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Crucial Role of Physical Modeling in Developing the Configuration of the Intake Headwork Structures

Nancy Sims – Associate1, Don Murray – Discipline Practice Lead - Hydrotechnical2, Amir Alavi – Hydrotechnical Engineer2, and Brian Hughes – Principal1

1 Northwest Hydraulic Consultants, Canada,2 Hatch Ltd., Canada,

Abstract

Understanding the natural sediment transport behavior contributed to the effective design and operation of the intake headwork structures for AltaGas’s 195 MW run-of-river Forrest Kerr Hydroelectric Project located on the Iskut River in northwestern BC, Canada. The Iskut River system carries a significant sediment load during the high flow season. The intake headwork structures had to be hydraulically designed to divert the heavy sediment load away from the power intake and direct the design flow into the power tunnel at the location of a major bend in the river channel.

The physical hydraulic model investigated the interaction of the hydraulic flow characteristics with sediment flow patterns. A range of river discharges and numerous design alternatives were investigated to determine the behavior of the flow as it entered the sluiceway approach channel. Ultimately, a solution was found whereby a box culvert was installed along the invert of the channel to extract the majority of the sediment bedload. The remaining flow was directed to an intermediate forebay through a desanding basin for settling and bypassing the finer, suspended sediments.

The authors contend that without the aid of the physical hydraulic model studies, the performance of the intake headwork structure would have jeopardized the operation of the hydroelectric facility. Funding for the engineering design and hydraulic model studies was provided by the owner, AltaGas Ltd.

Project Description

The Forrest Kerr Hydroelectric Project is a 195 MW capacity run-of-river hydroelectric generation plant located on the Iskut River in northwestern British Columbia as shown on Figure 1. The main project facilities consist of a water intake and diversion structure, power tunnel, underground powerhouse, tailrace tunnel, associated electrical substation and transmission infrastructure, and on-site operating and maintenance facilities. The project will operate at a net head of 86 m and a design flow of 252 m3/s.

The diversion structure will comprise a concrete weir approximately 10 m high, with two 10 m long by 5 m high Obermeyer gate sections, a gated sluiceway with a 6 m wide by 7 m high radial gate, and a riparian flow conduit with a minimum downstream flow release capability of 10 m3/s.

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Figure 1 - Project Location

Flow from the Iskut River will be diverted to a sluiceway channel located on the left side of the river; the discharge through this channel will be controlled by the Obermeyer gates at the weir and the setting of the sluiceway gate. Flow will be further diverted to the powerhouse through the forebay control structure and intake structure located on the left side of the sluiceway channel. The forebay control structure will contain a set of trashracks to prevent large debris and bounders from entering into the forebay channel. The forebay channel will contain a sediment extraction channel which is designed to further remove sediments from the flow and which will be flushed back into the sluiceway discharge channel.

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Water from the forebay channel will enter 9.6 m diameter, 3.15 km long drill and blast power tunnel through an intake structure. A vertical gate will be provided which can be closed to permit unwatering of the tunnel for inspection or repair.

The power tunnel will convey the diverted water to an underground powerhouse containing nine 21.7 MW horizontal Francis turbines and associated generation and control equipment. The project will develop approximately 96 m of gross head (~86 m of net head) between the normal operating level at the intake and typical tailwater levels. The powerhouse will be of the arched roof rock-cavern type, with a length of 144 m and a width of 17 m.

After passing through the power tunnel manifold and valves, the flow will pass through wicket gates which control the amount of flow into the turbine spiral case, onto the turbine runners and through the turbine discharge ring. The water will then pass through the draft tube water passages and return the flow to the Iskut River via a 300 m long tailrace manifold and tunnel.

Project Design Basis

The project site is located in the eastern portion of the Coast Mountains and has a watershed area of 6,820 km2. The watershed topography is relatively steep and rugged, and permanent snow and ice fields exist at higher elevations in the basin’s headwaters. The annual precipitation in the basin is roughly 1800 mm per year. Photo 1 shows an aerial view of the project area, before the onset of construction.

