RECLAMATION OF TIDAL FLATS

New Zealand Journal of Agriculture, Volume 74, Issue 1, 15 January 1947, Page 49

RECLAMATION OF TIDAL FLATS

TIDAL flats need not be useless areas abandoned to a jungle of mangroves. With suitable treatment they can be made highly productive. How it is done is clearly set out in the following article.

By

E. B. GLANVILLE,

Assistant Fields Superintendent, Auckland.

r T’HE reclaimed tidal flats of New Zealand consist, in the main, of very fertile soils. They form a wide range, varying from sand to sticky clays, and in their natural stqte the majority of them are covered with mangroves. For general purposes they can be classed in three broad groupssands, sandy loams to sandy clay loams, and sandy clay loams to sticky clays.

The sands are generally found at a slightly higher level, overlying beds of pipi shells, often quite near the surface. .They are very light in texture, dry out rapidly during the summer, and seldom show signs of cracking, even when thoroughly drained. The sandy loams to sandy clay loams are heavier in texture, the shell is generally further from the' surface, and often they do not overlie shell beds. The soils in this group are generally found at a slightly lower level, than the sands; they crack immediately after stopbanking, drain well, and can be grassed successfully two or three years after tidal water has been shut off.

The clay loams to sticky clays are found in the low-lying areas. They often overlie soft, blue clay mud, and because they are so wet and lowlying, drainage is a slower process. It generally takes from six to eight years before soils under this group can be brought to a suitable condition for grassing. These soils do not crack until they begin to dry out. The cracking after stopbanking and draining is important, as it assists to remove the excess salt from the surface soil.

Stopbanking

Before any attempt can be made to develop tidal flats it is first necessary to build stopbanks on the edge of the area to be reclaimed to prevent tide waters gaining further access. The material for building the stopbanks is obtained by digging a ditch inside the bank. The size of this ditdh depends on the height to which the ■ stopbank has to be built in order to hold tidewaters back. The ditch serves as the main outlet drain to the flood-gates for the . reclaimed area. Stopbanks should be built a few feet from the edge of the ditch to prevent weight causing the sides of the ditch to slip and block this outlet drain. Stopbanks should have a batter of at least one to * one, and should not be less than . 2ft. above the highest known spring-tide level. The size, therefore, has to be determined by the height necessary.

Banks with a rise of 6 to 7ft. require to be 15 to 18ft. wide at the base; this will allow sufficient batter with the top of the bank 3 to 4ft. wide. The chief difficulty in getting stopbanks to stand is the erosion caused by the tide-lap on banks exposed to the direct action of a wide expanse of water. In places it is often necessary to protect the banks by facing them with stone or fascines. It is also advisable to plant suitable grasses on the banks for binding purposes. Buffalo grass is quite suitable, but is somewhat slow in covering the banks. Another .grass which is used considerably for binding is kikuyu grass. It has much the same' growth habit as buffalo grass, but is more aggressive, spreads faster, and is more palatable to stock. Although it does not seed, there is a decided possibility of its gradually

spreading on to the flats, where it would become ; troublesome and make future cultivation of the land very difficult, if not impossible. Where there is a quantity of shelly material in the . stopbank strawberry clover establishes well. It will grow under salty conditions, and will cover the banks, but has not the same strong, rooting system as buffalo or kikuyu grass. Taking everything into consideration, even although buffalo grass is slow in covering the banks, it is considered to be the most suitable grass for this purpose.

Flood-gates

The ditch, or main drain, which is dug to provide material for the stopbank, is provided with outlets through the stopbanks. These outlets are fitted with flood-gates to stop effectively the inflow of water from incoming tides, and to let drainage water out when the tides recede. The outlets through the stopbanks should be concreted and provided with hinged flood-gates fitted close into the concrete, so that they become water-tight with the pressure of incoming tide waters. Concrete pipes, 1 to 2ft. in diameter (or larger if required), cemented at the joints, and set in a concrete bed, are suitable for outlets through the stopbank. Wing-walls are required for about 4 to sft. on each side of the outlet to prevent erosion from the flow of water. A fluming is also required at the outlet for about 20ft. to carry the water, into the tide, otherwise a large hole will form in the tidal side of the outlet, which ultimately will undermine the bank and cause it to slip away. Concrete flood-gates are preferable to wooden ones, as wood warps because of one side being wet and the other side dry. Flood-gates are constructed at the lowest level to provide outlets for the main drains, into which the internal drains flow, and they should be sufficient in number and size to carry drainage water away quickly between tides.

