This picture shows the overflow in operation. It was not part of the original construction but due to a severe flood soon after the reservoir was constructed, destroying the original Kings Mill, it was quickly built to take the surplus water into a dyke behind the dam to the river Maun. The original weir was much smaller and incapable of coping with fast thaws of deep snow that were very common during the winters of Victorian England. Historcal weather data, record snow drifts upto 50 feet deep in England during Christmas day 1836. The following winter the country experienced two continuous months of 24 hour frosts. London recorded -16c. The Thames was frozen solid such that it could be crossed on foot. Greenwich recorded -11c at midday. The lowest temperature recorded that year was -26c at Beckenham, Kent.

The overflow is now spanned by a bridge but as you can see here ramps were incorporated either side for cart access to the old route to Hermitage Lane that is shown on the 1835 Sanderson map on page 2 of this website. This old route to Hermitage Lane that exited opposite Hamilton Hill was redirected to its current position at Oakham roundabout probably in 1871 when the Midland Railway created the railway viaduct at the Hermitage. The new route shown below avoided this wet crossing over the overflow!

 

This picture (left) is taken from under Portland Bridge looking at the dam wall and the Maun running from the reservoir's weir. The picture below is from the 1887 OS map showing the reservoir's old sluice positions. The disused, horse-drawn Mansfield & Pinxton rail track over Portland Bridge can also be seen before the corner was realigned by the Midland Railway in 1871 taking the track over the wooden Hermitage Viaduct allowing faster steam trains to be utilised. During excavations in 1849 to upgrade the track, workman discovered 300-400 Roman denari from the 1st -3rd c AD found in a pot 2 feet below the surface in a field near to Kings Mill. The Duke of Portland claimed them as Treasuer Trove.