Field Research on the Performance of Various Drainage Materials in Lithuania

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Agricultural Water Management 68 (2004) 151–175

Field research on the performance of variousdrainage materials in Lithuania

A. Rimidis a, W. Dierickx b,∗

a Water Management Institute of the Lithuanian University of Agriculture, Parko 6,

LT-5048 Vilainiai, K edainiai, Lithuaniab Department of Mechanisation, Labour, Buildings, Animal Welfare and Environmental Protection, Agricultural

Research Centre, B. Van Gansberghelaan 115, B-9820 Merelbeke, Belgium

Accepted 10 March 2004

Abstract

Field evaluation of the performance of a drainage system or of various drainage materials is a

long-lasting research, very often with disappointing results. These disappointing results stem either

from a too wide a spread of the field data, or one or more necessary parameters which were notmeasured, or an insufficient knowledge of the flow occurrence in the vicinity of the drain resulting

in a wrong interpretation and evaluation of the research findings. Yet, a correct evaluation of the

performance of a drainage system is of primary importance to ascertain the real value of a drainage

system or to compare various drainage materials. As a result of the wide spread of field data within a

measuring season, a large amount of data from several seasons is required to obtain reliable results.

From discharge and total head loss measurements a second-degree polynomial results which only

permits a determination of whether the design criteria are met. The similarity with Hooghoudt’s

equation allows the determination of soil hydraulic conductivity and the equivalent depth of the

impervious layer. With this data the drain spacing can be checked. Too wide a spacing can be the cause

of malfunctioning of a drainage system, but the drainage materials can also affect its performance.

To evaluate or compare drainage materials it is necessary to measure the so-called approach-flowhead loss in the vicinity of the drain. The relationship between discharge and approach-flow head

loss should theoretically be linear and the regression coefficient gives the approach-flow resistance.

However, the relationship depends to a large extent on the approach-flow occurrence in the vicinity of

the drain. For non-linear relationships, the approach-flow resistance can be estimated from the slope

of the tangent line, which tends to a constant value at greater discharges for flow patterns covering

the whole drain circumference. The approach-flow resistance depends on the hydraulic conductivity

of the soil and does not permit a direct comparison of various drainage materials. Therefore, the

approach-flow constant or the dimensionless approach-flow resistance is calculated. This parameter

∗ Corresponding author. Tel.:+32-9-272-27-63; fax: +32-9-272-28-01.

E-mail addresses: [email protected], [email protected], [email protected]

(W. Dierickx).

0378-3774/$ – see front matter © 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.agwat.2004.03.004



152 A. Rimidis, W. Dierickx / Agricultural Water Management 68 (2004) 151–175

is related to the material itself and to the installation conditions reflected in the hydraulic conductivity

of the trench backfill. Consequently the comparison of the various drainage materials includes the

trench backfill. The approach-flow head loss fraction is that part of the total head loss which is

dissipated by the approach-flow. The relationship between approach-flow head loss and total headloss should theoretically be and is, indeed, linear so that the slope of the regression line gives the

approach-flow head loss fraction. This parameter combines the influence of drainage material and

flow pattern so that real flow situations in the vicinity of the drain, which are not necessarily the same

for all materials, can be compared.

© 2004 Elsevier B.V. All rights reserved.

Keywords: Drainage performance; Drainage materials; Drain discharge; Total head loss; Approach-flow head

loss; Approach-flow resistance; Approach-flow head loss fraction

1. Introduction

Subsurface drainage is one of the most important measures to maintain or enhance the

productivity of agricultural lands. In some areas of the temperate zone, drainage systems

continue to be an essential tool of groundwater control. To achieve that aim, drainage

systems should be properly designed and adequately installed using appropriate materials

(Stuyt et al., 2000). In Lithuania, about 3.4 million of the total agricultural area of 3.9 million

ha required some kind of drainage to improve agricultural productivity and, therefore, quite

a large area of agricultural land was already drained. To remove the excess water 62,722 km

of open ditches were excavated while the total length of subsurface drainage measures1590×103 km composed of 267×103 km of collectors and 1320×103 km of laterals. The

laterals have a diameter of 50 (clay drain tiles) and 63 mm (corrugated polyethylene pipes),

and the collector diameter ranges from 7.5 up to 160 cm. The larger-diameter-pipes are in fact

collectors which replace open ditches. The size of subsurface drainage systems varies from

a few up to more than 100 ha. Currently, the total area of drained lands covers 2980×103 ha,

2580×103 ha of which are provided with a subsurface drainage system. (Anonymous, 2002).

InLithuania,drainageworksstartedasearlyas1910,butthemainpartwasdoneduringthe

period 1956–1990. From 1991 to date, the construction of new drainage systems decreased

significantly. As the financial support of the state is limited (about 45 million Lithuanian

Litas or 13.08 million Euro), the available financial means go to ditch maintenance, repairand reconstruction of subsurface drainage systems (Mork unas, 2002; Smilgevicius, 2003).

Therefore, it is, more than ever before, important to know how to evaluate the real situation

of drained land and to follow a new course to monitor and maintain drainage systems.

