ThThe Role of Geosynthetics in Erosion and Sedimente Role of Geosynthetics in Erosion and Sediment

7/26/2019 ThThe Role of Geosynthetics in Erosion and Sedimente Role of Geosynthetics in Erosion and Sediment

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Geotextiles and Geom embranes

I I (1992) 535-550

T h e R o l e o f G e o s y n t h e t i c s i n E r o s i o n a n d S e d i m e n t

C o n t r o l A n O v e r v ie w

M . S . T h e i s e n

Erosion C ontrol M aterials, Synthetic Industries, Con struction Products D ivision,

4019 Industry Drive, Chattanooga, Ten nessee 37416, USA

A B S T R A C T

Th e u se o fgeosynthetic erosion an d sed im ent materials continues to ex pa nd at

a rap id pace. F ro m their early beginnings in the late 1950s geosynthetic

materials today are the bac kbo ne o f the erosion an d sedim ent control

industry. Geosynthetic com ponents are an integral par t o f erosion and

sedim ent materials ranging fr om temporary pro ducts such as hydraulic

m ulc h geofibers plastic erosion control mesh es an d nettings erosion control

blanke ts a nd si l t fence s to high perform ance tur f reinforcement mats

geocellular con finem ent systems erosion control geotexti les and abr ic or m ed

revetments. Th is pa pe r provides a b rie f overview of these materials an d

concepts.

I N T R O D U C T I O N

H o p e f u l l y w e a r e e n t e r in g a n e w e n v i r o n m e n t a l e r a w h e r e c o n c e r n f o r

t h e p r o t e c t i o n o f o u r p l a n e t 's n a t u r a l r e s o u r c e s w il l r e a c h g l o b a l

p r o p o r t i o n s . C o n t i n u e d t e c h n o l o g i c a l a d v a n c e s h a v e l ed to i m p r o v e d

m o n i t o r i n g o f E a r t h 's v i t al s ig n s. A s s u c h , p r i o r t h e o r e t i c a l m o d e l i n g o f

e n v i r o n m e n t a l c o n c e r n s s u c h a s t h e g r e e n h o u s e e ff ec t, o z o n e d e p l e t i o n ,

r i s i n g s e a l e v e l s , d e f o r e s t a t i o n , d r o u g h t , a c c e l e r a t e d e r o s i o n , s e d i m e n t

l o a d i n g o f w a t e r w a y s , sp e c ie s e x t in c t i o n a n d t h e e v e n t u a l d o w n f a l l o f

m a n k i n d a p p e a r c h i l l in g l y r ea li st ic .

535


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536 M S The i sen

Slogans such as 'Think globally, act locally', 'Love your mother' and

'Someone always lives downstream' are spearheading the efforts of

numerous preservation groups. With the continued demise of oppressive

governments, optimism for world peace and an unprecedented feeling

of global unity, a spirit of environmental cooperation is beginning to

prevail.

The term 'non-point pollution' hopefully is heading toward obsolescence

with groups such as Stream Watch sloshing their way up muddy creeks to

pinpoint sources of unchecked sediment. Improved methods to detect

and monitor rates of erosion and sedimentation via high tech satellite

imagery or even the actions of the Stream Watchers of the world lends

credence to the old saying 'you can run but you can't hide'. Generators of

sediment and other pollutants can and will be identified.

Cumulative research suggests excessive sediment in our waterways is

the planet's most prevalent contaminant. Increscent economic and

social losses from reductions in arable farmland, timber production,

fishery yields, species diversity and navigable waterways exceed those

caused by pollutants in the public eye such as nuclear and hazardous

wastes, smog or ground water contamination. Worse yet, the problem is

exasperated as one moves downstream toward our coastlines and

population centers.

Recently a number of laws have been manda ted in the United States to

combat excessive erosion. Such legislation ranges from local erosion and

sediment control ordinances to numerous state and federal agricultural,

waste containment and surface mining acts, to the broadly encompassing

Environmental Protection Agency's Clean Water Act and The Farm Bill

administered by the Department of Agriculture. These actions are only

the tip of the iceberg as more and more government agencies and entities

get with the program.

Just what is erosion control? To control erosion is to curb or restrain

(not completely stop) the gradual or sudden wearing away of soils. One

has seen extreme examples of excessive erosion such as gullied hill

slopes or stream channels choked with debris, but often erosion goes

unchecked on fiat to moderately sloping terrain. Soil loss is a continually

occurring process in natural ecosystems as well as successfully reclaimed

sites--without it our scenery would be very boring. The goal of any

revegetation or erosion control project should be to stabilize soils and

manage erosion in an economical manner (Theisen, 1988).

In this era of shrinking budgets, decision makers are hard pressed to

reclaim disturbed sites at minimu m costs. Given site conditions such as

slope angles, climate, runoff, soil profile and ultimate land use, a


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eosynthetics in erosion an d sed im ent control 537

specifier must select with confidence a technique he (she) feels will

perform up to expectations at the lowest cost. Over the past 25 years the

erosion control industry has experienced rapid growth and is becoming

more sophisticated. Materials developed for erosion and sediment

control (E & SC) are becoming increasingly effective. Improved design

and installation guidelines are directing the use of E & SC products

toward more specific and cost effective applications. The industry has

evolved from the seed drills, straw blowers, hydroseeders, excelsior, jute,

concrete channe l liners and rip rap of the sixties into a diverse hierarchy

of techniques and materials. It seems as if every month a new product is

introduced to control erosion and sediment in more specific situations.

Numerous materials have come and gone in the survival of the fittest,

most cost competitive products.

i s tor ical perspect ive

Many of us perceive the use of geosynthetic materials for erosion and

sediment control as a new horizon. However, geosynthetics have played

a major role in the E & SC industry for over 30 years, particularly in the

case of rolled goods. In 1958 a geosynthetic component was incorporated

into an erosion control system which has changed the course of slope,

channel and embankmen t protection. A'plastic cloth' was used in lieu of

a granular filter to prevent sand from washing out behind concrete

blocks used for shoreline protection (Richardson & Koerner, 1990). The

significant cost savings realized when a 0.4-mm thick plastic filter cloth

could replace up to a meter of soil peaked the interest of the US Army

Corps of Engineers. Subsequent successful installations of woven plastic

cloth filters in coastal Structures led to the birth of the geotextile industry

as is practiced today. Through the years millions of square yards of

woven and non woven geotextiles have been installed as part of hard

armor systems.