The annual hydrograph, as shown on Figure 2, at the intake site, is dominated by snow and glacier melt in the late spring and summer. Flows are lowest in the winter, reaching their minimum in February and March, and then begin to rise in response to snowmelt in April. Mean monthly flows decline in the autumn, but the largest floods can also occur at this time in response to frontal rainstorms. The approximate distribution of flow between the Iskut River and the Forrest Kerr Creek at the influence is approximately 90%-10%, respectively.

The Iskut River transports a large sediment load, ranging from clay-sized sediments to boulders. In most reaches of the river, the suspended load consists of sand, silt and clay, while gravel, cobbles and boulders travel as bedload. In the reach of the headworks, which is characterized by steep canyon walls and a narrow river section, the turbulent conditions, eddies and upwelling flows cause relatively coarse material to be entrained into suspension at higher flows. The annual suspended load of fine sediment (less than 2 mm) has been estimated to be in the order of 6.4 x 106 tonnes/year with an average bedload transport rate of 0.85 x 106 tonnes/year.

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Photo 1 - Project Area, Pre-Construction

Figure 2 - Average Daily Flow at the Intake

Iskut River

Forrest Kerr Creek

Design Discharge = 252 m3/s

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Sediment load at the headworks site varies from near zero during low winter flows to high values during the spring-summer snowmelt period and during autumn rainstorm events, as shown on Figure 3. Much of this material is too fine to be trapped by sediment excluding facilities incorporated into the headworks so it will be ingested with the diverted flows. It is anticipated that suspension of coarse material through the reach during high flow events will not be substantially altered following construction of the headworks. Bedload through the sluiceway approach channel during high flows will either settle out in the project forebay or be removed by the coarse sediment excluder facilities. The specified trashrack grill openings of 50 mm will limit the maximum size of particles from entering the intake forebay to 50 mm.

Figure 3 - Average Daily Sediment Loads at the Intake

Preliminary Design

The various structures that comprise the headworks are shown on Figure 4 and are comprised of:

Power tunnel intake structure; Forebay control structure and channel; Sediment extraction channel and riparian flow conduit; Sluiceway, approach and discharge channel; and Control weir and Obermeyer gates.

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Figure 4 -General Arrangement (Feasibility) The intake structure will provide a smooth transition of the flow from the approach channel/forebay into the power tunnel. The intake structure forms the water passage leading to the power tunnel. It will include a trashrack at the upstream end of the inlet and a vertical lift gate designed for closure of the power tunnel at full flow.

The purpose of the forebay control structure is to divert all river flows equal to or less than 262 m3/s, which is the maximum permitted diversion of 252 m3/s plus the riparian flow of 10 m3/s, into the intake forebay channel. The riparian flow will be released into the sluiceway discharge channel during power plant operation. River discharges greater than 262 m3/s will be passed through the sluiceway and/or over the control weir.

The forebay control structure will be oriented parallel to the flow and be sized to operate at a water velocity of approximately 1 m/s at its design discharge. The structure will accommodate a total of eight intake passages furnished with fine screen trashracks. The passages will also be furnished with stoplog slots to permit installation of stoplogs

OBERMEYERGATES

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to enable the dewatering of the immediate downstream forebay area between the forebay channel structure and the power intake for repair and/or maintenance.

The sediment extraction facilities at the headworks consist of two main components: the sluiceway on the upstream side of the forebay control structure; and, the sediment extraction channel and riparian conduit in the forebay channel upstream of the power tunnel intake.

The sluiceway structure at the downstream end of the sluiceway approach channel will incorporate a radial gate. The structure has been placed immediately adjacent to the downstream end of the forebay control structure to facilitate the passage of the bedload material as it enters the headpond and, in particular, the sluiceway approach channel. The sluiceway approach channel has been set below the invert of the forebay control structure to prevent the larger bedload material from collecting immediately against the trashracks. The sluiceway will also handle the Stage II river diversion during construction of the weir and once the project is in operation, the radial gate will provide additional spilling capacity to assist in passing the inflow design flood.

