Use of CFD Modeling for Creating Recreational Opportunities at the Calgary Bow River Weir

The Calgary Bow River Weir Project is intended to remove an existing safety hazard created by an ogee weir, while maintaining the weir’s ability to divert water for irrigation and not increasing upstream flood levels. The river reach downstream of the weir will be transformed into a high water channel and a low water channel, each comprised of multiple pool-and-drop features to provide recreational opportunities for boaters and improve fish passage at the site. Computational fluid dynamics models were developed to evaluate hydraulic conditions of design modifications to HWC Drop #1. In this study, a volume-of-fluid (VOF) model was employed to predict the water surface profile and to assess whether a hydraulic jump would form downstream of the drop structure. The CFD models were validated by comparing CFD results with qualitative and quantitative data collected in the physical models. The comparisons indicated that the CFD models were able to correctly predict hydraulic jump formation immediately downstream of the weir for the existing design, and demonstrated satisfactory hydraulic conditions for the proposed design at flows at which boat passage is expected to occur. This study demonstrated that CFD modeling is a viable tool for predicting flows involving highly deformed water surfaces, such as those associated with hydraulic jumps. Introduction The existing Western Headworks diversion weir (Calgary Weir), located on the Bow River within the City of Calgary, was constructed in 1975 and is operated by Alberta Environment for the purpose of supplying water to the Western Irrigation District (WID). The weir is located within the Inglewood District, adjacent to Pearce Estate Park on the right bank and Deerfoot Trail on the left bank. Left or right refer to directions as seen by an observer looking downstream. A Canadian Pacific Railway World Environmental and Water Resources Congress 2008 Ahupua'a © 2008 ASCE Copyright ASCE 2008 World Environmental and Water Resources Congress 2008: Ahupua'a 2 (CPR) bridge crossing is located approximately 350 m upstream from the weir and Cushing Bridge (17 Avenue) is located approximately 900 m downstream. The headworks facility includes the WID canal intake structure, a three-bay gated sluiceway, a fish ladder and the diversion weir, as shown in Photo 1. a) View looking north (flow is from left to right) b) View looking upstream (CPR bridge and downtown Calgary in background) Photo 1: Calgary Weir on Bow River – Existing Site Although the weir functions well to satisfy the intended purpose of diverting flow to the WID intake, the design of the weir creates a dangerous hydraulic condition that has claimed several lives over the years, and currently represents an impassable barrier to boaters and fish under most flow conditions. A pre-design study was undertaken to determine the feasibility of modifying the weir to allow small boat passage and to eliminate the dangerous hydraulic roller created by the existing structure (Golder, 2003). Other potential benefits include enhanced fish passage, channel restoration, and the creation of a whitewater recreational facility. The preferred alternative involves modifying the existing weir crest and replacing the existing concrete stilling basin with a series of smaller drops and pools to mimic a “natural rapids” in the reach downstream of the weir. It was proposed that the existing island downstream of the weir be raised and extended to create a separate high water channel (HWC), with multiple pool-and-drop features for whitewater recreational use, and low water channel (LWC) for downstream passage of beginner and novice boaters. Two physical models were utilized in developing the proposed design for the project: a “comprehensive model” was constructed at a scale of 1:50 to provide an overall assessment of how the facility will perform; and a “flume model” was constructed at a 1:15 scale to provide better insight into flow characteristics at specific in-channel drop structures (nhc, 2007). Although the proposed preliminary design met the study objectives, various options for filling in the downstream portion of the weir (HWC Drop #1) were conceptually developed, after the physical models were dismantled, to reduce construction costs. Additional modeling was deemed necessary to confirm whether hydraulic conditions remained satisfactory for the proposed modified design, and this modeling was carried out using computational fluid dynamics (CFD) simulations. World Environmental and Water Resources Congress 2008 Ahupua'a © 2008 ASCE Copyright ASCE 2008 World Environmental and Water Resources Congress 2008: Ahupua'a 3 Study Objectives The objectives of the CFD study were to: (i) assess whether possible design modifications to HWC Drop #1 create hydraulic conditions considered satisfactory from a boat passage perspective; and (ii) provide supporting information for assessing the need for additional erosion protection at the downstream end of the weir structure. CFD Model Description The CFD modeling software, FLUENT (version 6.3.26), was utilized to evaluate hydraulic conditions of the possible design modifications to HWC Drop #1. The model reproduced a narrow “slice” of river channel through HWC Drop #1 with simplified upstream and downstream bathymetries selected to be representative of the prototype. The CFD model employed the volume-of-fluid (VOF) model to predict the water surface profile and to assess whether a hydraulic jump forms downstream of the drop structure (FLUENT, 2007). The κ−e turbulence model was used in the numerical calculations of all simulations. The CFD model was capable of predicting the velocity distribution, water surface profile, vorticity, and boundary shear stresses throughout the model extents, which were used to assess the safety of boat passage and the need for additional erosion protection downstream of the weir. Although CFD models cannot be relied upon for the prediction of detailed standing waves in a river, they are capable of predicting the formation of hydraulic jumps (if present). In addition, qualitative data available from the physical models were used to provide guidance in interpreting CFD model results related to such a flow phenomenon. Geometries in the CFD model were comprised of approximately one million computational elements, which are considered sufficient for providing accurate predictions of flow patterns and velocities. Baseline Simulations (Model Validation) CFD model validation was conducted by comparing CFD modeling results for the existing weir and preliminary design to quantitative and qualitative observations on the comprehensive and flume physical models. Table 1 summarizes operating conditions for the two baseline simulations conducted for the existing weir. Table 1: Operating Conditions for Baseline Simulations Discharge (m/s) Unit Discharge Water Levels at Weir (m) Run No. Flow Condition Total River To WID Intake Below Weir (m/s/m) Upstream Downstream 1 Typical summer flow 150 13 137 0.91 1035.