ABSTRACT

Black, K.P.; Reddy, K.S.K.; Kulkarni, K.B.; Naik, G.B.; Shreekantha, P., and Mathew, J., 2020. Salient evolution and coastal protection effectiveness of two large artificial reefs. Journal of Coastal Research, 36(4), 709–719. Coconut Creek (Florida), ISSN 0749-0208.

Two offshore reefs constructed at Ullal, Karnataka, on India's west coast, are exhibiting coastal protection benefits. The reefs are large (275 m and 325 m long) and uniquely placed well offshore (600 m and 760 m) in intermediate depths (6 and 7 m below chart datum). Their crests are between low-tide and midtide level. Bathymetric and beach surveys show that sand has accumulated to form two evolving shoreline salients, which are currently 2–3 times the length of the reefs. The salients have grown underwater on the inner shelf to create a large sand-retention zone up to 1 m thick and stretching from the beach to the distant lee of the reefs. The reefs are the first protection structures that have restored the beaches and enhanced sand storage over this broad, severely eroded region, both inshore and on the inner shelf. Prior rubble-mound rock revetments (seawalls) and 50–70-m-long groynes failed to restore the beach in stormy monsoon conditions, while the inner shelf continued to erode. The reefs were designed using a combination of computer simulations, measurements of natural salients in India, and empirical relationships. The methodology that led to the reef shapes/sizes and their position offshore is discussed, along with monitoring results.

INTRODUCTION

Much of the Indian coast is suffering from severe erosion. There are numerous interruptions to littoral drift caused by port structures, dredging, and river training walls, while construction on primary sand dunes and steepening of the nearshore are changing the longshore and cross-shore sediment dynamics. Illegal and sanctioned sand mining is depleting the beaches, along with dams and changes to river flows due to irrigation and municipal water supply. When combined with sea-level rise from climate change, the future is bleak. Coastal protection solutions that are applicable and cost-effective, both now and in the future, need to be identified (Black et al., 2019).

Coastal protection measures in India also need to accommodate the tropical monsoon hydrodynamics and sediment dynamics, where the seasons of consistent wind and wave directions are several months long, rather than the more rapid weather and wave variations occurring in temperate zones. This gives the beaches more time to respond to a relatively constant condition. Along the west coast of India, the open beaches typically erode by 30–50 m during the southwest monsoon (June–September). The beaches naturally accrete back to their original state during the early phase of the dry season (October–December) (Baba, Hameed, and Thomas, 1988), but many degraded beaches are suffering net erosion each year (Rajawat et al., 2015). The annual signal is modulated inshore by changing wave directions, driving longshore and cross-shore currents, and by winds, driving inner-shelf near-bed currents (Black et al., 2008; Kumar et al., 2012). Both longshore and cross-shore sediment transport plays an important role, although very few studies have made direct measurements of sediment dynamics in India (Black et al., 2019).

In this paper, a case study is presented from Ullal (Mangalore, Karnataka), on India's west coast, where a succession of coastal protection measures were built, and long-term monitoring of the physical system was conducted. The site is highly informative because various structures were built in sequence over a period of 20 years, and so the effectiveness of each type can be assessed individually. The focus is on two offshore reefs built for coastal protection in early 2017. These reefs are uniquely larger and further offshore than any previously built for shore protection. Consequently, this is the first time that the evolution of sedimentation in the lee of such large reefs has been monitored at the shoreline and offshore.

The reefs induce wave dissipation and shorten the wave period (Lee and Black, 1978) to provide shelter and a local reduction in littoral drift, which creates a salient in their lee (Black and Andrews, 2001a,b). This new beach then protects the coast, rather than the reef itself. Offshore reefs provide a sensitive and effective solution for coastal erosion (Black and Mead, 2007), with the works well offshore, rather than on the beach. The reefs act on the waves, which are the primary cause of erosion at the shoreline, and provide shelter for boats, a good boat landing on the lee salient beach, and habitat for fish on the reef. Reefs also allow natural passage of sand along the coast after the salient comes into equilibrium with the sediment dynamics and hydrodynamics (Black et al., 2019). In contrast, with rock revetments, local fishermen in India complain that the revetments are slippery and dangerous, which stop them using small boats and nets. In many cases, the beach is lost in front of the seawalls, leaving them exposed to larger wave impacts and ultimately requiring bigger rocks and higher walls.

