suitability of recycled glass cullet as artificial dune fill along coastal environments

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Suitability of Recycled Glass Cullet as Artificial Dune Fill along CoastalEnvironmentsAuthor(s): Christopher Makowski, Charles W. Finkl, and Kirt RusenkoSource: Journal of Coastal Research, 29(4):772-782. 2013.Published By: Coastal Education and Research FoundationDOI: http://dx.doi.org/10.2112/12A-00012.1URL: http://www.bioone.org/doi/full/10.2112/12A-00012.1

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Suitability of Recycled Glass Cullet as Artificial Dune Fillalong Coastal Environments

Christopher Makowski†‡, Charles W. Finkl†‡, and Kirt Rusenko§

†Coastal Education and Research Foundation5130 Northwest 54th StreetCoconut Creek, FL 33073, [email protected]

‡Department of GeosciencesFlorida Atlantic UniversityBoca Raton, FL 33431, U.S.A.

§Gumbo Limbo Nature Center1801 North Ocean BoulevardBoca Raton, FL 33432, U.S.A.

ABSTRACT

Makowski, C.; Finkl, C.W., and Rusenko, K., 2013. Suitability of recycled glass cullet as artificial dune fill along coastalenvironments. Journal of Coastal Research, 29(4), 772–782. Coconut Creek (Florida), ISSN 0749-0208.

Coastal dune systems are an integral component of maintaining a sustainable, well-performing beach. With the aid ofdune-stabilizing vegetation, constructed foredunes provide a ‘‘natural’’ armorment behind the dry berm to help protectthe backshore from storm surge and intense overwash. However, as the costs of beach nourishment continue to inflate,the urgency to construct or restabilize the foredune area of the beach is often overlooked. Furthermore, to compound theproblem, suitable sand resources are becoming unobtainable because of regulatory restrictions, prompting engineers andcoastal zone managers to use all available dredged sediments for berm and beach face replenishment. In order to providean alternative method for dune construction, this study examined the suitability of recycled glass cullet as an artificialdune fill material. After construction of an artificial dune was completed, recycled (silica) glass cullet and natural beachsand were provided as growth mediums for dune-stabilizing vegetation. Immature transplants of sea oats (Uniolapaniculata) and panic grass (Panicum amarum) were planted in the artificial dune and evaluated over a 1-year growingperiod. Suitability of the recycled glass cullet was determined through the overall performance of the salt-tolerant plants,which included fresh and dry weight measurements, new shoot development, and root and stalk length. It wasdetermined that both species of dune-stabilizing vegetation planted in a recycled glass medium outperformed thosespecimens growing in the natural beach sediment controls. We postulate that the results may stem from a slight increasein the angularity of the recycled cullet vs. the natural sand grains. This minuscule planar difference in the surface area ofthe grains may contribute to more aggregated moisture content within a recycled glass cullet dune, allowing for optimalgrowing conditions for dune vegetation. By showing this positive suitability of recycled glass cullet as artificial dune fill, anew, innovative method for dune protection may now be considered.

ADDITIONAL INDEX WORDS: Beach protection, foredune construction, sea oats, panic grass, coastal hazards, dunevegetation, beach nourishment.

INTRODUCTIONSince a majority of the world’s population exists within close

proximity to some type of coastal environment (Crowell et al.,

2010), it becomes imperative to adequately protect and stabilize

these areas. It has been shown that coastal hazard ‘‘cascades’’

can potentially wreak havoc at these locations, where water

meets land, and can grossly impose physical, financial, and

physiological damage upon their inhabitants (Finkl and

Makowski, 2013). Without a complete beach system stabiliza-

tion, the coastal environment and all those that rely, visit, or

dwell there are in a state of continual risk from the impacts of

these hazard cascades.

One such method to provide a complete beach stabilization

entails the implementation of a coupled beach–dune system

that is developed and constructed in tandem (Sherman and

Bauer, 1993; van der Wal, 2004). Unfortunately, many coastal

engineers and beach managers concern themselves solely with

the task of replenishing the berm and beach face, while the

quintessential ‘‘backbone’’ of the entire beach system, the

foredune and secondary dune complex, is often ignored. The

dismissive attitude toward the rebuilding of these eolian

landforms can prove to be a costly pitfall within the zonal

management of coastal environments worldwide.

