Int. J . B i o m e t e o r . 1974, vol . 18, n u m b e r 2, pp. 121-127

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Drift Sand Reclamation of Walvis Bay, South West Africa

by PJ. Le Roux*

INTRODUCTION

Walv i s Bay is s i t ua t ed on the Wes t Coas t of A f r i c a in the de l t a of the Kuiseb R i v e r . P r o t e c t i v e m e a s u r e s aga ins t sand e n c r o a c h m e n t and f loods w e r e r e s p e c - t i v e l y in i t i a t ed in 1950 and 1961 as the town was f looded d u r i n g 1934. Dur ing t h e s a m e y e a r dune sand was p i led up to roof he igh t of bu i ld ings (Keet, unpub l i shed da ta) .

The d a n g e r of the town being f looded aga in was p r e v e n t e d d u r i n g 1961 when a 7 k m long wal l was bui l t a c r o s s the N o r t h e r n a r m of the Kuiseb R i v e r (Stengel, 1963). Al though v a r i o u s m e a s u r e s have been u n d e r t a k e n to p r o t e c t the town aga in s t sand e n c r o a c h m e n t t h e s e me thods have not yet been e n t i r e l y s a t i s f a c t o r y .

The sand dunes of the Namib D e s e r t c o v e r an a r e a of a p p r o x i m a t e l y 28 000 km 2 and ex tend in an u n b r o k e n s e a of sand fo r 460 k m f r o m Walv i s Bay to L t ider i t z . The width of the dune a r e a v a r i e s f r o m 50 to 120 km.

The dune a r e a which t h r e a t e n s Walv i s Bay is s i t ua t ed to the sou th of t h e town be tween t he lagoon and the At lan t i c Ocean on the W e s t e r n bounda ry and the s o u t h e r n a r m of the Kuiseb R i v e r on the E a s t e r n and Sou the rn boundary .

HISTORICAL REVIEW

Due to the e v e r e n c r o a c h i n g sand f r o m the d e s e r t and the t h r e a t of inunda t ion du r ing floods, the o r ig ina l h o u s e s in Walv i s Bay w e r e bui l t on p i l e s . As t h e t h r e a t of sand and w a t e r cont inued, Keet (1927) i n t e r a l ia , r e c o m m e n d e d the bui ld ing of s e a - w a l l s in the f o r m of soi l dykes , to p r o t e c t the town ag a i n s t f loods .

Af t e r the f lood in 1934 v a r i o u s indigenous p lan t s p e c i e s b e c a m e e s t a b l i s h e d in the Kuiseb R i v e r . Th i s r i v e r is only o c c a s i o n a l l y in f lood r e s u l t i n g in the s a l i n i t y of the u n d e r g r o u n d w a t e r to i n c r e a s e g r a d u a l l y t i l l a l eve l is r e a c h e d which k i l l s p a r t of the vege t a t i on .

As the influx of sand con t inued to be a p r o b l e m in the town, Keet (unpubl i shed data) r e c o m m e n d e d the e r e c t i o n of b a r r i e r dunes wi th t h e a id of po le s . Dur ing 1950 the W a t e r A f f a i r s b r a n c h of the Sou th -Wes t A f r i c a n A d m i n i s t r a t i o n c o m - m e n c e d wi th the e r e c t i o n of 12.8 km pole b a r r i e r s wi th t h e ob jec t of e s t a b l i s h i n g two a r t i f i c i a l dunes to p r o t e c t t he town and a c c e s s r o a d s a g a i n s t e n c r o a c h i n g sand .

The sand r e c l a m a t i o n work t h a t was done by the W a t e r A f f a i r s b r a n c h is d e - s c r i b e d by Stengel (1963). Since May 1970 t h i s work is be ing conduc ted by the D e p a r t m e n t of F o r e s t r y .