Another geosynthetic breakthrough was initiated about 10 years later.

In the mid 1960s only one type of erosion control blanket existed. A state

soil conservationist discovered the material used for wrapping cotton

bales could be used to prevent soil erosion. The material was jute, a

woven mesh of thick natural yarns, when applied on the soil surface

provided thousands of tiny check dams to help keep soil from washing

away. Jute blankets allow vegetation to become establ ished on steeper

slopes and in higher flowing swales than traditional straw and hay

mulches. A similar material remains in use today.

However, jute has its drawbacks: its open weave construction leaves


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538 M S Theisen

s o il e x p o s e d , th e o r g a n i c m a t e r i a l te n d s t o s h r i n k a n d s w e l l u n d e r f ie l d

c o n d i t i o n s , a n d it is e x tr e m e l y f l a m m a b l e . T o a c h i e v e o p t i m u m r es u lt s

s t ra w o r h a y m u l c h s ti ll m u s t b e p l a c e d b e n e a t h t h e j u te .

W h a t w a s n e e d e d w a s a o n e s te p , r o ll o u t m u l c h b l a n k e t . T h e f ir st

a t te m p t s i n v o l v e d a v e r y d e n s e m a t o f c u r le d , b a r b e d a s p e n w o o d

( e x c e ls i o r) f ib e rs . T h e m a t e r i a l s t a y e d t o g e t h e r b u t w a s t o o d e n s e t o a l l o w

v e g e t a ti v e g r o w t h . N e x t , a t w i s te d k r a ft p a p e r n e t w a s p l a c e d a b o v e a

t h i n n e r m a t o f e x c e ls i o r f ib er s. V e g e ta t io n g re w t h r o u g h t h e b l a n k e t b u t

p e r f o r m a n c e o f th e p a p e r n e t t in g w a s v e ry i n c o n s i st e n t; o ft e n b r e a k i n g

d o w n t o o q u i c k l y a n d b e i n g l i ft ed b y t h e v e g e t a t i o n or , w o r s e ye t, a l l o w i n g

t h e b l a n k e t t o b e w a s h e d a w a y b e f o re v e g e t at iv e e s t a b l i s h m e n t . A

s t ro n g e r , n o n - m o i s t u r e s e ns i ti v e , m o r e d u r a b l e n e t t i n g w a s n e e d e d .

P o l y p r o p y l e n e n e t t i n g w a s t h e a n s w e r .

C o m b i n i n g a d e n s e m a t o f e x c e ls i o r w i t h a p l a s ti c n e t l e a d t o th e f ir st

s u c c e s s f u l e x c e l s i o r e r o s i o n c o n t r o l b l a n k e t s . F i e l d tr ia l s w i t h v a r i o u s

n e ts , fi b e r le n g t h s a n d g l u e p a t t e r n s r e s u l t e d i n e s s e n t i a l ly t h e s a m e

b l a n k e t s w e s ee to d a y. T h e k e y to t h e i m p r o v e d p e r f o r m a n c e o f ex c e l si o r

o v e r j u t e b l a n k e t s is th e p l a s t ic n e t b a c k b o n e o f t h e p r o d u c t .

i a x i a l l y o r i e n t e d p r o c e s s n e t s

B i a x i a ll y o r i e n t e d p r o c e s s ( B O P ) n e t s a r e ty p i c a l ly m a n u f a c t u r e d f r o m

p o l y p r o p y l e n e o r p o l y e t h y l e n e re s in s . B O P n e t s a re e x t r e m e l y v e r s a ti l e

i n t h a t c o m p o s i t i o n , s t re n g t h , e l o n g a t i o n , a p e r t u r e s i ze a n d s h a p e , c o l o r

a n d u l t r a v i o l e t s t a b i l i t y c a n e a s i l y b e d e s i g n e d i n t o t h e p r o d u c t f o r

s p e c if i c s it e r e q u i r e m e n t s . S i n c e t h e y d o n o t a b s o r b m o i s t u re , th e s e n e t s

d o n o t s h r i n k a n d s w e ll l ik e k ra ft p a p e r n e ts a n d j u t e b la n k e t s. B O P n e t s

h a v e p r o v e n t o b e s o a d a p t a b l e t h e y a r e b e i n g u s e d t o c r e a t e m o r e

c o m p l e x p r o d u c ts a n d a re e v e n u s e d a lo n e t o a n c h o r l o o se f ib e r m u l c h e s

s u c h a s s t ra w , h a y a n d w o o d c h i p s. T h e l i g h t w e i g h t n e t t i n g s p l a c e d o v e r

m u l c h e s c o m e i n r o ll s w h i c h a r e 3 - 4. 5 m i n w i d th , w e i g h o n l y a b o u t

5 5 k g a n d w i l l c o v e r 0 .4 h (1 a c r e ) o r m o r e . I n s t a l l a t i o n o f t h e s e p r o d u c t s

is le ss l a b o r i n t e n s i v e t h a n t r a d i t i o n a l n e t t i n g p r o d u c t s .

E r o s i o n c o n t r o l m e s h e s

A s te p u p f r o m B O P n e t ti n g s a r e w o v e n p o l y p r o p y l e n e g e o te x ti le e r o s i o n

c o n t r o l m e s h e s . I n f ac t, t h e n e w e r t w i s t ed f ib e r e r o s i o n c o n t r o l m e s h e s

c a n p r o v i d e c o m p a r a b l e p e r f o r m a n c e t o e r o s io n c o n t r o l b l a n k e ts . T h e s e

p h o t o b i o d e g r a d a b l e , n a tu r a l l o o k in g , h i g h s t r en g t h p o l y p r o p y l e n e

m e s h e s p r o t e c t t h e s o i l s u r f a c e f r o m w a t e r a n d w i n d e r o s i o n w h i l e

a c c e l e r a t i n g v e g e ta t iv e d e v e l o p m e n t . F o u r m e t e r , l ig h t w e i g h t r o ll s

f a c i l i t a t e i n s t a l l a t i o n o n s l o p e s a n d c h a n n e l s . E r o s i o n c o n t r o l m e s h e s


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eosynthetics in erosion an d sedim ent control 539

m a y b e u s e d a l o n e , w i t h d r y m u l c h e s o r a s a s t a b i li z in g u n d e r l a y f or so d

r e i n f o r c e m e n t . T h e y a l s o s h o w p r o m i s e a s a n o p e n w e a v e g e o t e x t i l e

f a c i n g f o r fo s t e r in g v e g e t a t io n o n g e o s y n t h e t i c a l l y r e i n f o r c e d s t e e p e n e d

s l o p e s o r b i o e n g i n e e r i n g i n s t a l l a t i o n s .

