Appendix 2 Monitoring erosion and change in dune systems

  1. Introduction
  2. Aims and objectives for monitoring
  3. Priorities for measurement
  4. Frequency, timing and location of measurements
  5. Monitoring techniques – low cost approaches
  6. Monitoring techniques – aerial photography
  7. Measurements of the physical environment
  8. Data Storage and Analysis
  9. Data Sheets for Download

1 Introduction

Managing a beach-dune system effectively requires data on the initial character of the system, changes over a range of timescales and the factors which cause change. The data is required to:

There are several steps required to develop a useful database. A desk study must be completed to determine what information exists already, a baseline survey must be undertaken to establish the situation against which future change will be compared, and a programme of ongoing monitoring must be implemented to assess change and guide future management.

The desk study should include a review of maps, aerial photographs, surveys, published reports (see References) and any other documents that may reveal information relating to the past evolution of the shoreline, possibly including records of storms, shoreline construction works or periods of intense human activities impacting on the dunes (e.g. military exercises, sand mining, grazing). This information will help in understanding the present and may guide the monitoring programme.

The baseline survey will establish the existing physical character of the beach and dunes. It should be as detailed as possible and may take at least a year to complete.

Ongoing monitoring will include a subset of the baseline measurements and may include surveys at a range of different frequencies (e.g. hourly water levels, monthly walkover observations, seasonal profiling, annual aerial photography, five-yearly bathymetric surveys). The monitoring programme must be appropriate to the site, cost effective, flexible and must provide the amount and quality of data required by the shoreline manager.

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Monitoring can help determine whether erosion is cyclical or part of a long-term trend. The progressive exposure of this outfall, originally sited flush with the dune face, reflects on-going erosion of over 1m per year.

This appendix sets out the reasons for monitoring, suggests what should be measured, measurement methods and frequencies, approaches to analysis and discusses management responses. The methods include details of simple, low cost approaches and general guidance on more specialist techniques. It is assumed that shoreline managers will turn to competent consultants and contractors if there is a requirement to use specialist techniques. The appendix only covers monitoring of physical characteristics and does not cover any biological or social monitoring approaches, although these may be equally important to the planning and implementation of a dune-beach management programme.

2 Aims and objectives for monitoring

Monitoring must be driven by the need to provide appropriate data for shoreline management. There are several stages in this process:

3 Priorities for measurement

Each site will have specific aspects that require initial measurement and ongoing monitoring. In general the level of risk to fixed backshore assets will control the level of monitoring. Potential loss of footpaths, rough grazing or low value amenity areas may only justify investment in occasional visits by management staff to observe or photograph changes to the dune face. At the other end of the risk scale are high value assets such as industrial facilities or residential developments. For these sites it may well be worth developing a programme of accurate, frequent and high resolution measurements, including installation of wave/tide gauges and meteorological equipment, commissioning of topographic surveys and aerial photography, and development of computerised data storage and analysis systems. The possibilities for measurement will almost always exceed the budget, so priorities must be set.

The following are presented in their likely order of priority for baseline measurement and ongoing monitoring of an open coast situation:

These priorities do not include biological or social monitoring, although these may be equally important to the selection, planning and implementation of a management programme.

4 Frequency, timing and location of measurements

Dunes and beaches evolve continually in response to waves, water levels, winds, human activities, biological processes, etc. Rates of change vary from the micro-scale of wave periods, through tidal cycles, up to seasons and beyond to decades. Micro-scales are of little practical significance to dune management, so the most obvious changes will be as a result of storms (one or two days) or seasons (winter to summer). These relatively short term changes may mask the longer term evolution over decades. It is important that dune monitoring recognises these different scales. Baseline measurements must establish the existing situation, including the potential short-term variability of that situation. Once variability is understood, monitoring can concentrate on longer-term trends.

For example, water levels change continuously and the upper beach levels will change with every tide, while the position of the dune toe may only change during storms. Establishing astronomical tidal elevations requires several measurements per hour over a period of at least one month using self-recording equipment. Establishing the baseline for the upper beach may require frequent observations and elevation surveys every month for a year to define an envelope of variability. The baseline for the dune toe may require quarterly and post-storm surveys. Subsequent monitoring of the beach and dune face might both relax to twice yearly, while tide levels will not need to be measured again (continuous monitoring would reveal storm surges and allow predictions of extremes, but this is unlikely to be required for most dune management operations).

