Surface Water Management Plan: Better Mapping and Data is Key

The Government this week released its Surface Water Action Plan, aiming to tackle surface water flooding which affects up to 3.2 million properties in England. It seeks to bring our preparedness more closely into line with that from main rivers and the sea. We review the key points.

Improving Mapping

The Environment Agency (EA) has improved its surface water flood risk mapping but this latest plan recognises that it is still less robust than flood risks from rivers and the sea.

It identifies that mapping is limited by not accurately capturing:

• Drainage rate – A single drainage rate of 12mm/hr is given for all urban areas, unless Lead Local Flood Authorities (LLFAs) are able to give better local data. These where known are in the range of 6 to 20 mm/hr.
• Local validation – Many LLFAs have not yet submitted local surface water information or been able to validate the national modelling.
• Subsurface drainage features – Flood relief culverts and flood storage are frequently omitted.
• Outfall – It assumes water freely outfalls and does account for being withheld by tides or high river levels.
• Ground level data – LIDAR (light detection and ranging, a form of remote sensing) information provides a high degree of accuracy but works better in steeper rural areas versus flat urban areas. They do not take individual property threshold heights into account.

The EA is charged with improving national surface water mapping and risk assessments. This will be achieved through improved modelling approaches, better quality data and combining the effects of flooding from different sources. The first improved maps will be available by the end of winter 2020/2021.

We look forward to working with the EA on refining surface water risk assessment including the role that groundwater plays in surface flood modelling.

Our GW5 national groundwater flood risk map  delivers more granular data showing areas where  surface water flood risk may be elevated by a high groundwater table at the property level. 

Infrastructure resilience

LLFAs have the leadership role on surface water management.  Water and sewerage companies also have an important role to play, as they are responsible for much of the drainage network. Securing the long-term resilience of wastewater systems and providing for resilience against flooding and wider risk is a key priority.

Ofwat requires water and sewerage companies to meet the following performance commitments:

• Internal sewer flooding – The number of internal flooding incidents per year, including sewer flooding due to severe weather events per 10,000 sewer connections.
• Pollution incidents – per 10,000km of wastewater network, as reported to the EA.
• Sewer collapses – Number of sewer collapses per thousand kilometres of all sewers causing an impact on service to customers or the environment.
• Risk of sewer flooding in a storm – Percentage of population at risk of sewer flooding in a 1 in 50 year (2%) storm. This is a new commitment to help the water industry meet expectations about reducing the likelihood of service failures that lead to flooding.

Groundwater infiltration to sewers reduces their capacity to cope with surface water flooding. The role of groundwater is a key element to understanding surface water risk for sewer flooding.

We are engaging with water companies to integrate our infiltration and groundwater data as part of their asset management and resilience plans.

Surface Water Flood Forecasting

Surface water flooding is more difficult to forecast than flooding from rivers and the sea. Current meteorological methods cannot determine where this heavy rain will fall with the necessary levels of precision, and within a useful forecast lead time. The Met Office has launched an early warning for thunderstorms this summer as a trial. They, the EA and the Flood Forecasting Centre (FFC) are carrying out a review of how improvements can be made in surface water flood forecasting and communication.

We work closely with the FFC. Our real time groundwater flood risk and sewer infiltration forecasting is ensuring sufficient advance notice to utility companies, local communities and emergency responders to put contingency plans into operation.

Planning and SuDS

3 levels of management plans are now required to mitigate surface water flooding:

• Local Flood Risk Management Strategy (LFRMS) – LLFAs must publish an LFRMS assessing the local flood risk, setting out objectives for managing local flooding and describing measures to meet the objectives.
• Strategic Flood Risk Assessment (SFRA) – Local planning authorities (LPAs) must carry out a strategic flood risk assessment to inform local planning decisions. They must consider flood risk from all sources, including surface water and groundwater, accounting for climate change.
• Drainage and Wastewater Management Plan (DWMP) Water and sewerage companies, led by Water UK, will be developing DWMPs to improve the long term planning of drainage and wastewater services.

The 25 Year Environment Plan also includes specific commitments to strengthen surface water provisions, including putting in place Sustainable Drainage Systems (SuDS).

The Surface Water Management Plan commits to amending the National Planning Policy Framework (NPPF), Planning Practice Guidance and Building Regulations to encourage the uptake and long term maintenance of SuDS.

Draft revisions to the NPPF include a proposed requirement for all major development and all development in flood risk areas to contain SuDS unless there are clear reasons they would be unsuitable.

LPAs should also ensure maintenance arrangements of SuDS are in place over the lifetime of the development. The final revised NPPF is due to be published later this summer.

Better Data, Better Planning Decisions

There are clear management obligations across local government, national agency and utility companies to plan for these random, unpredictable surface flooding events. This extends to future new build development to ensure they do not exacerbate surface water flood risk.

A greater role for fully comprehensive Flood Risk Assessments and better data ensures that GeoSmart’s data and desktop reports will be at the heart of planning submissions and resilience planning in the future.

For more information, contact us on 01743 581 415 or email enquiries@geosmartinfo.co.uk