al-ansari and mcmanus, 1980)

8/19/2019 Al-Ansari and McManus, 1980).

1/9

A RE INVESTIGATION OF THE BATHYMETRY

f

LOCH EARN

N A

AL ANSARI d

J . Me

MAN

US

Dr McManus

is

Lecturer in Geology, University of Dundee, and

Mr

. AI-Ansari is from the

Department of

Geology, College

of

Science,

Uni

versity

of

Baghdad

Abstract: A newly prepared bathymetric chart

of

Loch Earn in the Grampian highlands

of

Scotland is

pr

esented.

The

continuously re

co

rded echograms reveal that the loch is

composed

of

several basins arranged in steplike progression shallowing eastwards from a

greatest depth

of

87.2 m. Seismic reflection profiling reveals over 22 m

of

sediments lining

the floor above tills. t is suggested that the watcrbody occupies a depression which

survived due to the presence

of

a detached block

of

stagnant but thick ice during deglaciation,

while areas to the east and west were filled with sediments.

INTRODUCTION

The

many freshwater lochs

of

Scotland provided

the

basis for major

pioneer limnological investigations

around

the turn

of

the century. Although

the water

ma

sses

had

been sources

of

interest in connection with recreation

and fisheries for many years it was not until the late nineteenth century that

detailed scientific examinations were carried

out

in order

to

determine their

physical and chemical natures and their behavioural patterns. Systematic

study was largely precluded until the boundaries of the lochs had been

established. In surface plan the extent of most

of

the lochs had been

determined by the early work of the Ordnance Survey, so that the

po

sitions of

the

shorelines were well known before

allempts to

provide information on

the

bathymetry began.

The

lochs vary from

deep

bodies

of

water

of

large areal

extent to many smaller shallow lochans, some in the highlands, and others

occupying sites in lowland areas.

At

the outset of the surveys to

produce

charts

of

the lochs the survey methods in use in oceanographic, coastal

and

fluvial investigations were based

on

the use

of

the lead line coupled with

position fixing by sextant. Problems inherent in the use of the lead line

include maintaining the line in a vertical orientation so that the true depth

may

be

determined,

and

the recognition

of

the depth at which the lead

encounters

the

bed.

On

a firm bed the impact

of

the lead on the

bottom

is

readily distinguished,

but in lo

chs with soft b

ottoms

the lead may penetrate

the bed sediment before resistance is detected by the remotely positioned

surveyor,

so

that overestimation

of

the water depth is

com

mon . The surveys

were

made

from rowing boats along traverses between known end points

Murray and

Pullar (1903)

introdu

ced a lead-lining recorder, in which the

line

ran

through a winch system which recorded on dials the length of line

paid out This introduced a substantial saving in time, especially for

soundings

in

the many deep-water lochs.

Their

device was readily mounted in

rowing cobles which were available on most inland waters at that time.

Uniform spacing of soundings was achieved by sampling the depth after every

30 pulls by the oarsman. The location of the

so

undings was_determincd by

sextant sightings from buildings

and

other prominent marks

around

the lochs.

Some

of the

earliest surveys of the lochs were produced by

Grant

Wilson,

whose charts were published in the Scollish Geogrnpltical Mngnziue in 1888.

Later

systematic surveys by Sir John Murray

and

F. P. Pullar initially covered


2/9

U..4"11W

'

Fig.

-Paths of traverses

along

whicll

ecl ograms

were

obtained.

..

Fig. Bathymcuic

chan

of Loeb Earn. Contours are given in metres.

I

U Wli

Lonf TW

0

l

§

:::

>

:

;

>

0

>

N

z

'


3/9

B THYMETRY

O LOCH

E RN

1 7

a few lochs each year,

but

following the tragic

and

heroic death of the latter,

the financing

of

the work was

put on

a more secure footing,

and

over one

hundred

lochs were surveyed

each

year for several years.

The

accounts

of the

loch surveys also appeared in the

Scottish eographic lMagazine

from

19 3

onwards, and ultimately the charts were gathered together

to

appear in the

series

of

six volumes by

Murray and

Pullar issued from the Challenger Office

in Edinburgh in 1910.

Since

that

time few attempts have been

made

to

re-survey the loch basins,

and the remarkable collection of bathymetric charts has remained largely

unchecked

and

unchallenged, despite the considerable advances in surveying

techniques which have occurred

in

the last 75 years. During this period the

sounding wire has been replaced by electronic echo-sounding devices which

work

on

a range

of

frequencies,

and

which

produce

continuous records

of the

depths beneath the moving research vessel.

