L A N D M A N A G E M E N T H A N D B O O K

Forest Management on Fans Hydrogeomorphic Hazards and General Prescriptions

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Ministry of Forests Forest Science Program

D.J. Wilford, M.E. Sakals, and J.L. Innes

Forest Management on Fans Hydrogeomorphic Hazards and General Prescriptions

Ministry of Forests Forest Science Program

Library and Archives Canada Cataloguing in Publication DataWilford, D. J. (David J.), 1950-

Forest management on fans : hydrogeomorphic hazards and general prescriptions

(Land management handbook, 0229-1622 ; 57)

Includes bibliographical references: p. 0-7726-5346-1

1. Mass-wasting - British Columbia - Forecasting. 2. Landslide hazard analysis - British Columbia. 3. Forests and forestry - Environmental aspects -British Columbia. 4. Forest management - British Columbia - Planning. 5. Hydrology, Forest - British Columbia. 6. Alluvial fans - British Columbia. 7. Colluvium - British Columbia. I. Sakals, M. E. II. Innes, John L. III. BritishColumbia. Forest Science Program. IV. Title. VI. Series.

424.54 2005 634.9'61 2005-960094-2

CitationWilford, D.J., M.E. Sakals, and J.L. Innes. 2005. Forest management onfans: hydrogeomorphic hazards and general prescriptions. B.C. Min.For., Res. Br., Victoria, B.C. Land Manage. Handb. No. 57.<http://www.for.gov.bc.ca/hfd/pubs/Docs/Lmh/Lmh57.htm>

Prepared byD.J. WilfordB.C. Ministry of ForestsSmithers, B.C.andM.E. SakalsBulkley Valley Centre for Natural ResourcesResearch and ManagementSmithers, B.C.and J.L. InnesFaculty of ForestryUniversity of British ColumbiaVancouver, B.C.

© 2003 Province of British Columbia

Copies of this report may be obtained, depending upon supply, from: Crown Publications521 Fort StreetVictoria, BC

(250) 386-4636www.crownpub.bc.ca

For more information on Forest Science Program publications, visit ourWeb site at: http://www.for.gov.bc.ca/hfd/pubs/index.htm.

ii

The use of trade, firm, or corporation names in this publication is for the information andconvenience of the reader. Such use does not constitute an official endorsement orapproval by the Government of British Columbia of any product or service to the exclusionof any others that may also be suitable. Contents of this report are presented for discussionpurposes only. Funding assistance does not imply endorsement of any statements or informationcontained herein by the Government of British Columbia.

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Forested alluvial and colluvial fans can be runoutzones for debris flows and debris floods, and are sub-ject to floods. Forest management activities on areasof fans with this hydrogeomorphic activity can exacerbate the effects of these events and lead to sub-stantial damage to infrastructure such as roads andbridges, productive forest sites, and fish habitat. Thishandbook presents a six-step hazard recognition

scheme that enables forest practitioners to prepareappropriate strategies and prescriptions. The six steps are: fan identification in an operating area, pre-typing watersheds, aerial photograph interpretation,fieldwork, prescription development, and monitoring.The scheme is applicable to forested fans throughoutBritish Columbia.

ABSTRACT

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This handbook represents 5 years of collaborative research involving many people throughout BritishColumbia. The initial call for research to improveforest management on fans came from Ted Wilson(Terrace) and Dave Rebagliati (Houston). Co-opera-tors in the research included Skeena Cellulose Inc.(Terrace), West Fraser Mills Ltd. (Terrace andSmithers), Fisheries and Oceans Canada (Smithers),Silvicon Services Inc. (Smithers), the B.C. Ministry of Water, Land and Air Protection (Smithers), theB.C. Ministry of Forests (provincially), Grainger and

Associates Consulting Ltd. (Salmon Arm), and theUniversity of British Columbia Faculty of Forestry.This handbook has gone through many editions overthe past 5 years as we have presented workshops, butthis version has had the benefit of very thorough re-views by Matthias Jakob, Bill Grainger, DavidMaloney, Tom Millard, and Steve Webb. We are in-debted to the B.C. Ministry of Forests, ForestRenewal BC, Forest Investment Account, and theUniversity of British Columbia Faculty of Forestryfor financial support.

ACKNOWLEDGEMENTS

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TABLE OF CONTENTS

Abstract. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2 Definition of Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

3 Forest Management Experience on Fans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

4 Fan Identification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

5 Pre-typing Watersheds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

6 Aerial Photograph Interpretation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126.1 Forest cover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126.2 Multiple channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126.3 High sediment load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126.4 Abruptly disappearing stream . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126.5 Abrupt change in stream direction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166.6 Major sediment source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

7 Fieldwork . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

8 Prescription Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

9 Monitoring Prescriptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

10 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

1 Forest practices and affected features associated with hydrogeomorphic events . . . . . . . . . . . . . . . . . . . . . . 3

2 A summary of forestry prescriptions that exacerbated hydrogeomorphic events on the 55 study fans with forestry activities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3 Predictive models for the dominant hydrogeomorphic process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

4 Watershed attributes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

5 Predictive models for power and disturbance extent of hydrogeomorphic processes in west-central British Columbia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

6 Predictive models for the number of hydrogeomorphic events during the past 50 years in west-central British Columbia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

7 Pre-logging aerial photographic features and relation to forest management issues on fans . . . . . . . . . . . . 12

8 Characteristics of hydrogeomorphic process deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

9 Field indicators of hydrogeomorphic activity within the hydrogeomorphic riparian zone . . . . . . . . . . . . . 20

10 Occurrence of selected field indicators of hydrogeomorphic activity on the 61 study fans with riparian forests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

11 A summary of hydrogeomorphically appropriate forestry prescriptions that address common problems encountered on fans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

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1 An example of a topographically defined watershed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2 The incised stream channels in this watershed are associated with extensive environmentally sensitive areas for slope stability that are producing sediment directly to the streams . . . . . . . . . . . . . . . . . 11

3 An aerial photograph of a 100-year-old cohort following a stand-level flood . . . . . . . . . . . . . . . . . . . . . . . . 13

4 Multiple channels are present on this fan. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

5 High sediment loads are being transported by this stream, as evidenced by a change in channel morphology of the main stream downstream of the fan, and by mid-channel bars and braided reaches in the stream channel on the fan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

6 The stream channel disappears from view on the aerial photograph, indicating a broadcasting of flows under the forest and/or multiple channels that do not have the power to clear a swath through the forest stand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

7 A large debris jam in the main channel led to the formation of a second channel, which initiates at an abrupt angle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

8 An example of major, natural sediment sources near the mouth of a watershed. . . . . . . . . . . . . . . . . . . . . . 18

9 Scars can be used to date the year and season of hydrogeomorphic events . . . . . . . . . . . . . . . . . . . . . . . . . . 21

10 Sediment deposited behind a log has formed a “log step” . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

11 An example of a buried tree. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

12 A log retaining wall storing a considerable volume of sediment and maintaining the stream channel location . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

13 A “recent” deposit of sediment within the hydrogeomorphic riparian zone with limited organic accumulation and vegetative cover. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

14 Roots provide reinforcement to the soil mass against the erosional effects of broadcast flows . . . . . . . . . . 24

15 Erosion of sediment deposited around trees can expose adventitious roots. . . . . . . . . . . . . . . . . . . . . . . . . . 24

16 Tree holes are the result of rotting following deep burial of tree stems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

17 A young cohort of spruce and hemlock growing on the sediment of a high-power stand-level disturbance debris flood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

18 A cohort of hemlock growing on sediment deposited by a low-power debris flood . . . . . . . . . . . . . . . . . . . 26

19 A streambank with two buried soil horizons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

20 Scattered boulders indicate debris flow activity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

21 This suspended boulder indicates a debris flow depth of >2 m. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

22 Sediment and debris on a tree indicate high flows in this area. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

23 A close-up of an increment core from a spruce showing abrupt reduced growth that began in 1903 and continued until 1915. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