E r o s i o n c o n tr o l b l a n k e t s E C B s )

B O P n e t ti n g s o r w o v e n m e s h e s o f v a r y i n g c h a ra c t er is t ic s a r e n o w p l a c e d

o n o n e o r b o t h s id e s o f f in e l y t u n e d e r o s io n c o n t ro l b l a n k e t s a d a p t e d t o

a n t i c i p a t e d s i t e c o n d i t i o n s . T h e s e 1 - 2 - m w i d e b i o d e g r a d a b l e f i b e r

e r o s i o n c o n t r o l b l a n k e t s ( E C B s ) a r e c o m p o s e d o f s tr aw , e x c e ls io r , c o t to n ,

c o c o n u t , p o l y p r o p y l e n e o r b l e n d s t h e r e o f . N e t t i n g s o r m e s h e s m a y

c o n t a i n U V s t a b i l i z e r s f o r c o n t r o l l e d d e g r a d a t i o n o r l o n g c h a i n

i n t e r ru p t e r s to a c c e l e r a te p h o t o d e g r a d a t i o n . C o l o r s v a r y f ro m c l ea r , t a n ,

g r e e n t o b l a c k . M e t h o d s o f h o l d i n g t h e f i be r s in p l a c e r a n g e f r o m g l u es

a n d g l u e s tr ip s t o m o r e s u p e r i o r p a r a l l e l l o c k s t it c h i n g w i t h c o t t o n ,

p o l y e s t e r o r p o l y o l e f i n t h r e a d s . A p p l i c a t i o n s f o r t h e w i d e v a r i e t y o f

b l a n k e t s r a n g e f r o m p r o t e c t i o n o f g r a d u a l t o s te e p s lo p e s to l o w o r

m o d e r a t e ly f l o w i n g c h a n n e l s . T h e s e b l a n k e t s m a y p r o v i d e te m p o r a r y

re s i s t ance t o f l ow ve loc i t i e s o f up t o nea r l y t h r ee me t e r s pe r s econd .

F i n a l l y a n d p e r h a p s o f m o s t c o n c e r n t o t h e e n v i r o n m e n t , t h e se

m e s h e s a n d n e t ti n g s m a y u l t i m a t e l y b e c o m e b i o d e g r a d a b le . A s p h o t o -

d e g r a d a t i o n p r o g r e s s e s th e p l a s t ic c h a i n s a r e c u t i n t o s h o r t e r a n d s h o r t e r

s e g m e n t s d o w n t o a p l a s t ic ' s a n d ' w h i c h b e c o m e s p a r t o f t h e s o il . T h e s e

s h o r t s e g m e n t s b e c o m e b i o l o g i c a l ly d e g r a d a b l e a n d a r e a tt a c k e d b y s o il

m i c r o o r g a n i s m s a n d c o n v e r t e d t o c a r b o n d i o x i d e a n d w a t e r ( G u i l l e t ,

1 97 4). I t is u n f o r t u n a t e t h a t e m o t i o n a l , u n i n f o r m e d a n t i - p l a s ti c s t ig m a s

s o m e t i m e s p r e c l u d e t h e u s e o f t h e s e e x t r e m e l y c o s t ef fe c ti v e t e m p o r a r y

ma te r i a l s .

T E R M s v er su s P E R M s

A t t h is p o i n t a n i m p o r t a n t d i s t in c t i o n m u s t b e p r e s e n t e d r e g a r d i n g t h e

i n t e n d e d u s e o f E & S C m a t e r ia l s. F o r m a n y i n s t a l l a t io n s v e g et a ti o n

a l o n e w i ll p r o v i d e a d e q u a t e l o n g t e rm e r o s i o n p r o t e c ti o n . H o w e v e r ,

g e t t in g v e g e t a ti o n e s t a b l i s h e d r e q u i r e s a v a r i e t y o f t e c h n i q u e s . M a t e r i a l s

o f a t e m p o r a r y n a t u r e w h i c h f a c i l i t a t e v e g e t a t i v e e s t a b l i s h m e n t , t h e n

d e g r a d e , m a y b e t e r m e d T E R M s ( t e m p o r a r y e r o s i o n a n d r e v e g e t a t i o n

mater ia l s ) .

B a s i c a l ly T E R M s c o n s i s t o f d e g r a d a b l e n a t u r a l a n d / o r s y n t h e t i c

c o m p o n e n t s w h i c h p r o v i d e t e m p o r a r y e r o s i o n c o n t r o l a n d f a c i l t a t e

v e g e t a t i v e e s t a b l i s h m e n t . T h e s e s h o r t t e r m m a t e r i a l s d e g r a d e l e a v i n g


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540 M S The i sen

TABLE 1

General TERM Techniques

Straw, hay and hydraulic mulches

Tackifiers and soil stabilizers

Hydraulic mulch geofibers

Erosion control meshes and nets (ECMNs)

Erosion control blankets (ECBs)

Fiber roving systems (FRSs)

o n l y v e g e t a ti o n f o r lo n g t e r m l o w t o m e d i u m f lo w r e si s ta n c e . Ta b l e 1 l is ts

v a r i o u s T E R M t e c h n i q u e s .

S it e c o n d i t i o n s r e q u i r i n g r e i n f o r c e d v e g e t a t i o n o r r e v e t m e n t sy st e m s

w i ll re q u i re P E R M s ( p e r m a n e n t e r o s i o n a n d r e v e g et a ti o n m a t e ri al s) .

