hazard assessment keys for evaluating site sensitivity to ... · e o rg an ics olm ped f th 40 w ,...

Hazard Assessment Keys forEvaluating Site Sensitivityto Soil-degradingProcesses — Interior Sites

Land Management HandbookFIELD GUIDE INSERT 8

ISBN 0-7726-7448-5

1993

Site Sensitivity InterpretationsWorking Group

Terence Lewis

William Carr

Ministry of ForestsResearch Program

Fgi08.qxd 2/22/2000 7:38 PM

ii

AUTHORS AND AFFILIATIONS

Field Guide Insert 8 has been prepared for the Research Branch, Ministry of Forestsby the Site Sensitivity Interpretations Working Group; Terence Lewis, B.S.F., Ph.D.,Soils and Land Use Consultant; and by William Carr, Ph.D. (CPESC), Terrasol.

ACKNOWLEDGEMENTS

The Timber Harvesting Subcommittee of the Site Sensitivity Interpretations WorkingGroup, which initiated and undertook production of the original field guide insert thatcontained the first hazard assessment keys, was chaired by Charlie Fur, Ministry ofForests, Kamloops, and included Rick Smith, Ministry of Forests, Vernon; BobMitchell, Ministry of Forests, Kamloops; and Adolph Koshke, Clearwater TimberProducts, Clearwater.

The Site Sensitivity Interpretations Working Group is responsible for the interpretivekeys in this field guide insert. Ministry of Forests working group members thatcontributed to this field guide insert include Mike Curran, Nelson (chairperson); BobMitchell, Kamloops; Marty Osberg, Victoria; and Bill Watt, Williams Lake.

We also appreciate reviews by regional pedologists and ecologists, other Ministryof Forests staff (particularly Peter Jordan, Nelson), forest industry personnel, forestryconsultants (particularly Greg Utzig, Kutenai Nature Investigations), and over700 participants of soil disturbance workshops held from 1989 to 1992.

Technical support from the Production Resources section of the Research Branchis greatly appreciated.

Fgi08.qxd 2/22/2000 7:38 PM

iii

PREFACE

This field guide insert is one in a series of technical support documents intended foruse by forestry staff to ensure high-quality data collection and interpretation for forestpractices in British Columbia. They are intended to be used together whendescribing map strata, delineating treatment units, and developing prescriptions.

These documents are not substitutes for education, training, and field workshopson soils, ecology, soil conservation, and management strategies. They are supportdocuments that complement these other activities and together constitute a goodframework for organizing soil conservation extension and training.

The keys in this field guide insert have been abstracted from Land ManagementReports No. 62 (1991), No. 63 (1991), and Field Guide Insert 2, and have beenrevised in response to recommendations provided by field scientists and forestrypersonnel following several years of use. This field guide insert is designed forpersonnel who collect data required to assess site sensitivity before thepreparation of forestry plans and prescriptions (e.g., Pre-Harvest SilviculturePrescription [PHSP]).

iii

Fgi08.qxd 2/22/2000 7:38 PM

iv

TABLE OF CONTENTS

AUTHORS AND AFFILIATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii

ACKNOWLEDGEMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii

PREFACE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii

OBJECTIVE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

SITE SENSITIVITY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

PRE-HARVEST DATA NEEDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

COMPACTION AND PUDDLING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Compaction and Puddling Hazard Key . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

DISPLACEMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Displacement Hazard Key . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

FOREST FLOOR DISPLACEMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Forest Floor Displacement Hazard Key . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

SURFACE EROSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Surface Erosion Hazard Key . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

MASS WASTING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Mass Wasting Hazard Key . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

TABLES OF RAINFALL (R) FACTORS FOR INTERIOR FOREST REGIONS . . . 14

Fgi08.qxd 2/22/2000 7:38 PM

1

OBJECTIVE

To provide those responsible for site data collection or the developmentof forestry plans and prescriptions with an objective process forassessing the sensitivity of sites to soil degradation.

Intended Use

The keys provided are intended to be used on a map stratum basis duringcollection of field data and to decide on treatment units. Site mapping, fielddata collection, and determination of treatment units are discussed in LandManagement Handbook No. 25.

