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DOCUMENT AMENDMENT RECORD

Client Mayo County Council Project North Mayo Landslide Title I Report on the Landslides at Dooncarton, , I Barnachuille and Pollathomais,

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EPA Export 25-07-2013:22:21:53 Report on the Landslides at Dooncarton. Glengad. Barnachuille and Pollathomais. County Mayo

TABLEOF CONTENTS

EXECUTIVE SUMMARY ...... 1 1. INTRODUCTION AND TERMS OF REFERENCE ...... 4 1.1 DRAVVINGS...... 5 2 . THE WEATHER CONDITIONS LEADING TO THE LANDSLIDES ...... 7 2.1 TIME AND EXTENT ...... 7 2.2 RAINFALL AND ANTECEDENT WEATHER STATISTICS FROM METSTATION AT .... 7 2.3 ESTIMATIONOF RAINFALL AMOUNT FROM RADAR IMAGERY AND OTHER STATION RECORDS 8 2.4 OTHER POINTERS TO THE RAINFALL AMOUNT...... 9 2.5 THEEVENT ON THE SHETLAND ISLANDS ...... 10 2.6 CONCLUSIONS AND PERSPECTIVE ON THE METEOROLOGICAL EVENT AND CAUSES OF THE LANDSLIDES ...... 10 3. THE LANDSLIDES AND INITIAL MECHANISM OF FAILURE ...... 13 3.1 INITIAL MECHANISMOF FAILURE...... 13 3.2 THELANDSLIDES ...... 14 4 . SITE DESCRIPTION...... 17 4.1 LAND TOPOGRAPHY...... 17 5 GEOMORPHOLOGY ...... 19 GENERAL...... 5.1 For inspection purposes only. 19 PREVIOUSSLOPE INSTABILI ConsentP/ ...... of copyright owner required for any other use. 5.2 19 6 . LAND USE ...... 20 7. SUPERFICIAL DEPOSITS ...... 21 7.1 BLANKETPEAT ...... 21 7.2 GLACIALTILL ...... 21 7.3 WEATHEREDROCK ...... 21 7.4 MARINE/ WINDBLOWNSAND ...... 22 7.5 DISCRETEZONES WITHIN WEATHERED ROCK...... 22 8. SOLID BEDROCK GEOLOGY ...... 24 9. WATER ON THE SLOPES ...... 25 9.1 WATERCOURSES ...... 25 9.2 GROUNDWATER...... 25 10. SURVEY AND INTERPRETATION OF ONGOING MOVEMENT ...... 26 10.1 REPORTON INSPECTION AREASA. Al. E AND B CARRIED OUT ON WEDNESDAY OCTOBER 22ND . 2003 ...... 26 I1. POST EVENT RISKS ...... 28

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EPA Export 25-07-2013:22:21:53 11.1 THENATURE OF RISK AND OUR APPROACH TO IT ...... 28 12. STABlLlSATlON IN SITU ...... 31 13. OTHER RISK MITIGATION OPTIONS ...... 32 I 13.1 CONTROLLED DESTRUCTION OF DESTABILISED PEAT ...... 32 13.2 INTERIMPROTECTION MEASURES ...... 32 I 13.3 REINSTATEMENTOF EXISTINGBARRIERS ...... 33 13.4 ROADINFRASTRUCTURAL WORKS ...... 33 13.5 PROTECTIONMEASURES - ADDITIONALWORKS - MEDIUMRISK AREAS...... 34 13.6 PROTECTIONMEASURES - LANDSLIDE BARRIERS- HIGH RISK AREAS...... 34 13.7 DRAINAGEWORKS:- DOONCARTONAND GLENGAD...... 35 I , 14. ESTIMATION OF COST OF STABlLlSATlON & PROTECTION WORKS ...... 37 15. SUMMARY ...... 39

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EPA Export 25-07-2013:22:21:53 Report on the Landslides at Dooncarton, Glengad, Barnachuille and Pollathomais, County Mayo

EXECUTIVE SUMMARY

On the night of September 19', 2003, intense localised rainfall caused multiple landslides on the slopes of Dooncarton and Barnachuille mountains. The combined effects of floodwater and landslides caused widespread damage to the public road infrastructure, to the old and new graveyards, and to private property, although fortunately without loss of life.

The primary cause of the more than 40 separate landslides in the Pollathomais area was exceptional rainfall, of such intensity as to overwhelm natural drainage systems in the peat and weathered rock, thereby mobilising sections of overburden through buoyancy and gravitational forces.

Analysis of the rainfall event from several perspectives supports the view that not less than 80 mm of rainfall fell on the slopes of the Dooncarton and Barnachuille mountains within a space of time less than two hours on that night. The expected frequency of such an event is once in 100 years. The localised nature of the event is underlined by the wide variation in the response of rivers in the area, with peak flood records being broken on the Bellanaboy catchment, even as the adjacent catchment, less than 6 miles away, recorded no impact whatever from the event.

For inspection purposes only. Once the extreme rainfall began,Consent water of copyright would owner requiredhave forrapidly any other use.penetrated to depth at cracked zones within the peat overburden, which zones would have been widespread following the dry weather in late summer. Given the intensity of the rainfall, water would have percolated rapidly to the base of the peat and weathered rock overburden. It would have spread laterally at shallow depth, and flowed downslope via the tubercular system of drainage conduits formed over the years within the peat. The result would have created zones, where the peat would have experienced buoyancy forces, lifting it off any impermeable subsurface deposits or the bedrock face below, while water at the failure surface would also have tended to lubricate the matrix of peat and weathered rock as a secondary effect.

Once sections of localised failure began to move, the slope gradient quickly imparted momentum to the sliding material, dislodging other material and liquefying the moving mass. Movement of the saturated debris away from the failure area was distinctly liquid in behaviour. What commenced as shallow translational sliding of peat and underlying weathered rock from the upper slope, continued as debris flows mixed with floodwater on the

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lower slopes, which also experienced rotational sliding of large masses of peat and weathered rock.

Mayo County Council have coordinated a progressive response to the event, concentrating initially on the safety status of the public infrastructure, the graveyards at Pollathomais as well as the LP1202 road, and in partnership with the Gardai and emergency services, framing appropriate guidelines to residents relating to their health and safety. Their emergency repair works have paralleled the work of surveying the disturbed deposits on the upper slopes over the entire affected area to assess their condition and the likelihood of further landslides. The Council have weekly coordinated the process of interpreting the ongoing survey findings, so as to adjust the guidelines on safe property occupancy in response to those findings.

It is clear that the decades old system of fencing and earthen ditches served an important function of absorbing some of the impacts of the debris flows on the mountain, and that the flood flows of debris-laden water gouged deep gorges in the natural drainage system approaching the public roads. The Council, assisted by the OPW, prioritised a programme of interim works to restore and make safe in a preliminary way, the damaged road infrastructure, the graveyards, and the damaged berms and ditches which had absorbed a significant part of the energy of impacting debris flows during the event.

This work can be characterised as immediate repair of the public infrastructure to a point which stabilises the position pending a considered response to residual risks.

Ongoing monitoring of the disturbed deposits on the mountain slopes indicates continuing For inspection purposes only. movement over the past month.Consent Weathering of copyright owner processes required for any have other use. commenced to break down the

failure faces into spalling blocks of debris of relatively small size, but recognisable blocks of disturbed overburden of several hundred square metres in extent are linked by tension cracks in the soil which continue to show movement. While the rainfall which caused the initial event was extreme, the destabilisation of remaining deposits is such that less extreme rainfall in future carries the risk of mobilising larger masses of material.

Technical options which attempt to anchor and stabilise this material in place are not recommended, because of the nature of the overburden and the ability of peat to creep around restraints. An approach of recognising that the disturbed material will inevitably continue to move, and of controlling that movement either by construction of earthen berms where appropriate, and kinetic energy absorbing rock-anchored barriers elsewhere, is recommended. The report identifies the locations where these works are required in order to protect the public infrastructure, and the adjacent properties, which are together at continuing risk from further landslides.

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This has identified three broad classes of necessary protection works, Le:-

Infrastructural works to the Graveyards, and the public roads, including replacement and upgrade of culverts, as well as wing walls, retaining walls, and protective barriers at hazards. This constitutes a package of measures to be implemented by the Council as Roads Authority, and as the Authority responsible for public infrastructure, and estimated to cost €2,835,500.00. Drainage channel remediation and stabilisation works, together with off-road catchpits to hold and prevent debris from being washed into road culverts, and the construction of a new earthen berm, with associated drainage works. This package of work, estimated to cost €685,589.20, is essentially earthworks and arterial drainage works in nature, and is suited to the expertise of an agency such as the OPW and could be carried out in partnership with Mayo County Council. Permanent kinetic energy absorbing barrier construction in designated areas, as a specialist package of works estimated to cost €1,539,075.00.