*) D e p a r t m e n t of F o r e s t r y , P . O . Box 333, Groot fon te in , South Wes t A f r i c a P r e s e n t e d at the Sixth I n t e r n a t i o n a l B i o m e t e o r o l o g i c a l C o n g r e s s , Noordwijk, The Ne the r l ands , 3-9 S e p t e m b e r 1972.

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PROBLEMS ENCOUNTERED

The loca l p r o b l e m s e n c o u n t e r e d in e s t a b l i s h i n g a v e g e t a t i v e c o v e r on the dune s ands can only be m e n t i o n e d br ief ly , and a r e : (1) The a v e r a g e annual r a in f a l l in the Namib D e s e r t n e a r Walv i s Bay is l e s s than 11 m m p e r annum; (2) due to the a r i d i t y and high sa l in i ty (ohm R 60°F = 238) of the dune sand and the s a l in i ty of the wet s i l t (ohm R 60°F = 8) only p lan t s w i th a v e r y h igh r e s i s t a n c e to s a l i n e cond i t ions wil l be able to t h r i v e h e r e ; (3) the p r e v a i l i n g S to SW winds which have an a v e r a g e ve loc i t y of 20 .2 k m / h and an a v e r a g e annual f r e q u e n c y of 3 435; (4) t ha t the p r e c i p i t a t i o n of the s a l ine fog on the dune sand and s i l t f ia t s i n c r e a s e s the s a l in i ty of the soi l and k i l l s any v e g e t a t i o n not r e s i s t a n t to t h e sa l t on i t s l e a v e s ; (5) the only w a t e r which is ava i l ab l e in u n l i m i t e d v o l u m e s for i r r i g a t i o n is s e a - wa te r , of which the e l e c t r o c o n d u e t i v i t y in m i c r o m o h m s / c m at 20°C is 4 7 . 5 (EC x 103), and u n d e r g r o u n d w ~ t e r of which the conduc t iv i ty r a n g e s f r o m 35 to 250 (EC x 103).

METHODS EMPLOYED IN COMBATING DRIFT SAND PROBLEMS

POLE BARRIERS. The pole b a r r i e r s (Fig . 1), which have been used s ince 1950, have b e e n employed wi th a f a i r amount of s u c c e s s . Al though l a r g e v o l u mes of sand have been depos i t ed at these b a r r i e r s , the sand has by no m e a n s been s t a b i l i s e d . To e n s u r e t ha t t h e s e b a r r i e r s a r e con t inua l ly ef fect ive , they m u s t be l i f ted r e g u l a r l y .

Fig. 1. E r e c t i n g a pole b a r r i e r .

A m a j o r p r o b l e m e n c o u n t e r e d at Walv i s Bay is t ha t the i n c r e a s e d wind ve loc i ty be tween t he po les r e m o v e s t he sand in t h i s a r e a f o r m i n g a V - s h a p e d gul ly . When th i s gul ly i s a p p r o x i m a t e l y 20 to 30 e m deep the poles topple r e s u l t i n g in the ad - jo in ing poles fol lowing.

To e l i m i n a t e t h i s d a m a g e (Fig . 2), a so lu t ion of 1 p a r t Sur faso l in 40 p a r t s w a t e r ha s been s p r a y e d wi th a w a t e r i n g - c a n in a 15 c m wide s t r i p on t h e windward s ide

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aga ins t , and between, the po les in the b a r r i e r . Th i s so lu t ion d r i e s r a p i d l y l e a v - ing a h a r d l a y e r which, to a l a r g e extent , p r e v e n t s wind d a m a g e to the b a r r i e r s .

Fig. 2. Seve re d a m a g e done to t he b a r r i e r s by s t r o n g winds .

DUNE STABILIZATION. Since 1964 the Mun ic ipa l i t y of Walv i s Bay h a s s t a b i l i s e d the dunes n e a r e s t the r e s i d e n t i a l and i n d u s t r i a l a r e a s wi th e i t h e r g r av e l , o ld m o t o r - c a r oil o r a sh f r o m the loca l power s t a t ion .

