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f l a r v e s t i n g Rainwater i n Semiarid Africa c o n s i s t s o f 6 M a n u a l s :

Manual No. 1 . W at e r T an ks w it h Guttering and flandpump.

Hanual No. 2 . S m a l l E a r t h Da m b ui l t b y AnimalTraction.

Manual No. 3 . R o ck C a t c hm e n t Da m w i t h selfclosing W a t e r t a p .

Nanual No. 4 . Shallow W e l l s w i t h B uc ke t l i f t .

Manual No. 5 . Subsurface and Sandstorage D a m s .

Manual No . 6 . S p r i n g P r ot e c t i o n s .

E a c h Nanual d e a l s w i th s i t in g c r i t e r i a , s t a n da r d d e s ig n s, b i l l s o fquantities and construction i n a simple t e xt a nd d r a w i n g s .

The Manuals a re b a s e d on p r a c t i ca l e x p e r i e n ce gained by building s o m e70 0 w a t e r s t r u ct u r e s f or r a i n w a t e r harvesting i n s e m i a r i d K e n ya overthe l a s t 1 4 y e a r s .

C o p y r i g h t

Permission i s hereby g i v e n for reproduction o f this m a t e r i a l , i n w h o l eor p a r t , f or e d u c a t i o n a l , s c i e n t i f i c , or development related p u r p o s e s ,e x c e p t t h os e i n v o l v in g c om m e r c i a l s a le o r s e r v i c e s , provided t ha t f ul lcitation o f t he source i s g i v e n .

Reproduction f or c om m e r c i a l p u r p os e s may n ot t a ke p l a c e without p r i o rpermission fro m:

ASAL Rainwater Harvesting

P . O . Bo x 14333

NAIROBI

K e n y a

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LIF~ATfl, INTERNATIONAL REFEPENCE

~N COMMUNITY WATER SUP~LY

AN~ SA~V~ON ((RC)

RO. Box 93190, 2~O9 AD The Hagu~TeI. (070) 814911 ext 141f142

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5/64

A C X N O W ! J E D G N E N T

Th e a u t h o r s o f t h e s e manuals ar e grateful t o DANIDA f or grants whichha s m a d e possible the coiupiling and publishing o f t h e i r e x p e r i e n c ew it h r a in wa t er harvesting f or selfhelp groups n semiarid a r e a s o fA f r i c a .

M uch g r at i tu de i s a l s o du e t o the Ministry o f Agriculture i n Ke nya ,w hi c h t o g e th e r w i t h DANIDA a f f o r d e d t he opportunity o f developing lowtech nology and labourintensive meth ods of harvesting r a i n w a t e r an dt he re by e na bl in g p eo pl e a nd livestock i n a semiarid region o f the

country t o h a v e access t o a steady w a t e r s u p p l y .

Thanks ar e a l s o du e t o t h e l o ca l inhabitants w i t h an d f or whom t h e s et e ch ni qu es we r e developed an d i mp le me nt ed . The ir understandableskepticism i n starting u p t he s e demanding a c t i v i ti e s g a v e the p r o c e s sa s o u n d and realistic foundation on which t o b ui l d .

Personal t h a n k s ar e very m u c h du e t o t he f o l l ow i n g p e o p l e:

P re be n E nh ar d o f Mutomo Soil an d W a t e r Conservation Project f or hi sconsiderable s u p p o r t .

Jan NissenPetersen for assistance i n the preparation o f the d r a w i n g s .

Kim NissenPetersen f or t he m a n y proofreadings and u s e f u l c o m m e n t s .

Mr . Moki, Mr . N g u n o a nd Mr. Dominic f or t he i r e x ce l l e n t w or k ondeveloping t he construction o f s m a l l e a r t h d am s b y a n i m a l t r a c t io n i nS o u t h e r n K i t ui , K e n ya .

E r i k NissenPetersen and Nichael ! J e e

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CONTENTS

PAGE

S U R V E Y O R S M A N U A L O N S M M J L E A R T H D A X S

1. Introduction 1

2. Standard Design 3

3. Siting Consideration 4

CONTRACTORS MANUAL ON SMALL EARTH DA X S 8

1 . Impermeable Foundation 9

2 . Building t he L ow e r P a r t o f a Da m W a l l 1 0

3. S l o p e s of Da m W a l l 1 2

4 . Designing t he Tw o S p i l l w a y s 1 3

5 . Th e C r e s t 1 4

6 . Determining t he H e i g h t o f a Da m W a l l 1 5

7 . Building a Spillway i n S t a g e s 1 6

8 . Protection o f Da m W a l l 1 7

9 . Protection o f S p i l l w a y s 1 7

10 . Protection o f Da m R e s e r v o i r 1 8

11. Extracting W a t er F r om a n E a r t h Da m 1 9

12 . Improving Inflow t o Da m R e s e r v o i r 2 1

13. Requirements f or Lab ou r, Materials an d T r a n s p o r t 2 3

14. B i l l s o f Quantities 2 6

15 . Naintenance 2 7

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SURVEYORS MANUAL O N

SMALL E A R T H D A N S BUILT B Y ANIMAL T R A C T I O N -

1. Introduction

Th e s m a l l e a r t h d a m s discussed i n this manual ca n s t o r e f r o m 5 0 0 cu b icmetres t o 10,000 c u b i c m e t r e s o f w a t e r i n b anks o f e a r t h n ot m o r e than5 metres h i g h an d 6 0 metres 1o~g.

Th e d a m s a r e b u i l t by m a n u al l a b ou r an d a n i m a l t r a c t i o n . u s i n g oxscoops.

Th e m a i n purpose o f a s m a l l e a r t h dam i s t o have w a t e r closer t oh omesteads during r a i n y s e a s o n s an d a f e w m o n t h s t h e r e a f t e r i n o r d e rt o t r a n s f e r l a b o u r f r o m f e t ch i n g w a te r t o a c t i v i t i e s i n t he f i e l d s .

