Storm Desmond – Scotland’s first red flood risk

Over the weekend of 5 – 6 December 2015 Storm Desmond – the fourth UK named storm of the winter season – brought damaging winds and heavy rain across Scotland, which experienced its most severe flood event since the advent of the forecasting service nearly 5 years ago. Impacts were widespread, and particularly severe in the southern Borders area. We issued our first ever Flood Guidance Statement containing a red (high) risk area, and also issued the first severe flood warnings since the service began. This is the first of two articles looking at the Desmond flooding. This article looks at the forecasting for this event; the second one will look at how this compared with a similar event in 2005.

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Areas of Hawick affected by flooding during 5 – 6 December 2015                                                                             Photo credits clockwise from top left: bbc.co.uk; Hawick Councillor Ron Smith; The Southern Reporter; Hawick RFC

Exceptional rainfall totals were recorded in a 48hour period from the 4th to the 6th of December as its associated weather fronts wriggled southward across Scotland and northern England.

Forecasters at the Met Office Aberdeen watched the developments of Desmond closely. The ground was already saturated after a wet and gloomy November – so the challenge was to predict as accurately as possible the intensity and distribution of the rainfall. Forecast models tend to provide more reliable guidance for the distribution of rainfall that is dynamic (frontal) rather than locally generated (e.g. thundery showers).

chartsIt was recognised at an early stage that the system would have a moist ‘river’ or ‘conveyer’ of winds ahead of the cold front. It was also important to consider the orographic enhancement of rainfall totals over windward mountain ranges.

Whilst the frontal structure of Desmond was complex, the initial weather predictions supported the view that the cold weather front would clear southward into England; and that the primary rainfall totals would be associated with the leading weather fronts. As Desmond moved towards Iceland, subsequent weather calculations indicated a trend to a slower and more erratic clearance of the cold front southward over the weekend.

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24 hour rainfall totals forecast on Friday for Saturday

Then finally; the cold front almost became stationary across the Borders, as further weather disturbances ran along it bringing further pulses of heavy rain.

The greater persistence and intensity of the moist conveyor, as well as the slower clearance of the dynamic rain associated with the cold front – lead to the prediction of exceptional rainfall accumulations across NW England and the Scottish Borders.

High flows were forecast throughout Scotland, as can be seen on the Grid-to-Grid (G2G) river flow model spatial plot, below. The widespread and prolonged nature of the expected rainfall meant that almost unprecendented quantities of run off were expected.

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4/12/15 07:30 Grid-to-Grid forecast for 5/12/15 18:30

Of particular concern were the Scottish Borders and Tayside, the G2G forecasts for which can be seen below. The forecasts for Hawick in particular were very consistent from two days before the event and closely matched what did occur. The forecast flows were comparable with those experienced in the extreme event of 2005, when there was extensive property flooding. The river Tay forecasts, whilst showing quite a range, all consistently indicated an expected flow approaching at least as great as that seen during the flood event of 2005, and possibly even the event of 1993, which was the largest in recent history. In the event it is clear that the model overestimated the Tay flows somewhat, possibly due to the non-inclusion of hydro schemes upstream which may have attenuated the peak of the flow.

desmond flowsThe heightened flood risk for the weekend was first signalled on the Flood Guidance Statement on Thursday, with a very low likelihood of significant impacts indicated. As confidence increased, on Friday the likelihood was increased to high, giving amber status to a large part of the country. Weather forecast models and G2G were giving consistent forecasts of very high rainfall totals and exceptional river flows. During Saturday it became clear that areas of the Scottish Borders, particularly Hawick and Newcastleton, were expected to experience severe impacts. These areas are not very far from the parts of northern England that were also going through extreme conditions.

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Risk maps from successive Flood Guidance Statements (left). Area of Concern map for peak of event (right)

A large amount of rainfall fell across Scotland from Friday through to Sunday. As can be seen below, the main areas affected in Scotland were the Borders and the centre of the country (Tayside area) plus the north west.

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Rainfall intensity from RADAR for the event

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Totals in mm recorded at SEPA raingauges during the event (note Teviothead is an underestimate due to the site flooding)

 

 

 

 

 

The impacts of this event were considerable. 15 flood alerts, 75 flood warnings and 2 severe flood warnings were issued across the country.  Over 150 properties were flooded in Hawick and other locations across the country. Significant travel disruption occurred. The Perth flood defence scheme was activated, entailing the closure of the main town bridge. Several railway lines were closed. Travel impacts were also exacerbated by the (unrelated) recent closure of the Forth Road Bridge for emergency maintenance.