Whi le the g r a v e l s t a b i l i s e d a r e a s a r e the m o s t durab le , t he cos t of RO-35 ($ 0.46) p e r m 2 fo r c o v e r i n g the sand wi th a 2 .5 c m laye r , i s p r o h i b i t i v e . A r e a s th3 t have b e e n c o v e r e d wi th c o a l - a s h s ince 1964 have been suit~ibly s t a b i l i s e d . The a s h m u s t have a t h i c k n e s s of about 1 .5 cm . The to ta l co s t i nvo lved when s t a b i l i s i n g sand wi th a s h i s RO-07 ($ 0. 093) p e r m2 .

The m o s t economica l me thod employed in dune s t a b i l i s ~ t i o n is to s p r a y t h e s and wi th old m o t o r - c a r oi l . The cos t of s p r a y i n g the s and wi th oi l i s RO-05 ($ 0. 067) p e r m 2. The dep th of p e n e t r a t i o n m u s t be at l e a s t 5 - 10 m m . The m i n i m u m r~ t e of app l i ca t ion i s 10 m3 o i l / h a .

A h i g h e r r a t e of app l i ca t ion m u s t be appl ied in a l l a r e a s l i ab l e to s e v e r e wind e r o s i o n eg. ~t the s ide s of b a r e h a n dunes . R e g u l a r m a i n t e n a n c e to oil s p r a y e d s u r f a c e s is e s s e n t i a l w h e r e a s c o a l - a s h o r g r a v e l c o v e r e d a r e a s a r e p r a c t i c a l l y m a i n t e n a n c e f r e e .

EXPERIMENTAL BARRIERS AND TRIAL PLANTINGS

PLASTIC BARRIERS. Due to the expense involved when e r e c t i n g po le b a r r i e r s , e x p e r i m e n t s w e r e conducted wi th v a r i o u s p l a s t i c b a r r i e r s to d e t e r m i n e w h e t h e r t hey would be m o r e e c o n o m i c a l and e a s i e r to e r e c t and to r a i s e t h a n t h e pole b a r r i e r s .

The p l a s t i c b a r r i e r s have a t h i c k n e s s of 3 to 5 ram, a r e 40 and 100 c m in he igh t and have a p e r m e a b i l i t y v a r y i n g be tween 30 and 53%. T h e s e b a r r i e r s have y i e l d - ed d i sappo in t ing r e s u l t s due to :

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(a) the wind u n d e r c u t t i n g the b a r r i e r s ; (b) t h e i r f l a b b i n e s s . They tend to c u r l r e s u l t i n g in a d e c r e a s e d he ight of the

b a r r i e r ; (c) the e x t r e m e di f f icul ty e x p e r i e n c e d in r a i s i n g t h e m if sand ha s been depos i t ed

aga ins t t h e m ; (d) t h e i r t e n d e n c y to t e a r .

VEGETATION

GENERAL. Though it wi l l be m o r e e x p e n s i v e to s t a b i l i s e dunes wi th v e g e t a t i o n t h a n s p r a y i n g t h e m with, e . g . oil, such a r e a s wil l not only be m o r e a g r e e a b l e to the c o m m u n i t y in an o t h e r w i s e d e s o l a t e a r e a , but wi l l s t a b i l i s e addi t iona l dus t and s and be ing blown onto t h e m .

Expe r imer l t a l r e s u l t s ind ica te t ha t the dune sand can be s t a b i l i s e d wi th v e g e t a - t ion p r o v i d e d the a r e a be tween the plar~ts a r e s t a b i l i s e d for about 2 y e a r s to en - ab le the p lan t s to b e c o m e w e l l e s t a b l i s h e d and if the p l an t s a r e i r r i g a t e d wi th w a t e r wi th a low sa l in i ty . As n o n - h a r d y plar~ts a r e d e t r i m e n t a l l y a f fec ted by t he a d v e r s e w e a t h e r and soi l cond i t ions p r e v a i l i n g at W a l v i s Bay, only h a r d y p lan t s , wh ich a r e a l so r e s i s t a n t to s a l i n e condi t ions , m a y t h r i v e u n d e r t h e s e cond i t ions .

