SEDIMENTATION STUDIES FOR DAM REMOVAL USING HEC-6T

Mobile Boundary Hydraulics, PLLC

SEDIMENTATION SEDIMENTATION STUDIES FOR DAM STUDIES FOR DAM

REMOVAL USING REMOVAL USING HEC-6THEC-6T


• HEC-6T isHEC-6T is

- a general purpose, Computational- a general purpose, Computational- Sedimentation Computer ProgramSedimentation Computer Program

- an enhancement and an extension of the an enhancement and an extension of the Corps of Engineers programCorps of Engineers program HEC-6HEC-6

- A proprietary computer program owned by A proprietary computer program owned by MBH Software, IncMBH Software, Inc

IntroductionIntroduction


It performs a continuous simulation of the It performs a continuous simulation of the movement of the water-sediment mixturemovement of the water-sediment mixture

- the water surface profile over timethe water surface profile over time- the bed surface profile over timethe bed surface profile over time- erosion and deposition subject to sorting and erosion and deposition subject to sorting and

armoring of the bed mixture, andarmoring of the bed mixture, and- the concentration and delivery of sediment the concentration and delivery of sediment

particles, by size class, for non-equilibrium particles, by size class, for non-equilibrium sedimentation processessedimentation processes

IntroductionIntroduction


“A. Introduction

1. General.—Sedimentation embodies the processes of erosion, entrainment, transportation, deposition, and the compaction of sediment. These are natural processes that have been active throughout geological times and have shaped the present landscape of our world. …”

ASCE Manual 54CHAPTER I.—Nature of Sedimentation Problems


1. When evaluating the applicability of a computer program, state the questions you want the program to answer.

2. By coupling computational features in HEC-6T with river morphology principles we can answer many questions that are pertinent to dam removal issues.

2 points …


The following examples show typical questions The following examples show typical questions that one can ask HEC-6T.that one can ask HEC-6T.

Removal of Washington Water Power DamRemoval of Washington Water Power Dam

Breach of the N1 Structure at Mt. Saint HelensBreach of the N1 Structure at Mt. Saint Helens

Pumping Spirit Lake at Mt. Saint HelensPumping Spirit Lake at Mt. Saint Helens

To illustrate point No. 1 …To illustrate point No. 1 …


690

700

710

720

730

740

750

760

770

780

0 1 2 3 4 5 6 7 8

River Miles

Ele

vati

on

in

Fee

t

Thalweg Elevation

Average Bed Elevation

Water Surface Elevation, Q=100,000 cfs

Fig 1. Removal of Washington Water Fig 1. Removal of Washington Water Power DamPower Dam


-6

-5

-4

-3

-2

-1

0

1972 1974 1976 1978 1980 1982 1984TIME (in years)

CHAN

GE IN

BED

ELE

VATION

(in feet

) CalculatedMeasured

Fig 2. Erosion of Reservoir Deposit at DamFig 2. Erosion of Reservoir Deposit at Dam


-5

-4

-3

-2

-1

0

1

1972 1974 1976 1978 1980 1982 1984

TIME (in years)

CHAN

GE IN

BED

ELE

VATION

(in feet

)

Calculated

Measured

Fig 3. Deposition Downstream of Dam, Fig 3. Deposition Downstream of Dam, River Mile 3.48River Mile 3.48



Questions to ask HEC-6T

– How much of the reservoir deposit will erode?

– Will the downstream channel transport the eroded sediment?

– How much time will pass before the river has returned to a state of equilibrium?


The following examples illustrate the questions The following examples illustrate the questions that one can ask HEC-6T.that one can ask HEC-6T.


Breach of the N1 StructureBreach of the N1 Structure

Point No. 1 (cont) …


was applied to first dam was applied to first dam removal study in 1977removal study in 1977

Breach of the N1 Structure


Fig 4. Breach of N1 StructureFig 4. Breach of N1 Structure(View from Right Bank Side to Left Bank Side, (View from Right Bank Side to Left Bank Side,

Downstream of Structure)Downstream of Structure)


Flow through Breach, N1 StructureFlow through Breach, N1 Structure N. Fork Toutle River - Mt. St. Helens, 1981N. Fork Toutle River - Mt. St. Helens, 1981


The following examples illustrate the questions The following examples illustrate the questions that one can ask HEC-6T.that one can ask HEC-6T.