P E R M s m a y b e s u b d iv i d e d i n to

b io techn ic l compos i te s

w h e n v e g e t a t io n

i s r e i n f o r c e d o r r m o r s y st e m s w h e n n o n - v e g e t a te d i n e r t m a t e r ia l s a r e

ins ta l l ed .

B i o t e c h n i ca l c o m p o s i t e s ar e c o m p o se d o f n o n - d e g r a d a b l e c o m p o n e n t s

w h i c h f u r n i sh t e m p o r a r y e r o s i o n p r o t e c t i o n , a c c e l e r a t e v e g e t a t i v e

g r o w t h a n d u l t im a t e l y b e c o m e s y n e r g is t ic a l lye n t a n g l e d w i t h l i v in g p l a n t

t is su e t o e x t e n d t h e p e r f o r m a n c e l i m i t s o f v e g e ta t io n . Th i s r e i n f o r c e d

v e g e t a ti o n p r o v i d e s 'p e r m a n e n t ' m e d i u m t o h i g h f lo w re s i st a n ce s o l o n g

a s b i o t e c h n i c a l c o m p o s i t es a r e p r o t e ct e d f r o m s u n l i g h t v i a s h a d i n g a n d

so i l c o ve r. Ta b l e 2 o u t l i n e s e x a m p l e s o f b i o t e c h n i c a l c o m p o s i t e s . H a r d

a r m o r s y st em s g e n e r a l l y e m p l o y i n e r t m a t e r i a ls u s e d t o p r o v i d e h i g h t o

m a x i m u m f lo w re s is t a n ce w h e r e c o n d i t i o n s e x ce e d p e r f o r m a n c e l i m i t s

o f r e i n f o r c e d v e g e t a t i o n sy s t e m s . S u m m a r i z e d i n Ta b l e 3 a r e sy s t e m s

u s e d t o p r o v i d e p e r m a n e n t e r o s i o n p r o t e c t io n o f a re a s s u b j e c t to h i g h

f lows , waves and /o r scour a t t ack .

Fiber roving systems FR Ss)

A n o t h e r g e o s y n t h e t i c c o n c e p t p r o v i d i n g m o d e r a t e e r o s i o n p r o t e c t i o n

a re f ibe r rov in g sys tems (FRSs) . D eve lope d in the l a t e 1960s rov ings a re

a p p l i e d i n a c o n t i n u o u s s t r a n d f o r p r o t e c t io n o f d r a in a g e s w a le s a n d

slopes .

F i b e r g l a ss r o v i n g is a m a t e r i a l f o r m e d f r o m f i b e rs d r a w n f r o m m o l t e n

g l a ss a n d g a t h e r e d i n t o s t r a n d s t o f o r m a s i n g le r i b b o n . P o l y p r o p y l e n e

r o v in g is f o r m e d f r o m c o n t i n u o u s s t ra n d s o f f ib r i ll a te d y a r n s w o u n d

o n t o c y l i n d r ic a l p a c k a g es s o t h a t t h e m a t e r i a l c a n b e f ed c o n t i n u o u s l y

f r o m t h e o u t s i d e o f th e p a c k a g e . U se o f fi b e rg l a ss r o v i n g h a s b e e n

d e c l i n i n g d u e t o it s c a r c i n o g e n i c p r o p e r t i e s a n d is b e i n g d i sp l a c e d b y

e n v i r o n m e n t a l l y f r ie n d l y p o l y p r o p y l e n e r o vi ng .


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Geosynthetics in erosion an d sediment control 541

T A B L E

Types of BiotechnicalComposites(PERMs)

UV stabilized fiber roving systems (FRSs)

Erosion control revegetationmats (ECRMs)

Tuff reinforcementmats fiRMs)

Sports turf geofibers

Vegetatedgeocellularcontainmentsystems(GCSs)

Vegetatedconcreteblock systems

T A B L E 3

Types of Armor Systems (PERMs)

Geocellular containmentsystems (GCSs)

Fabric formed revetments(FFRs)

Concrete block systems

Gabions

i p r a p

Composites and hybrids

Erosion control roving is unique because of the flexibility of

application, allowing for any width or thickness of material to be applied

(Agnew, 1991). Other erosion control materials, such as blankets or mats

require the user to apply the width or thickness of material supplied.

Fiber rovings may be viewed as an in-situ erosion control geosynthetic

with reduced labor and material costs over traditional blanket materials.

Using compressed air, roving is applied through a nozzle and then

anchored in place using emulsified asphalt or other natural or synthetic

soil stabilizers. The biodegradable polypropylene roving may be used for

temporary applications (TERM) or when UV stabilized is appropriate

for extended use situations (PERM).

T R M v e r s u s E C R M

Turf reinforcement is a me thod or system by which the natural ability of

plants to protect soil from erosion is enhanced through the use of

geosynthetic materials. A flexible three dimensional matrix retains seeds

and soil, stimulates seed germination, accelerates seedling development

and most importantly, synergistically meshes with developing plant

roots and shoots. In laboratory and field analyses, biotechnically

reinforced systems have resisted flow rates in excess of four meters per

second for durations of up to two days, providing twice the erosion

protection of unreinforced vegetation (Carroll e t a l . 1991). Such


8/16

542

M S Theisen

performance has resulted in the widespread practice of turf reinforce-

ment as an alternative to concrete, rip rap and other armor systems in the

protection of open channels, drainage ditches, detention basins and

steepened slopes.

Permanent geosynthetic mattings are composed of durable synthetic

materials stabilized against ultraviolet degradation and inert to chemicals

normally encountered in a natural soil environment. These mattings

consist of a lofty web of mechanical ly or melt bonded polymer nettings,

monofilaments or fibers which are entangled to form a strong and

dimensionally stable matrix. Polymers include polypropylene, poly-

ethylene, nylon and polyvinyl chloride.

Geosynthetic mattings generally fall into two categories: tu r f r e in force -

m e n t m a t s T R M s )

or

eros ion con t ro l r evegetat ion m a ts EC RM s) .

Higher

strength TRMs provide sufficient thickness and void space to permit soil

filling/retention and the development of vegetation within the matrix.