These works are necessary in order to reduce risks to public infrastructure and property to levels which will permit normal life in the community to confidently resume.

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EPA Export 25-07-2013:22:21:53 1. INTRODUCTION AND TERMS OF REFERENCE

On the night of September 19th, 2003, intense localised rainfall caused multiple landslides on the slopes of Dooncarton and Barnachuille mountains. The combined effects of floodwater and landslides caused widespread damage to the public road infrastructure, to the old and new graveyards, and to private property, although fortunately without loss of life.

TOBIN Consulting Engineers were requested on Sunday, September 2lSt,to assist Mayo County Council in assessing the condition of the slopes of the Dooncarton and Barnachuille mountains at Pollathomais, County Mayo, and to advise the Council in their response to widespread landslides and floodwater damage, which had threatened the lives and property of residents there.

We were requested to assess the risk environment for the public infrastructure, including roads, culverts, bridges, and the graveyard and also residential properties in the immediate aftermath of the event, and to keep that assessment under review as the survey information flow progressed.

Our instructions therefore required us to:-

0 make an appraisal of the conditions in the immediate aftermath of the landslides

0 advise on the risks of further landslides occurring, and the possible consequences of these

0 For inspection purposes only. determine the necessaryConsent protection of copyright and owner risk required mitigation for any other use. works

0 schedule and prioritise the works immediately necessary to stabilise the conditions on the slopes e make recommendations on long term protection works and precautionary measures

Our appraisal of the immediate conditions in the aftermath of the landslides lead us to advise that a number of residents in the area should be advised to leave their homes pending a proper assessment of the risk of further slides, and the text of this advice is included in Appendix No. 5.

Over the past month, Mayo County Council, assisted by the Office Public Works, have steadily progressed through a prioritised list of interim works, all of which have contributed towards restoration of normal life for residents and towards mitigating risks to public infrastructure and private property.

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EPA Export 25-07-2013:22:21:53 Our response to the Councils request for assistance on a broad front over the past month has therefore been a prioritised one, working in partnership with the Mayo County Council, and progressing in sequence from:-

* Advising the Council in a short term way on the probable condition of the disturbed slopes, in order that they might form a view of the safety status of the graveyards at Pollathomais as well as the LP1202 road, and so that they might frame appropriate guidelines to residents relating to their safety, Surveying the disturbed deposits on the upper slopes over the entire affected area to assess their condition and the likelihood of further landslides, Reviewing weekly the earlier advice, on foot of the ongoing survey findings, so as to adjust the guidelines on safe property occupancy in response to the survey findings, Prioritising and focusing the Councils’ programme of works in response to the perceived risks and the most effective mitigation measures, Reporting on the event and the residual risk environment, with recommendations related to necessary works to permit the normal life of the affected villages to be restored.

Our diary of work over the past month, at Appendix No. 1 indicates progress through the individual phases and this Report is the culmination of this work.

At the commencement of the survey work, the affected slopes were divided into several areas for identification purposes, namely A, AI, B and E (ref. Drawing No. 2033-1002). Points C and D marked the limits of the worst affected areas. We have retained this nomenclature For inspection purposes only. throughout this report. Consent of copyright owner required for any other use.

Surface deposits on the mountain consist of blanket peat on the upper slopes, of varying thickness. Beneath the blanket peat, the subsoil is a mineral soil, consisting of weathered rock, which has been affected over its history by processes of dissolution and redeposition of soluble constituents. Beneath this layer lies bedrock, which in places outcrops on the steeper slopes. Each of these features influenced the mechanism of failure, and the behaviour of the landslides once they had commenced.

1.1 Drawings The following drawings are referenced throughout this Report.

Drawing Title

I 2033-1001 I Layout Plan showing affected areas and surface water catchments I

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EPA Export 25-07-2013:22:21:53 Drawing Title No

2033-1002 Layout Plan showing slopes pre-landslides and limits of the survey areas ......

2033-1003 Layout Plan showing slopes post-landslides Layout Plan showing slopes post-landslide with tension cracks and monitoring 2033-1004 ...... points ...... Layout plan sho ied out by Mayo County Council & 2033-1005 ...... Office of Public ......

2033-1006 Layout Plan showing slope contouring post landslides

2033-1007 Layout Plan showing required extent of permanent works

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EPA Export 25-07-2013:22:21:53 2. THE WEATHER CONDITIONS LEADING TO THE LANDSLIDES

2.1 Time and extent The rainfall which fell on the Dooncarton, Barnachuille, Glengad and Pollathomais area on the night of Friday, September lgth 2003 was both extremely intense and exceptionally localised. The Met Eireann Station at Belmullet, 8 miles away approximately, did not effectively sense it as an event at all, and the streams in the area showed wide variation in response, reflecting the wide spatial variation in rainfall over short distances in their catchments.

Anecdotal evidence from householders in the area suggests that rainfall on that night did not commence before 8.45pm, and the rainfall event was effectively over at 10.45 pm that same night. The non uniformity of the rainfall, over such a small area in regional terms, makes the task of estimating the rainfall amount and peak intensity difficult, as the recorded event at the nearest Belmullet Meteorological Station is certainly unrepresentative of the rainfall as experienced on the upper slopes of Dooncarton and Barnachuille mountains.

We believe it to be significant that on the same day, exceptionally heavy and localized rainfall was also experienced in the Shetland Islands, causing landslides and infrastructural damage there on a similar scale, and indicating atmospheric instability on a wider regional scale as the mild late summer temperatures of early to mid September gave way to more typical autumnal conditions over a short space of time.

For inspection purposes only. Consent of copyright owner required for any other use. 2.2 Rainfall and antecedent weather statistics from Met Station at Belmullet Soil Moisture Deficit at Belmullet, as recorded by Met Eireann, rose steadily through the month of August last and had reached 54mm by September 4th. Soil Moisture Defecit describes the extent to which a previous period of dry weather has dried the upper levels of the soil, and it broadly provides an indication of how much rainfall would be required to redress the defecit, before significant runoff to streams and rivers would occur. Antecedent conditions on the mountain in late summer were therefore quite dry.

Met Eireann describe the situation on September lgfhas one where a cold front, with waves off the south coast, was slow-moving and affecting counties in Munster and Leinster. The situation over and Ulster was different, with showers developing during the morning and afternoon over land and showers also moving in from the Atlantic during the day. Their report also refers to evidence of a trough off the northwest coast during the day on the Atlantic surface charts.

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EPA Export 25-07-2013:22:21:53 Their interpretation of radar pictures taken on the day indicates shower activity over Connacht during the morning and afternoon with some intense echoes in one or two places at times.

Satellite thermal infra-red imagery taken from Dundee at 23:13 hrs, and referred to in the Meteorological Service report of the event, indicates shower activity over the Atlantic with a thicker band of shower cloud near the Northwest coast of Ireland.

Met Eireann have radar pictures available, at 15 minute intervals, from both Dublin and Shannon Airport Radar stations. Their view of these radar traces suggest that, whilst there were showers in the vicinity of Pollathomais Mountain during the day, it was not until the interval from 20:OO hrs and 23:OO hrs that intense shower activity occurred in this area. They remark that this intense activity was very localised. The Meteorological Service comment that individual radar echoes moved very slowly North Eastwards, but over this critical period in the Pollathomais area a tendency for cumulonimbus cloud formation, upstream of existing cumulonimbus cloud, meant effectively that this very local area was under the influence of active cumulonimbus cloud for a few hours.

2.3 Estimation of rainfall amount from radar imagery and other station records Met Eireann are of the view, from radar imagery, that rainfall amounts in the vicinity of Pollathomais Mountain on September 19th would have been much larger than amounts recorded in the vicinity of Belmullet itself, due to the highly localised intensity of the showers. This corroborates gauge readings at Belmullet and at Inver National School, which is approximately 5 miles ENE of Belmullet For inspection and purposes 3 miles only. west of Pollathomais. Consent of copyright owner required for any other use.

The total rainfall amount recorded at Belmullet on September 19fhwas only 5.0mm, with 3.6mm of this total falling between 1800 and 2400 hours.

On the moniing of Saturday, September 20th,the total rainfall amount read at Inver National School was 89.3 mm, indicating the extraordinarily localised nature of the event. This value refers to the period 0900 hours on 19thto 0900 hours on 20fh,however, the Meteorological Service consider that most of this fall was probably during the period from 2000 hrs to 2400 hrs hours on September 19fh.

A third rainfall gauge at Bunnahowen (Muinnachree) which is approximately 6 miles SE of Belmullet recorded a value of 8mm for the 24 hour period up to 0900 hours on 20fh,again indicating that the rainfall event was barely sensed at this location.

