Effects of Widening a Portion of a River

[sedimentnovice]

If I have a uniform river in equilibrium and I widen the width by a factor of 2 for about 500 ft of the length, what will be the sediment transport change(aggradation, degradation, or equilibrium) of the river? I was thinking that the upstream would stay in equilibrium, and the downstream would have aggradation while the widened area would have degradation. I'm new to sedimentation engineering and I'm having problems relating book knowledge to practical experience. Thanks.

 

[gbam]

The hydraulics of the system will determine the sedimentation. My first thought is that the velocity will drop in the wider section resulting in aggradation in this segment. Then there will be increased scouring downstream of this section. Also the change upstream will depend on the backwater. It all depends on the river flow regime before and after the widening. You can create specific energy diagrams and determine roughly what will happen as a result of your widening.

 

[Ryb01]

sedimentnovice,

What is the purpose of widening the section of river? Although the river seems as though it is in equilibrium, I would be surprised that it is. Rivers and streams often do not reach equlibrium as there is always changing paramters within the watershed (i.e. culverts, bridges, stormwater runoff, sediment transport, etc).

Widening the river would have a number of negative effects on the river upstream and downstream of your project area. gbam has touched on the main points. Unless the river restoration is being completed for increased aquatic habitat and or floodway control I would urge you to look further into the works before moving forward.

I would also be surprised if you could obtain the proper approvals without a watershed study.

Hope this helps.