470 Granville Street, Suite 630, Vancouver, BC V6C 1V5 Tel: 604-629-9075 | www.pecg.ca
Geohazard Assessment – Regional Overview of Debris Flow, Debris Flood, and Fluvial Geohazards
Regional District of Kootenay Boundary
Palmer Project # 1705004
Prepared For Ebbwater Consulting Inc. on behalf of Regional District of Kootenay Boundary
May 3, 2021
470 Granville Street, Suite 630, Vancouver, BC V6C 1V5 Tel: 604-629-9075 | www.pecg.ca
May 3, 2021
Robert Larson, M.Sc., P.H., P.Ag. Hydrologist Ebbwater Consulting Inc. 510 - 119 W Pender St Vancouver, BC V6B 1S5
Dear Robert Larson:
Re: Geohazard Assessment – Regional Overview of Debris Flow, Debris Flood, and Fluvial Geohazards Project #: 1705004
Palmer is pleased to present Ebbwater Consulting Inc., working on behalf of Regional District of Kootenay Boundary, with the results of our overview assessment of debris flow, debris flood, and fluvial geohazard susceptibility within Electoral Areas C, D, and E.
Appended to this report are maps depicting areas susceptible to debris flow, debris flood, and fluvial geohazards, for use in association with Ebbwater’s flood and geohazard risk assessment.
It has been a pleasure working with all parties on this interesting project. Should you have any questions, please do not hesitate to contact Dan McParland (226-979-8160, [email protected]) or Robin McKillop (604-355-8788, [email protected]).
Yours truly,
Dan McParland, M.Sc., P.Geo. Robin McKillop, M.Sc., P.Geo. Senior Fluvial Geomorphologist Principal, Geomorphologist
Geohazard Assessment – Regional Overview of Debris Flow, Debris Flood, and Fluvial Geohazards
Table of Contents
Letter
1. Introduction ...... 1 1.1 Key Definitions ...... 1 1.2 Background ...... 2 1.3 Study Area ...... 3 1.4 Objectives ...... 3
2. Physical Setting ...... 4
3. Methods ...... 6 3.1 Desktop Analysis ...... 6 3.1.1 Debris Flow and Debris Flood Susceptibility ...... 6 3.1.2 Fluvial Geohazards ...... 8 3.2 Field Reconnaissance ...... 11
4. Results ...... 12 4.1 Debris Flow and Debris Flood Susceptibility ...... 12 4.2 Fluvial Geohazard Susceptibility ...... 14
5. Discussion and Recommendations...... 17 5.1 Mitigation Options ...... 20 5.2 Assumptions and Limitations ...... 21
6. Summary ...... 23
7. Statement of Limitations ...... 23
8. Certification ...... 24
9. References ...... 25
List of Figures
Figure 1. Study area and excluded municipalities ...... 4 Figure 2. Schematic of the Active Stream Corridor (ASC) and Fluvial Hazard Buffer (FHB) that are combined to create the fluvial geohazard corridor for higher-order channels (adapted from Blazewicz et al., 2020)...... 11 Figure 3. A channel avulsion along Granby River about 1.5 km south of Niagara that occurred during the May 2018 flood (Left Panel: Google Earth, Right Panel: ESRI World Imagery)...... 18 Figure 4. Hierarchy of mitigation options for fluvial geohazards ...... 20
May 3, 2021 TOC i Geohazard Assessment – Regional Overview of Debris Flow, Debris Flood, and Fluvial Geohazards
List of Tables
Table 1. Dominant watershed process class boundaries...... 7 Table 2. Parameters used to determine propagation in Flow-R for debris flows and debris floods (adapted from BGC Engineering, 2019)...... 8 Table 3. Channel characteristics by stream order (as defined by the FWA) within the study area...... 9 Table 4. Standardized fluvial geohazard corridor widths for lower-order streams. The values represent the total corridor width. For instance, a 20 m total corridor width would be 10 m on either side of the mapped FWA watercourses...... 10 Table 5. Summary of debris flow and debris flood prone watersheds in RDKB (based on Melton ratio)...... 12 Table 6. Summary of combined debris flow and debris flood susceptibility1 in the study area...... 13 Table 7. Summary of fluvial geohazard susceptibility in the study area...... 14
List of Photos
Photo 1. Progressive bank erosion along Kettle River may undermine an abandoned building (looking downstream near Gilpin)...... 15 Photo 2. A channel avulsion that occurred in 2018 along Granby River about 1.5 km south of Niagara (looking upstream). The new channel position is on the left and the historic channel position is on the right...... 15 Photo 3. Undersized crossing along Granby River at 18105 North Fork Road (looking west)...... 19 Photo 4. Bank armouring and the road corridor (North Fork Road) locally perturb fluvial processes (looking downstream)...... 19
List of Appendices
Appendix D-1. Debris Flow and Debris Flood Geohazard Susceptibility Maps Appendix D-2. Fluvial Geohazard Susceptibility Maps
May 3, 2021 TOC ii Geohazard Assessment – Regional Overview of Debris Flow, Debris Flood, and Fluvial Geohazards
1. Introduction
Palmer was retained by Ebbwater Consulting Inc. (Ebbwater), on behalf of Regional District of Kootenay Boundary (RDKB), to complete a regional overview assessment of debris flow, debris flood, and fluvial geohazards within Electoral Areas C, D, and E. RDKB contains geohazard-prone areas due to mountainous terrain and hydroclimatic perturbations associated with climate and land use change. Recent unprecedented precipitation and flooding in the RDKB, including the May 2018 clearwater flood, has exacerbated fluvial and hillslope processes. The overview assessment of debris flow, debris flood, and fluvial geohazards described herein has been completed to help advance the initiative of RDKB to improve its understanding of the distribution of natural hazards, prioritize areas for follow-up, more detailed assessment, and ultimately help reduce associated risks. The results from this overview assessment form a key contribution to Ebbwater’s risk assessment (2021a).
Important background information, including the rationale and objectives for the assessment, key definitions, and details of the study areas (Section 1), is followed by an overview of the physical setting (Section 2). Section 3 describes the methods used for the assessment. Section 4 presents the results of the assessment, including summary tables. Section 5 discusses the implications of the results and provides high-level recommendations and mitigative strategies to manage natural hazard risk within RDKD. Section 6 summarizes key findings. Maps depicting areas susceptible to debris flow and debris flood geohazards as well as fluvial geohazards are contained in appendices.
1.1 Key Definitions
Geohazard: Geomorphological, geological, or environmental processes that have the potential to adversely impact human life, property, infrastructure and/or the environment (Komac and Zorn, 2013). The term ‘geohazard’ is used in association with debris flows, debris floods and fluvial processes, in this report, to avoid potential confusion with the stricter term of ‘hazard’ (probability x magnitude). ‘Geohazard’ is used to refer to the physical processes, themselves, as opposed to spatial or temporal characteristics of the processes.
Susceptibility: The spatial tendency, qualitative or quantitative, that a threat or natural hazard may occur in an area without considering the moment of occurrence or potential consequences of such an event (Domínguez-Cuesta, 2013). Susceptibility in this report refers to the regional, qualitative assessment of geohazard distribution. The susceptibility of an area to debris flow and debris flood geohazards was assessed with a focus on identifying areas prone to debris flow and debris flood propagation (e.g. transportation and runout zones) as opposed to specific initiation zones. Further technical explanation is provided in the corresponding methods below (Section 3.1.1). The susceptibility to fluvial geohazards includes areas that may be directly or indirectly affected by one or more fluvial processes (as defined below).
Clearwater flood: An extreme hydrologic event where sediment comprises less than 20% of the discharge by weight (Wilford et al., 2004). These events are commonly caused by moderate to heavy or prolonged rainfall, melting snow, or a combination of the two. This term is used herein instead of “flood” for clarity. Only contextual reference is made to clearwater floods in this report; a more comprehensive assessment of this process in RDKB is available in the associated report prepared by Ebbwater (2021a; 2021b).
