The Flood Came and Took Them All Away: A Sermon on the Holmfirth Flood (1852) by Rev. Joshua Fawcett
The immediate cause of the late catastrophe was the middle portion of the dam at Bilberry Reservoir being lower than the top of the waste-pit.
This waste-pit was designed to carry off the waste or flood water, but the top of the embankment having sunk below the top of the pit, and suffered to remain so, the flood-waters had no proper or sufficient escape, but went over the dam, which, as a necessary consequence, gave way.
In the evidence before you, mention has been made of a spring, of different leaks, and defective workmanship, but so long as the level of the dam was below the level of the waste-pit, and the flood suffered to pour over the top of an embankment of this kind, it would give way, though there were no springs, no leaks, and though the best quality of puddling was put in as water-tight as possible. It would give way, though not so simultaneously, from top to bottom; it would be slower in its operation, but still quick enough to form a flood of terribly destructive effect in its course.
To enable the jury to apprehend clearly the force of all the facts of the case bearing directly on the engineering part of the question, it is necessary first to give some idea of the principles on which these kinds of dams are designed, and how they are carried out. This I will do in as few words as possible, and equally concisely offer a few observations on the design of this reservoir and dam in particular, and draw your attention to the evidence given as to the manner in which that design was executed.
In constructing a reservoir of the nature of the one at Bilberry, the site being fixed upon, the extent or area of the district the surface water from off which will be drained into it, is ascertained. Calculations are also made from the most authentic records of the quantity of rain that falls upon and flows off this area in a given time, both on ordinary occasions and what may be expected in times of flood. In these calculations allowance is made for absorption and evaporation.
The capacity of the reservoir when full is estimated from levels taken at different depths.
To impound the water, an embankment is formed across the ravine, or valley, to the height that will contain the greatest quantity of water at a reasonable economical outlay.
The supply of water which may be needed for the manufacturer or other uses, and for which the reservoir is constructed, is led away from the interior, and nearly at the bottom, by what we call “supply-pipes,” or enclosed channels, constructed of metal or masonry according to the size. The quantity of water to be discharged is regulated by sliding-valves (called here “shuttles”) working in these, or at the extremities of these pipes. The sliding-valves in this case work vertically, and are placed one behind the other at no great distance, and in the same pipe, so that the water passes through the opening of both slides, and if either gets fixed by accident or injury when down, the passage of the water is stopped, and the reservoir must necessarily fill, rendering it a difficult operation to get at the slide to rectify, besides losing the use or service of the water. It must be understood distinctly and borne in mind always, that these channels or pipes are solely for the ordinary supply of water for the economical purposes alluded to, and for these alone. Their capacity is regulated accordingly. They should be protected from anything but water pressing upon or passing through, and this is generally done by iron grating, removed to a little distance and so designed that though obstructions may be intercepted, and for a time rest against them, there shall be space for the requisite supply of water to get into the pipes and through the slides.
To carry off the waste water, and the floods that may fall on the drainage area, other arrangements are made. In the case before you a circular pit of masonry was built up in the body of the embankment on the inner side. The ordinary supply pipes passed into the bottom of the pit, and a nearly horizontal culvert was constructed to convey away from the bottom of the pit the water coming through the ordinary supply pipes, as well as any waste or flood water flowing down into the pit. The culvert leads to a goit for the supply of mills down the valley.
When these waste-pits are adopted, I need scarcely impress upon you that they are so designed in height and capacity, and the culvert also in capacity, that the flood or waste-water shall freely fall down the pit, and pass off through the culvert in sufficient quantities to prevent the water in the reservoir ever rising to the height of the top of the dam.