At present, nobody can predict the functionality of drainage systems and their technical

state, even for the most widespread loamy soils. During the Soviet period the lifetime of

land reclamation structures was not evaluated. There was only episodic data with different

evaluation criteria. In the future, when land reform is over and drainage systems are passed

to the landowners, the problem of the functionality of the drainage systems will inevitably

be raised. Therefore, a unified way to evaluate drainage performance is necessary. In West

Europe where ownership is rather a long tradition, the monitoring of drainage performanceis put into practice immediately after installation (Dieleman and Trafford, 1984; Cavelaars

et al., 1994).



A. Rimidis, W. Dierickx / Agricultural Water Management 68 (2004) 151–175 153

Rimidis and Dierickx (2003) carried out some research work on the evaluation of the

performance of drainage systems at an experimental site in the central part of Lithuania

during the period 1994–1998. This site consisted of seven plots with various combinations of

drainage materials. To evaluate drainage performances, discharges and water heads on top of and midway between drains were measured. These observations allowed the determination

of whether design criteria were met or not, but they were largely inadequate to detect

the cause of drain malfunctioning and to compare the performance of the various drainage

materials. To solve these problems information on the approach-flow head loss in the vicinity

of drains is required. Therefore, a new experimental set-up was planned and installed, and

measurements were carried out during the period 1999–2003. On the basis of these results, a

monitoring programme on the performance of drainage systems can be drawn up to inform

future landowners on the real value of their drainage systems and on the measures to take

for maintaining or improving their functionality.

2. Experimental site

With a view to land reclamation, Lithuania is subdivided into three zones (Fig. 1): the west

(A), the centre (B) and the east (C). The experimental site is located in the central part (B) of

the country, at Pikeliai, a village in the Kedainiai district. This zone has a more continental

climate than the western zone (A) with an average annual rainfall of 568 mm ( Klimienie

and Buitkuviene, 1991) and an average annual temperature of 6.1 ◦C (Aloseviciene, 1992).

Fig. 1. Map of Lithuania with the three reclamation zones: the west (A), the centre (B) and the east (C).




Table 1

Rainfall (mm) and temperature (◦C) recorded at Dotnuva in the vicinity of the experimental site with field obser-

vations lasting the whole month (dark shading) and a fortnight only (light shading)

The monthly and yearly rainfall and temperature during the years of measurement as well

as the 50-year average monthly and yearly rainfall and temperature, measured at Dotnuvain the immediate vicinity of the Pikeliai village, are given in Table 1 which also indicates

the periods of measurement. The observations show that rainfall is somewhat higher during

the summer than during the winter months.

The relief of the central zone is a slight to moderate rolling plain, diversified by river and

stream valleys, where soggy gley soils of light to medium moraine sandy loam predominate

(Zelionka, 1967). These are the better soils of Lithuania which result in the largest increase

in productivity after being drained.

The site was first drained in 1959–1960. In 1994 the northern part of the site was re-

constructed because, in the early nineties, it was noticed that groundwater table response

was very slow and that the high groundwater table caused crop damage. With a view tothe construction of an experimental site, the old drainage system was rendered inoperative.

Instead of one large drainage system, covering the whole area of 9.07 ha, the total area was




Fig. 2. Lay-out of the experimental plots 2–6 and 10.

subdivided into smaller experimental systems which discharge through collector drains into

the open drainage ditch. The new drain lines were installed with a multibucket excavator.

Organic as well as synthetic envelopes were used to protect the drain pipes against sil-tation. In the southern part of the experimental site, an old drainage system with a total

area of 5.5 ha was still operating without any repair or reconstruction work. Plots 2–6 of

the northern part and plot 10 of the southern part (Fig. 2) were involved in this research.

The incorporation of plot 10 offered the possibility to compare the performance of an old

drainage system with the performance of various, more modern materials used at plots 2–6.

In these plots laterals consisted of both 33 cm long clay drain pipes with an inner diameter

of 50 mm and corrugated polyethylene pipes with an outside diameter of 63 mm protected

against soil particle invasion by various envelope materials. The laterals of plot 10 were

installed with a ditch plough and consisted of clay drain pipes with an inner diameter of

40 mm. Collector drains of all plots were clay drain pipes with diameters ranging from

75 to 200 mm depending on their length. Drains were installed at a depth of 0.90–1.10 m

and their spacing, which was based on the physical properties of the soil profile according

to Janert (1961) and slightly adapted to the Lithuanian circumstances (Ceicys, 1965), was

22 m at plots 2–6 while it was 20 m at plot 10. According to the classification system of

FAO (1990), the soil of the experimental site mainly consists of sandy to sandy clay loam

as can be seen in Table 2 which gives the soil texture from surface to drain depth for each

plot. The following material combinations were investigated:

• Clay drain pipes covered with a layer of 7–10 cm loose corn straw on top and along both

sides but without protecting the pipes from underneath (plot 2).• Corrugated polyethylene pipes pre-wrapped with corn straw with a thickness of 2–3 cm

and an average density of 0.1–0.2 g/cm3 (Rimidis and Kozhushko, 1995) (plot 3).