The

availability

of

such equipment

means that

it

is no longer necessary

for

the

boat to be

held stationary

during

sounding,

and

so motorised propulsion is used. Position

i ~ t i n g

by means

of

double sextant readings is still widely used,

but

the electronic range- range

position fixing

apparatus

using

VHF

radio waves provides simpler

and

more

precise location, often

to

within 2 metres in accuracy, albeit at substantially

greater cost. Since the completion

of the Murray and

Pullar surveys many

lochs have been re-examined individually, e.g. Loch Leven (Kirby, 1971)

and

Stormont

Loch {Fraser

and

Williams, 1969, personal communication) . While

both of

these lochs

are

essentially shallow,

the rather

simple methods used

in

the surveys showed little siltation, using comparison of

spot

height data

obtained.

In

the deeper water

of

Loch Leven the accretion was insignificant

but

in the very shallow Stormont Loch siltation coupled with increased

colonisation by vegetation

had

led

to

a substantial reduction in the surface

area

occupied by water.

In the last five years an examination of the behaviour of water in the

Tay

basin, involving principally gauging

and

sediment-movement studies, has led

to

an

interest

in

the lochs as natural reservoirs

in the

system. In consequence

surveys

of

several

of

the

major

lochs-have been undertaken, using modern

precision electronic-sounding devices and precision p o s t o

l ~ t i n g

apparatus.

The

first of these surveys has provided a reappraisal

of

Loch Earn which

had

hitherto been considered

to

consist

of

a single long

bath

-like depression with a

smooth

longitudinal profile

and

with steep lateral walls

of

uniform gradient.

Situated at 6

23 N

4 IO W, Loch Earn is one

of

the many lochs

of

the central

Grampians.

It has

a surface

area of

approximately 8 km

1

,

is 10 5

km in length

(running east- west), has

an

average

breadth

of

a little less than I km,

and

a

maximum breadth

of

1.24

km.

In

the

previous

major

survey

of

the bathymetry

Murray and

Pullar indicated

that

the loch

had the

form

of

a single basin

of

simple geometry, with a maximum depth of268 feet (87.5 m). It was bounded

to the

north and south

by steep sides, the west end shallowed steeply,

but

the

east

end

followed

an

apparently protracted rise towards St. FiJian s. Mainly

because

of

the simplicity

of

the plan

and bottom

configuration this body

of

water has been the subject

of

considerable mathematical interest in the

investigation

of

the seiche phenomenon.

The

calculated

and

observed periods

of

various harmonics failed

to

coincide, despite the apparently simple

geometry

of

the basin (Chrystal

and

Wedderburn, 1905; Chrystal, 1909;

Wedderburn, 1913; Wedderburn

and

Young, 1915).


4/9

1 8

SCOTTISH GIJOGRAPIIICAL MAGAZ Nil

NEW SURVEY OF

LOCH

E RN

In the preparation of the new chart of Loch Earn a total of

6

traverses

(Fig. I) were made

of

the loch during September 1973, using R.V. Mysella,

a motorised dory from the Tay Estuary Research Centre

of

the University

of

Dundee. Traverses were run

at

a constant speed of knots

and

continuous


5/9

BATHYMETRY OF LOCH EARN

109

traces obtained u ~ i n a Kelvin Hughes MS 26 Hydrographic echo-sounder.

The traverses were made between known start and end points, with inter

mediate position fixing using sextants in

the

horizontal mode after the fashion

of

a pelorus. The positions of all fix points were marked

on

the trace

electronically. Water surface level during

the

survey was constant

at

96.8 m

above Ordnance Datum, and depths were determined below this level. Points

of equal depth below datum were marked on the echograms at

10

metre

intervals and transferred

to

the plan

of the

loch on a scale

of I :

10,000,

and

based

on

existing Ordnance Survey maps. From the series

of

points so

obtained contour plots were constructed, yielding the bathymetric chart

presented in Figure 2.

The most outstanding feature of the bathymetry is the continuity of the flat

basin floors, a feature demonstrated by Murray

and

Pullar (op. cit) but

strikingly evident from the echograms (Fig. 3). The northern and southern

margins are steep, with varying gradients and ledges breaking up the slopes.

Some of

the

ledges appear

to

be continuous round the northern flank of the

loch

at

5

10

, and

18m

below the surface. The slopes vary

in

gradient,

but

are

normally steeper than I in 50, and off the Beich Burn they are commonly as

steep as I in 2. The steepest slopes are often associated with delta-mouth

areas, and gentler slopes occur away from sources of influent sediments.