History of Coastal Protection at Ullal

The confluence of the Gurpur and Netravati Rivers formed a wide natural entrance to the sea at Ullal (Figure 1). The distal ends of the two barrier spits composed of medium sands (D50 = 0.35–0.45 mm) were stabilized by entrance training walls and channel dredging in 1994 to provide access to Old Mangalore Port (Figure 1). With net transport coming from the north, Bengre Beach north of the inlet widened by 300 m, while erosion was severe to the south.

The early stage emergency beach protection structures to the south were sand-filled plastic bags, and then rock-filled gabion baskets. They failed, and then simple rubble-mound seawalls/ revetments, built near the high-tide line, were used extensively from 1996 to 2012 (Figure 2). The seawalls started with relatively low crest height, around high tide at 1 m above chart datum (CD). However, overtopping during large wave events, causing damage to houses, led to larger rocks and higher wall construction until the maximum elevation at the site is now up to 6 m above CD.

The high seawalls halted land erosion, but exacerbated erosion underwater due to toe scour and their imposition on the natural beach system, which prevented the seasonal two-way sand transfers between the beach and offshore. Measurements showed an immediate reduction of beach width in front of the seawalls; beach elevation dropped by 2 m in just 2 years after construction (NITK, 2007), while scour continued offshore underwater. This allowed larger waves to break on the seawalls and explains the need for higher walls with larger armor stones.

Eight groynes, each 50–70 m long (Euroconsult, 2015; Oberhagemann, 2012) (Figure 3), were then constructed between 2014 and 2018, initially using sand-filled geotextile bags of 1, 2.5, and 7 m3. The geocontainers were damaged during the first monsoon by waves, and the sealing caps were lost, both allowing sand to escape. The groynes were then rebuilt using rocks and tetrapod roundheads in 2016 (CWPRS, 2013) (Figure 3). However, the 2 tonne tetrapods on the head were damaged in the monsoon, and they are now being replaced with 5 tonne tetrapods. The groynes were constructed along the seawalls and along an indented beach within the erosion zone.

In front of the seawalls, the various outcomes with the groynes ranged from no beach forming in the compartments during monsoon to limited, intermittent sediment accretion (Figure 3). A wedge of sand formed intermittently on one side of some groynes, with no accumulation on the other side, while a narrow beach, evident at low tide, proved to be transient and not present during the monsoon. The security of local residents was still reliant on the presence of the large rock seawalls. Along the indented beach section, the beach widened during the dry season, but groyne compartments emptied quickly under the higher and steeper monsoon waves, sometimes cutting the beach to the lee of the groyne (Figure 3).

As the erosion progressed, several major programs were established to monitor waves, currents, and bathymetry. Some used numerical modelling to identify the causes and cures for erosion (ANZDEC, 2009; KERS, 1989; NITK, 2007). In 2009, the following hybrid coastal protection solution was proposed by ANZDEC (2009):

  1. three large offshore artificial reefs: two were constructed in early 2017 and are the focus of this paper;

  2. four nearshore reefs with shoreline attachment; these were not built, instead the 8 groynes discussed above were constructed;

  3. changes to the river entrance training walls to encourage natural sand bypassing: the southern training wall was shortened by 100 m in January 2016; and

  4. initial nourishment of the beach; not yet undertaken.

METHODS

Detailed bathymetric and beach profile surveys were undertaken seven times from September 2008 (prior to the reefs and groynes) until November 2018 (postconstruction) (Table 1). Postconstruction surveys used multibeam depth sounders with wave and tide corrections. Four surveys had narrow gaps in the nearshore (between 0 to 4 m depth) due to boat access issues, and so data from these zones are not considered here.

