Hydrology Study, Bow and Elbow River Updated Hydraulic Model Project, Rev

March 2010 HYDROLOGY STUDY, BOW AND ELBOW RIVER UPDATED HYDRAULIC MODEL PROJECT, REV. A Submitted to: Alberta Environment REPORT Report Number: 09-1326-1040 HYDROLOGY STUDY, BOW AND ELBOW RIVER UPDATED HYDRAULIC MODEL PROJECT Executive Summary Golder Associates Ltd. (Golder) was commissioned by Alberta Environment (AENV) to conduct a hydrology study for the “Bow and Elbow River Updated Hydraulic Model” project. The City of Calgary (the City), in partnership with Alberta Environment (AENV), plans to create a HEC-RAS (Hydraulic Engineering Center River Analysis System) hydraulic model of the Bow and Elbow Rivers through the City. The model implementation is primarily for supporting emergency response planning and operations through flood inundation mapping. It will provide additional perspective on current flood hazard area management and will provide a basis for increased understanding of fish habitat, river morphology and erosion, water quality and storm water runoff impacts. The scope of work for the hydrology study included: 1) Generation of naturalized daily flow series at the major storage facilities on the Bow River above Bearspaw Dam and on the Elbow River at Glenmore Reservoir; 2) Estimation of naturalized and regulated 1:2 to 1:1,000 year flood flows based on flood frequency analysis of naturalized and/or recorded peak flow series at relevant locations along the Bow River and its tributaries (including the Elbow River); 3) Development of synthetic inflow flood hydrographs for tributaries to the Bow River with hydropower developments and at the Water Survey of Canada (WSC) Bow River at Banff station; 4) Routing of the synthetic flood hydrographs through all major storage reservoirs along the Bow River above Bearspaw Dam and through the Glenmore Reservoir; 5) Commentary on the effects of climate change and the impact of stormwater runoff on flood estimates as well as on the seasonality of flood peaks; and 6) Comparison of the new flood flow estimates with those of AENV’s 1983 Calgary Floodplain Study (the 1983 study). Overview of the Bow and Elbow River Basins The Bow and Elbow Rivers are the primary water courses within the City. The river flows originate in the snowpack and glaciers of the Rocky Mountains and traverse the foothills before reaching the City on the edge of the Prairies. The upstream reach of the Bow River and its tributaries are controlled by several hydropower structures. Within the City, the Bow River is joined by several streams (Elbow River, Nose Creek, Fish Creek and Pine Creek). A short distance downstream of the Nose Creek confluence with the Bow River, water is diverted from the Bow to the Western Irrigation District (WID) canal. According to historical records, major floods occurred on the Bow River in 1879, 1897, 1902, 1929, 1932, 1995 and 2005. Records also indicate that major floods occurred on the Elbow River in 1915, 1923, 1929, 1995 and 2005. Major floods are commonly associated with high rainfall or rain-on-snow events in the May to July period. The study area includes the Bow River and its tributaries within the city upstream of its confluence with the Highwood River. The drainage area of the Bow River at Calgary (WSC Station 05BH004) is 7,868 km2. The drainage area of the Elbow River below Glenmore Dam (WSC Station 05BJ001) is 1,236 km2. March 2010 Report No. 09-1326-1040 ii HYDROLOGY STUDY, BOW AND ELBOW RIVER UPDATED HYDRAULIC MODEL PROJECT Hydrologic Analysis of Flood Peak Flows The hydrology of the Bow River basin was examined in detail by reviewing past work (e.g., the 1983 study), utilizing the latest methods of analysis and considering changes in flows due to flow regulation by hydropower infrastructure, water diversion and land use changes. Naturalized daily flow series were developed at all major storage facilities on the upper Bow River, including inflows to Bearspaw Reservoir, Ghost Lake, Lake Minnewanka, Spray Lakes, Barrier Lake, as well as Lower and Upper Kananaskis Lakes and Glenmore Reservoir on the Elbow River. The flow naturalization used the same project depletion method and the Streamflow Synthesis and Reservoir Regulation (SSARR) channel routing procedures used by AENV. Naturalization of daily flows was facilitated by the Natural Flow Computation Program (NFCP) developed for the Prairie Provinces Water Board (Optimal Solutions, 2009). Frequency analyses were completed based on flood flow series derived from naturalized daily flows for the Bow River and its tributaries above Bearspaw Dam and for the Elbow River above Glenmore Dam, based on the annual maximum daily flow series. Synthetic inflow flood hydrographs (shaped according to recorded flood events) at major storage facilities on the Bow River and its tributaries including the Elbow River at Glenmore Reservoir were developed for each flood flow obtained from frequency analyses of the naturalized flow series. The design flood hydrographs created for all return periods were routed through the storage facilities according to the existing flood control and operation