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~ ~ Z ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~~r - <br />~n I - <br />til:l)IA11:N~1-I~IJN~l1(UI-~I~f:C'HNI(2Ul:ti, CCN~I~NAL I'I:NNSYLVANIA j LItUS1UN ANU til:U1MI~N I-('UN~I~FtUl. A11:ASll lil~.ti l`I <br />. ~ ~~ <br />;~ I <br />}. i, $I <br />It <br />pp <br />~..ru. <br />flcuar 23.-The center and east offstream ponds es viewed from the west side of the <br />construction area, basin 2A, July I I, 1974. <br />than 100 percent would occur. The efficiency for each storm was determined <br />by the amount of mixing, by the detention time of the runoffwater in the <br />pond, by the amount of runoff water, and by particle settling velocities. <br />Detention time varied from storm to storm depending on the amount of <br />runolf and the period between storms. <br />During storms the first runoff from theconstructionarea normally carried <br />the highest sediment concentrations. As the storm continued, the runoff <br />water usually carried lower concentrations ofsuspended sediment. Late in a <br />storm, the pond could contain water with high sediment concentrations that <br />had been int^rcepted earlier. This water could then be displaced by runoff <br />with lower sedimetit concentrations. During a storm event, the instan- <br />taneoltstrap efficiency of the offstream ponds could change significantly and <br />could be either positive or negative: , <br />Figures 24 and 25 show the center pond duringa storm that occurred May <br />12, 1974. Figure 24, taken at the start of the storm, shows sediment-laden <br />water being discharged into the pond. The inflow is displacing relatively <br />sediment-free water already in the pond, the discharge point is at the farend <br />of the pond, right of center. At this time the trap efficiency of the pond is <br />shout 100 percent. Figure 25 shows the pond near the end of the storm. The <br />culvert is discharging a substantial amount of sediment-laden runolf that is <br />displacing sediment-laden water through the outlet. At this time the trap <br />efficiency of the pond is near zero. <br /> <br />Fwuae 24,-Clear water in the censer offstream pond m basin 2A being displaced by <br />sediment-laden inflow al the clan o(the storm on May 12.1974. Note the distinct dd(eren« <br />in color of water in this figure and that in figure 25. A nylon blanket filled with concrete <br />protects the culvert-outlet channel. <br />Ftcupe 25.-Srdimenl-laden water in the center nflsl ream pond in basin 2A brinE di, placed h~ <br />sediment-laden inflow near the <nd of the stnnn nn May 12, 1974 <br />