Hydraulic characteristics by weir type in a pool-weir fishway

Ecological Engineering 16 (2001) 425–433

Hydraulic characteristics by weir type in a pool-weir fishway

J.H. Kim *Department of Ci6il Engineering, Chung-Ang Uni6ersity, San 40-1 Nae-Ri, Daeduck-myun, Anseong, Kyeonggi-do,

456-756, South Korea

Accepted 12 July 2000

Abstract

This study deals with hydraulic characteristics for different weir types and effects on upstream migration of fish ina pool-weir fishway, and presents an optimal design of weir for an easy upstream migration. Experiments wereperformed to estimate hydraulic conditions for different weir types to determine the overall performance of the weirtested. The results showed that a rectangular weir with a notch installed in a straight configuration was preferable toone in a zigzag configuration or to a trapezoidal weir, since it made the flow stable and created a resting place forfish in the pool and thus made it possible for the upstream migration of fish. © 2001 Elsevier Science B.V. All rightsreserved.

Keywords: Pool-weir fishway; Upstream migration; Notch; Weir; Straight type; Zigzag type

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1. Introduction

At a weir or a dam, a fishway is installed toallow the upstream migration of fish. In manycountries, if a hydraulic structure poses potentialproblems for the passage of fish, a fishway isrequired by law (Hirose and Nakamura, 1991).For fish having strong swimming and jumpingability such as ayu and salmon, a pool-weirfishway is installed (Nakamura, 1995). The pool-weir fishway consists of successive weirs and poolsas shown in Fig. 1. It is called a fish ladder insome countries (Nakamura and Yotsukura, 1987).

A slightly different shape of weir can alter the

flow conditions in the fishway and hence can havean influence on the upstream migration of fish. Ifthe flow velocity over the weir exceeds the criticalvelocity for upstream migration, fish cannot moveagainst the flow direction. The occurrence of tur-bulent jets, eddies and circulating flow preventsfish from moving upstream (Kim, 1994; Kim andKim, 1994).

So, it is important to determine an optimal typeof weir for fish to migrate upstream easily withoutconsuming time and without being fatigued. Hy-draulic experiments on a fishway are very usefulfor estimating complicated flow phenomena suchas turbulent jets, eddies and circulation and fordetermining the optimal type of weir (Hirose andNakamura, 1991). The objective of this study is toexamine flow conditions and their effects on up-

* Tel.: +82-334-6703355; fax: +82-334-6751387.E-mail address: [email protected] (J.H. Kim).

0925-8574/00/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.

PII: S0925-8574(00)00125-7

J.H. Kim / Ecological Engineering 16 (2001) 425–433426

Fig. 1. The pool-weir fishway.

scale was attached to the side wall to see the flowfeatures well. A thick screen was laid at theentrance of the fishway model to reduce turbu-lence. The discharge which was controlled by avalve in a feed-back loop could be measured witha V-notch at the upstream end. The fishwaymodel was 30 cm wide, 30 cm long and on a slope1:10. The scale of the models was 1/10. Threefishway models were used in the experiment. Onewas a trapezoidal weir, another was a rectangularweir with a straight arrangement of the notch andthe orifice and the third was a rectangular weirwith a zigzag arrangement of the notch and theweir. The height of the trapezoidal and the rectan-gular weir was 8 cm or 9 cm at the end. Beneaththe weir, a rectangular orifice, of which the sizewas 4.0 cm×4.0 cm, was made for the purpose ofremoving deposited bed material in the pool andfor upstream migration of an eel.

Flow velocity was measured by using a two-di-mensional acoustic doppler velocity meter. Veloc-ity range was −1.0–1.0 m/s, respectively. Tocheck the flow pattern, dye injection and a videocamera with a strong light were used. Instanta-neous flow pattern was resolved using aluminumflakes with a neutral detergent illuminated by avertical light sheet using a slide projector in darkconditions (Kim and Kim, 1994).

