A Short Study of the Hydrography of the Estuary of the Avon and Heathcote Rivers, near Christchurch.
[Read before the Otago Branch, May 11, 1943; received by the Editor. September 9, 1943; issued separately March, 1944.]
The Estuary of the Avon and Heathcote Rivers, near Christchurch, is a large expanse of about seven square miles of tidal mud-flats, roughly taking the form of an equilateral triangle. The Avon River enters the Estuary in the northern corner, the Heathcote in the west, and the outlet to the sea is in the south-west. A sand spit about two miles long separates the estuary from the sea on its eastern side. At the point of entry of the Avon River, this spit is nearly half a mile wide and maintains approximately this width for another mile, when it suddenly narrows down to half that figure for the remainder of its length. This latter part is composed of loose sandhills sparsely overgrown with sand grasses and lupins and supports a few pine trees. The wider part of the spit is populated. The end is continually changing shape, due to the variable nature of the outlet channel. The depth of the bar is about six feet at high water. The north-east-south-west bank of the estuary between the inlets of the Avon and the Heathcote Rivers is composed of sand near the Avon and gradually changes to clay around the Heathcote. About three-quarters of a mile from the Pleasant Point bridge the Christchurch Drainage Board's sewage farm discharges its effluent into the estuary. This flows by a small channel to meet the main Avon channel in the centre of that part of the estuary. In the clay banks near the Heathcote River serious damage caused by crabs' burrows between the tide marks has necessitated the construction of concrete breastworks to combat the erosion. The remaining bank of the estuary for the most part skirts the trace of the hills and is thus composed of volcanic rock. About a mile from the outlet the road from Sumner crosses the estuary on an artificial causeway enclosing Macormacks Bay. This bay is further subdivided by a stone bank into two parts. The first is perpetually filled with water as the outlet is too small to allow it to drain completely between the tides. The other larger portion has two outlets to the main estuary, and almost completely empties except for a small pool around the larger outlet. Near the outlet of the combined channels to the sea, a massive rock stack, about 50 feet high, stands in the channel, always completely surrounded by water.
The disposition of the channels is shown in the map. It is seen that the two rivers open by means of two large funnels into a large, central portion of the estuary. The two main channels then coalesce and flow into another funnel which opens to the sea at its apex. Although the channels are continuously changing in shape and depth, these movements are such that the general form remains extremely constant. This map was drawn up in January, 1941, and differs but slightly from a survey made by the Lyttelton Harbour Board 20 years previously.
The volume of water flowing from the two rivers is by no means constant. The Avon and to a lesser extent the Heathcote, receives the effluents from the storm-water channels draining the City of Christchurch. When the area of roofing and tar-sealed roads is taken into account, it is at once apparent that with a very small fall of rain, the rivers will rise rapidly. Average figures would seem to indicate that the usual rate of flow of the Avon is approximately 100 cusecs and that of the Heathcote 75 cusecs.
Scope of the Work. The aim of this paper is to give a generalised picture of changes in the chemical nature of the water with the changes in the state of the tide in an estuary. For this purpose samples were collected from, different points in the two main channels at high and low water. These were analysed and the results of several such sets of analyses were averaged to give the results embodied in this report. These figures then, give a picture of the chemical nature of the water of most parts of the estuary at both high and low water. To give this picture continuity, series of samples were taken from two points of the estuary throughout all stages of the tide. These samples were similarly treated and compared with the former series. It is apparent that this procedure will not give exact data for any point in the estuary but should give a general picture of the changes in the estuary as a whole.
The constituents determined were the salinity, phosphate, nitrate, silicate, bicarbonate, hydrogen ion concenration, and carbon dioxide pressure. For the most part the water sampler described in the appendix was used for collecting the samples.
Height of the Tide and Rate of Flow of the Water
The height of the tide was determined by measurements made on posts and piles fixed in the bed of the estuary. Readings were made every half hour except around high and low water, when they were taken every five minutes. The rate of flow was determined by means of surface floats. The time taken to pass between two fixed points, one hundred feet apart, was found every quarter of an hour throughout the complete tidal cycle.
