APPENDIX A – SUPPLEMENTARY INFORMATION FOR THE GIS ASSESSMENT

10

• Granular superficial material – sand, gravel, boulders including silt and clay if they are minor components. Diamicton is assumed to be granular and capable of being mobilised.
• Loose material such as talus.
• Material that might reasonably be assumed to be so on a worst-case scenario such as made ground and fill
• If dense lodgement till could be distinguished from the above materials it could be assigned a lower rating, perhaps 5 might be appropriate on the basis that material in the near surface zone would be sufficiently weathered to become mobilised in the same way as a less dense melt out till would be mobilised.

9

• Materials at the finer end of the coarse materials with some silt and clay but not enough to stabilise the material if copious water were present.

8

• Materials with clay and silt listed as the major component.
• Probably sufficient fine material to stop debris flow mobilisation unless the components are present as discrete bodies that could be mobilised and the finer components then incorporated.
• Their potential for being mobilised may be overestimated at this score and subdivision and rescoring on geomorphological grounds may improve this. Possibly raised deposits go to score of 7 or 6 and the flat lying deposits go to a score of 1.
• Materials in the highest class of the accumulation model are assigned this score as described in Section 4.2.2

7

• Landslip and worked ground are included in this group on the basis that they are probably loose and at residual strength but may be fine-grained.

6

• No mapped materials are assigned to this score but accumulation materials as identified by the methodology described in Section 4.2.2 are assigned this value.

5

• This score has been assigned to bedrock lithologies that were considered the most likely to develop a significant regolith that could be mobilised by flowing water. Thus the regolith would be predominantly the result of physical weathering and comprise coarse material either through the induced fracturing along incipient discontinuities (schists, pelites semipelite etc) under the influence of freeze/thaw activity or lesser thermal effects or the break up of inter-mineral bonds by the break down of some of the mineral components (coarse grained igneous rocks, granites, migmatites etc).
• The working party report noted that schist and granite were associated with debris flows, an observation that supports this classification.

4

This score has been assigned to bedrock lithologies that appear less likely to generate a granular regolith because:

• They comprise mixed sedimentary rock with lithologies that contain some clay rich components that may soften and bind the regolith together e.g. undivided cyclic sedimentary rocks, ‘sandstone, siltstone, mudstone’, greywacke.
• Are mainly stronger and have a lesser propensity for breaking along discontinuities than the pelite/ semipelite lithologies. These lithologies are the more gneissose semipelites.
• Also included here are fine grained igneous rocks such as basaltic and andesitic lavas that are assumed to have large numbers of discontinuities due to cooling joints or a rubbly fabric that would assist their weathering, along with tuffs which are all known, in some instances to weather to a granular regolith.

3

• Sedimentary conglomerates are included on the basis that the individual components might weather out of a weaker matrix.

2

• These materials are assumed to have relatively few discontinuities that would allow them to form an extensive granular regolith and to be relatively resistant to chemical weathering. Although some of the basic igneous intrusions would be more likely to form clay-rich weathering products than the other lithologies in this group.
• These materials include sandstones, psammites, and minor igneous intrusions (both basic and acidic).

1

These materials are those which are considered unlikely to be mobilised as a debris flow because

• They are too silty or clayey.
• They are limestones that would dissolve rather than form a regolith.
• They are high-grade metamorphic psammite/gneiss and would be unlikely to form a regolith due to their strength and chemical stability.

0.0 to 0.025

8

0.025 to 1

6

10

• Formations including superficial and bedrock deposits of silts and clays with little permeability due to their fine particle size and bedrock formations of gneissic or plutonic formations whose low porosity and very widely spaced discontinuity spacing results in a low permeability.

9

• These formations comprise metamorphic rocks expected to have very low porosity and widely spaced, tight discontinuity spacing.

8

• These formations comprise fine-grained metamorphic rocks (pelite), uniform sandstone (quartzite) and mixed sequences of mudstone/siltstone/sandstone that might be expected to have slightly more discontinuities than the previous class.

