A request was made for the JBA Post Incident review regarding the October 2019 flooding on Glen Road, Laxey, and the authority released the final report with some information exempt. The disclosed document details the causes of the flood, including debris blockage at the MER Weir and the subsequent collapse of a highway wall.
Key Facts
The flood event occurred on the morning of 1 October 2019 in Laxey, affecting 62 properties.
51 of the flooded properties were located on Glen Road.
Flood waters entered Glen Road via a gap in the highway wall created for a fish pass construction.
Debris buildup behind the MER Weir caused elevated river levels, leading to the collapse of the adjacent highway wall.
The report was commissioned by Manx Utilities and prepared by JBA Consulting, with the final version issued in August 2020.
Data Disclosed
1 October 2019
62 properties
51 properties
August 2020
13 January 2020
29 January 2020
12 August 2020
Purchase Order 157500
September 1930
December 2015
5m
100mm
58g
73g
Original Request
I would like a copy of the JBA Post incident review conducted after the flooding in Laxey, particularly on Glen Road on 1st October 2019. My home was one of the properties destroyed in Laxey that day.
Data Tables (40)
RevisionRef/Date
Amendments
Issuedto
S3-P01 (DRAFT) -13January
2020
n/a
(ManxUtilities)
(ManxUtilities)
S3-P02 (DRAFT) –29 January
2020
Additional internal reviewcomments
and clientcomments.
(ManxUtilities)
(ManxUtilities)
A1-C01 (FINAL) –12 August2020
Final clientcomments.
(ManxUtilities)
(ManxUtilities)
Date andtime
(BST)
Description
1 October2019
06:00 –07:00
The model estimates flow began to emerge through ‘Gap A’ at 06:00 BST when
water levels exceed the levels of Glen Road (18.31mAD02). Flows continued to
flow through ‘Gap A’ throughout the peak of the event. The amount of flow
entering Glen Road was exacerbated due to the presence of the excavator and
building materials in the channel. The obstructions caused an approximate head
loss of 0.15m. Additional analysis was undertaken to assess the impact of the
excavator (Appendix E) due to the limitations of the a 1D modelling approach in
representing turbulence around the obstructions. The impact of the excavator is
exploredfurtherinScenario4(section5.8).
Flooded properties at this stage of the event = 18. The estimated
maximumflooddepthsforthepropertieswas0.35m.
1 October2019
07:00 –08:30
Flows leaving the river channel via ‘Gap A’started torecede following the peak of
the flood hydrograph. However, when blockage is applied to the MER Weir, water
levels are projected back up the river. The moment prior to the wall collapse
coincideswiththemodelledpeakflow(1.89m3/s)through‘GapA’.
Overtopping of the highway wall immediately upstream of the MER Weir is
estimated to occur from 07:45 BST until the collapse of the wall. A peak flow of
2.82m3/s was estimated to have occurred due to overtopping of the wall prior to
collapse.
Flooded properties at this stage of the event = 26. The estimated
maximumflooddepthsforthepropertieswas0.55m.
1 October2019
08:30 –09:00
Uponcollapseofthehighwaywall(‘GapB’)anapproximatepeakflowof13.24m3/s
entered the floodplain(approximately halfthevalue oftheflowhydrographatthe
same point in the event). Immediately after the wall collapse, upstream water
levels were reduced by approximately 0.5m. The model estimates following the
wallcollapsethewaterlevelsat‘GapA’hadreducedbelowthe level ofGlenRoad
(18.31mAD02).
Flooded properties at this stage of the event = 33. The estimated
maximumflooddepthsforthepropertieswas0.8m.
1 October2019
08:30 –11:30
Flows continued to spill from ‘Gap B’ and onto Glen Road. Flows continued to
recedeuntilemergencyactiontookplace. Atthetimestepbeforeemergencyaction
beganflowsleavingthechannelfrom‘GapB’hadrecededto 5.3m3/s.
Flooded properties at this stage of the event = 33. The estimated
maximumflooddepthsforthepropertieswas0.6m.
1 October2019
08:30 –13:00
Following the emergency action creating ‘Gap C’ and introducing the road block,
the model estimates a peak flow of 1.5m3/s re-enters the channel. However, the
modeldemonstratesthisdidnothavethegreatestimpactonreducingwaterlevels
alongGlenRoad. ThepartialremovalofdebrisfromMERWeirhadagreaterimpact
as water levels quickly receded to levels below Glen Road (17.1-17.4mAD02)
immediatelyupstreamoftheweir.
Flooded properties at this stage of the event = 33. The estimated
maximumflooddepthsforthepropertieswas0.6m.
