4/20/17. #31 - Coastal Erosion. Coastal Erosion - Overview

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1 Writing Assignment Due Monday by 11:59 pm #31 - Coastal Erosion Beach front property! Great View! Buy now at a great price! See main class web pages for detailed instructions Essays will be submitted in Illinois Compass (instructions later) No copying: Compass will check to see if you have copied sentences or phrases from internet sources. You MUST write the paper yourself, without borrowing fragments of more than a few words Penalties for plagiarism can be severe Problem - Many nations in world have coastal zones; 30 of 50 states have coastlines along major water body (Atlantic and Pacific Oceans; Gulf of Mexico; Great Lakes. ~ 50% of US population lives in coastal counties, population density increasing rapidly Rapidly rising population density in US coastal counties Coastal development in Oahu, Hawaii Coastal Development - Miami, Beach, FL Coastal erosion = ongoing, gradually developed geologic hazard in US and world. In US, much of Atlantic and Gulf coasts are severely eroding and other coasts (Pacific and Great Lakes) are moderately eroding. 1

2 Consequences of coastal erosion = Housing developments lost to sea. Storms and hurricanes can cause great erosion at specific localities, but by themselves cannot account for continuous, large-scale erosion. Coastal Erosion Damage - Examples Before and after photos of damage from Hurricane Ivan (2004) in Pine Beach, AL Before and after photos of damage from Hurricane Fran (1996) in North Carolina s Outer Banks Before and after photos of damage from Hurricane Ivan (2004) in Orange Beach, AL Damage after Hurricane Hugo (1989) in Pawley s Island, SC Before Hurricane Andrew (1992) in Raccoon Island, LA 9 months later - Bigger and better? After Hurricane Andrew (1992) in Raccoon Island, LA 2

3 Cliff retreated by >50 m in 105 years, Moss Beach, San Mateo Co., CA Lake Michigan coastline During and after storm on Great Lakes in Michigan Outdoor Deck Now Open in Isla Vista, CA Pacifica, CA in March, 1998; debris from collapsed homes at base of cliff Pacifica, CA in Dec., 2003, only 2 of 10 homes from 1998 photo remain 3

4 Large-Scale Coastal Erosion - Causes 1) Global rise in sea level - sea level rose through last century by 2-3 mm per year due to global warming. Since 1880, sea level has risen by ~20 cm. Large-Scale Coastal Erosion - Causes 1) Global rise in sea level - Small rise in sea level can cause large change in position of coastlines for flat areas. Models predict much faster rates of rising sea level (3-5x increase) due to future global warming. Large-Scale Coastal Erosion - Causes 1) Global rise in sea level - Possibility of flooding of major cities throughout world; southeastern US is extremely vulnerable to future rise in sea level (shallow sloping coasts) Large-Scale Coastal Erosion - Causes (2) Dams (loss of sediment supply) - sediment for beach drift is supplied mainly by rivers that enter lake or ocean (and eroding cliffs). Dam construction traps sediment before it reaches coastline. Many US rivers have been dammed, therefore more sediment is supplied from eroding cliffs, resulting in greater coastal erosion Malibu Canyon Dam filled with sediment within 13 years of being built Prediction of Future Coastal Erosion Complex, because it involves: 1) prediction of future sea levels, 2) sediment transport processes 3) amount of sediment supply (rivers, sea cliffs); 4) nature of wave energy (predominant direction, size); 5) sediment loss (amount of sediment loss, directions of offshore currents). Mitigation of Coastal Erosion Accelerating coastal erosion has lead to many efforts (often very costly) to prevent erosion and save property. Many efforts fail or transfer problem to another location (interfere with dynamic coastal environment) 4

5 (1) Seawalls - onshore wall of concrete or rock debris parallel to coastline. Purpose = protect land behind wall from wave energy, but commonly fail because wave energy comes around sides and is reflected downwards, causing erosion. For long seawalls sediment supply is cut off. Photo examples #1, #2, #3, #4 Lost cause? Example = Galveston, TX - City developed on barrier island. In 1900 large tropical storm washed away 2/3 of city buildings, killed 6,000. Example = In 1902 Galveston built largest seawall ever on barrier island (16 km long, 6 m high, $15 million). Seawall protected city from later storms, but beach gradually eroded. Continued development in hazardous areas. Sand is continually trucked in to replenish this beach Same story for seawall (5-6 m high, 8 km long) along Atlantic coast at Sea Bright, NJ (built to protect railroad and town). Mitigation of Coastal Erosion - Seawall Residents of Daufuskie Island, SC sued state to build seawall to protect their property (seawalls banned in SC). State basically won. 5

6 Mitigation of Coastal Erosion - Groin (2) Groin - wall of concrete, rock, wood, or sandbags built perpendicular to beach to trap moving sand and widen beach. Groins usually successful on upcurrent side, but sediment supply for downcurrent side of beach is cut off and erosion occurs there Clicker Question: Which direction is longshore sediment transport? A: Toward us B: Away from us. C. Neither Map View Started here on 4/14 (3) Jetties - pair of long groins that keep harbor channel from filling up with sediment Clicker question: How will creation of the Jetties show below effect beach growth and/or erosion? A. The beach to the left of the Jetties and the beach to the right side will both gain sand and grow. B. Both beaches will lose sand and erode (shrink) over time. C. The beach to the left will erode (shrink), while the beach to the right will grow. D. The beach to the right will erode (shrink), while the beach to the left will grow. Mitigation of Coastal Erosion - Jetties (3) Jetties - pair of long groins that protect harbor channel from sedimentation. Because of size, jetties completely cut off longshore sediment transport, problem of upcurrent deposition and downcurrent erosion much greater than for groins 6

7 Predominant direction of longshore current? right Where is the erosion problem? Mitigation of Coastal Erosion - Jetties Example = Ocean City, MD, located on highly developed barrier island next to jettyprotected harbor. Since jetty construction in 1930s, barrier island on downcurrent side (with little development) has retreated landward by ~500 m. Mitigation of Coastal Erosion - Jetties Example = Ocean City, MD Mitigation of Coastal Erosion - Breakwater (4) Breakwater - offshore wall parallel to coast, to absorb wave energy and provide quiet water for harbors or widen beach, photo Jetty at Ocean City, MD Problem = quiet water means deposition occurs behind endless cycle of dredging of harbor or sediment bypass begins; breakwaters widen beaches behind them but get erosion on downcurrent side 7

8 Mitigation of Coastal Erosion - Breakwater Mitigation of Coastal Erosion - Breakwater Mitigation of Coastal Erosion - Beach Nourishment (5) Beach nourishment - bring sediment from offshore (relatively cheap but commonly too fine-grained, quickly erodes away) or from other land location (expensive). Mitigation of Coastal Erosion - Beach Nourishment Examples of failed beach replenishment programs: Ocean City, NJ paid $5 million for replenished beach, storms washed it away 2 months later; Wrightsville Beach, NC = 10 beach replenishments over 25 years Replenishment at Wrightsville Beach, NC 3 years later Mitigation of Coastal Erosion Beach Nourishment Important to match characteristics of sediment already on beach. Miami Beach = enormous and expensive (~$60 million) beach replenishment program, largely successful Miami Beach in 1970s (before) Miami Beach in 1980s (after) Major Message: Beach sediments ALWAYS move Anything action taken to counter coastal erosion is temporary and will affect neighboring beaches 8