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Effects of Dam Removal: An Approach to Sedimentation October 1977 Approved for Public Release. Distribution Unlimited. TP-50

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4. TITLE AND SUBTITLE Effects of Dam Removal: An Approach to Sedimentation

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6. AUTHOR(S) David T. Williams

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7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) US Army Corps of Engineers Institute for Water Resources Hydrologic Engineering Center (HEC) 609 Second Street Davis, CA 95616-4687

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12. DISTRIBUTION / AVAILABILITY STATEMENT Approved for public release; distribution is unlimited. 13. SUPPLEMENTARY NOTES This paper was presented at the 1977 ASCE Annual Convention in San Francisco, California, Session No. 44, 19 October 1977. 14. ABSTRACT In recent years hydraulic structures such as dams have been removed due to deterioration, increased maintenance cost or obsolescence. Investigation of the hydraulic, hydrologic, and sediment transport consequences of the removal of these structures have been very limited, thus necessitating the establishment of analytical techniques and procedures to adequately predict these effects. To properly evaluate the development of techniques and procedures a model must be selected that closely simulates the actual behavior of the phenomenon being modeled. A mathematical model (HEC-6) was selected because of its success in the prediction of sediment transport when applied to a wide variety of cases. The removal of the Washington Water Power Dam on the Clearwater River near Lewiston, Idaho, was elected for study. Procedures and techniques of calibration and verification developed, comparison of actual and predicted volume of sediment transported, where the sediment scoured or deposited, and their rates are presented. There is discussion of the applicability of the model to this type of problem, limitations of a one-dimensional model, and interpretation of the results. 15. SUBJECT TERMS model studies, sediment transport, dams, Idaho, hydraulic structures, analytical techniques, analysis, rivers, hydraulics, mathematical models, computer models, on-site investigations, effects, data collections, sediment discharge, particle size, streambeds, dam removal 16. SECURITY CLASSIFICATION OF: 19a. NAME OF RESPONSIBLE PERSON a. REPORT U

b. ABSTRACT U

c. THIS PAGE U

17. LIMITATION OF ABSTRACT UU

18. NUMBER OF PAGES 46 19b. TELEPHONE NUMBER

Effects of Dam Removal: An Approach to Sedimentation

October 1977 US Army Corps of Engineers Institute for Water Resources Hydrologic Engineering Center 609 Second Street Davis, CA 95616 (530) 756-1104 (530) 756-8250 FAX www.hec.usace.army.mil TP-50

Papers in this series have resulted from technical activities of the Hydrologic Engineering Center. Versions of some of these have been published in technical journals or in conference proceedings. The purpose of this series is to make the information available for use in the Center's training program and for distribution with the Corps of Engineers. The findings in this report are not to be construed as an official Department of the Army position unless so designated by other authorized documents. The contents of this report are not to be used for advertising, publication, or promotional purposes. Citation of trade names does not constitute an official endorsement or approval of the use of such commercial products.

EFFECTS !IF DAM REMOVAL: A N APPROACl.1 TO SEDTFIE'PITATION

ABSTRACT

In recent years hydraulic structures such as dams have been removed

due t o deterioration, increased maintenance cost or obsolescence, Inves-

t i f fa t ion of the hydraulic, hydroloqic, and sediment transport consequences

of the removal of these structures have been very limited, thus necessitating

the establishment of analytical techniques and procedures to adequately

predict these e f fec ts ,

f o proper1 y eval uate the devel opment of techniques and procedures, a

model must be selected tha t closely simulates the actual behavior of the

phenomenon being modeled. A mathematical model (HEC-6) was selected because

of i t s success in the prediction of sediment transport when applied t o a

wide variety of cases. The removal of the Blashinqton !dater Pot?rer Dam on the

Cl earwater River near Lewiston, Idaho, was sel ected for stud,^.

Procedures and techniques of calibration and verification developed,

comparison of actual and predicted vol rime of sediment transported, where

the sediment scoured o r deposited, and the i r rates are presented, There

i s discussion of the appl icabi l i ty of the model to th i s type of problem,

l imitations of a one-dimensional model, and interpretation of the resul ts .

EFFECTS OF D4M RFF4QVAL :

4N APPR94CH TO SEDI AtENTATI 3Fi

nav i d T. i4i 1 l i ams

FOREWORD

The f inanc ia l , c le r ica l , and graphical support f o r t h i s paper was

provided by The H~/droloqic Engineering Center ( K G ) , U.S. Army Corps of

Engineers, 9avis , California, f n i t i a l l y a Corps sponsored research e f f o r t ,

i t :*!as a l so submitted as a Masters thes i s f o r the IJniversit~y of California

a t Davis. A more deta i led version of t h i s repor t (with technical data) i s

avai lable a t HE@ upon request

1 woul d l ke t o acknowledge Mr. Tony Thomas (lhfaterl.hjays Experimental

S ta t ion , Vickshurg, Yiss iss ippi) f o r h is technical and conceptual help,

Professors Flay B, Krone, Edward S. Schroeder, and 'dil liam i<. Johnson,

of the University of Cal i fornia , Davis, f o r t h e i r helpful comments and

guidance, and The klydrologi c Engi neeri ng Center s t a f f ,

This paper was presented a t the 1977 nSCE Annual Convention i n San Francisco, Cal i fornia , Session No. 44, 19 October 1977.

EFFECTS OF DAM REMOVAL: AN APPROACH TO SEDIMENTATION David T. Williams*

I . Introduction

In February of 1973, the Washington Water Power Dam (IrlWPD) on the Clear-

water River, Idaho, was removed because of increased maintenance costs and

obsolescence. As a r e su l t , changes occurred in the hydraulic and sedimentation

character is t ics of the river. The purpose of th i s study i s to determine how

the observed changes occurred, to develop a sui table analytical technique f o r

such s tudies , and to evaf uate and verify a mathematical model used t o predict

the observed changes. The changes in the r iver bed include the ra te of depo-

s i t ion or scour along the r iver bed, the magnitude o f deposition or scour a t

times subsequent t o the removal o f the dam, and changes in the hydraulic and

sediment transport character is t ics of the r iver fol lowing dam removal . In the past most of the studies related to dams have been in the area of

construction design and the hydraulic, hydrologic, economic, soc ia l , and envi-

ronmental impact of t h e i r placement. No assessments were made concerning the

measures (e ,g . , dam removal) that must be implemented a t the end of the i r

design l i f e (typical design l i f e of a dam i s 50 years) . During the Depression

Era (1930's) many dams were constructed that have a designed 1 i f e that will

terminate in the 1980's. Most of these dams require a high level of mainte-

nance as a d i rec t resu l t of age and are becoming increasingly cost ineffective.

