Houston, J.R., 2023. Measured fate of beach nourishment sand. Journal of Coastal Research, 39(3), 407–417. Charlotte (North Carolina), ISSN 0749-0208.
The fate of sand placed over a 20-year period in one of the largest series of beach nourishment projects ever conducted in the United States is determined by using direct measurements of elevation vs. distance from dunes to closure depth. Four projects placed 11.25 million m3 of sand from 1998–2017 on a 28.8-km-long shoreline at Panama City on Florida's Gulf of Mexico coast. Profile measurements were available in 1996 following Hurricane Opal and in 2018 after Hurricane Michael, one of the largest hurricanes to strike the United States, which came ashore only 30 km from Panama City beaches. The profiles were about 300 m apart within the project template and extended down to closure depth. After about 20 years, 87 ± 3% of the nourishment sand remained on profiles. Some sand used for nourishment was dredged within closure depth along a 3.6-km section and resulted in sand loss of 4% ± 1% as sand partially refilled the dredged holes. About 9% ± 3% was lost to longshore transport out of the project area. Calculations based on the equilibrium profile concept used in U.S. beach nourishment design predicted beaches should have widened 30.9 m ± 0.9 m and profiles raised 0.69 m ± 0.10 m due to the measured volume of sand remaining on profiles from the start of beach nourishment in 1998 to November 2018 after Hurricane Michael. The measured change in beach width was 32.4 m ± 4.0 m, and the rise was 0.66 m ± 0.05 m. The rise of 0.66–0.69 m is comparable to the 0.81 m mean sea-level rise by the year 2100 projected by the Intergovernmental Panel on Climate Change for its worst-case temperature scenario.
Beach nourishment has become increasing popular in the United States with the annual volume of sand that is placed having more than doubled since 1980 (Elko et al., 2021). Florida is the state in which the greatest volume has been placed, and 84% of Florida residents responding to a survey selected beach nourishment rather than structures or managed retreat as the best method to address beach erosion (Dornisch, 2015). Beach nourishment projects are usually cost shared by federal, state, and local agencies. The U.S. Army Corps of Engineers (USACE) is the major federal agency participating in these projects, and its primary interest is in building dunes and wide beaches to reduce storm damage to infrastructure with a secondary interest in supporting recreation and tourism. State and local agencies have the same interests, but usually their primary interest is in supporting recreation and tourism.
The fate of beach nourishment sand is an issue because nourishment is sometimes criticized as not lasting long, with comments such as “all the sand washed away” (Willson et al., 2017, p. 49), the “sand has gone missing” (Wilkinson, 2022), and the sand is “lost to storms” (Grubba, 2020). The primary reason for the perception that beach nourishment is not long lasting is that the USACE and other agencies place most of the sand on the subaerial beach because this is the least expensive method, enables sand to be placed uniformly along the shoreline, and the volume of the sand placed can be accurately measured for payment to contractors (USACE, 2008). The agencies assume in their project design that after 1 to 3 years (USACE, 2022; Willson et al., 2017) sand will have spread along a profile from the dune toe down to closure depth to produce an approximate equilibrium profile and to reach the design beach width (Dean, 1991). The profile after equilibrium is reached is basically the original profile that is translated laterally assuming a similar sediment-size distribution as the native beach. Figure 1 illustrates an approximate profile translation at Delray Beach, Florida, showing measured profile change over 22 years, starting before beach nourishment and extending three years beyond the last nourishment. Design beach widths are typically less than half of initial beach widths (USACE, 2022). Despite publicity to aid the public in understanding that the initial beach width is only transitory, the change is sometimes viewed as a loss of sand (Willson et al., 2017).
Another reason for the perception is that some beach nourishment projects have relatively short lengths and have significant lateral losses because of diffusive spreading, whereby the shoreline protuberance produced by the nourished beach results in lateral spreading of sand to adjacent beaches. The spreading is inversely related to the length squared of the sand placement, making the lateral spread of beach nourishment projects with short lengths relatively rapid (Campbell, Dean, and Wang, 1992). Using a database of beach nourishment projects in the United States, the average project has a length of only 3.2 km (American Shore and Beach Preservation Association, 2022). Campbell, Dean, and Wang (1992) showed that if beach nourishment for a typical Florida location was represented by a 3.2 km rectangular projection from a straight shoreline, about 20% of the nourishment would be lost from the lateral bounds of the nourishment in about 1 year.
