Surface water management planning: guidance (2013)

Appendix 4 - Further information on surface water flooding data

Potentially Vulnerable Areas

Owner - SEPA

Format - GIS shapefile

Potentially Vulnerable Areas ( PVAs) were identified as part of the National Flood Risk Assessment ( NFRA) required under the Flood Risk Management (Scotland) Act 2009. PVAs indicate where the risk of flooding is significant enough to justify further assessment of flood risk and further identification of measures.

SWMPs will be used to help understand and manage flood risk within PVAs.

It is important to note, not all properties within a PVA are at risk of flooding. PVAs are based on catchment units and will allow Scottish Government, SEPA, local authorities and Scottish Water to develop a planned response to manage flood risk effectively.

National pluvial flood extent (flood hazard)

Owner - SEPA

Format - GIS shapefile

DTM input - NextMap DTM, 5m resolution and a vertical accuracy of 0.7 - 1m.

Model type - ISIS-FAST Rapid Flood Spreading Method

Outputs - flood extents and depths for the 1 in 200 and 1 in 30 rainfall return periods.

These maps provide a national coverage of predicted surface water flooding but with a lower level of confidence than Regional pluvial hazard maps. They can be used by Councils considering flood risk outside of PVAs but provide no information about flow velocity.

Model parameters applied:

	Urban Areas	Rural Areas
Rainfall return period	1in 200 year, 1 in 30 year	1in 200 year, 1 in 30 year
Storm duration	1 Hour	3 Hour
Storm profile	50% summer	50% summer
Percentage runoff	70%	55%
Drainage allowance	12mm/hr	Nil

During post processing of the model results, flood depths of <0.1m were removed and these areas were shown not to flood. The same process was also applied to remove flood depths of <0.3m. These depth thresholds have been provided as two separate layers and have also had small flood 'ponds' of areas <200m ² removed.

The national pluvial extent map provides a strategic national overview of pluvial flood risk in Scotland. The pluvial extents should not be displayed on a background map that is more detailed that the 1:50,000 scale as this represents the level of modelling detail which can be derived from a national level study. It can be used for:

- A broad understanding of pluvial flood hazards

- Strategic assessment of the negative impacts (costs) of flooding

- An indication of those areas more susceptible to pluvial flooding and used to identify PVAs.

- Indication of where regional pluvial hazard modelling should be carried out

- Set high level objectives in the SFRMPs

- Potential use in land use planning development plans - may be of use in a limited manner at a strategic level by SEPA, working jointly with the LAs, to assist the consideration of catchment drainage issues at preliminary SFRA stage and assist scoping for Strategic Environmental Assessment. This would assist to highlight areas where further more detailed assessment may be required by the LA/ developer.

It does not provide enough detail to:

- Accurately estimate the flood risk associated with individual properties or specific point locations.

- Determine flood risk for insurance purposes or property enquiries.

- Identify or appraise structural measures to manage surface water flooding.

Pluvial flood extent Dundee, Carnoustie, Aberdeen and Inverness (flood hazard)

Owner - SEPA

Format - GIS shapefile

DTM input - LiDAR DTM, 1m resolution and vertical accuracy of approximately 0.15m.

Model type - JFlow 2D flood routing

Outputs - flood extents and depths for the 1 in 200 and 1 in 30 rainfall return periods.

These maps provide greater confidence than the national pluvial extent maps but in a limited number of locations. These will provide information on areas vulnerable to surface water flooding outside of the regions covered by regional pluvial hazard maps.

Model parameters applied:

	Urban Areas	Rural Areas
Rainfall return period	1in 200 year, 1 in 30 year	1in 200 year, 1 in 30 year
Storm duration	1 Hour	3 Hour
Storm profile	50% summer	50% summer
Percentage runoff	70%	55%
Drainage allowance	12mm/hr	Nil

During post processing of the model results, flood depths of <0.1m were removed and these areas were shown not to flood. The same process was also applied to remove flood depths of <0.3m. These depth thresholds have been provided as two separate layers and have also had small flood 'ponds' of areas <200m ² removed.

This project presents a pluvial flood risk map for Inverness, Dundee and Aberdeen using high resolution LiDAR ground models and a 2D hydraulic model. This approach is an improvement over the earlier mapping which used coarse resolution NextMap ground models and the ISIS-FAST Rapid Flood Spreading Technique.

Buildings were introduced as a +0.3m extrusion and roads were introduced as a -0.1m extrusion.

Modelling assumptions include:

• Filtered DTM to more accurately represent ground surface, streamlines and topographic lows.
• Flow is constrained along road by a notions 0.1m kerb height.
• Mannings 'n' is applied as a blanket figure of 0.1.
• Overland flow interacting with the edge of the model domain is lost.
• Extended model run time to allow for creation of final flood depths

Regional pluvial hazard maps

Owner - SEPA

Format - GIS shapefile

DTM input - combination of LiDAR and NextMap DTM, 2m resolution and 5m resolution.

