1-11 () FP - P13 

HYDRAULIC EVALUATION OF

TRAVELING BELT FISH SCREENS AT WEEKS FALLS

Donald P. Jarrett, P.

Sr. Mechanical Engineer

Hosey & Associates Engineering Co.

Fred Winchell

Fisheries Biologist

Hosey & Associates Engineering Co.

INTRODUCTION

This report presents the results of monitoring studies conducted to assess performance of

the fish screening facility at the Weeks Falls Hydroelectric Project. The FERC License required

monitoring and consultation with the involved resource agencies. The report presents field

velocity measurements made on two ~erent occasions. The first set of data was collected 

May 12- 13, 1988. The second measurement was made on November 3, 1988 following

modifications to the facility. On both occasions t.,e river flows were in excess of the maximum

capacity of the turbine. On both occasions sweeping and facing velocities were measured at

several depths.

FACILITY DESCRIPTION

The Weeks Falls Hydroelectric Project is located on the South Fork of the Snoqualmie

River, King County, Washington State. The project is a 5 MW run-of-river facility with a

maximum diversion of 750 cfs. The intake has 14 traveling belt screens arranged in two banks

of screen on each side (Figure 1). Water passes into the facility through a trashrack located at

the head of the structure, moves along two lateral channels, passes through the screens into a

central channel and enters into the power shaft. The width of the lateral channels taper down to

maintain a uniform sweeping velocity. Two fish bypass channels are located at the downstream

end of the lateral channels. The bypass channels join together and discharge into the river

downstream of the facility.

Each of the traveling belt screens is 8 feet wide and 20 feet tall. The water surface

elevation is normally maintained about one foot below the top of the screens. The effective

surface area of each screen is 125.7 square feet, excluding area obstructed by seals or above the

normal water surface elevation. The downstream two screens have surface areas which are
further reduced due to a ramp which slopes up to dle fish bypass channel. The effective

screening area of dlese screens are 103.2 and 56.2 square feet.

The facility has been in operation since May of 1987. During the fIrSt season of

operation a control system error allowed the screen motors to stop while there was a significant

debris influx. The screens were damaged by the resulting differential water level before the

turbine could be shut-down. Repairs were made in less than 3 days. The only other problem

was associated with accumulation of small debris at the downstream end of the lateral channels.

This accumulation resulted in the fish bypass orifices becoming plugged, which greatly reduced

the effectiveness of the fish bypass. As described below, the orifices were eliminated in

September 1988 and replaced with weirs to take care of the problem.

METHODS

Water velocities were measured using a Swoffer Model 210 current meter.

Measurements were made on the front side of the screens using a sensor assembly attached

directly to the screen face. The assembly was designed to hold one sensor perpendicular to the

screen face and another sensor parallel to the screen. The sensor assembly was lowered to the

measurement position by rotating the screen backwards.

Sweeping and facing velocities were measured at nine positions on the face of each

screen. Measurements were made at 0. , 0.5 and 0.8 of the total depth. At each depth,

measurements were made at 1 4 and 7 feet from the upstream edge of the screen.

The velocity of flow passing t.Ju'ough t.~e back side of the screens was measured two feet

below the water surface at three positions located 1.3, 4 and 6.7 feet from the upstream edge of

me screens. These measurements were made wit., I.,e sensor attached to a hand-held rod. The

fIXed assembly could not be used on the backside of the screens due to structural interferences.

Due to the force from the flow in the central channel, it was not possible to collect velocity

measurements at depths greater than a few feet.

RESULTS OF INITIAL MEASUREMENTS

Sweeping Velocities

Sweeping velocities measured at screens 1-7 generally ranged from 3 to 4.5 feet per

second. These rates varied little between the upper and mid-depth measurement positions

(Figure 2). Sweeping velocities generally diminished in a downstream direction across I.,e face

of each screen, but remained fairly consistent between screens.

Sweeping velocities measured on screens 8- 14 were more variable and somewhat lower
than those measured on the other side (Figure 3). Past measurements have indicated that the

total flow volume is greater on the left bank of screens than on the right, which may account for

the overall difference in sweeping velocities.

Facing Velocities

Facing velocities measured on screens 1-7 were consistently low (under 0.1 ft/sec), with

the exception of screens 1 and 7 (Figure 4). Screen 1 measurements may have been affected by

turbulence observed below the trashrack. Screen 7 had facing velocity in excess of the 0.5 ft/sec

state criteria.

Facing velocities measured on screens 8- 14 were generally higher than those measured

on screens 1-7. Screens 8 and 14 had facing velocities in excess of 0.5 ft/sec (Figure 5).

Velocities Measured Behind the Screens

On-site review of the data indicated that facing velocities measured were too low to

account for the volume passing tt'lrough dle screens. Furt.,ermore, visual inspection of I.,e flow

patterns indicated that more water was passing through the downstream end screens than the

upstteam end screens. It appeared that the measurement assembly was biasing the measurements

in some manner. Velocity data was collected behind each screen in order to provide a backup

estimate of t.~e flow t.~ough each screen so that average facing velocities could be calculated.

The velocities measured behind each screen showed a consistent increase across the face

of each screen and between screens in the downstream direction (Figure 6). Average facing

velocities for each screen were calculated as the average of the three measurements obtained.

The total calculated volume passing through the screen was computed as 660 cfs, as opposed to

the actual of 750 cfs. Since the calculated flow is reasonably close to the actual, it appears that

the velocities measured behind the screens are representative of the actual facing velocities.

