PREDICTIONS OF WAVES AND SURGE FROM SOUND TO EXE ESTUARY

SHUNQI PAN, DOMINIC REEVE, DAVE SIMMONS JOSE HEROLLIHORRILLO-CARABALLO

UNIVERSITY OF PLYMOUTH

MAY 2011

THESEUS Deliverable ID 1.11

1. DESCRIPTION OF THE STUDY SITE AND PRESENT CLIMATE CONDITIONS The study site: Plymouth Sound to Exe Estuary is located in southwest , see Figure 0(a), encompassing a 100 km stretch of coastline bordered by the . The site is one of the most diverse coastal settings in Europe and incorporates a range of habitats from exposed rocky and shingle coast to sheltered mud of flooded valleys or 'rias' together with densely populated urbanised and industrial zones of Plymouth Sound and . Therefore, it is a unique and representative site to involve the complex coastal and estuarine processes; interaction between coastal defence structures and coastal morphology; and significant economic, social and environmental impacts.

Within the study site, the research is focused on the Teign Estuary Figure 0(c). A major modifier of the coastline is the railway line, which started by the South Railway Company, running from St Davids to in 1846 and later extended to . Since then the railway have been modernised and operated to the present time. The railway line occupies considerable stretches of coastal frontage (Exeter to to Teignmouth to Newton Abbot). Coastal defence work to protect the railway line has modified coastal processes. Pressures also include physical disturbance, for example by trampling, dredging, fishing gear, land claim and adjacent coastal development through the construction of sea defences and potential for changes in the hydrological regime.

The study site also features a range of important and sensitive marine habitats including grazing meadows, salt marshes, mud flats, rocky and sandy seabeds, together with important biogenic habitats. Intertidal and marsh habitats are important for numerous species of birds, insects and plants. Collectively these habitats support a diverse flora and fauna including rare and endangered species. The estuary also supports commercially important species including oysters, cockles and crabs grazing marshes are important for cattle. Within the adjoining coastline are Slapton Ley, Torbay and the Exe Estuary. The Exe estuary is a RAMSAR site (wetland of international importance) Site of Special Scientific Interest, a Site of Special Protection (EC Birds Directive). Torbay, is one of 28 areas in England designated by English Nature as a Sensitive Marine Area. Its sheltered aspect, unusual geology and warm climate mean that underwater, just as on land, the area is host to an exceptionally diverse range of habitats. Sea grass and mussel beds, present throughout the area are important habitats and potential ecosystem engineers which stabilise the substratum and provide an important source of organic matter, and a surface for attachment by other species. This shelter makes them important nursery areas for flatfish and, in some areas, for cephalopods.

The spring tidal range at the study site is approximately 3.8 m, with the maximum tidal current up to 3.0 m/s, while the tidal current in the is relatively weak, of 0.5 m/s. The waves in the area are predominately the swell waves from Atlantic in the south-west direction, with yearly mean significant wave height of 2.0 m (max 4.0 m) and for 1 in 50 year return period, the mean significant wave height is 2.7 m (max 5.3 m).

The River Teign arises on at a height of 520m AOD (above Ordinance Datum at Newlyn) and flows in a south-easterly direction towards the Teign Estuary and the sea. The catchment covers an area of 550 km2. The principal sub-catchments are the Rivers Lemon and Bovey and the . The estuary is also influenced by the large river flow. The River Teign flows through a diversity

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THESEUS Deliverable ID 1.11 of landscapes and habitats, ranging from open moorland (Dartmoor) to ancient woodland, improved pasture land and broad valleys, before finally meeting its floodplain and the estuary.

(a) Plymouth Sound to Exe Estuary

(b) Exe and Teign Estuaries (c) Teign Estuary Figure 0: Study site – Plymouth Sound to Exe Estuary

The Teign Estuary is approximately 9km in length and less than 1 km wide at its widest Point and is defined as a ria by JNCC (1997). It is one of South Devon’s most valuable assets. The mouth of the Estuary is marked by a permanent spit "the Point", on the north bank at Teignmouth extending southwest, and the red cliffs at to the south.

The Exe Estuary starts just to the south of the city of Exeter, and extends south for approximately eight miles to meet the English Channel. The estuary is a ria and so is larger than would normally be

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THESEUS Deliverable ID 1.11 the case for a river the size of the , the main river feeding into the estuary, (Futurecoast 2002).