Large dam systems that have been implemented in l a t e r years have made many

minor dams obsolete. The disposition of an obsolete and/or decaying dam i s a

problem tha t will be addressed even more frequently in the future.

Removal o f a hydraulic s t ructure such as a dam causes changes in the

hydraulic and sedimentation character is t ics of a rfver which subsequently

"Research Hydraulic Engineer, The Hydrologic Engineering Center, U.S. Army, Corps of Engineers , Davis , Cal i fornia

caused adverse e f fec ts on both. man and t h e environment. If the e f fec ts o f t h e

dam removal a re p r e d i c t a b l e through the use o f an a n a l y t i c a l technique and a

mathematical model , measures (e.g. , gradual removal) cou ld be implemented t o

lessen the impact o f t h e adverse e f f e c t s .

The removal o f t h e Washington Water Power Dan (WWPD) on the Clearwater

R ive r was se lec ted f o r s tudy because o f t he a v a i l a b l e sediment da ta before

and subsequent t o t h e dam removal, Th is data was gathered by t h e U.S.

Geological Survey and t h e Walla Walla D i s t r i c t s f t he Corps o f Engineers and

was obta ined main ly f o r an ongoing study o f t h e Snake and Clearwater Rivers.

Clearwater R ive r i s a t r i b u t a r y o f the Snake R iver i n Idaho and serv ices

a drainage area o f 9,570 mi les . F igure 1 shows a map o f t he drainage basin.

The confluence o f t h e Snake and Clearwater R ivers was def ined as R ive r M i l e 0

and c ross-sec t ion l o c a t i o n s a re i n terms o f r i v e r m i l es upstream along the

C l earwater River .

Washington Water Power Dam Nas l oca ted a t R iver M i l e 4.62. I n ope ra t i on

s ince 1928, t he dam was approximately 35 f e e t h igh and 1100 f e e t long. The

dam was o f concrete cons t ruc t i on w i t h moveable gates which a1 1 owed submerged

f low. Dur ing the month o f February, 1973, t he dam was removed by l a r g e cranes

and minimal exp los ive demo1 i t i o n . The e n t i r e opera t ion was performed du r ing

a low flow per iod .

Located downstream o f t h e conf luence o f t he Snake and C l earwater Rivers

i s the Lower Gran i te Dam. The impoundment o f water behind t h i s dam began i n

February 1975. I n June 1975, the Lower Gran i te Reservoi r and Lock f a c i l i t i e s

became f u l l y opera t iona l . This impoundment has an impor tan t r o l e i n t he

sediment balance of t he study area because the backwater e f f e c t s o f t he Lower

Grani te Reservoi r extends i n t o the study area and the ana lys i s p e r i o d inc ludes

the t i m e o f impoundment.

I I . Basic Approach: Mathematical Model

The behavior of a 7.83 mile reach of the Clearwater River i s simulated by

a computer program and resul ts compared to f i e l d data to evaluate and verify

the computer program. These comparisons a re used to determine the sui tabi l i ty

of the analytical techniques t o studies of t h i s type. A1 1 computer appl ica-

t ions were conducted a t The Hydrologic Engineering Center (HEC) , Davis,

California.

The mathematical model selected for use in the analysis of dam removal

effects i s a generalized computer program ent i t led "HEC-6, Scour and Deposition

in Rivers and Reservoirs" and i s dis t r ibuted by The Hydrologic Engineering

Center, Corps of Engi neers (Computer Program Number 723-62-L2470). Thi s

computer program was selected because of the general success in i t s usage

over a wide variety of applications [8) and the accessibi l i ty of the program.

The sediment transport methods available fo r use in the program are those

developed by Toffalet i , Laursen, Duboys, and Yang. Toffaleti ' s transport

method was used in the study (10).

This simulation computer program i s designed to analyze scour and depo-

s i t i on by modeling the interaction between the water-sediment mixture, sedi-

ment material forming the stream's boundary and the hydraulics of the flow.

This i s not a sediment yield program per se. I t simulates the a b i l i t y of the

stream t o transport sediment and considers the fu l l range of conditions

embodied in Einstein's Bed Load Function plus s i l t and clay transport and

deposition, armoring and t he destruction of the armor layer. Figure 2 shows

a Functional Flow Chart of HEC-6 (9 ) .

The l imitations of the computer program are related t o the one-dimensional

aspect of the model, since i t i s a one-dimensional steady flow model with no

provision for simulating the development of meanders or specifying a la te ra l

dis t r ibut ion of sediment load across a cross section. The cross section i s

r- I

M o d u l e 1 L, r- i I M o d u l e 2 L-

r- I

I I I I t I I I I I I M o d u l e 4 f I I I t I I L- r- I I MENT LOAD AND, FOR EACH I C R O S S S E C T I O N , ARMORING, 1 E Q U I L I B R I U M D E P T H , GRADA- 1 T I O N O F MATERIAL I N A C T I V E I LAYER, TRANSPORT CAPACITY 1

M o d u l e s 5, 6 1 BY U S E R S CURVES

A 7 1 OR

S E D I M E N T LOAD LEAVING T H E I I

REACH, CHANGE VOLUME O F

1 BED MATERIAL T O R E F L E C T SCOUR + TRANSPORT

I OR D E P O S I T I O N , CHANGE C A P A C I T Y BY I DEPTH O F D E P O S I T TO LAURSEN L, L

R E F L E C T NEW VOLUME

I - -

Figure 2

FUNCTIONAL FLOW CHART OF HEC-6

subdivided into two parts with i n p u t data -- tha t part which has a moveable

bed, and that which does not; and the boundary between these parts remains

fixed for the study. The en t i r e moveable bed part of the cross-section i s

moved ver t ica l ly up and down. Bed forms are not simulated except that n-val ues

can be functions of discharge which indirect ly permits a consideration of bed

forms t o be made. Density currents and secondary currents are n o t accounted

fo r ( 9 ) .

