Polly and myself observed a team from SEPA yesterday on the Tromie where width and depth measurements were collected from seven transects established across the river downstream of the dam. The transects were set up and marked during August when the flow in the Tromie was below the compensation level due to the extended period of dry weather. Unfortunately we weren’t able to attend on that August day due to other committments but it was interesting to see the methodology used.
The reason of this detailed assessement of river dimensions is because of the proposal by SSE and SEPA to reduce the flow in the Tromie as part of the Garry/Tummel hydro scheme review. New draft guidance produced by UKTAG (http://www.wfduk.org/) to help regulators determine the flows required to meet the requirements of the Water Framework Directive has been consulted on recently. As the Tromie currently has a compensation flow above Q80 the flow can only be lowered below Q80 if there is no significant reduction in useable wetted area.
The compensation flow released from Tromie Dam is currently about Q60 (1.2 m3/sec) but there are two significant tributaries a short distance downstream which are also abstracted. The effective compensation flow in the Tromie where the two abstracted tributaries should have joined is actually equivalent to a Q80 flow.
SEPA are going to prepare width and depth profiles based on the measurements collected at the seven transect sites during the low flow conditions in August and the higher flows experienced yesterday. They will then have to decide whether there has been any change in the useable wetted area. The term useable is key. In our view and others habitat suitability for salmonids (there is little else present in these upland sites) depends on more than just depth. Current speed is critical as few parr are found in the still areas present in the margins of rivers. These still marginal areas can hold a lot of fry but the parr have a strong preference for flowing water. As flow speeds weren’t recorded during either survey a subjective assessment of whether an areas was classed as still margin or flowing water such as run/riffle has to be made.
So after about seven years of deliberation on the SSE application some width and depth measurements on the Tromie are becoming highly important. It will be very interesting to read SEPA’s report on these surveys. I think it is the first time this technique has been applied but there is a lot hanging on it! SEPA reckon there will be a draft decision made before the end of the year but we have heard that before. It all seems pretty clear to me but I suppose if you are a regulator or a hydro operator you see things in a different way.
It was a mixed day weather wise yesterday but this little colourful lichen? created its own little beam of light.