Do's and Don'ts
I have had a long discussion with David yesterday. As explained, his brother
has been taken ill and he won’t be able to make the trip over (at least as
far as he was aware last night). Having gone over the photos and our site
experience again, there are a number of things that must be given close
attention when the dams are reinstalled:
* Water depth is critical and you must know exactly what you are dealing
with. Need to take depths across the proposed lines of the dams before
installation and account for any potential increases once the dams are in
* The dams must be fully inflated to 8ft on the deepest sections. This needs
to be measured, not guessed
* The tubes must be filled at the same rate – David has some doubts about
what happened last time having seen the levels of dam 1 at the far end after
it had moved. Question – although both pumps were 4”, were their rated
capacities the same?
* The fill points for the dams should be at least 2 ft above the top of the
inflated dams and preferably more like 4 ft. This gives control over the
filling operation. David thinks the fill point for dam 1 was probably OK but
this should be confirmed by measurement.
* The dams need weight on both banks to help restrain movement ( the launch
point of the 1st dam is ideal) or must be securely tied back to trees etc if
this is not possible. The dams should not terminate ‘in the water’ as we did
on dam 1 unless they are lashed into position.
* The dams will move/lean and bend once the dewatering begins as the inner
tubes build friction, start to rotate and lock together. Dependant on
restrained water depth this can be anything between a couple of feet and a
couple of meters. The extent of movement you noted is not uncommon
* Dewatering must not start until the dams are fully inflated and secure.
This may be an issue because of potential water loss/leakage through the
sluice. Also, filling should be from water outside, not in-between the dams.
* We need portable pumps so that they can be easily moved around and used to
put more water into the dams if needed as they settle into silt etc and also
if back-up dams need to go into place
* The back up dams should be used
o where water depths are reasonably close to maximum and water build up is
possible after installation – eg if natural drainage into the canal is
making for any sort of flow which will be backed up when the dam seals off
o Where there are issues with ‘boils’ as water forces its way beneath the
dams through the sediment and the backup dams help minimise the effects of
scouring etc. This latter point very dependant on the particular
circumstances of the site
o for long term installations since retained water depths are likely to move
around by at least a few inches and this could be the difference between
success/failure (depending on the true rather than estimated water depth and
how close this is to 6ft) .
o There is a different methodology for installation of these which I will
explain to you on Monday.
* the fence to the car park must be removed and the dam allowed to track up
the path towards the station
* the dams must cross the banks at right angles. This will be an issue at
this location. We must either straighten the far bank (opposite tow path) or
make the dam bend in the canal.
* We must lift the tow path filling end well up into the air (unless you
have managed to clear the far bank and the height differential and available
space makes launch from here practical)
I aim to get to site before 11:00 on Monday – traffic on M4 permitting. Will
you be able to get the dams laid out on the forest track and the blower in
place so that we don’t loose time. I have rescheduled my Tuesday
appointments but must leave on Tuesday night.
Ian H Rees
Riverside Water Technologies Ltd
Riverside Centre, Pipehouse Wharf
Morfa Rd, Swansea SA1 1TD
Office 01792 655968
Mobile 07785 578372