Mobile Active Dams
A serious flooding scenario is the combination of a tidal surge bringing water up river into an
urban area while heavy rainfall brings water down from the inland catchment. Blocking the rain
coming into a city requires walls or storage basins upstream. A barrier which keeps the sea out
will also keep the rainwater in. But there is the alternative option of getting water out faster. It
can start to empty water channels before flood water arrive.
A mechanism which can do this is called a Mobile Active Dam and is shown in the attached
drawing. It has the same weight, dimensions and lifting points as a standard sea container and so
can be moved in large numbers by sea and anywhere on the road network very quickly. Most of
the container forms a straight-through passage with an area of 4.5 square metres, about 600 times
more than a standard 4 inch fire-hose. The passage is fitted with a vertical-axis, variable-pitch
rotor resembling a Voith-Schneider propeller. Above that is a 600 kW radial-piston turbocharged two-stroke Diesel engine geared down by a ring cam to the 80-rpm rotor speed. Above
the engine is a pancake electrical generator which can act as a low-head hydro system to produce
power up to 150 kW in normal conditions, depending on flow and head across it. Below the
passage are fuel tanks for 24 hours operation. If it is safe to do costing by weight with
comparisons to heavy goods vehicles, active dams should cost about £500,000 each in mass
production. With good flood forecasting (nine hours can be achieved today) a number of dams
at a central depot could be shared between several cities.
Active dams will be powerful enough to move 20 cubic metres of water a second against a head
of two metres and a higher flow, perhaps 30 cubic metres a second, against a lower head. They
can float, propel themselves and make attachments to pre-placed anchor points on a river bed to
lie at an angle up to 45 degrees. The sides of a dam module are fitted with reinforced textile bags
which can fill with water to seal the gaps between them. Banks of dams, anchored side-by-side
can block the incoming tidal surge but also pass the highest flow rates. A bank of fifty, costing
about £25 million plus the cost of river bed attachments, would pass the highest recorded flow of
the Clyde. An estimate for a passive Clyde barrier, which would block the escape of rain water,
is £800 million. The flow rate in the Thailand floods has been reported as 1.2 billion cubic
metres per day. This could be moved by 500 units . Active dams can also be used for dredging
and to prevent very low water levels exposing smelly river beds. Regular use for other purposes
is the best way to ensure reliability.
It will not be possible to use mobile dams at all sites. For example, the rate of flow over a
waterfall is not affected by anything below it. The way to assess feasibility is to assume that the
maximum flow is happening and to check that there is sufficient width for the number of dams to
pass it. The depth at the bank of dams has to be more than 2.4 metres. The design of the preplaced river-bed attachments requires geological information. We then assume that the active
dams are imposing an increase of head difference in the down-river direction. While the sea
itself is an ideal storage volume, an expanding estuary would be very good. If the levels down
river to the sea are known then the reductions at points in the up-river direction can be
calculated. If downstream water levels exceed present bank heights then we can decide whether
to raise banks or allow flooding of low value areas. Some cities further from the sea might need
more than one bank of active dams. Assessing the suitability of mobile active dams therefore
requires an accurate survey of the dimensions of the entire flow channel downstream of a high
value target.
This information should already have been collected by people responsible for flood prevention.
If it does not exist then its collection and analysis should be a matter of urgency.
Stephen Salter.
S.Salter@ed.ac.uk
KEY NUMBERS FOR MOBILE ACTIVE DAMS
(Note that Mathcad preserves all dimensions automatically).
mm
Suppose that rainfall RF  25
and that one mobile active dam can remove
day
30  m3
Qdam 
sec
The area that could be drained by each is Adrn 
Adrn  40.03  mile 2
This is a square of Sdrn 
Qdam
 103.68 km2 or
RF
Adrn  10.18 km or Sdrn  6.327  mile
Press reports are that that 62 pumps are at present moving Qnow  10 6 
Rate per second is Qnow  11.574
m3
day
m3
s
Note ratio one mobile dam to present pumps
Qdam
 2.592
Qnow
Press reports are that flooded area Afld  12  mile 2 or Afld  3.108  10 7 m2
If this is flooded to a depth of Zfld  1  m
Afld  Zfld
and if there is no more rain the drain time is Tdrn1 
 31.08 day
Qnow
If we had a dam fleet of Ndam  50 the drain time is reduced to
Afld  Zfld
Tdrn2 
 5.756 hr
Ndam Qdam
If container cross section is Acont  2.4  m 2.8  m  6.72 m2
Qdam
m
The water velocity at a nice Venturi exit is Uexit 
 4.464
Acont
s
kg
If water density is w  1000 
m3
1
The equivalent stagnation pressure is Pstg   w  Uexit 2  0.1 bar
2
1
Kinetic energy rate is Ekin   w  Qdam Uexit 2  298.9 kW
2
If water head is increased to H  1  m
the potential energy rate is Epot  Qdam w  H  g  294.2  kW
Kinetic energy will be transferred to potential if we can get the exit well designed.
Present choice of Diesel power is 600 kW. This could be increased if necessary by a small
reduction of the height of the flow passage.
The performance will be limited by internal cavitation where fluid passes through a throat of
reduced area and by erosion of material at entrance and exit. The system is a very powerful tool
and must be used with great care to avoid damage to river banks. We may need to reduce drive
at the ends of a row and wind out geotextile to reduce erosion. We can also use them for
dredging. If we set zero pitch and have electronics to convert the poly-phase generator to 3
phase 50Hz we can use the units for emergency generation.