CGU HS Committee on River Ice Processes and the Environment 19th Workshop on the Hydraulics of Ice Covered Rivers Whitehorse, Yukon, Canada, July 9-12, 2017.

Exploring flow operation schemes for sustainable ice-jam flood management along the Peace River in western Canada

Prabin Rokaya1, Apurba Das2 and Karl-Erich Lindenschmidt3 Global Institute for Water Security, University of Saskatchewan

11 Innovation Boulevard, Saskatoon, SK, S7N 3H5, Canada 1prabin.rokaya@usask.ca, 2apurba.das@usask.ca, 3karl-erich.lindenschmidt@usask.ca

Climate change and regulation have altered the natural flow regime of the Peace River in western Canada raising concerns over the river’s health, riverine species and riparian ecosystems. The major concern is over the reduced frequency of ice-jam floods which are particularly effective in replenishing the high-perched basins of the Peace-Athabasca Delta, a socio-economically and ecologically important delta in western Canada. Previous studies have suggested that releasing water at opportune times from the W.A.C. Bennett Dam could promote ice jam flooding in the delta; however, ice-jam flood events can also be severe and devastating as they often result in high water levels causing economic losses and human casualties. Historical records show that the communities in the Town of Peace River are prone to ice-jam flooding with ice-jam flood events recorded in 1973, 1974, 1979, 1992 and 1997 during the post-dam period. Thus, a critical and challenging question is how can we minimize ice-jam flood risks in upstream communities such as the Town of Peace River, Fort Vermillion and other smaller settlements, but still enable flooding in the downstream delta, where it is essential? This study reviews past approaches and explores possible reservoir operation strategies to minimize flood risk and maximize flood potential at desired locations. A preliminary hydrological modelling study complements the review to provide proof-of-concept of a modelling approach to address the above question.

1. Introduction The construction of W.A.C. Bennett Dam in the headwaters of the Peace River in 1968, the subsequent filling of the Williston Reservoir (1968-1971), and the energy demand driven flow release in the following years have altered the natural flow regime of the Peace River (Prowse et al., 2002a; Peters et al., 2006b); Toth et al. (2006) and raised concerns over the ecological health of the Peace-Athabasca Delta (PAD) due to prolonged dry periods, and considerable reduction in the area covered by lakes and ponds (Prowse & Conly, 2002; Beltaos, 2014). However, the changes in the hydro-ecology of the PAD is also equally attributed to change in climate (Wolfe et al., 2005; Wolfe et al., 2008) especially due to the decrease in the snowpack in the basin of the Wapiti-Smoky River, a major tributary (Prowse & Conly, 2002; Prowse et al., 2006; Romolo et al., 2006). The major concern is the reduced frequency of ice-jam floods which are particularly effective in replenishing the high-elevation ‘perched’ basins of the PAD (Prowse & Conly, 2000; Peters & Prowse, 2001) since the basins lose more water from evapotranspiration than is supplied by precipitation (groundwater being negligible) (Peters et al., 1999). Previous studies such as NRBS (1996), PAD-TS (1996) and NREI (2004) have found that, though open water floods might affect water levels in the lower portions of the PAD, only ice-jam flooding can inundate the perched basins (Prowse & Conly, 2000; Beltaos, 2014). This is because high staging during ice-jam events can block the northward drainage of the system resulting in reverse flows in the delta channels, and depending upon the elevation achieved, perched basins may be inundated (Peters & Prowse, 2001; Peters et al., 2006b). However, ice-jam floods can also be severe and disastrous for communities resulting in economic losses and human casualties (She, 2008; Lindenschmidt et al., 2015). Historical records show that the community of the Town of Peace River (TPR) are prone to ice-jam flooding with ice-jam flood events recorded in 1973, 1974, 1979, 1992 and 1997 during the post-dam period (Uunila & Church, 2015). The event of 1997 (April 19-23) alone resulted in estimated damages of $47 million and the evacuation of 4000 residents (Public Safety Canada, 2013). Although the town has a dike system to retain flood waters and reduce the risk of flooding, potential ice-jam flooding is still one of the main concerns for the community (Lindenschmidt et al., 2016). Thus, a critical and challenging question as concluded by Stetkiewicz et al. (submitted) is: how can we minimize ice-jam flood risks in upstream communities such as the TPR, Fort Vermilion and other smaller settlements, but still enable flooding in the downstream delta, where it is essential? There have been efforts to prevent flooding in the TPR (JTF, 2006; Jasek et al., 2007) as well as attempts to recharge the PAD (Prowse et al., 1996; Prowse & Demuth, 1996; Prowse et al., 2002b); however, an integrated approach to achieve both has not yet been presented (to the best of the authors’ knowledge). Thus, this study reviews previous approaches and explores possible reservoir operation strategies to minimize flood risk and maximize flood potential at desired locations. A preliminary hydrological modelling study complements the review to provide proof-of-concept of a modelling approach to address the above question.

