Geomorphology of Irish Estuaries: Inherited and Dynamic Controls

INTRODUCTION

The geomorphology of estuaries is known to vary spatially inresponse to a combination of inherited factors (geologicalsetting and bedrock framework) and process factors(contemporary dynamics. A number of regional syntheses ofestuary morphology have provided insights into the relativeimportance of various controlling factors (e.g. ,1977, ., 1989, and , 1988;

, 2003).In this paper, the geomorphology of Irish estuaries is assessed

and a preliminary categorization is presented that is basedprimarily on genetic factors (mode of origin) and secondarily oncontemporary dynamics. The Irish (Republic of Ireland andNorthern Ireland) coast, while small in extent, is highly variablein terms of inherited factors and contemporary dynamics andoffers an excellent opportunity to investigate the roles of bothsets of factors on estuary morphology. The aim is (a) to describethe spatial variability in potential estuary controlling factors andcontemporary dynamics, (b) describe the range of estuarinemorphology and (c) present a provisional classification basedon geomorphic parameters.

Against a highly variable geological and dynamicbackground, an estimated 400 transitional water bodies ofvarious sizes exist around the Irish coast. The coastal hinterlandis low-lying and humid. While there is perennial stream flow,the volume of sediment that is carried, is low because of thehinterland topography and vegetation cover. Marine factorsthat influence estuary morphology are highly variable, withstrong gradients in wave energy and tidal range around the Irishcoast. The entire island may be considered paraglacial and theinherited elements of the glacial landscape constrain thephysical setting of estuaries.

LANKFORD

DUFFY HUME HERDENDORF

COOPER

et al

ENVIRONMENTAL SETTING

Inherited Factors

The whole island comprises a bedrock-framed coast that lackscoastal plains. Estuaries occur in bedrock valleys, which formre-entrants on the coastline and in which coastal sedimentsoccasionally accumulate as barriers.

The island of Ireland has a 3000 km coast located between 50

and 55 N (Figure1). The entire island was affected bysuccessive glaciations, although the main Midlandian(=Wisconsinan) glaciation glaciation and ice limits formedduring deglaciation are the main constraints on contemporarycoastal geomorphology. The main ice limits (Figure 2) areimportant in coastal geomorphology in that they are associatedwith major sediment sources. Bedrock across the island hasbeen sculpted by glacial processes although fjards are bestdeveloped in the north and west of the island. Lough Swilly, forexample, has a 130m-deep bedrock valley, of which about100m is filled by glacial sediments. Glacial diamict wasdeposited on bedrock veneers according to sediment supplyduring glacial periods. In some areas, a paucity of sediment hasleft a sculpted bedrock surface while in others, glacial depositsprovide soft, coastal lithologies, vulnerable to erosion at presentsea level.

The distribution and variable sediment composition ofbeaches and barriers may be explained in large part by glacialinheritance. Areas located close to ice limits tend to have sandybarriers, the sand having been deposited initially by outwashand subsequently reworked by marine processes. Depositionalareas distant from ice margins are instead characterised bycoarser sediments (gravel and cobbles) derived largely fromreworking by wave action of glacial diamicts. Sediment supplyat the Irish coast occurs predominantly from reworking of shelfsands (themselves of glacial origin), locally from erosion ofbluffs of glacial sediments, and occasionally fromcontemporary fluvial sediment supply, although the latter ismainly confined to the steep NE coastal hinterland. Coastalsediment supply is thus strongly related to patterns of icemovement, stabilisation and decay during the last glacial cycle.

Contemporary rates of sea level change are equally variable.Tide guage observations in the north suggest a near stable orslightly falling sea level over the past 50 years, while in thesouth sea level appears to be rising at c3.0 mm/yr over the sameperiod.