The standard U.S. beach nourishment design assumes that most of the sand placed remains on project profiles with sand losses at lateral boundaries due to longshore transport, sand spreading, and interior losses because of inlet shoals. The time required for renourishments is determined by estimating these losses. Only losses to inlets are losses to the littoral zone because sand moving out of a project area because of longshore transport or spreading remains in the littoral zone as it moves to adjacent shorelines.
The beach width after nourishment has been placed and the profile has reached equilibrium is based on the equilibrium profile concept with variables related by Equation (1) (Dean and Dalrymple, 2002):
where, X is beach width gain, V is the volume of sand placed, h*is closure depth, B is the elevation of the beach berm, and L is the shoreline length of the nourishment. The notation is from Campbell, Dean, and Wang (1992) and has been used in many publications.
The Florida Department of Environmental Protection (FDEP) in the United States has long-term measurements of shoreline position (FDEP, 2022a). Using these measurements, Houston (2019) solved Equation (1) for V as a function of X to show that increases in beach width account for most of the 83 million m3 (108 million yards3) of beach nourishment placed on more than 170 projects on the Florida east coast since the early 1970s. Growth of nourished beach width accounted for about 68% of the sand placed, with 32% of the sand moving laterally and widening adjacent beaches that had never been nourished. Measured data on inlet shoal growth showed that longshore transport moved about 10% of the sand to inlets, where it was lost to the littoral system by shoal growth. Similarly, average beach widening of more than 30 m on nourished beaches in South Carolina, United States, from 1986–2006 accounted for all of the 21 million m3 of beach nourishment placed in more than 50 projects (Houston, 2021a).
Houston (2019, 2021a) indirectly determined only the fate of beach nourishment because Equation (1) was used to infer sand volume given beach width change. Measurements of sand volume change along profiles of a beach nourishment project are required for direct determination of the fate of beach nourishment sand. The measurements need to be on a beach nourishment project that is long enough to reduce lateral spreading, nourishment volume must be large enough so that its change can be accurately measured, and bathymetric measurements must extend to an approximate closure depth. Longshore and inlet losses must be relatively small, with estimates available.
There have been studies in the United States to track the fate of beach nourishment. For example, Do, Kobayashi, and Suh (2014) conducted profile measurements to monitor a beach nourishment project on the U.S. east coast, but it was monitored for only 2.5 years, had a length of only 1.8 km, and lost 30% of the sand during this period. Roberts and Wang (2012) monitored a nourishment that was 12-km long on the Florida Gulf coast for 4 years, but profiles extended only to a short-term depth of closure of 3 m, and the nourishment density was only 140 m3/m.
The most impressive beach monitoring has been for a series of nourishments along hundreds of kilometers of the Netherlands' North Sea coast. This long-term project is placing 12 million m3 of sand annually to raise the coastal foundation that extends from dune crests to water depths of 20 m to keep up with sea-level rise, thereby protecting the dunes from being undermined or overtopped during storms (van Slobbe et al., 2013). About 60% of the sand is placed in water depths from 8–20 m because it is the least expensive method. These nourishments have an average length of 4 km, with the remainder of nourishments placed on beaches and having an average length of 2.3 km (Brand, Ramaekers, and Lodder, 2022). The short lengths are not a problem because the lateral spreading helps move the sand along the hundreds of kilometers of shoreline in what is a national project. Similarly, there are other national projects in Europe, and “. . . in Europe, confidence has been established in shore nourishment as a central technique in the soft engineering strategy” (Hamm et al., 2002, p. 241).
The Panama City Beaches Shore Protection Project (FDEP, 2020a) in Florida is a good candidate to determine the fate of beach nourishment sand. It is located along the NW half of Bay County on Florida's Gulf of Mexico coast (Figure 2). It is one of the longest U.S. nourishment projects, with a length of about 28.8 km. Beach nourishment has been large, with a total of about 11.25 million m3 placed from 1998 through 2017 (FDEP, 2020a). The State of Florida has made highly accurate profile elevation measurements following two major hurricanes and has almost annual shoreline position measurements following beach nourishment starting in 1998. There are no inlets within the project area, and sand losses attributable to longshore transport are relatively small, with estimates available in the literature (Stapor, 1973; Stone and Stapor, 1996; Walton, 1973, 1976). About 8% of the Bay County shoreline is armored, typically with small wood and concrete bulkheads, with much of the armoring in the SE part of Bay County rather than Panama City beaches (Leadon, Nguyen, and Clark, 1998). The profile and shoreline measurements along Panama City beaches are made relative to monument locations numbered R001–R093, each separated by about 305 m (Figure 3).