Experience has shown t ha t o nc e a g r o u p o f p e o p l e h a v e b u ilt a s m a l l

e a r t h d a m , t h e y k ee p o n extending i t u n t i l i t ca n hol d wa te r f ornearly a l l d a y s i n t he y e a r . In m a n y p l a c e s people a ls o b ui ld asecond a n d t h ir d e a r t h da m t o c a t e r f or m o r e w a t e r n e a r t he h o m e s o fa l l members o f t he s e l f h e l p grou p.

A s m a l l e a r t h da m i s usually b ui l t b y a self-help g r o u p o f peoplec o n s i s t i n g o f about 2 5 f a m i l i e s living i n dry a n d r a t he r f l a t l a n dw i th u n r el i a b le r a i n f a l l s .

The members o f a g r o up w or k on t he da m f r o m 1 t o 3 d a y s a w e e k d u r i n gt he 6 dr y months o f t he y ear. On S a t u r d a y s w h e n schools a r e c l o s e d , 2c hi ld r en f ro m e ac h family work on t he d a m . B e s i d e s providing l a b o u rf or t he da m c o n s t r u c t i o n , t he c hi l d r e n a l s o l e a rn t ha t t he da m i s p a r to f t h e i r l i f e . T h a t i s i m p o r t a n t f or t he f u t u r e m ai nt en an ce a ndp o s s i b l e expansion o f t he d a m . -

Usually i t w i l l t a ke a b ou t t hr e e dr y s e a s o n s ( 3 y e a r s ) t o b u i l d a ndc o m p l e t e a s m a l l e a r t h da m which ca n h ol d w a t er f or about 1 0 months c ft he y ear.

As w i t h al l s i z e s a n d t y p e s o f e a r t h d a m s , t he s t o r e d w a t e r becomepolluted and i s t h er e f or e o n l y s u it a b l e f or l i v e s t o c k an d i r r i g a t i o n .The polluted w a t e r ca n o f c o u r s e be t r e a t e d f or domestic p u r p o s e s , butthat i s not economically v i a b l e f or people wh o l i v e o n v~rys m al l c as hi n c o m e s .

I n s t e a d s o m e people use t he polluted w a t e r a s i t i s, c l a i m i n g that t hew a t e r i s c l e a n e r t h a n t he w at e r t he y u s e d before t he dam was b u ilt.

M os t p e op l e, h ow ev e r , prefer t o h a v e c l e a n e r w a t e r by pulling i t upf r o m a shallow w e ll s u n k e i t he r i n t he s i d e o f t he r e s e r v o i r o r j u stdownstream o f the da m w all. The polluted w a t e r i s t hus t r e at e d byseeping t h r o u g h the underground i n t o t he shallow w ell.

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S m a l l e a r t h d a m s h a v e s o m e a d v a n ta g e s o v e r l a r g e e a r t h d a m s b e c a u s e :

( a ) T h e y require v e r y little c a pi t a l i n v e s tm e n t .

( b ) They d o n ot require mechanized e a r t h moving m a c h i n e r y .

( c ) T h e y ca n be designed and supervised b y l o c a l t e c h n i c i a n s .

( d ) They ca n b e b u i l d and maintained by t he u s e rs t h e m se l v e s .

( e ) T h e y d o n ot create erosion due t o o n l y s m a l l h e r d s o f l i v e s t o c k

b e i n g watered b y th e d a m .

( f ) A s m a l l e a r t h dam can h arvest rainwater f r om s m a l l s h o w e r s , w h i l ea l a r g e dam r e q u i r e s bi g s h o w e r s f or filling up .

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2 . S t a n d a r d Design

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3. Siting C o n s i d e r a t i o n s .

Construction o f s m a l l e a r t h d a m s b y manual l a b o u r an d a n i m a l t r a c t i o ni s o n l y p o s s i b le w he r e a determined and wellorganized s e l f he l p g r o upha s a r e a l n e e d f or having w a t er s i t u at e d nearer t h e i r h o m e s t e a d s .

Therefore a certain degree o f mobilization ha s t o b e carried out i nt he p r o c e d u r e o f siting an d d e s i g n i n g .

3.1. P h a s e 1 . Information C a m p a i g n .

Th e f i r s t s t e p i n promoting construction o f s m a l l e a r t h d a m s i s t om a k e g r o u p s realize t h e i r n e e d s and a d v a n t a g e s o f having w a t e r nearert h e i r h o m e s .

This message i s brought ab ou t i n a s o r t o f information c a m p a i g n a tpublic meetings w i t h o f f i ci a l s f r o m t he m i n i s t r i e s c o n c e r n e d . Duringor a ft er s uch meetings people w i l l organize t h e m s e l v e s i n t o g r o u p swi th s e le ct ed committees consisting o f a c h a i r m a n / l a d y , a secretaryand a t r e a s u r e r an d t h e i r d e p u t i e s .

Such c o m m i t t e e s w i l l r e g i s t e r t h e i r g r o up w i t h n a m e and l o c a t i o n , andf i l e a request f or t e c h n i c a l assistance t o t he dam b ui l d i n g w i t h t her e l e v a n t authorites.

3. 2 . P h a s e 2 . S e l e c t i o n Criteria f or S i te s .

Whe n s uch a r e q u e s t ha s b e e n a p p r o v e d , a n experienced s ur ve yor wi l lv i s i t t he g r o u p a t t h e i r l o c a t i o n f or t he purpose o f a s s i s t i n g t he

g r o u p i n t he difficult t a s k o f locating t he m o s t viable s i t e f or t h e i rs m a l l e a r t h d a m .

A s u r v e y o r w il l t e ll t he people t ha t t he y h av e t o s e l e c t 3 ~viab1es it e s a cc or d in g t o b e l o w mentioned c r i t e r i a , and t h a t t h e r e a f t e r hewi l l com e ba ck and select t he m o s t viable s it e o f t he 3 s it es t he yh a ve p r o p os e d .