This event proved a major challenge for the forecasting service. For the first time we had to decide on issuing a Flood Guidance Statement with an area on high (red) status. We also input to the decision to issue two severe flood warnings, also an unprecedented situation. Rainfall predictions, particularly the UKV 36 hour high resolution forecasts, proved to be very reliable for the most part. G2G forecasts were invaluable. They were particularly in demand for locations where no local model currently exists, and for lead times longer than that used for local models. Timely forecasts and warnings enabled mitigating actions to be taken. As will be shown in the next blog, improvements in forecasting and warning have led to a much improved outcome since the last time we saw this magnitude of flooding in Scotland.

By Richard Maxey and John Mitchell

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Decision making in times of uncertainty: the crying wolf

Nikéh Booister presents the second and final part of this series on decision making in times of uncertainty.

As part of her M.Sc. thesis research in Flood Risk Management Nikéh did a part of her research in SEPA. The research focussed on decision making under uncertainty with an outlook on improving decision making based on the cost-loss method. Nikéh continues working in the field of Flood Risk Management with her company FloodCom.

As part of her M.Sc. thesis research in Flood Risk Management Nikéh did a part of her research in SEPA. The research focussed on decision making under uncertainty with an outlook on improving decision making based on the cost-loss method. Nikéh continues working in the field of Flood Risk Management with her company FloodCom.

“The forecaster has to be the expert on knowing what uncertainties are and where they come from as we could have read in the previous post. The forecaster has to decide on what is relevant and not and to a certain extent already has to make a decision on the quantification of the uncertainty. Whether or not a forecaster understands and interprets correctly the uncertainty, the decision maker has a final call and responsibility to “pull a trigger” for action.

Currently most of the forecasts are presented in a deterministic way, which means a single valued number of a possible future. A part of the uncertainty is presented in a description of the future system state, nevertheless this is not quantified. A possibility is to present a forecast in a probabilistic way, where uncertainties are presented.

As mentioned in the blogpost of Paul Ryles; longer lead time makes the prediction of a future situation more challenging; the rate of uncertainty is higher. Probabilistic forecasts indicate possible future outcomes of a system and give the decision maker and responder more possibilities of responding (or at least the consideration of responding and with this more time to respond). This makes it possible to respond earlier in time and make decisions with a lower regret. Making a decision based on a deterministic forecast seems easier because it presents a binary solution (yes or no decisions). Nevertheless research has proven that decision making based on a probabilistic forecast gives better results (Ramos et al. 2013).

In possible flood situations Aberdeenshire Council can pull out the demountable defences to protect Stonehaven from flooding. Nevertheless at the moment a decision has to be made it is not sure if the event will actually materialise but due to time limitations the decision has to be made.  (Source: Aberdeenshire Council, 2014)

In possible flood situations Aberdeenshire Council can pull out the demountable defences to protect Stonehaven from flooding. Nevertheless at the moment a decision has to be made it is not sure if the event will actually materialise but due to time limitations the decision has to be made. (Source: Aberdeenshire Council, 2014)

When decision makers are introduced to the problem of uncertainty in forecasting it can appear to be too much; too complicated; and out of the scope of the decision maker to respond properly to this. This interacts with the fact that the decision makers and responders are the ones that have to deal with for example the uncertainty of citizen response. Thus, the responsibility is bigger. When a wrong decision is made, there is a threat of a “crying wolf” principle. Despite the efforts, there will always be dissatisfaction with the decision taken or damage occurred. And so, someone to be blamed. When a decision is based on a deterministic one, it is the model or the forecast to be blamed and not the decision maker – which makes the situation more acceptable.

The ultimate question in forecasting and decision making is who is responsible in the end? And who is to blame in the case of a wrong decision being taken? Don’t we all try as hard as possible?

