Laserfiche WebLink
<br /> <br />silt to bedrock. Bed ruck is shale. Gravel. content ranges from i <br />to 66 %, silt and clay d to l0 %, except in the sandy upper <br />layer, coarse sand content ranges from 18 to '~5 % and fine sand <br />content ranges from 11 Co 21 % . <br />Profiles of the streambed under the bridge r.ere taken in 19ti5 and <br />1985, fig. 'L. These show that except where a bank was built under U~~~.r <br />the bridge along the present channel --anc~-in the present. channel <br />the bed has degraded about 2 t't. The bank is about, 8 ft. trigh <br />and the stream bed has degraded about d ft. nn the avers@e.l- n <br />The degradation o£ the strr.am bed under the bridge is <br />understandable considering the fact that Jol,n Dtartin dam is only <br />lti miles upstream. The clear water releases t'rom the darn mould <br />pick up a new load of bed material and lower the elel•ation of the <br />bed. Also, confining ot• constraining the ricer channel in one <br />place b,y the construction of the dike downstream of the bridge <br />would cause it to degrade greater ttran the average. <br />PRESENT GRAVEL PIT SITUATION <br />The presence of a deep (45 to GO ft. deep) gravel pit only 'L DO <br />ft. down stream of the bridge presents a high rislr t'rum scour to <br />the bridge at both moderate and large. flood flows. Also, the <br />dike between the bridge and t}re gravel piC increases the risk to <br />the bridge. , <br />~f~`-'j~"Py~ ' <br />The dike is to protect the gravel pit from being flooded at <br />moderate floods but at large floras it is~to wash out to prevent <br />upstream flooding. It' the dike washes out the deep pit only 200 <br />ft, downstream will cause Dead cutting of the stream bed. This <br />head cut will under cut the present pier foundations. ~t.i,,lwl~~^i <br />If the dike is not overtopped during high flows it will <br />concentrate the flow onto the left (north) abutment, the left set <br />of piers and the north end of the dike. Tt,e dike is so high that <br />it will not be overtopped until a discharge of 20,000 cf:a. This <br />concentration of the flow will result in very high velocities su <br />that the north west corner of the dike will most likely mash out <br />and the flow willVgo'into the gravel pit. This increase :in local <br />slope will greatly increase the erosion of the bed there. A head <br />cut will then go up the present channel under cutting the piers <br />and the north abutment. <br />I would estimate that the degradation caused by the head cut <br />resulting from either the dike being overtopped or beinf; eroded <br />away at the north end would be from 15 to 25 ft. deep. 1'he sand ~ <br />and gravel bed of this stream is erodibTe:- "fhe gravel particles <br />would only armor plate the bed at the low flows. flow deep the <br />scour and how fast the head cut would move upstream would depend <br />on the magnitude and duration of the flood. It could take a <br />detailed computer simulation model sCud,v to determine ;lust huu <br />2 <br />