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Assisted Spillway and Stilling Basin Design

Computational fluid dynamics modeling was conducted by applying FLOW-3D to assist the design of spillway rehabilitation as well as the stilling basin for the lower outlet for a dam in the mid-west of USA. The purpose of the project was to evaluate various new spillway and outlet alternatives to meet updated hydrology requirements.

CFD modeling was adopted since the proposed designs are off the application limit of the empirical methods normally used for the conventional design. For this dam rehabilitation project, the FLOW-3D model was first validated against empirical methods in the literature when the design head is lower. The model was then expanded to develop the outflow rating curve for the control weir, and pressure and depth distribution down the spillway, which were subsequently for structure analysis. In the second phase of this design, FLOW-3D was used to assist the design of a stilling basin for the lower outlet.

Purpose of CFD Modeling

Application of FLOW-3D for this project includes the spillway and the outlet works. The purpose of the CFD modeling of the spillway was to:

  1. Confirm the discharge of the labyrinth weir for select lower reservoir levels based on
    published design guidelines (Falvey, 2003).
  2. Estimate the discharge of the labyrinth weir for select higher reservoir levels greater than
    El. 5398 in order to extend the elevation-discharge rating table beyond the head to
    structure height ratios provided in published guidelines (Falvey, 2003).
  3. Confirm the depth of flow in the spillway chute for the design outflow, including the
    generation of cross-waves due to the converging chute, in order to size the chute walls to
    prevent overtopping.
  4. Determine if any adverse hydraulic conditions could occur due to the proposed labyrinth
    spillway, spillway gate, and spillway chute configuration.
Figure 1. Proposed Labyrinth Weir in Replace of Ogee Weir for the Spillway
Figure 1. Proposed labyrinth weir in place of
ogee weir for the spillway
Fig 2: FLOW-3D Model Domain for the Labyrinth Weir and Spillway Chute
Figure 2: FLOW-3D model domain for the
labyrinth weir and spillway chute

The purpose of the CFD modeling of the outlet works was to determine the appropriate and economical energy stilling basin for the proposed 60-in lower outlet work. Two stilling basin alternatives were considered:

  1. Plunging pool energy dissipation stilling basin where the outflow from the outlet works discharge freely to a pool below. Proper pool size and lining were evaluated using CFD to make sure that downstream scour will not occur under the maximum design flow condition.
  2. Chute energy dissipation stilling basin where the outlet works discharges to a parabolic chute followed by a conventional Type III stilling basin. CFD was first used to confirm the hydraulics as compared to physical modeling results of similar outlet works. It was then used to confirm that the selected dimensions will result in the designed hydraulics in the stilling basin.

Figure 3. Isometric View of Chute Stilling Basin Alternative for the Outlet Works
Figure 3. Isometric view of chute stilling basin alternative for the outlet works

Summary of CFD Modeling Results

Spillway Rating Curve and Spillway Chute Hydraulics

FLOW-3D calculated discharges are 3% to 8% of the predicted discharges from the
methodology by Falvey (2003) for the same reservoir elevations as shown in Table 1. The differences in discharge are contributing to the inefficient flat topped profile, an additional headloss from the spillway gate structure, and a somewhat curved approach conditions to the spillway walls, as compared to the idea conditions in the Falvey’s model. The CFD model results were therefore used to refine the spillway rating curve, as shown in Figures 4 and 5 for the stages, respectively.

Raise Stage Reservoir El.
(feet)
H/P Ratio Discharge from
CFD model (cfs)
Predicted discharge
from Falvey
(2003) (cfs)
% Difference
in discharge
1 5398.4 0.9 8,580 9,312 -7.9%
2 5408.5 0.65 21,250 21,860 -2.3%
Table 1: CFD Model Results Comparison to Predicted Discharges

Figure 4: Stage 1 Spillway Rating Curve with CFD Model Results
Figure 4: Stage 1 spillway rating curve with CFD model results


Figure 5: Stage 2 Spillway Rating Curve with CFD Model Results
Figure 5: Stage 2 spillway rating curve with CFD model results

Outlet Works Stilling Basin Design

Free Jet Basin

The free jet basin was first sized according to the guidelines in Design of Small Dams (1987) and geotechnical assumptions for excavation and slope stability. The final dimensions of the basin were determined when appropriate design bottom velocity and other hydraulic characteristics are met for the site conditions. A three dimensional view of the free jet basin is shown in Figure 10. FLOW-3D modeling results are shown in Figures 11 and 12.

