Sicamous Narrows Hydraulic Conveyance Assessment April 12, 2017 KWL Project No

District of Sicamous

Sicamous Narrows Hydraulic Conveyance Assessment April 12, 2017 KWL Project No. 3239.016 Prepared for: Prepared by: Jeromy Schuetze AScT Dwayne Meredith, P.Ag. Joe McCulloch

DISTRICT OF SICAMOUS Sicamous Narrows Hydraulic Conveyance Assessment April 2017

Contents

Executive Summary ...... i

1. Introduction ...... 1-1

2. Field Work ...... 2-1 2.1 Field Quality Control and Assurance ...... 2-2

3. Assessment ...... 3-1 3.1 Geomorphic Assessment ...... 3-1 3.2 Hydrology and Flood History ...... 3-7 3.3 Hydraulic Model ...... 3-18 3.4 Climate Change Considerations ...... 3-25

4. Summary and Recommendations ...... 4-1 4.1 Summary ...... 4-1 4.2 Recommendations ...... 4-2

5. Report Submission ...... 5-1

Figures Figure 1-1: Location Plan ...... 1-1 Figure 3-1: Shoreline development along Sicamous Narrows in 1950, 1984 and 2007...... 3-3 Figure 3-2: 1928 Air Photograph of Mara Lake and Sicamous Narrows ...... 3-4 Figure 3-3: Grain Size Samples and Summary Characteristics ...... 3-6 Figure 3-4: Sicamous Narrows Hydrology ...... 3-9 Figure 3-5: Mara and Shuswap Lake Level Differences based on Shuswap Lake Level ...... 3-11 Figure 3-6: Annual Comparison of Shuswap River discharge ...... 3-12 Figure 3-7: Historic Flood Water Levels ...... 3-16 Figure 3-8: Lake Bathymetry from BC MOE ...... 3-19 Figure 3-9: 10 and 200 Year Flood Modelled Extents ...... 3-23 Figure 3-10: Sicamous Narrow Existing and Dredged Profile ...... 3-24

Tables Table 3-1: Historical Air Photographs Reviewed ...... 3-1 Table 3-2: Hydrology Data Sources & Descriptions ...... 3-8 Table 3-3: Shuswap Lake Return Period Water Levels ...... 3-13 Table 3-4: Flood Year Data ...... 3-13 Table 3-5: Return Period River Discharge ...... 3-17 Table 3-6: Unsteady Flow Model Input ...... 3-20 Table 3-7: Lake Level Records by Shuswap Lake Watch ...... 3-20 Table 3-8: Model Input for Calibration ...... 3-20

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DISTRICT OF SICAMOUS Sicamous Narrows Hydraulic Conveyance Assessment April 2017

Table 3-9: Peak Water Level for Existing Conditions ...... 3-21 Table 3-10: Peak Water Level for the Dredging Channel Conditions ...... 3-22 Table 3-11: Peak Water Level without Boat Docks ...... 3-22

Appendices None

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DISTRICT OF SICAMOUS Sicamous Narrows Hydraulic Conveyance Assessment April 2017

Executive Summary The District of Sicamous is located in an idyllic setting between Mara and Shuswap Lakes. Sicamous Narrows is the waterway channel between the two lakes around which the downtown is concentrated. It is a unique channel, which has the appearance of a river, but behaves differently at various times of the year due to elevation changes in the lakes at each end. Elevated water levels on Mara and Shuswap Lakes in recent years have caused high water in the Narrows and flooded District property and infrastructure. There are concerns that sediment is infilling Sicamous Narrows, reducing the efficiency of the watercourse to convey water downstream. The concern is that reduced conveyance (hydraulic capacity) could cause Sicamous Narrows and Mara Lake water levels to increase during freshet and storm events and remain high for extended periods of time. This study investigates potential causes of flooding in the District including:  channel changes including erosion and sedimentation;  higher inflows (e.g. long-term climate cycles and climate change impacts);  changes in the downstream water level at Shuswap Lake; and  riparian area development and construction of docks. Field work was done to collect data to determine the bathymetry and obtain sediment samples. A geomorphic assessment reviewed air photos from the earliest available in 1928 to current day, reviewed potential sediment sources and the likelihood of sediment transport. The regional hydrology was reviewed and a 1-dimensional hydraulic model was developed to examine potential channel improvement scenarios. In summary, the findings are that no significant channel morphological changes were found; the landforms from 1928 had not appreciably changed. The difference between high lake levels from Mara Lake to Shuswap lake was minimal, estimated to be 0.2 m, and there were no trends observed in the available data for lake elevations. That is, no trends in timing or volume of run-off could be found in the data; however, limited available data on Mara Lake introduces some uncertainty. Climate change is anticipated to cause a 10% increase in peak flows resulting in higher peaks occurring more frequently. The hydraulic model indicates that the relatively flat topography and small difference between the two lake elevations has significant effects in the District during high water conditions. The District may experience flooding to elevation 349 m (excluding freeboard) in less than a 10-year return period event. The model also indicated that the mitigation options of dredging or removing docks had little effect (0.05 m and 0.10 m, respectively) on water levels. Two recommendations were derived from the assessment: 1) develop an integrated flood hazard management plan to define hazards, consequences and risks, and engage First Nations, various stakeholders and the public to develop structural and non-structural flood mitigation measures and 2) implement consistent, reliable data monitoring platforms that would inform future technical analysis, planners and emergency responders at the District.

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DISTRICT OF SICAMOUS Sicamous Narrows Hydraulic Conveyance Assessment April 2017

1. Introduction Development in the District of Sicamous (the District) is concentrated near the shore of Mara Lake and along the channel that conveys flow from Mara Lake to Shuswap Lake. This channel is known as Sicamous Narrows, refer to Figure 1-1. It is a unique channel that has the appearance of a river, but behaves differently at various times of the year due to small elevation changes in the lakes at each end. In recent years, elevated water levels on Shuswap and Mara Lakes have flooded the surrounding District property, private lands, a provincial park, and infrastructure. In addition, sustained higher water levels on Mara Lake increase the likelihood for successive storms to impact the basin and may cause more frequent and more severe flooding. There are concerns that sediment is infilling Sicamous Narrows, reducing the efficiency of the watercourse to convey water downstream. The concern is that reduced conveyance (hydraulic capacity) could cause Sicamous Narrows and Mara Lake water levels to increase during freshet and storm events and remain high for extended periods of time. This study investigates potential causes of flooding in the District including:  channel changes including erosion and sedimentation;  riparian area development and construction of docks;  higher inflows (e.g. long-term climate cycles and climate change impacts); and  changes in the downstream water level at Shuswap Lake. The scope of this report provides the results of a bathymetric survey, a geomorphic assessment, a review of the local hydrology, and the development of a 1-dimensional hydraulic model to examine potential channel improvement scenarios.

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3 Reference: Orthophoto fromthe District of Sicamous. 2 3 \ Mara Lake p Copyright Notice: These materials are copyright of Kerr Wood Leidal Associates Ltd. (KWL).

