Understanding Reef Flat Sediment Regimes and Hydrodynamics can Inform Erosion Mitigation on Land

Coral reefs across the globe are affected by land-based sediment, which can increase turbidity and, in turn, smother corals and reduce light levels. Sedimentation may result in reduced coral growth [1,2] and recruitment [3,4,5], reduced photosynthesis rates in coral symbionts [6,7,8], and prevention of waste removal and gas exchange in corals [9]. Effects on reef fishes can include changes in physiological and ecological processes, including reduced foraging efficiency [10], prolonged larval development [11], and changes in use of coral reefs [12].

The likelihood of effects from sedimentation is higher for reefs in poorly flushed locations than in well-flushed locations [6]. Water circulation in coral reefs is highly variable in space and time, which leads to large variation in where land-sourced and reef-sourced sediments are deposited, how long sediment remains, how often it is resuspended, and its movement across the reef [13]. These spatial and temporal variations affect the distributions of coral species that have different levels of tolerance to sediment exposure [14,15 16].

In Hawai’i and other islands of steep, short montane watersheds, there is broad recognition that land and watershed management affects inputs of nutrients and sediments [17,18]. Requirements for mitigating effects of land-based sediments have been developed and implemented in such settings in Hawai’i through partnerships among state agencies, federal agencies, and local community watershed alliances[19]. These efforts identify major sources of terrestrial sediment and develop practices for land management to reduce sedimentation [20].

In north-facing Hanalei Bay, Kauai, for example, river plume sediments draining into the bay during the summer are displaced by wave driven resuspension, which intensifies and prolongs the potential effects of the plume itself. On the west-facing reefs of the western Maui, Hawai’i coast, waves and winds drive water flow in the shallow areas, and tides modulate currents in the deeper parts of the reef [21,22]. Along the south-facing coast of Molokai, Hawai’i, waves and tidally driven flows modulate the sediment advection across the reef flat; during rising tides, waves driven by trade winds blowing across the east-west trending reef flat lead to sediment resuspension, which sometimes can drain off the reef on ebb (falling) tides [23].

We focused on an east-facing reef and coastal area on Lāna’i Island, Hawai’i,where a local community organization (Maunalei Community Managed Makai Area) had developed a sediment remediation and stream restoration project in the ephemeral Maunalei stream that employed gabion dams. We also evaluated the efficiency of these mesquite check dams as sediment remediation mechanisms. This was a collaboration involving a non-governmental organization (Conservation International), community organization on Lāna’i Island (Maunalei Community Managed Makai Area), academic scientists (University of Hawaii-Manoa), and the Pacific Islands Ocean Observing System (a network of ocean monitoring systems for Pacific Islands). Our objectives were to assess spatial patterns of total suspended sediment (TSS) and investigate factors that affect the variability of TSS loads; to compare TSS and hydrodynamics at two reef flats in eastern Lāna’i, one of which receives substantial amounts of terrigenous sediment during the winter rainy season; and to estimate water and sediment residence time at those two sites. Such data are critical for informing local stream restoration and sediment remediation.

Lāna’i (20.8971°N, 156.8740°W) is the sixth largest of the main Hawaiian Islands, with an area of 364 km² (Fig. 1A). Lāna’i is located in the rain shadow of eastern Molokai and western Maui and receives about 270–1,400 mm of rain per year, with rainfall increasing as elevation increases; this rainfall is roughly 10 times less than that observed at the highest elevations of Molokai and Maui(Supplementary Material [SM], Section 1, Fig. S1) [24]. Across the Hawaiian Islands, flash floods occur most frequently in the winter and spring (October through April) [25]. Lāna’i has a long history of land use, from its early occupation by native Hawaiian societies to extensive agriculture during the colonial era. Currently, about 3,100 people live on the island [26]. Watersheds on Lāna’i and in Hawai’i, overall, are very short (on average < 10 km) compared to continental watersheds and catchments. Therefore, streams in semi-arid areas of this region are ephemeral and are prone to flash flooding, as rainfall can cause stream flow to initiate and increase rapidly.

