Landforms of Wave Deposition – Wave Erosion and Deposition (Senior 6 Geography) | Ganzaa

Wave Erosion and Deposition

Lessons

Senior 6 · Geography

Wave Erosion and Deposition

5 lessons

Lesson 3 of 5

Landforms of Wave Deposition

25 min

A harbour made of sand

Namibia has more than 1,500 km of Atlantic coastline, much of it open beach battered by heavy surf. Yet the country's main port, Walvis Bay, has calm water, because a long finger of sand called Pelican Point curls round the bay and shelters it from the ocean swell. Pelican Point is a spit, built grain by grain by waves carrying sand northwards along the coast, and it gives Namibia its only natural deep-water harbour. Depositional landforms like this are among the most useful features of any coast, and also among the most changeable. This lesson explains how beaches, spits, bars and tombolos form, and why people must manage them with care.

Definition

Beach: an accumulation of sand, shingle or pebbles on the shore, between the lowest tides and the highest point reached by storm waves.

Berm: a low ridge on a beach built at the limit of the swash, often one for each recent high tide. Storm ridge: the highest ridge on a beach, of the coarsest material, thrown up by storm waves.

Sediment cell: a stretch of coast, usually between two large headlands, within which sediment moves around but which gains or loses little sediment to the next cell. The sediment budget is the balance between the sediment entering and leaving it.

Beaches

Waves deposit where they lose energy, so beaches form where sediment is plentiful (from rivers, eroding cliffs and the seabed) and where constructive waves dominate, as in sheltered bays. The beach material decides the slope:

Shingle beaches are steep. Shingle is coarse and permeable, so much of the backwash soaks into the beach instead of running back down it. The weakened backwash cannot drag the pebbles back, and they pile up at the top of the swash.
Sand beaches are gentle. Sand is fine and less permeable, so the backwash runs back over the surface and carries sand down the slope.

A typical beach profile, from the land, has a storm ridge of large pebbles at the back, one or more berms marking recent high tides, and, on wide sandy beaches, ridges and runnels parallel to the shore on the lower beach. Where waves arrive parallel to the shore, as in a bay, the beach is swash-aligned and sediment moves mainly up and down it; where they arrive at an angle, the beach is drift-aligned and sediment moves along it by longshore drift. On Lake Kivu, the sandy beaches at Rubavu are swash-aligned beaches of this kind, built by the lake's own wind waves.

Worked example 1: a sediment budget

Problem. A sediment cell (illustrative figures) receives each year 45,000 m³ of sediment from eroding cliffs, 15,000 m³ from a river and 5,000 m³ washed in from the seabed. It loses 52,000 m³ to longshore drift leaving the cell, 10,000 m³ to deep water offshore and 8,000 m³ to sand mining. (a) Find the annual budget. (b) If the loss is spread over a beach 4 km long and 60 m wide, by how much does the beach surface fall each year? (c) Evaluate a ban on sand mining.

Solution. (a) Inputs $= 45\,000 + 15\,000 + 5\,000 = 65\,000\,\mathrm{m^3}$ ; outputs $= 52\,000 + 10\,000 + 8\,000 = 70\,000\,\mathrm{m^3}$ . Budget $= 65\,000 - 70\,000 = -5\,000\,\mathrm{m^3\,yr^{-1}}$ : a deficit, so the beach is shrinking.

(b) Beach area $= 4000 \times 60 = 240\,000\,\mathrm{m^2}$ .

\text{lowering} = \frac{5000\,\mathrm{m^3}}{240\,000\,\mathrm{m^2}} = 0.021\,\mathrm{m\,yr^{-1}} \approx 2.1\,\mathrm{cm\,yr^{-1}}

(c) Without mining the budget becomes $65\,000 - 62\,000 = +3\,000\,\mathrm{m^3\,yr^{-1}}$ , a small surplus, so the beach would slowly build up instead of lowering. The ban tackles the only output people control directly, but the surplus is small: if a dam upstream cut the river's 15,000 m³, the cell would be in deficit again.

Definition

Spit: a long, narrow ridge of sand or shingle joined to the land at one end and ending in open water at the other, built by longshore drift where the coast changes direction. A spit whose end curves landward is a recurved spit.

Salt marsh: an area of mud colonised by salt-tolerant plants, which forms in sheltered, shallow water such as behind a spit.

Spits

Figure 8: the formation of a spit across an estuary.

Drift. Waves driven by the prevailing wind carry sand and shingle along the coast by longshore drift.
A change of direction. Where the coast turns inland, for example at a river estuary, the drift does not turn with it. It carries on in its old direction into the deeper, calmer water across the estuary mouth, loses energy and deposits its load.
Growth. Over years the deposits build up above sea level into a ridge that extends further and further across the estuary.
Recurving. Waves refracted round the end of the spit, and storms from a secondary direction, curve its tip landward. As the spit grows, each old tip is left behind as a recurved ridge.
Behind the spit, the sheltered water traps fine mud brought by the river and the tide, and plants colonise it to form a salt marsh.
The limit. The spit cannot cross the estuary completely while the river's flow, and the tide in and out, keep a channel scoured open.

