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Hydrologic Engineering Center 

Emergency GateOperation(Induced Surcharge)

HEC-ResSim

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Outline

Terms and Background

Theory Basics

When to use the induced surcharge rule How to use the induced surcharge rule

What to look for in operations

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“Surcharge Storage” is water stored

above the top of the flood pool –

above the spillway crest

Surcharge happens naturally with any

overflow structure

We can “induce” surcharge storage in a

controlled way with a gated spillway

Terms and Background

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Background

Why induced surcharge operation?

Allows maximum use of the available flood

storage space in the reservoir

AND

Allows evacuation of flood waters in a

planned manner as the reservoir rises,before the flood pool has filled

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Gated Spillway Operation

Flood Control Objectives

1. Do not endanger the dam

Do not let water flow over the spillway

gates2. Do not make the flood worse: do not let

the peak outflow exceed the pre-project

natural peak discharge (or peak inflow)

3. Do not let the rate-of-increase of outflow

exceed acceptable limits

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Spillway Gate Section

Max Pool

Top of Flood Pool

Max Induced Surcharge

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Maximum

PermissibleElevation

Release With GatesFully Open

   R   e

   s   e   r   v   o   i   r   E    l   e   v   a   t   i   o   n

    [    f   t    ]

Top Of Flood Pool

Spillway Discharge [1000 cfs]

Minimum

PermissibleRelease

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Induced Surcharge Envelope

The Induced Surcharge Envelope is: the maximum permissible reservoir elevation at

any rate of release

OR

the minimum release at any extreme reservoirelevation

Steeper slope in more-used portion

 – required release increases more slowly

Flatter slope for more extreme events/elevations – required release increases rapidly w/elevation

 – save the dam!

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Emergency Spillway ReleaseDiagram

The Emergency Spillway Release Diagram(ESRD) provides instructions for emergencyoperation (a.k.a. Induced SurchargeOperation)

Required release is based on volume ofremaining inflow expected during the event

For a given pool elevation and inflow (or rateof rise), indicated release ensures the pooldoes not rise above the surcharge envelopecurve before the end of the event

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Decision

Point

Recession,Defined By Ts

Current

Release

Induced Surcharge Computation

Time

     S     t    r    e    a    m     f     l    o    w

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Decision

Point

Current

Release

Induced Surcharge Computation

Time

     S     t    r    e    a    m     f     l    o    w

Recession,Defined By Ts

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Induced Surcharge Computation

Decision

Point

Time

     S     t    r    e    a    m     f     l    o    w

Smaller

Value Of Ts

Larger Value

Of Ts

Recession,Defined By Ts

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Induced Surcharge Computation

Decision

Point

Recession,Defined By Ts

Time

     S     t    r    e    a    m     f     l    o    w

Later Decision

Points

Recession

Hydrographs

Larger Value

Of Ts

Smaller

Value Of Ts

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Time

     S     t    r    e    a    m     f     l    o    w

Decision

Point

Current

Release

Recession,Defined By Ts

Volume ToBe Managed

Induced Surcharge Computation

o

ieoiS 

Q

Qlog 1QQT 2Vol 

Qi 

Qo

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Time

     S     t    r    e    a    m     f     l    o    w

Decision

Point

Current

Release

Induced Surcharge Computation

volume of

release

inflow drops

below releasevolume to

be stored

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Questions:

Will the “volume to be stored” put the reservoir

above the allowable surcharge envelope (at current

release)?

What elevation is low enough that the volume willfit?

For many inflows and outflows, plot that elevation.

Plotting and connecting points for a given inflow

creates the surcharge diagram. For a real-time inflow and elevation, the surcharge

diagram provides the operating instructions of what

release to make.

Induced Surcharge Computation

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Induced Surcharge Envelope Curve

Spillway DischargeCurve (Free Overflow)

Reservoir Outflow [1,000 cfs]

   R   e   s   e   r   v   o   i   r   E    l   e   v   a   t   i   o   n

    [    f   t    ]

InflowLevel

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Induced Surcharge Envelope Curve

Reservoir Outflow [1,000 cfs]

   R   e   s   e   r   v   o   i   r   E    l   e   v   a   t   i   o   n

    [    f   t    ] Spillway Discharge

Curve (Free Overflow)

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How is the recession

constant Ts defined?

Plot recession limb of

project (or spillway) design

storm on semi-log paper,

and fit a straight line

Find time needed to

decrease from some flow

Q A to Q B, where

Q B= Q A/e (or Q B= Q A/2.71)

     l    o    g    e

    s     t    r    e    a    m     f     l    o    w

timeTS

Recession Constant, Ts

QAQA

QB =

QA /2.71QB

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 When is Induced SurchargeOperation Needed?

When one or more of the following conditions

are present…

Large to very large inflow event

Rapidly rising Multi-peak

Long duration

Small or rapidly decreasing flood pool

Dam stability concerns

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Top Of Conservation

Top Of Flood Pool

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Top Of Conservation

Top Of Flood Pool

Top Of Flood Dam

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Top Of Conservation

Top Of Flood Pool

Top Of Flood Dam

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 What’s wrong?

The problem:

The model doesn’t know what to do when the

reservoir is full

AND, is letting the reservoir become full

The Solution:We need a rule to allow a higher release when

the reservoir is close to full

Release as function of elevation?…but also depends on high inflow…

Use the Induced Surcharge Rule

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How do we produce induced surcharge

operation in the ResSim environment?

with the Induced Surcharge Rule

In which zones should we put the rule?in those zones that appear in the diagram

Does the IS Rule apply to the Pool or Dam?

the pool 

What type of limit is the IS rule?

the induced surcharge rule is a MINIMUM rule

Induced Surcharge in ResSim

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What information is needed?

Surcharge envelope curve

Time of Recession, Ts

Inflow (Time-series) Option

Falling Pool instructions

Decrease time

Transition elevation Falling pool release option

Induced Surcharge in ResSim

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Top Of Conservation

Top Of Flood Pool

Top Of Flood Dam

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More to know

The Induced Surcharge rule is a MINIMUM

release rule

It must be paired with a maximum rule at a

lower priority to produce the right operationA decreasing rate of change rule may also be

desirable to temper the transition between

the IS operation and normal flood control

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Top Of Conservation

Top Of Flood PoolTop Of Flood Dam

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Analyzing Results

Induced Surcharge operation can often bedifficult to analyze because…

The release decision report does not identify the IS

rule as the controlling rule

If the inflow event has multiple peaks, the IS rule

may turn on and off sporatically

Use DSSVue

Plot the FLOW-MIN and FLOW-MAX records forthe relevant rules together with the release (Pool,

FLOW-OUT) or FLOW-DECISION record.

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I ESRD C

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Input ESRD Curves

Originally, only the Induced Surcharge Function Rulewas available in ResSim.

The newest form of this rule now allows the user to

select either the original function method for

describing the rule or direct entry of the ESRD familyof curves.

The original function form is appropriate for designing this

operation from scratch.

The new ESRD form is appropriate for describing this

operation when the curves have been altered from those

produced by the function.

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