Diversion Systems and Spillways (22 Nov 2013)

Diversion Systems & Spillways

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Todd Lewis, MSCE, BSCE (Hons), P.E.

Water Resources Engineer

[email protected]

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)


• 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.”

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Probability Terminology (ARI)


• 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)


• 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)

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The “BIG” Picture


• Intensity / Duration / Frequency (IDF) Curves

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Hydrology – Inputs

• Hyetographs


• Digital Topographic Mapping

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Hydrology – Inputs (cont’d)

• Pictures

• Site Descriptions


• 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


• 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


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“Typical” Diversion Example


• 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


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“Advanced” Diversion Example (1 / 4)


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• 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)


• 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


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Inlet Design – Rating Curves (1 / 3)


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• 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)


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Chute Design - Example


• 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)


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Questions?

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Diversion Systems and Spillways (22 Nov 2013)

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