TRMs are installed first, then seeded and filled with soil. Seeded prior to

installation, ECRMs are denser, lower profile mats designed to provide

long term ground cover and erosion protection, By their nature of

installation TRMs can be expected to provide more vegetative entangle-

ment and long term performance than ECRMs. However, denser ECRMs

may provide superior temporary erosion protection.

Geoceilular containm ent system s G C Ss)

Geocellular containment systems work in a unique fashion in that

strength or stabilization by confinement is achieved by a series of three-

dimensional cells up to 20 cm deep. When expanded into position, the

polyethyleneor polyester cells have the appearance of a large honeycomb,

one of nature's most efficient structures. The cells are then backfilled

with soil, sand or gravel depending upon application. For revegetation,

the soil-backfilled cells are seeded, fertilized and covered with a variety

of TERM or PERM techniques. The mulches provide surface protection

while the cells greatly reduce the chances of subsurface failure and act as

a deeper rooted biotechnical composite. Vegetated GCSs are limited to

flow velocities of two to three meters/second because of the tendency of

the nearly impermeable cells to sustain scouring under high flow

velocities (Chen & Anderson, 1986).

For higher flow condit ions GCSs may be also filled with concrete or

gravel to create a hard armor system. Typically a geotextile will be placed

beneath the expanded web to provide separation and/or filtration.

Erosion control applications for GCSs are many including steep slope

revegetation, channel liners, shoreline revetments, retaining walls, boat

ramps, and low flow stream crossings.


9/16


F a b r ic f o r m e d r e v e tm e n t s

Fabric forming systems are mattresses typically constructed of water

permeable, double layer woven geotextiles which are posit ioned on the

area to be protected and filled with a structural grout. The two layers of

geotextile are joined at discrete points to create a form which when filled

with grout will conform to most subsoil conditions. Thickness and

geometry are determined by internal spacer threads woven into the

upper and lower sheets of fabric. In many cases the mattresses may be

installed for less cost than conventional armor systems since all

construction is conducted in place (Richardson & Koerner, 1990).

o n c r e t e b l o c k s y st e m s

Concrete block systems consist of prefabricated concrete panels of

various geometries which may be attached to and laid upon a woven

monofilament or non-woven geotextile. Bending and torsion are

accommodated by having the concrete blocks articulated with joints,

weaving patterns or connection devices. Concrete block systems may be

subdivided into three groups: non-tied interlocking blocks, cable-tied

blocks, or in-situ concrete (Hewlett

e t a l .

1987). In some cases the entire

erosion control section may be manufactured, trucked to the job site and

placed as a unit.

G a b i o n s

Gabions are compar tmented rectangular containers made of galvanized

steel hexagonal wire mesh and filled with hand-sized stone. Cells of

equal capacity are formed by factory-inserted wire netting diaphragms or

partitions which add strength to the container and help maintain its

shape during the placement of stone. In highly corrosive conditions a

polyvinyl chloride coating is used over the galvanized wire.

Advantages of gabions include flexibility, durability, strength, per-

meability and economy versus rigid structures. The growth of native

plants is promoted as gabions collect sediment in the stone fill. A high

percentage of installations are under lain by woven monofilament and

nonwoven geotextiles to facilitate sediment capture and prevent wash

out from behind the structure.

R i p r a p

Rip rap consists of stone dumped in place on a filter blanket or prepared

slope to form a well graded mass with a minimum of voids. Stone used for


10/16

544 M S The i sen

r i p r a p i s h a r d , d u r a b l e , a n g u l a r i n sh a p e ; r e s i s ta n t to w e a t h e r i n g a n d t o

w a t e r a c t i o n ; a n d f r e e f r o m o v e r b u r d e n , s p o i l , s h a l e a n d o r g a n i c

m a t e r i a l . Th e r i p r a p m a t e r i a l is g e n e r a l l y p l a c e d o n a g r a v e l b e d d i n g

l a y e r a n d / o r a w o v e n m o n o f i l a m e n t o r n o n w o v e n g e ot ex ti le fa b ri c.

e r f o r m a n c e o f e r o s i o n c o n t ro l m a t e r ia l s

S e v e ra l t es t p r o c e d u r e s h a v e b e e n p r o p o s e d t o q u a n t i f y p e r f o r m a n c e o f

e r o s i o n c o n t r o l m a t e r i a l s . I n i t i a l c o n c e r n f o r v e g e t a t e d sy s t e m s i s

t e m p o r a r y e r o s io n p r o t e c ti o n p r io r t o a n d d u r i n g s e ed g e r m i n a t i o n a n d

se e d l i n g d e v e l o p m e n t . Ty p i c a l ly , t h i s l e v e l o f p e r f o r m a n c e i s m e a su r e d

b y t h e m a t e r i a l 's a b i l i t y t o m i n i m i z e so i l l o ss w h e n su b j e c t e d t o v a r i o u s

f l o w r a t e s a n d / o r r a i n f a l l a m o u n t s . T e m p o r a r y e r o s i o n p r o t e c t i o n i s

i m p o r t a n t b u t t h e l o n g t e r m g o a l o f a n y e r o s i o n c o n t r o l m a t r i x i s t o

p r o v id e p e r m a n e n t e r o s i o n p r o t e ct io n v i a p e r m a n e n t v e g e ta t io n a n d / o r

s u b s e q u e n t r o o t r e in f o r c e m e n t . T h e m o r e r a p i d l y v e g e t a ti o n b e c o m e s

e s t a b l i s h e d t h e m o r e r a p i d l y l o n g t e r m e r o s i o n c o n t r o l m a y b e

a c c o m p l i sh e d . Th u s , t h e m a t e r i a l' s a b i l i t y to f a c i li t a te v e g e ta t iv e

e s t a b l i s h m e n t is e q u a l l y i m p o r t a n t . T o o m u c h e m p h a s i s o n a n e r o s i o n

c o n t r o l p r o d u c t' s t e m p o r a r y p r o t e c t i o n m a y i n h i b i t t h e g r o w th o f n e w l y

e m e r g i n g s e e d li n g s .