Timing and level uncertainty in flood prediction makes it hard to decide. The longer the lead-time the higher the uncertainty (Source: N.Booister M.Sc. thesis)

Timing and level uncertainty in flood prediction makes it hard to decide. The longer the lead-time the higher the uncertainty (Source: N.Booister M.Sc. thesis)

There are approaches that provide an ‘answer’ and provide the forecaster or the decision maker with a decision making perspective, but when we would like to provide the decision maker with a longer lead-time, as mentioned above, probabilistic forecast might be the solution. In order to avoid blaming of the decision maker; a decision making tool or method needs to be in place to support the decision. One of these methods is the cost-loss method. This method compares the probability of an event to occur with the cost of certain mitigation measures and the losses that they avoid. Although it presents a solution method, it seems too static in time and due to its economic point of view, does not take into account intangible effects.

Models are improving, more lead-time is being provided, but uncertainty will always remain. Decision making stays a difficult and also a sensitive topic.”

 

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Decision making in times of uncertainty: the forecaster

In a two-part feature, we cover various roles involved in decision making at times of uncertainty. The first is provided by Paul Ryles, Senior Scientist and operational flood forecaster at SEPA.

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Paul manages SEPA’s flood risk management and operational hydrometeorology trainee programmes and has been in flood warning service development since 2002

“Flood prediction is a science, especially in challenging situations such as providing long lead times (for example through our five day flood guidance statement) or flash flooding (surface water alerting). One of the primary roles of the flood forecaster in these situations is in communicating uncertainty in a way that is useful for responders. The forecaster must quantify uncertainties in the weather forecast (in terms of location, timing, intensities and amounts of rainfall) and any potential uncertainties in the resultant flood forecasting predictions and hazard footprint.

Meteorological and hydrological tools such as MOGREPS and Grid-to-Grid provide the forecaster with the opportunity to quantify uncertainty in flood predictions. Image of MOGREPS-G and probability of greater than 25mm in 24 hours ahead of ex-Hurricane Kate (Copyright Met Office)

Meteorological and hydrological tools such as MOGREPS and Grid-to-Grid provide the forecaster with the opportunity to quantify uncertainty in flood predictions. Image of MOGREPS-G and probability of greater than 25mm in 24 hours ahead of ex-Hurricane Kate (Copyright Met Office)

Understanding what type of information customers require is vital, as is an appreciation of how this information will be used and what type of decision will be made. The flood forecast will often be used to make crucial ‘yes or no’ decisions, such as committing resources in responding to a flood event, evacuations or deploying temporary flood prevention measures. Understanding these customer requirements is as vital as knowledge of meteorology and hydrology in producing successful flood forecasts.

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Flood Forecasters participating in Exercise Big Water, testing response and preparedness to new coastal flood forecasting capabilities.

Having competent and experienced flood forecasters is essential in delivering this service. To this end, the Flood Forecasting Service, with support from the Met Office College, has developed a formal qualification in Operational Hydrometeorology. The aim is for all of our flood forecasters to achieve this qualification, which assesses all aspects of producing and communicating our flood forecasts. The forecasters, who will already have completed training in flood forecasting, warning and response, will undertake this qualification after a period of on-the-job training.

Forecasting some flood situations remains a challenge, however ensuring all flood forecasters are trained and competent (and assessed) in accounting for, quantifying and communicating risk at times of uncertainty should maximise the benefit of flood predictions for responders when making difficult decisions.”

Posted in Forecasting, Hydrometeorology, Risk communication | 1 Comment

Flood forecasting: current challenges and future aspirations

How can flood forecasting best rise to the challenges and opportunities that communities and partners face; at what point does effort in flood forecasting become less effective than local adaptation; and what are the aspirations and capability of our future flood forecasting services – all questions posed for the speakers at this one-day conference in Birmingham.

Pilot capability for a risk-based coastal decision support tool. (source: Environment Agency)

Pilot capability for a risk-based coastal decision support tool. (source: Environment Agency)

The growth of flood forecasting or hydrometeorology as a science and its associated services is evident given the range of presentations. Products such as the five-day flood guidance statement – ground breaking when introduced 5 years ago – is now firmly a part of the flood response environment with a new appetite for longer lead times (Turner) and more challenging local applications such as wave overtopping forecasting at the community and asset scale (Wall; Lawless). So what are the key challenges and aspirations:

Coastal decider tool providing a 32-day look at the probability of coastal flooding related weather types. (source: Met Office)

Coastal decider tool providing a 32-day look at the probability of coastal flooding related weather types. (source: Met Office)

Who are the customers of our forecasts?
Forecasts are no longer there to purely support the issuing of flood warnings. The type and range of forecasts vary and agencies are grappling with the idea of what to forecast, where and for who given the growing desire for forecast data. Policies in England are now focused on making forecasts more accessible and usable (Anspoks), whilst at a community level these need to sit alongside local tool-kits and support the potentially powerful use of social media by flood groups (Barnes). The desire to receive ‘everything’ (Young) should also be offset with the need to facilitate and support users of the forecast information (Rowlands and Williams).

