(new information is still arriving
- please let us know if you find errors. Join the Forum
to discuss.)
See maps, tables and summaries from the Government consultation
paper here
Factor |
Severn
Estuary Tidal Reef |
Severn
Barrage |
Location |
Good |
Minehead-Aberthaw reduces
flood risk by breaking storm surge before it reaches low-lying
areas |
Poor |
Brean-Lavernock (Weston-Cardiff)
increases flood risk by damming the estuary above low-lying
areas |
Average output (TW-hrs a year) |
20*
7% UK elec |
likely appr. 15% higher
average output than barrage, also generated over a period
of 18+ hours per day, 85% usable |
17**
5% UK elec |
lower output, generated
over a period of 6 hours per day in 2 three-hour blocks,
thus only 25% usable |
Output Profile
(MW @ optimum) |
16+ hours per day |
4-5 hrs on, 1-2 hrs off, generating
on both ebb and flow tides. Output varies between 5000
MW at spring tides and 2000 MW at neap tides* |
8 -12 hours per day |
3 hrs on, 9 hrs off, generating on
ebb tide only. Estimated 8600 MW output when operating** |
Supply Issues |
Could be constant |
If a lagoon or tidal
coffer dam is included to kick in at the high & low
tides, supply could be constant at all times. |
Problematic |
Very expensive to provide
major backup from other sources for most of the cycle. |
Est. Build Costs
(£ bn) |
£19 bn* |
£2 bn less than the barrage, using
the same costing methodology. Saving is in weight of concrete,
foundations, and installation. |
£21 bn** |
Due to massive size and consequent knock-on
effects & time involved, Government would have to
subsidise it initially. |
Environmental Disruption |
Minimal |
Tides function; scouring / silting unchanged;
habitats are preserved - little overall change. |
Severe |
Habitats upstream are changed; clyces
will be needed; silting and scouring patterns will change
a lot. See Fundy
barrage failure |
Generation Start Date |
Soon |
2 years after construction
start - does not need to be complete to start production |
Delayed |
Estimated 7 years to build; expected to
start operating 2020 or even 2030** |
Longevity |
Excellent |
Likely to be upwards
of 100 years; no decommissioning costs |
Uncertain |
Silting could seriously
limit life span & cause other problems - see Fundy
barrage failure |
Visual Impact |
Little |
Very low profile as it rises and falls
with the tide |
Large |
Massive fixed structure - will cut right
across visual skyline. |
Shipping and Silting |
Easy |
Ships will be able to
pass through at all times. Channel will not become silted. |
Difficult |
Ships' passage times
will be severely restricted. Also, serious silting may
occur. |
Flood Alleviation |
Good |
This "early" barrier can inhibit
side-wash seiche effects, storm surges, and be adapted
to sea level rise issues if necessary. |
Bad |
May cause side-wash that could flood the
Somerset levels. No protection from storm surges or sea
rise. |
Practicality of design |
Likely to be good |
Needs funding for modelling
R&D. But standard turbines used in new overall design.
Little environmental protective work needed. |
Looks poor |
Massive funding needed
for R&D, environ't protection, and shipping locks.
More concrete, more road wear, no railways. |
Maintenance |
Feasible |
Many small turbines - can easily be repaired
at relatively low cost |
"Golden Handcuff"? |
There are many unknown questions, due
to severe environmental disruption: shipping, silting,
river sluices, etc. |
Manufacturing Diversity |
Fairly good |
Smaller units can be
made locally, by competing firms, and can be more easily
repaired |
"Golden Handcuff"? |
Such a monolithic and
disruptive scheme can play into the hands of distant multinationals
& monopolies |