May 3, 2021 1 Geohazard Assessment – Regional Overview of Debris Flow, Debris Flood, and Fluvial Geohazards
Debris flood: A channelized flood of sediment-laden water, where sediment concentration can range from 20-47% by volume (Wilford et al., 2004). Peak discharges of debris floods can be twice that of clearwater floods within the same hydrologic setting (Hungr et al., 2001). Debris floods are not considered a type of landslide.
Debris flow: A rapid, high-density mass movement of saturated debris. Debris flows are channelized events, typically within steep gullies, channels, or established flow paths (Hungr et al., 2013). A debris flow may initiate once a debris slide or rockslide on an open slope becomes channelized in a gully, for example, and enlarges through entrainment of surficial material, organic debris and water. Debris flows are commonly triggered by intense or prolonged precipitation and can have peak discharges up to 40 times greater than those of clearwater floods within the same hydrologic setting (Hungr et al., 2001). Debris flows may transition to debris floods through addition of water in tributaries (Wilford et al., 2009).
Fluvial geohazards: Morphological restructuring within and immediately adjacent to watercourses driven by a combination of erosion, transport, and deposition of sediment and wood (Blazewicz et al., 2020). The restructuring can occur gradually or abruptly and usually occurs during clearwater floods and, in some settings, during debris floods. Common fluvial geohazards include lateral channel migration, bed degradation, avulsions, reworking of alluvial fans, and localized failures along confining terraces and hillslopes.
Stream Order: A hierarchical structure to classify the relative size of a watercourse (Strahler, 1952). The smallest watercourses originating along the watershed margins are 1st-order streams. Stream order increases downstream of confluences of two watercourses of the same stream order. In general, higher- order streams have higher discharges and larger cross-sectional areas.
1.2 Background
Clearwater floods and associated fluvial processes have caused considerable damage in RDKB over the past decade. In particular, a large clearwater flood in May 2018 led to damage to private property and infrastructure and required the emergency evacuation of thousands of RDKB residents. The morphological restructuring of the valley bottom was so severe following the 2018 flood that previous flood mapping is no longer representative of the valley bottom. Climate change will likely increase the frequency and magnitude of large clearwater floods and the occurrence of associated geohazards.
To better understand and proactively manage natural hazards, RDKB retained Ebbwater to prepare a regional-scale clearwater flood risk assessment, which will ultimately inform clearwater flood mapping initiatives in RDKB. To properly understand clearwater flood-related risks in RDKB, geohazards that can affect, or be affected by, clearwater floods should also be documented. Palmer was retained to complete an overview assessment of debris flow, debris flood, and fluvial geohazard susceptibility in RDKB. Fluvial geohazards commonly occur in association with clearwater floods. Debris flows and debris floods often occur as a result of significant precipitation and snowmelt events, which may also produce clearwater floods. Debris flows and debris floods can locally pose greater risks to human life, infrastructure and property than clearwater floods (Hungr et al., 2013; Church and Jakob, 2020). Sediment and wood inputs
May 3, 2021 2 Geohazard Assessment – Regional Overview of Debris Flow, Debris Flood, and Fluvial Geohazards
from debris flows and debris floods into higher-order channels can also trigger lateral and/or vertical adjustments along the creek channel and in some cases form landslide dams.
1.3 Study Area
The study area for this assessment includes Electoral Areas C, D, and E of RDKB, a total area of 7,007 km2 (Figure 1). Electoral Area A and B are not included. As well, the City of Grand Forks, City of Greenwood, and the Village of Midway are completing their own clearwater flood mapping and risk assessments and, thus, are excluded from the study area of this project. Details regarding the physical setting of the study area are presented in Section 2.
1.4 Objectives
The overall objective of this geohazard overview assessment is to identify areas within the study area that are susceptible to debris flow, debris floods, or fluvial geohazards, as a basis for understanding any regional differences in their distribution, drawing attention to areas where more detailed, follow-up assessment is warranted, and supporting elements of the associated risk assessment completed by Ebbwater (2021a). To achieve this main objective, Palmer completed the following:
• Screening-level modelling and mapping of debris flow and debris flood susceptibility.
• Delineation of fluvial geohazard corridors along all mapped watercourses.
• Limited field reconnaissance to spot-check desktop-based characterizations and interpretations.
• Identification of locations where fluvial processes (and clearwater processes) may interact with hillslope processes.
Palmer also established a basis for prioritizing areas for more detailed, follow-up assessment, including field investigation. Common strategies for mitigating hazards and risks associated with fluvial, debris flood and debris flow processes are summarized for RDKB’s reference.
Strict hazard assessment involving establishment of site-specific probabilities (likelihoods) of geohazard occurrence, typically based on extrapolation from detailed spatio-temporal inventories of past events, was beyond the scope of this screening-level, regional-scale study. The likelihoods of fluvial geohazards were approximated in association with generalized annual exceedance probabilities for clearwater floods, such that fluvial geohazards were available inputs to Ebbwater’s risk assessment (2021a). Debris floods and debris flows could only be reliably characterized according to spatial susceptibility, at this scale of study and considering limitations in available data, so they did not undergo a formal risk assessment. Instead, the results of the mapping presented herein inform RDKB of the areas more and less susceptible to debris floods and/or debris flows to enable comparisons across electoral areas and prioritization of follow-up, site- specific hazard and risk assessment.
May 3, 2021 3 Geohazard Assessment – Regional Overview of Debris Flow, Debris Flood, and Fluvial Geohazards
Figure 1. Study area and excluded municipalities
2. Physical Setting
The study area is in a region of high relief in south-central British Columbia. Major drainage systems include the West Kettle, Kettle, and Granby Rivers, which dissect the landscape within approximately north-south trending valleys. Christina Lake occupies a wide, U-shaped valley upstream of the community of the same name. The lowest elevations are found along the Kettle River near Cascade and the U.S. border (approx. 440 m). The Columbia Mountains encompass the highest elevations in the study area, including the peaks of Mount Tanner (2,419 m) and Mount Cochrane (2,411 m). Major population centres are typically located on the plains, terraces, lake shores, and fans of major river valleys.
May 3, 2021 4 Geohazard Assessment – Regional Overview of Debris Flow, Debris Flood, and Fluvial Geohazards
Several physiographic regions within southern British Columbia are represented in the study area, including the Monashee Mountains, Okanagan Highland, and Thompson Plateau (Matthews, 1986). The highly mountainous Monashee Mountains comprise most of the study area, extending from the Kettle River Valley to the eastern edge of the study area; the remaining western side of the study area is almost entirely within the Okanagan Highlands, a subregion characterized by rounded mountains and ridges and gently-sloping uplands (Holland, 1976). The northwestern corner extends slightly into the gently-sloping uplands of the Thompson Plateau. Major valleys typically have steep bedrock walls partially covered by thin veneers of colluvium or glacial drift where slopes are gentler. Colluvial fans and aprons are commonly present where steep gullies and unstable slopes meet valley bottoms. Large alluvial fans occur at valley junctions.
The landscape has been shaped and modified by recurrent glacial and interglacial periods through the Quaternary period. During the most recent Wisconsinan (known locally as the Fraser) glaciation, most of the study area was beneath glacial ice of the Cordilleran Ice Sheet (CIS), which reached its maximum extent about 14,500 years BP (Ryder et al., 1991). Ice flowed generally southward across the study area, eroding, transporting, and depositing underlying material. High, alpine areas display the erosional history of glaciers, which has created landforms such as cirques, arêtes, and U-shaped valleys (Clague and Ward, 2011). The gently rolling uplands and valleys display both erosional and depositional glacial landforms. Valleys oriented roughly parallel with ice flow underwent the greatest degree of erosion (Church and Ryder, 2010), including the valleys now containing Kettle River, Granby River, and Christina Lake.
