Introduction
Many of you will have spent countless hours near or on the River Dee admiring its beauty and its grandeur. Well, back in the early 19th Century some of our great industrialists had other ideas and they agreed that harnessing some of its power was just what they needed.
At that time William Jessop had a vision to build a canal to connect the towns in north Shropshire and Denbighshire to our major waterways for the transportation of goods and materials. His original intention was to link the River Mersey in the north to the River Severn in the south, with a connection to the River Dee at Chester. Jessop gave Thomas Telford the challenge to design and manage the project which, amongst other things, resulted in two great aqueducts, tunnels and one weir on the River Dee in Llangollen.
The Horseshoe Falls (historically called Llantysilio weir) was built to control the flow of water down the Dee and allow for controlled abstraction to help maintain the level of the canal. Sadly, the canal was never completed but the Falls were, and water has been flowing down the canal now for over 200 years.
What was a boon for the industrialists though became another pressure point for the many river species who annually made their great migratory pilgrimage from the sea back to their spawning grounds in the upper catchment of the Dee and to this day the Falls has challenged even the strongest of swimmers.
Onwards and Upwards
LIFEDeeRiver (a project run by Natural Resources Wales, NRW) have for some time now been looking at the spawning habits of fish on the Dee and identified many weirs and other features that were hindering fish migration. These they have been steadily removing from the river, with 13 removed so far. This year they started a huge project that would allow fish to bypass the Horseshoe Falls completely.
Obviously with the heritage of the Falls this could not be modified in any way, but it was agreed that a second channel could be put in place for the fish and any other aquatic species.
This project is now well underway and shows the commitment that NRW has to our environment and to the marvellous features within it.
Initial Construction
This picture shows the work as it is progressing in June this year. The Falls themselves are to the top and to the left of the channel carefully hidden behind the trees. Indeed, when you visit the Falls after completion of the work there will be little indication that this work has even taken place. Thanks to the LIFEDeeRiver team for this excellent drone image.
As you can imagine a significant amount of planning was undertaken to ensure that the position, dimensions and gradient of the channel were such that it does not affect the volume of the water flowing over the Falls and more significantly into the canal as this canal now feeds drinking water to many homes in the Midlands as well as maintaining the level of the canal itself! Also, that it did not impact upon the beauty of the falls themselves.
Firstly a ‘coffer dam’ was built on the upstream side of the channel to ensure that water did not flood the site and once this was in place the channel was dug. The overall channel is a significant size to allow for easy access of the machinery and in the middle of this a smaller, deeper channel was excavated into which a ‘U’ shaped concrete culvert can be positioned to form the bed and sides of the fish pass itself. This will be contoured to resemble the bed of a river with built in boulders and hollows to give shelter and resting places for fish as they travel upstream. Great care and thought have been taken with this design always ensuring it meets the needs of the fish before anything else.
Once the concrete conduit has been put in place work will turn to where it connects to the downstream side of the river.
Attraction Flow
When constructing a fish pass, it is vitally important to co-locate the entrance to the fish pass as close as possible to the barrier. It is then vital to create an ‘attraction flow’ that fish will be able to easily find to allow them to then move up the fish pass. With the new channel this ‘attraction flow’ will be created through a slight steepening and narrowing of the downstream end of the bypass channel. This will create a stream of water that will draw the fish away from the difficult task of leaping the Falls to the easier task of swimming up the new channel.
Nearing Completion
Attention will then turn to the upstream end of the channel and similar tasks will be undertaken to grade and contour the exit of the fish pass and at this time the coffer dam will be removed, keeping everything crossed to ensure that all the initial calculations were accurate and a steady, gentle flow of water is achieved.
Work will then finally refocus onto the channel itself when it will be given a concrete lid and then covered in the original topsoil thus allowing the field to be returned to the landowner and making the field look like it has never been disturbed.
Conclusion
The project is progressing very well under the management of NRW Team Leader Joel Rees-Jones, and I must thank him for his time and patience explaining to me how all this will work.
I am hoping to go back later in the year when the project nears completion and will be happy to update you all as to how it has gone.
