Internal Training Workshop No. 2

Introduction to Modelling

1. Types of model

2. Overview of data requirements

3. Importance of survey data

4. Categories of hydrodynamic model

Types of Model

1. General categories of model for floods and water

allocation

2. Definition of Flood

3. Definition of Drought?

Flood definitions - types of flood

.

• Riverine/plains/fluvial floods• ??

Flood definitions – flash floods

.

• Flash floods are rapidly rising flood waters that are the result of excessive rainfall or dam break events. Rain-induced flash floods are excessive water flow events that develop within a few hours – typically less than six hours – of the causative rainfall event, usually in mountainous areas or in areas with extensive impervious surfaces such as urban areas. Although most of the flash floods observed are rain induced, breaks of natural or human-made dams can also cause the release of excessive volumes of stored water in a short period of time with catastrophic consequences downstream. Examples are the break of ice jams or temporary debris dams

WMO Guide to Hydrological Practices 2009

• A flash flood can be defined as a flood that threatens damage at a critical location in the catchment, where the time for the development of the flood from the upstream catchment is less than the time needed to activate warning, flood defence or mitigation measures downstream of the critical location. Thus with current technology even when the event is forecast, the achievable lead-time is not sufficient to implement preventative measures (e.g. evacuation, erecting of flood barriers).

ACTIF 2004

Drought definitions

.

Wilhite 2005

Types of Model – some typical

characteristics

Item Floods Droughts

Typical outputs

No. properties at risk, economic damages etc.

Yield/reliability, failure rates, operating rules etc.

Type Hydrodynamic,hydraulic, broadscale

Water allocation ( basin simulation, supply-demand)

Timestep Sub-daily (e.g. hourly) Monthly (or daily)

Forcing inputs

Unit hydrograph, CSM,Direct Rainfall

Observed flows and/or RR model outputs

Typicalscenarios

Climate change, floodrisk management strategies

Climate change, land use change, demands, water allocation etc.

Types of Model – examples

HEC-RAS, USACE

Types of Model – examples

AQUATOR, Oxford Scientific Software

Types of Model – examples

Types of Model – data

requirements

Types of Model – data

requirements

World Bank (continued overleaf)

Types of Model – model build

.

Item Floods Droughts

TopographicSurvey?

Hydraulic structures?

Storage?

Other?

Types of Model – model operation

.

Item Floods Droughts

Meteorologicalinputs?

Hydrological inputs?

Water demands?

Environmentalflows?

Types of Data – model calibration and

validation

.

Item Floods Droughts

Spatial?

Temporal?

Imagery?

Anecdotal?

Social Media?

Types of Model – scenarios

.

Item Floods Droughts

Climate change?

Land use change?

Socio-economicdevelopment?

Water Quality

Sediment Transport

Ratings

• What are they?

• Why are they important

• Who develops and maintains them

• How do they do it?

• Two UK Examples

Rating at Rothley Gauging Station, River Soar, UK

A good site for high flow rating?

Rating at Glenfield Gauging Station, River Soar, UK

This rating illustrates the importance of high spot flow gaugings

Rating at Glenfield Gauging Station, River Soar, UK

This rating illustrates the importance of detailed out of bank modelling

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

0 5 10 15 20 25 30 35

Stag

e (m

AG

D)

Discharge (cumecs)

Rothley Brook @ Glenfield: rating review

WISKI SFMs AMAX Bankfull 0.1% AEP EVY (2016a) EVY (2016b) EVY (2016c)

AMAX1 = 2.411 mAGD (6.41 m3/s)

Rating at Glenfield Gauging Station, River Soar, UK

This rating illustrates the importance of high spot flow gaugings

0.0

0.5

1.0

1.5

2.0

2.5

0 1 2 3 4 5 6 7 8 9 10

Sta

ge

(m

AG

D)

Discharge (cumecs)

Rothley Brook @ Glenfield: rating review

WISKI SFMs AMAX

AMAX1 = 2.411 mAGD (6.41 m3/s)

Rating at Rothley Gauging Station, River Soar, UK

A good site for high flow rating?

Rating at Rothley Gauging Station, River Soar, UK

This rating illustrates the importance of hysteresis and seasonality

Rating at Rothley Gauging Station, River Soar, UK

This rating illustrates the importance of out of floodplain flow modelling, hysteresis and bypassing

0

0.5

1

1.5

2

2.5

3

3.5

4

0 10 20 30 40 50 60 70 80

Stag

e (m

AG

D)

Flow (m3/s)

Rothley gauging station: Rating curve and SFM comparison

EA Rating Curve Low Flow Model Results v4.3 Winter Model Results v4.3

Summer Model Results v4.3 Winter SFMs Summer SFMs

Flood frequency analysis

.

Some desirable features:

• Long records (AMAX/POT)• “Instantaneous” rather than daily• Representative sample(s)

Pooling groups/regional analysis

Flood frequency analysis

.

Some potential issues:

• High flow rating problems• Non stationarity• Statistical dependence (serial correlation)• Short records• Naturalisation

Flood frequency analysis

.

??

Of course measurement uncertainty not constant!

Flood frequency analysis

.

?

Model Types

• Meteorological

• Rainfall Runoff

• Hydrological Routing

• 1D Hydrodynamic

• 2D Hydrodynamic

• 1D/2D Hydrodynamic

• 3D Hydrodynamic

1D Model Only Example

Limitations?

1D Model Limitations

• ???

1D/2D Model Only Example

Limitations?

1D/2D Model Example

Limitations?

3D Model Example

Limitations?

Recommended Reading Suggestion

Extension of Rating Curves at Gauging Stations

Best Practice Guidance Manual

R&D Manual W6-061/M

D M Ramsbottom and C D Whitlow

Model Selection Factors

• Cost

• Scope for Customisation

• Legacy

• ……….

Kelani Basin Flood RiskKey Factors for Modelling

• ……….

Attanagalu Basin Flood RiskKey Factors for Modelling

• ……….

Mahaweli Basin Flood RiskKey Factors for Modelling

• ……….

Direct Rainfall Modelling

• A “First Pass” Option for Flood Risk Modelling?

• Part of Risk Based Approach to Modelling?