September 2024

When I did my monthly bird count on the 17th I found that one of the four young Mute Swans standing by the main watersports jetty was trailing a length of fishing line from its leg. It was already tangled in twigs, waterweed and other debris, disabling the bird. Fortunately it was tame enough that I could catch it readily and remove the barbed hook. I saw all the swans in subsequent days, so it appears that the bird recovered.

On the night of the 22nd-23rd there was significant rainfall, which hit the national news because of the "sink hole" in the corner of the AFC Wimbledon pitch, some 1.5 kilometres away to the east. Unfortunately, I don't know just how much rain fell in the catchment of the lake (which includes much suburbia to the west and extends as far as the Windmill and Rushmere on the common), but there was a significant flood in the park. If anyone used a rain guage and got the figure, it would be of interest to know. The new outflow to the lake performed according to design and dumped the excess water straight into the stilling pool in Ashen Grove Wood, from where it rushed down to quickly flood areas all around the cafe and the crazy golf. That the flow went this way, means that the dam safety works were successful as there was no overtopping of the dam, so no risk it would fail. A very big rainfall would overtop the dam between the watersports building and the stadium entrance, but I heard of no overtopping there. Again, if anyone was there to check, please let me know. However, the flood revealed significant design faults in the new parts of the brook. The engineers had made a last minute design change to the part of the brook within Ashen Grove Wood, down to the toilet block, saying that this change should perform OK. But, it didn't. The bed of the brook was badly eroded, washing away much of the landscaping and wooden steps and eroding the underlying London Clay. Three pipes that had been buried below the bed where they crossed the brook are now exposed at the bottom of the brook. Clearly, this part of the brook is eroding its own course. Just how this can be remedied remains to be determined. 

I'm not sure what the pipes carry: perhaps sewage from the golf clubhouse and fresh water supply, possibly, gas or electricity. Whatever they carry, the intregity of the pipes may be in doubt as they are now subject to flood flows, where previously they were supported by the surrounding clay soil.

It appears that the flood in the public park was exascerbated by debris washed downstream and accumulating on the grille near the payment booth. This acted as a dam and the water backed up and overflowed the path and down beside the crazy golf to make its way back to the downstream part of the brook. As usual, when the park has a flood, the water entered the equipment store beneath the cafe. The debris that blocked the grille perhaps originated at the upstream eroded part of the brook, but anything loose would have joined it. At the other end of the large diameter pipe that runs under the access road to the cafe, recent works have allowed the grille to be removed. This is fortunate, as previously debris used to accumulate there where it was very difficult to remove. This second grille has been left off. It has long puzzled me why there needs to be grilles on this pipe at all. It was rumoured to be for public safety, but I don's see any special safety issue with a short length of large diameter pipe. With or without these grilles, the third grill, on the pipe that runs under the tube line embankment, would have blocked and the flood would have been just as great.

Flooding in the public park is entirely expected, as I pointed out at the LB Merton Council Meeting that approved the works to the dam and brook. The engineering reports made it clear that the new downstream works would prevent any worse flooding than before, not make the matter better. The reason for accepting this is that it was decided to maximise the depth of the lake and so sacrifice the ability to use the lake as a temporary storage for storm flows. The dam safety works, therefore, send much bigger volumes downstream in a flood peak than before, requiring more storage in the downstream brook to prevent much bigger floods there than before. Council Members were misled by their officers when the works were approved. Effectively, flooding in the public park is by design choice. This means that the park is used for temporary storage of flood water that would otherwise overcome the pipes downstream to the Wandle, or even flow under the railway bridge at Revelstoke Road to flood the grid. All this would be prevented were the weir at the lake outflow be lowered by 20cm or so. Then the lake, rather than the public park, would act as the temporary flood storage. One wonders, also, about the wisdom of effectively using the tube line embankment as a dam to hold the flood waters back in the park, so protecting residential areas downstream. That embankment was designed to carry trains, not as a flood control structure. Might it fail? 

The flood we saw was well within the designers' maximum flood. We will certainly have much bigger floods, it's just a matter of when. The maximum would flood most of the tennis courts and right up to the Revelstoke Road car park. It would rise nearly to the level of the floor of the cafe. 

Birds benefitted from the flood. I saw a Little Egret and two Grey Herons in the crazy golf area scavenging the fish that were carried over the weir by the flood flow.

Another interesting observation this month was confirming the identity of a large-leafed plant under the hedge on the south-east corder of the stadium. This was Deadly Nightshade, Atropa beladonna, and just the third record of this species I know of in a wide surrounding area. There was a plant on the tube line embankment a few years back, but not seen recently, and a large plant in the St Mary's Wimbledon graveyard. This species does well in disturbed, chalky soil and is doubtless spread by squirrels, or birds that are not susceptible to the poison. It favours disturbed chalky soils, in these places probably concrete as the local soils are not chalky.

There were several waterbird corpses and ill-looking birds this month, probably bird 'Flu. The blue-green bacterial bloom diminished over the month as temperatures cooled and was virtually gone by the end of the month. The predominant species this year was Aphanizomenon flos-cuculae, which seems not to have harmed any dogs this year. I was rather disturbed to see much watersports activity through the months when the bloom was obvious.