Rhaeadr y Bedol – David Tomlinson
Cyflwyniad
Bydd llawer ohonoch wedi treulio oriau di-rif gerllaw neu ar Afon Dyfrdwy yn edmygu ei phrydferthwch a’i mawredd. Wel, yn ôl yn gynnar yn y 19eg Ganrif roedd gan rai o’n diwydianwyr mawr syniadau eraill ac roeddent yn cytuno mai defnyddio rhywfaint o’i phŵer oedd yr union beth oedd ei angen arnynt.
Ar y pryd roedd gan William Jessop weledigaeth i adeiladu camlas i gysylltu’r trefi yng ngogledd Swydd Amwythig a Sir Ddinbych â’n dyfrffyrdd mawr ar gyfer cludo nwyddau a deunyddiau. Ei fwriad gwreiddiol oedd cysylltu Afon Merswy yn y gogledd ag Afon Hafren yn y de, gyda chysylltiad ag Afon Dyfrdwy yng Nghaer. Rhoddodd Jessop yr her i Thomas Telford ddylunio a rheoli’r prosiect a arweiniodd, ymhlith pethau eraill, at ddwy draphont fawr, twneli ac un morglawdd ar Afon Dyfrdwy yn Llangollen.
Adeiladwyd Rhaeadr y Bedol (a elwid yn hanesyddol yn morglawdd Llantysilio) i reoli llif y dŵr i lawr Afon Dyfrdwy a chaniatáu tynnu dŵr dan reolaeth i helpu i gynnal lefel y gamlas. Yn anffodus, ni chwblhawyd y gamlas erioed ond fe gwblhawyd y Rhaeadrau, ac mae dŵr wedi bod yn llifo i lawr y gamlas ers dros 200 mlynedd bellach.
Fodd bynnag, daeth yr hyn a oedd yn fendith i’r diwydianwyr yn bwynt pwysau arall i’r nifer o rywogaethau afonydd a oedd yn gwneud eu pererindod fudol fawr o’r môr yn ôl i’w mannau silio yn nalgylch uchaf Afon Dyfrdwy bob blwyddyn, ac mae’r Rhaeadrau hyd yn oed wedi herio’r nofwyr cryfaf.
Ymlaen ac I Fyny
Mae LIFEDeeRiver (prosiect a redir gan Cyfoeth Naturiol Cymru, CNC) wedi bod yn edrych ar arferion silio pysgod ar Afon Dyfrdwy ers peth amser bellach ac wedi nodi llawer o goredau a nodweddion eraill a oedd yn rhwystro mudo pysgod. Maent wedi bod yn tynnu’r rhain yn gyson o’r afon, gyda 13 wedi’u tynnu hyd yn hyn. Eleni, dechreuasant brosiect enfawr a fyddai’n caniatáu i bysgod osgoi Rhaeadrau’r Bedol yn llwyr.
Yn amlwg, gyda threftadaeth y Rhaeadrau, ni ellid addasu hyn mewn unrhyw ffordd, ond cytunwyd y gellid rhoi ail sianel ar waith ar gyfer y pysgod ac unrhyw rywogaethau dyfrol eraill.
Mae’r prosiect hwn bellach ar y gweill ac mae’n dangos yr ymrwymiad sydd gan CNC i’n hamgylchedd ac i’r nodweddion rhyfeddol ynddo.
Adeiladu Cychwynnol
Mae’r llun hwn yn dangos y gwaith wrth iddo fynd rhagddo ym mis Mehefin eleni. Mae’r Rhaeadrau eu hunain ar ben ac i’r chwith o’r sianel wedi’u cuddio’n ofalus y tu ôl i’r coed. Yn wir, pan fyddwch chi’n ymweld â’r Rhaeadrau ar ôl cwblhau’r gwaith, ychydig iawn o arwydd fydd bod y gwaith hwn wedi digwydd hyd yn oed. Diolch i dîm LIFEDeeRiver am y ddelwedd drôn ardderchog hon.
