Innovative means of assessing the
relative permeability of a soil type for
inclusion in a small catchment flood study
Presentation: 4th BHS International Conference
By David Price and Elmar Torenga
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© Copyright
September 2016 Jacobs
www.jacobs.com | worldwide
Talk contents
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1.
The hydrological issue
2.
Study objectives
3.
Prevalence of the issue amongst the reservoir catchments
4.
Detailed investigations
5.
Conclusions
What’s the hydrological issue?
Reservoir safety flood studies – FSR\FEH Rainfall-runoff Unit Hydrograph
model.
Essential is an understanding of the permeability of the catchment
Percentage runoff (PR)
Standard percentage runoff (SPR)
In the UK Hydrology Of Soil Types (HOST)
Some within the reservoir community have voiced concerns with HOST 4.
Welsh catchment that had a catchment SPRHOST of 2%
“This value was obviously wrong for the catchment …”
HOST 4 soils commonly found in west Pennines reservoir catchments
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“A visual inspection suggests that these values are too low for these
catchments”
Placed those with responsibility for reservoirs in an awkward position
Precautionary approach taken HOST 4 SPR = 60%
Project Objective:
How common is HOST 4 in the reservoir catchments
To assess the validity of using either 2% or 60% to represent the SPR of
HOST Class 4 soils in the catchments with reservoirs.
Give guidance on what value should be used in future flood reservoir flood
studies.
HOST 4:
Hydrogeology: Strongly consolidated,
non or slightly porous. By-pass flow
common.
Soil: Mineral soil. Groundwater > 2m.
No impermeable or gleyed layer
within 1m of the soil surface
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Prevalence of HOST 4
• Digital HOST dataset obtained from
Cranfield University
• Consists of polygons that represent
the soils on the national soil map
(Soil Survey of England and Wales
[1983]) - scale of 1:250,000
• For each polygon the percentage
coverage of each of the 29 HOST
classes is given
• Differs to FEH catchment descriptors
(1km grid) Caution in small
catchments
541g Rivington 2
83% HOST 4 (SPRHOST = 2%)
17% HOST 22 (SPRHOST = 47.2%)
Soil SPRHOST = 9.5%
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Reservoirs ranked by HOST 4 coverage
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Detailed site specific investigation
Highly ranked catchments targeted for detailed investigation
i) Desk based datasets
ii) Site visit interpretation
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Permeability evidence considered
• Soil descriptions
• Geological permeability (bedrock)
• Geological permeability (superficial)
• Vegetation
• Land use
• Channel characteristics
• Drainage density
• [Reservoir responsiveness]
• Presence of springs
• [Presence of wells]
• Presence of standing water bodies
• Presence of artificial drainage
• Soil wetness on day of visit
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Soil descriptions
Source: 1:250,000 scale soil mapping (Soil Survey of England and Wales,
1983)
Soil hydrology characteristics
“well drained”, “permeable” “moderately permeable”, “seasonal water
logging”, “slowly impermeable”
Developed by soil scientists prior to the development of HOST (1995)
NOTE: More detailed soil descriptions are available - LandIS (Land
Information System) website (Cranfield, 2016). Detailed descriptions
including “Soil Water Regime”. BUT unclear if these post-date HOST and
have been influenced by HOST thinking.
Not been used as evidence in this project.
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Soil descriptions (cont.)
Caution:
Soil mapping is inevitably somewhat imprecise.
At a scale of 1:250,000 small-scale (field size) variability will be lost plus there
will be a range of conditions that are grouped together under the same
mapping unit class.
Catchments of less than a few square kilometres may therefore not be so
well represented
Soil mapping at a more detailed scale in England is either absent or not
readily available …….
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Geological permeability descriptions
Source: BGS Permeability Index - Scale 1:50,000
Qualitative estimate of vertical permeability from ground surface through the
unsaturated zone down to the water table
Comprises a 3-part code
1. Predominant Flow Mechanism: intergranular, fracture, or mixed (intergranular
and fracture).
2. Minimum permeability
3. Maximum permeability
Index separately describes: bed rock & superficial deposits
“A scoping study to assess the potential effects of soils concluded that in almost
all cases the soils were a product of weathering of the underlying deposits and
that the resulting soils were of similar permeability to the deposits from which they
were derived……. The study concluded that since the permeability of the vast
majority of soils did not differ significantly from the underlying deposits, there was
little need for the creation of an additional ‘permeability’ layer for the soils.”