As only 6 800 m3 r e c l a i m e d sewage w a t e r wil l be a v a i l a b l e m o n t h l y f o r i r r i g a - t ion, only about 10 to 12 ha can be i r r i g a t e d . To u t i l i s e t h i s w a t e r e c o n o m i c a l l y only d rough t r e s i s t a n t s p e c i e s should be e s t a b l i s h e d on the dunes as w i n d b r e a k s whi l e the i r r i g a t i o n w a t e r should p r e f e r a b l y be i n j e c t e d into t he roo t zone of the p l an t s . Due to the neg l ig ib l e c a p i l l a r y m o v e m e n t of w a t e r in t he s a l ine dune sand (P. J . le Roux, unpub l i shed data) and the low r a t e of e v a p o r a t i o n of w a t e r f r o m the sand i t is e s t i m a t e d t h a t t h e s e p lan t s shou ld even tua l ly r e q u i r e an i r r i g a t i o n at about s ix week ly i n t e r v a l s .

TRIAL PLANTINGS. Since N o v e m b e r 1970 v a r i o u s t r i a l p l an t ings have been u n d e r t a k e n at Walv i s Bay. All p l an t s w e r e r a i s e d in t he n u r s e r y at Groot fon te in wi th n o n - s a l i n e w a t e r .

The v a r i o u s e x p e r i m e n t s and r e s u l t s ob ta ined a r e : (a) Var ious s p e c i e s w e r e p lan ted a t Mile 2 in s a l i n e s i l t w i th a r e s i s t a n c e of 8 ohm.. The u n d e r g r o u n d w a t e r has a conduc t iv i ty of 35 000t~ Q / c m .

The A T R I P L E X NUMMULARIA p lan t s a r e g rowing excep t iona l ly wel l whi le the fol lowing s p e c i e s d ied: ACACIA ALBIDA; ACACIA CYANOPHYLLA; CASUARINA EQUISETIFOLIA; LAGUNARIA PATERSONII and MYOPORUM SERRATUM.

(b) Dur ing Augus t 1971 v a r i o u s p lan t s w e r e p lan ted in 15 c m d i a m e t e r ho les d r i l l e d in the s i l t and f i l led wi th dune sand . T h e s e ho l e s w e r e d r i l l e d to a depth of a p p r o x i m a t e l y 2 m t i l l r e a c h i n g the s a l i n e u n d e r g r o u n d w a t e r .

The fo l lowing s p e c i e s a r e g rowing excep t iona l ly we l l : A T R I P L E X BREWERI; A. CANESCENS; A. HALIMUS and A. LENTIFORMIS.

The fo l lowing s p e c i e s d ied: A T R I P L E X SEMI-BACCATA; AGROPYRON DISTI- CHUM; CASUARINA EQUISETIFOLIA; DODONAEA VISCOSA; GALENIA SECUN- DA; LOTUS CRETICUS; POLYGONUM EQUISETIFORME and RETAMA ROE- TAM.

The A T R I P L E X BREWERI and A. LENTIFORMIS s e e m s to be the m o s t p r o m i s - ing plar~ts u n d e r t h e s e cond i t ions .

(c) A n u m b e r of 45 ga l lon oil d r u m s w e r e f i l led wi th dune sand and i r r i g a t e d wi th s e a - w a t e r to f ie ld capac i ty . The plarlts w e r e p lan ted in th i s sand and each d r u m was i r r i g a t e d wi th 9 l i t e r s e a - w a t e r at week ly i n t e r v a l s s i nce Augus t 1971. Ex - cept when t h e s e p lan t s w e r e r a i s e d in the n u r s e r y , t h e y have only been i r r i g a t e d wi th s e a - w a t e r .