Decay of the CIS occurred rapidly and was largely complete by 11,500 years BP (Ryder et al., 1991). Deglaciation occurred mainly through thinning and downwasting of the ice sheet. The first areas to become ice-free were the highest uplands as the ice margin moved to lower elevations in valleys (Clague and Ward, 2011). Continued ablation stranded glacial ice within valleys where it could no longer flow and led to the formation of ice-stagnation features, outwash plains, and thick accumulations of fine glacial lake sediments (Clague and Ward, 2011). Many of these deposits were later dissected by meltwater and early Holocene rivers, leaving terraces along valley margins.
The glacial and deglacial landforms were particularly unstable and subject to erosive forces until colonized by vegetation in the early Holocene paraglacial period (Church and Ryder, 1972). Much of the deep gullying of drift-mantled mountainsides underlain by weak bedrock occurred during this period. Several large, paraglacial fans in the study area are no longer geomorphologically active beyond the channels incised on their surfaces.
The study area has a humid continental climate, characterized by cold winters and warm summers. Annual precipitation varies across the study area from 400 mm/year to 750 mm/year (Chernos et al., 2020). Stream flows are generally low during the winter and peak in late spring as the result of snow melt and large rain events. Increases in forest harvest over the past 50 years in the Kettle River watershed, including peak annual harvests in early 1990s and early 2010s, have likely increased average annual flows and the magnitude of flood events (Chernos et al., 2020). Further details on the hydroclimatic conditions in RDKB and the potential influences of climate change are available in the risk assessment report (Ebbwater, 2021a).
May 3, 2021 5 Geohazard Assessment – Regional Overview of Debris Flow, Debris Flood, and Fluvial Geohazards
3. Methods
Regional debris flow, debris flood, and fluvial geohazard susceptibilities were assessed through systematic, desktop-based approaches (Section 3.1). The preliminary desktop results were spot-checked during field reconnaissance in October 2020 (Section 3.2) and updated, as required. The desktop and field methods are outlined in the following sections.
3.1 Desktop Analysis
3.1.1 Debris Flow and Debris Flood Susceptibility
Debris flows and debris floods are members along a continuum of steep creek processes involving the channelized mobilization of sediment, water, and woody debris. The distinction is made, in this study, to account for the difference in runout behaviours of these respective events when determining susceptibility. A debris flow may occur in a single surge or as a series of pulses, involving a peak sediment concentration exceeding 60% by volume, or a peak discharge of more than three times what would be expected in a major clearwater flood (Hungr et al., 2013). Debris flow deposits are generally localized in levées alongside confined channels (Wilford et al., 2004), and may spread out where the channel opens onto a fan, typically at higher slope gradients between 5 and 20° (Hungr et al., 2013). In contrast, the onset of a debris flood may build gradually in response to hydroclimatic events or may occur suddenly following a landslide into a channel or collapse of a natural or man-made dam (Church and Jakob, 2020). The sediment concentration of a debris flood may range between 20 and 47% by volume (Wilford et al., 2004). Debris floods typically travel further downslope and are found on fans gentler than 5° (Hungr et al., 2013). Debris floods differ from clearwater floods in that the former requires the entire stream bed to be mobilized for at least several minutes (Church and Jakob, 2020). Debris flood deposits may be found well beyond the main channel on a fan, while clearwater flood deposits are more localized around the active channel (Wilford et al., 2004).
Debris flow and debris flood susceptibility in the study area was assessed at a regional, overview level, through a systematic and multi-step process. The susceptibility assessment involved the qualitative investigation of the spatial extent of these events, which occur or may potentially occur in the study area. Consistent with the definition provided by Fell et al. (2008), the timeframe of potential events is not explicitly considered, although it may be expected that debris flows and debris floods will occur more frequently as relative susceptibility increases. Geospatial analysis was used to identify watersheds and sub-catchments prone to debris flow and debris flood processes throughout the study area. Desktop modelling was then used to determine debris flow and debris flood susceptibility through the transport and deposition zones.
Watersheds may be differentiated as prone to clearwater flooding, debris floods, or debris flows by analyzing key morphometrics in each catchment (Wilford et al., 2004). In the absence of mapped fans, a suitable classification of watersheds may be achieved by analyzing the Melton ratio and the total stream length (Holm et al., 2016). The Melton ratio provides a measure of catchment ruggedness and is defined as the watershed relief in kilometres divided by the square root of watershed area in kilometres (Melton, 1957; Wilford et al., 2004). Total stream length is measured in kilometres and provides an estimate of peak discharge generation (Wilford et al., 2004).
May 3, 2021 6 Geohazard Assessment – Regional Overview of Debris Flow, Debris Flood, and Fluvial Geohazards
The geospatial data used in this analysis were compiled and processed using Esri’s ArcMap (version 10.8). Modelling of debris flow and debris flood propagation was completed using the software package Flow-R, described in section 3.1.1.1. Watershed boundaries and stream networks were represented using the Freshwater Atlas of British Columbia (FWA), a standardized digital dataset of hydrologic features derived from the province’s 1:20,000-scale topographic base maps (Gray, 2010). Topography in RDKB was represented using the Canadian Digital Elevation Model (CDEM), provided at a resolution of 20 m in the study area (CDEM, 2016). A secondary LiDAR elevation model at a resolution of 1 m was used to help with validation where available.
Source areas for debris flows and debris floods were determined at the watershed level. Watersheds within the study area were classified according to the key morphometrics adapted from Wilford et al. (2004) and Holm et al. (2016) and limited to 1st- and 2nd-order watersheds (Table 1). The inventory of watersheds was overlain onto the stream network in RDKB, and streams were classified according to the dominant process. In the case of nested watersheds (i.e. 1st-order watershed within a 2nd-order watershed), morphometrics of the parent catchment considered the total area inclusive of sub-catchments; the dominant processes of sub-catchments were applied to the corresponding sub-catchment stream segments.
Table 1. Dominant watershed process class boundaries.
Dominant Process Melton Ratio1 Total Stream Length2 (km) Considered Stream Order Clearwater Floods < 0.2 Any Any 0.2 to 0.5 Any Debris Floods 1st-or 2nd-Order > 0.5 > 3 Debris Flows > 0.5 < 3 1st-or 2nd-Order Notes: 1 Melton Ratio is defined as watershed relief divided by the square root of watershed area. 2 Total Stream Length is calculated from the FWA stream network.
3.1.1.1 Flow-R Susceptibility Modelling
Debris flow and debris flood susceptibility was assessed using the Flow-R model developed by Horton et al. (2008, 2013). It functions in a two-step process: 1) source areas are defined by morphological and user-defined criteria; and 2) simulated landslides are propagated from source areas using directional and spreading algorithms in combination with friction laws to control path direction and runout extent (Horton et al., 2013). Selection of propagation algorithms and parameters is an empirical process whereby inputs may be adjusted to generate results consistent with real-world observations. Flow-R allows for the calculation of summed susceptibility or maximum susceptibility on a cell-by-cell basis (Horton et al., 2013). This study uses the results of the summed susceptibility (complete option), and the output reflects the contribution of every upslope source cell when determining susceptibility. High values indicate a greater contributing source area capable of reaching the target cell and/or higher susceptibility values from individual debris flow and debris flood propagation simulations.
Flow-R was run for the entire RDKB study area using the 20 m CDEM. Source areas were derived from FWA streams classified as either debris flow or debris flood prone in the previous section and are used as a proxy for the terrain susceptible to debris flow and debris flood initiation. Source areas were converted to
May 3, 2021 7 Geohazard Assessment – Regional Overview of Debris Flow, Debris Flood, and Fluvial Geohazards
20 m raster cells to match the CDEM. Calibrated debris flow and debris flood criteria for use in Flow-R were generated in a study in the neighbouring and physiographically similar Regional District of Central Kootenay (BGC Engineering, 2019). These parameters were applied in this study and determined to achieve satisfactory results through a series of spot checks of fans visible in available high-resolution LiDAR data and imagery in RDKB (Table 2).
Table 2. Parameters used to determine propagation in Flow-R for debris flows and debris floods (adapted from BGC Engineering, 2019).
