Geological Importance of Sand Compatibility for Sustaining Beaches (Economically Wasteful and Environmentally Damaging Beach Renourishment )

Transcription

1 Geological Importance of Sand Compatibility for Sustaining Beaches (Economically Wasteful and Environmentally Damaging Beach Renourishment ) Harold R. Wanless Katherine L. Maier (Donald F. McNeill) Department of Geological Sciences University of Miami hwanless@miami.edu

2 The Problem with Sand Sources Easily available sand (dredged( from offshore) tends to: Be mostly skeletal carbonate sediment that has never been abraded on a beach before; Be less dense than equivalent sized quartz; Be of shapes that move more easily than equant grains; Move in suspension more easily than equivalent (sieve) sized quartz; and Not be durable in beach environment

3 The Right Sand: why does it matter? Off Florida s s beaches are valuable coral, hard- bottom & fish habitats that are severely degraded or destroyed by sediment smothering, siltation, persistent turbidity & reduced light penetration Just off the proposed beach fill project in Ft. Lauderdale is a thriving coral reef

4 The Impacts - Environmental Rapid smothering and/or siltation of offshore sandy & hardbottom communities Dramatically increased turbidity & reduced light penetration in adjacent waters, stressing coral & other light dependent components of hardbottom & softbottom communities

5 The Impacts - Economic $$$ Not using sand of a proper size & durability so does not remain on beach $$$$ Not vegetating beach renourishment projects so moving sand behind beach face is lost shortening lifetime of project, shore is more easily storm eroded Siltation & turbidity to adjacent water is degrading or destroying adjacent marine communities (reefs and hardbottoms and fish) so recreational, tourism, & commercial activities are lost!

6 The Impacts - Human Safety Persistently murky water increases likelihood of shark attacks & increases difficulty of rescue of swimmers HURRICANE INEZ (Category 1) 1966 Key Biscayne Increased likelihood of severe shore erosion during major storms increasing risk to both inhabitant & structures

7 The Sand 1 Size Matters! Most SE Florida beaches have grains >250 µm (1/4 mm) particles <200 µm (1/5 of mm) move in suspension

8 The Sand 1 Size Matters! Most SE Florida beaches have grains >250 µm (1/4 mm) particles <200 µm (1/5 of mm) move in suspension 2 Composition Matters! Particle type is critical: can t use sieving because carbonate particles quartz particles (hydrodynamically)

9 The Sand 1 Size Matters! Most SE Florida beaches have grains >250 µm (1/4 mm), particles <200 µm (1/5 of mm) move in suspension 2 Composition Matters! Particle type is critical: can t use sieving because carbonate particles quartz particles (hydrodynamically) 3 Hardness Matters! Durability of offshore carbonate sand different than natural, not very durable & break down to form MUD

10 Sand isn t t just sand Size Density Shape Durability

11 ORIGINAL SAND Miami Beach NEW SAND

12 AFTER renourishment

13 Grains move by: 1) Bedload transport 2) Saltation 3) Suspension Fine sand, silt and clay moves in SUSPENSION Medium sand moves by SALTATION Prothero and Schwab (1996)

14 Grain finer than 200 microns tend to move in suspension Entrainment Diagram for Quartz Sand

15 <200 micron sand particles will move OFF the beach Boca Raton beaches courtesy Cry of the Water

16 Grains <200 microns in diameter tend to move in suspension Suspended mud The results of beach renourishment at John U. Lloyd State Park, southern Broward County

17 40% 40.0% 30% 30.0% PROPOSED BORROW AREA 3 Core BC ft Sieved grain size Proposed Borrow Area III - Core BC at 1.5 ft Bedload Moves on bottom Suspension transported 2-1 mm Weight Percent 20% 20.0% 10% 10.0% + breakdown mm 0% 0.0% >4mm 4-2mm 2-1mm 1-.5mm.5-.25mm mm mm <.062mm Sieved.5 Grain Size Grain size in millimeters mm

