Now that the safety works on the dam are complete, others have speculated what the effect has been on the lake water level. The approved plans for the works proposed a new outflow weir some 50mm lower than before, a marginal lowering of the controlled level of the lake. To prevent overtopping of the rest of the dam, the path at the waters' edge was to be raised by 50mm, with a slope up from there to 250mm across the lakeside prominade.
I am able to check whether this has been achieved because I took measurements of lake levels before and after the works at reference points. Before the works I had some 70 paired measurements of water depth at the old outlet weir and at my reference point beside the main watersports jetty, allowing me to convert measurements made at my reference point to depths at the weir.
As the safety works planned to raise the lakeside by watersports jetty, before the works began I took paired measurements at my reference point and in two places on the watersports jetties, which fortunately survived the engineering works. These allowed me to check the reference point after completion of the lakeside works in late February 2022. I had only six measurements and estimated from these that my reference point is now about 30mm higher, with a 95% confidence range of 13-44mm. This was surprising, because the approved safety works had this part of the lakeside raised by 50mm, significantly greater than my estimate.
On completion of the new outflow weir, I was then able to check its level by taking paired measurements there and at my recalibrated reference point. To date I have but five paired measurements. These give an estimated level of the new weir crest 120mm below the old crest, with 95% confidence range 114-126mm. Again, this was surprising, because the approved safety works were for a 50mm lowering.
Whilst my measurements are subject to sampling error, I'm confident that the lakeside at the watersports jetty has been raised less than in the approved plans, and the weir lowered more than in the approved plans. Further measurements will improve the precision of my estimates, but it appears that the regulated level of the lake has been lowered by around 120mm, or 5 inches. This is about twice what was planned. The planned level of the new weir was to be 17.42 metres above Ordnance Datum (AOD), but my estimate suggests it is nearer to 17.35m AOD. This will have two benefits. First, is that the lake level will average lower, so benefitting flooding upstream on the golf course and Wimbledon Club and reducing wave overtopping onto the lakeside prominade. Second, is that the the freeboard is slightly larger, so the lake can take a larger volume of flood water before the auxillary spillway between the Watersports Centre and the Stadium is overtopped.
At the height of the drought there was essentially no flow over the weir. Wave splash was minimal. The brook in the public park had a very low continuous flow, all of which issued from the old pipe near to the toilet block. This pipe now carries surface water drainage from Home Park Road, its catchment being the residential areas between Home Park and Arthur Roads. The reason for there being any flow from there during a long drought is speculative. It could come from leaks and misconnections upstream, or from a natural spring-fed flow intercepted by the drain. This drain was above ground adjacent to the play area before it was put into a pipe "to avoid children getting muddy" according to a resident who visited in the 1960s.
The drought meant that plant seeds applied to the new outfall and the sides of the new course of the brook did not establish well. Grazing by geese also affected the outfall. The low fall of the new sinuous brook in the Crazy Golf area meant that the flow was very slow. It was essentially a sinuous pond. There is a small step up to the pipe under the path near to the cafe that also held back the flow. A heavy growth of the blanketweed Spirogyra dominated this new brook, further holding back the flow.
The first thunderstorms sent no water down from the lake, but the Home Park Road drain pushed some of the blanketweed downstream to accumulate on the first grille downstream. The thunderstorm of 24-25th August, however, quickly overtopped the weir and flood flows went down the new couse of the brook. This revealed that criticisms of the design of the new brook were sound. There were coir rolls beside the new brook between the outfall basin and the toilet block. Many of these were ripped from their attachments and contorted by a turbulent flood flow. The gravel on the bottom of the brook washed downstream to accumulate behind the coir rolls and at the beginning of the old course of the brook, making it significantly shallower there.
Huge amounts of blanketweed were swept downstream in the flood to accumulate against the first grille. This blockage might have resulted in flooding around the cafe area were it not for the timely clearance of the grille by Idverde maintenance staff first thing in the morning. The new course of the brook was designed to provide temporary storage of floods to avoid worse overtopping and flooding than before. This is necessary because the great widening of the outflow weir leads to greater peak flood flows downstream. It would obviously be prudent to clear blanketweed and debris from the brook regularly to anticipate such difficulties. This has been recommended in the past by engineers inspecting the safety of the dam, but the message seems not to have got through to those who direct maintenance work in the park. This flood was the result of a 250mm rainfall event coming fortuitiously when lake level was 20mm below the weir crest. We can expect much more significant flood flows in future, resulting in significant flooding around the cafe area. This is a direct consquence of LB Merton giving priority to maintaining lake depth rather than protecting the public park and tube line embankment from flooding.
For the third year in sequence, there has been a significant bloom of blue-green bacteria in the lake. This is a direct consequence of high nitrogen concentrations in the lake water, perhaps originating from disturbance of the lake sediment or from contaminated water coming down the tributaries, exascerbated by high temperatures. In July and August I found the lowest water clarity ever over six years' measurements. In July the predominant blue-green was the same as in the previous blooms, Aphanizomenon flos-aquae, but by August another, Dolicospermum sp., predominated. Then, a low flow down the waterfall into the brook looked like a bright green soup. A third blue-green, Microcystis, was a minor component of the bloom. All three can be toxic and the Environment Agency advice is to avoid contact with the water. Dogs swimming or drinking from the lake are particularly at risk. Fortunately, there seems not to have been any fish kill from the bloom, so perhaps the strains of the bacteria are fortuitously non-toxic.
Compensation for the harm to Ashen Grove Wood from the new outfall has been planted whips of hedgerow plants in three areas: just inside the boundary fence in the public park; at the southern end of the lakeside prominade and beside the path near to the toilet block. The boundary fence planting was quite silly, being in a heavily-shaded area in the middle of an ancient woodland. Almost all of these have failed. All three areas suffered from being planted too late in the year and from the drought. Only regular watering by Idverde staff has ensured that the lakeside plot is surviving. The plants near the toilet block are in trouble.
A nice feature of the safety works was application of a cornfield annual seed mix to the large new bund near the Revelstoke Road entrance to the public park. This reflects the arable history of this sector of the park and it provided a grand show of colour during the drought, when most other plants were withering away. Sadly, this bund is not fit for purpose because it has not been keyed into the tube line embankment, so flood flows would escape readily there. Also, part of the bund has been lowered to permit access west into the park, so defeating the fail-safe design there also. One cannot rely upon rapid deployment of sand-bags when a huge flood event is threatening many other places.
Many of the trees were affected by the drought, with leaves drooping or dropping prematurely. This was obvious on the Lombardy poplars around the stadium and on London planes. The last surviving Balm-of-Gilead poplar in Horse Close Wood beside the western end of the car park had lost 2/3rds of its leaves. Leaves were also yellowing on Elm, Elder and Birch.
I did my monthly bird count on the 22nd. As before, Canada goose numbers are back up, with a count of 155, being the second highest count over the last 30 years, but numbers or both Greylags and Egyptian geese remained remarkably low. Grey herons seem to be increasing, and there's evidence that they may be about to establish a heronry on the island in the western arm of the lake.
The park opened around 07:00
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