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EPA Export 25-07-2013:22:21:53 I Met Eireann caution us on the difficulty of estimating rainfall amounts from radar echoes, . particularly where distance from Shannon would have occluded the lowermost 2km due to the curvature of the earth. Nonetheless, a “hot spot” of local intensity greater than 23 mm/hr clearly shows in the radar echo, reproduced as Figure 1, and for the reasons stated, the intensity on the slopes of Dooncarton and Barnachuille mountains, at 250m over sea level, is I likely to have been much greater than this. In terms of temperature fluctuations, the Meteorological Service indicate that temperature during the day reached a maximum of around 16 Degrees Celsius in the afternoon but were about 11 degrees during the period of heavy rainfall.

Met Eireann conclude that, “In the vicinity of Pollathomais, rainfall amounts of 82 mm would L be expected to occur only once every IO0 years in a 24-hour period, ” G.

The response of rivers in the area varied remarkably on a scale which reflects the variation in I

A report prepared on the hydrometric aspects of the event by the EPA indicates that the water : -- Q level at their Station 3301 1 on the Bellanaboy at the Bridge on the L1204 at Muingeroon arted to rise at 10:15pm on the night of 19 September 2003. It rose at a marked rate

until it peaked at 00: 15 hrs on 20 September For inspection 2003 purposes when only. the datalogger recorded a record high Consent of copyright owner required for any other use. water level for the site. It is always difficult to estimate such an extreme flow based on an - extrapolation of the rating curve established on much lower measured flows at‘the site, but, f with suitable caution, EPA estimated the peak runoff, corresponding to the recorded water t level on the staff of 3.06 myat 45 m3/s, and it is clear from their Photographs No. 1 and 2 that this flow completely filled the opening of the Bellanaboy Bridge. Their report is appended at

‘ Appendix No. 2 and is a most useful perspective on the event.

. The Gauging Station rqcords all of the water that passes it, and for a distinct rainfall event

such as that on September 19‘h, the Gauging Station at the bridge downstream of Bellanaboy I Bridge (i.e. Stn. No. i3011) provides a record of just how much water passed that point following the rainfall event. When the catchment area is known, and an estimat moisture deficit is made, the amount of causative rainfall can be estimated.

The volume of the flood in the Ballinaboy River at Station 3301 1 Muingeroon South totalled approximately 620,000 m3 of water over the 9.45 hour period from 22:15 hrs on 19

4. .f

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Consent of copyright owner required for any other use. other any for required owner copyright of Consent For inspection purposes only. purposes inspection For For inspection purposes only. Photo No. 1 Muingerroon South Bridge with Staff GaugeConsent for Station of copyright No. owner 3301 required 1 on for L.H.S. any other use.Flood Level at 3.060m on the staff is a Station record.

(Photograph courtesy of H. McGinley, EPA)

EPA Export 25-07-2013:22:21:54 Maximum Flood level = 3.060m.0.D. @ 00.15 Hrs 20/09/03

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Photo No. 2 Maximum Flood Level at Station 3301 1 filled the bridge opening (Photograph courtesy of H. McGinley, EPA)

EPA Export 25-07-2013:22:21:54 L:

September 2003 to 08:OO hrs on 20 September 2003, by which time the hydrograph had returned to baseflow levels.

From the watermark evidence collected on site, it can be established that most of the flood waters recorded by the data logger at Station 33011 (Muingerroon South) originated in the Ballinaboy River north of Ballinaboy Bridge (refer to Drawing No. 2033-1001). The catchment area to the secondary Station 33014, at Bellanaboy Bridge itself, is estimated at 10.8 km2. If the total runoff from this flood over the Bellanaboy River Catchment is spread over this active sub-section of the catchment above Bellanaboy Bridge, then a runoff in excess of 50 mm results. In further discussing the event with the EPA Hydrometric Staff, they formed the view that the small catchment marked No. 1 on Drawing No. 2033-1001 at Aghoos also showed little sign of high flows. If this is so, then the active subcatchment north of Bellanaboy Bridge may be even less than the 10.8 km2area referred to the Bridge, thereby increasing still further the estimate of runoff expressed as mm over the active area.

With due allowance for water held on the catchment in satisfying an estimated 10-15mm of Soil Moisture Deficit at September lgfh,this figure suggests that the rainfall which caused this event is unlikely to have been less than 80mm.

As final evidence of the remarkably localised nature of this event, the Gauging Station on the (Stn. No 33001), which is shown on Drawing No. 2033-1001 and is 8.8km distant, actually showed a declining flow hydrograph throughout the night of September 1gth/20fh,as can be seen from Figure 2.

For inspection purposes only. Consent of copyright owner required for any other use. 2.5 The event on the Shetland Islands During the early morning of the same day, Friday September lgth, 2003, landslides occurred on the southern side of Shetland off the Scottish coast. Heavy rain had started in the early hours of the morning, with the first landslides reported at 6.45am, followed by several others. Our investigations of this event suggest that 79mm of rainfall was recorded at low level there between 6am and 8am on that day. A rain gauge located 10 miles away recorded 18 mm. The similarities between the intensity and localised nature of the two events in Shetland and Pollathomais, which occurred hours apart, is striking, and is, we feel, indicative of a particular atmospheric instability, over a wide geographic region on that day, and which included local intense cells of rainfall at the locations affected.

2.6 Conclusions and perspective on the meteorological event and causes of the landslides It is clear that an amount of rainfall of the order of 80mm fell on the slopes of the Dooncarton and Barnachuille mountains in a period of two hours shortly before midnight on September

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EPA Export 25-07-2013:22:21:54 For inspection purposes only. Consent of copyright owner required for any other use.

EPA Export 25-07-2013:22:21:54 19th, 2003. This was an extreme rainfall event, with a recurrence frequency not less than once in 100 years.

Later in this report, and while they are not completely separable, we distinguish between risks associated mainly with the action of floodwater, and risks associated mainly with further landslides of destabilised material. Volumes of floodwater on the night of September 19th were extreme, just as the causative rainfall was extreme. The volumes of floodwater to be expected in a future event will continue to depend upon antecedent wetness condition in the catchment, and the severity of the rainfall event. In this sense, floodwaters such as those experienced that night, will broadly require similarly intense rainfall.

The position with future landslides is different. The rainfall event of September 19th has changed the stability of blanket peat deposits in the affected area, and has disturbed their drainage mechanisms, such that the threshold of rainfall event required to move previously destabilised peat deposits has now been lowered. It will not require a similarly extreme rainfall event to significantly increase the risk of landslide arising in the deposits of peat which have now been disturbed, and which remain on the upper slopes.

The primary cause of the event at Pollathomais was therefore exceptional rainfall, of such intensity as to overwhelm natural drainage systems in the peat and weathered rock, thereby mobilizing sections of overburden through buoyancy and gravitational forces.

Some of the residents of the area have expressed concern that previous movement of spoil For inspection purposes only. laden vehicles or rockbreakingConsent associated of copyright with owner requiredcivil engineeringfor any other use. works in the area may have been a contributory factor in the event. It has been our experience of many years of measurement of the vibrational effects of heavy traffic, and rockbreaking, that the peak particle velocities (ppvs) in rock or overburden, associated with such work, fall away rapidly in time, and also with distance from the source of vibration. At distances of the order of 50m from heavily laden vehicles traveling at speed, or from a rock breaker, ppvs will have dropped to barely measureable levels, well below levels accepted for minor vibrational damage to structures. Given that the landslides commenced at distances of the order of 300m- 400m from the LP1202, we can assure the Council that vibrational effects from past construction activities or related traffic were not a factor in this event. We can also be absolutely certain that bedrock geological structure played no part in this event, and the stability of bedrock following this event is not in question.

Others have expressed concern that the Radar Station may have contributed in some fashion to the event. In response, we note that the primary cause was intense, localized rainfall, but brought about by atmospheric conditions which were themselves regionally distributed, as is

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EPA Export 25-07-2013:22:21:54 evidenced by the Shetland event. Landslides at Pollathomais extended considerable distances from the Radar Station, and survey indications are that the worst of the rainfall centred to the east of the affected area, furthermost from the Radar Station. We can find no evidence that the construction methods used (which did not involve blasting) or the presence of the access road exerted any influence in terms of bias in the drainage networks in the area, and in all the circumstances we can adduce no mechanism by which the construction or presence of the Radar Station can have affected matters in any way.

