Flow structure at an asymmetrical stream confluence

ELSEVIER

Geomorphology 11( 1995) 273-293

Flow structure at an asymmetrical stream confluence Bruce L. Rhoads a, Stephen T. Kenworthy b aDepartment of Geography, Universio of Illinois, Urbana, IL 61801, USA b Department of Geography and Environmental Engineering, The Johns Hopkins University, Baltimore, MD 21218-2686, USA Received 23 July 1993; revised 20 May 1994; accepted 30 August 1994

Abstract Measurements of downstream and cross-stream velocities at a small, asymmetrical stream confluence show that the structure of low-stage flows is influenced by the tributary/main stem momentum flux ratio, the total discharge of the incoming flows, and the bed morphology. Flow accelerates through the confluence during all three measured events. This acceleration is associated with a downstream reduction in channel capacity caused in part by the presence of a large bar along the inner bank of the downstream channel. As the momentum ratio increases, flow from the lateral tributary increasingly deflects flow from the main stream toward the outer channel bank within the confluence. As a result, the mixing interface between the converging flows also shifts outward. The large bar in the downstream channel deflects flow along the inner bank toward the adjacent scour hole, enhancing flow convergence downstream of the confluence and producing a region of flow separation adjacent to, or in the lee of the bar. The loci of maximum topographic deflection and flow separation vary with momentum ratio and total discharge. Secondary circulation within the downstream channel is characterized by a single large helical cell when the momentum ratio exceeds one, and weak surface-convergent helical cells on opposite sides of the mixing interface when the momentum ratio is less than one. Curvature of the flow, and thus the strength of helical motion, is greatest on the tributary side of the mixing interface. Although the flow events measured in this study did not exceed the threshold for sediment movement, the bed

morphology at the confluence can be explained by the flow structure observed during these low-stage events, suggesting that formative flows may have similar downstream and cross-stream velocity fields.

1. Introduction Stream confluences are characterized by complex hydrodynamic conditions associated with the convergence of separate flows. Although knowledge of these conditions is essential for developing a general theory of confluence dynamics, at present, few field data are available to evaluate general conceptual (e.g. Best, 1987; Bridge, 1993) or mathematical (e.g. Weerakoon et al., 1991) models of flow through confluences. Conceptual models, based primarily on results of experimental research, indicate that hydrodynamic features at confluences include a zone of flow stagnation at the upstream junction comer, a shear layer between the 0169-555X/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDIO169-555X(94)00069-7

merging flows, twin surface-convergent helical cells on either side of the shear layer, and separation of flow from one or both channel banks immediately downstream of the confluence (Mosley, 1976; Best, 1987, 1988). Experimental work has also shown that the location, size, and intensity of these hydrodynamic features are a function of junction angle and the momentum flux ratio of the combining flows (Best and Reid, 1984; Best, 1987, ). Only a few field investigations have examined flow patterns at stream confluences and the results of these investigations have not been entirely consistent. The existence of twin surface-convergent helical cells at symmetrical (Y-shaped) confluences recently has been

confirmed by field measurements of downstream and cross-stream velocities in braided rivers ( Ashmore et al., 1992; Bridge and Gabel, 1992). However. Biron et al. ( 1993a) found no evidence of helical motion at a ( i.e.. a planconfluence with an asymmetricalplanform form where the downstream channel is a linear extension of the major tributary). These disparate tinding have helped to fuel controversy surrounding the mechanism responsible for the development of helical cells at confluences. Some investigators have attributed these features to separation of flow in the lee of avalanche faces that border a zone of scour within the confluence (Best. 1987, 1988; Best and Roy. 1991: Biron et al.. 1993b), whereas others have claimed that these cells are the result of curvature of the merging flows within the confluence (Ashmore and Parker. 1983; Ashmore et al., 1992). The relative importance of these two factors is unclear, but flow separation downstream of avalanche faces and curvature-induced secondary circulation may occur conjointly, particularly in confluences with deep scour holes and steep avalanche faces ( Bridge, 1993 ) Separation of flow from the channel banks downstream from the junction has been observed in flume experiments (Best, 1987). but has yet to be documented in the field. The absence of lateral flow separation at natural confluences has been attributed to reduction in the angle between confluent streams in the immediate vicinity of the junction (Roy et al.. 1988: Roy and Bergeron, 1990) and to disruption of the separation zone by distortion of the shear layer at confluences with channels of unequal depth (Biron et al.. 1993a). The influence of bed morphology on time-averaged downstream and cross-stream velocities at confluences has not been explored fully. Previous tield investigations have demonstrated that orientations of mean velocity vectors at junctions are influenced by changes in flow stage, suggesting that the effect of bed morphology on downstream and cross-stream velocities varies with changes in hydrologic conditions (Roy and Bergeron, 1990; Bridge and Gabel, 1992). As work in meandering rivers has shown, spatial variation in bed morphology can strongly influence downstream and cross-stream velocity fields, which in turn determine associated patterns of bed shear stress and sediment transport (Dietrich and Smith, 1983, 1984 )

This research demonstrates how time-averaged downstream and cross-stream velocity fields at an asymmetrical stream confluence vary with changes in the momentum flux ratio and the combined (total) discharge of the confluent tributaries. In particular, the study focuses on patterns of flow separation, flow convergence/divergence, and secondary circulation immediately downstream from the confluence, and the relationship of these patterns to the mixing interface between the combining flows.

2. Field site The tield site for the research is the confluence of the Kaskaskia River (KR) and Copper Slough (CS) in east-central Illinois, USA (Fig. 1). These confluent streams have been channelized for purposes of agricultural drainage and, therefore, are representative of headwater streams throughout the Midwest. Although the streams are not pristine, they are unconstrained and respond naturally to erosional and depositional processes. Because each stream flows within a ditch, identitication of bankfull stage is difficult. In the vicinity of the confluence, the portion of the banks below 96.3 m ( arbitrary datum) is not heavily vegetated and roughly defines an inset channel. Bottom widths of the inset channels within each ditch are similar (KR = 7 m; CS = 8.5 m). The two streams meet at an angle of 60” and. because the channels have been straightened artihcially. this angle does not change in the immediate vicinity of the junction. The planform of the confluence is primarily asymmetrical, but the downstream channel does curve slightly (Fig. 1 ) The drainage area of the Kaskaskia River at the confluence is slightly larger than that of the Copper Slough ( KR = 54 km”; CS = 41 km’). The Copper Slough, however, drains the western portion of Champaign, IL and, thus, has a flashy hydrologic regime relative to the Kaskaskia River, which, because it drains agricultural land, responds more slowly to precipitation events that produce runoff. This difference in hydrologic response produces wide variation in the momentum ratio at the site. The two streams also transport different types of sediment. Bed material in the Copper Slough consists mainly of fine gravel ( dsO= 3.5 mm), whereas the bed of the Kaskaskia River is comprised primarily of sand (cl,,, = 0.65 mm) (Mayer, 1994).

