Sediment Percentage in Cachuma Lake?

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By an edhat reader

Considering Lake Cachuma has been filled with sediment over the decades, I wonder what percentage full it is relative it's original capacity. Does anyone know? 

Has this been looked in to before? It's a shame so much sediment collects in the reservoir, it was a major source of sand for our local beaches! It's an issue that apparently no one is talking about.

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TheIVJackal Mar 27, 2019 02:02 PM
Sediment Percentage in Cachuma Lake?

Could you share the link to those or what conclusions were met in each? I asked about it years ago, haven't seen anything since, certainly nothing official from a government office. But I do remember the same excuses about how expensive it is, toxic, etc... There are many examples across the country of dredging to increase reservoir capacity. Yes, maybe we can't draw exact parallels between our lake and others, but there are plenty of examples we can draw from on how we could successfully dig/dredge out sediment! Don't forget the millions we spend shifting sand around already on our local beaches, all that sand at the bottom of the lake could greatly benefit the shores.

Luvaduck Mar 27, 2019 08:37 AM
Sediment Percentage in Cachuma Lake?

In this context, does 'dredging' mean sucking up the sediment & debris like you'd suck up sawdust and such off a floor with a shopvac, pretty much the same as they do at the harbor?

pstarSR Mar 26, 2019 04:06 PM
Sediment Percentage in Cachuma Lake?

It is actually something that has been discussed and talked about almost every year it rains here. I recall about 5 posts of the years asking the same question. with both suggestions to dredge the lake while it was so low last year. so, yes its been discussed. Quite a bit actually

JudiMagda Mar 26, 2019 03:12 PM
Sediment Percentage in Cachuma Lake?

Sedimentation is necessary and important to the ecological system. While too much of it can cause problems, too little of it would be devastating. If that doesn't do it for you, you can thank phytoplankton sedimentation for SB having the great weather that it has. The better question is, what can we do to keep and maintain our lakes and oceans healthy?

PitMix Mar 26, 2019 03:08 PM
Sediment Percentage in Cachuma Lake?

It costs about $15 per cubic yard to excavate and dispose of clean sediment. One acre-foot is about 1613 cu yd. So budget about $24,000 per ac-ft. So $410,00,000 to clean Cachuma out to design capacity. How much do we have to raise the hotel bed tax for this?

PitMix Mar 27, 2019 02:09 PM
Sediment Percentage in Cachuma Lake?

That 193,305 figure is for a lake surcharged to elevation 753. Rarely happens. Spillway level is elev 750.

BlueHorseshoe Mar 27, 2019 11:31 AM
Sediment Percentage in Cachuma Lake?

They're saying design capacity was 205,000 AF and current (2013) max capacity is 193,305 AF. Thus, 11,695 AF of sediment x $24,195 per AF to remove the sediment is $282.96 million to bring it to design capacity. Don't know where the $401mm number came from.

BlueHorseshoe Mar 27, 2019 09:28 AM
Sediment Percentage in Cachuma Lake?

This is an interesting point. The reason it hasn't been dredged is because they can build and operate a desal plant for far less. Using the math above (I dont know if those figures are correct but we'll go with it). Figure a 15 year depreciation for the assets, 7 years for the equipment, 15 to 25 for the infrastructure, and 40 for the buildings - $72mm cost to build / 15 years = $4.8mm per year in depreciation, plus the $4.1mm annual running cost to produce 3,125 AF = $2,848/AF. So you can spend $24k per AF to dredge & store it, which is only incremental when the dam is spilling, or spend $2,848/AF to make an endless supply. The real kicker here, is that an AF is 325,851 gallons. At the current MWD pricing of 1.2 cents per gallon (I dont know what SB rates are), the revenue per AF is 3,910.21. So you can dredge and sink money, or make water and make money. We all know the pensions need to be funded somehow, but at the end of the day the numbers dont work for dredging... YET. Someday when the lake has a capacity of 15,000 AF, they'll have to do something.

tagdes Mar 26, 2019 07:28 PM
Sediment Percentage in Cachuma Lake?

Don't have my calculator handy, so is it 41 mil and the coma's in the wrong place or 410 thou and missing a zero.

PitMix Mar 26, 2019 10:43 AM
Sediment Percentage in Cachuma Lake?

Wikipedia says "Cachuma Lake is a reservoir in the Santa Ynez Valley of central Santa Barbara County, ... design capacity of 205,000 acre⋅ft (253,000,000 m3), but it is currently limited to 188,000 acre⋅ft [at elevation 750] (232,000,000 m3) due to sediment accumulation." SB County Hydrology Report says "A reservoir capacity revision survey was conducted in December 2013, the result from which was an approximately 1% decline in capacity (-2300 AF) compared with the previous 2008 survey"

Flicka Mar 26, 2019 10:29 AM
Sediment Percentage in Cachuma Lake?

Dredging the Dam would be a monumental, expensive job. Not likely to happen. Gibraltar is loaded with silt.

a-1563288263 Mar 26, 2019 11:03 AM
Sediment Percentage in Cachuma Lake?

(you also need to consider where the sediment will go if they remove it, transportation is expensive. I don't think Goleta Beach can take any more.)

a-1563288263 Mar 26, 2019 11:02 AM
Sediment Percentage in Cachuma Lake?

It's also terrible for water quality. It's not just soil that settles out over time, but microscopic heavy metals. Dredging mixes these nasty things into the water again as it is flushed downstream. All contaminants are NOT removed through water treatment (especially metals). They could have taken advantage of the drought and bulldozed some sediment out, as this is much less "dangerous" than dredging, but god knows what environmental groups would have had to say about that.

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