The position of the entrance to the supply pipes, and the plan for carrying off the flood waters at Bilberry, are, I understand, very common in this part of the country, but I would not counsel their adoption in such sites as the one in which this is situated, high up in a hilly district at the junction of two deep ravines, with precipitous sides and rapid descent from above — obstructions of various kinds may be expected to be continually brought down, particularly in heavy floods of rain. They would be drawn by the set of the current towards the pit, and may impede the escape of the flood waters in sufficient volume, by getting into the waste-pit and choking up the passage. Even if there was a grating over the waste-pit, they would be gathered about it, and by the downward suction be kept upon it. It is stated in evidence that a tree once passed into the sliding valve and there remained fixed. When I caused the water remaining in the reservoir after the “burst” to be drawn off, a large stone was jammed against the entrance of the supply-pipe, and the whole bed of the swallow is deep in mud, and wreck, peat, ling, and stones, close up to the sliding-valve. Some arrangement at the entrance of the swallow to prevent this is always advisable, in addition to a proper description of grating in front of the valve.
I would prefer the byewash which is in more general use. It consists of a notch as it were, cut out at one or both ends at the top of the embankment. Through this notch the surplus water passes, and is conveyed away along the side of the valley on a broad open ditch or canal to a safe distance and then emptied into the valley lower down if allowed to run to waste. . Obstructions getting into the byewash could be removed with more facility, certainty, and expedition. It is also possible to widen the channel on the side of the natural ground in some cases, and greater room made for escape on any occasion quite beyond human foresight.
I think it will be more convenient to you, if, before I proceed to allude to the embankment, I connect the foregoing observations to this particular case.
I estimate the drainage area for Bilberry Reservoir at 1,920 acres, shown on the accompanying portion of the ordnance survey. The space drained is coloured red.
I find it very difficult to get good data for estimating the quantity of rain that passed off the surface. Very careful records are kept at Woodhead, in the rallies on the other side of the hill-range, where Mr. Bateman is constructing a series of reservoirs for Manchester. By the kindness of Mr. Bateman I have had access to these records, and calculating from them, and supposing equal quantities of rain to have fallen on both sides of the hill, on the day and night of February 4th, 1,920 acres would have given a supply of 500 cubic feet per second. I am under the impression, however, that a considerably greater quantity must have fallen on this side of the range at that particular time.
The accompanying drawings will give you the dimensions of the waste-pit, the slide-valves, and passage included between them, the dimensions of the culvert including its length.
[Captain Moody here laid before the jury several plans and drawings of the reservoir, which he explained to them, and then continued.]
The waste-pit is circular, 12 feet in diameter in the clear, the depth to the bottom of the culvert is 59 feet. The slide-valves are at the bottom and 18 inches in the clear, the space between them is two feet in diameter and 25 in length. The culvert is 6 feet 4 inches high, and 6 feet 6 inches wide, semi-circular at the top, with perpendicular sides, and is 180 feet in length. The sectional area of the waste-pit is 113.09; that of the culvert, 35.4; that of the slide-valves, 1.7.
From these dimensions, with the pressure due to the whole height, the culvert has capacity to discharge about 1,500 cubic feet per second. The quantity coming into the reservoir is assumed at above 500, so that proper allowance has been made for its escape had the waste-pit been so circumstanced as to fulfil the object designed in its construction.
I will now proceed to make observations on the dam.
The water is impounded in the reservoir by an earthen dam across the valley. The one which has been adopted in this case is of a common construction, and perhaps the most economical. It is formed of a wall of puddle with a mass of earth on either side. The puddle is 16 feet thick at bottom, and 8 feet at top. The inner slope of the earth has a base of 3 to 1, the outer slope a base of 2 to 1.
The length of the dam is 340 feet, and was carried up to 98 feet high, according to the original design. This mass, or rather that part of it on the outside including the puddle, acts by its weight, which should more than counterbalance the pressure or weight of water acting against it. The object of the puddle-wall is simply to prevent the water getting through to the outer portion. It is to keep the whole watertight, and is not to be considered as having any strength in itself. Such a dam answers extremely well, if the materials are carefully selected and the whole work well executed.
The heaviest portion of the materials (the heavier the better, stones, &c.) should be placed on the outside, and the more binding materials on the inside. Close also to the puddle-dam or wall, on both sides, the material should be very binding in its quality, and well rammed; the nearer it approaches to the effect of puddle the better.