Coastal dunes are cosmopolitan in their occurrence and

ecological diversity, as they are found in a wide-sweeping range

of latitudes and contain a highly variable assortment of flora

and fauna. Within the last few decades, coastal geomorpholo-

gists have begun to recognize and accept conceptual models

that confirm beaches and dunes are indeed an integrated,

coupled system with specific linkages and feedback loops over a

variety of spatial and temporal scales (Houser and Ellis, 2013;

Martinez, Psuty, and Lubke, 2004; Nordstrom, McCluskey,

and Rosen, 1986; Psuty, 1993; Sabatier et al., 2009; Sherman

and Bauer, 1993). Even though coastal dunes have uniformly

been accepted as part of the primordial shoreline sediment

budget, the notion that beach and dune systems are to be

considered a permanent, interconnected morphodynamic

framework was, in fact, a novel concept. Several models,

including morphodynamic-based projections, sediment-ex-

change pathway extrapolations, and beach–dune erosion

DOI: 10.2112/12A-00012.1 received 9 July 2012; accepted in revision25 November 2012; corrected proofs received 20 February 2013.Published Pre-print online 19 April 2013.� Coastal Education & Research Foundation 2013

Coconut Creek, Florida July 2013Journal of Coastal Research 29 4 772–782

process scenarios have given credence to the idea that beaches

and dunes have a process-bound, coupled interaction with one

another (Britton and Morton, 1989; Pries, Miller, and Branch,

2008; Priestas and Fagherazzi, 2010). This interaction becomes

most evident during an increase in coastal hazard storm

activity upon the shoreline. During calm sea state and weather

conditions, wave energy and swash exertions are limited to the

foreshore, which includes the low-tide terrace (i.e., inner surf

zone) and the beach face, virtually ignoring the coastal dune

component. However, during those times of heightened storm

action, the overall dynamics of the beach–dune coupled

interaction can be plainly seen, as dunes provide a crucial

coastal defense role by protecting the low, backshore areas from

surge and overwash (Psuty, Allen, and Starcher, 1988; Short

and Hesp, 1982). The contribution to the overall sediment flux

in the nearshore system during storms allows for a larger

modulated reflection of high energy waves, thereby providing

an increase in coastal stabilization through a lowering of swash

infiltration (Pries, Miller, and Branch, 2008; Sherman and

Bauer, 1993).

Even though their importance in the coastal system is

highly evident, many coastal dune ecosystems around the

world have either been ignored, degraded, or altogether

removed from the backshore. Much of this is attributed to

the exponential increase in both coastal development and

population totals along shoreline perimeters on almost every

continent. As a result, coastal dune stabilization and mainte-

nance is often overshadowed in favor of natural resource

exploitation, industrial growth, and population expansion

(Gomez-Pina et al., 2002; van der Meulen and Salman, 1996;

Williams et al., 2001). Hesp (2000) provided an example of

such an occurrence in New Zealand, where in just the past

century, approximately 115,000 ha of coastal dunes have been

removed and redeveloped for the sole purpose of agricultural

and forestry activities. On the other hand, in southeast

Florida, many of the natural coastal dunes have either been

excavated or allowed to be eroded away in favor of extensive

condominium development that now fronts up against the dry

berm, thereby offering no protection buffer in between the

anthropogenic structures and the beach (Finkl and Charlier,

2003). Similarly, in the Netherlands, the expansion of cities

into low, inland areas was made possible through the use of

sand from excavated coastal dunes (Carter, 1991). By

removing or severely degrading these coastal dune systems,

not only are coastal zone managers and regulatory personnel

putting communities at risk for extensive storm surge and

flooding impacts, but they are also altering an important

natural water catchment and agricultural area. It has been

shown that coastal dunes provide a source of groundwater

recharge, as they effectively buffer the onset of saltwater

intrusion through the retention of freshwater (Martinez,

Psuty, and Lubke, 2004). Additionally, farmers have found

that highly specialized agricultural crop yields can only be

grown within certain coastal dune environments (van der

Meulen and Salman, 1996). Through excavation, degradation,

or neglect, these coastal dune systems are becoming irrevers-

ibly altered, thus allowing for an imbalance in the beach–dune

coupled interaction relationship worldwide.