When the height of the water and the rate of flow are plotted against the state of the tide (time) the points tend to assume the form of a harmonic curve. At the outlet to the sea the two curves are nearly true, but as the inlets of the two rivers are approached, the variation becomes more and more apparent. The most noticeable fact is the lengthening of the ebb tide and the shortening of the flow. At the inlet of the Avon River, the flow tide occupies only four hours, while the ebb takes the remaining eight. In consequence of this, the average rate of fall of the water-level is twice as slow as the rise. Whereas the forms of the two curves are similar at the outlet to the sea, near the inlets of the rivers they tend to become separated so that the time between the slack water at low tide and slack water at high tide is not equal to the time between when the water is at its lowest and highest points (true high water and true low water) but greater. At all points the time occupied by incoming water is nearly balanced by the time occupied by outgoing water, but as the inlets of the rivers are approached, the difference between the maximum rate of flow in the two directions becomes more pronounced. Thus, at Pleasant Point bridge the maximum rate of flow on the incoming tide is about 1.5 ft./sec. and on the outgoing tide, about 1.0 ft./sec. In both the Avon and the Heathcote inlets it is seen that there are two maxima in the rate of the outflowing tides. When high water is reached at the outlet to the sea, the water in the main part of the estuary is still moving up stream. This fact and the fact that the rivers are unable to discharge during the incoming tide, cause an excessive banking up of water upstream, causing a great surge of water downstream as soon as the pressure is released. As soon as this pressure is released a fairly steady rate of flow is maintained for most of the remainder of the ebb tide. Then the second maximum is reached. This is the maximum rate of efflux of the outlet which has slowly travelled upstream. Then as the rate of efflux at the outlet diminishes, so does the rate of flow fall off here with a corresponding time lag.
And so the general movements of the water in the estuary may be summarised. Soon after low tide at the outlet, water begins to enter the estuary from the sea. The remainder of the water in the estuary is flowing downstream with the greatest velocity at the inlet of the rivers and with less and less speed as the outlet is approached until around this still water is found. The level here is raised slightly by the incoming tide-water tending to flow upstream, causing water higher up to come to a standstill, and finally to flow in the opposite direction. Thus the change in direction of the tide slowly passes upstream. The farther up the stream, the longer afterwards will this
point be affected. Exactly the opposite effect will be experienced with the outgoing tide, but the change will still be from the outlet upstream. This picture is complicated by two factors. Firstly, the rivers entering the estuary are continually tending to exude water. They will be exerting a constant pressure on the water at their inlets. On the incoming tide they will tend to resist the backward flow of water, thus making the flow tide shorter (water can not flow upstream until this pressure is overcome) and to slow up the process of emptying the estuary, due to the large volume of water banked up upstream which is unable to flow away while the tide has been coming in. The second complication is that of the momentum of the moving body of water. The water in the estuary, moving downstream at low water, or upstream at high water, will tend to continue in that direction until it is stopped by back pressure. This will result in a slight piling up of water. This helps to account for the unevenness of the rate of flow, the piled up water surging in the opposite direction as soon as the pressure is released. (Fig. 2, 3.)
Method of Determination. The total chlorides and chloride equivalents (bromides, etc.) were determined by titration of a known volume of sample with standard silver nitrate using chloride-free potassium chromate as an indicator. The density of the sample was determined with an immersion hydrometer and the salinity calculated on the assumption that 55.5% of dissolved salts were chlorides. The salinity is expressed in grams per 1000 grams of water.
The salinity is a direct result of the diurnal tidal movements and varies from a value which is that of the open sea to nearly zero. The values of the salinity in the Avon and Heathcote channels are summarised in table. It is seen that the greatest rate of change of salinity with distance occurs near the sea, and the inlets of the two rivers, while in the centre of the estuary it remains fairly constant. The horizontal distance between the high and low water graphs gives a rough indication of the distance upstream that the body of the water
will move on the incoming tide. Thus suspended or floating matter in the centre of the estuary at low water would, under favourable conditions, be expected to be carried about one and a-quarter miles up the Avon channel or about one and a-half miles up the Heathcote. The distance that it would be carried downstream on the flow tide would of course be greater.
Layering. Evidence pointed to a fairly extensive occurrence of this interesting phenomenon, but time did not permit of any quantative estimation and a boat, which would have been necessary, was not available. On the incoming tide, the denser salt water tended to displace the lesser dense fresh water from the channels, with the result that the salinity on the edges of the mud flats was considerably lower than that of the water in the channels at the same place. This also explains the unexpectedly sharp rise of the salinity of the water in the channels with the incoming tide which occurred in many places.
Macormacks Bay. The salinity of the permanently filled portion of Macormacks Bay showed an appreciable rise owing to evaporation during the summer months.
Method of Determination. The dissolved silicates were determined by Atkin's modification of Dienert and Wandenblück's Method, where the colour produced by ammonium molybdate in the presence of sulphuric acid is matched against picric acid solutions. Compensation tubes were used when the samples were at all cloudy.