Table 2

Soil texture of the experimental plots at various depths

Experimental

plot

Depth (cm) Clay fraction

(< 2m)

Silt fraction

(2–50m)

Sand fraction

(50–2000m)

Textural class

2 0–30 24.2 24.4 51.4 Sandy clay loam

30–50 18.3 25.4 56.3 Sandy loam

50–85 25.5 15.1 59.4 Sandy clay loam

85–110 13.9 16.1 70.0 Sandy loam

3 0–30 12.7 20.0 67.3 Sandy loam

30–50 24.4 31.8 43.8 Loam

50–70 12.9 15.7 71.5 Sandy loam

70–100 12.4 15.1 72.5 Sandy loam

4 0–30 13.9 23.5 62.6 Sandy loam

30–60 26.4 23.8 49.8 Sandy clay loam60–80 8.9 19.7 71.4 Sandy loam

80–130 10.2 17.8 72.0 Sandy loam

5 0–30 13.2 24.6 62.2 Sandy loam

30–50 28.1 15.8 56.1 Sandy clay loam

50–60 9.7 17.3 73.0 Sandy loam

100–135 11.7 17.2 71.1 Sandy loam

6 0–30 13.6 17.8 68.6 Sandy loam

30–70 15.9 18.0 66.1 Sandy loam

70–90 19.2 13.3 67.5 Sandy loam

90–130 15.2 16.6 68.2 Sandy loam

10 0–30 13.5 26.9 59.6 Sandy loam

30–50 13.2 23.7 63.1 Sandy loam

50–70 9.2 22.3 68.5 Sandy loam

100–120 3.9 10.4 85.7 Loamy sand

• Corrugated polyethylene pipes pre-wrapped with a sheet of locally made non-woven

synthetic fibre material, called Melita (plot 4).

• Clay drain pipes covered with a layer of 7–10 cm loose bulky flax boon on top and along

both sides but without protecting the pipes from underneath (plot 5).

• Clay drain pipes provided with a glass fibre sheet underneath and on top during installationof the pipes (plot 6).

• Clay drain pipes with an inner diameter of 40 mm and without any envelope material,

installed in 1960 (plot 10).

3. Experimental set-up

Measurements of drain discharge and hydraulic head or groundwater table midway be-

tween drains suffice to evaluate the performance of a drainage system. However, these

measurements are not sufficient to detect the cause in case of malfunctioning or to find dif-ferences between various materials (Rimidis and Dierickx, 2003). More information on the

possible cause of malfunctioning, or on differences between various materials, requires hy-




L/2 L/2 L/2 L/2

1 2 3 4 5 6 7

0.40 m 0.40 m

Fig. 3. Scheme of the row of piezometers consisting of two piezometers on top of the drains, two piezometers at

a distance of 0.40 m from the drains and three piezometers midway between drains.

draulic head measurements close to the drains, just outside the trench backfill. Furthermore,

hydraulic head measurements on top of the drain provide information on water standing

above the drain. Additional measurements of the hydraulic head in the drain are required to

find out whether standing water above the drain is dueto backpressure in the drain or entrance

resistance. In many cases, the hydraulic head in the drain is not measured and the drain is

considered as an equipotential with the head equal to the top of the drain. This assumption is

justified as long as the hydraulic head above the drain is zero or negligibly small which was

the case for the observations at the experimental site. Therefore, to evaluate the performancesof various material combinations, measurements were made of hydraulic heads midway

between drains, at a distance of 0.40 m from the drain and above the drain in addition to dis-

charge measurements. To measure these hydraulic heads, seven piezometers were installed

in one row in each of the experimental plots (Fig. 3). The piezometers were made of 1.50 m

long smooth polyethylene pipes with a diameter of 50 mm. The bottom part of the pipe

was perforated over a length of 30 cm with 5 mm holes. This perforated part was wrapped

with two layers of the locally made non-woven fabric “Melita” to prevent soil invasion. The

piezometer tubes were inserted in auger holes. At the surface, the soil around the piezometer

tubes was compacted to a depth of 0.40 m to avoid direct inflow of surface water along the

pipe wall. Water levels in the piezometer tubes were measured with an electric gauge.Each collector of the various plots discharged in a manhole where volumetric discharge

measurements with a graduated receptacle and stopwatch were carried out. Since the dis-

charge of the individual experimental drains was not measured separately, the average

discharge per unit surface area, or the average drainage coefficient of each plot, was calcu-

lated for each discharge measurement. The total discharge of the experimental drain results

from the average drainage coefficient, drain spacing and total drain length.

4. Data collection and processing

From 1999 to 2003 measurements of discharge and hydraulic heads were carried out

every 3–4 days, depending on rainfall amounts, in the autumn, during snowless winter




periods and in the spring. Table 1 gives the months in which measurements were done.

They were done during either the whole month or a fortnight (first fortnight in December

2001 and 2002; second fortnight in March 2003).