Dune nourishment, in tandem with beach nourishment,

allows for a complete beach stabilization. Without the influx of

a backshore dune sediment supply, the dry beach berm has no

foredune foundation by which it may perch itself in the face of

elevated storm events (Psuty, 1993). By nourishing the dunes

along with the beach, a comprehensive coastal system is thus

replenished. However, because of financial and logistical

constraints, dune nourishment is commonly eliminated from

the coastal construction plans in favor of increasing the fill

templates for the beach nourishment. While it is understood

that with any given beach nourishment, an adequate fill

length, density fill volume, and width must be used to prove

effective along the dry berm, those parameters should not be

met at the cost of completely eliminating the coupled dune

nourishment. Because offshore dredging activities prove very

costly, and regulatory agencies greatly restrict how and where

sand mining can be conducted, suitable fill alternatives for

dune nourishment must be considered. One such alternative is

the use of recycled silica glass cullet as a fill medium for

artificial dune replenishment.

Multiple studies have evaluated the use and suitability of

recycled glass cullet as alternative beach fill material (Edge,

Cruz-Castro, and Magoon, 2002; Finkl, 1996; Finkl, 1997;

Kerwin, 1997; Makowski, Finkl, and Rusenko, 2011; Makowski

and Rusenko, 2007; Makowski, Rusenko, and Kruempel, 2008;

Thomson et al., 2004). First proposed by Finkl (1996) and Finkl

and Kerwin (1997), recycled glass cullet has been found to

retain the same physical properties as natural silica sand and

can be mechanically processed to match any grain size of the

existing beach–dune coupled system. Geotechnical analyses

determined that recycled cullet was safe for human interaction,

since all contaminant standards for beach fill placement were

appropriately met (Thomson, Finkl, and Kruempel, 2004).

Furthermore, Makowski and Rusenko (2007) and Makowski,

Rusenko, and Kruempel (2008) showed that glass cullet does

not have an adverse effect on biological organisms cohabitating

within (i.e., interstitially) or upon the recycled substrate, and

the cullet does not pose risks to the embryo development of

endangered/threatened sea turtle eggs incubating within the

recycled matrix. However, if recycled glass cullet were to be

effectively used as an alternative dune fill material, it must

first be determined that sand-anchoring, salt-tolerant vegeta-

tion can grow in such a medium. Coastal sand dunes, being a

product of ever-occurring eolian processes (Ellis et al., 2012),

would continuously migrate with the passing winds if not

properly anchored in place. Therefore, salt-tolerant vegetation,

such as Uniola paniculata and Panicum amarum, provides a

natural anchoring system to keep the dune in place behind the

beach (Craig, 1991). Without the aid of these sand-anchoring

plants within a recycled glass cullet artificial dune, the

complete beach–dune coupled system would once again be

susceptible to impacts from coastal storm hazards.

The objective of this study was to determine whether recycled

glass cullet is a suitable alternative fill medium for artificial

dune renourishment. This was performed by monitoring the

growth success of planted dune vegetation within both recycled

glass cullet and natural sand mediums. A simulated dune was

constructed using alternating growing mediums of glass cullet

and natural sand grains, whereupon juvenile transplants of sea

Journal of Coastal Research, Vol. 29, No. 4, 2013

Suitability of Glass Cullet as Artificial Dune Fill 773

oats, Uniola paniculata, and bitter panic grass, Panicum

amarum, were planted accordingly. Growth patterns were

monitored, recorded, and compared to see how the dune

vegetation responded to a glass cullet growing medium vs.

natural sand controls. If recycled glass cullet proves to be a

suitable alternative material for artificial dune fill, coastal

scientists, regulatory agency personnel, and coastal zone

managers could potentially have another resource to help

maintain the beach–dune coupled system through dune

nourishment.

