TECHNICAL MEMO - MD of Bighorn
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TECHNICAL MEMO
To From
Robert Ellis, C.A.O. Roger Towsley, P.Geo, Senior Hydrogeologist
Company MCSL Branch
Municipal District of Bighorn No. 8 2131 – Edmonton, AB
Date
Re March 5, 2021
Community Questions File Number
2131-00114-00
1. INTRODUCTION
McElhanney Ltd. (McElhanney) has been retained by MD of Bighorn No. 8 to answer questions posed by the
Community of Exshaw for further explanation relating to some of the content and conclusions of the November 2020,
McElhanney Physical Hydrogeological Assessment. This memo provides answers to these question as presented
below. The community questions with the related data are attached in Appendix A at the end of the report.
1.1. Question 1
During the 2018 freshet, no groundwater flooding event was noted in the Exshaw community. Monitoring wells stayed
within normal ranges during the 2018 freshet also. Given the entirety of the data provided, can you explain why you
believe the snowpack was “abnormally high [to be] [t]he primary cause of the flooding in the Northern Community of
Exshaw in 2020” (Section 7, p15)?
One of the key elements to answering this question is being able to understand flood conditions in the Northern
Community in 2020 compared to other years. McElhanney was not present during the 2018 freshet and therefore has
no knowledge of the level of flooding or difficulties that residents in the community may have experienced with respect
to groundwater infiltration into their basements, although it was generally referenced that flooding in 2020 was above
the perceived average. Given the data the community has provided, please consider the following points.
The accumulated precipitation data over a seven-month period (November-June) is not the best indicator of
groundwater levels and the potential for flooding during a freshet. To illustrate this point, consider groundwater elevation
data provided on Figure 21 below for the Berm Irrigation Well (BIW) compared to the accumulated precipitation data for
South Ghost Headwaters and Seebee. The accumulated precipitation data in 2017 was lower than in both 2018 and
2020; however, the groundwater level in the BIW was the second highest of all the data recorded, only the 2020
groundwater levels were higher. As stated in the report, the effects of precipitation and snowmelt were observed to be
rapidly transferred to groundwater. As such, and in view of the information discussed above, the effects of precipitation
(and temperature) with respect to groundwater flooding events during a freshet should be considered within the
timeframe of days to a few weeks, and not months. In your question, it states that monitoring wells stayed within normal
ranges in 2018, but no information was provided. Again, the BIW data in 2018 shows the groundwater level was actually
the third highest compared to all other data and therefore was above average.
As shown by the snowpack graphs for the three meteorological stations (Sunshine Village, Three Isle Lake and Little
Elbow Summit), the snowpack was at or above the upper quartile. All of these stations are located more than 41 km
from Exshaw. It is possible that localized snowpack conditions may have been even higher closer to Exshaw, this is only
100 - 402 11 Avenue SE Tel 403 262 5042
Calgary AB Canada T2G 0Y4 www.McElhanney.com
TECHNICAL MEMO mentioned for consideration. It is worth reiterating from the McElhanney report that there was almost no rainfall [May 25-June 5, (5 mm)] during the period of peak flooding and in the absence of precipitation, high snowpack was the primary source of elevated groundwater levels during that time. The most relevant comparison of groundwater levels between 2018 and 2020 can be seen in Lafarge Well 05-15A (See Figure 8). This well is located 340 m west and hydraulically upgradient of Exshaw Creek; 05-15A was 5.8 m higher than the original creek elevation (EC-16-TP1) prior to flood mitigation project construction. The groundwater level around the similar dates in June was 1.83 m higher in 2020 than in 2018. The magnitude of this difference is viewed as very significant. The salient point is that while high groundwater levels and flooding were experienced downgradient of Exshaw Creek in 2020, groundwater levels was also very high in the upgradient areas from the creek relative to the 2018 data. In the absence of any significant rainfall during the freshet period in 2020, the evidence points towards above average snowpack as the source of the significantly higher groundwater elevation at the Lafarge well. Finally, the highest peak in the Bow River water level (around June 1) was roughly coincidental with the highest groundwater levels and flooding in East Exshaw (June 4) as shown on hydrograph BH-04 (Figure 13). The peak river level on June 1 exacerbated groundwater flooding as groundwater flow was stagnated by the high river condition; groundwater was not able to dissipate to downgradient areas from Exshaw. Additional groundwater monitoring wells are required to understand the hydraulic connection and groundwater flow between the Northern Community and the Bow River. Re: Community Questions | 2131-00114-00 Page 2 From: Roger Towsley | To: Robert Ellis
McElhanney ANSI AL - 2020-01-01
DATE: 2020-11-09, 16:50 FILE: G:\ENVIRO\Drafting\W5M\024-09\Sec23-24-9 W5M\DWG\18-271_Monitoring Well Base Map_October.dwg
THIS DRAWING AND DESIGN IS THE PROPERTY OF McELHANNEY AND SHALL NOT BE USED, REUSED
OR REPRODUCED WITHOUT THE CONSENT OF McELHANNEY. McELHANNEY WILL NOT BE HELD
RESPONSIBLE FOR THE IMPROPER OR UNAUTHORIZED USE OF THIS DRAWING AND DESIGN.