We conducted our study in the eastern portion of the island, which faces the trade winds. On land, we worked at two ephemeral streambeds (Site A-SS [Stream Site] and Site B-SS [Stream Site]),and in the ocean, we worked on two reef flats adjacent to the streambeds (Site A-RF [Reef Flat] and Site B-RF [Reef Flat]) (Fig. 1B).

Because no measurable sediment had been delivered by the stream into the reef flat at A-RF, we treated it and A-SS as controls. At B-SS, the community organization (Maunalei Community Managed Makai Area) constructed seven gabion dams of invasive mesquite (Prosopis pallida;kiawe [Hawaiian language]) logs and branches across the dry streambed (Fig. 2B). The gabions were staked in place from January–March 2014 to capture sediment that would otherwise flow into the coastal ocean.

We deployed seven Onset HOBO pressure sensors (Bourne, MA, USA) from January 10–March 17,2014 between the gabions in Maunalei stream (Site B). We deployed three sensors in the streambed at Site A at the same elevations as the upstream, midstream, and downstream sensors in Maunalei stream(Fig. 1C; SM, Section 2, Table S1). These pressure sensors were deployed to monitor the resulting stream flows during rain events. Here, we define rain events as precipitation over the course of 24 hours, and any resulting flash floods that flowed through the streambed. We took temperature and pressure readings at 300-s (5-min) intervals. After recovering the sensors, we recorded the distance between the streambed measurement markings on the sensors (see SM, Section 2) to estimate the sediment accumulation at each sensor. We aimed to test whether sediment accumulation varied as a function of distance from the furthest-upstream gabion dam.

We deployed two types of water quality sensors (a Sea Bird Electronics [Bellevue, WA, USA] 19plus CTD meter with a WETlabs C-Star transmissometer and WETstar flourometer) at Site A[20.895136°N, 156.87099°W] and a SBE16 with a Wetlabs ECOFLNTUS at Site B[20.89880°N, 156.87485°W]) at Site B-RF (Fig. 1B)from 10 January–17 March 2014 (wet season) and again from 5 June–4 August 2014 (dry season) to measure temperature, salinity, and light transmittance at a 600-s (10-min) interval. These were the instruments available for the study, and purchase of new equipment to duplicate instrument packages was beyond our budget. The transmissometer reports transmittance, and the ECOFLNTUS yields outputs in nephelometric turbidity units (NTUs). We took 10 measurements per sample and delayed 5 s before sampling. Light transmittance can be used as a proxy for turbidity. The sensor at the site A was deployed in a channelized, coral-dominated region; the sensor at site B was deployed in a wide sandy area. We deployed the sensors to monitor whether the variables changed on the reef as a result of a rain event recorded by the pressure sensors on land. We also deployed Nortek Aquadopp current profilers (Rud, Norway) at Site A-RF and Site B-RF from 5 June–4 August 2014 and recorded a 1-min average of current velocity once every 10 min (Fig. 1B). We deployed a solar-powered Davis Weather Monitor II station(Hayward, CA, USA) with an anemometer and wind-vane on an adjacent ridge (20.8943°N,156.8763°W) (Fig. 1B). Wind was recorded as an hourly average of measurements taken at 30-s intervals.

We installed tubular sediment traps (6.8 cm diameter with 1 cm diameter baffles on the open end)next to the other instruments at Site A-RF and Site B-RF from June 5–August 4, 2014. After recovery of the traps, we processed the sediments and measured their inorganic carbon(CaCO₃) and total carbon and nitrogen. To determine the potential sources of organic carbon, we measured δ¹³C and δ¹⁵N (refer to SM, Section 3, Fig. S6,Fig. S7).