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Th e people w i l l s e l e c t 3 proposed s i t e s according t o f o l l o w i n gcriteria w h i c h t h e i r secretary w i l l w r i t e do w n:

( 1 ) Th e m o st s ui t a bl e s it e s a re f o u n d at:

( a ) A s e a s o n a l watercourse n ot w i de r t ha n 3 metres a n d no t d e e p e r

t h a n 1 m e t r e .

( b ) Around the lower s i d e o f a n a t u r a l w a t e r h o l e .

( c ) Around a n excavation m a d e by r o a d b u i l d e r s .

( d ) Around t he downslope o f a s m a l l depression on gradually slopingl a n d .

( e ) A t t he f o o t s l o p e s o f a hillside w he r e r u n o f f a c cu m ul a t e s .

( f ) A t t he f oo t o f r o ck o u t cr o p s w he r e r u n o f f passes during r a i n s .

Fo r al l t he t yp e s o f s i t e s m e n t i o n e d , i t i s v er y i mp or ta nt t h a tr a i n w a t e r runoff flows o v e r t he s i t e several t i m e s during a r a i n ys e a s o n , otherwise t he da m r e s e r v oi r m i g h t no t be filled w i t h w a t e r .

( 2 ) Th e s i t e s m u s t not contain t he followinp f e a t u r e s :

( a ) B l a c k cotton so il.

( b ) Light s a n d y so il.

( c ) L i m e situated n e a r t he soil s u r f a c e .

( d ) An t h i l l s o r termite m o u n d s .

( e ) S t o n e boulders o r exposed r o ck s u r f a ce s .

( 3 ) Th e s i t e s should preferably contain a natural depression f or t he

purpose o f providinp f r e e storape such as:

( a ) A natural w a t e r hole.

( b ) A depression i n the l a n d s c a p e .

( c ) A widening o f a s e a s o n a l watercourse.

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3.3. P H A S E 3. Sol T y p e f or Dam S i t e s .

When a selfhelp g r o u p ha s selected 3 s i t e s w i t h r e g a r d s t o t he abovementioned c r i t e r i a , a s u r v e y o r will c a r r y out t he f i n a l selectionu s i n g t he following a p p r o a c h :

( a ) Decide which o f t he 3 s i t e s w i l l p ro vid e m axim um volume f orminimum e f f o r t o f labour.

( b ) Investigate whether t he s i t e chosen by him has a n impermeable( w at e r t ig h t ) l a y er situated i n t he underground o f t he chosen da m

s i t e .

Th e best way o f finding ou t ab ou t this i s t o di g a t r e n c h a l o n gt he length o f the planned dam w all. This t r e n c h w ill, a t a l a t e rs t ag e , b e utilized f or builinga w at e r ti g ht v e r ti ca l f ou nd a t io nu n d e r t he da m w a l l w hi c h w i l l prevent s e e p a g e underneath t he da mw all. Th e d e p t h o f this trench ha s t o be deeper than t he flooro f t he r e s e r v o i r a f t e r i t s e x c a v a t i o n .

( c ) Th e surveyor w i l l explain t he purpose o f digging t he t r e n c h an ds t a k e ou t t he outline o f t he trench which should b e a b ou t 4 0 cm .w i d e an d a s d e e p a s p o s s i b l e .

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( d ) Th e s u r v e yo r w i l l handover t he f o l l ow i n g e q ui p m e n t t o t hef or e x c a v a t i n g the t r e n c h an d building t he e a r t h d a m :

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1 0 k a r a i s ( i ro n p a n s )

1 ox-scoop2 harnesses for oxen or donkeys1 plough

5 wheelbarrows

0.60

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ox-scoop

( e ) I n s t r u c t the people t o p l a c e t he e x c a v a t e d soil d o w n s t r e a m o f thetrench.

( f ) T e l l the l e a d e r s t o call f or him t o i n s p e c t the trench w h e n the

instructed excavation i s c o m p l e t e d .

( g ) W h e n the excavation i s completed and a p p r o v e d b y t he s u r v e y o r , hew i l l handover the t e c hn i c a l a s s i s t a n ce t o a c o n t r a c t o r wh o willbe i n t r o d u c e d t o t he self-help g r o u p b y the s u r v e y o r .

To ge t he r t he y will pe g o ut t he outline o f the da m s i t e a nd thetwo s p i l l w a y s , an d i n s t r u c t t he people t o e x c a v a t e the d i t c h t o aw i d t h o f 1.5 m e t r e s .

0.,

4mm thick metol sheet

Front view

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CONTENTS

C O N T R A C T O R S M A N U A L O N S M A LL EARTH D A M S

P A G E

C O N T R A C T O R S M A N U A L O N SMALL E A R T H D A N S 8

1 . Impermeable Foundation 9

2 . Building t he L ow e r P a r t o f a Da m W a l l 1 0

3. S l o p e s of Da m W a l l 1 2

4. Designing t he Tw o S p i l l w a y s 1 3

5 . Th e C r e s t 1 4

6 . Determining the h e i g h t o f a Da m W a l l 1 5

7 . Building a Spillway i n S t a g e s 1 6

8. Protection o f Da m W a l l 1 7

9. Protection o f S p i l l w a y s 1 7

10 . P r o t e c t i o n of Da m R e s e r v o i r 1 8

11. Extracting W a t e r From a n E a r t h Da m 1 9

12 . Improving Inflow t o E a r t h Da m 2 1

13 . Requirements f o r l a b ou r , Materials and T r a n s p o r t 2 3

14 . B i l l s o f Quantities 2 6

15 . Maintenance 2 7

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1 . Impermeable Foundation

The supervision o f the s i t e ha s no w b e e n handed o v er f r om th e s u r v e y o rt o t he c o n t r a c t o r . The outline o f t he da m w a l l an d t he tw o s p i l l w a y sha ve b e en staked out w i t h wooden p e g s a n d all parties concerned h a v e

agreed on it.