On the east shore (from north to south) are the town of Topsham, the villages of Exton and and at the estuary mouth, the seaside resort of . Opposite Exmouth on the west shore is the village of with its sand spit extending across the mouth of the estuary. Above this there are fewer settlements on the west shore, with just the villages of and Cockwood, both adjoining the lower portion of the estuary. From the mid or late Holocene, following the submergence of the lower valley of the Exe to create the present day estuary, sedimentation of fine-grained material (clays and silts) has been continuous. The estuary therefore represents a sink for fine sediment, with sand and gravel being deposited closer to its entrance. Inputs have come from both marine and fluvial sources, with the latter probably of much greater relative significance in the past than now.

The sediment transport outside the Exe Estuary is predominately alongshore, sourced from cliff and shoreline erosion. The estuary is characterised by the meandering channel and large mud flat and sandbanks, which are partly exposed at low tides, with a narrow funnel shape. The estuary mouth is mainly confined by the sand spit from Dawlish Warren in the west. The estuary is highly dynamic and is bounded by offshore sandbanks and large ebb and flood deltas.

2. DATA AND METHODS For the present climate conditions analysis, various data sources have been used, as detailed as follows:

 Tides and surge: o Data: the British Oceanographic Data Centre (BODC – ww.bodc.ac.uk) o Locations: Devonport and Weymouth, two closest tide gauge stations to the study site over the past 19 years o Time series of tidal and surge levels (15-min intervals) and monthly extremes (maxima and minima) and monthly mean water levels. o Duration: 1991-2009

 Waves: o No measured wave data available in the study site o Model results from POLCOMS, forced by the wind data from ECMWF database o Locations: Devonport & Exe/Teign Estuaries o Duration: 1970 -1999  River discharge: o Data from National River Flow Archive, UK o Location: River Teign at Preston (station ID: 46002) o Duration: 1956 -2007 (daily)

In general, the extremes were calculated by fitting the annual maxima to a selection of candidate probability distribution functions. The 1 year lag correlation was calculated to ensure statistical

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THESEUS Deliverable ID 1.11 independence within the time series. Given the time-limited extent of the data sets, bootstrapping was used to re-sample the data and generate confidence limits based on the re-sampled populations following the methods described in Reeve (1996) and Li et al (2008).

3. RESULTS

3.1 MEAN SEA LEVEL The measured mean sea levels at the tidal gauges closest to the study site, Plymouth (Devonport) and Weymouth are shown in Figure 1. The average water level at Plymouth is 3.29 m above Chart Datum (CD) (0.07 m AOD) and 1.27 m above CD at Weymouth (0.34 m AOD). The yearly averages of the mean water levels indicate a tendency of a slight increase at both Plymouth and Weymouth in the range of 10 cm over the past 20 years. The mean sea level increase is more steadily at Plymouth than that at Weymouth.

Figure 1: Yearly mean water level at Plymouth (Devonport) and Weymouth

3.2 TIME SERIES & TREND OF ANNUAL MAXIMUM HIGH WATER Yearly maxima and minima of the measured sea levels from 1990 to 2009 are shown in Figure 2(a) for the Plymouth (Devonport) station and Figure 2(b) for Weymouth station. The results indicate that the high water levels fluctuate slightly over this period at both locations.

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7

6

5

4 Year Min 3 Year Max

2 Water Level(m) Water 1

0

-1 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 Year (a) 3.5

3

2.5

2 Year Min 1.5 Year Max 1

0.5 Water Level(m) Water 0

-0.5

-1 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 Year (b) Figure 2: Yearly maxima and minima at (a) Plymouth and (b) Weymouth

The measured surge levels, after the tidal components being removed from the measured water levels, over the same period are shown in Figure 3. At Plymouth, the storm surge was higher during 1992 and 1993, up to 1 m, and then decreased to 0.6 m for the following few years. Since then, the surge level has a tendency of increase from 0.6 m to 0.8 m in recent years. Yearly maximum surge levels at Weymouth have been constantly high around 1 m. In 2008, the surge level exceeded 1 m.

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(a)

(b)

Figure 3: Yearly maxima and minima of the surge levels at (a) Plymouth and (b) Weymouth

3.3 RETURN VALUES (TIDE+SURGE; SURGE ONLY) Analysis was carried out on the measured tide and surge levels using WeiBull, Gumbel and bootstrapping methods. Figure 4 shows the max and min extreme water levels for 2, 5, 10, 20, 50, 100, 200, 500, 1000, 2000 year return periods at both Plymouth and Weymouth. The predicted extreme water levels using Weibull distribution in general agree with those obtained from the Gumbel distribution. However, due to the lower/higher limits required in using the Weibull distribution, the former results are believed to be less reliable. Therefore, all predicted extremes using the Gumbel distribution are used in the extreme analysis hereafter.