Based on FDEP historical shoreline change measurements, the Panama City beach shoreline accreted about 0.3 m/y from 1855–1976 (Absalonsen and Dean, 2010). Beach nourishment started at Panama City beaches in 1998–99 (Table 1), largely in response to shoreline recession caused by Hurricane Opal, which was one of the most damaging hurricanes to strike Florida in the 20th century. Hurricane Opal came ashore in October 1995 as a Category 3 hurricane about 120 km west of the Panama City beaches, but it produced the highest high-water marks along these beaches and significant shoreline recession (Figure 4; Leadon, Nguyen, and Clark, 1998). The shore protection project was federally authorized in 1996 following Hurricane Opal and was initially conducted in 1998–99 by the local government with federal government reimbursement (Brenner, 2019).
Beach nourishment in 1998–99 widened beaches significantly (Figure 5). Sand came from offshore borrow areas and was similar to the natural beach sand but with a somewhat larger mean grain size of 0.35 mm vs. 0.28 mm on the beach (USACE, 2010). In 2004, Hurricanes Ivan and Dennis were Category 3 storms that came ashore about 120 km and 70 km, respectively, to the west of Panama City. Both resulted in shoreline recession, and a second large beach nourishment project was conducted in 2006. Hurricane Hermine in 2010 was a Category 1 hurricane that made landfall about 150 km east of Panama City beaches and produced shoreline recession, and there was a nourishment project in 2011 along about half of the shoreline length (Table 1). A 2017 project placed sand in “hot spot” erosional areas covering about 6.7 km of shoreline (Table 1; Armbruster et al., 2018). Hurricane Michael made landfall only 30 km east of Panama City beaches in SE Bay County in October 2018. Hurricane Michael was a Category 5 hurricane that was the strongest hurricane to ever impact the Florida Gulf coast and the fourth strongest to have hit the United States in recorded history (NOAA, 2019). A beach nourishment project was completed in May 2022 to address relatively limited erosion along part of Panama City beaches. Beach nourishment in 2022 is not included in analyses because the last available elevation measurements were made in November 2018.
The State of Florida usually conducts detailed elevation (topographic and bathymetric) measurements along profiles following major hurricanes. Measurements of elevation vs. distance were made along profiles extending from monuments R001–R093 (Figure 3) in October 1996 following Hurricane Opal and in November 2018 following Hurricane Michael (FDEP, 2020a). Although some profile changes may have occurred during the period of mild wave conditions from 1996 to the first beach nourishment starting in 1998, FDEP shoreline measurements between January 1996 just after Hurricane Opal and in 1997 before nourishment show a weak shoreline recovery from Hurricane Opal of only 0.6 m (2 ft) ± 0.3 m (1 ft). Therefore, the 1996 measurements are taken as prenourishment conditions in an analysis of the fate of beach nourishment placed from 1998–2017 and will be referred to as prenourishment. Parameters relating to volume and shoreline change are assumed independent with uncertainties determined in quadrature, as is standard, unless stated.
A year after Hurricane Opal occurred, elevation vs. distance measurements were made along profiles in October 1996, with a few measurements in September 1996 or March 1997 (FDEP, 2020a). The measurements were made starting at each monument R001–R093 (Figure 3) and moving perpendicular to the shore. Horizontal distances were relative to each monument location, and vertical elevations were relative to the North American Vertical Datum of 1988 (NAVD 88). These prenourishment elevation measurements extended to water depths of 12–15 m, with horizontal spacing finer in the nearshore, but averaging about 13 m. A complete set of measurements also occurred in November 2018 following Hurricane Michael in October 2018 (FDEP, 2020a). These elevation measurements extended to water depths of about 17 m with horizontal distances between measurements averaging about 4 m, making them closer together than prenourishment measurements.
Closure depth is a depth where net sediment transport is very small or nonexistent, with profile depths remaining essentially the same. Profile data from 1971–2006 are available to a depth of about 8 m at the Russell-Fields Pier near monument R040 at Panama City beaches (Figure 3; Clark, 2011). These data show an envelope of profiles during this period that still display some variation at a depth of 8 m. The envelope should have zero variation at closure depth, so the profiles have not quite reached closure depth. Figure 6 shows measured prenourishment and November 2018 profiles at monument R040, and little vertical variation occurs at a depth of 9 m. Other profiles consistently show little vertical variation at a depth of 9 m; for example, Figure 7 shows the profiles at R050, close to the middle of Panama City beaches. The mean difference in elevations between prenourishment and 2018 profiles at a depth of 9 m is calculated for monuments R001–R093 to be only 2 mm, with a standard uncertainty of the mean of ±2 cm. Notably, there is no sign of sand reaching or going beyond closure depth despite the beach having been struck by the largest hurricane to have made landfall on the Florida Gulf coast in historical times. Therefore, 9 m is selected as the closure depth to capture the sand gain from prenourishment to November 2018 that was found on all profiles.