Chute Stilling Basin Design

The chute basin was sized according to the guidelines by the USACE for supported jets, the Hydraulic Design of Stilling Basins and Energy Dissipators, Engineering Monograph No. 25 (USBR 1984) for Type III basins, and geotechnical assumptions for excavation and slope stability. Two basin widths were considered, 10 feet and 15 feet. The results from the FLOW-3D indicates that the outflow from the outlet works would spread gradually down the chute as confirmed by previous physical modeling studies by the USBR for similar projects. The flow at the toe of the chute becomes relatively fairly uniform with fins on both sides of the basin.

The FLOW-3D modeling results indicate that the 10-ft wide stilling basin resulted in extremely violent flow inside the basin and the desirable hydraulic jump could sweep out the basin for the maximum flow. As a result, the 10-ft wide basin was not selected as an alternative for further consideration.

The 15-ft wide basin, on the other hand, shows well behaved hydraulics inside the basin, and thus provides the desirable energy dissipations for the outlet works.

Figure 6: CFD Simulated  Surface Velocity Results
Figure 6: CFD simulated surface velocity results for Stage 1 crest; Reservoir at El. 5404.0
Figure 7: CFD Simulated Normal Water Depth Contours
Figure 7: CFD simulated normal water depth contours for Stage 1 crest; Reservoir at El. 5404.8
Figure 8. FLOW-3D cfd simulation at Stage 2 Crest with Reservoir Level
Figure 8. Stage 2 crest with reservoir level at El. 5408.5; Velocity magnitude looking towards left abutment along
labyrinth weir
Figure 9: CFD Simulated Water Depth Results
Figure 9: CFD simulated water depth results for Stage 2 crest;
Reservoir at El. 5408.5
Figure 10. Isometric View of the Free Jet Basin Alternative
Figure 10. Isometric view of the free jet basin alternative
Figure 11. Simulated Flow Velocity (half view) for the Plunging Pool Stilling Basin Alternative
(Q = 1133 cfs).
Figure 11. Simulated flow velocity (half view) for the plunging pool stilling basin alternative (Q = 1133 cfs).
Figure 12. Simulated  near-Bed Flow Velocity for the Plunging Pool Stilling Basin Alternative (Q = 1133 cfs).
Figure 12. Simulated near-bed flow velocity for the plunging pool stilling basin alternative (Q = 1133 cfs).
Figure 13. Simulated Flow Velocity (half view) for the 10-ft Wide Chute Basin Alternative (Q = 1133 cfs).
Figure 13. Simulated flow velocity (half view) for the 10-ft wide chute basin alternative (Q = 1133 cfs).
Figure 14. Sliced View of Simulated Flow Velocity for Chute Basin Option 2 – 15ft Wide Basin at Q = 1133 cfs with Tailwater Elev. 5292 ft.
Figure 14. Sliced view of simulated flow velocity for chute basin option 2 - 15ft wide basin at Q = 1133 cfs with
tailwater. Elev. 5292 ft.
Figure 15. Simulated near-bed Flow Velocity for the 15-ft Wide Chute Stilling Basin Alternative (Q = 1133 cfs, Final Plan)
Figure 15. Simulated near-bed flow velocity for the
15-ft wide chute stilling basin alternative
(Q = 1133 cfs, final plan)

Conclusions

CFD modeling based on FLOW-3D was successfully used for design of the new spillway and outlet works stilling basin for a dam in the mid-west of USA. With validation from documented empirical methodology or physical models, the models were applied to derive the rating curve for the complicated spillway weir, and to select the proper energy stilling basin for the lower outlet work.

References

Falvey, Henry. T., 2003. Hydraulic Design of Labyrinth Weirs. ASCE Press.Flow Science, Inc. 2008.
Flow Science, Inc., Santa Fe, New Mexico, USA. FLOW-3D Users Manual.
Peterka, A. J., 1984. Hydraulic Design of Stilling Basins and Energy Dissipators, A Water Resources Technical Publication, Engineering Monograph No. 25, U. S. Department of the Interior, Bureau of Reclamation.
USACE, 1980. Engineer Manual 1110-2-1602, Engineering and Design, Hydraulic Design and Reservoir Outlet Works, October 15, 1980, Department of the Army, Corps of Engineers, U.S.A.

Acknowledgements

This material was contributed by Frank Lan, Ph.D., PE, CFM, Principal Water Resources Engineer at URS, a runner-up in the 4th Flow Science 30th Anniversary Simulation Contest.

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