2. Site Description The Peace River begins in the eastern slopes of the Rocky Mountains of northern British Columbia (Toth et al., 2006), see Figure 1. The headwater streams, many of glacial origin, drain into the Williston Reservoir (70x109 m3) capturing 24% of the total basin area of the Peace River (Peters

& Prowse, 2001). The combination of the local geologic structure and sediment supply, coupled with the influence of in-channel flow and tributary inputs determines the overall morphology of the river (Prowse et al., 2002a). The river is 1923 km long and has a total drainage area of 293,000 km2 (Leconte et al., 2001). The average annual flow from 1960 to 2008, as measured at the Peace Point gauge, is 2090.4 m3/s which is approximately 62% of the total flow of the Slave River. The average annual precipitation is circa 467 mm (at Fort St. John) and typical summer (July) and winter (January) air temperatures are 160C and -180C, respectively (Prowse & Conly, 2002).

3. Data and Methods 3.1 Meteorological data A global meteorological forcing dataset for land surface modelling was developed at Princeton University to address time and space inconsistencies by providing high resolution forcing data of long temporal and global coverage (Sheffield & Wood, 2007). The dataset is constructed by combining a suite of global observation-based datasets with the NCEP/NCAR reanalysis and bias correction. The dataset is currently available globally at 0.25, 0.5 and 1.0 arc degrees and at 3-hourly, daily and monthly resolutions for the period 1948-2008 with an experimental update (version 2) for 1901-2012 (Sheffield et al., 2006). The data is very useful for understanding hydro-ecological processes and seasonal and inter-annual variability as well as to evaluate coupled models and other land surface prediction schemes (Sheffield & Wood, 2008). 3.2 Discharge data The discharge data were retrieved from the Water Survey of Canada through Environment Canada’s Hydat database (https://ec.gc.ca/rhc-wsc/default.asp?lang=En&n=9018B5EC-1). The data is available at a daily time step up to 2012 for standardized data sets whereas real-time raw data are available for the past 18 months at any current time. 3.3 Hydrological Model MESH The hydrological modelling in this study was carried out using Modélisation Environmentale–Surface et Hydrologie (MESH), a semi-distributed physically based land surface-hydrological model (Pietroniro et al., 2007). It uses the Canadian Land Surface Scheme for vertical exchanges and generation of lateral fluxes of energy and water balance for vegetation, soil and snow, and the WATFLOOD for flow routing (Haghnegahdar et al., 2014). For computational efficiency, it uses a Grouped Response Unit approach which combines areas of similar hydrological behavior to address the complexity and heterogeneity in the drainage basin (Kouwen et al., 1993). For large scale basins, this approach has been found more appropriate because of its operational simplicity while retaining the basic physics and behavior of a distributed model (Pietroniro & Soulis, 2003). 3.4 Model Set up The MESH model was setup for the Peace River basin with the outlet at Peace Point. The drainage database was prepared using GreenKenue, an advanced data preparation, analysis, and visualization tool (http://www.nrc-cnrc.gc.ca/eng/solutions/advisory/green_kenue_index.html). The digital elevation model was downloaded from Geogratis (http://geogratis.gc.ca/), land use data from GeoBase (http://www.geobase.ca/geobase/en/data/landcover/index.html) and soil data from Agriculture and Agri-Food Canada http://sis.agr.gc.ca/cansis/nsdb/slc/v3.2/index.html). The

model was calibrated using the parallel version of the Dynamically Dimensioned Search algorithm (Tolson & Shoemaker, 2007) using a multi-algorithm auto-calibration program, OSTRICH (Matott, 2005). As only a relatively short-term data set from 1959-1967 is available for pre-dam years at the gauging station, Peace River at Peace Point, the model was first calibrated for the 1959-1963 period and then validated for the 1963-1967 period. After achieving very good agreement (NS>0.8) for both periods, the validated model was run for the post-dam period from 1972 to 2011. For the post-dam simulations, a controlled reservoir release was added near Hudson Hope. Inserting a controlled reservoir option with a reservoir location (reach) in the drainage database neglects all the upstream generated flow above that reach and instead uses the user provided flow input from the reservoir release file. The discharge measured at the gauging station, Peace River at Hudson Hope, was used as the reservoir release.

