Historic Data Use – Porlock Bay Case Study Gemma Kaye Plymouth Coastal Observatory (PCO)

Introduction Breach Gravel Barrier Salt Marsh Porlock is a coarse gravel beach located in West Somerset and situated within the Bristol Channel; the bay is approximately 5 km long. The bay is subject to a tidal range of up to 9 metres.   Since the 1800’s Porlock has undergone several management schemes, however in the early 1990s active management plans were abandoned in favour of a minimal intervention approach. In addition to this the low lying grazing land behind the beach was awarded a biological site of scientific interest. In October 1996 a storm event breached the barrier and the low lying grazing land behind the beach was flooded. This led to the creation of a salt marsh and tidal lagoon. This breach was never repaired and as a result Porlock now represents an extremely valuable example of barrier beach evolution. The biological site of scientific interest designation has now been revised to geomorphological in order to reflect this.

Porlock area Porlock is a beach management plan site and as a result of this the south west regional monitoring program carries out baseline surveys annually here, rather than every 5 years (as is the case with none BMP sites).   Baseline surveys include full profile surveys carried out at 50 metre intervals and continuous spot height data collected at 1 metre intervals,. All PCO data is collected to MLWS level.

These are the historic topographic profiles. This is an illustration of how PCO topographic baseline profiles compares to the historical profiles. We can also see the continuous data I have just discussed.   The historic data is constrained to the gravel barrier area and doesn’t reach MLWS Before the South West regional coastal monitoring program there was no consistent monitoring of topographic beach changes in this area. We plan to do annual techinical reports for this area, so a substantial data set can be formed. A major part of our program has also been to try and integrate historic data as much as possible in order to help determine what long term changes may be. The historic data used in this case study was collected by the University of Portsmouth in 1999 and 2000. The historic data has been used as the baseline for topographic surveys. The data has then been analysed from the historic baseline to 2009 spring data Using this historic data has given us the advantage of seeing what some of the long term topographic changes for the bay could be. However are there are some constraints to using this data.

Area of profile analysis Therefore when looking at the data it should be kept in mind that the results do not extend along the whole of the profile.   For this case study the effect of this has been taken into account during analysis of the data. (not eroding!)

Not all profiles covered As you can see the historic data only covers a few of these profiles. Therefore analysis is limited to the profiles which are covered by historic data   Despite these issues the results that the historic data has given us is still very valuable.

Baseline 1999 to Spring 2009 (Historic levels) We can also look at the profile changes on this Cross sectional area map. The red represents erosion and the blue represents accretion. The colour of the profile line corresponds to how much in metres squared the profile has lost or gained. The legend can be found in the top left hand corner. We could see in the graph that this profile was eroding, and here we can see that it has lost volume greater than 30 metres square.

2009 data Historic This is a profile graph for one of the eroding profiles in the bay. The blue line represents the historic data and the green line represents the latest 2009 data. It can be seen that the gravel barrier has lowered and migrated landwards over the last 10 years.

Baseline 1999 to Spring 2009 (Historic levels)   This is the CSA map for the accreting profile we just looked at. We can see from looking at the legend that this profile has gained between 15-30 metres squared in volume since 1999.

2009 data Historic In contrast, nearer the breach area one of the profiles is accreting. We can see this profile has accreted material and can see that the gravel barrier has lowered very slightly in the last 10 years.

Baseline 1999 to Spring 2009 (Historic levels) This profile displays the same range of erosion as the first profile we looked at, a loss in volume greater than 30 metres squared. We can see that the majority of profiles within Porlock have eroded over the last 10 years. The exception being the two profiles accreting material near the breach area.

2009 data Historic This is a profile graph for another eroding profile. We can see very clearly in this example how much the gravel barrier has lowered.

Spring 2008 to Spring 2009 (Historic levels) Because of the value of the 1999 data we really need to make sure it feeds through to our modern analysis. We can do this by looking at the profiles in a further two ways. Firstly we can change our spring 2008 to spring 2009 levels to the same as those used for the historic data, so we can do some comparative analysis. Secondly we can look at the spring 2008 and spring 2009 profiles at the MLWS level.   Looking at the spring to spring data with the historic levels, we can see that there is a very different pattern to the long term one we have looked at. We can see that the erosion seen over the last ten years has reversed to be accretion over the short term. This pattern is evident in the three profiles we have just discussed.

Spring 2008 to Spring 2009 The second level of analysis is the spring to spring profiles which have been compiled using the full PCO data set and which reach the MLWS level. The cross sectional area change applies to the entire profile line and not limited to the gravel barrier. The profiles are also present over a much larger area of the beach.   All of the profiles for this data set show change less than 5% of the cross-sectional area.

Spring 2008 to Spring 2009 The historic Spring to Spring data showed much higher percentages of CSA change. As this data was concentrated to a smaller area of analysis.

Spring 2008 to Spring 2009

MLW 2007 MHW 2007 We also have some other data which we really want to use .We have some historic LiDAR data from the environment agency in this example. We can extract tide level contours from this historic data. The purple line represents the historic MHW LiDAR data. The turquoise line represents the MHW line derived from the 2007 topographic baseline data. Comparing this to the 2003 Lidar MHW line, we can see that the breach area has undergone a substantial amount of accretion in the last four years. This corresponds to the long term trends we saw earlier. We are now expecting some new LiDAR (which we should receive annually for this site). Once we have this we will be able to make difference models and in future be able to perform a more detailed analysis. MHW 2003 (LiDAR)

Summary All of the discussed analysis is available to view in the 2009 Porlock technical report This report should be considered as a preliminary assessment. The report is now available from PCO and the website Please contact PCO if you have any historic data, you would like us to use. All of the discussed analysis is available to view in the 2009 Porlock technical report.   The report also contains topographic difference models, a wave report, sediment distribution maps and bathymetric maps. We also have a technical report available for Mine head which uses historic data Both these reports are now available from PCO and the website. As we collect more data more detailed a more analysis will be available. We are really keen to adapt our analysis to include historic data, so if you have any historic data for an area, please let us know.