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1 INTRODUCTION

A fan is a cone-shaped deposit of sediment formedwhere a stream emerges from the confines of amountain (Bull 1977). Sediment originates in asource-area watershed and is transported to a fan byhydrogeomorphic processes (floods, debris floods,and debris flows) (Hungr et al 2001). In some cases,most deposited sediment is linked to glacial melt atthe end of the last Ice Age (Ryder 1971a, 1971b;Church and Ryder 1972; Ritter et al. 1993). In othersituations, it is apparent that fans are actively grow-ing (Beaty 1970). The cause of this growth is contemp-orary hydrogeomorphic activity, which is generallyneither rare nor extreme (Innes 1985; Jakob and Jor-dan 2001). To illustrate this point, a random sampleof 51 fans was investigated with regard to fan-shapingprocesses in an area with a mix of plateaus andmountainous terrain (the Bulkley Timber SupplyArea in west-central British Columbia). While thisarea would not be characterized as overly unstable,contemporary hydrogeomorphic activity was ob-served on 41 fans, indicating that 82% of the fans haddisturbances occurring on at least a portion of the fansurface in the last 50 years. Random sampling wascarried out on 55 fans in the southern Coast ForestRegion of British Columbia. Recent hydrogeomorphicactivity was found on 89% of the fans (T. Millard,B.C. Ministry of Forests, pers. comm.)

Forest management on areas of fans with hydro-geomorphic activity can exacerbate the effects ofnatural hydrogeomorphic processes and can lead tosubstantial damage to infrastructure such as roadsand bridges, productive forest sites, and fish habitat(Wilford et al. 2003). Recognition of hydrogeomorphic

hazards on fans is essential for cost-effective and en-vironmentally sustainable forest management. Inaddition, it is an offence under the British ColumbiaForest and Range Practices Act to destabilize fan sur-faces on the coast (Section 54), or to have an impacton forest soils or fish habitat on the coast and in theinterior (Sections 35 and 57). For forest managers toachieve compliance under the British Columbia Forestand Range Practices Act () it is necessary to rec-ognize hydrogeomorphic hazards on fans and developappropriate prescriptions.

This report is based on a detailed study of 65 fansin west-central British Columbia: 55 fans had somedegree of forestry activity and 10 were encountered in a natural state during the investigation (Wilford2003). A six-step hazard recognition scheme forforestry on fans is presented: identification of fans(Section 4), pre-typing watersheds (Section 5), aerialphotograph interpretation (Section 6), fieldwork(Section 7), prescription development (Section 8),and monitoring (Section 9). The scheme is applicableto fans throughout British Columbia. Experiencegained through 1-day training sessions held aroundthe province is that forest practitioners (includingseasoned forest technicians, foresters, geoscientists,and engineers) can apply the scheme with little addi-tional time, and that the cost is small compared tothe consequences of destabilizing fan surfaces or los-ing forestry infrastructure (e.g., roads and bridges).

Before proceeding, it is important to define someterms (Section 2) and recognize the degree to whichconventional forestry practices on fans can be prob-lematic (Section 3).

2

Fans can be runout zones for debris flows and debrisfloods, and be subject to floods. It is important todifferentiate these hydrogeomorphic processes be-cause debris flows can have peak discharges 40 timesgreater than anticipated peak streamflows (floods),and debris floods can have peak discharges up tothree times the anticipated peak streamflows (Hungret al. 2001; Jakob and Jordan 2001).

Hydrogeomorphic processes are differentiatedbased upon their water/sediment concentration dur-ing transport, and upon features of their deposits.Debris flows are complex, heterogeneous sediment–water mixtures with concentrations between 70 and90% by weight that resemble wet concrete whilemoving, and characteristically result in the formationof marginal levees and terminal lobes (Costa 1984,1988; VanDine 1985; Smith 1986; Pierson and Costa1987; Wells and Harvey 1987; Hungr et al. 2001). Debris floods, also referred to as hyperconcentratedflows, occur when essentially all material on thestreambed surface is mobilized but the mixing is notcomplete (there is a rapid increase in solids concen-tration towards the bed). Debris floods have sedimentconcentrations between 40 and 70% by weight, andsediment deposits are bars, fans, sheets, and splays(Costa 1988; Hungr et al. 2001). Floods in gravel, cob-ble, or boulder channels rarely mobilize the entirebed. Floods have sediment concentrations between 1 and 40% by weight, and sediment deposits are bars,fans, sheets, and splays (characteristically not as ex-tensive relative to channel dimensions or as thick asdebris floods) (Wells and Harvey 1987; Costa 1988;Hungr et al. 2001).

Where evidence of more than one process is foundon a fan, the dominant process should be determinedbased on the following order: debris flow, debris flood,and flood. The order is based on the relative peak dis-charges and the consequences for drainage structures.

Hydrogeomorphic events that clear a swath in the original forest cover are termed “high-power”events (Wilford 2003). If the clearings or cohorts(stands of trees established as a result of the disturb-ance from events) are wider than 20 m, the event istermed a “stand-level” event and should be visible on 1:20 000 aerial photographs. If the clearing is lessthan 20 m wide, the event is termed a “site-level”

event. Hydrogeomorphic events that have an impacton the understory while not immediately removingthe forest canopy are termed “low-power” events.Burial of trees by low-power events may eventuallylead to the death of a stand; however, unlike high-power events, debris from the original forest remainson the site. Thus, there are three power and disturb-ance extent categories of hydrogeomorphic events:stand-level, site-level, and low-power events. Whileall categories can lead to problems with forestry ac-tivities, the nature of the problems varies.

Stand-level and site-level events have the power to remove or significantly damage drainage struc-tures, roads, and riparian reserves. The difference isin the width of area influenced. Low-power eventscan plug drainage structures, cause localized damageto roads, and inundate riparian areas. All hydrogeo-morphic processes can produce stand- and site-leveldisturbances, although in our study area, only floodsand debris floods were found to be associated withlow-power disturbances.

The concept of riparian areas to maintain fishhabitat has been well established (Beschta et al. 1987;Bisson et al. 1987; Bilby and Ward 1991; Bjornn andReiser 1991; Murphy and Meehan 1991; British Co-lumbia Ministries of Forests and Environment 1995a;Naiman et al. 2000). However, for streams on fans,there is a need to expand the concept of riparianareas. Streams on fans are not confined by bedrockand, while reaches can be entrenched, it is commonfor streams to overflow their banks, particularlywhen hydrogeomorphic events occur. When waterand sediment leaves stream channels on fans, the riparian forests play a significant role in storing sedi-ment, limiting the potential for the development ofnew channels (avulsions), and maintaining the streamin its channel. We refer to these as the hydrogeomor-phic roles, and to the zone as the hydrogeomorphicriparian zone. Section 7 presents common site indi-cators for the zone. The width of this zone can rangefrom less than 10 m to hundreds of metres. Only onecase was found where the whole fan surface was in-cluded within the hydrogeomorphic riparian zone.1

Maintenance of the forest and its associated debris inthe hydrogeomorphic riparian zone is a critical ele-ment in maintaining fan stability.

2 DEFINITION OF TERMS

1 An implicit assumption is that within a forestry timeframe (e.g., 50 to 100 years) the hydrogeomorphic zone will not change; however,there is a risk that future hydrogeomorphic events could influence areas beyond the contemporary zone (as defined by trees that are atleast 100 years old).

