Seepage from Lake Burullus into the reclaimed Mansour and Zawia polder area

Agricultural Water Management, 7 (1983)411--424 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

411

SEEPAGE FROM LAKE BURULLUS INTO THE RECLAIMED MANSOUR AND ZAWIA POLDER AREA

J.H. B O U M A N S

I and A.M. M A S H A L I

2

IEuroconsult, P.O. Box 441, 6800 A K Arnhem (The Netherlands) 2Study and Design Department, Executive Authority for Land Improvement Projects, Ministry of Agriculture,Cairo (Egypt) (Accepted 23 February 1983)

ABSTRACT Boumans, J.H. and Mashali, A.M., 1983. Seepage from Lake Burullus into the reclaimed Mansour and Zawia polder area. Agric. WaterManage., 7: 411--424. The Zawia and Mansour irrigation areas, formerly part of Lake Burullus, were reclaimed, drained and developed in the 1960's. Reclamation was not fully successful, however, and the agricultural land is still highly saline. Recent studies carried out on behalf of the Executive Authority for Land Improvement Projects by Euroconsult and the Study and Design Department of the Authority concluded that the groundwater tables were still too high and that the drainage should be intensified and deepened. The question remained whether, and to what extent, improved drainage would increase seepage from the lake into the polder area. The studies reported here showed that geohydrological conditions were such that no seepage of importance can be expected. Piezometer observations of vertical pressure gradients confirmed these results. Calculations showed that an increase in seepage due to improved drainage would be very small compared with the capacity of the existing drainage system.

INTRODUCTION In 1 9 6 0 , a p a r t o f L a k e Burullus, s i t u a t e d w e s t o f t h e n o r t h - c e n t r a l p a r t o f t h e Nile Delta, a l o n g t h e M e d i t e r r a n e a n , was d i k e d in, w h i c h c r e a t e d t h e Z a w i a a n d M a n s o u r p o l d e r a r e a o f 65 0 0 0 f e d d a n (27 300 ha). T h e n e w land was r e c l a i m e d a n d e q u i p p e d f o r irrigated a g r i c u l t u r e in t h e 1 9 6 0 ' s . R o a d s , irrigation canals a n d drains w e r e c o n s t r u c t e d (see Fig. 1). T h e m a i n drainage s y s t e m c o n s i s t e d o f a series o f parallel drains s p a c e d a b o u t 2 k m a p a r t w h i c h d i s c h a r g e d i n t o t h e M o h e e t m a i n d r a i n or t h e B a h r a w i m a i n drain, t h e l a t t e r b e i n g s i t u a t e d j u s t inside t h e dike. F r o m t h e B a h r a w i drain, t h e w a t e r was p u m p e d i n t o L a k e Burullus b y t w o p u m p i n g s t a t i o n s w i t h a t o t a l c a p a c i t y o f 7 m m d a y - ~ f o r t h e drainage area. T h e soils o f t h e n e w l y r e c l a i m e d a r e a c o n t a i n e d u p t o 10% salt. T h e s e salts h a d t o b e l e a c h e d b e f o r e t h e l a n d c o u l d b e cultivated. This leaching p r o v e d t o b e m u c h m o r e d i f f i c u l t t h a n e x p e c t e d a n d , in 1 9 8 0 , m o s t o f t h e a r e a was

0378-3774/83/$03.00

© 1983 Elsevier Science Publishers B.V.

412

, a~

r _

,

PUMPINO O 5T.&TI ~

.... ; , , ' r ~

~ ..=,o

~/h-!

'X~A

I

I*R"A

:rC='~

#/\--

I

~,~.~

I l

~

/

X",,

~

~,

7o\

~OMP,NO / /~ -e

I

I

I

I:

I

\

\

i

\....x.~.. \

\

I

~

I,

". \

o o/eeo

I I

/ LEOEN

D MAIN CANAL

MAIN AND ,.~ECONDAI~Y D~AINA~E SYSTEM >>>>>>>> DIkE • ,0ooo• hECTOR 150UNDARY X 'IA

I I I

I z~,'A

\

I

~-"

I

i

'I

\

]

,

~. --~-~

I

5CCTOE

\

II

STATmN

I

I

M AN,5 0 U I~

~ /

I I

I

~r

~

LOCATION DEEP b O R I N 6 5 AND PIEZOMETER5

Fig. 1. Mansour and Zawia polder area.