guidelines and selected starting water levels. The arrival times of peak flows from each reservoir and from the natural contributing areas were based on travel times provided by AENV and estimated flood travel times developed by TransAlta as part of its Flood Action Plan. The peak daily flows, after routing, were transformed into peak instantaneous discharges based on relationships established between recorded annual maximum daily and annual maximum instantaneous flows. Historic flood peaks that pre-date systematic flow monitoring were available for the Bow River at Calgary for 1879, 1897 and 1902. The flow estimates for these floods were originally determined as part of the 1983 study, based on recorded high water marks, newspaper reports and information from local residents, and were adopted without change for the current study. A technique respecting the probability distribution of the systematic record was applied to in-fill missing annual peak flows in the historic period of the complete record. The historic floods were also considered in the estimation of flood flows on the Bow River at Bearspaw Reservoir. The flood flows for the lower reaches of the Bow River (the reach between its confluence with the Elbow River and the Highwood River) were derived using two approaches. In the first approach, the peak flow of a specific frequency for a specific tributary was added to the peak flow of the same frequency in the reach of the Bow River just upstream of the tributary to estimate the peak flow of the same frequency downstream of the confluence. The second approach was based on the methodology used in the 1983 study, and involved developing maximum instantaneous discharges for the Bow River downstream of its confluence with the Elbow River. The potential effect of climate change on flood flow estimates was analyzed based on recorded data and predictions from climate models. The result of this analysis showed that the recent patterns in the timing of these floods are similar to what were observed at the beginning of the century. Hence, there is no clear evidence that the patterns in magnitude or timing of flood peaks have changed significantly over the past 100 years. The naturalized peak flow series in the Bow River below Bearspaw Dam and the Elbow River below Glenmore Dam were analyzed to identify whether one flood mechanism (snowmelt or rainfall) dominates flooding. Based March 2010 Report No. 09-1326-1040 iii HYDROLOGY STUDY, BOW AND ELBOW RIVER UPDATED HYDRAULIC MODEL PROJECT on recorded data over the past 100 years, about 80% of the annual peak flows in the Bow River and the Elbow River occurred at the end of May and in June. This suggests that snowmelt or rain-on-snow events dominate generation of peak flows in the Bow River and Elbow River watersheds. However, it should be noted that the most significant historic floods are associated with high rainfall or rain-on-snow events. An analysis of the effect of stormwater runoff on flood flow estimates was conducted using the 2005 flood, the largest flood recorded in the Bow River and Elbow River watersheds since 1932. The result of this analysis indicates that the stormwater runoff contribution to peak flows in the Bow River was relatively small, and that flood flow estimates therefore do not require adjustment to reflect stormwater runoff. Conclusions The following main conclusions are supported by the results of this hydrology study: The review of the available hydrometric data indicated that data were missing at several locations on the Bow River and its tributaries for a number of years. The missing data may have a minor affect on the level of uncertainty in flow statistics derived from the naturalized flow series; Results of the statistical tests conducted for various locations on the Bow River and its tributaries indicate that most of the recorded flood flow series are independent, random and homogeneous and have no trend. The flood flow series recorded for the Bow River above Bearspaw Dam displayed some trend and non- homogeneity, primarily due to a combination of long-term variability in the climate regime and the construction of major facilities on the Bow River and its tributaries. However, the entire recorded flood flow series was considered appropriate for frequency analysis; The inclusion of historic floods (data that pre-date systematic flow monitoring) in the flood frequency analysis increases the magnitudes of peak flows for floods with low probability of occurrence (i.e., floods with long return periods), compared to the estimates obtained based on the systematically-recorded flood flow series only; The flood flow moderating capacity of the major storage facilities on the Bow River is relatively small and there is minor flood regulation capacity by Glenmore Reservoir on the Elbow River; and The flood flows estimated in this study for the Bow River and its tributaries through the City are smaller than those reported in the 1983 study, mainly due to inclusion of additional flood flow data from 1981 to 2008 in the frequency analyses.

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