THIS DRAWING AND DESIGN HAS BEEN PREPARED FOR THE CLIENT IDENTIFIED, TO MEET THE
STANDARDS AND REQUIREMENTS OF THE APPLICABLE PUBLIC AGENCIES AT THE TIME OF
PREPARATION. McELHANNEY, ITS EMPLOYEES, SUBCONSULTANTS AND AGENTS WILL NOT BE
DESTROY ALL PRINTS BEARING PREVIOUS REVISION
LIABLE FOR ANY LOSSES OR OTHER CONSEQUENCES RESULTING FROM THE USE OR RELIANCE
UPON, OR ANY CHANGES MADE TO, THIS DRAWING, BY ANY THIRD PARTY, INCLUDING
CONTRACTORS, SUPPLIERS, CONSULTANTS AND STAKEHOLDERS, OR THEIR EMPLOYEES OR
AGENTS, WITHOUT McELHANNEY'S PRIOR WRITTEN CONSENT.
INFORMATION ON EXISTING UNDERGROUND FACILITIES MAY NOT BE COMPLETE OR ACCURATE.
McELHANNEY, ITS EMPLOYEES AND DIRECTORS ARE NOT RESPONSIBLE NOR LIABLE FOR THE
LOCATION OF ANY UNDERGROUND CONDUITS, PIPES, CABLES OR OTHER FACILITIES WHETHER
SHOWN OR OMITTED FROM THIS PLAN. PRIOR TO CONSTRUCTION CONTRACTOR SHALL EXPOSE
LOCATIONS OF ALL EXISTING FACILITIES BY HAND DIGGING OR HYDROVAC AND ADVISE THE
ENGINEER OF POTENTIAL CONFLICTS.
Drawing No.
HAMLET OF EXSHAW
Suite 200
858 Beatty Street
2014 - 2020 21
Vancouver BC
1 NOV. 09/20 SL RT Canada V6B 1C1 GROUNDWATER ELEVATION
Project Number Rev.
T 604 683 8521
0
Rev
NOV. 05/20
Date Description
SL RT
Drawn Design App'd
MONITORING - BERM IRRIGATION WELL 213100114 1
EXSHAW, AB
M.D. OF BIGHORH No.8
PHYSICAL HYDROGEOLOGICAL ASSESSMENT
HAMLET OF EXSHAW
ROBERT ELLIS
0 1:2,000 100
.0
11332350.0
1320 .0
1335
.0
13 315.0
.0
10
1
5 .0
.0 130
55
13 50.0
1 5.0
3
4 130
13 40.0 0.0
13 5.0
133 .0
1330
5.0 5.0
132 .0
129
1320 0
.
1315 0 0.0
1 0 . 129
13 .0
5
130
1300.0
MW20-02
1295.0
12
90
.0
BH-02 BH-04
1295.0
MW20-05
EC-16-TP4
1300.0
MW20-03
MW20-01
.0
05
BH-03
13
Berm MW20-03
Irrigation Well
EC-16-TP3
EC-16-02
(VWP)
MW20-04
EC-16-TP2
EC-16-01
(VWP)
EC-16-TP1
Lafarge Well
05-15A
SITE LOCATION MAP -
MONITORING WELLS AND TEST PITS
8
McElhanney ANSI AL - 2020-01-01
DATE: 2020-11-05, 10:05 FILE: H:\ENVIRO\Drafting\W5M\024-09\Sec23-24-9 W5M\DWG\18-271_Monitoring Well Base Map_October.dwg
THIS DRAWING AND DESIGN IS THE PROPERTY OF McELHANNEY AND SHALL NOT BE USED, REUSED
OR REPRODUCED WITHOUT THE CONSENT OF McELHANNEY. McELHANNEY WILL NOT BE HELD
RESPONSIBLE FOR THE IMPROPER OR UNAUTHORIZED USE OF THIS DRAWING AND DESIGN.