We corrected sediment data for drift from biofouling by adjusting values on the basis of a linear fit of light transmittance over time. After conversion to TSS, the results of the linear correction were closer to what was measured in the field, i.e., through the bottle sample data. Therefore, we proceeded with linear fit. Furthermore, the most conservative way to address fouling was to assume that the fouling was linear. We calibrated between the transmissometer measurements of light attenuation and the sediment concentration in the reef waters (i.e., TSS), which allowed us to report data in TSS units (for more details, refer to SM, Section 4).

Because no velocity measurements were available, we used the Gauckler-Manning formula to estimate water velocity as $V=knRh2/3S1/2$

⁠, where V is velocity, k/n is a constant coefficient, k is a conversion factor between SI and English units, n is the Gauckler-Manning coefficient, Rh is the hydraulic radius(area: wet perimeter), and S is the slope. The constant k/n varies between 20-80 m^1/3s^–1 as a function of surface roughness; we used a mean value of 50 m^1/3s^–1 as our constant [27]. We used a slope of 0.1 (taken from a topographic map). Our initial calculation of water volume was based on data from sensor 13 and assumed a constant water height (21 cm), cross sectional area (0.38 m²), and velocity (1.06 m s^–1) over a 2-hour period.

For sediment volume calculations, we calculated the cross sectional area at a sensor with the measured channel width and maximum water level, and assumed that the channel was triangular. This allowed for calculation of a slope angle (which we assumed was constant), and for the area to be expressed as a function of water level. This method also allowed us to determine how much of the total sediment volume was captured between each of the gabion dams.

We calculated a spectral density for each dataset (i.e., salinity, temperature, current speed,pressure, and wind speed) with the Welch method [28]. We then calculated the correlation between the frequencies at which the peaks in each spectrum occurred in relation to the primary 12-hr tidal frequency. We tested the hypothesis that TSS varies as a function of temperature, pressure, chlorophyll a, salinity, current speed and direction, wind speed and direction. In addition, we tested whether TSS varied as a function of time (cumulative hours) to help capture temporal autocorrelation in the time-series, and as a function of site to represent any site-specific differences not captured in the other predictors. We fit a generalized linear model(GLM) with an inverse link function to allow for the positive, continuous response of the TSS variable. We also tested link functions of “log” and “identity” and assessed the behavior of the resulting residuals with reference to model assumptions. We used Pearson’s correlation and variance inflation factors (VIFs) to test whether predictors were colinear. We removed highly colinear predictors and refit the model until all VIF < 3 [29]. The final set of predictors was temperature, pressure, salinity, current direction, wind speed, wind direction, and time.

We fit models with all combinations of main effects before testing for 2-, 3-, and 4-way interactions. We selected models with Akaike’s Information Criterion adjusted for small sample sizes (AICc). We assessed error distributions and the linearity of relations in the best (lowest AICc) model and models with AICc values within 2 units of the model with the lowest AICc. We tested the assumption of linearity by comparing the best-fit GLM to a generalized additive model (GAM) that included the same main effects (interactions cannot be included in additive models). We used hierarchical partitioning to estimate the variance in all hierarchical models explained by each main effect. We conducted all analyses in R [30] with the“car” [31], “AICcmodavg” [32], “hier.part” [33], “dplyr” [34], “mgcv”[35], and “lmtest” [36] packages. Significance levels were 0.05.

We developed a box model to estimate sediment transport at each site. An imaginary 400 m ×400 m box (approximately the distance from the shore to the edge of the reef) was centered on each sensor location (Fig. 1B). We assumed an average water height of 1 m, no shear throughout the water column and homogeneous flow across the designated planar area,and that the sediment was carried across the box at the same velocity as the depth-integrated Lagrangian transport (i.e., the sum of the Eulerian and Stokes drift transports [37]). We assumed the dispersive flux was negligible and that the advective flux predominated [38]. For each site, we then calculated the flux across a 400-m² section perpendicular to the shore by multiplying the 400-m² area by the average sediment concentration and the average alongshore velocity component for that site. This is a first-order, rapid-assessment box model. The model could be improved if a greater number of current profilers were available. The data from these profilers could be used to quantify the cross-shore heterogeneity in along-shore flow speeds and the variation in suspended-sediment concentrations [39]. Hence, the simplistic box model developed here was based on the best data available for these sites. No other physical oceanography data have been published for Lāna’i Island reefs.