B.L. Rhoads, ST. Kenworthy /Geomorphology II (1995) 273-293

Fig. 1. Study area and measurement

cross sections.

215

3. Field data The field data consist of measurements of time-averaged downstream velocities, cross-stream velocities, and water temperatures immediately downstream from the confluence for three dates: March 20, April 24, and May 24, 1992. The temperature data provide information on the position of the mixing interface between the two flows, which had slightly different temperatures on all three dates (Table 1) . The configuration of the channel bed at the field site was similar for the three flow events, but momentum flux ratio, total momentum flux. and total discharge varied (Table 1) Thus, differences in flow structure on the three dates primarily reflect changes in hydrologic conditions, rather than changes in bed morphology. 3.1. Flow measurements Discharges and mean temperatures of the confluent streams were measured at cross sections located approximately 15 meters upstream of the confluence (Fig. 1) These measurements were used to compute the momentum flux ratio of the merging flows:

(1)

W=(P~Q~V~~(PIQ,V,)

where Q is discharge ( m3 s ’) , p is density of the water (kg me3 ), V is mean velocity (m s-l), and the subscripts 1 and 2 refer to the Kaskaskia River and Copper Slough, respectively. Flow velocities and temperatures were measured at four or five cross-sections located immediately downstream of the confluence and oriented orthogonally to the channel centerline (Fig. 1) . The path of the center-

line was defined by a line connecting a series of closely spaced ( = 2 m apart) points located halfway between the 96.3 m contour on each bank of the downstream channel (Fig. 2). This contour corresponds to the midbank position of the inset channel as defined on the basis of bank vegetation. The three flows measured in this study all had stages within L-O.15 m of 96.3 m. Although measurements at these cross-sections may not yield the cross-stream velocity field required to satisfy continuity (e.g. Dietrich and Smith, 1983), the data do illustrate how orthogonal components of the velocity field vary within a fixed frame of reference as momentum ratio or total discharge change. At each cross-section, vertical profiles of downstream (I’,) and cross-stream (c,) velocities were obtained using a Marsh-McBimey electromagnetic current meter (time constant: 0.2 seconds). The spherical sensor on this meter has a diameter of 3.8 cm and a sensing-volume radius (measured from the center of the probe) of 5.7 cm. A YSI thermistor probe positioned 25 cm behind the flow sensor provided information on water temperature at each position in the profile. This probe was connected to a YSI tele-thermometer with an accuracy of + 0.06”C. The electromagnetic sensor and temperature probe were mounted on a custom-built top-set wading rod. The amount of horizontal play in the mounting apparatus was less than + l”.The sensor and probe were oriented orthogonally to each cross-section by attaching the wading rod to a steel cable stretched tautly between metal fence posts at the ends of each crosssection. After the rod was attached to the steel cable, it was placed in a vertical position using a rod level. This

Table I Hydraulic conditions upstream of the contluence

KR

CS

May 24, 1992

April 24, I992

March 20, 1992 (cs+KR)

(CS/KR)

KR

CS

tCS+KR)

(CSIKR)

KR

cs

(CS+KR)

0.56 89.9 3.50 0.16

0.42 49.5 3.45 0.12

0.98 139.4 6.95 -

0.47 11.75

0.41

-

_

13.28

-

_

-

Q

V

0.29 46.9 1.83 0.16

0.51 170.6 I .53 0.33

0.80 217.5 3.36 -

D T

0.27 8.45

0.20 10.85

-

M A

0.47 81.1 2.68 0.17

_

0.3x I 1.25

0.84 281.6 2.50 0.34

1.31 362.1 S.18

1.81 3.47 0.93

0.30 13.50

flux (kg m s River, CS = Copper Slough, Q = discharge (m’5 ’1,M = momentum velocity (m s ‘), D=mean depth (m), T= temperature at IO:00 a.m. (“(2).

KR = Kaskaskia

V=mean

I .74 3.64 0.84 _

2). A

= cross-sectional

(CS/KR) 0.75 0.55 0.99 _

area of Row (m’),

B.L. Rhoads, ST. Kenworthy/Geomorphology

II (1995) 273-293

05

271

meters

Contour lntetvalis 0.1 meter. Arbitrary

Datum.

Scour hole

Fig. 2. Bed topography

at the field site in spring 1992 showing locations of main bar, secondary bar, and scour hole.

procedure ensured that flow velocities and water temperatures were measured in the same frame of reference on each date. Velocities and water temperatures at each depth in a vertical profile were sampled at 1 Hz and averaged over an interval of 60 seconds. The record length was selected as a compromise between obtaining measurements at a large number of locations in the flow field and accurately characterizing mean velocities in the mixing interface between the flows, which contains coherent eddies that produce periodic variations in velocities. Velocity measurements within the mixing interface indicated that, for the range of flows measured in this study, the periodicity of velocity fluctuations associated with the passage of coherent eddies is approximately 10 to 15 seconds and that 60-second mean velocities are within 5 to 10 percent of fiveminute mean values (Kenworthy, 1994).

3.2. Calculation of velocity components

The definition of flow structure in natural rivers is contingent upon the frame of reference in which velocity components are presented. A variety of frames of reference have been proposed (e.g. Bathurst et al., 1977; Dietrich and Smith, 1983; Bridge and Gabel, 1992; Markham and Thorne, 1992), but the appropriateness of a particular frame of reference depends upon the objectives of the study and the nature of flow conditions at a particular stream location. At stream confluences, the local path of the depth-averaged flow is often skewed in relation to the path of the channel centerline. In such cases, cross-stream velocities will consist of velocity components associated with helical motion, if present, and with skewness of the flow path in relation to the channel path (e.g. Chang, 1988, pp. 220-222).

