BC Ministry of Forests, Lands, and Natural Resource Operations and Rural Development

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1 Suite #3, 385 Baker Street Nelson, BC, V1L 4H BC Ministry of Forests, Lands, and Natural Resource Operations and Rural Development POST-WILDFIRE NATURAL HAZARDS RISK ANALYSIS ELEPHANT HILL FIRE (K20637, 2017) December 20, 2017 Report Number: Distribution: MFLNRO 1 copy SNT Geotechnical Ltd. 1 copy

2 Table of Contents 1. Introduction and Objectives Potential hazards as a result of the fire Methods Elements at risk Burn severity mapping Watershed mapping Reconnaissance risk analysis Sections Assessed Section 1 South of Cache Creek Section 2 Cache Creek and area to east Section 3 Lower Bonaparte River valley along Highway Section 4 Bonaparte valley along Loon Lake road Section 5 Maiden Creek to Clinton Section 6 Lower Hat Creek Section 7 Scottie Creek Section 8 Loon Creek and Hihium Creek/Lake Section 9 Middle Bonaparte River valley Section 10 North area, Young Lake - Pressy Lake Section 11 Deadman River valley Watershed management considerations Summary of Recommendations Closure Report Use and Limitations References Appendices... A1 Appendix A Map of entire fire area, Scale 1:80,000...AError! Bookmark not defined. Appendix B Maps of southwestern portion of fire (Maps 1-6)... BError! Bookmark not defined. Appendix C Reconnaissance Risk Analysis Tables... CError! Bookmark not defined. Appendix D Detailed Risk Analysis Summary and Tables...DError! Bookmark not defined. Appendix E Tables of burn severity by watershed... EError! Bookmark not defined. 1

3 1. Introduction and Objectives Elephant Hill Fire (K20637) PWNHRA Dec. 20, 2017 The Elephant Hill wildfire (#K20637) started near Ashcroft on July 6, 2017, and burned northward for the next two months, reaching an eventual size of 191,865 ha. It was fully contained as of September 29, Because the fire includes or is adjacent to several populated areas, it was considered by the Ministry of Forests, Lands, Natural Resource Operation and Rural Development (MFLNRO) to be a high priority to conduct a post-wildfire natural hazards risk analysis. SNT Geotechnical Ltd. was retained by the Thompson-Okanagan Region of the Ministry to do this work. The project terms of reference include two parts: First, a reconnaissance risk analysis which involved a review of operational maps and other information from the Kamloops Fire Centre, preliminary burn severity maps (Rapid Burned Area Mapping (RBAM) provided by Hatfield Consultants on contract with MFLNRO), and review of aerial imagery on Google Earth. Field work at the reconnaissance stage focused on identifying elements at risk, identifying potential hazards such as debris flow channels, and sampling burned areas in relevant areas to map soil and vegetation burn severity. Second, a detailed risk analysis was conducted based on field inspections of elements at risk, as identified in the reconnaissance phase of the study. The field work was supported by revised burn severity mapping, with a tabulation of high and moderate burn severity areas by watershed, to better assess the hazard level for each element. The detailed risk analysis included, where relevant, recommendations for mitigation measures. This portion of the report covers general methodology and the reconnaissance risk analysis. The detailed risk analysis is given in the tables and in the appendix which follows this report. Because of the large size of the fire, it was divided into eleven sections for the purpose of identifying and discussing risks. Figure 1 (following page) shows these sections and the general location of the fire. 2. Potential hazards as a result of the fire Debris flows and floods following wildfires can occur as a result of high-intensity rainfall on severely burned and/or water-repellent soils, typically in mid to late summer. Examples in BC include the Kuskonook Creek debris flow near Creston, and debris floods in Kelowna and near Falkland, which followed the 2003 wildfires. This hazard is greatest in the one to two years after the fire. Debris flows and floods can also occur during spring runoff as a result of rapid snowmelt in burned areas. Examples include several debris flows which followed the 2007 Springer fire near Slocan, and debris flows which followed a 2009 fire at Kelly Lake. They can also occur, although less commonly, during fall or early winter rain-on-snow events. The springtime hazards are due to increased snow accumulation, more rapid snowmelt, and higher groundwater levels in burned areas, and can persist for several years until revegetation occurs. Analysis of 36 debris flow and flood events which followed the 2003 and 2007 wildfires in southeastern BC (Jordan, 2015) identified several risk factors, including gentle-over-steep topography (gentler slopes or plateau draining into a steeper channel below), and high burn severity concentrated in the upper part of a watershed. Both these risk factors are present in several areas of the Elephant Hill fire, especially in the lower Bonaparte River and Cache Creek watersheds. 2

