Sand Dunes

Dune Hydrology

Dune Hydrology

Introduction to sand dune/shingle hydrology

Sand dune and shingle hydrology is a complex interaction between freshwater and salt water involving hydraulic gradients, topography, climate and weather and the underlying geology. In coastal regions, fresh water from rain flows out toward the sea while sea water penetrates some way inland. The interface between the two is determined by density differences and is quantified using the Ghyben-Hertzberg relationship (Putt). The factors mentioned above along with tidal fluctuation and abstraction/pumping regime complicate the relationship such that it is less of an interface than a transition zone of differing salinity. Freshwater tends to sit on top of brackish or sea water as a lens being less dense than either. This water table fluctuates seasonally, highest in Winter and Spring, lower in Summer and Autumn. This season fluctuation is subject to change and is reflected on a longer timescale as wetter or drier periods alternate over decades. Plants of the coastal zone are well adapted to water stress, particularly pioneer species such as Marram which adapts by reducing evapotranspiration and Crambe maritime which has exceptionally long tap roots. Fresh water habitats are visible on sand dune systems but not shingle, as slacks in ‘valleys’ between dune ridges which provide habitat for many plants and animals. Primary dune slacks are formed when newly formed dune ridges on accreting coasts cut off a section at the top of the beach, enclosing an area that may become wetted over time. Secondary dune slacks are a result of wind scour which erodes sand down to the water table.

Climate change impacts/threats

Clearly these habitats and their hydrology will be affected by climate change in terms of altered precipitation patterns and sea level rise. The risk of saline intrusion is further increased by a predicted growth in demand for water. Under natural conditions, habitats would move inland in response to sea level rise but coastal squeeze prevents this process meaning habitats are likely to be lost. There is a difficult choice involved in habitat conservation under the influence of sea level rise. Managed realignment schemes will protect the best brackish and intertidal habitats but possibly at the cost of freshwater habitats, choosing which habitats benefit will be an agonising decision.

Hydrology thematic group

The sand dune hydrology workshop took place in March 2010 and the many aims and ideas which were generated prompted the network to set up its first thematic group. This will allow experts from the field to meet as a sub group and discuss research directions, funding needs and policy developments among other things. For a full report on the hydrology workshop see newsletter 8. If you would like to be involved please email dunes@hope.ac.uk and we will put you in touch with Charlie Stratford of the Centre for Ecology and Hydrology who is heading up the thematic group.

Charlie has provided us with an update on the group’s activities as of July 2010. Along with Laurence Jones of CEH and Nick Robins of BGS, Charlie met with Mark Whiteman of the EA, Peter Jones of CCW and Graham Weaver of Natural England to identify and prioritise tasks that are both doable, in the context of available resources, and which help towards a better understanding of coastal dune ecohydrology. The focus is on collating and interpreting the existing time series water level data and working towards models which will lessen some of the uncertainties in current conceptual groundwater system understanding.  A first task is seeing whether statistical analysis of the dip well hydrographs identifies clusters, and to date, near beach and inland dip wells cluster very well, but whether forested and open warren will follow quite so nicely has yet to be tested. 

One of the key issues identified by delegates at the Southport workshop in March was securing long-term data sets and the need to reform our approach to collecting data. To this end, CEH is working to establish long term monitoring infrastructure with high frequency water level loggers at Braunton Sands and Whiteford Burrows complete with continuous rainfall monitoring. CEH is working with various organisations to uncover and access diverse datasets apart from the long-term dip well data sets such as the 20 year record created and maintained by John Breeds at Braunton. Charlie is on the trail of long-term botanical records at Whiteford which would be enormously useful coupled with the dip well records to investigate trends in biodiversity and species richness. Whiteford has also been identified as a candidate test case site for an EA led study examining the impact of climate change on wetlands. Being entirely rain fed and discharging to the coast provides a useful contrast to ‘typical’ inland wetlands and the long-term data sets available make it a robust case for the study.

CEH has been working with Jeremy Barker, a land management and conservation advisor for Natural England, and Nick Edwards of CCW to develop digital topographic models of slack floors at both Braunton and Whiteford.  Detailed mapping of the embryo dune and slack at Whiteford is planned for later this year.  Digital mapping at a fine scale compliments topographic studies at the site scale to give a better understanding of the hydrological regime, and subsequent repeat mapping will identify small morphological changes. The team has also being working in the lab to simulate the impact of different water levels on the soil moisture content of dune soils. This work will go towards understanding the role the depth to water table has on slack vegetation.