Because I think it’ll be nice to have more water stored
In: Engineering
you wouldn’t want to hold more water than the dam was originally designed to hold. Water is really heavy.
The gradual filling of the water side of a dam is referred to as “silting”, and it is an issue.
They do this to maintain the original capacity. All rivers move sediment downstream, but dams halt sediment transport and slowly fill the reservoir with sediment. They are digging out the San Gabriel Reservoir in LA right now for this reason. https://pw.lacounty.gov/wrd/Projects/Bobcatfire/sangabriel.shtml
What would they do with all the dredge spoil?
For anyone who wishes to answer the question, OP is wondering why not dig the reservoir deeper without doing anything to the edges.
Let’s see.
A modest reservoir is 100,000 acre-feet. Lets say you want to increase that by 10%.
That’s 10,000 acre-feet of mud, or (x1233) about 12,330,000 cubic meters. At 10 per truck, that’s 1.2 million truckloads, weighing about 30 million metric tons.
Now picture a big reservoir, like Shasta in California. 4,555,000 acre-feet.
There’s a good reason why reservoirs are built in naturally occurring valleys.
here are some videos of dam spillways opening to remove sediment
[Opening A Dam Spillway Gate after Years (youtube.com)](https://www.youtube.com/watch?v=GZXCcyZOyW0)
[Radial gate with flap clearing debris (youtube.com)](https://www.youtube.com/watch?v=JmxCwZ9wFbQ)
[50 million tons of sediment transportation (youtube.com)](https://www.youtube.com/watch?v=aggy_2n8qGI)
[Purging accumulated sediments in Hydroelectric dam (youtube.com)](https://www.youtube.com/watch?v=VUoHhyvY4_w)
Increasing the depth increases the water forces on the dam itself. It was likely built with a factor of safety based on as is condition. You’d have to strengthen the levee.
People rightly mentioning how much trouble and how they’re already dragging compensate for it, but there’s something more important missing.
Whats the dam for? If it’s for generating electricity, that new hole in the middle is useless, because it contains less energy and even if it still has some, will not reach the intake.
Similarly with water reservoirs. The intake is set a level above ground to get fresh water, not mud. If you ever got such a low level that you need to get water below ground level, this is going to be very expensive to pump out with specialty pumps. And will be mostly mud anyway.
Money. I’m in Iowa. Army Corps of Engineers says it isn’t feasible to dredge reservoirs that have lost 1/3 of the capacity and have had major floods 3 times in the past 30 years. They were each called 100 year floods
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