Regulation of Grand Coulee Reservoir is key to providing the 134
kcfs flow objective desired by the fishery agencies and tribes
at Priest Rapids for April 1997. Several reservoir regulation
scenarios were modeled to determine the likelihood for that flow
objective to be met on a monthly basis. Two groups of model runs,
Scenarios A and B, were made assuming the target pool elevations
shown in Table 1.
| Scenarios | End-of-March Elevation | Mid-April Elevation | End-of-April Elevation |
| A | 1238 | 1223 | 1208 |
| B | 1230 | 1218 | 1208 |
The end-of-April pool elevation (1208 ft) was dictated by flood
control requirement. Because Grand Coulee can only be drafted
by a maximum of one foot per day to avoid bank sloughing, Grand
Coulee pool cannot be any higher than 1208+30=1238 feet by the
end of March. This pool elevation of 1238 ft was used under Scenario
A. Under Scenario B, the reservoir was assumed to be at elevation
1230 ft by the end of March. The maximum draft rate of 1 foot
per day was used for the first 12 days of April to supplement
predicted low flows early in the month, ending at no lower than
elevation 1218 ft to meet navigation requirement. During the remainder
of April, a more conservative draft rate of about 0.6 foot per
day was used, to bring the pool down to its end-of-April flood
control elevation of 1208 ft.
Several reservoir inflow conditions were assumed based on selected
historical flow years with April-July runoff volumes comparable
to the runoff forecasted for the April-July of 1997 period. The
April daily flows for the selected 8 years, along with two sets
of predicted April 1997 daily flows (see Table 2), were regulated
by upstream storage projects using the same regulation criteria
as those in effect today.
| Nos. | Years | April Runoff Characteristics |
| 1 | 1933 | low runoff, late rise |
| 2 | 1950 | medium low runoff, no rise |
| 3 | 1951 | medium high runoff, no rise |
| 4 | 1954 | medium low runoff, no rise |
| 5 | 1956 | high runoff, high late rise |
| 6 | 1959 | medium runoff, moderate rise |
| 7 | 1969 | high runoff, high late rise |
| 8 | 1971 | medium low runoff, moderate late rise |
| 9 | 1997 | benign temperature |
| 10 | 1997 | hot wows (early hot, starting 8 April) |
The hydrographs of the regulated daily inflows to Grand Coulee
are shown in Figure 1. The results of those inflows regulated
by Grand Coulee under Scenarios A and B are summarized in Table
3, including half-monthly averages for the inflows to and outflows
from Grand Coulee, and the corresponding flows at Priest Rapids
(after addition of the local runoff inflow between Grand Coulee
and Priest Rapids).
| Runoff Years | Scenario A: Draft from 1230' to 1208' | Scenario B: Draft from 1238' to 1208' | Scenario A: Draft from 1230' to 1208' | Scenario B: Draft from 1238' to 1208' | |
| Average Inflows to Grand Coulee: April 1/April 2 | Average Outflows from Grand Coulee: April1/April 2 | Average Outflows from Grand Coulee: April 1/April 2 | Average Flows at Priest Rapids: April 1/April 2 | Average Flows at Priest Rapids: April 1/April 2 | |
| 1933 | 85/116 | 109/129 | 111/142 | *120/143 | *122/156 |
| 1950 | 102/129 | 126/142 | 128/155 | 141/161 | 144/174 |
| 1951 | 124/153 | 147/166 | 150/179 | 167/197 | 170/210 |
| 1954 | 81/116 | 105/129 | 107/142 | *118/147 | *121/159 |
| 1956 | 118/220 | 141/233 | 144/246 | 155/263 | 157/275 |
| 1959 | 116/136 | 140/149 | 142/162 | 158/171 | 161/183 |
| 1969 | 155/202 | 179/216 | 181/228 | 191/229 | 193/242 |
| 1971 | 101/140 | 124/153 | 127/166 | 135/165 | 137/178 |
| 1997 (benign) | 73/108 | 96/121 | 99/134 | *111/144 | *114/156 |
| 1997 (early hot) | 103/169 | 126/183 | 129/195 | 152/215 | 154/228 |
(*) Priest Rapids half-monthly average flow less than 134 kcfs
Conclusion. From the results listed in Table 3, the chance
for meeting the 134 kcfs flow objective set for Priest Rapids
on a half-monthly basis looks relatively good. Under Scenario
A, with a 20-ft draft in April (from 1230' to 1208'), the 134
kcfs flow objective was met in all but 3 runoff cases out of 10.
Under Scenario B, with a 30-ft draft in April (from 1238' to 1208'),
this flow objective was met in all 10 runoff cases tested. Even
under Scenario A, predicted flows at priest Rapids only missed
the 134 kcfs flow objective by at most 16 to 23 kcfs on a half-monthly
basis. Note that no potential exists to increase the inflow to
Grand Coulee because of limitations at upstream storage projects
(Arrow, Albeni and Hungry Horse).
Figure 1. Predicted Daily Regulated Inflow to Grand Coulee