Using Quantitative Precipitation Estimates in Flash Flood Forecasting Scott Sinclair, Geoff Pegram, Théo Vischel School of Civil Engineering, Surveying and Construction Management University of KwaZulu-Natal, Howard College, Durban, South Africa WRC

Why is flood warning important in South Africa? Large numbers of people live in informal settlements on flood plains in South Africa Formal developments also exist in areas where flooding might occur Disaster managers have limited existing structures to obtain relevant information Flood warning can save lives (and money) Check spelling and wording here. 29/06/2005

The cost of floods in South Africa Place Event Damage Ladysmith Floods, 1994 400 families evacuated R50 million in damages Pietermaritzburg Floods, 1995 173 lives lost Emergency shelter needed for 5500 people

Modelling the Hydrological cycle “What happens to the rain?”………Penman (1961) Measurement uncertainty http://www.cet.nau.edu/Projects/SWRA/research.html Modelling uncertainty Parameters & Processes

Discretization Scheme Types of Hydrological Model Lumped Semi-Distributed Distributed Discretization Scheme Entire Catchment Quaternary Catchment Grid Cell Control Volume

Rainfall data requirements for Flood Forecasting Real-time data Robust data transfer mechanisms Suitable temporal resolution Suitable spatial resolution Accurate validated accumulations in space and time Forecasts (Nowcasting & NWP models) Check spelling and wording here. 29/06/2005

Rain gauge sampling error Radar Gauge 100 60 80 40 20 mm Radar rainfall 24/01/1996 Gauge rainfall 24/01/1996 Indicate where the catchment is. Radar rainfall 05/02/1996 Gauge rainfall 05/02/1996

Radar sampling error Ignoring advection between scans Indicate where the catchment is. Accounting for advection between scans

Problems with Radar Volume Scan Data Parts of radar volume scan where data is unknown Rainfall estimates at ground level unknown Ground clutter contamination can be extensive Results in poor quality rainfall estimates

Radar Volume Scan Data After Data Infilling Radar Data repair Radar Volume Scan Data To ground level Radar Volume Scan Data After Data Infilling

satellite estimations Soil moisture from satellites Soil moisture estimation 1. Satellites SAHG products  Temperature gradient IR (MeteoSat) TUWIEN products  Scatterometer (ERS1-2, MetOP)  ASAR (ENVISAT) Check spelling and wording here. 29/06/2005 2. Local probes… 3. Hydrological modeling Comparison with satellite estimations

Relative Soil Moisture from ENVISAT Check spelling and wording here. 29/06/2005 http://www.ukzn.ac.za/sahg/share

Hydrological modeling Modelled processes TOPKAPI Model (Liu and Todini, 2002) 1. Soil Infiltration all the water infiltrates until soil saturation  preferential paths Transfer Distributed 2. Overland flows Production exfiltration or production on saturated areas Transfer 4. Channel flows Transfer 5. Evapotranspiration Check spelling and wording here. 29/06/2005

A real catchment Landscape - Hills to steep relief - Grassland South Liebenbergsvlei (4600 km2) South Africa Check spelling and wording here. 29/06/2005

Estimation of the model parameters a priori DEM from DLSI (1996) Land use/Land cover from GLCC (1997) Soil type from SIRI (1987) Soil texture from WR90 (1994) Resolution 1km² GIS treatment Liu and Todini (2002) Chow (1959) Maidment (1993) SIRI (1987) Slope n channel overland Soil depth Saturated moisture Residual conductivity Check spelling and wording here. 29/06/2005

Satisfactory flow simulations First results Season 1 CALIBRATION and VALIDATION Season 2 2 Satisfactory flow simulations  Soil moisture 1 Check spelling and wording here. 29/06/2005 2

First results Season 1 Season 2 At footprint scale R2=0.706 R2=0.791 Check spelling and wording here. 29/06/2005

What do the results tell us? Two different approaches of soil moisture estimation Hydrological modeling and Remote sensing Good correspondence! Very encouraging results for Hydrological modeling Remote sensing Check spelling and wording here. 29/06/2005 Use of remote sensed soil moisture - Validation - Initialization - Assimilation Use of hydrological models - Validation - Disaggregation

Using short term rainfall forecasts Rainfield forecasting model Observed Rainfields Forecast Rainfields Check spelling and wording here. 29/06/2005 Catchment model Forecast Flow

Using short term rainfall forecasts Check spelling and wording here. 29/06/2005 5 mins 10 mins 15 mins 20 mins

SBM forecast errors (absolute deviation) Using short term rainfall forecasts 5 mins 15 mins 30 mins 45 mins Check spelling and wording here. 29/06/2005 SBM forecast errors (absolute deviation)

S-PROG forecast errors (absolute deviation) Using short term rainfall forecasts 5 mins 15 mins 30 mins 45 mins Check spelling and wording here. 29/06/2005 S-PROG forecast errors (absolute deviation)

Using short term rainfall forecasts SBM S-PROG Uncertainty Check spelling and wording here. 29/06/2005

What about the difficult catchments? Check spelling and wording here. 29/06/2005 CAPPI at level 2

What about the difficult catchments? Ground clutter Beam blocking Check spelling and wording here. 29/06/2005

What about the difficult catchments? Ground clutter Beam blocking Check spelling and wording here. 29/06/2005

Hydraulic modelling and inundation levels Check spelling and wording here. 29/06/2005

Essential for organizations to collaborate meaningfully Summary Floods cannot be prevented (impacts can be managed given sufficient warning) Robust real-time rainfall estimation is critical to flood warning systems Hydrologic and Hydraulic models can only be as good as the available data and expertise Essential for organizations to collaborate meaningfully Important to maintain international links in an advanced technical field Check spelling and wording here. 29/06/2005

Credits Professor Geoff Pegram Dr Théo Vischel Stephen Wesson Mohamed Parak Nokuphumula Mkwananzi Ntokozo Nxumalo WRC