3

A detailed study was conducted in west-centralBritish Columbia, which included 55 fans withforestry activity, ranging from a single road to com-plete clearcutting (Wilford et al. 2003). Duringinterviews with forest practitioners regarding forestmanagement issues on fans, it was apparent that fewof them placed road or logging problems in a land-form context. While terrain stability maps havebecome a standard planning tool in British Columbia(B.C. Ministries of Forests and Environment 1999),landform maps (Howes and Kenk 1997) are rarelyused. Thus, it was a surprise to many forest practi-tioners that most road and riparian issues in theirmanagement units were on fans. In most cases, fansare considered as low-gradient, valley-bottom sites,and are not given any special attention for road orlogging planning. As a result, it is relatively commonfor forestry prescriptions to aggravate the extent ofdisturbance from hydrogeomorphic events (Table 1)(Wilford et al. 2003).

Impacts were classified as major if damage wassignificant; that is, if: • major drainage structures required replacement, • at least 100 m of road were eroded,

• streams were affected to the point where there wasno or very poor fish habitat (Johnston and Slaney1996), or

• more than 1 ha of plantations or forest stands wereburied in sediment or eroded.

Impacts were classified as limited if:• there was localized erosion of roads and limited

impacts to drainage structures, plantations, forestsites, and fish habitat. Drainage structures were present on all 55 fans,

and impacts were observed on 29 fans, primarily dueto the inability of the structures to carry bedload andwoody debris associated with hydrogeomorphicevents. Most drainage-related impacts were major (23 out of 29), with damage to the structures, roads,and fish habitat. Roads were present on 53 fans be-cause in two cases the roads climbed up the adjacenthillslopes to a drainage structure at the apex. Road-related impacts were observed on 24 fans, and 18 fanshad major impacts. The leading cause of road-relatedimpacts was roads that had a climbing grade tostream crossings. When drainage structures failed,water and sediment were channelled down roads,leading to erosion, the creation of new channels, and

3 FOREST MANAGEMENT EXPERIENCE ON FANS

Forest practices and affected features associated with hydrogeo-morphic events. Percentages are presented by the total in eachcategory.

Total in each Impactscategory identified

Fans with forestry activities 55 (100%) 41(74.5%)

Fans with specific forestry practiceDrainage structure 55 (100%) 29 (53%)Roads and ditches 53a (100%) 24 (45%)Riparian logging 24 (100%) 23 (96%)b removal or burned 4 (100%) 4 (100%)Channel excavation 4 (100%) 4 (100%)Mass wasting related to forestry activities 3 (100%) 1 (33%)

Affected featuresRoads 53 (100%) 32 (60%)Plantations and forest sites 37 (100%) 28 (76%)Drainage structures 55 (100%) 23 (42%)

Fish habitat 26 (100%) 15 (58%)

a While all study fans had drainage structures, two fans had drainage structures at the apex, and the roads were on the adjacent slopes, not the fans.

b = large woody debris.

4

the broadcasting of sediment beyond what wouldhave occurred without the road. Clearcutting of rip-arian forests was observed on 24 study fans, and 23had some degree of impact. The impacts consisted of damage to plantations from broadcast sediment,channel widening, and stream avulsions. While someof the logging predates the British Columbia ForestPractices Code () riparian guidelines (B.C. Ministries of Forests and Environment 1995a), therequirement for riparian reserves and managementzones is based on the presence of fish or connectionto fish habitat. Since only some study fans hadstreams with fish, it is likely that reserves would nothave been left even under the . As no directstreambank damage was observed, the impacts wereattributed to tree removal and large woody debris re-moval that reduced the sediment storage capability of the riparian zone and the surface roughness foroverbank (broadcast) water flows. The removal oflarge woody debris from stream channels was ob-served on only four fans, but in all cases there wereimpacts: three were major impacts, with mobilizationof bedload, widening of channels, and reduced fishhabitat complexity. Stream channels were excavatedon four fans to install drainage structures or as “pre-ventative maintenance,” and all resulted in majorimpacts to roads, fish habitat, channel stability, anddrainage structures. Even though fans were selectedthat had no or very limited forestry activities in their

watersheds, three watersheds had landslides due toforestry activities: one fan experienced major impactsto plantations and roads from a channel avulsion as aresult of the landslides.

By exacerbating hydrogeomorphic events, forestrypractices led to impacts to forestry infrastructure,productive forest land, and fish habitat. Impacts werenoted on 32 of the 53 fans with roads; 22 sustainedmajor damage that required repairs to at least 100 mof road length. Plantations were present on 37 fans,and impacts were noted on 28; impacts were majoron 22 fans, with more than 1 ha on each being buriedin sediment or eroded. Drainage structures were present on 55 fans and affected on 23; 19 structures received major damage and required replacement. Fishhabitat was present on 26 fans and impacts were sus-tained on 15; 14 had major impacts to the point therewas no or very poor fish habitat.

Clearly, on a significant number of the study fans,conventional forest practices are not cost-effectiveand are having an impact on natural resources (Table2). Similar observations have been made throughoutthe province, with four deaths attributed to a washed-out approach to a forestry bridge on a fan in thePrince George area. For forest managers to achieve compliance it is necessary to recognize hydro-geomorphic hazards on fans and develop appropriateprescriptions.

5

A summary of forestry prescriptions that exacerbated hydrogeomorphic events on the 55 study fans with forestry activities

Prescription Summary comment

Roads climb to stream crossings Road becomes an avulsion channel if drainage structure is blocked. Observed on 39 fans: aggravated events in 26 cases (66%) and roads were affected in 29 cases (74%).

Roads at slope break in stream/ Drainage structures fail due to sediment aggradation. Observed on nine fans and all hadfan (from steep to gentle) drainage structure failures.

Inadequate ditchblocks and Roads and ditches become stream channels with the lack of adequate cross-drainage.cross-drains as roads traverse fans Observed on 15 fans with problems: eight were linked to failure of main drainage

structures, five to broadcast flow interception, one to subsurface interception, and one eroded during a high flow.

Multi-span drainage structures Interfere with bedload movement, leading to sediment aggradation. Trap woody debris. Observed on four fans. All have been replaced due to blockage by bedload.

Inadequate drainage structures Drainage structures that are too small for, or damaged by, hydrogeomorphic events, and alter channel hydraulics (wrong cross-sectional shape), leading to channel scour. Observed on 30 fans (54% of the study fans and 73% of the fans with impacts).

Channel excavation - no rip-rap Unsupported channel excavations erode upstream, leading to sediment accumulation indrainage structures. Observed on five fans: major impact on two and localized impacts on three.

Inadequate drainage structures A common problem where roads cross in zones of multiple channels. At the time of in multiple-channel situations construction, drainage structures are installed based on the flow volumes. Volumes can

change significantly if the proportion of flow changes. Multiple channels were observedon 16 fans, with nine having road crossings in the zone of multiple channels. Five (55%)had impacts when the proportion of flow changed.

Non-engineered structures Dikes and berms built of local streambed materials can be readily eroded. Observed onfour fans and all have either failed, or will shortly fail, to achieve their objectives.

Roads on fans are not deactivated Roads on fans that are not used or maintained regularly and not deactivated exacerbatenatural hydrogeomorphic events by intercepting subsurface flows, redirecting and channelling surface flows, and creating avulsions due to plugged drainage structures. Observed on nine fans: major impacts on seven and localized problems on two.

Logging of the hydrogeomorphic Removes the forest influence: sediment storage, maintenance of channel location, andriparian zone reinforcement of the soil mass to avulsions. Observed on 24 fans: channel widening or

damage to plantations occurred on 23 (96%).

Skid trails on fans intercept and Particularly an issue with bladed skid trails that climb toward the stream. Observed on

concentrate flows seven fans: major disturbances on three and limited disturbances on four.