R District of Sicamous is permitted to reproduce the materials for archiving and for distribution to - third parties only as required to conduct business specifically relating to the District of D Sicamous Sicamous Narrows Hydraulic Conductivity. Any other use of these materials without X the written permission of KWL is prohibited. M \ S I G - 0 3 4

\ District of Sicamous 6 1 0

- Sicamous Narrows Hydraulic Conductivity Assessment

9 Project No. Date 9 2 3

- 3239-016 December 2016 0 u 0 a 2 L 3 150 0 150

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P A 1:10,000 Figure 1-1

DISTRICT OF SICAMOUS Sicamous Narrows Hydraulic Conveyance Assessment April 2017

2. Field Work The field work was performed on September 9, 2016. The field data were collected using a Real-time Kinetic (RTK) unit and an Acoustic Doppler Profiler (ADP) instrument deployed on a 5.5 m fiberglass boat. The ADP (Sontek M9) was used to measure flow depths throughout the channel and perform a channel discharge measurement. In conjunction, the RTK unit (Trimble R8) was used to obtain accurate position information (Northing, Easting, and Elevation) during the ADP data collection process. The ADP was configured to record a reading every second. ADP and RTK measurements were synchronized in the field using the Sontek RiverSurveyorLive software. The hydraulic survey was conducted to maintain integrity of the data of the survey throughout the data collection process. RTK base station data was collected and submitted to Natural Resources Canada for processing with their Precise Point Processing (PPP) software. The resulting three-dimensional solution was applied to the dataset and the corrected data compared to a Provincial Geodetic Control Marker (GCM 256033) elevation. Additionally, temporary control points were surveyed before and after the hydraulic survey to verify the RTK equipment maintained its accuracy throughout the survey.

Photo 1: Trimble R8 RTK unit mounted to the Photo 2: Sontek M9 ADP Unit vessel, right in photo.

In conjunction with the bathymetric survey, six sediment samples were collected using an Ekman Dredge and transported to the lab for sieve analysis. The sample locations were pre-determined to represent various channel reaches. Details regarding the sediment analysis may be found later in the report.

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DISTRICT OF SICAMOUS Sicamous Narrows Hydraulic Conveyance Assessment April 2017

2.1 Field Quality Control and Assurance The field quality control and assurance were as described below:  Position and elevation data from the RTK rover unit were checked by surveying a known point (i.e., GCM and a temporary benchmark) at the beginning and end of the survey day. The two measurements were within ±2 cm of each other to ensure that the RTK maintained its ±2 cm accuracy throughout the day.  The field crew visually checked the ADP unit to make sure that it was in good working order. Special attention was paid to the beams of the ADP which are integral to the collection of accurate measurements.  Once the ADP was deployed, a water depth calibration file was run to provide proper depth measurements and all equipment offsets were accounted for. The ADP unit does not require calibration because it has no moving parts and the manufacturer does not require or recommend a daily calibration.  During the bathymetric data collection, a visual ship track from the ADP RiverSurveyorLive software was used to monitor alignment of transects and data collection coverage.  During the bathymetric survey, operators monitored the signal from the RTK and bottom tracking values from the ADP. If signals were to fall below acceptable limits, the survey would be stopped to address the problem. Acceptable limits for RTK signal are within ±5 cm accuracy.  During channel discharge measurement, care was taken in selecting appropriate cross-sectional measurement location. For a discharge measurement to be valid, it is required that: 1) an equal number of left to right and right to left transects be surveyed (minimum of total of 4 transects) ; and 2) the standard deviation between transects should be within ±5%.

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DISTRICT OF SICAMOUS Sicamous Narrows Hydraulic Conveyance Assessment April 2017

3. Assessment

3.1 Geomorphic Assessment The geomorphic assessment was focused on two main issues: 1. Documenting changes in the channel condition over time; and 2. Developing an understanding of how the Mara Lake-Sicamous Narrows-Shuswap Lake system functions, in terms of sediment sources and deposition patterns. Sources of information included the following:  relevant background reports,  historical air photographs,  bathymetric survey of the Narrows, and  sediment samples collected from the channel and area. The following sections describe the results of the geomorphic assessment.

3.1.1 Air Photograph Review Historical air photographs (hard-copy stereo-pairs) of Sicamous Narrows were obtained from the UBC Geographic Information Centre1 and reviewed using a stereoscope. Additional, earlier photos were obtained from the National Air Photo Library to extend the photo series earlier into the 20th century. Photos reviewed for this project are summarized in the following table.

Table 3-1: Historical Air Photographs Reviewed Roll / Nominal Photo Date Notes Photo Numbers Scale 2007 15BCC07016 #176-177  Shuswap Lake el. 346.41 m to 347.72 m. 1:20,000 (May 7, Jul 7) 15BCC07010 #88-86  Eagle River flow 54.5 m3/s to 92.9 m3/s. 1994 30BCB94003 #88-85, 1:10,000  No lake level or discharge data available. (Mar 29) 59-68 1984 30BC84078 #85-86, 1:25,000  Photo date not available. (N/A) 218-217 (est.) 1974 BC5597 #45-43, 48-51,  Shuswap Lake el. 348.49 m. 1:10,000 (Jun 18) 34-36  Eagle River flow 259 m3/s.  Shuswap Lake el. 348.39 m to 346.54 m. 1967 (Jun 15) BC5246 #159, 162 1:32,000  Eagle River flow 163 m3/s to 24.7 m3/s. 1969 (Aug 2) BC5353 #208-207  Incomplete coverage of study area. 1959 BC2627 #13-11  No lake level or discharge data available, N/A (Jul 16-17) BC2615 #10-11 but Shuswap Lake relatively high.

1 Air photographs are supplied by the UBC Geographic Centre as a loan rather than a purchase.

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DISTRICT OF SICAMOUS Sicamous Narrows Hydraulic Conveyance Assessment April 2017

Roll / Nominal Photo Date Notes Photo Numbers Scale BC1092 #33-35, 1950 (Jun 27)  No lake level or discharge data available, BC1093 #6-3, 8-9 N/A 1951 (Jul 11 & 30) but Shuswap Lake relatively high. BC1292 #28-30 1928 A368 #86-88  Shuswap Lake about 1.5 m lower than 1:15,000 (Jul 25-26) A379 #11-13 average peak level. Overall, the historical photos show consistency over time in the landforms and features in the study area. Features visible in the 1928 photos are still discernible, and similar in appearance to the 2007 photos (or more recent imagery such as in Google Earth). This is relevant to the hydraulic study because it suggests that changes in morphology over the approximately 70 years of record that was reviewed are not sufficiently pronounced that they can be detected by visual comparison, i.e. the changes are likely to be modest and gradual. Additional observations from the air photo review that are pertinent to the Sicamous Narrows hydraulic study include the following:  The earliest photos reviewed (1928) show very little development along Sicamous Narrows, apart from the area immediately south of the railway bridge.  Progressing through the air photo series to later and later photos, development of the area surrounding the Narrows progressively increases. Changes include: conversion of forested areas to residential and agricultural uses, and construction of docks in the Narrows.  In 1928 there were about 5 dock structures in the Narrows, all concentrated in the developed area near the railway crossing. This had not greatly changed by the time of the 1950 photos. By the time of the 1969 photo, development extended all the way along the Narrows and onto the Mara Lake shoreline and docks were scattered along the length of the channel (about three times as many docks in the Narrows as in 1928). Subsequent decades show a general increase in the density of the docks over the Narrows shoreline, filling in an increasing proportion of the shoreline and the previously open water adjacent to the shoreline. Figure 3-1 presents a side-by-side contrast of the Narrows development between 1950, 1984 and 2007, for illustration of the trends described.  Erosion at meander bends on Eagle River can be clearly observed from air photos. This material is likely part of the sediment that is deposited at the mouth of the river, downstream of the Narrows.