To determine cross-sectional area differences between the prenourishment and 2018 profiles, the more closely spaced 2018 measurements are linearly interpolated to provide land elevations and water depths at the same horizontal distances from monument locations as the prenourishment measurements. Therefore, elevation differences occur between the prenourishment and 2018 profiles at each of the horizontal distance locations where prenourishment measurements were made. These elevation differences are multiplied by distances between horizontal locations to give cross-sectional areas. The cross-sectional areas are then summed down to closure depth for each profile. At 12 of the prenourishment profiles (R011, R018–019, R021, R032, R037, R052, R059, R066–067, R081, and R088) and one 2018 profile (R083), measurements did not start until water depths of about –2 m, therefore missing much of the nourishment sand placed above this level. For a normal profile, such as at R040 shown in Figure 6, almost three-quarters of the nourishment sand is above –2 m. For each of these profiles, the cross-sectional areas of profiles on either side are averaged and used as the cross-sectional area.
Longshore Sand Transport
Just to the NW of monument R001, Stone and Stapor (1996) estimated a longshore sand transport out of the Panama City beaches shoreline of 57,000 m3/y; Keehn et al. (2012) estimated 88,000 m3/y. Considerable uncertainty occurs in longshore transport rates, and Houston and Dean (2014) assumed an uncertainty of ±40%. This uncertainty is used, and adding the two estimates yields an average loss of about 73,000 ± 21,000 m3/y.
Panama City beaches are bounded on the SE by St. Andrews State Park that extends from monument R093 through R097, with St. Andrews Inlet starting at R097 (Figure 3). This inlet was created in 1934 after a channel about 4 km to the SE became too difficult to maintain. After the inlet was cut, it developed a very large ebb shoal from 1935–77, with a volume of about 18–21 million m3. Shoal growth has slowed and is presently estimated to be about 8000 m3/y (USACE, 2010). Maintenance dredging of St. Andrews Inlet is conducted an average of once every 2 years. Sand removed from the entrance channel (about 72,000 m3/y) is deposited on the shoreline on both sides of St. Andrews Inlet and moves back into the inlet (USACE, 2010).
Stapor (1973) concluded from analysis of historical shoreline change and dredging data that St. Andrews Inlet was a sand sink with sand transported toward the inlet from both sides. Stapor concluded that although there was large NW and SE sand transport, a net transport of only 9000 m3/y occurred away from the inlet toward Panama City beaches. Stone and Stapor (1996) reinforced this conclusion. Walton (1973) used the littoral-drift-rose method and also determined large NW and SE transport at St. Andrews Inlet but determined a net NW transport of only 5000 m3/y. Similarly, Walton (1976) used ship wave observations and determined a net NW transport of 21,000 m3/y. Therefore, the average NW sand transport toward Panama City beaches is a gain of about 12,000 ± 3000 m3/y, assuming a 40% uncertainty for each of the estimates.
Subtracting 12,000 ± 3000 m3/y from 73,000 ± 21,000 m3/y yields a net longshore transport away from Panama City beaches of about 61,000 ± 21,000 m3/y, with uncertainty adding in quadrature. Therefore, the sand loss to Panama City beaches for the 20 years from 1999–2018 because of longshore transport was about 1.2 ± 0.4 million m3.
Loss of Sand in Dredged Sand Holes
Sand used to nourish Panama City beaches was dredged from offshore. A few of the sand borrow-area locations were so close to shore that they affected the beach nourishment sand that was placed. Stauble and Gravens (2004, p. 21) noted that these borrow areas were close to the beach and “may have resulted in fill placed on the beach moving back to the borrow trough areas.” The nearest sand borrow area was only 560 m offshore, parallel to the shore, centered at R015, and extending from R011–R017 (Armbruster et al., 2018; URS Corporation, 2009). The borrow area had a width of about 300 m, so it extended from about 400 m to 700 m offshore from monument R015 (Figure 3). Therefore, part of the dredged hole was within closure depth. Figure 8 shows that the prenourishment profile (small-dash line) extending from R015 had smoothly increasing depths, much like the profiles in Figures 6 and 7; however, depths along the 2018 profile (solid line) begin plunging at a horizontal distance of 350–400 m and do not begin to merge with the prenourishment depths until a horizontal distance of about 700 m is reached. This rapid-depth change in the 2018 is attributable to a dredged sand hole in the borrow area. This also can be seen by comparing two 2018 profiles. The 2018 profile at R010 (large-dash line), which is outside the influence of the hole, is similar to the 2018 profile at R015 until the profile at R015 begins plunging. There also was a borrow area about 670 m offshore and centered on R020 that was within closure depth at R018–R022. The most significant holes are from R0013–R0017, with the holes decreasing until they are not discernable at R010 and R023.