We used data generated by the Simulating Waves Nearshore (SWAN) model [40] and provided by the University of Hawaii to assess wave direction over the summer deployment.

Stream depth at Site B-SS during a rain event on 3 February 2014 reached a depth of 0.3 m to 0.5 m in less than an hour; water flowed in the stream for a few hours during this event and during a 3 March event (Fig. 3). The water level at the peak of the rain event on 3 February was higher at the upstream sensors (1, 2, 3, 4), than the downstream sensors (5,6, 7, Fig. 3).

In-stream pressure data indicated that the rain events during February and March 2014 induced flash floods in Site B-SS (Fig. 4B), but no flow occurred in the control streambed, Site A-SS (Fig. 4A). The sediment deposition and maximum water depth during the rain event did not consistently decrease as distance downstream increased, but were highest in the furthest-upstream sensor (#1) at Site B-SS (Fig. 3; Table 1, SM, Section 5,Table S2). We estimated that 55.4 m³ (77.5 tonnes) of sediment was captured. Cumulatively, the first and second gabion dam trapped 88.5% of the total captured sediment (Table 2). Total water volume through the channel was estimated to have been ~350 m³ in under an hour for the 3 March 2014 event, and over 2800 m³ in less than 2h for the 3 February 2014 event.

During the rain events on 3 February and 3 March 2014, Site B-RF had higher levels of turbidity(NTU) than Site A-RF (SM, Section 6, Fig. S2, Fig. S3, Fig. S4). Site A-RF has higher current speeds, higher transmittance %, and lower TSS than Site B-RF, and similar salinity and temperature(Table 3).

The dominant winds were onshore, from the northeast, with a peed of 4 m s^–1± 1.1 (mean ± SD) (Table 3; SM, Section 7, Figure S5). On the eastern shore of Lāna’i, wind velocities during the day were higher than during the night, likely reflecting a solar insolation-driven pressure differential. The SWAN model indicated that wave direction in the study area from June–August 2014 was consistently from 40–80° (E-NE), generally perpendicular to the shore, and in line with trade winds. This wave directionality was consistent with the average current direction of 240° (towards shore)at Site A-RF with a speed of 0.3 m s^–1 ± 0.1 (mean ± SD). The sensor at Site A-RF was deployed in a channelized, coral-dominated region, which explains the relative unidirectionality of the flow (Fig. 5A). At Site B-RF, current velocity of 0.3 m s^–1 ± 0.02 (mean ± SD) in a 317° northwest direction (Table 3; Fig. 5B). The sensor at site B was deployed in a wide sandy area, unlike the sensor at Site A,and no single flow direction was dominant. The pressure record from the reef flat at Site A and Site B revealed patterns common to semidiurnal mixed tides. Depth and tidal ranges were 1.1 m(0.6–1.8 m) for Site A and 1.4 m (1–2 m) for Site B. Light attenuation peaked during spring tides (Fig. 6A,B).

The best model of TSS (no other models were within 2 AICc) included pressure, salinity, wind speed, time, and all possible 2-, 3-, and 4-way interactions. The large number of interaction effects is likely due to the high correlation typical of environmental time-series. The AICc value of the GLM was 89 units less than that of the GAM. The GLM was statistically significant(p < 0.001) and explained 81% of the deviance in TSS (Table 4).

Because we retained outliers on the basis of their ecological relevance to our hypotheses,residuals were significantly different from normal regardless of the link function used (results given used an inverse link function). In addition, autocorrelation was present in the residual time-series, particularly at Site A-RF. The autocorrelation seemed to represent tidally influenced processes with a period of about 12hrs.

