Do you really want to buy that beachfront house?

Recognize the different types of coasts Primary Coasts vs. Secondary coasts

Describe the processes that generate secondary coasts Beach anatomy Seasonal changes Longshore transport

How do the structures we build change the shoreline

This week we will study the types of coasts and beaches and the natural processes that create and maintain them

Here today, gone tomorrow: sediment transport and beach evolution

The shore takes on many forms along the WA coast and Puget Sound

Weather, waves and gravity continue to shape the shorelines

Beaches are dynamic

Anatomy of a beach Berm: ledge or terrace with flat tops

formed by wave-deposited material

Beach face: seaward slope, dependent on sediment type and wave energy

Troughs and bars run parallel to beach Change seasonally

Fig. 12.6

Seasonal Beach Changes

The small waves of summer move sand from offshore to the beach and form a summer berm

Summer Profile

The high waves and storms of winter erode sand from the beach and store it in offshore bars.

Winter conditions remove the summer berm, leaving only the winter berm on the beach.

Winter Profile

Fig. 12.19

Sands shift with the seasons

Most beaches in WA consist of a shallow covering of sand or gravel over rocks

Winter: storms remove most of the sand, exposing cobbles and underlying rock

Summer: sand returns

At South Whidbey State Park, sand bars cover the lower portion of the gravel beach. Sand moves with the seasons and storms, sometimes covering much of the beach, other times being drawn offshore

How do waves move sediments?

Wind = waves

Breaking wave = turbulence

Swash moves sediment onshore diagonally

Backwash moves sediment straight down beach

Swash + backwash = zigzag

http://www.youtube.com/watch?v=tN42EWWTP3w&feature=related

Fig. 12.20

How do waves move sediment? Winds blow, waves form

As a wave approaches shore, it feels the bottom (surf zone) speed changes, wave steepens and begins to break

The turbulence generate from the breaking wave tumbles beach material (beach absorbs some of the waves energy)

The “stronger” or more energetic a wave is, the more sediment it stirs up and tumbles

Waves moving toward shore transport sediment onshore (called onshore transport)

Wave angle results in the transport of sediment parallel to shore

Up-rush or swash from breaking waves moves sediment diagonally up and along beach in direction of longshore transport http://www.youtube.com/watch?v=U9EhVa4MmEs

Backwash moves sediment downslope

Sources & Sinks

Longshore transport is interrupted by headlands, inlets, and sharp bends in the shore contours and manmade obstacles

The shoreline is divided into "drift sectors,” each drift sector contains its own pick-up (source) and drop-off points (sinks)

Sources along the WA coast and Puget Sound include eroding bluffs, rivers, and streams

Sinks appear as spits, bars, tombolos, beaches (…can also be a source if supply is blocked or changes….)

Waves carry materials from eroding bluffs (source) and move them along the beach (drift path) to a spit, bar, tombolo, or beach (sink).

Sink or Source?

Source:

Sink:

Direction of drift or longshore transport:

bluffspit

Beaches are regions of coasts where sediments (usually sand or coarser particles) are accumulating.

The source of beach sediment is rivers and the erosion of bluffs and headlands.

Beaches are particularly dynamic environments, constantly changing in size and shape as a result of wave action and longshore transport.

Understanding Beach Evolution During the summer, waves tend to

be gentle and transport sediment landward. Beaches are at their widest during the summer.

During the winter, storms at sea produce larger waves that erode beaches and transport the sediment seaward.

Longshore transport also alters the shape and profile of a beach.

Where wave action is strong, sediment is eroded and transported.

Where wave action is reduced, sediment is deposited.

Wave action and longshore transport can erode beaches and be a hazard to boat harbors

People have taken measures to minimize those undesirable effects (with mixed results)

Human intervention in natural beach evolution

Build structures to stop beach erosion and stabilize beaches jetties, groins, seawalls

Build structures to protect harbors Breakwaters

Building dams

Beaches: River of Sand

To end