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Discussion Development
Environmental Impacts
Management inputs
Macro Design
In the suitable areas for Swamp Development there are five major environmental conditions determining five different design options at macro level. Each of these design options should focus on the effects for the on-farm conditions based on the physical characteristics of the area.
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Areas on the edge of Tidal and Non-Tidal Swamps. These areas are characterized by extended periods of high water levels in the upstream river sections adjoining the scheme, while gravity drainage depends on the most downstream river sections bordering the scheme. |
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Areas in Freshwater Tidal Swamps with no potential for Tidal Irrigation. These conditions are representative for the largest area in the Swamps. Land levels are equal or above mean high tide water levels during springtide periods. Often these areas are situated at least 3 km away from the nearest river. |
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Areas in Freshwater Tidal Swamps with a potential for Tidal Irrigation. These are often the traditionally developed areas along the rivers. Mostly they are situated within a distance of 3 km away from the nearest river. |
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Areas bordering peat domes. Peat domes need a buffer zone with the cultivated land. A collector drain parallel to the contourlines should be made which will be used as a collector of run-off water from the peat dome. |
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Areas bordering saline rivers most of the year. These areas should be mainly used for tree crops or fisheries. Tree crops are preferably grown in areas with a tidal range >3 m. |
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Areas
far away from the river (more than 10 km). These areas basically need
pumped irrigation before they could be made suitable for rice cultivation. They compare
with the Plain of Reeds in the Vietnam Mekong Delta in
problems and solutions. Design Options, based on hydrological and soil characteristics: GENERAL  For
Primary, Secondary and Tertiary canals: The flows should be able to flow
for drainage as well as for supply, depending on the needs in the
fields. That could include leaching of toxic elements. Also the type of
crops and the rainfall and the land elevations against flood
levels determine if drainage or supply is required.
This means they must be double connected
(No dead-ends of canals) are essential to be able to reach all parts of
the fields.
This
flow potential in primary and secondary canals is possible by making short-cuts to nearby rivers, or to arrange
upland flow including peat dome flow , depending on
local conditions
Double connection
always means need for water control including control of hazard of
“over-drainage”
For
each Lay-out and Scheme it would require investigations how best
drainage or supply can be obtained for the local conditions. Structures are required
at Primary or Secondary or Tertiary level, depending on conditions.
In many cases structures at tertiary level may be already sufficient to
reach flow potential in the area.Distance between tertiary canals should be not more than 200m and distance of tertiary double connected tertiary canal to nearest secondary canal should be not more than 1.5 km These recommendations for the water management infrastructure in Tidal Lowlands are essential to promote the required flushing potential to prevent stagnant water problems. For an example of a recommended design: see lay-out below on this webpage. It proved that the shortest distance from a field to fresh water in main canals is essential to enable optimum available water for flushing, irrigation or drainage. This relative short distance is the most important factor for deciding to recommend double connected tertiary canals with structures, instead of dead-ended canals. Dead-ended canals suffer from severe stagnant water at the dead-ends. Often these fields are abandoned. Double connection immediately solves the problem of stagnant water at the dead-ends. Exceptions on this rule might be found in areas on the edge of Tidal and Non-Tidal Swamps. See just below on this webpage. Examples are also found in the Mekong Delta where the main primary canal gets its water upstream and this water is used in a supply system lay-out, while the area is drained by a separate drainage system that is draining directly towards the coast/sea. Another exception might be found on areas bordering peat domes. See below on also on this webpage. |
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Areas
on the edge of Tidal and Non-Tidal Swamps. These areas are suitable for a lay-out with a separate supply and drainage system.