At Plymouth, the extreme water level is likely to increase approximately 1 m for 1 in 2000 year events. At Weymouth, such an increase is also in the range of 0.8 m. Table 1 gives the details of water level increase with various return periods.

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(a) Min water level at Plymouth (b) Max water level at Plymouth

(c) Min water level at Weymouth (d) Max water level at Weymouth Figure 4: Extreme analysis of maximum and minimum water levels with Weibull and Gumbel distributions for various return periods.

Table 1: Max water levels (m) for given return periods at Plymouth.

Return Period (Y) Weibull Gumbel 2 6.162 6.144 5 6.341 6.283 10 6.445 6.376 20 6.535 6.465 50 6.638 6.581 100 6.709 6.667 200 6.775 6.754 500 6.855 6.868 1000 6.913 6.954 2000 6.967 7.040

The extreme analysis has also applied to the storm surge level at Plymouth and Weymouth, as shown in Figure 5 for the max surge levels. With the Gumbel distribution, the extreme surge level at Plymouth can be as high as 1.6 m in 1 in 2000 year return period events, while at Weymouth, the extreme surge level can reach 1.8 m. Table 2 gives the detailed extreme values for the max surge levels at Plymouth at various return periods.

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(a) max surge level at Plymouth (b) max surge level at Weymouth Figure 5: Extreme analysis of maximum and minimum surge levels with Weibull and Gumbel distributions for various return periods

Table 2: Max surge levels (m) for given return periods at Plymouth

Return Period (Y) Weibull Gumbel 2 0.730 0.727 5 0.895 0.860 10 0.997 0.950 20 1.088 1.036 50 1.198 1.147 100 1.275 1.231 200 1.348 1.314 500 1.439 1.424 1000 1.505 1.507 2000 1.568 1.590

3.4 JOINT DISTRIBUTIONS OF WAVES & SURGES For the joint distribution analysis, wave and surge data were obtained from the POLCOMS/ProWam model, which was set up with nested grids centred at the study area. Model has been extensively tested and validated in the area (Chen et al, 2010). The present conditions were modelling using the wind and SLP from ECWMF for the period of 30 year slice from 1970 to 1999. Figure 6 shows the yearly maxima of significant wave height at Plymouth and Exmouth. Since the pre-dominate waves are from the south-west, the maximum wave heights at Plymouth are generally higher than those at Exmouth, which is more sheltered for the waves from south-west. The highest wave height at Plymouth is 11.34 m and 8.14 m at Exmouth. The average wave heights over the 30 year period are 7.82 m and 5.81 m at Plymouth and Exmouth respectively.

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Figure 6: Yearly maxima of significant wave heights at Plymouth and Exmouth

The hourly wave heights and surge levels from the POLCOMS model have been divided into 0.25 m and 0.1m intervals for the joint distribution analysis. The joint distributions of waves and surge at Plymouth and Exmouth are shown in Figure 7. The results indicate a clear correlation between waves and surge at both locations. However, at the Exmouth location, the surge more strongly depends on the wave conditions.

1.1 1.1

1 1

0.9 0.0001 0.9

0.8 0.8 0.0001 0.0001 0.7 0.0003 0.7 0.001 0.0001 0.0003 0.6 0.6 0.0003 0.0001

0.001 Surge /mSurge Surge /mSurge 0.5 0.0003 0.5 0.0001 0.001 0.0001 0.0003 0.003 0.4 0.4 0.0001 0.001 0.0003 0.003 0.3 0.3 0.01

0.03 0.01 0.2 0.003 0.2 0.01 0.003 0.001 0.001 0.03 0.1 0.01 0.1 0.0003 0.1 0.1 0.0003 0.0001 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 Hsig /m Hsig /m (a) at Plymouth (b) at Exmouth Figure 7: Joint wave and surge distributions: contours (in log scale) at Plymouth and Exmouth.