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EPA Export 25-07-2013:22:21:54 3. THE LANDSLIDES AND INITIAL MECHANISM OF FAILURE

3.1 Initial Mechanism of Failure Antecedent weather conditions in late summer were such as to dry the upper surface of the blanket peat to a point where shrinkage and dessication cracking would have been widespread. In. addition, numerous old failure surfaces and tension cracks were also probably opened during the dry summer. Once the extreme rainfall began, water would have rapidly penetrated to depth at cracked zones within the peat, and, given the intensity of the rainfall, which we estimate reached 60mm/hr for a short period, water would have percolated rapidly to the base of the peat / weathered rock, spread laterally at shallow depth, and flowed downslope via the tubercular system of drainage conduits formed over the years within the peat. The result would have created zones, where the peat would have experienced elevated pore pressures and buoyancy forces, lifting it off any impermeable subsurface deposits or the bedrock face below, while water at the failure surface would also have tended to lubricate the matrix of peat and weathered rock as a secondary effect.

Very thin impermeable zones were found extensively within the soil below the peat, in the C-r *- forq of thin iron or mineral pan surfaces, and are present in sections of Area A, B, E, in Carnhill and Seanbhaile, whereas an intermixed layer of pea# and rock acted as the failure surfqce in Area Al.

Once sections of localised failure began to move, the slope gradient would quickly impart kinGic energy to the sliding material, dislodging other material and liquefying the moving mass. Aerial Photographs, taken after the event, clearly show that, while material failed in For inspection purposes only. Consent of copyright owner required for any other use. tension and slipped on the upper side, movement away frob the failure area was almost immediately liquid in behaviour. I Based on visual inspection of failure scars the following primiry landslide failure types have been identified, and are illustrated in Figure 3: . i t (1) Shallow translational sliding of peat and underlying wd athered rock from upper slope. (2) Shallow rotational sliding of weathered rock on lower Slope. 1 Following primary failure, secondary failure types were initiatdd, namely: ! (3) Breakdown of failure debris and mixing with floodwater resulting in debris flow. Dislodgement of rock boulders resulting in isolated movement of boulders. (4) J

The most prevalent failure types are (1) and (3).

In terms of the pathways of the landslides, the following points have emerged:-

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EPA Export 25-07-2013:22:21:54 61 ParticleDebxis

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Figure 3 - Schematic Section through Slope

EPA Export 25-07-2013:22:21:54 0 Material, once mobile, behaved as a fluid due to the amount of surface and entrained water. e This feature of the slides brought the existing drainage channels and gullies into play as prominent material conduits on the middle and lower slopes.

0 Local small topographical features, both man made and natural, exercised a large influence on the path of the moving material. e The scouring effect of the mass of entrained material in the heavy water flows significantly eroded drainage gullies, causing secondary collapse of the sides of these gullies during the event.

3.2 The Landslides We have identified 41 distinct failure zones which occurred on the slopes during and following the intense rainfall, and these have been labelled on the post works aerial photograph of the affected area on Drawing No. 2033-1003. In addition to the northern and western slopes of Dooncarton / Barnachuile mountains, failures also occurred on the reverse or eastern slopes of Dooncarton mountain, affecting the Sheanbhaile road. Other failures occurred on the north western slopes of the Gortbrack mountain.

Landslides over the greater part of the affected area struck the berm fence separating commonage from individual land ownership, and this absorbed a significant fraction of its energy along much of the front, and was followed by heavy deposition of material behind the berm. The drain upslope of the berm redirected flow towards pre-existing drainage channels.

Where landslide debris filled the drain, For inspection and purposesat low only. points on the berm, landslide material Consent of copyright owner required for any other use. broke through the berm and flowed downslope.

It is evident, both from on site observation and from study of aerial photographs, that this old man-made boundary played a significant role in protecting the public road infrastructure, the residents and properties from the torrent of water and debris, streaming from all the landslides.

Along the southern side of Area Bythe commonage earthen berm trapped blocks of peat and vegetation so that it was primarily water which struck the upper side of the wood, dislodging small trees and scrub material. At locations where this channel was blocked by trees and other debris, two distinct streams of waterborne debris were generated, which cut through the low vegetation on the steep slope above the graveyard, gaining kinetic energy from the slope before it impacted on the graveyard and road below. Additionally, a NE trending drainage channel concentrated intercepted water at the crest of the berm, causing water to be deflected

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EPA Export 25-07-2013:22:21:54 eastward over the surface to a small drainage channel on the lower slope. This scouring flow caused the failure referenced as to SI (Refer to Drawing No. 2033-1003).

Three failure surfaces are present on the lower slope in Area Byone above the old graveyard and two to the rear of Properties 3 and 4. All three were examined in detail and it is considered that each localised landslide was triggered by either blocked drainage channels upslope or by the scouring effect of extremely high volumes of debris-laden surface water runoff. The momentum of the flow at the time is evidenced by the agricultural tractor which was swept downhill, and was rotated and deposited in the central part of the lower slope behind Property 3.

The landslides in Area E were intercepted by the earthen berm and ditch. Material which reached the lower road LP1202 was in the form of fluidised flows confined to, or in the vicinity of, existing drainage channels. The middle slope above the earthen berm is strewn with thousands of flat tabular boulders, dislodged and flushed down from the rock escarpment between the base and middle of upper slope.

The streams of debris deposited on the lower slope by the landslides, consisted of rounded peat blocks, some cobble / gravel sized rock fragments, vegetation, and utility poles.

The most severe property damage occurred in Area A where landslides referenced S22 and S23, laden with peat, flowed relatively unhindered onto and across the upper Glengad road (LT12025). The earthen berm is locally absent, or poorly developed in this section of Area A. Material from these two landslides partly demolished Property 37, and deposited large For inspection purposes only. amounts of debris against the externalConsent of copyright walls owner of Property required for any 38. other use.

It is evident that in the eastern section of Area A and Al, the failures on the upper slopes were quickly channelled by the ground profile, particularly by wide benches in the rock escarpment in the middle of the upper slope, into a common flow path across the plain below the rock escarpment. The entrained water and rock followed the drainage pattern in the area, taking the flow into the drainage channel which flows northwards to the sea. Residents in the area have advised that, prior to the event, this channel had typical dimensions of a few feet but the very high flowrate of material, and entrained rock, scoured the channel heavily over its entire length, from the upper road to the sea. Again the berm fence between commonage and private lands absorbed a significant part of the residual kinetic energy of the slides at the point of impact with the fence, and again the debris field behind the fence also filled the drains in that area, locally affecting lines of drainage during the event. This, and the local condition of the berm fence, resulted in breaches of the berm defence at the rear of Properties 37 and 38, and followed by overland flow of the debris right to the LP1202 and across it to the foreshore.

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In the area south of Point D, the debris from landslides on the northern face of Dooncarton mountain in Seanbhaile moved to the stream which drains the valley and became entrained in the torrent of surface water in the stream at the time. Housing in this area lies upgrade on the opposite slope, which was not affected by landslide, and was therefore primarily threatened by the debris/floodwater matrix, rather than landslide per se.

For inspection purposes only. Consent of copyright owner required for any other use.

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EPA Export 25-07-2013:22:21:54 ~ ~ ~~ ~~ ......

4. SITE DESCRIPTION

4.1 Land Topography Drawing No. 2033-1005 shows the contoured profile of Dooncarton and Barnachuille Mountain indicates ground levels ranging along the ridge between summits from 205 to 260 m OD. The location of most of the affected properties along the LP1202 resulted in a concentration of survey and analytical effort on the northern slopes from the coastline up to the ridgeline between Dooncarton mountain (260 OD) and the adjacent Barnachuille mountain (242 mOD).

The slope profile typically comprises all or some of the following elements:-

(1) Ridge Line. This consists of a narrow strip of gently sloping ground following the ridgeline. Peat and weathered rock are of variable thickness, with bedrock assumed to be at relatively shallow depth, evidenced by the bedrock at exposed failure zones. Some localised areas of old peat cuttings are present.

(2) Upper Slope. This zone comprises a steep upper slope with slope inclination typically 30 to 60 degrees and with peat blanket thickness varying from 0.2 to 1.2m. The depth of the underlying deposits of weathered rock is typically less than lm or may even be locally absent. Bedrock is again at relatively shallow depth and locally may directly underlie the peat cover.

Middle Slope. In this intermediate zone, land slope is typically 10 to 30 degrees, (3) For inspection purposes only. Consent of copyright owner required for any other use. and peat cover varies from 0.2 to lm in thickness. The depth of the underlying weathered rock layer is generally less than lm, so that bedrock is at relatively shallow depth of 1.4-2m.

(4) Lower Slope. These mountains depart from a profile of gradually diminishing slope toward the coastal lowlands. A steep lower slope with slope inclination in the region of 45 to 60 degrees, is a significant feature of lands immediately south of the LP1202 in Area B. On this lower slope, blanket peat is either present as a thin layer or is absent. As to the lower deposits, the weathered rock layer in this zone is thin, or is locally absent. Bedrock is therefore at shallow depth and is exposed in places. The Lower Slope has a benched appearance, which reflects the underlying bedrock structure.