Spurn, at the mouth of the Humber estuary in England, is a well-studied example: a sand spit about 5 km long. After a tidal surge in December 2013 broke through its narrow neck, the sea now covers that neck at high tide, so Spurn has become a tidal island. Spits are not permanent: they are built, breached and rebuilt as the balance between drift and storms changes.

Spurn, at the mouth of the Humber estuary, England, photographed from the air from the south in November 1979. The narrow ridge of sand leaves the mainland at the top and curves round to a hooked tip at the bottom, with waves breaking along its seaward side on the right and the calm estuary on the left. Photo: Stanley Howe, CC BY-SA 2.0, via Wikimedia Commons.

Worked example 2: will the spit close the estuary?

Problem. Old maps show that a spit has grown 1.1 km across an estuary in 220 years (illustrative figures). The estuary mouth is 1.8 km wide. At the same rate, how long would the spit take to reach the far bank? Is it likely to?

Solution.

\text{rate} = \frac{1100\,\mathrm{m}}{220\,\mathrm{yr}} = 5.0\,\mathrm{m\,yr^{-1}}, \qquad \text{time} = \frac{1800 - 1100}{5.0} = \frac{700}{5.0} = 140\,\mathrm{yr}

It is unlikely to close. As the gap narrows, the river and the tidal currents are squeezed through a smaller opening and flow faster, scouring the channel and carrying sediment away. Growth slows as the gap closes, and a channel stays open unless the river is very small.

Definition

Bar (bay bar): a ridge of sand or shingle that has grown right across a bay or river mouth, joining two headlands. Lagoon: the body of shallow, calm water cut off behind a bar.

Tombolo: a ridge of sand or shingle joining an island to the mainland. Cuspate foreland: a triangular projection of the coast, built where two sets of drift meet.

Barrier beach (barrier island when broken into islands): a long ridge of sand or shingle running parallel to the coast, separated from it by a lagoon or marsh.

Bars, tombolos and other features

Figure 9: (A) a bay bar and its lagoon; (B) a tombolo.

A bar forms when a spit grows right across a bay where no strong river flows to keep a gap open, sealing off a lagoon. Slapton Sands in Devon, England, is a shingle bar that separates the freshwater lake of Slapton Ley from the sea. Along the coast of Ghana, Keta Lagoon is separated from the Gulf of Guinea by a narrow sandbar. Some bars began as offshore ridges of sediment that were pushed landward as sea level rose after the last ice age.

A tombolo forms where an island lies close to the shore. Waves bend round both sides of the island, lose energy in its shelter and deposit sand behind it, until a ridge connects the island to the mainland. The ridge joining St Ninian's Isle to Mainland Shetland, Scotland, is about 500 m long and is the largest tombolo in the United Kingdom. Not every ridge that reaches an island is a tombolo, however: Chesil Beach in Dorset is often called one, but research shows it is a barrier beach that has been pushed landward.

Where two drifts from different directions meet, they may build a triangular cuspate foreland, such as the shingle headland of Dungeness in Kent, England.

The tombolo of St Ninian's Isle, Shetland, Scotland: a ridge of pale sand, with sea on both sides, runs from the dunes in the foreground towards the island in the distance. Photo: Richard Webb, CC BY-SA 2.0, via Wikimedia Commons.

Depositional landforms and people

Landform	Opportunities	Problems
Beaches	Tourism and recreation, fish landing sites, sand for building; a beach absorbs wave energy and is the cheapest natural sea defence	Sand mining and coastal building can starve a beach; erosion then threatens roads and houses behind it
Spits and bars	Sheltered harbours (Walvis Bay); calm lagoons for fishing, salt making and water birds	Drift may block a harbour or river mouth and must be dredged; a bar can trap floodwater behind it; lagoons are easily polluted
Salt marshes and mudflats	Nurseries for fish, feeding grounds for birds, store of carbon, natural flood buffer	Reclaimed for farming, ports or housing, losing those benefits
Tombolos	Road or footpath access to the island	Can be cut off at high tide or breached by storms

Because every depositional landform depends on a supply of sediment, actions far away can destroy it. The Aswan High Dam on the Nile traps silt that once reached the river's mouth, and the dam is blamed for erosion of the Nile Delta coastline. A harbour wall or a set of groynes that interrupts longshore drift builds the beach on its updrift side and starves the beaches downdrift. Good coastal management therefore plans for a whole sediment cell, not for one beach at a time.