7

• These formations comprise sandstone, minor igneous intrusions (i.e. not plutonic), limestone, conglomerate and lava which are likely to have moderately spaced discontinuities that might be expected to form a three dimensional pattern rather than a planar one and thus promote downward drainage.

6

• These comprise clay or silt rich superficial deposits that may have a small under drainage capacity if they contain discrete units of coarse material and a small number of lithologies with properties that are not easily predicted such as landslip, fault crush and worked ground.

5

• No materials are assigned to this score.

4

• No materials are assigned to this score.

3

• No materials are assigned to this score.

2

• No materials are assigned to this score.

1

• These materials are superficial deposits that are or may be expected to contain significant amounts of sand and/or gravel that would allow some under drainage of overlying material.

0.1

• These are superficial deposits that comprise primarily sand and gravel which would offer significant under drainage possibly to the extent that the passage of a debris flow on low slope angles could be slowed and pore water from antecedent rainfall might be dissipated relatively quickly.

1

Sea/estuary

Sea/estuary

Sea, estuary

22.1

Not applicable - effect neutral

1

Water (inland)

Water (inland)

Water (inland)

13.1

Not applicable - effect neutral

1

Littoral rock and sediment

Littoral rock

Rock and rock with algae

20.1

Bare coastal slope may promote debris flows otherwise not applicable

1

Littoral sediment

Mud, sand and sand with algae

21.1

Not applicable - effect neutral

1

Saltmarsh

Saltmarsh (Grazed/ungrazed)

21.2

Not applicable - effect neutral

1

Supra-littoral rock and sediment

Supra-littoral rock

Rock

18.1

Bare coastal slope may promote debris flows otherwise not applicable

1

Supra littoral sediment

Shingle, vegetated shingle, dune, dune scrub

19.1

Bare coastal slope may promote debris flows otherwise not applicable

1

Bog

Bog

Bog: shrub, grass/shrub, grass/herb Peat >0.5 m.

12.1

Not applicable separate assessment

0.85

Dwarf shrub heath

Dwarf shrub heath

Dwarf shrub heath (ericaceous/gorse) Peat <0.5 m thick

10.1

Some reinforcement by shrubs, better than grass.

0.9

Open shrub heath

Open shrub heath (ericaceous/gorse)

10.2

Some reinforcement by shrubs, better than grass.

0.9

Montane habitats

Montane habitats

Montane vegetation

15.1

Mixed, reinforcement depends on vegetation type - better than bare ground.

0.7

Broad-leaved/mixed woodland

Broad-leaved/mixed woodland

Scrub, open birch and deciduous mixed, broadleaved evergreen, yew

1.1

Good stabilising effect through root reinforcement and soil moisture demands

0.7

Coniferous woodland

Coniferous woodland

Conifers, new plantation and felled

2.1

Good stabilising effect through root reinforcement and soil moisture demands.

1.2

Arable and horticulture

Cereals

Barley, maize, oats % wheat

4.1

Bare ground - no root strengthening, loose condition

1.2

Arable horticulture

Bare, root crops, cropped legumes, linseed, rape, mustard, unknown.

4.2

Bare ground - no root strengthening, loose condition

0.9

Non rotational horticulture

Orchard, ley, set aside

4.3

Mixed, orchards 0.75 but ley and set aside 0.9. Will be mostly ley and setaside

0.95

Improved grassland

Improved grassland

Intensive grazing, hay/silage cut, grazing marsh

5.1

Slight reinforcement - better than bare ground.

0.9

Setaside grass

Grass set aside

5.2

Some reinforcement - better than bare ground.

0.9

Neutral grassland

Rough grass

Rough grass

6.1

Some reinforcement - better than bare ground.

0.9

Managed neutral grass

Grass (neutral/improved)

6.2

Some reinforcement - better than bare ground.

0.9

Calcareous grassland

Calcareous grass

Calcareous (managed, rough)

7.1

Some reinforcement - better than bare ground.

0.9

Acid grassland 

Acid grass

Acid

8.1

Some reinforcement - better than bare ground.

0.9

Acid with Juncus

8.1

Some reinforcement - better than bare ground.