From a chemical point of view, water will have a definite saturation value for silicate depending on the type of cation present. Thus sodium silicate is very soluble compared with calcium silicate. Water in a river will be saturated with silicate as it is thoroughly mixed with suspension of fine siliceous material derived from the bed. The lithological nature of the bed of the river thus determines the silicate content of the water. In the non-tidal reaches of Avon and Heathcote the silicate concentration was found to be constant at 200 parts/m3. It has been shown that the silicate content of the water of the open sea is dependant on diatom activity, these organisms absorbing the material and regenerating it into the water again. Harvey mentions a Spring maximum in the English Channel of 200 parts/m3, dropping in the Summer to 40 parts/m3. In the fully saline water at the mouth of the Estuary the concentration was found to be about 20 parts/m3 (November). In the body of the Estuary it was found that the silicate concentration decreased as the salinity increased. The decrease in concentration was plotted against the salinity and the points found to lie upon a curve which when solved gave the relationship:—
[SiO3]sat. - [SiO3]S% = 0.31 [S%0]0.56
where [SiO3]sat. is the saturation value for fresh water and [SiO3]S% is the silicate value for a given salinity [S%0].
The slope of the curve would be expected to change with the seasonal variation of the silicate in the open sea.
Carbon Dioxide Pressure.
Method of Determination. The carbon dioxide pressure of the water was determined by use of Saunders' formula:—
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
pH = 10.70 - 0.53√3c + log(Bik)/pCO2
Where pH is the hydrogen ion concentration, “c” is molality of the cations derived from a knowledge of the salinity, (Bik) is the normality of the excess base, pCO2 is the pressure of the carbon dioxide expressed in millimetres of mercury.
The pH was determined by the use of a comparitor colourimeter using Methyl Red as an indicator. The normality of the excess base was determined by carefully titrating 100 ml. of sample with N/100 acid. to a pH of 3.6 using Methyl Orange as an indicator and matching the colour against a buffer solution containing the same quantity of indicator.
Tables 3 and 4 give the relationship between these substances and the salinity and give the computed values for the carbon dioxide pressure. Samples collected October and November. It is seen that the pH and the normality of the excess base are proportional to the salinity. By comparison it is interesting to note that measurements taken in the open water of the Atlantic show the excess base to be from 0.0023 N to 0.0026 N with a pH of about 8.3 to 8.4.
Method of Determination. The phosphate was determined by Atkin's modification of Denige's Method. The colour produced by treating the sample with a solution of ammonium molybdate and a solution of stannous chloride (prepared by dissolving metallic tin in hydrochloric acid) and in the presence of sulphuric acid was compared with a similarly treated sample of standard phosphate solution.
The phosphate content of the sea water, like that of the silicate, is shown to undergo seasonal variation. Measurements made in the English Channel show 30 mgms./m3 in January falling to 2mgms./m3 in July. This variation has also been correlated with diatom activity. The phosphate content of the non-tidal reaches of the Avon and Heathcote, rivers which drain a heavily populated area, is spasmodically effected by the various industrial effluents received. Values of 20–30 mgms./m3 were usually encountered but frequently concentrations as high as 100 mgms/m3 occurred. The chief source of phosphate in the Estuary is from the Christchurch Drainage Board's Sewage Farm, which delivers approximately 10,000,000 gallons of treated effluent per day. An examination of a sample of this effluent showed phosphate to be present in the high concentration of 2 mgms. per litre (or 2000 mgms./m3). During the outgoing tide this sewage passes out into the centre of the estuary by a small channel. During the incoming tide this material is washed up towards the inlets of the rivers. This fact is in accordance with the sharp rise in the concentration of the phosphate at these points during that state of the tide. Values of 100 mgms./m3 were common. At the outlet to the sea an average concentration of 20 mgms./m3 was found during November.
Nitrate and Ammonia.
Attempts to prepare the reduced strychnine reagent for the determination of nitrate were unsuccessful, and it was necessary to resort to the tedious practice of evaporation to small bulk and a subsequent colorimetric estimation with phenyl disulphonic acid and standard nitrate solution. Due to the high concentration of organic matter, determination of ammonia by direct nesslerisation gave incomparable results. However it may be said that the same general trend of distribution seems to be displayed by these two constituents as by the phosphate probably due to the same sources of supply.
The Bed of the Estuary.
The bed of the estuary is a large tract of fine sands and muds which are completely uncovered at low tide except for the channels and a few patches of shallow standing water. In most places there is a gradual slope from high water mark down to the channel. These wide banks are not flat and continuous but frequently drained by small “consequent” runnels flowing more or less at right angles to the stream. In general, the bed of the estuary is fairly compacted except around the outlets of the sewers where it is very soft, composed of muds of an extremely fine nature. This and the liberation of quantities of objectionable gases from the decomposition of organic matter, most probably account for the complete absence of the usual fauna and flora—in fact, the only life there would appear to be a thin yellow surface scum of some form of Euglena and a nematode worm. The bottom of parts of Macormacks Bay is also fairly soft.
