The Dynamic Shoreline

1. The Dynamic Shoreline • The land/sea interface is a dynamic environment. Physical and geologic processes shape the sedimentary deposits of beaches, barrier islands, estuaries and deltas. In this chapter we will learn about: • Coastal water movement • Beaches • Barrier islands • Storm effects • Estuaries • Deltas • The impact of people on the coastline We will first consider coastal water movement.

2. Coastal Water Movement • This is how the shoaling of the coastline looks to a wave. • Most waves encounter the shoreline at an angle and are refracted.

3. Coastal Water Movement • Wave refraction is easy to see: notice the curved waves entering this cove:

4. Coastal Water Movement • This refraction focuses energy on headlands and disperses energy in coves:

5. Coastal Water Movement • Here’s how this works; watch the coast become straight:

6. Coastal Water Movement • This coast is being smoothed: • promontories are being eroded • coves are being filled in

7. Coastal Water Movement • Longshore currentsflow parallel to the shoreline. • Often caused by waves striking the beach at an angle. • Strength depends on the angle of wave approach

8. Coastal Water Movement • Where waves are higher, water piles up and flows longshore in the direction of “lower water” • Longshore currents converge result in water being forced away from the shoreline forming a rip current.

9. Beaches • Any time you see sand, think change: SAND MOVES • The reason sand exists is because it is moving enough to prevent the growth of vegetation. • The amount of sand at a given location can be described by a budget. • Sand sources = River input, Sea cliff erosion,Longshore and onshore sand transport • Sand losses = Longshore and offshore transport, Wind erosion - =

10. Beaches • Beaches are dynamic environments that expand and contract depending upon wave conditions. Typically sand is moved shoreward during the summer and seaward during the winter.

11. Barrier Islands • Barrier islands are large deposits of sand that are separated from the mainland by semi-isolated bodies of water e.g. lagoons, bays, and estuaries. They are common along the Atlantic and Gulf coasts of the United States.

12. Barrier Island Origins • Origin of barrier islands: • Submergence of sand ridges during the Holocene rise in sea level.

13. Barrier Islands • Origin of barrier islands: • From the longshore extension of sand spits that eventually become fragmented.

14. Barrier Islands • Origin of barrier islands.: • From the vertical enlargement of longshore bars.

15. Storm Effects • Storms cause a variety of effects which impact shorelines • Surgesare extreme high water events caused by high winds that pile water up along the shoreline.

16. Storm Effects • Storm surges lead to flooding of low lying coastal areas and contribute to the overwash of barrier islands.

17. Storm Effects • Here are before and after pictures • Hurricane Georges (October 1998) removed most of this island.

18. Estuaries • Estuaries are semi-enclosed bodies of water where a river meets the sea. They are distinct from lagoons which do not receive inflow from rivers. • Estuaries are protected from the dynamic ocean and thus provide good natural harbors. They are highly productive areas and serve as important nursery grounds for fish and other marine organisms.

19. Estuaries (Cont.) Origin of estuaries. Some are drowned river valleysthat have formed due to sea level rise.

20. Estuaries • The Chesapeake is this nation’s “classic” estuary • input from several rivers • definite salinity and habitat gradients • important fisheries • population centers

21. Deltas • Deltas form where the sediment input from rivers exceeds the sediment removed by waves and tidal currents.

22. DELTAS • this condition prevents the formation of deltas • as sediments are deposited at river mouths, the rising sea level keeps them submerged. • the only way you can form a delta is to have MASSIVE sediment input! The Mississippi River has it! That’s why we have a delta

23. Impact of People on the Coastline • Shorelines are dynamic environments that change due to natural events which occur on daily, seasonal, annual, and even greater time scales.

24. Impact of People on the Coastline • Humans have tried to modify the coastal zone to make it more stable and thus suitable for building homes, hotels, etc. Methods of stabilization include: • Jetties • Groins • Breakwaters • Seawalls • Beach nourishment

25. Impact of People on the Coastline • Jetties are built to diminish sediment deposition at the mouths of harbors, inlets, etc. They prevent dispersal of sand across the mouth. They cause erosion on the downdrift side.

26. Impact of People on the Coastline (Cont.) • This is because the flow of sand has been cut off • this region has lots of sand • this region is starved for sand Normal flow of sand

27. Impact of People on the Coastline (Cont.) • Groins are built on eroding beaches to trap sand and to promote accretion. Like jetties they also cause erosion on the downdrift side.

28. Impact of People on the Coastline (Cont.) • These groins are doing their job, but look at the consequences! Serious erosion Flow of sand Accumulation

29. Impact of People on the Coastline (Cont.) • Seawalls are built along the shore to protect beaches, roads, etc. from erosion by storms. • However, they increase turbulence and promote erosion at their base. • Seawalls and beaches don’t mix.

30. But when we build on the beach, they are often a necessity! • Look at what happened without them in these cases: “Move it or lose it!”

31. Beach nourishmentis the process of adding sand to beaches.

32. This is only a temporary solution because sand will always be removed by waves.

33. Money spent on beach nourishment must be spent again and again and again…….

34. Summary • The coastal zone is a dynamic environment that results from a variety of natural processes. • It is an extremely important region economically as evidenced by the fact that a majority of the human population lives within 50 miles of the coast. • Unfortunately, human perturbations to the coastal zone are increasing and interfering with the natural processes that are so important in shaping this environment.