GEOS 110
NATURAL DISASTERS
 
Coastal Hazards-I
 
1. Wave Refraction and Longshore Drift
 - Waves striking coast at an angle cause sand to be transported parallel to beach.   Zig-zag pattern of transport. The net transport is called longshore drift;   the current that carries the sand is called longshore current.
 
2. Concept of Sand Budgets
  - Inputs = rivers, seacliff erosion, longshore drift from adjacent beaches.    - Ultimate output is the deeper offshore environment.
  - Any disruption of the system will have consequences. For example, damming     rivers will cause loss of sand supply and thus cause beach erosion.
 
 
3. Coastal Dynamics (delicate balance between sediment supply, wave erosion, and sea level)
 1) Winter/Summer Beaches
  - response to seasonal differences in wave energy
  - winter beach is much smaller, sand is stored in offshore sandbars
  - summer beach is wider, sand is stored on beach and foreshore      (shallowest water).
 
 2) Sea-Level Fluctuations
  - causes landward or seaward beach migration, particularly in barrier     island systems. Can be quite dramatic (lateral change ~1000x     vertical change in sea level).
Prof. Gorring         Apr. 23, 1998
GEOS 110
NATURAL DISASTERS
 
Coastal Hazards-II
 
Engineering Techniques (attempt to protect property and stabilize erosion)
Two main philosophies
 
  1) Hard Stabilization
   - Seawalls: placed parallel to coastline near top of beach
    Adv. - may protect property behind beach, but usually leads to beach erosion
    Disadv - wave energy is reflected downward, removes sand producing a narrower       beach, ugly, older broken seawalls leave debris on beach
   - Groins/Jetties: linear structures built perpendicular to beach, into the       longshore current.
    Adv. - used to trap sand in the longshore current in order to widen the beach or to      maintain an open inlet, also reduces wave energy striking beach
    Disadv. - causes deposition on updrift side and erosion on the downdrift side;       severely disturbs natural longshore transport of sand
 
  2) "Soft" Stabilization
   - Beach nourishment: artificial addition of sand onto beach
    Adv. - much more desirable (in theory); produces a wider beach with no       structures
    Disadv. - cost, sand may be eroded quickly; also environmental issues of       dredging in sensitive areas (ie. tidal lagoons, inlets)
 
   - Dune stabilization: artificial stabilization of dune systems; usually by       putting up fences, planting dune grass (etc.)
    Adv. - protects beach from large storms, stabilizes migration
    Disadv. - none really, but not long term solution if beach is naturally eroding