The timing of some measurements is also important, particularly with respect to beach profile monitoring. If the intention is to measure the extremes of variability, surveys should be completed during or immediately after severe storms, when beaches will be drawn down, and also during periods of prolonged light swell and longshore winds when both beach and dune face are likely to be fully recovered. However, if the intention is to establish long term trends, the surveys should be undertaken when sea conditions are likely to show some consistency. For example, the monitoring programme might call for beach profiles to be completed within two days of the peak Spring tides of every second month – this would allow seasonal variations to be established and year on year trends to be identified with minimal observer bias.

Location of each measurement must be considered for the same reasons as timing. Establishing trends requires the surveyor to measure the same points or profiles on each occasion. Changes of even a few metres alongshore in the location of a beach-dune profile may introduce uncertainty into the analysis, particularly if there are any distinct features on the beach such as rock outcrops, streams or structures. Alternatively, if the surveyor wishes to establish potential variability during the baseline study, it may be preferable to shift profiles or point measurements by a small distance to record extremes of change, such as an area of maximum dune recovery or a scour channel caused by unusual wave induced currents.

5 Monitoring techniques – low cost approaches

5.1 Introduction

Dune systems are three-dimensional and irregular in form and do not lend themselves easily to analysis by standard surveying techniques. A set of x,y,z co-ordinates tells little about the health or evolution of a dune system, while a comparison of a number of such surveys to determine annual changes may be extremely difficult because of the very irregular and dynamic nature of a dune field. When managing a dune system a simple evaluation of the approximate extent and frequency of marine erosion or accretion is more important than an evaluation of the detailed volumes. This section directs the shoreline manager towards simple but effective techniques for monitoring using tape measures and cameras. Simple surveying equipment, such as levels or Total Stations, can be used for the same purposes if available, but this will require some additional training in field methods and data processing.

Erosion of the frontal dunes often manifests itself as a distinct scarp line that is easily identified if the dune/beach system is examined in cross-section. Changes over time to this cross-section, and the lateral extent of change, will provide useful information about the dune evolution. If the scarping is localised then the erosional processes must also be localised, possibly a funnelling of wave energy due to the construction of groynes, breakwaters or a revetment. If the scarping is widespread within an estuary then erosion may be due to a realignment of tidal flow channels. The intensity of erosion can clearly be related to the height of the scarp relative to the total height of the dunes and to the area of sand eroded to form that scarp. In terms of evaluating dune evolution a combination of techniques is necessary to locate the erosion or accretion areas, quantify the extent of damage, determine the likely causes and to evaluate the rate of change.

5.2 Establishing beach zones

Ideally, management operations require knowledge of tidal elevations, the extent of wave run-up under normal conditions and the potential maximum extent of run-up during storms. If data from a local tide gauge has been analysed to establish astronomical tidal characteristics (Highest Astronomical Tide, High Water Mark Ordinary Spring Tide, etc.), these can be related to an Ordnance Survey bench mark and used to establish beach face positions. However, in most locations around Scotland local tide data will not be available and the beaches may well be remote from any Ordnance Survey control. In these situation dune management must rely on interpretation of observations.

Seaweed and other flotsam will be left as a strand line along the upper beach after each high tide. Regular observations of the position of the strand line can be used to establish appropriate locations for any dune management schemes. The strand line is determined by the combined effect of the tidal elevation and wave run-up. The normal limit of wave run-up under high Spring tides, assuming moderate wave conditions, will define the seaward limit of shoreline vegetation. Foredunes will begin to form to landward of this line, but can easily be destroyed during storms.

The limit of storm wave run-up on an eroding beach will be defined by the elevation and position of the erosion scarp toe, though waves may actually splash even further up the face. Marram grass will not survive within this storm inundation area and therefore makes a useful indicator of the maximum run-up limit. Lyme grass and sand couch grass will survive occasional inundation and may grow further down the dune face.

Occasional extreme events, combining a storm surge water level and severe wave conditions, will cause run-up and erosion even further up the dune face. The potential importance of defining these extremes depends on the level of risk acceptable at the site. Sites with important and valuable backshore assets may need to be protected by substantial defences, and the services of specialist consultants will be required to establish appropriate design conditions.

5.3 Locating erosion areas

A reconnaissance survey should be made to identify obvious signs of dune retreat or areas of wind blown erosion (blow-outs). Sketches should be made on an Ordnance Survey map at 1:2500. Local topographic features such as footpaths, field boundaries, coppices or planted woodland should be used to provide an approximate indication of the location and extent of the eroded area. Details of the conditions in that area should be set out on a proforma, an example of which is attached. It is important to note the presence of any man-made features such as groynes, seawalls, outfalls, etc. as all of these could contribute to dune instability. Signs of trampling by livestock or by man should also be recorded.

