Reservoir #1 Dam Structure

To accurately model the river-reservoir system flows from Reservoir #1 to Reservoir #2, flows from Reservoir #2 to town and to the Lower Bull Run River need to be well characterized. In Figure 1 three flow pathways from Reservoir#1 downstream to Reservoir #2 can be identified; flow through the powerhouse, flow over the spillway and flow through needle values. Figure 2 provides an overview of the Reservoir #1 dam structure. Figure 3 illustrates the needle valve withdrawal structure cross-sectional profile and the withdrawal elevations. Note that needle valves at different elevations can be opened at the same time. Figure 4 shows the multiple withdrawal structure used to divert water through the powerhouse before sending it downstream. There are three spillway gates located in the center of the Reservoir #1 dam structure (Figure 2), each gate is 40 feet wide. The spillway crest is at an elevation of 1036 ft NGVD (315.78 m). This elevation can be raised 9.4 feet by the gates resulting in a raise pool elevation of 1045.4 ft.

Reservoir #2 Dam Structure

Figure 5 illustrates the various flow pathways from Reservoir #2 downstream or to town. Three flow pathways can be identified: flow through either of the two intake structures and flow over the spillway. Figure 6 provides an overview of the Reservoir #2 dam structure and spillway. Figure 8 illustrates the North Intake Tower cross-sectional profile and withdrawal elevations. Figure 9 shows the South Intake Tower cross-sectional profile with withdrawal elevations. Both Figure 8 and Figure 9 have arrows indicating the flow of water through the intake towers. The spillway is 460 feet wide with a crest elevation of 860 ft NGVD (262.13 m). Rating curves for both the spillway and the diversion pool dam are shown in Figure 10.

Figure 7 shows a drawing illustrating the flow of water from Reservoir #2 downstream to the Bull Run River or through the Headworks to Portland or Roslyn Lake. Traditionally, water flows through the North Intake Tower to Power House 2, then to the Diversion Pool where it is routed to the Headworks or overflows to the Lower Bull Run River. The water is then piped to Portland and Roslyn Lake from the Headworks. When Power House 2 is off-line, due to maintenance or debris jam, the Howell-Bunger valves are automatically switched on to bypass flow directly to the Diversion Pool. Then water starts flowing through the South Intake Tower and is increased until the Howell Bunger Valves can be shut off. As water is routed through the South Intake Tower it passes through a pressure-reducing valve (PRV) on its way to the Headworks. When Power House 2 comes back on-line, traditional flow resumes. Occasionally, flow is routed from the South Intake Tower through the PRV valve to the Diversion Pool, and then to the Headworks. This scenario does not occur often because it causes erosion on the north bank of the Diversion Pool.
 
 

Reservoir Operations, 1998

The two reservoirs in the Bull Run are operated in relatively the same way from year to year in order to maximize the amount of storage in the reservoirs when the summer season approaches. By reviewing data from 1998, general operations for the two reservoirs are illustrated in Table 1 and Table 2.
 

Reservoir #1 and #2 Outlet Hydraulics in W2 Model

In constructing the models of Reservoir #1 and Reservoir #2 the elevations and locations of the outlets for the withdrawals are needed. They are shown below in Table 3.
 

Outlet #	Elevation centerline, ft NGVD	Elevation centerline, m NGVD	Sink type	Upper layer limit (selective withdrawal constrained below this level)	Lower layer limit (selective withdrawal constrained above this level)
Reservoir #1
1	1025	312.4	Point	15 (336.5 m)	84 (267.5 m)*
2	995	303.28	Point	15	70
3	960	292.61	Point	15	70
Reservoir #2
1 (North Tower)	759.8	231.6	Point	15 (298.5 m)	84 (229.5 m)*
2 (South Tower)	755.5	230.28	Point	15	84
*Effectively the reservoir bottom; these values show the model computes selective withdrawal without any constraints based on model user inputs.

Spillway Rating Curves

In order to better characterize the flow from Reservoir #1 to #2 and to the Lower Bull Run River, an understanding of the rating curves for the spillways needs to be achieved. Figure 10 shows these rating curves for the two reservoir spillways and the diversion pool dam that leads to the Lower Bull Run River.

Using the rating curves developed above regression lines were developed for each. The Dam 1 Spillway can be related as (R² = 0.99999):

Dam 2 Spillway (R² = 0.99999):

Headworks Diversion Dam (R² = 0.99999):

Note that each of the rating curves follows the general relationship shown in Equation ( 1 ) where

( 1 )

H is the water height in meters. For the remaining parameters refer to Table 4.

Reservoir #1 Spillway Gate Rating Curve

Since the Reservoir #1 spillway has 3 gates which can be raised individually to allow water to pass downstream, rating curves were plotted as illustrated in Figure 11. A mathematical relationship was then developed for estimating the flow through each gate (R² = 0.9981):

Reservoir #1 Needle Valve Rating Curve

Traditional operation of the reservoir has consisted of opening the needle valves by turning a control wheel a given number of turns. The Water Bureau, City of Portland provided a rating curve relating the number of turns on the needle valve to the flow (cfs) for a given head, Figure 12. An equation was developed, similar to the spillways, relating the flow through one needle valve to the number of turns the valve was opened, the reservoir water level and the valve intake elevation. The equation for the flow through each needle valve is

The equations above can be characterized more generally with the following expression and variables described below:

( 2 )

This equation is similar to the expressions used to characterize flow over the spillways, Equation ( 1 ).
 

Reservoir Volume Elevation Rating Curves

Figure 13 shows the volume elevation rating curves for both reservoirs based on data provided by the City of Portland's Water Bureau. These rating curves were plotted to assess the volume of water available in each reservoir and to compare them with the reservoir bathymetry developed for the model to ensure the model accurately represents the reservoirs, Error! Reference source not found. and Error! Reference source not found.. The volume elevation curves also indicate that Reservoir #1 can store a significantly larger amount of water than Reservoir #2.

Reservoir Seepage

Based on conversations the Portland Water Bureau had with Cornforth Engineers, 0.1 cfs was measured in springs from Reservoir #2 during the summer. It was also noted that seepage flows could range from 0.5 cfs up to 10 cfs. The Water Bureau has installed drainage curtains and pressure relief wells to address the seepage problem. The Reservoir #2 dam is on the toe of an ancient landscape and that significant seepage is likely.