Where the reclaimed area is almost level with the outside water at low, or partly low, tides, drainage through flood-gates is not very satisfactory. Where this occurs other means have to be used to remove drainage water collected by the internal drainage system. The only satisfactory method of removing this water is with floodpumps. Even where flood-pumps are used, it is generally advisable to provide flood-gates, as a certain amount of drainage, water will escape from the reclaimed area at low tide. This will allow more economical use of the pumps, which would be in motion only when the tide waters prevented the escape of drainage water through the flood-gates. ■

It is very important for the successful development of reclaimed tidal areas that provision be made for drainage water to be removed' as quickly as possible. If the area is at a reasonably high level, this can be achieved by the use of flood-gates, but if all drainage water cannot escape through the flood-gates • between tides, it is necessary to remove it by flood-pumps. It is imperative to have a survey taken of levels of the area to be reclaimed beforework is commenced, . so that data are available which will indicate whether flood-gates can be constructed. at a level which will adequately . allow drainage water to escape through them when the tide recedes, or whether it is necessary to install flood-pumps to lift surplus drainage water over the stopbank into the tide.

. The use of flood-pumps for removing drainage water from farm lands has been dealt with previously in the ‘Journal of Agriculture.” In the issue of September, 1935, J. E. Bell dealt fully with this subject, and an article by E.-H. Arnold was published in the November, 1942, issue.

Internal Drainage

Internal drainage on reclaimed tidal areas is essential to bring the soil into a state fit for grassing.

The percentage of salt in the soil must be reduced before grasses and clovers will grow satisfactorily. This process is more rapidly' achieved by thorough underground drainage. If the soil cracks freely after stopbanking, open drains every five to six chains to a depth of 2ft. 6in. to 3ft. are usually sufficient to carry off the water. In sands and sandy loams overlying shell open drains every chain to a depth of 2ft. to 2ft. 6in. will serve ’ the purpose of eliminating salt rapidly, but the drains are much more effective if fascines are used and the drains put in two chains apart at a depth of 2ft. 6in. to : 3ft. The sticky clay types are the most difficult to drain and deal with generally.

Stock should not be allowed on stopbanks. Open drains one chain apart should be put in to a depth of 2ft. 6in. and, if possible, converted to fascine drains. Rushes and sedges usually come in on these types some time before they are ready to grass. The drains should be acting well for at least two to three years before an attempt is made to sow grass. Time Required The period which must elapse from the time the sea-water is shut off until a reclaimed tidal area is ready to be sown successfully to permanent pasture varies considerably with the type of soil and subsoil dealt with. After years of submersion by sea-water the

soil becomes saturated with salt. It may take two to ten years,' or more, before the percentage, of salt is reduced sufficiently for pasture grass and clovers to establish satisfactorily. While there is a large percentage of sodium salts in the soil, the clay types, particularly those with little or no shell near the surface, remain sticky. Ploughing or cultivating them too early may do a great deal of harm in rendering these types unsuitable for the growth of plants other than rushes and sedges. ' All types overlying shell beds are easily dealt with, and may be made ready by thorough drainage to carry permanent grass and clover within two or three years after stopbanking. Sandy loam to sandy clay loam types which crack freely after

stopbanking require least drainage, and are soon ready for grass. Sands with no shell which do not crack, and sticky clay types which take a considerably longer time to drain efficiently, are the slowest to come in. A thorough internal, underground drainage system working satisfactorily is essential before any attempt can be made to prepare these types for permanent grass. Indicator Plants As the drainage on reclaimed tidal flats begins to take effect, and the soil becomes sweeter through the percentage of salt decreasing, two plants, known as sea aster (Aster subulatvsi and fleabane (Erigeron canadensis). commence to establish. They are

known as indicator plants, and they act in the capacity of a soil chemist. When they start to come in and grow on reclaimed areas it is an indication that the soil is becoming sweeter through the salt content diminishing.

