10 Reasons Why …. 10 Rheswm Pam …. : 10

Tuesday 22nd December 2015

Blog post 10 of 10 about the geomorphology of Wales. Click on images to view larger version in a separate window. Parallel blog in Welsh at http://hywelgriffiths.blogspot.co.uk/

Reason 10. Successful environmental management needs geomorphological knowledge. Geomorphology can provide a key input to environmental management, including landscape conservation, ecosystem conservation and restoration, heritage conservation and carbon landscaping.

Sgwd GwladusOn account on spectacular natural and cultural landscapes, large areas of Wales have protected status. Collectively, the three National Parks in Wales – Snowdonia, Pembrokeshire Coast and Brecon Beacons – protect an impressive 20% of the country, including landscapes, habitats, and heritage sites (http://www.visitwales.com/explore/national-parks). The National Parks are complemented by other areas with varying levels of protection (e.g. Geoparks, a Biosphere Reserve, Sites of Special Scientific Interest, Special Areas of Conservation, National Nature Reserves) some of which overlap in space. For example, the Fforest Fawr Geopark (established 2005) is contained with the western part of the Brecon Beacons National Park, with some key objectives being to conserve and enhance the geological and geomorphological heritage, including by developing the area’s potential as an outdoor classroom and geotourism destination (http://www.fforestfawrgeopark.org.uk/). A complex history of tectonic and climatic changes have acted upon the varied lithologies in the area to form a diversity of landscapes and landforms (Reason 2), many of which form drawcards for tourists. The area around Pontneddfechan, Powys, is a case in point, as it is renowned for its high number of publically-accessible waterfalls that have developed on sedimentary strata, such as Sgwd Gwladus (Photo: Stephen Tooth). Although underutilised at present, geomorphology can play a key role in enhancing the tourist experience by providing information about the origins, development and significance of such landforms, and also can contribute to the development of sustainable management strategies for these popular but protected areas.

In National Nature Reserves, geomorphological processes and landforms provide the template upon which the valued wildlife habitats have developed. Some reserves have been established to protect near-pristine habitats, such as the peat bogs, estuary, and coastal sand dunes near Ynyslas, Ceredigion, while others have been artificially created in mitigation for loss of habitat elsewhere, a prime case being Newport Wetlands on the Severn Estuary. In such anthropogenic wetland landscapes (Reason 8), geomorphology can provide a key input to the design of management strategies, which may focus on maximising ecosystem services, including enhancing biodiversity, providing protection from coastal surges, and promoting carbon sequestration.

log jam 3Outside of protected areas, geomorphology can also play a role in developing strategies for restoration of degraded landscapes, including peatlands, hillslopes and river channels. For instance, in the Cwmparc catchment near Treorchy, Rhondda Cynon Taff, ongoing geomorphological research is helping to evaluate proposed sustainable flood management strategies, including the effectiveness of using engineered log jams to slow flood flows along heavily modified channels (Photo: Stephen Tooth). Geomorphology is also providing key inputs to the design of the coastal defence schemes that are being implemented along many parts of the Welsh coastline, such as at Borth, Ceredigion, where an artificially-constructed reef, rock groynes and breakwaters, and beach nourishment are parts of an overall strategy to protect properties from extreme coastal storms and longer term sea level rise (Reason 9). With mounting concern over habitat loss and likely future increases in the frequency and magnitude of geomorphological hazards (e.g. intense rainfall, river flooding, coastal surges), such trends are likely to continue in years to come.

Did you know? The importance of embedding geomorphological knowledge in environmental management sometimes only becomes apparent where engineering schemes and management strategies have failed. A classic example is provided by a reach of the middle Ystwyth River near Llanilar, Ceredigion. In 1864, the naturally meandering, gravel-bed river was artificially straightened to run adjacent to a railway track but historic maps show that meanders re-established during the next 100 years. In 1969, an artificially straight channel with a trapeizodal cross-section and flat bed was again engineered but without any bank protection works taking place. Within a few months, following a period of high winter flows, the channel had again transformed into a meandering channel with a more irregular cross section. Numerous gravel bars and pools had established along the bed and steep, vertical banks had formed locally (Source: Lewin, J. 1976. Initiation of bed forms and meanders in coarse-grained sediment. Geological Society of America Bulletin, v.87, pp.281-285). The local river authority made further several attempts to re-straighten the channel but such efforts also failed, and ultimately the authorities engineered a meandering channel. This example shows how a variable flow regime that responds rapidly to rainfall, a mobile gravel bed, and unstable banks can combine to give rise to a naturally dynamic, sinuous channel that confounds engineering efforts to artificially straighten and confine its course. In mid Wales and farther afield, this combination of factors is not uncommon (e.g. see Reason 4 and Reason 5). Nonetheless, the lesson seems to be a hard one to learn, for throughout Wales, there are numerous examples of channel re-alignment and bank protection schemes that have failed owing to an inability to take full account of the underlying geomorphological processes.

