Reclamation and drainage of tidal soils are closely related. Infact, during the initial stages of the process, they are largely synonymous.
In low-lying coastal soils, outflow from interior areas is blocked temporarily by high tides when it does not have free outlet to the sea. Thus, in these areas, draining water is ponded (stored) twice daily in drainage channels and adjacent low areas. This water may be fresh, salty, or a mixture of the two, depending on the hydraulic gradient between the sea and the ponded water. The basic purpose of reclamation in these areas is to prevent seawater from moving landward during high tide and to provide for the disposal of stored fresh water during periods of low tide.
Protection from overflow usually is achieved by enclosing such areas with dikes. Drainage may be obtained by establishing a system of internal drains with water discharged over the dikes by pumps, by gravity flow through gates, or by a combination of pumps and gates.
Pumps are necessary when storage for accumulating drainage water within ponding areas is not available; when flow through gates is prevented over long periods by tides, floods, or inadequate outlets into the sea; or when the construction and maintenance of foreshore channels are impractical.
The exact steps to be followed in the reclamation of a particular tidal area depend upon local conditions, but in all cases, when the area on the land side of the dike is wholly dried up, the initial overall appearance of the soils is that of soft mud. These soils, generally called "unripened soils" consist of loose alluvial deposits that have very little stability. Upon drainage, changes take place in the mud, which together are referred to as "ripening." Ripening precedes the common soil formation processes leading to full development of a soil profile .
Soil ripening of fresh subaqueous deposits starts with the loss of excess water from the mud, by evaporation and drainage. The water table falls and the soil above the phreatic surface becomes exposed to capillary forces that pull the soil particles into a closer packing arrangement. This induces a reduction of the soil volume, subsidence, and, in the end, the structural development of the soil. Together, these processes comprise the physical ripening of the soil.
The ripening process normally begins at the surface and slowly extends to the deeper soil layers. At the same time, chemical and biological changes occur in the soil (respectively referred to as "chemical" and "biological" ripening). Chemical ripening consists of oxidation processes and adjustments in the cation composition of the adsorption complex. Biological ripening involves the development of aerobic microbial life within the soil.
During the first steps of the ripening process, the hydraulic conductivity of the soil is usually very low and only artificial drainage keeps the surface clear of ponded water. To this end, the most suitable system is that of parallel field ditches, with spacing of the order of 10 m or more. The field ditches should be deepened gradually, in parallel with the progress of the ripening. Starting with depths of 30-40 cm, a depth of 60-70 cm normally is reached over a period of about five years.
By the time the ripening front has reached well into the subsoil (e.g., to about 60- to 70-cm depth) pipe drains also may be installed. In this way, during the rain period, the excess rainwater flows vertically through the cracks and then flows laterally, over the impermeable unripened layers, to the trenches and then down to the pipes.
The spacing to be used depends very much on the changes of the values of the hydraulic conductivity during the development of the ripening process. When a good number of ripening cracks are established, a spacing of up to 30-50 m may be used. On the whole, drainage can only remove the soil water above the field capacity. The removal of the pore water held below field capacity depends mostly on evaporative drying. This process is enhanced if there is a vegetative cover with a deep and extensive root system. Reclaimed land in northern Europe (the Netherlands, England, and parts of Germany) often is sown with reed just before emergence mainly for this reason. Reeds are followed in turn by the first crops. A grass ley, with legumes, may be sown and used for grazing sheep or cattle for some 10 years. During this time, soluble salts are leached from the surface layers to lower levels. Calcium gradually replaces sodium as the dominant ion in the cation exchange complex, organic matter accumulates, and soil structure is improved greatly. Sometimes, ripening may be considerably retarded by seepage of brackish water or seawater into the land. In such areas the process can be accelerated by intercepting the seepage in flow.
Coastal sands represent a valuable resource not only for their own sake as wild open country with an interesting vegetation, but also as a support area for beaches and other resort facilities. Recently, they have been subjected to an enormous pressure from recreation and even mining activities.
Sand dunes generally are made up almost entirely of silica particles and usually are lacking in water, organic matter, and nutrients. Moreover, they are unstable and shift under the influence of wind and waves.
When dune areas are being reclaimed, the first step is to ensure protection from sea erosion, by wave screens or groines. At the same time, the original vegetation must be reestablished in places where it has been removed or destroyed [29, 30]. Sea lyme grass is valuable for building foredunes because it can tolerate a high degree of salinity. A number of other species that are adapted to the unfavorable coastal environment also can be planted or sown. These include sea couch, red fescue, sea oats, and beach bean. After planting, the sand surface often requires stabilization. Bituminous stabilizers are suitable for this purpose and can be very effective if applied properly. To this end, cover crops of large-seeded annuals such as cereal rye and sorghum can be very valuable if sown at a low density along with fertilizer dressings. Experience from many different parts of the world shows that, in the first year, about 100 kg/ha of nitrogen and 25 kg/ha of phosphorus (given in two or three applications) are imperative. Where it is not essential to reestablish the native vegetation, attention should be focused on shrubs, mainly legumes, that grow rapidly and can act as slow releasers of nutrients for subsequent tree planting.
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