I 11, Data Coll ection and Processing

Each of the cross-section geometry measurements used in the study were

made by the Walla Walla Dis t r ic t of the Corps o f Engineers. A sonic fatho-

meter w i t h a resolution of + - .1 f ee t was used fo r the depth determination.

These measurements were transferred to a Cartesian coordinate system plot

and encoded into a format usable by the model.

All sediment load measurements were made by the U.S. Geological Survey

(1 , 2 , 5 , 6 ) through a cooperative program sponsored by the Wall a Ida1 l a

Distr ic t . The suspended load was measured by P-1 o r P-3 suspended-sediment

samplers ( 3 ) . Both point coll ec t i on and depth-i ntegrated samples were

collected for analysis of concentration and grain s i ze dis t r ibut ion. These

measurements were made fo r various discharges over a 5 year period. Conver-

sion of concentration to tons per day was made by the equation:

tonslday = ,0027 X concentration (mg/l) X discharge ( c f s )

These discharge vs, suspended sediment load data are plotted in Figure 4.

Bed load was measured using a Helley-Smith type bedload sampler ( 4 ) .

Figure 4 has a sca t t e r of data b u t a def ini te relationship ex is t s between

the sediment load and discharge. A least-squares f i t was developed f o r

suspended and bedloads by the USGS ( 6 ) The two sediment loads of the

least-squares curve were added to produce the total sediment load curve shown

750

745

- -I

740

Cr) C

736 w I>

730 m a

k 726 Lrl LLJ LL - 720

z 716 a n t- a 710

3 Ll-t

-l 706. w

700

696

UNIFORMLY CHANGED 690

BED ELEVATIONS

685 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 W 4 4 C1 0 T ID W h m B3

8 I

S T R T I O N ( F E E T )

CHANGED BED ELEVATION --o-. - -- -. --0---+-+ -- *.' 4

C L E R R W R T E R R I U E R , SECTION 2 .El9

10.000 P00.008

DISCHARGE ( In c f r

in Figure 4. From collection of many samples, an averaged grain s i ze d i s t r i -

bution of the inflowing sediment load was developed. The corresponding per-

cent o f the total load was determined for each grain s i ze class. Nith the use

of the total load curve in Figure 4, the percent of each s i ze class was

multiplied by the total load a t a certain discharge to obtain the sediment

load contributed by the grain s i ze fraction for tha t discharge, Using the

same procedure fo r other discharges , a di scharge-sediment 1 oad curve was

developed fo r each grain s i ze fraction. These re1 ationships determine the

grain s i ze distribution and weight of the inflowing sediment load fo r each

hydrograph discharge,

Bed material par t ic le s i ze dis t r ibut ion was determined by sieve analysis

of bul k samples one cubic foot in volume, Only three bed measurements were

made on the Clearwater River. The f i r s t measurement was made about two miles

above the upstream boundary of the model, the next a t about mile 4.74 and the

l a s t measurement was made near mile 2.0. In the i n i t i a l phase of the model

cal ibrat ion, the f i r s t measurement was considered to represent the bed from

mile 7.83 t o mile 5.56, The second measurement represented the bed from mile

5.39 to mile 4.62 ( s i t e of Washington Water Power Dam) and the l a s t measure-

ment, mile 4.61 t o mile -67.

Historic mean daily flows were obtained for the years 1966 t o 1975 a t the

USGS gage i n Spalding, Idaho (7) . Located near the upstream boundary o f the

model, the gage used a calibrated stream-flow gage which related the measured

water height to discharge. Future hydrology was predicted by assuming

(assumption was made by the author) tha t the h is tor ic events would occur in

the same sequence and intensi ty in the future, The hydrology of 1980 was

assumed t o be tha t o f 1970, 1981 assumed to be that of 1971 , etc .

The stage-di scharge rating curve a t the downstream model boundary (before

Lower Srani t e Reservoir impoundment) was determined by abserved stage heights

f o r a wide range o f f lows. Th i s r a t i n g curve was t h e downstream boundary

c o n d i t i o n f o r t he model and determined the s t a r t i n g e l e v a t i o n f o r water

sur face p r o f i l e c a l c u l a t i o n s f o r any p a r t i c u l a r discharge. A cons tant water

sur face e l e v a t i o n o f 738 f e e t was used a f t e r the impoundment o f Lower Gran i te

Reservoi r i n 1975.

A value o f .03 f o r n was se lec ted f o r t h e channel and overbanks, Th is

compares favorably w i t h o t h e r r i v e r s o f t h i s type w i t h c a l i b r a t e d n values o f

approximate ly ,03, Cal i b r a t i o n o f t h i s n was n o t poss ib le due t o t h e l a c k o f

water marks o r discharge vs. depth measurements upstream o f model boundary.

Model C a l i b r a t i o n and V e r i f i c a t i o n

Computer runs were made i n a " f i x e d bed" mode ( i . e. , no bed e l e v a t i o n

change) w i t h var ious discharges. The water sur face e l e v a t i o n was ca1 c u l a ted

a t each c ross-sec t ion w i t h an ope ra t i ng pool e l e v a t i o n o f 761 f e e t a t t h e

Washington Water Power Dam ( ~ i v e r M i l e 4.62). Checks were made t o i n s u r e

t h a t t he water sur face e leva t i ons d i d n o t e x h i b i t unusual c h a r a c t e r i s t i c s .