Process Spreading Algorithms and Friction Laws Method Value Directions Algorithm Holmgren (1994) modified dh = 2; exp = 1 Inertial Algorithm weights Gamma (2000) Debris Flow Friction Loss Function travel angle 5° Energy Limitation velocity < 15 m/s Directions Algorithm Holmgren (1994) modified dh = 2; exp = 1 Inertial Algorithm weights Cosinus Debris Flood Friction Loss Function travel angle 4° Energy Limitation velocity < 15 m/s
3.1.2 Fluvial Geohazards
Areas susceptible to fluvial geohazards were assessed at a regional level through a systematic and multi-step process. More specifically, fluvial geohazard corridors were delineated to define areas a watercourse has historically occupied or may occupy, or physically influence, as it transports and deposits water, sediment, and debris under the prevailing flow and sediment regimes (Blazewicz et al., 2020). The fluvial geohazard corridor accounts for the following geomorphological processes:
• Bed and/or bank erosion • Avulsions
• Deposition
• Growth and reworking of alluvial fans • Large wood jams
• Mass movements along confining terraces and hillslope due to fluvial erosion
Fluvial geohazard corridors were determined for all FWA mapped watercourses within the study area. The total length of the FWA watercourses is 12,542 km. The methods used to establish the fluvial geohazard corridors for this study were based on protocols recently established in Colorado State (Blazewicz et al., 2020) and, more broadly, protocols from Washington State (Olson et al., 2014). Colorado’s physiography and hydroclimatic conditions are similar to those in RDKB. The Colorado protocols are intended for reach- scale assessments (i.e. channel segment 10 km or less). Due to extensive channel length in the study area, simplifications and assumptions described throughout this section were required for this current project. The delineated corridors establish areas susceptible to fluvial geohazards, without specifying which of the
May 3, 2021 8 Geohazard Assessment – Regional Overview of Debris Flow, Debris Flood, and Fluvial Geohazards
various geomorphological processes is/are applicable, and they do not strictly establish probability or timescales of occurrence.
The desktop assessment primarily relied on topographic data, mostly high-resolution, and publicly available satellite imagery. LiDAR (2018 and 2019) derived contours and hillshade models were available for Electoral Areas C and D, and the majority of Electoral Area E. LiDAR data were not available for small portions of Electoral Area E within the Okanagan-Columbia Timber Sales Business Area. In addition to LiDAR data, the CDEM was available for the entire study area. Satellite imagery from Google, Bing, and Esri were available for the entire study area. The date of acquisition varied across the study area and by source. The majority of the study area had satellite imagery collected after the May 2018 flood from at least one of the three sources. Surficial geology mapping (Province of British Columbia, 2016) coverage was limited to about 35% of the study area, so the mapping only served to help calibrate interpretations where available.
Due to the length of watercourses within the study area, automated approaches were required for lower- order streams. For the purposes of this assessment, lower-order streams are 1st- through 5th-order streams as defined by the FWA. Lower-order streams have average channel slopes of 3% or greater and are partly or fully confined by surrounding hillslopes (Table 3). Many of the lower-order watercourses have ephemeral or intermittent flow. The higher-order streams (6th- through 8th-order) are alluvial or semi-alluvial watercourses that are partly confined or unconfined by surrounding hillslopes. The lack of confinement allows the channel to adjust vertically and laterally within the valley bottom. The methods used to delineate the fluvial geohazard corridors for lower-order and higher-order streams are described in the sub-sections below.
Table 3. Channel characteristics by stream order (as defined by the FWA) within the study area.
Stream Total Average Common Confinement Order1 Length (km) Slope2 (%) 1 7,681 19.8 Undefined channel or Confined 2 2,349 13.7 Confined 3 1,221 9.3 Confined 4 616 5.8 Confined or Partly Confined 5 297 3.0 Confined or Partly Confined 6 129 1.8 Partly Confined 7 172 0.8 Partly Confined or Unconfined 8 78 0.6 Partly Confined or Unconfined Notes: 1 – As defined by the FWA 2 – Determined from 20 m-resolution CDEM Definitions: Undefined – lacks discernible channel shape Confined – lack of accessible floodplain and the channel is unable to adjust laterally along both banks Partly Confined – lack of accessible floodplain and the channel is unable to adjust laterally along one bank or locally along both banks Unconfined – accessible floodplain and the channel can adjust laterally along both banks
May 3, 2021 9 Geohazard Assessment – Regional Overview of Debris Flow, Debris Flood, and Fluvial Geohazards
3.1.2.1 Lower-order Streams (1st- through 5th-order)
Due to notable channel confinement and localized lack of channel definition, fluvial geohazard corridors along lower-order streams were established using standardized widths (Table 4) in accordance with the Colorado protocols (Blazewicz et al., 2020). The standardized corridors were generated in GIS and centred along the mapped FWA watercourses. The selected corridor widths were based on representative spot measurements of channel and valley bottom widths using the LiDAR data throughout the study area. In general, the selected corridor widths encompass the valley bottoms and the toes of the adjacent hillslopes. The selected corridor widths may not fully encompass anomalous, unconfined sections of certain lower- order streams, but they are considered appropriately conservative for the majority of reaches.
Table 4. Standardized fluvial geohazard corridor widths for lower-order streams. The values represent the total corridor width. For instance, a 20 m total corridor width would be 10 m on either side of the mapped FWA watercourses.
Stream Order Standardized Stream Corridor Width (m) 1 20 2 20 3 40 4 60 5 80
The standardized fluvial geohazard corridors were manually adjusted where the lower-order streams reached an active alluvial fan (i.e. unconfined) within higher-order valleys. If the runout of the modelled debris flows or debris floods (Section 4.1) reached the alluvial fan, then manual adjustments on the fan were not made to the fluvial geohazard corridors as it was assumed that debris flows or debris floods would drive morphological restructuring of the fan.
3.1.2.2 Higher-order Streams (6th- through 8th-order)
Higher-order streams are partly confined or unconfined and can more freely adjust laterally and vertically. As such, a more robust method was required to delineate the fluvial geohazard corridors along these watercourses. In keeping with the ‘Fluvial Signature’ method of the Colorado protocols (Blazewicz et al., 2020), the fluvial geohazard corridors for these higher-order streams comprise two components: the ‘active stream corridor’ (ASC) and the ‘fluvial hazard buffer’ (FHB) (Figure 2). The ASC is the area of the valley bottom that the channel has historically occupied or may occupy in the future under the prevailing flow and sediment regimes. The ASC commonly encompasses areas of active (FAp) and inactive (Fp) fluvial plains, as defined in the Terrain Classification System for British Columbia (Howes and Kenk, 1997). Anthropogenic features such as highways and flood protection berms locally influence the configuration of the ASC. The ASC was manually delineated in GIS at a scale of 1:10,000 primarily based on interpretation of LiDAR hillshade models and satellite imagery.
May 3, 2021 10 Geohazard Assessment – Regional Overview of Debris Flow, Debris Flood, and Fluvial Geohazards
Figure 2. Schematic of the Active Stream Corridor (ASC) and Fluvial Hazard Buffer (FHB) that are combined to create the fluvial geohazard corridor for higher-order channels (adapted from Blazewicz et al., 2020).
The FHB was applied along both sides of the ASC. The FHB accounts for areas beyond the ASC that may be physically influenced by fluvial processes (e.g. mass movements driven by fluvial undercutting and oversteepening). Most notably, this included erodible confining features such as terrace scarps or hillslopes. Anthropogenic features such as highways, erosion protection and flood berms do not influence the position of the FHB because they may be compromised by fluvial processes in extreme events. Palmer initially applied a standardized 50 m FHB along the ASC and then locally manually narrowed and widened the buffer using the LiDAR data and satellite imagery, where required.