One consequence of highstand shorelines along the northcoast is the presence of highstand deltas, which providepotential sediment supply to rivers in the northeast. Anadditional consequence of this sea level history is the depth to

0

which river channels were incised during the Holocene

Journal of Coastal Research SI 39 176 - 180 ICS 2004 (Proceedings) Brazil ISSN 0749-0208

J.A.GCooper

Coastal Research GroupUniversity of UlsterColeraine BT52 1SANorthern IrelandUnited Kingdom

COOPER, J.A.G., 2006. Geomorphology of Irishe estuaries: inherited and dynamic controls. Journal of CoastalResearch, SI 39 (Proceedings of the 8th International Coastal Symposium), 176 - 180. Itajaí, SC, Brazil, ISSN 0749-0208.

The Irish (Republic of Ireland and Northern Ireland) coast contains a diverse range of estuary forms that havedeveloped in response to a combination of contemporary dynamic conditions and inherited factors. Most of thecoastline is bedrock-framed and estuaries typically occur in bedrock-confined valleys. The effects of the lastglaciation, however, largely control the relative sea-level history of the island. Thus the depths of bedrock valleysare substantially greater in the south, where sea levels fell to at least 60m below present levels compared to 30mbelow present in the north. Further inherited control is imparted through the distribution of glacially-derivedsediment on the coast and adjacent shelf. Major sediment sources for contemporary estuarine barriers coincide withknown glacial limits and retreat phase stabilisation. Contemporary dynamics vary substantially around the coastwith the main contrast being between the exposedAtlantic western coast and sheltered eastern, Irish Sea coast. Tidalrange varies around the island from micro- to macrotidal and this too exerts a strong control on estuary morphology.This paper presents a threefold classification of estuary type in Ireland (drowned river valley estuary, river-dominated estuary and bar-built estuary) and attributes the variation in morphology primarily to inherited controlsrelated to the island's paraglacial setting.

ADDITIONALINDEX WORDS: Paraglacial coast, barrier, lagoon.

ABSTRACT

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lowstand. Much deeper incision is expected in the south thanthe north. Against this, valleys throughout the island werescoured by ice and contain overdeepened channels.

The main process variables identified as affecting estuariesinclude tidal range, wave energy and river discharge( ., 1992; , 1993). Around the Irishcoast there is marked variation in tidal range. The west coastcomprises meso- to macrotidal environments, while the eastcoast ranges from microtidal to macrotidal settings. It is swell-dominated with modally high wave and wind energy andcharacteristically dissipative sand beaches, or more reflectivegravel systems. Large sand dunes characterize many beachsystems and these appear to be related to major deglacialdepocentres. Superimposed on this variability is a strong east-west gradient in wave energy (Fig.3). The west coast is exposedto high-energy swell waves while the east coast is dominatedby locally generated waves produced within the Irish Sea. Theeast coast is thus more prone to energetic short period waves andlongshore sediment transport than the west coast whererefraction is typically near complete as swell waves approach

the shoreline.River discharge is an important variable in estuary dynamics.

In Ireland most estuaries are of comparatively small catchmentsize because of the small drainage area of the island as a whole.

Most estuaries are fed by rivers with <200km catchment areas.

Only a few rivers have catchment areas greater than 1000 km .By far the largest catchment is that of the Shannon (>11,000

km ). The large rivers drain low-lying areas and supply littlecoarse-grained sediment to the coast. High vegetation in thehinterland cover also reduces the suspended sediment load

This group of estuaries may be subdivided into those thathave barriers and those that lack barriers. The barrier in suchcases is not essential to the existence of the estuary, but simplyreflects the availability of non-cohesive sediments in thecontemporary coastal zone. Examples of such estuariesinclude the estuaries of Lough Foyle ( and ,2002), SW Donegal ( and , 1998), andWexford Harbour 1988) (Fig.4A). Common to all ofthese estuaries is a local supply of relict sediment for barrier

Dynamic Variables

Irish Estuary Types

Drowned Valley Estuaries (Fjards, RiverValleys)

Barred Drowned Valley Estuaries

DALRYMPLE COOPER

COOPER GAULT

BURNINGHAM COOPER

(ORFORD,

et al

2

2

2

Within this highly variable environmental context a range ofestuary types may be identified on the basis of contemporarygeomorphology and consideration of the contemporarygeomorphology in the context of inherited and processvariables. Here the term 'estuary' is considered in its broadestcontext as a transitional fluvio-marine coastal water body.