The model was v e r i f i e d by ana lyz ing known h i s t o r i c a l stream bed condi-

t i o n s and us ing them as performance c r i t e r i a . F igures 5 , 6 and 7 show t h e

change i n bed e leva t i ons upstream o f t he WWPD. Looking a t t he l i n e s repre-

sen t i ng the dam i n p lace cond i t i on , t h e f i g u r e s show o n l y s l i g h t bed e l e v a t i o n

change. This was expected because the WWPB has been i n opera t ion f o r a long

time, A s l i g h t o v e r a l l depos i t i on t r e n d was c a l c u l a t e d upstream o f t h e dam,

i n d i c a t i n g t h a t t he r e s e r v o i r was n o t complete ly f i l l e d w i t h sediment a t t h e

t ime of dam removal.

F igure 8 shows the change i n bed e l e v a t i o n downstream o f t h e WWPD. I t

shows s l i g h t degradat ion o f the bed e l e v a t i o n w i t h the dam i n p lace. This

again i s reasonable because the i n f l o w i n g sediment load t o these sec t ions i s

d e f i c i e n t (caused by t h e WWPD) compared t o the sediment l oad under n a t u r a l

cond i t ions . The model a l so showed s l i g h t o v e r a l l scour downstream o f t he dam.

It i s d i f f i c u l t t o determine whether v a r i a t i o n s i n sediment y i e l d a re

caused by v a r i a t i o n s i n discharges o r changes i n t h e h y d r a u l i c regime such

as those caused by dam removal , For example , i f a dam i s removed, t he down-

stream sediment y i e l d should be g rea te r because o f t h e scour ing o f t h e sed i -

ment pool behind t h e o r i g i n a l dam. However, t he sediment y i e l d would be

d i s t o r t e d i f , du r ing t h e p e r i o d of ana lys is , h igh water discharges w i t h the

associated h igh sediment discharges occurred. To compensate f o r t h i s v a r i a -

t i o n caused by the f low, t h e r a t i o o f sediment ou t f l ow t o sediment i n f l o w was

used t o measure scour and depos i t i on t rends, Th is r a t i o i s dimensionless and

i s independent of water d ischarge v a r i a t i o n . A r a t i o of 1 i n d i c a t e s no change

( i . e . , e q u i l i b r i u m : what goes i n , goes ou t ) , l e s s than 1 i n d i c a t e s sediment i s

being accumulated (depos i t i on ) , and g rea te r than 1 i n d i c a t e s sediment i s being

removed f rom the bed (scour) .

F igure 9 shows the computed change i n t h e in f low/out f1ow r a t i o over a

10 yea r p e r i o d w i t h the dam i n p lace, Without t he Lower Gran i te Reservoi r

impoundment, t h e curve i n d i c a t e s s l i g h t depos i t i on ( r a t i o < 1) w i t h a tendency

t o 1 (equ i l i b r i um) . Th is leads t o the conclus ion t h a t t he WWPD was s t i l l

causing s l i g h t depos i t i on bu t t h a t r e s e r v o i r had almost reached i t s capac i ty

o f sediment. M i t h t he Lower Gran i te Reservoi r impoundment, g rea te r depos i t i on

occurs w i t h i n t h e study area because the i n f l o w i n g sediment l oad i s being

dropped because o f the i n f l uence o f the downstream rese rvo i r . There i s a

tendency towards equi 1 i b r i urn as t h e C l earwater arm of t h e Lower Gran i te

Reservoi r begins t o f i l l . I t appears t h a t e q u i l i b r i u m w i l l occur sometime

a f t e r 1983. The i n f l o w / o u t f l o w r a t i o a t 1974 i n F igure 9 shows i n i t i a l scour

and reason i n d i c a t e s t h a t i t should be depos i t ion . This discrepancy i s

a t t r i b u t e d t o s l i g h t e r r o r i n the i n i t i a l c o n d i t i o n which the model ad jus ted

as c a l c u l a t i o n s were made over t ime.

F igure 10 shows the p red i c ted volume of sedimentat ion w i t h i n the model

boundaries w i t h t h e WWPD i n place. M i thou t the Lower Gran i te jmpoundment,

15

TI M

E (

in y

ears

)

FIG

URE

9

CALC

ULAT

ED R

ATIO

OF

SEDI

MEN

T

OUTF

LOW

TO

INF

LOW

WAS

HING

TON

POW

ER D

AM I

N P

LACE

t he model showed t h a t an a d d i t i o n a l depos i t i on o f approximate ly 40 acre- fee t

would have occurred in the WWPD pool i f the \dWPD dam had remained i n p lace f o r

10 more years. With the Lower Gran i te Reservoi r impoundment, depos i t i on i n

t he s tudy area would have been about 650 ac re - fee t over 10 years. A l l t he

depos i t i on occurs downstream o f t h e WWPD. These t rends are t o be expected

under the ope ra t i ng cond i t i ons s p e c i f i e d ,

V. Dam Removal Resul ts and Discussion

Measured bed e l e v a t i o n changes were determined by ana lys i s o f measured

cross-sect ion geometry f o r February 1973, September 1973, A p r i l 1975, November

1975, and August 1975. Bed e l e v a t i o n change from February 1973 t o September

1973 was determined by ove r lay ing t h e c ross-sec t ions (must be o f t h e same

sca le ) , p lan ime te r i ng t h e area below a common e l e v a t i o n datum where bo th

cross-sect ions meet, f i n d i n g t h e d i f f e r e n c e i n t h e areas, and d i v i d i n g the

r e s u l t i n g area by t h e w id th o f t h e bed p o r t i o n t h a t moved. Th is procedure

was a l so used t o o b t a i n t h e bed e l e v a t i o n changes f o r o t h e r t ime increments.

