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1

ERODIBILITY OF

HILL PEA
T

1


2

J. Mulqueen, M. Rodgers,
N. Marren

and M.G. Healy


3

Department of Civil Engineering, National University of Ireland, Galway, Ireland.

4



Corresponding author. E
-
mail: mark.healy@nuigalway.ie. Tel: +353 91 495364

5


6

Abstract

7



8

The ener
gy nec
essary to entrain soil in water depends on
the

soil
strength
.

Once
9

entrained, the settling velocity of
the
eroded
soil in water

is of fundamental importance to
10

the processes of

sediment

transport and deposition
.

In this paper,

stream p
ower
theory
11

and

transport concepts coupled with the equation of continuity
were used to derive a
12

transport
-
limi
ted peat

concentration.
The ratio of

the log of the actual

sediment

13

concentration
in surface run
-
off
to

the

lo
g of the transport
-
limited
sediment

concentration

14

was the index of erosion

used
.

The

value of this index is a measure of the sensitivity of
15

peat to erosion by
sheet flow
.
Four

peats were subjected to a range of overland flow rates
16

under tw
o slopes in a laboratory flume.

The peats represented peat farmed i
n a
17

sustainable manner

(Leenane)
, overgrazed peat

(Maam)
,

peat undergoing erosion

18

(Newport)

and peat which had undergone weathering following exposure by a landslip
19

(Croagh Patrick)
. B
oth
in
-
situ

and surfa
ce damaged slabs were studied.

The results
20

indicate

that
s
hearing and remoulding of a wet peat surface (e.g. by animal treading)
,

and
21

weathering of exposed drained peat surfaces predispose

peat

to erosion. Defoliation by
22

overgrazing is considered to be of secondary importance.

23



2


24

Keywords
: peat erodibility
; sedimentology; sustainable farming


25


26

Introduction

27


28

Soil erosion by water results in
(i)
the

depletion of soil
in
-
situ
,

and
(ii) the trans
port of

the
29

resulting

sediment to dow
nslope and downstream areas.
When sufficient energy is no
30

longer available to t
ransport soil particles in

suspension

or by

saltation
,

net
deposition
31

occurs. D
epletion

of soil
in
-
situ

is caused by

the following
erosion
processes:

detachment
32

and
re
-
detachment

by raindrops
,

entrainment

and

re
-
entrainment

by overland flow,
33

accompanied by

transport

in sheet and rill flow (Rose, 1993)
.

Detachment refers to the
34

removal of soil from the o
riginal soil matrix by raindrop
-
induced shear stresses in the
35

absence of any flow (Torri and Borselli, 2000). Some of thi
s soil
sediment settles back
36

close
-
by and some may be splashed into the air to be

captured in a shallow

water layer

as
37

it falls back down
.

Re
-
detachment refers to
rainfall
detachment of already detach
ed and
38

deposited soil sediment.

In
-
situ

soil always has some cohesion while the deposited
39

s
ediment is loo
se and much more easily eroded

as it does not have time to build up
40

cohesive links with neigh
bouring particles (Rose, 1993).

A similar reasoning applies to
41

the
entrainment of original soil
,

and its re
-
entrainmen
t by overland flow following
42

de
position
.

43


44

Flow
-
driven e
rosion is commonly

differentiated into sheet erosion and
rill er
osion. Sheet
45

erosion
can be caused by

r
ainfall detachment/re
-
detachment and/or run
-
off
46



3

entra
inment/re
-
entrainment

on a land surface
.

Detachment/re
-
detachment are domin
ant
47

where the thickness of the water layer on the soil is less than 3 rain drop diameters (Rose,
48

1993). As the water layer thickens, the streampower increases in accordance with SDV,
49

where S is the land slope and the product DV is the flux of water per un
it width of plane
50

surface, D being the thickness of
the overland flow layer and V

the water velocity. With
51

an
increase in
the
thickness of the water layer
,

erosivity of run
-
off increases and rainf
all
52

effects become unimportant.

The erosive effects of rain
fall and run
-
off d
epend on the soil

53

cohesion
.

In erodible soils, a

combination of heavy rainfall and run
-
off produce
s

a greater
54

soil loss than run
-
off alone due to the increase in turbulence of the run
-
off produced by
55

the rainfall (Proffit
t a
nd Rose, 1991)
,

except on

steep

(e.g.
>
5%) slopes.

Where soil
56

strength is dominant, due to soil type or reinforcement by a dense mesh of strong roots,
57

the effects of a surface cover or canopy of low growing vegetatio
n in reducing soil loss is
58

secondary (Rose, 1993); i
n erodible soils a vegetation cover or canopy near the soil
59

surface can

limit rainfall effec
ts.

Likewise, a surface

cover such as a mulch, by
60

intercepting rainfall and slowing down run
-
off rate, is effective against
both rainfall and
61

flow
-
driven erosion.