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10 Reasons Why …. 10 Rheswm Pam …. : 9

Monday 30th November 2015

Blog post 9 of 10 about the geomorphology of Wales. Click on images to view larger version in a separate window. Parallel blog in Welsh at http://hywelgriffiths.blogspot.co.uk/

Reason 9. The Earth’s landscapes are becoming more hazardous. Both global environmental change and human activities are increasing the magnitude and frequency of geomorphological hazards, which occur wherever and whenever land surface stability is affected and adverse socio-economic impacts are experienced.

Throughout Welsh history, various geomorphological processes – many of them related to extreme events – have represented hazards to local communities. Being located in the temperate mid latitudes far from tectonic plate boundaries, in recent millennia Wales has been largely unaffected by the various hazards posed by extreme events such as glacial outburst floods, tsunami and volcanic eruptions, while earthquakes are a relatively infrequent occurrence (Reason 2). By contrast, given frequent intense and/or prolonged rainfall, a lengthy coastline that is exposed to Atlantic swells, and much steeply sloping terrain, extreme events such as river and coastal flooding, rockfalls, and landslides are common hazards. While such events may result from entirely natural causes, human factors can enhance their severity, possibly increasing the damage to infrastructure and/or the loss of human lives.

IMG_4450 reducedThe events that took place in the small village of Dolgarrog, Conway, provide a case in point. Following a period of heavy rainfall that had started in mid October, on the evening of 2nd November 1925, the dam wall of the Llyn Eigiau reservoir in the Carnedd Mountains was breached. The breach released water that flowed downstream and then overtopped and breached the dam wall on the Coedty reservoir. Collective failure of the two dams caused a flood that continued along the Afon Porth Llwyd, rapidly cascading down its steep escarpment course towards Dolgarrog. The water and many thousands of tonnes of transported debris inundated part of the village, forming a fan with imbricated (stacked) boulders up to several metres in diameter on the western margin of the River Conwy floodplain (Photo: Stephen Tooth). 16 people lost their lives in the disaster, a figure that would have been much higher had many villagers not been watching a film in the local theatre (Source: Fearnsides, W.G. and Wilcockson, W.H. 1928. A topographical study of the flood-swept course of the Porth Llwyd above Dolgarrog. The Geographical Journal, v.72, pp.401-416). In 2004, a memorial trail was created through the boulder fan, and this serves as a sobering reminder of the lasting impacts that such extreme events can have on local communities.

On 21st October 2016, the 50th anniversary of the disaster in Aberfan, Merthyr Tydfil, will also provide pause for reflection. In this instance, a toxic combination of several days of heavy rainfall and negligent management practices contributed to failure of a local colliery spoil tip on the side of Mynydd Merthyr, liberating over 150 000 m3 of water-saturated debris. Some of the debris was re-deposited on the lower slopes of the mountain, but some 40 000 m3 continued as a viscous flow more than 10 m deep, rapidly inundating parts of the village, including the classrooms at Pantglas Junior School. 144 people, the majority of them schoolchildren, died in the disaster (Source: https://en.wikipedia.org/wiki/Aberfan_disaster; http://www.nuffield.ox.ac.uk/politics/aberfan/home.htm).

