MEMORANDUM 1 INTRODUCTION. Comox Road Dyke Slough Tide Gate Modifications Numerical Modelling and Conceptual Design Report (DRAFT)

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1 3 100 Wallace St Nanaimo, BC V9R 5B MEMORANDUM TO: Craig Wightman, R.P.Bio. (BCCF) DATE: October 14, 2013 FROM: Graham Hill, P.Eng. NO. PAGES: 13 CC: Esther Guimond, R.P.Bio PROJECT NO.: RE: Wayne White (Project Watershed) REF. NO.: Comox Road Dyke Slough Tide Gate Modifications 1 INTRODUCTION In 2012 Northwest Hydraulic Consultants Ltd. (NHC) prepared a hydrotechnical overview of the existing culverts and tide gates that convey flow from Glen Urquart and Mallard Creeks into Dyke Slough and then under Comox Road through three culverts with tide gates in the Courtenay River Estuary (NHC, 2012) (Figure 1). The NHC (2012) report concluded that a low cost method to improve fish passage through the culverts was to raise the downstream control (downstream riffle) to backwater the culverts; and, to install a permanent hole in the tide gate on Culvert #1 to provide continuous juvenile fish passage. A more detailed hydrotechnical analysis was required to confirm and refine the NHC (2012) conclusions. For this assessment, high and low flow models were developed that used one-dimensional hydrodynamic modelling software run in the unsteady flow mode. The models were used to compare the water levels in Dyke Slough for the existing culvert/tide gate configuration with a combination of permanent hole and tailwater modifications; these included: Increasing the tailwater riffle elevations downstream of the culverts to -0.6 or -0.4 m; and Installing permanent holes (40 x 80 mm or 200 x 300 mm) in one of the tide gates. Hydrometric input included a summer base flow of 0.02 m 3 /s (20 L/s) adapted from other studies (Riddell and Bryden, 1996; Guimond, 2010) for the low flow assessment. For the high flow assessment a storm hyetograph was developed using regional data from January 2010, and then rainfall-runoff software was used to transform the hyetograph into a hydrograph for input into the flow model. A stage-storage relationship was developed for the slough using topographic data and air photo interpretation. The high and low flow models were calibrated to the January 8 to 13, 2010 and July 1 to 17, 2010 periods. The combination of design changes that caused the largest increase to water levels in Comox Slough were the elevation of the tailwater to -0.4 m and the 200 x 300 mm permanent hole in the tide gates. For low discharge conditions, water levels in the slough ranged from no change to 0.4 m higher during flood tides. At higher discharges the effects of the design modifications diminished, and during large floods (e.g. 5-year flood) the change in water levels between the existing conditions and the design conditions were negligible.

2 Page 2 of 13 Figure 1. Project location and precipitation gauge locations. 2 AVAILABLE DATA Data analyzed for this study included topographic surveys, LiDAR data and aerial photos (courtesy of the City of Courtenay), rainfall records, water level recorders, and tide data. 2.1 TOPOGRAPHIC DATA The site was surveyed by Foster Surveying & Mapping on October 16, 2010 and September 4, The horizontal and vertical datums for the survey were determined using GPS. The elevations were provided in Canadian Geodetic Datum (CGD). The survey included a cross section about 110 m upstream of the culverts, a cross section about 60 m downstream of the culverts, the culvert inverts both upstream and downstream, and a number of other data points (Appendix A). The invert elevations for the three culverts are provided in Table 1. For this report the west culvert is Culvert #1, the center is Culvert #2, and the east culvert is Culvert #3. The road surface elevation is approximately El. 3.0 m (CGD).

