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.., . HYDRAULIC ENGINEERING '94 738 front of the boulder becomes more pronounced. ~e wake ~n~ is more disturbed in tbe form of a ripple patlem.' The pattern is termed side hydrauhc Jumps (8). d) F &. B Hydraulic ~jump ~, super super sub sub , e) B Higber flow line Hydraultc ~?-'-~ : super so ~ o D Higher flow line Hydraulic L ....,....,. jump "- ....... ,... .,..."./ l..~'''<, ...4 .j..............;'/:.l i a) LD Flow separation ~t sub b) S Flow Hydmulic separatIon jump :E?r c) F &. S Flow Hydraulic separatioo j~ump/ (\0Ij ... \... super super sub b sub sub super su Flsure I. Flow pattem c1assificalion developed from field observations (sub . subcrltical now. super = supercritical flow). 3 ) Progression to the third pattern is seen when the stage is high enough to ca~ the b~ild up of water on the leading edge of the boulder 10 break ~ dev~op I: · h dr' The shape depends on the boulder shape: . flat .ronl ge · rm:"~t I~i::.:::iic jump while a rounded front has a CUlVed hydraulic ~ump, At ~e breafing point of the jump there is a slight dip in the water surface e1evatlo~. !he ~I~ hydraulic jumps are present, with the widest point of the boulder detemllmng Ii ear location The wake zone between the two side jumps has a l~wer w~ter sur ace elevatio~ than the jumps and shows some disturbance or up~elhng_ Higher stag: show that the side hydraulic jumps become less curved asd!h~:c~~ g= ;::m them downstream faster and the wake zone becomes more IS e. . (F & S) (figure Ic) is referred to has hydmulicjumpson the front edge and both s.des dr' 4 ) In the fourth pattern the trailing edge disturbance becomes a pennanent .h~ rau ~~ j';mp while there is still one at the leading edge (figure ld), The rea~drauh~!~:~ very straight and is attached to the trailing edge of the bhouIder- II~ Pht d' ce~ a t~: water wake zone. The approach flow to the boulder may a~e.a S Ig Ip 10 surface The flow pattern is called front and back hydrauhCJumps (F & B), " S ) Th~ flow goes over the boulder into a well developed trailing edge hydrauhc Jump (figure Ie) fonning the fifth pattern, The hydmulicjump is still stmight,and at'i:r: the boulder At the lower flow stages of this pattern the water surface nses .at.. A edge befo"'; it gels washed downstream 10 fnnn the tmiling edge h~drauhc Jumt.. S the flow stages get higher there is a big build up of water on the leading e~8e,;. and of water, before it flows over the boulder. In some cases there. ~as no Side Ip acceleration. The name of this flow pattern is back edge hydraulic Jump (B). , . FLOW PATfERNS--STREAM tl9 6.) As Ihe stage increases further the hydraulic jump finally detaches from the bailing edge of the boulder and moves downstream. producing the sixth flow pattern. The jump is no longer straight, but is more curved as the edges are affected by the free s1ream flow. As the stage increases the hydraulic jump moves further downstream but the Froude numbers do not appear to change much. The approach flow also shows a big dip in the water surface elevation before it rises to flow over the boulder (figure 10. In the first five flow patterns there is a decease in water surface elevation from upstream of the boulder to downstream. In this pattern the water surface elevation is aboutlhe same and even seen to be higher than the approach when the stage is really high, This is the downstream hydraulic jump flow pattern (0), As the hydraulic jump move;s further downstream with a stage increase there could come a point at which the boulder has much less effect and where there is a dip in water surface elevation downstream of the boulder before it goes into a standing wave. This was seen on the fall River once but no measurements Were taken. 1.) The possible seventh flow pattern occurs when the stage is high and the boulder, is drowned ou1 (DO). This is very similar to what is seen in small scale roughness rh'e~. This pattern was seen at one stage on the Fall River. To analyse these flow patterns the relative submergence, Froude number relationship was used. Various methods of finding tbe approach velocity and depth wefe tested to see which gave the best results. Two ways of measuring the vertical height or Ihe boulders, the 'c' axis and the exposed height of the boulder, were also fonsidered. The best relationship was found by averaging, with some interpretation, .he approach velocity and depth, and the exposed roughness height, It can be seen from figure 2 that the Froude number appears to have little effect on .he flow pattems, It seems that the depth of flow mther than the velocity and I. roude number has a greater effect on the generation and location of hydraulic jumps. I he depth dictates at what level the flow strikes the boulder and, therefore, how it is dc:necled. The observed sequence of flow patterns seems to support this finding. When intetpreting figure 2 little can be said about the Orst two flow patterns, .he low disturbance pattero (LO) and the flow with side hydmulic jumps (S), as only three data points are seen for each. Generally, the relative submergence is higher for the ~ide hydraulic jumps and they do have some of the lowest Froude numbers. The .hird Oow pattern displays hydmulic jumps on the front and side of the boulder (FS) illld appears to be positioned mostly below a relative submergence of 0.8. The next ilc\'clopmen1 of the flow has the hydraulic jumps on the front and back of the boulder II- II), requiring a higher flow depth and so appearing in a higher relative submergence h.tnd between 0.8 to 0.9. When this patterns changes to the fifth, wi1h only one h)drdUlic jump on the trailing edge (B), the relative depth has to be greater and the hand lieen is from 0.9 to 1.0. The boulder is right at the submergence level for this n.IW pallem. When the hydraulic jump detaches from the trailing edge of the boulder dud moves downs1ream (D), the band is seen for relative submergence values of 1.0 and up 10 2.0 which is the upper limit for this data set, but it can probably go higher. I he po!lSible seventh pattern may be seen when the relative submergence is high C'llough 1hat the boulder's effect on the flow becomes drowned out. It could be c ___peeled to have a relative submergence greater than those seen for the previous flol."cm but the one lime it was seen it plotted in the midst of the previous pattern. ().lcuaaioD or now pattern analysis , A .heory of flow patleros was presented by Bathurst et ai" (1979) and can be \('cn 111 figure 3. Concentrating firstly on the flow pattern desCriptions it is apparent 'Ilallhe field data does not support the majority of the theory presented by Bathurst et ill . (1979). The firsl two categories are seen in the field, the low disturbance pattern I r.8ul< ,2. & 3.) and the appearance of side hydraulic jumps (figure 2b & 3b), ..\ccordmg to theory 1he jump then moves to the leading edge only while in the field _I " ,1' Ii ,I .~ , :\~ t f , 'I ; ~f ........