METHODSThe following section details the methods and materials that

were implemented to achieve this study’s objective. This

included acquisition of the dune-stabilizing vegetation (i.e.,

Uniola paniculata and Panicum amarum), the construction

and maintenance of the simulated artificial dune, planting of

the dune grass specimens, data collection over the duration of

the study, and data analysis and statistical interpretation. The

study lasted for one year. The vegetation was planted in the

month of June, which was one of the recommended planting

months for these particular species of dune grasses, and then

removed for analysis the following June.

Dune VegetationDune-stabilizing vegetation was acquired through a dona-

tion from Aquatic Plants of Florida, Inc.� (APF; Sarasota,

Florida, U.S.A.; http://apofl.com/). A quantity of 300 juvenile

transplants were donated, of which 150 transplants were sea

oats (Uniola paniculata) and 150 transplants were bitter panic

grass (Panicum amarum), for use in this study. Sea oats are

perennial dune-stabilizing grasses that can propagate exten-

sive root mats (up to 5 m in length), which aid in the anchoring

of foredune sediments (Craig, 1991). Having a tolerance for

salt, overwash, drought, and direct sunlight, the sea oats used

in this study are commonly found growing in dense patches

along coastal dunes and are known to recruit vegetatively

within Hardiness Zones of 7 and higher (Barnett and Crewz,

1997). Similarly, the donated panic grass transplants consisted

of rhizomatous perennial coastal dune grasses that have a

tolerance for salt, direct sunlight, poor soil, and droughtlike

conditions (Craig, 1991). The panic grass specimens used in

this study are known for their wide green blades and numerous

pelletlike seed heads and are also found to proliferate within

Hardiness Zones of 7þ (Acosta, Carranza, and Izzi, 2009).

Artificial Dune ConstructionConstruction of the artificial dune took place at the Gumbo

Limbo Nature Center in Boca Raton, Florida (http://www.

gumbolimbo.org/). Natural sand was collected from the fore-

dune along native beaches in southern Palm Beach County,

Florida (Figure 1), and the recycled glass cullet was obtained

through Glass Aggregate Systems (GlassAGG; Faribault,

Minnesota, U.S.A.; http://www.glassagg.com/). The natural

sand and the glass cullet both had a mean grain size between

0.38 and 0.40 mm.

The artificial dune consisted of a cinder block foundation

with a cushioning layer of natural foredune sediments. The

orientation of the constructed dune faced west to east, with

more blocks placed at the base of the western end to simulate a

natural tilting effect commonly found in foredune areas.

Natural dune sediments were then added on top and around

the blocks to complete the artificial dune construction. Twelve

test trays were then embedded on the top of the artificial dune

and used as growing enclosures for each one of the experimen-

tal (cullet) and control (natural sand) series. Each tray (61.0 cm

3 43.2 cm 3 25.4 cm) was allocated to house a specific growing

medium and dune vegetation grass species, as shown in Figure

2. Experimental series involving sea oats and bitter panic grass

contained a 100% recycled glass cullet growing medium, while

control series of the same two species consisted of 100% natural

foredune sands.

Planting and Data CollectionEach growing tray, whether a control or experimental series,

held 15 specimen transplants of either sea oats (Uniola

paniculata) or bitter panic grass (Panicum amarum). Both

species were never intermingled within the same tray. Within

each tray, three growing rows were delineated for planting

(Figure 3). Both the sea oats and bitter panic grass transplants

were planted in three separate control trays of 100% natural

foredune sediments and three separate experimental trays of

100% glass cullet. The root crown (i.e., area just below where

the leaves emerge) of each transplant was planted approxi-

mately 15.0 cm below the sand or cullet surface. Each

transplant was spaced approximately 10.2 cm apart, for a total

of five transplants per row. A time-release 10-10-10 NPK

fertilizer was applied around the base of each transplanted

grass and worked into the growing medium. Fertilizer was

applied at the time of the planting and then reapplied on a 6-

week interval. Watering of the transplants occurred immedi-

ately after planting and continued on a weekly schedule for

several months.