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Monitoring of reprofiled dune face with simple levelling equipment.

Fixed aspect photographs should be taken from features which are easily identifiable so that repeat monitoring can be carried out (all photos must be dated and annotated to provide a meaningful record). Photos should show the dune crest, face and toe in profile, plus any other interesting features observed during the visit. The location of the photographer’s position should be noted and, if possible, clearly marked on site to allow re-establishment on subsequent visits.

At this stage it is worth setting up several markers to be used as datums for subsequent surveys. Marker posts should ideally be made of 2m long pressure treated timber posts (60mm diameter) or scaffold pipe driven at least 1m into the beach at about 50m intervals along the toe of the dunes; the spacing may be greater for systems with uniform alongshore features or less for smaller or more complex systems. Another line of markers should be set out 30m or so landwards of the first line in positions that are safe from erosion. The posts should be located as accurately as possible on a 1:2500 OS map, with the landward set intended to be used to reposition the beach posts if they are lost. The distance and bearing from one to the other must be noted along with position relative to adjacent sets of markers or fixed points within the dunes (pill boxes, fences, trees, etc). Each marker must have a unique reference. Figures A1 and A2 illustrate the set up of marker posts.

Surveys can be conducted in two ways. The simplest approach, applicable to small dune systems with minor erosion problems, is to hand measure offsets from the marker posts using a 50m fabric tape and a compass. The distance and bearing to the dune toe, the highwater line and to other distinguishable features (berms, spits, trash lines etc.) should be measured. A simple spreadsheet can be used to plot and analyse repeat surveys. Where available a simple surveyor’s level with stadia lines can be used effectively by a two person team with very limited training. Elevations and distances relative to a fixed point can be recorded rapidly and accurately (+/-2mm vertically, +/-200mm horizontally).

fig-a01

Figure A1 Dune profile showing possible locations for markers

More extensive dune systems with important backshore assets at risk should be surveyed in greater detail. A Total Station can be used to survey profiles by a competent dune manager and an assistant after limited training. Cross-shore profiles from the grey dunes down to the low water mark can be measured relative to a line defined by a pair of well-established marker posts of known position and elevation. Survey lines should generally be approximately shore normal to limit the complexity of analysis. Spacings of 50m to 100m should be sufficient to define changes along the dune system, with small bays and estuaries requiring closer spacing than extensive open coast systems

fig-a02

Figure A2 Plan of dune face showing possible profile line layouts

Assistance from a trained surveyor may be required to establish the control marker positions and elevations accurately, but follow on surveys can be undertaken by a team of two non-specialist personnel. Results can be plotted, stored and analysed using a simple spreadsheet, or more complex survey analysis software.

Hand held GPS (Global Positioning System) equipment is not yet sufficiently accurate to allow for adequate surveying, although future developments may alter this view. Differential GPS can be very accurate but will certainly require the services of trained surveyors.

5.4 Equipment

Once the marker posts have been fixed the minimum required survey equipment is simple and portable, requiring only one person (although it is recommended that two people work together for speed and safety, particularly in remote locations):

Improved data can be obtained using a simple surveyor’s level or a Total Station to replace the tape and clinometer. The cost of staff training and the additional time spent setting up, processing data and transporting equipment may not be justified for many sites.

If marker posts are displaced (by waves, livestock or people) they must be carefully re-established and their new position noted. Markers used for Total Station Surveys should be re-surveyed relative to each other during every field operation. This does not take long and ensures that analysis will be based on a valid datum.

5.5 Determining the likely causes of erosion

Having mapped out the erosion areas some preliminary evaluation should be made as to the likely causes of erosion. Wind data can be purchased from the nearest anemometer station and any periods of high winds from the main onshore wave generating directions highlighted. Records from the nearest tide gauge station should be examined for any periods of higher than normal water levels. This simple data review should reveal the recent periods of likely dune erosion.

If the records fail to identify any such events, other reasons for erosion should be considered. Natural reasons for dune toe erosion should be obvious from the site inspection. For example, the undercutting of the dune foot by meandering tidal channels within an estuary should be evident. If erosion has been caused by man, and is of a short term nature, this too should be obvious from the site inspection. Longer term erosional trends may be caused by structures interfering with the natural supply of sand to the dune area. Such structures could be groynes located some distance from the site or possible underwater obstructions to sand movement. Evaluating the causes of erosion may therefore entail discussions with adjacent landowners, local authorities, harbour authorities, etc.