6

The first step in the hazard recognition scheme is to identify fans in an operational planning area. If terrain (landform) maps are available, fans are iden-tified as either Ff (fluvial fan) or Cf (colluvial ordebris flow fan) (Howes and Kenk 1997). If landformor soil association maps are not available, refer totopographic maps and identify streams that flow intothe main valleys of the planning area. Fans formwhere streams lose confinement unless streams di-rectly drain into higher-order streams with sufficientpower to remove the deposited sediment. Refer to

aerial photographs and check for cone-shaped de-posits (although the classical fan shape is not alwayspresent). Steep-gradient floodplains may also be pre-sent in wider reaches of confined streams, and thefield features presented in Section 7 may prove help-ful in identifying zones of activity beyond thechannel. If no fans are identified in the planning area, confirm this in the field: there should be no evidence of an active hydrogeomorphic riparian zone beyond the confined banks.

4 FAN IDENTIFICATION

7

Watersheds are pre-typed using morphometric(topographic) measurements from topographic mapsor a Geographic Information System () to predictthe hydrogeomorphic process influencing a specificfan. The classification of hydrogeomorphic processesinto floods, debris floods, and debris is based on therelative water and sediment concentrations, andcharacteristics of movement and deposits. The prac-tical significance of the different processes is thatdebris flows can produce peak discharges up to 40times greater than anticipated flood flows, and debrisfloods can have peak discharges 3 times flood flows(Hungr et al. 2001). This is of major significance sincethe common approach to designing drainage struc-tures in the forest sector of British Columbia is to usecalculated peak water flows (50- or 100-year designfloods) (B.C. Ministries of Forests and Environment1995a). Recognition of the hydrogeomorphic processinfluencing a fan is a key step in developing appro-priate forest management prescriptions.

The first step in determining the hydrogeomorphicprocess influencing a fan is to delineate the watershedboundary on a topographic map. Figure 1 presents anexample. The lowest point in the watershed is alsothe apex (top) of the fan.

Identifying the hydrogeomorphic process involvescalculating the Melton ratio (Melton 1957). The Meltonratio is watershed relief divided by the square root ofthe watershed area (with measurements commonlyin km and km2). The Melton ratio is a surrogate forwatershed slope, a factor that is directly related tostreampower and debris flow propensity. The Meltonratio has been used in several areas of British Colum-bia to differentiate flood and debris flow watersheds(Jackson et al. 1987; Bovis and Jakob 1999; Boyer1999). It is likely that for much of British Columbia,watersheds with Melton ratios of less than 0.3 areflood watersheds. In west-central British Columbiaan additional measurement, watershed length, wasused to differentiate debris flood and debris flow wa-tersheds (Wilford et al. 2004b). Watershed length isthe planimetric straight-line distance from the fanapex to the most distant point in the watershed(measured in km) (in some cases portions of this line are not contained within the watershed). Table 3presents class limits for differentiating the hydrogeo-morphic processes.

5 PRE-TYPING WATERSHEDS

An example of a topographically defined watershed.

8

Limitations to predictive hydrogeomorphic processmodels: • the predictive models were developed for specific

geographic areas and may not universally apply. • the Melton ratio has limited applicability for

watersheds in plateau terrain or hanging valleys,where the lowermost reaches are much steeperthan the average channel gradient, and

• the Melton ratio has limited applicability in situ-ations where steep tributaries discharge into themainstem near the fan apex.A case in point is Yard Creek near Salmon Arm,

B.C. It has a 113 km2 watershed with a Melton ratio of 0.086, and a large low-gradient fan, suggesting thatflooding is the principal fan-forming process. How-ever, debris flow lobes and scattered boulders up to 4 m in diameter were observed in the upper portionof the fan. The obvious reason for the apparent dis-crepancy between the Melton ratio and field observa-tions is a steep tributary just upstream of the fan apexthat is prone to debris flows. This example shows thatmorphometric variables such as the Melton ratio andwatershed length can be used as tools for pre-typingwatersheds for hydrogeomorphic process hazards,but that field verification is required.

Having identified the potential hydrogeomorphicprocess, the next step involves determining the power,disturbance extent, and frequency of events. Modelsto predict the power and disturbance extent cate-gories for hydrogeomorphic processes and the numberof events in the past 50 years were developed forwest-central British Columbia (Wilford 2003). Themodels use variables that can be determined from a and were selected to enable forest practitionerswithout a strong background in geomorphology or

hydrology to generate reasonably accurate estimatesof the characteristics of hydrogeomorphic processes(Table 4). The models are presented in Tables 5 and6. While these models are not applicable to otherareas without further testing, the variables are worthyof consideration when pre-typing watersheds.

Watershed area is the most significant variable for predicting the power and disturbance extent offloods and is the most important variable in all re-gional hydrological models. This is also intuitive:larger watersheds produce greater volumes of waterthat will be delivered to the apex of the fan (Murpheyet al. 1977). The density and length of stream chan-nels in a watershed can be observed on maps, andthese directly influence the routing of water and sedi-ment (Carlston 1963; Patton and Baker 1976). Thehypsometric integral relates elevation to watershedarea (Strahler 1952) and can be used as an indicator of a range of factors from peak flows, through snowaccumulation and melt, to overall channel steepness.A well-graded watershed has an integral close to 0.5.A watershed that is on a plateau and drops to a lowerelevation at its mouth has a high hypsometric inte-gral (e.g., closer to 1.0), while a watershed that isgenerally at the same elevation except for a limitedarea at high elevation has a low integral (e.g., closerto 0.0). The predictive model presented in Table 6for the number of stand-level debris floods indicatesa direct relationship between the hypsometric inte-gral and the number of events. The relief ratio iswatershed relief divided by watershed length (Strahler1958). Along with other factors such as the extent ofexposed bedrock and vegetative cover, watershedswith high peak flows are characteristically short andhave high relief (i.e., a high relief ratio) (Costa 1988).

Predictive models for the dominant hydrogeomorphic process

Hydrogeomorphicprocess Class limits Source

Flood Melton < 0.30 Jackson et al. 1987;Wilford et al. 2004b

Debris flood and Melton > 0.30 Jackson et al. 1987;debris flow Wilford et al. 2004b

Debris flood Melton 0.30–0.60 and Length ≥ 2.7 km Wilford et al. 2004b

Debris flow Melton > 0.60 and Length < 2.7 km Wilford et al. 2004b

Debris flow Melton > 0.52 Bovis and Jakob 1999

9

Watershed attributes

Hydrogeomorphic Watershedcategory attribute Abbreviation Description Units

Peak flow Area Area Topographically defined area of the watershed. km2

generation Watershed length Length The planimetric straight-line length from the fan kmapex to the most distant point on the watershed boundary.

Length of channels Channels The total length of stream channels identified on km maps.

Drainage density DrainDen The total length of stream channels (km) divided km/km2

by watershed area (km2).

Hypsometric Hypso The hypsometric curve is a plot of the percent %/%integral watershed area above a relative elevation (the

maximum elevation is 100% relative elevation and the minimum elevation is 0% relative elevation). The hypsometric integral is the area under the curve.

Sediment Relief Relief The elevation difference between the highest and kmproduction lowest points in a watershed.

Environmentally s are forest cover map attributes that are %sensitive areas for identified by forest classifiers or terrain specialists.soil stability s are map polygons that contain the initiation

sites for natural mass wasting. The extent is expressed as a percent of the total watershed area.

Commercial forest Comm The percent of watershed area with commercial %cover forest cover, defined as areas of mature and

immature forest, and areas that are not satisfac-torily restocked as a result of logging or natural disturbances (e.g., wildfire).

Extent of terrain G35 The percent of watershed area that has slopes %> 35° or 40° G40 greater than 35° or 40°.

General Minimum elevation MinEl The elevation above sea level of the fan apex kmmorphometric (the lowest point in a watershed).

Ratios Melton ratio Melton Watershed relief (km) divided by the square root km/kmof watershed area (km).

Relief ratio ReliefRatio Watershed relief (km) divided by watershed length km/km

(km).