Sea aster (Aster subulatus) is the first indicator plant which makes its appearance. It is an annual with quite smooth stems and leaves free from hairs. The plant seldom reaches more than 3ft. in height. The lower leaves may be up to Bin. long and fin. wide, slightly fleshy and smooth to the touch, becoming thin when dry. The margins of the leaves may be somewhat wavy, but lack distinct teeth. The upper leaves are shorter and very narrow, widest near the bases. The branched inflorescence is more spreading and open than that of fleabane. The flower heads are purplish in the centre, and the white rays are usually more distinct than in the fleabane. Sea aster will rapidly become general over the reclaimed area ,as soon as the salt Content of the surface soil is slightly reduced. This plant definitely indicates that the salt is commencing to go from the soil, but it is not in a fit condition to prepare for permanent pasture until fleabane , (Erigeron canadensis) commences growth and begins to replace sea aster. Fleabane (Erigeron canadensis) is an annual, with erect, rather wandlike, finely-grooved, bristly-hairy stems. On good soil it may reach a height of 10ft., but on poor soil may flower when only a few inches high. When cut, it stools freely, assuming a bushy habit. The lower leaves are

about 3in. long and up to fin. wide, the widest part being above the middle. They are rough to the touch, owing to the bristle-like hairs, and usually show several distinct, distant teeth along the margins. The upper leaves are much narrower, usually

lacking teeth, and produced in abundance. The flower heads are very small and numerous, more or less clustered on branching stems from among the upper leaves. Small white rays in the flowers can be seen only on very close inspection. Fleabane is often known as horse-weed, butter - weed, fire-weed, or bitter-weed, but is generally known as fleabane. It is rejected by stock, and can be classed as a harmless weed. The leaves are bitter and the plant is objectionable if occurring in any quantity ■ of hay. On reclaimed areas in the process of sweetening it grows on the higher parts, and does not invade the flats till the salt content has considerably decreased. As the sweetening process proceeds fleabane comes in first along the edges of drains, on drain banks and stopbanks, and gradually invades the lower areas. Fleabane does not become general until the land is well enough drained and free from salt to take grass well. When the area reclaimed reaches this stage, and fleabane has practically taken charge, preparation may commence for sowing permanent grass, but it is a mistake to make any attempts at grassing until this stage is reached.

Cultivation

Considerable areas of reclaimed flats have been grassed satisfactorily after surface cultivation. This applies especially to soil types which come under the heading of sand and sandy loams. Even although fairly successful pasture can be established after

this type of cultivation, better results are obtained after ploughing and a fallow before sowing. Ploughing should be done during the late autumn, before the land becomes wet, and the furrow x slices should be broken down roughly. If the land is left in this condition through the winter, the rains will assist to wash out more salt to the lower layers. or into the drainage system. It is generally not necessary to plough a second time, but the land should be constantly worked with a cultivator and penetrating harrows during the summer. This sweetens the soil considerably.

and allows adequate aeration while a good seed-bed is being prepared which will be consolidated from the bottom up. The seed-bed should be ready for sowing during late February, and one stroke of the Cambridge roller just prior to sowing the seed should be sufficient. Clay soil types cannot be handled satisfactorily without ploughing, on account of rushes, , and they should be subjected to thorough summer cultivation before sowing to grass. It is necessary, on the clay types, to clear the mangrove trees before any progress can be made with ploughing and cultivation. If mangroves are present, they rapidly die when the sea water is shut off the area, but they entail a considerable amount of work in clearing where the trees are large. Stumping is often necessary to prepare the land for ploughing.

Grassing

Where the drainage is efficient, and the cultivation is thorough, no difficulty is experienced in establishing a high-producing sward of perennial ryegrass -and clovers. Strawberry clover does well on reclaimed flats. It will establish before other types, and should be sown in all mixtures. White clover establishes soon after strawberry, but is checked considerably during the summer, when reclaimed tidal flats become very dry. Perennial ryegrass will thrive only when the clovers have made good establishment and are. thriving. Paspalum, although slow to establish, does well when established, and forms a good combination with perennial ryegrass and white and strawberry clovers. Italian ryegrass, alsike, and red clover can be used, but they form only temporary elements. A mixture which

has proved satisfactory on well-drained reclaimed areas consists- of—lb. per acre. Perennial ryegrass (Certified) 25 Paspalum (Australian) .... 5 White clover (Certified) 2 ' Strawberry clover .... 2 Red clover .. 3 37 Note: Paspalum is included for North Auckland, but further south, where climatic conditions are cold, it should be deleted.