Specific objectives of this research are to identify the separate contributions of skewed flow and helical motion to the cross-stream velocity field and to determine how these separatecontributions vary with changing hydrologic conditions. These aims can be achieved by first calculating primary and secondary velocities and then determining the cross-stream components of these velocities. Primary (l’,,) and secondary (I’,) velocities, as defined by Bathurst et al. C1977). are components of the resultant velocity (11,)at a particular depth in the flow column that are oriented parallel and orthogonal, respectively, to the depth-averaged velocity vector at that vertical. These velocities are computed as: I’p=l’, cos(cp-4) I’, = IS,sin( cp- 4) l.r=J(l.f+t,;). cp=tan ‘((3,/c,,). where d= tan ’( VJ L’,), V, is the depth-averaged cross-stream velocity at the vertical, and V, is the depth-averaged downstream velocity. The orientations of depth-averaged velocity vectors (4) at different verticals across the channel define the pattern of skewed flow over the cross-section, whereas individual values of 13~at each vertical define the strength of skewed flow at particular locations in the water column. Secondary velocities (t‘\) define circulation in a plane normal to the depthaveraged velocity vector at each vertical and, thus, indicate the strength of helical motion within the skewed flow (e.g. Ashmore et al., 1992). The cross-stream components of ~‘r and I’, are:

where the sum of 11~~and c,, is equal to the measured cross-stream velocity (13,). Values of I’,,~and L’,,.represent the separate contributions of skewed flow and helical motion, respectively, to the cross-stream velocity field. Because the pattern of I’,~preserves the overall pattern of secondary circulation, the method allows helical motion to be identified within strongly skewed flow. The procedure also can be used to determine how various types of fluid motion contribute to changes in the cross-stream velocity field as hydrologic conditions at the confluence change.

4. Results 3. I. Bed morphology Bed morphology on the three flow-measurement dates consisted of a scour hole extending from the central portion of the confluence downstream along the west bank, and a bar complex comprised of a main lateral bar along the east bank of the channel immediately downstream of the confluence and a secondary lateral bar in the lee of the main bar (Figs. 2 and 3 ) The slopes bordering the scour hole ranged from a minimum of 4” at the upstream end of this feature to a maximum of 26” immediately adjacent to the main bar. The crest of the main bar marked the highest point of a wedge of sediment that protruded into the confluence from the Copper Slough and wrapped around the downstream junction comer. The surface of the main bar was covered with an armor layer of fine to medium gravel; this material was underlain by coarse sand and fine gravel (Mayer, 1994). The scour hole adjacent to the main bar was incised into dense, highly cohesive glacial till. The secondary bar was composed primarily of unconsolidated silt and clay and fine organic material. The bed of the channel was clearly visible on all three measurement dates. No bed-material transport was observed within or upstream of the confluence on any of these dates. In addition, no inactive bedforms were present at the site. Between March 20 and May 24, 1992, flow stage rose slightly above the surface of the bar several times

Fig. 3. Bar complex along the inner bank of the downstream channel at the confluence of the Kaskaskia River (top) and Copper Slough (right) (view looking upstream). Main bar is the light subaerial surface along the inner bank of the downstream channel. Secondary bar is dark subaerial surface immediately downstream of the main bar.

B.L. Rhoads, S. T. Kenworthy / Geomorphology II (I 995) 273-293

279

m Fig. 4. Variations in flow stage between early March and late May 1992 at the Kaskaskia l&s indicate times of flow measurement.

(Fig. 4), resulting in minor erosion of the upstream portion of the main bar (cross-section A), deposition on the tail of the secondary bar (cross-section E), and a slight shift in the position of maximum depth of scour at cross-section C (Fig. 5). Maximum changes in bed elevation at the flow cross-sections were less than 0.10 meters. No detectable erosion of the western bank of the downstream channel occurred, except at cross-section A where the bank moved gradually to the west by 0.4 meters. This lateral movement was caused primarily by piping-related bank failures associated with seepage Cross-section A

Cross-section E

5m

!-

lrn

----------

April 24,1992

March20,1992

-

May 24,1992

Fig. 5. Comparison of inundated portions of channel cross sections on the three flow measurement dates.

River-Copper

Slough confluence.

Dashed vertical

outflow from a layer of glacial outwash deposits near the base of the channel bank. 4.2. Hydraulic conditions The three flows measured at the field site include two events with M,> 1.0 and one event with M, < 1.0 (Table 1) . Also, the two events with high momentum ratios have similar values of M,, but substantially different total discharges. Together, these three events provide a basis for evaluating the influence of changes in momentum flux ratio and total discharge on flow structure at the confluence. Hydraulic conditions at the site are sensitive to changes in the combined discharges of the two confluent streams, but also are controlled in part by the momentum ratio. When the Copper Slough is dominant (i.e., M, > 1.O) , flow entering the confluence has a low mean depth and high mean velocity; conversely, when the Kaskaskia River is dominant, flow at the site is deep and mean velocities are low (Table 1) . This change in hydraulic conditions occurs because the Copper Slough has a steeper gradient than the Kaskaskia River (0.00112 versus 0.00043). Mean hydraulic conditions downstream of the confluence primarily reflect the influence of channel morphology on flow conditions. On all three dates the flow accelerates through the confluence; mean velocities at cross-section A are 1.1 to 1.5 times greater than the mean velocity of the dominant upstream tributary (Table 2). Flow acceleration at the site is associated with a 35 to 45% reduction in total cross-sectional area

280

Table 2 Hydraulic conditions downstream of the confluence March 20, I992

,\pril 24. I YY7

May 24. 1992

Cros\ section

C‘ro\\ wctlon -______

Cross section

A

B

B’

(‘

E

A

H

C’

E

A

B

C

E

vivu

I .07

1.10

I .02

0.87

0.x7

1.16

I

I.3

I.21

I 37

I .47

I.61

I.63

I .4X

A ~A.,,,,

0.57

0.55

0.60

0.6’)

0.69

(137

0 5x

0.54

0.6 I

0.62

0.56

0.56

0.6 I

X.1

8.9

WID

31.2

V= mean velocity (m 5 area of flow in tributaries

IS.0

29.7

20 5

II.1

’). Vu= mean velocity of dominant upstream tributary (m \ ’1.A ( I?), W= flow width (m ). D = mean flow depth ( III),

of the merging flows between the upstream cross-sections and cross-section A (Table 2). Downstream ot cross-section A the flow continues to accelerate as the cross-sectional area of the channel decreases adjacent to the main bar. Downstream of the bar complex the channel area increases and the flow decelerates. 4.3. Depth-averuged