For each experimental plot and every measurement, the collected data at similar locationswas averaged so that only one value of head midway between drains, one value of head

at a distance of 0.40 m from the drain and one value of head on the drain corresponded

with one discharge measurement. As no piezometers were installed to measure water heads

inside the drain pipes, it could not be stated whether the drains were flowing under pressure

or not. However, the water heads in the piezometers on top of the drain, which may result

from either backpressure in the drain or entrance resistance, were zero to negligibly small

and, therefore, it may readily be accepted that the drains were not flowing under pressure.

Considering drain midpoint as reference level and assuming the drain just flowing full

without backpressure, the drain circumference can be considered as an equipotential with

the head equal to the radius of the drain pipe. Consequently, the total head loss ht (m) of

flow towards a drain is given by

ht = hm − ro (1)

with hm (m) the head in the piezometer midway between drains, or the height of the ground-

water table midway between drains above the reference level and r o (m) the outside drain

radius.

The approach-flow head loss, hap (m), or the head loss in the vicinity of the drainage

system due to drain pipe, envelope material and surrounding soil results from:

hap = hv − ro (2)

with hv (m) the head in the piezometer 0.40 m from drain midpoint. From the approach-flow

head loss, the approach-flow resistance, W ap (day per m), and the approach-flow constant,

αap, can be calculated:

W ap =hap

qL=

hap

q1(3)

and

αap = W apK (4)

with q (m per day) the specific discharge, q1 (m2 per day) the discharge per unit drain

length, L (m) the drain spacing, and K (m per day) the hydraulic conductivity of the soil.

Theoretically, αap only depends on the materials used if the soil and trench backfill have the

same hydraulic conductivity. If not, αap also includes the positive or negative effect of the

trench backfill. The approach-flow head loss as a fraction of the total head loss is given by

F ap =hap

ht(5)

with F ap the approach-flow head loss fraction.With this data the performance of a drainage system can be evaluated, the functioning of

various envelopes compared, and in case of drainage malfunctioning the cause ascertained.




From the relationship between ht and q the performance of a drainage system can be

evaluated. To draft this relationship the same procedure used by Rimidis and Dierickx

(2003) was followed. A second-degree polynomial of the form

q = Ah2t + Bht (6)

gave the best fitted curve through the data points and is, moreover, similar to the well-known

equation of Hooghoudt (1940):

q =4Kah2

t

L2+

8Kbdht

L2(7)

with K a and K b (m per day) the hydraulic conductivity, respectively above and below drain

level, and d (m) Hooghoudt’s equivalent depth.

The best fitted curve was drawn through the set of points and forced through the origin

because it may readily be assumed that the discharge equals zero for a head equal to zero.

A small positive or negative intercept at ht = 0 could imply a small calibration error of the

piezometer midway between drains to the reference level and/or inaccuracies inherent to the

experimental work. Observation points beyond the 95% confidence limits of the individual

values were considered as erratic and a new regression line was calculated without these

observations.

The approach-flow resistance as a result of the head loss of drain pipe, envelope, trench

backfill and surrounding soil determines the functioning of the drainage material in the

immediate surrounding of the drain pipe. The approach-flow resistance, W ap, can be derivedfrom

hap = W apq1 + b (8)

which gives the relationship between the approach-flow head loss, hap, and the discharge

per unit drain length q1. A small positive or negative value of the intercept b may be

attributed to small calibration errors of the piezometers to the reference level and/or to

inaccuracies typical of measurements. In principle W ap is constant but many factors affect

it such as: (a) the flow pattern around the drain which may not be fully radial; (b) the water

entry through a section of the drain circumference only; and (c) the hydraulic conductivity

of the surrounding soil among others (Dierickx, 1980; Stuyt et al., 2000). Plotting the

approach-flow resistance against the total head loss shows indeed a linear decrease in the

approach-flow resistance with increasing total head loss and confirms that the approach-flow

resistance is a parameter which depends on the flow pattern and eventual variations in soil

properties. In spite of the small coefficient of determination, a reliability test proves the

existence of a linear relationship with 95% confidence.

The approach-flow head loss fraction, F ap, is given by the regression coefficient of the

linear relationship between ht and hap according to

hap = F apht + c (9)

As for the other observations, small positive or negative values of the intercept c are typical

of experimental work.




5. Research results

5.1. Performance of the drainage system

The q–ht relationship determines the performance of a drainage system. Rimidis and

Dierickx (2003) already determined this relationship for a number of plots, including plots

2–6, during the seasons 1994/1995 to 1997/1998. As total head loss and discharge measure-

ments were continued from 1999/2000 to 2002/2003 for plots 2–6, the q–ht relationships

of the latter measurement period could be compared with the available relationships of

the former period. The results of plots 2–6 for each of the four seasons of the measuring

period 1994/1995 to 1997/1998 are shown in Fig. 4, and the coefficients A and B of the q–ht

second degree polynomials and their respective coefficient of determination r 2 are given in

Table 3. The table also contains these parameters for the same plots 2–6 and plot 10 for the

measuring period 1999/2000 to 2002/2003 while Fig. 5 depicts the results.

