Copyright 2004, by J. Johnson (firstname.lastname@example.org).
The full report by the "Board of Engineers" (Joseph Jacobs, E. H. Baldwin, and Glover F. Perrin) on the Cedar River Project is lengthy and contains much material not pertinent to the Boxley Blowout. Here I have assembled pertinent excerpts and comments.
The Board of Engineers were given the following questions to consider:
(a) Is proposed location of the dam suitable for such a structure, and if not, what location should be selected? (b) Do the general plans and design for the pro- posed structure conform to the best engineering practice for dams of this magnitude; if not, what type should be adopted? (c) Is the foundation of the dam sufficient to stand the load to be placed upon it, and sufficiently impervious to prevent undue leakage? (d) Is there danger of seepage through the north bank with water at an elevation of 1590 feet if the lake be raised by gradual stages covering a series of years? (e) Is the proposed plan for the pressure tunnel and the location of the new power house to be preferred to the construction of additional pipe lines down the canyon of the river to the present power house site? (f) Is the plan proposed by the City Engineer for taking the pipe lines from the vicinity of Cedar River be- low Landsburg by the construction of a tunnel, the best and most practical method of securing the protection re- quired, or should steel pipes be constructed beneath the bed of the river crossings? (g) What method should be adopted by the City in future construction of pipe lines or other facilities for bringing water into the city from intake?
Of these seven questions four pertained to the dam itself: location, design, foundation, and seepage through the north bank.
Regarding the plan proposed by the City Engineer and the modifications suggested by the Board of Public Works, the Board of Engineers had quite a few comments and suggestions. However, they cautioned (page 12):
We would here, however, specifically emphasize that the above comments are not to be construed as a recommendation by us that a dam be built either to the height or at the location above proposed, and that it is intended merely to show, in event the City does proceed with the present plans, how a modified treatment would in our opinion secure a more rational design and effect a material saving in cost.
This is particularly noteworthy given the Board of Public Works claim a few days later that the "present site, the board holds, is indorsed by the three experts in their report as a safe foundation for a permanent dam and that the plans are in accord with the best engineering practice." As we will see below, the Engineers did endorse the plans (though with suggestions for improvement), and found that, regarding the foundation, a "stable and secure" dam could be built at the proposed site. But as seen above, these did NOT constitute an endorsement for the site itself. As will be seen, the Board actually recommended against proceeding at the proposed site. Why the members of the Board of Public Works--all experienced engineers--construed endorsement despite an express statement to the contrary seems to derive from not wanting to write-off the $300,000--a later estimate was $700,000-- already spent as a mistake. "Insane" was one later judgment.
The Board of Engineers had plenty to say about the proposed location (most of which is excerpted here). In particular they leaned strongly towards another site higher up the river:
Question (a):- "Is the proposed location of dam suitable for such a structure, and if not, what location should be selected?" This question must be considered not only with respect to phys- ical fitness, but as well from the view point of securing the most economical storage obtainable on the stream. Concerning physical fitness would say that the matters of type of dam, character of foundation and danger of seepage through the north bank, are discussed in detail under inquires "b", "c" and "d", which follows. We may say here, however, that we do not by any means regard this site as an ideal one for the height of dam proposed, not only because there must be an excessive amount of stripping of unsound rock from the dam foundation and that extensive grouting will be required to secure an impervious cut-off around the dam, but chiefly because the situation at the north end is one that requires very careful treatment. It is our opinion,however, that a stable and secure dam can be built at this site even to the height proposed by the City Engineer. [....] We examined Cedar River from the present crib dam to the present power house and believe that the first available dam site above the power house that could be considered is the one that has been selected by the City Engineer. Between this site, however, and the present crib dam, there is at least one site that appealed to us strongly, and which we believe should be surveyed and carefully considered in the light of well prepared comparative estimates before adopting the lower site as the ultimate location for a dam to impound water on this stream. The site referred to is at a point about 2000' below the present crib dam, where a spur from the hills on the south side extends down to the river, this being opposed on the opposite side by an abrupt bank which rises approximately to elevation 1590.
(It should be explained here that the "crib" dam is the original structure built at the outlet of Cedar Lake. Below the spur of rock mentioned above the river canyon widens to form the Masonry Pool behind the dam. This is the section with the troublesome "north bank", where the moraine of gravel and sand left by the glacier filled in the ancestral canyon and forced the river over the spur of rock where the dam is now situtated.)