When the trench i s du g t o a w i d t h o f 1.5 m e t r e s , i t i s t o b e filled upw i t h compacted clayish soil t o f o r m a watertight foundation o f t heda m w all. This i s d o n e a s f o l l o w s :

( a ) S h o v e l i n t o t he t r e n c h a 1 5 cm . l a y e r o f clayish soil an d c o m p a c ti t w e l l wi th wa te r and stamping b y f e e t . I f w a t e r i s no tavailable f or this p u r p o s e , t he s o i l ca n be compacted by stampingi t w i th s h or t l e n g t h s o f t r ee t r un k s.A l te r n a ti v e l y, l i v e st o ck ca n b e driven f o r t h an d b ack ov er thes o i l i n the trench fo r c o m p a c t i n g .

( b ) Th e whole trench i s t o be filled u p t hi s wa y o f 1 5 cm . thickl a y e r s o f compacted clayish s o il u n ti l i ts t op surface i s e v e nw i t h t he surrounding s o i l s u r f a c e .

( c ) Neanwhile o t h e r members o f th e g r ou p w i l l c l e a r t he s i t e fo r theda m w a l l and the two s p i l l w a y s f or v eg et at io n a nd p l a c e thedebris downstream o f t he pegged out da m w all.

I t i s v e r y i m p o rt a n t t h a t r o ot s o f t r e e s an d b u s h e s f r o m the s i t ear e removed completely f r o m th e s o i l t o a v o i d regrowth o f roots

with in the dam w a l l .

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2. Building the Lower Part of the D am W all.

Th e s o il f or building t he da m w a l l i s t a ke n f i r st f r o m t he s i t e o f t her e s e r v o i r f or t he purpose o f making t he r e s e r v o i r a s d e e p a s p o s s i b l ei n or d er t o l i m i t e v a p o r a t i o n , an d t h e re a f t er f r om t he e x c a v a t i o n o fthe s p i l l w a y s .

Th e d e p t h o f t he r e s e r v o i r m u s t not, h o w e v e r , b e l ow e r t h a n the d e p t ho f t he i m pe rmeable f o u n d a t i o n o f t he da m w all, because t ha t cou lda l l o w s e e p a g e f r o m t he r e s e r v o i r t o e s c a p e u nd e r t he da m w all.

W h i l e transporting s oi l t o the da m w all, it i s i m p o r t a n t t o b e a r i nm i n d t ha t t he m o s t clayish s o i l should be placed a l o n g t he middle o fthe dam w a l l a s a n extension o f the clayish f o u n d a t i o n i n t he f i l l e d u p t r e nc h.

Th e m o s t s a n d y so il, which i s the poorest f or building ashould b e placed on t he downstream p a r t o f the da m w all.

da m w all,

S o i l s b e i n g a mixture o f c l a y an d soil s h a u l d b e placed o n t heupstream s i d e o f t he da m w a l l w h e r e i t w i l l l i m i t infiltration o fw a t e r i n to t he w all.

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Th e procedure f or bulding a da m w a l l by draught a n i m a l s i s found t obe m o s t viable i n t he f o l l o w i n g way:

( a ) Remove al l vegetation a nd t re e stumps f r om t he s i t e o f t her e s e r v o i r . Thereafter plough the s i t e w it h t he ai m o f l o o s e n i n gthe to p l a y e r o f the soil . A c h i s e l plough i s t he be s ti m p l e m e n t , bu t a n ordinary plough can b e u s e d a s w e l l .

( b ) Transport s o i l f r o m t he ploughed a r e a t o t he s i t e o f t he da m wallw i t h the ox scoop and wheelbarrows. T he ox -scoop i s driven i ncircular patterns o v e r the da m w a l l a nd t he excavation pit o f ther e s e r v o i r . E v e r y h ou r o r s o the direction i s turned o p p o s i t e ,that i s c lo ck wi se a nd a n t i c l o c k w i s e , i n o r d e r t o g e t thedeposited soil compacted ev enly by t he d r oug ht a n im a ls an dp e o p l e .

Loading an d off-loading o f t he ox-scoop i s d o n e by manouveringthe handles o f t he ox scoop eith er upward o r downward w h i l e

d ri vi ng a ro un d o v e r the respective places o f loading a n d o f f l o a d i n g .

( c ) Offloading should take p l a ce a l on g t he centre l i n e o f t he lowestp a r t o f the da m w a l l an d t h e r e a f t e r be e x t e n d e d upward an do u t w a r d , s o t ha t t he da m wall i s a l w a y s highest a t i t s middles e c t i o n .

This i s i m p o r t a n t t o k e e p i n m i n d during the whole construction

p e r i o d , because i f t he da m w a l l i s not highest a t the middle a tal l t im es , t h e n a single unex pected r a i n s h o w e r c an p ro du ces u f f i c i e n t r u n o f f t o w a s h o u t t he middle section o f the da m wallinstead o f passing h a r m l e s s around t he e n ds o f the da m w all.

I n fac t, by making t he m id dl e s ec ti on o f the da m w a l l a bout 1.5m et re s h ig he r t h a n i ts e nd s a t a ll t i me s o f the constructionperiod an d by placing a fe w l a r g e s t on e s a t the e n d s o f t he w all,i t w i l l function a s a completed da m w h i c h ca n withstand h e a v y

thunderstorms and s t o r e r u n o f f w a t e r f or q ui t e s o m e t i m e .

( d ) T he offloaded soil should placed i n l a y e r s b e i n g about 1 5 cm .t h i c k i n o r d e r t o g et i t compacted we l l by e i t h e r l i v e s t o c kdriven o v e r t he s oi l d e p o s i t s or b y c o m p a c t i o n by s hor t t r e et r u n k s operated by a person or two.

( e ) Th e p i t i n t he r e s er v oi r a r e a f r om w he r e t he s oi l i s removed will

h a v e t o b e ploughed now a nd t h e n i n d i f f e r e n t directions f orl o o s e n i n g t h e s o i l t o be transported t o the da m w all.

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3. Slopes of D am Wall.

(a) As the dam wall is build higher i t i s a l s o m a d e w id e r b y o f f l oa di ng a nd c om p a c t in g s o il on i ts t wo s i de s , s t il l bearing i nm in d t ha t clayish soil i s offloaded a l o n g t he upstream s i d e o ft he dam wall.