There are considerable water level differences during several weeks and often longer,
between mean water levels in the upstream sections and mean water levels in the downstream
sections in the canals. These height differences could promote an one-way flow through the
system and the fields when the water supply system is completely separated from the
drainage system. However in presence of acid sulphate soils there could be major problems
at dead-ends of these canals. It is recommended in that case to make double connected
canals. That means the supply system should be connected with the drainage system by using
overflows on the transition. In that case the rice might be grown partly outside the
flooding season to be sure sufficient leaching will be applied. In these conditions pumped
irrigation should be used to promote leaching and keep down the toxic groundwater during
dry spells. Recommendations: Complete separate supply and drainage canal system. In case of presence of acid sulphate soils: double connected canals with overflows on the transition of supply and drainage canals. Control Structures. Flood protection. Adaptation of the cropping season for low lying, deeply flooded, areas. Pumped irrigation when to grow rice in low lying areas, especially for acid sulphate soils. |
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Areas
in Freshwater Tidal Swamps with no potential for Tidal Irrigation. (The largest and most important part of the Swamp Schemes)
This area should be subdivided in two areas:
Interchangeable Supply and Drainage system. A separated supply and drainage system should not be made in these areas.It will be difficult to generate one-way-flow in the fields. (no level differences in upstream and downstream levels). In practice, excess water will often disappear to the nearest tertiary canal. This can be concluded by analysing isohypses (contours of groundwater levels). For that reason there will be in many cases no difference in water quality for the supply canal and the drainage canal. Most important is that during low rainfall periods there should be direct access to the nearest canal for supply. Therefore it is recommended that each tertiary canal should have the potential to function both as a supply or drainage canal and each rice field should be connected to the nearest tertiary canal to guarantee maximum accessibility of the field by the canal system. Avoid dead ends in main canals. Dead ends in Primary, Secondary and Tertiary canals generate slackwater conditions which are a source of excessive weed growth, acidity accumulation in canals and increased sedimentation.That means all canals should be double connected. If possible, short-cut connections should be made to the nearest river or main canal. In most cases control structures in main canals will be required to promote one-way flow in these canals. This will avoid slackwater conditions and will promote the tidal flows throughout the Scheme. An example of a suitable lay-out is found below. Control structures and on-farm watermanagement system. The proposed combination of control structures and an on-farm watermanagement system is the key to control water levels in the rice fields. The existing control structures in the tertiary canals are for instance essential to avoid excessive seepage losses during dry spells towards the tertiary canals. However, for the present system, with control structures, but without an on-farm water management system, the conclusion can be made that there is still insufficient water control in the rice fields. During the El Niño extended dry season, the groundwater levels in the rice fields will drop, regardless of the levels in the canals. The on-farm watermanagement system should consist of quaternary ditches of 70 cm deep and a ditch spacing of 100 m. As an improvement a subsurface drainage system is proposed which reduces the workload for the farmers for maintenance and increases the water supply to the field during dry spells. For the type and reasoning of the use of subsurface drainage see WebPages System. Box culverts. Box culverts are primarily required to reduce construction costs for double connected canals (at side where the canal was originally dead-ended) and to make connections between canals when they pass a (field)road. Besides box culverts are required for individual operation of a field. Topographical level differences of rice fields could vary within a tertiary unit as much as 30- 40 cm. This requires adaptations of water control at an individual level. The combination of box culverts with control structures remains essential to make use at high tide of the extra storage capacity for supply in tertiary canals and field ditches.This is important in areas with no potential for tidal irrigation. For an example for such a design, see below. Recommendations: Each rice field should border directly a tertiary canal, design should maximize the supply potentials. Design should avoid slack-water conditions in Primary, Secondary and Tertiary canals. Control structures combined with field ditches or a sub-surface drainage system will determine the potential for an appropriate soil and water management system in the fields. Individual Operation of water levels in the field, should be based on a box culvert connecting the Tertiary canal with the field ditches or the subsurface drainage system. |
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Areas in Freshwater Tidal Swamps with a potential for Tidal Irrigation. There is no need for a separated supply and drainage system. However a separate supply and drainage system could be an option if sufficient watersupply can be guaranteed. In these conditions it is extremely important that each rice field has direct access to the nearest tertiary canal for supply at high tide. It is still a point of discussion if control structures are required for these areas. For a second crop in the dry season control structures may be recommended to minimize seepage losses at low tide. For individual operation of each rice field a box culvert between tertiary canal and the field is essential in all circumstances.In areas with deep flooding during high tide flood protection is a must along the tertiary canals. |
Recommendations: Each rice field should border directly a Tertiary canal, design should maximize the supply potentials. Flood protection required in many cases. Individual Operation of water levels in the field, based on a box culvert connecting the Tertiary canal with the field ditches for supply in the rice field.
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Areas bordering peat domes. Peat domes are ombrogenous peat soils of more than 3 meter thickness. The peat dome should be bordered by a collector drain parallel to the contour lines. In case of a large peat dome bordering the area, it is often required to make a dike between the peat dome area and the rice growing area to prevent deep flooding in the rice fields. The collector drain will collect surface run-off water from the peat dome. This collector drain should also have a function as a buffer for the peat dome with the bordering cultivated land. Perpendicular to this collector drain the secondary/tertiary canals will be made to drain the cultivated land and use the water in the collector drain as a gravity- irrigation source for the rice fields . Downstream near the river there might be another collector drain with an outlet control structure towards the river. |
Recommendations: The collector drain on the edge of the peat dome should be used as a supply canal for the cultivated land. For that reason control structures are made in the secondary canals. The control structure is usually a stoplog structure or a temporary weir made from nylon bags filled with clay. Along the secondary canal several weirs might be required to control the water levels, depending on the slope from the peat dome towards the river.