P e r h a p s t h e m o s t c r it ic a l p a r a m e t e r i n a n e n g i n e e r i n g d e s ig n i s fl o w

r e s i s ta n c e b e f o r e , d u r i n g a n d l o n g a f t e r v e g et a ti v e e s t a b l i sh m e n t . S o m e

e r o s i o n c o n t r o l m a t e r i a l s m a y b e w a s h e d a w a y b e f o r e t h e v e g e t a t i o n

t a k e s h o l d w h i l e o t h e r s m a y t e m p o r a r i l y e x h i b i t e x c e l l e n t f lo w r e s is t a n c e

o n l y t o lo se t h e i r e f fe c ti v e n es s a s t h e y d e g r a d e o r d e c o m p o se o v e r t im e .

S p e c i f i e r s m u s t t a k e i n t o a c c o u n t i m m e d i a t e a n d l o n g t e r m f l o w

r e s is t a n ce b a s e d u p o n l o n g e v it y o f t h e m a t e r ia l w h e n d e s i g n i n g g ra s s ed

s l o p e s a n d w a t e rw a y s .

T w o b a s ic d e s i g n c o n c e p t s a re u s e d t o e v a lu a t e a n d d e f i n e a c h a n n e l

c o n f i g u r a t i o n t h a t w i l l p e r f o r m w i t h i n a c c e p t e d l i m i t s o f s ta b i li ty . Th e se

m e t h o d s a r e d e f i n e d a s th e p e r m i s s ib l e v e l o ci ty a p p r o a c h a n d t h e

p e r m i s s i b l e tr a c ti v e f o r ce ( sh e a r s tr es s ) a p p r o a c h . U n d e r t h e p e r m i s s i b l e

v e l o c it y a p p r o a c h t h e c h a n n e l is a s s u m e d s t ab l e i f t h e a d o p t e d v e l o c it y is

l o w e r t h a n t h e m a x i m u m p e r m i s s i b l e v e lo c it y . Th e t r a c ti v e f o rc e

( b o u n d a r y sh e a r s t r e s s ) a p p r o a c h f o c u se s o n s t r e s se s d e v e l o p e d a t t h e

i n te r f ac e b e tw e e n f lo w i n g w a t e r a n d t h e m a t e r ia l s f o r m i n g t h e c h a n n e l

b o u n d a r y ( C h e n & C o t to n , 1 988).

T h e p e r m i s s i b le v el o c it y a p p r o a c h u s es M a n n i n g ' s e q u a t i o n w h e r e

w i t h g i v e n d e p t h o f flo w , D , t h e m e a n v e l o c i ty m a y b e c a l c u l a t e d a s :

V = R 2 3 S l 2 n


11/16

eosynthetics in erosion an d sediment co ntrol 5 4 5

w h e r e

V =

n - -

R

a v e r a g e v e l o c i t y i n t h e c r o ss s e c t i o n

M a n n i n g ' s r o u g h n e s s c o e f f ic i en t

h y d r a u l i c r a d iu s , e q u a l t o t h e c r o s s - se c t io n a l a r e a ,A , d i v i d e d b y

t h e w e t te d p e r i m e t e r , P

S = f r ic t io n s l o pe o f t h e c h a n n e l , a p p r o x i m a t e d b y t h e a v e ra g e b e d

s l o p e f o r u n i f o r m f lo w c o n d i t i o n s

Th e t r a c t iv e f o rc e a p p r o a c h u se s a s i m p l i f i e d sh e a r s tr es s a n a l y s i s

w h i c h i s e q u a l t o :

w h e r e

=

y =

D

S

yDS

t rac t ive force

u n i t w e i g h t o f w a t e r

m a x i m u m d e p th o f f lo w

a v e r a g e b e d s l o p e o r e n e r g y s lo p e

D e s i g n c r it e ri a b a s e d o n f lo w v e lo c i ty m a y b e l i m i t e d b e c a u s e

m a x i m u m v e lo c it ie s v a r y w i d e ly w i th c h a n n e l l e n g t h ( L), s h a p e (R ), a n d

rou ghn ess coe f f i c i en t s (n ). In rea l i ty i t i s the fo rce deve lo ped by the f low,

n o t t h e f lo w v e l o c it y i ts e lf , t h a t c h a l l e n g e s t h e p e r f o r m a n c e o f e r o s i o n

c o n t r o l sy s te m s . T r a c t iv e f o r ce s c a u se d b y f l o w i n g w a t e r o v e r t h e g r o u n d

su r f a c e c r e a t e sh e a r s t r e s se s w h i c h c a n b e u se d a s a d e s i g n p a r a m e t e r

i n d e p e n d e n t o f c h a n n e l s h a p e a n d r o u g hn e s s. M o r eo v e r, th e h i g h e r

s tr es se s d e v e lo p e d i n c h a n n e l b e n d s o r o t h e r c h a n g e s i n s t r e a m c h a n n e l

g e o m e t r y c a n b e q u a n t i f i e d b y s i m p l i f i e d sh e a r s t r e s s c a l c u l a t i o n s ,

p r o v i d i n g a h i g h e r l e v el o f d e s i g n c o n f i d e n c e t h a n o t h e rw i s e p o s s ib l e

(Chen & Cot ton , 1988) .

C r i t ic a l s h e a r s t re ss d e t e r m i n a t i o n s a r e m e a n t t o b e u s e d w i t h v e lo c i ty

c a l c u l a t i o n s f o r p r e s c r e e n i n g o f c h a n n e l l i n i n g d e s i g n s . M a n n i n g ' s

e q u a t i o n r e m a i n s t h e p r i m a r y h y d r a u l i c r e s e a r c h a n d d e s i g n t o o l .

H o w e v e r , a s e v e r y d a y p r a c t i c e h a s d e t e r m i n e d , a s i m p l i f i e d s c r e e n i n g

c r it e ri a s u c h a s m a x i m u m s h e a r s tr es s i s n e c e s s a r y t o e n s u r e p r o p e r l y

e n g i n e e r e d d e s i g n o f c h a n n e l l i n i n g e r o s i o n c o n t r o l sy s te m s . F i g u r e 1

c o m b i n e s c u m u l a t i v e re s e a r c h f o r s e ve r al e r o s i o n c o n t r o l m a t e r ia l s a n d

a t t e m p t s t o g r o u p c a t e g o ri e s o f e r o s i o n c o n t r o l m a t e r i a l s in t o t h e i r c o s t

e f fec tive des ig n n iches .