Table 1 shows the experimental conditions inthe trapezoidal weir with an orifice in a plungingflow and a streaming flow. Here, the overflowdepth and the overflow velocity were the values

stream migration of fish for the existing typicalweir types in a pool-weir fishway and to provide,through hydraulic experiments, an optimal shapeof the weir for an easy upstream migration of fish.

2. Experimental set up and measurements

The experiments were performed to examinethe flow conditions of several fishway models. Fig.2 shows the experimental arrangement. The typi-cal fishway model made of a waterproof plywoodwas installed in a recirculatory tilting flume 0.3 mwide, 0.4 m deep and 20 m long. The side wall ofthe flume was made of glass and a transparent

Fig. 2. Experimental arrangement.

J.H. Kim / Ecological Engineering 16 (2001) 425–433 427

Table 1Experimental conditions

Overflow velocity (m/s) Orifice velocity (m/s)Overflow depth Discharge (m3/s per m)RN Remark(m)

1 0.014 0.62 1.11 0.01 Plunging flow0.69 1.100.020 0.012 Plunging flow

0.0253 0.68 1.08 0.02 Plunging flow0.72 1.104 0.020.030 Plunging flow0.71 1.140.035 0.035 Plunging flow0.74 1.156 0.030.040 Plunging flow0.72 1.120.045 0.047 Plunging flow

0.0478 0.76 1.05 0.04 Plunging flow

1.44 1.211 0.300.200 Streaming flow2 0.185 1.42 1.20 0.28 Streaming flow3 0.170 1.32 1.20 0.25 Streaming flow

1.20 1.190.165 0.224 Streaming flow1.21 1.155 0.200.150 Streaming flow1.20 1.020.140 0.186 Streaming flow

0.1357 0.98 0.95 0.13 Streaming flow0.78 0.760.120 0.108 Streaming flow

0.1109 0.76 0.74 0.08 Streaming flow0.10010 0.75 0.74 0.07 Streaming flow

Fig. 3. Plunging flow in a trapezoidal weir. (a) Orifice installed beneath the front weir. (b) Orifice not installed beneath the frontweir.

measured at the middle section of the trapezoidalweir. The orifice velocity was defined as the veloc-ity measured in front of the orifice and the dis-charge was defined as the discharge per unitwidth. These values were scaled to the equivalentprototype values.

3. Flow characteristics in a trapezoidal weir

The flow in the pool-weir fishway changes froma plunging flow to a streaming flow with anincrease in discharge (Hirose and Nakamura,1991). A transition zone exists between these two

flows, where the two flows occur alternately. Ac-cording to Katopodis (1987), these two flows aredetermined by the following dimensionless flowrate equation:

Q. =Q/(bSL3/2g1/2) (1)

where Q. is the dimensionless flow rate, Q is theflow discharge, b is the width of the fishway, S isthe slope of the fishway, L is the length of thepool, and g is the gravitational acceleration. Ka-topodis (1987) found through experiments thatthe transition zone existed at Q. $0.25. In thisstudy, transition occurred between Q. =0.20 and0.27, which agreed well with the suggested valueof Katopodis (1987).


Fig. 3 shows a plunging flow at Q. =0.15 in thepool in a trapezoidal weir. In Fig. 3(a), where arectangular orifice was installed beneath the frontweir, the flow passes out of the orifice in the formof a turbulent jet whose velocity was about 1.1 m/s(converted prototype value), which was in excess ofa critical swimming velocity for an eel. Here, thecritical swimming velocity is defined as the maxi-mum velocity when a fish can migrate upstreamwithout being pushed out by the flow, which isknown to be about 1.6 m/s for an ayu and 0.5 m/sfor an eel (Yu, 1986; Katopodis, 1987; Kim, 1993).It can be seen that the orifice, which originally wasmade for the upstream migration of an eel, was noteffective because it produced a turbulent jet andeddies with a velocity exceeding the critical flowvelocity. The flow separated at point A and re-es-tablished at point B, forming a vertical circulating

flow. In Fig. 3(b), where the orifice was not installedbeneath the front weir, a large vertical circulatingflow also existed, but its area was larger than thatin Fig. 3(a). There was no orifice and hence noturbulent jet. A smaller circulating flow near theupstream corner was generated with flow directionopposite to the larger one.

