FIG. 8. Basic farm size, tidal lowlands,
Indonesia.
water in the polder has a high salinity content due to saline seepage and the receiving
water body has to be kept fresh, special measures have to be taken to prevent ‘pollution
of the receiving water body’.
4.3 Design of the Drainage System
When designing the water management system of the polder, two design approaches may
be followed: empirical design and optimization. Empirical design is normally based on
the conditions that have occurred in the past from which general data have been deduced.
In the optimization approach the investments and maintenance costs of the water
management system are compared with the yields and damage that can be expected in
relation to the functioning of the system.
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The requirements of the land drainage system form the starting point in the design. For
rice, the normal depth of flooding should not be exceeded too many times by a certain
depth. For dry food crops the depth of the ground water table below the surface has to be
controlled.
From the hydraulic characteristics of the land drainage system the amount of water to
be evacuated can be derived to obtain the required degree of water control. Outlet
capacity depends very much on the admissible storage in the polder.
Storage may be on the field, in the soil and in the open water system. Criteria for
storage on the field, e.g. for rice crops, will largely depend on the type of crop and the
stage of crop development. For old rice varieties a regular variation of the water layer on
the field of 0·05–0·10 m is allowed. In the case of high-yielding varieties of rice the
admissible surface storage on the fields is about 0·10 m with a return period of 5–20
years. Storage in the soil is important for dry food crops in temperate climates and may
amount from 50 to 200 mm after a dry period, without raising the ground water level
above a required depth. Especially significant is surface storage in the open canals, pools
and lakes of the polder area. In areas under cultivation a permissible rise of the polder
water level may vary from 0·10 to 0·40 m.
The design of drainage systems requires geological, topographic and soil maps, and
climatic and agricultural data. Generally the investigations require a sequence of studies
with increasing intensity. Therefore two or three phases in the investigations can be
considered: (a) reconnaissance level (pre-feasibility study)—the main objective is to
identify the feasibility of the proposed project, first of all on technical, but also on
economic grounds; studies at this level are mainly based on existing information but may
also include some field work; (b) semi-detailed level (feasibility study) —alternative
plans obtained from the reconnaissance study are worked out to a preliminary plan so that
the competent authorities can make a decision; the data are the same as for the
reconnaissance level, but are needed in more detail; (c) detailed level (project designs)—
design of the selected project, including a list of quantities and preparation of tender
documents.
Polders 193