4 Elephant Hill Fire (K20637) PWNHRA Dec. 20, 2017 CLINTON CACHE CREEK ASHCROFT Figure 1. Index map. 3

5 3. Methods Field work in the burned areas was conducted October and October by Peter Jordan, P.Geo. and Steve Thompson, P.Ag. Although a helicopter flight was planned, it was unable to proceed due to bad weather and snow cover. A snowstorm on October 12 delayed field work. During this field work, a number of locations in the valley bottoms, mostly near Cache Creek and Ashcroft and in the lower Bonaparte River valley, were identified to be at risk of possible flood or debris flow hazards. A follow-up field trip was made on November 7-9 by Doug Nicol, P.Eng., Mike Walsh, P.Eng., and Michael Burnett, EIT, to do detailed investigations of sites at risk. Most of the potential risk sites are alluvial fans at the base of steep creek channels, or the floodplains of major streams such as Cache Creek. Before the final field trip, these sites were mapped and given identifying numbers as shown on the maps in Appendices A and B. Mapping of potential post-wildfire hazards and the analysis of risks followed the general methodology outlined in Hope et al. (2015). In the simplest terms, risk is the product of hazard and consequence. For the purpose of post-wildfire risk analyses, only partial risk is considered; this is the probability that a hazardous event (e.g. a debris flow) will occur and that it will reach or affect the site of the element at risk (e.g. a house or highway) with consideration to the spatial and temporal probability but not the value or vulnerability of the elements at risk. Detailed risk analysis involves inspections of individual sites. Subjective terms (low, moderate, high) are used to describe hazard and risk, based on generally accepted definitions used in other risk analysis studies and mapping projects in British Columbia (Hope et al., 2015). The qualitative risk matrix used for the determination of partial risk in this study is shown Figure 2. Figure 2. Qualitative risk matrix for partial risk (Wise et al, Land Management Handbook 56) 4. Elements at risk Field work focused on areas where there is property or infrastructure that might be at risk from postwildfire hazards, such as flooding or landslides. These areas or sites are referred to as elements at risk and include the following: 4

6 houses and other occupied structures such as public buildings (schools, hospitals, etc.) and businesses highways and other transportation infrastructure domestic water supplies and municipal infrastructure industrial facilities, including pipelines and hydro transmission lines campgrounds, parks, and tourism facilities agricultural land and infrastructure. Some general information on potential risks is given here, and more specific analyses of risks are described in the following sections and the accompanying tables. If any particular elements are not given any further mention, it is because no moderate or high risks were identified. Houses and other buildings are shown on maps produced by the Fire Centre, based on a GIS layer provided by the Thompson-Nicola Regional District. Further information was obtained from aerial photos and satellite images (using Google Earth and imagery on the Regional District mapping website). Houses and other structures were considered to be at risk if they are located on alluvial fans or near stream channels, or at the base of steep slopes below burned areas. Such sites comprise most of the risk elements in this study. Using maps and aerial imagery, a number of potential risk sites were identified to be examined in more detail during subsequent field work. The Village of Cache Creek includes areas which are at potentially at significant risk. The headwaters of the channel of Cache Creek were extensively burned, and the creek runs through the centre of town. Part of the Village of Ashcroft is located at the base of Elephant Hill, which was burned by the fire. The Ashcroft Indian Reserve was severely affected by the fire, and includes areas which may be exposed to runoff from burned slopes above. The Bonaparte Indian Reserve is partly on alluvial fans downstream from burned areas. The Village of Clinton is not at risk, as it is on the other side of the Clinton Creek Valley from the burned slopes, but some areas nearby have potential risks. Most houses of potential risk are in rural areas dispersed throughout the region. The greatest concentration of affected houses is at Loon Lake. The Loon Lake area, including lower Loon Creek, is covered by separate contracts to other companies, so will not be addressed here. The second greatest concentration of rural houses is along Highway 97 north of Cache Creek, in the lower Bonaparte River valley. Highway 1 through the Semlin Valley, the Village of Cache Creek, and the Boston Flats area, crosses a number of alluvial fans draining burned areas. Highway 97 was the lower edge of the fire for much of its length from Cache Creek to Clinton, and burned areas directly border the highway on the upslope side. In addition to creek crossings and alluvial fans described in the detailed risk analysis, there are many locations where small-scale local erosion of burned slopes above the highway could fill in ditches or plug culverts. The CN railway follows the right bank of the Thompson River and is exposed to runoff from burned areas at three locations: Battle Creek, an unnamed creek nearby (C9), and at the base of the steep slope downstream from the Highway 97C bridge in Ashcroft. Water license information was obtained from the BC Government water license query page, and from the POD (points of diversion) shown on imap BC (the two sources give somewhat different information). 5