In the construction of the Bilberry dam this careful selection has not been made. The material is similar on both sides, and loose in its nature. The inner portion is permeable throughout, and instead of the part next to the puddle-dam being closely rammed and almost puddle in its character, a dry, open, rubble wall or backing appears to be carried up from the bottom, on both sides of the puddle-dam, inviting the water, as it were, to act on the whole inner surface of the puddle, and to escape with greater ease at any leaks or fissures arising from settlement or bad execution of the work. In flowing over the top of the dam (which it ought not, if the waste-pit was in a position to act), the water would flow down through this dry rubble to the very bottom, and, acting on any cavities or porous, or weak portions at that part of the embankment, would act with immense hydraulic pressure, — in fact, on the principle of an hydraulic ram. In the case before us you have it in evidence that the water, before passing over the outer surface of the dam, did pour down thus for half an hour, and also acting on the water which was forcing its way through leaks, and a spring at the bottom; the dam boiled up in the centre, as the witness stated, and burst out from the bottom, almost simultaneously with breaking away in masses from the top. It was thus the whole dam gave way, and the volume of water in the reservoir burst forth at once.
The construction and materials of the earth-work in the slopes of the dam are comparatively of minor importance to the puddle in the centre. The trench extending down the sides and bottom of the valley to receive the ends and base of the puddle-wall, should, as the specification before you provides, go down to the solid rock or impervious strata. All springs should be carefully led away, and even every fissure got past or through until all is safe, firm, and solid, clear of water, or what might be channels for it when the reservoir is filled, after the completion of the work. This observation applies equally to the sides as well as to the bottom. In executing this, it sometimes happens that very heavy and quite unforeseen expenses have to be incurred. The excavations are sometimes obliged to be extremely great in depth, and if the rocks are shaky or open in their stratification at the sides, it may be found necessary to puddle all over the ends or junction of the dam with the sides of the valley. It may be necessary to puddle part of the sides of the reservoir itself. In short, no care can be too great, and no expense withheld to make all perfectly water-tight. Leakages or springs are continuous and continuously injurious, reaching eventually, perhaps to very heavy expenses, if not to disastrous effects.
The puddle should also be of the best quality. But puddle should always be excellent. There are different opinions as to the best mixture. In this instance gravel and clay are mixed together, and it is unequal, though what is now seen in the embankment may be considered good. You have evidence of much which we cannot at present see, being bad, and the effects which are to be observed seem to confirm that evidence. To be water-tight and not liable to crack or settle unequally are the conditions good puddle should fulfil.
The trench to receive the puddle-wall at Bilberry Dam was cut down to a depth of 9 feet in the centre, in consequence of coming on a soft place. At this depth it appears that a very strong spring was tapped in the lower strata of shale. The section before you shows the stratification of the rocks (millstone, grit, and shale), and from the dip it will be seen that water might be expected to rise where it did rise. The stratification of the rocks immediately above the dam, are full of fissures and very shaky. From the runs showing themselves lower down than the dam, and the leaks at each end of the dam, when there was much water in the reservoir, it is to be inferred that the openness of the strata was not sufficiently regarded.
It appears in evidence that the spring at the bottom of the puddle-trench was not led away by any of the usual modes. I think it proper to observe that the expense of doing this would have to be borne by the contractors. It however was not done, but very objectional plans resorted to in hope of choking it up, or “weighting it down,” to use the words of the evidence. But it was not to be “weighted down,” it rose as the work rose, materially infusing the lower portion of the puddle; making it weak and bad, of a nature to be easily worked away with the water of the spring, as the latter forced itself through the outer part of the embankment like a little rill of water issuing from the foot. At times this rill was clear, and at times muddy and yellow. The muddyings varied with the head of water in the reservoir. To the weak nature of the puddle at the base, and the washing away from time to time by the continuous run of water from the spring under the bottom of it, the great settlement of the puddle-dam in the centre is to be attributed, a settlement which continued to go on during the construction, and after the dam had been raised to the height required in the specification. Of late years the settling down appears to have gradually ceased; doubtless the soft puddle had been nearly all squeezed out, and then would probably commence a different mode of action, leaks increasing in size and unequal settlements causing fractures.