SITE SENSITIVITY

Many factors influence the overall sensitivity of a site to timber harvesting.Any or all of these factors may influence a harvesting prescription:

Site Degradation Watershed

Economic FisheriesOVERALL

SENSITIVITYWildlife Visual/Aesthetic

Soil Degradation is the focus of this field guide insert.

SOIL DISTURBANCE — is any abrupt change in the chemical, physical, orbiological properties of the soil.

SOIL DEGRADATION — is the reduction of long-term soil productivity bymanagement activities.

Soil Disturbance does not always equal Soil Degradation.

Therefore, only soil disturbance that reduces long-term productivity isconsidered soil degradation. Soil conservation guidelines recognize that sometypes of soil degradation are necessary for forest management (e.g., haulroads) and define disturbance types that are of concern because theyrepresent degradation or because they put other resource values at risk(e.g., water quality). Soil conservation guidelines are available from Ministry ofForests offices.

➤

➤

➤

➤

➤

➤

1

Fgi08.qxd 2/22/2000 7:38 PM

2

The five keys provided in this field guide insert with the assessment of theinherent sensitivity of a site to five major soil-degrading processes:

1. Compaction and puddling2. Displacement (including exposure of unfavourable subsoil and slope

hydrology changes)3. Forest floor displacement4. Surface erosion (by water and wind)5. Mass wasting

Definitions, controlling site factors, management considerations, and a hazardassessment key are provided for each process.

Management considerations influence the level of impact that actually occurs;the objective is to minimize soil degradation. Users should consult theappropriate field guide insert for assistance in formulating strategies tominimize impact (field guide inserts 1 for harvesting and 2 for site preparation).

Fgi08.qxd 2/22/2000 7:38 PM

3

PRE-HARVEST DATA NEEDS

Site and soil data must be collected to characterize map strata conditionsbefore treatment units for Pre-Harvest Silviculture Prescriptions are delineated.The data required to use the hazard assessment keys are summarized below.Methods for collecting and recording these data are described in regionalEcosystem Classification Field Guides; Describing Ecosystems in the Field,Ministry of Environment Manual 11, 2nd Edition (1990); and Site Mapping, DataCollection and Sensitivity Evaluation for Pre-Harvest Silviculture Prescriptions:Interior Sites, Ministry of Forests Land Management Handbook No. 25 (1993).

Climatic information: biogeoclimatic subzone/variantoccurrence/depth of dry soilsnow depth/compressibilityoccurence/depth of frozen ground

Slope information: slope gradientslope length/uniformityslope continuitypresence of slope stability indicators

Site hydrology information: gully spacingwater course spacingsoil moisture regimeocurrence/depth of seepage

Soil information: forest floor depth and dominant horizonAh horizon depthsoil texture and changes with depthcoarse fragment content (%) and changes with depthdepth to carbonatesdepth to bedrockdepth to unfavourable subsoildepth to restricting layer

The evaluation of degradation sensitivity of mineral soils involves workingthrough five hazard assessment keys — for compaction and puddling,displacement, forest floor displacement, surface erosion, and mass wasting.

Organic soils comprised of more than 40cm of wet, organic material orpeaty forest floors >40cm, are susceptible to rutting and puddling bydisplacement of their very low load-bearing strength materials.Consequently, organic soils have a high displacement hazard and a veryhigh puddling hazard.

3

Fgi08.qxd 2/22/2000 7:38 PM

4

COMPACTION AND PUDDLING

Compaction is the increase in soil density that results from the rearrangement ofsoil particles in response to applied external forces.

Puddling is the destruction of soil structure and the associated loss ofmacroporosity that results from working the soil when wet.

Site factors determining hazards: Management considerations:*

texture applied forcescoarse fragments - equipment (ground pressure)moisture regime - number of passesforest floor H horizon ≥20cm scheduling of operationsorganic soil scalping

slope (adverse, favourable)frozen soil >15cm deepcompressibility/depth of snow >1mseasonal soil moisture content(moist soil compacts, wet soil puddles)

Soil texture abbreviations:

S - sand LS - loamy sandSL - sandy loam fSL - fine sandy loamL - loam Si - siltSiL - silt loam SC - sandy claySCL - sandy clay loam SiC - silty claySiCL - silty clay loam C - clay

CL - clay loam

The COMPACTION AND PUDDLING HAZARD KEY (facing page) derives a hazardrating from an assessment of the combined influences of soil texture, coarsefragment content, moisture regime, thickness of the forest floor H horizon and soiltype (mineral or organic) on the load-bearing capacity of the soil.