Breach of the N1 StructureBreach of the N1 Structure

Pumping Spirit LakePumping Spirit Lake

Point No. 1 (cont) …


was applied to first dam was applied to first dam removal study in 1977removal study in 1977

Pumping Spirit Lake


Fig 5. Channel from Pumping Spirit LakeFig 5. Channel from Pumping Spirit Lake

(Formed by a discharge of 182 cfs flowing for 1 Year)


Pumping Spirit LakePumping Spirit Lake

WhereWhere

C = bank resistance coefficientC = bank resistance coefficient

Q = channel forming water dischargeQ = channel forming water discharge

W = top width of channelW = top width of channel

W C Q


What questions did we ask HEC-6 about What questions did we ask HEC-6 about Mt. Saint Helens watersheds?Mt. Saint Helens watersheds?

– How much sediment, by particle size, How much sediment, by particle size, will be eroded from that network of will be eroded from that network of streams as a function of time? streams as a function of time?

– How much of that volume will be How much of that volume will be transported to the Columbia River?transported to the Columbia River?

– What will the range of concentrations What will the range of concentrations be over the next 50 years?be over the next 50 years?


What questions did we ask HEC-6 about What questions did we ask HEC-6 about Mt. Saint Helens watersheds?Mt. Saint Helens watersheds?

4.4. How much of the volume in transport How much of the volume in transport could be stored in a sediment retention could be stored in a sediment retention basin?basin?

5.5. How much time would be available to How much time would be available to construct the basin before sediment construct the basin before sediment deposits blocked the bypass channel.deposits blocked the bypass channel.


Special Features in HEC-6TSpecial Features in HEC-6T

Point No. 2 …


Physical properties of interest in channel Physical properties of interest in channel evolution studies areevolution studies are

1.1. channel widthchannel width no width equation in HEC-6Tno width equation in HEC-6T

2.2. channel depthchannel depth computed automaticallycomputed automatically

3.3. channel slopechannel slope computed automaticallycomputed automatically

4.4. channel roughnesschannel roughness bed roughness equationsbed roughness equations

5.5. bank line migrationbank line migration inferred from computed shearinferred from computed shear

6.6. plan-formplan-form inferred from computed inferred from computed sediment deliverysediment delivery


Features that allow one toFeatures that allow one to- - determine channel widthdetermine channel width

The conceptual, Channel Evolution Model of The conceptual, Channel Evolution Model of Schumm, Harvey and Watson.Schumm, Harvey and Watson.

–Shows 5 stages of development

–Stage II is predominantly erosional

–Stage IV is highly depositional


NOTE:hc= critical bank height

Figure 18-1.

Stage IIStage II

Bed ErosionBed Erosion

h < hh < hcc

h = bank heighth = bank height


NOTE:hc= critical bank height

Figure 18-1.

Stage V

Flood Plain Evolution

h bank heigh t


- - determining channel widthdetermining channel width

Pumping Spirit Lake convinced me:Pumping Spirit Lake convinced me:

– It is not necessary to model the details of It is not necessary to model the details of channel evolution from one stage to the next.channel evolution from one stage to the next.

– The hydraulic geometry equation for channel The hydraulic geometry equation for channel width gives a reasonable top width even in width gives a reasonable top width even in highly disturbed watersheds.highly disturbed watersheds.

– We need only to convert that top width to a We need only to convert that top width to a bottom width and assign it to the original bottom width and assign it to the original cross section. cross section.


NormalPool1

2 36

8 1312

14

Ele

vatio

n

Station

InitialCross Section

FinalFinalCross SectionCross Section

Features in HEC-6TFeatures in HEC-6T- coding bottom width- coding bottom width

Erosion Limits

– By By restricting erosion to an assigned bottom restricting erosion to an assigned bottom width, erosion and deposition computations width, erosion and deposition computations will establish the new bottom slope and the will establish the new bottom slope and the new bank height.new bank height.