7 The Bonaparte River has 13 domestic licenses and one waterworks license (Village of Cache Creek). Several large tributaries have one or more domestic licenses, including Scottie Creek, Hat Creek, Cornwall Creek, and Fly Creek. (Loon Lake has several domestic water licenses, but this area is not included in the present risk analysis.) It is not known how many of these licenses are actually used as drinking water sources. Cornwall Creek is the only community watershed in the area; only a very small part of this watershed is in the fire perimeter. There are a large number of irrigation licenses on the Bonaparte River and its tributaries. The Bonaparte River and its tributaries have many sediment sources agriculture, cattle grazing, logging roads, highways, and local bank erosion. Erosion from burned areas is unlikely to result in significant adverse impacts to water quality, except in a few locations where high severity burn occurred in riparian areas. However, increased peak flow in some extensively burned watersheds could result in additional sediment entrainment from bank erosion. In the first few rainstorms and snowmelt events after the fire, ash and soot can be washed downstream and enter water intakes. There are many small dams throughout the burn area, most of which serve to retain water into the dry season in support of ranching operations, or to enhance bird habitat. Dam locations were examined on aerial imagery (Google Earth or satellite photos). None of these small dams appear to be in high-risk locations. However, one somewhat larger dam, the Tsotin Lake dam in the Battle Creek drainage, is in a high-risk location. It is described in the detailed risk analysis report. There is a major oil and gas pipeline corridor running from northwest to southeast through the centre of the fire. Small lateral gas pipelines branch off to supply Clinton and Cache Creek. The pipelines are owned by Pembina Pipeline Corp. and Spectra Energy Ltd. (according to signs on the right-of-way). Only one location was noted where the main pipeline corridor might be at risk from post-wildfire erosion; this is noted in the risk analysis tables. A hydroelectric transmission line corridor runs through the southwest part of the fire. Generally, transmission lines are not at significant risk from post-wildfire processes, as the towers are normally located on high ground. There are five known campsites operated by BC Recreation Sites and Trails in the fire area. These are small, primitive campgrounds located on lake or river shores. Two of these sites were inspected (Pressy Lake and Bonaparte River Bridge) and found to be in good condition and not at risk from post-wildfire hazards. From examination of air photos, the others appear to be on flat ground at locations not exposed to any risks. There are four provincial parks in the burn area, but these have no facilities. 5. Burn severity mapping Burn severity maps are used to assess the potential hydrologic effects of the fire in each watershed due to forest cover and soil changes. The definitions of high, moderate, and low vegetation burn severity (VBS) are given in Hope et al. (2015) and Parsons et al. (2010), as well as the procedure for preparing a burn severity map. Briefly, the burn severity categories are: High trees dead, needles, twigs, and understory consumed Moderate trees dead, scorched needles remain on trees, understory burned Low canopy mostly unburned, understory lightly burned. Soil burn severity (SBS) is similarly classified as high, moderate, or low, and is based on the extent of consumption of the forest floor and fuels on the ground, and on the extent of exposed bare soil: 6

8 High forest floor and near-surface roots consumed, mineral soil structure altered Moderate litter consumed, duff partly consumed or charred, mineral soil unaltered Low litter scorched or partly consumed, often with patchy forest floor burn. Vegetation and soil burn severity are usually, but not always, fairly well correlated. For example, a high VBS site may have moderate SBS or vice versa. Water repellency is often, but not always, present on high SBS sites. Where SBS is high, the infiltration capacity of the soil can be greatly reduced, and overland flow may be generated during heavy rain. If this occurs over large areas, soil erosion and downstream flooding can occur. If water repellency is present, the amount of overland flow can be considerably greater. Where VBS is moderate, dead needles remain on the trees. These soon fall, covering the ground with an effective mulch which promotes infiltration and reduces the likelihood of overland flow. Therefore, flood and debris flow hazards from rainstorms are high only where both VBS and SBS are high. Increased flood hazard during spring snowmelt is due to loss of the forest canopy, which results in both a higher winter snowpack and more rapid snowmelt. The effect is similar to that of clearcutting. However, in the first one or two years, the effect of fire may be greater than clearcutting, due to the black colour of burned tree trunks and the soot and debris which falls on the snow. Also, because there is no longer transpiration from trees and understory vegetation, the water table and soil moisture may be higher when winter comes. The flood hazard in a watershed is a function of the area burned at high and moderate VBS; low VBS sites (in which many trees and shrubs have survived) do not contribute much to flood hazard. A burn severity or Burned Area Reflectance Classification map (BARC) was prepared by the Kamloops Fire Centre, based on Landsat 8 satellite images taken on August 19, 2016 (pre-fire) and August 22, 2017 (post-fire). The northernmost part of the fire was still burning at this time. The Cariboo Fire Centre prepared another BARC map using a later Landsat image. Data for the north area was added to the earlier map to prepare the burn severity map used in this report (Appendix A). BARC maps are based on change detection, using satellite images taken at about the same date on consecutive years. There are some anomalies in the data, several which are due to continued fire activity after the date of the second image. As such, there are a few areas of burn that are underrated or not shown on the BARC map (for example, east of Hihium Lake). Some areas are overrated, especially in ranching areas north of Young Lake apparently differences in moisture between the two years are misinterpreted as burn in wetlands, pastures, or irrigated fields which were not burned. Some more site-specific information on burned areas can be obtained from the high-resolution post-fire satellite images (Photo 10 is an example of these is images). Cutblocks which were logged in the year before the fire are likely to be misinterpreted as high-severity burn. 7

