On the Estuary of the River Severn

300 ESTUARY OF THE RIVER SEVERN.

March 24, 1846. SIRJOHN KENNIE, President, in the Chair.

NO.736. " Onthe Estuary of theRiver Severn." By William Parkes, Assoc. Inst. C.E.* The author having been engaged in making observations on different parts of the Severn, below Gloucester, for Mr. Walker, when projects for the erection of bridges and other works connected with proposed railways were referred to him by the Admiralty, has laid them before the Institution, in pursuance of the advice often given by Mr. Walker,when President, that the younger members of the profession should communicate the result of their observations when engaged upon surveys, even though the information so obtained may not be perfectly novel ; and this, he hopes, will be sufficient apology for offering no more than a mere sketch, which is all that surveys, extending only over a few weeks, and made for particular purposes, can enable him to give. The river Severn takes its rise at the foot of Plynlymmon, in the county of Montgomery. At Welshpool, inShropshire, it becomes navigable;and gradually increasesin dimensions, in its course of 140 miles, between that point and Gloucester. About a mile above Gloucester the river separates into two chan- nels, which meet again about a mile below the city. One channel is nearly direct ; the other, which is smaller, passes by Gloucester, and is about a mile longer. For about 10 miles below Gloucester (as far as Longney) the river isgenerally deep and narrow; in some partsthere is a depth of nearly 20 feet at average low water. In general, the surface of the water is below the level of the adjoining land; but the suddenfloods, to which the Severn is particularly subject, owing to the hilly character of the country through which it passes, frequently raise the water above the level of the land, which would be inundated, but for the embpnkments on each side; From Longney, the river assumes the character of a tidal estuary, rather than of afresh-water stream. The effect of the land-floods, though still verysensible, is much diminished, and the rise of the tide, which above is of comparatively little importance for the navigation, is here the only means by which it is carried on.

* The discussion of this paper extended over four evenings, but an abstract of the whole is given consecutively, for the sake of facility of reference.

As far as Longney, the inclination of the surface of the water is slight, and follows the general rule observed in other rivers, of gra- dualdiminution inits declivitytowards the outlet, the freer and more enlarged channel allowing of a less surFace velocity, and, con- sequently, less fall, with a much greater discharge. The fall between Gloucester and thispoint is about lt inch permile. It is pretty regular; for although there are some obstructions of rocks and shoals, they are so low as not to offer any great impediment to the course of the stream, when there is an average quantity of water in the river. Below Longney the river becomes of greater width ; its low-water channel is irregular and shallow, so that in many places there is not more thanone foot in depth,and it frequently shifts among the extensive shoals of mud and sand, of which the bed of the river presents an almost continuous series. Thestream is ratherrapid at all times, but varies considerably, according to the quantity of fresh water coming down. The fall in the surface here increases to a most remarkable extent. For the first 2 miles, from Longney to Framilode, the fall is 9 inches pe.r mile; for the next 8 miles, to Hock Crib, 14 inches per mile; thence 5 miles to Sharpness Point, 24 inches per mile. Uponthe supposition, that below SharpnessPoint the highwater line is comparatively level, and calculating from the difference of the lift of the tide, the fall in the next 12 miles, to the Old Pas- sage, is about 15 inches per mile, below which point it becomes nearly level;for at Kingroad, 8 miles lower down, the lift is nearly the same as at the Old Passage. This lowerportion of theSevern, from Longney to theOld Passage, is, in fact, a great natural weir, which keeps the water of the upper portion of the river at a higher level than would be other- wise due to its distance from the sea. The greater part of the shipping avoid this natural weir, by taking the Gloucester and Berkeley Ship Canal, which, being 13 feet 6 inches above low water at Gloucester, carries that level down to Sharpness Point, 16 miles distant, or 25 miles in the course of the river, where the low water is 34 feet below it. What has been said willgive some idea of the capacity of the Severn, for draining a large portion of the central counties of Eng- landand Wales. The area of countrydrained by this river and its tributaries above the Wye, is 4,400 square miles. That drained by the Thames is 6,000 square miles; but from the Severn passing through a hilly country, a larger proportion of the water is removed by drainage, and asmaller proportion by evaporation,thau in the district of the Thames, theTrent, andother largerivers of this country.

