Are you in the U.S.?

Building a sizeable dam in a floodplain will almost certainly require multiple layers of government approval. That will be quite expensive to pursue.

A groundwater pond (no dam) in a floodplain would be much more likely to be approved (depending upon your state). If your excavated material was moved OUT of the floodplain then your project would not be worsening the flood height. However, your pond would definitely have the problems of fish going in and out during floods.

Water pressure on both sides of the dam is not a problem. A normal dam has water pushing on one side and air pushing on the other side. This is obviously unbalanced, so the dam has to be built strong enough so the water side does not push downstream.

Water on both sides would be more stable. However, rapidly moving floodwaters are highly erosive. Rising flood waters moving parallel to the long axis of your dam could erode the dam and cause catastrophic failure.

P.S. The link to your pond site did not come through in your post.

I think you need to post a lot more info to get more definitive help from the Pond Boss experts.

Good luck on your project!