10

A series of map-based variables is related to sedi-ment production in a watershed:• Relief is the elevation difference in a watershed,

which, when combined with watershed length(e.g., relief ratio) or area (e.g., Melton ratio), canbe used as a measure of watershed steepness.

• The stability of slopes in a watershed can be easilydetermined if terrain stability maps are available(although generally the map coverage is limited toareas with commercial forest cover and excludesalpine areas, which can be a significant source ofsediment to a stream channel) (B.C. Ministries ofForests and Environment 1999). Environmentallysensitive areas for slope stability ( or x)are an attribute on forest cover maps (B.C. Min-istry of Forests 1992). These polygons contain theinitiation sites for natural mass wasting (i.e., ClassV terrain). The location of unstable sites with

relation to a stream channel provides an indica-tion of sediment delivery to a channel. Figure 2presents an example of a watershed with a deeplyincised stream channel network that has steep, ac-tively failing streambanks that are classed as s.This watershed has frequent high-power floodsthat deliver considerable volumes of sediment tothe fan. In this case the unstable terrain is “con-nected” to the stream channel. In other cases,where there is gentle terrain between the unstableareas and a stream channel, there is a potential forless eroded material to enter a stream channel.

• The percentage of a watershed with commercialforest cover is defined as areas of mature and im-mature forest, and areas that are not satisfactorilyrestocked as a result of logging or natural distur-bances (e.g., wildfire). Commercial forest coverwas selected as a variable due to the role of forests

Predictive models for power and disturbance extent of hydrogeomorphic processes in west-central BritishColumbia (Wilford 2003)

Hydro- geomorphic Watershed Watershed Disturbanceprocess attribute Power attribute extent Class limits and models

Flood Area Low-power – – < 15 km2

High-power – Site 15–38 km2

Stand > 38 km2

Debris flood Relief and Low-power – – Comm > -109 + 182 (Relief)Comm High-power ReliefRatio Site G35 < 34.6 – 68.4 (ReliefRatio)

and G35 Stand G35 ≥ 34.6 – 68.4 (ReliefRatio)

Debris flow Comm High-power – Site > 45%

Stand < 45%

Predictive models for the number of hydrogeomorphic events during the past 50 years in west-central British Columbia (Wilford 2003)

Hydrogeomorphic Model to predictprocess Category the number of events

Flood Stand and site-level 0.36 + 1.05 ()Low-power –5.64 + 2.80 (Length)

Debris flood Stand-level –17.88 + 8.31 (MinEl) + 36.98 (Hypso)Site-level –3.04 + 3.98 (DrainDen)Low-power 13.23 – 0.97 (Length)

Debris flow Stand-level 29.97 – 0.76 (Comm)

Site-level 9.72 – 0.22 (G40)

11

in moderating runoff and enhancing slope stabili-ty (Sidle et al. 1985; Hetherington 1987). Withinthe study area, the extent of commercial forest in a watershed is a strong indicator of potential de-bris flow hazards (Tables 5 and 6).The pre-typing of a watershed should provide

an indication of the dominant hydrogeomorphicprocess influencing a fan. The overview of watershed

morphometric measurements and forest covershould provide an indication regarding the powerand disturbance extent of events, and perhaps thenumber of events in the past 50 years. These predic-tions and estimates will be checked during thefieldwork, but provide an early indication of the po-tential hydrogeomorphic hazards influencing a fan.

The incised stream channels in this watershed are associated with extensive environmentally sensitiveareas for slope stability (ESAs or Class V) that are producing sediment directly to the streams.

12

Six fan and watershed aerial photograph featuresprovide indications of hazards for forestry activities:1. forest cover, 2. multiple channels, 3. high sediment load, 4. abruptly disappearing stream, 5. abrupt change in stream direction, and 6. major sediment source near mouth of watershed.

A summary of these variables and their relation tomanagement issues in the study is presented in Table7, but note that these features were observed on fanswith forest management issues throughout BritishColumbia.

6.1 Forest cover

Aerial photographs are used to determine if there aredisturbances in the forest canopy on a fan wider than20 m (stand-level disturbances) that are linked to thestream channel. The disturbances can range fromareas of bare sediments to swaths of well-establishedcohorts (groups of trees established at approximatelythe same time). If these are apparent then the fan ismost likely subject to stand-level hydrogeomorphicevents. An example is presented in Figure 3.

6.2 Multiple channels

Multiple channels radiating out from either themouth of the watershed or at points along the streamchannel indicate a distributary stream system (Figure4). The channels may be visible as open channels orstrips of deciduous or riparian forest cover. Multiplechannels represent a hazard for forestry activities because the proportion of flow in any channel canchange quickly from 0 to 100% of the total flow.

6.3 High sediment load

Evidence of high sediment load in stream channelson fans and in the contributing watershed includesmid-channel bars, braided reaches, fans building intowater bodies or across valleys, and changes to chan-nel form downstream of the confluence of the fanwith a larger stream (Figure 5). In these situations,high sediment loads are being transferred to and potentially across the fan. It is common for drainagestructures on these fans to be under-designed to passhigh flows and high sediment volumes. Riparianforests characteristically maintain the stream channelalignment and store sediments.

6.4 Abruptly disappearing stream

When stream channels on fans abruptly disappearfrom view on aerial photographs it is an indicationthat the stream channel has lost confinement and hasdeveloped multiple smaller channels or flows that arebeing broadcast under the forest canopy (Figure 6).Below the point of disappearance, the hazard is thatstream locations can be difficult to determine, partic-ularly when the streams flow over the forest floorwithout a defined channel. Recognizing where cross-drains are required and determining their ap-propriate sizes can be problematic because watervolumes can change due to the multiple channel situ-ation. Forest harvesting can be problematic where theforests are storing sediments and limiting the devel-opment of new channels.

6 AERIAL PHOTOGRAPH INTERPRETATION

Pre-logging aerial photographic features andrelation to forest management issues on fans

Fans Fans related towith management

Feature feature issue

Forest cover (stand-level 29 17 (57%)clearings or cohorts)

Multiple channels 16 5 (31%)

High sediment load 10 10 (100%)

Abruptly disappearing 7 1 (14%)stream

Abrupt change in 3 3 (100%)stream direction

Major sediment source 17 15 (88%)

13

An aerial photograph of a 100-year-old cohort following a stand-level flood. Scale 1:11000 (approx.).

14

Multiple channels are present on this fan. Scale 1:11 000 (approx.).

15

High sediment loads are being transported by this stream, as evidenced by a change in channel morphology of themain stream downstream of the fan, and by mid-channel bars and braided reaches in the stream channel on the fan.The arrow indicates flow in the main channel. Scale 1:19 500 (approx.).

16

6.5 Abrupt change in stream direction

Abrupt changes in stream direction on fans are gen-erally not controlled by bedrock. The cause is some-times migrating channel bends but is usually log jams(Figure 7). The hazard for forestry activities is a chan-

nel avulsion (rapid lateral shift or change in directionof the stream), with the stream re-occupying an oldchannel, creating a new channel, or cutting off a bendin the original channel (Allen 1965). This action canlead to water and sediment arriving at the road at un-expected locations and/or flow discharges.

The stream channel disappears from view on the aerial photograph (arrow), indicating a broadcasting of flows underthe forest and/or multiple channels that do not have the power to clear a swath through the forest stand. Scale 1:4200 (approx.).

17

6.6 Major sediment source

Major sediment sources that are visible on aerialphotographs and that are located near the mouth of awatershed (lower 25% of the watershed) can providedirect delivery of sediments and debris to a fan (Fig-ure 8). There may be limited opportunity for channel

and bank sediment storage, or gradual routing ofsediments to the fan. Characteristically, hydrogeo-morphic events have increased power when majorsediment sources are located close to the mouth ofthe watershed. Sediment sources found near themouths of watersheds represent a significant hazardfor forestry activities on fans.