3.2 Field Reconnaissance
Palmer completed limited, vehicle-based field reconnaissance in collaboration with Ebbwater on October 20-22, 2020. Weather conditions during the field reconnaissance were clear and mild. Recorded discharges at the West Kettle (08NN003), Kettle (08NN026), and Granby (08NN002) River Water Survey of Canada stations during the field program were slightly higher than the long-term mean for October but well below mean annual discharge. Field reconnaissance was completed along the following road corridors: Highway 33, Highway 3, Christian Valley Road, Boundary Creek Road, North Fork Road, Granby Road, and Burrell Creek Forest Service Road.
The purpose of the field reconnaissance was to spot-check desktop-based interpretations and collect additional information that cannot be reliably determined remotely. Particular attention was given to spot-
May 3, 2021 11 Geohazard Assessment – Regional Overview of Debris Flow, Debris Flood, and Fluvial Geohazards
checking the preliminary fluvial geohazard corridors at representative locations along different reaches and watercourses. Characterization of materials comprising banks and terrace scarps was also completed to inform the FHB. Factors contributing to documented fluvial geohazards at key sites (e.g. channel geometry, bar distribution, large wood, anthropogenic alteration, etc.) were noted. Following field reconnaissance, the ASC was locally adjusted where necessary.
4. Results
4.1 Debris Flow and Debris Flood Susceptibility
The results of the Flow-R susceptibility modelling are presented as a series of maps in Appendix D-1. A total of 9,127 watersheds were identified as either debris flow or debris flood prone through geospatial analysis of their respective Melton ratio and total stream length (Table 5). These watersheds highlight regions where debris flows and debris floods can potentially initiate.
Table 5. Summary of debris flow and debris flood prone watersheds in RDKB (based on Melton ratio).
Dominant Process Stream # of Order1 Watersheds 1st-Order 3,788 Debris Flow 2nd-Order 343 1st-Order 3,720 Debris Flood 2nd-Order 1,276 Notes: 1 2nd-Order watershed count excludes overlapping 1st Order watersheds.
The summed susceptibilities have been classified into four categories in line with the debris flow and debris flood percentile thresholds used for the neighbouring Regional District of Central Kootenay (BGC, 2019). High susceptibility is defined as values greater than the 95th percentile and typically includes the main debris flow and debris flood channels and their margins. Moderate and Low susceptibility are defined as values from the 85th to 95th percentile, and from the 70th to 85th percentile, respectively. These classes generally reflect increasing distance from the main channel toward open slopes and more distal areas of defined fans. Very Low to Negligible susceptibility is defined as values less than the 70th percentile and represents the least susceptible areas that are still potentially reached by modelled debris flows and debris floods. The Negligible term was added to reflect spot-checked scenarios in high-resolution LiDAR data where CDEM- based propagations are not plausible based on actual microtopography (e.g. spreading outside a deep canyon only represented in the LiDAR data). In general, the simulated propagation extents from the Flow-R model are typically larger than deposits that may be observed in the field, especially on some upper slopes. This is an intentional result of the model in an effort to conservatively represent all possible events, including those which may yet occur (Horton et al., 2013).
In total, 5.43% of the study area (380 km2) has been classified as Low to High susceptibility to debris flows, and 8.13% (570 km2) has been classified as Low to High susceptibility to debris floods. Combined, 11.9% (836.6 km2) of the study area is susceptible to both geohazards, owing to overlaps in the propagations.
May 3, 2021 12 Geohazard Assessment – Regional Overview of Debris Flow, Debris Flood, and Fluvial Geohazards
Debris flow and debris flood susceptibility zones are relatively well confined to active stream channels and adjacent slopes within gullies and minor valleys, although debris floods extend further downstream along lower gradient streams. Where steep channels reach floodplains and broad valley floors, the constraining effect of the terrain is lost, and the processes are permitted to spread out into fan-shaped susceptibility zones. Debris flood susceptibility extends further both laterally and downstream along fans in comparison to debris flow susceptibility. The larger areal extent of debris flood susceptibility can be explained by the combination of more numerous source areas and greater spatial spreading of individual propagations.
At an overview level, both debris flow and debris flood prone watersheds tend to be reasonably well distributed across the landscape. The density of debris flow susceptible terrain is appropriately higher in the comparatively rugged Monashee Mountains physiographic region, in the eastern half of the study area. Very steep watersheds are more prone to debris flows than to debris floods, such as those associated with the creeks above the Granby River valley near Mount Cochrane and Mount Tanner, and the gullies surrounding McRae Creek and Highway 3 east of Christina Lake. Debris flow susceptibility extends to valley floors at the outlets of most steep, 1st- and 2nd-order creeks in these areas. The density of debris flood susceptible terrain is slightly higher in the comparatively gentler Okanagan Highland physiographic region, in the western half of the study area. The U-shaped Boundary Creek valley and the rolling uplands along the western edge of the study area are examples of areas where debris flood susceptibility far outweighs debris flow susceptibility. Overall, there is a slightly higher proportion of terrain susceptible to either debris flows or debris floods in the eastern Monashee Mountains than in the western Okanagan Highlands.
The total area susceptible to either debris flows or debris floods was summarized by Electoral Area in Table 6. Where both debris flow and debris flood susceptibilities are present (e.g. a fan downstream of both debris flow and debris flood dominated catchments), the higher susceptibility class of the two processes was used. At an overview level, all three electoral areas have comparable proportions of terrain classified as Moderate or Low susceptibility to debris flows and floods. Electoral Areas C and D have higher proportions of terrain classified as high susceptibility to debris flows and debris floods (2.5% and 2.2%, respectively) compared to Electoral Area E (1.5%). Electoral Area E is mostly located in the Okanagan Highlands physiographic region, while Electoral Areas C and D are located entirely within the comparatively rugged Monashee Mountains physiographic region, which may explain the slight difference in the proportion of High susceptibility terrain.
Table 6. Summary of combined debris flow and debris flood susceptibility1 in the study area.
RDKB Electoral Area High Moderate Low Combined Susceptibility Susceptibility Susceptibility Susceptibility RDKB C Susceptible Area 14.0 km2 25.6 km2 32.3 km2 71.9 km2 (556 km2) % of Electoral Area 2.5 % 4.6 % 5.8 % 12.9 % RDKB D Susceptible Area 47.1 km2 94.8 km2 124.0 km2 265.9 km2 (2,113 km2) % of Electoral Area 2.2 % 4.5 % 5.9 % 12.6 % RDKB E Susceptible Area 64.3 km2 184.7 km2 249.8 km2 498.8 km2 (4,338 km2) % of Electoral Area 1.5 % 4.3 % 5.8 % 11.5 % Total Total Susceptible Area 125.4 km2 305.1 km2 406.1 km2 836.6 km2 (7,007 km2) % of Electoral Area 1.8 % 4.4 % 5.8 % 11.9 % Notes: 1 The higher susceptibility class has been used in areas susceptible to both debris flow and debris flood.
May 3, 2021 13 Geohazard Assessment – Regional Overview of Debris Flow, Debris Flood, and Fluvial Geohazards
4.2 Fluvial Geohazard Susceptibility
The fluvial geohazard corridors throughout the study area are presented in a series of maps in Appendix D-2. The total area susceptible to fluvial geohazards is 407.9 km2 (Table 7), which represents 5.8% of the entire study area. The proportion of total area susceptible to fluvial geohazards is similar (within 1%) for the three electoral areas. Christina Lake is a large waterbody that occupies the largest valley in Electoral Area C. Christina Lake does not have an associated fluvial geohazard, which leads to a smaller proportion of total susceptibility for Electoral Area C relative to the other two electoral areas. Furthermore, the proportion of the susceptibility along lower-order channels is very similar among electoral areas, which likely reflects similar physiography and drainage densities. The proportion of susceptibility along higher-order channels is notably less for Electoral Area C relative to the other two electoral areas, simply due to its paucity of higher-order channels.
Table 7. Summary of fluvial geohazard susceptibility in the study area.