Scenario
Number of flooded properties
1
34
2
19
3
33
4
0
Date
Studyname
Description andconclusions
2012
Flood Risk to CoastalTowns
A 1D HEC-RAS model (steady state) was developed in this
strategic study to assess flood risk from Laxey River and Glen
Roy. Thestudyidentifiedareasatfloodriskandsomelimitations
of a 1D modelling approach in Laxey, specifically, that using
sectionsdidnotaccountforcomplexoverlandflowroutesinthe
floodplain. The study concluded that to fully understand flood
mechanisms in Laxey a more detailed model with better
representationofout-of-bankflowrouteswouldberequired.
2014
Isle of Man (IoM) Surface
Water FloodMap
Laxey was covered by the island-wide surface water flood map.
Based on the results from broadscale surface water flood
modelling this identified overland flow routes within the Laxey
catchment. The model was used to identify areas shown at
greatest flood risk to give MU a priority list for future
investigation.
2017
Laxey FloodModelling
update
The1DHEC-RASmodeldevelopedaspartofthe2012studywas
converted to 1D-2D. The objective was to produce a single
seamless model that could be used to assess not only complex
overlandflowroutesinLaxey,butalsofloodriskfromallsources
(fluvial, pluvial and tidal (still water)) and the interactions
between the different types of flooding. Hydrological data was
alignedwithbestpracticeandconsideringlocalgauged data.
Date
Photographs
Description
September
1930
MER Weir, GlenRoad
Heavy rainstorm on the night of 17 September
1930resulted inflashflooding,exacerbatedbya
severe accumulation of debris behind the weir of
theLaxeyPowerStationhydroelectricplant(MER
Weir). The flood caused destruction to many
partsofLaxey.Thewaterturbine andpartof the
steam workings were also flooded, requiring
repairs which were undertaken in October and
November19301.
Photographic evidence shows the wall on the left
bank of the river collapsed downstream of the
MER Weir, and a large section of Glen Road was
washedaway.
December
2015
Old Laxey Bridge post flood event
(Source: )
On3December2015theIsleofManexperienced
widespread flooding. The Mountain Box rain
gauge recorded 69.2mm of rainfall between
10:30GMTand18:00GMT,withpeakintensities
around 15:30 GMT. Over the same period, the
Brandywell Corner rain gauge recorded 50.2mm
ofrainfall.UsingtheFEHDDFmethod(discussed
in section 4.2.2), this suggests a rainfall return
period of 10 to 20 years. At 16:26 GMT, the
Laxey River gauge recorded a depth of 2.14m.
Immediately after the gauge malfunctioned and
madenofurtherrecordings,soitislikelythetrue
peak was not recorded. Anecdotal evidence from
property flooding suggests the peak occurred at
around 17:00 GMT. The recorded level of 2.14m
isestimatedtocorrespondtoaflowof37m3/s.
Flooding was caused by a combination of fluvial
and surface water mechanisms. Increased flow
in the Laxey River caused river bank and wall
collapses,whichallowedout-of-bankflow.Debris
blocked culverts and bridges. At least 39
properties were flooded. Properties on Cranleigh
Road reported the highest depths of water,
describing four feet of water externally and two
to four feet of water within the properties. Ten
residents were evacuated from their homes on
Back Shore Road due to concerns about soil
erosion near the cliffs, given there had been
landslides elsewhere in Laxey. The Old Laxey
Bridgecollapsedataround16:00GMT,causinga
bustocrashintotheriver.
Date
Photographs
Description
January
2016
-
Over a 24-hour period on 22 January, 109.8mm
of rainfall was recorded at Mountain Box rain
gauge. Damage to walls and embankments on 3
December 2015 which had not been repaired
meant that river water was able to leave the
channel. The Laxey River was reported to have
overflowed in places, causing flooding. The
primary school was closed2 and several roads
were shut3. During this flood event, the river
gaugeremainedoutofuseafteritsdisplacement
during the 3 December 2015 event, therefore no
levels were recorded. However, at Corrany Weir,
the peak levels on 22 January 2016 were higher
thanthoserecordedon3December2015.
November
2017
Surface water flooding on Minorca Hill
(Source:MU)
On 22 November 2017, a total of 71.8mm of
rainfallwasrecordedatMountainBoxraingauge
over a period of 22 hours (starting at 03:15
GMT), and 60.4mm was recorded at Brandywell
Corner for the same period. Higher than normal
rainfall was recorded for the 3 days prior to the
event. The peak intensity was at 09:45 GMT,
with a smaller secondary peak at 20:15 GMT.
The river gauge recorded a peak river depth of
1.868m at 10:30 GMT, which is estimated to
correspond to a flow of 26m3/s. The river level
was observed to be elevated for about 4 days
afterthispeak.
There is less anecdotal evidence relating to this
event than 3 December 2015. However, roads
were closed due to surface water flooding, with
GlenRoadreportedlyfloodedfrom09:00GMTto
13:00 GMT. The only reported infrastructure
damagewasthecollapseofariverwallupstream
oftheShoreHotelwhichwascrackedpriortothe
floodevent.