Before planting commenced, measurements of fresh

weight, root crown length, and top stalk length were taken

for all transplants. The number of shoots present at the time

of the planting was also recorded for each transplant. In

addition, those grasses not used in the study (n¼60 for each

species) were dried in a combustion oven at low heat

(~37.78C) for a period of 24 hours and weighed to obtain

an average dry weight. Measurements of stalk length and

the recorded number of new shoots continued throughout

the study. Upon completion of the study, all the specimens

were extracted from their growing medium and immediately

transported to a laboratory for analysis. Data recorded for

each specimen included individual fresh weights, maximum

root length, maximum stalk length, and number of new

shoots. After following the drying procedure previously

mentioned, dry weights of each specimen were also ob-

tained.

Data Analysis and Statistical InterpretationData analysis included descriptive statistics (i.e., mean [m],

standard deviation [r], range [r]) of prestudy and poststudy

fresh weight, maximum root length, maximum stalk length,

and number of new shoots for each of the sea oats natural dune

sand control series (n¼ 45) and experimental cullet series (n¼45), as well as the bitter panic grass natural dune sand control

series (n¼45) and experimental cullet series (n¼45). One-way


774 Makowski, Finkl, and Rusenko

analysis of variance (ANOVA) statistical tests were performed

on each of the data categories to determine whether the

experimental cullet series differed significantly from the

natural dune sand controls. ANOVAs were carried out with a

95% confidence and a significance threshold of p , 0.05.

Additionally, nonstatistical average comparisons were per-

formed between the prestudy measurements and the poststudy

measurements, as well as control vs. experimental poststudy

measurements.

RESULTS

The following section presents the experimental and control

results from the one year study. Prestudy measurements are

presented first (Table 1), followed by each of the data categories

recorded poststudy (Table 2). A summary of the overall

poststudy results can be found in Table 3.

Prestudy MeasurementsBefore starting the experimental procedures, several pre-

study measurements of the dune-stabilizing grass transplants

from both species were recorded. Fresh weights of the control

sea oats (i.e., those allocated for planting in natural dune sand)

had a mean of 8.68 g (r¼ 1.30, r¼ 6.47–12.17), while the fresh

weights of the experimental sea oats (i.e., those allocated for

planting in recycled glass cullet) recorded a mean of 8.59 g (r¼1.57, r ¼ 5.97–12.94). Even though no roots extended beyond

the crown before planting took place, the root crown itself was

measured for the control sea oats (m¼ 4.5 cm, r¼ 0.4, r¼ 3.7–

5.3) and experimental sea oats (m¼4.4 cm, r¼0.5, r¼3.6–5.3).

Top stalk length at the time of the sea oats planting was

recorded for both control (m¼ 21.0 cm, r¼ 3.3, r¼ 12.5–28.6)

and experimental (m ¼ 20.7 cm, r ¼ 3.1, r ¼ 15.7–27.5)

transplants. The last prestudy measurement of the sea oats

included the dry weight of those transplants (n¼ 60) not used

Figure 1. A typical dune system located in southern Palm Beach County, Florida. Salt-tolerant vegetation, such as sea oats (Uniola paniculata) and bitter panic

grass (Panicum amarum), can be seen anchoring the dune sediments for increased stability. Sand from this dune system was extracted and used for the growing

medium in this study’s control series.



either in the control or experimental series, which contained a

mean of 2.55 g (r¼ 0.38, r¼ 1.90–3.57).

Similar prestudy measurements were recorded for the

bitter panic grass transplants. Fresh weights of the control

series transplants recorded a mean of 38.18 g (r ¼ 3.74, r ¼28.44–44.73), while the experimental series transplants

recorded a mean of 37.43 g (r ¼ 4.21, r ¼ 28.19–43.71). The

root crown was measured for both the control bitter panic

grass series (m¼5.8 cm, r¼0.3, r¼5.2–6.3) and experimental

bitter panic grass series (m¼5.7 cm, r¼0.3, r¼5.2–6.4). Top

stalk length at the time of the bitter panic grass planting was

recorded for both control (m¼ 32.9 cm, r¼ 1.6, r¼ 29.8–35.9)

and experimental (m ¼ 32.8 cm, r ¼ 1.7, r ¼ 29.8–35.9)

transplants. A control prestudy dry weight was recorded (m¼23.86 g, r ¼ 2.34, r ¼ 17.78–27.96) from those bitter panic

grass transplants (n¼ 60) that were not used in the planting

phase of this study.