IMG_9074 reduced

IMG_9065 reducedAround Wales’s coastline, extreme coastal storms represent some of the biggest geomorphological hazards. Recently, this was illustrated to dramatic effect along the seafront in Aberystwyth, Ceredigion, which was subject to a succession of high tides and high-energy wave events in the winter of 2013/14. In the most extreme event (early January 2014), the ground floors and basements of many seafront properties were flooded, paving slabs were eroded, and large volumes of sand and gravel were deposited across the promenade, roads and car parks (Photos: Stephen Tooth). While many parts of the seawall survived unscathed, in at least one place, the wall was breached, and erosion of the backfill lead to subsidence and partial collapse of a seafront shelter (for an animated 3D laser scan of the damaged shelter, see https://www.youtube.com/watch?v=8S9YFmPNTHI). As a Grade II-listed structure, the building material was salvaged and the shelter later reconstructed, and other parts of the seafront were quickly cleaned up and repaired. While such events are dramatic, and had never before been witnessed by many local residents, they are certainly not unprecedented, for significant damage to Aberystwyth’s seafront also had been caused by extreme storms in January 1938 and October 1927, as well as in earlier decades. Moreover, despite much of the media’s appetite, it may never be possible to attribute any individual extreme event to the impacts of global climate change. But in a world with rapidly rising sea levels, warmer average air temperatures and a more unstable atmosphere, such coastal storms may provide insights into the types of geomorphological hazards that in future may become more common along the Welsh coastline.

Did You Know? According to a Welsh government-sponsored report, 1 in 9 people in Wales live in properties that are at risk of flooding from rivers or the sea. In total, this represents 357 000 people and 220 000 properties (Environment Agency Wales, 2010. Future Flooding in Wales: Possible Long-Term Investment Scenarios. Available at: http://webarchive.nationalarchives.gov.uk/20140328084622/http://www.environment-agency.gov.uk/static/documents/Research/Flooding_in_Wales_Flood_defences_ENGLISH_V5.pdf). Many of these properties are located in towns and cities in the low-lying, densely populated coastal areas of north Wales and the post-industrial valleys and lowlands of the southeast, having been constructed when regulations on building in flood-prone areas were lax. With projections of future increases in inland and coastal flood frequency and magnitude, the number of flood-prone properties is likely to increase, with flood management likely to become one of Wales’s most pressing environmental management problems. Some of the impacts of extreme floods are obvious, such as the inundation and damage or destruction of property and infrastructure. Other impacts are less obvious but may be just as severe. For example, in the headwaters of many Welsh river catchments, historical mining activities have led to significant concentrations of heavy metals in floodplain sediments. During extreme floods, many of these sediments are re-mobilised by bank erosion, and then transported downstream and re-deposited on lower-elevation floodplains. This can result in widespread pollution of agricultural land and even domestic gardens. For example, levels of lead, zinc and cadmium locally have been found to be above recognised guideline values, and may pose significant risks to the health of grazing animals (Foulds, S.A., Brewer, P.A., Macklin, M.G., Haresign, W., Betson, R.E. and Rassner, S.M.E. 2014. Flood-related contamination in catchments affected by historical metal mining: an unexpected and emerging hazard of climate change. Science of the Total Environment, v.476, pp.165-180). Such studies show that despite the end of most metal mining over a century ago, its legacy continues to affect the Welsh population. Due to the largely invisible nature, however, this particular geomorphological hazard is often overlooked.

10 Reasons Why …. 10 Rheswm Pam …. : 8

Thursday 29th October 2015

Blog post 8 of 10 about the geomorphology of Wales. Click on images to view larger version in a separate window. Parallel blog in Welsh at http://hywelgriffiths.blogspot.co.uk/

Reason 8. Human activities are influencing landscape dynamics. Increasingly, many geomorphological processes and landform/landscape developments are influenced by human activities.

Across Wales, human activities influence geomorphological processes, landforms and landscapes, both indirectly and directly. Indirect influences include human-induced changes to animal populations or vegetation covers that may have an influence on hillslope runoff and sediment transfer, such as badger control programmes (Reason 2), afforestation or woodland felling. Direct influences include deliberate manipulation of geomorphological processes, and can either enhance natural rates of change, such as by promoting river meander cutoffs as part of channel straightening projects (Reason 5), or suppress natural rates of change, such as through river bank or coastal protection works (Reason 7).