IYY

veloci~

vectors

The general spatial pattern of depth-averaged flow is similar on all three dates (Fig. 6). This pattern consists of flow convergence immediately downstream of the confluence (cross-sections A and B ) , separation of flow from the inner bank of the downstream channel (cross-sections B, B’ or C), and flow divergence as water moves past the bar complex along the inner (east) bank of the downstream channel (cross-sections C and E). Although the general pattern does not change, the strength and loci of flow convergence, divergence, and separation vary for the three dates. On March 20, the flow is shallow because M, > 3 and total discharge is the smallest for the three dates. As a result, a large portion of the bar complex is exposed and the flow is forced to move around this feature. Progressive shoaling of the flow at the head of the main bar deflects flow outward (west) toward the adjacent scour hole, resulting in strong flow convergence at cross-section A. The outward-directed, high-momentum flow detaches from the bar margin between cross-sections A and B. The region of separated flow along the inner portions of cross-sections B and B’ is characterized by small depth-averaged velocities with vectors oriented upstream. Between cross-sections B’ and C the effective channel width (width excluding the

23.9

21.7

16.3

IO.1

= cross-sectional area of flow (m’).

18.1 A,,,,, = total

region of how separation) increases and the velocity vectors diverge as the flow expands. Divergence of the flow continues at cross-section E as the flow moves past the downstream end of the exposed bar complex. On April 24 the flow is deeper and less of the bar complex is exposed than on March 20. Slight convergence of the flow occurs at cross-section A, but convergence is strongest at cross-section B, reflecting a downstream shift in the position of maximum topographic deflection of the flow relative to conditions on March 20. Flow separates from the bar margin downstream of cross-section B. The presence of a separation zone along the inner bank at cross-section C limits expansion of the flow at this location. On May 24 hydraulic conditions at the confluence are controlled by the Kaskaskia River (i.e., M, < 1.O) ; thus, flow is deepest on this date and the entire bar complex is submerged. Depth-averaged velocity vectors are nearly parallel at each of the four cross-sections, indicating that topographic steering of the flow is less effective on this date than on the previous two dates. Locally, topographic deflection of the flow by the submerged crest of the main bar produces a zone of recirculating fluid at cross-section C. The bar crest is located immediately upstream of cross-section C and on this date is covered by emergent plants. Shoaling flow moving downstream toward the crest is slowed by the plants and forced laterally towards the center of the channel, producing a zone of recirculating flow downstream from, rather than adjacent to, the main bar. Petts and Thorns ( 1987) reported a similar zone of recirculating fluid downstream from a submerged confluence bar in the North Tyne River, UK.

B.L. Rhoads, S.T. Kenworthy/Geomorphology

-

bAanwllumRolb=3.64 TdalMunenlumFC1x=217~ma~

-. -_______

TdalDim%qe=0.2Ods”

-

Momentum

Ratii

= 1.31 d

281

273-293

1 Fa%$wrhee m@,*

0 -

4 metsIx

.25md

1

= 3.47 -.

TdalMomanlumFlux=363kgms~ Total Dkharge

lI(1995)

8.’

____--_-

Xi: m*r*

Lfaca

0 4 M14R

.25md

Momedun Rslb z 0.53 TolalMometlwnflm=130lqm& Told

Diirge

* O.oB d

8”

Fig. 6. Vertically averaged velocity vectors on March 20, April 24, and May 24. Also shown is the center of the surticial position of the mixing interface hktween the merging flows as determined from temperature data and field observations.

In sum. variation in the strength and loci of flow convergence and divergence is controlled by topographic forcing of the flow by the bar complex. which in turn is a function of flow stage and spatial variation in bar morphology. Topographic deflection of flow also appears to enhance the potential for the development of flow separation downstream of the confluence. particularly when M, > I .O. However, even when M, < I .O. topographic forcing of the flow produces a local pocket of separated flow along the inner bank of the downstream channel. 4.4. Water tempercrture The Kaskaskia River is approximately 2°C cooler than the Copper Slough on all three dates (Table 1). Diurnal variations in water temperature produced changes in absolute temperature from cross-section to cross-section, but the temperature difference between the two streams remained fairly constant ( I .S to 2.W ) throughout each of the three days. This temperature difference corresponds to a density contrast of less than 0.0396, but is large enough to reveal the location of the mixing interface between the two flows and patterns of mixing downstream of the confluence ( Fig. 7). The mixing interface is expressed as a zone of‘ closely spaced isotemps within the channel cross-section. This interface clearly separates water from each confluent stream. On each measurement date, the surficial position of the mixing interface was marked by distinct Kelvin-Helmholtz instabilities that were made visible by slight differences in turbidity between the tributaries (Fig. 6). At cross-section A the mixing interface is vertical or nearly vertical on all three dates (Fig. 7 ) As expected. the location of the mixing interface shifts outward along this cross-section as momentum ratio increases. Downstream from cross-section A a similar pattern of distortion of the mixing interface is evident for the three flows, but the spatial scale of this distortion varies. The general pattern of distortion consists of progressive inward movement of cool water from the Kaskaskia River along the channel bed in the downstream direction. Penetration of cool water extends upward near the inner bank and eventually reaches the surface. In other words, cool water from the Kaskaskia River appears to wrap around a core of warm water from the Copper Slough. As this process occurs. the bottom part of the

mixing interface becomes increasingly distorted as the isotemps spread apart. Conversely, the near-surface portion of the mixing interface remains well-defined until the wrap-around process is complete. This upper portion of the mixing interface shifts progressively outward in the downstream direction on all three dates, but outward displacement is most pronounced on March 70 and April 24 when M, > I .O (Figs. 6 and 7). The spatial scale of mixing varies both with total discharge and with the momentum ratio of the confluent flows. Mixing occurs most effectively on March 20 when the momentum ratio is high and total discharge is small. On this date, the bulge of cool inward-moving water reaches the surface of the inner portion of the tlow at cross-section B’. An increase in total discharge on April 24 expands the length scale of mixing; inward movement of the bulge of cool water is still evident at cross-section C. The pattern of mixing on May 24, when M, < I .O, is similar to that which occurs on April 24. The total discharge on this date falls between those for March 20 and April 24.

