User menu

Tower Surface Mine Restoration Proposed Drainage

2 1 Introduction 2 Strategy 3 Catchment Details 4 Watercourses 5 Maintenance Plan 6 “Betterment” Improvements t 3 1 Introduction Tower Regeneration Limited (TRL) operates a large surface mine site at Hirwaun Aberdare in the county of Rhondda Cynon Taf. Due to coal no longer being required by RWE Aberthaw mining at the site ceased in March 2017, at which point the mining operations gave way to the main restoration phase involving overburden removal and the filling of the former mining void in accordance with the approved Restoration Plan. It is projected that the overburden backfill will be completed by September/October 2018 with site drainage works taking place in line with progressive restoration. Some of the site lies within Rhondda Cynon Taf Councils (RCTC) local development plan National Strategic Area 8 (NSA 8), an area identified for mixed use development. The Restoration landform has been modified to meet the requirements of NSA 8 with the creation of a development plateau 20Ha in area., All other areas are to be landscaped in accordance with the 2011 Planning Consent. The restoration of the site will need to be efficiently drained in accordance with local and national standards, together with technical guidance such as the SUDS manual to prevent flooding and aid water flows within various systems throughout the site and to discharge without affecting receiving waters and flows downstream. 2 Strategy The Land Drainage Strategy employed is based, as far as practicable, on replicating the pre-mining catchment areas for the various discharge points into the site restoration landform providing flood attenuation where practicable. This will ensure that the water flows leaving the site through the various discharge points maintain, in terms of water volumes and quality, the condition of the receiving waters. Catchment No. Pre-Mining Area (Ha) Post Mining Area (Ha) Diff. (Ha) Discharge Co-ords 1 31.0 31.0 0.0 E.293441 N.205197 E.293642 N.205337 2 99.0 113.0 + 14.0 E.294735 N.205742 3 68.0 59.0 - 9.0 E.294810 N.205442 4 219.0 220.0 +1.0 E.295315 N.205074 5 18.0 18.0 0.0 E.295047 N.205732 Table 2. Catchment area increase/decrease between pre and post mining operations. 4 3 Catchment Details The project area straddles the watershed between the Neath Valley and the Cynon Valley. There are 5 locations where water discharges from the area into the various catchments. The 5 principal catchment areas have site discharge point locations that remain the same from pre to post mining operations, are described below and are shown on The Pre & Post Mining Catchment Plans. 1. Western Discharge ( Ref. 1a & 1b ) ( Neath Catchment ) 2. Old Settling Ponds Discharge ( Ref. DP 2 ) ( Cynon Catchment ) 3. Closed Tip No. 901 Discharge Point ( Ref. DP 3 ) ( Cynon Catchment ) NRW Sample Point 4. Nant -y- Bwlch Stream ( Ref. DP 4 ) 5. Closed Tip No. 107c Discharge ( Ref. DP 5 ) ( Cynon Catchment ) NRW Sample Point A catchment study has been undertaken to ascertain the greenfield runoff for these principle catchments and the individual discharges from the site (Appendix 1). Within these principle catchments there are sub catchments. These have been included in the principle catchments as they have no individual impacts on receiving waters upon leaving the site. 3.1 Pre Mining Catchments and Drainage Systems ( Pre Mining Catchment Plan 1 ) 3.1.2 Catchment 1, Western Discharge ( Ref. 1a & 1b ) (River Neath Catchment) This catchment which is 31 Ha in size lies to the western extremity of the site and lies at elevations between 245m and 495m. AOD with a down slope length of 1100m. The catchment contains many unlined natural channels which show many erosional features with a general flow path towards the north west into a main channel that runs alongside Treherbert Road, from here water flows through 2 pipes, 600mm (Ref 1a) and 400mm (Ref 1b) in diameter which run under the road and into the River Neath Catchment. Catchment 1 has no influence on flows to other drainage systems throughout TRL`S land boundary. The area draining through the Western Discharge points into the Neath Catchment, due to the early cessation of mining, has not been disturbed. No changes to the landform or drainage layout is proposed for this catchment during Restoration. 3.1.3 Catchment 2, Old Settling Ponds Discharge ( Ref. DP 2 ) (River Cynon Catchment) This catchment which is 99 Ha in size lies to the western extremity of the site and lies at elevations between 220m and 500m AOD with a down slope length of 1700m. The catchment contained many natural unlined channels which showed erosional related features in the upper steeper terrain area near Twyn Canwyllyr The mid and lower parts comprised of channels that varied in size, some natural others man made, all unlined and showed erosional related features. Water treatment areas also occurred which were associated with the former Tower Colliery Washery. There were also channels associated with field/land drainage and internal access roads. The general flows here are to the north and north east into an unlined channel at the rear of a factory before running via a 600mm pipe 17m in length with a gradient of 1v30h (Ref 1C) under Rhigos road (A4069) and into a concrete lined channel before entering a culvert 700mm in size approximately 80m in length (Ref.1D) near the conveyor before eventually flowing into the pond drainage system of Tip 107, leaving the site at (Ref. DP 2) and then under the A465 dual carriageway with eventual discharge into the River Cynon. 