(5) Coastal Strip. This consists of a narrow strip of moderately sloping ground, varying from 10 to 30 degrees, adjacent to , to the northeast of the LP1202.

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EPA Export 25-07-2013:22:21:54 1

The ridgeline between the summits of Dooncarton and Barnachuille mountains dip into a local saddle point. Below the saddle, the backwall of the drainage basin is notably steep, about 60 degrees, with bedrock exposed. Several small streams drain this basin and flow northwards into .

For inspection purposes only. Consent of copyright owner required for any other use.

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EPA Export 25-07-2013:22:21:54 5 GEOMORPHOLOGY

5.1 General The slopes of DooncartodE3arnachuille mountain show some evidence of previous slope instability, which is typical of mountainous terrain, and is visible elsewhere in the Pollathomais region and in North Mayo.

5.2 Previous Slope Instability At the base of the upper slope and lying on the middle slope are ridges, healed tension cracks and masses indicative of previous landslide activity. Some of this debris is possibly related to the periglacial period, that is immediate post-glacial, and represents soliflucted-type deposits, or those generated through freeze-thaw cycles over time.. Rockfall is likely to have occurred from the steep upper slope, particularly from the backwall of the drainage basin. Large rock pieces are present at the base of the slope particularly in the basin area. More recent landsliding is evident by the presence of irregular topography, and numerous scarp features observed on the upper slope. This likely represents localised slippage or creep movement of overburden deposits.

In recent years a peat slide occurred on the southern slopes of Dooncarton, below the 205m OD summit. Inspection of this slide showed failure of peat on mineral soil. Anecdotal information suggests that the slide occurred in the late 1970s.

For inspection purposes only. Consent of copyright owner required for any other use.

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EPA Export 25-07-2013:22:21:54 6. LANDUSE

The affected area consists of rough commonage grazing on the higher slopes, with improved grazing on the lower slopes in those locations where the ground has been drained, fenced and fertilised in private holdings. The coastal strip contains the LP1202 public road, adjacent houses in mainly linear village development and both the new and old cemeteries.

The area of Barnacuille has a total of five dwellings, adjacent to the road on the middle slope of the mountain. The lands to the immediate south of these properties comprise mainly of commonage grazing directly behind the drainage berm on the top slope. To the north east of these properties, on the middle slope of the mountain, private agricultural land is farmed. The lands in this area are maintained and drainage and fencing work is evident. These lands would have made up the landowners primary agriculturally productive holdings in this area and these have been affected by water and debris flow during the event.

As one progresses westward, the LP1202 road traverses the middle slope where there is further housing. Again, this area has developed in a linear village extension from the original dwellings. Most properties along the road LP1202, on the lower slope, are constructed with agricultural buildings and yards attached to, or near the main dwelling. The lands in this area are farmed as pasture, and clearly have been maintained by improved drainage and fencing works over the years.

The Glengad Road (LT12025) has a total of eleven properties adjacent to the road and again all lands to the north of this road would For inspection be considered purposes only. to be improved agricultural lands. The Consent of copyright owner required for any other use. lands to the south of this road are again primarily commonage with some small plots of land having been reclaimed from the open areas. These small plots have had drainage works and new post and wire fencing added, to separate them from the open commonage areas.

The LP1202 continues across the lower slope of the GlengadDooncartan area with dwelling houses constructed in a linear village on the southern side of the road only. Again the majority of the dwellings in this area have associated agricultural buildings and yards adjacent to, or near to the main property. The lands in this area have been used for primarily agricultural uses, such as grazing or fodder harvesting (silage).

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EPA Export 25-07-2013:22:21:54 7. SUPERFICIAL DEPOSITS

The superficial deposits within the Dooncarton-Barnachuille area can be briefly classified into the following main types:-

(1) Blanket Peat (2) Glacial Till (3) Weathered Rock (Hill Creep Deposit) (4) Marine /Windblown sand

In general terms all superficial deposit types oLer than peat may be referred to as mineral soils. Discrete layers of material within the weathered rock observed in failure scars are also discussed separately below.

7.1 Blanket Peat Blanket peat covers most of the slopes. In certain areas, such as the lower slope, peat is thin or absent, and blanket peat thickness varies from 0.2 to 1.2m. Examination of peat exposures shows humification of peat at the base. In most cases peat lies directly on weathered rock.

Examination of the interface between the overlying peat and underlying weathered rock showed no indications of a relict ground surface (palaeosol), such as a buried forest which may have been inundated by peat development. Relict tree stumps, which have been

identified from lower-lying sites in the For inspectionvicinity, purposes were only. not present. Consent of copyright owner required for any other use.

7.2 Glacial Till Till deposits were not identified in the failure scars, though till is present towards the lower part of the slope. Till is commonly found as silty/clayey sandy gravelly matrix, of low plasticity, with varying amounts of coarser material. The coarser material ranges from sub- angular to sub-rounded fine to coarse gravel, through to cobbles and boulders.

7.3 Weathered Rock Weathered rock underlies the blanket peat and lies above bedrock. Most of the slopes have a varying cover of weathered rock with a thickness ranging typically from 0.2 to several metres, as evidenced from outcrop exposed in some of the drainage lines on the lower slopes. Locally weathered rock is absent on the steeper slopes, notably parts of the upper and lower slopes, and in these areas bedrock underlies the peat or is exposed.

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I

Weathered rock on the slopes is likely to have been affected by mass movement, such as solifluction (freeze-thaw mechanisms) or by creep. Where solifluction is the main form of movement, then the term 'head' is commonly applied to such material. Up to 2m of cohesive hill creep deposits are exposed on the Barnachuile Road.

The apparent absence of any residual structure in the disintegrated weathered rock, as exposed in the failure scars, suggests that most of the weathered rock debris, particularly on the upper slopes, has been affected by creep movement over geological time extending over millions of years. This type of slow creep movement of weathered debris downslope affects all mountain slopes to varying degrees, but would not have been a primary causative factor in the landslides of September 1gth 2003.

7.4 Marine / Windblown Sand Light brown sand is exposed in cuttings on the Barnachuille Road. The sand deposit is thought to have originated from past Marine / Coastal depositional processes and it forms the core of the lower slope in Area B.

7.5 Discrete Zones within Weathered Rock Within the weathered rock, as observed in failure scars from the upper slope, there is a zone aligned parallel to the ground containing several discrete layers. This zone forms the basal shear surface of most failures on the upper slope. The zone is typically located at 200 to 300 mm depth below the top of the weathered rock with a typical thickness ranging from a few millimetres to 1OOmm. This zone contains:- For inspection purposes only. Consent of copyright owner required for any other use.

(1) Hard pan comprising a layer, varying from several millimetres to 50mm thick, of nodular hard black internally red amorphous silthlay into which is cemented angular gravel to cobble-sized rock fragments. The hard pan is considered a precipitated layer, of iron mainly, but also including manganese minerals, leached from the overlying deposits. This pan layer is known locally as the Red Pan.

(2) Above the hard pan is a layer of irregular thickness typically 50 to +100mm, which can be locally absent, of soft to very soft brown to dark brown slightly organic sandy silthlay.

(3) The top of the zone comprises a thin layer (typically 3 to 5 mm) of stiff black to dark brown amorphous organic silt/clay. The thin layer is polished and striated on the upper surface, which may represent slickensides, or relative movement between layers of material.

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EPA Export 25-07-2013:22:21:54 Not all parts of each zone are present at all failure scars. Within the eastern failures, layer (1) appears to be prevalent. Where layer (2) and (3) are absent there is sometimes a very soft black amorphous siltlclay. In certain areas zone (2) is thin. Due to erosion of the basal shear surface by water and the movement of failed material, it has not been possible to assess the actual extent ofthe zone.

For inspection purposes only. Consent of copyright owner required for any other use.

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EPA Export 25-07-2013:22:21:54 8. SOLID BEDROCK GEOLOGY

The Geological Survey of Ireland report entitled "Geology of North Mayo" (1 992) indicates that the area consists of rocks of the Dalradian Supergroup. The rocks are Late Precambrian in age (700-600 million years old). The Dalradian rocks of North Mayo preserve a variety of deformational and metamorphic features which were developed during several episodes of orogenesis'. Folding and accompanying formation of schistosities, and metamorphism intermittently spanned an interval of about 200 million years from Late Proterozoic to Devonian times.

