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Have Water levels fallen? :

Presentation by Martin Hollingham on the hydrology of the warren for the habitats and species group meeting of the Newborough Liaison partnership on 13 th Oct 2005

 

 

•  What evidence is there for a lower water table?

•  Historic recollections usually remember the unusual and extreme events and are not reliable

•  Drainage ditches and ploughed areas in forest areas still flood in 2001 - 2008 and footpaths in warren pass through flooded areas.

•  Ranwell records pond depth of 1.5m in winter during the 50s, and Bristow recorded this too in 2000-2001

•  Other dune systems, Braunton and Sefton, also suffering from falling water tables.

 

 

•  Given the CCW aim of enabling the sand dunes to become mobile, is a lower water table a cause for concern?

•  Less soil water, less vegetation, more unstable and mobile the sand dunes

•  Dunes need to be mobilized for pools to form as the watertable controls the height of the dune slacks, wind erodes the sand to the new lower height of capillary water above the water table.

•  When changes are made to increase recharge, like removing forest and changing drainage, water levels will rise in the newly deepened pools and if dunes are still mobile, will cause filling of the dune slacks.

  Climate change and Global warming effects

•  Rising sea levels, coastal erosion and their effect on the water table is unknown, but rising sea levels will raise water levels and result in consequent shrinking of freshwater lens under the warren

•  Changes in rainfall pattern, tendency for wetter winters, drier summers (fig. 1) which increases the tendancy for winter flooding.

•  Past Rainfall

•  Dry 40s, Wet 50s & 60s, Dry 70s and 90s, Average 80s, wet since 2000. (Fig.1)

 

 

•  8 monthly rainfall seems to match t he sum of the average piezometric head data plotted over the period Jun 89-Dec 96 (Fig 7.1b in Bristow's NW Hydro report) (Fig. 2)

 

   

•  Water levels seem to stabilize when rainfall is 60-67mm, which represents the average monthly losses from Et and aquifer drainage.

 

•  Land Drainage

•  Main source of recharge for the warren west of Llyn Rhos Ddu is east of the forest from the barrier to the caravan park and is directed by drains into Llyn Rhos Ddu.

•  The height of Llyn Rhos Ddu sluice has been lowered by 2m and then recently raised again by 1 m. This has reduced recharge to the warren but also reduced flooding at Pen Lon

•  Drainage from Llyn Rhos Ddu joins drains to Pen Lon drainage system

•  Ditches to control flooding at Pen Lon drain into the Braint.

•  East side of the warren underlain by clay and also the first area to flood

•  Drainage from a spring in the field below the barrier is directed into Llyn Rhos Ddu instead of the forest area SW of Gwnhingar, which occupies higher ground adjacent to the highest dune slacks.

•  This spring is supplied from a perched aquifer in the field north of the forest barrier.

•  There is a Palaeo valley and drainage ditch along the main track which directs drainage directly to the coast, and so the forest west of the main track is unlikely to affect the main bulk of the warren.

•  However, Bristow's Groundwater modelling indicates that removing forest from higher areas is the most effective way of increasing recharge.

 

 

•  Trees

•  In general hydrological research indicates that forests evaporate and intercept more rain than dune vegetation, however this is a generalization and Newborough is an unusual forest planted with Corsican, Scots and a variety of other pines, spruces, cedars and broadleaves on a sand dune system.

 

•  How good is the CCW commissioned hydrological research?

 

•  Results used in Bristow reports are using Et and Interception estimates based on early Plynlimon research- an upland area planted with sitka spruce.

•  Different trees, different interception rates (table 1). A mature Pinus Pinaster stand seems to intercept and evaporate less water than Sitka Spruce, Scots Pine, oak and mixed broadleaf forest (Arnell, N. (2002) Hydrology and global environmental change. Prentice Hall, London , New York .).

 

Tree type
Evap loss
(% gross P)
Interception
(% gross P)
Total Loss
(% gross P)
Scots Pine
31-42
42-33
64-75
Sitka Spruce
27-32
27-31
54-63
Maritime pine
12-18
13-27
25-45
Oak
26
26
52
Mixed hardwood
19
23
42

•  Later Plynlimon research ( Hudson , J.A, Crane, S.B. and Blackie, J.R. (1997) The Plynlimon water balance 1969-1995: the impact of forest and moorland vegetation on evaporation and streamflow in upland catchments. Hydrology and earth systems sciences, 1(3), 409) showed:

•  Mature forest E and I is less

•  Et from the immature forested of the Severn was 60% more than grassed Wye catchment in 1972

•  Et From the mature forest crop was 18% more than the Wye just prior to clear felling in 1985

 

•  FC research indicates less Transpiration on sandy soils, and for mixed and older stands. (http://www.forestry.gov.uk/pdf/fcin065.pdf/$FILE/fcin065.pdf)

•  It is impossible to check the SWAP model results in the CCW ADAS report as not all model parameters are stated. In particular it is unclear if interception is calculated using early Plynlimon results.

•  Also stated in appendix B in the CCW ADAS report, forest PET (800mm) is nearly the same as forest AET (780mm) yet table 5 in the same report (table 2) indicates this difference should be closer to 100mm.

 

 

Parameter
France
Belgium 1
Belgium 2a
Belgium 2b
Newborough
Rainfall (mm)
804
655
672
1042
967
Max. Mean Temp.( ° C)
13.8
12.4
10.4*
10.7*
12.9
Min. Mean Temp. ( ° C)
6.8
6.6
7.3
Soil
Sand
Sand
Sand Loam
Sand Loam
Sand
Tree type
Pinus nigra
Populus nigra
Pinus sylvestris
Pinus sylvestris
Pinus nigra
PET (mm)
653
636
AET (mm)
559
509
658 / 567*
632 / 629*

 

 

•  Dune Vegetation

•  ADAS report also indicates that Recharge is also reduced by established dunes. Stable dunes are in part also responsible for the reduction of the water table.

 

•  Options for raising water levels

•  Remove vegetation to encourage mobile dunes then allow wind to deepen pool hollows, before raising water levels

•  Re direct existing drainage to Llyn Rhos Ddu into the warren

•  Use surface water drainage from Newborough village or STW

•  Raise water levels in Llyn Rhos Ddu

•  Fill in forest drains

•  Chop trees

•  May be plant them where flooding is a problem by Pen Lon?

 

 

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Copyright © Martin Hollingham