I have done quite a few sedimentation surveys, which is basically a hydrographic survey that is compared against an initial survey to compute the volume of sediment that has accumulated in a reservoir since inception.
Now I have a request from a privately owned lake, built to create a lake to sell houses around it. They want to dredge, and the state requires an estimate of the quantity of silt for some kind of permit.
But, they claim to have no map or other information on the original conditions (i.e. a topo of the bottom of the reservoir prior to impoundment).
I would think that such a map would have been required at the time of construction (mid 60's, I believe). Either by aerial or planetable methods.
Any other ideas on how to determine the volume? A quad is not accurate enough. Any ideas on where to look for the map? I assume they have contacted the state, but I am not sure if a federal permit would have been required. I think the lake size is about 400 acres.
Has any agency flown lidar? Or would that be too new?
Here's where NOAA has the coastline stuff. (Well more or less, my bookmarks were to dud pages so I found what I could to get here)
http://csc.noaa.gov/digitalcoast/dataregistry/#/
Try determining who the engineering firm was that designed the dam. They would be the most likely source of original information. Or that may lead you to the mapping firm that did topo.
How to do this? Check with the county engineers office? Or maybe the state office that handles dam safety.
Or another possibility - if it is near a highway of consequence the state DOT may have some mapping from long ago that covers the area.
Or check with a local, long time surveying firm. They may not have done the work, but we all seem to have a long memory for things like that.
We did a ton of similar surveys in the 1980's. The process where we were may not work for you, but the silt formed a loose layer of mud lining the bottom of the reservoirs. The engineers wanted to know the volume of mud and the only way to get that was to measure to the top, feeling with a 25' rod (some of the reservoirs were deeper but only a few) then take screw together range poles, push the pole until it got to solid ground, note the difference for each shot. Slow but worth every penny and they said we were a bit low with the volumes but not too bad.
I told him one way was to probe to solid, but that would best be done on a flat boat with a small drill rig or something similar.
I did do a silt sampling survey once, we used a metal pipe, open at one end and with a weight at the top. Tried to drop it vertically, but the objective was to recover some tubes full of silt rather than to reach solid ground. A messy job, and, if I recall correctly they decided to do it in the winter in PA (more than 30 years ago).
At this point, it is just inquiries, so I have not been there. I have no idea how deep the lake is, just that they are having problems at the upper end with homeowners hitting their props in the mud, so they want to remedy that at least.
Seems to me it's going to be a pre-dredge and post dredge survey. How will you determine the extent (depth) of the suspended silt?
It's a design decision once you find out the current situation.
Dtp
I think that some of the hydro survey systems can determine the depth of the silt. This might be a job where you need to sub the data collection, our you may be able to rent the equipment to do it.
I thought about that, Steve. It would have to be a frequency that penetrates the silt but not the solid ground.
Good place to start seeking info about dams:
It should be inventoried and these folks are the experts.
Contact these folks. If there is a way to do it with soundings, they will know. Ask for John Tamplin.
http://www.seafloorsystems.com/index.html
Good luck.
I have found that the Odom/Teledyne Hydrotrac is very good at depicting different material depths.
Mud is pretty easy to identify compared to actual hard bottom.
What I have found out is that a dual frequency system will give the depth to various layers. We have a single frequency only system. And there are "sub-bottom profiler" systems available.
All of these are probably more than a homeowner's association can afford. But, in any case I am giving them contacts who have this capability.
imo, the best way would be to use a single beam, dual frequency fathometer (we use an Odom Echotrac CV200). the high and low frequencies will pick up the hard bottom and the "fluff". this is how dredge volumes are usually calculated.
The more I think about this the more I think the dual frequency fathometer is the way to go.
I say this based on personal experience. Back in the early 70's I was in charge of stakeout for a large earth fill dam - over 1/2 million yards of fill. A lot of the fill was taken from the bottom of what would turn out to be the lake. If you try to use original topo this would not be taken into account. The fathometer would identify the bottom post construction.