The navigation of the Severn is equally important, and, perhaps, more remarkable, as it depends entirely upon thetide, from Kingroad to near Gloucester. Viewed in reference to its tides, the portion of the river now undernotice may be considered part of the Bristol Channel ; for it is from the peculiar form andposition of the Channel,. that it derives its facilities for navigation. The Bristol Channel is a funnel-shaped arm of the sea, directly open from thewestward to the Atlantic. The width at the mouth, from St. Gowan's Head to Hartland Point, is 43 miles; and it contracts, gradually and pretty regularly, in a distance of 90 miles, to a breadth of 7 miles at Kingroad. The consequence of this form is, that although the tide-waveon the coast of Pembrokeshire is not more than 24 feet high, the water, running up with considerable momentum, is, as it were,choked, and is lifted above its natural sea level. Thus the lift of a high tide at Kingroad is 47 feet ; at the Old Passage it is 45 feet; and it has been generally believed, that at Chepstow, which is upon the Wye, 3 miles above its mouth, the lift of tide was most ex- traordinary, having been stated to be56 feet, and some accounts have raised it as high as even 70 feet; but. the author, by Mr. Walker's direction, measured a tide, and found it to correspond exactlyat high-water level with the same tide at the mouth of the Wye ; the river fall at low water being at the latter spot 2 feet 8 inches. The lift at Chepstow was 37 feet 10 inches, and at Beachley,near the confluence of the Wye with the Severn, it was 40 feet 6 inches; so that atide, corresponding to a45-feet tideat the AustPassage, would rise 42 to43 feet, and an extraordiuary tide 4 to 5 feet higher at each place. At a distance of 2f miles below the confluence of t.he Wye, are the English Stones,rockya shoal of greatextent. The low-water channel throughit, which is called :he Shoots, is narrow and tortuous. Immediately above this is the Dun Sand, which lies in the middle of the river, having a channel on each side of it. .The width from shoreto shore, ishere 2 miles.Above the Wye is the promontory of Beachley, a high rocky headland, jutting into the middle of the river, and contracting its width to little more than a mile. At this point is situated the Old, or Aust Passage, a ferry which is on the main road from Bristol, and the southern and south-western parts of England, to South and North Wales and Liverpool. From the contraction of the river at this spot, the bed is nearly clear of mud and sand; in fact, with the exceptionof a smallbank called the Dod Sand, the bottomis hard limestone-rock. The main set of the tide

is through the channel for the navigation, which is close to the north, or Beachley shore. Thefirst set of flood tideis over towards the southern shore, owing to the influence of the land waterfrom the Wye, and when there is a heavy flood in that river, this direction of the current lasts longer, until, in fact, its power is overcome by the greater volume and force of the tidal water, when the latter takes a more direct course. The rise of a good spring tide at this point is, as has been before stated, about 45 feet; the flood lasts 44 hours, and the ebb 73 hours ; the flood-tide current is very rapid, generally about 64 miles an hour. There is scarcely any slack water either at high, or low water ;but the current turns, and thewater begins to rise, or fall, immediately after it has ceascd ebbing, or flowing. Small vessels begin to pass up at13 hour after the flood commences, driftiug up with the tide merely with a sail set, or in calm weather, with a boat ahead, to keep sufficient way on the vessels to make them answer the helm. Larger vessels, whichare alwaystugged by steam-power, pass up soon after half-flood, which gives them time to reach Sharpness Point, the entrance to the Gloucester and Berkeley Canal, before high water; whereas smaller vessels, whichnavigate the course of the river to Gloucester, are obliged to take an earlier part of the flood. Abouta mile above theAust Passage are the extensive banks known as the “ Slimeroad,” ‘‘ Narlwood,” and “ Oldbury,” Sands, which occupy the greater part of the width of the river, leavittg a channel on each side of them. The principal channel, both for the set of the tide and thenavigation, is on the uorthern side of the sands. The river there widens from 1 + mile to23 miles, andthen graduallynarrows, as faras Sharpness Point, where it ishalf-a-mile wide. At that point is the entrance to the Gloucester and Berkeley Canal. To form asafe and certain entrance to a basin, for ships of large burthen, from such an extraordinarily rapid current,was a work requiring great judgment and skill. In this portion of the river, large vessels, which have been stranded on the shoals, have been known to be completrly rolled over hy the force of the flood. At this point there is,as at the Old Passage, a contraction, above which the river again widens. The lift of the spring tideis about 30 feet, the duratio~~of the flood 29 hours. The flood-current continues running in the centreof the stream 10 minutes after high water, except in time of freshes, when it stops suddenly. There is a projecting point of rock on the south side of the river, immediately be!ow the entrance, which sets the current off; and a ship coming up with all available sail set till she is past this point,steers into the eddy behind it and letsgo an anchor, wphich

