One of the management scenarios under consideration for meeting water demand and fish habitat needs in the Bull Run River is the construction of a third reservoir in the Bull Run Watershed, Figure 1. Based on siting work conducted by Squier Associates (1994) for the Water Bureau, the location of Reservoir #3 was identified with a full pool elevation of 2000 ft. NGVD.

Subbasins that would contribute flow to the new reservoir were identified along with their basin areas using a geographic information system, Figure 2. This information was later used to estimate incoming flows to the reservoir.

Reservoir #3 Bathymetry

Since the reservoir location has not been surveyed, the bathymetry of the reservoir was obtained by using a Digital Elevation Map (DEM) from the U.S. Geological Survey. The horizontal spatial resolution of the elevation topography was 30 m. The elevation data within the proposed reservoir boundary were then isolated and converted to a data set of coordinates and elevations. The data set was then used to generate a contour plot using SURFER, Figure 3. The centerline of the reservoir and several branches within the reservoir were identified and volume and area slices were done for various elevations to obtain a volume elevation curve for the proposed reservoir as illustrated in Figure 4 and Figure 5. The centerline of the reservoir was then used to "slice" through the elevation contour data to get the longitudinal profile of the reservoir, Figure 6. Although the longitudinal profile is coarse it provides an indication of how steep the topography is at the reservoir site. The centerlines of the river and reservoirs were obtained from the bathymetry analysis and used to generate a longitudinal profile for the river-reservoir system. Figure 7 shows the overall profile of the system including Reservoir #3 and the Main Stem of the Bull Run River between Reservoir #1 and #3. The locations of several landmarks such as bridges and tributary inflows along the lower river and the reservoirs are also shown. The river mile locations were estimated using a geographic information system to map the watershed. The profile shows an overall slope of 1% for the Lower Bull Run River and more gradual slopes along the bottom of the reservoirs. The contour plot and the centerline data were then used to develop the model geometry for the Bull Run River -Reservoir Model.

Reservoir #3 Inflows

The subbasin inflows contributing to the proposed Reservoir #3 are relatively unknown except for two gaging stations, the Main Stem of the Bull Run River below Reservoir #3 and Blazed Alder Creek which measures the stream flow from subbasin 10, Figure 2. All of the subbasin flows contributing to this reservoir can be accounted for in the flows measured at the Main Stem gage station. Similar to characterizing the distributed flows into Reservoir #1 and #2 for the ungaged subbasins, the subbasins contributing to Reservoir #3 were related to the Main Stem of the Bull Run River. A comparison was done between the Main Stem Bull Run River and Blazed Alder Creek, Figure 8.

Additionally, the flow relationship used to calculate the distributed flows into Reservoir #1 and Reservoir #2 was then used on Blazed Alder to determine if the flows at the Main Stem gage station could approximate the Blazed Alder flows using a relationship between the basin areas. The equation relating the two is:

Using the Main Stem flows, Blazed Alder Creek flows were calculated and compared to measured Blazed Alder Creek flows to determine the coefficient, Figure 9. The coefficient is close to unity illustrating a close correlation between the two flows.

Using the subbasin map for Reservoir #3, Figure 2, areas were associated with each subbasin using a geographic information system with results shown in Table 1. The subbasins were then grouped into logical area sets which would be contributing flow to Reservoir #3 as either a point source such as a creek flowing into the reservoir or as a distributed flow into the reservoir. Table 2 shows the subbasins that were grouped together and their surface area summed and divided by the ratio of the basin area contributing flow to the Main Stem gage station. The ratios of area were then used with the flows at the Main Stem gage station to calculate the flows for each group of subbasins.
 
 

Reservoir #3 Inflow Temperatures

There is no temperature data for the inflows to Reservoir #3. Temperature files were created for each subbasin group by using the temperatures recorded at the North Fork.

Main Stem Bull Run River, Reservoir #1 to Reservoir #3

Incorporating Reservoir #3 in the model system grid also required modeling the stretch of the Bull Run River between Reservoir #1 and Reservoir #3. Since there were no detailed topography data available on this stretch of the river, USGS Digital Elevation Maps were used to characterize the topography. Using a geographic information system the centerline of the river channel was digitized and incorporated with the DEM data and used in SURFER to generate a contour plot of the river channel topography, Figure 10. Using the grid file generated in SURFER and the centerline points of the river channel, the river geometry could be generated for the Bull Run River -Reservoir Model.