Fel y gallwch ddychmygu, gwnaed cryn dipyn o gynllunio i sicrhau bod safle, dimensiynau a graddiant y sianel yn gyfryw fel nad yw’n effeithio ar gyfaint y dŵr sy’n llifo dros y Rhaeadrau ac yn bwysicach fyth i’r gamlas gan fod y gamlas hon bellach yn bwydo dŵr yfed i lawer o gartrefi yn y Canolbarth yn ogystal â chynnal lefel y gamlas ei hun! Hefyd, nad oedd yn effeithio ar harddwch y rhaeadrau eu hunain.
Yn gyntaf, adeiladwyd ‘argae coffr’ ar ochr i fyny’r afon o’r sianel i sicrhau nad oedd dŵr yn gorlifo’r safle ac unwaith yr oedd hyn yn ei le, cloddiwyd y sianel. Mae maint y sianel gyffredinol yn sylweddol i ganiatáu mynediad hawdd i’r peiriannau ac yng nghanol hon cloddiwyd sianel lai, ddyfnach lle gellir gosod cwlfert concrit siâp ‘U’ i ffurfio gwely ac ochrau’r llwybr pysgod ei hun. Bydd hwn yn cael ei gyfuchlinio i debyg i wely afon gyda meini mawr a thyllau wedi’u hadeiladu i roi lloches a lleoedd gorffwys i bysgod wrth iddynt deithio i fyny’r afon. Cymerwyd gofal a meddwl mawr gyda’r dyluniad hwn gan sicrhau bob amser ei fod yn diwallu anghenion y pysgod cyn unrhyw beth arall.
Unwaith y bydd y ddwythell goncrit wedi’i gosod, bydd y gwaith yn troi at y man lle mae’n cysylltu ag ochr i lawr yr afon.
Llif Denu
Wrth adeiladu llwybr pysgod, mae’n hanfodol lleoli mynedfa’r llwybr pysgod mor agos â phosibl at y rhwystr. Yna mae’n hanfodol creu ‘llif denu’ y bydd pysgod yn gallu dod o hyd iddo’n hawdd i ganiatáu iddynt symud i fyny’r llwybr pysgod wedyn. Gyda’r sianel newydd, bydd y ‘llif denu’ hwn yn cael ei greu trwy serthhau a chulhau ychydig o ben i lawr yr afon o’r sianel osgoi. Bydd hyn yn creu nant o ddŵr a fydd yn tynnu’r pysgod i ffwrdd o’r dasg anodd o neidio’r Rhaeadr at y dasg haws o nofio i fyny’r sianel newydd. Bron â Chwblhau
Yna bydd y sylw’n troi at ben i fyny’r afon o’r sianel a bydd tasgau tebyg yn cael eu gwneud i raddio a chyfuchlinio allanfa’r llwybr pysgod ac ar yr adeg hon bydd yr argae coffr yn cael ei symud, gan gadw popeth wedi’i groesi i sicrhau bod yr holl gyfrifiadau cychwynnol yn gywir a bod llif cyson, ysgafn o ddŵr yn cael ei gyflawni.
Yna bydd y gwaith yn ailganolbwyntio o’r diwedd ar y sianel ei hun pan roddir caead concrit iddi ac yna’i gorchuddio â’r pridd uchaf gwreiddiol gan ganiatáu i’r cae gael ei ddychwelyd i berchennog y tir a gwneud i’r cae edrych fel nad yw erioed wedi bod. n wedi’i aflonyddu.
Casgliad
Mae’r prosiect yn mynd rhagddo’n dda iawn o dan reolaeth Arweinydd Tîm Cyfoeth Naturiol Cymru, Joel Rees-Jones, a rhaid i mi ddiolch iddo am ei amser a’i amynedd yn egluro i mi sut y bydd hyn i gyd yn gweithio.
Rwy’n gobeithio mynd yn ôl yn ddiweddarach yn y flwyddyn pan fydd y prosiect yn agosáu at ei gwblhau a byddaf yn hapus i roi’r wybodaeth ddiweddaraf i chi gyd ynghylch sut mae wedi mynd.