Transcript DESIGN OF BORDER & BASIN IRRIGATION PRABHU.M BTE-06-024

DESIGN OF BORDER & BASIN
IRRIGATION
PRABHU.M
BTE-06-024
BORDER IRRIGATION
• With border irrigation, water is diverted in a
pre-constructed border, which is between 100
and 1 000 m long and 3 to 30 m wide.
• The borders have a uniform slope away from
the water canal so that the water flows into
the borders by means of gravity while it
infiltrates into the soil
DESIGN OF BORDER IRRIGATION
BORDER SPECIFICATION AND STREAM SIZE
• Width of border strip: The width of border usually
varies from 3 to 15 meters,depending on the size of the
irrigation stream available and the degree of land
levelling practicable.
• Border length: The length of the border strip depends
upon how quickly it can be wetted uniformly over its
entire length.
1. Sandy and sandy loam soils: 60 to 120 meters
2. Medium loam soils
: 100 to 180 meters
3. Clay loam and clay soils
: 150 to 300 meters
Cont..
• Border slope: The border should have a uniform longitudinal
gradient.
1. Sandy loam to sandy soils : 0.25% to 0.60%
2. Medium loam soils
: 0.20% to 0.40%
3. Clay to clay loam soils
: 0.05% to 0.20%
• Size of irrigation stream: The size of the irrigation stream needed
depends on the infiltration rate of the soil and the width of the
border strip.
1. Sandy soil : 7 to 15 ( LPS)
2. Loamy sand : 5 to 10 ( ,, )
3. Sandy loam : 4 to 7 ( ,,)
4. Clay loam : 2 to 4 ( ,, )
Design
• When border irrigation design as 2 types;
1.design of open end border system
2.design of blocked end border system
Cont..
1.Design of open end border system
• The first four design steps for open-ended
borders are the same as those outlined under for
traditional furrow systems
(1) assemble input data;
(2) compute maximum flows per unit width;
(3) compute advance time; and
(4) compute the required intake opportunity time
Cont…
• Hart et al. (1980) also suggest computing a
minimum flow, Qmin, based on a value that
ensures adequate field spreading. This
relationship is:
• Qmin = 0.000357 L So.5 / n
Where,
Qmin is the minimum suggested unit discharge
in m3/min/m and
L, So, and n are variables
Cont…
• The depth of flow at the field inlet to ensure
that depths do not exceed the dyke heights.
For this:
• where yo is the inlet flow depth in m.
CONT…
• After completing the first four design steps, as
with furrows, open-ended border design
resumes as follows:
• v. Compute the recession time, tr, for the
condition where the downstream end of the
border receives the smallest application is,
tr = rreq + tL
Cont…
• vi. Calculate the depletion time, td, in min, as
follows:
1. Assign an initial time to the depletion time,
say T1 = tr;
2. Compute the average infiltration rate along
the border by averaging the rates as both ends
at time T1:
Cont…
3. Compute the 'relative' water surface slope:
Cont…
4. Compute a revised estimate of the depletion time, T2:
5. Compare T2 with T1 to determine if they are within
about one minute, then the depletion time td is
determined. If the analysis has not converged then let
T1 = T2 and repeat steps 2 through 5.
Cont…
• The computation of depletion time given above is
based on the algebraic analysis reported by Strelkoff
(1977).
vii. Compare the depletion time with the required intake
opportunity time. Because recession is an important
process in border irrigation, it is possible for the
applied depth at the end of the field to be greater than
at the inlet. If td > rreq, the irrigation at the field inlet is
adequate and the application efficiency, Ea can be
calculated using the following estimate of time of
cutoff:
tco = td - yo L / (2 Qo)
Cont…
• If td < rreq, the irrigation is not complete and the
cutoff time must be increased so the intake at the
inlet is equal to the required depth. The
computation proceeds as follows:
tco = rreq - yo L / (2 Qo)
• and then Ea is computed
• Since the application efficiency will vary with Qo
several designs should be developed using
different values of inflow to identify the design
discharge that maximizes Ea.
Cont….
viii. Finally, the border width, Wo in m is computed
and the number of borders, Nb, is found as:
• Wo = QT/Qo and,
• Nb = Wt/Wo
• where Wt is the width of the field. Adjust Wo until
Nb is an even number. If this width is
unsatisfactory for other reasons, modify the unit
width inflow or plan to adjust the system
discharge, QT.
2.Design of end block borders
The suggested design steps are as follows
• i. Determine the input data as for furrow and border systems
already discussed.
• ii. Compute the maximum inflows per unit width using with p1 = 1.0
and p2 = 1.67. The minimum inflows per unit width can also be
computed using
• iii. Compute the require intake opportunity time, rreq.
• iv. Compute the advance time for a range of inflow rates between
Qmax and Qmin, develop a graph of inflow, Qo verses the advance
time, tL, and extrapolate the flow that produces an advance time
equal to rreq. Define the time of cut off, tco, equal to rreq. Extrapolate
also the r and p values found as part of the advance calculations.
• v. Calculate the depletion time, td, in min, as follows:
td = tco + yo L / (2 Qo) = rreq + yo L / (2 Qo)
Cont….