Event
Photographs
Description
Hydrological
context
-
On 30 September, a yellow weather warning
was issued by Ronaldsway Met Office for 30
September 17:00 BST to 1 October 10:00 BST,
with 20-35mm of rainfall predicted over the
hills. Two rain gauges on the perimeter of the
Laxey catchment recorded over 100mm from
14:00BSTon30Septemberto14:00BSTon1
October2019.
Pre-event
Source:MU
Prior to the event, a fish pass was under
construction at the Woollen Mills Weir. In order
to access the river channel with plant
machinery, a 5m section of the highway wall
dividing Glen Road from the river was
demolished and a ramp created to access the
river. The highway wall is not classed as a
formal flood defence due to its construction
(masonry with concrete joints and no water
barrier) but it is known to provide some flood
protection by means of containing flow in the
channel,soisconsideredasadefactodefence.
The excavator was in the channel on the night
of the event and based on photographic
evidence it is unclear if provision was made to
fillthegap.
During
Event
Source:MU
Source:MU
Peak river levels were recorded around 06:30
BST. River levels were sufficiently elevated to
overflow through the gap created for the fish
pass construction. Flows travelled along Glen
Roadandinundatedproperties.
Debris was observed by residents and
emergency response teams to collect on the
MER Weir. The build-up led to increased water
levels upstream thus forcing more flow out
through the gap created for the fish pass
construction.
Water levels also exceeded the level of the wall
immediately upstream of the MER Weir.
Subsequently, a section of the highway wall
collapsedupstreamoftheMERWeirresultingin
a sudden release of water. This caused
increased flooding to properties along Glen
Road.
Emergency action, coordinated by Silver
Command, was undertaken after 11:00 BST.
This included removing a section of wall
downstream of the weir to allow flows to re-
enter the channel and clearing debris from the
MERWeir.
Event
outcomes
In total, 62 properties (59 residential) were
flooded within the village, of which 51 were
locatedonGlenRoad. Theother11weremainly
isolated properties in the village with flooding
attributed to surface water runoff rather than
floodingdirectlyfromtheriver.
On 2 October, Silver Command coordinated
further removal of debris behind the MER Weir,
and on 3 October the central pillars of the MER
Weir were removed, and modifications made to
the weir to reduce future risk from debris
blockage.
Date(2019)
Time(BST)
Comment
Prior to 30
September
Fish pass construction began –‘Gap A’ created to allow access to the
riverchannel.
30September
11:00
Yellow Weather Warning issued by Ronaldsway Met Office for 30
September 17:00 to 1 October 10:00. Forecast was for 10-20mm of
rainfallacrosstheisland,with20-35mmpossibleoverthehills.
30September
Approx.15:00
Beginning of rainfall event, two hours earlier than forecast.
30September
Approx.17:00
Excavator and building materials for fish pass construction left within
riverchannel.
1October
Approx.6:00
Flows exceed level of Glen Road and water flows out of ‘GapA’.
1October
06:21
Peak flood level recorded at Laxey Rivergauge.
1October
Before07:00
Blockage starts to build up behind MER Weir.
1October
08:30
Amber Weather Warning issued by Ronaldsway Met Office. Expected
conditionswereheavyraintocontinueforthemorningbringingafurther
15-20mm for low ground and potentially another 30-35mm over the
hills.
1October
Approx.08:40
Breach occurred in section of highway wall upstream of MER Weir.
Overtopping of the wall was experienced prior to collapse. The surge
from ‘Gap B’ inundated properties on Glen Road and evidence shows
largevehiclesbeenmovedbytheforceof water.
Sandbags placed on Glen Road to divert water through ‘GapC’.
1October
12:55
Incident downgraded to recoveryphase.
1October
13:00
End of rainfallevent.
2October
n/a
Remaining debris behind MER Weirremoved.
3October
n/a
Central pillars of MER Weirremoved.
Type
Comments
SiteContractor
The individual was a member of the team constructing the Fish Pass, which had
includedcreating‘GapA’. Theyhadalsoassistedwithcreating‘GapC’andremoving
the MER Weir central pillars. After the event, they were part of the team building
the new wall on Glen Road to fill in ‘Gap C’. They had taken numerous videos and
picturesoftheeventandprovidedthefollowingcomments:
Thecontractorsweretoldthattherewasgoingtobe50mmofrainfallovernight;
They placed a board over ‘Gap A’ behind the excavator to stop water flowing
ontoGlenRoad;
They arrived on site at 07:00 BST – water was flowing through ‘Gap A’ and
alongGlenRoad. Thehighestdepthsoffloodwaterswereatabout11:00inthe
vicinityoftheMER Weir.
Emergency action to remove the MER Weir was delayed due to confusion in
SilverCommandaboutownershipofthestructure.