THIS DRAWING AND DESIGN HAS BEEN PREPARED FOR THE CLIENT IDENTIFIED, TO MEET THE
STANDARDS AND REQUIREMENTS OF THE APPLICABLE PUBLIC AGENCIES AT THE TIME OF
PREPARATION. McELHANNEY, ITS EMPLOYEES, SUBCONSULTANTS AND AGENTS WILL NOT BE
DESTROY ALL PRINTS BEARING PREVIOUS REVISION
LIABLE FOR ANY LOSSES OR OTHER CONSEQUENCES RESULTING FROM THE USE OR RELIANCE
UPON, OR ANY CHANGES MADE TO, THIS DRAWING, BY ANY THIRD PARTY, INCLUDING
CONTRACTORS, SUPPLIERS, CONSULTANTS AND STAKEHOLDERS, OR THEIR EMPLOYEES OR
AGENTS, WITHOUT McELHANNEY'S PRIOR WRITTEN CONSENT.
INFORMATION ON EXISTING UNDERGROUND FACILITIES MAY NOT BE COMPLETE OR ACCURATE.
McELHANNEY, ITS EMPLOYEES AND DIRECTORS ARE NOT RESPONSIBLE NOR LIABLE FOR THE
LOCATION OF ANY UNDERGROUND CONDUITS, PIPES, CABLES OR OTHER FACILITIES WHETHER
SHOWN OR OMITTED FROM THIS PLAN. PRIOR TO CONSTRUCTION CONTRACTOR SHALL EXPOSE
LOCATIONS OF ALL EXISTING FACILITIES BY HAND DIGGING OR HYDROVAC AND ADVISE THE
ENGINEER OF POTENTIAL CONFLICTS.
Drawing No.
HAMLET OF EXSHAW
Suite 200
858 Beatty Street
BH-04 GROUNDWATER LEVEL 13
Vancouver BC
Canada V6B 1C1 MAY 14 - SEPTEMBER 17, 2020
Project Number Rev.
T 604 683 8521
0 NOV. 05/20 SL RT
Rev Date Description Drawn Design App'd
213100114 0
TECHNICAL MEMO 1.2. Question 2 Given that your report suggests an exceptional snowpack was the cause of the flooding, would you not expect to see more flow in the lower reaches of Exshaw creek during the groundwater flooding event and not less? Additionally, given that the groundwater flow direction was determined in your study to be SE (ie from roughly from the creek towards E. Exshaw) how was reduction in stream length reconciled with the conclusions that influence of the mitigation work was “minor” and “likely coincidence and not cause” (section 7, p.14)? Given the stream bed was significantly lower post- construction, combined with the weir limiting outflow of Exshaw creek, would you not expect increased surface water- ground water interaction? The images presented by the community are of a small scale making it difficult to see the actual creek flow; however, it appears that the creek flow does extend further on July 15, 2018 compared to July 27, 2020. In answer to the first part of this question as it relates to the exceptional snowpack: Would you not expect to see more flow in the lower reaches of Exshaw Creek during the flooding event, not less when comparing the 2018 and 2020 images? McElhanney would like to first point out that the alpine snowpack had disappeared in late June and that by early July at the latest, the freshet period had ended. Thus, the Google image provided July 15, 2018 shows show creek flow after the spring freshet period and other factors need to be considered for the length of streamflow. McElhanney is not an expert on streamflow but it is reasonable to assume that precipitation, temperature and evaporation would be factors affecting streamflow after the spring freshet. Moreover, the image provided at the end of July (July 27, 2020) is 12 days after the 2018 image for the same month, and certainly well after the spring freshet had ended. As such, the same factors affecting streamflow mentioned above would need to be considered over an even longer time period for an equivalent comparison. As noted in the groundwater report (McElhanney, 2020), creek bed and subsurface materials are highly permeable with the potential to dissipate large volumes of water, and thus creek conditions may differ significantly between the period of mid-July to late-July for any given year. McElhanney maintains this is a very complex system, and given all the variables and timelines, cannot provide a comment on stream length from the information presented by the community. Furthermore, as noted during the freshet in 2020, the sediment pond did fill nearly to the point of the weir. The original creek bed in this area was greatly altered to incorporate a sediment pond with a holding capacity of 49,000 m3. This structure will obviously have an effect on the length of streamflow as it will impound water and debris during a significant runoff event. McElhanney was not involved in the design of the sediment pond and directs the community to Golder Associates to answer any further questions on how the sediment pond governs streamflow, and under what circumstances streamflow can be expected to reach the lower reaches of Exshaw Creek. With respect to the other questions regarding construction activity in the streambed, the report does state that the engineered creek is wider than the original creek bed and deeper than the original thalweg. It was also said that there is potential that alteration of the creek bed may have facilitated increased infiltration of surface water from the creek to the permeable subsurface gravels. What is not clear is the effect that the engineered debris flood mitigation structure on Exshaw Creek may have on local groundwater levels in the North Community, and as stated in the report, additional investigation is required to assess this situation. Re: Community Questions | 2131-00114-00 Page 3 From: Roger Towsley | To: Robert Ellis