Poststudy Measurements

Immediately following the end of the study, measurements of

all the control and experimental series transplants took place.

Control series sea oats recorded means for fresh weight (m ¼27.16 g, r¼ 3.06, r¼ 21.41–31.73), maximum root length (m¼19.3 cm, r¼1.9, r¼16.2–23.9), maximum stalk length (m¼95.6

cm, r¼6.7, r¼84.9–111.2), new shoot growth (m¼15, r¼2, r¼11–21), and dry weight (m ¼ 7.71 g, r ¼ 0.84, r ¼ 5.95–9.07).

Likewise, sea oats used in the experimental series recorded

means for fresh weight (m¼29.62 g, r¼2.99, r¼23.02–34.25),

maximum root length (m ¼ 22.0 cm, r ¼ 2.5, r ¼ 18.1–26.9),

maximum stalk length (m¼ 99.7 cm, r¼ 7.5, r¼ 89.1–117.5),

Figure 2. Final construction of the simulated artificial dune at the beginning of the study. Growing trays contained an alternating growing medium of either

recycled glass cullet (experimental series) or natural dune sand (control series). For the purposes of photographic documentation, experimental cullet trays were

saturated with water and not yet exposed to the sun, allowing them to give off a yellowish tint. Each tray housed either sea oats (Uniola paniculata) or bitter panic

grass (Panicum amarum), exclusively.



new shoot growth (m¼20, r¼3, r¼14–26), and dry weight (m¼8.57 g, r¼ 0.88, r¼ 6.59–9.70).

Bitter panic grass control series transplants recorded means

for fresh weight (m ¼ 108.93 g, r ¼ 9.51, r ¼ 88.61–121.59),



new shoot growth (m¼30, r¼5, r¼22–37), and dry weight (m¼61.17 g, r ¼ 5.77, r ¼ 49.23–71.5), whereas bitter panic grass

experimental series transplants recorded the following means:

fresh weight (m ¼ 112.73 g, r ¼ 10.10, r ¼ 92.28–124.85),



new shoot growth (m¼34, r¼5, r¼19–42), and dry weight (m¼68.85 g, r¼ 8.41, r¼ 53.61–89.18).

Figure 3. Within each tray, three growing rows of dune grass transplants were delineated at the start of the study. Above, in the center, bitter panic grass

transplants have been planted in a 100% recycled glass cullet matrix. To the left, bitter panic grass transplants have been planted in a natural sand control series

tray, while to the right, sea oats have also been planted in a control series tray.

Table 1. Summary table of prestudy and poststudy measurement means for

the sea oats control vs. experimental series.

Data Category

Prestudy

Sea Oats

Control

Series

Poststudy

Sea Oats

Control

Series

Prestudy

Sea Oats

Experimental

Series

Poststudy

Sea Oats

Experimental

Series

Fresh weight (g) 8.68 27.16 8.59 29.62

Max. root length

(cm)

4.5 19.3 4.4 22.0

Max. stalk length

(cm)

21.0 95.6 20.7 99.7

Number of new

shoots (n)

0 15 0 20

Dry weight (g) N/Aa 7.71 N/Aa 8.57

a The prestudy dry weight of those sea oats specimens not used in the

study was measured with a mean of 2.55 g.