• vi. Assume that at td, the water on the surface of
the field will have drained from the upper
reaches of the border to a wedge-shaped pond at
the downstream end of the border and in front of
the dyke.
• vii. At the end of the drainage period, a pond
should extend a distance l metre upstream of the
dyked end of the border. The value of l is
computed from a simple volume balance at the
time of recession:
Cont…
where,
• Zo = k tda + fo td
• ZL = k (td - tL)a + fo (td - tL)
• If the value of l is zero or negative, a downstream pond will
not form since the infiltration rate is high enough to absorb
what would have been the surface storage at the end of the
recession phase.
• In this case the design can be derived from the open-ended
border design procedure.
• If the value of l is greater than the field length, L, then the
pond extends over the entire border and the design can be
handled according to the basin design procedure outlined
in a following section.
Cont..
• The depth of water at the end of the border, yL, will be:
yL = l So
viii. The application efficiency, Ea, can be computed. However,
the depth of infiltration at the end of the field and at the
distance L-l metres from the inlet should be checked as
assumes that all areas of the field receive at least Zreq.
• The depths of infiltrated water at the three critical points
on the field, the head, the downstream end, and the
location l can be determined as follows for the time when
the pond is just formed at the lower end of the border:
Z1 = k (td - tL-1)a + fo (td - tL-1)
Cont…
where,
tL-1 = [(L-l) / p]1/2
• It should be noted again by way of reminder that one
of the fundamental assumptions of the design process
is that the root zone requirement, Zreq, will be met over
the entire length of the field.
• If, therefore, in computing Ea, one finds ZL-1 or ZL less
than Zreq, then either the time of cutoff should be
extended or the value of Zreq used should be reduced.
• Likewise, if the depths applied at l and L significantly
exceed Zreq, then the inflow should be terminated
before the flow reaches the end of the border.
Design of basin irrigation
Basin irrigation
• Basin irrigation is as good choice in cases
where the natural gradient is relatively flat
and even.
• Permanent orchards and grazing crops are
especially well suited to basin irrigation.
• The farmer has to be prepared, however, to
check that all the basin remain level
throughout the season
Cont…
•
First, the friction slope during the advance phase of the flow can be
approximated by: Basin irrigation design is somewhat simpler than
either furrow or border design.
• Tailwater is prevented from exiting the field and the slopes are
usually very small or zero.
• Recession and depletion are accomplished at nearly the same time
and nearly uniform over the entire basin.
• However, because slopes are small or zero, the driving force on the
flow is solely the hydraulic slope of the water surface, and the
uniformity of the field surface topography is critically important.
Sf = yo / x
• in which yo is the depth of flow at the basin inlet in m, x is the
distance from inlet to the advancing front in m, and Sf is the friction
slope.
Cont…
• Utilizing the result of in the Manning
equation yields:
or,
C ont…
• The second assumption is that immediately upon cessation
of inflow, the water surface assumes a horizontal
orientation and infiltrates vertically.
• In other words, the infiltrated depth at the inlet to the
basin is equal to the infiltration during advance, plus the
average depth of water on the soil surface at the time the
water completes the advance phase, plus the average
depth added to the basin following completion of advance.
• At the downstream end of the basin the application is
assumed to equal the average depth on the surface at the
time advance is completed plus the average depth added
from this time until the time of cutoff.
Cont….
• The third assumption is that the depth to be
applied at the downstream end of the basin is
equal to Zreq.
• Under these three basic assumptions, the
time of cutoff for basin irrigation systems is
(assume yo is evaluated with x equal to L):
Cont…
• The time of cutoff must be greater than or
equal to the advance time.
• Basin design is much simpler than that for
furrows or borders.
• Because there is no tail water problem, the
maximum unit inflow also maximizes
application efficiency.
Cont…
As a guide to basin design, the following steps
are outlined:
• i. Input data common to both furrows and
borders must first be collected. Field slope will
not be necessary because basins are usually
'dead level'.
• ii. The required intake opportunity time, rreq,
can be found as demonstrated in the previous
examples.
Cont….
iii. The maximum unit flow should be calculated along
with the associated depth near the basin inlet. The
maximum depth can be approximated:
• and then perhaps increased 10-20 percent to allow
some room for post-advance basin filling.
• If the computed value of ymax is greater than the
height of the basic perimeter dykes, then Qmax needs to
be reduced accordingly.
Cont…
• As a general guideline, it is suggested that
Qmax be based on the flow velocity in the basin
when the advance phase is one-ninth
completed.
• Usually the design of basins will involve flows
much smaller than indicated .
Cont….
iv. Select several field layouts that would appear to yield a well organized field system
and for each determine the length and width of the basins. Then compute the unit
flow, Qo for each configuration as:
Qo = QT / Wb
•
where Wb is the basin width in m. As noted above, the maximum efficiency will
generally occur when Qo is near Qmax so the configurations selected at this phase
of the design should yield inflows accordingly.
v. Compute the advance times, tL, for each field layout as discussed .the cutoff time,
tco, from (if tco < tL, set tco = tL), and the application efficiency .
•
The layout that achieves the highest efficiency while maintaining a convenient
configuration for the irrigator/farmer should be selected.
Thank u
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