Representing Logjams in HEC-RAS 2D

[MikeU]

Hello,

I am currently carrying out a study which is modelling the effects of natural flood management measures (NFM) (e.g. woody debris dams/logjams, runoff attenuation, woodland creation) on flood peak flows.

My modelling strategy is to create a fully 2D model using 1m LiDAR DTM and the ‘rain on grid’ approach using net precipitation and use the hydrograph profile tool to extract the flow profiles at various locations in the catchment.

I propose to represent the majority of NFM measures by changing manning’s n roughness and manipulating DTM cells to provide storage areas and bunds. However, I am currently unsure on how best to represent wood logjams in the 2D model.

Does anyone have any suggestions as to how to represent logjams in a fully 2D model? there is likely to be 20-30 of these features in the catchment to make a difference to flows.

Any help is appreciated. Many thanks.

[cameron]

What cell size are you using? It may be best to incorporate the logjams in the terrain and use breaklines and smaller grid cells near the logjams.

[Anonymous]

The DTM has a 1m cell size. The 2D mesh currently has a blanket 4m cell size. However, I intend to increase the 2D cell size in the channels to 0.5m or less to pick up the flow patterns.

Thanks for the idea. I assume adding as terrain feature would in effect form a weir type structure in the channel with flows going over the top. Some of the flow goes under the logjam as well – could this be represented by a culvert or sluice gate? or would adding lots of these structures potentially introduce instabilities in the model?

[Anonymous]

I would represent them all as lateral structures with culverts for low flows if they occur, change the weir coefficient to 0.2 and may consider using a different loss value in the culvert data editor.

Thats the easiest way to do it other wise, terrain manipulation may take some time if you have 20-30

[Author]

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