TECHNICAL MEMO 1.3. Question 3 Do you have an explanation for the reversal in the gradient between the two MD wells? Monitoring wells located near constructed water features such as lagoons are typically not used in groundwater flow assessments. Ponded water in these features and anthropogenic activity (pumping into or out of?) related to the feature itself can influence groundwater, resulting in inaccurate groundwater levels. McElhanney only used data from properly installed monitoring wells that were surveyed to a high degree of accuracy for groundwater elevation assessments. It is important to note that during the flooding event, there was a significant volume of ponded water that was trapped in the vicinity of the berm irrigation well and MW20-01; a berm runs north from MW20-01 and also east-west along the highway. It was observed that MW20-01 was completely submerged starting in early June, and that the flooding extended into areas south across the highway and in the ditches around the berm irrigation well. The exact duration of flooding in the area is not exactly known but it was likely several weeks. Localized ponded water controlled by berms and ditches will affect shallow groundwater levels as there is a connection between permeable surface materials and the upper water-bearing unit. For all the above reasons, it’s not possible to provide a definitive explanation for the apparent reversal in gradients between the Berm Irrigation Well and Lagoon Well, there are too many complicating factors. In future, a proper groundwater network with wells installed away from manmade features such as lagoons and other ponded water areas should be used to verify groundwater levels in the Northern Community. Re: Community Questions | 2131-00114-00 Page 4 From: Roger Towsley | To: Robert Ellis
EXSHAW, AB
M.D. OF BIGHORH No.8
PHYSICAL HYDROGEOLOGICAL ASSESSMENT
HAMLET OF EXSHAW
ROBERT ELLIS
0 1:2,000 100
.0
11332350.0
1320 .0
1335
.0
13 315.0
.0
10
1
5 .0
.0 130
55
13 50.0
1 5.0
3
4 130
13 40.0 0.0
13 5.0
133 .0
1330
5.0 5.0
132 .0
129
1320 0
.
1315 0 0.0
1 0 . 129
13 .0
5
130
1300.0
MW20-02
1295.0
12
90
.0
BH-02 BH-04
1295.0
MW20-05
EC-16-TP4
1300.0
MW20-03
MW20-01
.0
05
BH-03
13
Berm MW20-03
Irrigation Well
EC-16-TP3
EC-16-02
(VWP)
MW20-04
EC-16-TP2
EC-16-01
(VWP)
EC-16-TP1
Lagoon Well
Lafarge Well
05-15A
SITE LOCATION MAP -
MONITORING WELLS AND TEST PITS
8TECHNICAL MEMO 1.4. Question 4 Given the ground water flow direction, could temporary mitigation strategies applied to Exshaw Creek to limit infiltration, result in a reduced magnitude of the groundwater flow as shown in the report? The report was unable to conclusively rule out the mitigation structures as a cause or contributing factor to the flooding. The direction of ground water flow underneath the North community as determined from the groundwater monitoring well network to be broadly from Exshaw Creek towards the North community. Given the assumed lack of effective dewatering from pumping from basements into ditches, pipe and culverts onto open land, what temporary mitigation strategies do you think could be applied to reduce the severity of future groundwater flooding events. Would temporarily diverting some or all water from Exshaw creek directly to the Bow river through pipe or sealed channel be a possible mitigation strategy by reducing infiltration? A water management company out of Red Deer gave a rough estimate based on the flow rate as estimated by a resident who rented a flow meter on June 15th (not peak flow rate). Estimated flow at top of alluvial fan was 517 L/s. Estimate: $3000 mobilization and $300 /day rental rate for such equipment + fuel. Pumps would be on vacuum once the line/lines are full due the gradient of the creek. Temporary surface water and groundwater diversion strategies should be considered over the period of future freshets in an effort to mitigate flooding in the North Community. Before considering these strategies, the main issue with all of them