62


63

Rills are small streams eroded out by water flow, fed by run
-
off from sheet flow. Erosion
64

from rills is due to entrainment and re
-
entrainment by running water aided by mass
65

movements of soil into the rill due to sidewall sloughing and slips, undercutti
ng of
66

sidew
alls and head cutting of rills.

Generally
,

the erosive power of flowing water in rills
67

is greater than in sheet flow. This is due to the greater streampower in the rill

(Marshall
68

et al., 1996)
.

69



4


70

The sedimentology of peat silt from milled peat f
ields has been investigated arising from
71

concerns about its impact on salmonid spawning grounds in Ireland (Migniot
et al
.,

72

1969). In windy weather, wind
-
blown milled peat from storage piles and harvesting
73

grounds is trapped in drainage trenches and later
re
-
entrained and transported by water in
74

wet weather to

streams
,

rivers

and sedimentation basins

where it settles out.

The mean
75

velocity for re
-
entrainment of peat sediment in the bottom of a river is about 0.15
m
/
s

for
76

0.4 m depth of water; depending on d
epth of flow
,

this value should be adjusted upwards
77

or downwards;
e.g.,

for a 1 m depth of water
the value would be 10% greater

(Migniot
et
78

al.
, 1969)
.

The critical shear stress




for re
-
entr
ainment was estimated at 0.05 N
/
m
2

79

using measured veloc
ities at different depths in a

flume and applying the logarithmic
80

velocity law; a



for 0.1 mm diameter sand grains is 0.1
N
/
m
2

(Migniot
et al.
, 1969).


81


82

Mulqueen
et al
. (2000)

quantif
ied hill peat erosion from a south
-
facing slope of a
83

predominantly peat covered hillside catchment at Leenane, Co. Mayo
, Ireland
.
They
84

report
ed

a mean annual sediment loss of 278
kg
/
ha (equivalen
t to 0.4 mm
/
y
r

loss), which
85

approximately balanced

the build
-
up of peat from the accumulation of plant

remains.

86

They
also reported

on the erodibility

of hill peat from four diverse sites in a laboratory
87

flume under various degrees of remoulding, simulating treading d
amage (poaching) by
88

hill sheep.

They found that

re
moulding
and weakening of the peat

predispose it to
89

detachment, entrainment and transport in flowing water
.

90


91



5

Using theory developed b
y Rose (1993) and Yang (1996),
theoretical developments in
92

erosion
are reviewed
and

a transport
-
limit
ed peat sediment conce
ntration

is derived
.

T
he

93

ratio of the

log of the actual peat sediment concentration
released
from a flume or a
small
94

catchment to

the log of the transport
-
limi
ted peat sediment concentration

gives an
95

erodibility index

(

)
.

The numerical value of this index is a measure of the sensitivity of
96

the peat to erosion b
y
surface run
-
off, and

may be a useful tool in environmental
97

management.

98


99

Erosion theory

100


101

E
ntrainment and re
-
entrainment by overland flow.

102


103

Water flowing overland

or in a channel exerts a shear stress on the soil surface. This is
104

expressed for a channel by

105


106








=

e

g
R
h
S











[1
]

107


108

where



is the
shear stress (N
/
m
2
)
;

e
, the
dens
ity
(kg/
m
3
)
of sediment
-
laden water
; g, the
109

acceleration due to gravity (m
/
s
2
);
R
h
, the hydraulic radius

(i.e.

the
cross sectional area of
110

a
channel divided by
its
wetted perimete
r (m)
)
; and S

the
s
lope of

the
channel (m
m
-
1
)

111


112

For sheet

flow
,

R
h

is replaced by D,
the depth of the flowing water.

The sediment
-
laden
113



6

fluid density is

(Marshall et al., 1996)

114





c
s
s
e



















[2
]

115






c
62
.
0



(mineral soil);












116







c
29
.
0



(peat soil)

117


118

where



is the
density of clean water
;

s
, the
density of

solids

in the soil

(2650 kg
/
m
3

for
119

mineral soil; 1400 kg
/
m
3

for peat soil); and c, the

concentration

(kg/m
3
)

o
f sediment in
120

sheet

flow
.
The

density
of peat
is within the range quoted by Bell (1981).

121


122

Bag
nold (1977) defined the streampower (

) that may cause erosion as

123


124







=

V












[3
]

125


126

where

V is the

mean velocity
(m/
s)

of flow

and
Ω

is measured in W/m
2
.

127



128

Streampower combines the effects of slope, water flux and the flow concentrat
ing effects
129

o
f rills. For sheet flow from Eq. [
2
] and Eq. [
3
] and with


substituted for

e
, the
stream
130

power is given by

131


132







=

g
DSV











[4
]







133











134

wher
e SV is the

unit streampower, i.e. the
rate of decline of potential ener
gy of a unit
135



7

wei
ght of water (Yang, 1996).