The formation of the pits or craters along the top of the embankment admit of speculation as to their cause. It is not a matter of certainty. The lowest point of the pits now remaining are exactly at the edge of the puddle-dam, and immediately over the dry rubble backing described before. In one instance the lowest point is at the inner edge, and in the other at the outer edge of the puddle-dam. They might have been formed at the time of the high water or freshet alluded to in the evidence, and which filled the reservoir soon after its completion. At this time it is not unlikely that some water poured over the edge of the puddle-dam into the dry rubble backing carrying with it some of the earth, and leaving a crater-like formation. At the time of the catastrophe it poured into this portion, at the centre of the dam for half-an-hour. At the time to which I am alluding, it might have been only for a very short period. I am induced to think they must have been formed at an early period, when the bank was at its full height, because there is a similar formation on the right flank of the dam, at an elevation above the level of the top of the waste-pit. The top of the dam, nearer the centre, but close to this little crater or pit, has sunk bodily all across; and in the sunken part is a larger crater formed, I conceive at the same time as the higher one; both are shown in the plans and sections before you. This sunken part is over the culvert, and is no doubt due to the washing away of the bad puddling over and about the culvert where it passes through the puddle-wall below. This bad work, and the fruitless attempts to remedy it, are detailed abundantly in the evidence you have had before you. That evidence does not bear directly on the bursting of the dam, but proves the inferior execution, the misunderstandings, and faulty management and control that has marked the whole construction of the dam.
From the description of a leak half way up the middle of the dam, as well as the great depth and width of the centre pit, as described in evidence, but now washed away, I would infer it is probable the puddle was shaky at this part, and had some fissure near the upper portion, through which water leaked when the head of water was high. This shakiness would be caused of late by the continual wearing of the spring beneath.
I will not take up more of your time by alluding to other points that have attracted my notice; I would first observe they would simply serve to show, in addition to what I have already stated, that the execution of the work was not what it ought to have been, and bad execution in works of this kind, or any works connected with water, is fatal. The work must be good and water-tight, or they will be dangerous, and their destruction must come sooner or later.
In the evidence there appears to have been much stress laid on the great cost of this reservoir. I think it therefore right to observe to you that in hydraulic engineering generally it is extremely difficult, if not impracticable, to say with certainty what the final cost may be. It has been shown in evidence that eminent engineers have tendered estimates of different amounts to put the reservoir in an efficient and safe condition; the plan proposed being to cover the inner slope of the dam with puddling, and repitch it with stones, also to puddle a portion of the sides, and thus make it, if possible, water-tight. Still, if that had been done, it might not have answered so long as that full spring existed where it was, and unknown to the engineers also, runs of water round the flanks of the dam. They might have had to execute other works and incur other expenses. I am speaking of really eminent men skilled in their profession and well knowing what they were proposing. I do not mean "unprofessional” men who are unskilled, who do not know what they are proposing and had better, much better leave hydraulic engineering, and all engineering to engineers. I conceive it quite possible that it might have been necessary to extend the puddling and pitching far up the sides of the valley, making it almost like a tank. It might have been necessary to do this. The stratification is extremely full of fissures and shakes. In this neighbourhood there are many mountain reservoirs receiving floods of water, impounded by mill dams or lofty dams; pray don’t look upon them and treat them like fish ponds. They are engines of mighty force, strong in aid of your industry to augment your wealth, and terrible in their power to destroy if mis-managed or neglected. This fact must be indelibly impressed on the minds of all the dwellers in Holmfirth.
The conclusion of the above statement was marked by a burst of applause from the parties present.
The Foreman. — Is it your opinion, Captain Moody, that if the waste-pit had been sufficiently low, and although the embankment was in an unsafe state, would it have saved it from bursting?
Captain Moody. — I cannot but believe it would. I ought to add that it would still be a dangerous embankment; but I do not, in my own belief, think that if the waste-pit had been lower it would have burst now. It would ultimately have burst, because the mischief was going on.