Wave Conditions

Data from the Wavewatch III model (Tolman and Chalikov, 1994; WAVEWATCH III Development Group, 2019) over 13+ years show that three different wave conditions predominate along the Karnataka coast (Figure 4). In the dry season, waves with typical heights of <1 m and periods >12 s come from the SW quadrant (215°T), mostly generated in the Southern Indian Ocean. These arrive all year, but they are largest during the Southern Hemisphere winter in June to August. During the monsoon (from June to August inclusive), waves with 2–5 m height and periods of 8–10 s dominate from the W (265°T), associated with local monsoon winds. Winds off Oman and Yemen create the “Shamal” waves, with 1 m height and periods of around 5 s, from the NW (315°T) from November to May (Aboobacker, Vethamony, and Rashmi, 2011; Glejin et al., 2013).

While not presented here, long-term wave conditions from the WAM model were found to be compatible with local Acoustic Doppler Current Meter (ADCP) measurements made for 40 days during the 2017 monsoon and again in the premonsoon in May–June 2018 at nearby Someshwara (6 km south) at 8 m depth. The instruments additionally provided time-averaged flows in 0.35 m layers through the vertical water column (discussed below).

Reef Description

The reefs were constructed using a rock core covered by 7.2 tonne concrete blocks at the crest and 4 tonne tetrapods as armor layer (CWPRS, 2013) (Figure 5). The north reef is 275 m long in 6 m depth at 600 m offshore. The south reef is 325 m long in 7 m depth at 760 m offshore. Both have concave-seaward “cup” shapes (discussed below). Their orientation was set to be perpendicular to the large monsoon waves, which come from 265°T, thereby maximizing the size of the wave shadow during the erosive period. Their effective lengths perpendicular to the wave direction are 250 m and 295 m, respectively, in the alongshore direction.

Mean crest heights on the north and south reefs are 0.75 and 0.35 m above CD, respectively (Table 2). The original design crest height was set to 1.8 m above CD (ANZDEC, 2009), which included an allowance for settlement on the fine sand and clay substrate (Table 2). Based on 20-m-deep boreholes, the settlement was estimated to be 0.65 m (Sarathy Geotech, 2010). The construction designers suggested 1.0 m of rapid settlement (CWPRS, 2013) and ultimately set the construction crest height to 0.5 m after settlement. Since construction over 2 years ago, measured crest levels have subsided by ∼0.5 m on the north reef and ∼0.2 m on the south reef, both of which are within the estimated ranges.

Tidal heights at the site are 0.25–1.75 m, with mean sea level at 1 m above CD, which puts the crest between mid and low tide on the north reef and at low tide on the south reef (Table 2). The crest elevations are both less than the original design, but they are close to the construction design elevation of 0.5 m.

RESULTS

An overview of the shoreline changes is first provided by examining historical Google Earth images. The detailed changes are then presented in map and transect form using the combined offshore and concurrent beach profile measurements.

Aerial Observations

Aerial and Google Earth images show that by November 2018, 1.5 years after reef construction, a new salient beach up to 40 m wide had formed in the lee of both reefs (Figure 6a,b). The northern salient was 600 m long, which is 2.4 times the alongshore reef length, while the southern salient was 890 m long, which is 3.0 times the effective alongshore reef length. Unlike prior works, the salient beach actually grew larger during the monsoon. Moreover, on the beach section, the groynes were buried in sand, i.e. no longer sitting proud of the beach as seen prior to reef construction (Figure 6c,d). In front of the seawalls in the gap between the reefs, more accretion was present in the groyne compartments, formed by sand spilling from the reef salients (Figure 6e), compared to preconstruction conditions.

Bathymetric Surveys

Preconstruction surveys show a similar seabed bathymetry from 2008 to 2014 (Figure 7a). Postconstruction, new accretion began to extend across the inner shelf beyond the beach salient out to the immediate lee of the reef (Figure 7b). The offshore accretion was already present by October 2017, one monsoon after construction. By November 2018 (1.5 years after construction), a large underwater salient had grown from the shore to the lee of both reefs. The accretion zones cover some 500,000 m2 in total and have a total volume gain of order 100,000 m3 yr–1.

