FLOOD10363 ‎(2)‎

• County: Gunnison
• Community: Gunnison County
• Stream Name: East River, Cement Creek, Alkali Creek, Ohio Creek
• Title: Gunnison County Flood Hydrology, Contract # W99-D-0018 Final Report
• Date: 11/1/2004
• Prepared For: US Army Corps of Engineers, CWCB and Gunnison County
• Prepared By: Tetra Tech, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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1. Avoid spilling reservoir (spilling a Division of Wildlife concern due to mysia shrimp).
2. Fill to within 1 to 2 feet of 9,330 feet if possible.
3. Winter releases of approximately 100 cfs desirable to maintain the reservoir level at 2 to 3 feet
below spillway.
4. Operations cannot affect the irrigation supply operation of the dam.

The outflow discharge is based on the water surface elevation in the reservoir for the time step, the inflow
into the reservoir, and the operational limits of the low-level gates and spillway. The outlet works are
operated in the model so that the discharge from the dam mimics pass-through conditions as long as
possible. This would prevent any loss in reservoir storage while helping to meet the goal of maintaining
the reservoir elevation at the desired of 9,328 feet. The following operation would be used.

a. For inflow below the maximum discharge of the outlets works at a given stage and the stage is
below the spillway crest, the discharge would be set equal to the inflow until the maximum
capacity of the outlet works for the given stage is achieved.
b. For inflows meeting the maximum capacity of the low level outlet works at the given stage and the
reservoir stage is below the spillway crest, the gates would be opened to the full outlet capacity.
c. For stages at or above the spillway crest elevation, as the spillway discharge increases, the
discharge from the outlet works is decreased in proportionally in such a manner to that a flow
equal to the maximum discharge of the outlets works is maintained. Once the spillway discharge
meets the maximum discharge of the low-level outlet works, the outlet gates will be closed
completely and all remaining discharges will be based on the spillway capacity.

6.3.3 Model Methodology

The input data is processed using the following methodology:

1. Determine the Hydrograph for East River at Almont. The East River at A1mont hydrograph was
created by subtracting the 1 percent chance exceedence balanced hydrograph Taylor River at A1mont
from the 1 percent chance exceedence balanced hydrograph Gunnison River at Almont

2. Determine the Hydrographs for the Local Flow Contribution. The local inflow contribution
between Taylor River at Almont and Taylor River below Taylor Park Reservoir hydrograph was
created by subtracting the 1 percent chance exceedence balanced hydrographs for the Taylor River
at A1mont from the 1 percent chance exceedence balanced hydrograph for Taylor River below Taylor
Park Reservoir.

The OhIO Creek (local inflow to Gunnison River between Almont and Gunnison) hydrograph was
created by subtracting the 1 percent chance exceedence balanced hydrograph from the Gunnison
River near Gunnison from the 1 percent chance exceedence balanced hydrographs for Gunnison
River at A1mont.

3. Determine the Starting Storage in the Taylor Park Reservoir. The starting reservoir storage was
determined using the starting water surface elevation (see Section 6.3.4) supplied as model input and
the stage storage relationship for the Taylor Park Reservoir.

, 4. Determine the Starting Outflow from the Taylor Park Reservoir. The starting outflow discharge is
based on the starting water surface elevation and outlet operation scenario discussed in Section
6.3.2.

5, Determine the Reservoir Outflow and Storage. The Taylor Park Reservoir outflow and storage for
each time period was computed using the Modified Puis Routing methodology. The Modified Puis
Routing uses the following equation:

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