4. Results and Discussion 4.1 Hydrological Model Calibration and Validation Figure 2a and 2b shows that the model performed reasonably well when simulations are compared with observations at both Peace River at Peace River and Peace River at Peace Point with respective Nash Sutcliffe (NS) efficiencies of 0.87 and 0.84, and log(NS) of 0.89 and 0.82 for calibration years. In validation years, NS of 0.88 and 0.79, and log(NS) of 0.89 and 0.81, respectively, were observed. High NS values show that the model can capture peak flows whereas high log(NS) values reveal the model capability to accurately simulate low flows. Then, the reservoir feature was activated with the controlled reservoir option for the post-dam period. With the controlled reservoir option, all the runoff generated upstream of the reservoir location is neglected and only the outflow from the reservoir is then routed downstream, incorporating other lateral flows downstream of the reservoir. Figure 2c and 2d shows the post-dam simulated and observed flows at the TPR and Peace Point, respectively, for the following 8 years. Though there are some biases in the summer flows, the model is able to capture the spring flows well. 4.2 Revised flow operation One of the major recommendations of the final NRBS hydrology report (Prowse & Conly, 1996) was a modification in flow operation of the Bennett Dam to increase the probability of ice-jam formation and subsequent breakup events near the PAD (Prowse & Conly, 2000). In fact, in the spring of 1996, when all the hydro-meteorological conditions were deemed suitable, increased flows were released from the dam which resulted in the first major flooding in over two decades in the PAD (Prowse et al., 2002b). During this event, the flow was increased by roughly 50% (from 1100 to 1600 m3/s) between April 25 and May 3 which resulted in an additional staging of 0.27 m in the water level downstream of Peace River near the PAD. Considering that the river was already in a flood crest state, it represented a significant additional flow available to flood the PAD (Prowse et al., 2002b). For the perched-basins in the PAD, recharge occurs primarily through over-bank flooding whereas water loss occurs mainly through open water evaporation and evapotranspiration (Peters et al., 1999). Since evaporation is larger than precipitation and groundwater flow is considered negligible, perched basins are estimated to dry up within 5-7 years (Prowse et al., 1996). Peters et al. (2006a) performed a detailed study of persistence of water within these isolated perched basins.

Their findings show that 0.8m deep ponded water lasted for about 5 years in a cool-dry period of the 1920s whereas in the post-1974 flood era, it lasted slightly longer. For the wet period during the 1940s and 1950s, water persisted for up to 9 years. Adopting the findings from the above studies into the modelling as scenarios, flow release at Hudson Hope was increased by 50% for a continuous 10-day period during spring ice cover breakup every five years. The travel time of flow from Hudson Hope to Peace Point (~7 days) was considered in the routing while releasing additional flow to increase discharge at breakup near the PAD. The breakup flow was considered the largest discharge near the last ‘B’ value as provided by Environment Canada’s discharge data. As the dam operated from 1972, flow was modified for eight events with five year intervals, i.e. 1976, 1981, 1986, 1991, 1996, 2001, 2006 and 2011. The results of adjusted flows at Hudson Hope with additional flows at the TPR and Peace Point are provided in Figure 3, 4 and 5. The hydrological modelling results show that the 50% flow increase at Hudson Hope translated to a 12.4-20.8% increase at the TPR with an average of 16% flow increase over the eight events (details at Table 1). Similarly, at Peace Point, the average flow increase over the eight events was 11.7%, ranging from 8.8-15.5% for different events. Similar results were reported by Prowse et al. (2002b) who modelled the 1996 ice-jam flood event. An approximate 50% increase in discharge from the dam that year translated into an 11% flow increase at the PAD. An increased flow release does not necessarily mean it will result in an ice-jam flood since ice-jam floods are highly complex and unpredictable phenomena. Ice jams are primarily governed by fluvial geomorphology, water level at freeze-up, ice characteristics, meteorological conditions and spring flows (Beltaos, 1997; Andrishak & Hicks, 2008). For the PAD, Beltaos (2003) has estimated a discharge of at least 4000 m3/s is required for ice-jam flooding to occur. Interestingly, for three events, i.e. 1981, 2001 and 2011, when simulated flows were lower than 4000 m3/s, the modified flows (with 50% increase from the reservoir) resulted in more than 4000 m3/s of flow at Peace Point. 4.3 Pre-conditions for consideration While promoting periodic ice-jam flooding in the PAD, it is also of utmost importance to ensure that the upstream communities such as the TPR and other towns like Fort Vermillion are not flooded. There is already a memorandum of understanding between Alberta and British Columbia, which guides operating procedures for monitoring and controlling flows during freeze-up and breakup conditions of the Peace River at the TPR to prevent any possible ice-jam flooding. The breakup water level is aimed at a water level elevation of 314 m with a flow threshold of 3200 m3/s at the TPR as a threshold to protect against the calculated 1:100-year flood in spring (Jasek et al., 2007). In our numerical experiment, the discharge surpassed 3200 m3/s at the TPR for all flow adjusted years except 1981. However, this would not increase the risk of ice-jam flooding significantly especially if the receding ice front had already passed the TPR in the downstream direction. Each year, ice jam progression and recession fronts are monitored and all historical records are available at Alberta Government (2017), thus, the increased flows from the reservoir can be released once the ice-jam front has receded downstream past Fort Vermillion, given suitable hydro-meteorological conditions. However, there are some smaller First Nations communities downstream of Fort Vermilion (see Figure 6), which requires a flood risk assessment to examine what magnitude of discharge under ice jam conditions can be handled.