During the two postmonsoon surveys (October 2017 and November 2018), the apex of the salient was located in the lee of monsoon waves coming from 265°T. However, the premonsoon April 2018 survey shows the salient further north in the lee of the SW waves of the dry season. This shift occurs right across the inner shelf, not just at the beach, indicating that sediment transport is occurring offshore in depths up to 7 m in both seasons. Indeed, the 2018 survey reveals scour at >8 m depth in the immediate lee of both reefs (Figure 7b) (discussed below).

Bathymetric Transects

Figures 8 and 9 show cross-shore transects taken through the north reef, south reef, and in the gap between the reefs. For clarity, Figure 8 shows the nearshore zone with one preconstruction survey (from September 2008) plus the three postconstruction surveys. The 2008 survey had no gaps near the beach. The difference in depths between 2008 and November 2018 across the full transect from the shore to beyond the reef is shown in Figure 9.

In the lee of the north reef (Figure 8A), growth of the beach and adjacent subaerial seabed over the 1.5 years since reef construction was very large, with 5.7 m of vertical sedimentation at the toe of the new beach. The same occurred in the lee of the south reef (Figure 8B), where sedimentation above the 2008 condition was >4 m over much of the profile. Beach elevation was 1.3 m higher in November 2018 than in October 2017. The beach in April 2018 was lower because the salient had migrated to the north, away from the selected transect. In the lee of both reefs, the greatest rate of accretion occurred during the first monsoon before October 2017, while slower sedimentation is still ongoing.

In both lee transects, the exponential shape of the profile in 2008, indicative of erosion, has been replaced in the nearshore by a subtidal platform-shaped “diffusion bar” at 1 m below CD (Black, Gorman, and Bryan, 2002). The bar, evident after the first monsoon, is continuing to grow up and seaward, especially on the offshore foreslope. While the public cannot see this feature, its presence is a strong indicator that the full beach system (above and below water level) is being repaired.

In the 750-m-long gap between the reefs in front of the seawalls, the pattern is different. There was minimal sedimentation after the first monsoon in October 2017 (Figure 8C). The beach crest was highest in April 2018, but the offshore bar was still mostly absent. By November 2018, the beach crest level had risen, and a diffusion bar had formed below CD. The results indicate that the profile is being repaired, albeit slower than in the direct lee of the reefs. Sedimentation in front of the rock seawalls after the monsoon has no precedent at this site. The accretion is therefore due to the reefs, which are growing the beach beyond their own length. The new beach then provides the coastal protection.

Figure 9 presents the seabed depth changes between September 2008 and November 2018 along cross-shore profiles that extend beyond the reefs. Depths offshore of the reefs have eroded slightly, possibly due to the continuing long-term erosion after 2008 caused by the construction of the updrift river training walls. In the lee of the reef, there is accretion across most of the two transects, with highest gains near the reef (about 1 m of shoaling) and inshore at the beach. Scour in the immediate lee of both reefs is evident. In the gap between the reefs, the changes are small, except for the accretion at the shoreline and the unfinished repair of the profile immediately offshore (Figure 9C).

DISCUSSION

The Ullal shoreline has been protected by sandbags, gabions, rock seawalls, groynes, and offshore reefs in a succession of projects. An assessment has provided insights into the performance of the various coastal protection measures, which have been subject to the same sediment dynamics and wave/ wind climate on the west coast of India.

The groynes have not produced a permanent beach. However, the groynes are short, relative to the monsoon surf zone width, and so the compartments are leaky and on a steep, eroding shoreline. Longer groynes, which better compartmentalize the beach and allow for natural beach rotations between the seasons, may have been successful. However, these structures block littoral drift, and the resulting downdrift erosion can be prevented only by constructing a groyne field covering the full sediment cell or by construction of seawalls on the downdrift beaches (Black et al., 2019).