\

\

.I

J

~ ~ "~

~

//

3 :/ "1 :1 ..~

:1..-" .~ :;~

/

~---,\.

"\

~

~,

~,

~,

~,

413 still too salty for crop production. Table I shows the land use in 1980. The estate farms are on land which was given as a concession to the Delta Sugar Company for growing beets for its Zawia factory. In view of this very unsatisfactory situation, the Executive A u t h o r i t y for Land Improvement Projects (the Authority), which was in charge of the rehabilitation of the area -- including the improvement of saline soils -- started a detailed study in 1980 on soil, groundwater, and salinity conditions. This study, which was financed by the European Economic C o m m u n i t y , was carried out by a combined team of workers from Euroconsult and the study department of the Authority. The study included an analysis of the difficult and slow desalinization process in order to determine appropriate measures for a more successful and more rapid reclamation of the soils. The conclusion of the study as reported by Euroconsult and the Authority in 1981 was that for a major part of the area successful and accelerated leaching of salts is possible, provided that the groundwater table is lowered by means of a properly designed and constructed deep drainage system (Euroconsult/EALIP, 1981). The question arose as to whether deepening the drainage system and lowering the groundwater table could cause an important increase in the seepage of salty water from Lake Burullus into the reclamation area, all the more because the clay soils in the delta are underlaid by an extended aquifer of sand and gravel deposits. The possiblity of a considerable increase in seepage by underground flow, therefore, had to be assessed. This paper reports studies of the effects of deep drainage on the seepage inflow based on the analysis of geohydrological conditions and groundwater and piezometer data. Flow calculations for a number of alternatives were performed.

TABLE

I

Land use in the Mansour and Zawia irrigationareas in 1980 (%)

Cultivated land Uncultivated saline land Fallow Fish farm/leaching

Estate farms

Small farms

Total

13

16

29

53 9

7 2

60 11

GEOHYDROLOGY A general view of the geohydrological situation in the area is given in Fig. 2. The figure shows a lithological cross-section south of Lake Burullus. The section is based on the logs o f three wells. A clay cover overlies the upper part of the sandy aquifer. Near the lake this covering layer is about 30 m thick; the thickness decreases towards the south. The presence of such a thick clay cap near the lake reduces the risk of increasing seepage into the

2'5

ZO 1'5 DISTANCE FROM BURULLU5 (kin)

VELL

~ig. 2. Lithological longitudinal section in the north of the delta.

tO

WELL ~9)

1'0

o12000

~

~ALINITY

5AND

s

ppm)