To determine the difference in cross-sectional areas from prenourishment to 2018 for profiles with obvious holes, differences are calculated until the profiles cross (e.g., about 350 m from monument R015 [Figure 8]). The quantity of beach nourishment sand that flowed into the holes is estimated by comparing the average gain in sand for R011–R022 to the greater average gain in sand from the averages of five bounding profiles on each side at R006–R010 and R023–R027. The difference in averages is used to estimate a sand loss to the holes of about 0.4 ± 0.1 million m3 using a standard uncertainty of the mean.
Volume and beach-width changes and profile rise are determined based on the measured data and the equilibrium profile concept.
From Table 1, the total sand placed on Panama City beaches from 1998–2018 was 11.25 million m3, or about 390 m3/m. The uncertainty in the volume is not known, but it is almost always assumed that the volume of beach nourishment placed is completely accurate. Typically, profiles with spacings as little as about 30 m are measured almost immediately as each section of nourishment is placed. Because most of the sand is placed on the subaerial beach, measurements should be accurate.
Cross-sectional area differences between prenourishment and 2018 profile measurements are summed for monuments R001–R093 and yield a mean area of 340 m2 or a volume of 340 m3/m with a standard uncertainty of the mean of only ±9 m3/m. The uncertainty also is calculated by assuming that the uncertainty in the depth difference at closure depth of ±2 cm applies along entire profiles that have an average length of 520 m. Multiplying the uncertainty by profile length yields a cross-sectional uncertainty of about ±10 m2 or ±10 m3/m, which is very close to the uncertainty determined by considering the measured variation in cross-sectional areas. Therefore, the average volume per meter is taken as 340 ± 10 m3/m.
Multiplying 340 ± 10 m3/m by the 28.8 km length of the nourished shoreline gives a volume of about 9.8 ± 0.3 million m3. Therefore, about 87% ± 3% of the 11.25 million m3 of beach nourishment sand placed since nourishment began in 1998 was still on profiles in 2018 following Hurricane Michael. The volume of nourishment sand lost to the sand-borrow holes was estimated in the previous section to be 0.4 ± 0.1 million m3, or about 4% ± 1% of the nourishment sand placed. The sand lost through longshore transport is estimated to be the total nourishment sand placed minus sand on the profiles and minus sand lost to borrow holes. This equals about 1.0 ± 0.3 million m3, or about 9% ± 3% of the sand placed. This loss fits within the estimated longshore transport loss determined from the literature as 1.2 ± 0.4 million m3. The estimated loss over the 20 years from 1998–2018 based on measurements would be 50,000 ± 15,000 m3/y vs. the 60,000 ± 20,000 m3/y mean estimate based on the literature. The total loss due to the holes and longshore transport is 1.4 ± 0.5 million m3, or about 70,000 ± 25,000 m3/y.
The FDEP (2022b) has historical shoreline change measured almost yearly following the 1998–99 beach nourishment that provide the distance of the intersection of mean high water with a profile relative to each of the fixed monuments R001–R093. The difference in shoreline position from prenourishment to November 2018 equals the beach-width change at each monument. Beach-width changes at monuments R001–R093 are averaged to give an average beach-width change for the entire shoreline of 32.4 m ± 4.0 m from prenourishment to November 2018. Hurricane Michael reportedly produced surprisingly minor erosion along Panama City beaches except at the project's SE end (FDEP, 2019, 2020a). From prenourishment to November 2018, the only significant areas with lower than average shoreline gain were the SE end that was affected by Hurricane Michael (6.1 m ± 1.6 m lower) and the shoreline opposite the holes from monuments R011–R022 (8.7 m ± 3.5 m lower).
