By Charles Evan Fowler *
The first Cedar Lake Dam was a log-crib structure used to impound water for supplying the City of Seattle, Washington. (ENGINEERING NEWS, June 17, 1909, p. 647.) This dam was also used to supply water for the municipal hydro-electric plant. The construction of a massive concrete dam at a point about one and one-third miles below the former outlet of Cedar Lake has just been completed at a cost of approximtely $1,500,000. This dam has a length of on the crest of 795 ft., a height of 160 feet above the old stream bed, 200 ft. above bedrock in midchannel and about 217 ft. above the deepest portion of the foundation. The dam is curved in plan, although it has a gravity section.
It abuts the rock wall of the mountain at the south end, but at the north end the masonry section terminates in a gravel plateau or morainal bank which lies at right angles to the dam and is in appearance a saddle east of the dam. This morainal bank or ridge is not a true moraine, the bank being morainal material crowded over into the old channel when the new was formed. The topographical condition may be noted in the sketch in Fig. 1 and in the view in Fig. 2. As originally designed the water face of this saddle was to be puddled and an earth dike with a concrete core wall built from the north abutment of the dam easterly to the 1620 ft. contour of the mountain, providing there a crest 20 ft. higher than the dam proper. As actually built there was no north abutment to the dam, but its crest stood some 10 ft. higher than the top of the moraine, as shown in the section in Fig. 1. No cutoff wall was built nor was any puddle applied to the reservoir face.
The reservoir created by this dam has a capacity of 146,500 acre-ft. up to a crest elevation of 1600, to which the dam has been completed.
[Fig. 1 Sketch Plan and Section of Cedar Lake Dam -- omitted]
[Fig. 2 Downstream Face of Cedar Lake Dam, Seattle, Wash. -- omitted]
When the plan was first proposed, a Board of Geologists, consisting of Henry Landes, a State Geologist, now president of the University of Washington, and Milnor Roberts, professor of mining engineering at the same institution, made a report in July, 1910, calling attention to the fact that Cedar Lake has a pre-glacial subterranean channel to the Snoqualmie Valley on the north, and to the possibility that under the great head proposed by the new dam, the waters might be forced out this way.
This was, however, disregarded and plans were proceeded with for building the dam as designed, but no definite plan was adopted for rendering the north abutment water-tight, further than to propose in case the use of puddle upon the reservoir face failed to make it tight, that a cutoff wall of some type could be constructed for a distance of 1000 ft. or more through this north bank or terminal moraine of loose rock, sand and gravel. The data in the City Engineer's office, including the test pits shown on Fig. 3 in which water stood, convinced the city's engineers that the dam had been located at the proper site.
Public clamor became so intense against the building of the damn at this location that the City Council, by ordinance on Mar. 5, 1912, caused a Board of Engineers to be appointed to investigate and report on the entire project. This Board consisted of Joseph Jacobs, E. H. Baldwin and Glover F. Perin, and their report dated May 6, 1912, recommended that the whole project be delayed for a sufficient period to enable a thorough investigation to be made not only at this location, but of a location they suggested for an earth-fill dam at a point 1800 ft. further upstream and 2000 ft. below the old timber-crib dam.
[Fig. 3 General Location of Cedar Lake Dam, Showing Weirs Measuring Leakage -- omitted]
This report was very exhaustive and among other things called attention to the porous character of the rock on which the dam was to be founded, and also of the same character of rock composing the south wall or abutment. The morainal bank which formed the north abutment and bank was as fully investigated as was possible form the data available, and further borings recoommended to determine more closely what the leakage would be and the best means to reduce it. Their deductions from the best data available indicated a probable leakage of from 50,000 to 65,000 acre-ft. per annum, or a 20% loss in storage and power possibility, although it was thought highly probable that the loss would exceed the above figures.
The recommendation was made that the foundation and south abutment be carefully grouted and a cutoff made at the north abutment by excavating a trench down to El. 1550, driving sheetpiling down to El. 1490 and filling up the trench with selected material.
Work at this time had been in progress by day labor in making borings, clearing, stripping and preparing the foundation. This method of conductiong the work was proving very expensive, over $600,000 having been expended on foundations, and as the city authorities had decided to ignore the work of the Board of Engineers and proceed with the work without making further examinations, the Chamber of Commerce of Seattle, through its Taxation Committee, instituted a further investigation.
Among reports received by them was one from Joseph Jacobs, a member of the former board, and R. H. Ober, Superintendent of Buildings and former Assistant City Engineer in charge of certain features of the dam and reservoir. Mr. Jaocbs called attention to the recommendations made by the previous Board of which he was a member and reiterated his belief that a more thorough investigation should be made.