anthropogenic influences

In recognition of the widespread influence of human activities on the Earth’s surface, the term ‘Anthropocene’ has been proposed as a new geological time interval. Have human activities become the dominant influence on the shaping of the Earth’s surface, and if so, will these activities leave an imprint in the long-term, future geological record? In Wales, as elsewhere, vigorous debate surrounds the merits of this proposed new time interval, in part because it is not easy to assess the relative importance of natural extreme events (e.g. earthquakes or rare floods – Reason 2 and Reason 4), internal landform adjustments (Reason 5), and human activities as influences on landform/landscape development and the long-term geological record. Natural extreme events, for instance, can accomplish rapid geomorphic change, perhaps undoing many decades of human engineering and infrastructural developments, as was seen in the winter 2014/2015 coastal storms and floods that damaged many parts of the Welsh coastline (see forthcoming Reason 9). Nevertheless, there is no doubt that many human activities in Wales – indirectly or directly – involve the movement of mass (rock, sediment and water) at rates that vastly exceed natural rates, and in ways that will persist far into the geological future. This is most visible in the case of mining activities, such as Electric Mountain near Llanberis, Gwynedd, where vast quantities of high-quality slate have been removed to leave a terraced hillside that towers above Dinorwic Power Station (upper photo: Denis Egan – www.flickr.com/photos/theancientbrit/288064095/, reproduced under Creative Commons licence). Other spectacular examples include the copper mines on Parys Mountain, Anglesey, or the open cast coal mines of the south Wales valleys. Other highly visible human impacts on natural hydrological and sedimentary cycles occur as a result of activities such as dam building, reservoir construction and inter-basin water transfer schemes, as exemplified in the Elan valley, Powys (lower photo: Stephen Tooth), while many coastal areas have been affected by estuary dredging, beach replenishment, and coastal dune landscaping (see ‘Did You Know?’ below).

glass & plastics

Other indirect and direct human activities are far less visible, but nonetheless may still be impacting on geomorphological processes, landforms and landscapes across Wales. For instance, along many parts of the Welsh coastline, a variety of human-made materials such as glass, house bricks, ceramics, metals and plastics now form a small, but perceptible and still growing, component of beach sediments. The long-term significance of these materials, both geomorphologically and in wider environmental terms, is open to debate. Glass ‘pebbles’ recovered from South Beach in Aberystwyth, Ceredigion (upper photo), are relatively benign, as glass is made primarily from commonly-occurring natural elements (mainly silica, sodium carbonate and calcium carbonate). Although persisting in sediments, glass will undergo rapid physical break down under the influence of high-energy or extreme wave and swash conditions. By contrast, many plastics, such as the beads recovered from Whitesands beach, Pembrokeshire (lower photo), are human-made polymers derived from petrochemicals (hydrocarbons). Some plastics will persist in sediments but also may represent a more insidious problem, as it is thought that their physical and chemical break down may facilitate ingestion by marine organisms. In Wales and farther afield, this is leading to fears that plastics are entering the food chain, with as yet poorly understood implications for human health (Photos: Stephen Tooth).

Did You Know? Kenfig National Nature Reserve in Glamorgan is one of the last remnants of a large 3300 ha (33 km2) dune system that once stretched more-or-less continuously along part of the south Wales coastline from the Gower Peninsula in the west to the River Ogmore in the east. Over the last century, however, many of these coastal dunes have been lost to urban, industrial and recreational developments, including caravan parks and golf courses, while others have become overgrown and stabilised by vegetation such as marram grass (Ammophila arenaria). Some estimates suggest that over the last 50 years, 64% of areas of open, mobile sand dunes have been lost from the Welsh coastline as a whole, eliminating the conditions necessary for a variety of rare plants and insects to flourish. These include the threatened fen orchid (Liparis loeslelii), marsh helleborine (Epipactis palustris), the vernal bee (Colletes cunicularius) and the dune tiger beetle (Cicindela maritime). Loss of mobile dunes has occurred even in protected sites such as Kenfig, so in recent years, heavy vehicles have been shifting tons of sand at the site to recreate ‘natural’ dune blowouts and slacks, which are characterised by regular wind disturbance and more mobile, open sand surfaces (http://www.bbc.co.uk/nature/17339061). Given Kenfig’s protected status, creating new habitat from the deliberate loss of some existing habitat might raise some eyebrows, but the policy has been deemed ‘destructively constructive’; in other words, a necessary risk to provide the habitat essential for ensuring the survival of rare species. From this example, the influence of human activities on Welsh landscape dynamics can be seen clearly, and both now and in the future, such deliberate earth moving is likely to form an increasingly important part of much environmental management.