The general pattern of downstream velocity at the confluence differs for events with momentum ratios greater than and less than one (Fig. 8) On March 20 and April 24, a single core of high-velocity flow exists near the surface at cross-section A (Fig. 8). This core of high-velocity fluid is located inward of the mixing interface between the two flows, indicating that it is associated with flow from the Copper Slough (Figs. 7 and 8 ). Even though the flows on March 20 and April 24 have similar momentum ratios, the high-velocity core is closest to the outer bank on March 20. Strong outward deflection of flow at the head of the main bar allows water from the Copper Slough to penetrate farther into the confluence on this date than on April 24. Downstream of cross-section A, the complexity of the downstream flow field increases. The high-velocity core gradually shifts outward, mirroring the outward displacement of the mixing interface. The core of high velocity also extends downward toward the base of the scour hole. In addition, a zone of high-velocity flow develops next to the lateral separation zone along the inner margin of the flow field; the separation zone is characterized by a region of upstream flow (negative downstream velocities). The complicated pattern of

B.L. Rhoads, XT. Kenworthy/Geomorphology

ll(1995)

273-293

283

284

m

a

B.L. Rhoads, ST. Kenworthy /Geomorphology

downstream flow is best illustrated by the velocity field at cross-section B’ on March 20 (Fig. 8). This pattern consists of: ( 1) a core of high velocity that extends downward and inward from the surface to the base of the scour hole along the outer bank, (2) a zone of reduced velocity near the surface in the center of the channel, (3) an adjacent region of relatively high velocity, and (4) recirculating flow in the separation zone along the inner bank. A similar pattern exists at cross-section C on April 24. The upward-bulging isovels over the inner portion of cross-section C on this date suggest that the development of the high-velocity region next to the separation zone is associated with inward and upward transport of high-velocity near-bed fluid by secondary currents. In other words, downstream momentum associated with the high-velocity core over the outer part of the flow is transferred downward, inward, and upward around a zone of low-velocity flow in the center of the channel. This “wrap-around” pattern of downstream velocity is similar to the pattern of water temperature described above. The secondary high-velocity core is at the surface at cross-section B’ on March 20, whereas the upward bulging isovels do not reach the surface at cross-section C on April 24. Thus, the length scale of lateral transfer of downstream momentum increases with increasing total discharge. The downstream velocity field becomes more uniform at cross-section E, but two cores of highvelocity near-surface flow, separated by an intervening zone of low-velocity near-surface fluid, are still apparent. On May 24 two cores of high-velocity flow straddle the mixing interface at cross-section A (Figs. 7 and 8). These cores represent the separate thalwegs of the converging streams. Downstream of cross-section A, the downstream velocity field is less complex than on March 20 and April 24. At cross-section B the two cores have merged into a broad zone of high-velocity flow, and a small secondary core has developed beneath this zone over the scour hole. The velocity field is relatively uniform at cross-section C. Here, a narrow high-velocity core extends downward toward the bottom of the scour hole. Flow separation occurs over the inner portion of this cross-section, which is in the lee of the main-bar crest. The high-velocity core is skewed toward the inner bank at cross-section E, suggesting that downstream momentum is being redistributed lat-

I I (1995) 273-293

285

erally by secondary currents. The role of these currents is examined below. 4.6. Cross-stream

velocity

The data on cross-stream velocity confirm that substantial cross-stream movement of water occurs at this confluence and that the cross-stream velocity field, as well as its primary and secondary components (up? and LI,,), vary with momentum ratio and total discharge. On March 20 and April 24 strong outward movement of water occurs over the inner portion of cross-section A (Figs. 9 and 10). The larger cross-stream velocities on March 20 reflect stronger topographic deflection of the flow at the head of the main bar on this date than on April 24. Over the scour hole, outward cross-stream flow occurs only near the surface and near-bed flow is oriented inward, suggesting that helical motion exists within the flow. Decomposition of the cross-stream velocity field into its primary and secondary components reveals that secondary circulation occurs within converging flow at cross-section A (Figs. 9 and 10). The secondary circulation is strongest near but slightly inward from the mixing interface between the flows (Figs. 7 and 9). This circulation moves in a counterclockwise direction (looking upstream) and extends well into the zone of net outward movement of water over the inner portion of the channel (Figs. 9 and 10). On both dates, the pattern of secondary circulation is weak and poorly defined on the west side of the mixing interface. On March 20, counterclockwise secondary circulation is well-developed at cross-section B and except for the zone of lateral flow separation, extends over most of the flow width (Fig. 9). Values of uSYconstitute nearly the entire cross-stream velocity field at this cross-section. On April 24, shoaling of flow and a reduction in flow width between cross-sections A and B produce strong outward movement of water over the inner portion of cross-section B (Fig. 10). Counterclockwise secondary circulation occurs within the outward-directed flow and the strength of this circulation is strongest inward of the mixing interface (Figs. 7 and 10). On both dates, the strength of the secondary circulation gradually decreases downstream of cross-section B (Figs. 9 and 10). As flow moves past the main bar it diverges. On March 20, the diverging flow is skewed toward the inner bank at cross-sections B’ and