Concerning the nature of the sediments composing the floor of the estuary, it may be said that the material nearer the sea tends to be coarser than that near the inlets of the rivers and the material higher up from low water mark is generally coarser than that nearer the channels. Due to the more or less homogeneous nature of the sediments, the writer has found that the moisture content of the mud gives results that are directly proportional to the average grain size of the mud, if the samples are taken from the same vertical height above low tide mark and at the same state of the tide. Figure (15) shows the moisture content of samples taken at low water from high water mark at various points along the eastern bank of the estuary, the mud which is at first very sandy near the outlet, becomes continuously finer as the inlet of the Avon River is approached. A similar series of samples taken from low water mark at the same time demonstrate a significant discordance of results. It has been stated that the capacity of a river for carrying material in suspension varies as the sixth power of its velocity. Now, with the decrease in velocity downstream from the inlets of the rivers, first the coarser, and gradually the finer materials would settle out. This would be the case in the channels near low water mark, while at high water, the water flowing in an opposite direction would cause the phenomenon to be reversed, depositing first its coarse sands near the outlet and its finer materials higher upstream. Remembering that high moisture content represents fine deposits and vice versa, it is seen that this hypothesis is well substantiated by fact. The same state of affairs occurs in the Heathcote channel, but it is not so pronounced, partly
because of the more complicated nature of the channel and partly because the high water mark is the rocky base of the hills, so that there is not the same shallow belt of slowly moving water where deposition should occur.
Due to the evaporation from the exposed surface of the sands uncovered between the tides, the dissolved salts of the moisture held in the sand will tend to increase in concentration. Thus the salinity of the water contained in the sediments at high water mark will be very much greater than those at low water mark at low tide, firstly because they have been uncovered for a greater time than the latter and secondly because they have been saturated with water that is more saline than that of the latter. Thus in all cases it was found that the salinity at high tide mark was greater than the maximum salinity that passed that point at high tide. Figure 16 demonstrates the distribution of the moisture of the salinity in the sands of a typical transit from high water mark to low water mark, taken at low water near the Heathcote Bridge, where the maximum salinity of the water in the channel is 27.0% and the minimum nearly zero.
The difficulty of obtaining a sample of water from the centre of a channel and from a known depth, when the collector is standing on the bank, was overcome by the construction of the sampler described below.
The instrument consists of a brass cylinder of some 200 c.c. capacity fitted at the top and bottom with two brass cocks. A float chamber, a light sealed tin, is attached loosely to the top of the cylinder by three brass strips. The top cock is provided with a long arm having an eye at its extremity and two rings are securely brazed on to the sides of the cylinder near the top. The sample is taken in the following manner:—A long length of thin, strong rope is secured to the instrument through the eyes on the sides of the cylinder. A piece of thin cord is fitted at one end to the eye on the extremity of top cock-arm and to the other end to an eye soldered to the base of the float. The length of this cord gives the depth from which the sample will be subsequently taken (when depths of several fathoms are required it is essential that the cord be loosely wound about itself so that it will readily fall apart without releasing the cock). The top cock is closed and the bottom opened. Air is withdrawn as completely as possible from the cylinder. The writer used a bicycle pump with the washer reversed and well greased, and a strong clip for a stop-valve. This clip was opened on the extension stroke and closed prior to compression. A little practice will produce the right technique, when very good vacuums may be achieved. The tap is then closed. The float being in position in its clips and the float-cord loosely wound and securely placed, beneath the float, the whole apparatus is thrown (as the fisherman throws his line) towards the centre of the channel. As it strikes the surface and begins to sink, the float-chamber becomes detached from its clips. When the cylinder has sunk to the limit of the length of the float-cord, it pulls on the float and opens the top cock and is immediately filled with water. The apparatus
5.Top Cock Arm.
is then drawn to the shore by means of the rope attached to the cylinder and the water is removed by opening the lower cock. With the type of pump described above, 190 cc. were collected with a cylinder of 200 cc. capacity. Both the cocks must of necessity be well fitting and great care must be taken to grind the top tap well into its seat, as it must be able to move easily under the slight tension produced by the pull on the float and at the same time remain perfectly airtight under a considerable vacuum. The use of a good vacuum-cock lubricant is recommended. See Figure 6.
The writer wishes to express his indebtedness to Professor E. Percival, who extended the facilities of the Canterbury College Biology Department, for his stimulating criticism and for reading the proofs; to Dr. H. N. Parton for checking the chemistry involved in this paper; and to those who kindly assisted in the laborious task of collecting samples.
Harvey, H. W., 1928. Biological Chemistry and Physics of Sea Water. Cambridge Press.
Saunders, J. T., 1926. The Hydrogen Ion Concentration of Natural Waters…. L. Relationship of the pH to the Pressure of Carbon Dioxide. British Journal of Experimental Biology, 4, pp. 46–72.
|Avon Channel||High Water||Low Water|
|Heathcote Bridge.||Pleasant Point Bridge.|
|Pleasant Point Bridge.|