel morphology.
This was a pilot scheme associated with the Environmental Protection Agencies (EPA) Reconnect project and the European Adaptive Management of Barriers in European Rivers (AMBER) project. The scheme contributed to a database of examples which will be used to establish best practice guidance for channel restoration to improve the integrity of Irish freshwater rivers and aid decision making. The project also contributes to the objectives of the Water Framework Directive (WFD) and potentially improve the status of the river.
The Shanrahan weir was identified by Inland Fisheries Ireland and the Irish Environmental Protection Agency as having a significant impact on habitats along the river Duag and impeding the movement of several important fish species, limiting the biodiversity of the river. The removal of the weir was proposed to improve connectivity by restoring the natural functioning of the river and provide additional social, environmental, and economic benefits to the local area.
Although the removal of the structure provides ecosystem benefits, there were significant consequences in terms of sediment dynamics and flood risk which needed to be assessed. There are several bridges nearby and there were concerns about the potential impact on bank stability and the system of riffles downstream, which provide valuable habitats for fish. Furthermore, upstream of the weir was a bifurcation channel used for agricultural purposes. Therefore, the scheme needed to ensure that this was not starved of flow.
A hydro-morphological assessment was undertaken by Sweco’s Geomorphology team and involved both desk and field-based techniques. An initial desk-based review of background information related to the geomorphology of the site was undertaken utilising a variety of data including aerial imagery and geology mapping, as well as topographic data provided by the client. A River Hydro-morphology Assessment Technique (RHAT) Survey was undertaken by a trained field geomorphologist to document the current morphological condition of the river, and to inform the sediment transport model. Grain-size distributions were also estimated in the field by conducting Wolman pebble counts both upstream and downstream of the weir.
The second component of the study involved the construction of a sediment transport model in HEC RAS, to predict how patterns of erosion and deposition may change following the weir removal. Sweco’s river modellers built the model using survey data and hydrological inputs calculated using the Flood Studies Update methods. The estimated grain-size distributions from the field survey along with the local geological information was integrated into the model to better represent changes in the sediment and bank characteristics along the channel. The model was then used to evaluate the effects of several weir removal scenarios, at varying timescales, for both high and low flow conditions.
The Result and Benefit to the Client
Sweco used the results to inform an assessment of the potential for increased flood and scour risk downstream, as well as the impacts on wider habitats. The weir was found to be resulting in aggregation of material upstream of the weir and depletion of sediment downstream of the weir. Removal of the weir was predicted to enhance connectivity and increase habitat diversity, but result in local, short-term channel instability, which could result in channel incision and impact nearby habitats, such as riffles, pools and gravel beds. The modelling also indicated that following the weir removal the channel over time would revert to its narrower natural width of 15m and result the bifurcation channel (Mill Race) upstream of the weir becoming cut off from the main channel. This resulting in a slight increase in flows and velocities downstream.
Sweco provided the client with advice and practical mitigation measures to reduce negative impacts, which included progressively lowering the weir to reduce the impact on fish habitats; incorporating measures to redirect flow to ensure that the bifurcation channel is maintained; and the use of green bank protection (in the form of willow spilling) to provide additional habitat whilst allowing the river to naturally adjust its morphology to the new conditions.