Coastal Landforms, Materials, and

Processes

MODULE 1

First of three (3) modules

By Sagip Baybay PH

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

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

p r o c e s s e s . U n d e r s t a n d i n g w h e r e s e d i m e n t s c o m e f r o m a n d h o w

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

m a n a g e m e n t . "

IntroductionCoastal erosion is a growing concern in many coastal areas worldwide especiallyon island systems and archipelagic countries like the Philippines. It affects about70% of the world’s sandy beaches, which comprise a mere 20% of the earth’scoastline (Bird, 1985). Coastal erosion is prevalent in many areas in thePhilippines. About half of the 625-km coastal segments studied so far hasexperienced erosion since the 1970s, including many highly developed andpopulated coastal areas (e.g., Manila Bay, Davao City) as well as tourism sites(e.g., Boracay Island, Bantayan Island). With sea level rising at about 1 cm/y inthe Philippines, erosion is expected to worsen in retreating coasts and affectpresently accreting or stable coasts. Thus, coastal erosion poses a serious threatto the 60% of Philippine population that live near the coast. The rapid loss ofcoastal lands can displace coastal residents, undermine vital coastalinfrastructures and facilities, and disrupt livelihood and other economicactivities along the coast.   Despite the prevalence and severity of coastal erosion in many places in thePhilippines, it is not yet recognized as a threat to coastal communities at thenational level. Part of the reason is the lack of awareness about this hazard.Often, the community becomes aware only when lives and properties becomeendangered as coastal erosion occurs abruptly after strong typhoons orworsens over time. Thus, most of the interventions addressing this hazard aread hoc, oftentimes a response to storm events, and implemented on acommunity level.. A common solution is the placement of seawalls to protectthe affected community. But seawalls can worsen, rather than solve, theproblem of coastal erosion by promoting the loss of beach sediments. Bydisrupting the sediment transport, addressing coastal erosion by buildingdefenses such as seawall has led to chronic erosion in many places.   To better understand the problem of coastal erosion, this three (3)-moduleseries will first introduce coastal processes, landforms and materials (Module 1),followed by current trends and causes of coastal erosion in the Philippines(Module 2), and different solutions to address this problem (Module 3). Module 1 presents the local coastal landscapes, beach materials, and processesthat affect coastal stability.   

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The Philippine archipelago and its origin

The Philippine archipelago is composed of 7,107 (NAMRIA’s latest count is 7,641)islands with a total land area of about 300,000 km2 and an aggregate coastlinelength of 36,289 km. Sixty four (64) out of 79 provinces, more than half of themunicipalities (822 of 1502), and 25 major cities are considered coastal. Thisconstitutes about 60% of the nation’s 2005 population of 87.8 million(WorldBank, 2005).

The archipelagic nature and landscapes of the Philippines result mainly from itsactive tectonic setting. T Three large structures are associated with thearchipelago, namely: (a) the discontinuous Manila-Negros-Sulu-Cotabato Trenchon the west, (2) the East Luzon Trough and Philippine Trench on the east, and (3)the 1200-km long strike-slip Philippine fault cutting from Northern Luzon toMindanao (Allen, 1962). These three structures, along with numerous smalleractive faults, are responsible for high seismic activity of the Philippine archipelago.

(Credits: Mary Rose Gabuyo &  Keanu Jershon Sarmiento)

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What are the different coastal landscapes in the Philippines?

Owing to its geologic history, the Philippine archipelago has a variety of coastallandscapes. The islands are generally hilly or mountainous which are typicallydissected by short steep rivers with associated small fan-deltas along the coast.Additionally, volcanoes abound in the country and contribute to its mountainouslandscape.

Rocky cliffed coastlines are abundant with pocket beaches in small embayments(A, B). Occasionally, the steep coastline is broken by a relatively broad coastalplain associated with a delta or a strandplain where beaches can be extensiveand well-developed (C). Muddy coastlines (D) are more common in embaymentswithin the internal seas. These are associated with mangroves but numerousmuddy coastlines have already  been converted to fishponds.

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Coral reefs, mangroves, and seagrass meadows abound in the Philippines.These three (3) ecosystems are typically associated with each other.  Most of thelarge islands are lined with fringing reefs except in areas close to river dischargepoints. Mangroves and seagrasses are also found in muddy coastlines. Manysmall islands have uplifted reefal limestone edifices or are low-lying sandy cays.Thus, a large proportion of Philippine beaches have materials derived directlyfrom the adjacent coral reef. 

Among the different coastal landforms, beaches, which are composed of loosesediments, are most susceptible to erosional processes. Such coastal landformis discussed further in the succeeding sections. 