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DISTRICT OF SICAMOUS Sicamous Narrows Hydraulic Conveyance Assessment April 2017

1950 1984 2007

Figure 3-1: Shoreline development along Sicamous Narrows in 1950, 1984 and 2007.

3.1.2 Sediment Sources and Sediment Transport From review of air photographs coupled with knowledge of sediment transport processes, it is inferred that sediment that forms the channel bed in the majority of the Narrows is likely from two sources:  original post-glacial sediment deposits (i.e. Eagle River floodplain material), and  material transported from the beach deposit at the north end of Mara Lake. The large deposit at the outlet of the Narrows has a third source, which is Eagle River; however, transport of Eagle River sediment into the Narrows is not likely to penetrate far up the channel (as this movement would be against the dominant flow direction), and therefore it is unlikely to affect the majority of the Narrows. The 1928 air photograph shows that the sediment deposit at the north end of Mara Lake (the beach) extends into the Narrows: essentially this is one continuous feature (Figure 3-2). The materials surrounding the lake and therefore composing the lakeshore would be expected to be primarily composed of post-glacial sediments, most recently Eagle River floodplain deposits. In addition, some material is likely deposited on the Mara Lake shoreline from Gillis Creek, which enters the lake at the east end of the beach (see Figure 3-2). Other sediment sources to Mara Lake such as Sicamous Creek and Hummingbird Creek are not likely to be important sources to the beach deposit at the north end of the lake for two reasons: 1. The sediment delivery from these creek sources is episodic, occurring in conjunction with the highest flows of the year, which is a relatively short proportion of the year. For the remainder of the year it is anticipated that the water from the creeks would not be carrying much sediment. 2. Aerial photos show no evidence of longshore transport occurring along the east shore of Mara Lake to connect the creek sources to the beach deposit at the north end of the lake. The typical mechanism whereby coarser sediments (e.g., sand and fine gravel) are moved along a lake (or ocean) shore is by wave action re-working material that makes up the shoreline. The force of the waves is most effective in the upper-most few meters of the water column: material that is deposited deep on the lake bottom cannot be mobilized by surface waves. There is little photo evidence of a

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DISTRICT OF SICAMOUS Sicamous Narrows Hydraulic Conveyance Assessment April 2017

shoreline deposit of sediment along the east shore of Mara Lake between Sicamous Creek (or Hummingbird Creek) and the beach at the north end of the lake that would indicate that longshore transport between the creeks and the north end of the lake is occurring. Plumes of turbid water generated by the creek flood events are unlikely to be able to move significant volumes of sediment of the size fraction that would be deposited in the beach. As an example, average measured water velocity measured at 20 m to 35 m depth in Mara Lake is in the order of 0.01 m/s (Self and Larratt, 2013). Comparing this velocity to the settling velocity for different grain sizes, a current of 0.01 m/s potentially would be capable of keeping in motion material finer than about 0.15 mm (Robert, 2003) (i.e. very fine sand, silt and clay). Coarser material would be not be mobile.

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Figure 3-2: 1928 Air Photograph of Mara Lake and Sicamous Narrows (A368, #87; flight date: July 25, 1928).

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DISTRICT OF SICAMOUS Sicamous Narrows Hydraulic Conveyance Assessment April 2017

There is evidence of longshore transport of material from Mara Lake into the Narrows, particularly at the eastern end of the beach, as evidenced by the pattern of deposition and erosion around the dock structures: there is more sediment on the east side compared with the west side. Given the beach geometry compared with the prevailing wind/wave climate, longshore transport is also more likely at the east end of the beach where the waves are impacting the shoreline in a more parallel orientation. Further to the west the waves would impact the shoreline almost perpendicularly, leaving little energy to drive longshore transport; however, there could be a small amount of longshore transport into Sicamous Narrows. Given the expected lower energy of the waves, it is anticipated that transport of material into the Narrows would be a relatively slow process and would likely see entrainment of relatively smaller- size sediments. The deposit of sediment at the north end of the Narrows is likely material from Eagle River that has been pushed by waves. Winds oriented with Shuswap Lake geometry (i.e. from the south-west or from the north), would generate waves that would push some of the sediment from Eagle River toward the mouth of the Narrows. Six sediment samples were collected at various points along Sicamous Narrows channel and at the deposit at the north end of the channel as part of this project. Samples were collected using an Ekman dredge. Sample material was collected for later laboratory sieve analysis. Figure 3-3 shows the locations of these samples and also summary grain size distribution information. The samples indicate a pattern of size that is finest at the south end of Sicamous Narrows, coarsest near the bridge crossings, and then finer at the mouth of the Narrows (but still coarser than the south end samples). Grain size variation can be explained by two main factors:  the source of the materials, and  the energy of the environment in which it is located. Fine sediments typically cannot persist in very high-energy environments because they would be washed away. As such, the samples at the south end of the Narrows are consistent with what would be expected from a relatively low-energy longshore transport environment, such as was described above. The coarser sediments near the bridge and at the outlet may be affected by local hydraulics around the bridge, and/or sediments from Eagle River. Sediment samples were also collected in 2015 as part of a bathymetric survey project, but were not intended for detailed grain size distribution characterization.

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3 Mara Lake Reference: Orthophoto fromthe District of Sicamous. 2 3 \

p Copyright Notice: These materials are copyright of Kerr Wood Leidal Associates Ltd. (KWL).

R District of Sicamous is permitted to reproduce the materials for archiving and for distribution to - third parties only as required to conduct business specifically relating to the District of Sicamous D Sicamous Narrows Hydraulic Conductivity. Any other use of these materials without the written X permission of KWL is prohibited. M \ S I G - 0 3 4