"~CAJRO

WELL 1

.AKE 6 U R U L L U S

~D rq

t10

fO0

8O

70

v

~0~

T

50 ~

4O

30

20

I0

0

415 polder area, b u t the a m o u n t of the reduction depends on the exact nature of the clay cap and on whether the cross-section is representative for the whole area. Therefore, a more detailed study was made of the upper part of the clay cap. Sixty-eight borings were made, most were 5 m deep but some were 10 m deep. In all of these, soil characteristics were recorded, hydraulic conductivity measured and piezometers observed. Some typical profiles are presented in Fig. 3. The soil profile to a depth of 5--10 m is very stratified. The predominant texture is clay. Layers o f sand or loam occur in some places. These more permeable horizons are due to the sedimentation patterns of the flood plain where the lighter-textured materials were deposited at the levees of former river channels. Due to the frequent shifting of these channels, the permeable sand and loam layers are probably n o t continuous b u t are located in pockets. The hydraulic conductivity for horizontal flow (Kh), measured with the H o o g h o u d t method in the 68 holes, showed a close correlation with the soil texture. The K h for the clay soil was 0.06 m d a y - ' , and 10 m d a y - ' for the sandy and loamy soil, although higher values of up to 20 m d a y - ' were also found for the latter layers. The average Kh for the t o p 10 m of the clay cap was 1.0 m d a y - ' . The hydraulic conductivity for vertical flow (Kv) is much less than for horizontal flow, due to microstratification of the separate soil layers and macrostratification relating to the sequence of different soil layers in the profile. On the basis of the results of infiltration and leaching tests: an average Kv of the clay cap of less than 0.005 m day -1 was found. This implies that the hydraulic resistance, C, which is the thickness of the layer divided by Kv, would be 2000 days for a clay cap of 10 m and 6000 days for a clay cap of 30 m. In later calculations, however, a value for Kv of 0.05 m d a y - ' , corresponding to C equal to 200 and 600 days, respectively, has been used. In summary, the data shows that the clay cap is at least 10 m thick and is probably 30 m thick. The cap is stratified and contains pockets of sand and loam. The average Kh is 1 m day -~. The hydraulic conductivity for vertical flow is, however, small, so that vertical water movement is restricted. The latter will limit the seepage flow into the drainage area. GROUNDWATER DEPTH AND PIEZOMETER STUDIES The depth to the groundwater table in the present situation is roughly 0.5 m in cultivated land and 1.0 m in fallow land; some o f the land is flooded. The average depth for the three land use classes mentioned in Table I is 0.75 m. For successful leaching of salts the water-table should be kept below 1 m , which implies that, in practice, it will fluctuate between 1 and 1.5 m, with an average depth of 1.25 m. For a water depth in Lake Burullus of 0.25 m above ground level, the difference in hydraulic head between the lake and the polder area is a b o u t I m in the present situation and would be a b o u t 1.5

416 tA

6tA

2

dcba

D

dcbo

EC

D

dcbo

EC

D

EC

;ii_ ,j

5.

I" C~ RI C)

6.

8.

10J

0-

70

6tB

tE~ dc

dcbo

-- D [ EC

D

dc ba

FC

D

EC

I///X

~

• r,,,

,///I

L/5 _75_

.........

,,

7_o 37

3.

Itl

%()

tq

~q' uJ I I--

hJ ,~i

0

l:i

.

.

.

.

ii]], , ~q

Ill I iq~]

I

I I JIIq q ill' V ~ l 11 I q l 4

I,' '~

~II;llxhJ .....

~t~' SI,~t~I /

'St~.I

• ":.c ;." {..%'.',v

I!;!I;|!;IXi!I:!2I'/J !'.~

.

~ I~ ~I)~ I ~ I

I

~0J CLAY

~--~--

SILTY LOAM

SAPID

SAPIDY LOAM

51LTY CLAY

5OFT

SILTY CLAY LOAM

PEATY CLAY

D

DEPTH TO O~OUNDWATP-.P- "I"AI~LP-

EC

ELECTRICAL CONDUCTIVITY OF G ~ 0 U N D W A T E ~

(cm)

CLAY OR 50FT CLAY LOAM

) } 0~SERVATION5 ( m ~ c t ~ -1

~2

AND ~/3-19~1

417 m in the future, with improved drainage. The increase is 0.5 m or 50%, which roughly implies an increase in the seepage of 50%. The present seepage, although n o t known quantitatively, has n o t caused serious groundwater or drainage problems. Already for that reason alone it is not likely that improved drainage will lead to an unacceptable increase in seepage inflow. Information on the actual seepage inflow was deduced from piezometer observations. Six sets of piezometers were installed, three near the main drain, and three at a distance of 500 m. Each set had observation wells with filters at depths of approximately 1.5, 3, 5, and 10 m below the ground surface (Fig. 3). In the period 19/12/80 to 16/03/81, depth to the water table and electrical conductivity of the groundwater were recorded. Piezometers 1A, 2 and 61A, situated near the main drain, showed a downward pressure gradient, hence a downward movement of groundwater, which indicates that near the drain the seepage from Lake Burullus is intercepted b y the drain. Piezometers 1B and 70, at 500 m, were outside the influence o f the drain. They showed a small upward gradient of the order of 0.01 m per metre, indicating seepage inflow. Assuming a vertical permeability of 0.05 m day -1, the corresponding flow would be 0.5 mm day -1. Piezometer 61B, although also 500 m from the main drain, did show a downward water movement, indicating drainage. In summary, piezometer readings showed that seepage exists, b u t rates are low, and it is local. Some of the seepage is intercepted by the main drains. SEEPAGE CALCULATIONS