As no approach-flow head loss measurements were carried out during the measuring

period 1994/1995 to 1997/1998, Rimidis and Dierickx (2003), for want of anything better,

attempted to explain the performance of the drainage materials by the evolution of the

yearly relationship. However, considering the spread of the measurements of the total head

loss with discharge within 1 year and the difference in drainage performance between the

four measuring seasons of the various plots of both the previous and current measuring

period (Figs. 4 and 5), a single overall q–ht relationship for the measurements of the 4

years of both measuring periods can be calculated. Thus combining the measurements of

a whole period of 4 years, a single and more reliable relationship between total head lossand discharge results for each plot over the whole measuring period. Fig. 6 shows the q–ht

relationship of each plot for the whole measuring periods 1994/1995 to 1997/1998 (Fig. 6a)

and 1999/2000 to 2002/2003 (Fig. 6b) while Table 4 gives the coefficients A and B of the

regression equations and their respective coefficient of determination r 2.

Differences in the q–ht relationships of the various plots between both measuring periods

are rather limited and, therefore, the measurements of both periods were combined to a

single relationship for each plot. The spread of the data points of the various plots (Fig. 7)

clearly illustrates that this way of processing was justified and so did the regression equations

which coefficients A and B and the coefficient of determination r 2 are also given in Table 4.

From Eqs. (6) and (7) it follows that

A =4Ka

L2(10)

and

B =8Kbd

L2(11)

The q–ht relationship will be quadratic for drains on the impervious layer since d = 0 and

consequently B = 0. A linear relationship emerges for deep soils (soils with the impervious

layer at great depth) since the contribution to the flow of the zone above drain level, given by

the first term Ah2t of Eq. (6), becomes insignificant compared to the zone below drain level,represented by the second term Bht. The stronger the curvature of the q–ht relation, the

larger is the contribution of the layer above the drain. From the curvature of the regression



Table 3Coefficients A and B of the regression equation q = Ah2

t + Bht with the coefficient of determination r 2 of the various plot

measuring seasons of both the measuring period 1994/1995 to 1997/1998 and 1999/2000 to 2002/2003

Measuring period 1994/95–1997/98

Plot 1994/95 1995/96 1996/97

A

(×10−3 m−1

per day)

B

(×10−4 per

day)

r 2 A

(×10−3 m−1

per day)

B

(×10−4 per

day)

r 2 A

(×10−3 m−1

per day)

B

(×10−4 per

day)

2 2.71 1.05 0.872 1.80 3.12 0.878 2.59 0.99

3 3.52 1.64 0.878 3.63 −1.40 0.975 2.51 6.45

4 2.95 11.3 0.947 0.39 20.5 0.701 0.47 26.5

5 1.88 0.701 0.776 2.87 −1.31 0.824 3.07 −2.68

6 1.76 0.982 0.958 0.56 8.08 0.838 1.36 3.81

Measuring period 1999/2000 to 2002/2003

1999/2000 2000/01 2001/02

2 2.86 0.947 0.785 3.09 0.192 0.882 3.51 0.032

3 2.73 −0.788 0.811 3.38 1.16 0.918 2.08 0.989

4 2.48 10.4 0.907 2.24 15.6 0.899 1.72 13.9

5 3.52 −9.12 0.879 1.64 −0.755 0.836 3.01 −5.09

6 1.72 7.85 0.905 1.24 7.49 0.821 2.00 1.64

10 3.07 33.2 0.967 1.97 19.4 0.783 4.01 31.0




Fig. 4. Relationship between the total head loss ht and the discharge per unit surface area q of the plots 2–6 for

the individual seasons of the measuring period 1994/1995 to 1997/1998 (after Rimidis and Dierickx, 2003).

lines (Fig. 7), it can readily be accepted that the water mainly comes from the soil layer

above drain level, and that the soil layer below drain level contributes little to the discharge

of the drains. Considering the obtained regression equations (Tables 3 and 4), it is obvious

that equations with a negative second term are not in agreement with Hooghoudt’s equation.

They may be the result of a high approach-flow resistance since one solution of these second

degree polynomials gives a positive head for q = 0, while the second solution, ht = 0 forq = 0, is in fact imposed by forcing the regression line through the origin. Another possible

explanation for this anomaly is a small error in reference level. Too deep a piezometer




Fig. 5. Relationship between the total head loss ht and the discharge per unit surface area q of the plots 2–6 and

plot 10 for the individual seasons of the measuring period 1999/2000 to 2002/2003.

compared to drain midpoint also gives a positive head for q = 0. The main cause, however,

may be the averaging of the data at similar places to reduce the amount of data and the

rather wide spread of the data points. From Tables 3 and 4 it is obvious that the number

of negative second terms decreases when the amount of data by combining seasons and

periods increases and so does the value. This indicates that a large number of observations

is required to obtain reliable relationships between discharge and total head loss.

From the second degree polynomialq–ht relationship and Eqs.(10)and(11), the hydraulic

conductivity K and the equivalent depth d of the impervious layer can be calculated, provided




Fig. 6. Relationship between the total head loss ht and the discharge per unit surface area q of each plot for the

whole measuring period 1994/1995 to 1997/1998 (a) and 1999/2000 to 2002/2003 (b).



