This document provides an introduction to 1D, 2D, and coupled 1D-2D flood modeling. It discusses the differences between model types in terms of data requirements, preprocessing, setup, and output. 1D models are suitable when flow is primarily uni-directional, while 2D models are needed when flow spreads widely. Coupled 1D-2D models represent the channel in 1D and overflow areas in 2D, avoiding the need for a fine mesh in the channel. The document provides guidelines for selecting a 1D, 2D or coupled model based on study area characteristics.

1. 1D & 2D Modeling-
An Introduction
Rajesh Kumar Mahana

2. Flood Modelling
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 How do the models differ in terms of data requirements, pre-
processing, model set-up, and output?
For what applications could 1D, 2D or 1D-2D models be
suitable?
How sensitive is flood inundation to computational mesh
construction?
How should the computational mesh be designed around the
river and on the floodplain respectively?
How does the parameters affect the model Stability?

3. Flood Modelling
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Hydrologic
Hydraulic- 1D, 2D and Coupled 1D and 2D, 3D

4. Modelling
• Data: Geometric Data, Flow Data, Structure,
Stage
• Boundary Condition
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5. Types of Flood
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 Fluvial (River Flood)
Excessive rainfall over an extended
period of time causes a river to
exceed its capacity (Overbank
Flooding & Flash Flooding)
Pluvial (Surface Flood/Urban
Flood)
Heavy rainfall creates a flood event
independent of an overflowing water
body
Coastal (Surge Flood)
Extreme tidal conditions caused by
severe weather

6. When 1D?
1. Locations where flow isn’t required to ‘spread’ significantly
(flow maintains primarily uni-directional flow patterns).
2. Well-defined channel/overbank systems (channel is
bounded by steep slopes, constricting the lateral
expansion of flows).
3. Simply-connected floodplains where flow in main channel
is well connected to flow in the overbank and that flow in
both is primarily uni-directional in nature.
4. When elevation data of only limited quality/quantity are
available.
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7. When 2D?
1. Anywhere flow is expected to spread
2. Urbanized Areas
3. Wide Floodplains
4. Downstream of Levee Breaks
5. Wetland Studies
6. Lake or Estuary Studies
7. Alluvial Fans
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8. 1D & 2D Coupled
• 1D and 2D Coupled
• Channel flow is modeled purely in 1D, and the channel
is only represented in the cross sections.
• No need to use a finer mesh cell size in the river to
accurately model river flow
• Main Disadvantage: 1) Stability Issue
• Changing Boundary Condition leads to Stability Problem
• choice of time step and calculation method as well as to the
geometry of the lateral connection.
2) parameters describing the coupling have
such large impact on model results.
3) The 1D domain of the model cannot
model any momentum in the lateral direction
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9. Rule of Thumb
1. The general rule of thumb is that if the length-to-width ratio is
larger than 3:1, a 1D model can possibly be used; otherwise, a 2D
model is needed
2. Features such as a narrow bridge crossing causes significant
expansion/contraction are best modeled using 2D capabilities.
3. If knowing the flood patterns around buildings and other discrete
features is important, a 2D model will be necessary.
4. Detailed animations showing floodwave progression in multiple
directions at a local scale is best represented using a 2D model. If
simple water surface elevation graphics are needed, both 1D and
2D models can be used to produce these results.
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10. What to Use?
• No Definite Answer.
Depends on Study Area,
Data Availability, Budget &
Time Schedule
• 1D and 2D approach more
suitable where the 1D and
2D domains are separated
by a clear and smooth levee,
and where no tributary
streams enters the 1D main
channel from the 2D domain
• If modeling an area with
tributaries, the tributaries
should probably either be
added as 1D channel
• Main Channel in 2D
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11. A Comparison
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12. # 12