NWRT - Nant Garred Ddu Rocky Ramp NGarrDdu-Rramp-0001

Tender description

Please contact the above email/telephone number to arrange a site visit or visit this open site yourself and contact us with any questions.Reinstate a rocky ramp/stepped pool that was previously installed. Please consult the above drawings.Scour pool infill:The scour pool immediately below the weir is too deep (1-1.5m depth depending on flow) to install all of the rocky ramp/stepped pool. This will require filling in partially (50%) with gravel/pebble material (0.05-0.25m sizes; ~12.5m3). The pool is approx. 5 x 5 m and is sloped into the centre of the pool. Once filled, this will need to be topped with a geotextile membrane which will be secured in place. The newly implemented rocky ramp/stepped pool will sit on top of this section. Gravel, pebble and cobble should be placed on top to form a new base layer.Rocky ramp:Large 0.5-3 tonne boulders will be required for the works (please provide an estimation of the amount required upon quoting, we estimate requiring 5-8 large boulders per “step”) as they need to be large enough to not move during high flow. Over a 25m distance from the weir working downstream, arrange boulders in a downstream facing arc at 5 metre intervals (see drawings above). One or a few of the boulders in each arc should be lower (into substrate) to allow water to chute over the top and provide an entry point for fish into the pool. The head difference (height of the water) between the lowest boulder(s) in each arc to be no more than 0.3m to the next consecutive pool up- or downstream. Boulders should be keyed into the bank and substrate where possible to increase stability and 12.5mm rebar can be used if needed to connect boulders. Boulders may be required around the edge of the scour pool downstream of the weir. Once boulders have been placed into the channel, gaps should be infilled with 0.05-0.25m gravel/pebbles to secure the boulders in place. Channel width is approx 5-6m over the 25m length. Normally the ramp is constructed in a downstream to upstream direction. Care must be taken to secure the downstream end of the ramp to prevent the toe from being removed in high flows. ‘Flow control’ rocks must also be used between the slots in each step (see image).Water diversion:The utilisation of a coffer dam (or similar) may be required to divert flow from each bank (to one half of the channel width) at a time in order to carry out the works on each side in a dry area.PLEASE consult the attached document for more details.NOTE: To register your interest in this notice and obtain any additional information please visit the Sell2Wales Web Site at https://www.sell2wales.gov.wales/Search/Search_Switch.aspx?ID=142951. This notice has been sent as a region specific notice. If you did not receive an alert about this you are not based in the specific region selected by the buying authority. Questions on why this was sent as a region specific notice should be sent to the buying authority.(WA Ref:142951)