Table 4

Coefficients A and B of the regression equation q = Ah2t + Bht with the coefficient of determination r 2 of each plot for both the

and 1999/2000 to 2002/2003 and for all measurements together (period 1994/1995 to 2002/2003)

Plot Period 1994/1995 to 1997/1998 Period 1999/2000 to 2002/2003 Perio

A (×10−3 m−1

per day)

B (×10−4

per day)

r 2 A (×10−3 m−1

per day)

B (×10−4

per day)

r 2 A (×

per d

2 2.69 −0.422 0.846 3.31 −0.339 0.873 2.75

3 3.39 0.906 0.868 2.47 2.96 0.852 2.82

4 3.49 10.6 0.913 1.66 15.4 0.857 2.28

5 1.78 1.54 0.819 2.94 −5.58 0.878 2.11

6 1.62 2.23 0.920 1.68 5.18 0.824 1.69

10 – – – 4.66 18.3 0.866 4.66




Fig. 7. Relationship between the total head loss ht and the discharge per unit drain surface area q of each plot for

both measuring periods 1994/1995 to 1997/1998 and 1999/2000 to 2002/2003 together.

that Ka = Kb which may readily be accepted, considering the textural classes at successive

depths (Table 2). When done for each year, the calculations may give strongly differing

results for the same plot (Rimidis and Dierickx, 2003). Therefore, it is better to consider

the q–ht relationship for each measuring period which only gives one specific value of

K and d for each plot for the considered measuring period. The results of both measuring

periods 1994/1995 to 1997/1998 and 1999/2000 to 2002/2003, given in Table 5, are far fromidentical in spite of the fact that each measuring period contained the measurements of four

seasons. Yet, similar trends can be found with regard to the magnitude of the hydraulic




Table 5

Hydraulic conductivity K (m per day) and equivalent depth d (m) of the impervious layer resulting from the

coefficients A and B of the regression equations for the measuring periods 1994/1995 to 1997/1998 and 1999/2000

to 2002/2003 and for both measuring periods together (period 1994/1995 to 2002/2003)

Plot Period 1994/1995 to

1997/1998

Period 1999/2000 to

2002/2003

Period 1994/1995 to

2002/2003

K (m per day) d (m) K (m per day) d (m) K (m per day) d (m)

2 0.33 −0.01 0.40 −0.01 0.33 0.02

3 0.41 0.01 0.30 0.06 0.34 0.04

4 0.42 0.15 0.20 0.46 0.28 0.32

5 0.22 0.04 0.36 −0.09 0.26 −0.01

6 0.20 0.07 0.20 0.15 0.20 0.10

10 – – 0.47 0.20 0.47 0.20

conductivity and the equivalent depth of the impervious layer. Only one single value of K

and d is obtained for each plot when both measuring periods are combined. These values,

also given in Table 5, are more reliable and more suitable for further analysis. The negative

equivalent depths result from the negative second term of the second degree polynomial

equations for which a small error in reference level or more likely the averaging of data

may be responsible. In every case, they indicate that the impervious layer is at or near to

drain depth.

5.2. Approach-flow resistance, W ap

According to Eq. (8) a linear relationship between discharge q1 and approach-flow head

loss hap should exist, and the slope of the line, or the coefficient of regression, is W ap. This

means that, in principle, W ap should be a constant. The experimental paired data q1–hap of

the whole measuring period 1999/2000 to 2002/2003 for each plot is given in Fig. 8 and

the best fitted relationship was determined, based on the coefficient of determination r 2. A

linear regression was only obtained for plots 4 and 6 while a power function better fits the

experimental data of plots 3 and 10 and a logarithmic function better fits data of plots 2

and 5. The best fitted equation for each plot and its coefficient of determination is given in

Table 6.

Table 6

Equations giving the best fitted relationship q1–hap and ht–hap for each plot and their coefficient of determination

r 2

Plot q1–hap r 2 ht–hap r 2

2 hap = 0.066 ln q1 + 0.416 0.798 hap = 0.272 ht 0.828

3 hap = 2.248q0.6731 0.701 hap = 0.311 ht 0.755

4 hap = 3.079q1 + 0.046 0.651 hap = 0.251 ht 0.780

5 hap = 0.078 ln q1 + 0.558 0.878 hap = 0.355 ht 0.8926 hap = 6.373q1 + 0.105 0.709 hap = 0.460 ht 0.873

10 hap = 1.277q0.5721 0.817 hap = 0.415 ht 0.845




Fig. 8. Relationship between the discharge per unit drain length q1 and the approach-flow head loss hap over the

whole measuring period 1999/2000 to 2002/2003, for each experimental plot.