3.1.4 Catchment 3, Closed Tip No. 901 Discharge ( Ref. DP 3 ) NRW Sample Point (Cynon Catchment) This catchment which is 68 Ha in size and collected drainage from Tower Colliery Tip 901 at elevations between 220m and 285m AOD with a length of 630m, comprising various lined and unlined channels together with various water treatment areas (Ref 1F) which were essentially de-silting ponds which vary in size and capacity. Flows were generally towards the north although the low gradient berm drainage flowed east to west before entering the “Jim Davies and S Pond” (Ref 1G) treatment areas into concrete lined channel and discharge out of site at ( Ref. DP 3 ). 3.1.5 Catchment 4, Nant–y-Bwlch ( Ref. DP 4 ) (Cynon Catchment) This catchment which is 219 Ha in size lies to the east of the site and is the largest catchment with elevations between 220m AOD near the Hirwaun Welfare to a maximum at Tarren Y Bwlch of 505m AOD with a length of 1800m. 5 The upper catchment is a glacial cwm where drainage consists of natural unlined channels with various erosional features which flow into a large saturated peat area characterised by reeds. West of Nant-y-Bwlch Stream There are 3 main channels that flow in a north east direction from the cwm area (Ref. X, Y, Nant-y-Bwlch Stream). The 2 smaller channels, ( Ref. X & Y ) to the west enter the site and flow into various tip/land drainage before discharging back into the Nant Y Bwlch, which shows in places deep scouring in excess of 10m, has a gradient of 1v:10h and varies in width from 1m in the upper part to excess of 14m in the lower sections. Any run-off waters not collected by channels (Ref. X, Y) are caught by Tip No. 901 perimeter intercept channel (Ref. 1I) which formed the southern lower drainage limit flowed towards the east and into the Nant-y-Bwlch Stream at (Ref 1J). East of Nant-y-Bwlch Stream A further unlined channel enters the catchment and flows from the east to west this enters the Nant Y Bwlch at (Ref 1K). There is also a drainage channel that enters the south east boundary area and flows off site to the east this forms part of an old opencast drainage system The downstream area of Nant Y Bwlch flows into a 700mm concrete pipe (Ref 1L) which is in a poor condition under an access track ( which is deteriorating badly ) before eventually discharge into a 1600mm pipe 8m in length that runs under a public footpath ( DP 4 ). 3.1.6 Catchment 5, Old Washery Lagoons ( Ref. DP 5 ) NRW Sample Point (Cynon Catchment) South of Rhigos Road A4059 This catchment is 18 Ha in size and lies to the east and north of catchments 2 and 3 respectively. The unlined Nant Y Cnapiau watercourse, to the east tip 901, runs through and transmits water to the north east before turning north west, alongside the old railway track in an unlined channel to cross Rhigos Road through a culvert with pipe diameter of 600mm, 28m in length at 1v:30h gradient (Ref 1M). The gradients on the unlined channels are not steep enough to cause scouring, the channels remaining in good condition. Water is then transmitted through a series of ponds associated with Tip 107 (Ref 1N) with eventual flow into the river Cynon at ( Ref. DP 5 ) . 3.2 Post Mining Catchments and Drainage Systems ( Post Mining Catchment Plan 2 ) 3.2.1 Catchment 1, Western Discharge ( Ref. DP 1 ) (River Neath Catchment) This catchment which is 31 Ha in size lies to the western extremity of the site and lies at elevations between 245m and 495m. AOD with a down slope length of 1100m. The catchment contains many unlined natural channels which show many erosional features with a general flow path towards the north west into a main channel that runs alongside Treherbert Road, from here water flows through 2 pipes, 600mm (Ref 2a) and 400mm (Ref. 2b) in diameter which run under the road and into the River Neath Catchment. Catchment 1 has no influence on flows to other drainage systems throughout TRL`S land boundary. The area draining through the Western Discharge points into the Neath Catchment, due to the early cessation of mining, has not been disturbed. No changes to the landform or drainage layout is proposed for this catchment during Restoration. 3.2.2 Catchment 2, Old Settling Ponds Discharge ( Ref. DP 2 ) (River Cynon Catchment) Post Mining The catchment is 113 Ha and lies at elevations between 220m and 500m. AOD with a down slope length of 1700m. There is an increase in area of 14 Ha compared to pre mining, although the catchment downslope length is similar to pre mining length. A water intercept channel, track and power line diversion (SHIC) was constructed along the southern limit of the site, is approved under planning and will be retained. This track and intercept channel transmit flows to the west from the upper steeper parts directly above the diversion route. The catchment water originally flowed into the Cynon Catchment – the track and intercept channel continue to direct water flows into the Cynon River Catchment. A second intercept channel is proposed to capture water emanating from the unmined old tip system, effectively the area remaining unmined within the approved mining limit of excavation. 