Mica Schist of the Inver Schist Formation (Appin Group) form the upper slopes of the mountain. These schists are locally folded and deformed. However there is an observed strike trend between 072" and 145" with an average trend of 115" with an average dip of 40" to the south. All the landslides in area B and E occurred over these schists and weathered tabular boulders formed part of the resultant debris field. Geological information indicates there is a limestone (marble) band within the schist although this was not observed in the areas B and E during the survey. On the lower slopes of the mountain the schist is observed overlying older psammites, quartzites and semi-pelites of the Grampian Group. The boundary between the two lithologies is considered to be ductile faulting. The weathering profile of the schists varies from 0.2m to 2m whereas the psammites and quartzites typically have a weathered zone between 0.4m and 2m.

For inspection purposes only. Consent of copyright owner required for any other use.

' Orogenesis - Mountain building due to continental collision.

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EPA Export 25-07-2013:22:21:54 9. WATER ON THE SLOPES

9.1 Water Courses A number of natural minor streams drain the higher slope. In most cases, these streams originate in shallow topographic depressions on the ridge line and/or upper slope where surface water concentrates, and thereafter the streams follow the contours downslope. Several failures were noted to form below areas of topographic depressions on the upper slope.

Streams on the higher slopes are typically narrow gullies less than a metre wide with the stream bed located in weathered rock. Further downslope, these streams join a ditch and associated berm. The ditch and berm run transversely to the slope and form the boundary between commonage land upslope and privately owned land downslope. The ditch therefore intercepts discharge from minor streams that flow into it across the slope. At several locations the ditch is cut through by larger streams which flow down slope into Sruwaddacon Bay and Broadhaven Bay. Within most of the larger streams bedrock is exposed in the streambeds.

9.2 Groundwater Near-surface groundwater movement is likely to have been a significant contributory factor in landsliding, which was of a shallow nature. The following observations are based on inspection of failure scars in the upper slope.

For inspection purposes only. The hard pan located at shallowConsent of copyrightdepth ownerwithin required the for any weathered other use. rock effectively impedes downward migration of groundwater due to the relatively low permeability of the pan.

The zone of soil immediately above the pan would therefore represent a preferential drainage path to groundwater movement downslope. Above the hard pan natural conduits are present in the weathered rock, as observed in most of the failures. These pipes range in width from about 0.1 to lm and are generally aligned downslope or slightly across the slope. It is likely that these tubercular conduits exploited depressions and undulations within the pan, which would reflect the pre-existing topography of the weathered rock surface.

A plane of seepage was also observed above the hard pan. This was evident from seepage emanating from the head scarp of many failures.

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10. SURVEY AND INTERPRETATION OF ONGOING MOVEMENT

Observations, measurement and surveying of the upper slopes, which began on September 22"d 2003, have recorded significant movement of disturbed material above and between failure scars.

Numerous Tension Cracks have opened along pre-existing cracks (reactivated Tension Cracks) and in previously undisturbed material. Monitoring of the Tension Cracks recorded movements of 2m and more, where peat blocks have detached and slid downslope along failure surfaces. The migration of tension cracks upslope was noted on a daily basis. All material downslope of a Tension Crack is unstable and 4,000 tonnes was detached and free to move as sizeable masses in Areas B and E up to October lofh2003. Wedges of material between failure scars did not show obvious signs of movement during the site surveys with the exception of Area B where a tension crack opened, widening and lengthening between the failure zones B3 and B4, upslope of the graveyard.

Surveyed positions of tension cracks are shown on Drawing No. 2033-1004.

Over 100 monitoring points have been established on the upper slope, above the failure scars, on and between Tension Cracks. The condition of the failure scars and tension cracks will need to be monitored visually, once a week, for a minimum period of sixteen weeks. GPS surveys have mapped the positions of all monitoring points over the past month. Monthly GPS surveys will need to be conducted for a minimum period of 4 months. For inspection purposes only. Consent of copyright owner required for any other use.

10.1 Report on Inspection Areas A, Al, E and B carried out on Wednesday October 22"d,2003 At the time of writing this report, the most up to date position related to residual movements of overburden on the mountain is as follows:

10.1.1 AreaA There are no obvious further movements on the majority of tension cracks, compared to the October 1Oth record, except:-

The tension crack upslope of Property 32 had widened to 0.2m due to downslope movement and showed signs of further lateral propagation. One tension crack in Area A showed signs of having been affected by recent surface water flows, with between 0.2m and 0.3m of grey gravelly sand deposited at the base of cracks. This process of washing of surface debris of small particle size into tension cracks will continue over the coming winter.

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EPA Export 25-07-2013:22:21:54 10.1.2 Area A1 On the failure surface in Area Al, detached peat blocks continue to disintegrate under the action of weathering. Blocks around the edges of the failure scar had torn free and moved downslope by various amounts, generally less than half a metre.

Tension Crack 2 (Re$ Drawing No. 2033-1 004) Locally, the aperture of tension crack 2 had further widened to 0.90m. There is also evidence of water borne deposition activity at the base of the tension crack, with grey sandy gravel deposits being formed at the base of the crack. It is clear from this inspection that surface water is utilising the tension cracks as irregular drainage channels, and these cracks will introduce water rapidly to depth in the peat in future rainfall events.

Tension Crack 1 (Re$ Drawing No. 2033-1 004) The previous monitoring point in this area has been damaged, with a monitoring peg knocked over. The aperture of the tension crack at the location of this peg is now 0.25m. At a location approximately 10m above failure scar, tension crack 1 has widened to 0.50m, and has extended in the overburden to a depth of 0.80-0.90m. The ground surface downslope of tension crack 1 has clearly dropped 0.1 m since the October 1Ofh inspection.

10.1.3 AreaE No obvious movements have been recorded in the tension crack in this area.

10.1.4 Area B (Ref. Drawing No. 2033-1004)

Tension cracks continue to form and For inspectionpropagate purposes between only. the two failure scars on the upper Consent of copyright owner required for any other use. slope above the graveyard. A new crack, currently less than O.lm wide, and 0.3m deep, was noted on the edge of B3, apparently migrating towards B4. The ground surface upslope is covered by reeds and silt, and was saturated under foot on October 22"d.

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EPA Export 25-07-2013:22:21:54 11. POST EVENT RISKS 11.1 The nature of risk and our approach to it Living involves risk-taking daily, risks from travelling, from sport, from accident or disease. Given that daily living cannot be risk free, our approach to risk management in the affected areas is one of trying to mitigate risk to reduce it as near as possible to pre-September 19th levels, so that where possible, normal life can proceed as before the event took place.

In deciding what level of risk we must aim to achieve, this is always judged against what is at stake, and tolerable risks to life or health differ in order of magnitude from other risks deemed tolerable in daily living.

In defining the risks associated with the condition of the land on the Dooncarton and Barnachuille mountains post September 1gth, we may partition these as follows:

0 Risks to lives, public infrastructure and property arising predominantly from further landslides

0 Risks to lives, public infrastructure and property predominantly from effects of floodwater

0 Risks to drivers using the LP1202 road

0 Risks to farmers working on the slopes of Drumcarton and Barnachuille mountains

Broadly speaking, it can be accepted that the seriousness of risks are related to the numbers of people who are exposed to these risks, and to the duration over which they are exposed to For inspection purposes only. such risk. Consent of copyright owner required for any other use.

Our approach to quantifying these risks is one of firstly examining the risk of large volumes of blanket peat and weathered rock failing and sliding downslope, given that extensive areas of blanket peat have been confirmed to have moved over the four week period following the initial event. This risk must then be taken through a further probability screen represented by the questions:

0 If the material slips, what is the risk that the resulting material flow reaches public infrastructure, or a residence, or other building used by people?

0 If such a slip reaches a road, or a property, what is the sequent risk of causing serious injury or fatalities there?

0 If the fluidized flow of a further slip in wet conditions is managed to avoid properties, but it reaches the LP1202, what are the consequences and risks?

As an interim classification, we defined the following risk categories.

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EPA Export 25-07-2013:22:21:54 -. . .

Low Risk (L):- Notwithstanding that further land slippages take place, public infrastructure and residential properties in this zone have a low risk of injury to persons or damage to property, and can be used and occupied subject to monitoring of conditions on the slopes.

Medium Risk (M):- Some hazards in these areas require remediation works, and ongoing maintenance of these works, and monitoring thereafter, and on the basis of these works proceeding without delay, residences can be occupied. Re-opening of the public road can proceed due to the repair of roadside fences and the reduced time of exposure to risk of those travelling on the road.

High Risk (H):- Areas designated High Risk require immediate preventative and protective control measures, and the public infrastructure and residences within these areas are at elevated risk until these works are in place.

Our analysis of the event of September lgth shows that the influence of raised features in the debris field was strong in directing what was a fluidized flow on the lower slopes. Similarly the ditch separating commonage from privately owned lands served to absorb some of the kinetic energy of the material which struck it and the pre-existing drainage channels also absorbed some of the volume of debris material reaching them.