Simplified flow model A schematic flow model for seepage from Lake Burullus into the reclaimed area is presented in Fig. 4. The hydrological profile consists of a stratified clay cap, into which the Bahrawi main drain cuts, overlying a sand and gravel aquifer. The seepage flow may be split into three components: Flow A is the seepage intercepted by the main drain. Flow B is the seepage through the clay cap passing underneath the drain. For the quantitative approach it will be assumed that the clay layer extends to the impervious barrier. In doing so the flow will be slightly over-estimated b u t compensation is made for this in the calculation of Flow C. Flow C is the seepage through the aquifer. Flow C has to be corrected for the flow already included in Flow B. It will be assumed that the three flows are interdependent and may be superimposed. Actually the flows, particularly A and B, are interrelated. The result will be that the seepage is over-estimated, and that the results and conclusions are on the safe side. Flow A depends on the difference in water level b e t w e e n the lake and the main drain, which is n o t affected b y the proposed lowering of the water table in the polder area. For assessing the increase in seepage due to the Fig. 3. Soil profiles and piezometer observations.

418 025

DA

FLOW C

Fig. 4. F l o w model.

proposed drainage measures only Flows B and C have to be considered. An analytical solution for the calculation of Flow B has been given by Burgers {1926) and for assessing Flow C by Mazure (1936) and Verruyt (1970). The formulae and their applications are given in the appendix. Selection o f parameters

The parameters have been selected in accordance with the available geohydrological data. The thickness of the clay cap and that of the underlying aquifer are variable, as shown in Fig. 2. Therefore, for each layer two values have been selected, one extreme corresponding to maximum seepage and one believed to be more realistic. The values for the depth of the clay cap are 30 m and 10 m, the latter is considered to be the minimum value. The selected values for the thickness of the aquifer are 10 m in accordance with the thickness of the shallowest aquifer in Fig. 2, and 50 m which is thought to be the maximum thickness which could occur in this area. The most unfavourable combination with respect to seepage is a 10 m clay cap overlying a 50 m aquifer. The hydraulic conductivity of the aquifer is determined at 20 m day -1 for horizontal and vertical flow. The clay cap, however, is considered anisotropic with respect to hydraulic conductivity, the conductivity for horizontal flow (KCh) being 1 t o d a y -1 and that for vertical flow (KCv) being 0.05 m day -~ . To allow calculation of seepage, the actual anisotropic flow system may be transformed into an imaginary isotropic flow system using the following equations: Xt = X Z t = Z~/(Kh/Kv)

K t = x/(Kh - Kv)

419

.......'-I .......... S

......

~c~

Kc t

~~8 IllllllllllIHllJHI

I Illll]ll]

I III II III Illl

Ill[

III Illl

I Illl

Illl

Kc~ Illl

I till

lill

Illil

III I I I Illllll

III I Illll

Illlll

Ill ] Ill Ill

I ] I l i l t I KL l l I

FI6.5 ~

I~CllllUllllllllll

II Illlllll

I llllll

IlUl lllllll

1 Ilnl

Wit III I Ul HlllllllllllllIPllllll

FIO. 5 b Fig. 5. Seepage through the clay cap, Flow B: (a) schematic flow pattern; (b) geometry an(] symbols used in Burger's (1926) analyses.