will be discussed below. Interception and pumping of surface water from Exshaw Creek directly to the Bow River would certainly prevent some surface water from entering groundwater storage. Similarly, selective groundwater extraction in the Northern Community to the Bow River would have the effect of reducing groundwater storage and water levels. The issue is directly transferring surface water to the Bow River, that would otherwise naturally infiltrate to groundwater, will have consequences considering the large volume of water in streamflow at the peak of the freshet. The concern with this activity is it may greatly increase Bow River levels in the Exshaw area which could result in localized flooding (backflow) in the Northern Community. In 2020, peak Bow River levels around June 1st were believed to have caused impounding of water in the Northern Community, resulting in upwelling of the groundwater to surface and localized flooding. Surface water/groundwater modelling should be completed to determine the effects of direct streamflow/groundwater transfers to the Bow River. As mentioned, interception and pumping of surface water within Exshaw Creek somewhere above the North Community would be one method to divert streamflow from groundwater storage. This method will involve renting large pumps to attain the desired flow rate and hoses to direct the water to an appropriate location. The most practical place to run hoses and discharge the water would be to the lower reaches of Exshaw Creek. The pumps would require constant supervision to keep them running on a 24-hour basis and to prevent theft. It should be realized that regardless of capture efforts, some of the streamflow will infiltrate to groundwater given the high permeability of creek bed materials, and therefore, it is unclear how effective this method will be. Another method that would effectively reduce groundwater storage and remove the uncertainty of the infiltration losses described above would be to install a series of groundwater extraction wells on the downgradient side of the creek. For illustration purposes, two pumping wells (PW1 and PW2) are located at the estimated interception points of groundwater flow between Exshaw Creek and the Northern Community (See Figure 10). The wells should be large diameter (10-12 inch) capable of producing very high flow rates. The wells could be joined with a common header and Re: Community Questions | 2131-00114-00 Page 5 From: Roger Towsley | To: Robert Ellis
TECHNICAL MEMO groundwater piped to some point downgradient on the creek bed outside of the capture zone of pumping, and perhaps all the way to the Bow River if necessary. Without getting into the specifics of all the testing requirements here, differing pumping rates should be explored to achieve overlapping drawdown cones, with the ultimate goal of reducing groundwater flow to the Northern Community. Analytical and numerical groundwater modelling would be required to 1) assess aquifer properties (transmissivity and storage) and establish an effective pumping rate(s) for dewatering, 2) assess the impact of groundwater abstraction on the Bow River and existing water well users in the community and 3) determine the potential for back flooding into the Exshaw Community (and other areas) given the significant increase of water volumes to the Bow River. The key would be to establish a pumping rate sufficient to reduce groundwater levels in the community, while concurrently, not resulting in excessively high water levels in the Bow River. Please note these are conceptual ideas for consideration by the community and require validation through surface water and groundwater modelling. Re: Community Questions | 2131-00114-00 Page 6 From: Roger Towsley | To: Robert Ellis
EXSHAW, AB
M.D. OF BIGHORH No.8
PHYSICAL HYDROGEOLOGICAL ASSESSMENT
HAMLET OF EXSHAW
ROBERT ELLIS
0 1:2,000 100
.0
11332350.0
1320 .0
1335
.0
13 315.0
.0
10
1
5 .0
.0 130
55
13 50.0
1 5.0
3
4 130
13 40.0 0.0
13 5.0
133 .0
1330
5.0 5.0
132 .0
129
1320 0
.
1315 0 0.0
1
13 .0
0 . 129 1287.67
5
130
1300.0
MW20-02
1295.0
12
90
.0 1287.80
1288.33 BH-02 BH-04
1295.0
MW20-05
EC-16-TP4
1288.15 1300.0
MW20-03 1287.51
MW20-01
7.7
.0
BH-03
05
13
1286.99
128
7.9
MW20-03
PW1
128
Berm 1286.99
.1
Irrigation Well
88
1287.1
12
PW2
EC-16-TP3
EC-16-02
(VWP)
MW20-04
1288.10
EC-16-TP2
EC-16-01
(VWP)
EC-16-TP1
Lafarge Well
05-15A
GROUNDWATER FLOW - JUNE 26, 2020
10TECHNICAL MEMO
We trust the above provides the necessary information for your review. Please contact the undersigned should you have
any questions.