In addition to the interim works carried out by Mayo County Council and the Office of Public Works, which will require to be maintained, the construction of a new berm in Area E will permit the reclassification of medium risk properties in the central Glengad area to low risk, by reducing the likelihood of further landslides reaching these properties, given always that For inspection purposes only. they can occur. Passage of timeConsent and of weathering copyright owner required of the for failureany other use. scars will continue the process of natural disintegration of smaller masses of material at the failure front, but the berm is needed to reduce the risk of a larger amount of material moving as a unit, under the influence of heavy, but unexceptional, rain over the next few years.

Our assessment of the likely path of further landslides, in Area B particularly, indicates that a permanent barrier will be required in the regions shown on the drawings in order to supplement the interim works already done, so as to reduce the risk of these interim works being overwhelmed by further falls of material in unit masses of 500 tonnes or more, again under the influence of heavy but unexceptional rain over the next few years.

With due precautions during periods of exceptionally heavy rainfall, when the LP1202 should be closed to non essential traffic, the foregoing measures will be sufficient to reduce risk to infrastructure and lives, to low risk category, for all properties except those currently high- risk and these will be reduced to low risk on completion of the proposed Works. We recommend that the Council agree with Met Eireann to include intense rainfall forecasts for

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EPA Export 25-07-2013:22:21:54 the Belmullet area within their service of severe weather alerts to the Council, and that procedures for precautionary closure of the LP1202 in the affected area during such conditions are agreed with the Gardai.

In terms of permanent works, we have considered the two broad options of stabilisation in- situ or barrier based protective measures in the following sections.

For inspection purposes only. Consent of copyright owner required for any other use.

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EPA Export 25-07-2013:22:21:54 12. STABILISATION IN SITU

Stabilisation options seek to fix the blanket peat in place by methods which anchor it to the stable rock surface below. Such options might include a grid of rock anchors drilled and grouted into solid bedrock, with the blanket peat between grid points held in place by a mesh of braided stainless steel cable spread parallel to the slope and tied to the anchors.

We cannot recommend reliance on such methods in this instance because:-

There are wide variations in gradient of the material to be stabilised Peat is a difficult material to hold, being capable of gradual creep and extrusion around and over a holding cable or anchor The degree of destabilization is such that intense, but unexceptional rainfall, will induce ongoing movement of a saturated mass between anchor points Ongoing movement and relaxation of the peat, even within the anchored grid, will tend to gradually redistribute load from one anchor point to another over time Such load redistribution increases the risk of overload at any one anchor point, and sequential failure of the anchors on the overloaded side

We accordingly recommend against reliance on options which seek to fix in place material which we believe will continue to move over time towards an equilibrium which may well see much of the blanket peat on the upper slopes denuded.

For inspection purposes only. Consent of copyright owner required for any other use.

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EPA Export 25-07-2013:22:21:54 13. OTHER RISK MITIGATION OPTIONS

Given that an approach based on stabilizing the blanket peat in situ is unlikely to be successful at Dooncarton / Barnachuille, our attention focuses on how to minimize the risk from material which will fall in future.

13.1 Controlled destruction of destabilised peat If the peat deposits which carry a risk of sliding as a mass can be broken up in a safe and controlled fashion, then the risk can be locally mitigated to some degree.

This would involve the cutting of the peat into blocks typically 10 cu.m in size, along the front of the failure surfaces over the steeper slopes, and inducing falls of manageable size to lower levels where weathering processes can be allowed to fbrther break up the masses not destroyed in the fall. Cutting of the peat blocks would be carried out by water- or compressed air-lances, with further movement induced by inserting inflatable diaphragms in the tension cracks and filling these with compressed air.

These methods will reduce the risk from material which is located on a slope where failure can be induced in a controlled fashion, under gravity, with some mechanical assistance, but without the action of significant rainfall.

Clearly this would be hazardous work for those involved, operating with the most unstable material at the edge of the steepest slopes,For inspection and purposes anchored only. harnesses and anchored tethers for Consent of copyright owner required for any other use. plant would have to be employed, working to method statements and a timetable driven primarily by considerations of worker safety.

This approach will leave residual deposits which will not move under the forces which can be mobilised as described, but which would move under the action of severe rainfall. Risks associated with this material could be removed by removal of the remaining blanket peat cover and weathered rock material itself, but this would be a major and costly material transport operation, with significant visual and other environmental impacts.

13.2 Interim Protection Measures The purpose of interim protection measures is to reduce the risk to infrastructure and properties to levels as near as possible to those pertaining pre-September 19th2003. However, the whole area has experienced significant landslides and no individual or combination of protection measures will guarantee 100% elimination of risk.

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EPA Export 25-07-2013:22:21:54 The significant role of natural and man made features in absorbing impact and directing the course of the landslides has been noted and their influence on the flow paths of debris and water streams has been taken into account in framing our proposed protection measures.

13.3 Reinstatement of Existing Barriers Mayo County Council have, with the assistance of the OPW, reinstated the ditch and berm between commonage and cultivated land on the middle slope. This work enabled the risk rating for many of the properties on the LP1202 to be reduced over the past month.

13.4 Road Infrastructural Works The LP1202 is crossed by a number of drainage culverts which are intended to take the runoff from the mountain streams beneath the road in a controlled fashion to the seaward side. A consequence of the utility of the drains and berms in intercepting material washed from the mountain on September 19fhis now an increased tendency of material to be washed further along in wet weather and to block these culverts. If the capacity of these culverts is not I available when called upon to pass runoff from future wet weather, then overland flow onto and across the public road will result. Depending on the intensity of the rainfall, these flows will have more suspended material than heretofore, and will tend to scour and rut the road surface, presenting a hazard to traffic using the road during such weather.

In terms of mitigation, increased frequency of clearing roadside drains is firstly required. In addition, it will be necessary to provide upstream sumps likely to catch larger boulders which

would otherwise enter and block culverts. For inspection purposes only. Consent of copyright owner required for any other use.

Runoff from the mountain streams during the events of September 19fhlast would have been orders of magnitude over the normal design flows for road culverts and minor bridges. This is perhaps best illustrated by Photographs 3 and 4, which show the bridge culvert at Pollathomais, and the adjacent debris field.

It is clear from the photographs that debris was deposited at the level of the upper rail in the roadside concrete fence, and this by peak flows from a subcatchment area as small as 4 square kilometres.

It is not practical to propose culvert systems capable of handling such flows, rather the culverts would be designed to accommodate flows from floods with lesser return periods, in the knowledge that more extreme events will result in flows greater than the culvert capacity. The path of overtopping, more extreme flood flows would then be kept free of roadside development, and cross-carriageway flows would be anticipated, and designed for, by protective roadside barriers which would protect vehicles during an extreme event.

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Debris Level at concrete fence along LP1202 Road

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Photo No. 4 Debris Level at Pollathomais Bridge (Photograph courtesy of H. McGinley, EPA)

EPA Export 25-07-2013:22:21:54 On this basis, and subject to structural investigation of the integrity of the bridges, we would not foresee extensive widening works at bridges as a design response to the recent extreme floods. We would however recommend that culverts along the L1202 be upsized to accommodate the expected peak flood of a 20 year, or other appropriate return period to be decided by the Council, and that each culvert have an upstream catchpit, regularly maintained, to intercept boulders and debris from blocking such culverts over the next few years.

Mayo County Council, as the Roads Authority, have both the capacity and the expertise to carry out the required works to the road, including associated culverts, wingwalls, retaining walls and protective barriers. This work has been separately itemised and estimated by Mayo County Council and included in Appendix No. 6 to this Report.

In the overall context of remedial works, we would see the roadside fence as a natural boundary between what is essentially roadworks, and what is earthworks and arterial drainage works, and we suggest that this delineation of the different expertise required might guide the partitioning of the necessary works into packages for delivery by the different agencies assisting in the effort overall, primarily the Office of Public Works and Mayo County Council.

13.5 Protection Measures - Additional Works - Medium Risk Areas We propose that an open V-ditch drain, 2m deep by 3m wide, should be constructed roughly parallel to the 50m contour in Area For E inspection and thepurposes 70m only. contour in Area A. Peat and topsoil Consent of copyright owner required for any other use. immediately downslope of the ditch should be removed and surplus material from the excavation should be placed and compacted to form a berm 1.5m high. The north facing slope of the berm will be topsoiled and seeded. The proposed length of the ditch and berm in Area B is 500m and 70m in Area A. Where surplus materials from the ditch are poor quality soft or silty materials, crushed rock will need to be imported to strengthen the berm in these areas. These works are referenced in the Estimate in Appendix No. 3 and on Drawing No. \ 2033-1 007.

It will be necessary to include drainage works through this berm which allows natural drainage to continue along existing paths.

