Blocking ceramic rainwater sewer pipes?

[Drellingore]

Does anyone have any suggestions for how to temporarily block 3-4" diameter ceramic pipes?

 

On the site of my prospective barn conversion I've found a gravity/silt trap for surface water. It's about L120xW90xD65cm, and has five ceramic inlet pipes of either 3" or 4" internal diameter which protrude into the trap by about 1". There is one outlet of 4" diameter, the ultimate destination of which is unknown. It was put in by the local drinking water company (the local supply is abstracted 300m away) in the 60s, is owned by them, and doesn't show up on any public register.

I thought it'd be simple to block the pipes, fill the trap, unblock the outlet, measure how long it takes the water level to drop, and then use that to work out the maximum outflow rate. I wanted to make use of this surface water sewer in my drainage strategy to reduce the volume of soakaways required on site. We've got at least 400sqm of plan-area roofs that we'll need to dispose of rainwater from.

 

I tried covering the pipes with black plastic pond liner that I had knocking about, and wrapping elastic bands around them. It turns out this isn't a good enough seal and water is seeping through making it impossible to fill the trap quicker than it leaks (I'm filling it from an IBC using a garden hose).

 

Any clever ideas on how else I could seal them? Expanding foam would work for the inlets, but for the outlet I need to be able to 'unplug' it to run the outflow test. The only other option I can think of is to get a bigger hose for the IBC, so I can hopefully fill it much more quickly than the seals leak.

[Drellingore]

Here's a really naff photo of some of the inlets.

[dpmiller]

drain test bungs?

[Drellingore]

38 minutes ago, dpmiller said:

drain test bungs?

 

Googles
Discovers that drain test bungs are a thing
doffs cap to @dpmiller

Edited July 20 by Drellingore
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