9 Photo 1. Low burn severity. (Plot 63, B8 creek) Photo 2. Low-moderate burn severity. (Plot 2, lower Tsotin Creek) Photo 3. Moderate vegetation burn severity with needle fall; Moderate-high soil burn severity with root burnout. (Near Plot 2, lower Tsotin Creek) Photo 4. Moderate-high soil burn severity with water repellency note water drops on mineral soil in shallow soil pit. (Plot 23, near Young Lake) Photo 5. High vegetation and soil burn severity. (Plot 64, south of Scottie Creek) Photo 6. Overland flow during rainfall; high soil burn severity. (Near Tsotin Lake) 8

10 Field data on vegetation and soil burn severity were collected at 81 plots throughout the fire (shown on the burn severity maps, Appendix A and B). The field observations were used to check the accuracy of the high, moderate, low, and unburned categories on the BARC map. The field observations were found to correspond reasonably well to the BARC map categories, and no adjustments to the map were required. On the average, soil burn severity at most plots was somewhat lower than vegetation burn severity (by about half a category; many high VBS sites have moderate to high SBS) although this varied somewhat in different areas of the fire. Considering the very large size and varied forest types of the fire, the number of observation plots was insufficient to draw a separate map of soil burn severity. Water repellent soils were found at many, but not all, plots with high SBS; however, strong water repellency was not usually present over large areas. Photos 1 to 6 show examples of typical low, moderate, and high burn severity sites. 6. Watershed mapping Watershed boundaries in the area had previously been mapped by regional MFLNRO staff. The watershed tables used here are based on this mapping. In areas where flood and debris flow risks might be present, the watersheds were subdivided into smaller units, using 1:20,000 topographic maps, to delineate drainage basins above alluvial fans. (Many streams are not named on published maps, so some names used for watersheds are arbitrary.) In the reconnaissance risk analysis tables (Appendix C), the percentage of high and moderate vegetation burn severity is given by watershed for those watersheds where a significant risk might be present. A complete table of burn severities by watershed is given in Appendix E, and the watersheds are shown on the accompanying maps (Appendix A and B). As an approximate guide, if more than about 10% of a watershed is burned at high soil burn severity (which is correlated with the area of vegetation burn severity, but is likely to be somewhat lower), then there is likely to be a significant hazard of overland flow and rapid runoff during high-intensity rainstorms. If more than about 30% of a watershed is burned at high or moderate vegetation burn severity, then there is likely to be a significant increase in runoff from spring snowmelt, especially if the burn is concentrated in the upper elevations. These are very rough guidelines only; they are based on examples in BC and elsewhere, where changes in hydrologic response have been observed after forest cover changes. An approximate index of potential debris flow hazard is the relief ratio (which is the elevation range divided by the square root of watershed area). If the relief ratio is greater than 0.6, then the watershed is likely to be susceptible to debris flows; if between 0.3 and 0.6, to debris floods. Again, this is a very rough guideline, and debris flow susceptibility depends on many other geologic and hydrologic factors. The relief ratio is included in Appendix E for relevant watersheds. 7. Reconnaissance risk analysis For the reconnaissance part of the project, a map of the fire was obtained from the Kamloops Fire Centre, which showed the locations of all known structures (from Regional District data), private lots and other land status, the fire perimeter, roads, and topographic information. Preliminary burn severity maps were available, including a BARC map from the Cariboo Fire Centre which could be viewed in Google Earth. These maps were used to identify locations where elements at risk, primarily houses, were present 9