The main basin lies in the western section of the loch and reaches a

maximum recorded depth of 87.2 m near the centre about I km south-east of

the

Beich Burn delta . A second large basin occurs further east and its flat

floor is at about 72 m below datum in the area 2 km west of the Glentarken

Burn delta. Further subsidiary basins, also with flat floors, occur at pro

gressively higher levels eastward, and these are separated by ridges, some

symmetrical and some strongly asymmetrical, frequently curving in plan,

and

standing

up to

10 m above the upper basin floor (Fig. 4 . The sill and tread

pattern steps the floor upwards

to

about 6 m south of Glentarken Burn, to

35

m south of Alit an Fhionn, and to about

2 m

ofT St. FiJians. Although

more gentle and regular in profile, without marked ridges disrupting the

upward slope, the western rise also contains small basins west of the Burn of

Ample. The western end

of

the loch shallows fairly steadily over a distance

of

2.5 km towards

the

rivers entering from Glen Ogle and the Kendrum Burn.

Deltas of several rivers e.g., Ample Burn, Beich Burn, and Glentarken Burn,

form positive features which cause radical deflections in the plan

of the

coastline, but yet produce relatively minor modifications to the general

bathymetry. Within 50 m

of

the shore at the head of a delta there is

frequently over 30 m

of

water present.

Towards the bed

of

the loch in summer and autumn the suspended

sediment concentrations rise substantially and the SCUBA diver experiences

considerabledifficulty in distinguishing the bed from waters with high sediment

content. Indeed, so little strength does the bed material possess that

on

many

occasions it is possible for the diver

to

fin completely into

the

bed. This coultl

raise major problems when attempts are made

to

compare the old and new

charts

or

the loch. The presence

of

a fluid mud layer ofover I metre thickness

presents a problem in the definition of the bed or the loch, and offers no

obvious reflective surrace

to

the signals of the echo-sounder used during the

bathymetric surveys. Nevertheless, there exists

at

some depth a layer with

sufficient rigidity

to

have been identified consistently by the lead-lining survey


6/9

110

SCOTTISH GEOGRAPIIJCAL MAGAZINE

1 1

p ' ,

;·

w

·

f

il

i r

I

\

. .. . ·

I . . . 41 ..A

N

Fig.

4-{a)

Part of the echogram

(MN)

obtained along the centre

of Loch

Earn running

eastwa1ds from l

oc

hearnhead (left) towards St. Fillans. Note the regular steep initial slope

in the west, the hummocky central section

and

· the very flat profile of the main basin flo

or

.

(b) Part of the echogram (PQ) obtained along the centre of Loch Earn running east towards

St

. Fillans (right).

of Murray and Pullar (op. cit.), and also to enable release of the trigger

mechanism

of

a Van Veen sediment sampler grab. Precisely what this layer

represents is not clear but in the absence

of

any higher bed with resistance it is

taken to define the

solid

floor of the l

oc

h. Using the echo-sounder and

leadline sounding simult

aneo

usly it was established

that

bolh techniqu

es

seem

to identify the same surface.

During the bathymetric surveying samples

of

sediments from the floor

of

the loch were collected using a small modified VanVeen grab. These revealed

that

whilst coarse sed

im

ents

are

restricted to the shallow nearshore regions,

fine sediments

co

ver the major

part of

the loch floor. A thin strip

of

gravels

associated with sand runs parallel

to

the shore of the loch. Beyond this is a •

slightly broader belt

of

medium to fine sand associated with silt, and the rest of

the basin is

fl

oored with layered silts, as is normal in most large lakes. Near

the mouths

of

influent streams sediments of a wide range of grain sizes are

deposited, and marginal deltas have been built

up

by sands and gravels. The

sediments, which are largely of ve ry fine particles of quartz, feldspar,

muscovite and c

hl

orite of local origin, contain much diatom debris. No clay

minerals have been identified from the floor of this lake. Seismic reflection

pr

ofiling with a Giffi-ORE pinger system revealed that in the central bas

in at


7/9

BATHYMETRY

OF LOCI

EARN

l l

least

two

phases of deposition

of

layered sediments followed accumulation of

the glacial moraines which mantle the bed rock beneath the loch. The

greatest thickness oflayered silts observed on 25 t r a v e r s e ~

w ~

22 metres. On

the steep basin sides the silts

are

rarely more than 2- 3 m e t r e ~ thick but loca l

p

oc

kets

up

to 4 metres thick occur on some of the submerged ledges.

DISCUSSION

Comparison of the chart of Murray and

Pullar

(1910) wi th

that

presented

in this

paper

reveals ve ry strong overall

s i m i l a r i t i e ~

There

arc

differences,

most

of

which may be attributed large

ly

to the availability of continuously

recorded echograms for the survey traverses. Differences between the two

techniques may lead to differences in the estimated position of the bed.