With the reefs, growth of salients and beach restoration were observed after just one monsoon. The reefs are therefore providing the first solution that is restoring the beach at Ullal. No permanent accumulation had occurred prior to reef construction along the seawalls, while the sandy beach area was subject to annual erosion each monsoon. Using the zones beyond the influence of the reefs as a control base case, these zones have not exhibited the same sedimentation; severe scour is still occurring on other nearby beaches.

In the simplest terms, the reef creates a zone of reduced longshore transport in its lee. Littoral drift moving on the open beach is larger than in the shadow zone, which leads to accumulation in the lee of the reef, where sediment inputs are greater than outputs. At the scale of the reefs presented here, wave diffraction around the tip of the reef is not the primary mechanism for salient formation (Black and Andrews, 2001b).

The accretion in the lee of the reefs may be surprising given the highly adverse environment:

  1. The region has been suffering erosion for decades due to sediment shortages from the north.

  2. The salient and offshore accretion are forming in a broad zone that extends offshore underwater where the original sediments were muddy/clay.

  3. The salient is forming in front of a rock wall where beaches are normally disturbed by turbulence at the toe and wave reflections.

  4. Sand extraction is ongoing from the river, thereby reducing or eliminating delivery of new sand to the beaches.

At odds with normal behavior in India, the salients grew larger during the monsoon, instead of the usual beach erosion that occurs under short-period storm waves and onshore winds. This may be occurring because the monsoon is mobilizing the updrift beach and inner shelf sands, which are then accumulating once they enter the wave shadow of the reefs.

Reef Design Parameters

The Ullal reefs are larger and further offshore than any previous offshore protection structures. They were designed using a combination of computer simulations, observations of natural salients in India, and empirical relationships. Black and Andrews (2001a,b) showed that the size of the salient in the lee of natural reefs and islands was primarily dependent on the alongshore length of the reef (B) and the distance offshore (S). They obtained the following empirical relationship to predetermine the size of the salient at its apex (widest midpoint):

 
formula

where, Xoff is the distance between the tip of the salient and the offshore reef.

This relationship was developed using cases from temperate wave climates, and so Indian cases were examined to determine if the formula remained valid in the tropics. Adopting the same methods as Black and Andrews (2001a), Equation (1) was confirmed (Figure 10) against measured reef and salient sizes (Table 3). The data included one case of a modern salient formed in the lee of a shipwreck just north of Ullal on Bengre Beach. Mostly, the reefs or islands were several hundred meters offshore (Table 3), and all of them had very well formed salients in their lee. This gave confidence to place the new reefs well offshore.

As a function of the reef's offshore distance, Equation (1) predicts a rapid increase in salient size initially, but the rate slows at larger distances offshore; e.g., for a theoretical 250-m-long reef, the salient size stops increasing beyond about 1200 m offshore (Figure 11). Because reef construction costs rise with depth, and there are diminishing shore protection benefits for greater offshore distances (Figure 11), the reefs were ultimately placed at 600–760 m, with the larger reef further offshore.

Equation (1) indicates that longer reefs produce a larger salient. However, a series of smaller reefs, rather than a single long reef, has been shown numerically to provide more effective coastal protection (Black and Mathew, 2015). In confirmation, Equation 1 indicates that halving the reef length does not halve the salient apex size. For example, changing the reef length from 200 m to 100 m (placed 500 m offshore) reduces the salient apex width by 63%, rather than 50%. Consequently, two reefs were constructed, rather than a single larger reef, to increase the benefits per unit reef length.

Sediment Inputs

Around the surf zone, longshore sediment transport calculations (using the 5 year wave time series shown in Figure 4 for boundary conditions) predicted net transport to the south is 116,000 m3 yr–1 on Bengre Beach (located north of the river entrance) and 131,000 m3 yr–1 at Ullal (Table 4). As a validation, approximately 1.4 million m3 of sediment has been trapped adjacent to the river training wall on the northern beach over 10 years, i.e. 140,000 m3 yr–1 from the north (Black and Mathew, 2017), which is close to the prediction from the empirical longshore transport formulae.

