1. Diversion Systems & Spillways
1
Todd Lewis, MSCE, BSCE (Hons), P.E.
Water Resources Engineer
h2o_engr@msn.com

2. Session 7: Water Management and Practical
Design Concepts
• Annual Exceedance Probability (AEP): the % chance
that a given discharge is exceeded within a period of
one year.
• Correct usage: “The spillway was rated for 500 m3/s,
which has an AEP of 1 in 100 (or 1%).”
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Probability Terminology (AEP)

3. Session 7: Water Management and Practical
Design Concepts
• Average Recurrence Interval (ARI): the average period
between years in which a given discharge is exceeded,
whether once or more than once, NOT the average
period between times a given discharge is exceeded,
as is commonly (incorrectly) assumed.
• ARI = 1 / AEP
• Correct usage: “The spillway was designed for a
100 year ARI storm discharge of 500 m3/s.”
3
Probability Terminology (ARI)

4. Session 7: Water Management and Practical
Design Concepts
• These probability terms are important because they
can be used to quantify the RISK that a structure will
encounter a discharge exceeding its design capacity.
• RISK = 1 - e(-L / ARI)
 RISK: the % chance that a given discharge is exceeded
at least once over the design life of the structure.
 L: design life of structure (years).
 ARI: average recurrence interval (years).
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Probability Terminology (RISK)

5. Session 7: Water Management and Practical
Design Concepts
• Water conveyance (diversions, spillways) design
entails a MODULAR application of two sciences
 Hydrology – estimate runoff from design storm(s):
• Peak flow rate (a “snapshot” in time) or
• Hydrograph (the entire “movie”)
 Hydraulics – determine response of structure layout /
sizing to design runoff:
• Water surface level
• Flow velocity (and other variables)
5
The “BIG” Picture

6. Session 7: Water Management and Practical
Design Concepts
• Intensity / Duration /
Frequency (IDF) Curves
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Hydrology – Inputs
• Hyetographs

7. Session 7: Water Management and Practical
Design Concepts
• Digital Topographic
Mapping
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Hydrology – Inputs (cont’d)
• Pictures
• Site Descriptions

8. Session 7: Water Management and Practical
Design Concepts
• Peak Flow Estimation Equations (i.e. a “camera”)
 Rational Method
 Regional Regression Equations (e.g. Index Flood Method)
• Runoff Routing Models (i.e. a “camcorder”)
 Software: HEC-HMS, HydroCAD, CivilStorm, RORB, etc.
 Specify loss and routing methods used -w- software
• Loss: SCS Curve Number (CN) Method, Initial Loss /
Continuing Loss (IL-CL) Method, etc.
• Routing: Kinematic Wave, Muskingum-Cunge, etc.
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Hydrology – Methods

9. Session 7: Water Management and Practical
Design Concepts
• Inputs: peak flow, slope, channel dimensions and
roughness (Manning’s n)
• Solve for Normal depth with Manning’s equation and
add an allowance for freeboard (using Excel)
• Key results are flow depth and velocity
 Velocity ≤ 0.8 m/s: too slow – siltation will occur!
 0.8 m/s < Velocity ≤ 1.5 m/s: earth or grass-lined
 1.5 m/s < Velocity ≤ 6.0 m/s: riprap
 Velocity > 6.0 m/s: concrete, gabions, etc.
• Riprap sizing if required (e.g. FHWA HEC-11SI, etc.)
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Hydraulics – “Typical” Diversions

10. Session 7: Water Management and Practical
Design Concepts
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“Typical” Diversion Example

11. Session 7: Water Management and Practical
Design Concepts
• Necessary when the catchment is large, consequences
of failure are high or when the interaction of multiple
elements: channels, ponds, culverts, etc. has to be
considered.
• Inputs: more detail needed: cross sections, runoff
hydrographs, etc.
• Solve for peak flow rate (depth, velocity etc.) which
changes throughout the system due to routing and
attenuation effects.
• 1-D flow profile software (HEC-RAS, etc.) may be used.
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Hydraulics – “Advanced” Diversions

12. Session 7: Water Management and Practical
Design Concepts
12
“Advanced” Diversion Example (1 / 4)

13. Session 7: Water Management and Practical
Design Concepts
13
“Advanced” Diversion Example (2 / 4)

14. Session 7: Water Management and Practical
Design Concepts
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“Advanced” Diversion Example (3 / 4)

15. Session 7: Water Management and Practical
Design Concepts
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“Advanced” Diversion Example (4 / 4)

16. Session 7: Water Management and Practical
Design Concepts
• Peak outflow influenced not just by runoff, but also
by the spillway inlet and the volume of the pool!
• Must consider storms of different duration at the
design frequency (not just the time of concentration)!
• Design involves use of two models:
 Hydrologic model: runoff estimation, reservoir pool
routing, peak outflow estimation (e.g. HEC-HMS)
 Hydraulic model: assess performance of spillway chute
and outlet under peak outflow (e.g. HEC-RAS)
• … and three design areas (Inlet, Chute and Outlet)
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Hydraulics – Spillways (Key Points)

17. Session 7: Water Management and Practical
Design Concepts
• Goal: empty the pool fast enough to keep the max
water surface below the embankment crest elevation
(minus total freeboard)
• Inputs: freeboard criteria, catchment, hyetographs,
stage / storage / discharge relationships, etc.
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Hydraulics – Spillways (Inlet Design)
 Assume configuration;
estimate S/D curve
 Route different duration
storms with HEC-HMS
 Find critical duration,
check, repeat if req’d

18. Session 7: Water Management and Practical
Design Concepts
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Inlet Design – Rating Curves (1 / 3)

19. Session 7: Water Management and Practical
Design Concepts
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Inlet Design – Rating Curves (2 / 3)

20. Session 7: Water Management and Practical
Design Concepts
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Inlet Design – Rating Curves (3 / 3)

21. Session 7: Water Management and Practical
Design Concepts
• Goal: convey peak runoff (generally in the
supercritical regime) from inlet to outlet without
spilling or eroding the chute – similar to a diversion…
• Must consider water surface roughness, wave action,
air bulking, splash and potential roll wave formation
when setting required freeboard
• Chute lining (riprap, gabions, concrete) must be
specified to resist erosion by (typically) high velocity
flows
• Uses outputs from HEC-RAS hydraulic model
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Hydraulics – Spillways (Chute Design)

22. Session 7: Water Management and Practical
Design Concepts
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Chute Design - Example

23. Session 7: Water Management and Practical
Design Concepts
• Goal: dissipate excess energy from high-velocity water
in chute and transition back to the subcritical flow
regime
• Typical approaches: hydraulic jump basins, rock riprap
aprons, flip buckets, etc.
• Required inputs and methods vary based on the
approach selected (HEC-RAS output + ???)
• Alternative approach: employ a stepped cascade
(gabion basket or other) spillway – this requires
specialist expertise
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Hydraulics – Spillways (Outlet Design)

24. Session 7: Water Management and Practical
Design Concepts
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Questions?