\ District of Sicamous 6 1 0

- Sicamous Narrows Hydraulic Conductivity Assessment

9 Project No. Date 9 2 3 - 3239-016 December 2016 Grain Size Samples and 0 u 0 a 2 L 3 150 0 150 \ J :

: R r Summary Characteristics

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P A 1:10,000 Figure 3-3

DISTRICT OF SICAMOUS Sicamous Narrows Hydraulic Conveyance Assessment April 2017

3.1.3 Comparison of 2015 and 2016 Channel Surveys Sicamous Narrows channel survey (bathymetry) was compared between the 2015 (MoTI) and 2016 (KWL) surveys to evaluate change in that period. Detection of changes was challenging given some important differences between the two surveys, both in terms of timing of the surveys and in instrumentation. The MoTI survey was conducted in April 2015, while the KWL survey was conducted in September 2016. Substantial aquatic vegetation growth was noted by KWL during the September survey, with plants rooted on the channel bottom but extending to near the water surface. This vegetation growth has the potential to produce false bottom readings that would be artificially high, as the survey instrument can get a signal return from the plant material that is suspended in the water column. It is expected that vegetation growth would be less in April. MoTI used a multibeam ‘swath’ survey instrument which produces a high density of data, while KWL used an instrument which has a single beam to detect bottom depth. The comparison of the 2015 and 2016 surveys indicated many areas with very little change (elevation differences of ±0.1 m). Areas where greater differences are indicated are difficult to interpret given the factors listed above, and may simply be the result of vegetation growth, rather than sedimentation. 3.2 Hydrology and Flood History Key hydrologic features in the study area are Mara Lake, Sicamous Narrows, Shuswap Lake, and Eagle River. The hydrologic analysis required an assessment of water levels and flows over a broader area, which is described in brief below and is presented in Figure 3-4:  Shuswap River watershed discharges to Mara Lake and accounts for approximately 80% of the total drainage area into the Lake.  Mara Lake flows into Shuswap Lake via Sicamous Narrows. The Mara Lake watershed is over 40% of the total watershed area of Shuswap Lake.  Eagle River also discharges to Shuswap Lake just northwest of the mouth of Sicamous Narrows.  Shuswap Lake has several arms, which ultimately flow to the South Thompson River. Salmon Arm flows north past Sicamous and then west through Cinnemousun Narrows along with flow from Anstey Arm in the north. Discharge from the Narrows joins Seymour Arm from the north and flows west past Sorrento.  Northwest of Sorrento, the Adams Lake watershed (3,380 km2) discharges to Shuswap Lake just above Little Shuswap Lake which discharges to the South Thompson River near Chase. There are three Water Survey Canada (WSC) lake level gauges at three locations on Shuswap Lake at Sicamous, Canoe and Salmon Arm. Only the gauge at Salmon Arm remains active. WSC also has hydrometric stations that measure flow on Shuswap River at Enderby, Eagle River upstream of Sicamous, and the South Thompson River at Chase below Little Shuswap Lake. Information from the WSC gauges noted above as well as additional manual gauge data collected by Shuswap Lake Watch (shuswaplakewatch.com) was used to complete the historical hydrologic analysis. The missing Shuswap Lake at Sicamous data was estimated based on the following relationship: Sicamous water levels equal Salmon Arm water levels less 0.03 m. The relationship has a Nash- Sutcliffe efficiency of 0.999, which is very good.

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For comparison, Shuswap Lake at Canoe data was used to fill the data gap from 1986 to 2010, to create a second composite dataset. However, as there was no overlapping data, no relationship could be developed to adjust Canoe data to Sicamous data. As a result the Canoe data was used directly in the second composite dataset. A summary and description of the gauges is provided in Table 3-2 below. The gauge locations are shown on Figure 3-4 along with the predominant direction of flow.

Table 3-2: Hydrology Data Sources & Descriptions WSC Data Gaps & Drainage Name Data Range Station No. Comments Area (km2) Shuswap River Near Enderby 08LC002 1911-2013 Missing 1935-1961 4,270 Mara Lake near Sicamous 08LC037 1961-1974 No significant gaps Manual readings n/a from a staff gauge; Mara Lake (Shuswap 1994-2012 gauge relocated to 5,470 Lake Watch) Shuswap Lake in 2012 Shuswap Lake near 08LE053 1923-1974 Missing 1929-1961 Sicamous Manual readings n/a Shuswap Lake near from staff gauge; (Shuswap 2012-Present Sicamous gauge relocated in Lake Watch) 2012 Missing 1985-2011; Shuswap Lake near Salmon 1951-2014 08LE070 2011-2014 GSC Arm (Active) datum Missing portions Shuswap Lake at Canoe 08LE109 1986-2010 1997-2010 Missing 1916-1955 1913-2014 Eagle River near Malakwa 08LE024 & 932 (Active) 1956-1965 Missing portions South Thompson River at 1911-2014 08LE031 from 1911-1913; all 15,800 Chase (Active) of 1959-1970

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DISTRICT OF SICAMOUS Sicamous Narrows Hydraulic Conveyance Assessment April 2017

3.2.1 Lake Level Analysis The water levels in Mara and Shuswap Lakes were analyzed to better understand the relationship between the lake water levels and assess if there were any trends over time. There is a long record of water levels on Shuswap Lake collected at various locations: Salmon Arm, Canoe, and Sicamous. Data is available for the period of 1951 to present, with some limited data in the 1920s. Typically there are some differences between lake levels at separate locations, which need to be considered when assessing the data. The data available from WSC has some significant data gaps as described below and summarized in Table 3-2.  Mara Lake WSC data is only available from 1961 to 1974. An additional dataset for Mara Lake is available through Shuswap Lake Watch (shuswaplakewatch.com) from 1994 to 2012. This data was manually recorded as observed from a staff gauge. The gauge was relocated in June 2012.  Water levels on Shuswap Lake near Sicamous were collected by WSC between 1961 and 1974, with some data available from the 1920s. Since 2012, Shuswap Lake Watch has been manually collecting lake level data at the highway bridge.  Shuswap Lake at Salmon Arm has the longest data set from 1951 to 2014, but with a gap from 1985 to 2011.  There is some overlap in water level records for Salmon Arm and Sicamous (1961-1974), which allows the record at Sicamous to be extended based on the Salmon Arm record.  Shuswap Lake at Canoe only has data from 1986 to 2010 filling some gaps in the general Shuswap Lake data, but there is no overlapping data with the other two Shuswap Lake gauges. Overlapping data at separate locations is required to reconcile differences in the levels and develop a relationship between them to develop a composite long-term dataset. A Shuswap Lake composite data set was created to enable the estimation of return period lake levels with a longer dataset. Figure 3-5 presents a comparison of the difference between Mara Lake and Shuswap Lake at Sicamous water levels plotted against the Shuswap Lake elevations (WSC data). Positive values mean that Mara Lake is higher than Shuswap Lake at Sicamous. The data indicates that 90% of the time Mara Lake is higher, which is expected since Mara Lake drains to Shuswap Lake. The data shows considerable scatter in the relative differences in the two lake levels at low water levels, but at high water levels (349 m or more), the differences are small (<10 cm). At lower lake levels it is possible for Shuswap Lake to flow in the direction of Mara Lake, which is evidenced by negative differences on Figure 3-5.

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Mara and Shuswap Lake Level Differences 0.30

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Figure 3-5: Mara and Shuswap Lake Level Differences based on Shuswap Lake Level

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Figure 3-6 shows the annual peak lake levels for Mara Lake, Shuswap Lake and Shuswap River discharge on the same day for each year from 1961 to 1974. Generally speaking, at higher lake levels, the water level difference between the lakes is slightly larger, but the differences are relatively small.

Mara Lake Peak Water Level Comparison 350.0 600

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1972 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1973 1974 1975 Year Figure 3-6: Annual Comparison of Shuswap River discharge Shuswap Lake level and Mara Lake level on peak Mara Lake level days

Water Level Trend Analysis A trend analysis was completed for lake levels using the Mann Kendall trend test in order to determine if peak lake levels have been changing over the years. The trend analysis was completed for the longest data set, Shuswap Lake at Salmon Arm. It was found that there are no statistically significant trends in water levels. However, it is important to note that continuous lake level data from the 1920s to present day is not available at this location. A trend analysis was also completed using Mara Lake level near Sicamous data set (1961-1974 and 1996-2011). Similarly, no statistically significant trend was found in the Mara Lake level analysis.

