Just completed a small bathymtery survey for a 5 hectare lake/pond inside of a private subdivision. It was a small job done in less than 1 day of field work using 2 base + 1 rover attached to an echo sounder.
So I have created the bathymetry contour map that has several depressions within the lake itself. The lake has an intake & outfall pipe that circulates the water from a nearby river.
The lake manager (?) runs the water from the river once every 2 weeks or when water starts to stagnate or stale. I saw that the pressure from the intake pipe is quite strong when valve is fully opened.
My concern is after I submit my bathymetry map, how long will the data on them be valid? One of the depression that was present on the map is near the intake pipe which I assume is caused by the water pressure when it get released into the pond.
I am just concerned that the next surveyor who will do the semi-annual bathymetry survey will compare his results to mine and say that my work is quite different from his. How do you defend it? I placed a note that my map is valid when it was surveyed as of xx xx date. I was called in to do the survey as a 3rd check because the 1st & 2nd surveys were "different".
Is there a standard time period after which I could safely say that any difference from my survey data is caused by water currents & sedimentation?
Anytime you do a field survey, it can be assumed that the map depicts the conditions of the area surveyed as they were at the time of the survey. If the conditions changed since then, then they changed -no problem. You're certifying that on XX date that this is how the conditions were. You're not guaranteeing that it will always be as you have mapped it. Just make sure you have a place on your survey that specifies the "Field survey date".
Sounds like the lake isn't very deep. Take some manual shots with a rod to verify that your soundings are accurate (to whatever accuracy standard you are required to meet). As mentioned above, if the bottom profile changes later-not your problem.
We had a situation recently where we were trying to take soundings in a stilling basin when the air temperature was about 10°F. I was told that they suspected there might be a 20' hole there due to erosion. This was just below a dam, so the operators shut down the flow and we began the survey using a fathometer mounted on a kayak (too small an area for a motorized vessel). As soon as the flow stopped, a thin layer of ice began to form on the surface, and I had to break it using the paddle. Easy to break, very thin. But, when I looked at the data, I was getting some normal depths (6 feet) and some 30 foot depths. Since I had crossing lines, and big differences in the same spot, I knew something was not right. I believe that the echo was bouncing off the bottom, then off the thin ice at the surface, and doing this a few times before the transducer was picking it up. So we wound up doing the sounding manually with a total station (robotic) and pole with prism on top.
Here is the stilling basin below a weir below a larger stilling basin:
With the dam gates opened:
The fathometer on a kayak:
We then made a long rod by attaching several rigid extensions to a fixed height pole:
We also had to do the area downstream (with the flow running):
it was 7°F when this picture was taken. Just my hands were a little cold:
Here is the final contours (powerhouse in green). This was a survey to establish the conditions prior to commencing hydro power operations (which started the week after), so that they could determine scour rates. Hopefully the next survey will be in the summer!
Actually, the ground around the powerhouse will be different than what we surveyed. There was a small dam built across the tailrace so that they could finish construction. When we came back the next week that area had been changed. But they wanted it surveyed that way.
> I am just concerned that the next surveyor who will do the semi-annual bathymetry survey will compare his results to mine and say that my work is quite different from his. How do you defend it? I placed a note that my map is valid when it was surveyed as of xx xx date. I was called in to do the survey as a 3rd check because the 1st & 2nd surveys were "different".
A couple of ways:
Clearly state on your map and be able to explain the coordinate system that was used for the survey. Are the heights orthometric, ellipsoidal or arbitrary; for example based on a BM @ 500ft. State the control points on that map too.
Have you incorporated check lines running perpendicular to your (surveyed) planned lines? Some programs such as Hypack will provide statistical analysis showing variation between both data sets. Some suggest a ratio of 10:1 (10 planned lines / 1 check line).
You mentioned making use of two RTK GPS bases. How does the final data relate? Where both vectors used for the solution of the points, or one as a check vector?
Finally, I would suggest that you could have done a few check shots with a leadline. This would have confirmed that the echosounder is giving you result comparable to the conventional method and no major systematic error was present in your set-up. Those manual checks could have been incorporated on your map.
Do a search "USA Corps of Engineers" and bathymetry. Those folks have a very good reference manual, easy to read from start to end.