For the period from prenourishment to November 2018, beach-width gain based on using the measured volume change in Equation (1) is consistent with the measured beach-width gain. Substituting the sand volume of 9.8 ± 0.3 m3 that remained on profiles into Equation (1) with h* = 9 m and B= 2 m (USACE, 2020a) yields X = 30.9 m ± 0.9 m vs. measured beach-width change of 32.4 m ± 4.0 m.
The volume of sand lost in the borrow holes also can be estimated using beach-width change because volume and beach width are related through Equation (1). The average beach-width gain from prenourishment to 2018 for monuments R011–R022, which were affected by the borrow holes, is only 22.7 m ± 3.4 m. The average beach-width gain for the bounding monuments at R006–R010 and R023–R027 is 35.3 m ± 1.7 m. Substituting the difference of 12.6 m ± 3.8 m into Equation (1) with the shoreline length of about 3660 m yields about 0.5 ± 0.2 million m3. This compares favorably with the estimate of the volume of sand lost to the borrow holes based on profile measurements.
As seen in Figure 1, placing beach nourishment not only widens beaches but also raises profiles. The mean rise is just the mean cross section of 340 ± 10 m2 divided by the mean length of profiles down to closure depth of W* = 520 m ± 35 m, giving a mean rise of about 0.65 m ± 0.05 m. A second method is used because the rise also equals the increase in beach width (X = 32.4 m ± 4.0 m) multiplied by the approximate profile slope of (h* + B)/W*, as seen in Equation (2). Substituting values into Equation (2) yields a mean rise in profiles of about 0.69 m ± 0.10 m:
A mean rise of 0.66–0.69 m is comparable to the mean sea-level rise of 0.81 m by the year 2100, which was determined by the Intergovernmental Panel on Climate Change (IPCC) for its worst-case temperature increase scenario (SSP5-8.5) of 5°C (IPCC, 2021). The mean rise in the profile shown in Figure 1 was 1.3 m from 1994–2016 despite significant lateral losses attributable to the short 2.7-km length of the nourishment (FDEP, 2020b). Beach nourishment can raise profile amounts comparable to the sea-level rise projected for many years into the future.
Measurements clearly show that beach nourishment sand at the Panama City beaches has not “washed away” or “gone missing,” but its fate is accountable with almost 90% of it still on profiles, widening beaches at least as much as expected in the project design. The remainder of the sand moved back into dredged holes within closure depth or moved beyond the lateral boundaries of Panama City beaches to adjacent beaches, providing similar storm-damage-reduction, recreation, and tourism benefits as for Panama City beaches. The Netherlands has a good national perspective that sand placed in the littoral zone that moves laterally is just nourishing a different portion of the country's littoral zone. Since 1970, about one-third of beach nourishment placed on the Florida east coast has moved laterally to adjacent beaches that have never been nourished (Houston, 2019). Sand moving laterally is not lost to the littoral zone.
As noted in the “Introduction,” agencies generally assume it will take 1 to 3 years for a profile to reach an approximate equilibrium after beach nourishment, with the time depending on the wave climate. Panama City beaches are located along a shoreline with a relatively low-wave climate with large waves striking only during episodic storms such as hurricanes. As a result, it takes longer than 1 to 3 years for profiles to reach equilibrium. This is illustrated by comparing measured beach widths with beach widths computed using Equation (1).
Postnourishment shoreline positions minus prenourishment shoreline positions at monuments R001–R093 give beach-width change because of beach nourishment at each monument. These widths are summed and averaged to give an average beach-width change for the entire Panama City shoreline by year. Figure 9 shows these beach widths based on measured data at the times shown, with trends also shown.
Figure 9 also shows average beach-width change since prenourishment based on Equation (1). The first point is computed using the 1998–99 nourishment volume of V = 6.97 million m3 (Table 1), h* + B= 11 m, and L= 28.8 km and results in a beach width of X = 22 m. This is the design beach width used in the United States that is expected after profile equilibrium. Dots are shown in Figure 9 at the midpoint at each year a nourishment was completed (Table 1). Between each nourishment, dashed lines in Figure 9 show reductions in average beach width due to losses determined in the “Results” section of 70,000 m3/y. The second nourishment was in 2006, so seven years (1999–2006) had losses of 70,000 m3/y, yielding a total shoreline loss of 1.5 m using Equation (1). Therefore, just before the 2006 nourishment that was justified by erosion caused by Hurricanes Ivan (2004) and Dennis (2005), the predicted average equilibrium beach width, including losses, would have been 22 m – 1.5 m = 20.5 m. The hurricanes hastened movement of the sand toward equilibrium, but the average measured beach width at this time was still about 4 m greater than the design beach width. Using the same parameters, the 2006 nourishment of 2.5 million m3 would add 7.9 m to beach width after equilibrium was reached (Figure 9). The next nourishment was in 2011 and was justified by numerous storms in 2010 and before. Using Equation (1), predicted equilibrium beach widths after each nourishment are plotted as dots in Figure 9, along with dotted lines showing the annual sand loss rate until the next renourishment. The predicted equilibrium beach width after Hurricane Michael was 31.1 m. Therefore, the predicted equilibrium beach width in November 2018 is 9.1 m greater than the original design beach width following the 1998–99 nourishment. Figure 10 shows the differences between measured beach widths and calculated equilibrium beach widths at the same times. When the difference reaches zero, the beach width has reached the equilibrium beach width. Differences reach minimums just before each renourishment in 2006, 2011, and 2017, with each minimum lower than the previous one as profiles approach equilibrium. Both measured and calculated equilibrium beach widths increase following nourishments.