Fig. 4 shows the streaming flow at Q. =0.39. InFig. 4(a), where the rectangular orifice was locatedbeneath the front weir, a turbulent jet resulted withits velocity reaching 1.2 m/s, which was not suitablefor the upstream migration of an eel. A verticallycirculating flow could be seen but its direction wasopposite to that in Fig. 3(a). In Fig. 4(b), where theorifice was not located beneath the front weir, acirculating flow larger than that in Fig. 4(a) wasproduced which covered the whole section of thepool.

Fig. 4. Streaming flow in a trapezoidal weir. (a) Orifice installed beneath the front weir. (b) Orifice not installed beneath the frontweir.

Fig. 5. Three-dimensional schematic flow in a trapezoidal weir.


Fig. 6. Two types of rectangular weir with rectangular notches. (a) Straight type. (b) Zigzag type.

A three-dimensional schematic of streamingflow at Q. =0.39 is shown in Fig. 5. The flowvelocity and flow pattern were measured by usinga two-dimensional acoustic doppler velocity me-ter, dye injection and a video camera with stronglight. Here an arrow gives the flow direction andits thickness gives the magnitude. Large flow oc-curred over the lower part of the trapezoidal weir.Its maximum velocity was about 1.44 m/s, whichdid not exceed the critical velocity for fish movingupstream such as ayu, perch and salmon. Sincethe lower part of the successive trapezoidal weirwas installed reciprocally to the adjacent weir, thelarge flow formed an S-shape. There occurredlarge eddies throughout the pool and hence therewas no resting place for fish. A turbulent jet alsooccurred from the orifice.

4. Flow characteristics in a rectangular weir withnotches

A rectangular weir with a notch was furtherclassified into two types by the location of thenotch. It was either a straight or a zigzag typerelative to an adjacent weir as shown in Fig. 6.The width of the notch was 10 cm, one-third ofthat of the weir, and the depth of the notch was2.0 cm. The length between weirs was 30 cm andthe slope was 1/10. The small rectangular orifice,of which the size is 4.0 cm×4.0 cm, was madebeneath the weir. The location of the orifice wasreciprocal to that in the adjacent weir in thezigzag type, but it was one-sided in the straighttype.

4.1. Rectangular weir with notches in zigzag type

Fig. 7 shows the three-dimensional schematicflow characteristics in a plunging flow at Q. =0.15.Primary flow occurred through the alternatingnotches forming an S-shape. Its velocity wasabout 0.7 m/s on the notch and 0.5 m/s in thepool. Secondary horizontal circulating flow oc-curred at the water surface due to flow separationbehind the weir side and its velocity was about 0.3m/s. Fig. 8 is the plan view of the plunging flow.

Fig. 7. Three-dimensional schematic flow in a rectangular weir.

Fig. 8. Plunging flow in a rectangular weir.


Fig. 9. Values of discharge coefficient with relative depth(plunging flow).

horizontal circulating flow occurred behind theweir. The direction of the circulating flow wasreciprocal to that in an adjacent pool.

The discharge equation in a plunging flow canbe expressed as follows:

Qp=Qpw+Qpn=Cpwbpwhpw3/2g1/2+Cpnbpnhpn

3/2g1/2

(2)

where, C is the discharge coefficient, b is a widthand h is a overflow depth. Subscript pw means theweir side and pn means the notch side in aplunging flow. So, Cpw means the discharge coeffi-cient at the weir side and Cpn means that at thenotch side in a plunging flow, and so on. Qpw andQpn are the discharge at the weir side and at thenotch side, respectively, and QP is the total dis-charge. The discharge coefficient is known to beabout 0.61 in the case of the simple rectangularweir with no notch (Katopodis, 1987).