Apparently no detail survey has yet been made at this site, nor have any borings or test pits been put down. So far as may be judged from surface indications there is a con- siderable depth of sand and gravel in this river bed and probably bed rock lies at a great depth, though this can only be ascertained by actual borings. The indications, however, are that the type of dam to be constructed here would be either an earth dam or an earth and rock fill dam, both of which have ample precedent and both of which may be regarded as safe types of dam construction. They can be made reasonab- ly tight against seepage losses, and the principal feature to be consid- ered in their adoption for this location is that of securing a proper cut-off in the river channel. The advantage of this location is, that to secure the same amount of storage as would be obtained by the construction of a dam at the lower site, i.e., with flow line of reservoir at elevation 1605, it would require a maximum height of only 100' as against a maximum height of not less than 225' and possibly 240' at the lower site. It seems to us almost unquestionable that this upper site will prove far cheaper than the lower, such rough estimates as we have been able to make from data now available indicating, in fact, that there might be a difference of $1,400,000 in first cost of the two structures, in which estimate an allowance has been made of $350,000 for the extension of power tunnel from the lower to the upper site. A further advantage of this upper site is that it would in measure reduce seepage losses resultant from high storage by reason of the long exposure of high river bank between this site and the proposed lower site, this being a bank having an average height of 90' and a length of about 5000', or, excluding lenght of proposed dike at lower site, which would cut off some seepage, a length of at least 4000'. Our present opinion is that a safe, substantial and permament dam of earth, or of earth and loose rock combined, can be constructed at this point, though the exact method of treatment of the foundation and cut-off would need be matter of further and more careful study, based upon the re- sults shown by actual borings. The final phase of this entire question that must be considered, is the actual height to which any dam should be built on Cedar River to secure the maximum or at least the most [?? missing in the source]. From our general study of storage conditions, as hereinbefore dis- cussed and our deductions as to seepage losses, as hereinafter discussed, it is clear to us that some height less than that now proposed is the most economic and every change in the direction of lowering the height of this dam renders the upper site more advantageous and indicates a greater disparity in its favor in the matter of first cost.
Question. (b):- "Do the general plans and designs for the pro- posed structure conform to the best engineering practice for dams of this magnitude; if not, what type should be adopted?" In our opinion the general type of structure proposed, namely, a masonry dam of gravity type, does conform to the best engineering pract- ice for dams of this magnitude and upon the character of foundation that here obtain. [Various detailed comments and suggestions.] In this connection we would again call attention to the fact that we are not recommending that a dam be built at this location, or to this height, but are now only considering type of dam, should the city decide to proceed with its present plan to build at this site.
Tell 'em, and tell 'em again!
Regarding the underlying foundation:
Question (c):- "Is the foundation of the dam sufficient to stand the load to placed upon it, and sufficiently impervious to prevent undue leakage?"
Curiously, they didn't really answer the question. But the trend of their comments is that the foundation at the proposed site will suffice, given certain caveats and concerns. And indeed it has sufficed.
However, the big question was that of seepage through the north bank:
QUESTION (d):- Is there danger of seepage through the north bank with water at an elevation of 1590 feet if the lake be raised by gradual stages covering a series of years?" This is indeed one of the important questions to be considered in connection with this development, for the amount of seepage loss that may be suffered, directly effects not only the amount of storage that need be provided, but as well the amount of power that can be developed. The right bank of Cedar River between the present crib dam and the proposed new dam, rises abruptly from the river bed to a general elevation of 1600, which means a bank approximately 80' high at crib dam and 150' high at side of proposed new dam. The brow of this bank forms the southerly edge of a morainal plateau extending northerly to the Snoqualmie River, the general elevation of 1600 being maintained for a distance of about one and one-half miles, thence dropping rapidly to the Snoqulamie River at an elevation of approximately 600' feet,which it reaches in another one and one half miles. So far as is disclosed by surface exposure along the creek bank along the cuts of the C.M. & P.S.Ry., which cross it and from such test wells as have been put down at Camp 2, this entire plateau was formed by glacial action and is made up of intermixed sand, gravel and clay of a highly porous character. That the formation is highly porous is indicated in several ways, as follows: 1st. By visual observation and examination of the material itself in so far as that was possible, there being no general deep borings from which samples could be obtained. 