III t l I f I I I I I I I III l~l I I I l I [ l l l l l t l l l +

l

I t l l l l l LIIIILL I I I I H I I l l l l J I l l l l l l t l l +

i

J, +.mu.m.l..umll.l.lu

FIO. 6 b

, ..mLl..|,i.LiL|.m

I~

I I I I I 111 II I I I I I l I I II I I I I | 1111 l t l I l l

I I I ] I I I l l I I I 1111

, ~

I

L"ml m'~|LU~I m'"

~"

Fig. 6. Seepage through the aquifer, Flow C: (a) schematic flow pattern; (b) geometry and symbols used in Mazure (1936) analyses.

420 TABLE II Alternatives and corresponding parameters Alternatives Ia

Ib

IIa

IIB

10 10 0.75 1.25 20 1 0.05 3.25 1.0 1.5 15

50 10 0.75 1.25 20 1 0.05 3.25 1.0 1.5 15

10 30 0.75 1.25 20 1 0.05 3.25 1.0 1.5 15

50 30 0.75 1.25 20 1 0.05 3.25 1.0 1.5 15

45 32 3.4 5.6 0.22

45 32 3.4 5.6 0.22

134 121 3.4 5.6 0.22

134 121 3.4 5.6 0.22

A c t u a l values

DA DC

DD DD KA Kah Key ZA ZB Zs B

(m) (m) (m) present (m) future (today -~ ) (mday -I) (mday -1 ) (m) (m) present (m) future (m)

T r a n s f o r m e d values

Dct DBt DDt DDt Kct

(m) (m) (m) present (m) future (mday -z)

X t a n d Z t r e f e r t o t r a n s f o r m e d h o r i z o n t a l a n d vertical d i m e n s i o n s , respectively. T h e n f o r K h = 1.0 a n d K v = 0 . 0 5 m d a y -1, t h e vertical d i m e n s i o n s axe expanded by a factor x/20, the horizontal dimensions remain unchanged and K t becomes x/0.05. T h e p e r t i n e n t p a r a m e t e r s are i n d i c a t e d in Figs. 4, 5a a n d 6a a n d are listed in T a b l e II. T h e w a t e r d e p t h o f t h e lake is 0.25 m ; t h e c u r r e n t a n d f u t u r e m i n i m u m w a t e r level in t h e m a i n drain is 3 m b e l o w g r o u n d level. H e n c e , in Fig. 4, t h e h y d r a u l i c h e a d , ZA, is 3.25 m a n d t h e h y d r a u l i c head, ZB, bet w e e n t h e lake a n d t h e g r o u n d w a t e r t a b l e in t h e p o l d e r is 1.0 m in t h e p r e s e n t s i t u a t i o n a n d 1.5 m f o r t h e f u t u r e s i t u a t i o n . R E S U L T S A N D DISCUSSION

T h e results o f t h e c a l c u l a t i o n s axe s u m m a r i z e d in Tables III and IV. I t is t o be n o t e d t h a t t h e length o f t h e p r o t e c t i o n d i k e is 28 k m and the t o t a l drainage area is 33 500 h a (80 000 f e d d a n ) . T h e p r e s e n t seepage i n f l o w n o t i n t e r c e p t e d b y t h e m a i n drain ranges f r o m 178 t o 4 3 8 ls -1 f o r the d i f f e r e n t alternatives. This seepage affects the w a t e r tables in t h e p o l d e r . T h e effect, h o w e v e r , is l i m i t e d t o a strip o f a f e w k m wide, as s h o w n in T a b l e IV. T h e t a b l e also s h o w s t h a t t h e seepage r a t e o f 0.5 m m d a y -1 derived f r o m p i e z o m e t e r readings a t 5 0 0 m is within t h e calculated rates f o r t h a t distance.