Any coastal works in the vicinity of the dune system should be examined carefully to see if they may have triggered off downdrift erosion. Changes in the land use should also be considered, including the possibility of trampling, wind induced scour or overgrazing. If the causes of erosion have not been identified a revisit should be made. Discussions should be held with local conservation volunteers, Council engineers, golf course managers, fishermen etc. to obtain background information about the area.

5.6 Measuring the amount of erosion

If erosion is not threatening to create a permanent breach in the dunes, or a serious loss of dune area, no capital works should be considered until a number of site surveys have been made over a period of at least two winters. As noted in the introduction, change to the beach and dunes can occur over a range of timescales. The frequency of surveying must recognise this. Initially surveys should be undertaken bi-monthly, with additional surveys following severe storms. Regular surveys should be completed at the same point in the tidal cycle, e.g. during Spring tides, and post-storm surveys should be completed as close to the event as possible. After the first year (say, six to ten surveys) the frequency can drop to quarterly, plus post storm (say, four to eight surveys). After several years the surveys will have revealed the potential extent of short term change, and the frequency can drop once more to early spring and early autumn, plus extreme storms (say, two to four surveys). The timing of these ongoing surveys should be consistent in terms of the month and the tidal state, perhaps ideally the March equinox tide (erosion expected) and the first Spring tide in September (accretion expected). At this frequency the long term trends can be monitored, and can be analysed with a well founded knowledge of short term variation.

Measurements should give the bearing and distance to identifiable features from each datum. These features will be some or all of the following:

Having identified the various geomorphological zones, any change to their extent between successive surveys should be mapped. The horizontal rates of change of the various boundaries can be calculated and put into context. Simple spreadsheet software or survey analysis packages will allow changes to be stored and presented effectively.

5.7 Erosion Checklist

An indication of the severity and possible causes of erosion may be obtained from the following checklist. Note: the values given are indicative only.

Measured Change/Observation Comments/Possible Causes of Erosion
Less than 1m over a year at all points. Erosion likely to be insignificant.
Over 1m between surveys. Erosion may be significant or may be episodic (cyclic).
Erosion of the dune face sustained at 1m per year. Erosion significant and possibly severe.
Erosion of the dune face at 1m to 10m per year. Serious erosion requiring immediate attention.
Dunes badly trampled at individual access points. Localised pedestrian damage.
Dunes badly damaged over large dune area. Serious damage due to tourist pressure, livestock or vermin.
Discrete areas of dune face erosion. Wind damage (blow outs).
Dune face erosion over wide area. Damage by wave action.
Dune toe and upper beach erosion. Damage caused by waves or tidal currents.
Upper and lower beach accretion. Change of profiles in response to wave conditions.
Upper and lower beach erosion. Severe wave erosion/meandering of tidal channel/impact of coastal defence.
Dune scarp/exposure of marram roots. Sustained recession of dunes.
Dune scarp covered by fresh sand accretion. Intermittent dune recession.
Beach ridges/runnels moving landwards. Recent beach build up, hence dune erosion may be temporary.
Beach ridges/runnels moving seawards. Beach erosion continuing, hence dune recession also likely.
Beach salients. Unstable conditions in an alongshore direction may result in local and episodic recession in adjacent areas.
Groynes/breakwaters located updrift. Beach may be affected by downdrift erosion.
River/training walls updrift. Beach may be affected by a change in sediment supply.
Eroded dune surface but beach stable/backshore sand covered. Wind induced erosion/pedestrian damage/livestock or vermin damage.
Seawall remnants. Evidence of long term erosion.
Groynes/seawall, etc. on lower foreshore. Strong evidence of long term erosion.
Groynes/seawalls/sills at toe of dune but recession continuing. Inadequate defences/unusually severe wave/tidal conditions.
Groynes/seawalls/sills at toe of dunes but displaced or tilted over. Unusually severe waves/tidal currents causing structural failure. Possibly poor initial design or poor foundations.
Erosion in one area of beach or sediment cell matched by accretion in adjacent area. Change in dominant wind/wave direction.
Erosion throughout frontage including non dune areas. Adjustment to changed wave climate/tidal prism, not related to human activities.
Severe erosion at structure tailing off downdrift. Lee side erosion caused by coastal works.

