1. 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

2. 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

3. 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

4. Modelling the Hydrological cycle
“What happens to the rain?”………Penman (1961)
Measurement uncertainty
Modelling uncertainty
Parameters & Processes

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

6. 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

7. 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

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

9. 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

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

11. 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

12. Relative Soil Moisture from ENVISAT
Check spelling and wording here. 29/06/2005

15. 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

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

17. 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

18. 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

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

20. 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

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

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

23. 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)

24. 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)

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

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

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

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

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

30. 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

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