( b ) Th e s l o p e o f the upstream s i d e o f t he da m wall should be a bout

3:1, w h i c h i s 3 m e t r e horizontal w i d t h f or e v e r y 1 m e t r e h e i g h t .

( c ) Th e s l o p e of t he downstream s i de of t he w a l l should b e a b ou t 2:1,w h i c h i s 2 m e t r e horizontal with f or e v e r y 1 m e t r e h e i g h t .

Th e a b o v e relations between w i d t h a n d height ar e o f a s o m ei m p o r t a n c e , bu t s i n c e a trained c o n t r a c t o r ca n d e t e r m i n e t hec o r r e c t relationships b y ey e, m o r e sophisticated measurements aren ot needed fo r t he t i m e b ei n g .

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4. Designing the T w o Spillways

To a v o i d a washout o f a da m w a l l b y h e a v y r a i n s , i t i s m o s t i m p o r t an tt ha t t he s p i l l w a y s can d i s c h a r g e surplus r u n o f f safely when t he

r e s e r v o i r i s filled up . Experience w i t h s m a l l e a r t h d a m s ha ve s hownthat:

( a ) T h e r e should a l w a y s b e t wo s p i l l w a y s b ui l t , t h at i s one s p i l l w a ya t e a c h end o f a dam wall.

( b ) Th e f r e e b o a r d , which i s t he vertical d i s t a n c e between t he heighto f t he s p i l l w a y s an d t he height o f t he e n ds o f t he da m wall,

s h o u l d a t a ll t im es a nd a l s o during the construction p e r i o d , b ea t l e as t 1 .2 metres or preferably 1 .5 m e t r e s.

( c ) Th e t o t a l length o f t he t wo s p i l l w a y s should correspond t oapproximately 1 /3 o f t he length o f t he da m wall, t h a t i s 1/3 f or

e a c h s p i l l w a y .

( d ) T hi s m e a n s t h a t w h e r e a da m w a l l i s no t b u i l d t o i ts f i na lbefore a r a i n y s e a s on , t he s p i l l w a y s c a n n o t b e b u i l d t of i n a l h e i g h t s , because t h e i r height m u s t b e a t l e a s t 1.2b e l o w t he e n d s o f t he dam wall.

A s explained e a r l i e r , t he height o f t he middle o f t he dam w a l lm u s t b e a t l e a s t 1.5 metres higher t h a n t he e n d s o f t he wall.

Therefore t he height between t he s p i l l w a y s an d t he middle o f t heda m w a l l w il l b e a bo ut 1 .5 metres + 1.2 metres = 2 .7 m e t r e s .

( e ) Th e height o f t he s p i l l w a y s ca n b e extended upwards by b u i l d i n g as t o n e masonry w a l l a c r o s s t he spillway o r i f f un d s d o n ot a l l o wthat, s o m e bi g b o u l d e r s packed i n s oi l ca n b e placed a c r o s s t hes p i l l w a y s a n d i n te r planted w i t h creeping g r a s s .

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5. The Crest.

As mentioned above, t he u p p e r outline o f a da m wall, called t he c r es t ,m u s t a l w a y s a t l e as t 1 .5 metres higher t h a n t he e n ds o f a dam wall,t he reasons f or t hi s a p p e a r a n c e o f t he b e i n g that:

( a ) I n c a s e t he s p i l l w a y s i s n ot b e a b l e t o d i s c h a r g e a n e xt ra he av yf l ood i ng f r om a t h u n d e r s t o r m , a w a s h o u t o f a section o f t he da mw a l l ca n occur. S i n c e i t i s much e a s i e r t o r e p a i r a w a s h o u tsection a t the low w a l l s a t t he e nd s t ha n a t t he h i g h middle

s e c t i o n , t he middle section m u s t b e t he h i g h e s t p l a c e i n t he da mw a l l .

( b ) Also s i n ce m o r e s o i l i s placed a t t he middle s e c t i o n t h a n a t t helower e n d s , t he middle p a r t w i l l s i n k m o r e t h a n t he e n d s when t her e s e r v oi r w i l l b e filled w it h w at e r a n d s a t u r a t e t he s o il i n th eda m w a l l .

I f t he c r e s t o f t he middle o f t he da m s i n k s t o l e s s t ha n 1. 0metres a bo ve t he e n d s o f t he dam wall, i t m u s t b e raised t o i t sformer h e i g h t , otherwise the middle section i s i n danger o f b e i n gwashed ou t i n h e a v y r a i n s .

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6. Determining the Height of a D am Wall.

Th e f i n a l height o f a da m w a l l i s determined by:

( a ) T h e owners a nd b u i l d e r s demand f or w a t e r .

( b ) Th e c a t c h m e n t s ability t o supply s u f f i c i e n t r u n o f f t o f i l l t hed a m r e s e r v o i r .

A da m wall ca n b e b u i l d i n s t a g e s during dry s e a s o n s , but before t heo n s e t o f a r a i n y season t he da m w a l l a n d s p i l l w a y s s h o u l d b e c o m p l e t e da t those heights t h e y h a ve r e a c h e d , otherwise h e a v y r a i n s m i g h t d a m a g eo r s w e ep a w a y totally t he w h o l e s t r u c t u r e .

A da m wall b u i l d i n s t a ge s should t h e r e f o r e be c o m p l e t e d before ar a i n y s e a s o n a s f o l l o w s : -

C a ) The w i d t h o f t he c r e s t should be 1. 5 m e t r e s w i d e a l o ng i t s whole

l e n g t h . It s s u r f a c e should c u r v e u p w a r d s t o a v oi d p ool s o f w a t e rc o l l e c t i n g on t he c r e s t , because that ca n c r e a t e p a r t s o f the da mw a l l t o s l i d e d o wn w i t h d i s a s tr o us r e s u l t s .