Downstream of the trashracks, the water velocity in the forebay channel will be limited to approximately 1 m/s in order to facilitate the settling of the larger suspended material before it reaches the raised concrete sill placed diagonally across the forebay channel. This sill will vary in height and form the downstream side of a cross channel in the forebay invert which will slope down towards the downstream side of the forebay. This channel is called the sediment extraction channel. This channel will lead to the riparian flow conduit, which will serve a dual purpose; one to flush the accumulation of fine sediment from upstream of the sill and secondly, to provide a minimum riparian flow downstream of the headworks at all times.

The weir will consist of a broad crested diversion structure with two Obermeyer gates to create and maintain a headpond to provide adequate submergence and approach conditions at the headworks. The weir will be designed to pass excess flood flows and will allow river bed sediments and floating debris to pass over the structure.

Description of the Physical Model

A detailed comprehensive physical hydraulic model of the site area at the headworks including all of the component structures was recommended in order to confirm/refine the hydraulic design of the structures and channels. The physical model was used to test the effectiveness of the configuration of all the structures while at the same time maximizing the hydraulic efficiency of each structure to minimize the hydraulic losses for power generation and to ensure the adequacy of the desilting and sediment sluicing capability to minimize the entrainment of sediment into the power tunnel. A separate 1:20 scale sectional model study was conducted to develop the concept for flushing

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sediment from within the forebay control structure. The sectional model will not be discussed further in this current paper.

At an undistorted scale of 1:40, the comprehensive physical model was constructed to reproduce the confluence of Forrest Kerr Creek with Iskut River, including approximately 575 m of the Iskut River and approximately 225 m of the Forrest Kerr Creek. The model scale was set sufficiently large to reproduce details of the intake facilities, and diversion structures, allow accurate measurements of discharge rates and velocities, and provide sufficient dimensional control. Photos 2 and 3 and Figure 5 show the initial design installed in the model at the onsetof testing.

The model topography was based on a LiDAR survey. Synthetic bathymetric data was developed for initial testing based on the results of a HEC-RAS numerical model and review of site photographs during a low water event.The model topography was built as a fixed surface using concrete and the riverbed sediments were represented using geometrically-scaled sands and gravels placed in accordance with the estimated bed profile. The banks of the model were roughened to approximate the estimated roughness of the banks in the prototype (Manning’s n-value of 0.07 to 0.08).Selection of model sediment material was based on sediment samples collected in the field and site photographs. A washed sand with a D10 of 0.18 mm, D50 of 0.48 mm and D90 of 2 mm (model) - equivalent to a D10of 7 mm, D50of 19 mm, and D90 of 80 mm (prototype) - was fed into the upstream end of the model at at a rate that approximated the estimated loading rate in the prototype. Utilizing this sediment in the model should provide a good indication of the behavior of the material expected in the prototype.

Scenarios Investigated

Initial Design Configuration The intial design intake was located along the inside of a sharp 120-degree bend in the Iskut River. Centrifugal effects at the bend force the surface flow to move along the right bank opposite the intake; while at the same time near-bed flow moves toward the left bank, where the intake will be located, carrying bed sediments with it. Initial design testing was conducted to evaluate the general hydraulic performance of the initial design approach channel and forebay control structure with the powerhouse operating.

Morphology testing was conducted to evaluate the performance of the sluiceway approach channel with sediment being continuously fed to the upstream end of the model at a river discharge of 550 m3/s with the powerhouse operating at 252 m3/s. Before the onset of the test, a submerged weir was placed near the upstream end of the approach channel in an attempt to reduce the volume of sediment entering the approach channel.

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 Figure 5 – Model Layout

Photo 2 – View looking downstream showing the model approach channel and the Obermeyer gates in the fully inflated position.

Photo 3 – View looking upstream showing the general arrangement of the model including portions of Forrest Kerr Creek and Iskut River, the intake structure, sluice gates and Obermeyer gates.  

ObermeyerGates

Intake Structure 

Sluice Gates 

Approach Channel 

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While the test was intended to be operated over a duration of 24 hours (model) or 6.3 days (prototype), the approach channel and forebay control structure became inundated with sediment after operating for only a few hours (model), as shown in Photos 4 and 5.