W ap is constant and minimum for fully radial approach-flow conditions or approach-flow

conditions which cover the whole drain circumference and increases with increasing devi-

ation from that (Dierickx, 1980; Stuyt et al., 2000). The deviating approach-flow decreases

with increasing discharge and W ap tends to the constant minimum value at higher dis-

charges. W ap can be estimated by determining the slope of the tangent line to the curve at

the maximum measured discharge being 0.08 m2 per day for plot 2, 0.07 m2 per day for plot

3, 0.075 m2 per day for plot 5 and 0.055 m2 per day for plot 10. The slope of the tangent

line results from the first derivative dhap /dq1. For the logarithmic equation

hap = a ln q1 + b (12)




Table 7

Approach-flow resistances W ap (day per m), approach-flow constant αap and approach-flow head loss fraction F ap

for the various drainage materials

Plot W ap (day per m) αap F ap

2 0.83 0.27 0.272

3 3.61 1.23 0.311

4 3.08 0.86 0.251

5 1.04 0.27 0.355

6 6.37 1.27 0.460

10 2.53 1.19 0.415

W ap is given by

W ap =dhap

dq1=

a

q1(13)

and for the power equation

hap = aqb1 (14)

W ap is given by

W ap =dhap

dq1= abqb−1

1 (15)

It is evident that W ap for the linear equation

hap = aq1 + b (16)

is given by

W ap =dhap

dq1= a (17)

W ap obtained according to either Eqs. (13), (15) or (17) is given in Table 7. The dimension-

less approach-flow constant αap, calculated by applying Eq. (4) using the K -values of the

combined measuring period 1994/1995 to 2002/2003, given in Table 5, is also presented inTable 7.

5.3. Approach-flow head loss fraction, F ap

F ap, given by Eq. (9), results from the regression coefficient, or slope of the linear rela-

tionship between ht and hap. From Fig. 9 it can be seen that the relationship between ht and

hap for the whole measuring period is clearly linear for all plots. It may be accepted that

hap = 0 when ht = 0, and consequently the linear relationship can be forced through the

origin. These linear regression equations and their coefficient of determination are part of

Table 6 while F ap for each plot or each combination of drainage materials forms a part of Table 7. For plot 6 with clay drain pipes provided with a glass fibre sheet underneath and

on top and for plot 10 with clay drain pipes of an inner diameter of 40 mm and without any




Fig. 9. Relationship between the total head loss ht and the approach-flow head loss hap over the whole measuring

period 1999/2000 to 2002/2003, for each experimental plot.

envelope material, more than 40% of the total head is dissipated as approach-flow head loss

within a distance of 0.40 m from the drain centre.

6. Discussion

6.1. Performance of the drainage system

The results clearly illustrate that differences between the yearly q–ht relationship withinboth measuring periods are rather small for the same plot or the same drainage material

(Figs. 4 and 5). These differences should rather be attributed to the large variability of the




Plots 2 and 5, clay drain pipes respectively with loose bulky corn straw and loose bulky

flax boon both on top and along both sides, represent a logarithmic relationship between

q1 and hap. Such a relationship has a steep slope and consequently a high W ap at small

discharges, and tends to a rather small slope and a small W ap at greater discharges. Thedifference in W ap between smaller and greater discharges has to be attributed to the absence

of envelope material underneath the drains on the one hand, and that some of the organic

material was still present on top of the drain as was observed after excavations.

W ap for the various plots and hence also for the various combinations of drainage ma-

terials, as given in Table 7, can only be compared if K of all plots is the same. As this is

not the case (Table 5), the dimensionless approach-flow constant αap is a better parameter

to compare the combined effect of material and trench backfill (installation conditions) on

the drainage performance. However, it should be kept in mind that the approach-flow con-

stants, given in Table 7, are values valid for flow at which the whole drain circumference

is involved. From these values it is evident that the clay drain pipes with the bulky mate-

rials (plots 2 and 5) give the smallest αap followed by the corrugated polyethylene pipes

pre-wrapped with the locally made non-woven ‘Melita’ envelope (plot 4). The clay drain

pipes of 40 mm inner diameter without any envelope material (plot 10) come next and then

the corrugated polyethylene pipes originally pre-wrapped with corn straw (plot 3). The clay

drain pipes with a glass fibre sheet underneath and on top (plot 6) exhibit the highest αap.

The difference in αap of the last three materials is rather limited. Since drain pipes are of

secondary importance when used in combination with envelope materials (Dierickx, 1980),

the envelope materials, including the effect of the trench backfill, can be divided into three

classes based on αap and taking into account that minor differences are not important:• Class 1 (good, αap ≤ 0.50): The loose bulky corn straw and flax boon materials (plots 2

and 5).

• Class 2 (moderate, 0.50 < αap < 1.00): The locally made synthetic non-woven ‘Melita’

envelope (plot 4).

• Class 3 (poor, αap ≥ 1.00): The pre-wrapped corn straw; the glass fibre sheet and no

envelope material (plots 3, 6 and 10).

If the drainage materials were arranged according to W ap, a wrong classification and

evaluation should result because W ap is not only related to the material used but also to the

hydraulic conductivity of the soil which differs from plot to plot. The above classificationof the envelope materials was based on flow occurrences at which the whole drain circum-

ference is involved. An estimation of the real performance of the materials in their actual

situation taking into account all possible influences is given by the F ap.