brings her to, just above the pier, whence s6e is warped round the pier-head into the basin. As thegreat force of the flood-current diminishes,there are various counter currents immediately in front of the entrance, which, towards the end of the flood, bring vessels closer in ; and sometimes an experienced pilot is enabledto steer his vessel in without bringing up at all. Notwithstanding the apparently dangerous nature of this entrance, not one accident, which could be attributed to its peculiar form, has occurred since the opening of the canal in 1826; and it appears to be generally allowed, that no situation in the river could have been more advantageously adopted, notwithstanding the great expense at whichthe entrance basin, the lock, andthe canal approach toit, were constructed.The workswithin, consist of asmall half-tide basin, a large ship-lock, and another lock for smaller vessels. The canal level is about 3 feet 6 inches above ordinary high water, and this level continues up to Gloucester. A permanent change of the low-water channel was effected about 3 years ago, by means of a remarkably heavy land-flood, when it was diverted from the south to the north side of the river : this prevents the full ebb of the water by 3 feet, at the entrance. Above Sharpness Point the river again widens, and, at low water, presents an immense extent of dry sand and mud for a distance of 5 miles. In this portion of the river there is, as has been stated, the greatest declivity of the lowwater surface,namely, 24 inches in a mile. Hereis first observed thecurious phenomenoncalled the ‘bore,’ well knownin this and some otherrivers in this country, such as the Parret and the Great Ouze. The low-water channel is small and shallow, and the current is rapid; while the flood comes in with such impetuosity, that the resistance of the fresh water, and the friction uponthe bottom, are too great to allow itto make a gradual rise, and like a breaker of the sea upon a flat sandy shore, it rises, as it were, in a mass, and, roning over and over itself, presents a foaming, almost vertical surface, generally 2 or 3 feet, and sometimes 5 or 6 feet high; reaching across the river, rising higher, but moving more slowly, as it passes over a shallow bottom, and where the water is deeper, diminishing in height and increasing its speed ; being almost lost in deep water. The ‘ bore ’ moves at a considerably greater speed than the flood-current which follows it, but the rise is very rapid. It is a curious sight to watch the tide in this portion of the river : at one moment the bed of the river is a vast extent of sand, and the small channel can hardly, in many places, be distinguished

from the shore; but in an instant many of the shoals are covered by the ‘bore,’ and an immense body of water rushes up with the utmost impetuosity. In 25 minutes after the ‘bore’ has passed, a crowd of vessels, some of 150 tons burthen, are sailing safely over shoals, which only a short time previously were high and dry; and in 13 hour there is a current in the oppositedirection of almost equal velocity, the water having in that short periodrisen 18 feet, and sometimes as muclras 20 feet. At Hock Crib, theriver, which immediately below is 14 mile wide, takes a suddenbend to thenorthward, and contracts in width to three-eighths of a mile. This is the most difficult part of the aaviga- tion of the whole river; for in addition to the strong current of the tide, the channel is so subject to rapid changes, that in some states of the river, the pilots going up are obliged to inquire of those coming down, what coursethey can safely take. On the north side of the river, below the bend, lies the Nooze Sand, a shoal of considerable extent, about 4 feet above low water. The channel lies to the north, or south, of this, according to the relative power of the land-floods and tidal-waters. Duringthe spring months,after the land-floods have been prevalent, the eouthern channel, which is the most direct course for them, is keptopen; while the tendency of the flood-tide isto close it, and open the northern channel, which is more direct for the upwardcurrent. The Nooze is formed of a quicksand,which is verydangerous to shipping. It is covered in 10 minutes of the flood, at spring tides; and the current very soon sets entirely over it, leaving the siteof the southern channelcomparatively still. Above Hock Crib the river takes a circuitous course for 8 miles, in the form of a horse-shoe, the other extremity of which is at Frami- lode, only 14 mile from HockCrib. In this part, the river, though considerably narrower, is generally of the same character as below; the bed consisting of shifting sand of a thickness of only a few feet generally, while here and there the substratum of lias rock crops out. The current is tending to make the course still more circuitous, by wearingaway the concaveshore, asd depositing the debris on the opposite side. It may not be uninteresting to notice, that near the town of Newn- ham, atthis part of the river,a tunnel was commenced about 30 years ago, and was carried under the bed of the river through the lias rock, until, upon entering a stratum of sand, the water burst in and the work was abandoned. The tunnel is now completely filled with sand, and is almost forgotten in the neighbourhood. At Framilode is the entrance to the Stroud Canal. The lift of tide there is 10 feet, and the timeof flood one hour.