Water Level Frequency Analysis A water level frequency analysis was completed using HYFRAN software for Shuswap Lake water levels, using the extended dataset described earlier. The resultant return period water levels for both datasets are presented in Table 3-3. There is very little difference between the two datasets, so the composite dataset that includes Canoe was used for the hydraulic modelling since the 200-year water level was slightly higher.

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Note that the assessment was based on annual maximum daily water levels. The peak instantaneous water levels were found to be the same as the daily maximums. A climate change allowance was not included.

Table 3-3: Shuswap Lake Return Period Water Levels Shuswap Lake Level at Shuswap Lake Level at Return Period Sicamous Composite Sicamous Composite (years) Excluding Canoe Data Including Canoe Data (m) (m) 2 348.4 348.3 5 348.8 348.7 10 349.1 349.0 20 349.2 349.2 50 349.5 349.5 100 349.6 349.6 200 349.7 349.8

Historic Flood Levels Historic flood levels were considered to be lake levels greater than a 10-year return period level, which was taken to be 349.0 m based on Table 3-3. In the approximately 60 years of available data, there were 6 years of historic flood levels: 1928, 1972, 1974, 1997, 1999, and 2012. Of these, 1972, 1997, and 2012 were anecdotally reported as flood years in the historic documents referenced. The largest flood year based on historic documentation, 1948, was not available in the water level record. A summary of the water levels and peak flow in Shuswap, Eagle and South Thompson Rivers is provided in Table 3-4. Approximate return periods are given as well. Flood frequency return periods presented in the table are based on the analysis of flow data summarised in the next section.

Table 3-4: Flood Year Data Maximum Maximum Daily Return Period Year / Gauge Lake Level Flow (m) (m3/s) (years) 1928 Shuswap Lake at Sicamous 349.06b - 13 (composite incl. Canoe gauge) Mara Lake near Sicamous NA - NA Shuswap River at Enderby - 626 105 Eagle River at Malakwa - NA NA South Thompson River at Chase - 1310 14 1948a Shuswap Lake at Sicamous NA - NA (composite incl. Canoe gauge) Mara Lake near Sicamous NA - NA Shuswap River at Enderby - NA NA Eagle River at Malakwa - NA NA South Thompson River at Chase - 1610 117 1972a Shuswap Lake at Sicamous 349.70b - 140 (composite incl. Canoe gauge)

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Maximum Maximum Daily Return Period Year / Gauge Lake Level Flow (m) (m3/s) (years) Mara Lake near Sicamous 349.74 - NA Shuswap River at Enderby - 566 41 Eagle River at Malakwa - 286 15 South Thompson River at Chase - 1480 44 1974 Shuswap Lake at Sicamous 349.27b - 27 (composite incl. Canoe gauge) Mara Lake near Sicamous 349.32 - NA Shuswap River at Enderby - 535 25 Eagle River at Malakwa - 281 13 South Thompson River at Chase - 1310 14 1997a Shuswap Lake at Sicamous 349.14c - 17 (composite incl. Canoe gauge) Mara Lake near Sicamous 349.32e - NA Shuswap River at Enderby - 508 16 Eagle River at Malakwa - 251 5 South Thompson River at Chase - 1280 11 1999 Shuswap Lake at Sicamous 349.26c - 26 (composite incl. Canoe gauge) Mara Lake near Sicamous 349.22e - NA Shuswap River at Enderby - 476 10 Eagle River at Malakwa - 294 19 South Thompson River at Chase - 1420 30 2012a Shuswap Lake at Sicamous 349.42d - 42 (composite incl. Canoe gauge) Mara Lake near Sicamous NA - NA Shuswap River at Enderby - 509 16 Eagle River at Malakwa - 259 7 South Thompson River at Chase - 1450 37 Notes: a. Reported Flood Year b. Data source: Shuswap Lake at Sicamous gauge c. Data source: Shuswap Lake at Canoe d. Data source: Estimated Shuswap Lake at Sicamous e. Lake Level recorded by Shuswap Lake Watch Program at Mara Lake 1 (Lat 50.810556, Lon -118.973352) Extents of flooding and spot locations of the flood extents for reported flood years have been assembled from anecdotal reports and a report titled Sicamous Then and Now, 1894 to 2015 Volume 1 – A Visual History of our Community. Historic Flood extents/spot locations are shown on Figure 3-7. Figure 3-7 also shows the LiDAR based elevations in the District. A significant area of the gently sloping western part of Sicamous (south of BC Hwy 1 and west of BC Hwy 97A) is at an elevation between 349 m and 350 m, which corresponds approximately to between a 10 and 200-year return period water level. This indicates that the District is relatively low-lying compared to water levels in the

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Narrows and impacts of high water levels can be expected in the community with an annual exceedance probability (AEP) of 10% (or on average every ten years over many decades)..

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District of Sicamous555 is permitted to reproduce the materials for archiving and for distribution to third parties only as required to conduct business specifically relating to the District of Sicamous Sicamous Narrows Hydraulic Conductivity. Any other use of these materials without the written permission of KWL is prohibited.

District of Sicamous

DISTRICT OF SICAMOUS Sicamous Narrows Hydraulic Conveyance Assessment April 2017

Historic Floodplain Mapping Floodplain maps were developed for Eagle River in 1979 (MELP, 1979). The downstream boundary of the mapping on Sheets 1 and 2 of 7 includes Mara and Shuswap Lakes. The maps indicate an estimated 200-year Shuswap Lake level of 351.0 m at Sicamous, and 351.13 m on Mara Lake, including freeboard. The freeboard amount is not provided on the maps and the design brief was not available to provide background and rationale. This 0.13 m difference in water level between Mara Lake and Shuswap Lake during a 200-year event is in reasonable agreement with the relatively small differences in lake levels noted at very high water levels on Figure 3-5. The 1980 floodplain maps of the Shuswap River provide a slightly lower estimated 200-year Mara Lake level of 351.1 m, which includes 0.9 m of freeboard (see Sheet 1 of 17 of the floodplain maps, MELP 1985). This elevation result is approximately 0.4 m higher than the estimated 200-year elevation of 349.8 m on Shuswap Lake from the frequency analysis (without freeboard), but it is reasonable to assume that there could be on the order of 0.2 m higher water levels on Mara Lake (see hydraulic modelling section below). So the 1980 floodplain maps level (less freeboard) is only approximately 0.2 m different than the present estimate. The floodplain maps are over 35 years old and are based on a shorter period of record, so differences with the present analysis are expected.

3.2.2 Flood Flow Analysis Peak flows were analysed for the three WSC gauges presented in Table 3-2, which included:  Shuswap River at Enderby which is the primary tributary watershed to Mara Lake;  Eagle River at Malakwa which discharges to Shuswap Lake near Sicamous; and  South Thompson River near Chase, which includes the entire Shuswap Lake watershed and some additional drainage from Adams Lake. All three gauges have long periods of record with some gaps (see Table 3-2).

Flood Flow Trend Analysis Trend analyses were completed for all three gauges using the Mann Kendall trend test. No statistically significant trend in peak flows was found for the rivers.