A threefold classification based initially on genetic (mode oforigin) characteristics is presented here. The inherited factorsare considered to be the dominant geomorphological control asthey set the physical limits within which contemporaryprocesses operate.

Valleys, both of glacial and fluvial origin, have beeninundated during the Holocene rise in sea level to form estuarybasins. In terms of contemporary geomorphology the nature ofthe drowned valley has limited significance. The drowning wasaccompanied by and succeeded by sedimentation that acts toreduce estuary volume. Because typical fluvial sedimentationrates are low, most of these estuaries are still infilling. Typicalestuarine circulation patterns exist in these estuaries and a rangeof salinity is noted.

Geomorphology of Irish Estuaries

Figure.1 Map of Ireland showing place names and estuariesreferred to in the text.

Figure 2. Main ice limits in Ireland (after and ,2003) at c.21k yr BP and at c.14k yr BP and relative sea levelcurves for the north and south Irish coast (based on

. 1988, and 2003 and ., 2002).

MCCABE CLARKE

TAYLOR

MCCABE CLARK, COOPER

etal et al

Figure 3. Tidal range and 1:50 yr wave heights around the Irishcoast.

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construction. All of these examples are

nsported to act as the main infilling mechanism for theseestuaries ( , 1999), coast, theAntrim Glens.

Within this genetic characterisation s

al flats in its upper reaches, developed throughdeposition of fluvial sediments. Its lack of a barrier relates to alack of local sediment supply as much as the strength of tidaland fluvial currents which prohibits barrier development onmacrotidal estuaries elsewhere.

Common to all of these estuaries is a substrate dominated bymud or bedrock. Contemporary geomorphological variationswithin this group arise from the volume of fine-grainedsediment deposited. Belfast Lough is rich in fine sediment dueto a relatively large inflowing river and potential anthropogenicenhancement of the sediment supply ( ., 1997).Strangford Lough is sediment-poor due to the small catchmentsof inflowing rivers.

r-built estuaries. Typically, sediment sources areeroding glacial bluffs ( ., 1984) which yield coarseclasts (gravel). Such enclosed water systems have smallvolumes and are not associated with major terrestrial drainagesystems. (1984) studied several such lagoons inSE Ireland and observed two main forms of interaction betweenmarine and terrestrial waters. Many systems lacked an outletand communicated via seepage andsufficient tidal prism to enable semi-permanent inlet/outlets tobe maintained.

Several examples exist with inlets. Of these, Tramore inCounty Waterford is a good example. Here a barrier encloses a

adjacent to sites of icemargin stabilisation followed by outwash, which providedabundant sandy sediment that was reworked during theHolocene to form estuary barriers. The early formation ofbarriers is inferred to have caused constricted inlets with flood-tidal dominance and through which shelf sands weretra

ome variability can beseen that is related to contemporary dynamics. Ebb-tidal deltasare well developed in Castlemaine harbour, Lough Foyle, andthe SW Donegal estuaries, despite high wave energy. Thissuggests that the tidal currents generated by large tidal prisms

produce semi-fixed ebb-deltas. At Wexford Harbour, an ebb-delta is present but is less well developed as a result of vigorouswave reworking. Flood-tidal deltas are not easily discernible inthe SW Donegal, and Foyle examples where the estuarinebasins have been largely infilled by flood-tidal deposition thuslimiting accommodation space for distinctive delta features. InWexford and Castlemaine, flood-deltas attain typicalmorphologies with a flood ramp, ebb shield and spillover lobes.This development is probably linked to lower sedimentationrates in the back-barrier.