6 These 2 channels will flow into the proposed pond 6 then into a 1500mm pipe 13m in length at 1v:40h (Ref. 2C) under a access track and into a lined channel 2m deep with 2m invert width at 1v:25h (WHIC) before flowing into the pre mining system at (Ref. 2D). The 1500mm pipe and lined channel were constructed during the surface mines operational phase. There are also 2 proposed agricultural drainage channels that will flow to the west into the lined channel. These unlined channels will be constructed at a gradient of 1 in 100 to reduce stream velocity and limit channel erosion (Ref.2E). 3.2.3 Catchment 3, Closed Tip No. 901 Discharge ( Ref. DP 3 ) (River Cynon Catchment) NRW Sample Point The catchment is 59 Ha and lies at elevations between 220m and 330m AOD. There is a decrease in area of 9 Ha compared to pre mining, the catchment length will also increasing to 1000m. The Restoration Landform, in order to accommodate any potential development within the Local Development Plan Area NSA8, is profiled to create a plateau area. In order to prevent any scouring on the northern faces of the plateau it has been constructed with a backfall to the south. Drainage off this plateau will be enabled by a secondary fall from the centre of the plateau east and west. Run-off will be collected in 2 wet swale channels which will transmit water east and west. The western swale entering a 1500mm pipe (Ref.2F) and flowing into a channel which then runs into Pond 2 ( Jim Davies ) and eventually into the pre existing “S Pond” treatment areas to discharge point DP3. 3.2.4 Catchment 4, Nant–y-Bwlch ( Ref. DP4 ) (Cynon Catchment) The catchment is 220Ha with similar elevation and catchment length to pre mining conditions, 80 Ha lie within the site boundary and make up the restoration area. There is an increase in area of 2 Ha compared to pre mining, the catchment length remains at 1800m, with elevation of 500 to 200maod. The proposed east swale enters pond ( Ref.1A) then flow is directed into a channel before entering pond 3 before discharge into pre existing 1600mm pipe (Ref.2G) at the Nant Y Bwlch discharge point ( Ref. DP 4 ), this structure is subject to change as part of the A465 upgrade drainage. For the reasons detailed below (Watercourses, N-y-B Stream) it is proposed to retain the Nant-y-Bwlch stream in the diverted position. There were 2 main phases of works carried out during mining operations: (i) the first being the lined Nant Y Bwlch (NYB) diversion which is 1600m in length wider and with shallower gradients compared to the original 800m pre mining configuration. The former smaller channel that runs through the south site boundary into the NYB has also been lined (Ref 2H) (ii) The second phase of drainage works referred to as EHIC involved the interception of the 2 main channels that lie to the west of NYB (Ref. X and Y) and drain to the north east from land that makes up the saturated upper glacial cwm area, this channel runs along the site perimeter to the east into where NYB enters the site via a concrete/blockstone cascade (Ref.2I) the channel is also lined to minimise erosion. It is proposed to retain EHIC to control water flows through the site. A section of the NYB is piped here 3 no. 900mm diameter size pipes transmit flows along the 101m pipe length to outfall at a pond which contains large blockstone features that reduce the energy of water prior to re-entering the N-y-B diverted channel (Ref 2J). 3.2.5 Catchment 5, Old Washery Lagoons NRW Sample Point South of Rhigos Road A4059 This catchment is 18 Ha in size and lies to the east and north of catchments 2 and 3 respectively. The unlined Nant Y Cnapiau watercourse, to the east tip 901, runs through and transmits water to the north east (Ref 2k). A proposed drainage channel will tie into the existing unlined ditch alongside the old railway track and then into a pond which runs under Rhigos Road through a pipe with diameter of 600mm, 28m in length at 1v:30h gradient, pipe (Ref.2L). The flow is the transmitted through a series of lagoons (Ref M), before eventual discharge through the Sample Point 5 before continuing to the river Cynon. These lagoons, as part of normal operational practice are regularly desilted to maintain capacity. Following the cessation of mining, when weather conditions allow, lagoon (Ref. 2N ) will be enlarged and deepened to provide additional flood attenuation. 7 4 Watercourses The restoration of the site affords the opportunity to rectify issues that existed pre-mining and to improve various aspects of: Principle watercourses (i) Nant-y-Bwlch Stream (ii) Western Channel Land drainage watercourses within the site boundary 4.1 Nant-y-Bwlch Stream Pre-mining the N-y-B stream flowed on full dip , almost due North across the mining/overburden mound area which necessitated the diversion around the eastern site boundary of site operations. On the original course the N-y-B fell from a level of 305m AOD to a level of 215m AOD over a distance of 900m. An overall gradient of 1 in 10 – locally 1 in 4. These gradients resulted in a deeply incised channel offering no opportunity for flow control or flood attenuation. The diverted location reduces this overall gradient to 1 in 18 over a distance of 1600m. The major difference in level (261.99 to 240.46 – fall of 1 in 5 ) is catered for by the use of as previously agreed culvert 2295-c14a Dec 2013( 3 no. 900mm diameter pipes ), gradients indicated on the Nant-y-Bwlch Plan with hydraulic calculations in the Appendix. These gradients significantly reducing the potential for erosion and creating flood attenuation areas. This benefit is aided further by the fact that the channel, when constructed , was oversized ( evidenced by the lack of any problems since constructed). There are also ecological benefits to retaining the N-y-B in the diverted location with better access to the stream. The construction of the stream diversion using a soft construction of mudstones and shales avoiding the use of sandstones and concrete has resulted in a channel with a fully developed stream bed which has naturalised to a position of high ecological value. 4.2 Western Channel This channel was positioned to convey water around the operational site and constructed to the same specification as the N-y-B diversion. The pre-mining drainage of this area, Catchment 2, consisted of a series of interconnected natural channels of varying width and depth with no flood attenuation features draining the area Post mining this channel will be retained, As a result of the dimensions and gradient this channel provides flood attenuation previously absent in addition to the advantages identified for the N-y-B channel. The field drainage channels connecting into this channel are located such that the gradients are approx.. 1 jn 100 which minimises erosion, reduces flow velocity providing additional buffering at times of inclement weather. 5 Management of Drainage Features The proposed drainage systems will have an access track constructed so that channels, ponds etc can be easily accessed by machine in order to carry out maintenance works such as repair, desilting and clearing vegetation which could have the potential to cause flooding through blockages. These works will form part of the site aftercare plan. 8 6 “Betterment” Improvements to water flows from pre to post mining drainage systems Flood Control / Attenuation There was very little opportunity for flood water attenuation pre-mining other than the operational settling lagoons whose primary function was the storage of water to allow the settling of suspended solids to take place. Whilst this afforded some attenuation it was applicable only to limited operational areas , the lagoons were designed for sedimentation not flood attenuation and did not significantly assist with controlling water flows through the site. Erosion Control The re-designed site drainage has significantly reduced the potential for stream and drainage channel erosion by: (i) Wherever possible the drainage channel gradients are reduced to 80 / 100:1. On site experience has shown that drainage channels and tip berms suffer no erosion, indeed where localised shallow ponding tales place sediment settling occurs. (ii) At gradients up to 1: 40 the channels are lined. (iii) Where channels are lined mudstones and shales are used on the banks, breaking down to form a naturalised stream bed with vegetation developing further impeding water flows. (iv) The introduction of water swales and ponds further reduces water velocity and consequently the potential for erosion. Pollution Control Groundwaters Prior to surface mining in January 2010 consultants FWS prepared a report titled “Hydrological and Hydrogeological baseline report for the proposed Tower surface mine” reference 1359. The report concluded that leaching of opencast backfill mobilizing list 1 and 2 substances (EU Directive/Groundwater Regulations 1998) would be unlikely and that any leachate would contain iron concentrations lower than the regional groundwater /minewater system. Surface Waters All the backfill and restoration materials used are derived from the site and are essentially composed of coal measure sandstones, mudstones, and siltstones with some coal. The only monitoring historically required by NRW are for: (i) pH (ii) Suspended solids To reduce the potential for pollution: (i) Intercept ditches at the foot of areas with topography gradients in excess of 1 in 10 and with intercept ditch gradients steeper than 1 in 40 will be lined to reduce the impact of erosion and mobilisation of substances that have the potential to cause pollution.. (ii) Field and agricultural drainage ditches will be laid to gradients less than 1 in 80. Site experience has shown that at these gradients ditch lining will not be required. (iii) Suspended solids together with total/dissolved iron can be managed efficiently with the construction of ponds and wetland areas these structures are known as an effective control measure. Analysis of surface water on the site prior to surface mining by FWS generally showed water chemistry not to exceed consent. Surface water monitoring during site operations also show no exceedance and water is below the NRW discharge consents. This daily monitoring will be continued over the approved Restoration period, Agricultural 5 yrs. And Nature Conservation & Public Access land 25 yrs. Overflows Where practical most ponds have an overflow channel constructed to prevent the potential for flooding caused by surcharge due to blockage of the drainage system, these flows would be directed away from the pond to other parts of the sites drainage