After‘GapC’wascreated,thewateronGlenRoadflowedback intotheriverin
afewhours,andtheroadwasdrybetween13:00and14:00BST;
As well as removing the MER Weir, 60 tonnes of gravel was removed from the
riverbehindtheweirfollowingtheevent.
Cooil RoiSheltered
housingmanager
The managerexpressed aconcernaboutthe gap inthe wall onthe leftbank of the
Laxey River. However, they indicated the water level on the right bank during
previous flood events. For 2019, the water level reached a white cobble visible in
the wall. This level was higher than that observed in 2015 at this location. She
suggestedthatCooilRoiShelteredHousinghadnotbeenfloodedbyeitherthe2015
or 2019 flood events, and that flood risk occurs downstream of the confluence with
GlenRoy.
Localresident
A local resident who lives on Glen Road confirmed the timings of the different
occurrences in the flood event. There was a general concern about the debris in
the river prior to the floor event. A resident had undertaken several walkovers of
Glen Roy catchment prior to the flood event and in the days after the flood event
to identify debris sources and was convinced that the debris that collected on the
MER Weir was a result of tree felling from the 2015 floods that have been left on
the sides of the Glen Roy.
Date and
time(BST)
Photograph
Description
1October
08:16
Photo 1: The gap in the wall was created
prior to 30 September to allow construction
ofafishpassatLaxeyWoollenMillWeir.
Prior to 07:00 BST on 1 October, water was
flowing through this Gap. This photo shows
that flows were impeded by the excavator
resultingin moreflowenteringGlenRoad.
Video evidence of this mechanism is
providedinthedigitalAppendixA.1.
Source:FireandRescueService
1October
08:17
Photo 2: A downstream view of ‘Gap A’,
further empahsising the reduction in flow
caused by the excavator and subsequent
raising of water levels. This photo also
demonstrates the volume of water which
entered Glen Road (depth approximately
0.4-0.5m). Further downstream, debris had
builtupbehindtheMERWeirbutpriortowall
collapse(‘GapB’).
Source:FireandRescueService
1October
11:33
Photo 3: This photograph shows the
position of the excavator and size of ‘Gap
A’. At this time the river level had
considerably reduced, so water was no
longer flowing through ‘Gap A’. At this
time, emergency action was yet to be
taken.
Source:MU
Date and
time(BST)
Photograph
Description
1October
08:45
Photo 4: This is a downstream view of ‘Gap
B’ immediatley after the wall collapsed at
approximatley 08:40 BST.Itshows the level
of water on Glen Road where depths
exceeded 1m at properties in the vicinity of
thewallcollapse.
Source: Fire and RescueService
1October
approx.
11:00
Photo 5: This photograph shows the
blockage behind the MER Weir, and the
breachofthewall(‘GapB’)upstreamofthis.
Water is shown to back up behind the
blockage and flow through the gap. The
photograph also shows the level of water
flowing down Glen Road and entering
properties. The estimated peak flood levels
on Glen Road occurred around this time.
Further upstream, the volume of water
flowing through ‘Gap A’ had reduced
comparedtoearlierinthemorning.
Source: MU, taken from Laxey Central
FacebookGroup
1October
11:14
Photo 6: This shows a downstream view of
water backing upbehind the blockage atthe
MER Weir, and flowing through the gap into
Glen Road. Comparison with Photo 4 shows
that water levels on Glen Road had reduced
slightly from 8:45 BST to 11:14 BST, using
the garage door and blue car as reference
points.
Source:MU
Date and
time(BST)
Photograph
Description
1October
11:36
Photo 7: Another excavator was used to
create ‘Gap C’at this time.Comparisonwith
Photo6showsthatwaterlevels fellbetween
11:14BSTand11:36BST.InPhoto7,
rubble is visible upstream of the parked car,
and the level of water on the garage door
andcarislower.AsshowninPhoto3,water
wasnolongerflowingthrough‘GapA’atthis
time.
Source:MU
1October
11:55
Photo8:After‘GapC’wascreated,someof
thedebriswasclearedbehind theMERWeir.
Water can be seen flowing through ‘Gap C’
from Glen Road back into the Laxey River.
Water was still flowing through ‘Gap B’ (on
therightofthephotograph),butwaterlevels
onGlenRoadhadfallen.
Source:MU
Date and
time(BST)
Photograph
Description
2October
17:00
Photo 10: The day after the flood event (2
October), all debris was cleared from
behind the MER Weir and gravel extraction
began. This occurred after the flood event
to allow flow conditions to subside for safe
access, and because Silver Command had
to decide if the benefits outweighed the risk
of passing debris downstream to Swales
Bridge.
Source:MU
3October
14:29
Photo 11: Two days after the flood event
(3 October), the central pier of the MER
Weir was removed and parts of the weir
crest werelowered.
Source:MU
Date
Grade
Comments/recommendations and photographs made at the
time of theinspection
20November
2014
Weir: 4 –Poor
Remedial repairs to central part of the weir required.
