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London Gateway inquiry inspector's report - volume 1

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9 Inspector's conclusions on marine and coastal processes

9.1 I consider that the main considerations on the subject of marine and coastal processes are

(i) would the proposed dredging cause any significantly adverse effects on the erosion and deposition in the estuary; and,

(ii) whether the introduction of the proposed container vessels would lead to unacceptable risks to river traffic.

9.2 Whereas the Applicants, the Environment Agency (EA), the Port of London Authority (PLA), BP and SPEAC submitted documentary evidence for the inquiry, only the Applicants and SPEAC presented any and the latter was non technical. The EA, PLA and BP reached agreement with the Applicants that their concerns could be met through the modification of the HEO to include certain provisions.

9.3 Accordingly, the evidence presented by the Applicants has been the subject of detailed consideration by the EA, PLA and BP. They became satisfied that, subject to monitoring, there would be no damaging effects caused by erosion or deposition in the Thames Estuary as a result of the capital and maintenance dredging operations and the construction of the port. The agreements were reached outside the inquiry. Nevertheless, the process has enabled the scientific evidence of the Applicants to be the subject of critical peer review on the basis of advice from their own or independent experts in that particular topic and thus constitute "sound science". I have not examined such evidence further. The EA submitted a statement outlining the basis on which the agreed position has been reached. 166

9.4 Therefore, on that basis and in the absence of any technical evidence to the contrary, I do not consider there would be any significantly adverse effects on either the erosional or depositional processes in the river. However, the evidence identified the following consequences which have a bearing on fishing and nature conservation which are dealt with in Chapters 10 and 11 below.

Points emerging from agreed evidence

9.5 The main morphological changes over the last century are that about 13ha of salt marsh has been eroded from the northern end of Mucking Flats since 1867. Large quantities of ballast dredging and disposal have been undertaken in Lower Hope Reach. Mucking Flats has demonstrated net accretion. Since 1979 there has been a raising of the intertidal surface and a slight loss in total area above CD. The western end of Blyth Sands has increased in area above CD and has reduced in average level. The eastern end of Blyth Sands has increased in area above CD and has increased in average level. There has been subtidal accretion in the entrance to Holehaven Creek. 167

9.6 The dredging for the port would, through removal of the sea bed, directly influence 112 ha of seabed in the manoeuvring and berthing area, 1520ha for deepening of the navigation channel, and 94ha in the Zulu 3 Anchorage Area. This would be a total of 1726ha. The area of the bed of the estuary covered by the reclamation would be 93ha, of which 68 are subtidal and 25ha intertidal. 168

Construction

9.7 The combination of port reclamation and dredging of the navigation channel and manoeuvring and berthing area would maintain the existing cross-sectional area of the Estuary in Upper Sea Reach. The construction of the bund and dredging needs to progress together to avoid significant changes to the existing cross-sectional area of the estuary at the location of the reclamation. 169

9.8 As a result of the capital dredging some fine material would be suspended into the water column. It is predicted that this would amount to about 4.55Mm3 of fine material and about 2.62Mm3 of fine sand. Of the fine material about 1.0Mm3 would be suspended whilst dredging between 0km and 13km from the upstream end of the London Gateway Port site. 170

9.9 There would also be losses from the reclamation activities. For purposes of impact assessment these have been estimated to be up to 1.2Mm3 of fine material. The highest rates of loss would occur during open hydraulic filling of the reclamation bund by trailer suction hopper dredger. This rate of loss would be comparable to the present maintenance dredging undertaken at the existing berths at Shell Haven, Coryton and Canvey Island by water injection dredging. Thus short term impacts associated with the losses from the reclamation would be similar to those presently associated with maintenance dredging. Persistent effects (i.e. net accumulation) would occur at higher rates than that experienced as a result of present maintenance dredging because the total quantities of fine material lost from the reclamation are greater than those released from present maintenance dredging. 171

9.10 The modelling showed that for a worse case scenario of continuous dredging of silty sand 3-5km from the upstream end of London Gateway Port there would be no significant effect on intertidal processes over the sensitive intertidal areas to the east of Canvey Island with mean temporary slackwater deposition on the intertidals being less than 1mm and average depths of build up predicted to be less than 1mm per year. On Mucking Flats the persistent effects would be greater, with average depths of build up predicted to be up to 100mm per year. 172

9.11 The main physical effects of the losses of fine material from the reclamation and dredging activity would be to locally increase suspended sediment concentrations and slack water deposition in the vicinity of the construction activities. 173

9.12 A comprehensive monitoring programme would be set up during the capital dredging as outlines in the Dredging and Reclamation Strategy. 174 The principles of this monitoring would be the establishment of a boundary around the dredging footprint such that increases in suspended solids concentrations at this boundary would be controlled with appropriate caution and stop thresholds. 175

Completed Works

9.13 It is predicted that there would be a small impact on tidal propagation in the Thames Estuary. Tidal range is predicted to slightly increase. The net effect of this between Tilbury and Southend is that about 5ha of low intertidal would be converted to shallow subtidal. The effect on surge tides would be minimal. Generally wave conditions would be little changed by the dredging and reclamation. 176

9.14 The application of 3D mud transport modelling predicted that siltation in the berthing and manoeuvring area would be in the range 0.8Mm3/year to 2.55Mm3/year. The upper limit of this range is conservative and likely to be more representative of winter conditions. 177

9.15 The mud transport modelling also predicted that, because of changes to the suspended sediment regime, there would be increases in siltation at the adjacent berths at Coryton (an increase of up to 20,000m3/year) and Canvey Island (an increase of up to 2,000m3/year).

