I recently completed a 5 acre pond that is holding water great and is about 12-16" from the spillway overfow elevation. Part of the watershed is a 15 acre pasture hill and we installed a 16" culvert x 100' long along with a berm to catch the water flowing from this hill and route to the pond. My contractor was supposed to install the culvert invert (bottom) elevation of the outlet at the same elevation as the spillway.

Since the water level has risen with recent rains, I now find the water level is at the outlet invert elevation and stil have at least a foot, probably around 18" to go before reaching the spillway overflow elevation. He tells me there is an 18" drop in elevation from inlet invert to the outlet invert elevation of the culvert.

It seems to me that as long as the inlet invert elevation is higher than the top of culvert elevation at the outlet, that water will stil flow but it sounds like it will be very close. Even if it does flow, I'm wondering if having the culvert outlet almost or completely below water at full pool will cause any other issues? Any input is much appreciated. Although time will tell with more rainfall, I'd like to have a preliminary plan.

Sorry if this gives you a headache to try and understand!