F igures 5, 6 and 7 show t h e observed change i n bed e levat ions upstream

of t h e Washington Dam s i t e . The comparison between the measured and observed

bed e l e v a t i o n change f o r R ive r M i l e 4.74 (F igure 5 ) , the sec t i on immediately

upstream of t he dam s i t e , shows good c o r r e l a t i o n f o r bo th t i m i n g and magnitude

of bed e l e v a t i o n change. F igure 6 i n d i c a t e s general agreement between the

measured and computed magnitudes o f bed e l e v a t i o n change over an extended

t ime per iod, The f l u c t u a t i o n s o f t he measured bed e leva t i ons i n F igure 7

l i m i t any ana lys i s o f t he o v e r a l l tendencies o f t he bed. Any f u r t h e r ana lys i s

would r e q u i r e more data p o i n t s bo th w i t h i n t h e p o i n t s a c t u a l l y shown and

beyond 1976. The o v e r a l l magnitude o f the computed bed e l e v a t i o n changes

were i n f a i r l y acceptable agreement w i t h t h e measured changes, P r ~ j e c t i ons

i n t o the f u t u r e revealed t h a t t he sec t ions would s low ly cont inue t o scour.

The timing of the measured and computed bed elevation changes fo r

Figures 6 and 7 appear t o lag by approximately 10 months. The r a t e of

scour decreases s ignif icant ly near the end of 1973 fo r the measured data

and the middle of 1974 for the computed.

Adequate information to determine bed elevation changes for 1973 to

1975 were not available fo r the sections downstream of the dam s i t e . For

the purpose of analysis, the April 1975 bed elevation was assumed to be tha t

of the computed elevation a t tha t time and measured bed elevation changes a re

determined by changes from the April 1975 measured bed elevation.

Since very l i t t l e bed change occurred downstream of the dam s i t e fo r

both the measured and computed cross-sections, i t i s rather hard t o discuss

the accuracy of timing and magnitudes of change. Discussion i s then limited

t o tendencies shown by the model and prototype. As shown in Figure 8, the

model calculated i n i t i a l scour a f t e r the WWPD was removed, and both the

computed and measured sections showed deposi t i onal tendencies a f t e r impound-

ment of Lower Granite Reservoir. Projections into the future indicate

continued deposition a t these sections. This deposition wi 11 probably

continue until Lower Granite Reservoir reaches an equi 1 ibrium condition.

No suspended and bed load measurements were available to determine the

sediment load. Because of t h i s lack of f i e ld data, analysis must be concen-

t ra ted on the reasonability of the model resul ts when compared to what i s

actual ly expected to happen.

As shown in Figure 11, the r a t io of outflow to inflow indicates rapid

scour in the f i r s t year a f t e r dam removal. Without the Lower Granite Reservoir

impoundment, the ra t io indicates a decrease in the scour ra te a f t e r the f i r s t

year. The projection into the future shows continued b u t decreasing scour

w i t h equilibrium eventually being achieved. With the impoundment there occurs

a t rans i t ion from scour to deposition and then a decrease in the deposition

w i t h a tendency toward equi 1 i bri um,

19

FIG

URE

i 1

CALC

ULAT

ED R

ATIO

OF

SEDI

MEN

T

OUTF

LOW

TO

INFL

OW

WAS

HING

TON

POW

ER D

AM R

EM

OV

ED

The quantity of sediment deposition or scour within the model l imits i s

presented i n Figure 12. This plot shows tha t calculated volume changes were

as expected, The cumulative volume became negative (scour) a f t e r the dam was

removed and continued th i s trend f a i r l y uniformily f o r the case of no impound-

ment. With the impoundment, the graph shows an immediate increasing trend and

eventually became positive in 1979. Figure 10 shows tha t the predicted volume

of deposition a f t e r ten years with the WWPD in place and Lower Granite Reservoir

impounded i s 646 acre-feet. In Figure 12 the predicted volume of deposition

a f t e r ten .years, with the WWPD removed ar,d the Lower Granite Reservoir

impounded, i s 400 acre-feet. This indicates tha t the total e f fec t of the

dam removal a f t e r ten years i s the removal of 240 acre-feet of sediment from

the model 1 imi t s . Figures 13, 14, 15 and 16 show the observed par t ic le s i ze dis t r ibut ions

of the sediment on the stream bed before WWPD removal and the calculated

par t ic le s i ze dis t r ibut ion eight years a f t e r removal. Figure 13 shows the

bed becoming f iner a f t e r the dam removal. This i s expected because i t i s the

section closest to the Lower Granite Reservoir impoundment. Figure 14 shows

the section immediately downstream of the dam s i t e and exhibits a s l igh t ly

f i n e r par t ic le dis t r ibut ion a f t e r the dam removal. The most abrupt bed change

i s expected a t the dam s i t e . a s shown in Figure 15. Coarsening of the bed i s

expected due to the increased veloci t ies a f t e r dam removal. Figure 16 shows

very l i t t l e bed change because i t i s out of the influence of the former

WWPD pool,

A t the end of 10 years of flow through the model w i t h the dam removed,

the bed profi le was determined and a r t i f i c i a l flows were input to determine

the new water surface profi les . These new bed and water surface profiles are

presented i n Figure 17, Thalweg and average bed profi les show a lowering of

the bed immediately upstream of the dam s i t e and deposition in the scour hole

* N

EO

AT

IVE

V

OL

UM

E

IND

ICA

TE

S R

EM

OV

AL

O

F S

eD

lME

NT

F

RO

M T

NE

M

OD

EL

. T

IME

(in

yea

rs)

FIG

URE

12

CALC

ULAT

ED V

OLUM

E OF

SED

IMEN

TATI

ON

WAS

HING

TON

POblE

R DA

M R

EM

OV

ED

LL OQ:

W m z i - -0e

-2z W C Q : P L ' 3 . 4 3 m w EZd L L t

V) - U C U DCO

imedia te ly downstream of the dam s i t e . The influence of the Lower Granite

Reservoir impoundment can be seen by the higher elevation from River Mile

.67 t o 2.0.

V I , Sensi t ivi ty Tests

Several sens i t iv i ty t e s t s were made to determine the e f fec ts of i n p u t

changes on scour and deposition rates and eventual bed elevations. Changes

were made in each of the following: Manning's "nu , bed grain s i ze d-istribu-

t ions , cross-section distance.