M axim um pe rmiss ib le ve loc it ie s fo r vege ta tive t echn ique s a re i l lu s t ra t ed

u n d e r v e g e t at e d a n d n o n - v e g e t a t ed c o n d i t io n s . T h u s a d e s i g n e r w i ll h a v e

p e r f o r m a n c e g u i d e l i n e s f r o m t h e t i m e a m a t e r i a l is i n s t a l le d to w h e n i t

b e c o m e s f u l l y v e g e ta t ed .


12/16

546 M S The i sen

6'0

E

~ , 4 . 5

.90

:>

r

. ~ 3 - 0

u~

1 0

1-5

17D

E

0

._ l

Hi g h v e l o c i t y b l a n k e ts

_ f i b e r r o v in g s y s t e m s h i g h

r a t e ? ~

M e d i u m v e l o c i t y b l a n k e t s a n d m e s h

f i b e r r o vi n g s y s t e m s l o w rate?X - ~

L o w

v ~o cit~ ; ~ l ; ~ ; P . . . . .

I I

2 5

F l o w d u r a t i o n , h

H a r d a r m o r s y s t e m s

~ t e d

TR

S o f t a r m o r z o n e

N n v e g e t a t e d T RM o r E CR M

1 0 0 1. C o v e r . . . . . . . . . .

L i m i ts o f n a t u r a l v e g e t a t i o n

Poor cover

are soil erosion

1 0 2 0 5 0

Fig. 1. Recommended maximum design velocities for various erosion control materials.

*No known published data for flows exceeding 0.5 h.

l o w d u r a t io n

Of key importance is the significance of duration of flow. Note from

Fig. 1 that allowable flow velocities decrease with flow duration. This is a

critical point. Manufacturers of both organic and synthetic erosion

control products often express the erosion resistance of their materials in

terms of maximum allowable flow velocity. Though unstated, these flow

limits are typically for very short durations (minutes rather than hours).

They do not reflect the potential for severe erosion damage that results

from moderate flow velocities over a period of several hours. Ironically,

many manufacturers, designers and users do not consider duration of

flow when evaluating and selecting erosion control measures (Theisen &

Carroll, 1990).

Typically, a major precipitation event will produce significant flow

velocities with durations lasting hours or days--not minutes. The two

day design duration was selected because in grass waterways, high

velocity flow events should be no longer than about two days in duration,

following which grass recovery and subsoil drainage should be able to

take place (Hewlett

e t a l .

1987). As Fig. 1 illustrates, duration of flow will

reduce the erosion resistance of a grassed surface. It is critical that design

of grassed waterways take this into account.

Flow values for the various temporary erosion control mulches,


13/16


blankets, meshes and rovings have been truncated and placed into the

lower left area of the graph because extended flow duration trials for

these materials have not been reported. Their long term performances

may either go up or down as vegetation becomes established and

ultimately will fall into the niche for natural vegetation as the product

degrades.

The Soft Armor Zone begins just above the limits of natural

vegetation. Performance data for reinforcing mats ranges from un-

vegetated TRMs and ECRMs (which exceed performance of natural

vegetation) to the upper curve which delineates maximum recommended

design velocities obtained from field and laboratory evaluation of

vegetated TRMs (Western Canada Hydraulic Laboratories Ltd, 1979;

Hoffman & Adamsky, 1982; Hewlett e t a l . 1987; Theisen & Carroll, 1990;

Carroll

e t a l .

1991).

The upper end of the graph is comprised by the niche for hard armor

materials. The graph is not intended to establish upper performance

limits for these materials, but rather to define the upper limits of 'soft

armor' (reinforced vegetation). Performance for hard armor materials

will be considerably higher and upper limits are beyond the scope of this

paper.

ed i m ent contro l

Going hand in han d with aggressive erosion control measures should be

a well conceived sediment control plan. Erosion control measures are an

offensive strategy to attack potential sedimentation while sediment

control practices are a stop gap defensive strategy. In erosion and

sediment control planning, the old sports axiom that a strong offense is

the best defense is certainly apropos. Vegetation is clearly the finest

sediment control product on the planet

Geosynthetic silt fences have become a standard construction practice

over much of the United States replacing straw and hay bales, brush

layers and rock check dams. Silt fences are generally installed at the

beginning of the construction project and usually consist of woven slit

tape geotextiles mounted on a prefabricated fence.

A well designed silt fence must initially screen silt and sand particles

from runoff. A soil filter is formed adjacent to the silt fence and reduces

the ability of water to flow through the fence. This leads to the creation of

a pond behind the fence which serves as a sedimentat ion basin to collect

suspended soils from runoff water. To meet such needs, the geotextile

must have properly sized openings to form the soil filter and the storage


14/16

548 M S T h e i s en

c a p a c i t y o f t h e f e n c e m u s t b e a d e q u a t e t o c o n t a i n t h e v o l u m e o f w a t e r

a n d s e d i m e n t a n t i c i p a t e d d u r i n g a m a j o r s to r m ( R i c h a r d s o n & K o e r n er ,

1990).

P o r o u s s e d i m e n t c o n t r o l s t r u c t u r e s a r e t h e n e w e s t g e o s y n t h e t i c

a p p r o a c h t o s e d i m e n t c o n t r o l . A t h r e e - d i m e n s i o n a l m o l d a b l e m a s s o f

c r i m p e d p o l y p r o p y l e n e f i b e r s m a y b e p l a c e d i n f i ll s o r g u l l ie s t o p r o v i d e

p a s s i v e s e d i m e n t c o n t r o l . P l a c e d b y h a n d w i t h i t s s i z e a n d s h a p e

d e t e r m i n e d b y t h e i n s t a ll e r , a p p l i c a t i o n s i n c l u d e f il l a n d g u l l y r e p a i r,

d i t c h c h e c k s , s e d i m e n t t r a p s , a n d p e r i m e t e r b e r m i n g . M o r e o v e r t h e

f i b e r s m a y b e e n c a p s u l a t e d i n a p o l y p r o p y l e n e m e s h t o c r e a t e

p r e f a b r i c a t e d c h e c k d a m s f o r sw a l e a n d d i t c h p r o t e c ti o n . T a b l e 4 li s ts a

f e w s e d i m e n t c o n t r o l t e c h n i q u e s .

the r geosynthet ic opportun i ti e s

I d e a s f o r g e o s y n t h e t ic e r o s io n a n d s e d i m e n t c o n t r o l m a t e r i a ls a b o u n d .