Sincerely,
McElhanney Consulting Services Ltd.
Report prepared by:
Roger Towsley, P.Ag., P. Geo.
Senior Hydrogeologist
APEGA Member No. M77642
rtowsley@mcelhanney.com
STATEMENT OF LIMITATIONS
This technical memorandum was prepared by McElhanney Ltd. ("McElhanney") for the particular site, design
objective, development and purpose described in this report and for the exclusive use of the client identified in this
report (Municipal District of Bighorn No. 8). The data, interpretations and recommendations pertain to the Project
and are not applicable to any other project or site location and this report may not be reproduced, used or relied
upon, in whole or in part, by a party other than the Client, without the prior written consent of McElhanney. The
Client may provide copies of this report to its affiliates, contractors, subcontractors and regulatory authorities for
use in relation to and in connection with the Project provided that any reliance, unauthorized use, and/or decisions
made based on the information contained within this report are at the sole risk of such parties. McElhanney will
not be responsible for the use of this report on projects other than the Project, where this report or the contents
hereof have been modified without McElhanney’s consent, to the extent that the content is in the nature of an
opinion, and if the report is preliminary or draft. This is a technical report and is not a legal representation or
interpretation of laws, rules, regulations, or policies of governmental agencies.
This report was prepared under the direction of a professional geoscientist registered in the Province of Alberta
with the degree of care, skill, and diligence as would reasonably be expected from a qualified member of the
same profession, providing a similar report for similar projects, and under similar circumstances, and in
accordance with generally accepted geoscience and scientific judgments, principles and practices. McElhanney
expressly disclaims any and all warranties in connection with this report.
Re: Community Questions | 2131-00114-00 Page 7
From: Roger Towsley | To: Robert EllisAppendix A
Community Questions
2131-00114-00Section 6.1.3, p14 examines snowpack using the little elbow snow pillow. The Little Elbow snow pillow station is located in the upper, westernmost reaches of the watershed at 2225 m a.s.l. in an area of relatively high snowpack accumulation (Hopkinson et al, 2012). For reference, the rough elevation of some points in the Exshaw Creek drainage are: Town 1320m, Mount Fable 2700m, Exshaw Mountain 1760m. Accumulation tends to be greater in sheltered areas. This suggests that the data from Little elbow is more representative of the further reaches of the drainage, in areas of higher elevation, sheltered from the winds. Below are plots of Seebe & South Ghost Headwaters precipitation data and snow pillows of Sunshine Village, Little Elbow & Three Isle Lake. These are the closest provincial meteorological stations to the Exshaw town site. Precipitation in 2020 at South Ghost was similar to 2018 with the precipitation at Seebee being slightly less than 2018. On the 3 year plot at Little Elbow, the peak snow water equivalent (SWE) is only marginally lower in 2018. At both Sunshine Village and Three Isle lake, the peak SWE was either similar or higher in 2018 than it was in 2020. Out of the three snow pillow sites referenced here, Little Elbow was the only one of the 3 closest snow pillows to have fallen above the upper quartile in 2020. Your report mentions that as of May 1, 2020, the Alberta River Basins Website reported mountain snowpack was significantly (146%) above average. The same forecast for May 2018 was 141%, also significantly above average. A chart of Bow river levels by year for the last 20 years is also attached. The levels observed in 2020 appear to fall within normal trends. Although the peak flows were above upper quartile at some points in 2020 the duration was short and the magnitude was not exceptional when compared to peak flows on other years. Question: During the 2018 freshet, no groundwater flooding event was noted in the Exshaw community. Monitoring wells stayed within normal ranges during the 2018 freshet also. Given the entirety of the data provided, can you explain why you believe the snowpack was “abnormally high [to be] [t]he primary cause of the flooding in the Northern Community of Exshaw in 2020” (Section 7, p15)?
Overview of the snow pillow sites closest to Exshaw townsite. Distances noted with each site, graph represents 2019/2020 snow data (red line) and lower and upper quartile (grey area).