Nevertheless, with the exception

of

recognising the eastern and their

intervening lips there is very good agreement between the surveys. Since a

period of over 70 years had elapsed between the surveys some evidence of

si ltation might be detected in suitable sites. Some min

or

accretion is certainly

present

at

the mouths

of

delta-forming streams, but elsewhere the changes

are

so slight that they a re recognised only with difficulty.

The

body

of

standing water serves

to entrap

sediment in transit through

the river system. In studies of the River Earn catchment Al Ansa ri and

McManus (1975) have demonstrated that bed-load transport accounts for

3- 5% of the total solids load passing through the system. Above the

uppermost gauging station, at Kinkell, the average annu:tl sediment yield was

determined as 180 tonnes per km

1

.

This estimate is a minimal value, for

it

takes no account of the fact that 127 km

1

of the catchment above that point

(of a total area of 591 km') drains directly into Loch Earn. Assuming that

this

rate is

valid for the area above the loch a total

of2286

tonnes should

enter

the loch basin each year, having a

dry

volume

of

approximately 860 m

3

assuming a density

of

quartz.

If

the entire sediment load

we

re to be trapped

within the 10 km" surface area loch, a maximitl accretion

rate of

0.086 mm p..a.

would be achitved if the material were to be uniformly distributed throughout

the basin floor. Thus in 70 years since the Murray and Pullar survey accretion

of

at most 6 em might be expected over the entire floor.

Seveml of the assumptions used here may be quest io

ned,

for the deposited

sediment would not be dry and therefore would occupy initially a far greater

volume than

that of

dry sediment. Permitting the settled sediment to

h:t

ve

initially

half

the dry weight density would give

double

the volume on the loch

floor, so that the estimate might legitim:ttely be raised to

12

em of average

thickness increase. In add ition to the inorganic m:tterial on the bed some

of

the sediments i n c o r p o r ~ t e cons

id

er

ab

le

quantities of

o r g ~ n i c

m:t ter der ived

from the woods beside the loch as leaves, twigs, etc. and also as diatoms,

which locally account for over

5

of the sediment. These further components

are liable to s

uffer

from chemical decomposition after deposition, but

also

serve to enh:tnce the possib

le

loch floor accretion rates. The deposi

ti

on of

perhaps 2 em of sediment on the bed of the l

oc

h is close to the limit of

detection using such widely different techniques as on the two surveys

concerned.

The generally close similarities of the charts pr

od

uced leaves the modern

surveyors with a profound admir:ttion for the work of their predecessors, but

the impo

rtant

discovery that Loch E:trn does not occupy one single simple


8/9

112 SCOTIJSII GI OGRAPJIICAL MAGAZINI

basin, but a series

of

flat-floored steep-sided basins progressively stepping

upwards in an easterly direction throws light on hitherto unsuspected short

comings of the field

area

selected for investigation

of

the seiche

phenomenon.

Seiches are oscillations

of

the surface level

due

to the passage ofan

atmo

spheric

front or the sudden cessation

of

a wind which has piled up water at

one end

of

the loch. Their characteristics depend on the geometry of the basin.

GEOMORPHOLOGICAL MODEL

The rock basin now occupied by Loch Earn was deepened during the

several Pleistocene glaciations experienced by the highlands. Its position

below the confluence of

the

Ogle

and

Kendrum Burns,

and

between confining

walls

of

relatively strong metamorphic rocks to

north and

south, lends support

to

the

idea

of

its development

and

excavation

y

dynamic ice (Sissons, 1967).

Overdeepening

due

to scouring by the ice below the confluence may indeed

have led to the creation of the hollow and repetition of scouring

during

repeated glaciations would have led to .accentuation of the feature once

formed. The great depths

of

the

main basin may be justified in these terms,

and

a progressive shallowing eastwards in the general direction

of

flow

of

ice

confined within the valley

may be

expected.

However, detailed examination

of the echograms has revealed the presence

of low ridges separating relatively flat-floored basins at progressively higher

levels eastward, and such a configuration is not as readily explained purely in

terms of the mechanisms

of

scour. Likewise, the very survival

of

the major

water-bearing basins which have not undergone total infilling by sediments

during ice retreat poses problems. In areas in which steady ice retreat occurs

the backfilling of valley floors with sand and gravel of fluvioglacial origin,

often to very considerable depths,

is

normal.

In

alpine valleys infill by over

100 metres

of

unconsolidated drift material is

common.

