basically, you cannot do this by gravity (unless you could set up a siphon situation, which would be tricky to engineer/justify). even a siphon would probably not meet regulatory requirements.
pipe through the berm is the most common way, typically with a concrete structure within the pond to control the flow from the different design storms (i.e. 1-yr, 10-yr, 100-yr storms). what jurisdiction (country/state) is the project located?