ANALYSISOne-way ANOVA tests were carried out to determine if any

significant differences existed within the measured data

categories between the natural dune sand control series and

experimental recycled glass cullet series. Even though the

experimental sea oat series recorded a higher measurement

total in almost every data category, statistical analysis

determined that there were no significant differences (p ,

0.05) between the dune grasses growing in the recycled glass

cullet and those growing in the natural dune sand. With

consideration that different baseline measurements were

recorded, nonstatistical poststudy comparisons revealed that,

on average, sea oats that grew in a recycled glass cullet medium

had a heavier fresh weight by more than 2.00 g, a larger root

length by over 2.0 cm, a larger stalk length by more than 4.0

cm, a more abundant new shoot count by five, and a heavier dry

weight by just less than a gram. Similarly, nonstatistical

poststudy comparisons revealed that, on average, bitter panic

grass that grew in a recycled glass cullet medium had a heavier

fresh weight by more than 4.00 g, a larger root length by over

3.0 cm, a more abundant new shoot count by four, and a heavier

dry weight by over 7.00 g. The only category in which the bitter

panic grass natural sand controls outperformed the experi-

mental series was maximum stalk length, where the controls

recorded a larger stalk length by only 0.7 cm. Even with these

comparisons, statistical analyses determined that none of the

differences were to be deemed significant (p , 0.05).

DISCUSSIONDunes are an integral part of the coastal system and should

be a permanent consideration when discussing a complete,

coupled beach–dune interactive complex (Martinez, Psuty, and

Lubke, 2004; Sabatier et al., 2009; Sherman and Bauer, 1993).

A common pitfall of coastal zone management and beach

nourishment stabilization is that the dune nourishment

component is often dismissed when fill templates are being

assessed and planned. By ignoring dune nourishment, coastal

managers and engineers are denying the beach its ‘‘backbone’’

support in the form of a foredune, making the dry berm and any

coastal development much more susceptible to flooding and

overwashing surge in the event of a storm (Clements et al.,

2010; Pries, Miller, and Branch, 2008; Priestas and Fagherazzi,

2010; Sherman and Bauer, 1993). In order for dune nourish-

ment to be a permanent fixture with beach renourishment,

available fill resource materials for dunes must be made

available. This entails alternative sources of dune fill material

such as recycled glass cullet, which is the manufactured

byproduct of silica (SiO2) to any grain size desired. However,

for recycled glass cullet to be considered suitable for dune

nourishment, it must first be determined whether dune-

stabilizing vegetation can grow in such a medium. The

objective of this study was to test the hypothesis of whether

dune-stabilizing grasses (i.e., sea oats, Uniola paniculata, and

bitter panic grass, Panicum amarum) could successfully grow

after being planted in 100% recycled glass cullet. Without the

aid of these kinds of dune-anchoring vegetation, a foredune

nourished with recycled glass cullet would become unstable

and provide no addition support to the beach–dune coupled

system.

Through the construction of an artificially simulated dune,

this study was able to show that sea oats and bitter panic grass

not only have the capability to grow in a medium of 100%

recycled glass cullet (Figure 4), but can also outperform those

control transplants growing in 100% natural dune sands. Even

though statistical analyses determined that significant differ-

ences between the natural dune sand control series and the

experimental recycled glass cullet series did not exist, direct in

situ measurements showed an outperformance of those grasses

growing in the recycled glass cullet in almost every data

category when compared to those transplants growing in

natural dune sands. The elevated performance that was

recorded included heavier fresh weights, longer maximum root

lengths, longer maximum stalk lengths (except for the bitter

panic grass transplants, where the control series was longer by

0.7 cm), larger abundance of new shoot growth, and heavier dry

weights. Both the control and experimental series in this study

recorded growth rates for sea oats and bitter panic grass that is

consistent with established literature (Hester and Mendels-

sohn, 1991; Hitchcock, 1950; Lonard and Judd, 2011; Tiner,

1993).

We postulate the reason that those dune-stabilizing

grasses planted in a 100% recycled glass matrix can

outperform those planted in natural dune sands may stem

from the subtle difference in angularity among the grains of

the two fill materials. Previous geotechnical analyses have

shown that even though it is an insignificant difference,

Table 3. Summary table of poststudy measurement means for the sea oats

control vs. experimental series and the bitter panic grass control versus

experimental series.

Data Category

Sea Oats

Control

Series

Sea Oats

Experimental

Series

Panic Grass

Control

Series

Panic Grass

Experimental

Series

Fresh weight (g) 27.16 29.62 108.93 112.70

Max. root length

(cm)

19.3 22.0 44.53 48.4

Max. stalk length

(cm)

95.6 99.7 104.8 104.1

Number of new

shoots (n)

15 20 30 34

Dry weight (g) 7.71 8.57 61.18 68.85

Table 2. Summary table of prestudy and poststudy measurement means for

the bitter panic grass control vs. experimental series.

