C150341 Feasibility - Laserfiche WebLink

Home Browse Search

ENGINEER'S INSPECTION REPORT DATE. 6/28/2011 DAM NAME: MOUNT PISGAH DAM I.D.: 120129 SEEPAGE PROBLEMS NOTED ❑(30) NONE ❑(31) SATURATED EMBANKMENT AREA ❑(32) SEEPAGE EXITS ON EMBANKMENT ❑(33) SEEPAGE EXITS AT POINT SOURCE Q(34) SEEPAGE AREA ATTOE Wj (35) FLOW ADJACENT T0OUTLET [:J(36)SEEPAGE NCREASED / MUDDY Show location of drains on sketch and indicate DRAIN OUTFALLS SEEN %*:]No ❑Yes amount and quality of discharge. [:J(37) FLOW INCREASED / tiumY ❑(38)DRAIN DRY/ OBSTRUCTED ,rV (39) OTHER - Ponded water was observed downstream of the old outlet conduit endwall. This is a historical seepage area. We found the V -notch weir to be in poor condition - bowed clogged with weeds and needs a staff ga-ge. The weir should be reset and this seepage should be monitored reaulariv for flow quantity and turbidity and recorded alona with reservoir gage height, especially at high reservoir levels. - Thank you for providina our office with the specifications from the dam rehabilitation after the 1928 upstream slope failure. Accordina to the specifications the upstream damaged portion of the two original 16" dia. C.I. conduits were removed and the "sound" portion of the pipes were left in place and was either sealed with cast iron stoppers leaded in or the upstream ends were filled with concrete plugs dependina on which way the bell ends were found to be facing We have no record of which was actually done. Regardless It sounds as if the remainder of the two C.1. pipes were left inside the dam and they are now around 100 years old. According to the C -55 plans the original upstream slope was 1 5H•1V The present slope was extended out at - 3H:1V. The C -55 plans show the crest at 76 -ft above the orinainaf outlet pipes Therefore we can say that the plugs in the old conduits would be a minimum of 114 ft (76'x 1.5) into the dam from the present upstream slope and likely more because some sections of the conduits may have been removed if they were damaaed in the 1928_ slope failure This would still put the upstream end of the old conduits at about the upstream 113 of the dam and there are likely significantly Pore pressures along the old conduits The conduits should be monitored carefully for increased seepage and external signs of distress. Consider inspectina the conduits and planninq to grout them or otherwise properly abandon them in the Iona term. - Very dry conditions have left the grass on the dam brown and would make seepage very obvious Besides the areen wet area mentioned above at the old outlet outfalls no indications of seepaae were observed on the embankment or at the downstream toe or the aroins. CONDITIONS OBSERVED: M Good MR Poor PROBLEMS NOTED [J(40)NONE ',❑ (41) NO OUTLET FOUND ❑ (42) POOR OPERATING ACCESS ❑ (43) INOPERABLE 7j (44) UPSTREAIs1 OR DOWNSTREAM STRUCTURE DETERIORATED (45) OUTLET OPERATED DURING INSPECTION OYES E,/]NO INTERIOR INSPECTED W(120) NO 71(121)YES ❑(46) CONDUIT DETERIORATED OR COLLAPSED ❑(47) JOINTS DISPLACED E] (48) VALVE LEAKAGE LrJ (49) OTHER - There was a 5 cfs release through the tunnel so we were not able to inspect it. - Valves and valve house are old and in poor condition Owner is working with contractors to have the valves, operators and valve house CONDITIONS OBSERVED: ❑ Good M Acceptable ❑ Poor PROBLEMS NOTED ❑ (`0) NONE ❑(51) NO EMERGENCY SPILLWAY FOUND [:J(52) EROSION WITH BACKCUTTING ❑(53) CRACK - WITH DISPLACEMENT (54) APPEARS TO BE STRUCTURALLY INADEQUATE ❑ (55) APPEARS TOO SMALL ❑ (56) INADEQUATE FREEBOARD ❑ (57) FLOW OBSTRUCTED ❑(58) CONCRETE DETERIORATED / UNDERMINED FVI(59) OTHER - we surveyed a cross - section approximately at the spillway control section and found it to agree closely with the 1988 C -1690 as -built plans: approx 11.4 ft freeboard (design freeboard was -10.9 ft per C4690) spillway top width ^90.6 ft and bottom with -57 -ft. The bottom width is about 4 -ft narrower than shown on the C4690 plans and may be because of talus material that has fallen from the left rock slope. Talus material should probably be cleared from the left side depending on the results of the Spillway Hydrolooy study and future plans for enlargement. -Based on the C -1690 sections we computed a saillwav capacity of -7,000 cfs at 11 -ft of head (0.4 feet residual freeboard). However, during the inspection we shot the low point of the downstream dike along the riaht side of the spillway at El. 7990.74 -ft Accordina to our hydraulic model the dike may overtop at - -6500 cfs Depending on the result of the Spillway Hydrology study and future plans for spillway enlargement the dike may need to be raised -0.5 ft to prevent overtopping The dike should be monitored durna spillway events. - We surveyed a cross- section through the spillway return channel at the downstream toe of the dam. Currently the channel is overgrown with willows using a roughness of 0.20 for thick willows and slope of 7.5% (from USGS topo) the return channel capacity is -1900 cfs before overtoppina the stone wall at the toe of the dam Assumina the willows are cleared and using rouahness of 0.035 the capacity would be -4000 cfs Willows should be cleared from return channel in the short term. Long term the return channel needs to be designed to convey the inflow design flood. CONDITIONS OBSERVED: El Good X❑ Acceptable 11 Poor Page 2 of 5