11 downstream or downslope from burned areas. Other sources of information, used after the initial field work, included high-resolution post-fire satellite imagery, and maps available on the Thompson-Nicola Regional District web site. Where potential risks were identified, as many areas as possible were visited or viewed in the field. Because of the large size of the fire, the project area was divided into sections based on access. These are shown on Figure 1. A summary of the reconnaissance risk analysis is given in table form by section (Appendix C), and some background information and details on each section are given below. Sections 1 to 5 include the areas for which a detailed risk analysis was done, and further information is provided in that part of the report. 8. Sections Assessed 8.1. Section 1 South of Cache Creek This section includes the area to the south of the mouth of Cache Creek, including a small part of the Village of Cache Creek, and the area south to the Ashcroft Indian Reserve. The most significant hazards are flooding and possible debris flows from steep creeks on the east side of Campbell Hill. Above 800 m elevation, the slopes are forested, and were burned with high and moderate soil and vegetation burn severity. Below 800 m, the area is grassland, and soil burn severity is generally low. The Cornwall Creek FSR climbs through the watersheds of S2, S3 and S4 creeks, crossing several tributary gullies. Generally, this road and the spur road accessing the communications towers on Campbell Hill have inadequate drainage control, with few culverts and no water bars. Therefore, there may be a hazard of accidental drainage diversions between watersheds, which increases the likelihood of debris flows or debris floods. This area includes the Cache Creek landfill, a large industrial operation which processes garbage from Greater Vancouver. Most of the landfill infrastructure is in or adjacent to the S2 drainage. The creek has been diverted through ditches to bypass the facility. The diversion ditches, the road, and possibly some parts of the facility could be at risk in the event of flooding Section 2 Cache Creek and area to east The Village of Cache Creek experienced damaging floods during spring runoff in 2015 and The headwaters of Cache Creek have been extensively logged, and a high ECA (equivalent clearcut area) may have been a factor in causing increased rates of snowmelt runoff. The fire has burned most of the higher elevation areas in the watershed which weren t logged, including the Lopez Creek watershed and the protected area of Arrowstone Creek, so the resulting ECA is very high. Therefore, the incremental hazard due to the fire of flooding along lower Cache Creek is high. In addition to houses and the road along lower Cache Creek, the gas pipeline could be at risk from further flood events where it is adjacent to the creek channel. 10

12 To the east, the Tsotin Creek watershed, which drains into the Tsotin Lake reservoir, has extensive high and moderate severity burn, as well as recent logging at the highest elevations, and therefore has a significant hazard of increased peak flows. The rest of the Battle Creek watershed is not extensively burned Section 3 Lower Bonaparte River valley along Highway 97 This section includes ten or more steep creeks, tributary to the Bonaparte River, each with a large alluvial fan. Six of the fans have residential development; these were subject to detailed risk analysis, and further information is found in that part of the report. The bedrock in this part of the valley is weak, and therefore there are a lot of gullies, each with an ephemeral channel and a small fan bordering the highway and river floodplain. The highly erodible terrain probably leads to frequent small debris flows and lesser erosion events. The fire, which has burned almost all the forested slopes at mid and high elevations, has led to a high incremental hazard of flooding in several watersheds. The relief ratios of the watersheds are from 0.28 to 0.47 which are in the range of debris floods, not debris flows. However, due to the friable rock and abundant colluvium, debris flows are possible under high runoff conditions, especially if a small landslide or debris flow in a steep gully enters the main channel. In September of this year, small debris flows occurred in the B8 and B9 channels, originating in small side gullies at low to mid elevation. An additional risk factor is the logging road which traverses at mid-elevation in watersheds B5 to B9, and several spur roads leading to logging near the ridge tops. Generally, drainage control is substandard on these roads, which creates the chance of accidental drainage diversions from one watershed to the next. Such drainage diversions are a common cause of debris flows, so upgrading the drainage control on these roads should be a high priority. There are several large slump-earthflow type landslides in weak bedrock along the Bonaparte valley, which probably occurred during or shortly after deglaciation. One of these, at site B10 (Photo 7), appears to have had some movement into the river floodplain since deglaciation, and possibly could be subject to a small amount of renewed movement under conditions of high groundwater levels. The upper part of this landslide has been burned at high to moderate severity, so there is a small chance (low hazard) of renewed landslide activity. 11