Mr. Ober's report placed the possible leakage at a much larger percentage than that previously quoted and stated his belief that for this reason the new dam would not increase the storage and power possibilities over what then existed with the old log-crib dam, which had held a less head of water and had consequently a much less loss from leakage thorugh the porous north bank. Mr. Ober had proposed a plan for tapping the water from the bottom of the old storage lake, by means of a tunnel, constructing a permanent dam near the old log-crib dam, thus increasing the storage capacity up to about the theoretical capacity of the proposed new concrete dam to El. 1555. His belief as to the great loss from leakage was based primarily on the fact that test pits dug before the first reservoir was built developed the fact that when water from the river was turned into certain of these pits, it disappeared into the pervious material very rapidly, in one case disappearing with a roar. With the new dam the head would be increased and with the more porous nature of the formation at the north end of the dam this loss would make the project nearly prohibitive.
The Chamber of Commerce, however, approved the project and the net result of this investigation was to cause the stopping of the work by day labor and a contract to be let for building the dam.
While the dam has been practically completed to El. 1600, a center section was left out down to El. 1555, as shown in Figs. 1-2. Water was impounded in the new reservoir in November, 1914, and with a depth at the dam of 80 ft. above the bed of the stream, continued to leak steadily away, largely through the moraine, into the Snoqualmie Valley. No puddling had been attempted nor cutoff wall built.
One spring which has increased to a large amount is below the dam (Fig. 4). This flows back into Cedar River and is presumably from leakage thorugh the moraine and not through the seamy rock. The total fall in elevation at the head was approximately one inch per hour, and after lowering to a depth of 70 ft., the level is still steadily disappearing at the same rate of about one inch per hour, or over 30,000,000 gal. per day.
Weirs are maintained in the streams on the north slope of the saddle and an endeavor made to locate the leakage by comparison with the readings of former ones before the leakage began. The farthest leak outlet so far found is in the Snoqualmie watershed over one mile north of the dam.
[Fig 4. Spring near Cedar River below Dam, Showing Leakage Flow -- omitted]
The report of the assistant engineer engaged in these observations is as follows:
Nos. 1, 2, 3 and 4 weirs, which have been read for three years, showed no undue increase until Dec. 10, 1914, when Nos 1, 2 and 3 showed an increase of 50, 225 and 165%, respectively, two new springs appearing to increase the flow of No. 2. No. 5 weir was installed Dec. 3, on which date there was a flow of 19.7 cu.ft. per sec. A maximum flow of 26.8 cu.ft. per sec. was reched on Dec. 7, receding gradually. The present flow of this stream is estimated at three or four times its normal volume. No. 7 stream has weir readings for part of 1911 and all of 1912. Readings taken Dec. 8, 9 and 10 show an increase of 2.9 times the maximum flow of any period. The maximum flow of this stream occurs normally in February. From Dec. 3 to 15 no precipitation has occurred, the weather remaining cold and dry. Frost is in evidence in many parts of the ground, indicationg an increase in the above streams to be not due to local weather conditions.
The leakage in and under the dam itself is practically of no account, and in only one place in the drainage galleries of the great monolith is any water coming through the masonry and this is a very small seepage.
The Snoqualmie watershed is now being thoroughly surveyed and examined to determine whether or not there are other leakages showing up, and to make possible a summary as to the total loss going this way.
The city does not at present contemplate building a cutoff wall in the moraine, nor have any borings been made to locate the bedrock or old pre-glacial channel of the Cedar River to the Snoqualmie Valley, but a plant is now being installed to sluice material into the reservior in an attempt to stop the leakage by silting up the entire porous northern wall of the reservior up to the old crib dam for a probable distance of 6800 ft. In the meantime, however, test pits and borings will be put down in the morainal bank to locate bedrock and the old pre-glacial channel, preparatory to putting in a cutoff wall if the silting up does not obviate the cutoff wall, which would cost a comparatively large sum. On Jan. 4, the leakage at that time being considerably greater than was at first anticipated, $15,000 was appropriated to add to the repair plant.
R. H. Thomson, M. Am. Soc. C. E., former City Engineer, and who planned and constructed the first dam and reservoir at Cedar Lake and who planned, located and began the construction of the present system, estimates that it may take from one to five years to make the reservoir tight, either by the silting-up method or by grouting. This latter method would undoubtably be quicker if it proved effective, but would be very expensive.
The design and construction of the dam have been carried out under A. H. Dimock, the present City Engineer, with T. H. Carver, Assistant City Engineer, in direct charge of the designing.
* Consulting Civil Engineer, Seattle, Wash.