RESULTS OF MEASUREMENTS FOLLOWING MODIFICATIONS

Modifications to the Facility

While review of the initial measurements indicated a need to re-distribute flow to

cti:.o

upstream screens, the facility had1been experiencing problems with small debris. The original ,

V\iVt/ ( (r, -IP-yJ,LJ j,

design had orifice plates which regulated the water into the fish bypass channels) small debris.

tended to clog the orifices-. Furthermore, it was believed that an increase in fish bypass flow

would increase the sweeping flow on the downstream screens. The facility was modified such

that flow to the fish bypass channel is provided by a weir and the flow was increased from 13 to

25 cfs.
Swee Velocities

Sweeping velocities were measured at 1 , 4 and 7 feet from the upstream edge of screens

6 and 7. Sweeping velocities measured 3.8 feet below the water surface were very similar to

those obtained prior to I.l:1e modifications (Figure 7). An area with low sweeping velocity is

apparent at the downstream end of the downstream screen in both data sets. Some difficulties

were encountered with leaves and other debris accumulating on the velocity sensors while

measurements were being obtained.

Facing Velocities

Facing velocities measured in front of screens 6 and 7 were more evenly distributed than

t.~ose measured prior to modification of I.,e bypass channel (Figure 8). Excessive facing

velocities appeared to still be present at screen 7.

Velocities Measured Behind the Screens

Velocities were measured behind each of t.~e 14 screens at 3 points perpendicular to the

screen; 1.3, 4 and 6.7 feet from its upstream edge. Each measurement was taken 2 feet below the

water surface. The total flow passing through each screen was estimated using the average

velocity measured at I.l:1ese t'1ree locations. The data showed a relatively high volume passing

through screens located in the downstream end of the facility (Figure 9 and 10). The

modifications appear to have moderated the amount of flow passing through screens 7 and 14.

At the time that the measurements were taken screen 12 was not operating due to a

broken gearbox. This screen had become partially clogged with leaves as evidenced by the

relatively low volume passing through the screen. The increased resistance to flow through the

screen appears to have increased the flow I.'1rough screens 10 and 11. It is expected that baffles

would have a similar effect.

CONCLUSION

Measurements made to date indicate that I.l:1e facility, following minor modifications, will

perfonn generally in accordance with state screening criteria. The overall performance of the

facility has been good, with the major problem associated with debris management.

Baffles will be installed on screens 3-7 and 10- 14 during the summer of 1989.

Measurements will be made during November 1989 to determine the effectiveness of the baffles

and to detennine if further adjustments will be required.

June 13, 1989
.. 

FLOW 

r::J bJ r::: r:J

CENmAL 

r~ J (~"l".lN' r~ j r~ 

PLAN VI EW

NORMAL WSEL

ASH EXIT

SECTION

. ,.

Weeks Falls Hydroelectric Project

Fish Screening Facility

Figure 1
DIST. FROM TRASHRACK (FEET)

DEPTH .. 3.8 FEET DEPTH .. 9.5 FEET

DEPTH .. 15.2 FEET

Weeks Falls Hydroelectric Project

Sweeping Velocities Measured at Screens 1-

Figure 2
;:.

DEPTH.. 3.8 FEET

3.4

2.8

D!ST. FROM TFtASHRACK (FEET)

DEPTH.. 9.5 FEET

DEPTH.. 15.2 FEET

Weeks Falls Hydroelectric Project
;:..

DEPTH a 3.8 FEET

DIST. FROM TRASH RACK (FEET)

DEPTH.. 9.5 FEET

DEPTH - 15.2 FEET

Weeks Falls Hydroelectric Project
....j ::

DEPTI-I .. 3.8 FEET

Weeks Falls Hydroelectric Project

DIST. FROM TRASH RACK (FEET)

DEPTH .. 9. 5 FEET

DEPTH .. 15.2 FEET

Facing Velocities Measured at Screens 8-

Figure 5
-oJ

;::.

2.0

1.2

DISTANCE FROM TRASH RACK (FEET)

SCREENS 1- SCREENS 8-

Weeks Faf/s Hydroelectric Project

Facing Velocities Measured Behind the Screens

Figure 6
3.0

0.5

0.0

Screen #6

411

3.8 FT, MAY 88

3.8 FT. HeN 

Screen #7

DIST. FROM TRASHRACK (FEET)

8.5 FT. MAY 88

8.5 FT. HeN 

Sweeping Velocities Measured In Front of Screens 6 & 

Before and After Bypass Modification

Weeks Falls Hydroelectric Project Figure 7
;:.

0';

0.8

0.8

0.3

0.2

3.8 FT. MAY 88

3.8 FT, NCN 88

Weeks Falls Hydroelectric Project

Screen #6 Screen #7

DIST. FROM TRASHRACK (FEET)

-+- 115 FT. MAY 88 6. 115 FT HCN 

Facing Velocities Measured in Front of Screens 6 & 7

Before and After Bypass Modification

Figure 8
110

100

:I:

I/)

IS::! MA Y 1 888

SCREEN NUMBER

!22:d NOV. 1988

Weeks Fal/s Hydroelectric Project
110

'00

110

IS:! MAY 1881!

SCREEN M-'EER

NOV. 11188

Estimated Volume of Flow Passing Through Screens 8-14

Before and After bypass Modification

Weeks Falls Hydroelectric Project Figure 10