Warning

Common mistakes

Drawing longshore drift the wrong way along a spit. The spit grows in the direction of the drift, away from where it joins the land.
Saying the spit's end curves because of the river. The recurve comes from waves refracted round the tip and from secondary winds; the river keeps the channel open.
Calling any ridge to an island a tombolo. Check how it formed: Chesil Beach is a barrier beach.

Tip

Exam technique. For any depositional landform, name three things: the source of the sediment, the process that moves it (usually longshore drift) and the reason the waves lose energy at that place (a change in coastline direction, shelter, deeper water). Draw a plan view with the prevailing wind, the drift arrows and a north point.

Geographer Online: how coastal spits are formed, with a labelled diagram. Watch for the change in coastline direction and the recurved end, and compare them with Figure 8.

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Practice

Define a spit, a bar and a tombolo.
Explain why shingle beaches are steeper than sand beaches.
Distinguish between swash-aligned and drift-aligned beaches.
A sediment cell gains 30,000 m³ a year and loses 34,500 m³ a year. Its beach is 3 km long and 50 m wide. Calculate the budget and the mean lowering of the beach each year, in centimetres.
With the help of a labelled plan diagram, explain how a recurved spit forms across an estuary.
Explain why a bar can form across a bay but a spit across a large river's estuary usually leaves a channel open.
Explain how a dam built far inland can cause erosion of a beach on the coast.
"Depositional landforms bring more benefits than problems to coastal communities." Evaluate this view, with examples.

Show answersHide answers

Spit: a ridge of sand or shingle joined to the land at one end, ending in open water. Bar: a ridge that has grown right across a bay, joining two headlands and enclosing a lagoon. Tombolo: a ridge joining an island to the mainland.
Shingle is permeable, so the backwash soaks in and is too weak to drag pebbles back; sand is less permeable, so the backwash flows over the surface and carries sand down.
Swash-aligned: waves arrive parallel to the shore and move sediment up and down the beach. Drift-aligned: waves arrive at an angle and move sediment along the shore.
Budget $= 30\,000 - 34\,500 = -4\,500\,\mathrm{m^3\,yr^{-1}}$ . Area $= 3000 \times 50 = 150\,000\,\mathrm{m^2}$ . Lowering $= \frac{4500}{150\,000} = 0.030\,\mathrm{m} = 3.0\,\mathrm{cm}$ a year.
Drift carries sediment along the coast; where the coast turns inland at the estuary, deposition continues across the mouth in calmer, deeper water; the ridge grows; refracted waves and secondary winds curve the tip landward; salt marsh forms behind (diagram as Figure 8).
In a bay with no strong river, nothing removes the sediment at the end of the spit, so it can reach the far headland. In an estuary, river flow and tidal currents speed up as the gap narrows and scour a channel.
The reservoir traps the sediment the river used to carry to the sea, so the coast's sediment budget falls into deficit and waves erode the beach (as on the Nile Delta after the Aswan High Dam).
Benefits: sheltered harbours such as Walvis Bay, tourist beaches, lagoon fisheries, salt marsh habitats, natural sea defence. Problems: shifting bars block harbours and need dredging, breaches cut access (Spurn), lagoons pollute easily, and the landforms are fragile when sediment supply falls. A balanced answer judges that the benefits are large but last only if the sediment cell is managed as a whole.

Summary

Waves deposit where they lose energy and sediment is plentiful; shingle beaches are steep and sand beaches gentle because of how the backwash behaves.
Spits grow by longshore drift where the coast turns; their tips recurve; a river or tide keeps the channel open.
Bars close bays to form lagoons; tombolos join islands to the mainland; cuspate forelands form where drifts meet.
Every depositional landform depends on the sediment budget, so dams, sand mining and harbour walls can destroy it.

Sources and credits

Walvis Bay, Spurn, St Ninian's Isle, Slapton Ley, Chesil Beach, Keta Lagoon, Aswan Dam, Dungeness (headland) and Gisenyi (for the beaches at Rubavu): Wikipedia contributors, CC BY-SA 4.0, adapted by Ganzaa.

Photos: Spurn in 1979 (Stanley Howe, CC BY-SA 2.0); Slapton Sands From The Top Deck, Devon (Andrew, CC BY 2.0); Tombolo, St Ninian's Isle (Richard Webb, CC BY-SA 2.0); all via Wikimedia Commons.

Figures 8 and 9, the worked examples and the exercises were drawn and written by Ganzaa.

Video: "How Coastal Spits are formed - labelled diagram and explanation", Geographer Online, embedded from YouTube.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1A harbour made of sand

2Beaches

3Spits

4Bars, tombolos and other features

5Depositional landforms and people

A harbour made of sand

Beaches

Spits

Bars, tombolos and other features

Depositional landforms and people