136



137

A model of the entrainment and re
-
entrainment process
es

by overland flow is presented
138

in Ma
rshall et al. (1996). A

threshold

stream power (

0
)
is required before any sediment
139

is moved

by water flowing over it.

F

is

the fraction of

the excess streampower (


-


0
)
140

available to drive re
-
entrainment of deposited sediment or entrainment of
in
-
situ

soil
141

leaving the fraction (1

F) to dissip
ate in heat and noise; F has

v
alues of 0.2 for laminar
142

flow and 0.1 for turbulent

flow (Rose,1993).
Flow is laminar when the Reynolds number,
143

Re, is less than about 500 and is turbulent when Re is larger than about

2000 (van Dort
144

and Bos, 1974).

H is

the

fraction of original soil shielded from ent
rainment by deposited
145

sediment and

(1


H)

the fraction exposed.


146



147

At equilibrium, the rate of deposition equals the rate of re
-
entrainment of t
he deposited
148

layer, yielding the following relationship (Marshall et al., 1996):

149


150







d
di
O
i
i
M
m
D
g
HF
I
c
v




















[
5
]

151


152

where v
i

(m/s)
is the settling

velocity of the ith class, c
i
, the concentration
(kg/m
3
)
of the
153

ith class sediment in the run
-
off water,
σ
, the submerged density of the soil (1050 kg
/
m
3

154

for a peat soil),
I, the number of class sizes into which the original in
-
situ soil may be
155

distribute
d for water erosion,
m
di
,

the mass of sediment class
i

per unit area
(kg/m
2
)
of
156

deposited layer and M
d
, the total mass of sediment per unit area of deposited layer
157

(kg
/
m
2
).
Summing
Eq.
[
5] over all i
-
size classes and since

( m
di
/M
d
) = 1 yields

158



8


159





















gD
I
v
HF
c
O
i











[
6
]

160


161

Since H has an upper limit of 1, then c
(kg/m
3
)

has an upper limit or maximum
162

concentration (c
t
) for given flow conditions.

For

sheet flow,

substituting
Eq.
[
4
]

into
Eq.
163

[
6
]
, and neglecting
Ω
o

in comparison with
Ω
,

yields

t
he transport
-
limited sediment
164

concentration

(kg/m
3
)
, c
t


165


166







SV
I
v
F
c
i
t

















[
7
]

167


168

For non
-
cohesive sediments, Yang (1996) found sediment concentration closely
169

p
roportional to SV.

Eq.

[
7
] may also

be used to evaluate F
.

In rill erosion, c
t

is def
ined by
170

(Marshall et al., 1996).

171


172







D
W
W
gD
I
v
F
c
b
b
o
i
t
2

























[8
]

173


174

where W
b

is the width of the rill
, D is the depth of flow,

and W
b
+2D its wetted perimeter.

175


176

In cohesive soil, the
specific
energy
, J,

required for entrainment increases with soi
l
177

strength and, as a result, the sediment concentration is less than the transport
-
limited
178



9

concentration

(Marshall et al., 1996)
.
If c is
plotted against streampower for

parti
cular
179

valu
es of J,

a family of positive response curves starting from the origin
and tending
180

toward asymptotes

(c
t
)

with incre
asing streampower
is obtained
(Rose, 1993).

A similar
181

suite of curves can be obtained from

Eq. [9
] (Marshall et al., 1996
)
:

182


183





c = c
t


(


< 1)










[9
]

184


185

where


is an empirical or approximate erodibility parameter (closely related to J)
and
186

can be determined from

187


188





t
c
c
ln
ln















[10
]

189


190

where

c is the
flux weighted concentration, determin
ed from run
-
off plo
ts or flumes
.



191

will only exceed unity if other erosion mechanisms, such as rainfall impact or bed
-
load

192

transport, add sediment to that from flow
-
driven erosion.

193


194

Materials and Methods

195


196

A laboratory flume comprising a 150 mm x 150 mm galvanised steel ch
ann
el 3 m long
197

was built to accommodate

relatively undisturbed 150 mm wide slabs of peat. Peat slabs
198

at least 150 mm wide and 600 mm long were carefully excavated in the field from
4

sites.

199

They were transported to the laboratory, trimmed and placed in the f
lume; each slab was
200

butted against its adjacent slab or the head weir to form a continuous peat surface, 2.4 m
201



10

long and the slab was prevented from sli
ding out by a retainer plate.
Overland flow
was
202

applied to

this flume and the effects on

sediment concent
ration of
flow rate, slope of
203

surface and surface disturbance of the peat examined. The slope of the flume
was set at
204

either 5
o

or

10
o
. Water was supplied from a constant head tank from which the flow rate
205

was controll
ed by two 12.5 mm lever valves.

Wate
r from the tank flowed into a small
206

chamber at the head of the flume and then over a wei
r onto

the surface of the peat.