A large debris jam (the white area between the arrows) in the main channel (white arrow) led to the formation of asecond channel (black arrow), which initiates at an abrupt angle. Flow occurred in the second channel for a period ofyears but is now totally in the main channel. Scale 1:6000 (approx.).

18

An example of major, natural sediment sources near the mouth of a watershed. Scale 1:9400 (approx.).

19

Fieldwork should be undertaken to check the pre-typing classification, explore the implications offeatures identified on aerial photographs, identify sitecharacteristics, and collect dendroecological data.Once forest practitioners learn to recognize site fea-tures, very little additional time is required to notetheir presence. The nature of sediment deposits is di-agnostic of hydrogeomorphic processes (Table 8).

A particularly important aspect of the fieldwork isto identify zones where forests are storing sediment,maintaining the stream location, and reinforcing thesoil mass (Tables 9 and 10). This is termed the

“hydrogeomorphic riparian zone,” and the featuresare common on forested fans throughout British Co-lumbia. Human disturbance in the hydrogeomorphicriparian zones, including the removal of trees orwoody debris or the disturbance of other riparianfeatures, has the potential to increase the transportdistance of sediment across the fan and increase theprobability of channel avulsions. Maintenance of thehydrogeomorphic riparian zone is one of the keys tomaintaining historic levels of stability on fans. Thiszone does not have a pre-determined width; it mayrange from 1 m to several hundred metres wide.

7 FIELDWORK

Characteristics of hydrogeomorphic process deposits (after VanDine 1985; Smith 1986; Wells and Harvey 1987; Piersonand Costa 1987; Costa 1988; Hungr et al. 2001)

Characteristics Flood Debris flood Debris flow

Mode of deposition Grain-by-grain, dominated Rapid grain-by-grain En masseby traction processes aggradation from both

suspension and traction

Stratification Massive or horizontal None or horizontal Nonestratification (with cross- stratificationstratification)

Grading Variable: as a result of Frequently distribution None; reverse; reverse tosequential processes rather normal-graded (coarse on normal, coarse-tail normalthan a single process bottom, fine on top)

Sediment characteristics Clast-supported with an Clast-supported, with Matrix-supported; rarelyand texture open framework or predominantly coarse sand, clast-supported; very poor

distinctly finer-grained moderate to poorly sorted, to extremely poor sorting; matrix of infiltrated sand; bmax

a typically <80 cm but extreme range of particlerounded clasts; wide range may be larger sizes; bmax

a 60–230 cm, of particle sizes; sorting may contain megaclastsfrom front to tail; bmax

a >400 cm<10 cm to >20 cm

Orientation of clast Always perpendicular to Large cobble to boulder - Variable, based on locationlong-axis (A); flow; usually well usually perpendicular to within flow; parallel to flowimbricationb imbricated flow; pebbles to small is most prominent; weak to

cobbles - usually parallel to no imbricationflow; weak imbrication and collapse packing

Landforms and Bars, fans, sheets, splays, Similar to water flood but Marginal levees, terminaldeposits channels have large width- deeper deposits lobes, trapezoidal to

to-depth ratio U-shaped channel

a A clast (e.g., pebble, gravel, cobble) has three axis: “a” is the longest, “b” is the second longest, and “c” is the shortest. The “b” axis determines the sieve size that the clast will pass through. The term “bmax” refers to the “b” axis of the largest clast that can be found in a deposit.

b Imbrication is the shingling or overlapping of clasts with the upper edge of each clast inclined downstream, similar to a deck of tiltingcards. Clasts are moved into this position by stream flow.

20

Field indicators of hydrogeomorphic activity within the hydrogeomorphic riparian zone

Feature Description

Scars on trees Scars on the upstream or sides of trees can be the result of hydrogeomorphic events. Rocks may be(Figure 9) imbedded in trees and sediment can be present on the bark at least 6 years after an event. Examine

the area for other causes of scars (e.g., animals, windthrow, human activity).

Log steps Downed woody debris traps broadcast sediment, forming a step. There can be a range of ages with(Figure 10) the most recent having exposed soils and the oldest being just a flat deposit with the log rotted away.

Buried trees Sediment deposits around the base of trees obscure the basal or butt flare. The flare redevelops as (Figure 11) adventitious roots become established (decades).

Woody dikes Riparian vegetation is supporting individual logs or woody debris along the edge of the channel—generally <1 m in height.

Log retaining walls Multiple horizontal logs supported by trees are storing sediment—generally >1 m in height. Found(Figure 12) on the lateral edges of a channel or deposit, and tend to maintain stream location. In contrast, debris

or log jams are generally perpendicular to flow and tend to block channels, forming avulsions.

Levee enhancers Trees and understory vegetation along a stream channel provide channel roughness that reduceswater and debris flow velocities and results in obvious enhancement of levee formation along banks.

Recent sediment Sediment deposits in the forest that are unvegetated or have limited accumulation of organic mattersplays (Figure 13) () are considered to be “recent deposits” (colonization times are site-specific).

Soil reinforcement Roots are protecting the soil mass from erosion on streambanks and in avulsion channels.(Figure 14)

Exposed adventitious After burial, trees develop adventitious roots. With subsequent erosion of the sediment, the roots areroots (Figure 15) exposed. The exposed roots indicate that the area is actively storing and transporting sediment.

Tree holes Some buried trees die and rot, leaving a hole in the ground. These are not found in actively(Figure 16) aggrading areas. Some holes may be >2 m deep.

Cohorts These are groups of trees growing on sediment from hydrogeomorphic events. High-power events (Figures 17 and 18) clear a swath in the forest, while low-power events spread sediment under the canopy.

Buried soils Soil horizon development during a period of hydrogeomorphic activity followed by burial will(Figure 19) produce a buried soil (Paleosol). Examine streambanks for evidence of this feature.

Scattered boulders Scattered boulders indicated debris flow activity. Ensure that the boulders did not roll off the (Figure 20) adjacent hillslope.

Elevated sediment Sediment (fines, pebbles, cobbles, and boulders) and debris may be deposited on trees and otherand debris riparian vegetation. These features may provide information on the flow depth of events.

(Figures 21 and 22)

21

Occurrence of selected field indicators of hydrogeomorphic activity on the61 study fans with riparian forests (four fans were completely clearcut)

OccurrenceFeature (of 61 fans with riparian forests)

Buried trees 97%Log steps 93%Recent sediment splays 88%Levee enhancers 84%Scars from events on trees 79%Woody dikes 49%Exposed adventitious roots 26%Soil reinforcement 24%Log retaining walls 13% (primarily debris flow fans)

Tree holes 8% (requires deposition followed by inactivity)

Scars can be used to date the year and season of hydrogeomorphic events. The scar on this tree was caused by a debrisflow approximately 20 years ago.

22

Sediment deposited behind a log has formed a “log step.” Note the person standing beneaththe log step.

An example of a buried tree. Note the lack of buttflare.

23

A log retaining wall storing a considerable volume of sediment and maintaining the stream channellocation (visible in the middle right of the photo).

A “recent” deposit of sediment within the hydrogeomorphic riparian zone with limited organicaccumulation and vegetative cover.

24

Roots provide reinforcement to the soil mass against the erosional effects of broadcast flows.Depending on the volume and duration of flow, this protection will delay or prevent the formation of new channels.

Erosion of sediment deposited around trees can expose adventitious roots.

25

Tree holes are the result of rotting followingdeep burial of tree stems. This hole is 35 cmwide and 2 m deep.

A young cohort of spruce and hemlock growing on the sediment of a high-powerstand-level disturbance debris flood.

26

A cohort of hemlock growing on sediment deposited by a low-power debris flood.

A streambank with two buried soil horizons (Paleosols).

27

Scattered boulders indicate debris flow activity.