The antecedent morphology of the drowned river valleyexerts constraints on the contemporary morphology in that (i)less sediment is needed for barrier development if the valley isnarrow, and (ii) those estuaries developed in wide valleys havelarge internal fetch distances in which estuary waves maymodify the shoreline. The latter phenomenon is welldemonstrated in Lough Foyle where extensive intertidal flatshave developed through erosion and planation on the downwindmargin of the estuary.

A large number of valleys that were drowned during theHolocene persist as elongate inlets more or less dominated byseawater. They lack barriers and have fully marine salinity. Inthe upper reaches of some, freshwater discharge reducessalinity and deposits deltaic sediments, which may formintertidal flats. Examples include Belfast Lough, StrangfordLough, Killary Harbour and Cork Harbour (Fig.4B). Theseexamples exhibit marked morphological differences as a resultof their mode of origin (fjards are common in the north whilerias characterise the south of the island) and surroundinggeology but they have in common, their origin as drownedvalleys. None has a barrier and they are dominated by seawater.In the upper reaches of Belfast and Strangford Loughs are minordeltaic deposits from the inflowing rivers. Low fluvial andmarine sedimentation rates render these valleys still in animmature stage of development. The largest estuary in Ireland,the Shannon, also falls in this category. It too has extensiveintertid

Several estuaries have developed in low-lying coastal areaswhere inflowing rivers are small or non-existent. Theseestuaries exist because of the local availability of sediment fromwhich barriers have been constructed (Fig.4C). They may thusbe termed ba

overwash, while others had

BURNINGHAM

ORFORD

CARTER

CARTER

Non-barred Drowned Valley Estuaries

Bar-Built Estuary/Lagoon

Barriers With Inlets

et al

et al

et al.

Cooper

Figure 4. A. Drowned valley estuary with barrier, WexfordHarbour; B. Drowned valley estuary without barrier, BelfastLough; C. Bar-built estuary/lagoon, Lady's Island andTacumshin; D. River-dominated estuaries of the northeast.

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shallow embayment located between two bedrockpromontories. The size of the enclosed embayment produces asufficient tidal prism to maintain an inlet in the barrier, and asmall ebb-tidal delta is present. A similar system exists inBallyteige Bay while the adjacent lagoon at Tacumshin,fluctuates between seepage and channelised water exchange.Bar-built estuaries with surface channels typically exhibit smallebb-tidal deltas and limited or no flood-tidal deltas as a result ofthe small tidal prism and limited accommodation space forflood-tidal deposition.

Several small enclosed systems exist in which the barrierlacks a natural inlet. These systems facilitate exchange of watervia percolation through the barriers ( ., 1984). Thebarriers of the south Wexford coast originate through erosion ofadjacent glacial bluffs and the high porosity afforded by theirgravelly texture assists in freshwater exchange in a seawarddirection. Similar barrier-enclosed systems are developed atseveral other localities on the south coast including BrandonBay, and the west coast north of Killary Harbour (Delaney andDevoy, 1995). These systems are dominated by freshwaterseepage and barrier overwash and are probably best termedlagoons as they lack typical estuarine salinity patterns.

At Lady's Island Lake, (1997) noted that such lagoonsundergo wide fluctuations in salinity related to barrieroverwashing and freshwater discharge volumes. Freshwaterdischarge in winter caused water levels to rise by over 2m andwater levels in the enclosed lagoons were perched severalmetres above sea level.

In the northeast of Ireland in the Antrim Glens, several smallrivers discharge across a steep, uplifted hinterland on whichhigh-level, coarse-grained fluvio-glacial deltas are present(Fig.4D). They thus deliver appreciable quantities of sand andgravel to the coast, which is reflected in the coarse-grainedchannel sediment and associated coastal barriers and beaches.These systems discharge fluvial sediment directly into the seaand have minimal tidal prisms as a result of fluvial infilling.