10 Reasons Why …. 10 Rheswm Pam …. : 7

Monday 28th September 2015

Blog post 7 of 10 about the geomorphology of Wales. Click on images to view larger versions in separate windows. Parallel blog in Welsh at http://hywelgriffiths.blogspot.co.uk/

Reason 7. Global change is influencing landscape dynamics. Ongoing global environmental change, which includes atmospheric warming and sea level rise, is currently driving landform development, including desert lake desiccation, ice sheet and glacial retreat, and coastline erosion.

mires

Being located in the temperate mid latitudes far from desert lakes, ice sheets and glaciers, some of these landscape dynamics appear to be of little direct relevance to Wales. But sea level rise and coastal erosion is certainly an issue along large parts of the Welsh coastline (see below), and global environmental change is driving other forms of landscape development. For instance, many parts of the Welsh uplands are covered by blanket mires (‘bogs’), which are characterised by peat that is draped across the underlying topography. Mires are major carbon sinks, being formed from partly decomposed plant material that has built up over many thousands of years under typically cool, waterlogged, oxygen-starved conditions. Today, however, many areas of blanket mire are degraded and actively eroding, as seen from Bwlch Y Groes in Gwynedd, one of the highest public road mountain passes in Wales (left photo: eroding mire is visible on the hilltop in the far distance). Mire degradation can be driven by a combination of factors, including overgrazing, overburning, and air pollution but global atmospheric warming is almost certainly playing a role, particularly through subtle changes to upland water balances. A warmer, more variable climate can lead to longer and/or more frequent dry periods, thereby reducing the extent and duration of waterlogging. This enables oxidation of the peat and promotes increased rates of microbial breakdown of the dead plant matter, with some of the solid carbon being converted to carbon dioxide gas. Drier mires are more vulnerable to extreme fires, leading to loss of peat through combustion, a process that also generates carbon dioxide. Drier, fire-affected mires may also be susceptible to gully erosion (right photo), especially during subsequent extreme rainfall events. Gully erosion may be associated with rapid lowering of water tables, leading to further peat oxidation and carbon dioxide generation. Hence, rather than remaining as a carbon sink, extensively degrading and eroding blanket mires may ultimately become a net source of atmospheric carbon dioxide, so adding to the ever-increasing burden of greenhouse gases and providing a positive feedback in an already warming climate (Photos: Stephen Tooth).

Llyn cliff erosion

While a large percentage of the Welsh landmass lies well above sea level (Reason 1), the country nonetheless possesses a lengthy coastline. Much of this coastline is characterised by headlands formed of resistant rock, and intervening bays formed in more erodible rocks or sediments, as exemplified in Pembrokeshire (Reason 3) or along the Llŷn Peninsula. These erodible rocks and sediments may be vulnerable to increased erosion under rising sea levels, particularly if higher seas are accompanied by an increased frequency of extreme storm events. For instance, in Aberdaron Bay, located near the western tip of the Llŷn Peninsula, coastal erosion is pronounced where the 20 m high cliffs are formed in weakly-consolidated glacial sediments that are comprised principally of sand, silt and clay. Erosion of the base of the cliffs is leading to slumping and collapse of the entire cliff face (upper left photo). Along the eastern part of the bay, the cliff is retreating mainly into agricultural land but farther west, the erosion threatens a nearby road, a church cemetery and other buildings. In response to this threat, an elaborate coastal defence scheme has included sea wall construction and chevron-style drainage on the cliff face (upper right and lower photo) (Photos: Stephen Tooth. Aerial imagery from Google Earth, 0.72 km across, with north oriented to the top).

Did You Know? How long is the coastline of Wales? The answer partly depends on the scale of the map that is used for the measurement, as the larger the map scale, the more ‘wiggly’ the coastline appears to be, and so the longer the length that can be measured. Whether measurements are made at high tide, low tide or somewhere in between could also make a difference. Using 1:10 000 maps and measuring along the mean high water mark, mainland Wales’s coastline is said to be about 1317 miles (2120 km) long, while adding the islands of Anglesey and Holyhead increases the figure to 1680 miles (2740 km) (Source: The British Cartographic Society – http://www.cartography.org.uk/default.asp?contentID=749). But with rising sea levels, even that figure may change over time, as some beaches may narrow or disappear owing to inundation or erosion, while others perhaps get wider as retreating cliffs generate greater sediment supply. Around the Welsh coastline, recognition of a rapidly changing coastline is leading to serious debates about future priorities for coastal defence schemes. While some communities have benefitted from extensive investment in coastal protection works in recent years (e.g. http://www.bbc.co.uk/news/uk-wales-mid-wales-16105700), there is tacit acknowledgement that not everywhere can continue to be protected, with other communities possibly facing the prospect of abandonment as sea levels rise further (e.g. http://www.bbc.co.uk/news/uk-wales-26495935).