Fig. 9 shows the values of the discharge coeffi-cient, Cpw and Cpn with relative depth, hpw/H andhpn/H, where H is the height at the weir side.Values of discharge coefficient were 0.46–0.71and increased with an increase in relative depth.Their mean value was about 0.57, which wasalmost the same as the value of Katopodis (1987).Values of the discharge coefficient at the weir sideand at the notch side were almost the same at thesame water level, the reason for which seemed tobe that flow velocity became large with an in-crease in overflow depth. Values of dischargecoefficients varied widely and some of their distri-butions were non-linear, the reason for whichseemed to be that the primary flow and the circu-lating flow occurred simultaneously due to flowseparation. Moreover, the position and directionof the circulating flow became reciprocal to thatin an adjacent pool and hence the flow becameunstable.

Figs. 10 and 11 show the flow characteristics ina streaming flow over the notch side at Q. =0.39.Primary streaming flow occurred through the al-ternating notch, forming an S-shape, which wasthe same result as that in a plunging flow in Fig.7. Its maximum velocity was about 1.4 m/s on thenotch and 0.7 m/s in the pool, which did notexceed the critical velocity for fish such as ayu,perch and salmon. The horizontal circulating

Fig. 10. Three-dimensional schematic flow in a rectangularweir.

Fig. 11. Streaming and plunging flow in a rectangular weir(plan).

It could be seen that the primary flow of S-typeoccurred through the notch and the secondary


flow, which occurred in a plunging flow behindthe weir side, did not occur in this case. But theplunging flow occurred behind the weir side dueto the effect of the jet flow from the orifice asshown in Fig. 11, so, the streaming flow and theplunging flow occurred simultaneously in thesame pool. Moreover these two flows becamereciprocal to those in an adjacent pool and hencethe flow became unstable. In this case, the fishmoving upstream would be confused and wouldnot swim over the notch. There occurred largeeddies throughout the pool and hence there wasno resting place.

The discharge equation in the streaming flowover the notch side and in a plunging flow behindthe weir side can be expressed as follows:

Q=Qpw+Qsn

=Cpwbpwhpw3/2g1/2+Csnbsnhsn(gSL)1/2 (3)

Parameters g, S and L were mentioned in Eq.(1) and parameters C, b and h were mentioned inEq. (2). Subscript sn means the notch side in thestreaming flow and pw means the weir side in theplunging flow. So, Csn means the discharge coeffi-cients at the notch side in the streaming flow andCpw means one at the weir side in the plungingflow, and so on. Qpw is a discharge in the plungingflow at the weir side, Qsn is one in the streamingflow at the notch side, and Q is the total dis-charge. The discharge coefficient, Csn and Cpw areknown to be about 1.50 and 0.61, respectively, inthe simple rectangular weir with no notch (Ka-topodis, 1987).

Fig. 12 shows the values of discharge coeffi-cient, Csn with relative depth, hsn/H. Values ofdischarge coefficient increased with an increase inrelative depth, which was a similar result to thatin the plunging flow. The mean value was about1.43, which is almost the same as the value ofKatopodis (1987).

4.2. Rectangular weir with notches in straighttype

Fig. 13 shows the three-dimensional schematicflow in a plunging flow at Q. =0.39. There was noflow over the weir side. Primary flow occurredthrough the notch side forming a plunging flow,although Q. was relatively large. Note that thestreaming flow occurred at Q. =0.39 in a rectan-gular weir of the zigzag type. The reason theplunging flow occurred through the notch sidewas that the orifice was installed beneath thenotch side and not beneath the weir side. Jet flowoccurred from the orifice, and this flow pulled theprimary flow over the notch side into the bottomof the pool and hence made it a plunging floweven if Q. was large. So, the primary flow becamestable and made the flow conditions suitable forthe upstream migration of fish. The maximumvelocity of the primary flow was about 1.3 m/s,which did not exceed the critical velocity. Thelarge eddies which occurred in the rectangularweir of the zigzag type did not occur in this case.

Fig. 12. Values of discharge coefficient with relative depth(streaming flow).

