Commentary on the Wimbledon Park Lake Project
Flood Study Stage 2
Summary
The London Study gives
options for preventing the Wimbledon Park dam from failing.
Extreme rainfall leads to Wimbledon Park Lake rising so
high as to overtop the dam. Overtopping risks failure of the dam, which could quickly
release the water of the lake to flood the public park and residential areas
between the park and Earlsfield. At worst, the flood would be 3.5 metres (4 yards)
deep in the public park and some 1700 buildings would be at risk of flooding. The
risk to life requires works to prevent any chance of the dam failing. The study
outlines four options to achieve this. Five other options were rejected as
impractical.
Option 1 involves reinforcing the lakeside path and the
downslope of the dam. To do this, most trees and shrubs on the downslope of the
dam would be removed. Much arboretum planting and a singificant part of the
ancient Ashen Grove Wood would be lost. This would replace Capability Brown’s
18th century design with a characterless short sward and would harm
the habitat of birds, bats and other wildlife.
The other three options all have a large new outfall
structure in Ashen Grove Wood to cope with floods from extreme rainfall. Even
larger floods would be accommodated by allowing overtopping of a 150 metre (165
yard) long strengthened section of the dam into the area between the
watersports centre and the athletics stadium. In these three options,
overtopping of the rest of the dam would be prevented by lowering the level of
the lake by 35cm (14 inches) and raising the dam by 110cm (43 inches). The height
of the dam above the water level of the lake (“freeboard”) would be increased
from the present 23cm (9 inches) to 168cm (66 inches). The difference between
the three options is in how the dam would be raised. In Option 2, this would be
done by adding a 110cm-high wall along the eastern edge of the lakeside path.
In Option 3, the lakeside path itself would be raised by 110cm, and in Option 4
a new, low path would be created by infilling a three-metre-wide strip of the
edge of the lake, allowing a 110cm-high earth wall (“bund”) to replace the
existing path.
All four options are essentially to prevent the dam from
failing. They all continue to pass floodwater down into the public park, just
in a more controlled way. Reinforcing the path on the top of the dam and
lowering of the lake would result in somewhat less (and less frequent) flooding
in the public park. In this way, the regular floods that occur would be reduced.
However, the downstream effects of extreme rainfall events would hardly be
affected.
In Options 2, 3 and 4, replacing the present outfall with
the big new one would allow rare floods up to twenty times the capacity of the
present outfall to flow into Wimbledon Park Brook. These flows would far exceed
the capacity of narrow or blocked pipes in the public park and of the pipe
taking Wimbledon Park Brook under the Tube line embankment. There are no
proposals in the study for dealing with these downstream problems, nor with the
much rarer high floods across the strengthened part of the dam. The brook would
need to be opened out and surrounded with a floodplain to cope with these
exceptional floods. This would be most readily done if the outfall was moved to
near the position of the waterfall. Increasing the capacity of the pipe taking
to Brook underground to Earlsfield is the only way to reduce the risk and
volume of flooding in the low parts of the public park, but this would transfer
the problem to the River Wandle at Earlsfield.
In options 2, 3 and 4 it’s proposed to lower the level of
the lake by about 35cm (14 inches). This would take the level down to
Capability Brown’s original 1765 design. Brown’s steepened lake edge survives
around most of the perimeter of the lake, so lowering the lake by this amount
would hardly affect its shape and area. To return fully to Brown’s design,
however, would require restoring the lost 120 yards of the southern arm of the
lake, and a smaller area lost at the eastern end. The lake has always been
shallow, so the effect of the reduced level on angling and watersports would be
small. A great benefit of lowering the level of the lake would be to more than
double the flood storage capacity of the lake and so reduce the severity and
occurrence of regular flooding of the public park by about 90%. The report is
deficient in not pointing out that this lowering could be accomplished soon,
and cheaply, so providing major benefit and lowering the risk to people and
buildings whilst the detail of other measures is being worked up. The report
also fails to note that lowering the lake would result in a considerable
benefit to the drainage of the golf course and The Wimbledon Club and would virtually
eliminate the regular nuisance caused by minor flooding in parts of the public
park.
Option 1 and the strengthened section of the dam in the
other three Options preserve Brown’s design of the dam. However, Options 2, 3
and 4, raise the dam elsewhere. This would affect Brown’s design and there is
little to choose between the three options for this aesthetic harm. Infilling
part of the lake for the new path in Option 4 would lose around 1% of the area
of the lake.
Merton Council’s Wimbledon Park Masterplan suggests replacing
the waterfall with a new “Brownian cascade” outfall. This proposal would preserve
the best part of Ashen Grove Wood from a disruptive new construction and would
enable the restoration of part of Brown’s lake that was lost when the present
outfall was constructed. But this is not considered in the report and the
sketch of the replacement outfall in the report would need substantial
modification to approximate to a Brownian design. The Masterplan also proposes
to move the watersports building and equipment storage to the south corner of
the athletics area. This would remove visually intrusive structures that may
not be resistant to overtopping floods and enable the replacements to be
designed with full consideration of flood safety.
The engineers see such problems with trees on the slopes of
the dam that they require them to be removed in Option 1. This is strange,
given that built structures on the slopes present similar problems, but there
are no proposals in the report to modify or remove them.
The adverse effects of all the proposals on Ashen Grove
Wood would remove a woodland screen from one of the few parts of the lake that
are distant from light pollution, so affecting bat species that are found there,
such as Daubenton’s bat. All options require extensive works on the top of the
dam so the existing lakeside trees would be removed. This, also, would harm both
amenity and bats.
A detailed design will not be drawn up until an option has
been chosen. However, there is no mention in the report of the need to design
an outfall structure to enhance the access of elvers to the lake, so as to
conserve this threatened species. A crucial detail of Options 2, 3 and 4
appears to have been missed. Raising the existing dam would leave an area on
the golf course beyond each end which is not raised. High floods would pass
around both ends of the dam and enter the public park in an uncontrolled manner,
risking erosion and release of the lake water. I estimate that the erosion
protection might need to be extended some 20 metres (22 yards) up towards the
golf clubhouse and some 130 metres (140 yards) beside the fence between the
golf course and athletics stadium. This would entail an additional cost,
especially as it would require land not under the control of LB Merton.
Option 4 is supposed to aid delivery of a project unrelated
to flood safety: de-silting the lake. This would deepen the lake and could more
than compensate for the lowering of lake level but would be very expensive and
intrusive. The report describes this proposal as helping to achieve amenity,
ecology and heritage “aspirations” but there is no detail to back up this
assertion and it has been challenged. Should costs be saved by using the silt
from the middle of the lake to infill the margins of the lake, there would be
great harm to the “aspirations”. This would also reduce significantly the
storage capacity of the lake, adversely affecting flood safety. The report is seriously
deficient in failing to consider this flood storage aspect of Option 4.
Sadly, the stringent engineering requirements cannot be
delivered without significant changes to Capability Brown’s dam, so harming landscape
amenity, biodiversity and heritage as well as recreational use. I propose an
Option 5 that minimises this harm.