How good are the forecasts?
Investment in our services is under increasing scrutiny especially with continued funding pressures in the public sector, therefore prioritising effort where it’s most needed for improvement is important. An example was presented of a framework to integrate flood model forecast assessment studies for local and national scale models to provide an evidence base for flood forecasting model development (Moore) – one to assist in future decision making.

Are they understood?
The case for the difficulties in effectively communicating flood forecasts was made by a number of speakers with the recent severe flooding in the south of France being highlighted (Mitchell). When the Somerset levels flooded in 2014, the impacts were felt for several months with the Environment Agency incident room being staffed continuously for 11 weeks and 3 days. One response from someone affected was that ‘the water came so quickly’; however it’s interesting to note that the first Flood Alert predicting the impacts was issued some 16 days before the first property was impacted (Rowlands and Williams).

Prototype fluvial flood risk forecast maps using G2G hydrological ensembles in conjunction with NFRA flood impact data presented for flooding affecting Prestwick Airport in 2012. (Photos left source: Prestwick Airport; Maps right source: CEH Wallingford)

Prototype fluvial flood risk forecast maps using G2G hydrological ensembles in conjunction with NFRA flood impact data presented for flooding affecting Prestwick Airport in 2012. (Photos left source: Prestwick Airport; Maps right source: CEH Wallingford)

Do they reflect the impacts well enough?
One speaker referred to this area as ‘making the forecasts come to life’. Effort is now being put into translating flood forecasts into risk-based flood impact assessments, with frameworks being developed for inundation mapping (Whiting), coastal flood risk decision support (Anspoks) and our own prototype forecast river hazard mapping in Scotland (above).

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Heavy Rainfall Alerts – latest advances

In order to provide probabilistic forecasts of flooding from surface water the forecasting service has been using a Heavy Rainfall Alert (HRA) tool since 2013, as discussed in a previous article. The tool consists of a spreadsheet that is updated four times a day and emailed to the forecasting duty officer, and a series of maps. After two years of use we have reviewed the tool and made some improvements to the spreadsheet, adding new functionality and making it more user friendly.

As before, the tool uses MOGREPS-UK forecasts subjected to neighbourhood sampling (over 60km) to calculate the probability of exceeding depth duration thresholds (that have been defined in terms of expected impact), within specified operational areas. The new tool now has two key innovations.

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Areas assessed by the tool. Point maxima (left) and catchment averages (right)

  1. Urban areas

In the new tool an assessment of depth-duration threshold exceedence is made for each of seven urban areas. These areas are each around 1000km2, which is believed to be the optimum size for trading off event probability and usefulness of forecast. Now, by looking at the spreadsheet, the forecaster can see at a glance which urban areas might be affected. Previously, she would only have had information on exceedences within a larger area, and there might not have been any information immediately to hand to confirm whether or not the urban area was at risk.

2. Fast responding catchments

The other innovation relates to a new assessment of 16 catchments. These are all small, rapidly responding, flood prone catchments, where early warning of potential river flooding is required. Catchment average depth duration thresholds for various levels of impact have been calculated. The tool calculates exceedence of these thresholds by the areal average of these catchments, as opposed to the point maxima used previously. This is quite an innovation, and is the first time that the HRA process has been used in this way within the UK.

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Example excerpt from the tool

  1. Other improvements

In addition, a first guess risk tool has been built in to the spreadsheet, combining impact and likelihood. Maps, lists of thresholds and user guidance are also included to make the tool user friendly.

As has been seen more than once this summer, the ability to focus risk based forecasts for surface water and rapid response catchments is critical to enable effective emergency response, and so the improved tool should go a long way towards helping us to meet that challenge.