5. Conclusion and way forward In this study, we show that the hydrological model, MESH is capable of simulating streamflows of a large and heterogenous basin. We also demonstrated that, by increasing reservoir release in the breakup period, it is possible to increase the likelihood of ice-jam flooding in the PAD without necessarily resulting in ice-jam floods in the upstream communities. The timing of the release, taking into account the ice cover breakup recession, must be considered. However, this study does not estimate the potential water level that may result from revised reservoir operations every five years. In case of ice-jam flooding, it is well known that even a small discharge can result in higher water levels depending on other hydro-meteorological conditions. Thus, further research is necessary to assess the impacts of increased water levels, both at the TPR and the PAD, from the additional water releases using a 1D hydrodynamic river ice model such as RIVICE. Once water levels and discharges have been calculated, inundation modelling can be carried out to estimate the extent and scale of flooding that can be expected in the PAD. Acknowledgments The authors are thankful to Dennis Lazowski from Water Survey Canada in Alberta for providing the water level data. This work is supported by the Canadian Excellence Research Chair in Water Security through the Global Institute for Water Security, University of Saskatchewan. References Alberta Government. (2017). River Ice Observation Report: 2016-2017 Peace River, Report No.

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List of Figures:

Figure 1. The Peace River basin.

Figure 2. Calibration and validation of the MESH model at the TPR (Peace River gauging station, 07HA001) and at Peace Point (Peace River gauging station, 07KC001).

Figure 3. The increase in flow release at Hudson Hope and simulated flow increase at the TPR

and Peace Point for 1976, 1981 and 1986.

Figure 4. The increase in flow release at Hudson Hope and simulated flow increase at the TPR

the Peace Point for 1991, 1996 and 2001.

Figure 5. The increase in flow release at the Hudson Hope and simulated flow increase at the

TPR and Peace Point for 2006 (left panel column) and 2011 (right panel column).

Figure 6. First Nations communities along the Peace River.

List of Tables:

Table 1: The adjust flows at Hudson Hope and subsequent flow increases at the TPR and Peace

Point. 1976 1981 1986 1991 1996 2001 2006 2011 Hudson Hope

Avg. manual flow increased (m3/s)

552.3 458.4 504.9 458.0 630.0 605.4 468.8 542.7

Avg. percent increased

50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0

TPR Avg. simulated flow

increased (m3/s) 519.8 431.1 473.7 444.1 603.9 569.6 439.7 524.4

Avg. percent increased

15.9 16.9 12.4 12.7 17.8 18.4 13.8 20.8

Peace Point

Avg. simulated flow increased (m3/s)

489.8 414.2 462.3 436.1 602.3 534.0 408.5 513.8

Avg. percent increased

10.8 12.5 9.4 8.7 13.9 13.9 8.8 15.5