RDKB Electoral Area Lower-Order1 Higher-Order2 Total Susceptibility Susceptibility Susceptibility RDKB C Susceptible Area 24.7 km2 3.5 km2 28.2 km2 (556 km2) % of Electoral Area 4.5% 0.6% 5.1% RDKB D Susceptible Area 90.8 km2 34.0 km2 124.8 km2 (2,113 km2) % of Electoral Area 4.3% 1.6% 5.9% RDKB E Susceptible Area 182.1 km2 72.8 km2 254.9 km2 (4,338 km2) % of Electoral Area 4.2% 1.7% 5.9% Total Total Susceptible Area 297.6 km2 110.3 km2 407.9 km2 (7,007 km2) % of Electoral Area 4.2% 1.6% 5.8% Notes: 1 stream order 1st through 5th 2 stream order 6th through 8th
Higher-order streams, which are more prevalent near the southern portions or the study area, are generally single-thread, irregular meandering watercourses with pool-riffle bed morphology. As expected, higher- order streams exhibit the greatest variability in the width of the fluvial geohazard corridor ranging form less than 30 m in confined or canyonized settings (e.g. Kettle River at Cascade Falls) to well over 1,000 m in broad, low gradient sections of valley (e.g. Granby River at Grand Forks Golf Course). Along these higher- order segments, channel migration as a result of bank erosion is common in unconfined sections (Photo 1). Channel avulsions, although infrequent, have drastically altered portions of the valley bottom (Photo 2).
May 3, 2021 14 Geohazard Assessment – Regional Overview of Debris Flow, Debris Flood, and Fluvial Geohazards
Photo 1. Progressive bank erosion along Kettle River may undermine an abandoned building (looking downstream near Gilpin).
Photo 2. A channel avulsion that occurred in 2018 along Granby River about 1.5 km south of Niagara (looking upstream). The new channel position is on the left and the historic channel position is on the right.
May 3, 2021 15 Geohazard Assessment – Regional Overview of Debris Flow, Debris Flood, and Fluvial Geohazards
Along the valley margins of the higher-order streams, alluvial fan growth/reworking has locally altered their planforms and longitudinal profiles by increasing channel confinement and altering sediment inputs. Locally, higher-order streams have contributed to erosion and mass movements along confining, erodible features (e.g. high terrace scarps). Large wood has less influence on channel hydraulics and sediment transport along higher-order streams relative to lower-order segments due to larger channel widths and higher discharges. Localized channel confinement and valley narrowing along higher-order segments can act as hydraulic ‘pinch-points’ and lead to localized morphological instability immediately upstream and downstream of the confinement. For example, valley narrowing along Granby River between Niagara and the Grand Forks Golf Course has exacerbated upstream and downstream morphological instability.
Lower-order streams have a broad spectrum of planforms and associated bed morphologies. Step-pool and plane bed morphologies are common (Montgomery and Buffington, 1997). Along lower-order streams, lateral channel migration and large avulsions are moderated by widespread channel confinement. Excess deposition, large wood jams, and mass movements along confining terraces and hillslope are more common processes along these lower-order streams relative to higher-order streams. Episodic sediment and wood inputs from debris flows and debris floods can exacerbate these processes. The general stability of the lower-order streams can influence the stability of the downstream alluvial fans in the higher-order valleys. Confluences, regardless of stream order, are morphologically dynamic areas.
May 3, 2021 16 Geohazard Assessment – Regional Overview of Debris Flow, Debris Flood, and Fluvial Geohazards
5. Discussion and Recommendations
The results of this regional overview assessment demonstrate that many of the slopes and valleys within the study area may be susceptible to debris flow, debris flood, and/or fluvial geohazards. Existing property, infrastructure and/or human lives are most likely to be exposed to one or more of these processes where they occur within the limits of a fluvial geohazard corridor, or the areas susceptible to debris floods and/or debris flows. Establishment of what risks one or more of these geohazards pose to property, infrastructure or human lives requires more detailed, local- to site-specific assessment to determine the probability (likelihood) of occurrence within a particular timeframe of interest. Determination of annual probability for debris flows and debris floods typically involves development of an inventory of past events, in space and time, as a basis for extrapolating encounter probability into the future. Application of the documented recurrence to a particular area requires at least broad demonstration of a similarity in overall conditions.
Several important implications of the results warrant consideration, especially without formal establishment of risks:
• Prioritization of follow-up assessment – Ebbwater (2021a) has compared the mapped distribution of existing property, infrastructure and land uses to the limits of the areas most likely susceptible to fluvial, debris flood and debris flow geohazards as part of the risk assessment. Priority for follow- up assessment should be given to assets located within or near the highest propagation susceptibilities for debris flows and debris floods, as well as assets within fluvial geohazard corridors that are in particularly close proximity to the margins of active channels. • Fluvial processes – Numerous fluvial processes can contribute to fluvial geohazards in RDKB. Along higher-order unconfined streams, channel migration and channel avulsions can lead to progressive (Photo 1) or rapid morphological restructuring (Photo 2), respectively. Additional analyses using historical aerial photographs could help determine channel migration rates and trajectories at individual meanders. As well, critical analyses of LiDAR data and clearwater flood mapping could help determine individual meanders that may be susceptible to avulsions such as the avulsion that occurred along Granby River in 2018 about 1.5 km south of Niagara (Photo 2, Figure 3). Along lower-order streams, fluvial processes are highly ‘coupled’ with hillslope processes. Mass movements along hillslopes can exacerbate fluvial processes along receiving watercourses.
May 3, 2021 17 Geohazard Assessment – Regional Overview of Debris Flow, Debris Flood, and Fluvial Geohazards
Figure 3. A channel avulsion along Granby River about 1.5 km south of Niagara that occurred during the May 2018 flood (Left Panel: Google Earth, Right Panel: ESRI World Imagery).
• Interaction of fluvial and hillslope processes – The results of the fluvial geohazard corridor mapping were presented separately from the results of mapping of areas susceptible to debris flows and/or debris floods to ensure clarity during final map production. In a digital environment, however, RDKB can overlay the debris flow and debris flood susceptibility mapping onto the fluvial geohazard mapping to identify and examine areas of overlap where fluvial and hillslope processes are most likely to interact. Fluvial geohazards – downstream, locally and/or upstream – may be influenced by one-time or recurrent delivery of fine- to coarse-grained sediments from debris flows and/or debris floods. Such areas may be more dynamic than sections of channel further from hillslope sediment inputs. Fluvial processes can also initiate or exacerbate slope instability where channels abut steep terrace scarps or hillsides. In some cases, the effects of fluvial erosion can extend many tens of metres above water level by undercutting and destabilizing slopes. Generally, the interaction between fluvial processes and hillslope processes are in confined, lower-order segments where the hillslope and watercourse are ‘coupled’. Understanding where fluvial and hillslope processes coincide can help draw attention to areas where risk assessment must account for multiple, interacting geohazards.
• Anthropogenic influence on fluvial geohazards – At a regional scale, forestry has likely altered the flow regime of watercourses throughout RDKB (Chernos et al., 2020). Forestry can also increase sediment supply to watercourses through a variety of mechanisms, including landsliding (Jordan, 2006). As well, climate change has and will continue to affect hydrological processes at a regional scale (BC Agriculture and Food Climate Action Initiative, 2019). Perturbed flow and sediment supply regimes affect fluvial processes and can exacerbate fluvial geohazards. For instance, increased landsliding in the headwaters can increase the sediment supply to a downstream alluvial fan. At a reach-scale, infrastructure such as flood protection berms and highway corridors can affect fluvial processes through increased channel confinement and reduction in local sediment supply. At a local scale, undersized road crossings and local bank armouring can perturb channel hydraulics and fluvial processes and exacerbate erosional processes immediately upstream or downstream of the crossing/hardening (Photo 3, Photo 4).
May 3, 2021 18 Geohazard Assessment – Regional Overview of Debris Flow, Debris Flood, and Fluvial Geohazards
Photo 3. Undersized crossing along Granby River at 18105 North Fork Road (looking west).