A rather curious circumstance may here be remarked, with regard to the velocity of the tide-wave in this part of the river. It has been stated,that at Sharpness Point the time of the flood-currentis 2% hours, and at Hock Crib 13 hour. The first of the flood appears at the former place an hour before it is seen at Hock Crib, so that the time of highwater is nearlythe same. At Framilode,which is 8 miles higher up (following the course of the river), the first of the flood is 14 hour later than at Hock Crib, and the time of high-water is 40 minutes later. Thus in the former 5 miles, there is no difference in the time of high water, while in the latter 8 miles, there is a difference of 40 minutes. There is, however, at Hock Crib, rather more slack-water, at the top of the tide, than at Sharpness, so that the commencement of the ebb is rather earlier at the latter place; but the cessation of the flowiug appears to be nearly simultaneous : the difference betrveen a wide andstraight channel, suchas that from Sharpness to Hock Crib, and acomparatively narrow and crooked one, such as that in the Horse-shoe, may account for this ; the effect of theslackening of the force of thecurrent being more quickly communicated in the former case. Above Framilodethere is little toremark upon the tides. The ' bore ' travels with considerable speed in the more level parts of the river. By some observationsmade on the7th May, 1845, upon R very low spring tide, the ' bore' was shown to havetravelled from Sharpness to Gloucester,a distance of 25 miles, in 3 hours 55 minutes, or at the average rateof 6 miles per hour; while the velocity of the flood-water itself was no where greater than 4%miles per hour, and nearGloucester, where the ' bore ' was fastest, not more than 12 mile per hour. The ' bore' of course increases in height wherever it meets with an obstruction. At Stone Bench, 5 miles below Gloucester, where there is a rock, about 3 feet under low water,across the river,it is said sometimes to rise instantaneously higher than the subsequent high water, the lift of the tide there being 8+ feet. The lift at Gloucester is 8 feet, the duration of flood is three-quarters of an hour, and the time of highwater is 1% hour after high water atSharpness Point. The relative levels of high water in different parts of the river are worthy of notice. By some simultaneuus observations made in 1836, under the direction of Mr. Rhodes, it appears, that the high water was higher at Minsterworth, 6 miles below Gloucester, than at Sharp- ness Point, hy 2 feet 6 inches, and that it fell again 6 inches towards Gloucester. By the observations made for Mr. Walker in May, 1845, upon a much lower tide, the summit was at, or near, Nexn-

ham, where it was 1 foot 8 inches above the high waterat Sharpness ; and at Gloucester it was 13 inches lower than at Newnham. Thus, as might be supposed, the higher the tide, the further up the river was the sumrpitlevel of high water, and the greater the inclination. It is unnecessary to say much of the neap tides. As the great lift in the Bristol Channel is owing to the momentum of the great body of water flowing iu, it is natural to suppose, that the difference of the rise at the spring and neap tides should be very great, as this cause is SO much diminished; and, accordingly, the lift at the Old Passage, which at the spring tides is 45 feet, is only 25 feet at the neap; at SharpnessPoint it is 14 feet;at Hock C rib 5 feet ; and 4 mile higher up it is not felt at all. In the lower part of the portion of the river under notice, as the momentum of the water in one direction causes a high flood, so in the other direction it occasions a low ebb at spring tides ; and this cause being wanting at neap tides, the ebb is not so low by 10 feet at theOld Passage;hut from a few miles below SharpnessPoint the reverse is the case, for the river water being there an important part of the discharge, which is not retarded at neap tides to the same extent by the powerful flood, the low water is generally somewhat loaer than at springtides. After the above sketch of the nature of the Severn, it may not be out of place to offer a few remarks upon its capahilities for improvement as a navigation. By far the most important part of the river, in this point of view, is that below Sharpness Point, for there the whole trade to and from Gloucester has to pass. There, however, little improvement is neces- sary, for the greatlift of tideinsures, at the proper times, ample depth of water. But above that point (where only the smaller class of vessels pass, the cargoes of which will not admit of the expense of the Gloucester and Berkely Canal) the navigation is by no means good. There, as below, itis dependent on the tide only, and it is only the spring tides for five or six days that are available. It is not difficult to perceive, that on account of the great inclination, thelow- water channel can never be made entirely available for navigation, by any system of regulating or contracting. If the fall were made regular from Gloucester to Sharpness Point, along the present course of the river, the distance being 25 miles, and the fall 20 feet, there would be an inclination of 93 inches per mile. In the portion of the river for 2 miles above Framilod~,or 10 or 12 miles below Gloucester, where the present fall nearly agrees with this, there is a sufficient quantity of water for the navipion of small C1846.1 Y