Seagrass beds occupy  a large portion of the reef flat in Caramoan and Samalislands. Mangrove stands front the adjacent sandy beaches.   

http://www.reefbase.org/gis_maps/gallery.aspx

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A beach is a landform built by the accumulation of wave-deposited sedimentssuch as sand, gravel, pebbles, cobbles, or a combination of these, and fragmentsof littoral materials (e.g., corals, shells, foraminifera, or marine plants) on thecoast. It is formed where there is an excess in sediment supply.

The cross-section of a beach, known as the beach profile, shows distinct zonesshaped by different processes, namely: (a) backshore, (b) foreshore, and (c)nearshore/offshore (Komar, 1976).

The backshore is the zone extending from the high-tide line to the landward limitof a beach such as vegetation line, dune field, or sea cliff. It includes the berm, anearly horizontal to landward sloping portion of the beach formed by thedeposition of sediment by waves (Komar, 1976). A beach can have more thanone (1) berm; storm waves form the higher berm referred to as a storm berm. Incontrast, deposition of sediments by waves and tides during normal conditionsforms the lower berm called the fair-weather berm.     Located below the fair-weather berm crest is the inter-tidal zone, or the areabetween the high-tide and low-tide lines, which delineates the foreshore. Acharacteristic feature of this zone is the beach face, a seaward sloping regionformed by the swash and backwash of waves; hence, the name swash zone. 

What is a beach?

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The angle of the beach face is a function of wave parameters and grain size (e.g.,Komar, 1976; Pethick, 1984). Gently sloping beaches are typically associated withfine-grained sediments and high-energy waves. In contrast, steeply slopingbeaches are associated with coarse-grained sediments and low-energy waves.

The nearshore is the zone seaward of the low-tide line, extending all the way tothe shoreface, a depth at which there is little movement of sediments. A narrowstrip from the waterline up to the breaker zone is the surf zone, where longshorecurrents are generated and transport large amount of sediments along thecoast. Where the waves break is the breaker zone. The shoreface marks theboundary between the nearshore and offshore. The latter is no longerconsidered part of the beach profile. 

Beach sediments are derived from a wide variety of sources including adjacentwatershed, nearby cliffs, offshore sources (i.e. coral reefs), and a combination ofthese as shown below:

Sediments originating from land are produced by processes  such as weatheringand soil erosion, landslides, and volcanic eruptions. These are transported byrivers to the coast. Beach sediments reflect the characteristics of the parent orsource rock. Sediments may also come from cliff or bluff erosion. Repeatedpounding of waves may cause a portion of a cliff or bluff to collapse, breaking itdown into smaller pieces and transported by waves and currents onto nearbybeaches.

Where do beach sediments come from?

(a) River-derived sediments known as terrigenous, siliclastics orlithics

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Sediments may come from deposits in the littoral depths, or from biogenicprocesses. Siliciclastic materials can come from deposits within theshoreface (e.g., longshore bar) that can be transported onshore by waves.‘Biogenic’ sediments are particles produced by living organisms. These aremostly coral reef-derived materials and are known as carbonates since theyare composed of calcium carbonate (CaCO3). Some examples are coralfragments, flakes of calcareous algae Halimeda and foraminifera.

Carbonate sediments Coral reef-derived sediments (a) are generally lighter in color and are typically composed of the following:foraminifera (b), Halimeda flakes (c), coral fragments (d), mollusk shells (e), echinoderm spines (f), tube coralfragments (g) and fine grains (h) which are most likely bits from coralline algae and spines. Scale bar = 0.5 cm

(b) Offshore sources

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Beach sediments can also be a combination of siliciclastic and carbonatematerials. Immediate sources can be sandflats or mudflats where mollusks areabundant (e.g., bayhead of San Miguel Bay in Camarines Sur) or where a thrivingseagrass community is present. Mixed sediments are also common in areaswhere coral reefs and rivers co-exist within the same coastal cell (e.g., Malay,Aklan as seen in the photo below).  

The color of beach sediments is determined by its source rock. Siliciclastics(river-derived) are typically darker than carbonate (coral-reef derived) materials.Siliciclastics  are mostly derived from volcanic rocks having hues of gray, brown,and black. Moreover, degree of coloration reflects the abundance of certainminerals (i.e. darker sediments contain higher levels of iron or chromium). Theblack sands in Camiguin de Babuyanes Island, for example, are iron rich; hence,are called magnetite sands. Some siliciclastics are light-colored due to theabundance of quartz and plagioclase crystals; giving the sediments a glassyappearance.  Beaches fed by rivers draining Mt. Pinatubo such as in Cabangan,Zambales have glassy sediments.

Why do beach sediments have different colors?

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The following are examples of river-fed beaches or beaches with siliciclasticsediments (left), and the common types of sediments (middle and right panels).

Carbonates are typically light-colored materials. Instead of glassy sediments,carbonates are composed of coral fragments and reef-associated organismssuch as molluscs, echinoderm, foraminifera and Halimeda flakes. The individualcomponents reflect the biogenic sources of beach materials. The abundance ofcoral fragments in beach sediments, for example, suggests the presence ofbranching corals in the nearby offshore area.  Pinkish sands are made up offragments of the red organ pipe coral, Tubipora. 

