Worst Case flood event.
The worst possible combination of flood events is a deep depression over the North Sea producing a
tidal surge tidal up the Thames estuary following weeks of heavy rainfall upstream. Operating the
Thames barrier to stop the water coming in to London will also stop it getting out. When the barrier
was designed the assumption was that it might be used once or twice a year. It was used 24 times in
2001 and 17 times in 2013. Deep depressions are frequently associated with heavy rainfall.
Mobile active dams placed downstream of a valuable target can reduce water levels upstream before
floods arrive to give a buffer volume. They can also keep the upstream water level below sea level
after the floods arrive and move a higher flow rate than any observed so far in the UK. They would
need river bed attachments and mechanisms to attach and remove them quickly.
It may be possible to fix mobile active dams to the upper edges of some of the existing segments of
the Thames barrier. These segments would be raised to a slightly smaller angle than needed for
blocking an incoming flow and the active dams would pump water over them. If the hoped-for flow
rate of 30 cubic metres a second can be achieved for each container module we could move 1000
cubic metres a second over just one of the four barrier sections used for navigation. Putting them in
the centre of the channel will reduce the risk of damage to the downstream banks but there is a web
mention that these have been reinforced.
The dam containers will be subject to a large upstream force which interestingly reduces the stress
on the barrier. This force must be passed through a hinged plate on the forward underside of each
container. The attached calculations show that there are at least three ways that quick-release
connections could be done.
Groups of five containers can be bolted together like dominoes and will form a stable and very agile
block which can be moored on the river bank close to the barrier. A barrier segment would be raised
so that bollards on its top edge were just awash. Ropes made of a buoyant material can be passed
through fairleads at the lower forward corners of the containers at the ends of group. A bight in each
rope can be passed round one of the row of bollards and back through second fairlead. Two bights
would operate in the fore and aft direction and two obliquely to give side-to-side control. By
careful winching in of the ropes the five groups of five containers can be moved to the correct
position at the barrier segment starting with the central group and followed by the two on each side.
The barrier segment will then be lowered to bring the downstream tops of the containers to the
downstream water level. Trained people with speed and pitch control of the vertical-axis rotors and
perhaps with side-thrusters should be able to ‘moor’ five of the five-container groups in a few
minutes. The end gaps can be plugged with water bags like Avon inflatables.
The barrier owners will certainly be anxious about drilling any holes or welding any lugs for bollard
attachment. It may be possible to use a strong and flexible adhesive to hold plates along the
downstream face of the segments.
A sketch with dimensions taken by eye from http://en.wikipedia.org/wiki/Thames_Barrier is
attached.
[email protected]. 16 February 2014
A sketch of a group of mobile active dams mounted on a barrier segment pumping 1000 cubic metres a
second against an incoming tidal surge.
WORST CASE EVENT. (Note Mathcad preserves units.)
3
9 L
m
The site http://en.wikipedia.org/wiki/1947_Thames_flood gives Qmax  61.7  10 
 714.12
day
s
The very worst case is if this flow coincided with a deep depression over the North Sea and a tidal surge
3
m
which required the closure of the Thames barrier. The target is to remove Qwst  1000 
or more
sec
through a closed barrier so the water levels in London will be reducing even when the barrier is closed. One
way to do this is to mount the ISO dam containers on one or two of the barrier sectors which would be raised
to a slightly smaller angle than normal to bring the top of the dam exit to the downstream water level.
3
m
 Qmax   24
If one mobile active dam can move qdam  30 
the dam number is Ndam  ceil 

 qdam 
sec
If module width is wdam  2.4  m the total fleet width is Wtot  Ndam wdam  57.6 m
The site http://en.wikipedia.org/wiki/Thames_Barrier says that the barrier has four segments which are

Wseg  61  m We can fit Ndam2  floor 
Wseg 
  25 This is very neat.
 wdam 
Small gaps between dams can be filled by expanding rubber bags.
We need a quick-mate connection between the dams to the flat surface of the segments. The connection must
be strong enough to take the hydrostatic pressure equivalent to the container height of Hdam  2.8  m
The force is Fcon 
1
 w  g wdam  Hdam 2  9.226  104  N
2
5
Apply a factor of safety for wave loads, clumsiness etc. so Fcon  2  Fcon  1.845  10 N
If shear stress on one of two hinge pins pin  80  MPa pin diameter
dpin 
4  0.5  Fcon
 38.3  mm
  pin
Obvious connection methods are vacuum, permanent magnets and electromagnets. Assume that the vacuum
is not quite perfect so its holding 'pressure' is Pvac  0.9  bar
Fcon
 854.3 mm
wdam  Pvac
 7 henry
The permeability of free space is 0  4    10 
m
The width of a vacuum pad is Zvac 
2
If electromagnet pole flux density Bem  1.2  tesla The 'pressure' is Pem 
Width of poles is Zem 
Bem
 5.73  bar
2  0
Fcon
 134.2 mm Note some width is needed for copper windings.
wdam  Pem
Neither the hinge plate or the segment face will be perfectly flat but we can arrange for multiple poles with
some compliance. Permanent magnets with neodymium boron and some flux concentration can achieve
similar flux densities. The number of ampere turns or the length of permanent magnet material depend to a
small extent on the length of the flux path in iron but mainly on the length of the flux path in air, water, rust,
paint and bio-fouling all of which are uncertain.
Lpth Bem
If the non-iron path length is Lpth  2  mm the ampere turns AT 
 1.91  103 A
0
If current density is KI  6 
amp
mm
2
the side of a square winding is Acop 
AT
 17.8  mm
KI
A major problem with permanent magnets is disengaging them. This can be done by fitting a flat fire-hose
in the gap between the hinge plate and the barrier segment needing extra width. Vacuum pads with soft rim
sealing would be most tolerant of bad barrier sector surfaces. Groups of, say, five dams with variable-pitch
contra-rotating rotors attached to each other side-to-side form a stable and very agile self-installing vessel