421 TABLE III Calculated seepage rates for different assumptions Alternatives

Ia

Ib

IIa

Hb

Present situation QB ( m3 d a y - ' per m dike) Qc (ms day -~ per m dike) Qtotal ( ms day -~ per m dike) Qtotal (1 s -1 for whole polder)

0.20 0.49 0.69 224

0.24 1.11 1.35 438

0.27 0.28 0.55 178

0.29 0.64 0.9,3 301

Future situation QB ( m3 d a y - ' per m dike) Qc ( m3 day -x per m dike) Qtotal (m 3 d a y - ' per m dike) Qtotal (1 s-' for whole polder)

0.30 0.74 1.04 337

0.36 1.69 2.05 664

0.40 0.43 0.83 269

0.43 0.96 1.39 450

Increase Q (m s d a y - ' per m dike) Q (1 s -t for whole polder)

0.35 113

0.70 226

0.28 91

0.46 149

TABLE IV Seepage inflow in relation to the distance from the main drain (Flows B and C, present situation) Distance from drain (m)

0 100 500 1000 2000

Alternatives Ia

Ib

IIa

IIb

(1)

(2)

(1)

(2)

(1)

(2)

(1)

(2)

0.69 0.29 0.04 0.00

1.5 0.2 0.01

1.35 0.88 0.35 0.11 0.01

2.0 0.8 0.3 0.02

0.55 0.21 0.07 0.02 0.00

0.6 0.2 0.04

0.93 0.56 0.33 0.17 0.05

0.7 0.4 0.2 0.06

(1) Inflow (m 3 day -1 per m dike) passing at the given distance. (2) Upward seepage rate (mm d a y - ' ) in relation to distance.

The future seepage reaching the polder underneath the 28 km dike protecting the 8 0 0 0 0 feddan polder area is 664 ls -1 for the most unfavourable alternative. This flow represents only 3% of the available drainage capacity of the pumping station and canal n e t w o r k of 7 mm day -~, corresponding to 22 000 1s -1. Even a much higher seepage could easily be handled b y the existing drainage system. The increase in seepage ranges from 91 to 226 ls -~. These rates are negligible compared with the existing drainage capacity of 22 000 1s -~. Even if the seepage rates increased b y 10 to 20 times, it would n o t present a real problem to the drainage and salinity control of the polder area.

422 CONCLUSIONS Thickness, n a t u r e and p e r m e a b i l i t y o f the clay cap overlying the e x t e n d e d sand and gravel aquifer are such t h a t t h e r e is little risk t h a t seepage will increase greatly a f t e r lowering the water-tables in the p o l d e r area. P i e z o m e t e r studies indicated t h a t seepage rates u n d e r the p r e s e n t c o n d i t i o n s are very limited, which implies t h a n an increase a f t e r i m p r o v e d drainage w o u l d remain very restricted. These findings were c o n f i r m e d b y calculations o f the seepage flow for p r e s e n t and f u t u r e c o n d i t i o n s . T h e calculated seepage increase and the t o t a l f u t u r e seepage i n f l o w reaching the p o l d e r are m i n o r and practically negligible c o m p a r e d with t h e available drainage capacity. Even rates five t o t e n times higher t h a n t h o s e calculated c o u l d easily be handled b y the existing drainage system. As t o t a l seepage is small, the salinization e f f e c t o f an increased seepage w o u l d be m i n o r or negligible.

REFERENCES Boumans, J.H., 1978. Drainage calculations in stratified soils using anisotropic soil model to simulate conductivity conditions. In: J. We~eling (Editor), Proc. International Drainage Workshop, 16--20 May 1978, Wageningen. ILRI Publ. 15, International Institute for Land Reclamation and Improvement, Wageningen, The Netherlands, pp. 108--123. Burgers, J.M., 1926. Groundwater flow near canal network. Ingenieur (The Hague), 32: 657--665 (in Dutch). Euroconsult/EALIP, 1981. Study on a programme of soil improvement in the Kafr el Sheikh governorate. In: Final Report (Report of the studies carried out on behalf of the Executive Authority for Land Improvement Projects, Ministry of Agriculture, Egypt by Euroconsult and the Department for Study and Design of the Authority), Vol. 2, pp. 122--127. Mazure, J.P., 1936. Seepage and salt intrusion into the Wieringermeer (in Dutch). In: Rapport en Mededelingen betreffende de Zuiderzeewerken No. 5, Bijlage 10. Rijksuitgeverij, The Hague, pp. 84--88. Verruyt, A., 1970. Theory of Groundwater Flow, 1st Edition. In: E.M. Wilson (General Editor), Civil Engineering Hydraulics Series, MacMillan, London, pp. 29--33.

