A new modelling study and investigation into how annual burning on the Walshaw Moor Estate (WME) may affect high river flows in Hebden Bridge has shown that the most effective way of managing the moors in order to reduce the peak flow at Hebden Bridge is to eliminate burning and encourage conservation and restoration of Sphagnum. Over time, this will allow sphagnum to replace some or much of the cotton and moorland grasses and heather.

Practiced over the whole Hebden Water catchment, this management for sphagnum restoration would reduce the peak flow at Hebden Bridge by between 5%-10%.

This kind of management on WME only, leaving the rest of the Hebden Water catchment unchanged, causes a reduction in the flow peak at HB of about 2.5-5%.

More generally, the study indicates strongly that management focused on such conservation and restoration is likely to assist and reinforce the effects of other measures introduced on the moorland to reduce peak flows at HB.

ANY burn patches on Walshaw Moor Estate increase flow peak at Hebden Bridge

This new study by Nick Odoni, Honorary Fellow in the Department of Geography at Durham University, has found that any arrangement of burn patches on WME, wherever situated, increases the flow peak at Hebden Bridge.

This finding shows that burning should be banned everywhere – not just on blanket bog and deep peat. And it reopens the question of why Natural England has permitted burning on Walshaw Moor Estate

It must have considerable implications for the EU Commission’s investigation into the UK government’s failure to enforce environmental protection laws to protect the WME Natura 2000 site.  It also bears out Natural England’s own 2012 report that found:

“the voluntary [Heather and Grass Burning] code, Natural England management agreements, and site designation are having little demonstrable effect in protecting either bog or blanket peat areas from fire use.”

Burning sphagnum has bigger effect on increasing peak flow than burning grass

A supplement to the study found that Burning Sphagnum has a bigger effect on increasing the flow peak than does burning grass.  Similarly, the more Sphagnum there is in the catchment generally – whether it is dominant or provides part of the ground cover in some areas and is absent in others – then the more likely it is that annual burns will have to include some areas of Sphagnum. As a result, the increase in the peak flow will be greater than would be expected if the burn was applied to a moorland vegetated by grass and heather alone.

Patch burning works against other moorland management measures to reduce peak flow in Hebden Bridge

The new study by Nick Odoni also finds that patch burning on Walshaw Moor Estate is likely to work against any measures carried out on the moor to reduce the peak flood at Hebden Bridge. This includes rewetting the blanket bog by blocking grips and drains, which Walshaw Moor Estate has been doing under the terms of its Higher Level Environmental Stewardship agreement.

The study also found a clear positive correlation between the area burnt each year and the increase in the flow peak at HB.  So the bigger the annual burn area, the higher the increase in the flow peak is likely to be, compared with the base case.  This implies that for the rainfall-runoff scenario modelled here, patch burning on the WME is likely to work in opposition to any measures implemented on the moor to reduce the flood peak at HB.

When burning sphagnum is also taken into account, the effect of modelling the burns over a long rotation is roughly to double the increase in the flow peak at HB compared with the increase predicted under one year’s burn in isolation.

Rotational burning  increases Hebden Bridge flow peak

A third key finding of the study is that rotational burning – which the study calls “long term annual burning at a given percentage rate” – roughly doubles the increase in the HB flow peak, compared with a burn of that percentage area for one year only.

This is significant, because the  Walshaw Moor Estate Higher Level Environmental Stewardship Agreement permits rotational  burning and pays Walshaw Moor Estate for carrying it out.

Nick Odoni’s study explains that this result is to be expected, because long term rotation burning will increase the overall area of the Walshaw Moor Estate which is to some extent affected by burning, whether a particular patch has only just been burnt or is in partial recovery of the vegetation.  Depending upon the density of present and previous burning, therefore, the number of patches so affected by burning may range from between about 20% and 100% of the moor’s area.

The Odoni study’s final key finding is that longer vegetation recovery times also raise the increase in the peak flow predicted at Hebden Bridge, although the effect is about 1/10th of the burn rotation effect.  This implies that provided the vegetation in any patch is able to recover fully from previous burns, the increase in the flow peak at Hebden Bridge caused by burn rotation should broadly stabilise over the longer term.  This raises the question as to whether repeated burns, over a rotation cycle, themselves affect vegetation recovery times.  This is possibly significant if the cycle of burning leads to a change in the species cover of burn patches which have been repeatedly burned over decades or longer, although this aspect of the ecology and hydrology of the moor-peatland system has not been explored here.

“A modelling study and investigation into how annual burning on the Walshaw Moor estate may affect high river flows in Hebden Bridge” is by Dr Nicholas A. Odoni, Honorary Fellow, Department of Geography, Durham University and was delivered in July 2016.  It was commissioned by Treesponsibility,  a partner in the SOURCE project that is carrying out ecological restoration work in the headwater catchment area of the River Calder.

You can read the study online here.