13.6 Protection Measures - Landslide Barriers - High Risk Areas Infrastructure and residences currently situated in the area classified as high risk require kinetic energy absorption fences to be constructed upslope of individual road sections and residences. These fences are an adaptation of rockfall / landslide barriers used in Europe

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EPA Export 25-07-2013:22:21:54 which have the capacity to trap landslide debris but allow the free unrestricted flow of water through the barrier. A drainage ditch would be excavated immediately downslope of the fence to collect and re-direct surface water contained within the landslide. The fence will comprise 3 - 5m high steel or “reinforced concrete” posts, concreted in pockets cut in bedrock, at 6m to 12m intervals, angled downslope with upslope anchor ropes. The mesh will consist of a corrosion net of elastic rings designed to absorb and deform on impact.

The requirements of an effective kinetic barrier are that it absorb the energy of momentum of material striking the barrier, and that it retains the material safely in place until it can be subsequently removed. The fence must be high enough to capture loose boulders, having regard to. the upslope surface and the likely bounce height of the boulders on that surface. The barrier must itself be anchored in a stratum which cannot itself slide.

In the areas where we consider a kinetic barrier is required, we would expect that it need not exceed 2.4m in height over the greater part of its length, because the up slope material is such that bounce heights are not expected to exceed this design height. An exception would be the area immediately above the graveyard, where the fence height will need to be locally increased to 3.5m.

Figure 4 shows the main features of the proposed barrier. The elastic high grade steel wire ring net spanning between the posts will deform under the weight and energy of sliding material. Anchor cables are typically 16mm in diameter, and are tied to rock head anchors drilled and grouted into bedrock. The cables themselves include ring brakes, which deform and permit the cable to elongate under load, absorbing the energy of the impacting material as For inspection purposes only. it does so. The steel posts Consent are anchored of copyright owner in requiredbedrock for any against other use. shear failure, but can rotate slightly as the ring brakes on the stay cables require, so that the entire fence, chain mesh, posts and cable stays, can deform under impact while retaining the material.

Our recommendation is that the fence would be coloured and landscaped with native vegetation to merge with background colours on the mountain, and corrosion resistant materials would be used throughout.

13.7 Drainage Works:- Dooncarton and Glengad The impacts of debris-laden floodwaters on the drainage system at Dooncarton and Glengad can be seen from Figures 5 and 6, which show the pre-, and post-landslides, positions respectively. A deep ravine has been cut in this area, whose side slopes remain unstable and which, without intervention, will be at risk of collapse of its sides in wet weather conditions, leading to downstream risks of blockage of culverts. We anticipate that stabilization of the sides of this ravine over its length from the debris field to the public road, by grouting works,

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i gabion systems and rock armouring at points liable to scour will be necessary, together with additional improvements to the drainage network in the basin area of Area A and A1 and have included this work in the estimate of cost.

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EPA Export 25-07-2013:22:21:55 14. ESTIMATION OF COST OF STABILISATION & PROTECTION WORKS

The construction of landslide energy absorbing barriers on the slopes of Dooncarton and Barnachuille mountains requires safe access and a safety-oriented working methodology for specialist plant to break out founding pockets in the bedrock and to grout in the steel or reinforced concrete posts.

The General Items covering insurances, method related charges and mobilization costs to Pollathomais will therefore be a larger fraction of the overall cost of carrying out the work than would normally be the case in construction contracts.

In addition to the permanent barrier works proposed here, there is a requirement to construct a new earthworks berm, and to stabilise the sides of the various gullies and stream channels which conduct water from the mountain slopes to the culverts across the LP1202.

The Council have separately estimated the cost of a third category of works, that is the infrastructural works associated with the public roads, including replacement of some, and the upgrading of other culverts along the LP1202, together with retaining works, wingwalls and protective barriers needed to restore the drainage infrastructure in the public road. It will be recalled that upsizing of culverts is required to accommodate the expected peak runoff from the slopes during less severe flood events in future. This separate estimate is appended to this report as Appendix No. 6.

For inspection purposes only. Consent of copyright owner required for any other use. Our Estimated Cost of the Permanent Protection Works and off-road drainage channel works is €2,224,664.20, partitioned as follows and detailed in Appendix No. 3 to this Report:

General Items € 773,500.00 Measured Works *€ 1,027,050.00

+€ 1,800,550.00 VAT ut 13.5% 15' 243,074.25 Subtotal Including VAT € 2,043,624.25

Engineering Design (2 98,039.95 Supervision 1; 50,000.00 Legal costs (2 18,000.00 Travel & Out of Pocket e 15,000.00 Total estimated Works Cost € 2,224,664.20

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EPA Export 25-07-2013:22:21:55 The road infrastructural works which are separately required, are estimated in Appendix No.6 to cost €2,835,500.00.

The combined estimate may in turn be partitioned between the proposed permanent barrier works to protect the road infrastructure and roadside residences, and the essentially earthworks-based and drainage related work, the latter in the in the channels and gullies draining towards the LP1202 road, with the former being the proposed new berm shown on the drawings. This breakdown of estimated cost is as follows:

I Protection Works Infrastructure Permanent Works Off-Road Drainage Mayo County & Berm works Council Infrastructure Works (App 6) € 615,000.00 € 158,500.00

€ 630,000.00 € 397,050.00

€ 1,245,000.00 € 555,550.00 VATat 13.5% 15 168,075.00 6' 74,999.25 Subtotal Including VAT € 1,413,075.00 € 630,549.25 € 2,835,500.00

Engineering Design € 75,000.00 € 23,039.95 incl Supervision € 35,000.00 € 15,000.00 incl Legal costs € 8,000.00 € 10,000.00 incl & For inspection purposes only. € incl Travel Out of Pocket Consent of copyright owner€ required8,000.00 for any other use. 7,000.00

Total estimated Works Cost E 1,539,075.00 € 685,589.20 € 2,835,500.00

The above partitioning of the overall estimate of €5,060,164.20 identifies on the one hand a category of works which includes earthworks, land drainage, drain sides stabilisation and grouting work, and which might fall within the expertise of an agency such as OPW or Mayo County Council, with specialist protection barrier construction costs separately estimated in the event that the Council decide to procure that element separately, either by contract or by direct labour works.

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EPA Export 25-07-2013:22:21:55 15. SUMMARY

The landslides of September 19*, 2003, were preceded by exceptional and extreme rainfall, and resulted in such flows of floodwater and overburden material that it was indeed fortunate that lives were not lost in the event. The immediate impacts of floodwater and debris flow from more than 40 landslides have been addressed by a comprehensive response by Mayo County Council, assisted by the OPW, focusing on public infrastructure and those works which were essential to reopen the public road and graveyard and to permit residents to return to their homes at the earliest possible date. Further works to road culverts will be necessary in the medium term.

The exceptional conditions induced by the September 19* rainfall have lowered the threshold of weather conditions now likely to remobilise disturbed material on the mountain slopes. There remains, therefore, elevated residual risks of further landslides of material. An approach which recognizes this, and attempts to direct the likely path of travel of material by earthwork ditch and berm construction will suffice to reduce risks to low levels in the central, mid slope areas. Energy absorbing landslide barriers will be needed to protect properties currently designated high risk on the upper roads off the LP1202.

The overall protection and infrastructural works are estimated to cost €5.06m, and these works are required in order to permit the people of the community of Pollathomais, Dooncarton, Glengad and Barnaehuille to confidently resume their normal lives.

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Michael F Garrick BE, MEngSc, MBA, CEng, FIEI, MCIWEM, MConsEl Project Director TOBIN Consulting Engineers

Date: 10 November, 2003

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EPA Export 25-07-2013:22:21:55 Report on the Landslide at Dooncarton, Glengad, Barnachuille and Pollathomais, County Mayo

APPENDICES

Appendix No. 1 Diary of work over the past month

Appendk No. 2 EPA Report

Appendix No. 3 Estimate of Costs

Appendix No. 4 Photographs

Appendix No. 5 Statements issued to Mayo County Council

Appendix No. 6 Inventory of Inpastructural Rehabilitation Works required (prepared by Mayo County Council

Appendix No. 7 Drawings

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EPA Export 25-07-2013:22:21:55 Appendix No. 1

Diary of work over the past month

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EPA Export 25-07-2013:22:21:55 Appendix No. 2

EPA Report

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Appendrjr No. 3

Estimate of Costs

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Appendix No. 4

Photographs

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EPA Export 25-07-2013:22:21:55 Appendix No. 5

Statements issued to Mayo County Council

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EPA Export 25-07-2013:22:21:55 Appendix No. 6

Inventory of Infiastructural Rehabilitation Works required (prepared by Mayo County Council

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EPA Export 25-07-2013:22:21:55 Appendix No. 7

Drawings

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