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C, whereas on April 24 flow over the inner portion of the channel moves toward the inner bank and flow over the outer portion of the channel moves toward the outer bank (Figs. 6, 9 and 10). Between cross-sections C and E the flow expands and spills over the tail of the secondary bar, producing strong inward flow over the inner portion of cross-section E. On May 24, flow convergence dominates the crossstream flow field at cross-section A and values of u,, are small ( <0.02 m/s) (Fig. 11). A comparison of individual velocity vectors within the flow field reveals, however, that a subtle, but distinct, pattern of secondary circulation occurs at this cross-section. Differences in the orientations of these vectors ( cps) indicate that weak surface-convergent helical cells are present on each side of the mixing interface. The temperature data show that the mixing interface at cross-section A is centered between the fifth and sixth verticals from the west side of the channel (Figs. 6 and 7). At the third, fourth, and fifth verticals flow within 0.10 m of the bed is directed outward relative to flow within 0.10 m of the surface. Angles of deviation between the near-bed and nearsurface velocity vectors range from 4.2” (vertical three) to 11.2” (vertical five). Conversely, at verticals six through twelve near-bed flow moves inward relative to near-surface flow with angles of deviation ranging from 6.6” (vertical eight) to 1.7” (vertical eleven). These data suggest that the mixing interface separates a clockwise-rotating cell within the Kaskaskia River flow from a counterclockwise-rotating cell within the Copper Slough flow. A similar pattern of secondary circulation occurs at cross-section B. Here, the center of the mixing interface lies between the fourth and fifth verticals from the west bank (Figs. 6 and 7). At the second, third, and fourth verticals flow near the surface moves inward relative to flow near the bed. Angles of deviation range from 2.5” (vertical two) to 8” (vertical four). At the third and fourth verticals the maximum angle of deviation occurs at 0.20 meters above the bed. Below this point the flow direction becomes oriented inward relative to the flow above it, suggesting that a small counterclockwise-rotating cell underlies the clock-wise rotating cell at these two verticals. At verticals five through ten, near-surface flow moves outward relative to near-bed flow with angles of deviation ranging from 22.8” (vertical six) to 2.5” (vertical eight). This counterclockwise-rotating cell is embedded within outward-moving

289

flow over the inner portion of the channel. The outward movement of water, produced by shoaling of the flow and narrowing of the channel between cross-sections A and B, is also responsible for the development of the pocket of flow separation along the inner portion of cross-section C. On May 24, counterclockwise secondary circulation extends over most of cross-section C, including the portion of this cross-section to the west of the mixing interface (Figs. 7 and 11) . The secondary circulation, however, is strongest inward of the mixing interface. The strength of secondary circulation diminishes between cross-sections C and E. Expansion of the flow between these cross-sections results in net inward movement of water over the inner portion of crosssection E.

5. Discussion The results of this investigation support findings of previous research and also provide new insight into the dynamics of flow through asymmetrical confluences. Acceleration of flow through the confluence on all three dates is consistent with findings of previous experimental and field studies of hydraulic conditions at confluences (Roy et al., 1988; Roy and Bergeron, 1990; Mamedov, 1990). At this particular confluence, flow acceleration is related primarily to the presence of a large bar in the downstream channel, which dramatically reduces the total cross-sectional area of the combining flows. This study is the first field investigation to directly confirm the occurrence of flow separation at a natural confluence. Previous field studies have attributed the lack of separation at natural confluences to gradual changes in bank orientation at the downstream junction comer as opposed to the sharp downstream comers in most laboratory channels (Roy and Bergeron, 1990; Ashmore et al., 1992). The flow separation associated with the low-stage events examined in this study is produced mainly by outward deflection of shallow flow along the inner portion of the downstream channel by a large bar. Additional investigations currently are being conducted at the site to determine whether flow separation occurs during high-stage flows when topographic deflection of the flow is reduced.

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This study also supports the view that flow structure at confluences can be compared to flow through meander bends (e.g. Bridge, 1993). As separate streams enter a confluence either one or both of the flows must curve to become aligned with the downstream channel. This curvature should produce superelevation of the water surface and resulting helical flow (Bridge, 1993). The degree of curvature of each stream is controlled by the planform of the confluence, the junction angle, and the momentum ratio. At symmetrical confluences both flows curve as they mutually deflect one another along a mixing interface. If the momentum ratio is close to one, the zone of superelevation will occur near mid-confluence and twin surface-convergent helical cells of roughly equal size will occur within the flow immediately downstream of the confluence (e.g.. Ashmore et al., 1992). At asymmetrical confluences, such as the one examined in this study, how from the lateral tributary must curve abruptly to become aligned with the downstream channel. Initially the mixing interface will be positioned near mid-confluence as the two flows meet at the upstream junction turner. Superelevation of flow in this region may generate twin surface-convergent cells within asymmetrical confluences ( Weerakoon et al., 199 1) . Field evidence of such cells at the Copper Slough-KaskaskiaRiver confluence has been reported by Kenworthy ( 1994). The field data described above show that when M, > 1.O, flow immediately downstream of the confluence resembles movement of water through a meander bend. Under these conditions, a single large helical cell exists within the flow in the downstream channel. The large helical cell lies inward of the mixing interface between the converging streams, which is located close to the outer bank. This cell is produced by curvature of flow from the lateral tributary (Copper Slough) as it moves around the downstream junction comer and main lateral bar. The helical motion associated with this cell entrains water from the main stream (Kaskaskia River) and transports it inward along the bed. It also transfers downstream momentum downward and inward from a core of high velocity in the outer part of the flow, thereby distorting the distribution of downstream velocity. Constriction of the flow as it moves downstream past the main bar and adjacent separation zone compresses the helical motion and forces inwardmoving water upward toward the surface. Upwelling of water from the main stream into flow from the trib-

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utary downstream of confluences has been observed at other confluences and has been attributed to distortion of the shear layer by flow separation in the lee of steep avalanche faces (Roy and DeSerres, 1989; Best and Roy. 1991; Biron et al., 1993b). This study indicates that upwelling over the inner portion of the downstream channel can also be produced by strong helical motion associated with how curvature. When M, < 1.O, flow from the lateral tributary cannot penetrate far into the confluence and the mixing interface is positioned near the center of the channel immediately downstream from the confluence. Under these conditions, flow through this asymmetrical confluence is analagous to flow through two meander bends placed back to back; it is also similar to llow through a symmetrical confluence. Weak, but distinct, surface-convergent cells occur on each side of the mixing interface. The cells are weak because flow velocities within the confluence are relatively small when the main stream is dominant (Table 1) and, thus, the effect of curvature Because on the llow is not as great as when M,> 1.0. flow from the lateral tributary must curve to a greater extent than how in the main stream, the helical cell inward of the mixing interface is stronger than the adjacent cell on the other side of this interface. This pattern of helical motion is consistent with results of threedimensional mathematical modelling of flow through an asymmetrical confluence with M, < 1.O, which show that the cell associated with the tributary flow is stronger than the cell associated with flow from the main stream (Weerakoon et al., 1991). This mathematical model also predicts that the strong inner cell produces substantial upwelling adjacent to the separation zone along the inner margin of the flow and that secondary circulation increases near-bed downstream velocities, conditions that have been documented at the held site. Previous research has shown that transverse advective transport associated with curvature-induced secondary currents enhances rates of transverse mixing in natural streams (Fischer, 1969; Young and Wallis, 1993 ) . The temperature and velocity data indicate that curvature-induced secondary circulation plays an important role in transverse mixing of the two flows at this confluence. Strong circulation over the inner portion of the downstream channel entrains water from the main stream along the mixing interface and transports it inward along the bed and upward into the adjacent

