Resumé of the St James’ Weir Assessment Report (carried out for the Environment Agency by Halcrow Group Ltd, May 2013)

Introduction

St. James’ Weir was built in the C14 by the Courtenay family to raise the river level in order to feed the St. James’Mill Leat. The weir is considered to have significant archaeological, recreational and amenity value. The weir is at the tidal limit of the Exe and is the first barrier for migratory fish going upstream (Exe and tributaries) to spawning grounds. The fish pass on the right hand side was refurbished by the Environment Agency 11 years ago, but is not considered effective. The weir is disintegrating but as it does not serve flood defence purposes the Environment Agency does not consider it has a prima facie remit to repair the weir. A new fish pass (designed by Halcrow) would cost around £500,000 but a collapse of the weir would render expenditure on a new fish pass redundant. Building a new fish pass would imply repairing the weir.

Weir condition assessment

The weir is on a sharp right hand bend in the Exe. Residential properties in Od Abbey Court overlook the weir on the left hand side upstream of the weir. The entrance to St James’ Leat is immediately upstream of the weir. The right bank opposite the housing is a mix of recreational land and deciduous woodland. The bank itself is owned by the Environment Agency. There is a footbridge at the head of the leat connecting to Duckes Marsh (owned by ECC) The weir is 97 m. long by 10-12 cm wide, with a sloping glacis. Halcrow carried out its survey in May 2013 and located 5 significant areas of damage.

Location 1

FIsh pass area. The baulk is expected to remain intact for 10-20 years.

Location 2

A section of the weir toe, left of the fish pass baulk has broken away (8 x 1 m.) No significant deterioration has been noted since 2003.

Location 3

A section (12 x 3 m.) of the weir toe 2/3 across from the right bank has collapsed. Damage in this area is not a short term (-10 years) threat but progressive failure in the medium term (10 – 20 years) could lead to a breach in the weir.

Location 4

A large horseshoe-shaped hole in the glacis is situated 2/3 way across the weir from the right hand side. It has grown rapidly since 2003 and now extends half way up to the crest of the weir. The deterioration is exacerbated in flood conditions. A full breach could be expected within 9 years. Water levels would then drop and the St James’ Leat would run dry. Improved fish passage would render the leat redundant.

Location 5

In the lower half of the weir and adjacent to the left bank there is leakage through the surface of the weir as concrete has broken away. However, no significant deterioration has been detected in the past 17 years. This damage should not have any significant consequence in the short term.

Other remarks on the weir’s condition

The weir crest is in generally good condition and also the weir glacis apart from those locations identified above. The weir toe is undercut in several places, especially at location 4. Some mortar loss is apparent, and if several blocks were dislodged this could lead to significant structural damage.

Options

(i) Do nothing.(It is worth reproducing the text of the report in full here)Assuming no intervention is made a breach in the weir is expected within 10 years which would quickly increase in size and result in a move towards equalization of water levels upstream and downstream, the following resulting consequences should be expected:

  • Mobilisation of large amounts of fine sediment and silt that have built up behind the wear
  • Change in the tidal limit of the River Exe;
  • Scour and collapse of river banks upstream of the weir due to lowered water levels;
  • Drying out of St.James’ Mill Leat leading to significant loss of habitat and negative effect on the setting of the remains of the mill building complex at Countess Wear;
  • Negative effects on the archaeological, cultural heritage, amenity and recreational value of the area;
  • Increased velocity in the river upstream of the weir would result in the erosion of the left bank of the river , eventually leading to loss of Duckes Marsh playing field land and the bridge over Mill Leat; and
  • Complaints from the public

(ii)Do the minimum. Repair the hole at location 4. The estimated cost is £135,000. Flooding and tidal conditions could delay this repair.

(iii)Comprehensive repair. Repairs to be carried out at locations 2,3 and 4. Scour holes to be filled under weir toe. Repointing of exposed blocks would be carried out using fibre reinforced concrete or repair mortar. The cost is estimated at £430,000 but to this would be added the cost of a new fish pass bringing the total cost to close to £1,000,000.

(iv)Remove the weir. This would cost as much as a new fish pass, and would be unacceptable to local residents and the public.

Conclusions and recommendations

(Thetext of the report given in full here)

St James’ Weir is in poor condition and a breach is expected within 10 years.

A breach would ease fish passage at the weir but would, however have some significant negative impacts for the amenity, heritage, recreational and environmental value of the area which are likely to cause public concern.

Should the Environment Agency choose to invest in a fish pass for the weir, it is essential that repairs to the weir are carried out to ensure the value of the fish pass is not compromised by a failure of the weir in the short term.

As a minimum, the large hole at location 4 on the left of the weir should be repaired. This will ensure the weir’s integrity for at least another 10 years. The weir could then be monitored and repairs carried out on a reactive basis. Alternatively the Environment Agency may choose to carry out more comprehensive repairs to the weir at the same time that the fish pass is constructed.

NB. This is a resumé and much technical detail has been omitted. Italics indicate words not in the reportor my remarks. “Right hand” and “left hand” side are taken to mean looking downstream.

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