Bengre Beach has now accreted to the tip of the northern breakwater, and bypassing may be occurring again. In addition, the shortening of the southern breakwater wall has led to diversion of the river entrance channel to the south (not shown here). This is assisting transport across the entrance. As such, the net accumulation of order 100,000 m3 yr–1 in the lee salients is compatible with expectations.

Offshore on the inner shelf, currents measured at Ullal in 8 m depth during the 2018 monsoon were southward, under winds with a southward net vector (Kumar et al., 2012). In the cross-shore direction, downwelling associated with onshore wind-driven surface currents induced a measured offshore current at the seabed. With wave orbital motion assistance, eroded beach sands can be carried well offshore on the inner shelf (Black et al., 2008). Concomitant alongshore currents carry this sand into the lee of the reef, where it can settle due to the reduction in bed orbital motion in the sheltered zone, as confirmed by measured sand accumulation during the monsoon.

Initial and Equilibrium Phases and Downdrift Impacts

In the initial phase after reef construction, sand was captured to grow the salient and so downdrift impacts could occur. After the initial phase, when the salient stops growing and there is no further demand for net capture of sediment, sand inputs/outputs must be balancing, albeit within a variable dynamical system or “pseudo-equilibrium” (Black and Andrews, 2001b). At this equilibrium stage, the beach is wider, the natural passage of longshore sediment transport comes into equilibrium with the salient's sedimentary environment, and the demand for net capture of sand ceases. Specifically, reefs and their salients do not cause net downdrift erosion once the equilibrium has been reached.

Nourishment was recommended to prevent downdrift impacts during the initial phase when the salient was developing, but the nourishment has not been undertaken. The impact depends on the fraction of sand captured from the net sediment flux. Surveys already show a salient apex width of 40 m, with a subtidal bar extending 100 m further offshore and accretion out to the lee of the reefs at 600–760 m offshore, suggesting that the fraction being captured may be large. Consequently, initial nourishment at Ullal is especially important, and new interruptions to the natural sediment supply must be prevented.

Effects of Circulation and Dynamics over Submerged Reefs

The 2018 survey shows systemic localized scour in the immediate lee of both reefs (Figures 7b and 9). Wave breaking drives strong currents over the reef crest (Lee and Black, 1978; Symonds and Black, 2001; Symonds, Black, and Young, 1995), and the return currents in the immediate lee of the reef induce the scour (Black and Mead, 2007; Ranasinghe, Turner, and Symonds, 2006). A simple calculation shows that for a typical crest current of 1 m s–1 (Symonds and Black, 2001) with depth of 1 m over a 300-m-long reef, the currents will bring ∼4 × 106 m3 over the crest in a single high tide of 4 hour duration. The lee scour is the equivalent of a rip current channel adjacent to a sand bank.

Lee scour can undermine the reef structure, which may justify the greater expenditure at construction time for a higher crest. In addition, if a submerged reef is too close to shore, this current can scour the beach and prevent the salient from forming. The lee scour is not present on emerged reefs because wave-driven currents across the crest are absent. Thus, a submerged reef should be placed further offshore than an emerged reef.

The concave-seaward “cup” shape at Ullal was designed to beneficially utilize the wave-driven flow over the reef crest. Feeder currents from offshore should bring sand into the cup to help stabilize the reef foreslope. Although theoretically anticipated, there is not yet any clear evidence of seabed accumulation (Figure 9), possibly due to the dominance of muddy clays offshore of the reef. A second benefit of the cup shape is the increase in wave scattering by refraction over the crest, which broadens the reef wave shadow at the shoreline. The cup shape has two negatives. First, the bowed crest shortens the effective reef length perpendicular to wave propagation. Second, breaking waves run down the offshore side of the reef arms to meet in the central part of the cup. The higher waves may damage the reef crest at this location if not accounted for at the engineering design stage.