APPENDIX

Flow calculations Calculation of Flow B Burgers ( 1 9 2 6 ) analysed t h e flow f r o m a lake i n t o a p o l d e r t h r o u g h a phreatic aquifer u n d e r n e a t h an i m p e r m e a b l e dike f o r t h e situation given in Fig. 5b. T h e water-table in the p o l d e r is assumed t o be h o r i z o n t a l and t o coincide with g r o u n d level. F o r t h e flow, Q~ p e r m dike, Burger, using the c o m p l e x variable t e c h n i q u e , d e v e l o p e d the relationship:

423

Q = KZ (Fa/Fb)

with Fa = fo ~'T (1 -- a sin2x) -~ dx

and

F b = fo ~'r (1 -- b sin2x) -v' dx

a = e -("B/H)

b = (1 - a) K = hydraulic conductivity H, B, Z, see Fig. 5b

Fifty percent o f the flow, Q, reaches the surface within the distance: Ls0% = ( H / ~ ) in (1 + b °'s) For 0 < P < 0.9, F p can be approximated within 0.5% by the empirical relation:

Fp = 0.3431 tan (1.3806p rad) + 1.5703 The Burgers formula is used to calculate Flow B by taking: Z = ZA

B =15m

(see Fig. 5a)

K ffi Kct = 0.22 m day -1 H = D o t + D A - - 1/2 D D t

H is used as the average of the upstream and the downstream depth of the phreatic aquifer, and is assumed to reach the impervious barrier. Results are shown in Table V. The effect of the seepage is limited to a narrow strip along the dike. Fifty percent of the flow reaches the surface within 9 to 22 m from the dike. We m a y safely assume t h a t at 100 m the effect of the seepage is negligible. TABLE V C a l c u l a t e d seepage flow, Q, a n d 50% r e a c h ( < 50%) Alternative

la

Ib

IIa

Lib

52.75 0.20 9.6

92.75 0.24 14.4

141.75 0.27 19.2

181.75 0.29 22.6

51.65 0.30 9.4

91.65 0.36 14.3

140.65 0.40 19.1

180.65 0.43 22.5

P r e s e n t s i t u a t i o n Z = 1 In H (m) Q ( m 3 day -~ p e r m ) Lso~ ( m ) F u t u r e s i t u a t i o n Z = 1.5 m H (m) Q ( m 3 d a y -l p e r m ) Lso% ( m )

424 Calculation o f F l o w C

Mazure (1936) and later Verruyt.(1970) studied the seepage into a polder through a semi-confined aquifer. The infiltration upstream of the impervious dike is laterally transported through the aquifer and seeps up into the polder. The flow through the aquifer is assumed to be horizontal, and vertical through the overlying confining layer (see Fig. 6b). For the situation shown in Fig. 6b, Mazure and Verruyt give the following solution: KHZ Q0 =

~1 +)~2+B

Qx = Qo e<-X/~2) Sx = (Qol)~2) e¢-XJ~2) )~1 = x/ ( K H C , ) )~2 = ~/ (KHC2)

C1 = vertical resistance D I / K v upstream C2 = vertical resistance D2/Kv downstream Sx = upward seepage rate at distance x.

The Mazure approach has been used to assess the seepage flow through the aquifer (Flow C) by using: Z -- difference between water level of the lake and the downstream water level H =DA

K = K A -- Kct (As part of the aquifer flow was already included in Flow B, KA is reduced by Kct. The effect of this correction on the calculated flows is otherwise very minor and it is not actually necessary.) C1 = D c t / K c t

C2 = ( D c t - DDt)/Kct DDt is the depth below ground surface of the water-table. This depth (Table II) differs in the present and future situations. The C values could also be assessed by using the actual values for D c and Kv.