10 Reasons Why …. 10 Rheswm Pam …. : 6

Saturday 29th August 2015

Blog post 6 of 10 about the geomorphology of Wales. Click on images to view larger versions in separate windows. Parallel blog in Welsh at http://hywelgriffiths.blogspot.co.uk/

Reason 6. Landscapes are archives of the past. Landscapes contain histories of their development that potentially can be deciphered and reconstructed from study of the associated landforms and sediments.

glaciated landscapes

Many Welsh landscapes bear the unmistakable signature of activity by ice sheets, ice streams or valley glaciers during past intervals of extreme cold. Much evidence is derived from erosional landforms, many of which represent large-scale landscape elements. The armchair-shaped hollow of Cadair Idris in Gwynedd (top left) is a classic cirque (‘cwm’) resulting from glacial erosion in the Welsh uplands, while parabolic (‘U-shaped’) valleys such as those near Lake Vyrnwy (top right) and Llanwrtyd Wells (bottom left) in Powys reveal significant widening and deepening by ice that was conveyed from the uplands to the lowlands. The present-day rivers that occupy the valley floors are too small to have carved these large features by themselves and are said to be ‘underfit’. Scoured, striated and streamlined bedrock surfaces are smaller-scale erosional landforms that can provide indications of the nature and direction of past ice movement, and can be found around Cadair Idris (bottom right) and in many other parts of Wales. Other evidence for past glacial activity is derived from sediments and depositional landforms. For instance, low relief, poorly sorted sedimentary deposits that range from mud through to boulders are known as ‘diamicton’ (also referred to as ‘till’ or ‘head’), and can be found in many Welsh landscapes, commonly being well exposed in coastal cliffs. Elevated (up to ~10 m tall), arcuate ridges of poorly sorted sediment commonly represent ‘terminal moraines’ and, where present, have been used to reconstruct the former extent of glaciers in places such as the Nant Ffrancon valley, Snowdonia (Photos: Stephen Tooth).

Cors Fochno

Besides the landforms and sediments resulting from past glacial activity, landscape histories also can be reconstructed from other forms of evidence. For instance, Borth Bog (Cors Fochno) in Ceredigion is one of the largest surviving raised estuarine bogs in western Europe, and is important not only ecologically but also as an archive of past landscape and wider environmental changes, including the frequency of extreme events (Photo: Stephen Tooth). The lower photo shows part of a core recovered from the margins of the bog, and reveals the transition from grey estuarine clays (lower part of core on the right) to brown peat (upper part of core on the left) that occurred about 6000 years ago in response to rising sea levels (Photo: Henry Lamb). Using a 4 m long core from the bog, variations in the sand content and the bromine deposited from sea spray have been used to reconstruct a decadal-resolution record of late Holocene storminess. Twelve episodes of enhanced storm activity have been identified during the last 4500 years, and these extreme events have been related to changes in ocean and air temperatures and their influences on the intensity of westerly airflow and atmospheric circulation (Source: Orme, L.C., Davies, S.J. and Duller, G.A.T., 2015. Reconstructed centennial variability of Late Holocene storminess from Cors Fochno, Wales, UK. Journal of Quaternary Science, v.30, pp.478-488).

Did You Know? Although there is an impressive array of erosional and depositional evidence for multiple episodes of past glacial activity across Wales, the nature, timing and extent of that activity remains a topic of considerable debate. Even for the last glacial cycle, which peaked at around 20-25 000 years ago, many uncertainties surround the thickness, direction of movement, and extent of ice in places like Snowdonia, the Cambrian Mountains, the Brecon Beacons, and the Gower Peninsula. Ongoing accumulation of field evidence, coupled with increasingly sophisticated numerical models for ice sheet growth and decay (see Numerical Modelling of the Last British Celtic Ice Sheet and Reconstructing the Last Ice Cap Over Wales), should help to resolve some of these uncertainties in the years to come. Past episodes of deglaciation have also left their mark across many parts of Wales; with the decay of ice at the end of glacial cycles, large volumes of meltwater were generated, which contributed to the formation of many lakes in low-lying terrain, as well as the carving of interconnected valley networks, such as in the Cardigan-Fishguard region in South Ceredigion and Pembrokeshire. Following the peak of the last glacial cycle, the global decay of ice sheets and glaciers also resulted in a substantial (~120 m) rise of sea level, drowning many parts of the Welsh coastline and forcing the transition from estuarine to wetland environments in places like Borth Bog. (Source: Glasser, N.F., Etienne, J.L., Hambrey, M.J., Davies, J.R., Waters, R.A. and Wilby, P.R., 2004. Glacial meltwater erosion and sedimentation as evidence for multiple glaciations in west Wales. Boreas, v.33, pp.224–237).