Column 1 of the key is used for mineral soils with xeric to subhygric moistureregimes where the forest floor H horizon (the well-decomposed humic layer) is<20cm thick. Column 2 is used for mineral soils where the soil moisture regime issubhygric and the forest floor H horizon is ≥20cm thick; or the soil moisture regimeis hygric or wetter. Organic soils with >40cm of wet, organic material or peaty forestfloors >40 cm are rated as Very High Puddling Hazard.

* Management considerations are discussed further in field guide inserts 1 (forharvesting strategies) and 2 (for site preparation strategies).

Fgi08.qxd 2/22/2000 7:38 PM

5

COMPACTION AND PUDDLING HAZARD KEY

HAZARD RATINGb

SOIL TEXTUREa Xeric-subhygricc Subhygricd-subhydric(0–30cm) c(H horizons <20cm) d(H horizons ≥20cm)

Fragmentalcoarse fragments >70% L M

SandyS, LS LLoamySL, L M VHe

SiltyfSL, SiL, Si HClayeySCL, CL, SiCL,SC, SiC, C VH

a Use dominant soil texture and coarse fragment content of the upper 30cm ofmineral soil to assess compaction hazard. If a pronounced textural change occurswithin the upper 30cm (e.g., silty over sandy soil), then use the more limiting soiltexture, providing it amounts to 5cm of the top 30cm.

b L=Low; M=Moderate; H=High; VH=Very High.c Use this column for subhygric sites with forest floor H horizons <20 cm thick.d Use this column for subhygric sites with forest floor H horizons ≥20 cm thick.e Organic soils comprised of more than 40cm of wet organic material, or peaty

forest floors >40cm, are susceptible to rutting and puddling by displacement oftheir very low load-bearing strength materials. Consequently, organic soils have ahigh displacement hazard and a very high puddling hazard.

Coarse fragments <70%

Coa

rse

frag

men

ts <

70%

Fgi08.qxd 2/22/2000 7:38 PM

6

DISPLACEMENT

Displacement is the mechanical movement of soil materials by equipment andmovement of logs. It involves excavation, scalping, exposure of underlying materialsand burial of surface soils.

Three aspects of displacement can produce soil degradation:

• exposure of unfavourable subsoils

• redistribution and loss of nutrients

• alteration of slope hydrology


slope steepness logging systemslope complexity ground vs cablesoil depth to: - equipment size

- bedrock - use of snow- unfavourable subsoil - skid road spacing,- seepage pattern, gradient

soil chemistry site preparation- carbonates depth of scalping

Displacement Hazard Key Definitions:

Close gully spacing — 2 or more >2m deep, sharp-edged gullies occur per 100malong the contour. Gentler, rounder gullies are not a concern, since extraexcavation would not be involved in crossing such gullies with a bladedstructure.

Hummocky terrain — broken terrain with small, but steep-sided knolls or ridges,(e.g., eskers, rocky knobs and drumlins).

Unfavourable subsoils — includes subsoil conditions that produce unfavourablegrowing conditions when exposed by displacement. Does not includecarbonates, seepage or bedrock, which are rated separately in the key.Unfavourable subsoils include:- dense parent materials — compact glacial till, silt or clay glaciolacustrine

materials that cannot be readily dug into with a shovel (i.e., a pick or pulaski isrequired to loosen before digging).

- dense, clayey Bt horizons — clay-enriched subsoils of Luvisols >5cm thickand with clayey textures that cannot be readily dug with a shovel.

- granular materials with sand or loamy sand texture — granular materials with alow content of silt and clay, and low water-holding and nutrient-storing capacity.

- fragmental materials — subsoils comprised of >70% coarse fragments(i.e., fragments >2mm diameter).

Carbonate — a soil layer containing appreciable calcium carbonate (lime) in whichthe soil particles <2mm in diameter fizz when contacted with 10% HCl (muriaticacid); may have white coatings on coarse fragments; may have powdery whitedeposits in the soil.