13 Photo 7. Large bedrock landslide in lower Bonaparte River valley, site B10. Highway 97 Loon Lake Road junction at right. (Google Earth oblique view) 8.4. Section 4 Bonaparte valley along Loon Lake road Similar to Section 3, there are several small tributary watersheds with substantial burn areas. These have large alluvial fans occupied by ranches, which are subject to moderate to high hazards from flooding, or possibly from debris flows. The intervening slopes have many complex gullies with small, steep fans built by small debris flows Section 5 Maiden Creek to Clinton Northwest of the Bonaparte River valley, Highway 97 follows the valleys of Maiden Creek and Allen Creek, and the interior-draining Alkali Lakes valley. Several alluvial fans along these valleys have 12

14 ranches or other habitation which may be exposed to possible debris flows from burned slopes above. The Village of Clinton is on the northwest side of the Clinton Creek valley, and is therefore separated from burned slopes on Hart Ridge to the southeast. There are two locations along the southeast side of the Clinton Creek valley (noted in the tables) where properties are below patchy moderate and high burn severity, however there are no obvious creek channels or alluvial fans, so flooding risks are likely to be low Section 6 Lower Hat Creek Several houses on the Lower Hat Creek Reserve are located between Hat Creek and Highway 99. The highway effectively protects the properties from minor flood events or debris flows, and most of the slopes are burned at low or moderate severity. The H4 watershed is severely burned; the highway crossing of the creek is vulnerable, but nearby houses are not at risk Section 7 Scottie Creek The Scottie Creek watershed was severely burned, especially the lower part of the watershed (about 1/3 of the total area) and its northern tributary, Chrome Creek. The lower watershed has steep slopes and gullied terrain above the creek, which may be subject to erosion and small debris flows. The creek is likely to experience increased peak flow in spring, and as well as increased sediment load. Therefore, properties near the mouth of the creek, and the Highway 97 crossing, could be at risk of flooding and possible aggradation of the channel (Site B11 in the detailed risk analysis). The upper part of the watershed is less severely burned, and slopes are mostly gentle. However, it has been heavily logged, and there may already be increased peak flow effects Section 8 Loon Creek and Hihium Creek/Lake The lower Loon Creek-Loon Lake area is not included in this report, as it was assessed under a different contract. However, we did include the watershed and its main tributaries in our burn severity calculations, including several field plots in the watershed. Upstream from Loon Lake, the slopes above the ranch on upper Loon Creek are not severely burned. The main upper tributary, Brigade Creek, mostly escaped the fire. North of Upper Loon Lake, there is one isolated property (DL 7385), on gently sloping terrain, which does not appear on post-fire satellite imagery to be significantly affected by the fire. The lower watershed of Hihium Creek, including steep slopes above the channel, were burned at high severity, so increased peak flow and sediment load could affect lower Loon Creek. The area around Hihium Lake, at the upper end of the watershed, is low-relief terrain. Most of the developed area on the east and north shores of the lake escaped the fire, and is not at risk or at low risk from post-fire processes. (We did not visit this area; our assessment is based on aerial and satellite imagery.) 13

15 8.9. Section 9 Middle Bonaparte River valley From the Rayfield River to Loon Creek, the Bonaparte River flows through a narrow valley which is incised about 200 to 400 m into the plateau. From Chasm Creek to below Clinton Creek, there are a series of large, probably inactive, bedrock landslides on the left (southeast) slope of the valley, where lava flows of the Chilcotin group overlie the much weaker, older rocks of the Cache Creek group (Photo 8). It is very unlikely that these landslides would be affected by increased runoff or groundwater levels due to the fire. Over the last 12,000 years the area has experienced many repeated cycles of forest fires followed by regrowth of the forest, as well as climatic variations. C B A Photo 8. Google Earth oblique view looking southeast over middle Bonaparte River valley. A large bedrock landslide (A) has blocked the valley. Mound-Loon Lake Road ascends the lower part of another landslide complex (B) which has disrupted the drainage of a small creek (left edge of landslide). A residential property is located alongside this creek (C). The flat-bottomed valley of the Bonaparte River, formed when the river was dammed behind one of these landslides, is occupied by farmland in the Mound Road area. Upstream from Chasm Creek, the river valley is bordered in places by glaciofluvial terraces, several of which are occupied by ranches. 14