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Areas bordering saline rivers most of the year. These areas are mainly suited for tree crops or fisheries. Rice is completely depending on rainfall and can never be supplied by pumped irrigation. Tree crops are preferably grown in areas with a tidal range of >3.0 m, fishponds are preferred in areas with a tidal range of about 1.0 m. Large tidal ranges will promote severe scouring and erosion along the edges of the ponds. Tree crops require flood protection to avoid saline intrusion and damage to the trees. Areas with mangroves should not be developed for cultivation, degraded mangroves should be replanted with mangrove trees. |
Recommendations: Mangroves should remain protection area. Other areas might be suitable for tree crops or fisheries. For tree crops is required Flood control along the saline water course with a collecter drain inside of the floodprotection dike. Control structures should be built in the dike at regular intervals with as main objective controlled drainage for tree crops. In cases the control structures should be made high enough for the passage of boats for transport of tree crop products at high tide.
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Areas far away from the river (more than 10 km). These areas are characterized by slackwater conditions in the adjoining canals. The water levels in the canals are close to the groundwater levels and are hardly not anymore influenced by the tides. These areas are presently often very acid. There is stagnant water and there is hardly no percolation in the soil. With the present applied scheme designs, these areas have no potential, except for Melaleuca plantations. Solutions for rice cultivation are only found when supply is based on a large canal in the backswamps, running parallel to the rivers. From this major supply canal, the adjoining lands, far away from the river, should be irrigated by pumping. Because of the low percolation and water supply potential, these areas can only be developed with pumped irrigation. A separate supply and drainage system is recommended in that case. |
Recommendations: Large supply canal from the river with a supply structure (one-way) near the river. The supply canal should run parallel to the river. Pumped irrigation in the backswamps far away from from the river. Separate supply and drainage system. The On-farm Water Management system should consist of a supply inlet and drainage outlet.
This hazard exists for all rice areas far away from the river outlet and with a good to reasonable production level. These rice fields are characterized by either A) areas having a delicate balance between rainfall, leaching and sufficient wet conditions for rice, usually with soils with few acidity problems and/or B) areas receiving extra water from higher lying areas. (such as peat-domes). An open canal system without control structures with large canals and with short-cuts to the main river may totally destroy this delicate balance and may cause excessive dry conditions, acidity increase without an increased leaching potential. This may cause the total loss of the rice fields. A lay-out/design should recognize these areas and the possible source of the extra water influx. An intensive system of canals and ditches is OK but Control of water levels in the whole system with gates is essential; both at macro and micro level of the system. Control of water levels will be also important during the construction period of canals and gates in these rice areas. See also Areas bordering peat domes.
A recommended Lay-out for Macro- and Micro system
(For a special sub-surface micro-system, see webpage Design Micro)
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Example Standard System TAM (Tata Air Mikro=On-Farm Water Management) and its cost per hectare With recent experience it is proved that movable flapgates in the tertiary structure (for drainage position of flapgate or for supply position) better suits the flow potentials to each field than the sliding gate structures
This example is based on the conditions in Pamusiran Scheme in Jambi and may be used as an example for required works for Tata Air Mikro (TAM)= On-Farm water management. Farmers Input will include the construction of the sub-tersiers, the quaternaries (kuarter) and the field ditches (saluran cacing dangkal) and mini-culvert (mini-gorong-gorong) which connects the farmer’s field with the Tertiary canal.
1 hectare with full TAM, which can be implemented by the farmer, consist of:
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sub-tersier/jalan usaha tani= 50-m with a volume of 0.48 m3 /m sub-tersier
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kuarter=300-m with a volume of 0.30 m3 / m kuarter
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kuarter guladan with a volume of 0.14 m3/m kuarter guladan
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cacing=700-m with a volume of 0.04 m3 / m cacing
1 hectare with construction to be implemented by Contractor consist of
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gorong2 besar= 1/32 gorong-gorong besar per hectare
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gorong2 sedang=1/16 gorong-gorong sedang per hectare
It is assumed that one farmer needs about 25 working days per hectare to implement the TAM except for the gorong² besar and gorong2 sedang. When he owns 2 ha he can complete the works in 2 months. The costs for the gorong² besar and sedang is estimated at about Rp.12,800,000 (US$1,280) per 32 Ha (1 Tersier) which means about Rp.400,000/ha. (US$40)
Tersier=Tertiary canal; Kuarter=Quaternary canal; saluran cacing dangkal= shallow field ditch; gorong2=box culvert; besar=large; sedang= medium; jalan usaha tani=field road, guladan= field bunch
The most common environment for Swamp Schemes is the freshwater Tidal Swamp Scheme with no potential for Tidal Irrigation. The thumbnail image shows a typical example for a design of the macro and micro system for these areas. Important is the new proposal for a sub-tertiary canal in between the tertiary canals at 200 m distance from the tertiary canals. The sub-tertiary canal is essential to have sufficient access to all fields via the micro-system
Note also that all canals are double connected. The tertiary canal is operated for one-way flow with supply at the secondary canal near the village (SPD side) and controlled drainage at the other secondary canal (SDU side). The sub-tertiary canals have open connections with the secondary canals and are mainly used for shallow drainage.
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