Why does the track take so long to drain?

Thanks to Andrew's last post showing how flooded the track still is. I agree this will take a while to drain (typically at least a month when this stretch floods)

But why?

I observe as follows:

With reference to this RSPB leaflet with map:

The stretch that takes ages to drain is where the busway passes between the Trout Pond and Ferry Lagoon.
Typically this section is not the first to flood. Rather the river has to very high before it does flood. But when it floods the water gets trapped.

Covills drain is not marked on the map - it lies just east of the Trout Pond, and runs south to north and drains into the Great Ouse through a flood control gate. Cyclists can identify it by the wedge shaped chunk of concrete across the path where it crosses the east bank of the drain.

When the water gets very high, Covills drain overflows its west bank, pouring water into the Ferry Lagoon. This happened 2 weeks ago.
The observant will note that the west bank of Covills drain is deliberately lower than the east bank (there is only one concrete wedge, not two). This is so the lakes flood in preference to Swavesey.

There is a culvert under the busway just west of the west bank of Colvills drain, balancing the Ferry Lagoon and the Trout Pond. So when water flows into the Ferry Lagoon, it passes through this culvert, and raises the Trout Pond which overflows and floods this stretch of the path.

But how does it drain?  Once the level has got back down to the top of the Colvills drain bank - very slowly.

If you walk along the east bank of Colvills drain about 50-100m north of the busway, you will see across the drain the end of a pipe, which drains the surplus water from Ferry Lagoon through the west bank back into Colvills Drain.

As far as I know (but I am happy to be corrected if anyone knows better) this is the total drainage from the Ferry Lagoon + Trout Pond back towards the river.

This is not very big to drain all that trapped water down by about 1m.  And as the draining continues, the difference in height falls, so the flow through the pipe drops too.

The above photo, taken 31 December, shows water coming out of the pipe - which is completely submersed under water.

This photo, I took a few years ago, looking down on the outlet, with the water lower, shows the flap over the end of the pipe, to prevent water flowing back into the Ferry Lagoon from Covills drain. The flap is pushed open when the water flows in the intended direction, as it is here.

But why is this flow so restricted?