Leaky Woody Dams have been used successfully at Pickering and elsewhere to successfully attenuate the flow of water down the stream and river network. These dams were constructed by spanning logs above and across the stream bed and securing them in the banks with stakes or by using existing trees. To install successful LWD’s here in the Upper Calder catchment several technical challenges have to be overcome if they are to remain in place during a flood event.
Our early work on the best places to install LWD’s concludes that the upper reaches of our river systems are most favourable for slowing the flow if we are to avoid delaying flows to the point where the flood peaks then later coincide with those from other tributaries in the valley bottoms. However, many of the stream beds in the upper reaches have rock in the stream beds and it is not possible to knock stakes into either the stream bed or the banks. Conventional ways of drilling rock to install retention systems for LWD’s cannot be carried out practically using normal plant, as even small drilling rigs will struggle to reach many of the places we have in mind for locating LWD’s.
In Scotland in 2012/13 a new overhead transmission line was constructed from Beauly near Inverness to Denny north of Glasgow, 600 pylon towers were erected many of them in inaccessible places on mountain tops and in remote valleys. A substantial portion of the final contract value was spent constructing temporary access roads for plant which at additional cost were later reinstated back to their natural state. We can avoid these pitfalls by engineering LWD’s so that they can be constructed with handheld equipment that can be transported by quad bike and trailer and even wheelbarrow.
A LWD is effectively a tier of logs (the sketch shows three, this could vary).
The LWD is secured in position on a rock stream bed by using steel dowels installed by coring (rather than drilling) the rock with a small coring rig which typically are used for coring masonry or concrete and are easily transportable.
The dowels are sized and spaced to carry the bending and shear forces from the stream flow and the forces transferred into the rock in shear and tension. Cement grout is ideal for securing the dowels, made from Ordinary Portland Cement and water mixed at site in a barrel using a simple drill powered paddle mixer. In river the holes are cored by working from wooden baulks placed in the river bed and the coring rig fixed to them, or hand held coring rigs can be used.
The grouting operation uses a short length of plastic tubing or casing that fits snugly into the cored hole, the bar is placed with centraliser spacers in the centre of the cored hole. Grout is poured into the casing through a short plastic pipe introduced to the base of the cored hole (a tremie), this displaces the water out of the rock socket and up through the top of the casing. Grouting ceases just before reaching the casing brim. Once the grout is cured the casing can be removed exposing the dowel or alternatively the plastic casing could be left bonded to the dowel thereby disguising it. Either way this construction process avoids contaminating the river with grout and is a well proven approach used in bored pile construction below the water table.
These dams are intended to allow the normal river flows to pass underneath along with any fish and attenuate water only in higher flows, with the intention that they also create slower over bank flow, so ideally they will be placed in areas where there are wide, low, flat river banks. Such interventions are cost effective and can play a significant role in managing downstream flood risk.
Terrain Geotechnical Consultants