At the
207

tail end, the water flowed over the end of the peat surface and retainer plate into a tank
208

which was plac
ed on a balance. The ba
lance

was read and 250 ml samples of the run
-
off
209

were taken every 1 or 2 minutes for the first 10 minutes and thereafter every 10 minutes
210

until

equilibrium
sediment concentrations

were obtained.

211


212

Slabs of peat were taken
for erosion investigation
from
4
si
tes: Leenane
,

where
the
field
213

studies

were conducted; Maam;
Newpo
rt

and Croagh Patrick
.

At Leenane
,

the
field
214

measurements

(Mulqueen
et al
.
, 2000)

were carried out at the scale of the sub
-
c
atchment
215

(0.5 to

20 ha) to include the effects of soil cover and ro
ck outcrop, slope and breaks in
216

slope, rilling and channelised flow and land management
,

which would not be evident at
217

a plot scale of up to 1000 m
2
.

The sub
-
catchment was 7.68 ha and located on the Teagasc
218

250 ha Hill Farm at Glendavock townland
, Leenane,

Co. Mayo
. It was also used as the
219

source of peat sediment for sizing and measu
rements of settling velocity.
The peat depth
220

varie
d

from a thin veneer over most of the catchment to a maximu
m of 2.7 m in a
221

concave valley.

A canopy of grassy plants gave a 70%

ground cover and there was

also a
222

more

or less continuous layer of gelatinous

algae on the surface.
The entire far
m
,

223



11

including the sub
-
catchment
,

wa
s grazed by Scottish Blackface shee
p stocked at a rate of
224

0.9 ewes
/
ha under a sustainable management system

(Hanrahan and O’Malley, 1999).


225


226

Maam peat was also sampled

as it was overgrazed and devoid of a vegetation cover in
227

winter; Newport peat was taken from a sub
-
catchment of Lough Feeagh near

Furnace,
228

some of which was

undergoing
significant erosion.

Croagh

Patrick peat was used as it
229

had developed on a surface left bare and subject to weathering after a landslip. This peat
230

had no plant growth and had developed a blocky structure due to weathering under
231

ambient conditions.

It was not possible to retrieve sla
bs or blocks of Croagh Patrick peat
232

due to its blocky and brittle nature; instead a 25 mm layer of the blocky peat was placed
233

on an existing peat slab and compacted lightly to a level surface, resembling on
-
site
234

conditions.

With the exception of Croagh Pat
rick peat, a
ll peats selected

we
re very slow
235

draining with
hydraulic conductivities <10 mm/
d

when

tested in the saturated state.

236


237

The rainfall at Leenane is among the highest in Ireland; annual average rainfall as
238

measured on the lower slopes at abo
ut 30
m OD amounting to 2500 mm
/
y
ea
r

is
239

distributed throughout the year wit
h lowest monthly values of 100 to
200 mm in April
240

through Septembe
r and the highes
t (up to 465 mm
/
month) in October through January.

241

For example, the rainfall

in October 1995 was 435.8 mm

with a maximum daily of 53.4
242

mm and there were 7 days with daily rainfalls in e
xcess of 25.4 mm.

Rainfalls at Maam,

243

Newport
and Croagh Patrick
are of a similar order.

244


245



12

The surfaces of the peats
in the flumes
were disturbed by pushing steel rods into them
to
246

simula
te punching failure of the peat

by sheeps’ hooves.
Four

degrees of disturbance,
247

including zero disturbance, were employed

(Table 1)
.
The applied disturbance

caused
248

compaction and remoulding of the peat, reducing its cohesion at the surface and
249

ren
dering the peat more liable to erosion by flowing water.

250


251

[Table 1 here]

252


253

Flow rates were
set at 0.05, 0.1, 0.2 and 0.3 L
/
s over the

undistur
bed peats

at slopes of 5
o

254

and 10
o
,

and at 0.2 L
/
s
only
over the disturbed peats at a slope of 10
o
. The
variables

255

m
easured in each test were the slope of the flume, the rate of flow (m
3
/s
) and

the
256

sediment concentration (mg
/
L).
The depth of flow
, D,

was derived using the Manning
257

equation

(Chow, 1959)

258


259





w
D
S
n
Q
66
.
1
5
.
0
1












[
11
]

260


261

whe
re

Q is the

measured

rate of discharge (m
3
/s
); n, the Manning roughness number; S,
262

the

tangent of the slope angle
(5
o

or 10
o
)
of the flume
;
D
, the depth
(m)
of flow; and w,
263

the

width of flume (0
.15 m
)
.

The Manning roughness num
ber was assigned to the soils
264

after

visual inspec
tion

of the soils using values from Chow (1959)
.

265


266

Having derived D, the velocity
of flow was computed from
V=Q/
A where A is the cros
s
267

section of the flow area (= w
D).

F, the fraction of excess streampower available to drive
268



13

entrainment or re
-
entrainme
nt, w
as assigned a value of 0.2, as all calculated Reynolds
269

numbers were less than 2000.

