SecretLeeds - History, culture and architecture in Leeds

jim wrote:One needs to know where the datum level is taken as being.

Hi Jim . Thanks for that interesting information.

I've looked again at the Environment Agency website and in the 'Leeds Holbeck Flood Alleviation Scheme' information it states "Site datum 0.00mAOD". In the website it states each Environment Agency monitoring station "has its own datum - a height in metres fixed relative to mean sea level. This height is expressed as metres above ordnance datum (mAOD). The river levels we provide for each monitoring station are all relative to its site datum. If we didn't do this, we'd show some very high measurements, which could cause confusion. By using a local site datum, the river levels are easier to understand, especially by users familiar with the data from a particular monitoring station. A river's height above ordnance datum can be seen by adding the site datum to the river level". I am though confused by that as the Water Lane site must surely be above sea level (unless it has been dug very deep!).

I notice for the very nearby Leeds Crown Point location it states:- River name: River Aire. Typical range: 0.53m to 1.35m.
Highest level on record: 2.96m on 27 December 2015. Site datum: 23.13mAOD".

Your previous post said "highest recorded level is 26.10 AOD". Add the site datum (23.13) to the highest level on record 2.96 from your latest post and sure enough the result is 26.10. From this we can deduce that the site datum is a particular nearby point on the bed of the River Aire expressed as its height above sea level. In flood, the level of the Hol Beck along Water Lane will inevitably be the same as that of the river.

jim wrote: the level of the Hol Beck along Water Lane will inevitably be the same as that of the river.

If the levels were the same, then there would be no flow at all!

True if the river and beck were static. However it would be flowing downstream and would therefore allow some discharge from the beck. Of course given their relative sizes the amount from the beck would be miniscule. Also if there were no flow and the beck was being topped up then the levels would not be the same for long.

Well put tyke bhoy. Also the level is governed by the capacity of the river cross section and the flow rate over the weir at Crown Point. So long as the Hol Beck outfall is at a lower level than that weir, the levels will equalise.

tyke bhoy wrote:True if the river and beck were static. However it would be flowing downstream and would therefore allow some discharge from the beck. Of course given their relative sizes the amount from the beck would be miniscule. Also if there were no flow and the beck was being topped up then the levels would not be the same for long.

For there to be any flow at all, there has to be a difference in level - the greater the difference in level the faster the flow. Zero difference, zero flow. Water, unless pumped, can only ever flow downhill/downstream until levels equalise, then the flow stops. Check the benchmark ASL's

Contarct for further work has just been awarded.

http://www.constructionenquirer.com/201 ... l-for-76m/

chemimike wrote: ↑

Wed 26 Jun, 2019 5:41 pm

Contarct for further work has just been awarded.

http://www.constructionenquirer.com/201 ... l-for-76m/

Thanks chemimike for that interesting link.
I notice that in its information it states "This will lift the whole scheme up from a one-in-100 year event to a one-in-200-year level of protection".