9.16 The sand transport modelling predicted that there would be accumulation of 0.24Mm3/year) of fine sand at the western end of the manoeuvring area. In addition to this a similar quantity of fine sand was also predicted to accumulate in the navigation channel. 178

9.17 None of the predictions of sediment accumulation take into account the effects of vessel movements that would be expected to reduce overall accumulations and modify the distribution of sediment build up on the bed.

9.18 In the seaward parts of the navigation channel only small areas of capital dredging would be required. However, these areas of the existing channel are subject to change associated with long term migration of the subtidal and intertidal bank system and the presence of sandwaves. 179

Maintenance dredging

9.19 Maintenance dredging would be required at the western end of the London Gateway Port manoeuvring and berthing area when the western berths become operational and would also be required within the navigation channel on the basis of navigational needs and safety. 180

9.20 There would be no long term effects on the fine sediment regime upstream of Coalhouse Point or seawards of Canvey Island. The maintenance dredging may have an effect on the local sediment regime, and it is predicted that up to about 5% of the material mobilised from the London Gateway Port could accumulate at the existing berths at Coryton and Canvey Island. The potential increase in siltation in these areas would thus be between 40,000m3/year and 130,000m3/year. 181

9.21 Sand that would accumulate in the manoeuvring and berthing area could be removed by the hydrodynamic dredging. Any residual accumulation would be removed by trailer suction hopper dredger. The navigation channel would also be maintained with trailer suction hopper dredger. During the period of the development of London Gateway Port (up to 12 years after completion of the reclamation) materials dredged by trailer suction hopper dredger would be used on the London Gateway Port site. 182

9.22 The positioning of the navigation channel in the offshore area relative to the deepest water and pattern of bank movements would define the requirements for future maintenance. The capital dredging required would not be expected to significantly affect the slow migration of sandbanks or the formation of sandwaves. A combination of channel rebuoying and maintenance dredging would be used to maintain safe navigation in this area as is the case today. 183

Assessment of longer term change

9.23 The principle area of change predicted by the modelling is on the northern end of Mucking Flats. Here predicted changes were fed back into the mud transport model to provide a means of predicting the nature of the morphological evolution that would occur on the mudflats. It is predicted that over a period of 7-12 years up to 1Mm3 of accretion would occur on parts of the mudflat raising the surface in places at the northern end by about 1 metre. The main area of change is the northern most 60ha of the mudflat. The changes would arise as a result of the changed regime once the western bund has been completed. On a precautionary basis it has been estimated that up to 10ha of the mudflat could be converted to saltmarsh as a result of the accretion. 184

9.24 Minor changes to the ongoing evolution of Blyth Sands have been predicted by the modelling. The present pattern of evolution is growth in area of Blyth Sands at an average rate of about 7.2ha/year (measured above Chart Datum). It is predicted that this rate of growth would reduce to about 5.8ha/year on completion of London Gateway Port. The upper intertidal at the western end of Blyth Sands is presently eroding. This erosion is predicted to continue. 185

9.25 No significant changes are predicted to the morphological evolution of the intertidal areas to the east of Canvey Island as a result of London Gateway Port. The London Gateway port proposals are predicted to lead to increased accumulation of material in the subtidal area at the mouth of Holehaven Creek. 186

166 EA/0/4

167 APP/10/1 paras 4.2.2 – 4.2.18

168 APP/10/1 paras 5.1.1 – 5.1.2

169 APP/10/1 paras 5.2.1 – 5.2.3

170 APP/10/1 para 5.3.2

171 APP/10/1 paras 5.4.1 – 5.4.8

172 APP/10/1 para 5.5.15

173 APP/10/1 para 5.5.17

174 CD/612

175 APP/10/1 para 5.5.19

176 APP/10/1 para 6.1.1

177 APP/10/1 para 6.3.6

178 APP/10/1 paras 6.4.1 – 6.4.2

179 APP/10/1 para 6.4.4

180 APP/10/1 para 6.5.1

181 APP/10/1 para 6.5.4 – 6.5.6

182 APP/10/1 para 6.5.8 – 6.5.9

183 APP/10/1 para 6.5.10

184 APP/10/1 para 7.5.2 – 7.5.5

185 APP/10/1 paras 8.9 – 8.11

186 APP/10/1 paras 8.7, 8.13

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