These results initiated a series of tests and design refinements to identify an effective sediment control design to remedy the inundation of the intake structure. The design refinements included relocating the entrance of the sluiceway approach channel, adding culverts within the sluiceway approach and discharge channels, refining the width and shape of the sluiceway approach and discharge channels, and changing the planned operational procedures.

Relocated Entrance of the Sluiceway Approach Channel

In order to avoid having the entrance located on the inside of a bend where sediment deposition normally occurs, it was decided to relocate the entrance of the approach channel to a location on the Iskut River, upstream of the confluence with Forrest Kerr Creek. Figure 6 contains a sketch showing the modified approach channel entrance in red.

Photo 5 -View looking upstream showing the sediment deposition in the forebay control structure following the initial design testing.

Photo 4 -View looking downstream at the approach channel at the conclusion of the initial design test.

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Figure 6–Relocated Approach Channel Entrance Upstream of Confluence

The entrance to the revised approach channel was set at an elevation of El. 242.5 m. It was envisioned that this elevation would be high enough to limit the amount of bedload entering the approach channel and also to facilitate maintenance of the approach channel during winter low flows (utilizing the Obermeyer gates to lower the water level in the river).

Testing demonstrated that the relocated approach channel resulted in less sediment entering the approach channel and improved flow conditions at the forebay control structure. Testing indicated that the head losses (from the river upstream of the Obermeyer weirs to the entrance of the tunnel portal) were approximately 1 m less than those recorded for the initial design configuration. However, the momentum in the Iskut River flow combined with the geometry of the left bank entrance resulted in flow separation along the left side of the approach channel which generated sediment deposition on the left half of the approach channel, as shown in Photo 6.

Relocated Approach Channel Entrance 

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Addition of Culverts

Testing was conducted to evaluate the addition of a box culvert installed along the floor of the approach channel in front of the forebay control structure in an attempt to convey heavier bed load sediment through the approach channel and minimize the amount of sediment entering the intake and forebay control structure, as shown in Photo 7. In addition, a box culvert was also installed downstream of the sluice gate to convey sediment through the discharge channel.

The addition of the box culvert was a significant improvement in bypassing bedload sediment through the approach channel and discharge channel, and substantially reduced the volume of coarse sediment entering the intake structure. Three different culvert heights were examined in the model consisting of 1 m, 1.5 m, and 2 m. Testing demonstrated that the intermediate box culvert height of 1.5 m was most effective at maintaining sediment movement.

Slotted openings were added to the top of the box culvert in the approach channel in an attempt to draw sediment from along the full length of the sluiceway approach channel and divert it through the box culvert. Testing demonstrated that the slotted openings did facilitate sediment scour from along the culvert length; however, the openings also resulted in a notable decrease in the capacity of the culvert. Based on this, the slotted openings were not incorporated into the final design of the box culvert.

Photo 6 - View looking downstream showing the flow conditions entering the approach channel at a river discharge of 1000 m3/s. 

Flow 

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Refinement of the Width and Shape of the Approach Channel

A number of refinements to the left and right walls of the approach channel were developed in the model in an attempt to improve sediment movement through the approach channel. Specific refinements included reshaping the left bank approach channel entrance in an attempt to decrease flow separation entering the approach channel (as shown in Photo 8) and reshaping the left approach channel into an “S-curve” in an attempt to further reduce the amount of sediment that accumulated along the left wall and facilitate the transport of sediment through the box culvert (as shown in Photo 9). Further refinements to the left and right walls of the approach channel including increasing the width at the entrance of the approach channel and modifying the transition to the box culvert.

The width and shape of the approach channel and discharge channel were refined through the design process to the final arrangement in which the bottom width of the approach channel converged from 45 m at the entrance to 6 m wide near the upstream end of the forebay control structure and then maintained at 6 m wide width to the downstream end of the discharge channel (with only one 6 m wide radial gate within the sluiceway structure), as shown in Photo 10. Testing indicated that at a river discharge of 800 m3/s with the intake operating at 252 m3/s (with the water level at the tunnel portal at El. 246 m and the tailwater level at El. 242.3 m), the box culvert performed well at flushing sediment downstream. However, testing demonstrated that significant quantities of sediment may deposit on top of the box culvert and within the intake

Photo 7 -View looking downstream showing a box culvert installed within the approach channel.