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The fol lowing p l an t s have been i r r i g a t e d wi th s e a - w a t e r and show r e a s o n a b l e g rowth . The f igure in b r a c k e t s deno tes the p e r c e n t a g e of the o r i g i n a l n u m b e r of p l an t s s t i l l a l ive : A T R I P L E X BREWERI (100); A. CANESCENS (60); A. HALI- MUS (89); A. LENTIFORMIS (100); A. NUMMULARIA ( 1 1 ) a n d A . S E M I - B A C - CATA (66).

The fol lowing s p e c i e s have shown the b e s t g rowth : A T R I P L E X BREWERI and A T R I P L E X HALIMUS.

The fol lowing s p e c i e s d ied in t h i s e x p e r i m e n t : AGROPYRON DISTICHUM; AMMOPHILA ARENARIA; ARTEMISIA MONOSPERMA; CASUARINA EQUISETI- FOLIA; DODONAEA VISCOSA; EHRHARTA VILLOSA; EUCALYPTUS CAMAL- DULENSIS; GALENIA SECUNDA; LOTUS CRETICUS; MYOPORUM SERRATUM; POLYGONUM EQUISETIFORME; PROSOPIS J U L I F L O R A E and RETAMA ROE- TAM.

F u r t h e r e x p e r i m e n t s a r e be ing conduc ted wi th SALSOLA NOLLOTHENSIS p l an t s to d e t e r m i n e w h e t h e r ~hey can be g rown in dune sand and i r r i g a t e d wi th s e a - w a t e r .

(d) Dur ing D e c e m b e r 1971, 590 p lan t s of 21 s p e c i e s w e r e p lan ted wi th in the u p p e r a r e a r e a c h e d by the waves d u r i n g h igh t ide . Due to the s e v e r e ac t ion of t h e waves d u r i n g n e a p t ide , al l t he p l an t s w e r e e i t h e r w a s h e d out by the w a v e s o r c o v e r e d by sand to a dep th of 60 cm.

(e) Dur ing J a n u a r y 1972, 39 l a r g e oil d r u m s w e r e f i l led wi th dune sand whi l e e a c h d r u m was i r r i g a t e d wi th 65 l i t e r r e c l a i m e d sewage w a t e r to d r a i n the ex - c e s s s a l t in the sand . One hund red and s e v e n t y e ight p l an t s of 22 s p e c i e s w e r e p lan ted in t h i s sand . The 5 p lan t s in e a c h d r u m w e r e i r r i g a t e d wi th 4 . 5 l i t e r r e c l a i m e d sewage w a t e r a t week ly i n t e r v a l s .

All p l an t s which w e r e not s c o r c h e d a r e g rowing excep t ioua l ly wel l . As t h e s a l i n - i ty con ten t of the sewage w a t e r is v e r y low (conduct iv i ty = 3 0 0 0 f , ~ / c m ) , i t is an t i c ipa t ed tha t the p lan t s which a r e i r r i g a t e d wi th t h i s w a t e r could p r o b a b l y r e s i s t a h i g h e r s a l i n i t y con ten t .

A p a r t f r o m the p l an t s which a r e shown in T a b l e 1 to be i r r i g a t e d wi th r e c l a i m e d sewage wa te r , t he fol lowing s p e c i e s a r e g rowing excep t iona l ly we l l : A T R I P L E X BREWERI; A. CANESCENS; A. LENTIFORMIS; A. NUMMULARIA; A. SEMI- BACCATA; LOTUS CRETICUS; POLYGONUM EQUISETIFORME. T h e s e s p e c i e s can w i t h s t a n d a h i g h e r s a l i n i t y l eve l as shown in Tab le 1.

