Charles D. D. Howard (1), LM ASCE, and Irena M. Netik (2)

1. Charles Howard & Associates Limited, 1350 Rockland Avenue, Victoria, BC, V8S 1V8, Canada; PH (250) 381-2722; FAX (250) 381-5800; email: cddhoward@shaw.ca
   
2. Powel Group, Inc., Suite 210, 239 Menzies Street, Victoria, BC, V8V 2G6, Canada; PH (250) 385-0206; FAX (250) 385-7737; email: inetik@powelgroup.com

Abstract

The mountain water supply of the City of Nanaimo (population 78,000) in British Columbia, Canada, provides the potential for generating hydroelectric energy. The hydro plants would be connected directly to the two supply pipelines - head available for power therefore will vary inversely with water consumption. The hydraulic analysis made effective use of the capability of the Shamir and Howard network analysis program SDP to determine the hydro energy by explicitly solving for unknown conductance, and combinations of head and consumption at the same node (Shamir and Howard, 1968). Optimization of the installed capacity also considered opportunities for power sales, variability in present and future water consumption, hydrology of the ungauged river sites, instream flow requirements to protect aquatic habitat, and hydroelectric licensing requirements.

Introduction

The City of Nanaimo (population 78,000) is located on the east side of the mountains of Vancouver Island, British Columbia, Canada. The Greater Nanaimo Water District (the District) provides the water supply to the City. The main source is the Jump Creek Reservoir on a tributary of the Nanaimo River. Releases from Jump Creek Reservoir flow downstream to the South Fork Dam. The dam provides the head for delivering water to the City through the 23-kilometer long pipeline system. At the terminus of the pipeline the surplus head is dissipated through a hollow jet valve discharging into a small terminal reservoir, Reservoir No. 1. This provides an opportunity to capture the wasted energy for hydroelectric power. Hydroelectric generation at Reservoir No. 1 will be in step with the time varying pattern of water consumption by the City.

There are additional opportunities for power operation along the route of the pipeline system. Winter flows in the river are normally higher than required for water supply and instream purposes so there is a surplus that can be used to generate power and be returned to the river before reaching the City. Head losses in the two water supply pipelines will depend on the combination of flows supplied to the City and flows supplied to one or more hydro plants located along the pipeline route.

The hydraulic analysis and power study made effective use of the flexibility of the Shamir and Howard network analysis program, SDP. It determined the hydro energy by explicitly considering specified heads and discharge at the City supply nodes by solving for combinations of unknown heads and consumptions at the same node (power site), and unknown conductance of the elements connecting the power sites to the pipeline system.

Optimization of the installed capacity considered hourly and seasonal variability in electricity prices and present and future water consumption, hydrology of the ungauged river sites, instream flow requirements to protect aquatic habitat, pipeline hydraulics, conceptual hydroelectric design, and costs. This was an office study of the proposed hydroelectric site developments. More detailed studies including fieldwork are required to establish technical and environmental feasibility and to design the facilities.

Water Supply System and Consumption

The System Layout

The main storage reservoir on Jump Creek is 10 kilometers upstream from the intake for the City water supply pipelines at South Fork Dam. This dam is a concrete arch structure with a crest elevation of 250 meters above sea level. The releases from South Fork Dam are used to meet instream flow requirements in the Nanaimo River downstream of the dam and water supply for the City of Nanaimo. The intake located inside the dam is connected to parallel 762-mm and 1200-mm diameter supply mains. The supply mains are cross-tied at various points to form a ladder type of network and the 762-mm diameter pipeline terminates at Reservoir No. 1 at Elevation 107 meters. The 1200-mm diameter pipeline continues for an additional 2 kilometers to terminate at College Park Reservoir at Elevation 182 meters. The water supply is provided entirely by gravity.

From the South Fork Dam to approximately 14 kilometers downstream, the two supply mains run adjacent to the river where there are potential sites for a diversion from the pipelines. Figure 1 illustrates the network schematic and the locations of potential hydroelectric sites.

Figure 1. Network Schematic Water Consumption

In 2002, the total average summer demand was approximately 12 MGD. In winter, the consumption averaged approximately 7.5 MGD. From an analysis of average monthly consumptions, it was determined that 50 to 70-percent of the total water consumed during the summer passes through the 1200-mm pipeline leading to College Park. The 762-mm pipeline to Reservoir No. 1 carries the remainder.