Culvert 

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structure at river discharges greater than 300 m3/s as a result of the relatively high velocities generated in the approach channel.

Photo 8 - View showing the approach channel operating at a river discharge of 600 m3/s.

Photo 9 – View looking downstream showing the approach channel operating at a river discharge of 1000 m3/s.

Photo 10 – View looking downstream showing the final approach channel operating at a river discharge of 900 m3/s with the head difference between the tunnel portal and the downstream water level of 3.5 m.

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Modifications to the Sluiceway Discharge Channel

Model testing demonstrated that the initial design arrangement of the sluiceway discharge channel resulted in sediment deposition at its downstream end (as shown in Photo 11), which restricted flow through the sluiceway approach channel and contributed to sediment depositing and blocking the approach channel.

Testing indicated that narrowing the discharge channel had an initial clearing effect as the increased channel velocity carried sediment farther downstream (as shown in Photo 12); however, the clearing of the upstream portion of the discharge channel was not enough to maintain sufficient transport of sediment through the full length of the discharge channel at river discharges greater than 600 m3/s due to backwater effects generated in the downstream river channel.

The addition of the culvert in the discharge channel resulted in an improvement to the sediment conveyance downstream of the sluice gates. However, sediment still deposited at the downstream end of the channel and backed up flow in the culverts at a river discharge of 1000 m3/s. Based on this, the discharge channel was realigned to include an exit in the main river flow to reduce sediment accumulation at the downstream end of the box culvert, as shown in Figure 7 and Photo 13.

Photo 11 – View from above showing flow patterns in the sluiceway discharge channel. Note the sediment deposition at the downstream end of the channel.  

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Figure 7– Plan View showing the revised alignment of the discharge channel

Photo 12 - View looking upstream showing the sluiceway discharge channel at a river discharge of 550 m3/s.

Revised alignment of the discharge channel 

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The width of the discharge channel was refined through the design process to the final arrangement in which the bottom width was maintained at 6 m to the downstream end of the discharge channel (with only one 6 m wide radial gate within the sluiceway structure). Testing demonstrated that the realigned and narrowed discharge channel resulted in improved sediment flushing through the approach channel as the Iskut River flow effectively mobilized sediment at the discharge channel exit.

Operational Changes

Model testing indicated that at a river discharge of 800 m3/s with the intake operating at 252 m3/s, the sluiceway box culvert performed well at flushing sediment downstream. However, at river discharges ranging from 900 m3/s to 1200 m3/s the water levels downstream of the control weir increased significantly, which reduced the driving head and sediment conveyance through the culvert to the point where the culvert would plug.

Testing was conducted by modifying the tailwater level (and maintaining the water level at the tunnel portal) and also by modifying the water level at the tunnel portal (and maintaining the river tailwater level) to determine the required differential head for adequate flushing of the box culvert at river discharges ranging from 900 m3/s to 1200 m3/s. Model testing demonstrated that a minimum head difference between the tunnel portal and the tailwater of 3.5 m was required to maintain sediment movement through the culvert for river discharges up to 1200 m3/s. Therefore, it was determined that for intake operation at river discharges greater than or equal to 900 m3/s, the minimum head difference at the site will have to be maintained at 3.5 m.

Photo 13 – View from above showing the flow conditions at the exit of the box culvert at a river discharge of 900 m3/s with the intake operating at 252 m3/s.

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Summary

AltaGas’s Forrest Kerr Hydroelectric Project is a 195 MW capacity run-of-river hydroelectric generation plant located on the Iskut River in northwestern British Columbia. The project will operate at a net head of 86 m and a design flow of 252 m3/s. Sediment load at the intake site varies from near zero during low winter flows to high values during the spring-summer snowmelt period and during autumn rainstorm events. The initial proposed entrance for the approach channel was located along the inside of a sharp bend in the Iskut River, where sediment deposition normally occurs.

A 1:40 scale mobile-bed physical hydraulic model study was conducted to evaluate the performance of the Forrest Kerr Hydroelectric Project intake facilities. A primary focus of the study was evaluating methods for bypassing sediment to minimize the amount of sediment that would enter the power tunnel. The physical model study was considered a key design aid for sediment management given the significant sediment loads expected at the intake location.