3 Page 3 of 13 Table 1. Culvert parameters (based on data from Guimond (2010) and Foster Survey and Mapping (2010)). Culvert # Diameter Downstream Upstream Invert (m) invert (m) (m) Tide Gate Type Side hinge Side hinge Top hinge Contour data was generated from a LiDAR survey commissioned by the City of Courtenay in An ortho photo of the project area was captured during the LiDAR work. 2.2 WATER LEVELS Continuously operating water level recorders were installed by Guimond (2010) on the upstream and downstream sides of the culverts. The water levels were recorded in 15minute intervals from September 5, 2009 to September 3, Water levels at the outlet of the culverts are controlled by a riffle about 60 m downstream, and the minimum water level at the culvert outlets is m. Tides below m do not affect the culvert hydraulics (i.e., there is no backwatering effect at the culverts). 2.3 PRECIPITATION During the period of water level recording (September 2009 September 2010), the largest storm occurred in early January Half-hour precipitation data was obtained for this storm from the UBC Fluxnet Denman Ferry (HDF88) gauge and daily precipitation data was obtained from Environment Canada s Courtenay- Puntledge (Climate ID ) and Comox Airport (Climate ID ) gauges (UBC, 2011; EC, 2013). Figure 1 shows the locations of the precipitation gauges relative to the project location. 3 HYDROLOGY Two runoff conditions were modelled for this study; the first was with low discharge from the slough and the second was with high slough discharges. For the low flow discharge estimate, a constant base flow of 20 L/s was adapted from estimates provided by Riddell and Bryden (1996) and Guimond (2010). For the high slough unsteady discharge a rainfall runoff model was developed using the US Army Corps of Engineer s HEC-HMS 3.5 software (USACE, 2013). 3.1 HIGH FLOW RUNOFF MODEL A synthesized storm hyetograph was developed using precipitation data from three nearby gauges for the January 8 to 13, 2010 storm. The gauged data were half-hourly records from the UBC Fluxnet Denman Ferry (HDF88) gauge and daily records from Environment Canada Courtenay-Puntledge (Climate ID ) and Comox Airport (Climate ID ) (UBC, 2011; EC, 2013). The Denman Ferry precipitation distribution was linearly scaled by a factor of 0.5 such that the daily totals were similar to the Courtenay-Puntledge and Comox totals (Table 2).

4 Page 4 of 13 Table 2. Daily precipitation summary for the January 2010 storm event. Date Courtenay Puntledge (mm) Comox (mm) Denman Ferry (mm) Adopted Comox Dyke Slough (mm) Jan Jan Jan Jan Jan Jan HEC-HMS was used to develop a rainfall-runoff model of the Dyke Slough watershed which is comprised of Glen Urquart Creek and Mallard Creek. Table 3 summarizes the model inputs. Table 3. HEC-HMS rainfall-runoff model parameters for the January 2010 storm event. Parameter Glen Urquart Creek Mallard Creek Watershed area km km 2 Canopy method None None Surface method None None Loss method Initial and Constant, impervious area of 2%, loss rate of 1.2 mm/hr Initial and Constant, impervious area of 2%, loss rate of 1.2 mm/hr Transform method SCS Unit Hydrograph, lag time of 102 min SCS Unit Hydrograph, lag time of 55 min Baseflow method Constant rate of 0.05 m 3 /s Constant rate of 0.05 m 3 /s Meteorological input Scaled January 8 13, 2010 precipitation Scaled January 8 13, 2010 precipitation 1 Watershed areas were determined from TRIM mapping contours in ArcGIS. The peak flow at the culverts was 9.4 m 3 /s at 23:30 on January 10, Comparison to the instantaneous peak flow estimated with the multivariate method (NHC, 2012) indicates that this flow was on the order of a 5-year runoff event; this is consistent with regional flooding associated with this storm (NHC, 2011). The adopted January 2010 hyetograph and Dyke Slough hydrograph are shown in Figure 2. The hydrograph was used for the flow model input for the high flow culvert analysis.

5 Page 5 of 13 Figure 2. The hyetograph and Dyke Slough hydrograph for the January 2010 storm. 4 HYDRAULICS 4.1 MODEL DEVELOPMENT A high flow and a low flow model were developed to assess the effects of varying flood discharges, varying tides, and the effects of raising the culvert tail waters and constructing a permanent hole in the tide gates. An unsteady-state one-dimensional hydrodynamic model, HEC RAS 4.1, was chosen for this study. The software is produced by the Hydrological Engineering Center US Army Corps of Engineers (USACE, 2013). The model extended from approximately 110 m upstream of the road to approximately 40 m downstream of the road. Cross sections, culvert inverts, riffles, data logger locations and road elevations were surveyed by Foster Survey and Mapping on October 16, 2010 and September 4, LiDAR data for the project was supplied by the City of Courtenay. The land area upstream of the culverts is low gradient, and there is significant flood storage at higher water levels. To assess the storage effects upstream of the culverts a stage-storage relationship was developed using LiDAR data, topographic surveys, site observations, and air photo interpretations (Figure 3). There was limited underwater data available thus the stage-storage curve is considered a coarse approximation. Additional detailed surveys would be required to refine the curve.