18 Student Katie Maier looked at the durability of proposed Broward County renourishment sand

19 Calibration Sand 1-22 mm sieved size fraction used 1 mm

20 Sample split A mixed with coarse sand 1 mm

21 Sample split E mixed with ooids 1 mm

22 Sample split E after one week tumbling 1mm 1 mm

23 Sample split A after one week tumbling 1mm 1mm

24 Durability tests were run on 5 proposed borrow site samples (mixed with = weight of 1 mm glass spheres & tumbled in water for one week) 40.0% 30.0% 20.0% 10.0% 0.0% Proposed Borrow Area III - Core BC at 1.5 ft >4mm 4-2mm 2-1mm 1-.5mm.5-.25mm mm mm <.062mm Sieved Grain Size 40.0% 30.0% 20.0% 10.0% 0.0% Proposed Borrow Area II - Core BC-96-38B at 3.3 ft >4mm 4-2mm 2-1mm 1-.5mm.5-.25mm mm mm <.062mm Sieved Grain Size Core BC ft Core BC ft Core BC ft Core BC-96-38B 3.3ft Core BC-96-1A 4.3ft Weight Percent Weight Percent 2-1 mm 2-1 mm 2-1 mm 2-1 mm 2-1 mm mm mm mm mm mm mm mm mm mm mm

25 Sand from Proposed Broward Borrow Area IN 1 WEEK OF TUMBLING 1 to 3% TURNED TO MUD!! Percent gain after tumbling 3.00% 2.50% 2.00% 1.50% 1.00% 0.50% 0.00% -0.50% -1.00% Tumbling of Borrow Site Samples In 1 week of tumbling natural beach sand only 0.03% mud produced new fine sand & mud BC BC-96-38B BC BC-96-1A BC % -2.00% >2mm 1-2mm <63 size fraction (microns)

26 Sand from Proposed Broward Borrow Area IN 1 WEEK OF TUMBLING 1 to 3% TURNED TO MUD!! BC ft (tumbled one week) In 1 week of tumbling natural beach sand only 0.03% mud produced percent of sample Before tumbling After tumbling Series1 Series < grain size (microns)

27 Durability Results Within one week of tumbling abrasion, 3% of the sand had turned to mud! Sand from proposed borrow areas creates fine sand & mud at rate up to two orders of magnitude (100 times) faster than adjacent natural beach sand!

28 This is why the water off John Lloyd State Park looks like this

29 What we end up with. A beach filled with offshore-derived carbonate sand invariably becomes a long-term source of persistent fine-sediment release & persistent degraded nearshore water quality

30 What to do? Demand that renourishment projects contain only excellent quality, durable sand with NO material that is <200 microns NO longer permit borrow areas offshore Florida All samples should be analyzed wet for grain size Demand that settling analyses & durability analyses be conducted on any sands with >5% carbonate particles (and establish procedures & criteria) follow FAC Rule 62B " shall" be similar in color and grain size distribution (sand grain frequency, mean and median grain size, and sorting coefficient) c to the material in the existing coastal system "

31 There are places to get good sand all the stripes are fossil beaches from other sea levels (quartz)

32 But, even upland sources (non carbonate) are not being properly evaluated prior to placement Old beach re-exposed exposed (tan). New fill just added and rapidly eroding (gray). St. Lucie fill project completely eroding away as it is being placed on the beach Spoonful of St. Lucie beach fill 24 hours after stirring into water

33 The Bahamas may now be willing to sell

34 EXAMPLE OF A SUITABLE CARBONATE BEACH NOURISHMENT SAND Core BC ft Sand is between 1mm mm Carbonate particles are rounded & polished skeletal fragments (already worn). Will not be prone to rapid abrasion and release of silt- and clay-sized particles. Carbonate skeletal particles are solid,, not full of open pore space. Will behave as indicated by sieved size. Will not be prone to rapid abrasion and breakdown on beach. Carbonate particles are fairly equant shaped, not thin rods & flat plates. Will move as indicated by sieved size. Will not be prone to rapid abrasion and breakdown on beach. 2-1 mm mm mm

35 1 Size Matters! In the End it's the Sand Most SE Florida beaches have grains >250 µm (1/4 mm) particles <200 µm (1/5 of mm) move in suspension 2 Composition Matters! Particle type is critical: can t use sieving because carbonate particles quartz particles (hydrodynamically) 3 Hardness Matters! Durability of offshore carbonate sand different than natural, not very durable & break down to form MUD