This suspended boulder, scarring and mud deposits on the stems indicate a debris flow depth of >2 m.

28

If preliminary forest development plans call for astream crossing, measure the channel gradient at sev-eral locations above and below the proposed roadcrossing. A decrease in the gradient at the site or justupstream usually indicates a depositional environ-ment with a high probability of causing drainagestructure problems (in-filling and constant mainte-nance). Descriptions of other features (e.g., potentialfor upstream avulsions, description of cohort width,meander bends upstream) are also required for astream crossing design.

When trees are buried, scarred, or tilted, whenthey become established, or their neighbour trees areremoved or when their water supply is radicallychanged, the events can be “recorded” in their treerings (e.g., changes in ring width and wood anatomi-cal features such as compression wood). The study oftree response to environmental conditions is calleddendroecology (Fritts and Swetnam 1986; Schwein-gruber 1996). Methods and analysis techniques fordating hydrogeomorphic events are simple andstraightforward, and can be easily learned by forestpractitioners (Wilford et al. 2005). Dendroecologysamples are taken to determine the frequency of hy-

drogeomorphic events (Schweingruber et al. 1990;Jakob 1996; Strunk 1997). Samples should be takenfrom areas with evidence of previous events (i.e.,within the hydrogeomorphic riparian zone) and alsofrom areas not influenced by events that can be usedas controls to identify other factors influencing treesat the stand level (e.g., weather, insects). The site evi-dence of previous events includes deposits (as des-cribed in Table 8) and site features (as described inTable 9). In most cases it is possible to determine theapproximate ages of cohorts and scars in the field.Determining the dates of abrupt growth changes dueto either sediment burial or clearing of adjacentstands may be possible in the field; however, samplepreparation (mounting and sanding) and microscopeexamination may be necessary (Figure 23) (Wilfordet al. 2005). Although not always feasible, microscopeexamination of samples will allow the dating of woodanatomical changes (e.g., compression wood, trau-matic resin canals) that may be linked to hydrogeo-morphic events. Sampling sites and site observationsshould be located, either with a Global PositioningSystem () or on a base map of the fan.

Identification of hydrogeomorphic features by sea-soned forest practitioners requires limited additionalfield time. Undertaking dendroecology sampling re-quires increment bores and hand or power saws, andlimited training to develop proficiency. Field analysisof 15 samples is estimated to take less than 2 hours. Ifsamples require laboratory analysis, the additionaltime would approximate 20 minutes per sample(Wilford et al. 2005). The time required for dendro-ecology interpretations is of minor importance com-pared to the information generated and the degree ofconfidence that can be placed in the identification ofhydrogeomorphic hazards influencing a site.

Sediment and debris on a tree indicate high flowsin this area.

A close-up of an increment core from a spruce(Picea glauca) showing abrupt reduced growththat began in 1903 and continued until 1915.Growth was normal in 1916, then increased.These growth responses were due to deep burialby flood sediments, subsequent establishment ofadventitious roots, and reduced competition(because the event resulted in a 200 m wideswath cleared adjacent to the tree).

29

The fifth step in the hazard identification scheme isto incorporate the office and fieldwork componentsinto appropriate prescriptions for forestry activities.The guidance provided here should assist in the de-velopment of appropriate prescriptions; however,this is not intended to be a “cookbook.” There aretwo reasons for this: combining differences in fansand management plans/objectives results in toomany combinations to present a list of “perfect” pre-scriptions, and the science of forest management onfans is too young (inappropriate prescriptions areknown but there are limited operational trials or ex-periments to test “best management practice” ideas).When forest practitioners develop prescriptions toaddress hydrogeomorphic hazards on fans it is im-portant to refer them to a qualified professional,particularly when the practitioners have limited ex-perience with forest management prescriptions forfans. There is more evidence to support caution thanrisk taking (Wilford et al. 2003).

Defining the hydrogeomorphic riparian zone high-lights where special prescriptions are required. Insome cases this area can be avoided (e.g., roads crossthe stream below the zone). If the zone must becrossed, it is important for the design of drainagestructures to have information regarding the hydro-geomorphic process, its power, and the frequency ofevents. For high-power events, cohorts or bare sedi-ment deposits should provide evidence of the “trimline,” and thus peak discharge, of these events. Thecohorts will also provide evidence of the frequency ofevents. If events occur on a regular basis (e.g., every10 years) and the road is expected to provide long-term, year-round access, the drainage structureshould be designed to pass the anticipated peak dis-charge. An alternative is to design the structure sothat events can pass over it without increasing thedisturbance caused by the event, but this alternativerequires hazard signage or road closure during peri-ods of hydrogeomorphic activity.

Although less destructive, low-power events canpresent challenges for forest management. The com-mon issues are related to the in-filling of drainagestructures and channelled erosion of roads. Erosionof roads on fans can occur relatively quickly due tothe high erodability of loosely deposited sandy andgravelly sediments. To limit these impacts it is appro-priate to install frequent cross-drains with armoured

ditch blocks (i.e., not of local material) and avoidstream crossings where there is a marked drop instream gradient (Table 11).

Consideration should be given to the location ofroad crossings on fans. On debris flow fans it is ap-propriate to cross below the deposition zone (e.g.,lobes and levees) or at the fan apex, particularly if the bridge can be founded on bedrock and providesufficient clearance to pass the design event. In caseswhere these approaches are not feasible, two optionsshould be considered. The first is seasonal access withdrainage structures that handle seasonal flows andarmouring of the road to accommodate higher flowsor events (reinforced fords). With this option it isnecessary for the road to drop to the crossing and tomaintain streambanks or levees that influence thepath of debris flows (a major consideration with thisoption is to define the “debris flow season”). The sec-ond approach is to design drainage structures androads that withstand events, either by allowing thewater and debris to pass through the structure orover the road, and continue down the channel (sig-nage and seasonal use must be considered). Ifmultiple channels are present on a fan it is frequentlybest to cross in the zone with the fewest number ofchannels. Low positions on fans may have lower po-tential for being influenced by high-power events,but finer-textured soils and multiple stream crossingscan lead to high road construction costs. Low posi-tions on fans are frequently zones of sedimentaccumulation within the channel; this can result inchallenges for drainage structures. Roads shouldcross above any abrupt gradient reductions in thestream channel to reduce the impacts from sedimentaccumulation.

When considering forest harvesting, defining hy-drogeomorphic riparian reserves and managementzones depends on the specifics of an individual fan.Several examples are given to illustrate a range ofconditions. Stream channels that are entrenched orincised in the fan surface should not be considered as an indicator of stability on a fan surface withoutfurther exploration. Search for evidence of a hydro-geomorphic riparian zone along the bank. Also beaware that channels entrenched more than 4 m canbe filled in one single debris flow event (Osterkampand Hupp 1987). Some channels that are entrenchedon the mid or lower portion of a fan can lack bank

8 PRESCRIPTION DEVELOPMENT

30

A summary of hydrogeomorphically appropriate forestry prescriptions that address common problems encountered on fans

Common problems Solutions

Roads climb to stream crossings. If drainage structures become Design road profiles to drop to stream crossings or avoid theplugged, water runs down climbing roads and ditches, creating fan altogether with a crossing at or above the apex.erosional problems. When hydrogeomorphic events encounter Do not excavate streambanks.a climbing road, the result is generally a re-directing of the event If roads must climb to the stream crossing, design features to away from the channel, increasing the disturbed zone on a fan. allow water and sediment to cross the road: robust ditchblocks

and over-sized cross-drains; armoured rolling dips; outslope roadwith no ditchline.

Roads built at gradient break in stream/fan (from steeper to less Cross streams in uniform slope reaches or design drainagesteep). This is a zone of sediment deposition, and as a result structures to accommodate a rising streambed.drainage structures are generally under-sized. Constant mainte- Do not excavate streambanks.nance is required to drainage structure to avoid channel avulsions.