Strawberry clover (.Trifolium fragiferum) has proved a very valuable plant for establishing pastures on re-

claimed tidal flats. Without strawberry clover it is practically impossible to obtain successful pasture establishment on soils such as are found on tidal flats. It is a perennial plant, and is like white clover in its habit of growth, spreading over the ground by means of stolons. Like white clover, the stolons strike roots at the nodes, but it differs from white clover in that it has a very deep rooting system; its roots will penetrate up to 2ft. into the soil.- This deep rooting system explains why strawberry clover grows so successfully and produces highly through ' the summer on soils such as these, which dry out so rapidly during hot summer weather.

The leaflets of strawberry clover are hairless, and similar in appearance to those of white clover, only they are a true oval, instead of being heart-shaped or narrower at the base as compared with the leaflets of white clover. The flower is very similar to the white clover flower, but the petals usually have a. pinkish tinge. Later it . forms a strawberry-like seed head from which it receives its name. Strawberry clover establishes best in damp situations. Although it will exist under very wet conditions,, it grows best where the drainage is good. It will exist in drier, more salty, and in wetter conditions than white clover, but it demands high fertility for vigorous growth. In the winter, and in wet summers, it grows less vigorously than white clover. In dry summers, when white clover ceases to grow, strawberry clover produces abundantly. Without clover, perennial ryegrass will not thrive. A good clover ' establishment in laying permanent pasture is the secret of success.; Strawberry clover is the pioneer plant which paves the way for other high-producing grasses to grow on a soil type where it is difficult for other clovers to establish. As strawberry clover establishes, perennial ryegrass commences to grow through it. and the pasture sward becomes a ryegrass-strawberry clover one. Such a pasture is capable of high butterfat production per acre, the ryegrass producing most of the feed in the autumn, winter, and spring, while the strawberry clover maintains production during the summer. In areas where climatic conditions are suitable for paspalum, strawberry clover greatly assists this grass to make the sward a better producing one, especially during the summer.

Fertilisers

Soils on reclaimed tidal flats are very fertile, and manorial trials conducted on them have given little information. The most promising results have been obtained from superphosphate, basic slag, and potash. When in good condition the land produces a strong growth of clover which

causes a considerable amount of bloat in dairy stock. Topdressing with phosphates or potash only increases this trouble by producing more clover when a response is given. It is the practice of many who are farming these areas to use a topdressing such as ammoniated superphosphate, or sulphate of ammonia, to encourage a stronger growth of ryegrass. It appears advisable to dress the land with

a light dressing of phosphate at the time of sowing, and on the sticky clay soil types it would be an advantage to use, in addition to phosphate, ground limestone at wt. per acre to assist the rapid establishment of strawberry and white clover. Subsequent topdressing depends entirely on the sward established. If the sward does not ■develop into a high-producing one without topdressing, it is necessary to build up the fertility with phosphates, and perhaps lime or potash in addition.

Crops

These soils grow good crops of •mangolds, turnips, and maize, but unless a normal season is experienced, the crops are grown with difficulty. The soil is difficult to cultivate, and if wet weather is experienced, it becomes very heavy and soft.. Attempts have been made to grow lucerne, but few successful stands have been e-stab-lished, as the winter conditions are generally too wet for the - production of good crops. Maize can be grown successfully, and the grain is valuable •for pig and poultry feed. With the prolific summer growth of pasture, adequate supplies of hay and silage can be saved, and farmers on these areas are inclined to rely on all-grass farming, using the hay and silage during periods of lean grass growth, rather than face the difficult cultivation necessary to produce good ■crops. References Allan,- Dr. H. H. “Fleabane and Sea Aster.” (Their identication and incidence in tidal reclamation.) "N.Z. Journal of Agriculture,” .'June, 1933. Bell, J. E.—“ Reclaimed Tidal Land at Pipiroa.” (The value of strawberry clover.) “N.Z. Journal of Agriculture,” October, 1937.

Hill, R. P.—" Small Farm Settlement on Reclaimed Tidal Flats, Whangarei Harbour.” "N.Z. Journal of Agriculture,” September, 1937.

flamblyn, 0. J.“ Pasture Establishment on Reclaimed Tidal Flats.” (Some experimental work at Whangarei.) “N.Z. Journal of Agriculture,” August, 1932.

Rowan. R.—“ Reclamation of Tidal-Swamp 'Lands in North Auckland.” “N.Z. Journal of Agriculture,” , December, 1917.

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