Accumulated Precipitation. Nov 1-Jun 24
700
600
500
400
300
200
100
0
year 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
South Ghost Headwaters Seebee
Accumulated precipitation from the South Ghost and Seebee Dam stations, 2006-2020
Little Elbow Summit SWE graph from 2017/2018 to 2019/2020 winter seasonThree Isle Lake SWE graph from 2017/2018 to 2019/2020 winter season Sunshine Village SWE graph from 2017/2018 to 2019/2020 winter season
20
00
100
150
200
250
300
350
400
450
500
0
50
-0
2 0 5-0
00 1
-0
2 0 5-0
00 3
-0
2 0 5-0
00 5
-0
2 0 5-0
00 7
-0
2 0 5-0
00 9
-0
2 0 5-1
00 1
-0
2011
2000
2 0 5-1
00 3
-0
2 0 5-1
00 5
-0
2012
2001
2 0 5-1
line represents 2020.
00 7
-0
2 0 5-1
00 9
-0
2013
2002
2 0 5-2
00 1
-0
2 0 5-2
00 3
-0
2014
2003
2 0 5-2
00 5
-0
2 0 5-2
00 7
-0
2015
2004
2 0 5-2
00 9
-0
2 0 5-3
00 1
-0
2016
2005
2 0 6-0
00 2
-0
2 0 6-0
00 4
-0
2017
2006
2 0 6-0
00 6
-0
Flow on Bow River at Banff
2 0 6-0
00 8
-0
2 0 6-1
2018
2007
00 0
-0
2 0 6-1
00 2
-0
2 0 6-1
2019
2008
00 4
-0
2 0 6-1
00 6
-0
2 0 6-1
2020
2009
00 8
-0
2 0 6-2
00 0
-0
2 0 6-2
2010
00 2
-0
2 0 6-2
00 4
-0
2 0 6-2
00 6
-0
2 0 6-2
00 8
-0
6-
30
Historic Bow river levels measured at Banff by Environment Canada. Thicker redReferences Chris Hopkinson, Tim Collins, Axel Anderson, John Pomeroy & Ian Spooner (2012) Spatial Snow Depth Assessment Using LiDAR Transect Samples and Public GIS Data Layers in the Elbow River Watershed, Alberta, Canadian Water Resources Journal / Revue canadienne des ressources hydriques, 37:2, 69-87, DOI: 10.4296/cwrj3702893 Water Office Banff, https://wateroffice.ec.gc.ca/mainmenu/historical_data_index_e.html
Your report shows images from 2020 of infiltrating water and subsequent reduction in streamflow due to the infiltration. (Section 3, P 7-8). The dated google images below show that on July 15 2018 Exshaw creek flowed to the Bow River. However, on July 27, 2020 it stops significantly short. Another image from 2019 (caltopo date unconfirmed) also shows the creek running to the bow river. These images are the most recent images of Exshaw that can be found on Google Earth and not chosen for any other reason. Pictures of the sediment pond included in your report (Appendix D) confirms residents’ observations that Exshaw creek did not overspill the wear and therefore stream length was significantly reduced through the entire freshet and summer flow period. Question: Given that your report suggests an exceptional snowpack was the cause of the flooding, would you not expect to see more flow in the lower reaches of Exshaw creek during the groundwater flooding event and not less? Additionally, given that the groundwater flow direction was determined in your study to be SE (ie from roughly from the creek towards E.Exshaw) how was reduction in stream length reconciled with the conclusions that influence of the mitigation work was “minor” and “likely coincidence and not cause” (section 7, p.14)? Given the stream bed was significantly lower post- construction, combined with the weir limiting outflow of Exshaw creek, would you not expect increased surface water-ground water interaction? Google Earth image from July 2018, showing the creek flowing to the Bow River.
caltopo.com Global Imagery Layer (Copernicus satellite) from 2019, showing the creek flowing to the Bow River.
Google Earth image from July 2020, showing the creek infiltrating partway down the creek bed.
During 2020 freshet, one of the first signs of an unusual ground water event was the different order that houses started seeing evidence of a rising groundwater table compared to previous years hinting there may have been a change in the flow pattern of the groundwater. In an email sent on July 29, 2020 it was mentioned that we had plotted the heights of water in the Berm and Lagoon wells relative to each other. As can be seen below, we believe that it showed a change in the direction of flow in the groundwater under Exshaw in the freshet of 2020 compared to the same period in previous years. Note that in the previous years, the Lagoon well level was consistently above the Berm well level throughout the spring freshet and into the summer. In 2020, this trend is reversed until June 22. The data used for these graphs is the municipal well data, as available to McElhanney and the MD. Please note in 2018, for the June 1 data point, it was noted the value was an estimate due to technical difficulties. Another way to represent this data is as a gradient; the height difference of the water table relative to the fixed horizontal distance between the wells. Groundwater flow direction is related to gradients between well points, consistent with your analysis of lateral hydraulic gradients (section 5.2 p 10). A reversal in the gradient could be a shift in the direction of flow and therefore a change in the relative magnitudes of the different sources of groundwater (Bow river, Exshaw Creek and Jura Creek). The graph outlining the gradient between the Lagoon well and Berm well for the 2015-2020 period is included below. Question: Do you have an explanation for the reversal in the gradient between the two MD wells?