Figures 9 and 10 show that it can take several years after beach nourishment for average beach width to begin approaching the design beach width given by the equilibrium profile concept. It is likely that the local community becomes accustomed to the greater transient beach width following each the nourishment. Then when a major storm strikes and reduces beach width, moving it toward the design beach width, the storm is blamed (“lost to storms”), and the beaches are renourished despite being wider than the design width. The net effect is an upward trend in measured beach width following the low just before the 2006 nourishment (Figure 9). Measured beach width never falls below the original design width of 22 m, yet subsequent renourishments further widened beaches.
The agreement between the measured beach-width gain of 32.4 m ± 4.0 m from 1998–2018 with the beach-width gain of 30.9 m ± 0.9 m based on Equation (1) supports use of beach-width change to determine the fate of beach nourishment sand. Florida has shoreline change measurements almost yearly (Figure 9) at thousands of monument locations throughout the state. However, elevation measurements along profiles are generally made only for beaches impacted by major hurricanes.
Results emphasize the importance of having beach nourishment projects extend over as long a shoreline as possible. The USACE nourishment projects are longer than the U.S. average, but cost-sharing policies make it challenging to have long nourishment projects in the United States because several communities must agree to fund the local cost-sharing portion of a project. The USACE projects require perpetual easements for the public to use the entire beach from the dunes to the ocean, despite private ownership in the United States extending down to mean high water in 44 states and down to mean low water in six states (Houston and Gordon, 2022). It is often difficult to get beach property owners to agree to perpetual easements that allow the public to use their land. This limits the number of property owners who agree and, therefore, the lengths of projects. Moreover, owners of property adjacent to short beach nourishment projects benefit from lateral sand spreading without having to pay part of the cost. Because mobilization and demobilization of dredges are significant parts of the overall costs of beach nourishment projects, long beach nourishment projects also reduce unit costs.
The Panama City Beaches Shore Protection Project also illustrates the problem of borrow sites being too close to shore. In this case, some borrow sites were within closure depth. The holes are still in place and will cause sand loss from beaches for many years. This is not a sand loss to the littoral zone, but a return of sand to where it started in the littoral zone.
The Panama City Beach Shore Protection Project satisfied its primary function from the USACE perspective of reducing storm damage. Hurricane Opal (1995) was a marginal Category 3 storm (Mayfield, 1995) when it came ashore about 130 km from Panama City, although it produced a significant surge at Panama City beaches of about 2.5 m. It caused massive “damage to 471 buildings and numerous seawalls along Panama City Beach” (FDEP 2019, p. 20; Figure 11). The USACE estimated that had the 1998–99 beach nourishment project for Panama City beaches been in place prior to Hurricane Opal, it would have prevented 70% of the damages (Leadon, Nguyen, and Clark, 1998). Beach nourishment was tested when Hurricane Michael struck in 2018. Michael was a Category 5 hurricane at landfall about 30 km from Panama City (FDEP, 2019; Janssen, Lemke, and Miller, 2019). Panama City beaches were on the left side of Hurricane Michael when it made landfall, so the surge was only 1.9 m. The beach nourishment did better than preventing 70% of the damages, it “protected all beach fronting development and infrastructure along Panama City Beach,” with no coastal damages except those due to wind (FDEP, 2019, p. 20). Spurgeon (2022, p. 64) noted that “nourishment that has taken place in the past at Panama City Beach has built it into a robust and resilient system that responds to adverse events better than ever before.”