16 At the mouth of Clinton Creek, there are two properties which are at the base of a slope which has patchy high and moderate severity burn; however, risks to buildings appear to be low. There is a residential property below the Mound-Loon Lake Road (DL 7795; see Photo 8) alongside a creek which drains a high-severity burn area on the plateau. We inspected the creek valley above (we did not inspect the property itself) and found that there is no channel which could carry water there are a series of depressions and ridges formed by the ancient landslide, and water from the contributing watershed above must flow underground. The risk to the property is therefore assumed to be low. The ranches on terraces of the Bonaparte River upstream appear to be at low risk from runoff or erosion from nearby burned slopes. However, on the furthest ranch upstream (DL 786) there are buildings on a small fan at the mouth of Fly Creek, which could possibly be at risk from flooding of the creek. The hazard and risk are considered moderate, based on the extent of burn in the Fly Creek watershed Section 10 North area, Young Lake - Pressy Lake Most of the northern part of the burned area is a level to gently rolling plateau, underlain by the basalt lava flows of the Chilcotin group. There are many lakes, some with no outlet, and others connected by small, seasonal streams. The Rayfield River has cut a narrow canyon through the lava flows. Young Lake, on the Bonaparte River, is in a zone of glaciofluvial outwash, terraces, small lakes, and esker-like ridges, which fill the valley for 10 km from the lake to the mouth of the Rayfield River. These glaciofluvial deposits are very permeable, and tributary streams such as Campeau Creek disappear into them, with no surface water connection to the river. There is a subdivision with many homes on small lots along the north side of Young Lake. There are some severely burned slopes north of the lake, drained by several small streams; however, these probably disappear into the glaciofluvial terraces without reaching the developed area along the lakeshore. There are two properties on level ground along the Bonaparte River above Young Lake, which also do not appear vulnerable to runoff from burned slopes to the north. At the Pressy Lake subdivision, most lots are on a glaciofluvial ridge or terrace between the lake and a wetland parallel to the lake, and are therefore are not exposed to runoff from burned slopes to the south. At the west end of the lake, however, there is a group of about 24 lots which are on a small alluvial fan complex, formed by several small, ephemeral creeks which drain the slopes and plateau to the north (Photos 9 and 10). Their drainage basins, a combined area of about 7 km 2, are burned at high vegetation burn severity and moderate to high soil burn severity. Increased peak flow is probable on these streams, and they could carry sediment from erosion of their channels along their lowest few hundred metres. The channels are probably not steep enough to produce debris flows however. The North Bonaparte Road follows the base of the slope above the lots, and it would likely catch a lot of sediment; however, it also could divert floodwaters to unpredictable locations. Most of the homes in this area were burned. If rebuilt in their original locations close to the lakeshore, they are far enough from the base of the slope that they probably would experience only minor flooding. 15

17 Photo 9. View from over Pressy Lake, looking southwest. Properties at west end of lake (centre) are on an alluvial fan complex at the mouth of creeks draining the plateau to the right. (Photo by Rita Winkler.) Photo 10. Pressy Lake, as seen in post-fire false-colour infrared satellite image (north is up). The creeks feeding the alluvial fan complex (centre) drain an area of high severity burn on the plateau to the north. 16

18 There are several ranches on the plateau between Young Lake and North Bonaparte Road (DL's 4964, 4908, 1387, 1383, 1393, and 1382). These are on flat to gently rolling terrain, and none of them appear to be in locations where flooding from nearby streams is likely. However, some of the properties could be affected by high water in adjacent lakes or wetlands, due to increased snow accumulation and reduced evapotranspiration in burned areas. The south shore of Green Lake has many homes, and there is some burned area on the slopes to the south and east. However, the burned area is not extensive, the terrain is gentle, and the small creeks draining the area pass through small lakes or wetlands before reaching Green Lake. Risks due to the fire are therefore very low. At Sheridan Lake, the situation is similar only a small area of the watershed has been burned, the area is relatively flat, and there are small lakes and wetlands along tributary streams Section 11 Deadman River valley On the east side of the fire, several watersheds drain east into the Deadman River valley, where there are ranches at the mouths of several of the tributary creeks. This area was not visited in the field, but was reviewed on aerial imagery (Google Earth) and satellite images. The only watershed with extensive high and moderate severity burn is Barricade Creek. The ranch buildings near the mouth of the creek are below the Deadman-Vidette road, close to the river and not near the apex of the fan. The risk to the buildings from flooding, therefore, is believed to be low. Most of the watersheds draining the east side of the fire have had extensive logging in their headwaters, and flood hazard due to the clearcut area in these watersheds is probably more important than effects of the fire. 9. Watershed management considerations The following watersheds have high pre-fire ECA s from logging in their headwaters, as well as large burn areas and potential downstream risks from flooding. Therefore, salvage logging of low and moderate burn severity stands is not recommended, as this will increase the ECA. Any salvage logging plans in these watersheds should be reviewed for potential downstream impacts. Cache Creek Scottie Creek B7 Creek B8 Creek Hihium Creek Upper Clemes Creek Barricade Creek 10. Summary of Recommendations Recommendations from the reconnaissance stage of the risk analysis, with land status or responsibility where known, include: 17