Data Category

Prestudy

Panic Grass

Control

Series

Poststudy

Panic Grass

Control

Series

Prestudy

Panic Grass

Experimental

Series

Poststudy

Panic Grass

Experimental

Series

Fresh weight (g) 38.18 108.93 37.43 112.70

Max. root

length (cm)

5.8 44.53 5.7 48.4

Max. stalk

length (cm)

32.9 104.8 32.8 104.1

Number of new

shoots (n)

0 30 0 34

Dry weight (g) N/Aa 61.18 N/Aa 68.85

a The prestudy dry weight of those bitter panic grass specimens not used in

the study was measured with a mean of 23.86 g.



there is a slightly higher angular property to the individual

glass grains of recycled cullet vs. the weathered oblong-

shaped grains of natural dune sands (Makowski, Finkl, and

Rusenko, 2011; Makowski et al., 2007; Thomson, Finkl, and

Kruempel, 2004). While this minuscule increase in angu-

larity poses no adverse cutting risk to flora, fauna, or people

interacting with the recycled glass cullet sediment grains

(Makowski and Rusenko, 2007; Thomson et al., 2004), it

may serve to provide a slightly elevated interstitial water

retention capability. Because water would drain at a slower

Figure 4. Above, the artificial dune at the start of the study. Below, after a one year study, dune-stabilizing vegetation growth within the experimental recycled

glass cullet trays was successful and showed no significant differences from the natural dune sand controls.



rate within the pore spaces of more angular grains, a higher

moisture content could develop within the growing medium

and sustain the dune vegetation, especially in times of high

sun or drought. Furthermore, Makowski, Rusenko, and

Kruempel (2008) showed with the aid of time-log data

recorders that the moisture content of beach test plots

containing some proportion of recycled glass cullet was

slightly higher than those controls containing 100% beach

sand. This lends credence to the belief that dune sediments

containing a portion of recycled glass cullet may elevate the

moisture content slightly for stabilizing vegetation to use

for growth and rhizome establishment (Figures 5 and 6).

It is understood that this study is only the first step in the

process of using recycled glass cullet as artificial dune fill

material. Further research must be conducted to see how well

the performance of glass cullet is within a ‘‘true’’ beach–dune

coupled system. While previous studies have shown that

recycled glass cullet is suitable for beach and surf-zone

placement, it is recommended that future studies involving

glass cullet placement on a natural foredune be carried out.

Even though further research is recommended, it is evident

from the growth response of the dune-stabilizing vegetation

used in this study that recycled glass cullet is a suitable

artificial fill material for dune nourishment.

CONCLUSIONThis study marks the first attempt to show the suitability of

recycled glass cullet as artificial dune fill material. Through the

experimental growth of dune-stabilizing vegetation (i.e., sea

oats, Uniola paniculata, and bitter panic grass, Panicum

amarum) in a glass cullet medium, it can be concluded that

there are no significant differences between the growth

patterns of transplants growing in recycled glass cullet when

compared to those growing in natural dune sands. The facts

that (1) no mortality was recorded in any of the experimental

Figure 5. Successful rhizome proliferation within the recycled glass cullet was shown for the experimental bitter panic grass (Panicum amarum) transplants.

Above left is a bitter panic grass transplant at the start of the study, while above right shows the same transplant after being extracted from an experimental

recycled glass cullet growing medium. The maximum poststudy root length for this experimental transplant measured over 45 cm.



series and (2) the transplants growing in the cullet outper-

formed those growing in natural dune sands lead to the

conclusion that recycled glass cullet is a safe, inert, suitable fill

material to be used by coastal zone managers and coastal

engineers for dune nourishment.

ACKNOWLEDGMENTSAcknowledgments are bestowed upon the Coastal Educa-

tion and Research Foundation (CERF), Aquatic Plants of

Florida, Inc.� (APF), the Gumbo Limbo Nature Center, and

Florida Atlantic University (FAU). Special individual ac-

knowledgments are given to Dr. Michael Simini and Dr. Jean

Ellis for extensive peer review of this paper.

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