Volume calculation in DEM with wather surface

[GrinGEO]

I need to do a volume calculation of a gravel with a lake in the mid of it.

The water level is 513,5m. I have tried to clear up the densepoints within the shoreline as good as possible, but obviosly you cannot delete every point... so probably this part is not very perfect.

Whe calculating DEM it creates also high values within the lake. But if you define the shoreline shape as a inner border for exported DEM it creates something like a hole.... please see this image



Unfortunately when trying to do a calculation over the lake it seems like there are some errors



So Im wondering how to deal with this sitation. Is there any way to have something like a cutting plane on the sealevel so that all points within this cutting plane or the shoreline Shape could be set to the right value of 513,5m Altitude?

Becouse otherwise the volume calculations "below" will give wrong values.

Thanks for helping

[GrinGEO]

Can anyone help please? Does Agisoft calculate the volume based on the DEM real surface or only based on the points of the designed polygon?

Thanks