270


271

Sediment Settling Velocity

272


273

Samples of peat sediment were taken from a collection c
hamber at Leenane.

Particle size
274

distribution was measured by sieving under water using
a nest of sieves with decreasi
ng
275

mesh size

in accordance with the British Standards (BS 1377, 1990)
.

The peat retained on
276

each sieve was washed off into a beaker, filtered a
nd dried.

Particle size distribution was
277

ex
pr
essed as a grading curve

(Fig. 1)
.


278


279

[Figure 1 here]

280


281

Particle settling velocities
were determined using the

modified
bottom withdrawal tube
282

(
BWT
) method

(Lovell and Rose, 1988
)

and incorporating constructional and operational
283

details from Vanoni (
1975).

Settling velocity is the terminal velo
city attained by a
284

particle settling under gravity in a fl
uid (Torri and Borselli, 2000).

It depends on the size,
285

density, shape and surface texture of the particle, on the viscosity, density and degree of
286

turbulence of the fluid
,

and on the distribution a
nd concentration of other particles in the
287

fluid (Lovell and Ro
se, 1988).

A BWT tube was made from polyethylene tube and the
288

bottom was drawn down to an 8 mm no
zzle by welding a funnel to it.

A 75 mm length of
289

15 mm bore rubber tube was fitted to the nozz
le and closed off using a ‘snap lock’ pinch
290

clam
p to enable quick withdrawals.
The BWT was calibrated by marking off 0.9 m on the
291



14

straight portion and measuring the volume contained between the calibration marks. One
292

ninth of this volume was then added to

the drawn
-
down end of the tube making the
293

bottom of the

water meniscus the 0.1 m mark.

The BWT was then marked of
f every 0.1
294

m to the 1
-
m mark.

An estimated amount of wet peat sediment

and water

to give the
295

desired concentration approximately

and volume
-

to bring the meniscus to the 1 .0 m
296

mark in the BWT

-

w
ere

placed in a 5L

flat
-
bottomed round flask. The flask was then
297

agitated to produce a uniform dispersed suspension that was poured quickly and
carefully
298

into the sloping BWT
, which was corked when fu
ll.
In order to dislodge peat particles
299

that would have settled in the tube nozzle during pouring, the
corked
BWT was tilted to
300

about 30
0

with the nozzle u
pwards (Lovell and Rose, 1988).

Part
icles from the nozzle
301

then slid

along the length of the tube and
with additional tapping and rotating of the tube
302

about the ho
rizontal, a uniform suspension wa
s obtained.

The BWT was

uncorked and

303

then
quickly mounted vertically

at time t
0
.

The ‘snap lock’

was opened ten times, e.g. at
304

0.5,1, 2, 4
, 8, 16, 32, 64,
128
and
greater than 128
minutes;

approximately
0.1 of the
305

initial volume

was dischar
ged each time
.

Since i
t was not found possible to withdraw

an
306

exact 0.1 m at each opening,

withdrawals were collected in numbered conical flasks
307

which were measured
for volume and

filtered
.

The f
iltered sediment was dried at
5
0
o
C
for
308

24 hours
.
The volumes of suspensions and masses of peat silt were accumulated to give
309

the total volume and mass respectively, from which
the
concentration of the suspension
310

was derived.
The settling ve
locity was then calculated using the improved approximate
311

method of Anon (1943), detailed in Lovell and Rose (1988)

(Fig. 2)
.

312


313

[Figure 2 here]

314



15


315

Results

316


317

Particle size

and settling velocity


318


319

About 50
% of the eroded
Leenane field study
peat particles was

f
iner than 0.2 mm and
320

the D
10

wa
s about 0.035 mm (Fig
.

1
).
The mean settling velocity (v
50
) of the
Leenane
peat
321

sediment for
sediment
concentrations of 225 and 907
mg
/
L

were simi
lar and varied from
322

2.2 to 2.5 mm/
s

(Fig. 2)
compared with 0.55 to 1.5 mm/
s for

wind
-
blown
milled
peat
323

sediment from the Mongagh

r
iver (Migniot
et al.
, 1969), reflecting the larger size of the
324

water eroded peat sediment
.

The v
50

for the 1946 mg/
L

sediment

co
ncentration increased
325

to 5.5 mm/
s, indic
ating aggregation of the sediment con
centration
.

326


327

Erodibility

328


329

Results in
Table
2

show

that u
nit streampower (SV) was lowest for the Leenane peat
330

reflecting the greater (70%) ground cover by grassy vegetation and resulting surface
331

roughness causing an increase in the depth of water flowing
downslope. The mean
332

(C
mean
) and maximum (C
max
) concentrations of sediment from the Leenane and Maam
333

peat surfaces were low

compared to
the

Newport and Croagh Patrick

peats
, showing that
334

the latter two

peats are erodible. While

mean

values for Leenane and
Maam were

0.
17
,
335


mean

values for Newport

were in excess of 0.30

for three of the flow tests.

max

followed
336

similar trends with three high values from Newport

varying from 0.53

to

0.70
. The results
337



16

show that both the grassy Leenane peat and the defoliated
but gelatinous algae
-
covered
338

Maam peat are entrainment
-
limited due to their high shear strengths (26 and 13 kPa,
339

respectively).
The
Newport and
the strongly weathered
Croagh Patrick peats are

340

erodible
.



values are not quoted for the Croagh Patrick peat
in

Table 2
as the settling
341

velocity curves were calculated from the Leenane peat, which was considerably different
342

in texture to the Croagh Patrick peat.