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Beach sediments have different grain sizes depending on their proximity to itssource and exposure to waves. There is greater concentration of large clastsnear river mouths, while it becomes finer as one moves farther away from theriver. Additionally, sediment sizes are proportional to wave energy: coarse-grained (fine-grained) beaches with steep (gentle) slopes are usually associatedwith high-energy (low-energy) setting. Thus, sediment sizes are important whenconsidering beach nourishment-- artificial replacement of sediments on aneroding beach.

The following are examples of carbonate beaches (left), types (middle)  and theirimmediate source of  sediments (right) in the offshore area: 

Why do beach sediments have different sizes?

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How do materials reach and get distributed along the coast?

Sediments from land are delivered by rivers to the coast. In places with nearbysand dunes, wind (aeolian) is an important transport agent (Figure 5). On reef-fringed coasts, reef sediments are moved onto the shore via onshore-offshoretransport. From the offshore reef area, carbonate sediments are carried andredistributed by currents and waves onto the shore.

Where waves push sea water at an angle onto the shore, the currents tend torun along (parallel) to the shore. These wave-generated longshore currents thenredistribute both land- and reef-derived materials to other coastal segments.Longshore currents follow the direction of wave approach, or the angle createdby waves as they reach the shoreline.

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What are some indicators of longshore currents?

The direction of longshore currents depends on the direction of the winds. Sinceprevailing winds can change directions, then the longshore currents directionsalong the coast may also change. However, within an annual cycle, there can bea certain direction that can predominate. The predominant longshore driftresults in the formation of coastal features such as spit,and tombolos. It mayalso result in asymmetrical deltas.

Longshore drift may form spits across river mouths or downdrift of a delta oracross embayments. A spit is a narrow,linear, fingerlike landform built throughsediment deposition that extends out tosea. The spit elongates in the direction ofthe net longshore drift. 

In places where there is a nearby island,waves refract around the island,generating bi-directional longshorecurrents. This movement sweepssediments together from both sides andeventually forms a tombolo, sedimentdeposits that connect an island to themainland.

The asymmetry of river deltas may alsopoint to the predominant direction oflongshore currents. Greater sedimentaccumulation south of the Bauang andAringay rivers is due to greater delivery ofsediments to the south by  longshorecurrents.  

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Tombolo

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How important are longshore currents for maintaining

sediment supply along the coast?

A coastline may have several coastal compartmentswhere sediment source transport and sinks areconfined. A coastal compartment may consist of alinear beach bounded by rivers or cliffs, and an embayedbeach or a pocket beach bounded by cliffs. Within acompartment, longshore currents are responsible fortransporting sediments to different coastal segments.Smaller littoral cells may form in coastal compartments.Such cells are defined by uninterrupted longshoretransport.  

How important are longshore

currents for maintaining sediment

supply along the coast?

Interruption of longshore currents candisrupt the transport of sediments ,which in turn, triggers erosion in coastalsegments  starved of sediments. Suchinterruption is commonly observed inareas with solid-based perpendicularstructures such as piers and groins.While accretion (or widening of beaches)occur updrift of these structures, theyblock the passage of sediments causingerosion of the downdrift segment. Forthis reason, hard protection structuressuch as groins may not be the bestsolution in addressing coastal erosion. 

Longshore transport

Sediments are trapped in the updrift side of arubble groin in Samal Island in Davao Gulf,causing the opposite segment to erode,where the tree line is almost at the waterline.

Samal Island, Davao

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Images were downloadedfrom Google Earth

Solid-based piers, 150-m long,in La Union and BantayanIsland in Cebu blocks thepredominant longshore drift to the south (yellow arrow).These piers trap sedimentsupdrift, or near the source ofsediments, of the pier, causingthe shoreline downdrfit of thepier to retreat by several tensof meters. This resulted inshoreline offset of more than100m on the opposite sides ofthe piers. 

How can interruption in alongshore transport be prevented?

In building piers or ports, one may consider  having these structures perched onstilts or piles. The placement of groin, as a protection strategy, will be discussedin more detail in Module 3 of this series.

Note that along linear coastlines, the influence of longshore currents may gobeyond political boundaries. In such cases, management plans shouldincorporate the natural coastal divisions defined by littoral cells instead of theadministrative boundaries of local government units (LGUs). Cooperation andclose coordination among stakeholders and LGUs  will ensure that activities orprojects in each municipality will not harm other municipalities located within thesame littoral cell. In other words, this calls for a regional approach to coastalmanagement, which considers the connectivity of coastal processes within alittoral cell. 

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For additional information, read theaccompanying book Coastal ErosionManagement in the Philippines: A Guidebook Room 318, Marine Science Institute University of the Philippines, Diliman, QuezonCity 1101

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