Manning's "nu was changed from .03 to .024 which resulted in only a small

change in the scour and deposition rates . Resulting bed elevation change was

insignif icant ,

The bed par t ic le dis t r ibut ion upstream of the Washington Power Dam s i t e

was made f ine r by inputting a bed par t ic le dis t r ibut ion having a D50 of .1

millimeter, This resulted, as expected, in a s l igh t increase in the scour r a t e ,

The bed elevations ten years a f t e r the dam removal were about 20% lower than

with the original bed par t ic le dis t r ibut ion. The timing of the computed bed

elevations in Figure 6 could have approximated the measured elevations i f the

bed par t ic le dis t r ibut ions were made f iner . However, the dis t r ibut ions

required to do t h i s were s ignif icant ly different from any of the observed

dis t r ibut ions,

Additional cross-sections were inserted in the model with same geometry

as the immediate (measured) downstream cross-section. These sections were

inserted such tha t the model had a geometry section every 100 f e e t along the

r iver axis , This resulted in no appreciable change in scour o r deposition.

VI I . k n c l usions

The comparison of measured and computed final bed elevations, with the

dam removed, was very sat isfactory. Overall long range trends for each

operating condition was as expected. The cqlculated ra te of scour was

accurate a t the WWPD s i t e (River Mile 4.62) b u t lagged by approximately ten

months a t other upstream sections. This difference can be a t t r ibuted t o

local ized scour and "layering" of the bed par t ic le dis t r ibut ion. Neither

can be modeled by HEC-6.

Some o f the variations in the ra te of scour and deposition can be

at t r ibuted t o the l imitations of a one-dimensional model. Dam removal i s a

mu1 tidimensi onal phenomenon and would best be modeled using a two o r three

dimensional model. These types of models are limited by the amount and type

of data available, computer time/cost, and input requirements. If only the

1 ong range average bed elevation changes are desired, a one-dimensional model

would be suf f ic ien t . I f scour and deposition rates are of concern, a one-

dimensional model may not fu l ly simulate the physical occurrence and a two-

or three-dimensional mode1 may be needed. To the author's knowledge, dam

removal has not been modeled using a two- or three-dimensional model. Because

of t h i s , going t o a multidimensional model does not guarantee a s ignif icant

increase in accuracy.

Possible sources of errors were previously mentioned, I f these errors

were minimized by more accurate data and transport relationships, the one-

dimensional model may be suf f ic ien t ly accurate fo r the objectives of a sedi-

ment study, Many of the errors mentioned are also applicable to the two- and

three-dimensional models ; therefore, any errors from these sources would a1 so

occur i n these models. HEC-6 can be used confidently on dam removal studies

i f :

a, Bed par t ic le s ize dis t r ibut ions are available upstream and down-

stream of the dam,

5, Only long term scour and deposition rates are of concern.

c. Average bed elevation changes are desired.

d. No unsteady flow phenomena occurs.

e , Cohesive sediment i s insignif icant .

VI I I , Recommendations

The r e su l t s of the usage of a one-dimensional model for predicting the

effects of dam removal were very encouraging. Further study should be made

with a one-dimensional model w i t h the use of measured bed par t ic le s i ze

dis t r ibut ions a t each cross-section, Another case study should be made with

bed measurements upstream and downstream of the dam s i t e made before and

a f t e r dam removal, This would help to determine the appl icabi 1 i ty of a one-

dimensional model and the expected errors due to the 1 imitations of such a

model, The resu l t s should then be compared with the resul ts of a two- or

three-dimensional mode1 to determine i f accuracy i s increased enough to

warrant the use of a multidimensional model and i t s associated costs and input

requirements.

Sensi t ivi ty t e s t s have indicated tha t bed elevations were very sensi t ive

to bed par t ic le s i ze dis t r ibut ions. I t i s recommended that thi.s type o f

measurement be made a t each cross-section in subsequent studies.

I t i s recommended that when dam removal studies a re made using a one- - 3

dimensional model , calculated scour rates be closely examined and interpreted

with consideration of the effects of secondary currents, localized scour, and

"1 ayered" bed par t ic le s i ze dis t r ibut ion.

APPENDIX I REFERENCES

Emmett, W .W, , and Sei t z , H, R. , "Suspended and Bed1 oad-Sediment Transpor t i n t h e Snake and Clearwater Rivers i n the V i c i n i t y o f Lewiston, Idaho," Data sumnary o f J u l y 1973 through June 1974, U.S, Geological Survey. Emmett, W. W. , and Se i t z , H. R. , "Suspended and Bedload-Sediment Transpor t i n the Snake and Clearwater R ivers i n the V i c i n i t y o f Lewiston, Idaho," Data summary of March 1972 through June 1973, U.S. Geological Survey. Guy, H.P., and Norman, V.W., " F i e l d Methods f o r Measurement of F l u v i a l Sediment ," Techniques of Water Resources Inven to ry o f t he U.S. Geological Survey, Book 3, Chapter C2, 1970, pp. 59, Hel ley, E,J,, and Smith, W, , "Development and C a l i b r a t i o n o f a Pressure- Dif ference Bedload Sampler ," U.S. ~ e b l o g i c a l Survey open- f i l e r e p o r t , 1971, pp. 18. Sei t z , H, R. , "Suspended and Bed1 oad-Sediment Transport i n t h e Snake and Clearwater Rivers i n t h e V i c i n i t y o f Lewiston, Idaho," Data summary o f August 1975 through J u l y 1976, U.S. Geological Survey. Se i t z , H.R., "Suspended and Bedload-Sediment Transport i n t h e Snake and Clearwater R ivers i n t he V i c i n i t y o f Lewiston, Idaho," Data s u m a r y o f August 1974 throush Jut v 1975. surface Water ~ u p i l y o f t h e Un i ted Sta tes , U.S. Geological Survey Water Supply Paper 2134, P o i n t 13, Snake R ive r Basin, 1966-1970. Thomas, W.A., and Prasuhn, A.L, "Mathematical Modelina o f Scour and ~ e p o s i t i o n , " - ~ o u r n a l of t h e ~ y d r a u l i c s D i v i s i o n , A S C E ~ Vol. 103, No. HY8, August 1977, pp. 851-863. Thomas, W.A. , "Scour and Deposi t ion i n R ivers and Reservoirs ," HEC-6 Users Manual of General ized Computer Program 723-G2-L2470, Hydro log ic Engineering Center, U.S. Army Corps of Engineers, March 1976. T o f f a l e t i , F.B., "A Procedure f o r Computation o f To ta l R i ve r Sand Discharge and D e t a i l e d D i s t r i b u t i o n , Bed t o Sur face ," Committee on Channel S t a b i l i- zat ion , U.S. Army Corps o f Engineers, November 1968.