Photo 4. Bank armouring and the road corridor (North Fork Road) locally perturb fluvial processes (looking downstream).
May 3, 2021 19 Geohazard Assessment – Regional Overview of Debris Flow, Debris Flood, and Fluvial Geohazards
5.1 Mitigation Options
Many municipalities, resource industries and community members across Canada are exposed to the same kinds of geohazards that are described in this report. Various strategies to mitigate associated risks have been applied and fine-tuned over decades or more. Which strategy, or combination of strategies, is expected to be most effective for mitigating risk depends on site-specific conditions and access to resources. This regional-scale assessment is a screening-level tool that informs the risk assessment (Ebbwater, 2021), but it does not provide sufficient spatial detail to determine appropriate mitigation options at a site (i.e. property) or reach scale. However, general options for mitigation are identified below.
Site-specific geohazard assessment is necessary to determine if mitigation is required or whether monitoring is a more appropriate approach, at least initially (Figure 4). If risks associated with a geohazard are determined to be unacceptable, the proponent can explore options that spatially separate the geohazard from the at-risk element (i.e. give space). This can be completed through land acquisition, moving infrastructure, or actions associated with implementation of ‘green infrastructure’. If spatial separation of the geohazard from the at-risk element is not feasible, indirect or direct structural mitigation options should be considered. Indirect structural mitigation options for fluvial geohazards involve management of energy and re-directing forces away from the at-risk element through options such channel realignments, dredging, flow training, or grade-control. Direct structural mitigation involves combatting the tractive forces ‘head on’ through armouring of channel banks and bed and stabilization of slopes. Direct mitigation provides the proponent with the most mitigation options but may also require more maintenance. Incorporation of direct mitigation and, to a lesser extent, indirect mitigation can initiate or exacerbate geohazards elsewhere along the watercourse.
Figure 4. Hierarchy of mitigation options for fluvial geohazards
May 3, 2021 20 Geohazard Assessment – Regional Overview of Debris Flow, Debris Flood, and Fluvial Geohazards
A variety of opportunities also exist for the effective management of unacceptable risks associated with debris floods and debris flows. A strong preference should be given to avoiding the placement of property or infrastructure within areas at risk from debris flows and debris floods or relocating existing at-risk property or infrastructure. Underground infrastructure that may be at risk could be buried deeper. Where avoidance and relocation are not possible, risk mitigation can be accomplished through structural measures. Debris flow and debris flood risks can be mitigated through deflection berms, designed to convey runouts in a controlled manner away from elements at risk. Robust grade control (e.g. large, interlocked riprap) may be able to withstand scour during a debris flow to protect underlying, buried infrastructure (e.g. utilities, pipes). Various forms of robust, steel posts, grates and nets have been used in some constrained environments in an attempt to trap or slow and disperse debris conveyed along a channel. Sedimentation basins, sized and shaped to accommodate an appropriately conservative debris flow, can be constructed to mitigate downslope risks from a particular source gully or channel. In some cases, multiple measures may be required to satisfactorily mitigate risk.
5.2 Assumptions and Limitations
The fluvial geohazard mapping and debris flow and debris flood modelling were completed at a regional scale to draw attention to areas more and less susceptible to these geohazards and to inform the risk assessment (Ebbwater, 2021). The assessment was almost entirely desktop-based. The vehicle-based field reconnaissance was limited to three days along major road corridors. The results of this assessment may be used to identify potential areas for additional geohazards investigation, but they should not be used to understand property-scale hazards and associated risks.
The following assumptions and limitations should be considered when interpreting the results of the debris flow and debris flood susceptibility mapping:
• Debris flow and debris flood parameters were calibrated empirically at a regional scale, and the model provides a range of possible events. The susceptibility modelling should not be used as a substitute for detailed modelling of individual events (Horton et al., 2013).
• Debris flow and debris flood modelling was completed using the 20 m-resolution CDEM, which provided complete study area coverage at a suitable scale for regional modelling. Susceptibility results may show local inaccuracies when compared against high-resolution elevation data (e.g. 1 m-resolution LiDAR). • Debris flows and debris floods have only been modelled for areas with an FWA-mapped stream, which is used as a proxy for terrain prone to debris flow and debris flood initiation. Failures are possible in some areas without mapped streams, so not all areas susceptible to debris flows are necessarily represented in the results of the modelling (e.g. slopes above a debris flow dominated stream; zero-order gullies without a mapped stream).
• Debris flow and debris flood susceptibility is determined by modelling failures that propagate from source areas. Likelihood of initiation may vary across the study area due to different geomorphic and hydroclimatic conditions, which could not be accounted for in this study. Furthermore, the very limited coverage of surficial geology mapping (Province of British Columbia, 2016) within the study area precluded refinement of source areas based on sediment availability.
May 3, 2021 21 Geohazard Assessment – Regional Overview of Debris Flow, Debris Flood, and Fluvial Geohazards
• Modelled debris flow and debris flood propagation did not account for interactions with large water bodies. Lakes and rivers may have attenuating and transformative effects on runout behaviour, and susceptibility may be locally overestimated where overlying water features.
• Debris flow and debris flood runout behaviour is complex and may be influenced by a multitude of factors not accounted for in the modelling (e.g. debris flow volume and mass, material characteristics, vegetative cover).
• Watershed morphometrics used to classify each catchment describe the potential process type, but do not consider the geomorphic or hydroclimatic conditions needed to generate a debris flow or debris flood. Therefore, watersheds may be identified as prone to debris flow or debris flood processes without evidence for prior events (Holm et al. 2016) or without the availability of source material to generate such processes.
The following assumptions and limitations should be considered when interpreting the results of the fluvial geohazard susceptibility mapping:
• The methods used to establish the fluvial geohazard corridors for this study were based on protocols recently established in Colorado (Blazewicz et al., 2020). Due to extensive channel length in the study area, simplifications of the protocols and assumptions were required.
• Fluvial geohazard mapping of higher-order segments was manually delineated in GIS at a scale of 1:10,000, for regional consistency. Site-specific fluvial geohazards may not be accurately represented at this scale.
• Due to the large size of the study area (7,007 km2), fluvial geohazard corridors along lower-order streams were established using standardized widths. The chosen corridor widths may not be large enough to encompass anomalous unconfined sections along these lower-order streams but are considered conservative for the majority of segments. • The standardized fluvial geohazard corridors for lower-order streams were centred along the mapped FWA watercourses. The mapped FWA watercourses may be outdated or incorrectly positioned, at least locally. • Interpretation of fluvial processes and calibration of fluvial geohazard mapping was mostly completed without surficial geology mapping, so the FHB may be locally over- or under- conservative where erodibility of channel banks or confining scarps was misinterpreted. • Historical aerial photography was not reviewed as part of this assessment, even along higher-order channels, so channel dynamics within the ASC and fluvial influence along the adjacent FHB were inferred mainly based on microtopography and vegetative indicators visible in the LiDAR hillshades and satellite imagery, respectively.
May 3, 2021 22 Geohazard Assessment – Regional Overview of Debris Flow, Debris Flood, and Fluvial Geohazards
6. Summary
Palmer completed a geohazard overview assessment to identify areas within RDKB Electoral Areas C, D, and E that are susceptible to debris flow, debris floods, or fluvial geohazards, as a basis for understanding any regional differences in their distribution, drawing attention to areas where more detailed, follow-up assessment is warranted, and supporting elements of the associated risk assessment completed by Ebbwater (2021a). The total area classified as Low to High susceptibility to either debris flow or debris flood geohazards is 836.6 km2 (Table 6), which represents 11.9% of the entire study area. The proportion of the total area susceptible to debris flood and debris flow geohazards is similar within the three electoral areas. The total area susceptible to fluvial geohazards is 407.9 km2 (Table 7), which represents 5.8% of the entire study area. The proportion of total area susceptible to fluvial geohazards is similar within the three electoral areas. The approaches used in this study allowed for the regional assessment of debris flow, debris flood, and fluvial geohazard susceptibility. The results of this assessment cannot be used to understand property- scale hazard and risk.