10 Reasons Why …. 10 Rheswm Pam …. : 5

Thursday 23rd July 2015

Blog post 5 of 10 about the geomorphology of Wales. Click on images to view larger versions in separate windows. Parallel blog in Welsh at http://hywelgriffiths.blogspot.co.uk/

Reason 5. Landscape dynamics are often complex. In addition to changing tectonic, geological, climatic or ecological conditions, internal readjustments can also drive landform and landscape development.

hillslopes

Many Welsh landforms and wider landscapes have developed, and continue to develop, in response to external factors, including changing tectonic, climatic, sea level or ecological conditions (see Reasons 1-4). Nonetheless, some landforms and landscapes can develop as a result of internal readjustments that occur independent of changes to external factors. The alternating stability and instability of debris on hillslopes provides a good example, as shown in a photograph looking north along the upper Twymyn valley, Powys. Here, weathering of rocky outcrop (mainly shale) on the upper part of the western valley slope has generated abundant coarse, angular debris, and this has been transported downslope under the action of gravity and running water to accumulate on the mid slope. The debris has gradually built up over time, forming a ‘scree slope’ that tends to reach a stable inclination (‘the angle of repose’) that is governed by the size and angularity of the debris (typically 30-45°). Continued accumulation of debris, however, oversteepens the slope, raising the inclination above the angle of repose and making the scree unstable. Small-scale mass movements (e.g. shallow debris slides) therefore occur periodically, leading to further downslope transport of debris and reducing the inclination back to or below the angle of repose. These changes can occur more-or-less independently of changes to external conditions, although the mass movements on the oversteepened, unstable slopes may be triggered by extreme rainfall events. On the lower slope, the debris is more stable owing to a slightly lower inclination and the binding action of grass, shrub and tree roots.

Carno_blog_arrows

Along some Welsh alluvial river channels, meander bend cutoffs can result from internal readjustments that can also occur independent of changes to external factors. Even under conditions of approximately steady flow and sediment transport, ongoing bank erosion (see Reason 4) can result in adjacent parts of a meander bend eventually meeting. When this happens, the channel straightens, and subsequent flow and sediment transport effectively bypasses and abandons the former bend. This process is known as a ‘neck cutoff’. Abandoned bends may remain full of water for many years and are termed ‘oxbow lakes’, but gradual infilling with fine sediment and organic material means that the lakes eventually disappear. The photograph (taken in April 2008) shows an example from the Afon Carno, a tributary of the upper River Severn near Caersws, Powys. Ongoing bank erosion had left only a narrow (1-2 m wide) strip of floodplain between two adjacent parts of a meander bend (red arrows indicate flow direction), so a neck cutoff was imminent. In such instances, the final cutoff event may occur during a large or extreme flood, but high flows are not essential, as the process is simply the inevitable end result of ongoing bank erosion. On the Afon Carno, this cutoff has now occurred.

Did You Know? Neck cutoff is just one of the many processes by which meandering rivers abandon former bends. Chute cutoff is one alternative process, and occurs when overbank floodwaters take a more direct path across the floodplain between adjacent parts of a meander bend. These floodwaters carve a straighter channel into the floodplain, and over time this channel deepens to take more and more of the flow and sediment transport, eventually also leading to bypass and abandonment of the former bend. While bends abandoned by neck cutoff tend to be tightly curved and form nearly complete circles in plan view, chute cutoffs tend to result in abandonment of more open, less circular bends. More than 30 years ago, an extensive survey of 964 km of major river valleys in Wales and the Borderlands identified 145 cutoffs. 16% involved simple neck cutoffs, 55% involved simple chute cutoffs, while the remaining cutoffs were the result of more complex processes involving ‘multiloop’ (13%) and ‘mobile bar’ forms (11%), or were due to artificial channel straightening or realignment (5%). For individual rivers, cutoff rates were found to be highly variable but the dataset as whole indicated that one cutoff occurred every 5 years in the period 1880-1900 and nearly every other year in the period 1950-1970 (Source: Lewis, G.W. and Lewin, J., 1983. Alluvial cutoffs in Wales and the borderlands. Special Publication of the International Association of Sedimentologists, v.6, pp. 145-154). Given recent concerns over alterations to flow regimes in Welsh rivers resulting from climate change and human impacts, it would be an interesting exercise to update this study and compare cutoff types and rates from the 1970s to the present.