18 May2016
Control Gates
Central Pier: 4-
Poor
Control Gates
Frame: 5 –Very
Poor
Scour pool (Approx. 1.6m deep) developing at the downstream toe
of the structure. Repairs should be undertaken, or removal of the
structureshouldbeconsidered.
16 July2019
N/a
Structuredeemedasahistoricalassetandnolongerfulfilsitsoriginal
function. Consider removal of the remaining structure from the
channeltoavoidconveyanceissuesandimproveflow conditions.
Weir needs remedial work and the whole structure requires review
andeitherrepairingorremoving.Urgencyclassedas routine.
10October
2019
N/a
Structure has now been removed. Further work required to
improve the condition of other aspects of theweir.
Grade
Comments/recommendations and photographs made at the time of the
inspection
3 -Fair
Lots of vegetation growing through the wall. Some mortar loss, but overall in fair
condition. Some sections of scour and undermining thewall.
4 -Poor
Scour and undermining should be repaired. Missing blocks should be replaced.
Monitor and maintain vegetation in channel side and exposed face.
Descriptor
Definition
Value
Area
Catchment drainage area derived using an Integrated Hydrology
Digital Terrain Model (IHDTM) developed at UKCEH Wallingford.
25km2
BFIHOST
Base Flow Index derived using the UK Hydrology of Soil Types
(HOST)classification.
0.36
SAAR
Standard Average Annual Rainfall (SAAR) for the period 1961-
1990 /1941-1970.
1625mm/
1394mm
FARL
Index of flood attenuation attributable to reservoirs and lakes.
0.992
DPLBAR(km)
Mean of distances between each node (on regular 50 metre grid)
and the catchment outlet (km). Characterises catchment size
andconfiguration.
4.49
DPSBAR(m/km)
Mean of all the inter-nodal slopes for the catchment (m/km).
Characterises the overallsteepness.
202.8
Raingauge
Total rainfall(mm)
CorranyIntake
89.4
MountainBox
96.2
BrandywellCorner
111.6
Duration
(hours)
CorranyIntake
MountainBox
BrandywellCorner
Rainfall
(mm)
Return
Period
(years)
Rainfall
(mm)
Return
Period
(years)
Rainfall
(mm)
Return
Period
(years)
1
13.6
Less than2
12.2
Less than2
6.4
Less than2
2
25.2
2 to5
21.2
Less than2
10.8
Less than2
3
34.6
2 to5
26
Less than2
15.2
Less than2
6
57
5 to10
45.8
2 to5
26.6
Less than2
12
83.6
5 to10
87.8
5 to10
45.8
Less than2
24
100.8
5 to10
108.2
5 to10
60.4
Less than2
Event
Observed peak depth
(m)
Estimatedflow
(m3/s)
Correspondingreturn
period (years)
December 2015
2.14*
37
10 to20
November2017
1.88
26
2 to5
October2019
2.26
42
10 to20
Totalflow
volume
(m³)
Baseflow
volume
(m³)
VF(m³)
Area of
catchment
(m²)
Meantotal
rainfall
(m²)
VR(m³)
Percentage
runoff(%)
1,183,852
378,616
805,236
22,810,000
0.0898
2,048,338
39.31
Date andTime
Tide
Depth(mAD02)
29/09/201905:41
Low
-3.07
29/09/201911:21
High
3.82
29/09/201917:55
Low
-3.08
29/09/201923:42
High
4.02
30/09/201906:20
Low
-3.39
30/09/201912:08
High
3.82
30/09/201918:37
Low
-2.97
01/10/201900:24
High
4.17
01/10/201907:01
Low
-3.20
01/10/201912:45
High
3.68
01/10/201919:13
Low
-3.23
02/10/201901:10
High
3.61
02/10/201907:44
Low
-3.14
19 00:00 30/09/20
19 00:00 01/10/20
19 00:00 02/10/20
Date andtime
(BST)
Associated
figures
Description
1 October2019
06:00 –07:00
Figure 5-3(Flood
Extent)
Figure 5-4(Long
section)
The model estimates flow began to emerge through ‘Gap A’ at
06:00 BST when water levels exceed the levels of Glen Road
(18.31mAD02). Flows continued to flow through ‘Gap A’
throughout the peak of the event. The amount of flow entering
GlenRoadwasexacerbatedduetothepresenceoftheexcavator
and building materials in the channel. The obstructions caused
an approximate head loss of 0.15m. Additional analysis was
undertaken to assess the impact of the excavator (Appendix E)
due to the limitations of the a 1D modelling approach in
representing turbulence around the obstructions. The impact of
theexcavatoris exploredfurtherin Scenario4(section 5.8).
Flooded properties at this stage of the event = 18. The
estimated maximum flood depths for the properties was
0.35m.