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The July Perthshire floods: hydrometeorological predictions

Figure 1: Flooding impacts in Alyth.  Clockwise from the top: Cars swept along Commercial Street in Alyth (source: Angus Forbes Photography); flood waters into properties (source: Michelle Macleod); and debris and damage caused by the Alyth Burn (source: Perthshire Picture Agency)

Figure 1: Flooding impacts in Alyth. Clockwise from the top: Cars swept along Commercial Street in Alyth (source: Angus Forbes Photography); flood waters into properties (source: Michelle Macleod); and debris and damage caused by the Alyth Burn (source: Perthshire Picture Agency)

A major clean-up operation is underway following flooding that affected some Perthshire communities on the 17th July 2015. Villages such as Bankfoot were affected and surface water flooding was seen across much of the road network; however the flooding impacts were most critically witnessed in the town of Alyth. During the morning of the 17th July, serious flooding from the Alyth Burn resulted in the evacuation of 70 homes and businesses, significant impacts on bridges, with cars being swept along due to the force of the water, and over 750 properties being left without power. The spell of heavy rain originated from an occluding frontal system associated with a low centre near North West Ireland which tracked northwards across Perthshire on Friday morning. It was an active frontal system with the rainfall amplified by uplift over the hills with embedded heavy showers. Numerical weather prediction models were highlighting this area well; the UKV model was estimating a broad area of 30 to 40mm in 6 hours and MOGREPS-UK (ensemble forecasting system) producing a greater than 80% likelihood of more than 30 mm in 6 hours across central Perthshire.

Figure 2: UKV model estimates for rainfall accumulation in 6 hours to midday on the 17th July with a lead time of 24 hours (left) and MOGREPS-UK probability of exceeding 30mm in 6 hours with a lead time of 35 hours (right)

Figure 2: UKV model estimates for rainfall accumulation in 6 hours to midday on the 17th July with a lead time of 24 hours (left) and MOGREPS-UK probability of exceeding 30mm in 6 hours with a lead time of 35 hours (right)

Although the investigation into the cause of the flooding continues, the relatively short duration of the intense rainfall has certainly been a contributing factor. Previous research into flooding impacts carried out by the flood forecasting service suggests that this depth of rainfall over this short duration has the potential to lead to significant flooding. Although there are no raingauges in the immediate vicinity of the Alyth Burn catchment, some raingauges in the area did record significant 6-hour accumulations including Kindrogan (40mm) and Perth (43mm). Radar estimates across the small catchment provide the opportunity to estimate the distributed rainfall depths for the Alyth Burn catchment with totals being estimated in the range of 30mm in Alyth to 42mm in the headwaters.

Figure 3: G2G (a 1km distributed hydrological model for Scotland) highlighting areas of greater than 60% of ensemble members above the 2-year warning level (left) and model predictions and observed flows for the Craighall gauging station.

Figure 3: G2G (a 1km distributed hydrological model for Scotland) highlighting areas of greater than 60% of ensemble members above the 2-year warning level (left) and model predictions and observed flows for the Craighall gauging station.

Likewise the hydrological model Grid-to-grid (G2G) was also suggesting likely response in the river systems across Perthshire and Angus. The lack of gauged data for smaller catchments, such as the Alyth Burn, in the area makes it difficult to assess the accuracy of G2G estimated flows, however for some gauged records it appears that the severity of the flows may have been underestimated in places (River Ericht at Craighall observed peak flow estimate 335 and UKV deterministic G2G prediction 226 cubic metres per second).

Figure 4: The Flood Guidance Statement area of concern map issued to emergency responders on the 16th July.

Figure 4: The Flood Guidance Statement area of concern map issued to emergency responders on the 16th July.

These forecasting tools were able to allow for the prediction of potential flooding on the 17th. Whilst minor flooding impacts were expected, the Flood Guidance Statement published on the 16th July did indicate the potential for some significant impacts, although the likelihood was very low. Some immediate lessons could be taken away from the experiences of forecasting this event, specifically:

  1. With meteorological models indicating a medium to high likelihood of rainfall depth-duration leading to significant impacts in the area (from historical analysis), could we have been more confident in our assessment of the flooding impacts scenario. This may have resulted in a higher overall flood risk status.
  2. The hydrometeorological forecasting tools were all indicating a spell of potential flood disruption yet pinpointing exact locations of the more significant impacts is always challenging. More direct links to SEPA’s national flood risk assessment and potentially vulnerable areas might help steer future resources to at risk locations, especially where no flood warning schemes exist.