343


344

[Table 2 here]

345


346

Effect of disturbance on erodibility
.

347


348

Table
3

shows a comparison of the erodibility

of peats from Leenane, Maam,

Newport
349

and Croagh Patrick
sites

after they were disturbed
.

Both
Leenane and Maam peats
350

showed reduced resistance

to erosion in this disturbed state.

T
he mean

sediment

solids
351

content of the run
-
off waters from
the
Newport
and
Croagh Patrick
peat
s

were still
352

substantially greater than those from the Leenane and Maam peats.

Complete
remoulding,
353

as might happen at

sheep feeding facilities, would be expected to yield
sediment

354

concentrations closer to the transport
-
limited value as
indicated by the behaviour of the
355

disturbed Newport peat.

356


357

[Table 3 here]

358


359

Distribution of

sediment in an erosion event
.

360



17


361

Fig. 3 shows sediment run
-
off measurements in the flume.

Each run
-
off tes
t took place 24
362

hours after the previous event so that the
peat was in a similar saturated st
ate at the start
363

of each event.

It was noticeable tha
t the sediment

concentrations were highest by orders
364

of magnitude in
the first minute of each event.

The much higher initial sediment
365

concentrations for Newport and Croa
gh Patrick
peats
are evident.
Further work is
366

required to understand

the mechanics of this erodibility
.

367


368

[Figure 3 here]


369


370

Discussion

371


372

Settling velocities for eroded hill peat and water
-
transported milled peat

were similar,
373

with eroded hill peat ha
ving settling velocities for the 50%
(v
50
)
slower than
2.2 to

2.5

374

mm
/
s

for sediment

concentrations
less than 1000 mg
/
L.

In comparison, a
s would be
375

expected from a consideration of their specific gravities, mineral soils have much h
igher
376

settling velocities
:

the v
50

for a dispersed soil (e.g. a sodic soil) is about
15 mm
/
s; for a
377

well aggregated vertisol soil about 40 mm
/
s
;

and for a sea sand about 65 mm
/
s

(Lovell
378

and Rose, 1988). As a result of its low density
, the critical shear stress for entrainment of
379

p
eat is less than that for mineral soils (Migniot
et al
.,
1969).

380


381



18

S
treampower values employed in the flumes were high and were wel
l in excess of the
382

values required

to impart the critical shear stresses to induce erosion, confirming the high
383

resistance of

virgin fibrous peats to erosion.

384


385

The calculation of


assumes an approximate steady
-
state has been achieved in the
386

erosion event.
The

mean

value
s, reported

in Tables 2 and 3
,

are

in line with the theoretical
387

origin of

.
Calculating


using the maximum value of sediment concentration achieved
388

early in the
erosion experiment, whilst not strictly valid in terms of basic theory, does
389

have practical utility.
The use of

max

does quantify the
rapid early loss of sediment

in the
390

erosion event (Fig. 3)
. As
the sediment lost early in a runoff event is

high in sorbe
d
391

nutrients (Rose and Dalal, 1988),

max

is a useful parameter to emphasise soil erodibility.

392


393

Conclusions

394


395

Flume studies were used to derive indices

of erodibility,

, of four peats representative of

396

(i) peat

farmed in a sustainable
manner (Leenane) (ii)
overgrazed peat (Maam) (iii)

peat
397

undergoing erosion (Newport) and
(iv) peat that

had undergone erosion following
398

exposure by a landslip (Croagh Patrick).

Mean erodibility values,


mean
, ranged from 0.07
399

(Maam) to 0.38 (Newport).

Erodibility values rose si
gnificantly
after controlled
400

disturbance
.

The studies

indicate
d

that the physical state of the peat is the primary factor
401

predisposing to erosion. Shearing and remoulding of the peat by animal treading
402

(poaching)
-

as simulated in the flumes by driving thr
eaded rods through the surface of
403

the peat
-

and weathering
-
induced cracking of exposed peat surfaces along sheep tracks
404



19

and land slips predispose the peat to erosion. Defoliation is thought to be of secondary
405

importance in promoting erosion due to the res
istance offered by the strongly fibrous
406

nature of the top layer of many peats, as indicated by the low value of the erodibility
407

parameter,


, for defoliated Maam peat.

408


409

Acknowledgments

410


411

The authors are grateful to Mr L. O’Malley (Teagasc)

for technical hel
p.