6 Page 6 of 13 Figure 3. Stage-storage curve for Dyke Slough upstream of Comox Dyke Road. Two separate models were created for the high flow and low flow scenarios. This modelling approach was taken to reduce instabilities where, for some scenarios, small changes in gate coefficients resulted in null solutions. For the high flow model the culverts were modelled with headwalls and square cut ends of pipes. For all culverts the estimated entrance loss coefficients were 0.5 and the exit loss coefficients were 1.0. The tide gates were modelled using inline structures and sluice gates with an orifice flow discharge coefficient of 0.65 (Brater et al., 1912). The gate opening stage differentials were set to model the tide gates within the limits of the model s capabilities and based on water level differentials between the slough and the estuary. Setting the gate opening and closing stage differentials and rates to simulate the actual observed tide gate characteristics caused model instability issues. Accelerated opening and closing rates allowed the model to run; however the slough did not reach the same stage as the measured levels. Several additional macro scale model input uncertainties factor into the actual versus modelled water levels in the slough; these include the hyetograph input and the stagestorage curve. The low flow model had simplified geometry which increased the model stability and allowed fine tuning of the gate parameters. The culverts were entirely removed from the model and the tide gates served as the primary hydraulic controls for the simulations. Gate coefficients for the two models are shown in Table 4.

7 Page 7 of 13 Table 4. Gate coefficients for the high and low flow models. Culvert / Gate 1 Culvert / Gate 2 Culvert / Gate 3 High flow Low flow High flow Low flow High flow Low flow Stage differential for opening (m) Stage differential for closing (m) Opening rate (m/min) Closing rate (m/min) Max opening (m) Max closing (m) Initial opening (m) MODEL CALIBRATION The high flow model was calibrated and verified with the January 8 to 13, 2010 water levels in the slough and the low flow model was calibrated and verified with the July 1 to 17, 2010 water levels. Comparison of the stage hydrographs showed that the models reasonably simulate the water levels in the slough (Figure 4 and Figure 5). Figure 4. Comparison of the modelled and measured water levels in the slough for the Jan 8 to 13, 2010 storm.

8 Page 8 of 13 Figure 5. Comparison of the modelled and measured water levels in the slough for the low flow period of Jul 1 to 17, MODEL SIMULATIONS The low flow and high flow models were used to assess the following eight scenarios: a. 40 x 80 mm permanent hole, high flow (Jan 8 to 13, 2010 storm) with various tides: i. tailwater to -0.6; and, ii. tailwater to b. 40 x 80 mm permanent hole, low flow with various tides: i. tailwater to -0.6; and, ii. tailwater to c. 200 x 300 mm permanent (fry / parr / adult) hole, high flow (Jan 8 to 13, 2010 storm) with various tides: i. tailwater to -0.6; and, ii. tailwater to d. 200 x 300 mm permanent (fry / parr / adult) hole low flow with various tides: i. tailwater to -0.6; and, ii. tailwater to -0.4.

9 Page 9 of MODEL RESULTS The modelled Dyke Slough stages immediately upstream of the culverts were compared with the modelled existing conditions for each scenario. The high flow analysis is separated into small runoff events and large runoff events. The peak flow near midnight on January 10, 2010 was comparable to a 5-year runoff event. Prior to this storm, a series of smaller rainfall events occurred. During the smaller runoff events (for instance, January 9-10, 2010), hydraulic modelling of the 40 x 80 mm hole showed an increase in the slough water levels of 0.04 m or less. The 200 x 300 mm hole increased water levels up to 0.3 m with the -0.4 m tailwater scenario. For large runoff events there were negligible changes in water levels for either hole option (Figure 6). Figure 6. Dyke Slough stage comparison for the Jan 8 to 13, 2010 high flow scenario combinations. For the low flow analysis the 40 x 80 mm hole combined with the -0.6 m tailwater raised the slough water levels up to approximately 0.03 m. The small hole combined with the -0.4 m tailwater raised the slough water levels up to approximately 0.25 m (Figure 7). The 200 x 300 mm gate increased water levels up to 0.35 m above the modelled existing conditions with the tailwater raised to -0.6 m. The -0.4 m tailwater increase water levels approximately 0.4 m (Figure 8).

10 Page 10 of 13 Figure 7. Dyke Slough stage comparison for the 40 x 80 mm gate and varied tailwater elevations for Jul 1 to 17, Figure 8. Dyke Slough stage comparison for the 200 x 300 mm gate and varied tailwater elevations for Jul 1 to 17, 2010.

