Hydronia 2D or Not 2D

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2D or not 2D, that is the Hydraulic Question!Reinaldo Garcia, PhDWebinar 10/06/2011

Webinar Outline Model vs Reality 1D and 2D hydraulic model assumptions When the 1D approach is not enough Myths about 2D models 2D model advantages and limitations When 2D?

Models only approximate reality

Models perceive only their own reality

All models are not created equal

There is nothing wrong with the 1D approachIf it is properly used

The BIG problem is that almost always models give an answer regardless of the incompatibility between model assumptions and reality

1D model assumptions On a given cross section: Constant Water Surface Elevation Uniform (almost) velocity

Unidirectional flow Negligible exchange of momentum between main channel and floodplain Many unsteady 1D models created primarily for subcritical flows (e.g. HEC-RAS)

1D Equations

1D model perspective

When the 1D approach is fine? Confined flow Floodplain storage is negligible Flow is mostly unidirectional No need of detailed velocities

When the 1D approach is not OK? Threaded rivers and poorly confined flow Floodplain storage is important Non-uniform flow patterns Complex channel-floodplain interaction

Make everything as simple as possible, but not simpler

2D model assumptions Negligible vertical accelerations Uniform velocity throughout depth

2D Shallow Water Equations

2D model perspective

2D model advantages Accounts for lateral variation in WSEs without Ad hoc artifacts Better schematization of distributed flow in threaded rivers or unconfined flows More realistic computation of velocities and water surface elevations

2D model limitationsNo-secondary flow (bends)

2D model limitationsNo-vertical accelerations No-vertical velocity

1D vs 2D

Myths and half truths about 2D models Require more data Only used for elaborate and detailed models Difficult to understand Difficult to use Slower than 1D models

Require more data?If the flow pattern is truly two-dimensional, then a 1D approach will be useless as a predictive tool.

In such cases 1D models are unable to predict unobserved events. J. Cunge

Require more data?Data 1D Model 2D Model

Elevations

Surveyed cross sections

LiDAR, DTM, DEM

Mannings n

1D Spatial distribution

2D Spatial Distribution

Boundary Conditions

Inflow Hydrograph Outflow WSE or rating curve

Inflow Hydrograph Outflow WSE or rating curve

Difficult to understand?

Difficulties lie in the physical processes 2D model may only be a better approximation to complex hydraulics

Are 2D models slower than 1D models?

YES!

+43,000 elements 1 hour simulation 3 minutes runtime

How difficult is it to setup a 2D model?

2D or RiverFLO-2D

Simplicity or computational economy should not justify using a modeling approach that ignores fundamental aspects of the flow hydraulics!

If the flow is truly 2D, you will not get good results, no matter how much you tweak a 1D model. Use 2D when: Storage areas are important WSE is not constant on cross sections Threaded rivers Flow attenuation is important

Hydronia.com

2D or not 2D?

Where do we go from here?Next webinar:

Effective and practical transition from 1D to 2D modelingOctoberth 27