6. Monitoring techniques – aerial photography

As a compliment to the observational and low cost approaches set out in Section 5, the most useful method of monitoring dune morphology changes is by aerial photography. Professionally flown, survey quality, aerial photos provide excellent records of change and are useful for obtaining an overall understanding of dune systems as well as monitoring marine erosion. Hand held photography or video taken from a small plane at low altitude can produce very useful qualitative information. As an alternative there are a number of companies offering the services of model aeroplanes equipped with survey cameras.

Photogrammetric measurements of erosion or accretion require the services of specialist aerial surveyors. Colour photos at a scale of about 1:5000 taken with appropriate overlaps will allow photogrammetric analysis to produce digital ground models, from which changes can be measured (+/-100mm resolution). Photogrammetry requires ground control points that are carefully surveyed for position (x, y, z), visible from the air and can be replaced at the exact position for subsequent flights. There are numerous aerial survey companies around the country who are able to provide a professional service, although it may be difficult to obtain competitive quotes for work in more remote areas. Flights should be undertaken annually and close to the same dates and tidal conditions to allow easy comparison. Clear weather and good light are required for best results. Tide levels should be below mean low water to allow important process information to be derived for the beach as well as the dunes. Where small dune systems are to be monitored it may be sensible to survey several sites during a single flight.

7 Measurements of the physical environment

Monitoring morphological change is useful to maintenance operations, but does not allow for improved forward planning of dune management. Additional meteorological and sea state information is required to understand the processes leading to change.

Some processes are active over a very long term. These include changes in relative sea level, changes in the total volume of sand available to the coast (due to particle degradation, cliff erosion or losses to offshore sinks), or changes in climate. Measurement of these processes is undertaken under regional and national research programmes and can not be effectively addressed at a local level.

Other processes occur over time scales that can be measured, including tides, storm surges, winds and waves. Some, or all, of these may be measured regionally and managers may be able to access the information from port managers, coast guard stations or the Meteorological Office. However, dune systems can be influenced by processes that are site specific: winds are influenced by local topography and exposure directions; storm surges measured at the nearest port tide gauge may be quite different in character within an embayment 60km away; and waves are strongly affected by local wind, bathymetry, currents, and by exposure to the open sea.

If the shoreline manager wants to measure local conditions to help understand specific dune responses, it is quite possible to deploy tide gauges, anemometers and wave monitors. However, the costs for a low maintenance system that can telemeter real time data direct to a shore based PC for processing, analysis and storage will be high, while the quality of data retrieved from a low technology, labour intensive system may be too poor to make the effort worthwhile on a long term basis. Specialist advice on appropriate technology must be sought on a site by site basis.

As an alternative to deploying wave monitors, which are the most costly and difficult of the instruments, it is possible to get high quality predicted wave information from the Met Office for offshore points all around the UK (and Europe). The offshore data can be transferred inshore using a numerical transformation model for analysis in conjunction with observations and aerial mapping.

Tide information can be derived from a regional tide gauge provided that local information is available to determine appropriate time and range corrections. This local information is best obtained by deploying a tide gauge close to the site for a period of at least 4 weeks (two tidal cycles) for comparison with the regional gauge or for computer analysis to determine harmonic constituents. The former is a relatively simple operation and the analysis can be done without specialist help, while harmonic analysis requires appropriate software and knowledge. Analysis will provide adequate data to determine local tide levels, but will not be reliable for storm surges; only long term (i.e. 10 years plus) local measurements will provide information on potential maximum surge elevations.

8 Data Storage and Analysis

Data collected must be stored effectively if it is to serve any long term purpose. Clear records that can be retrieved and interpreted by others should be kept. Spread sheets should contain sufficient header information to be of general use, rather than simply presenting a series of numbers. Full use should be made of comments recorded in field note books, as data analysis may not take place for months or even years after collection, when memories of site observations are no longer reliable.

GIS (Geographical Information Systems) packages offer convenient data storage, analysis and display systems. However, setting up an effective GIS is a specialist and costly task, and requires ongoing management to obtain a useful return in the future. It is likely that a GIS would only be cost effective if maintained on a regional or even national basis, including all of the dune systems of interest in Scotland.

Having collected and processed field data on an apparently eroding dune system, the manager may feel that action is vital to prevent further deterioration. As discussed earlier, unless there is strong evidence that erosion is ongoing over at least several years, it will be prudent to wait before committing to any more than minor maintenance work. The shoreline is a dynamic zone, and erosion can revert to accretion, making the managers efforts redundant.

Data Sheets for Download