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Vegetation
Some plants only grow in wet conditions whilst others prefer well-drained
soils
Use with care to infer soil moisture conditions
Rushes indicate poorly drained, wet soils
Other useful plants:
Gorse, bracken, certain types of broad leaf woodland
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Channel characteristics
Groundwater dominated streams - flow regimes with relatively little
variation.
Impermeable catchments - flashy flow regime (low flows not sustained
by groundwater. Flood flows are high [majority of the rainfall shed from
catchment]).
In nutrient poor upland landscapes - groundwater carries more nutrients
than runoff from the impermeable landscape where the residence time of
the water in the catchment is much less.
Flood evidence included here:
Flood debris, flood flattened vegetation etc
Site visit during the 2015\16 winter after the notable flood period
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Channel characteristics
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Drainage density
Impermeable landscape = high runoff – naturally forms many drainage
channels (streams) to accommodate the large amounts of rapidly shed water
Permeable landscape = low runoff – less pronounced drainage network
Soil association 541p (Malham 2)
SPRHOST = 2%
Permeable limestone geology
Interestingly classified as 100%
HOST Class 4
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Presence of springs
Springs indicate permeable geology - (slow response path) rainfall infiltrates
and percolates down to groundwater - emerges from a spring or as a
baseflow sustaining the flows in a watercourse.
Presence of standing water
Natural ponds and lakes suggest an impermeable substrate.
Presence of artificial drainage
Artificial drainage (surface ditches or sub soil agricultural drainage) points to
excessive wetness and waterlogging. Such soils are less likely to be freely
draining.
Also ditches along roads and tracks
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Soil wetness on day of visit
After significant rainfall the soil will exhibit different degrees of wetness due to
its permeability and position in the landscape
1.
Highly impermeable ground - tend to be saturated and evident from
walking on the surface . (Water readily rises to the surface when trodden
on. Surface water in small depressions amidst the sward or as ponded
water on flat ground).
2.
Highly permeable ground - rapidly drains (Ground relatively firm,
presence of excess water not obvious). (Exception where groundwater
rising to surface).
Antecedent rainfall helps interpretation. 2015/16 winter has been notably wet.
Same day, same catchment two adjacent soils show markedly different soil
wetnesses, a good steer to relative permeabilities.
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Permeability scoring system and summarising method
Approach:
Assess the Target soil together with a Reference soil in each catchment
Make a relative qualitative assessment of permeability
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Information
Information gathered
from site visits (field
observation)
and
desk based digital
data:
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Reservoir A relative soil permeability
Res A
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Reservoir L relative soil permeability
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Summary of Rivington 2 target soils (83% HOST Class 4)
Res D
Res G
Res A
Res L
Res C
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Permeability summary of the various reference soils
Res D (541j)
Res G (1011b)
Res A (713g)
Res L (721c)
Res C (721c)
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Conclusions
Soils mapping scale (1:250,000) is rather approximate, may lead to appreciable
uncertainty in catchment SPR estimates for small catchments.
Use of 1km gridded SPRHOST in FEH catchment descriptors particular uncertainty for
small catchments.
1.
The study concludes that the Rivington 2 soil is a fairly permeable soil.
2.
Strong evidence to discount the practise of assuming a SPR value of 60% for the
HOST Class 4 component of the Rivington 2 soil.
3.
Use of reference soils gives good grounds for the Rivington 2 SPR to be < 30%.
The aggregated evidence suggests an SPR somewhat lower than this.
4.
The qualitative approach followed does not allow precise SPR estimates to be
reached. To achieve this would require hydrometric monitoring.
5.
In light of this uncertainty the client may wish to consider the use of a precautionary
HOST Class 4 SPR value of 30%. This is close to the upper bound value found in
this study. It is suggested that this would provide a defensible basis for determining
Category A spillway PMFs consistent with concerns regarding dam safety.
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The HOST Class 4 project
Key Contacts:
Lead technical:
David Price
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© Copyright
September 2016 Jacobs
[email protected]
0141 243 8446
www.jacobs.com | worldwide
HOST (in general) appreciable uncertainty!
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HOST (in general) appreciable uncertainty!
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