7. Statement of Limitations
This report has been prepared by Palmer for Ebbwater, working on behalf of RDKB, in accordance with the agreement between Consultant and Client, including the scope of work detailed therein (the “Agreement”). The report and the information it contains may be used and relied upon only by Client, except (1) as agreed to in writing by Consultant and Client, and (2) as required by-law.
The extent of this study was limited to the specific scope of work for which we were retained and is described in this report. Palmer has assumed that the information and data provided by the client or any secondary sources of information are factual and accurate. Palmer accepts no responsibility for any deficiency, misstatement or inaccuracy contained in this report as a result of omissions, misinterpretations or negligent acts from relied-upon data. Judgment has been used by Palmer in interpreting debris flow, debris flood, and fluvial geohazards based on desktop analyses and limited field reconnaissance. The results of this regional-scale overview assessment of geohazard susceptibility do not strictly represent hazards or risks and should not be applied to local or site-specific evaluations without follow-up, more detailed investigations and analyses. This work is not a substitute for a Legislated Landslide Assessment (APEGBC, 2010).
Palmer is not a guarantor of site conditions or projected characteristics of fluvial, debris flood or debris flow geohazards but warrants only that our work was undertaken, and our report prepared, in a manner consistent with the level of skill and diligence normally exercised by competent geoscience professionals practicing in British Columbia. Our findings, conclusions and recommendations should be evaluated considering the limited scope of our work.
May 3, 2021 23
Geohazard Assessment – Regional Overview of Debris Flow, Debris Flood, and Fluvial Geohazards
9. References
BC Agriculture and Food Climate Action Initiative. 2019. Regional Adaption Strategies: Kootenay and Boundary. https://www.bcagclimateaction.ca/regional/rap/kootenay-boundary.
BGC Engineering Inc. (BGC). 2019. Flood and Steep Creek Geohazard Risk Prioritization. Report prepared for the Regional District of Central Kootenay. Blais-Stevens, A. and Behnia, P. 2016. Debris flow susceptibility mapping using a qualitative heuristic method and Flow-R along the Yukon Alaska Highway Corridor, Canada. Nat. Hazards Earth Syst. Sci., 16, pp 449-462. Blazewicz, M., Jagt, K., and Sholtes, J. 2020. Colorado Fluvial Hazard Zone Delineation Protocol Version 1.0. Prepared for the Colorado Water Conservation Board. Canadian Digital Elevation Model (CDEM), 2016. [computer file]. Sherbrooke, QC: Natural Resources Canada. Edition 1.1. http://www.geogratis.gc.ca/.
Chernos, M., MacDonald, R.J., Green, K. 2020. The hydrological effect of forest disturbance on the Ketlle River Water, BC. MacDonald Hydrology Consultants Ltd., Aped Geoscience Consultants Ltd., Report prepared for British Columbia Ministry of Forests, Lands, Natural Resource Operations and Rural Development.
Church, M. and Jakob, M. 2020. What is a debris flood? Water resources research 56(8). Church, M. and Ryder, J.M. 1972. Paraglacial Sedimentation: A Consideration of Fluvial Processes Conditioned by Glaciation. Geological Society of America. Bull. 83(10), 3059.
Domínguez-Cuesta, M.J. 2013. Susceptibility. In: Bobrowsky, P.T. (ed.), Encyclopedia of Natural Hazards. Encyclopedia of Earth Sciences Series. Springer, Dordrecht.
Ebbwater Consulting Inc., 2021a. Boundary Region Flood and Geohazard Risk Assessment. Prepared for the Regional District of Kootenay Boundary, February 2021. Ebbwater Consulting Inc., 2021b. RDKB Flood Hazard Assessment – Appendix to the Boundary Region Flood and Geohazard Risk Assessment. Prepared for the Regional District of Kootenay Boundary, February 2021. Fell, R., Corominas, J., Bonnard, C., Cascini, L., Leroi, E., and Savage, W.Z., on behalf of the JTC-1 Joint Technical Committee on Landslides and Engineered Slopes. 2008. Guidelines for landslide susceptibility, hazard and risk zoning for land use planning. Engineering Geology. 102, pp 85-98.
Gray, M. 2010. Freshwater Atlas User Guide. Victoria, BC: Integrated Land Management Bureau, British Columbia Ministry of Forests and Range. Holland, S.S. 1976. Landforms of British Columbia: A Physiographic Outline. B.C. Dep. Mines Petrol. Resour., Victoria, B.C. Bull. No. 48.
Holm, K., Jakob, M., and Scordo, E. 2016. An inventory and risk-based prioritization of Steep Creek Fans in Alberta Canada. FLOODrisk 2016 - 3rd European Conference on Flood Risk Management, E3S Web of Conferences 7, 01009.
Horton, P., Jaboyedoff, M., and Bardou, E. 2008. Debris flow susceptibility mapping at a regional scale, in Proceedings of the 4th Canadian Conference on Geohazards, Quebec, Canada, 2008, pp 399-406.
May 3, 2021 25 Geohazard Assessment – Regional Overview of Debris Flow, Debris Flood, and Fluvial Geohazards
Horton, P., Jaboyedoff, M., Rudaz, B., and Zimmermann, M. 2013. Flow-R, a model for susceptibility mapping of debris flows and other gravitational hazards at a regional scale. Nat. Hazards Earth Syst. Sci., 13, pp 869-885. DOI:10.5194/nhess-13-869-2013.
Howes, D.E. and Kenk, E., 1997. Terrain Classification System for British Columbia (Version 2). Province of British Columbia, Resource Inventory Branch, Ministry of Environment, Lands and Parks; Recreational Fisheries Branch, Ministry of Environment; and Surveys and Resources Mapping Branch, Ministry of Crown Lands, 102 p. Hungr, O., Leroueil, S., and Picarelli, L. 2013. Varnes Classification of landslide types, an update. Landslides, 11, 167-194.
Jordan, P. 2006. The use of sediment budget concepts to assess the impact of watersheds of forestry operations in the southern interior of British Columbia. Geomorphology 79(1-2): 27-44.
Komac, B., and Zorn, M. 2013. Geohazards. In: Bobrowsky, P.T. (ed.), Encyclopedia of Natural Hazards. Encyclopedia of Earth Sciences Series. Springer, Dordrecht. Mathews, W.H. 1986. Physiographic map of the Canadian Cordillera. Geol. Surv. Can., Ottawa, Ont. Map 1701A.
Montgomery, D.R., Buffington, J.M. 1997. Channel-reach morphology in mountain drainage basins. GSA Bulletin 109(5): 596-611. Olson, P.L., Legg, N.T., Abbe, T.B., Reinhart, M.A., and Radloff, J.K. 2014. A Methodology for Delineating Planning-Level Channel Migration Zones. Shorelands and Environmental Assistance Program, Washington State Department of Ecology. Province of British Columbia, 2016, Terrestrial Ecosystem Information (TEI) Spatial Data SurficialGeology_NonPEM_Okan_Koot. Data available from the British Columbia Ministry of Environment, Ecosystem Information Section at: http://www.env.gov.bc.ca/esd/distdata/ecosystems/TEI/TEI_Data/
Ryder., J.M., Fulton, R.J., and Clague, J. 1991. The Cordilleran Ice Sheet and the Glacial Geomorphology of Southern and Central British Columbia. Géographie physique et Quaternaire, 45, pp 365-377. Strahler, A. 1952. Dynamic Basis of Geomorphology. Geological Society of America Bulletin, 63, 923 938. Wilford, D.J., M.E. Sakals, J.L. Innes, R.C. Sidle, and W.A. Bergerud. 2004. Recognition of debris flow, debris flood and flood hazard through watershed morphometrics. Landslides 1(1):61–66.
May 3, 2021 26
Appendix D-1
Debris Flow and Debris Flood Geohazard Susceptibility Maps
Appendix D-2
Fluvial Geohazard Susceptibility Maps