A number of design modifications to the approach channel and discharge channel at the Forrest Kerr intake facilities were evaluated. Testing demonstrated that the initial arrangement of the approach channel (with the entrance located downstream of the confluence with Forrest Kerr Creek) was fairly effective at excluding sediment at river discharges less than 550 m3/s; however, as the river discharge increased above 550 m3/s, the approach channel and intake structures were inundated with sediment.

Based on these results, the entrance of the sluiceway approach channel was realigned to a location upstream of the confluence with Forrest Kerr Creek. The width and shape of the approach channel and discharge channel were then refined through the design process to the final arrangement in which the bottom width of the approach channel converged from 45 m at the entrance to 6 m wide near the upstream end of the forebay control structure and then maintained at 6 m wide width to the downstream end of the discharge channel (with only one 6 m wide radial gate within the sluiceway structure), as shown in Figure 8.

The addition of a 1.5 m high box culvert in the approach channel and discharge channel effectively conveyed coarser bedload sediment through the approach channel and discharge channel for all river conditions up to 1200 m3/s in which a 3.5 m head difference between the tunnel portal and tailwater was maintained.

The modifications, as developed in the physical model, were used in finalizing the design of the facility which is now currently under construction and scheduled to be online as of July 1, 2014. Photo 14 shows an aerial view of the project area during construction.

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Figure 8 - General Arrangement (Final)

Photo 14- Project Area During Construction

Approach Channel 

Power Tunnel Portal Entrance 

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Acknowledgements

The Forrest Kerr Hydroelectric Project Physical Hydraulic Model Study investigations were conducted by Northwest Hydraulic Consultants in conjunction with the final design engineers Hatch Ltd. The authors wish to recognize the funding provided by the developer AltaGas Ltd. In addition, the authors wish to acknowledge the acceptance of all organizations for the publication of this paper.

Authors

Nancy Sims, P.Eng., M.Sc.(Eng.) started as a hydraulic engineer for Northwest Hydraulic Consultants in 1999, and became an Associate of the firm in 2010. Her areas of expertise include hydraulic design, physical modeling, and numerical modeling. She has been involved in numerous assignments ranging from small run-of-river to major hydroelectric developments, navigational studies, intake structures, bypass systems, influent pump stations, drop structures, and dam spillway projects.

Don Murray, P.Eng., M.Sc. is a Project Manager and the Hydrotechnical Discipline Practice Lead Engineer at the Hatch Ltd. office in Vancouver, BC. having responsibility for hydrotechnical engineering services. He has more than 40 years of experience in major electric utility resource projects. He has worked on some 80 hydroelectric projects ranging in size from 70 kW to 1800 MW. His experience ranges from initial feasibility investigation through to generation planning, energy evaluation, final design, procurement, and preparation of construction drawings and technical specifications.

Amir Alavi, P.Eng.,M.A.Sc. is a senior Hydrotechnical Engineer (water resources) at Hatch and has more than ten years of experience in water resource projects. He has extensive experience in hydraulic numerical modeling (CFD) using FLOW3D software as well as studies and design implementation of hydro power facilities, irrigation and water supply projects. He has also developed models for hydro power generation optimization simulation projects. He received his Masters Degree from UBC in Hydro-technical Engineering in 2004. He has authored four technical articles and for one of them received an award from the World Energy Committee. 

Brian Hughes, P.Eng.,M.A.Sc. has been a hydraulic engineer for Northwest Hydraulic Consultants since 1988, and a Principal of the firm since 1994. He has acquired strong managerial and technical knowledge in the areas of hydraulic design, physical and numerical hydraulic modeling, and river hydraulics. Physical and numerical modeling experience includes hydroelectric developments, fish passage projects, spillway design/rehabilitation, a variety of river processes, navigation and sediment transport investigations, and intake and pump station design studies. Since joining NHC, Mr. Hughes has participated in over 200 models. Mr. Hughes is currently the Director of NHC’s modeling operations and sits on the firm’s Board of Directors.