6.3. Approach-flow head loss fraction, F ap

The ranking of F ap of the various plots or materials, given in Table 7, differs from that

for αap because of differences in the approach-flow occurrence. F ap is the smallest for the

pre-wrapped locally made synthetic non-woven ‘Melita’ envelope (plot 4) as only 25.1% of

the total head loss dissipates within a distance of 0.40 m from the drain centre. This resultsfrom a rather low αap combined with an impervious layer which lies about 0.30 m below

drain level. Then, the loose bulky corn straw (plot 2) follows with a very low αap, negatively




affected, however, by the absence of material underneath the drain and the vicinity of the

impervious layer. The originally pre-wrapped corn straw (plot 3) with a rather high αap but

with the drain level slightly above the impervious layer comes next. Then follows the loose

bulky flax boon (plot 5) with a very low αap but without any material underneath the drainand which is, moreover, situated near the impervious layer. The drain without envelope

(plot 10) and with a rather high αap that may be slightly compensated by the depth of the

impervious layer is next. Despite that the impervious layer is 0.10 m below drain level, the

glass fibre sheet (plot 6) with the highest αap shows also the highest F ap which represents

almost half of the total head loss. A classification of the materials based on F ap can also

consist of three classes:

• Class 1 (good, F ap ≤ 0.30): The loose bulky corn straw and the locally made synthetic

non-woven ‘Melita’ envelope (plots 2 and 4).

• Class 2 (moderate, 0.30 < F ap < 0.40): The pre-wrapped corn straw and the loose bulkyflax boon (plots 3 and 5).

• Class 3 (poor, F ap ≥ 0.40): The glass fibre sheet and no envelope material (plots 6 and 10).

This classification is based on both the material and the actual flow situation while the

classification on αap only considers the material.

7. Conclusions

This research shows the simplicity of determining whether a drainage system meets the

design criteria or not, just by measuring discharge and total head loss. Such measurements,however, do not tell more about the cause in case of malfunctioning when design criteria

are based on soil composition and not on hydraulic conductivity and depth of the imper-

vious layer. Such measurements also do not explain differences in performance between

various drainage materials. To know more about the possible cause of malfunctioning or

the differences in performance between various materials, hap needs be measured.

Since the performance of the drainage systems in the various plots was inadequate to vari-

ous degrees, and since each plot consisted of a different drainage material, a further analysis

of the functioning of the various drainage materials could explain the differences in perfor-

mance. Such an analysis permitted a ranking of the investigated materials, to determine the

actual value of the materials and to select the most appropriate material(s) for future use.

This research also shows the complexity to evaluate the performance of drainage materials

because the approach-flow pattern can be completely different from one material to the other.

A correct interpretation requires a good understanding of the approach-flow occurrence and

can only be done for similar flow conditions at the same hydraulic conductivity. Therefore,

the dimensionless αap is an important parameter. F ap includes the actual situation in which

the drainage materials operate and does not always yield the most suitable materials.

This research learns that:

• A large number of measurements is imperative to obtain reliable relationships between

q and ht; between hap and q1; and between hap and ht.• The design criteria are not met as the result of a too wide a drain spacing and the W ap of

the drainage systems.




• W ap is in principle a constant as long as the approach-flow pattern does not change with

discharge.

• Envelope materials which do not completely surround drain pipes may also affect the

approach-flow pattern and consequently W ap, especially at low discharges.• F ap does not depend on discharge despite the fact that W ap may vary.

• From the comparison of drains with and without envelope it follows that envelope mate-

rials promote fully circumferential flow.

• An envelope material with a small αap may exhibit a relatively high F ap due to the presence

of an impervious layer in the immediate vicinity of the drain and an incomplete envelope

surround.

• No envelope material and glass fibre sheet both exhibit a large αap and a substantial F ap.

• To compare the performance of drainage materials, it is essential that the approach-flow

pattern and the hydraulic conductivity are the same.

• αap is higher for small flow rates as a result of incomplete circumferential flow and

decreases with increasing flow rates till a constant value at full circumferential flow.

• Loose bulky envelope materials are clearly better than all the other investigated materials,

and much better than the thin glass fibre sheet.

• The locally made non-woven ‘Melita’ envelope performs less good than the bulky mate-

rials but it is the best pre-wrapped envelope material.

• The poor performance of organic materials is indicative of a material decay.

From this research a monitoring programme on the performance of drainage systems

can be derived to inform future landowners on the real value of their drainage system. This

monitoring programme should at least include the measuring of drain discharges and totalhead losses. A comparison of the relationship between discharge and total head loss with

the design criteria will reveal the functionality of the drainage system. Furthermore, if the

approach-flow head loss is measured, a further analysis of the approach-flow constant can

be made. Also as a result of the measurement of the approach-flow head loss, the combined

influence of drainage material and flow situation can be derived from the approach-flow

head loss fraction.

Design criteria usuallydo not consider the approach-flow resistance because it is supposed

that the drainage materials together with the loose backfill of the excavated trench do not

exert an additional flow resistance. However, the drain spacing should be reduced by either

40 or 25% if the approach-flow resistance results in an approach-flow head loss fraction of

respectively 50 or 30%.

From the additional information on hydraulic conductivity and depth of the equivalent

impervious layer, it can then be decided whether another drain midway between two drains

or the replacement of the whole drainage system will resolve the problem.

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Field Research on the Performance of Various Drainage Materials in Lithuania

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