This analysis considered current (2002) levels of demand and future projected demands for year 2025 for the summer, May through September, and winter, October through April. The projection by the District Planning Department estimated that in 2025 water consumption would reach 1.64 times the 2002 level of demand. Table 1 summarizes 2001-2002 observed water demands that were used in the hydraulic network model.

Table 1. 2001-2002 Observed Water Consumptio

Demand Patterns

The hourly dimensionless pattern of consumption was determined from an analysis of hourly readings at College Park and Reservoir No. 1, Figure 2. The analysis determined hourly demand patterns for different days of the same week in each season. The hourly average demand patterns for each week were used for modeling the pipeline hydraulics under the summer and winter levels of demand. The Village of Extension was assumed to have a constant demand.

Figure 2. Normalized Demand Patterns

Water Sources and Hydrology

The availability of water for power generation depends on instream flow requirements, water consumption by the City, hydrological variability of the South Fork of the Nanaimo River, and opportunities for fine-tuning with the storage behind Jump Creek Dam.

During 2001 and 2002, the District measured the depth of water discharging over the 49-meter long spillway crest of South Fork Dam. From these measurements a brief period of discharge record was determined, Table 2. However, a longer record was required for estimating the economic viability of the hydro sites.

There are no long-term records of inflow for South Fork Dam. A previous study estimated the unregulated daily inflow to the Jump Creek Dam (Kenward, et al, 1998). For that study, a conceptual hydrologic model, HSPF, was used to simulate the daily inflow time series (U.S. Department of Commerce, U.S. Environmental Protection Agency, June 1984). The drainage basins of the South Fork Dam and the Jump Creek Dam are very similar in topographic and other hydrologic features, except for scale. Therefore inflows to Jump Creek Reservoir provided a basis for estimating daily inflows to South Fork Dam from the ratio (203/51) of the contributing drainage areas. The result was a time series of daily inflow corresponding to the 1950-1995 period of weather records.

The simulated South Fork daily inflow time series was used to estimate the discharge that would have been available for power production during each day of 1950-1995. The monthly estimated discharge from South Fork Dam by the two methods is summarized in Table 2.

Table 2. South Fork Dam Monthly Average Discharge

Hydraulic Network Modeling for Power

The System Development Package (SDP) was used to perform the hydraulic analysis of the network. SDP has evolved since it was first developed for a 24-hour simulation of Boston Water Distribution System (Maguire, 1967). SDP solves directly for combinations of knowns and unknowns such as heads, demands, and resistances under time series loading conditions. Inputs are easily modified through a graphical interface to analyze design options.

The network model was setup to represent the existing system initially with unknown heads at all nodes except the point of supply at South Fork Dam and calibrated with the District data. Calibration was by comparison with pipeline pressure data provided by the District, who also provided estimates of the conductance for each element of the pipe network. The power output at the hydro sites was provided by the solution of unknown heads and discharge for the corresponding nodes. Hydro plant tailwater was set to the estimated elevation of the river water surface at the various sites.

The output from SDP determined the discharge and net head available for power by accounting for head losses in the piping while delivering the specified flows and heads to points of connection to the City water supply system. A series of runs were made with the model to develop functional relationships between hydro plant discharge and power production. The result was a curve of power versus discharge for each site, Figure 3 below.

The river flows were used with the site-specific discharge-power relationships to estimate the average annual energy from the hydro plants for a range of installed capacities. This provided the basis for the relationship between average annual energy versus cost for each site. As instructed by the District, it was assumed that for reasons of water supply operations and environmental considerations power will not be produced from the river sites during the summer months, May through September. The hydro plant, or plants, discharging back to the river will operate only in winter while runoff is high. The plant located at the pipeline terminus at Reservoir No. 1 will operate all year round since it uses only the water that is being consumed within the City.

Reservoir No. 1 Site

Reservoir No. 1 Site would operate only with water that is supplied for consumption within the District. The powerhouse would discharge directly from the existing pressure pipeline into the reservoir at Elevation 107 meters. The head available for generation at this site depends on the pipeline head losses associated with water supply to Reservoir No. 1 and with an upstream power plant located along the pipeline system. At times of high pipeline discharge the head and the power generation capability drops rapidly at the Reservoir No. 1 site. As the consumption of water in the City increases into the future, electricity generation at the Reservoir No. 1 site will increase in response to the increase in municipal water consumption but the increase will be less than proportional because of non-linear head losses in the pipeline.

The power potential at Reservoir No. 1 is significantly affected by friction losses in the pipeline system and operation of the upstream power sites. Over the full range of operation during a typical day, with operation of an upstream site, the power potential at the reservoir will vary by a factor of two, from slightly more than 100 kW to more than 200 kW.