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/

Fig. 12. Changes in channel cross sections downstream of the confluence produced by high-momentum ratio flows on July 12, 199 1 and August 6, 1991.

flow. Because conlluences are locations where streams with different water quality characteristics join, the occurrence of secondary currents at these sites may have important implications for the dispersion of solutes and suspended solids throughout river networks (Kenworthy and Rhoads, 1994). Although the flows on all three dates did not exceed the threshold for sediment motion, the bed morphology of the confluence can be explained by the general patterns of flow observed during these low-stage events, suggesting that formative flows may have similar velocity fields. The locations of the scour hole and main bar conform with patterns of bed morphology in experimental confluences with M, > 1 .O (Best, 1988). Daily visits to the confluence during the summer of 1991 indicated that the main bar developed largely during flows on July 12 and August 6,199 1. Discharge measurements obtained upstream of the confluence during the receding stages of these events showed that both the July 12 and August 6 flows had momentum ratios slightly greater than 40. The flow on July 12 accreted sand and gravel onto the face of a small preexisting bar and also deposited a distinct ridge of sandy sediment on top of this bar (Fig. 12). The lee slope of this ridge of sediment faced the inner channel bank. The flow on August 12 elongated the bar by depositing additional sediment on downstream portions of the bar face and bar top (Fig. 12). Lateral growth of the bar reduced the size and depth of the scour hole; however, neither event deposited sediment over the entire downstream channel and glacial till remained exposed on the bed adjacent to the toe of the bar face. Once the main bar

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was in place, a region of recirculating flow developed immediately downstream. Deposition of suspended sediment and organic material within this zone of recirculating flow between August 1991 and March 1992 resulted in the formation of the secondary bar. The strong helical flow observed during highmomentum ratio, low-stage events and the patterns of erosion, deposition, and resultant bed morphology produced by the formative flows suggest that the fluvial dynamics of this confluence are similar to those in a meander bend when M,> 1 .O. From a morphologic perspective the locations of the main bar and scour hole are consistent with the expected positions of the point bar and pool, respectively, if the downstream junction comer is viewed as the apex of a meander bend defined by flow moving into the downstream channel from the Copper Slough (Nelson and Smith, 1989). The dynamics of bar growth during the two formative events are also similar to those in a meander bend. Accretion of sediment on the face of the bar along the inner bank and exposure of till over the outer portion of the channel conform with the pattern of erosion and deposition associated with helical flow through a meander bend (Thome and Rais, 1984; Anthony and Harvey, 1991). At a confluence, erosion over the outer portion of the downstream channel will be enhanced by large turbulent intensities in the mixing layer between the flows (e.g. Biron et al., 1993a), which for M,>>1.O is located along the outer bank. The helical flow can also transport suspended sediment into a zone of separated or lowvelocity flow along the inner bank downstream of the bend apex where it is deposited as a linear ridge, or scroll bar, aligned subparallel to the inner channel bank (Hickin, 1974; Nanson, 1980). The ridge of sediment on top of the main bar appears to be a small scroll bar. Like this ridge of sediment, scroll bars typically have lee faces that face the inner channel bank and are composed of sediment that is finer than that on the adjacent bar platform (Jackson, 1976a; Bridge and Jarvis, 1982). Best ( 1988) has shown that bars with distinct ridges often develop along the margin of separated flow at the downstream junction comer in experimental asymmetrical confluences with M,> 1.O. The potential for scroll bar development is probably enhanced by high-stage conditions that reduce topographic steering of the flow along the inner bank and allow the helical cell to expand over most the channel cross-section (Thome et al., 1985). These conditions were probably

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fulfilled during the initial phase of the July 12 event when the main bar was still small but the flow was deep ( > 1 m) .During the August 6 event, topographic steering at the head of the bar was more effective than on July 12 and accretion of fine sediment on top of the bar occurred farther downstream. This pattern of bar elongation is characteristic of scroll-bar development in meander bends (Nanson, 1980; Jackson, 1976b).

6. Conclusion This field study has demonstrated that flow conditions during low-stage events at an asymmetrical confluence are strongly influenced by momentum ratio. total discharge, and extant bed morphology. Acceleration of flow through the confluence is primarily the result of constriction of the flow by a large bar along the inner bank of the downstream channel. As the momentum ratio increases from values less than one to values greater than one, flow from the tributary penetrates farther into the confluence, forcing the mixing interface between the two streams toward the outer bank. As a result, the helical cell produced by curvature of the tributary flow around the downstream junction comer increases in size and the counter-rotating cell on the other side of the mixing interface diminishes in size. The velocity of flow in the lateral tributary increases. enhancing the effect of curvature and the intensity of helical motion within flow from the tributary. Flow along the inner portion of the downstream channel is deflected outward by the main lateral bar, producing a region of flow separation along the inner bank. As the bar becomes increasingly submerged because of an increase in total discharge and/or a decrease in momentum ratio, the locus of maximum outward deflection of the flow, and, thus, the position of the separation zone. shifts downstream towards the bar tail. Further work is needed to document flow patterns at this confluence during formative events. The correspondence between flow patterns of low-stage events and the extant bed morphology at the site suggests that similar patterns may occur when hydraulic conditions exceed the threshold for sediment movement. In any case, this study has shown that transport-effective events in which one stream dominates over the other are capable of producing a persistent bed morphology

that influences the structure and mixing of subsequent low Hows.

Acknowledgements Mark Welford and Dan Mayer assisted in the collection of the field data for this project. We thank Jane Domier for her assistance in designing and producing the tigures. John Bridge and an anonymous reviewer provided comments that inspired new ideas and greatly improved the clarity of the discussion. This research was supported by grants from the National Science Foundation (SES-9024225 and SES-9242848) and the University of Illinois Research Board.