3.5 MOST DANGEROUS EXTREME SEA LEVELS Based on the tidal gauge measurements, the most dangerous extreme sea level recorded at Plymouth is 6.35 m (above CD – 3.13 m AOD) occurred in 2004 and the highest sea level recorded at Weymouth is 3.04 m (above CD – 2.11 m AOD) occurred in 2008.

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3.6 EXTREME RIVER DISCHARGES Figure 8 shows the time series of the measured river discharge for River Teign at Preston, approximately 10 km north to Newton Abbot, from 1956 to 2007. The measurements include the main catchment of the estuary, but additional run-off and contributions from small tributaries could also increase the total river discharge at the mouth of the estuary. With the data available at the Preston Station, yearly maxima and averages of the river discharges are shown in Figure 4.3.10. Over the past 52 years, there have been only two occurrences for the river discharge exceeding 200 m3/s and 8 times for exceeding 150 m3/s, as shown in Figure 10. With Gumbel distributions, the extreme analysis shows that the yearly average river discharge will increase from the present value of 10 m3/s to 23 m3/s for 1in 2000 year return period events and the predicted yearly maximum river discharge reaches 426 m3/s, as shown in Figure 9 and Table 3.

Figure 8: Measured time series of discharge for River Teign.

Figure 9: Yearly maxima and averages of measured discharge for River Teign.

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300 283

250

200

150

Occurence 100 79

50 32 12 8 2 2 0 > 50. > 75. > 100. > 125. > 150. > 175. > 200. Discharge Exceeding (m^3/s)

Figure 10: Occurrences of the river discharge exceeding the given values.

(a) Yearly average (b) Yearly maxima

Figure 11: Predicted yearly averages and maxima of discharge for River Teign.

Table 3: Predicted extreme river discharge (m3/s) for given return periods for River Teign

Return Period (Y) Weibull Gumbel 2 92.637 91.054 5 136.138 134.128 10 162.706 162.752 20 186.327 190.233 50 214.529 225.820 100 234.193 252.493 200 252.746 279.070 500 275.891 314.135 1000 292.534 340.636 2000 308.545 367.128

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4. HISTORICAL FLOODING INDUCED DAMAGES Examples of the historical damages caused by flooding events in 2004 are shown in Figure 12, Figure 13, and Figure 14.

Figure 12: Damage due to wave overtopping to the seawall

Figure 13: Damage due to wave overtopping to the seawall

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Figure 14: Damage due to wave overtopping and flooding to the South-West main railway line

5. COASTLINE POSITIONS: Due to the South-west main railway line at the Exmouth site, and other permanent coastal defence structures, such as seawalls and groyns, the coastline position has not been altered significantly. However, beach profiles at the various locations have significant alternations under the storm conditions.

6. FUTURE SCENARIOS The future scenarios were considered according to IPCC suggest greenhouse gas emission models A1B (IPCC AR4 2007), and the wind and SLP data from Max Planck Institute for Meteorology WDCC/CERA database (WDCC 2009). Three time slices from 2010 to 2100 were modelled. Figure 15 shows that yearly maxima of the significant wave heights at Plymouth and Exmouth, together with the present wave conditions.

The mean significant wave heights at Plymouth and Exmouth for the three future time slices, together with those for the present conditions are shown in Figure 16. At both locations, the mean significant wave heights under A1B scenarios for future time slices exhibit a cyclic pattern. The wave heights for the periods of 2010-2040 and 2070-2100 are higher than that at present conditions, but the wave height during 2040-2070 is lower the present wave height.

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(a) Plymouth

(b) Exmouth Figure 15: Yearly mean water level at Plymouth (Devonport) and Weymouth.

Figure 16: Mean significant wave heights at Plymouth and Exmouth for three future time slices and present conditions.

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The joint wave and surge distributions for 3 time slices future scenarios are shown in Figure 17 at Exmouth. The distribution shapes are very similar, indicating insignificant changes for the A1B greenhouse gas emission scenario.

1.1 1.1 1.1

1 1 1 0.0001 0.0001 0.0001 0.9 0.0001 0.9 0.9

0.8 0.8 0.8 0.0001 0.0003 0.0003 0.0001 0.0003 0.7 0.7 0.7

0.0001 0.001 0.001 0.001 0.6 0.6 0.6

0.0001

0.0003 0.0003

0.0003

Surge /m Surge /m Surge Surge /m Surge 0.00030.001 0.5 0.003 0.5 0.5 0.0001 0.003 0.001 0.0003 0.0001 0.0003 0.001 0.003 0.0001 0.4 0.4 0.0001 0.4

0.3 0.003 0.3 0.3 0.01 0.003 0.01 0.01 0.2 0.2 0.2 0.03 0.03 0.03 0.0001 0.003 0.001 0.001 0.1 0.1 0.001 0.1 0.0003 0.0003 0.0003 0.1 0.01 0.1 0.01 0.1 0.01 0.0001 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 Hsig /m Hsig /m Hsig /m (2010-2040) (2040-2070) (2070-2100) Figure 17: Joint wave and surge distributions: contours (in log scale) at Exmouth for future scenarios.