Fig. 13. Three-dimensional schematic flow in a rectangularweir.


Fig. 14. Plunging flow in a rectangular weir (plan).

Fig. 14 is the plan view of a plunging flow.Here, the dotted area is a stagnant region wherethe flow velocity was almost zero. It could be seenthat the resting place was created behind the weirside.

The discharge equation in the plunging flowover the notch side can be expressed as follows:Qp=Cpnbpnhpn

3/2g1/2 (4)The parameters in Eq. (4) have already been

explained in Eq. (2). Values of discharge coeffi-cient, Cpn with the depth, hpn/H are shown in Fig.15 together with the values, Cpn and Cpw for thezigzag type. These values were in the range of0.47–0.79 regardless of the overflow depth, whichwas a similar result to that occurring in the plung-ing flow as explained in Section 4.1.

5. Conclusions

Hydraulic characteristics of different weir typesand their effects on the upstream migration of fishin a pool-weir fishway were studied using thehydraulic experiments. From these results, thefollowing conclusions were obtained:1. The primary flow in a trapezoidal weir forms

an S-shape and a turbulent jet occurs from theorifice. Large eddies occur throughout thepool and hence there is no resting place forfish in the pool. So, this configuration is notsuitable for the upstream migration of fishsuch as an ayu, perch and salmon.

2. The primary streaming flow occurs throughthe alternating notch in the rectangular weirwith a notch installed in a zigzag configura-tion, and the secondary plunging flow occursbehind the weir. Since the streaming flow andthe plunging flow occur simultaneously in thesame pool and these flows become reciprocalto those in an adjacent pool, the flow becomesunstable and there is no resting place for thefish in the pool. This type is also unsuitable forthe upstream migration of fish.

3. The primary flow formed a stable plungingflow through the notch side and the secondaryflow created a resting place for fish behind theweir in the rectangular weir with a notch in-stalled in a straight configuration.

Fig. 15. Values of discharge coefficient with relative depth.

The weak secondary flow occurred behind theweir side and this flow proceeded to an upstreamdirection. Its maximum velocity was about 0.3m/s, so creating resting places. According toNakamura (1995), the fish swims over each notchusing its burst speed, or in some cases by jumpingover them. The fish then rests in the pool for awhile before passing over the next notch and soon, till it swims into the upstream region. Sincethe straight type gave the resting places behindthe weir side and the fish could take a rest thereafter swimming over the notch side, this typecould be a good structure for the upstream migra-tion of fish.


4. Since the primary flow formed a stable plung-ing flow and the secondary flow created aresting place, the rectangular weir with a notchinstalled in a straight configuration is an opti-mal design for the upstream migration of fish.

References

Hirose, T., Nakamura, N., 1991. Design of Fishway.Sankaido, Tokyo, p. 376.

Katopodis, C., 1987. A Guide to Fishway Design. FreshwaterInstitute, Department of Fisheries and Oceans, Winnipeg,Manitoba, Canada.

Kim, S.J., 1993. Experimental report on Saemankeum compre-hensive tideland reclamation project, Rural DevelopmentCorporation, Anyang, Korea.

Kim, J.H., 1994. Analysis of hydraulic characteristics andupstream migration of fish by the shape of fishway septum:Proc. Conf. Kor. Soc. Civil Eng. KSCE 2, 151–154.

Kim, J.H., Kim, C., 1994. Study on hydraulic characteristicsfor upstream migration of fish in a pool-weir fishway. J.KAHS 27 (2), 63–72.

Nakamura, S., 1995. Topics on Fishways, Guideline for Fish-way Design. Sankaido, Tokyo, p. 275.

Nakamura, S., Yotsukura, N., 1987. On the design of fishladder for juvenile fish in Japan: Proc. Int. Symp. onDesign of Hydraulic Structures, Fort Collins, pp. 499–508.

Yu, B.S., 1986. A study on fishway in Geum River, Agricul-tural Development Corporation, Anyang, Korea.

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Hydraulic characteristics by weir type in a pool-weir fishway