VEGETATION AND ITS RESISTANCE TO SALINE UNDERGROUND W A T E R

The d i s t r i b u t i o n of t he ind igenous v e g e t a t i o n in the Kuiseb R i v e r and i t s e s t u a r y is d e t e r m i n e d by t he s a l in i ty of the soi l and u n d e r g r o u n d w a t e r . The s a l i n i t y l eve l s (conduct ivi ty) of t h i s u n d e r g r o u n d w a t e r have been r e c o r d e d and v a r y f r o m 2 300 to 254 0 0 0 t ~ / c m .

Indigenous vege t a t i on grows p r o l i f e r o u s l y in t h i s a r e a w h e r e the conduc t iv i ty of the u n d e r g r o u n d w a t e r v a r i e s be tween 2 300 and 16 000. ARTHROCNEMUM A F - FINE and SALSOLA NOLLOTHENSIS can t o l e r a t e u n d e r g r o u n d w a t e r wi th a con - duc t iv i ty of 76 500 and 61 000 /~D/cm r e s p e c t i v e l y . F o r c o m p a r i s o n , the co n d u c - t iv i ty of s e a - w a t e r i s 47 5 0 0 / ~ / c m .

The loca l ind igenous spec i e s , and the h i g h e s t r e c o r d e d conduc t iv i ty of t h e u n d e r - g round w a t e r wh ich t hey can t o l e r a t e , a r e p r e s e n t e d in Tab le 1. Excep t TAMARIX USNEOIDES which is a t r e e , a l l o t h e r p l a n t s a r e g r a s s e s , s m a l l s u c c u l e n t s o r s h r u b s . The m o s t p r o m i s i n g indigenous p lan t is SALSOLA NOLLOTHENSIS (F ig .3 ) which con t inues to grow even if p a r t l y c o v e r e d by sand . Th i s p l an t h a s b e e n ob- s e r v e d to grow in s and 10 m above the r i v e r bed f r o m wh ich i t Obtains i t s m o i s -

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ture. It grows to within 120 m of the sea where the saline fog and spray wets the plants and where it is regular ly sandblasted.

TABLE 1. Provisional salt tolerance of indigenous and exotic vegetation Walvis Bay

Elect r ica l conductivity recorded (max.) of underground or i r r igat ion water

EC x 103

76.5 ARTHROCNEMUM AFFINE* Moss ex Adamson 61 SALSOLA NOLLOTHENSIS* Aellen 47.5** ATRIPLEX BREWERI, A. CANESCENS, A. HALIMUS, A. LENTI-

FORMIS, A. SEMI-BACCATA 35 ATRIPLEX NUMMULARLA, POLYGONUM EQUISETIFORME 30 PHRAGMITES AUSTRALIS* (Cav.) Steudel,

TAMARIX USNEOIDES* E. Meyer ex Bunge 29.5 LYCIUM TETRANDUM* L. fil. 24.5 ODYSSEA PAUCINERVIS* (Nees) Stapf 18 ACANTHOSYCIOS HORRIDA* Welw.,

CAPPARIS HEREROENSIS* Sehinz 10 CROTALARIA COLORATA* Schinz,

HELIOTROPIUM CURASSAVICUM* L, SCIRPUS DIOICUS* (Kunth) Boeck.

3*** AMMOPHILA ARENARIA, ARTEMISIA MONOSPERMA, CASUARINA EQUISETIFOLIA, EHRHARTA VILLOSA, EUCALYPTUS CAMALDU- LENSIS, E. GOMPHOCEPHALA, GALENIA SECUNDA, LEPTO- SPERMUM LAEVIGATUM, PROSOPIS JULIFLORAE, RETAMA ROETAM

*) -Indigenous species (The pH of the groundwater varied between 8.1 and 8.4). **) Irr igated with seawater. ***) Irr igated with sewage water .

