Flow Recovery Following Abstraction Reduction

Pictured above: the flow gauge at Redbourne on the River Ver. It may not look like it sometimes but following a significant abstraction reduction in 1993 at Friar’s Wash on the upper river, an average of approx 80% of the water returned as surface flow. The River Ver, however, is still heavily over-abstracted.

The Chalk Streams First coalition has CSF has commissioned an independent investigation (CLICK HERE) into flow recovery following abstraction reductions in the Colne and Lea chalk streams.

This is an important piece of work because currently overly precautionary and unrealistically low estimates of flow recovery are shaping WRSE and Affinity Water plans.

Everyone acknowledges that if you switch off groundwater abstraction a lot of that water comes back as river flow. Not all: some stays under the ground as aquifer throughflow. But about 80% on average comes back as river flow: known as flow recovery.

The flow recovery is not evenly distributed through the year, however. You get a much higher % back at high flows than low flows (see my post “something we should all agree on”). The return at low flows becomes critical, because this is the time when London’s supplies are most threatened and under stress.

Therefore, it is useful if we can find extra water to underwrite the lower returns you get at low flows. And of course, quite how much you get back makes a big difference to cost.

We need about 150 Ml/d to re-naturalise flows in the Colne and Lea chalk streams. And that has been recognised in WRSE plans. So far, so good.

Currently the Grand Union Canal transfer is set to underwrite about 50Ml/d of reductions by 2030-ish. Also, so far, so good.

BUT … the majority of the proposed reductions are not scheduled until after 2040 and are framed as being dependent upon a large strategic resource option such as either the Abingdon Reservoir or the Severn to Thames transfer.

This would push the ecological recovery of the chalk streams decades down the line. We think that is a really bad idea. And unnecessary.

The contingency / delay appears to be based on an estimate of 17% flow recovery from chalk stream abstraction reduction at very low flows, Q95 – Q100, meaning the strategic resource is necessary to underwrite the abstraction reductions. The 17% figure derives from a triangulated process of analysis conducted by Affinity Water and consultants, summarised in Technical Appendix 5.6 “Deployable Output Benefits from Abstraction Reduction”.

Our independent investigation into flow recovery from abstraction reductions suggest that the 17% figure is unjustifiably conservative and that average flow recoveries at the relevant percentiles are considerably higher: in the region of 50% to 60% of upper catchment reductions translates into increased deployable output in downstream reservoirs at the average percentiles through the 1921 and 1933/34 droughts.

The delay in implementing the reductions is therefore unnecessarily precautionary.

It’s a long report, so maybe just read the summary. Once again, it is really important that as many of us as possible get our heads around the nitty gritty of all this, so we can be well informed in our discussions with the water companies, WRSE, the EA and government.

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