Model type - JFlow+ 2D spreading approach

Outputs - flood extents, depth, flow pathways (velocities) and hazard ratings for a range of return periods.

These maps provide greater confidence than the national pluvial extent maps across specific urban locations. These will provide information on areas vulnerable to surface water flooding outside of the regions covered by regional pluvial hazard maps.

14 scenarios being run including rainfall return periods 1in 10, 1 in 30, 1 in 30 plus climate change, 1 in 50, 1 in 100, 1in 200, 1 in 200 plus climate change. Two storm durations are being applied for each scenario. More information on areas with regional pluvial hazard maps and all scenarios can be seen in Appendix 5.

The regional pluvial extent map will be derived via a two-dimensional (2D) modelling to provide pluvial flood extents, depths and velocities for both ponded areas of water and pluvial flow pathways. These model runs are required for selected areas of Scotland identified within Appendix 1.

This work will build upon SEPA's national pluvial project (Derivation of National Pluvial Hazard Database) to provide a more refined representation of pluvial flood hazard and will deliver a range of additional return periods, model loss scenarios and storm durations as detailed in Table 1 of Appendix 5.

The outputs of this contract will feed into the development of flood hazard and risk maps as required by the FRM Act.

The areas to be included in the regional pluvial modelling area shown in Figure 1 of Appendix 5.

Limitations include:

• Degraded LiDAR in areas which include NextMap data.
• Blanket loss parameter
• Limited rural coverage
• Topographic influence in vicinity of watercourses

Flood risk from sewerage systems (Section 16 of the FRM Act)

Owner - Scottish Water

Format - GIS shapefile

DTM input - combination of LiDAR and NextMap DTM, 2m resolution and 5m resolution.

Model type - InfoWorks CS

Outputs - Location, volume, extent and depth of flooding from sewerage systems in areas with a risk of surface water flooding.

This information can be used to assess the local capacity of sewer systems and understand flooding mechanisms in areas with large transfers of flow underground through sewers. Pluvial flood mapping and sewer flood mapping can be used in conjunction to explain flood mechanisms.

1 km ² grid of impacts on receptors

Owner - SEPA

Format - GIS shapefile

Produced as part of the National Flood Risk Assessment ( NFRA) required under the Flood Risk Management (Scotland) Act 2009.

The Risk Grid Cells show the adverse impact on receptors (see table below) from the 1 in 200 year flood event. The impact of pluvial flooding on receptors used the national pluvial flood extents and flood extents for Dundee, Carnoustie, Aberdeen and Inverness.

Receptor Group	Description
Human Health A - People	number of residential properties and the social vulnerability of the area
Human Health B - Community	Important facilities that could cause community disruption if affected e.g. schools, hospitals.
Economic Activity A - Business	number of business properties and the estimated weighted annual average damage related to the property
Economic Activity B - Transport	Roads, railways and airports
Economic Activity C - Agriculture	Agricultural land and forestry
The Environment	Areas designated for natural heritage purposes and their vulnerability to flooding
Cultural Heritage	Cultural heritage sites such as UNESCO World Heritage Sites.

Regional pluvial baseline impacts

Owner - SEPA

Format - GIS shapefile

Produced for the development of SFRMPs / SWMP and LFRMP and for use in the strategic cost - benefit appraisal of measures to address surface water flooding.

Shows the adverse impact of pluvial flooding on receptors for a range of rainfall return periods. The receptors assessed using the regional pluvial flood hazard maps are:

Category	Receptors	Flood Risk Indicators
Economic	Non residential properties ( NRPs)	Direct economic impacts expressed in monetary damages (£s)
	Residential properties ( RPs)	Direct and indirect economic impacts expressed in monetary damages (£s)
	Transport	Direct damages to transport (road) infrastructure (£s)
	Vehicles	Direct damages to vehicles (£s)
	Emergency Services	Direct damages already considered under RPs and NRPs. Additional assessment of indirect impacts expressed in monetary damages (£s)
Social	Human Health	An assessment of risk to life
		Number of people at risk of flooding adjusted for social vulnerability
	Community Facilities	Count of community facilities
	Utilities	Count of utilities
	Disruption to Transport	Score based disruption to roads, rail and airports
	Cultural Heritage	Score based on importance of site at risk of flooding and vulnerability to flooding.

Historical flood events national database

Owner - SEPA

Format - Excel spreadsheet

SEPA have collated information on historical flood events. This datasets provides a comprehensive list of historic flood records from a number of sources including:

• Local Authority data
• Newspaper chronicles
• Archived data
• SEPA data
• Information from the public
• Academia
• Literature

There are approximately 15000 records within the database which vary in reliability and detail. Screening has been undertaken to isolate the records that are of most use. Records have been classified into the source of flooding (i.e. fluvial, pluvial, coastal or sewer).

Map of watercourses and SUDS

Owner - local authority

Format - to be determined

Section 17 of the FRM Act requires local authorities to map watercourses and SUDS in their area.