0.9

Acid Nardus/Festuca/Molinia

8.1

Some reinforcement - better than bare ground.

0.85

Bracken

Bracken

Bracken

9.1

Stoloniferous roots reinforce ground.

1

Fen, marsh, swamp

Fen, marsh, swamp

Swamp, fen/marsh, fen willow

11.1

Not applicable separate assessment

1.1

Built up areas, gardens 

Suburban/rural developed

Suburban/rural developed

17.1

General infiltration impeded but potential for focused drainage

1.1

Continuous Urban

Urban residential/commercial

17.2

General infiltration impeded but potential for focused drainage

1.1

Inland bare ground

Inland bare ground

Despoiled/semi-natural

16.1

Bare ground - no root strengthening

0.5

0-7

Generally stable and only influencing the run-out characteristics of a debris flow.

1

8 - 15

Slopes within this range that occurred between a road and an area of debris flow hazard were likely to maintain the movement of the debris flow and facilitate its impact on the road although it was unlikely to be sufficiently steep to allow the initiation of a debris flow within it.

6

16 - 30

It appears that debris flows may be initiated on slopes within this range but it would be equally likely that additional material would be incorporated within this zone.

9

31 – 45

This slope range is considered the most likely to initiate debris flows based on the experience of the working group. This would appear to be sensible in that the peak angle of shearing resistance of dry granular material might be expected to be in this range (BS8002:1994).

10

Slope > 45

It is logical that slopes in the >45⁰ class should have a factor or weighting greater than the 31- 45 class in recognition of the increased driving force associated with the increase in the down slope component of shear stress.

x 1

10

1

9

6.68

x 1

10

0.1

9.9

4.33

x 0.75

1.2

0

1.2

0.92

x 0.75

10

0

10

0.88

x 1.25

10

0.1

9.9

2.08

0-12.0

12.1-15.0

15.1-16.5

16.6-18.0

>18.1

Group of statistics calculated
by road section

Number of points along section (only valid for trunk road network)

POINT_NO

X-coordinate of start point of road section

START_X

Y-coordinate of start point of road section

START_Y

X-coordinate of end point of road section

END_X

Y-coordinate of end point of road section

END_Y

Length of section

S_LENGTH

Highest point along section

S_HIGH

Lowest point along section

S_LOW

Average height along section

S_MEAN

Group of statistics calculated
on catchments intersecting
the road section. These are
the areas that would be likely
to yield material for a debris
flow. (from intersection of
NEXTMap DTM, NEXTMap
slope model and catchments)

Number of catchments intersected

C_COUNT

Total catchment area (m²) intersected

C_AREA

Highest point in catchment

C_HIGH

Lowest point in catchment

C_LOW

Average height in catchment

C_MEAN

Maximum slope

C_MAX_SLOPE

Minimum slope

C_MIN_SLOPE

Average slope

C_AVE_SLOPE

Group of statistics calculated
within the intersected
catchments (from debris flow
hazard grid and NEXTMap
DTM)

Maximum debris flow hazard score

H_MAX

Maximum debris flow hazard class

H_MAX_CLASS

Minimum debris flow hazard score

H_MIN

Minimum debris flow hazard class

H_MIN_CLASS

Average debris flow hazard score

H_MEAN

Average debris flow hazard class

H_MEAN_CLASS

Group of statistics calculated
within the intersected
catchments (from component
hazard grids)

Maximum lithology score

H_LITH_MAX

Minimum lithology score

H_LITH_MIN

Average lithology score

H_LITH_MEAN

Maximum water conditions score

H_WATER_MAX

Minimum water conditions score

H_WATER_MIN

Average water conditions score

H_WATER_MEAN

Maximum vegetation score

H_VEG_MAX

Minimum vegetation score

H_VEG_MIN

Average vegetation score

H_VEG_MEAN

Maximum stream channel score

H_STREAM_MAX

Minimum stream channel score

H_STREAM_MIN

Average stream channel score

H_STREAM_MEAN

Maximum slope angle score

H_SLOPE_MAX

Minimum slope angle score

H_SLOPE_MIN

Average slope angle score

H_SLOPE_MEAN