6-
M 1-
M
1,285.40
1,285.60
1,285.80
1,286.00
1,286.20
1,286.40
1,286.60
1,286.80
1,287.00
1,287.20
1,287.40
1,285.00
1,285.50
1,286.00
1,286.50
1,287.00
1,287.50
1,288.00
ay ay
-1
9 -
8- 17
13 M
-M ay
15 -1
ay -M 7
-1
9 ay
20 2 2 -17
-M -M
ay ay
-1 2 9 -17
9 -M
27 ay
-M -1
ay 5- 7
-1 Ju
9 n-
12 17
Berm
Berm
3- - Ju
Ju n-
n- 19 17
19
2019
2017
- Ju
n-
10 26 17
- Ju - Ju
n- n-
19
Lagoon
Lagoon
17
3-
17 Ju
- Ju l-1
n- 10 7
19 - Ju
l -1
24 17 7
- Ju - Ju
n- l -1
19
24 7
- Ju
1- l -1
Ju 31 7
l-1
Berm and Lagoon well elevations 2015-2020.
9 - Ju
l -1
7
3-
M
1,285.80
1,286.00
1,286.20
1,286.40
1,286.60
1,286.80
1,287.00
1,287.20
1,287.40
1,287.60
1,287.10
1,287.20
1,287.30
1,287.40
1,287.50
1,287.60
1,287.70
1,287.80
1,287.90
1,288.00
1- ay
Ju -1
n- 8
20 10
3- -M
Ju
n- ay
20 -1
5- 8
Ju 17
n- -M
20 ay
7- -1
Ju
n- 8
20 24
9- -M
Ju ay
-1
n- 8
11 20 31
- Ju -M
n- ay
-1
13 20
- Ju 8
n- Berm 7-
Ju
Lagoon
15 20 n-
- Ju 18
2018
2020
n- 14
17 20 - Ju
- Ju n-
n- 18
19 20 21
Lagoon
Berm
- Ju - Ju
n- n-
21 20 18
- Ju 28
n- - Ju
23 20 n-
- Ju 18
n-
25 20 5-
- Ju Ju
l-1
n- 8
27 20 12
- Ju - Ju
n- l -1
29 20 8
- Ju
n-
20Gradient from Lagoon well to Berm well for all data points during freshet: May - July, 2015-2020. Positive gradient indicates the level in the lagoon well is higher than the berm well.
The report states the “importance of the effects that the […] mitigation structures on Exshaw Creek have on local groundwater levels in the North Community of Exshaw are unclear, but effects appeared to be minor as compared to the impact from the melting snowpack.” (Section 7, p. 16). In the Figures section (p. 17-22), the report includes several groundwater contour maps, outlining East to Southeasterly groundwater flow during various dates in 2020. Additionally you mentioned during you presentation to council the pumping water from basements was largely ineffective and that the pooling water visible between the north community and the bow river likely reentered the groundwater table. Question: Given the ground water flow direction, could temporary mitigation strategies applied to Exshaw Creek to limit infiltration, result in a reduced magnitude of the groundwater flow as shown in the report? The report was unable to conclusively rule out the mitigation structures as a cause or contributing factor to the flooding. The direction of ground water flow underneath the North community as determined from the groundwater monitoring well network to be broadly from Exshaw Creek towards the North community. Given the assumed lack of effective dewatering from pumping from basements into ditches, pipe and culverts onto open land, what temporary mitigation strategies do you think could be applied to reduce the severity of future groundwater flooding events. Would temporarily diverting some or all water from Exshaw creek directly to the Bow river through pipe or sealed channel be a possible mitigation strategy by reducing infiltration? A water management company out of Red Deer gave a rough estimate based on the flow rate as estimated by a resident who rented a flow meter on June 15th (not peak flow rate). Estimated flow at top of alluvial fan was 517 L/s. Estimate: $3000 mobilization and $300 /day rental rate for such equipment + fuel. Pumps would be on vacuum once the line/lines are full due the the gradient of the creek.
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