This preliminary assessment of estuary geomorphology inIreland reveals the presence of several distinctive types ofcontemporary estuary, differentiated in terms of their mode oforigin and contemporary morphodynamics. The dominantcontrols on contemporary morphology appear to be inheritedfeatures related to the nature of the original valley, and thenature of sediment supply. Barriers across drowned valleyestuaries are developed only where relict glacial sediment isavailable in the immediate vicinity. Barrier development setsup tidal asymmetry in the inlet and may enable estuarineinfilling from marine sources. Where sediment is abundant onthe shelf, such estuaries are largely infilled and contain broadintertidal flats. In areas of lower sediment abundance,distinctive flood-tidal deltas may be distinguished. Drownedvalleys in sediment-poor locations produce essentially marineembayments within which reduced wave energy may permitsuspension settling of fine sediments from inflowing rivers.Both types of estuary appear to operate as sediment sinks in thecoastal sediment budget.

Barrier-built estuaries occupy pre-existing topographic lowsthat may or may not be related to fluvial or glacial incision.Their presence is related to the availability of sediment ofglacial origin in most instances, and a coastal re-entrant inwhich barrier sediment may accumulate. The contemporarygeomorphology and hydrology of these systems is a function ofthe relationship between seepage through the barrier,freshwater discharge and frequency of marine overwashoverwashing and lack of return flow also mean that thesesystems act as net sediment sinks river-dominated estuaries are

distinctive forms that have developed through fluvial infillingof their valleys and which now act as sediment sources for theadjacent coast. The frontal limits of the resulting channeldeposits are reworked to form estuary mouth barriers. Thebeaches of the northeast coast owe their existence to this fluvialsediment supply.

There is a generalised geographical distribution of estuarytype that relates largely to inherited, glacial characteristics(Fig.5). In the northeast, river-dominated estuaries owe theirexistence to isostatic uplift which gave rise to shallow estuarinevalleys, and provided both a ready sediment source (fluvio-glacial deltas) and steep rivers in which the sediment could betransported to the adjacent coast.

Barred, drowned valley estuaries are located close to majorice limits that occurred during the deglacial phase. At suchlocations outwash was responsible for depositing the now relictsand and gravel in the vicinity of contemporary barred estuarymouths. On the northwest coast a major ice limit at c14k yr BPproduced high volumes of sand and gravel outwash from theadjacent crystalline basement outcrops. Further south,temporary stillstands in the retreating ice front produced similarconcentrations of outwash. A north-south increase in the depthof the incised valleys may be responsible for the relativeimmaturity of development of barred estuaries southwards inthat greater volumes of sediment are required to infill the largervolume drowned valleys.

Drowned valleys distant from ice margins have remained asmarine indentations in which contemporary fluvialsedimentation may produce bayhead deltas and tidal flatsand, if wave action is reduced, fine-grained sedimentation inthe drowned valley. Tidal asymmetry is less developed thanin barred estuaries. Bar-built estuaries tend to be small involume and rely on a local supply of erodible glacial diamictthat typically supplies downdrift barriers with necessarysediment.In all cases, inherited factors are a dominant control incontemporary estuary morphology. Patterns of uplift have beenimportant in their control on high-level sediment sources in thenortheast and in locating relict glacial outwash in the Holoceneactive wave zone. The influence of relative sea level history hasalso determined the depth of Holocene valley incision.

Contemporary dynamics exert a secondary variability on thispattern, which is best investigated by comparison of geneticallysimilar estuaries in contrasting dynamic zones. Clearly furthersubdivision of estuary type is possible and the physico-chemical characteristics of these estuary types remain to beinvestigated. This paper, however, presents a preliminarygeomorphological classification in which the influence ofInherited factors can be seen to be dominant over contemporaryprocesses in controlling contemporary estuary morphology.

Closed/Percolation Lagoon

River-dominated Estuaries

CARTER

HEALY

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DISCUSSION

Figure 5. Gross distribution of estuary types around the Irishcoast.

Geomorphology of Irish Estuaries

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