2nd. By the fact, that no surface drainage channels are revealed until the lower and steeper stretches of the plateau toward the Snoqual- mie River are reached, although the mean annual precipitation at its upper end is 110" and the mean annual precipitation for the entire plateau about 93", thus indicating the highly absorbent nature of the materials upon which this moisture falls. 3rd. The fact that water moves freely through the material, as can be plainly seen along the excavations of the C.M.& P.S.Ry. already referred to, and more definitely in the little stream which empties into the Snoqualmie River at the North Bend Lumber Co's mill which stream is in fact the gathering channel for all the water which ultimately comes to the surface from this plateau. Weir measurements made of the discharge of this stream during the progress of our inves- tigation indicate its present flow to be about 20 cu.ft. per second, which flow we are informed by the mill people is at times somewhat smaller than this, but also at times materially larger. Assuming 20 second feet as the mean runoff its per annum yield would amount to 14600 acre feet, which for a precipitation of 93" per annum over its small water shed of about 6 square miles means a ratio of seepage runoff to precipitation of 50% which may be regarded as high. It may be admitted at once that any attempt short of actual test measurements to determine the rate of ground water movement through these soils is more or less a conjecture. In the absence, however, of any such actual experiments, and in view of the many physical indica- tions of high porosity of the soil, it becomes necessary to make evalua- tions of the rate of such ground water movement, basing our deductions of course upon actual experiments in other localities and upon such theory as has been already been deduced in relation thereto. [....] we are led to believe that not less than from 50,000 to 55,000 acre feet of storage would be lost per annum, depending on the actual stage of water maintained in the reservoir. A loss of such an amount means we conceive it to be entirely possible that the seepage losses may be greater than above estimated. We regard the matter of such vital importance, in any event, that in our opinion no definite work should be done in the way of actual dam construction until this matter has been satisfactorily solved by actual tests and by actual tests we mean the putting down of a series of test wells to determine the position of the ground water plane between the proposed dam site and Snoqualmie River, actual measurements of the move- ment of ground water between these test wells and numerous mechanical analyses of the materials derived at different depths from said wells. Such a series of experiments would form a fairly secure basis for esti- mating probably seepage losses from the proposed reservoir and should by all means be made before further construction expense be incurred. As to the suggestion that the exposed right bank of the river would gradually silt up under a plan of raising the water surface in reservoir by slow successive annual stages, we would say that in our opinion no such result may be reasonably expected under the condi- tions which prevail here. We are entirely cognizant of the fact that in canals and rivers charge with silt there is a tendency to seal up sides and bottom of such channels, often to the complete prevention of seepage losses. It should be noted, however, that the head under which such sealing usually occurs is extremely small. And furthermore, it is unlikely that any appreciable quantity of silt bearing water would each this particular bank, all such material having practically been precipitated in the main Cedar Lake above, which acts as a large settling basin. Practically the only silt that could be picked up for such a process would be the silt contained in the bank itself and it is not believed that the velocity which seepage water would attain in moving through this soil (perhaps 30' per day) would be such as to either pick up or deposit sufficient silt to form an effective seal. Under the high pressure which would obtain against this bank and with the apparent steep hydraulic gradient of the water plane toward Snoqualmie River, we believe nothing short of a very thick and a very well compacted mat of clay puddle would be effective against seepage and such provision would be abnormally expensive. When one considers that in the construction of an earth dam, whose base width is usually five times its height, that undue seepage can be prevented only by first carefully selecting the earth materials of which the dam is built, and then compacting these material in 6" layers under a process of heavy rolling, the fallacy of the notion of sealing up this bank under heads at full reservoir varying from 80' to 150', by a mere surface coating of silt, becomes apparent.
"Fallacy"! "No such result may be reasonably expected". That seems clear, definite, and well-founded. Yet despite these plain words City Engineer R. H. Thomson remained unshaken in his belief that the reservoir could be sealed, and the City continued attempting to seal the reservoir through 1919 (see articles).
Several alternatives were considered, in regards of height of dam, location, and type ("masonry" [concrete] or earth-filled).