Ditches on fans intercept broadcast flows, leading to Avoid the construction of ditchlines, or if necessary provide:significant erosional events. armouring; rolling dips in the road grade; and frequent

cross-drains. Ditchblocks must be robust. Limiting road width will reduce the height of cutbanks and the size of ditches.

Lack of cross-drainage as roads traverse fans leads to concentration Design rolling grades with armoured road shoulders throughof broadcast flows or excess accumulations of water in ditches. old channels, provide frequent cross-drains with armoured ditch-The result is generally severe erosion due to the non-cohesive blocks and design cross-drains to handle bedload and woodynature of soils on fans. debris (e.g., favour log culverts over pipes).

Drainage structures are narrower than the stream channel or Select structures with rectangular cross-section and openhave less hydraulic roughness than the original channel bed, bottoms that will maintain the channel width.changing channel hydraulics and leading to streambed scour.

Drainage structures are designed to accommodate peak flows Identify situations where streams are carrying a high sedimentbut not bedload, debris, or the substantial sediment loads load. Use the dendroecological information (scars and cohorts) associated with debris flows and debris floods (Jakob to determine the frequency of hydrogeomorphic events, and theand Jordan 2001). cohorts to identify peak discharges. Recognize the site features of

hydrogeomorphic events, particularly the power and disturbanceextent. Options: design structures to pass these events; removestructures seasonally; design “fail-safe” structures; or designstructures that remain following an event.

Excavation into stream channel for drainage structure Place rip-rap to maintain channel stability if excavation is required,installation and removal of large woody debris—channel but, as a rule, avoid excavations into the stream channel. Clearingdestabilization. woody debris from stream channels will increase the volume of

bedload transported during even normal runoff events.

Inadequate drainage structures in multiple channel situations Design all structures in multiple channel situations to handle allleads to damage to structures and roads, and usually to channel the flow, debris, and bedload, or design the structures to beentrenchment with down-fan broadcasting of sediments. “fail-safe.”

Roads on fans are not deactivated. Deactivate roads and structures as soon as possible andbuild roads with deactivation in mind.

Breached streambanks provide opportunities for channel Do not breach banks but, if necessary, reconstruct as soon asavulsions. possible during deactivation.

The “hydrogeomorphic riparian zone” is logged, leading to a Recognize and maintain the hydrogeomorphic riparian zone. broadcasting of sediments, channel avulsions, and widening of Retain large woody debris to maintain an important hydrogeo-the stream channel. morphic role. Partial cutting maintains a degree of the

hydrogeomorphic role and may be appropriate on fans withlow-power events.

Bladed skid trails on fans intercept and concentrate flows, leading Limit bladed skid trails, deactivate, and place large woody debristo entrenchment. on the surface.

31

confinement near the apex, and thus pose a potentialavulsion hazard. Where a fan has old-growth forests(e.g., 250 years old) growing on debris flow leveesand the stream is confined between the levees rightfrom the fan apex, a forested reserve extending be-yond the levees should be prescribed only if the areais providing large woody debris that is important forchannel stability. On some fans the hydrogeomor-phic riparian zone is confined by a slight slope break,and an argument could be made to log on the upperfan surface to the break in slope. A consideration inthese situations is the role played by large woody de-bris originating above the slope break. Depending onthat assessment, the prescription could differ: a 30-mreserve from the slope break, partial harvesting to theslope break, or clearcut harvesting to the slope break.On some low-power fans it may be apparent that thehydrogeomorphic riparian zone has been increasinglaterally in recent years. Depending on the slope direction, a reserve should be considered for 30 m beyond the current extent of deposits. This reservewould continue to provide large woody debris to trapsediment and thus serve to limit expansion of the deposition zone. An alternative strategy would be toundertake partial cutting in the area extending 30 mbeyond the hydrogeomorphic zone, with attentionbeing paid to leaving large woody debris across theslope to trap sediment. A key to any harvesting strat-egy adjacent to the hydrogeomorphic zone is to avoidcreating bladed trails that could serve to channelwater.

As with all prescriptions, operational considera-tions may lead to changes. For example, a fan

prescription may call for winter road access withfords that do not result in excavation of streambanks,but, due to ice conditions on the road, it may be nec-essary to do the excavation. Operational staff shouldunderstand the reasons for the prescription, and, inthis case, reconstruct the banks immediately follow-ing logging (e.g., prior to spring runoff). Operationalstaff should appreciate that hydrogeomorphic haz-ards on fans must be considered seriously and whatmay appear to be slight modifications of prescriptionscould have significant implications for fan surfacestability. Prescriptions for fans should be consideredin the same league as designs for major drainagestructures in that “as built” certificates are appropriate.

As guidance, a summary of hydrogeomorphicallyappropriate forestry prescriptions that address com-mon problems encountered on fans is presented inTable 11. While fans can present hazards, they are nodifferent from other hazards and issues that requiredecisions by forest managers. A key aspect of deter-mining trade-offs regarding forest managementstrategies for fans is the recognition of potential haz-ards, costs, and impacts as well as the benefits. Anexample is presented by Wilford et al. (2004a) wheredecisions were made to construct seasonal accessacross a debris flow fan for winter logging and retaina wide forested reserve in an active hydrogeomorphicriparian area. The decision was based on a relativelyhigh long-term specific risk of damage to the planta-tion, and was made after considering associated risksto the environment and potential economic loss.

Fans can have high-productivity forest sites and offerlow-cost forest harvesting opportunities due to easyaccess and relatively gentle gradients. However, envi-ronmental and long-term financial costs can be highbecause forestry activities on fans can exacerbate im-pacts from hydrogeomorphic processes. The six stepsnecessary for appropriate forest management on fansare:1. identification of fans in an operating area, 2. determination of potential hydrogeomorphic

hazards prior to fieldwork through pre-typing watersheds,

3. review of aerial photographs, 4. recognition of key features in the field, 5. development of prescriptions that deal with

hydrogeomorphic hazards, and6. monitoring of results.

Very few cases were found where the entire fansurface is subject to hydrogeomorphic hazards thatcan affect forestry activities. The key point is thatzones of hydrogeomorphic activity can be identified,

and, once identified, managed appropriately to mini-mize the impact of forestry operations. An implicitassumption is that hydrogeomorphic riparian zoneswill not change within a forestry timeframe (e.g., 50to 100 years); however, there is a risk that future hy-drogeomorphic events could influence areas beyondthe contemporary zone (as defined by trees that areat least 100 years old). Professional expertise shouldbe sought where the practitioner has limited experi-ence with forest management on fans.

It is critical for all people involved in the logging or road-building operation to be aware of the haz-ards of undertaking forestry on fans and to followprescriptions closely. Deviations from the prescrip-tion should be discussed and accommodations madeif the hazard level is considered too high. If done ap-propriately, forest harvesting and road constructionon fans can be undertaken with limited risk to invest-ments and the environment. It is intended that theinformation presented will be beneficial to the prac-tice of sustainable forest management on fans.

32

The body of scientific and operational knowledge regarding forest management on fans is currently relatively small. A way to improve the operationalknowledge base is to monitor prescriptions and share the results with other forest practitioners (e.g.,through technical and trade publications, and work-shops). The key is adequate documentation. It iscommon for road or drainage structure issues to bedocumented as a certain kilometre on a road system,

rather than on a fan. Small changes in documenta-tion would greatly improve both the awareness andmonitoring of forest management issues on fans.Since the hazard classification scheme is based on theavailable scientific and operational knowledge, it isanticipated that the extra time and cost incurred inimplementing the scheme will result in overall lowerfinancial and environmental costs. Monitoring willprovide the proof.

9 MONITORING PRESCRIPTIONS

10 CONCLUSIONS

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