1 October2019
07:00 –08:30
Figure 5-5(Flood
Extent)
Figure 5-6(Long
section)
Flows leaving the river channel via ‘Gap A’ started to recede
following the peak of the flood hydrograph. However, when
blockage is applied to the MER Weir, water levels are projected
back up the river. The moment prior to the wall collapse
coincides with the modelled peak flow (1.89m3/s) through ‘Gap
A’.
Overtopping of the highway wall immediately upstream of the
MERWeirisestimatedtooccurfrom07:45BSTuntilthecollapse
of the wall. A peak flow of 2.82m3/s was estimated to have
occurredduetoovertoppingofthewallpriortocollapse.
Flooded properties at this stage of the event = 26. The
estimated maximum flood depths for the properties was
0.54m.
1 October2019
08:30 –09:00
Figure 5-7(Flood
Extent)
Figure 5-8(Long
section)
Uponcollapseofthehighwaywall(‘GapB’)anapproximatepeak
flowof13.24m3/senteredthefloodplain(approximatelyhalfthe
value of the flow hydrograph at the same point in the event).
Immediately afterthe wallcollapse, upstreamwater levelswere
reduced by approximately 0.5m. The model estimates following
the wall collapse the water levels at ‘Gap A’ had reduced below
thelevelofGlenRoad(18.31mAD02).
Flooded properties at this stage of the event = 33. The
estimated maximum flood depths for the properties was
0.79m.
1 October2019
08:30 –11:30
n/a
Flowscontinuedtospillfrom‘Gap B’and onto GlenRoad.Flows
continued to recede until emergency action took place. At the
timestep before emergency action began flows leaving the
channelfrom‘GapB’hadrecededto5.3m3/s.
Flooded properties at this stage of the event = 33. The
estimated maximum flood depths for the properties was
0.6m.
Date(2019)
Time(BST)
Comment
Prior to 30September
Fish pass construction began –‘Gap A’ created to allow access
to the river channel. ‘Gap A’ was located opposite the southern
limit of the Woollen Mills building (243,454mE484,329mN)
30September
11:00
Yellow Weather Warning issued by Ronaldsway Met Office for 30
September 17:00 BST to 1 October 10:00 BST. Expected
conditions were 10-20mm of rainfall across the island, with 20-
35mm possible over thehills.
30September
Approx.15:00
Beginning of rainfall event.
30September
Approx.17:00
Excavator and building materials for fish pass construction left
within river channel overnight at the location of ‘GapA’.
1October
Approx.06:00
Flows exceed level of Glen Road and flow out of ‘GapA’.
1October
06:21
Peak flood level recorded at Laxey River gauge (243,688.3mE,
484,029.3mN;11.70mAD02).
1October
Approx.07:00
Debris blockage starts to build up behind the MER Weir.
1October
08:30
Amber Weather Warning issued by Ronaldsway Met Office.
Expected conditions were heavy rain to continue for the
morning bringing a further 15-20mm for low ground and
potentially another 30-35mm over thehills.
1October
Approx.08:40
Breach occurred in section of highway wall upstream of the MER
Weir on left bank. Overtopping of the highway wall was
experienced prior to collapse. The surge of water through ‘Gap
B’ inundated properties along Glen Road and was even seen to
move large vehicles (fireengine).
1October
Approx.09:00
Water no longer flowing through ‘GapA’.
1October
11:30-11:50
‘Gap C’ created as part of emergency action to allow water back
into the channel from Glen Road. Consequently, the level of
water on Glen Road decreased. Debris from behind MER Weir
began to beremoved.
1October
12:00-15:00
Sandbags placed on Glen Road to divert water through ‘Gap C’.
1October
12:55
Incident downgraded to recoveryphase.
1October
13:00
End of rainfall event.
2October
n/a
Remaining debris behind MER Weir removed.
3October
n/a
MER Weirremoved.
Date andtime
(BST)
Description
1 October2019
06:00 –07:00
Themodelestimatesflowbegantoemergethrough‘GapA’at06:00BSTwhenwater
levels exceed the levels of Glen Road (18.31mAD02). Flows continued to flow
through ‘Gap A’throughoutthe peak of the event.The amount of flowentering Glen
Roadwas exacerbatedduetothepresenceoftheexcavatorandbuildingmaterialsin
the channel.The obstructions caused an approximate head loss of 0.15m. Additional
analysis was undertaken to assess the impact of the excavator (Appendix E) due to
the limitations of the a 1D modelling approach in representing turbulence around the
obstructions. The impact of the excavator is explored further in Scenario 4 (section
5.8).
Flooded properties at this stage of the event = 18. The estimated maximum flood
depthsforthepropertieswas0.35m.
