Rapid response catchment forecasting – challenges and solutions · 2015-01-08 · 28 20 October 2010 BHS South West Section Meeting on Rapid Response Catchments Conclusions •

Rapid response catchment

forecasting – challenges

and solutions

Murray Dale, Halcrow, Associate Hydrometeorologist

COPYRIGHT HALCROW GROUP LTD. 2010

20 October 20102 BHS South West Section Meeting on Rapid Response Catchments

Contents

• Background & RRC definitions

• Challenges in flood forecasting

for RRCs

• Solutions for addressing these

challenges

3 BHS South West Meeting on Rapid Response Catchments20 October 2010

Background & Definitions


Pitt Review of 2007 Floods

• 3: The Met Office should continue to improve its

forecasting and predicting methods to a level which

meets the needs of emergency responders.

• 6: The Environment Agency and the Met Office

should work together, through a joint centre, to

improve their technical capability to forecast, model

and warn against all sources of flooding.

• 34: The Met Office and the Environment Agency

should issue warnings against a lower threshold of

probability to increase preparation lead times for

emergency responders.

• 35: The Met Office and Environment Agency should

issue joint warnings and impact information on

severe weather and flooding emergencies to

responder organisations and the public .


Key Challenge: Lead time and accuracy

Pitt Review, 2007


What is a Rapid Response Catchment?

• Term RRC was defined in 2006 through studies undertaken by HR

Wallingford for the Environment Agency that developed a register

of RRCs in England & Wales – used number of criteria

• “All communities where the upstream catchment has a Time to

Peak (Tp) that is less than or equal to 3 hours, require a rainfall

detection capability. You can provide this by an adequate

coverage of radar or an adequate coverage of rain-gauges.”

(Agency operational instruction Flood warning levels of service Operational

instruction 137_05, issued 18 December 2008)

• Forecasting Options for Rapid Response Catchments – R&D Scoping

Study (Halcrow, 2009) used Tp </= 3 hours

• This then incorporates Fast Responding Urban Catchments (FRUCs)

– many in Thames Region that were not on original RRC Register


RRC Characteristics

• Small catchment size

• Steepness of catchment slopes

• Relatively impermeable soils

and underlying geology

• A high degree of urban land

cover in the catchment,

increasing impermeability

In many cases, RRCs have a combination of

these characteristics, and sometimes a

combination of all these characteristics.



Traditional forms of forecasting don’t provide required lead times for effective action

Inflow 1

Inflow 2

Inflow 3

Town

Forecast

Routing

HD


Forecasting Challenges

Forecasting convective rainfall

Ottery St Mary October 2008

Images courtesy of the

Environment Agency

Probability of exceeding 100mm in the 24-

hour period 15 UTC 5th to 15 UTC 6th September from the UK 4km

model

Forecast starting at 03 UTC 5th





Forecasting Solutions


Forecasting Options for RRCs

• Study in February 2009 identified 6 options, split into two ‘phases’

• ‘Alert Phase’ & ‘Warning Phase’

• Alert Phase uses forecast rainfall

• Warning Phase uses measured hydrometric data (data actuals)


Options Evaluation




Depth-duration alerts

• Recommendation from Forecasting Options Scoping Study

• What rainfall depth causes what response in each catchment – know

what you’re dealing with

• Use historic event data to define thresholds



Operational DDA tool (Thames Region)

01/04/2010

Area 1hr 2hr 3hr 6hr Total Event

West

North East

South East

Area 1hr 2hr 3hr 6hr Total Event

West

North East

South East

Area Catchment Flood Watch Flood Watch Areas Flood Warning Flood Warning Areas

West Cole 0 1 0 4

West Ray (Wilts) 0 1 0 3

South East Beverly Brook 0 1 0 8

South East Hogsmill 0 1 0 2

South East Ravensbourne 0 1 0 11

South East Wandle 0 1 0 7

North East Brent Brooks 0 1 0 6

North East Pinn 0 1 0 7

North East Crane 0 1 0 2

North East Lower Lee Tribs 0 1 0 11

Region Total 0 10 0 61

Summary of Issues Expected

Forecast Issued:

Heavy Rainfall Alert Forecast (mm) for 01/04/2010

Heavy Rainfall Alert Forecast (mm) for 01/04/2010

Summer

Winter

Forecast & Warning Decision Making


Specific Developments - ERAs

• Alerts for Surface Water

Flooding

• Uses probabilistic forecasting

technology

• Uses latest meteorological

science, methods and models

to estimate likelihood of

exceeding: 30mm in 1-hr,

40mm in 3-hr, 50mm in 6-hr

• Forecast technology can be

used for RRCs


Flood sirens – a flood warning option

• Option for alerting in rapid response

catchments

• Requires reliable upstream level

trigger

• Lead time is very short, can

facilitate more immediate actions if

those at risk are aware of

procedures


Probabilistic Flood Forecasting

• Can increase lead time

• Can be used in a risk-based

framework

• Capture & quantify forecast

uncertainty


Conclusions

• Rapid Response Catchments present particular challenges, pushing

technical and operational limits

• However, rainfall forecasting has improved dramatically – options

are becoming possible

• Combining rainfall forecasts with other information (e.g. catchment

state, flood vulnerability, rainfall actuals) can give rise to risk-

based alerts in RRCs

Documents

Rapid response catchment forecasting – challenges and solutions · 2015-01-08 · 28 20 October 2010 BHS South West Section Meeting on Rapid Response Catchments Conclusions •