19 Section 1 In the watersheds of creeks S2, S3, and S4, inspect drainage structures on the Cornwall Creek FSR (Patterson Rd), and install additional culverts where necessary. (Provincial forest) Section 3 Upgrade drainage on logging roads in upper watersheds of B5 to B8, or deactivate roads. (Provincial forest) Section 4 Monitor creek crossings on highway, especially H4; clean culvert inlets as necessary. (MOTI) Road drainage on transmission line and logging road (Veasy Lake Road) above should be inspected, and upgraded as necessary to prevent drainage diversions between watersheds. (Provincial forest, BC Hydro) Section 9 Pipeline operator should inspect pipeline in the vicinity of Chasm Road after significant snowmelt or rain events, and improve drainage control as necessary on pipeline right-of-way and adjacent access roads. (Pipeline right-of-way) Section 10 Ensure that ditches and culverts along North Bonaparte Road are adequate to carry increased flows from small streams along Pressy Lake. Flow from culverts should be directed to minimize risks to houses below. (MOTI) On rebuilding homes, owners should plan adequate drainage on their lots to minimize exposure of the homes to possible floodwaters. (Private land) More detailed site-specific assessments were conducted where dictated by the reconnaissance stage of the risk analysis. Recommendations from the detailed site-specific risk analysis are shown in Appendix D. The recommendations are provided for general and site-specific risk reduction strategies. The recommendations are not intended as an evaluation of the acceptability of either the present risk or residual risk given the implementation of a risk reduction strategy. In addition, some risk reduction strategies may result in increased risks for downslope values so any measures implemented must consider the potential for both positive and negative consequences. The detail provided in Appendix D for site specific risk reduction measures is considered preliminary and has not undergone the necessary analyses and does not include the detail required for construction. For example, where berms may reduce the hazard and risk, their location, width, height, shape, length, and composition requires more detailed site surveys and design to ensure the implemented measures satisfy the desired risk reduction. 11. Closure Report Use and Limitations This report was prepared for the exclusive use of the MFLNRO. The material in it reflects SNT Geotechnical Ltd. s best judgment and professional opinion in light of the information available to it at the time of preparation. Any use which a third party makes of this report or any reliance on or decision to be made based on it are the responsibility of such third parties. SNT Geotechnical Engineering Ltd. accepts no responsibility for damages, if any, suffered by any third party as a result of decision made or action based, or lack thereof, on this report. No other warranty is made, either expressed or implied. 18

20 The report and assessment have been carried out in accordance with generally accepted professional practices in B.C. The discussion and recommendations presented are based on available information and limited field investigation and inferences from surficial features. No subsurface investigation was carried out as part of this assessment or development of conclusions or recommendations. Inherent variability in local precipitation, run-off conditions, soil and vegetation burn severity, surface and subsurface conditions may create unforeseen situations. Property boundaries (private, municipal, reserve, crown) referred to on maps and in the text were obtained via publically available cadastral layers overlain onto orthoimagery (e.g.: Regional District of Thompson-Nicola web site) and is approximate and may not be accurate for the purposes of locating risk mitigation strategies. Boundaries should be confirmed prior to design and implementation of risk mitigation strategies. Prepared by: ORIGINAL SIGNED AND SEALED Peter Jordan, P.Geo., PhD. ORIGINAL SIGNED AND SEALED Doug Nicol, P.Eng. ORIGINAL SIGNED AND SEALED Mike Walsh, P.Eng. ORIGINAL SIGNED AND SEALED Michael Burnett, EIT 19

21 12. References Hope, G., P. Jordan, R. Winkler, T. Giles, M. Curran, K. Soneff, and B. Chapman Post-wildfire natural hazards risk analysis in British Columbia. Province of B.C., Victoria, B.C. Land Management Handbook Jordan, P Post-wildfire debris flows in southern British Columbia, Canada. International Journal of Wildland Fire, 25(3), pp Parsons, A, Robichaud, PR, Lewis, SA, Napper, C, and Clark, JT Field guide for mapping post-fire soil burn severity. USDA Forest Service, Rocky Mountain Research Station, General Technical Report RMRS-GTR Wise, M.P., G.D. Moore, and D.F. VanDine (editors) Landslide risk case studies in forest development planning and operations. B.C. Min. For., Res. Br., Victoria, B.C. Land Manage. Handbook No. 56. < 20

22 Appendices A1