10 Reasons Why …. 10 Rheswm Pam …. : 4

Tuesday 30th June 2015

Blog post 4 of 10 about the geomorphology of Wales. Click on images to view larger versions in separate windows. Parallel blog in Welsh at http://hywelgriffiths.blogspot.co.uk/

Reason 4. The Earth’s landscapes are dynamic. Landscapes and landforms are not static and unchanging, but are dynamic and develop through time.

bedrock reaches

While there is a tendency to view many Welsh landforms as fixed in form, size and position, in reality all landforms are subject to change over time. Nevertheless, because the rates and timescales of landform change can vary widely, the full extent of this dynamism is not necessarily widely appreciated. Variability in dynamism is well illustrated in Welsh river valleys, many of which alternate between relatively short, steep, fast-flowing reaches in bedrock, and longer, gentler, slower flowing reaches in alluvium (e.g. gravel or sand). In bedrock river reaches, illustrated by these photos from the upper River Severn in Hafren Forest, Powys, changes typically occur so slowly that they are effectively imperceptible during a single human lifetime. During low flows and regular flood events, the shale bedrock remains highly resistant to erosion by the water and any transported sediment particles. Nonetheless, the sharp, angular bedrock faces exposed in the river bed and banks provide clear evidence of local erosion of entire joint-bounded blocks, a process known as hydraulic plucking. Most of this erosion only takes place over short intervals (hours to days) during the rare extreme floods that may occur only once every few decades. Over many thousands or tens of thousands of years, however, the cumulative effect is a lowering of the river bed and upstream retreat of rapids (upper photo) and waterfalls (lower photo). As these rapids and waterfalls retreat upstream, albeit erratically, steep-sided bedrock gorges may be generated downstream, many of which also form spectacular elements of the landscape (Photos: Stephen Tooth).

alluvial reachIn many alluvial river reaches, changes tend to occur much more rapidly. The photo from the sinuous upper Severn River near Caersws, Powys, shows that deposition on an inner bend has formed a gravelly point bar, while erosion on the outer bend is maintaining a steep bank. Much of this depositional and erosional work takes place during regular flood events that perhaps occur a few times a year, and cumulatively this leads to lateral migration of the entire bend. Similar erosional and depositional processes characterise numerous other bends in such alluvial reaches (see Google Earth image) and, over time, collectively this results in river migration back and forth across the valley. Although the rare extreme floods may also play a role, they are not an essential part of the dynamics in such reaches, and so significant changes to river size, shape and position can be effected on timescales of years to decades (Photo: Hywel Griffiths. Aerial imagery from Google Earth, 2.5 km across. Blue arrow indicates flow direction (lower left to upper right) and red arrow shows direction of photograph).

Did You Know? The reach of the River Severn at Caersws is one of the most dynamic in Wales. Measured bank erosion rates at this site are typically 35 cm per year, but can be as high as 60 cm per year. Measured bank erosion rates on other Welsh rivers include maximum rates of 31 cm per year on the River Ilston and 0.96 cm per year on the Afon Trannon. These rates are low in comparison to rates on larger rivers; for example, 792 m per year on the Brahmaputra, Bangladesh, and 48.2 m per year on the Mississippi, USA (for these and other rates, see the compilation by Lawler, D. M. 1993. The measurement of river bank erosion and lateral channel change: a review. Earth Surface Processes and Landforms, v.18, pp.777-821). Nonetheless, they can cause significant management problems, including the loss of agricultural land, undermining of bridges and pipelines, threats to roads and railways, and introduction of high sediment loads. In some parts of Wales, sediments may be contaminated as a result of historic metal mining, and the increased loads can raise river bed levels, which may cause flooding problems downstream.