Seepage — consider seepage only for subhygric, hygric and subhydric sites, asindicated by vegetation (site series). For these sites, estimate typical depth ofseepage by direct observation of seepage or water table (make allowances forrecent weather and spring break-up); or by inference, using soil colours, eithermottling or gleying.


Fgi08.qxd 2/22/2000 7:38 PM

7

DISPLACEMENT HAZARD KEYa

Mineral Soil1. Slopeb

Slope gradient (%) Points Slope gradient (%) Points

0 0 40 65 1 45 8

10 1 50 1015 2 55 1220 3 60 1625 3 65 2030 4 70 2635 5 75 32

2. Slope complexity

Terrain feature Slope Points

Close gully spacing:two or more >2 m deep sharp-edged gullies occur per 100malong the contour

<30% 2

30–45% 4

>45% 6

Hummocky terrain:broken terrain with small, but steep-sided knolls or ridges,(e.g., eskers, rocky knobs and drumlins) . . . . . . . . . . . . . . . . . . . +2

3. Subsoil conditions

USE THE MOST LIMITING FACTOR

Depth from bottom of LFH to: <30cm 30–60cm 61–90cm >90 cm

POINTSUnfavourable subsoil 8 4 2 0

Carbonate 12 8 4 0

Seepage 12 8 4 0

Bedrock 12 8 4 0

DISPLACEMENT HAZARD RATING: (point total)

Low Moderate High Very High<7 7–14 15–24 >24

Organic Soil

Organic soils with >40cm of wet, organic materials or peaty forest floors>40 cm thick have a High Displacement Hazard.

a The Displacement Hazard Key involves adding the points for slope gradient, slopecomplexity (gullied and/or hummocky terrain), and substrate conditions; the totaldetermines the rating.

b Use average slope to determine rating, but also consider the upper end of theslope range in formulating prescriptions.

Fgi08.qxd 2/22/2000 7:38 PM

8

FOREST FLOOR DISPLACEMENTForest floor displacement is the mechanical movement of the upper organicmaterials by equipment and movement of logs. It involves excavation, scalping,mineral soil exposure, and burial of the forest floor.Effects range from beneficial to detrimental, depending on site factors (e.g., mineralsoil characteristics) and degree of forest floor displacement (e.g., how far thedisplaced forest floor is from the seedlings).Two aspects of forest floor displacement can produce soil degradation:• redistribution and loss of nutrients• exposure of unfavourable rooting medium

Site factors determining hazards: Management considerations:*forest floor harvesting system

- type silvicultural system- depth site preparation

soil texture - typecoarse fragment % - patternslope steepness - prime moverslope complexity - implementsoil depth to: operating gradient

- unfavourable subsoil depth of scalping- bedrock operator experience- seepage instructions- carbonates seasonal soil moisture content

ground freezingcompressibility/depth of snow

Forest Floor Displacement Hazard Key Definitions:Close gully spacing — 2 or more >2m deep, sharp-edged gullies occur per 100m

along the contour. Gentler, rounder gullies are not a concern, since extraexcavation would not be involved in crossing such gullies with a bladed structure.

Hummocky terrain — broken terrain with small, but steep-sided knolls or ridges,(e.g., eskers, rocky knobs and drumlins).

Unfavourable subsoils — includes subsoil conditions that produce unfavourablegrowing conditions when exposed by displacement. Does not include carbonates,seepage or bedrock, which are rated separately in the key. Unfavourable subsoilsinclude:- dense parent materials — compact glacial till, silt or clay glaciolacustrine

materials that cannot be readily dug into with a shovel (i.e., a pick or pulaski isrequired to loosen before digging).

-dense, clayey Bt horizons — clay-enriched subsoils of Luvisols >5cm thick andwith clayey textures that cannot be readily dug with a shovel.

-granular materials with sand or loamy sand texture — granular materials with alow content of silt and clay, and low water-holding and nutrient-storing capacity.

- fragmental materials — subsoils comprised of >70% coarse fragments (i.e.,fragments >2mm diameter).

Carbonate — a soil layer containing appreciable calcium carbonate (lime) in which thesoil particles <2mm in diameter fizz when contacted with 10% HCl (muriatic acid);may have white coatings on coarse fragments; may have powdery white depositsin the soil.