1 October2019
07:00 –08:30
Flows leavingtheriverchannelvia‘Gap A’startedtorecedefollowing thepeakofthe
flood hydrograph. However, when blockage is applied to the MER Weir, water levels
are projected back up the river. The moment prior to the wall collapse coincides with
themodelledpeakflow(1.89m3/s)through‘GapA’.
Overtopping of the highway wall immediately upstream of the MER Weir is estimated
to occur from 07:45 BST until the collapse of the wall. A peak flow of 2.82m3/s was
estimatedtohaveoccurredduetoovertoppingofthewallpriorto collapse.
Flooded properties at this stage of the event = 26. The estimated maximum flood
depthsforthepropertieswas0.54m.
1 October2019
08:30 –09:00
Upon collapse of the highway wall (‘Gap B’) an approximate peak flow of 13.24m3/s
entered the floodplain (approximately half the value of the flow hydrograph at the
same point in the event). Immediately after the wall collapse, upstream water levels
werereducedbyapproximately0.5m. Themodelestimatesfollowingthewallcollapse
thewaterlevelsat‘GapA’hadreducedbelowthelevelofGlenRoad (18.31mAD02).
Flooded properties at this stage of the event = 33. The estimated maximum flood
depthsforthepropertieswas0.79m.
1 October2019
08:30 –11:30
Flows continued to spill from ‘Gap B’ and onto Glen Road. Flows continued to recede
until emergency action took place. At the timestep before emergency action began
flowsleavingthechannelfrom‘GapB’hadrecededto 5.3m3/s.
Flooded properties at this stage of the event = 33. The estimated maximum flood
depthsforthepropertieswas0.6m.
1 October2019
08:30 –13:00
Following the emergency action creating ‘Gap C’ and introducing the road block, the
model estimates a peak flow of 1.5m3/s re-enters the channel. However, the model
demonstrates this did not have the greatest impact on reducing water levels along
GlenRoad. ThepartialremovalofdebrisfromMERWeirhadagreaterimpactaswater
levels quickly receded to levels below Glen Road (17.1-17.4mAD02) immediately
upstreamoftheweir.
Flooded properties at this stage of the event = 33. The estimated maximum flood
depthsforthepropertieswas0.6m.
Scenario
Number of flooded properties
1
34
2
19
3
33
4
0
Duration
(hours)
2-year
rainfall
(mm)
5-year
rainfall
(mm)
10-
year
rainfall
(mm)
20-
year
rainfall
(mm)
30-
year
rainfall
(mm)
50-
year
rainfall
(mm)
75-
year
rainfall
(mm)
100-
year
rainfall
(mm)
1
13.62
19.52
24.15
29.58
33.21
38.24
42.57
45.79
2
20.74
28.34
34.57
41.68
46.44
52.71
57.88
61.63
3
26.26
35.23
42.68
51.23
56.66
63.76
69.51
73.62
6
38.22
50.55
61.01
72.49
79.23
87.64
94.31
99.03
12
52.77
70.11
84.53
98.57
106.58
116.33
123.92
129.21
24
68.76
92.49
110.42
126.9
136.01
147.02
155.49
161.37
Duration
(hours)
2-year
rainfall
(mm)
5-year
rainfall
(mm)
10-
year
rainfall
(mm)
20-
year
rainfall
(mm)
30-
year
rainfall
(mm)
50-
year
rainfall
(mm)
75-
year
rainfall
(mm)
100-
year
rainfall
(mm)
1
13.93
20
24.66
30.19
33.83
39.01
43.44
46.71
2
21.53
29.29
35.57
42.88
47.64
54.14
59.49
63.34
3
27.42
36.58
44.1
52.89
58.44
65.78
71.7
75.91
6
40.3
52.8
63.49
75.22
82.19
91.03
97.9
102.71
12
56.05
73.59
88.31
103.05
111.42
121.7
129.59
135.08
24
73.04
97.29
116.09
133.46
142.95
154.63
163.51
169.63
Duration
(hours)
2-year
rainfall
(mm)
5-year
rainfall
(mm)
10-
year
rainfall
(mm)
20-
year
rainfall
(mm)
30-
year
rainfall
(mm)
50-
year
rainfall
(mm)
75-
year
rainfall
(mm)
100-
year
rainfall
(mm)
1
13.8
19.69
24.32
29.85
33.55
38.73
43.15
46.41
2
21.3
28.8
34.92
42.38
47.35
53.8
59.01
62.75
3
27
35.83
43.12
51.88
57.51
64.72
70.52
74.65
6
39.27
51.1
61.36
72.75
79.56
88.14
94.84
99.56
12
53.83
70.21
84.1
98.23
106.23
116.06
123.7
129.05
24
69.53
92.42
109.79
125.87
134.7
145.82
154.39
160.33
Duration
(hours)
2-year
rainfall
(mm)
5-year
rainfall
(mm)
10-
year
rainfall
(mm)
20-
year
rainfall
(mm)
30-
year
rainfall
(mm)
50-
year
rainfall
(mm)
75-
year
rainfall
(mm)
100-
year
rainfall
(mm)
1
12.1
17.29
21.49
26.42
29.71
34.27
38.21
41.13
2
18.67
25.52
31.18
37.88
42.23
47.99
52.7
56.1
3
23.74
31.88
38.66
46.62
51.68
58.15
63.39
67.14
6
34.65
45.64
55.22
65.65
71.81
79.53
85.65
89.97
12
47.49
62.85
75.72
88.5
95.74
104.59
111.5
116.34
24
61.55
82.32
98.21
112.92
121.09
131.06
138.75
144.12
Date
Description
February1823
Very heavy rain. Worst floods for 20 years. Recent addition to Old Laxey Bridge washed
away.