Flood Flow Frequency Analysis A flood frequency analysis was completed for all three gauges using Hyfran software. The results are reported in Table 3-5.

Table 3-5: Return Period River Discharge Return Discharge (m3/s) Period Shuswap River Eagle River near South Thompson (years) near Enderby Malakwa River at Chase 2 352 212 960 5 430 252 1155 10 480 276 1273 20 525 297 1378 50 582 323 1505 100 624 341 1595 200 665 359 1682

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The values in Table 3-5 are the maximum daily discharge. Peak instantaneous values are estimated to be 1% higher than the daily values. Estimated instantaneous values were used in the modelling discussed in Section 3.3. These estimates do not include a factor for climate change (see Section 3.4). There are several years in which a 10-year return flow was recorded, but was not identified as a high water year according to peak lake levels as presented in Table 3-4. Peak flow does not always coincide with peak lake water levels on the Shuswap Lake system, which is expected since the volume of runoff in the hydrograph also influences flood levels on a large lake system with significant potential to attenuate peak flows.

3.3 Hydraulic Model A computer model of Sicamous Narrows was developed to assess the channel hydraulics under existing conditions and the relative effects of changes to the channel geometry and the presence of docks. Other factors affecting lake water levels such as wind set-up were not assessed as part of this study. The GeoHEC-RAS software, developed by CivilGeo Engineering Software, was used to model Sicamous Narrows. GeoHEC-RAS allows the user to enter channel bathymetry, water storage and flow in steady state or two-dimensional unsteady flow conditions in order to assess the effects of changes to flow, channel roughness and geometry on water levels. The following sections provide a brief description of the hydraulic model and summarize the methodology and results of the hydraulic assessment.

3.3.1 Model Development

Channel Geometry The channel geometry of the Sicamous Narrow was represented by 18 cross sections from the inlet of the Narrows at Mara Lake to the outlet at Shuswap Lake. The cross sections were created perpendicular to the flow direction based on 2015 MOTI survey, which provides denser data points and greater coverage than the 2016 KWL survey. Cross section locations were selected to capture the bends and unique channel bathymetry where humps or abrupt elevation changes were found. Each cross section was extended above typical water levels using LiDAR topographic data to include the overbank areas up to an elevation of 352 m. This is the approximate elevation of the Vernon- Sicamous Highway (BC Hwy 97A), and is at least 1 m above high water levels from historical flooding events. The overbank section was checked to ensure its consistency with the expected ground elevations. In several cases, Lidar data had picked up the building elevation as opposed to the ground elevation. These erroneous data points were manually removed from the GeoHEC-RAS cross section surface and the ground surface was estimated as a straight line between two known ground elevations. Two bridges, namely RW Bruhn Bridge and the CN Railway Bridge, are located at the downstream end of Sicamous Narrows. They are parallel, approximately 58 m from each other. In the absence of as- built drawings, bridge deck and pier geometries were estimated using site photos and the river cross section surveyed immediately upstream and downstream of the bridges. The underside of the bridge decks were set well above the 200-year water elevation to avoid hydraulic impact on the flood profile. At the cross sections located immediately upstream and downstream of the bridge sites, the channel contraction and expansion factors were increased from 0.1 and 0.3 to 0.3 and 0.5, respectively, to account for the flow restriction at the piers.

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Mara Lake was modelled as a storage area. Lake bathymetry in the vicinity of Sicamous Narrows, as shown in Figure 3-8, was obtained from the Ministry of Environment website. The lake area above El. 348.0 m was set to be a constant 17.66 km2 for simplicity.

Figure 3-8: Lake Bathymetry from BC MOE

Unsteady State Flow Data The hydraulic analysis of Sicamous Narrows was performed using unsteady flow. An inflow hydrograph was routed through Mara Lake and discharged into Sicamous Narrows. Shuswap Lake was represented by a single cross section, in which lake level was assigned as the downstream boundary condition of the model. As listed in Table 3-6, design flow events ranging from 2 to 200-year return periods were selected for the hydraulic analysis. The inflow hydrograph to Mara Lake was obtained from the Shuswap River at Enderby hydrometric station. An adjustment factor of 1.12 was applied to the selected hydrographs to adjust the peak flow to the drainage area of Mara Lake. The adjustment factor was based on the BC regional estimate developed by Obedkoff and Coulson (1998). The Shuswap Lake level, corresponding to each return period, was computed using the composite lake level record from 1923 to 2014 (including Canoe data). A list of unsteady flow model input is provided in Table 3-6.

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Table 3-6: Unsteady Flow Model Input Shuswap Lake Peak Inflow to Return Period Level (m) Mara Lake (m3/s) 2 348.3 399 10 349.0 544 20 349.2 596 200 349.8 754

3.3.2 Model Calibration

Calibration Event Model calibration was conducted using the lake level and inflow data from 2012, which is the most recent flood event in Sicamous. In 2012, the inflow to Mara Lake, based on the Shuswap River at Enderby hydrometric station, had a return period of approximately 16 years. The outflow from Shuswap Lake, measured at the South Thompson River at Chase hydrometric station, had a return period of approximately 37 years. Channel bathymetry is expected to be similar to the 2015 survey and existing conditions in terms of sedimentation and the presence of dock structures. The 2012 Mara Lake level data was obtained from the Shuswap Lake Watch program (shuswaplakewatch.com). An elevation datum was established by Water Survey of Canada and Fisheries and Oceans Canada. Water levels were manually read by staff from the Shuswap Lake Watch program on a daily basis. The precision of the manual recordings is estimated to be less than a centimetre based on a conversation with Bernhard Kramer of Shuswap Lake Watch. Table 3-7 lists the gauge locations and periods of record.

Table 3-7: Lake Level Records by Shuswap Lake Watch Name Location (lat, long) Period of Record Mara Lake 1 50.810556, -118.973352 1994/12/10 - 2000/12/31 Mara Lake 2 50.823953, -118.981472 2001/1/1 - 2012/6/15 Shuswap Lake 50.836341, -118.993521 2012/6/15 - present

The maximum water level at Mara Lake 2 recorded on June 15, 2012, was used for the model calibration. The gauge was moved to Shuswap Lake at the RW Bruhn Bridge during the rising limb of the lake level in 2012, which peaked by end of June. Table 3-8 lists the input for the model calibration run.

Table 3-8: Model Input for Calibration Peak Shuswap Lake Peak Mara Lake Peak Inflow to Mara Calibration Event Level a (m) Level b (m) Lake c (m3/s) June 15, 2012 348.68 348.84 463 Notes: a measured by WSC on June 15, 2012. b measured by Shuswap Lake Watch Program on June 15, 2012. c Max inflow to Mara Lake from January 1 to June 15, 2012 based on scaled Shuswap River data.

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Boat Docks Boat docks occupy nearly the entire length of the Sicamous Narrows, serving residents, tourists, and marinas. The majority of the docks are located along the right bank (looking downstream). In certain locations, the dock structures span roughly half of the channel width. To account for the head loss caused by the boat docks, channel roughness coefficients were increased to a Manning’s n value of 0.14 for areas under the docks. This Manning’s n value is typically used for a floodplain with a dense stand of timber, which is similar to the dense piers of the docks. The calibrated channel roughness coefficients ranged from 0.036 to 0.038. A roughness value of 0.10 was selected for the overbank areas on both sides of the channel. The inflow hydrograph was modelled as an unsteady state, clear water flow with no allowance for sediment or bridge blockage. The downstream boundary condition was specified as the peak water level in Shuswap Lake at El. 348.68 m. Based the above assumptions, the model was calibrated to match the peak recorded Mara Lake level of El. 348.84 m.

