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Home / News / Derwent Reservoir Water Levels - Present and Historic
Home / News / Derwent Reservoir Water Levels - Present and Historic

Derwent Reservoir Water Levels - Present and Historic

Published 14:40 on 19 Sep 2023

As many of you are aware, the hot topic amongst many members at the moment is the water level in the reservoir, which directly impacts on the distance we have to traverse on the slipways when launching and recovering our boats. To try and answer this burning question on our reservoir water levels, I had a look at the Environmental Agency's website and managed to find the daily water level data since 26th November 2016 to the present day from their monitoring station at the dam, which rather helpfully can be downloaded into an Excel spreadsheet and from there prepare graphical presentations of the data.

The Environment Agency has a disclaimer about the figures but I think you would agree that broadly the peaks and lows are around the time frames we would expect.

The results will no doubt surprise you as they did me so here goes:

The figure above shows the readings at the dam end in metres below full capacity (reading 0 metres), which I believe means that the water is virtually at the top of the slipway. The two peaks you see represent the dam at full capacity December 2019 March 2020 and January 2021 March 2021 (which coincides with the Covid period), thereafter it falls. But what is interesting is if you look at the period from November 2016 - July 2019, the levels are pretty much the same as September 2021 onwards. So if anything, the halcyon days of a full reservoir could be regarded as the outliers and that the present levels are more representative of days gone by.

The chart below shows just 2022 with the vertical axis registering the metres below full capacity at the dam end. As you can see it has virtually flatlined at about 6.1 metres below full capacity from the 30th May onwards. Notice also that the maximum water level in March 2022 was circa 3.6 metres below the full capacity level. Please note the scale on the vertical axis has been changed to give a more detailed picture.

In terms of 2023, this is shown below. It didn't fill up as high as last year (maximum water level 4.1m below the capacity level) but as you can see from June, again we hit our low point this year at 5.9 metres below the full capacity level. Please note the scale on the vertical axis has again been changed to give a more detailed picture.

The shape of the reservoir bottom is also critical. The dam end is the deepest at 30 metres and it shelves upwards from there towards the nature reserve which is shallowest. As a result, any draw-off from the dam has a magnified effect on water receding from the banks further up. Indeed, the difference between the lowest water levels recorded at the dam in 2022 at 6.1 metres and 5.9 metres at 2023 probably makes the difference of the water level being off the slipway and into the mud as seen in 2022 and still just being on the slipway currently.

In terms of fun facts, the capacity of the reservoir is 50,000 megalitres and the sustainable daily draw-off claimed by NWA is 138.6 megalitres which equates to just over 50,000 megalitres a year. I have done some very crude/back of a fag packet calculations of runoff based on the size of the catchment area NWA say feeds the reservoir and this works out at around 60,000 megalitres. The overall effect is about neutral. As rainfall and draw off will never be in sync we will get fluctuations in levels.

Overall, I don't think the position will get better, but by the same token it's certainly no worse than it was pre-July 2019 and it would appear that they seem to be sticking to a policy where the lowest point fluctuates around 6 metres below the full capacity level for the reservoir so we have some consistency.

Kev Brown

Last updated 17:34 on 28 October 2023

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