With an installed capacity of 275 kW the Reservoir No. 1 site will produce approximately 1,760 MWh annually under present levels of demand without the operation of the upstream power sites. An installed capacity of 350 kW may be installed at the Reservoir No. 1 Site to account for future increase in demand if an upstream site is not in operation.

River Site A

River Site A is located just downstream of the confluence of the South Fork and the Nanaimo Rivers. It is approximately 4 kilometers downstream on the pipeline from the South Fork Dam. At this site, the pipelines will carry more than the municipal and industrial water supply. The surplus will be diverted for power and pass back into the river at the location of the hydroelectric plant.

The studies for this site, which is close to the source, showed that pipeline head losses during energy production are only weakly dependent on the level of water consumption by the City. The head available at the site is controlled by the requirements for total heads of 182 meters at College Park and 107 meters at Reservoir No. 1. The interconnected pipelines from the dam at Elevation 250 meters to the power site provide a time varying residual head (pipeline pressure at the hydro site) of more than 60 meters, depending mainly on the flow to the powerhouse.

The maximum installed capacity at all of the river sites is controlled by the pipeline hydraulics, not by the availability of river flows during the high runoff season. The maximum capacity at River Site A is approximately 1,540 kW. With this installed capacity this site will produce a long-term average winter season output of approximately 7,565 MWh. As future demands increase, less water will be available for energy generation because more will be required downstream for consumption in the City.

River Sites B and C

River Site C is located 11 kilometers down the pipeline from South Fork Dam; River Site B is 12 kilometers downstream from the dam. River Site C will experience lower pipeline head losses because it is upstream from River Site B. However, at River Site C the riverbed is approximately 11 meters higher than at River Site B. The net head at River Site C works out to be less than the net head at the downstream River Site B.

The network modeling studies showed that the lower tailwater condition at River Site B offsets the higher head losses compared to River Site C. An installed capacity of 1,760 kW at River Site B will have a long-term average winter season output of approximately 8,710 MWh. At River Site C an installed capacity of 1,670 kW will have a long-term average winter season output of approximately 8,260 MWh.

Figure 3. Comparison of Power Sites on the Nanaimo River

Site Comparison

As shown by the comparison in Figure 3, Site B will support a capacity of approximately 1,760 kW - this site has the highest power potential at all rates of flow. On the basis of its power potential, River Site B is the preferred option. Table 3 shows that the energy that could have been generated during the winter months of 2001-2002 is similar to the generation indicated by the long-term simulated hydrologic record.

Table 3. Comparison of Winter Generation

Conclusion

Construction at the Reservoir No. 1 Site will be entirely contained on the District property. The water for power is already committed to urban water supply and a diversion from the river is not required. Reservoir No. 1 power station will operate all year round and have no effect on the river.

Based on its power potential, River Site B is the preferred option. However in selecting the preferred alternative site, and its capacity, the District will consider factors that were outside the scope of this screening analysis, including environmental implications, the site-specific advantages and disadvantages, and hydraulic transients associated with powerhouse operations.

The effect of the power operation will be to reduce flows from just below the dam to the point where the powerhouse returns the flow to the river. For River Site A, this bypass reach encompasses a distance of approximately 4 kilometers - for the sites further downstream the bypass reach is approximately 12 kilometers. Below these points the power operation will not affect the flow in the river. But distance alone is not the sole criterion for comparing the environmental effects - the habitat affected may be more or less valuable from one place to another along the river. The downstream River Sites B and C produce more power than the upstream Site A, and because of their higher heads, for the same amount of power, they would have smaller diversions from the river. Subsequent studies will include environmental concerns and determine feasibility and costs for the alternatives investigated here.

References

Kenward, T., Netik, I. and Smith, D.I. (1998), "Estimated Frequency of a Fuse Plug Washout at
Jump Creek Dam", 51st CWRA Conference, Victoria, BC, Canada, June.

Maguire C.A. and Associates (1967), "The Boston Water Study", City of Boston, Massachusetts.

Shamir U. and Howard, C.D.D. (1968), "Water Distribution Systems Analysis", ASCE Jn., Hyd.
Div., Vol. 94 No. HY1. Jan.

U.S. Department of Commerce, U.S. Environmental Protection Agency (1984). Application
Guide for Hydrologic Simulation Program - FORTRAN (HSPF). National Technical
Information Service, Springfield, Virginia.