2016 Low Flow Event During the channel survey on September 9, 2016, a discharge of 41.6 m3/s and a water level of El. 345.47 m were measured in the Sicamous Narrows channel. The measurements were not used for model calibration for the following reasons:  A low flow of 41.6 m3/s is not large enough for calibration of the channel roughness near the top of the banks and overbank areas; and  During low flows, the weeds on the channel bottom have a greater effect in reducing flow velocity under the docks. During high flows, the weeds are expected to be somewhat flattened and have a lesser effect on flow velocity.

3.3.3 Hydraulic Modelling Results Inflow hydrographs with return periods of 2, 10, 20 and 200 years (Table 3-6), were input to the calibrated model to compute the flood profiles under three scenarios:  Existing channel conditions;  A dredged channel scenario with uniform channel bottom slope; and  Existing channel geometry without docks scenario. The resulting water level differences between Shuswap Lake and Mara Lake are provided in Table 3-9 to Table 3-11). The 10-year and 200-year return period flood extents (without freeboard) for existing conditions are shown in Figure 3-9.

Table 3-9: Peak Water Level for Existing Conditions Estimated Peak Shuswap Lake Computed Mara Water Level Return Period Instantaneous Level (m) Lake Level (m) Difference (m) Inflow (m3/s) 2 399 348.30 348.44 0.14 10 544 349.00 349.18 0.18 20 596 349.20 349.39 0.19 200 754 349.80 350.03 0.23

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Dredged Channel Scenario A dredged channel scenario was developed to assess the impact on water levels in Mara Lake. The channel thalweg profiles for the existing and theoretical dredging conditions are presented in Figure 3-10. The dredged cross section was assumed to have a 10 m bottom width with 1 horizontal to 1 vertical side slopes. A constant channel bottom slope of 0.05% was assumed along the entire length of Sicamous Narrows.

The dredging scenario is expected reduce the peak water level by less than 5 cm, as shown in Table 3-10. The 200-year return period flood extents (without freeboard) for the dredging scenario are presented in Figure 3-9.

Table 3-10: Peak Water Level for the Dredging Channel Conditions Estimated Difference Shuswap Computed Return Peak Water Level from Existing Lake Level Mara Lake Period Instantaneous Difference (m) Conditions (m) Level (m) Inflow a (m3/s) (m) 2 399 348.30 348.41 0.11 -0.03 10 544 349.00 349.15 0.15 -0.03 20 596 349.20 349.36 0.16 -0.03 200 754 349.80 349.99 0.19 -0.04

This dredging scenario would have very little impact on peak lake levels, so would not be an effective means of flood mitigation.

Impact of the Boat Docks

Scenario 2 was simulated for Sicamous Narrows to assess the impact of the boat docks on water levels by removing them from the model. Channel roughness under the dock area was reduced from 0.14 to 0.038 to reflect natural channel conditions. The channel contraction and expansion coefficients were reset to 0.1 and 0.3, respectively. As shown in Table 3-11, the design water levels would be 0.09 to 0.15 m lower without the boat docks. The 200-year flood extents (without freeboard) for the no docks scenario are shown in Figure 3-9

Table 3-11: Peak Water Level without Boat Docks Estimated Difference Shuswap Computed Water Level Return Peak from Existing Lake Level Mara Lake Difference Period Instantaneous Conditions (m) Level (m) (m) Inflow (m3/s) (m) 2 399 348.30 348.35 0.05 -0.09 10 544 349.00 349.07 0.07 -0.11 20 596 349.20 349.27 0.07 -0.12 200 754 349.80 349.88 0.08 -0.15

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10-year Flood Extents - Existing Conditions

200-year Flood Extents - Existing Conditions

200-year Flood Extents - No Docks Scenario Model Extent

200-year Flood Extents - Dredging Scenario Ma ra Lake

Reference: Orthophoto fromthe District of Sicamous.

Copyright Notice: These materials are copyright of Kerr Wood Leidal Associates Ltd. (KWL). District of Sicamous is permitted to reproduce the materials for archiving and for distribution to third parties only as required to conduct business specifically relating to the District of Sicamous Sicamous Narrows Hydraulic Conductivity. Any other use of these materials without the written permission of KWL is prohibited.

District of Sicamous

© 2016 Kerr Wood Leidal Associates Ltd. Sicamous Narrows Hydraulic Conductivity Assessment Project No. Date 3239-016 December, 2016 80 0 80 200-year Modelled Flood Extents (m) 1:6,000 Figure 3-9 2016/05/17, 2:36 AM 2016/05/17, SAVED

Existing Bridge Column 360 360 At Full Size, this border measures 260 mm x 400 mm this border measures At Full Size, Paper Size = ANSI B Paper Size 50

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Elevation (m) Dredging

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Scale H: 1:5000 | V: 1:500

Sicamous Narrow Existing and Dredged Profile

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DISTRICT OF SICAMOUS Sicamous Narrows Hydraulic Conveyance Assessment April 2017

3.4 Climate Change Considerations There is a large degree of uncertainty regarding the specific effects of climate change on lake levels and peak flows in BC. However, as discussed in Appendix H of the Professional Practice Guidelines - Legislated Flood Assessments in a Changing Climate in BC prepared by APEGBC in 2012:  Temperatures are expected to rise by about 2.8°C on average by the end of the 21st century, which would result in a change in the distribution of runoff in a freshet driven system such as the Shuswap Lake watershed.  It is estimated that average annual precipitation could increase by about 10% in BC by 2100.  Spring snowmelt freshet floods may become more severe due to a more rapid snowmelt. This could possibly result in an increase on the order of 10% in extreme spring flood flows. If peak flows and runoff volumes were to increase by on the order of 10% in the Shuswap Lake watershed as indicated above, this could result in higher water levels occurring more frequently. For example, a 20-year return period flow may become a 10-year return period flow in the future. This is of concern since the community is already vulnerable to flooding, and climate change could result in more frequent and more severe flooding.