For information on the how flooding hazard impacts are forecast please read our guide to the Flood Guidance Statement.

Posted in Flood, Forecasting, G2G, Hydrometeorology, Risk communication, Weather prediction | 1 Comment

Surface Water Flooding in Aberdeen

As noted in previous articles surface water flooding can be challenging to forecast. On Tuesday 7th July a period of intense rainfall over Aberdeen and the surrounding area led to significant surface water flooding. The forecasting service was able to provide timely guidance and alerts.

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Synoptic situation leading up to the event

The threatening weather situation leading up to the flooding in Aberdeen was initially forecast the preceding weekend. In the build up to Tuesday, an associated low centre was expected over Aberdeenshire and Aberdeen City. The airmass was relatively moist and potenitally very unstable. Many of the conditions for localised downpours were evident. Namely, unstable air, warm daytime temperatures, slack flow and some changes in wind direction aloft. On the day, temperatures quickly reached the values required to set off the heavy and thundery showers. Initially over high ground to the southwest of Aberdeen, gradually drifting northeast over the city. The showery procession may also have been reinforced by ‘convergence’ along the coast. A sea breeze effectively adding impetus to an already fraught situation. A process called ‘back-building’ may also have played a part. This is when a shower cloud sets of ‘daughter cells’ behind it as it moves with the main steering flow. Met Office weather predictions models were suggesting some high rainfall totals over a short period of time and these predictions were reinforced by associated probability forecasts.

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Met Office forecasts in the run up to the event. Heavy Rainfall Alert probabilities of 30mm/3hr (significant impacts) from MOGREPS-UK, left, and deterministic UKV, 3hr totals, right

The exact location, intensity and timing of heavy showers are notoriously difficult to forecast. Scottish Flood Forecasting Service (SFFS) meteorologists and hydrologists did however decide there was enough evidence to warn of the risk of significant ‘surface water’ impacts on the preceding Monday. This risk was repeated in the SFFS Flood Guidance Statement and accompanied by alerts on the day of the flooding.

Area of Concern map from 7 July Flood Guidance Statement

The event caused significant disruption. Impacts included flooded properties, roads and commercial premises, including a shopping centre. The airport terminal was flooded, as was a dogs’ home. Children had to be evacuated from a nursery.

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Flooding impacts in Aberdeen, from Press and Journal

The rainfall was of the order of 10mm in 1 hour, or 20mm in 3 hours, as recorded at various gauges and by the nearby Hill of Dudwick rainfall radar. At individual locations, this would indicate a return period of around 5 to 10 years, however given the relatively widespread nature of the event, with these totals being recorded over several square kilometres, the actual return period of the event could be much higher. Taken as isolated spot values, and comparing with our depth-duration guidance thresholds, these figures would normally indicate expected impacts in the minor category, but given the widespread nature of the rainfall, it is unsurprising that impacts were significant. The cumulative effect of this event following another heavy rainfall event on the preceding Saturday, and the high tide restricting drainage from some areas, may also have increased the severity of flooding.

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3 hour rainfall accumulation from radar. Widespread 20mm totals.

Given the current availability of tools and resources the forecasting service performed very well. However the less predictable and smaller scale event (with mainly minor impacts) in Aberdeen just three days earlier did not get the prominence it perhaps deserved, meaning there is still quite a lot to do to if the Scottish Flood Forecasting Service is to have a comprehensive, consistent and accurate capability in the difficult and challenging area of surface water flood forecasting.

The forecasts (particularly the Heavy Rainfall Alert tool) suggested there was >40% probability of significant impacts for the 32x32km grid square in which Aberdeen is located at one corner. This would have yielded a medium (amber) flood risk on the Flood Guidance Statement, however as it is sometimes felt that extreme event probabilities can be overdone, a lower probability was assumed for the urban area. A new version of the HRA tool, which comes online for use by the forecasting service later this summer, will better target urban areas and make this kind of decision making a little easier. A detailed surface water flood risk mapping tool, similar to that used in Glasgow for the Commonwealth Games last summer, would also improve forecasting of this type of event, and we plan to further develop the scope of this model in future.

Posted in Flood, Forecasting, Surface Water | 2 Comments