Fig. 3. SALSOLA NOLLOTHENSIS plar~ts with deposited sand having been transported f rom the coast over the wet silt a rea in the background.

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Whi le c e r t a i n p l a n t s p e c i e s can r e s i s t h igh s a l i n i t y l e v e l s (Tab le 1) the s a l i n i t y of the u n d e r g r o u n d w a t e r in ex t ens ive a r e a s of the e s t u a r y i s too h igh (between 80 000 and 250 000/~ ~ / c m ) and can p r o b a b l y not be r e c l a i m e d by e s t a b l i s h i n g vege t a t i on .

SUMMARY

The sand dunes of the Namib D e s e r t c o v e r an a r e a of a p p r o x i m a t e l y 28 000 k m 2. Th i s sand was o r i g i n a l l y t r a n s p o r t e d f r o m the i n t e r i o r to t he Sea m a i n l y by the Orange R ive r , f r o m w h e r e i t m i g r a t e d a long the s h o r e wi th the aid of the Bengue l a c u r r e n t .

The m a i n p r o b l e m s e n c o u n t e r e d in sand con t r o l a r e : (a) the low r a i n f a l l of 11 m m p e r annum; (b) the a r i d i t y of the dune sand; (c) the p r e v a i l i n g s t r o n g S to SW winds ; (d) the f r equen t o c c u r r e n c e of s a l i n e fog; and (e) the h igh sa l in i ty of the so i l and u n d e r g r o u n d w a t e r .

A few dunes have b e e n s u c c e s s f u l l y s t a b i l i s e d w i th e i t h e r g r ave l , c o a l - a s h o r oi l .

Po le b a r r i e r s have b e e n u s e d wi th s u c c e s s to e n s u r e t ha t l a r g e v o l u m e s of sand a r e depos i t ed in p r e d e t e r m i n e d a r e a s . The d a m a g e c a u s e d to t h e s e b a r r i e r s b y the wind had been m a i n l y e l i m i n a t e d by s p r a y i n g a p l a s t i c / w g t e r e m u l s i o n on the w indward s ide of the b a r r i e r .

Va r ious e x p e r i m e n t s a r e be ing conduc ted to d e t e r m i n e the m o s t s u i t a b l e s and b a r r i e r and the t o l e r a n c e of p lan t s p e c i e s to loca l s a l i n e cond i t i ons . T h e s e e x p e r i m e n t s i nd i ca t e t h a t : (a) p l a s t i c sand b a r r i e r s a r e unsu i t ab l e ; (b) f ive A t r i p l e x s p e c i e s a r e ab le to grow in v e r y s a l i n e soi l (conduct iv i ty =

8 ohms) and w h e r e the u n d e r g r o u n d w a t e r has an EC x 103 of 35; (c) f ive A t r i p l e x s p e c i e s p lan ted in dune sand and i r r i g a t e d wi th undi lu ted s e a

w a t e r show r e a s o n a b l e g rowth ; (d) m o r e t han 15 p lan t s p e c i e s wi l l grow in dune sand which i s i r r i g a t e d wi th

r e c l a i m e d sewage w a t e r wi th an EC x 103 v a r y i n g be tween 2 and 3.

Ten ind igenous p lan t s p e c i e s have b e e n co l l ec t ed n e a r Walv i s Bay wh ich can t o l e r a t e s a l i n e u n d e r g r o u n d w a t e r wi th an EC x 103 v a r y i n g b e t w e e n 10 and 67 .5 .

R E F E R E N C E S

KEET, J . D . M . (1927) : A f f o r e s t a t i o n and C o n s e r v a t i o n in South Wes t A f r i c a . J o h n M e i n e r t L td . , Windhoek.

STENGEL, H.W. (1963) : P r o t e c t i o n f o r t he town of Walv i s Bay-In W a t e r A f f a i r s in S . W . A . A f r i c a - V e r l a g d e r Kre i s , Windhoek . 347-355.