Coastal issues: Beaches and Estuaries

Transcription

1 Coastal issues: Beaches and Estuaries Definitions: Coast = broad zone where both marine & terrestrial processes act It includes the beach, lagoons, marshes, bars etc. Coastal processes act to shape the coastline, these include: Tectonics, erosion, waves, tides, storms Changes in sea level Beaches: What is the source of the sediment (sand an gravel) on beaches? Sediment from offshore is moved onto the beach. Off-shore sediment is derived from coast or inland Sediment is constantly in motion and the beaches are continually reshaped Hawaii Cusps at Point Reyes, CA Refraction focuses energy and causes erosion: Erosion from waves: by force of pounding water and abrasion of sediment. Results in sea cliffs, sea caves, points, etc Generates sediment form th eerosion of the coast W. Ireland near Galway Bay 1

2 Sediment Transport along the coast Waves approach from a dominant direction and result in: Beach Drift = sediment transported down the beach Swash & backwash Longshore current produced by waves approaching at an angle This transports sediment along the coast in large bars within the offshore and near-shore regions Longshore and drift results Why do waves approach beaches dominantly from one direction? A. Wave refraction results in waves coming from a single direction B. Tidal forces are relatively consistent and thus waves are too. C. Atmospheric circulation patterns are consistent and thus the direction of wave approach is too. 2

3 Large Coastal Circulation Cells: Sediment is supplied to the coast by rivers or erosion of coast Cell = length of coast where sediment supply balances sediment loss Disruption of transport results in an imbalance and possibly loss of sediment along the coast (beaches shrink) Deflection of L.S. current Deposition in canyon Dam rivers and trap sediment Which is not true of Beaches? A. Beach sediment can be derived from rocks on continents B. Beach sediment can be derived from skeletal remains of marine organisms C. The physical character of any single beach changes very little over short periods of time (year) D. Building dams on rivers can alter the characteristics of beaches on the coast Coastal Issues: Problem with Beaches People like them We try to keep them We want to protect investments Problem: Ocean is more powerful than we are. 3

4 Attempts to protect beaches from erosion usually cause problems! Our efforts are Expensive & Unsuccessful Hard stabilization of beaches Attempts to protect beach from erosion Groins, Jetties and Sea Walls Groins and jetties ( perpendicular to the beach) Trap longshore transport of sand. Starve down-current beaches of supply (causing erosion) Seawalls Intended to prevent erosion of beach Increase wave E at base of wall causing erosion in front of wall (Miami)! Replenishment EXPENSIVE Temporary Fine sand washes away Storms destroy the replenished beach Groins & Jetties Miami Beach In general, beach replenishment is expensive and temporary: costing $3.3 to $17.5 million/mile, repeated every 3-7 years Program for Developed Shorelines Largest beach replenishment project in US Began in 1981, cost $65 million by 1989 Slow rate of erosion keeps sand on the beach In places the beach has eroded despite groins and replenishment. Seawalls built to protect property. In some place there is now no beach at all! 4

5 Btw: what s an unusual storm? Defined by the engineers as any storm that destroys the beach! Ultimately, all stabilization efforts fail. Replenishment will always fail, because the coast is removing the beach in response to changes in (1) sea level, (2) sediments supply, (3) changes to coast line, (3) natural, complicated perturbations of the physical system. Sometimes bad things happen, even to natural beaches. This beach will recover. It may move on-shore as sea level rises, it may move downshore, but it will recover. The houses will not. If a seawall had been constructed prior to the storm, then the storm might ve permanently damaged the beach. Given our discussion of beaches: Why do beaches change over time? A. The supply of sediment may change, thus beaches may either grow or shrink in size. B. Changing sea level causes beaches to move with the changing shoreline C. Strong storms impact beach size and character D. All of the above E. None of the above 5