August1836
Flooded river flows into Laxey Mine and five minersdrowned.
October1841
Considerable floods. Lower village completely inundated. Paper mill weir and
embankment damaged. Livestock swept out to sea. 200 yards of walls washedaway.
December1841
Rivers inflood.
November1860
Heavy rains and severe floods. Hundreds of tons of mines waste swept out to sea. Laxey
harbour scoured and remains of ancient pieruncovered.
July 1872
Island wide flooding. Glen Roy river in flood destroys Glen Roy Mine damhead.
June1876
Call for bridge at Creers Mill (now Salmon Centre). Complaint by farmers that river
frequently in flood and cattle cannot be herded across toAgneash.
September1883
Great flood. Glen Roy river swollen to height not seen in previous 5 to 20 years.
Undermined wall outside Woollen Mills. Wall collapsed for 30 feet allowing water onto
Glen Road. Part of Mine tramway washed away. Temporary measures put in place to
repair breach.
October1883
Terrific rainfall like of which not seen in years. Landslide to rear of Snaefell View (Greens
Garage). 100 tons of earthslipped.
November1890
Laxey River overflowed its bank in severalplaces.
October1891
Heavy Floods. Great body of water rushing down Laxey River damaging new bridge at
Creers Mill and the bridge at OldLaxey.
October1896
Floods. River swollen to such extent that fears it would flow into the mine. Miners
brought to the surface as a precaution. Severe damage at GarwickGlen.
January1899
Severe storms. Several houses in Glen flooded and two landslides. Old inhabitants say it
was the severest storm they canremember.
November1901
Severest storm for many years past. River overflowed its banks at the MER Weir flooding
several houses in the vicinity. MER turbine race damaged.
Beginning of
October1909
The biggest flood for many years. River reached an abnormal height. MER boiler room on
Glen Road flooded. Considerable damage to Laxey harbour at low water by swollenriver.
Middle of
October1909
Heavy rains. Many houses in Glen Road flooded. House at South Cape had water running
in the back and out thefront.
October1918
Laxey Village Commissioners meeting. Clerk instructed to draw the attention of the
Highway Board to the recent flooding on GlenRoad.
September1922
Considerable damage to Laxey harbour by floods due to heavy rain. Newly built wall and
weir designed to keep river from harbour sweptaway.
January1928
Downpour resembling a tropical rainstorm. Worst floods in living memory. Several
houses evacuated. Men working up to their waists in water while clearing drains. Bridge
across Laxey River at Creers Mill (now the Salmon Centre and Corn Mill) washed away.
Bridge on road to Cronk e Chule (now Laxey Wheel car park) washed away. River
overflowing onto road at Woollen Mills. Houses and gardens on Glen Road flooded and
several outbuildings swept away. Bridge across river to MER power house washed away.
Landslide on Church Hill. Great damage to Snaefell Mountain Railway. Water flowed from
line flooding several houses on Dumbells Terrace.
September1930
Worst floods in living memory causes flooding and damage on GlenRoad.
December1930
More floods. River burst its bank on Glen Road, through gap not repaired from
September 1930 flood, and flooded severalhouses.
February1931
Severe rain fall. Temporary repairs on Glen Road wall after September flood held back
the swollenriver.
November1931
Severe rain fall. Laxey River burst through temporary repair and flooded cottage of Mrs
Lewin who suffered much damage in 1930. Rain was spread over a longer period of time
than September 1930, but river was up to the top of the arches at Old Laxey Bridge in a
way never before seen by the older inhabitants. Ground water logged and water coming
off fields flooded a number of houses including one at the foot of MinorcaHill.
August1932
Severe flooding of river but Laxey escaped with littledamage.
November1940
Fairy Cottage river in flood flows onto main road and Old Laxey Hill causing flooding to
several houses. Landslide on cliff onto end property.
April1947
Bridge washed away at Agneash due to flood, washing away water supply to thehamlet.
January1948
Floodwater causes wall on Captains Hill tocollapse.
October1949
Heaviest rainfall for seven years. Storm water swept down the glen. People had to
barricade themselves into theirhouses.