Seepage — consider seepage only for subhygric, hygric and subhydric sites, asindicated by vegetation (site series). For these sites, estimate typical depth ofseepage by direct observation of seepage or water table (make allowances forrecent weather and spring break-up); or by inference, using soil colours, eithermottling or gleying.


Fgi08.qxd 2/22/2000 7:38 PM

9

FOREST FLOOR DISPLACEMENT HAZARD KEYa

LFHb <6cm 6–10cm >10≤20cm <10cm ≥10≤20 cm >20cm allAh <3cm <3cm <3cm 3–10cm 3–10cm <10cm >10cm(points) 12 8 6 6 4 4 0

Dominant Very Coarse Coarse Medium FineSoil Matrix(top 30 cm)(points) 8 4 2 8

Depth tounfavourablesubsoil,bedrock, <15cm 15–30cm 30–60cm >60cmseepage, orcarbonates(points) 12 8 2 0

Slopec >60% 30–60% <30%Gullied or hummocky terrain

(points) 6 3 0

RATINGS: >25 16–25 5–15 <5Very High High Moderate Low

a The Forest Floor Displacement Hazard rating is determined from the total of pointsadded up for: forest floor LFH depth/Ah depth; soil ‘‘matrix’’ (texture/coarsefragments); depth to unfavourable subsoil, bedrock, seepage, or carbonates; andslope/hummocky terrain.

b Not including rotten wood.c Use average slope to determine a rating, but in formulating precriptions also

consider the upper end of the slope range.

SOIL MATRIX

TEXTURE COARSE FRAGMENT CONTENT

<30% 30–70% >70%

S, LS, SL Coarse Very Coarse Very CoarsefSL, Si Medium Coarse Very CoarseSiL, Loam Medium Medium CoarseSC, SiC, SCL, Fine Medium MediumSiCL, CL, C

Soil texture abbreviations:

S - sand LS - loamy sandSL - sandy loam fSL - fine sandy loamL - loam Si - siltSiL - silt loam SC - sandy claySCL - sandy clay loam SiC - silty claySiCL - silty clay loam C - clay

CL - clay loam

Fgi08.qxd 2/22/2000 7:38 PM

10

SURFACE EROSION

Surface erosion is the wearing away of the earth’s surface by water and includessplash, rill and gully erosion. ‘Accelerated’ erosion is erosion that results from humanactivities, in excess of ‘geologic’ erosion. It causes on-site impacts (soil loss,nutrient loss, lower productivity) and off-site impacts (water quality, sedimentation,habitat impacts).


climate (R factor) logging system- rain intensity/duration ground vs cable- snowmelt - soil exposure

topography extent of forest floor removal- slope % road systemslope length site preparation

soil properties erosion control- texture soil moisture during operations- structure- coarse fragments- restricting layers

The Surface Erosion Hazard Key rates the susceptibility of exposed soil to watererosion (i.e., when protective vegetation and florest floor are removed).

Surface erosion of forest soils remains low when surface organic layers are intact.

After exposure of mineral soil, rates of erosion are initially high and drop off after thefirst year. To be effective, erosion control measures must be promptly applied.

Surface Erosion Hazard Key Definitions:

Rainfall (R) factor — integrates precipitation type, frequency, intensity and durationand the biogeoclimatic subzone/variant. Extract appropriate value from tables onpages 14 to 18.

Short slopes — <150m unbroken slope length between level or adverse slopes thatwill impede the continued flow of water.

Long slopes — ≥150m unbroken slope length between level or adverse slopes thatwill impede the continued flow of water.

Broken slopes — variable, complex or benchy slopes.

Water-restricting layer — restricting to downward flow of water, but not necessarily toroot growth. Includes impermeable, dense, compact or cemented layers;bedrock; or permanent water table.