( b ) A s explained e a r l i e r , t he cr e st m us t b e a t l e a s t 1. 5higher a t t he middle than a t the e n d s o f the da m wall t o

s tor m wa te r f rom s pi ll in g o ve r a t t he middle o f t hew hi c h w i l l r e s u l t i n a w a s h ou t d i f f i c u l t t o r e p a i r .

( c ) Th e s l o p e s o f t he s i d e s o f t he da m wall s h o u l d b edownstream s i d e an d 3:1 on the upstream s i d er e s e r v o i r .

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The e a s i e s t wa y o f measuring s u c h s l o p e s i s t o n a i l t o g e t h e r 3p i e c e s o f t i m b e r o f 4 x 1 a s s h o w n b elow.

By placing a spiritlevel horizontally o n t he u p p e r e d g e o f t het i m b e r t he c o r r e c t s l o p e ca n be s e e n .

In ca s e t he s l o p e s a re t oo s t e e p , t h e y should b e brought up t ot he c or r e ct s lo pe b y l a yi n g a blanket o f t o p s o i l on t he s l o p e s .

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7. Building a Spillways in Stages.

Since i t i s o f v i t a l i m p o r t a n ce t ha t t he height o f t he tw o s p i l l w a y sm u s t b e a t l e as t 1 .2 metres b e l o w t he e n d s o f t he da m w all, thes p i l l w a y s ar e raised accordingly t o t he present height o f t he da mw a l l .

N e v er a t te m p t t o r a i s e t he s p i l l w a y s before t he da m w all, because thatc oul d a l lo w unexpected s t o r m w a t e r t o w as h a wa y m o st o f t he s t r u c t u r eo v e r n i g h t .

A s p i l l w a y ca n be raised by placing s t o n e s on t he floor an d s i d e ssloping downstream f r o m t h a t p o i n t o f t he s p i l l w a y be i n g 1. 2 m e t r e sb e l o w t he e n d s o f the da m w all, that i s w h e r e the discharge o f s u r p l u sw a t e r w il l t a ke p l a ce .

Th e c r e s t s o f the tw o s p i l l w a y s should be h o r i z o n t a l a nd o f t he s a m e

h e i g h t .

To prevent t he stones b e i n g washed a w a y by overflowing w a t e r , to p soilca n b e packed around the s t o n e s a nd creeping g r a s s planted i n t he so il.

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8. Protection of D am Wall.

( a ) A new dam w a l l ha s t o protected a g a i n s t splash erosion f r o mr a i n d r op s w hi c h can e r o d e seriously t he s i d e s o f a d a m .

C r e e p i n g g r a s s f o u n d locally and planted o n t he dam w a l l a tf i r s t o n s e t o f r a i n s i s v e r y suitable f or preventing erosionr a i n and w i n d .

( b ) To prevent l o n g t e r m erosion w hi ch w il l w ea r d o wn t he da m w a l l b ypeople and animals walking up an d d o w n on it, a da m w a l l ha s t o

b e fenced of f permanently.

I t ha s b e e n f o un d t h a t thorny bushes planted a l o n g th e e d g e s o f adam w a l l ar e m o r e e f f e ct i v e t h a n barbed w i r e on p os t s . Toprotect n e w l y planted thorny bushes f rom b ei ng destroyed b ya n i m a l s , t he p la nt s s ho ul d b e covered w i t h c u t - o f f thornybranches u n t i l t he plants h a v e reached a certain h e i g h t .

9. Protection of the Spillways.

( a ) The two s p il l wa ys m us t b e protected f r o m erosion caused b ys u r p l u s w a t e r flowing o v e r t h e i r f l o o r s a n d a l o n g t he ir s id esduring d a ys w it h a f ul l r e s e r v oi r b e i n g r e c h a r ge d .

As mentioned e a r l i e r , t he f l oo r an d s i d e s o f t he s p i l l w a y s shouldb e covered w i t h s t o n e s packed i n s o i l and interplanted w i t hcreeping g r a s s .

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10. Protection of the D am Reservoir

Inflow o f r u n o f f w a t e r originating f r o m c u l t i va t e d f i e l d s an d e r o d e dsoil s u r f a c e s brings du st, debris and a s m a l l portion o f t op s oi lw i t h it. U n l e s s checked t h i s m a t e ri a l w i ll s e t t l e on t he bottom o fthe r e s e r v o i r a s s i l t a t i o n .

( a ) A s m al l amount o f s i l t a t i o n , sa y a 5 cm . l a y e r , i s d e s i r a b l ebecause t h a t w i l l ac t a s a s e a l l i m i t i n g s e e p a g e through t hefloor o f the r e s e r v o i r .

( b ) H o w e v e r , i f siltation i s n ot k ep t a t t h a t minimum l e v e l i t willgradually s il t u p t he r e s e r v o i r r e s u l t i n g i n reduced o r e v e n n os t o r a g e f or w a t e r .

( c ) S i l t a t i o n a nd s ee pa ge t h r o u g h t he f l oo r o f a da m r e s e r v o i r i sg r e a t e s t i n a r e a s w i th s a n d y s o i l s

F o r t u n a t e l y , t he se t wo disadvantages c an b e c o m b i n e d t o onea d v a n t a g e by utilizing t he finegrained s i l t a t i o n d e p o s i t s a s asealing a g e n t m i xe d w it h cattle d u n g an d c om p act ed i nt o t hes a n d y floor o f a r e s e r v o i r .

Th e m ix in g a nd c om pa ct in g work i s c a r r i e d out by l e t t i n g

l i v e s t o ck g r a z e t he da m r e s e r v o i r during dry s e a s o n s .

( d ) Inflow o f s e d i m e n t s ca n be reduced by building s i l t t r a p s o fl a r g e stones packed w i t h s oi l a nd interplanted w i t h g r a s s a c r o s st he inflow c h a n n e l supplying r u n o f f w a t e r t o the da m r e s e r v o i r .

F or reinforcement o f s uc h s t r u c t u r e s i n l a r g e r c h a n n e l s l o n g

rooted t r e e s ca n be planted n e x t t o the l owe r s i d e o f t hestone/soil/grass b u n d s .