11 Page 11 of 13 5 SUMMARY For the high flow simulations, results were summarized into small runoff events and large runoff events (i.e. significant floods). For small runoff events the water level in the slough was raised up to 0.04 m for the 40 x 80 mm hole, and for the 200 x 300 mm hole the water level was raised in the slough up to 0.3 m; the effect of the elevated tailwater ranged from 0 to 0.05 m. For significant floods the modifications did not affect the water levels in the slough. The model results show that the permanent holes will improve tidal flushing in the slough during low flow periods. In each tide cycle the slough water levels were elevated during the flood tide and yet returned to the same level as the existing conditions during the ebb. For the low flow simulations the 40 x 80 mm hole increased the water levels by approximately 0.03 m in the slough for the -0.6 m tailwater condition and by approximately 0.25 m for the -0.4 m tailwater conditions; the elevated tailwater had the largest effect on slough water levels. For the 200 x 300 mm hole the water levels were increased approximately 0.35 m for the -0.6 m tailwater and 0.4 m for the -0.4 m tailwater; the flow through the hole had a larger effect on the slough water levels than the elevated tailwater. The upstream extent of the inundation caused by the assessed design options is unclear. Figure 9 shows the longitudinal profile for the slough for 100 m upstream of the culverts (Station 1+75). At Station 1+75 the channel invert elevation is approximately m. The modelled slough water levels for the existing and design conditions are regularly higher than the surveyed channel invert; additional survey data is required to assess the estimated upstream extents of the inundation in the slough caused by the proposed modification at the tide gates. 5.1 FISH PASSAGE The permanent hole would allow upstream fish passage through the tide gate during incoming tides when the tide gates are closed. Juvenile fish could navigate the 40 x 80 mm hole but adult salmon could not. Juvenile and adult salmon could navigate the larger 200 x 300 mm hole. Potential fish utilization of the hole should be assessed by fisheries biologists. The tide gates close during a flood tide when the tailwater elevation is higher than the slough water level. Raising the tailwater to -0.4 m will allow the tide gates to remain open longer which should improve upstream and downstream fish passage potential. The pet door option would see the hole close at about the same time as the main tide gates, thus the water levels upstream are not expected to vary from the existing conditions. The advantage of the pet door over the existing conditions is that it can open more easily during small head differentials across the structure which would allow fish to exit from the slough more readily. The pet door option combined with the elevated tailwater to -0.4 m would see an increase in the slough water levels of approximately 0.25 m during low flow periods.

12 Page 12 of 13 Figure 9. Dike Slough plan and longitudinal profile.

13 Page 13 of 13 REFERENCES Brater, E.F., King, H.W., Lindell, J.E. and Wei, C.Y Handbook of hydraulics for the solution of hydraulic engineering problems. Published by McGraw-Hill with various updates to Guimond, E Courtenay River Estuary (Dyke Slough) Biophysical Assessment Prepared for Living Rivers Georgia Basin / Vancouver Island and BC Conservation Foundation. Northwest Hydraulic Consultants Ltd Tsolum River Flood Hydrology Investigation Final Report. Prepared for City of Courtenay, Comox Valley Regional District and TimberWest Forest Corp. June Northwest Hydraulic Consultants Ltd Comox Road Dyke Slough Tide Gates Hydrotechnical Overview. Prepared for BC MoTI (Sean Wong, R.P.Bio.). June 28, Riddell, A. and Bryden, G Courtenay River water allocation plan. BC Ministry of Environment, Lands and Parks, Regional Water Management, Vancouver Island Region. Nanaimo, BC. 80 p. UBC, from Nicholas Grant, M.Sc. Researcher, Biometeorology and Soil Physics Group, University of British Columbia, Main Mall, Vancouver BC V6T 1Z4. Environment Canada USACE, HEC-HMS USACE, HEC-RAS * * * * * Sincerely, northwest hydraulic consultants ltd. Graham Hill P.Eng. Project Engineer DISCLAIMER This document has been prepared by Northwest Hydraulic Consultants Ltd. in accordance with generally accepted engineering and geoscience practices and is intended for the exclusive use and benefit of the client for whom it was prepared and for the particular purpose for which it was prepared. No other warranty, expressed or implied, is made. Northwest Hydraulic Consultants Ltd. and its officers, directors, employees, and agents assume no responsibility for the reliance upon this document or any of its contents by any party other than the client for whom the document was prepared. The contents of this document are not to be relied upon or used, in whole or in part, by or for the benefit of others without specific written authorization from Northwest Hydraulic Consultants Ltd. and our client.