Fgi08.qxd 2/22/2000 7:38 PM

11

SURFACE EROSION HAZARD KEYa

DEGREE OF CONTRIBUTION OF FACTORSSITEFACTORS LOW MODERATE HIGH VERY HIGH

CLIMATERainfall factor (R) <25 25–49 50–100 >100(points) 3 6 9 12

TOPOGRAPHYSlope gradient (%) 0–15 16–30 31–60 >60(points) 2 4 6 8

– – – – – – – – – – – – – – – – – – – – – – – –and Length/Uniformity short broken short uniform long broken long uniform(points) 1 2 3 4

DEPTH TO WATER-RESTRICTINGLAYER (cm) >90 61–90 30–60 <30(points) 1 2 3 4

SURFACE SOILDETACHABILITYb

(0–15 cm)Texture SC, C, SiC SiCL,CL,SCL SL, L Si,SiL,fSL,LS,S(points) 1 2 4 6

SUBSOILPERMEABILITY(16–60cm)Texture S,LS,SL,fSL L,SiL,Si CL,SCL,SiCL C,SC,SiC(points) 1 2 3 4

SURFACE EROSION LOW MODERATE HIGH VERY HIGHHAZARD RATING(point total) <16 16–22 23–29 >29

a The Surface Erosion Hazard Key involves adding the points for the six site factors;the total determines the rating.

b If two contrasting textures occur in the depth, use the most limiting texture with thehighest point rating.

Fgi08.qxd 2/22/2000 7:38 PM

12

MASS WASTING

Mass wasting is erosion due to gravity.

• Dry ravelling occurs on oversteepened slopes underlain by non-cohesive,granular and fragmental materials.

• Shallow, rapid failures — debris slides and debris avalanches — are of concernon slopes that have shallow soils overlying impermeable material (i.e., bedrock,till) above which zones of saturation are present or develop during periods ofrainfall or snowmelt.

• Deep-seated, slow failures — slumps and slump-earthflows — occur on deepercohesive materials (i.e., clayey till, lacustrine).


climate (R factor) logging system- rain intensity/duration - ground vs cable- spring break-up - extent of cut and fill

topography roads- slope % - width, cut height, sidecast- slope length - drainage structures- slope continuity - maintenance

soil properties - deactivation- texture- coarse fragments- soil moisture regime- restricting layers

Excavation-related slides may be triggered by removal of support, oversteepening offill/sidecast slopes, overloading, and concentration of drainage waters. Clearcut-related mantle failures may be caused by loss of root strength and changes in slopehydrology.

Mass Wasting Hazard Key Definitions:

Rainfall (R) factor — integrates precipitation type, frequency, intensity and durationand the biogeoclimatic subzone/variant. Extract appropriate value from tables onpages 14 to 18.

Continuous slopes — ≥150m slope length between slope segments at least 20mwide and <30% slope gradient, or between ridge crests and valley bottoms.

Discontinuous slopes — <150m slope length between slope segments at least 20mwide and <30% slope gradient (i.e., variable, complex or benchy slopes), orbetween ridge crests and valley bottoms.

Gullied — two or more >2m deep sharp-edged gullies occur per 100m along thecontour.

Water-restricting layer — restricting to downward flow of water, but not necessarily toroot growth. Includes impermeable, dense, compact or cemented layers;bedrock.

Texture groupings — Sandy = S, LS; Clayey = SC, SiC, SCL, SiCL, CL, C. Usepredominant textural group overlying the restricting layer or the most limiting soiltexture in the profile.


Fgi08.qxd 2/22/2000 7:38 PM

13

MASS WASTING HAZARD KEY

A. If the site contains active slides or the initiation zone of historicslope failuresa, THEN mass wasting hazard = VERY HIGH

B. If the site is underlain by non-cohesive materials – i.e., sands (S,LS), gravels,volcanic pumice or fragmental material with >70% coarse fragment content(e.g., rubbly talus), THEN dry ravelling is the process of concern.b

SLOPE (%): <30 30–45 46–60 >60

MASS WASTINGHAZARD RATING: LOW MODERATE HIGH VERY HIGH

C. OTHERWISE, the risk of slope failures and excavation-related cut and fillslope failures for the more cohesive materials is assessed by adding thepoints determined by the following site factors:c

R FACTOR <25 25–49 50–100 >100(points) 0 5 10 20

SITE Very xeric- Subhygric-MOISTURE submesic Mesic hygric Subhydric(points) 0 5 10 20

SLOPE (%) <30 30–40 41–50 51–60 61–70 >70(points) 0 2 4 8 16 32

Slopes <30% Slopes≥30%

Discontinuous Continuous GulliedLENGTH/CONTINUITY(points) 0 4 6 8

SOIL Sandyd SL, L, fSL Si, SiL ClayeyTEXTURE(points) 3 5 8 12

DEPTH TOWATER-RESTRICTINGLAYER (cm)