For the predicted extreme significant wave heights for time slices: 1970-2000 for present conditions; 2010 -2040 for short-term; 2040-2070 for mid-term; and 2070-2100 for long-term climate scenarios, the wave heights at Plymouth are shown in Table 4 and a similar table can be produced for Exmouth.

Table 4: Predicted (Gumbel) extreme wave height at Plymouth for 4 time slices.

Return Period (Y) 1970-2000 2010-2040 2040-2070 2070-2100 2 7.668 7.923 7.458 7.923 5 9.085 9.358 8.827 9.277 10 10.029 10.314 9.739 10.179 20 10.936 11.232 10.615 11.046 50 12.111 12.422 11.750 12.169 100 12.991 13.314 12.601 13.011 200 13.868 14.202 13.449 13.850 500 15.026 15.375 14.567 14.956 1000 15.901 16.261 15.412 15.793 2000 16.776 17.147 16.257 16.629

7. SUMMARY The detailed analysis of the present climate conditions at the study site can be summarised as follows:

 Mean sea level at the study site increases slightly at the rate of about 10 cm over 19 years (~ 5mm/year)  Average high sea level at Plymouth is 6.15m (ACD) and 2.81m (ACD) at Weymouth.  The most dangerous sea levels are 6.35 m (ACD) at Plymouth and 3.04 m (ACD) at Weymouth.  The highest surge level is about 1 m at Plymouth, but 1.1 m at Weymouth.

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 The joint wave and surge distributions indicate a strong correlation between waves and surge at both Plymouth and Exmouth.  The predicted extreme water levels using Gumbel Distribution at Plymouth for 1 in 50 and 1 in 2000 year return period events are 6.58 m (ACD) and 7.04 m (ACD) respectively. Those at Weymouth are 3.21 m (ACD) and 3.64 m (ACD).  The predicted extreme surge levels using Gumbel Distribution at Plymouth for 1 in 50 and 1 in 2000 year return period events are 1.47 m and 1.59 m respectively. Those at Weymouth are 1.26 m and 1.74 m.  The yearly average river discharge at upstream of River Teign is around 10 m3/s over the last 51 years, while yearly extreme maximum discharge is as high as 250 m3/s.  The predicted extreme yearly average discharges for 1 in 50 and 1 in 2000 year return period events are 16.1 m3/s and 23.5 m3/s. The predicted extreme yearly maximum discharges are 228.8 m3/s and 373.3 m3/s respectively.  There are clear evidences that storm conditions have caused significant damages to the coastal structures and shoreline changes.  Model results for A1B scenario indicate an increase of wave height at both Plymouth and Exmouth for the short-term (2010-2040), and slight reduction for mid-term (2040-2070) and recovery to the short-term for the long-term (2070-2100). Insignificant changes in the joint wave and surge distribution are found.

8. REFERENCES Futurecoast (2002), Predicting Future Coastal Evolution for Shoreline Management Plans. Defra 2002. JNNC (1997), An inventory of UK estuaries, Vol 1: Introduction and Methodology, Joint Nature Conservation Committee, NHBS Environment Bookstore, , UK. Li,Y. Simmonds, D. J. & Reeve, D.E. (2008), Quantifying uncertainty in extreme values of design parameters with resampling techniques”, Ocean Engineering, Vol 35(10), p1029-1038. Reeve, DE (1996), Estimation of extreme Indian monsoon rainfall, International Journal of Climatology, 16 (1), 105–112 Chen, Y., Pan, S., Hewston, R. and Cluckie, I. (2010), Ensemble modelling of tides, surge and waves, Proceedings of the 20th International Offshore (Ocean) and Polar Engineering Conference, CD- ROM WDCC (2009). http://cera-www.dkrz.de/ World Data Centre for Climate, CERA-DB. IPCC AR4 (2007). Climate change 2007. Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, UK

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