Features in HEC-6T Features in HEC-6T for channel evolution computationsfor channel evolution computations

(1)(1) channel widthchannel width no width equation in HEC-6Tno width equation in HEC-6T

(2)(2) channel depthchannel depth computed automaticallycomputed automatically

(3)(3) channel slopechannel slope computed automaticallycomputed automatically

(4)(4) channel roughnesschannel roughness bed roughness equationsbed roughness equations

(5)(5) bank line migrationbank line migration inferred from computed shearinferred from computed shear

(6)(6) plan-formplan-form inferred from computed inferred from computed sediment deliverysediment delivery


Features in HEC-6TFeatures in HEC-6T-fail the banks-fail the banks

If the computed height becomes greater than a If the computed height becomes greater than a stable bank height, HEC-6T will fail the banksstable bank height, HEC-6T will fail the banks

The bank sediment will slump into the channel The bank sediment will slump into the channel where it will be removed by the flow as where it will be removed by the flow as transport capacity becomes available.transport capacity becomes available.

All computations are made by particle size.All computations are made by particle size.


Features in HEC-6TFeatures in HEC-6T-dam removed in stages-dam removed in stages

• The dam can be removed in stages by placing The dam can be removed in stages by placing cross sections in the simulation hydrograph at cross sections in the simulation hydrograph at the times when notches will be constructed.the times when notches will be constructed.

• If tributaries are present, they can be coded If tributaries are present, they can be coded into a stream network.into a stream network.

• HEC-6T will increase transport of the sands HEC-6T will increase transport of the sands and gravels due to the high concentration of and gravels due to the high concentration of fines.fines.


Summary and ConclusionsSummary and Conclusions

Sedimentation Studies For Dam Removal Sedimentation Studies For Dam Removal Using HEC-6TUsing HEC-6T


HEC-6T is a general purpose computer program that calculates changes in cross sections resulting from the 5 basic sedimentation processes.



5 Basic Sedimentation Processes5 Basic Sedimentation Processes– ErosionErosion– EntrainmentEntrainment– TransportationTransportation– DepositionDeposition– Compaction of Deposited SedimentsCompaction of Deposited Sediments




It can be applied to study channel It can be applied to study channel development and sediment delivery resulting development and sediment delivery resulting from the removal of a dam by coupling it withfrom the removal of a dam by coupling it with

– the Channel Evolution Model, and the Channel Evolution Model, and

– river morphology principles.river morphology principles.


1. How deep will the bed erode? 2. How deep will the sediment deposit

downstream? 3. How wide will the top width become due to

bank failure from channel invert erosion? 4. What will the average boundary shear

stress be? 5. What will the gradation of the surface

layer be and how will it change over time?

Typical Questions one might ask HEC-6T.Typical Questions one might ask HEC-6T.((Examples, not an exhaustive list)Examples, not an exhaustive list)


6. How will the concentration of sediment in the water column vary over time?

7. How many tons of sediment will be delivered downstream?

8. How much sediment will be transported out of the reservoir?

9. What will the average bed profile become, upstream and downstream of the dam, and how will it change over time?

Typical Questions (cont) … Typical Questions (cont) …



• Observations at Mt. Saint Helens convinced me Observations at Mt. Saint Helens convinced me that river morphology principles can be used to that river morphology principles can be used to predict channel width even in highly disturbed predict channel width even in highly disturbed watersheds.watersheds.


NormalPool1

2 36

8 1312

14

Ele

vatio

n

Station

InitialCross Section

FinalFinalCross SectionCross Section


Erosion Limits

• The channel width can be converted into a The channel width can be converted into a bottom width and coded into the initial cross bottom width and coded into the initial cross section using the “erosion limits” feature in section using the “erosion limits” feature in HEC-6T.HEC-6T.



• The dam can be removed in stages by placing The dam can be removed in stages by placing cross sections in the simulation hydrograph at cross sections in the simulation hydrograph at the times when notches will be constructed.the times when notches will be constructed.

• If tributaries are present, they can be coded If tributaries are present, they can be coded into a stream network.into a stream network.

• HEC-6T will erode silt and clay, and the HEC-6T will erode silt and clay, and the transport of the sands and gravels will be transport of the sands and gravels will be increased if the concentration of fines increased if the concentration of fines becomes significant.becomes significant.



•My presentation today shows features in My presentation today shows features in HEC-6T that are not in HEC-6.HEC-6T that are not in HEC-6.

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SEDIMENTATION STUDIES FOR DAM REMOVAL USING HEC-6T