The

project was
412

funded by

the European Union Structural Funds EAGGF distributed under the
413

Department of Agriculture and Food Stimulus Fund
.

414



415


416


417


418


419


420


421


422


423


424


425


426


427



20

References

428


429

Anon. 1943. A study of new methods of size analysis of suspended sediment samples.
430

Rep
. No 7. St. Paul, U.S. Engineer District Sub
-
Office, Hydraulic Lab., Univ. of Iowa,
431

Iowa City, USA.

432

Bagnold, R.A. 1977. Bedload transport by natural rivers.
Water Resources Research

13
:
433

303


311.

434

Bell, F.G. 1981.

Engineering properties of soils and rocks
.


Butterworth & Co., London.
435

P. 149.

436

BS 1377. 1990.
Method of test for soils for civil enginee
ring purposes.

British Standard
437

Institution.

438

Chow, V.T. 1959.
Open
-
channel Hydraulics. McGraw
-
Hill, London. 680pp.

439

Han
rahan, J. P. and O’Malley, L. 1999.


Hill s
heep production system

.

End of Projects
440

Report:
Sh
eep Series No. 4, Project 4041.

Teagasc Resear
ch Centre, Athenry, Co.
441

Galway, 36 pages
.

442

Lovell
, C. J. and Rose, C. W. 1988.

Measurement of soil aggregate settling ve
locities. 1.
443

A modified
bottom withdrawal tube method.

Australian Journal of Soil Research

26:
55


444

71.

445

Marshall, T. J., Holmes, J. W. and Rose, C. W.
1996
.
Soil

Physics, 3rd ed. Cambridge
446

U
niversity Press, Cambridge, UK.

274
-

307.
(See Chapter 11).


447

Migniot, C. T., Bellessort,

B., Gerlier, P.
and Dooge, J. 1969.

Sedimentolgy of peat silt.
448

Trans.
Institution of Engineers of I
reland
96
:

3
-

12.

449

Mulqueen,
J., Rodgers, M. and Marren, N. 2000.

Erosion of hill

peat in Western Ireland.
450



21

Proceedings of the 32
nd

International Conferen
ce of the

Erosion Control A
ssociation, Las
451

Vegas, NA
:

5


14.

452

Proffitt, A. P. B. and
Rose, C. W. 1991.

Soil erosion processes.

1. T
he relative
453

importance of rainfall detachment

and run
-
off entrainment.
Australian Journal

of Soil
454

Research

29:

671


683.

455

Ros
e, C. W. 1993.

Erosion and Sedimentation. Chapter 14 in Hydrology and Water
456

Management in the Humid Tropics (M. Bonell, M. N. Hufschmidt and J. S. Gladw
ell,
457

eds.).

Cambridge University Press, Cambridge, UK
, pp.

301
-

343.

458

Rose, C.W. and Dalal, R.C. Erosio
n and runoff of nitrogen. In: Wilson, J.R. (Ed.).
459

Advances in nitrogen cycling in agricultural ecosystems, CAB International, Wallingford,
460

pp. 212
-
235.

461

Rose, C. W., Hairsine, P. B., Proff
itt, A. P. B. and Misra, R. K. 1990.

Interpreting the role
462

of soil s
trength in erosion proc
esses.

Catena Supplement
17:
153


165.

463

Torri, D. and Borselli, L. 2000.

Water Erosion. Section G Chapter 7 in Handbook of Soil
464

Science (M. E. Sumner, ed.). CRC Press, Boca Raton, FLA, USA. G
-
171


G
-
194.

465

Van Dort, J.A. and Bos,
M.G.1974. Main Drainage Systems. Chapter 29 in Drainage
466

Principles and Applications, Volume IV. International Institute for Land Reclamation and
467

Improvement, Wageningen, The Netherlands. 123
-
222.

468

Vanoni, V. A. 1975.


Sedimentation Engineering
”. Manual no
. 54. American Society of
469

Civil

Engineers, New Yor
k, 745 pages
.

470

Yang, C.T. 1996.

Sediment Transport: Theory and P
ractice

. Mc
Graw Hill Inc., New
471

York. 396 pages
.

472


473



22

Captions for figures.

474


475

Figure 1. Grading curve for
eroded
hill peat

(Leenane site)
.

476

Figur
e 2. Settling velocity curves of
Leenane site
hill peat suspensions for three
different
477

concentrations.

478

Figure 3. Sediment c
o
ncentrations
from flume tests on four peats after disturbance 4.

C
max

479

= 5440mg SS/L (Newport) and 10,042 mg SS/L (Croagh Patrick).

480


481


482


483


484


485


486


487


488


489


490


491


492


493


494


495


496



23

Table 1. Disturbances used in
flume
tests.