Crest Height

A crest around high-tide levels, plus an additional allowance of up to 1 m for other sea-level factors, will produce optimal performance (Black et al., 2019). However, construction costs are less with a lower crest, and the Ullal results show strong coastal protection with the crest at low to midtide. At Ullal, the tidal range (∼1 m) is of the same order as typical wave heights. If the tidal range is large, then a smaller fraction of the waves will break over the tidal cycle, and so the crest must be higher in these locations. In general, the designer will balance costs and budget, risk of subsidence or damage, offshore depths, tidal ranges, typical wave heights at the site, and expectations for beach widening. At one extreme, an emerged breakwater will give absolute protection and a resilient salient, but factors such as visual impact and extra cost may not warrant such a high structure.

Coastal Rotation

In India, where river and dune/cliff sediment supplies have been significantly reduced, a long-term option is to reorient the beaches onto a more neutral sediment transport alignment. “Grand schemes,” which reduce the need for new sand on beaches, may become more important under climate change, particularly with unabated demands for construction sand (Black et al., 2019).

The reef salient acts as a natural headland, which beneficially reorients the updrift shoreline onto a more neutral angle. Coastal stabilization can then occur over a distance that is much greater than the reef length (Black and Mead, 2001). The new beach provides the required protection at the shoreline, not the reef itself. Accordingly, two reefs were designed, with the smaller reef updrift to initiate coastal rotation, and the larger reef downdrift to further rotate the beach onto a more neutral orientation.

The alongshore length of the salient is normally 5–8 times the length of the reef, mostly due to variations in wave orientation (Black and Andrews, 2001a,b). By creating additional beach to protect the coast, this 5–8 factor helps to offset the higher costs of reefs over other forms of coastal protection. Moreover, in India, where many beaches are threatened, large offshore islands may return costs through tourism, lower maintenance, deep ports, and/or real estate. These may be preferred in the future under climate change conditions (Black et al., 2019). The Ullal results demonstrate that reefs/islands placed well offshore can protect the coast and rotate beach orientation onto a more neutral sediment transport alignment to maximize the length of protected coast.

CONCLUSIONS

Two large reefs with low to midtide crest elevations were constructed well offshore in 6–7 m depth at Ullal in Karnataka, west coast of India. The offshore reefs are providing successful restoration of the beach, with rapid and substantial sediment accumulation in their lee. The reefs have induced permanent beach widening plus sand accumulation on the inner shelf, including during the monsoon. The low crest height of the reefs was shown to be effective at Ullal, where the tidal range is small.

For reef design, the empirical relationship of Black and Andrews (2001a), which was developed for temperate beaches, was shown to be also suitable for tropical Indian beaches. The reefs were placed 600–780 m offshore, where optimal salient apex size was predicted. Further distance offshore would have increased the cost without proportional coastal protection benefit. Shorter multiple reefs, rather than a single long reef with the same total length, were shown to provide a stronger coastal protection benefit for the same costs, and so two reefs were constructed. Design decisions needed to balance costs and budget, risk of subsidence or damage, offshore depths, tidal ranges, typical wave heights at the site, and expectations for beach widening.

The Ullal results confirm that large reefs or islands placed well offshore beneficially protect the coast. In India, large offshore reefs/islands, which can return costs through tourism, ports, and other ventures, may be a preferred option under climate change due to sea-level rise and future demands for construction sand in India (Black et al., 2019). One long-term goal is to reorient the beaches onto a more neutral sediment transport alignment, so that new sand supplies from updrift are reduced or potentially no longer required.

ACKNOWLEDGMENTS

The Sustainable Coastal Protection and Management Investment Program (SCPMIP) is a loan project funded by the Asian Development Bank. The authors thank the Government of Karnataka and Asian Development Bank and the Project Management Unit, SCPMIP, for the privilege of undertaking this study, funding support, and permission to publish this study. CWPRS, Pune, India, undertook the wave flume studies and engineering reef construction design. Many scientists and academics were part of the Artificial Reefs Program, including Professor Terry Healy, Department of Earth Sciences, University of Waikato, New Zealand, who helped so diligently and provided additional momentum for the reef program initiated by Professor Black.

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