C e r t a i n l y n e w i d e a s h a v e b e e n o m i t te d o r a r e b e i n g d e v e l o p e d a t t h e ti m e

o f th i s p u b l i c a ti o n . O d d s a r e h i g h t h a t g e o s y n t h e ti c s w i l l w o r k t h e i r w a y

d o w n t h e l a d d e r i n t o m o r e t r a d i t i o n a l a p p l i c a t i o n s s u c h a s h y d r a u l i c

m u l c h e s a n d e r o s i o n c o n t r o l b l a n k e t s . H y d r a u l i c m u l c h g e o f i b e r s

( w h i c h i m p r o v e t h e t e n a c i t y o f w o o d f ib e r a n d r e cy c le d p a p e r m u l c h e s )

a n d r e c y c le d p l a s t ic f ib e r b l a n k e t s a n d m a t s a re a l r e a d y e n t e r i n g t h e

m a r k e t . G e o f i b e r s a r e b e i n g u s e d a s p a r t o f s p o r ts t u r f s y s t e m s in m a j o r

a t h l e t i c s t a d i u m s p r o v i d i n g b o t h g r o u n d s t a b i l i z a t i o n a n d a r o o t

r e i n f o r c i n g m a t r i x .

T h e f u t u re o f g e o s y n t h e t i c s fo r E & S C l ie s p a r t l y i n t h e r e c y c l i n g O f

w a s t e p l a st ic s g e n e r a t e d f r o m o t h e r a p p l i c a t i o n s . P o l y m e r s p e c i fi c a t io n s

f o r E & S C a p p l i c a t i o n s a r e n o t a s s t r i n g e n t a s o t h e r g e o s y n t h e t i c

m a t e r i a l s s u c h a s g e o m e m b r a n e s , g e o g r i d s a n d g e o t e x t i l e s . I t i s a n

e n v i r o n m e n t a l l y f r ie n d l y g e s tu r e i n a n e n v i r o n m e n t a l l y f r ie n d l y in d u s t ry .

And r ecyc l ed p l a s t i c s a r e cos t e f f ec t i ve .

TABLE 4

Examples of Sediment Con trol Techniques

Vegetation

Straw and hay bales

Brush layers

Silt fences

Porous sediment control structures (PSCSs)

Rock check dams

Sediment traps, basins and ponds


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Geosynthetics in erosion an d sediment control 549

More research on E & SC effectiveness of the myriad of materials is

mandatory. Questions regarding resistance to extended flow durations

and long term performance for all materials must be answered. Systems

must be developed for standardizing product descriptions, sanctioning

uniform test and evaluation procedures, and creating a market reporting

system to insure broad acceptance of E & SC industry. Organizations

such as the Internat ional Erosion Control Association (IECA), Erosion

Control Technology Council (ECTC), American Society for the Testing of

Materials (ASTM), Industrial Fabrics Association International (IFAI),

and the Geosynthetics Research Institute (GRI) must lead the way.

Another hurdle to overcome or trail to blaze, depending upon how one

looks at it, is the issue of biodegradable plastics for short term

applications. This is an area of very important research. Members of the

industry as well as the general public must be educated. The performance

advantages of man made fibers over natural fibers is recognized in many

sectors of the textile industry. The E & SC industry is quite possibly a

sleeping giant for man made fibers. Keep your ear to the ground and your

eyes wide open because 'you ain't seen nothin' yet'. The future of

geosynthetics in erosion and sediment control is bright

REFERENCES

Agnew, W. (1991). Erosion control product selection.Geo technical Fabrics Report

9 (3), 24-27.

Carroll, O. Jr, Rodencal, J. & Theisen, M. S. (1991). Evaluation of turf

reinforcement mats and erosion control and revegetation mats under high

velocity flows.Proceedings o f the X X II Ann ual Conference of the International

Erosion Control Association Orlando, FL.

Chen, Y. H. & Anderson, B. A. (1986). Development of a methodology for

estimating embankmentdamage due to flood overtopping. USDOT Federal

Highway Administration and USDA Forest Service, report number

DTFH61-82-C-00104, Ft Collins, CO.

Chen, Y. H. & Cotton, G. K. (1988). Design of roadside channels with flexible

linings. Federal Highway Administration report HEC-15/FHWA-1P-87-7,

McLean, VA.

Guillet, J. E. (1974). Plastics, energy and ecology--a harmonious triad. Plastics

Engineering 30 (8), 48-56.

Hewlett, H. W. M., Boorman, L. A. & Bramley, M. E. (1987). Design of re-

inforced grass waterways. Construction Industry Research and Information

Association, Report 116, London.

Hoffman, G. L. & Adamsky, R. (1982). Nylon erosion control mat. Presented at the

Transportation Research Board sixty-first annual meeting Washington DC.

Theisen, M. S. (1988). Cost-effective techniques for successful erosion control.

Proceedings High Altitude Revege tation Work shop No. 8 Ft Collins, CO.

Theisen, M. S. & Carroll, R. G. (1990). Turf reinforcement--the 'soft armor"


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550 M. S. Theisen

al ternat ive .

Proceedings of the XXI Annual Conference of the International

Erosion Control Association,

W a s h in g t o n D C .

R icha rds on , G . N . & Ko em er , R . M. (eds ) (1990).A Design Primer. Geotextiles and

Related Materials,

In d u s t r i a l F a b r i c s A sso c i a t i o n In t e rn a t i o n a l , S t P a u l, M N .

W e s t e rn C a n a d a H y d ra u l i c L a b o ra t o r i e s L t d (1 9 7 9 ) .

Cove Country Club:

Experimental Studies on Permissible Shear Stresses of Bermuda Grass.

P o r t

C o q u i l a m , B r it is h C o l u m b i a , C a n a d a .

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