Technical Paper Series TP-1 Use of Interrelated Records to Simulate Streamflow TP-2 Optimization Techniques for Hydrologic

Engineering TP-3 Methods of Determination of Safe Yield and

Compensation Water from Storage Reservoirs TP-4 Functional Evaluation of a Water Resources System TP-5 Streamflow Synthesis for Ungaged Rivers TP-6 Simulation of Daily Streamflow TP-7 Pilot Study for Storage Requirements for Low Flow

Augmentation TP-8 Worth of Streamflow Data for Project Design - A

Pilot Study TP-9 Economic Evaluation of Reservoir System

Accomplishments TP-10 Hydrologic Simulation in Water-Yield Analysis TP-11 Survey of Programs for Water Surface Profiles TP-12 Hypothetical Flood Computation for a Stream

System TP-13 Maximum Utilization of Scarce Data in Hydrologic

Design TP-14 Techniques for Evaluating Long-Tem Reservoir

Yields TP-15 Hydrostatistics - Principles of Application TP-16 A Hydrologic Water Resource System Modeling

Techniques TP-17 Hydrologic Engineering Techniques for Regional

Water Resources Planning TP-18 Estimating Monthly Streamflows Within a Region TP-19 Suspended Sediment Discharge in Streams TP-20 Computer Determination of Flow Through Bridges TP-21 An Approach to Reservoir Temperature Analysis TP-22 A Finite Difference Methods of Analyzing Liquid

Flow in Variably Saturated Porous Media TP-23 Uses of Simulation in River Basin Planning TP-24 Hydroelectric Power Analysis in Reservoir Systems TP-25 Status of Water Resource System Analysis TP-26 System Relationships for Panama Canal Water

Supply TP-27 System Analysis of the Panama Canal Water

Supply TP-28 Digital Simulation of an Existing Water Resources

System TP-29 Computer Application in Continuing Education TP-30 Drought Severity and Water Supply Dependability TP-31 Development of System Operation Rules for an

Existing System by Simulation TP-32 Alternative Approaches to Water Resources System

Simulation TP-33 System Simulation of Integrated Use of

Hydroelectric and Thermal Power Generation TP-34 Optimizing flood Control Allocation for a

Multipurpose Reservoir TP-35 Computer Models for Rainfall-Runoff and River

Hydraulic Analysis TP-36 Evaluation of Drought Effects at Lake Atitlan TP-37 Downstream Effects of the Levee Overtopping at

Wilkes-Barre, PA, During Tropical Storm Agnes TP-38 Water Quality Evaluation of Aquatic Systems

TP-39 A Method for Analyzing Effects of Dam Failures in Design Studies

TP-40 Storm Drainage and Urban Region Flood Control Planning

TP-41 HEC-5C, A Simulation Model for System Formulation and Evaluation

TP-42 Optimal Sizing of Urban Flood Control Systems TP-43 Hydrologic and Economic Simulation of Flood

Control Aspects of Water Resources Systems TP-44 Sizing Flood Control Reservoir Systems by System

Analysis TP-45 Techniques for Real-Time Operation of Flood

Control Reservoirs in the Merrimack River Basin TP-46 Spatial Data Analysis of Nonstructural Measures TP-47 Comprehensive Flood Plain Studies Using Spatial

Data Management Techniques TP-48 Direct Runoff Hydrograph Parameters Versus

Urbanization TP-49 Experience of HEC in Disseminating Information

on Hydrological Models TP-50 Effects of Dam Removal: An Approach to

Sedimentation TP-51 Design of Flood Control Improvements by Systems

Analysis: A Case Study TP-52 Potential Use of Digital Computer Ground Water

Models TP-53 Development of Generalized Free Surface Flow

Models Using Finite Element Techniques TP-54 Adjustment of Peak Discharge Rates for

Urbanization TP-55 The Development and Servicing of Spatial Data

Management Techniques in the Corps of Engineers TP-56 Experiences of the Hydrologic Engineering Center

in Maintaining Widely Used Hydrologic and Water Resource Computer Models

TP-57 Flood Damage Assessments Using Spatial Data Management Techniques

TP-58 A Model for Evaluating Runoff-Quality in Metropolitan Master Planning

TP-59 Testing of Several Runoff Models on an Urban Watershed

TP-60 Operational Simulation of a Reservoir System with Pumped Storage

TP-61 Technical Factors in Small Hydropower Planning TP-62 Flood Hydrograph and Peak Flow Frequency

Analysis TP-63 HEC Contribution to Reservoir System Operation TP-64 Determining Peak-Discharge Frequencies in an

Urbanizing Watershed: A Case Study TP-65 Feasibility Analysis in Small Hydropower Planning TP-66 Reservoir Storage Determination by Computer

Simulation of Flood Control and Conservation Systems

TP-67 Hydrologic Land Use Classification Using LANDSAT

TP-68 Interactive Nonstructural Flood-Control Planning TP-69 Critical Water Surface by Minimum Specific

Energy Using the Parabolic Method

TP-70 Corps of Engineers Experience with Automatic Calibration of a Precipitation-Runoff Model

TP-71 Determination of Land Use from Satellite Imagery for Input to Hydrologic Models

TP-72 Application of the Finite Element Method to Vertically Stratified Hydrodynamic Flow and Water Quality

TP-73 Flood Mitigation Planning Using HEC-SAM TP-74 Hydrographs by Single Linear Reservoir Model TP-75 HEC Activities in Reservoir Analysis TP-76 Institutional Support of Water Resource Models TP-77 Investigation of Soil Conservation Service Urban

