TSS Study in county drains 1

[VanBurenSoilGuy]

We have a sedimention problem in a large network of county drains that outlets to a large lake. The lake is filling up slowly with sediment. We are trying to pin point the locations of the highest sediment by using a flow-meter, rain guage, and taking samples and having them anaylized for suspended solids during rain events. Once we have the largest contributors of the sediment recognized we will work on installing BMP's.
Does anyone know of a more scientific means that would aid us with the tools we are already using in trying to find where the most sediment is coming from?
Or possibly pointing us in the right direction for reference materials?

 

[gbam]

Just a few questions:
Sampling is a good (Best) method; however, how long do expect to sample until you decide on the worst contributor?

Have you investigated each contributing watershed and conveyance channel?

You mention that you are only measuring suspended load; have you ruled out bed load?

Are there signs of deltas at each of the outfalls?

Dont forget about the watersheds. You may want to evaluate the impact due to development (clear water discharge, etc...).

Just some thoughts.

 

[VanBurenSoilGuy]

I am only concentrating on suspended solids, the soils washing downstream are smaller in size from muck farming in the upper branches of watershed. The watershed is almost entirely farm use. Yes, there is a large delta, when the water finally does slow down enough.

 

[gbam]

VanBurenSoilGuy;
Have you compaired historic aerial photos for each Delta? You may be able to determine the rate of sedimentation from measurements of the deltas and provide insight into which drainage is contributing the most sediment. Then you may want to concentrate your sampling efforts on those drainages identified by the photos.

Are you certain that the upstream channels are stable? I am not that familiar with "muck farming". If the channels are stable, then I would agree that washload would be a valid investigation. However, if you find that there is movement of the channel alignment both vertical and horizontal, I think you need to re-access the washload only theory. You may want to test the bedload also.

 

[beej67]

In terms of "science," you want to take a look at the USLE or one of its many revisions:

http://en.wikipedia.org/wiki/Universal_Soil_Loss_Equation

http://www.ars.usda.gov/Research//docs.htm?docid=10626

http://fargo.nserl.purdue.edu/rusle2_dataweb/

The USLE is built to do what you're talking about - predict soil loss in agricultural watersheds given inputs of soil information, climate, and agricultural practice. You could predict soil loss rates for every parcel of land in your basin, if you had good enough information.

HOWEVER, I caution you against using it as a predictor for accuracy, just use it to plan where to look with your testing. It's the best thing going for your application, but it's still not extremely accurate.




Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[beej67]

Something else to have in the back of your mind while you're undergoing this exercise:

Sediment transport via bed and bank erosion is a natural process. A stable stream doesn't necessarily mean it's not eroding, "stable" just means that whatever material that's taken away from the stream bed is replaced by new material advected from upstream. Your lake could easily fill with sediment due to natural sediment transport processes even if your channels upstream are stable and there's no major TSS contributor in your basin. In fact, most lakes do exactly that over time, particularly if their contributing basins are hilly. Lake Meade is a pretty famous example of this.

Using hydrologic principles, you could establish the flow regimes in the contributing streams, and doing some stream sediment testing you could back your way into what the "normal" stream sediment load is for your basin. If you have an estimated rate of sedimentation for your lake, you could compare that rate with what the normal sediment load is from the upstream reach, to determine whether the load you're seeing is unusual, or whether it's anything you can reasonably do anything about. (shoot me an email if you want to discuss this off the forums)

My recommendation would be pick up Sturm's "Open Channel Flow" and read through Chapter 10, although I there are some other texts that cover it fairly well also.

Albert Einstein's son (Hans Albert) actually did quite a bit of research in this field.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com