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11 . Extraction o f W a te r f r om an Earth D a m .

S o m e people prefer to take their water for domestic use directly fromthe dam reservoir, while at the same time some people are bathing andwashing clothes in the very same reservoir.

To make matters e v e n worse, d o m e s t i c an d w i l d a n i m a l s a r e s ha ri ng t hewater by being allowed to enter into the reservoir for drinking ande m p t y i n g t h e i r s t om a c hs i n t o t he r e s e r v o i r .

By u s i n g relatively simple m e a s ur e s s u ch a situation ca n be i m p r o v e dg r e a t l y , i f th e o w n e r s an d u s e r s o f such ~ da m would s ee t he irb e n e f t s o f having cleaner w a t e r a t t he s am e t im e protecting t h e i r da mf r o m d e g r a d a t i o n .

( a ) Domestic w a t e r should be d r a wn f r om a shallow w e l l situatede i t h e r i n t he r e s e r v o i r o r j us t downstream o f t he da m w all. Th epassage o f w a t e r through the underground w i l l c l e a n i t t o a n

acceptable s t a n d a r d .

A few metres downstream o f t he shallow w ell, a washing s l a b an d abathing p l a c e for men and w o m e n c o ul d b e b u i l t and fenced in.

T o i l e t f a c i l i t i e s an d a cattle trough should be installed furthera w a y downstream o f t he shallow w e l l i n o r d e r t o a v o i d pollutiono f t he w a t e r f or t he shallow w e l l .

( b ) W at e r f r om l i ve s to ck should be d r a wn f r om t he shallow w e l l t o adrinking trough situated a t l e as t 5 0 t o 100 metres downstream o ft he da m w all. W a t e r ca n b e delivered by gravity f r o m t he shalloww e l l t o the trough via concrete c h a n n e l .

( c ) W a t e r for gardening an d a t r e e nursery can b e d r a wn f r om eithert he recommended shallow w e l l or livestock trough.

( d ) W i l d a n d domestic a n i m a l s ca n b e k e p t ou t o f t he da m r e s e r v o i r bya l iv e f en ce o f thorny v e g e t a t i o n planted around t he s i t e .

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Shallow Well.

Livestock Trough

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12. Improving Inflow to D a z a Reservoir.

It may h appen that a s u r v e y o r can misjudge the ability o f a c a t c h m e n ta r e a t o provide r a i n w a t e r r u n o f f f or filling up a da m r e s e r v o i r . I tca n a l s o happen that a r a i n y season i s s o p oo r t ha t i t cannot f i l l a

da m r e s e r v o i r .

In s u c h c a s e s th e inflow t o a da m r e s e r v o i r ca n be m a d e b i gg er byenlarging t he catchment a r e a f or t he purpose o f collecting m o r e ru n-

off.

Th e m e a n s o f enlarging a c a t ch m e n t a r e a consist o f building g u t t e r sstretching o ut w a rd s f r om t he r e s e r v o i r and surroundng a c a t ch m e nt a r e aw h i c h drains r u n o f f a w ay f r o m t he r e s e r v o i r . Such gutters s l o p eslightly u p w a r d s f r o m a r e s e r v o i r i n o r d e r t o d r a i n t he collected ru nof f w a t e r t o w a r d s t he reservoir by m e a n s o f g r a v i t y .

The materials f or th e gutters depend on t he t y p e o f c a t ch m e nt a r e a an dt he availability o f local materials on t he s i t e .

( a ) On gently sloping l a n d o f s oi l e .g . f a rm l an d , t he g ut te rs c anc o n s i s t o f a d i t ch w i t h the e x c a v a t e d soil placed a s a b u n d j u stdownhill o f the d i t c h . Th e b u n d should b e planted w it h c re ep i ngg r a s s t o prevent i t from b e i n g demolished b y l i v e s t o ck .

Almost similar t y p e s o f guttering can b e f ou nd i n c u t o f f drainson l a nd s w i t h proper soil c o n s e r v a t i o n , an d f r o m ditches a l o n gr o a d s .

Cotlecting run-off from s L o p L Q fl d

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( b ) O n s t o n y h i l l s i d e s , e.g. grazing l a nd o r f or es t, t he g u t t e r s ca nc o n s i s t o f s t o n e s sa t i n so il. The u p h i l l s i d e o f t he g u t t e r i n gi s covered w i t h clayish s oi l f or w a t e r p r o o f i n g . Nonpalatable

v egetation sh ould b e planted on t he d o w n h i l l s i d e o f theg u t t e r i n g t o a v oi d l i v e st o ck kicking t he s t o n e s out o f l i ne

ColLecting run-off from rocks

( c ) I n p l a c e s w i th r o ck o ut c r op s , g u t t e r s ca n b e b u i l d o f f l a t s t o n e ss at i n c e m e n t mortar ( see manual on r o c k c a t c hm e n t d a m s ) . Th ecollected r u n o f f w a t e r c o ul d be d i v e r t e d t o a n e ar t h da m byd i t c h similar t o t he one described a b o v e u n d e r ( a ) .

Roc k

) n e s sut in mortur

sut in soil

CotLecting run-oft from hilLside

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1 3. Requirements o~l a b o u r , materials an d t r a n s p o r t

1 . Labour requirements

( a ) A s u r ve y or w i l l v i s i t a s i t e 4 t i m e s i f t he f i r s t selected s i t e

has proved s u i t a b l e , o t h e r w i s e h e w il l h a v e t o m a ke m or e v i s i t s .O n t he s i t e visits he w il l c ar r y ou t th e f o l l o w i n g t a s k s :

1 . Inform t he selfhelp g r o u p concerned o f t he s e l e c t i o n criteriaf or proposing 3 s i t e s f or t he da m site.

Investigate t he proposed 3 s i te s t he n an d s e l e c t t he m o s t viableo f them . S t a k e o ut t he trench t o b e dug and distribute t o o l s f ore x c a v a t i o n .