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4. Summary and Recommendations

4.1 Summary a) The historical photos show consistency over time in the landforms and features in the study area. Features visible in the 1928 photos are still discernible, and similar in appearance to the 2007 features. b) The Sicamous Narrows are a relatively low energy environment and sediment recruitment or movement was not discernible in this study. c) There is a long record of water levels on Shuswap Lake collected at various locations: Salmon Arm, Canoe, and Sicamous. Data is available for the period of 1951 to present, with some limited data in the 1920s. Typically, there are variations between lake levels at different locations, which were considered when assessing the data. d) Water levels on Shuswap Lake near Sicamous were collected by WSC between 1961 and 1974, with some data available from the 1920s. Since 2012, Shuswap Lake Watch has been manually collecting lake level data at the highway bridge. There is some overlap in water level records for Salmon Arm and Sicamous, which allows the record at Sicamous to be extended based on water levels at Salmon Arm. e) There is limited Mara Lake level data available from two sources: WSC for the period 1961 to 1974; and Shuswap Lake Watch between December 1994 and June 2012. f) There is WSC lake level data available for both Shuswap Lake at Sicamous and Mara Lake between 1961 and 1974; this allows a comparison of the lake levels over that period. Mara Lake normally flows to Shuswap Lake, and 90% of the time Mara Lake is higher. The data shows considerable scatter in the differences in the two lake levels at low water levels, but at high water levels the differences are small (<10 cm). g) Regional flow data was assessed at key locations: Shuswap River upstream of Mara Lake; Eagle River at Sicamous; and the South Thompson River downstream of Shuswap Lake. h) The hydrological analysis revealed no significant trends that lake levels or river discharges have been increasing or decreasing over time. The limited data available for Mara Lake introduces some uncertainty to water level trends on Mara Lake, however, with the present information; there is no indication of a trend. i) A frequency analysis was conducted on Shuswap Lake level data. There is insufficient Mara Lake level data to justify a frequency analysis. j) Other factors affecting lake water levels such as wind set-up and waves were not assessed as part of this study. k) The District is relatively low-lying compared to high lake levels, particularly on the east bank of the Narrows. Based on hydraulic modelling, impacts of high water levels could be experienced in the community in an event of less than a 10-year return period, which has an estimated water level of approximately 349 m (excluding freeboard).

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l) The gentle slope of the east bank of the Narrows means that relatively small changes in lake levels during flood conditions could have a significant effect on the level of flooding experienced by the community. An estimated 200-year water level is expected to cause widespread flooding on the east bank. m) Under existing channel conditions, the 200-year water level difference between Shuswap and Mara Lakes is estimated to be approximately 0.2 m. This suggests that the hydraulics of the Sicamous Narrows has only a limited effect on flooding in the community. This result agrees with the historic observations previously noted and existing floodplain mapping. n) Historic floodplain maps of Eagle River (1979) indicate an estimated 200-year Shuswap Lake level of 351.00 m at Sicamous, and 351.13 m on Mara Lake (including unspecified freeboard). This 0.13 m difference in water level between Mara Lake and Shuswap Lake during a 200-year event is in reasonable agreement with the hydraulic modelling results. The 1980 floodplain maps for Shuswap River provide a flood level for Mara Lake similar to the 1979 mapping, which includes a 0.9 m freeboard. o) Approximate 200-year flood extents (excluding freeboard) were derived from the modelling but this does not constitute a floodplain map for use in the development of flood construction levels (FCLs). p) The 1948 flood location point and 200-year water level extents roughly coincide. q) A hydraulic model of Sicamous Narrows was used to assess the effects of dredging and the presence of docks. As the modelling was based on the LiDAR topography, the elevations of individual buildings and areas should not be considered precise. r) The potential flood impacts to existing infrastructure, whether above ground or buried, were not assessed. s) The hydraulic model indicates dredging would have very little impact on peak lake levels. The dredging scenario considered reduced peak lake levels by less than 5 cm, which would not be an effective means of flood mitigation. t) The hydraulic model indicates that the docks within the channel cause a 10 to 15 cm elevation increase in the Mara Lake end of the channel. u) Based on present estimates, climate change could increase runoff and peak flows by on the order of 10%. This could result in higher water levels occurring more frequently. The values presented in this report do not include an adjustment for climate change.

4.2 Recommendations 1. Given that flooding occurs on a relatively frequent basis, we recommend that the District consider the development of an Integrated Flood Hazard Management Plan (IFHMP) that would:  Include the development of updated inundation and flood hazard mapping for Shuswap Lake, Mara Lake, and Eagle Creek in the vicinity of the District. This should include an allowance for climate change.  Consider the effects of various degrees of flooding on residents, businesses, infrastructure and the environment by developing flood risk maps.  Identify potential flood mitigation options, both structural and non-structural.

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Traditional structural flood mitigation for the community would likely be a challenge since there is little room for a standard dike, but innovative structural flood protection solutions may be possible. Other mitigative measures could include development restrictions, covenants, and bylaws (FCLs and site specific flood-proofing) as well as emergency management. An integrated approach for flood hazard management would help the District develop mitigation and management solutions. 2. Since flooding in the District is a function of high lake levels, accurate water level information at the inlet and outlet of Sicamous Narrows would be beneficial to the community. Shuswap Lake Watch has been providing a service of manual lake level collection since 1994; however it is dependent on the owner and operator of Shuswap Lake Watch to manually collect the information. We recommend installing automated hydrometric gauges to record lake levels near Sicamous Narrows at both the inlet and outlet. This data would be useful for technical analyses in connection with any future consideration of flood mitigation and also for navigation, planners and emergency responders at the District.

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Statement of Limitations This document has been prepared by Kerr Wood Leidal Associates Ltd. (KWL) for the exclusive use and benefit of DISTRICT OF SICAMOUS for the Sicamous Narrows Hydraulic Conveyance Assessment. No other party is entitled to rely on any of the conclusions, data, opinions, or any other information contained in this document. This document represents KWL’s best professional judgement based on the information available at the time of its completion and as appropriate for the project scope of work. Services performed in developing the content of this document have been conducted in a manner consistent with that level and skill ordinarily exercised by members of the engineering profession currently practising under similar conditions. No warranty, express or implied, is made. Copyright Notice These materials (text, tables, figures and drawings included herein) are copyright of Kerr Wood Leidal Associates Ltd. (KWL). DISTRICT OF SICAMOUS is permitted to reproduce the materials for archiving and for distribution to third parties only as required to conduct business specifically relating to Sicamous Narrows Hydraulic Conveyance Assessment. Any other use of these materials without the written permission of KWL is prohibited. Revision History Revision # Date Status Revision Author

A December 14, 2016 Draft Draft for Client Review DWM

0 April 12, 2017 Final DWM

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References Obedkoff and Coulson, 1998. British Columbia Streamflow Inventory, Province of BC, Ministry of Environment Lands and Parks. Robert, A. 2003. “River Processes: An Introduction to Fluvial Dynamics”. Arnold. 214 pp. Self, J., and H. Larratt, 2013. Source Assessment of the District of Sicamous Mara Lake Drinking Water Intake. Report prepared by Larratt Aquatic Consulting Ltd. for the District of Sicamous. 69 pp + appendices. APEGBC, 2012 Professional Practice Guidelines - Legislated Flood Assessments in a Changing Climate in BC Association of Professional Engineers and Geoscientists of British Columbia. MELP, 1979. Floodplain Maps of the Eagle River, Drawing No. A5187 1 to 7 - Designation Date 87/12/03. Canada-British Columbia Floodplain Mapping Program, Ministry of Environment, Lands and Parks. MELP, 1987. Floodplain Maps of the Shuswap River, Bessette & Duteau Creeks Drawing No. 96 - 7 Sheets 1 to 8 - Designation Date 98/09/30; Shuswap River: Mara Lake to Mabel Lake Drawing No. A5241 Sheets 1 to 10 - Designation Date 87/09/30; Drawing No. A5241 Sheets 11 to 17 - Designation Date 87/09/30. Canada-British Columbia Floodplain Mapping Program, Ministry of Environment, Lands and Parks.

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