6 Storms and natural variations change the character of any beach. We try to stop this change to protect or preserve the beach. Doing so cost lots of money, is temporary at best, and damages the sediment circulation along the entire coastal circulation cell. Do you see the cycle? Jersey shore Additional problem: Rising sea level is going to impact coastal communities. What do we do? We could retreat, and allow the beach to move on-shore. Or, we can stand and fight the ocean, ultimately loose, and then retreat. Recent Sea Level: Over past 2 million years sea level has varied by ~130 m Today sea level is high 6 m below maximum Why? We are in an interglacial period - reduced glacier volume Recent low 18,000 years before present Recent glacial maximum Coasts not yet in Equilibrium with current high! See how global warming is extra problematic today. Baltimore D.C. Patomac R. Chesapeake Bay True or false: all attempts to stabilize beaches are destined to either fail or negatively impact the beach. A. True B. False C. Unknown 6

7 Estuaries: Body of water partly enclosed by land & replenished by river Include wetlands & marshes Protected from waves, storms by land enclosure Highest primary productivity of all marine environments Nutrient input from river & recycling from circulation Crab, clams, oysters, shrimp, fish Juvenile stages thrive Many species reproduce here Civilization too (Tigress, Euphrates, Nile, Po) Chesapeake Bay Susquehanna River Potomac James River Chincoteague Bay, MD Mixing Fresh & Sea Water makes for an extremely fertile ecosystem Mixing in the ideal estuary Net Circulation Flow is seaward on top of estuary (low density river water) Flow is landward at depth (return flow of denser seawater) Result: Waste & juveniles transported to sea Nutrients & clean seawater flow into sound RECYCLING nutrients! 7

8 RECYCLING nutrients: Photosynthesis at surface These organisms die and accumulate at bottom Dead organisms decompose Release Nutrients from bottom Back into land-ward flow feeding surface waters Estuaries are important to marine life and coastal communities Nutrient recycling makes them rich with life Algae and plants are abundant These serve as base of food web Serve as nursery for many marine organisms Protect the coastline Grasses and mangroves prevent storm damage to coast Marsh plants trap contaminants The Benefit of Wetlands and Estuaries They clean polluted river input - 1 acre can filter ~1/4 million gallons/year. They serve as nurseries for >0.5 commercial fish in SE of USA They protect coastlines from storms We currently have lost ~50% of wetlands worldwide Expected sea level rise this century (50 cm) to cause ~40-50-% loss of existing wetlands. 8

9 Environmental Issues: Estuaries & Wetlands Should we protect them from this? Why Over developed and industrialized estuaries cannot provide the benefits of healthy estuaries. A - Marked decline in number of Taxa since industrial revolution Lower biodiversity = greater chance of extinction B - correlation: as biodiversity decreases, the fisheries collapse Cause: (1) water quality declining due to human activity, and/or (2) over fishing causing decline in biodiversity and this results in collapse of ecosystem. Coastal & Estuary biodiversity declining Regional Scale Declining Extinct Science, 2006 Biodiversity of large marine ecosystems declining too (large marine ecosystems are those with area >150 km 2 areas) Triangles = cumulative loss Diamonds = annual loss Black = all systems Blue = species- pore systems Red = species-rich systems Map shows the large marine systems color coded to fish species richness Science, 2006 Conclusions: biodiversity of important marine ecosystems is declining. This trend is projected to result in complete collapse of commercial seafood fisheries by Time remains to fix this problem. >50% of world s population lives w/in 50 miles of the coast, and >30% of food protein comes from fish alone. 9

10 Do you impact a coastal ecosystem? Which one? A. Gulf of Mexico B. Gulf of California C. San Francisco Bay D. All of these E. None of these, we live in Illinois! Review Questions Describe how a groin works to trap sediment and why they are problematic. How are seawalls detrimental to beaches? Why doesn t beach replenishment work in the long-run? From evidence offered by observations of New Jersey, Miami and Carolina coastlines, what is the best way to insure healthy, sustainable beaches? Why are estuaries so fertile? Why are healthy estuaries so important? Why are there fewer healthy estuaries each year? How is the decline in healthy estuaries tied to the decline in biodiversity of Large Marine Ecosystems? 10