Laserfiche WebLink
<br />'.H. <br />}~ <br />d~ <br />.~.~. <br />___ PRE -FU)W BANK~ <br />- POST-FLOW BANK" <br />."lI; <br />-co- MAX. SCOUR (CML~. <br />SCOUR-COR,O..'~~j <br /> <br />. ',--;Jf;~ <br /> <br />,:.;1 <br />~------~---~ ~'80~1 <br /> <br />\ /' ,,.. <br /> <br />\ (/ ---PRE:FLOW BEot'i <br />Zr.5 - POST~FLOYI BED ;:\", <br />- PRE- a POST-FLOW BED b \ / _._ MAX. SCOUR . ,;-;//j <br />-.- MAX. SCOUR '....,,'. . 'J <br />Figure 4. Maximum scour map for (a) January 1974 flood :lad (b) December 1974 flood. \10;1 <br /> <br />~ <br /> <br />i' <br /> <br />562 <br /> <br />W. Emmett (1976 written commun.) and orhers have shown that <br />all eypes of rope tend to float when exposed by scour and hence give <br />unreliable scour indications. Flume and tank experiments on the <br />cord used in these experimems, however, showed that it floated <br />only in nearly still water, particularly a&er it had become saturated <br />wich water. The high-velociry shallow flows to be expeCted -in a <br />steep channel such as Quaeal Creek do nm allow these cords to <br />float. All cords recovered where scour had occurred were turned <br />sharply from the vertical to a horizontal position. In S(:vera! in- <br />stances, the right-angle bend in such 'cords was found as much as <br />0,5 m below layers of cobble "dropout anno," (Foley, 1977), This <br />suggests that subsequent burial of the parr of the cotd that had been <br />exposed by smur rook place before the flow declined from the <br />maximum, since recovery was made before any more discharge <br />events could disturb the stream bed. <br />Laboratory experimeOls (Foley, 1975) also showed that the <br />nylon cord was dimensionally stable under all moisture conditions. <br />Rep~ted surveying of cords not subjected to scour showed that <br />sagging or other changes of cord-tip elevation were less than the <br />surveying error. Laboratory flume experiments with minature <br />scour chains (Foley, 1975) .showed that scour chains or cords will <br />faU straight downstream when exposed by scour only during flows <br />in the uppet flow regime. Chains exposed by ripples became tan, <br />gled by seoondary flows in the crough, rarely were horizontal or in- <br />clined downstream, and occasionaUy were inclined nearly upstream <br />(Foley, 1975, Fig. 6-16). All cords recovered in Q'latal Creek were <br />inclined straight downstream except two in the lee of boulders, <br />which supports the suggestion to be made below that all flows were <br />in the upper flow regime. - <br /> <br /> <br /> <br />. 'f:l~ <br />-, 5 <br />_'0 <br />::::--;; . <br />_____ ,0 <br /> <br />.~q <br /> <br />'0 <br /> <br />/~ <br /> <br />-<s <br /> <br />~~ <br />.~ <br />~, <br /> <br />a <br /> <br />2~ METERS <br /> <br />- - - PRE - FLOW BANK <br />- POST-FLOW BANK <br />- /5 _ MAX. SCOUR (eM) <br />SCOUR - CORD <br /> <br />o <br />. <br /> <br />, <br />. <br /> <br />KJ <br />, <br /> <br />'s <br />. <br /> <br />'0 <br />. <br /> <br />A <br /> <br /> <br />"'jt <br />~ ~ J <br />~ 28.0 \...._ _._,_._........... . ......;' <br />.......,/ <br /> <br />A' <br /> <br />f <br />Z8_0 <br /> <br />a <br /> <br />M. G, FOLEY <br /> <br />Experimental Results <br /> <br />January 1974 Runoff Event. All scour.cords were recovered i <br />place shordy after storms recorded at Parriway on January 4 an <br />21, 1974. A shovel was used to remove small increments of be <br />material until the scour-cord was exposed. The length of SCOur <br />tord turned over indicated the maximum scour di recdy since th <br />end of the cord had been flush with the former stream-bed surfacl <br />Depth of maximum scour below rhe new scream-bed surface WJ <br />measured to obtain the amount of fill. These dara were used [0 dt, <br />tennine the elevation of the postrunoff channel surface relative [, <br />the preflood surface. <br />Figure 4a is an enlarged view of the area with the SCour-Con <br />array (shown in Fig. 3), with maximum-scour COntours shown fo <br />flow in the main channel. The south bar was not overtopped, anl <br />the array there is not included in Figure 4. No signs of overban. <br />flow on the stream banks in the study area were noted. The anioun <br />of maximum scour contoured in Figure 4a is the difference betweef <br />initial bed elevation and devario~ of the bottom of reworked be, <br />materiaL The difference between initial and final bed elevation wa~ <br />minor (shown as the same line in cross section AA' in Fig. 4a), am <br />scour can be measured equally well from either bed surface. <br />Decembc:r 1974 Runoff Event. AU scour-cords were recoveree <br />after a storm recorded at Pattiway on December 4, 1974. save one <br />that was presumed lost because SCour depth exceeded emplaceme.m <br />depth. Maximum scour from the preflow bed was at least 88 ~, <br />and possibly in excess of 1 m at the missing scour-cord locariol1. <br />The contoured map of maximum scour (Fig. 4b) is drawn relativ'e <br />to the preflow surface. MoaIbcacion of the souch bar was more ex- <br /> <br />, <br /> <br />OVERBANK FLOW (~. <br /> <br /> <br />.~,j <br />'~o~~~i <br />Jof ~ -:'~~ <br />;..~.~, <br />/~-.- <br />". '0__'; <br />". ~ <br /> <br /> <br />~ <br /> <br />"\(-:- <br />\['" <br /> <br />s <br /> <br />KJ <br /> <br />IS <br /> <br />20 <br /> <br />2.~ METERS <br /> <br />A <br />28.Sl- <br /> <br />28.0 <br />