>90 61–90 30–60 <30

(points) 0 2 4 6

MASS WASTING HAZARD RATING: e

LOW = <15 MODERATE = 15–30 HIGH = 31–45 VERY HIGH = >45

a Evidence of old slides includes contrasting strips of seral vegetation not associatedwith snow avalanching, tilted or jack-strawed trees, tension cracks, headscarps,rotational failures — see Land Management Handbook 25.

b Part B of the Key rates dry ravelling hazard directly from slope steepness.c Part C of the Key involves adding points for the six site factors; the total determines

the rating.d Where sandy (especially finer sands) soils have moisture conditions that vary

seasonally from wet (moist) to dry, it may be necessary to use Parts B and C of theKey.

e Where evidence of potential slope instability exists, such as accelerated soilcreep, buried or absent soil profiles, curved trees, etc., the hazard rating should beincreased by one level — see Land Management Handbook 25. If Mass WastingHazard is Very High, seek further geotechnical advice before proceeding withdevelopment.

Fgi08.qxd 2/22/2000 7:38 PM

14

Rainfall (R) Factors for the Cariboo Forest Region

<25 25–49 50–100 >100

all BGPPxh

IDFxm IDFmwIDFxwIDFdk

MSxk MSxc

SBSdw SBSmhSBSmc* SBSmc*

SBSmwSBSwk

SBPSxc SBPSmcSBPSdc SBPSmk

ESSFxv ESSFwcESSFwk* ESSFwk*

ICHdkICHmkICHwk* ICHwk*

* These subzones encompass two R factor ranges. Use local experience in decidingthe appropriate R factor to apply in the keys.

Fgi08.qxd 2/22/2000 7:38 PM

15

Rainfall (R) Factors for the Kamloops Forest Region

<25 25–49 50–100 >100

all BGPPxh

IDFxh IDFmwIDFxw IDFwwIDFdkIDFdm

MSxk MSdcMSdmMSmm

SBSdh SBSmmSBSdw

SBPSmk* SBPSmk*

ESSFxc ESSFdc ESSFwc ESSFvcESSFdv ESSFwm ESSFvv

ICHmk ICHvkICHmwICHwk* ICHwk*

CWHdsCWHms

* These subzones encompass two R factor ranges. Use local experience in decidingthe appropriate R factor to apply in the keys.

Fgi08.qxd 2/22/2000 7:38 PM

16

Rainfall (R) Factors for the Nelson Forest Region

<25 25–49 50–100 >100

PPdh1PPdh2

IDFdm1IDFdm2IDFunIDFxh1

MSdkMSdm1

ESSFdc1 ESSFwc1 ESSFvcESSFdk ESSFwc2

ESSFwc4ESSFwm

ICHdw ICHvk1ICHmk1 ICHwk1ICHmw1ICHmw2ICHmw3ICHxw

Fgi08.qxd 2/22/2000 7:38 PM

17

Rainfall (R) Factors for the Prince George Forest Region

<25 25–49 50–100 >100

SBSdh SBSmh SBSvkSBSdk SBSwkSBSdw SBSmwSBSmk1 SBSmk2SBSmc* SBSmc*

BWBSdkBWBSmw2 BWBSmw1BWBSwc3 BWBSwc1&2

ESSFmm ESSFwc ESSFvcESSFmvESSFwk* ESSFwk*

all SWB

ICHmc ICHvkICHmmICHwk* ICHwk*

* These subzones/variants encompass two R factor ranges. Use local experience indeciding the appropriate R factor to apply in the keys.

Fgi08.qxd 2/22/2000 7:38 PM

18

Rainfall (R) Factors for the Prince Rupert Forest Region

<25 25–49 50–100 >100

SBSdkSBSmc* SBSmc*

SBPSmc

BWBSdk2BWBSmw2

ESSFmc ESSFwvESSFmk

all SWB

ICHmc ICHvc

CWHws1&2

MHm

* These subzones/variants encompass two R factor ranges. Use local experience indeciding the appropriate R factor to apply in the keys.

Fgi08.qxd 2/22/2000 7:38 PM

hazard assessment keys for evaluating site sensitivity to ... · e o rg an ics olm ped f th 40 w ,...

Documents