In

the

Scottish

Highland valleys few deep boreholes or deep seismic surveys of the fill have yet

been accomplished,

but

it is evident from preliminary findings

that

very

great

thicknesses

of fill are

present to the east

of

Loch Tay.

The

eastern approaches

to Loch

Earn are studded with ro hes mouto1mees and rock exposures occur

intermillently along the present river bed, so there

is no

question

of

a now

Jilled basin occurring at this point. Rather, the features of the floor

and

the

presence

of

water in the basin today must

be

explained in more local terms.

t is

suggested

that

during deglaciation the basin floor was occupied by

the

lowermost reaches of the glacier. As retreat and surface wasting occurred, so

the easternmost extremity

of

the ice migrated westward across the basin floor.

Meltwaters carried

sand

and

gravel sediments to mantle the

ground

moraines

on

the newly exposed basin floor

to the

east. This basin floor may have been

flooded,

and

early lake sediments accumulated

in

this area. In front

of

the ice

margin a recessional moraine mark

ed the western limit

of the

lake basin floor.

Around the margins of the glacier lateral moraines and kame terraces

developed, marking the recessional phases further west in the ice-occupied

sector

of

the basin. t is suggested

that

repeated stages

of

ice-retreat,

the

formation

of

further fluvio-glacial deposits overlain by lake-basin sediments

and bounded to both east and west by recessional moraines may. account for

the stepped topography of the present lake floor. The lake-margin ledges

so

common in the western end, but rarely observed in the east, are interpreted as

ice-marginal depositional features. Shallow seismic profiling

Pinger

surveys


9/9

BATHYMETRY

OF

LOCH EARN

113

show that many of these

are

underlain by bouldery material such as typifies

many tills or morainic deposits. Finally it is suggested that the lake basin

remained open largely as a result

of

the survival

of

the snout of the glacier

within a deep hollow, even after its nourishment from higher parts

of

the

catchment had ceased. Buried by a veneer of moraine, such stagnant ice may

survive for many years. The presence

of

the block occupying the basin floor

ensured that sediments laid down during the final removal

of

the valley glacier

failed to accumulate in the hollow, so that when the final

_melting

occurred

the kettle Jake occupying the extensive rock basin became a relatively

permanent feature of the landscape.

AI-Ansari, N A. (1976), Sediments and sediment transport in the River Earn, Scotland.

Ph.D. thesis (unpubl.), University of Dundee.

AI-Ansari, N. A. and McManus,

J.

(1975), Calculation

of

sediment discharge in the River

Earn, Scotland. Proc. 9th Internal. Congress on Sedimentology, Nice. Vol. 3, pp. l21-129.

Kirby, R P. (1971), The bathymetrical resurvey

of

Loch Leven, Kinross.

Geog. Jnl.

137,

372-8.

Chrystal, G (1909), An investigation

of

the seiches

of

Loch Earn

by

the Scottish Lake

Survey.

Trans. Roy. Soc. Edinb.

46,

455-517.

Chrystal, G and Wedderburn,

E.

M. (1905), Calculation

of

the periods and nodes

of

lakes

Earn and

Treig, from the bathymetric

data of

Scottish Lake Survey.

Trans. Roy. Soc.

Edinb.

41, 823-50.

Grant-Wilson,

J

S. (1888) A bathymetrical survey

of

the chief Perthshire lochs

and

their

relation

to

the glaciation of that district.

Scott. Geogr. Mag.

4, 251-8.

Murray, Sir J. and Pullar, L. F. (1903), Bathymetrical survey

of

the Freshwater Lochs

of

Scotland.

Scott. Geogr. ag .

19, 449-79.

Murray, Sir

J and

Pullar, L.

F

(1910),

Bathymetrical Survey o f the Scottish Freshwater

Lochs.

Challenger Office, Edinburgh, 6 vols.

Sissons,

J B.

(1967),

The Evolution of Scotland s Scenery.

Oliver Boyd, Edinburgh,

259 p.

Wedderburn, E. M. (1913), Temperature observations in Loch Earn with further contri

butions

to the

hydro-dynamical theory

of

the temperature seiches.

Trans. Roy. Soc.

Edinb.

48, 629-

95

.

Wedderburn, E. M.

and

Young, A. W. (1915), Temperature observations in Loch Earn.

Part

11 Trans. Roy. Soc. Edinb.

50, 741-67.

CHURCIDLL TRA YELLING FELLOWSHIPS

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100 awards are made annually

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cover all expenses

up to

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to

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ca

tegories

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1981

awards

(made in January) send name and address on a postcard between August and mid-October

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15

Queen s Gate Terrace, London SW7 5PR

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