The Panama City Beach Shore Protection Project also satisfied its primary function from the local and state perspective of supporting tourism and recreation. Tourism is Bay County's primary industry, generating more than $3 × 109 in annual revenue and supporting more than 20,000 local jobs (Bay County Chamber of Commerce, 2022; Bente, 2022). Panama City Beach has a population of 15,000, which increases to 100,000 during peak summer months (Panama City Beach, Florida, 2022); it is almost completely dependent on tourism, which is almost completely dependent on the condition of its beaches. Had beaches remained in poor conditions (Figure 4), the economy of Panama City beaches would have been ruined. Although Bay County tax revenues from tourists in hotels and rental property dropped in 2020 during the first 5 months of the COVID-19 pandemic, tourism at Panama City beaches then exploded, with revenues up more than 50% in 2021 vs. prepandemic 2019 revenues and further up in 2022 despite reaching hotel and rental capacities (Bay County Clerk of Court and Comptroller, 2022).
Maintaining the November 2018 beach width into the future requires beach nourishment to compensate for the measured volume losses of 70,000 ± 25,000 m3/y. Interestingly, a recent publication that considered beaches in the SE United States, including those in Florida, used a different approach to estimate future annual beach nourishment needs (USACE, 2020b). The approach uses the following equation:
where, VA = Annual volume, VT = Total volume placed 11.25 million m3, N = Number of nourishments = 4, YL = Year of last nourishment = 2017, and Y1 = Year of first nourishment = 1998. Solving Equation (3) yields VA = 440,000 m3/yr. This annual volume is at odds with measured losses of only about 70,000 m3/y. If 11.25 million m3 were required from 1998–2017, it may seem reasonable to assume the same volume would be required for the next 19 years. However, this approach neglects the fact that Panama City beaches did not return to their prenourishment beach widths over the 19 years, but instead beach widths increased 32.4 m over 20 years despite longshore and borrow-site losses. Placing another 11.25 million m3 would widen beaches an additional 32.4 m (additional losses would need to be considered that are attributable to increased sea-level rise for the next 20 years compared with the previous 20 years).
The longevity of nourishment sand at Panama City beaches is reminiscent of the Harrison County, Mississippi, United States, beach project. That project has a length of 43 km and is the longest U.S. beach nourishment project. Similar to Panama City, the shoreline is fairly straight, not interrupted by inlets, has relatively low longshore-sand transport, and the wave climate is relatively benign, except for hurricanes. It differs from the Panama City nourishment in that only 4,600,000 m3, or 107 m3/m, of sand was placed. After 7 years, profile measurements along a section of the nourishment showed that 98% of the sand was still in place on profiles (Watts, 1958). A renourishment in 1972–73 added only 1,500,000 m3 of sand. The nourished shoreline has been successfully in place for more than 60 years from 1951 to at least 2013 (Houston and Dean, 2013). Neither Hurricane Camille (1969) or Hurricane Katrina (2005), both of which were Category 5 storms that directly struck the project, caused significant shoreline recession. Hurricanes generally move rapidly, and they sometimes do not cause as much shoreline recession as slow-moving extra-tropical storms. This also may explain why Hurricane Michael did not cause greater beach erosion at Panama City beaches.
Sand has not gone missing at Panama City beaches. Most of it is still located on profiles, widening beaches by at least the amount predicted in the design of the beach nourishment project. None of the sand that left the project area appears to have been lost to the littoral zone because it either moved to NW shorelines or refilled borrow-site holes within closure depth.
The equilibrium profile concept works quite well in describing beach change at Panama City beaches. Beach nourishment sand spreads along offshore profiles such that they retain their basic original shape but are translated seaward. Sand remains on profiles down to closure depth except for sand lost at lateral boundaries. Storm waves and surge move profiles toward equilibrium. The very simple Equation (1) reasonably predicts beach width change vs. volume of sand placed, and Equation (2) reasonably predicts profile rise vs. beach-width change. Of course, the sand that was placed was of similar size to the natural beach sand. Equilibrium beach profiles would need to be modified for a markedly different sand size (Dean, 1991).
The Panama City Beach Shore Protection Project is not typical because it stretches over a long distance, is not intersected by inlets, longshore transport losses are not great, and borrow sites contain sand similar to beaches. However, equilibrium profile concepts may work reasonably well for beach nourishment projects on other shorelines. As noted in the “Introduction,” Equation (1) has been shown to work well for a wide variety of beaches on the Florida east coast and South Carolina coast (Houston, 2019, 2021a).
© Coastal Education and Research Foundation, Inc. 2023