Captain Moody said he had been requested to offer to them a few observations on the state of the Holme Styes reservoir. He had inspected it. It was a hill reservoir similar to the Bilberry Mill reservoir. It appeared to him that they should not delay sending for some superior engineer well acquainted with these works. Take his advice, and carry it into execution. Don’t think too much about making an economical bargain. They remembered what he told them just now respecting the grating which should guard the entrance to the ordinary supply-channel. There was one in this case, but not of a good design. It was vertical, and close to the channel, if not a little within it. It might therefore get choked up, and the ordinary supply of water not be able to pass through it. There was also the same arrangement in the sliding-valves, so that if either got down, the water could not get through and the reservoir must fill. When he saw the reservoir the water was 45 feet high, and there was a leakage in the valve-pit, and there being evidence of one outside the embankment of the culvert, the leakage in the valve-pit was very considerably increased. When he saw it, it was about the segment of a circle, 2 feet in the chord, and 4J inches deep, and by the stop-watch the water was running in at the rate of 16 inches per second. When he saw it the water running from the leakages alluded to was very muddy, and he watched it for a considerable time, in company with two brother officers. But these flaws were not so great as another. They remembered how he had recommended a bye-wash in preference to a waste-pit. There was a bye-wash to the Holme Styes; but when he went up to the reservoir the bye-wash was not only walled up, but firmly puddled! If the water had only risen a few feet higher in the Holme Styes reservoir on the night of the late terrible calamity, the inhabitants of Holmfirth would have had a flood down the other valley, meeting the one from the Bilberry reservoir at right angles, and the destruction of life and property would have been most awful. (Great sensation.) He assured them that when he saw the byewash built across, he said, “these people are insane.” (The audience unable to restrain their feelings, here gave vent to them in most unmistakeable terms.) He could not have believed it possible, that mill-owners who had property in the valley below could have suffered such a thing to exist. (Sensation.) But he saw it with his own eyes, otherwise he certainly could not have believed it. He took it upon himself, having the consent of the magistrates, to order the instant removal of this wall. (Loud applause.) There were sinkings in the reservoir. He would not tell them how he advised a remedy for the defects he saw, but would advise them to send at once for an hydraulic engineer and act according to his advice. He would just add one word about the men who had charge of these reservoirs: these men were not paid enough, and they had no right to expect from a man, who had only £5 per annum, that knowledge which the "duties of his office required." They ought to have competent men; and he would conclude by advising them that whenever these reservoirs got out of order not to attempt to remedy the defect themselves, but send for a competent engineer. (Applause.)
A Juror. — Would you state Captain Moody, at what depth you consider this reservoir safe?
Captain Moody. — The water ought not to be suffered to rise to the height of the sinking which is near the valve pit.
A Juror. — Did you then notice a sinking near the valve pit?
Captain Moody. — Yes.
A Juror. — Then do you think it would be safe up to that point?
Captain Moody. — I think it would be well to avoid it. I have already stated that the bye-wash is out of order and should be repaired. I noticed to you that the water should be carried by a bye-wash to a point of safety down the Talley, and I don’t think the point of safety has been reached in this distance.
A Juror. — Do you think the principle a safe one in permitting the water from the bye-wash to drop down the valley?
Captain Moody. — I do not think it a safe one (great sensation, and cries of “we’ll have it down.” “Let’s pull it down.”)
Mr. Superintendent Heaton lustily cried out “silence, silence,” but the feelings of the indignant audience could not easily be suppressed, and a working man, who evidently spoke the sentiments of others, said " how can we be silent; we cannot sleep in our beds, we are so afraid of this reservoir bursting.”
Mr. John Hinchliffe proposed some rambling questions to Captain Moody, and concluded by a gratuitous assertion that he was quite wrong in what he had said!
The Coroner, in dismissing the jury, begged to thank them for their attention to the long enquiry upon which they had been engaged. He cordially agreed in the verdict which they had returned, which he considered not at all too strong, and he hoped that their verdict, along with the very judicious remarks, arid the able report of Captain Moody, would have the desired effect upon the Commissioners of the Holme reservoirs, and would teach them that the affairs of the corporation should be conducted in a different manner to what they had been. If that would not accomplish the object, he was at a loss to know what would.