Hydrology Techniques TP-78 Potential for Increasing the Output of Existing

Hydroelectric Plants TP-79 Potential Energy and Capacity Gains from Flood

Control Storage Reallocation at Existing U.S. Hydropower Reservoirs

TP-80 Use of Non-Sequential Techniques in the Analysis of Power Potential at Storage Projects

TP-81 Data Management Systems of Water Resources Planning

TP-82 The New HEC-1 Flood Hydrograph Package TP-83 River and Reservoir Systems Water Quality

Modeling Capability TP-84 Generalized Real-Time Flood Control System

Model TP-85 Operation Policy Analysis: Sam Rayburn

Reservoir TP-86 Training the Practitioner: The Hydrologic

Engineering Center Program TP-87 Documentation Needs for Water Resources Models TP-88 Reservoir System Regulation for Water Quality

Control TP-89 A Software System to Aid in Making Real-Time

Water Control Decisions TP-90 Calibration, Verification and Application of a Two-

Dimensional Flow Model TP-91 HEC Software Development and Support TP-92 Hydrologic Engineering Center Planning Models TP-93 Flood Routing Through a Flat, Complex Flood

Plain Using a One-Dimensional Unsteady Flow Computer Program

TP-94 Dredged-Material Disposal Management Model TP-95 Infiltration and Soil Moisture Redistribution in

HEC-1 TP-96 The Hydrologic Engineering Center Experience in

Nonstructural Planning TP-97 Prediction of the Effects of a Flood Control Project

on a Meandering Stream TP-98 Evolution in Computer Programs Causes Evolution

in Training Needs: The Hydrologic Engineering Center Experience

TP-99 Reservoir System Analysis for Water Quality TP-100 Probable Maximum Flood Estimation - Eastern

United States TP-101 Use of Computer Program HEC-5 for Water Supply

Analysis TP-102 Role of Calibration in the Application of HEC-6 TP-103 Engineering and Economic Considerations in

Formulating TP-104 Modeling Water Resources Systems for Water

Quality

TP-105 Use of a Two-Dimensional Flow Model to Quantify Aquatic Habitat

TP-106 Flood-Runoff Forecasting with HEC-1F TP-107 Dredged-Material Disposal System Capacity

Expansion TP-108 Role of Small Computers in Two-Dimensional

Flow Modeling TP-109 One-Dimensional Model for Mud Flows TP-110 Subdivision Froude Number TP-111 HEC-5Q: System Water Quality Modeling TP-112 New Developments in HEC Programs for Flood

Control TP-113 Modeling and Managing Water Resource Systems

for Water Quality TP-114 Accuracy of Computer Water Surface Profiles -

Executive Summary TP-115 Application of Spatial-Data Management

Techniques in Corps Planning TP-116 The HEC's Activities in Watershed Modeling TP-117 HEC-1 and HEC-2 Applications on the

Microcomputer TP-118 Real-Time Snow Simulation Model for the

Monongahela River Basin TP-119 Multi-Purpose, Multi-Reservoir Simulation on a PC TP-120 Technology Transfer of Corps' Hydrologic Models TP-121 Development, Calibration and Application of

Runoff Forecasting Models for the Allegheny River Basin

TP-122 The Estimation of Rainfall for Flood Forecasting Using Radar and Rain Gage Data

TP-123 Developing and Managing a Comprehensive Reservoir Analysis Model

TP-124 Review of U.S. Army corps of Engineering Involvement With Alluvial Fan Flooding Problems

TP-125 An Integrated Software Package for Flood Damage Analysis

TP-126 The Value and Depreciation of Existing Facilities: The Case of Reservoirs

TP-127 Floodplain-Management Plan Enumeration TP-128 Two-Dimensional Floodplain Modeling TP-129 Status and New Capabilities of Computer Program

HEC-6: "Scour and Deposition in Rivers and Reservoirs"

TP-130 Estimating Sediment Delivery and Yield on Alluvial Fans

TP-131 Hydrologic Aspects of Flood Warning - Preparedness Programs

TP-132 Twenty-five Years of Developing, Distributing, and Supporting Hydrologic Engineering Computer Programs

TP-133 Predicting Deposition Patterns in Small Basins TP-134 Annual Extreme Lake Elevations by Total

Probability Theorem TP-135 A Muskingum-Cunge Channel Flow Routing

Method for Drainage Networks TP-136 Prescriptive Reservoir System Analysis Model -

Missouri River System Application TP-137 A Generalized Simulation Model for Reservoir

System Analysis TP-138 The HEC NexGen Software Development Project TP-139 Issues for Applications Developers TP-140 HEC-2 Water Surface Profiles Program TP-141 HEC Models for Urban Hydrologic Analysis

TP-142 Systems Analysis Applications at the Hydrologic Engineering Center

TP-143 Runoff Prediction Uncertainty for Ungauged Agricultural Watersheds

TP-144 Review of GIS Applications in Hydrologic Modeling

TP-145 Application of Rainfall-Runoff Simulation for Flood Forecasting

TP-146 Application of the HEC Prescriptive Reservoir Model in the Columbia River Systems

TP-147 HEC River Analysis System (HEC-RAS) TP-148 HEC-6: Reservoir Sediment Control Applications TP-149 The Hydrologic Modeling System (HEC-HMS):

Design and Development Issues TP-150 The HEC Hydrologic Modeling System TP-151 Bridge Hydraulic Analysis with HEC-RAS TP-152 Use of Land Surface Erosion Techniques with

Stream Channel Sediment Models

TP-153 Risk-Based Analysis for Corps Flood Project Studies - A Status Report

TP-154 Modeling Water-Resource Systems for Water Quality Management

TP-155 Runoff simulation Using Radar Rainfall Data TP-156 Status of HEC Next Generation Software

Development TP-157 Unsteady Flow Model for Forecasting Missouri and

Mississippi Rivers TP-158 Corps Water Management System (CWMS) TP-159 Some History and Hydrology of the Panama Canal TP-160 Application of Risk-Based Analysis to Planning

Reservoir and Levee Flood Damage Reduction Systems

TP-161 Corps Water Management System - Capabilities and Implementation Status