497

________________________________________________________________________

498

Disturbance

Hole diameter


Spacing along flume


Spacing across flume

Rod

type

499





mm




mm






mm

500

_______________________
________________________
_____________
__________________

501

1




3





100







30






smooth

502

2




3





30







25






smooth

503

3




6





30







50






threaded

504

4




20





40







50






threaded

505

______________________________________________________
_________________

506









507


508


509


510


511


512


513


514


515


516


24

Table 2
. A comparison of erodibility for four

in
-
situ

peats.

517

_____________________________________________________________________
________________________
_______
_____
__________

518

Site



Slope


Flow


n



Depth


SV



C
max
a


C
mean
b


C
t




max



mean

519




Degrees


10
-
4

m
3

s
-
1




10
-
3
m


m/
s



mg/
L


mg/
L


10
4
mg/
L

520

_________________________________________________________________________
________________________
________
_____
_____

521

Leenane


5



0.5



0.042


2.5



0.011


4.3



2.3



1.1



0.16


0.09

522




5



1



0.042


3.8



0.015


11.5



4.2



1.5



0.25


0.15

523




10



0.5



0.042


2.0



0.028


12.7



3.4



2.8



0.25


0.12

524




10



1



0.042


3.1



0.038


13.0



5.0



3.7



0.24


0.15

525




10



2



0.042


4.7



0.050


5.8



3.4



4.8



0.16


0.11

526




10



3



0.042


6.0



0.058


8.4



4.3



5.7



0.19


0.13

527

Maam


5



0.5



0.019


1.6



0.018


13.4



5.5



1.8



0.2
6


0.17

528




5



1



0.019


2.4



0.024


6.0



2.0



2.3



0.18


0.07

529

10



2



0.019


2.9



0.080


18.
5



4.4



7.7



0.26


0.13

530




10



0.
5



0.019


1.3



0.046


12.6



3.6



4.5



0.2
3


0.12

531




10



1



0.019


1.9



0.060


48



4.9



5.9



0.35


0.14

532




10



3



0.019


3.8



0.094


6.0



3.8



9.1



0.16


0.12

533

Newport


5



0.5



0.019


1.6



0.018


117.5


44.5



1.8



0.4
8


0.3
8

534




5



1



0.019


2.4



0.024


462



21.9



2.4



0.6
0


0.3
0

535




10



1



0.019


1.9



0.060


366



12.8



5.9



0.5
3


0.23

536




10



3



0.019


3.7



0.094


3170


73.6



9.1



0.7
0


0.3
7

537

C. Patrick

5



0.5



0.010


1.1



0.027


295



135.6


-



-


-

538




5



1



0.0
10


1.6



0.036


1243


458



-



-


-

539




10



1



0.010


1.3



0.089


356



36.9



-



-


-

540




10



3



0.010


2.6



0.138


3910


264.3


-



-


-

541


_________________________________________________
______________________
__________________________
_
_
_____
_____
_____
_

542

a

ma
ximum concentration of

sediment in run
-
off.

543

b

weighted

mean concentration of

sediment in run
-
off.
544


25

Table
3
. A comparison of the erodibility parameter (

) for the 4

disturbed peats.

(Disturbance 4:
20 mm
545

diameter rough
-
walled holes, 20 mm deep, fo
rm
ed

in rows 40 mm apart along the length of the flume and
546

spaced 50 mm apart across the flume).

Slope and flow
were
10
o

and 2x10
-
4

m
3
s
-
1
, respectively.

547

________________________________________________________________________

548

Site



SV




C
max
1



C
mean
2



C
t





max



mean

549





m/
s



mg/
L



mg/
L



mg/
L

550

________________________________________________________________________

551


552

Leenane


0.050



146



14




4.8
x10
4


0.46


0.24

553

Maam



0.080



1326



106



7.8
x10
4


0.6
3


0.4
1

554

Newport


0.080



5440



585



7.8
x10
4


0.76


0.57

555

C. Patrick


0.117



10042



950



-




-



-

556

________________________________________________________________________

557

1

maximum
sediment concentration

over a 1
-
hour run
-
off event.

558

2

w
eighted mean
sediment concentration

over a 1
-
hour run
-
off event.

559


560





561


562


563


564


565


566


567


568


569


570


571


572


573


574


575


26

Figure 1.

576


577


578


579

0
10
20
30
40
50
60
70
80
90
100
0.001
0.01
0.1
1
10
Particle size (mm)
Percentage passing

580


581


582


583


584


585


586


587


588


589


590


591


592


593


594


595


596


597


598


599


600


27

Figure 2.

601


602

0.0
10.0
20.0
30.0
40.0
50.0
60.0
70.0
80.0
90.0
100.0
0.001
0.010
0.100
1.000
10.000
100.000
Velocity (mm/s)
% sediment slower than
225 mg/L
907 mg/L
1946 mg/L

603


604


605


606


607


608


609


610


611


612


613


614


615


616


617


618


619


620


621


622


623


624


28

Figure 3.

625


626


627


628


629


630


631


632


633


634


635


636