2 . Investigate t he s oi l profile i n t he t r e n c h an d i f v i a b l e , s t a k eou t t he f in al d i m e n s i o n s o f t he t ren ch and distribute ox

i m p l e m e n t s f or e a r t h m o v i n g . I f t he s o il s t r uc tu re s ho ws t h a tt he s i t e c a n n o t hold wa te r, a ne w s i t e ha s t o be s e l e c t e d , peggedout an d e x c a v a t e d .

3. I n t r o d u c e t he contractor t o t he g r o u p a nd h an d ov er t hesupervision o f t he construction work t o h i m .

Measure t he s i t e f or production o f d r a w i n g s an d b i l l s o fq u a n t i t i e s .

4. Produce d r a w i n g s an d b i l l s o f q u a n t i t i e s , and distribute t h e m t o

t he parties c o n c e r n e d .

Requirement o f working d a y s f or a S u r v e y o r :

4 f i e l d v i s i t s t o a s it e = . . 4 d a y s

Producing and distributing drawings 3 d a y s

Total 7 d a y s

( b ) A C o n t r a c t o r should v i s i t a dam construction g r o u p a b o ut t w i ce am o n t h f or t he purpose of :

- Teaching t he g r o u p i n moving e a r t h b y manual l a b o u r an d a n i m a ld r a u g h t , including t r a i n i n g t he a n i m a l s .

Instructing t he g r o u p ho w t o b u i l d t h e i r da m i n accordance w i t hthe design g i v e n .

Informing t he r e l e v a n t authorties o f any c h a n g e s i n design orimplementation p r o c e d u r e s .

Assisting t he g r o u p i n a p p l y i n g f or a d d i t i on a l a c t iv i t i e s s u ch a sconstruction o f shallow well, trou gh, washing slab, t r e e n u r s e r y ,f e n ci n g m a t er i a ls , etc.

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Requirements o f working d a y s f or a Contractor

E x a m p l e : 2 visits x 6 months x 3 y e a r s = 3 6 day s.r

( c ) Skilled lab ou r fo r building shallow w ell, trough, etc. ca n b e

estimated a s f o l l o w s :

Cattle trough, s t a n d a r d design : 5 d a y s pe r t r o u g h

Shallow w e l l w i t h h e a d wo r ks : 4 d a y s pe r m e tr e d e p th

Washing slab , : 4 d a y s

( d ) U ns ki ll ed l ab ou r p ro vi de d f r e e by s e lf -he l p g r ou p ma y b e

estimated a s f o l l o w s f or t he purpose o f having a v a l u e .

E x a m p l e : 2 5 people x 3 d a y s weekly x 6 months x 3 y e a r s = 1 3 5 0 day s.

E n t e r the f o u n d requirements on the b i l l s o f q u a n t i t i e s .

2 . Naterial requirements.

( a ) Th e e a r t h fo r building a n e a r t h da m i s n ot valued i n t e r m s o f

cost, because i t ha s n o s a l es v a l ue a s such.

( b ) The materials f or building the installations a t a n e a r t h da m ca n

be estimated as :

B a g s o f Tonnes Ton n e s Ton n e s

cement o f s a n d o f s t o n e s o f w a t e r

4 1

Shallow w e l l

6 m e tr e d e ep 2 4 6 3 4

Trough 6 3 9 1

Washing s l a b 4 2 3 1

E n t e r t he f o u n d requirements o n the b i l l s o f q u a n t i t i e s .

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3. Transport requirements

Transportation o f materials i s divided i n t o t wo c a t e g o r i e s :

( a ) T r a n s p o r t o f local m a t e r i al s , s u ch a s s oi l, s an d, stones a n d

wa te r, wi ll be transported t o t he s i t e by t he selfhelp g r o u p su s i n g oxen, donkey an d h a n d c a r t s , an d ar e t h e r e f o r e d i f f i c u l t t oe s t i m a t e h e r e .

( b ) T r a n s p o r t o f purchased m a t e r i a l s , e.g. c e m e n t , etc . a r e estimateda c c o r d i n g t o t o n n a g e, d i s t a n c e an d cost p er km .

Example for category ( a ) :

1 2 t o nn e s s a n d = 2 4 c ar t l oa d s x Shs = Shs

Example f or category ( b ) :

T r a n s p o r t of s u r v e y o r : 4 t r i p s x km x S hs = Shs

T r a n s p o r t o f c o n t r a c t o r : 1 t r i p x km x S hs = Shs

T r a n s p o r t o f c e m e n t : 5 t o n n e s x km x S hs = Shs

E n t e r t he fou nd requirements on t he b i l l s o f q u a n t i t i e s .

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14. BiUs of Quantities aM Costir~

T~bill of quantities are needed, because abctit half the ite~will be delivered by adrsrjr/ministry aM the other half will be delivered free of charge by the cc*~mityccticerned. Since the catiminity is supposed to ccritribute abcut half the st of theproject, a value of their input has to be estimated.

~camp1e:

Bills of Quantities for itei~to be delivered by dc*rir/ministry

SJcilledlabour : lsurveyorforlday xShs =

lccctractorfor36days xshs =

1~rtisanfor10days xshs =

F~uipnent: 5 pick-axes x Shs =

Sshovels xShs =

5springj~nbes xshs

lkarais xshs =

lc-scocipwthhanesses xshs =

5wheelbarrcMs xshs =

Materials: bags of c~nent x Shs =

Otherit~ xshs =

Thansprt: tri~x k m xShs = _______

Ibtal st by dc*rir/rninistry ____________

Bills of Quantities for itei~to be delivered free by the grctip

UnskiUedlabour days xShs =

SaM tounesx2= cartsx4=....wbeelbarrowsxShs

Stoues... totmesx2= cartsx4=....wheelbarrowsxshs =

Water.... totnesx5= dnmis xshs = _______

Thtal value of self-help Shs

~aM tota1 cost and value of project Shs

Shs

Shs

Shs

Shs

Shs

Shs

Shs

Shs

Shs

Shs

Shs

Shs

Shs

Shs

Sbs

Shs

Shs

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Small earth dam built by animal traction

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