barges and trows drawing 4 or 5 feet, and it may be presumed, that this would be equally the case up to Gloucester ; but even then, only the downward navigation could be carried on : for the velocity of the current would be too great for anyvessels tostem it, and thus theupward navigation would bedependent, as at present, upon the spring tides. But further, the consequence of removing the obstructions at and below Longneg,decreasing the declivity inthat direction, and increasing it towards Gloucester, must be to increase the velocity, and so to diminish the sectional area of water at and near Gloucester, and therefore to lower the surface ; the effects of which would be to draw offthe water above Gloucester and injure the navigation there. And if the bed were raised, to keep up the surface, an additional obstruction would be presented to the free passage of the land-floods, the consequences of which would be most serious to the landed interest. It mayalso be fairly doubted, whether the present sandy bed of the river, in most parts, is sufficiently tenacious to be more permanent than it is at present, under so great a current as would be produced ; and the flood-tide rushing up, highly charged with solid particles in suspension, is a constant disturbing force. It appears, then,:that a general system of improvement is not easily applicable to this portion of the Severn, so as to increase the useful- ness of its river-water navigation. It is possibIe that, in some places, local improvements might be effected, rendering the channel, such as it is, more permanent, by confiniug the ebbing and flowing waters to the same line. Any such improvement would be chiefly of advantage to the dow-nward trade ; for the upward.bound vessels float upon the tidal water only, being entirely raised above the low-water level, and they meet with no obstruction until the tide turns. Sometimes they can run from King-road to Gloucester, nearly 50 miles, in one tide. A great local improvement might be effected by cutting a canal (as was proposed last year by the South Wales Railway Company, in connexion with their bridges over the river) across the neck of the Horse-shoe, fromFramilode to Hock Crib. This,though it would be of little or no advantage to the upward trade,would much facilitate the downward trade; for thecraft, at present, in their downward passage make two stoppagesin the Horse-shoe, ateach of which they are detained a tide for want of water. By the canal, one tide at least would be saved, and sometimes two, for they would be able to enter at.Framilode, very soon after high water, and to be discharged at Hock Crib while there is sufficient water to float them in the river. A vessel leaves Longney (where there is always a stoppage) at high

water, and arrives at Framilode, 3 miles distant, at about 1% hour ebb at that place: the time of this period of the ebb, on '7th May, 1845, was 47 minutes past 10 ; allowing three-quarters of an hour to pass along the canal, it would leave the canal at half-past eleven, when, on that day, the tide had yet 46 feet to fall, which would give ample depth. This canal would also enable trows and barges to reach the Stroud Canal at neap tides.* The paperis illustrated by Thomas's Chart of the Severn, No. 4320, and by a diagram showing a longitudinal section of the river from Gloucester to Sharpness Point.

Mr. WALKERcordially concurred in the vote of thanks to Mr. Parkes for his paper, which evinced much observation, and a laud- able desire to contribute to the interest of the meetings, by commu- nicating the result of an examination of a locality to which he had been seut, for purposes connected with professional labours. Mr. Walker had formerly strongly enforced, from the Chair, the advan- tages that would result to the Institution, from such a course being pursued by its members of all classes, and he was gratified to find his advice had been followed so successfully by one of his assistants. The paper was full of interesting matter, and it was not possible to examine the phenomena described, without allowing, that the river Severn afforded a wide field for observation, and for discussion, upon subjectscomparatively untouched, and which, although of great importance, were but imperfectly understood. AtWorcester the rivernearly resembled a canal; a few miles below Gloucester it wae pent up by natural weirs; while the lower

* It has been suggested to the Author by Mr. J. R. McClean (M. Inst. C.E.), that the present opportunity should not pass, without calling the attention of the Institution to the necessity of having one datum for tidal reference, all round the coast of England. The observations on the tides have hitherto been insufficient to determine the cause of the great variation between high andlow water, at many places on the coaet; but if all the registers referred to the same horizontal line, much valuableinformation might be afforded, bothas regards the effects of the various forms and depths of estuaries, and of the operation of the prevailing winds on the tidal wave. In many cases it might befound, that the great rise was only relating to the low mater of the place, and that the low mater was under the level of places less affected by the tides, and, ir, fact, during part of the time of ebb, running up an incline, when referred to the level of these places. v3