References Anthony, D.J. and Harvey, M.D., 1991. Stage-dependent cross-section adjustments in a meandering reach of the Fall River, Colorado. Geomorphology, 4: 187-203. Ashmore. P.E. and Parker, Cl., 1983. Confluence scour in coarse braided streams. Water Resour. Res., 19: 392402. Ashmore, P.E., Ferguson, RI., Prestegaard, K.L., Ashworth, P.J. and Paola, C., 1992. Secondary flow in anabranch confluences of a braided, gravel-bed stream. Earth Surf. Process. Landforms, 17: 299-3 I I. Bathurst. J.C.. Thome, C.R. and Hey, R.D.. 1977. Direct measurements of secondary currents in river bends. Nature, 269: 504506. Best, J.L.. 1987. Flow dynamics at river channel confluences: Implications for sediment transport and bed morphology. In: F.G. Ethridge, R.M. Flores and M.D. Harvey (Editors), Recent Developments in Fluvial Sedimentology. Society of Economic Paleontologists and Mineralogists, Spec. Publ. 39, Tulsa, pp. 2735. Best, J.L., 1988. Sediment transport and bed morphology at river channel confluences. Sedimentology, 35: 48l-l98. Best. J.L. and Reid, I., 1984. Separation zone at open channel junctions. J. Hydraul. Eng., 110: 1588-1594. Best, J.L. and Roy, A.G., 1991. Mixing layer distortion at the confluence of channels of different depth. Nature, 350: 4 1 I-4 13. Biron. P., De Serres, B., Roy, A.G. and Best, J.L., 1993a. Shearlayer turbulence at an unequal depth confluence. In: NJ. Clifford, J.R. French and J. Hardisty (Editors), Turbulence: Perspectives on Flow and Sediment Transport. Wiley, New York, pp. 197-213. Biron. P.. Roy, A.G., Best, J.L. and Boyer, C.J., 1993b. Bed morphology and sedimentology at the confluence of unequal depth channels. Geomorphology, 8: 115-129. Bridge, J.S.. 1993. The interaction between channel geometry, water flow, sediment transport and deposition in braided rivers. In: J.L. Best and C.S. Bristow (Editors), Braided Rivers. Geological Society of London Spec. Publ. 75, pp. 13-71.

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Bridge, J.S. and Gabel, S.L., 1992. Flow and sediment dynamics in a low-sinuosity river: Calamus River, Nebraska Sandhills. Sedimentology, 39, 125-142. Bridge, J.S. and Jarvis, J., 1982. The dynamics of a river bend: a study in flow and sedimentary processes. Sedimentology, 29: 499-550. Chang, H.H., 1988. Fluvial Processes in River Engineering. Wiley, New York, NY, 432 pp. Dietrich, W.E. and Smith, J.D., 1983. Influence of the point bar on flow through curved channels. Water Resour. Res., 19: 11731192. Dietrich, W.E. and Smith, J.D., 1984. Bedload transport in a river meander. Water Resour. Res., 20: 1355-1380. Fischer, H.B., 1969. The effect of bends on dispersion in streams. Water Resour. Res., 5: 496-506. Hickin, E.J., 1974. The development of meanders in natural riverchannels. Am. J. Sci., 274: 414-442. Jackson, R.G., 1976a. Depositional model of point bars in the lower Wabash River, J. Sediment. Petrol., 46: 579-594. Jackson, R.G., 1976b. Large scale ripples of the lower Wabash river. Sedimentology, 23: 593-623. Kenworthy, S.T., 1994. Hydrologic and morphologic influences on confluence flow structure. MSc. thesis, Department of Geography, University of Illinois. Kenworthy, S.T. and Rhoads, B.L., in press. Hydrologic control of spatial patterns of suspended sediment concentration at a stream confluence. J. Hydrol., in press. Mamedov, A.S., 1990. Hydraulic calculation of a confluence. Hydrotechnical Construction HYCOAR, 23: 553-556. Markham, A.J. and Thome, C.R., 1992. Geomorphology of gravelbed river bends. In: P. Billi, R.D. Hey, CR. Thome and P. Tacconi (Editors), Dynamics of Gravel Bed Rivers. Wiley, Chichester, pp. 433456. Mayer, D.R., 1994. Hydrologic control of spatial patterns of sutlicial bed-material characteristics at a stream confluence. M.Sc. thesis, Department of Geography, University of Illinois.

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Mosley, M.P., 1976. An experimental study of channel confluences. J. Geol., 84: 535-562. Nanson, G.C., 1980. Point bar and floodplain formation of the mean dering Beatton river, northeastern British Columbia, Canada. Sedimentology, 27: 3-29. Nelson, J.M. and Smith, J.D., 1989. Evolution and stability of erodible channel beds.In: S. Ikeda and G.Parker (Editors), River Meandering. American Geophysical Union Water Resources Monograph 12, pp. 321-377. Pens, GE. and Thorns, M.C., 1987. Morphology and sedimentology of a tributary confluence bar in a regulated river: north Tyne, U.K. Earth Surf. Process. Landforms, 12: 43340. Roy, A.G and DeSerres, B., 1989. Morphologie du lit et dynamique des confluents de tours d’eau. Bull. Sot. Geogr. Liege, 25: 113127. Roy, A.G., Roy, R. and Bergeron, N., 1988. Hydraulic geometry and changes in flow velocity at a river confluence with coarse bed material. Earth Surf. Process. Landforms, 13: 583-598. Roy, A.G. and Bergeron, N., 1990. Flow and particle paths at a natural river confluence with coarse bed material. Geomorphology, 3: 99-l 12. Thome, CR. and Rais, S., 1984. Secondary current measurements in a meandering river. In: C.M. Elliot (Editor), River Meandering, American Society of Civil Engineers, New York, NY, pp. 675-686. Thome, CR., Zevenbergen, L.W., Pitlick, J.C., Rais, S., Bradley, J.B. and Julien, P.Y., 1985. Direct measurement of secondary currents in a meandering sand-bed river. Nature, 315: 746-747. Young, P.C. and Wallis, S.G., 1993. Solute transport and dispersion in channels. In: K. Beven and M.J. Kirkby (Editors), Channel Network Hydrology. Wiley, New York, pp. 129-173. Weerakoon, S.B., Kawahara, Y. and Tamai, N., 1991. Three-dimensional flow structure in channel confluences of rectangular section. Proc. XXIV Congr. International Association for Hydraulic Research, A, pp. 373-380.