Dredging and sedimentation... 1

[SMIAH]

I’m in charge of reviewing a project that includes dredging of an existing channel to increase its hydraulic capacity. The existing canal has a very mild slope gradient. Actually some portions of the reach are set on an adverse slope and therefore this is reducing its capacity.

The proposed bottom slope shown on the plans seems practically impossible to excavate precisely (i.e. It has a slope of about 4 inches for a mile).

The water is almost always turbid on the reach. From the results of more than 100 sampling (from almost 5 years) in the existing canal, the Total Suspended Solids (TSS) is averaging a value of 30 mg/L (with peak up to more than 220 mg/L).

During a flood, after the dredging, the velocity in the canal will be of 1.3 fps (maximum value associated with the peak flow).

I have some concern about the long-term viability of this project as the sediment will probably deposit in the reach and therefore reduce the height of the canal. Not forgetting that there will still be some portion of the reach on adverse slope...

Has someone been through a similar project? Am I missing something? Thanks in advance.

 

[Ryb01]

I've been involved on several natural channel design projects and the question of long term viability seems to arise quite frequently at the start of projects. The feasibility of the project would depend on what your projects "long term viability" time line looks like. What would be calssified as a sucess or failure? These questions could be based on the functionality of the channel after a particular storm event or even say a period of 10-years for example. Questions like these should be discussed and understood and or agreed upon by all interested parties at the start of the project.

These type of projects are not cheap to complete and do not come with a warranty. As the engineer, project manager, geomorphologist,etc we can not predict the future. It is possible to encounter say the 100-year storm within a few weeks of the channel rehabilitation, after which the newly constructed channel may not look the same as it did just after construction. The feasibility of the project is defined and verfified by the objectives you set at the start of the project.

Your specific location of the reach seems to be in a deposition zone. Is there historic aerial photos that give you an idea of what kind of time frame the sedimentation has occured? The aerial photos will also give you an idea of the migration and meander patterns in that area.

Dredging the channel will provide an interm relief of that area, however, over time, this section of reach will more than likely return to it's current condition.

You may want to look at your reach downstream of your work area. What's the current condition say 1-2 miles downstream? Is it stable? Are there potential erosion sites? Is the vegetation sloping into the channel? etc...

By dredging the channel u/s, the channel now has more energy (velocity) and often a results in increased erosion downstream until at such time the channel reaches an equilibrium state once again.

Hope this helps.

 

[SMIAH]

Thanks for your comments.

I will try to find aerial photos.

Long term viability is not easy to define and this pre-project step hasn't been fully addressed. As much as the criteria to define this project as a success.

And I will try to find some answers.

Thanks again for your time.

 

[gbam]

SMIAH - Ryb01 hits on some key points. You may want to have the consultant conduct a sediment transport study for the long term. This approach can be used to predict the life for this project. A single event will not necessarily determine the future configuration of the canal. We all understand that economic plays a role in these projects and if your consultant can provide you with the ave anual cost for this canal you will be able to evaluate whether a better solution (design) may be necessary at this time.