Cedar River Alternatives:- In our general investigation of this project, a number of alternative possibilities suggested themselves to us, most of which have already been referred to and all of which, in our opinion, should be given due consideration before actual construction is decided upon. These several alternatives may be recapitulated, as follows: Scheme 1. To build a masonry dam at the site now proposed and to an elevation of 1615, either along the lines proposed by the City Engi- neer or the modifications therof suggested by ourselves. Scheme 2. To construct a solid masonry dam to an an elevation of 1605 thereby securing all the water which we believe can be secured, seepage losses considered, such storage providing an ample supply to take care of years of minimum runoff, thereby securing continuous power from a single hydro-electric plant. Scheme 3. To build a solid masonry dam at site now proposed to elevation 1575, such storage to provide only for years of mean runnoff, the deficiency in years of minimum runoff to be cared for by the con- struction of an auxiliary steam plant located in the City of Seattle. Based upon our assumption as to probable seepage losses from the reservoir, we find this scheme will in fact develop more continuous power by reason of there being less seepage loss than will Scheme 2, though it will of course, involve as already indicated, the operation of the steam plant during such seasons as the runoff falls below the mean. Scheme 4. To consider the same alternatives as listed above with reference to an earth or earth and rock-fill dam as suggested by us for the upper site. In this connection it should be stated that the adoption of the upper site will involve the extension of the proposed power tunnel a distance up-stream of about 5200' and in weighing the difference be- tween the two locations the cost of such extension should operate as a credit to the lower site or a charge against the upper site. The possible difference in value between the least expensive and the most expensive of these alternative schemes, considering actual first cost of construction and the power that may be developed there- from, may in our opinion reach as high a figure as $1,500,000, and therefore indicates clearly how unwarranted it would be to proceed with this development without having first thoroughly investigated the possibilities suggested above. The whole subject, in fact, of power development in this region, whether municipal or private, is of such vital importance to Seattle's future growth as to be worthy of far more careful study than it has yet received. As we can conceive of no better investment the City could make than to place at the disposal of such department as is best equipped to handle the work, funds sufficient to institute and to carry on for several years if necessary, exhaustive surveys and investiga- tions of all the as yet undeveloped power possibilities tributary to the City. The results of such surveys and investigations, properly compiled, would be invaluable, not only to indicated what the City could and ought to do in the matter of future power development, but, what is equally essential, to indicate what it ought not to do, and to point the way for securing the maximum sufficiency from its future expenditures for those purposes.
And finally some recommendations:
Recommendations:- Our final recommendations concern- ing this project are as follows: 1. [Regarding crib dam repairs.] 2. [Regarding pipelines.] 3. That a series of well drill holes be put down along the line extending from Camp 2 easterly to foot of moun- tains just opposite present crib dam, for the purpose of deter- mining character of material penetrated, position of ground water plane, and, if practicable, the depth to bed rock. 4. That a series of well drill holes be put down along the line extending from a point say midway between Camp 1 and Camp 2 across the morainal plateau between that point and Sno- qualmie River, for the same purpose named in the paragraph above, and for the additional purpose of determining by actual experi- ments the rate of movement of ground water between those holes. 5. That further diamond drill borings be made imme- diately up-stream and down-stream from the line of of holes now put in at Camp 2 dam site. 6. That the upper dam side, hereinbefore described, be explored by putting down a few test pits at the higher eleva- tions along a line of dam and sufficient test drill wells and if need be diamond drill borings in river bed to determine the prox- imity or absence of bed rock. 7. That an accurate topographic survey of upper dam site be mde and that careful comparative estimates be made of the several alternative possibilities for dams of different heights, both at this location and at the Camp 2 location before adopting any scheme for final construction. 8. Our chiefest recommendation, and this is without qualification, is that constructive operations in connection with the building of a dam at Camp 2 be suspended until such time as a thorough investigation of the possibilities suggested in this report shall have demonstrated the best plan to pur- sure and that such investigation be commenced at once.
There it is, the "chiefest recommendation": suspension of current construction until a thorough investigation be made. This was rejected by the mayor; see the various news articles for details, and subsequent consequences.
But perhaps it wasn't quite clear just what was to be investigated, and why. After Moncton flooded Mayor Cotterill said the only possibilities that had needed investigation were regarding the type of dam; this was some what at variance with an earliar statement that the results of these investigations would affect only " storage capacity and advantageous height of dam, not location." Never mind that seepage was "one of the important questions to be considered", of "such vital importance" that "no definite work should be done in the way of actual construction" until the matter was resolved: the engineers did not contemplate the consequences of the seepage other than reducing power capacity, they anticipated neither the flooding of Moncton nor the Blowout. (Although the City of Seattle later suggested that the residents of Moncton had been "incautious" in not recognizing certain indications that Rattlesnake Lake had flooded before.)
For all that the Engineers failed to predict the Blowout, yet they were right on being most concerned with the very problem that was later so troublesome. The City's decision making, technical as well as political, was flawed, but it is an enduring question as to what the experts might have done to be more effective.
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