The diel range in temperature within each site during both seasons (6–9°C) exceeded the seasonal difference between the sites (1.1°C), which is typical of shallow tropical reefs(Fig. 7A; Table 3). Salinity at both Site A-RF and Site B-RF was close to oceanic with the exception of the two storm events (3 February and 3 March), during which salinity values declined by as much as 2.5 psu. (Fig. 7B). We recorded simultaneous high winds and high turbidity at both sites during the winter (SM, Section 6, Fig. S3, Fig. S4). Even in the summer dry season, TSS was higher at Site B, which received a higher sediment load during the wet season. Mean TSS at the Site A-RF was 4.59 mg L^–1 (range 2.55–12.04 mg L^–1), whereas the mean TSS at Site B-RF was 6.46 mg L^–1 (range 4.69–31.84 mg L^–1) (Fig. 7C).

We used the longer time series from the summer as input to the one-dimensional box model, which yielded an outward flux of 0.1 kg s^–1 at Site A-RF and 0.03 kg s^–1at Site B-RF. The amount of time it would take to move 77.5 t of sediment (the amount estimated to have been trapped by the gabions during a winter rain event) out of the box at the control site(Site A-RF) (i.e., the rate of sediment removal from the site) would be 8 d, whereas the removal rate for the same amount of sediment across the same distance at the gabion site (Site B-RF) would be 35 d (Table 5). Thus, the removal rate at Site B-RF was about 4.5 times slower than that at Site A-RF for the same theoretical input of sediment.

Over the course of the 60-d deployment, Site B-RF collected 152.5 cm³ (wet volume) of sediment, and the control site, Site A-RF, collected 214.3 cm³. From the total amount of sediment collected at Site A-RF and Site B-RF, we estimated a gross annual accumulation rate of 1,286 cm³ (455 g) and 915 cm³ (324 g) per 36.3 cm² (planar area of sediment traps) for Sites A-RF and B-RF, respectively. This translates to 126.4 kg m^–2 y^–1 (346.2 g m^–2 d^–1) for Site A-RF and 90 kg m^–2 y^–1 (246.6 g m^–2d^–1) for Site B-RF.

We estimated net accumulation rates from 11.5–17.3 g m^–2d^–1 for Site A-RF and 8.2-12.3 g m^–2 d^–1 for Site B-RF. Percent weight of calcium carbonate (CaCO₃) in the sediments, which we assumed to be marine-derived, was 39.1% for Site A-RF and 39.3% for Site B-RF (for more details, SM, Section 3). The fractions of inorganic carbon and organic carbon, 4.7% and 1.8%, respectively, were the same at both sites. Terrigenous sediment was not deposited at either site during the dry summer season,which could explain their similar chemical compositions. Although we did not deploy sediment traps during the wet winter season, we expect the organic carbon content in the sediment to be higher at Site B-RF during that time of year due to substantial sediment influx.

Our research suggests that 1) the rate of sediment removal from the reef, i.e., reef exposure to sediment, depends on proximity to terrestrial sediment sources and coastal current dynamics;sediment resuspension can increase the time of local sediment retention from 35 days to 18 months,2) the rate of net sediment accumulation at Site B-RF is higher than that in some other locations in the Hawaiian Islands, 3) reef sites in east Lāna’i within 1 km can have very different sediment and current dynamics, and 4) streambeds adjacent to the east Lāna’i coastline and within 1 km can have different hydrological characteristics, including exposure to winter flash floods.

We found that a community-level effort to remediate sediment measurably reduced the amount of sediment reaching reefs. Our partnership among a community organization (Maunalei Community Managed Makai Area), environmental nonprofit organization (Conservation International), the University of Hawaii, and the Pacific Islands Ocean Observing System demonstrated that insights from studies of sediment movement on reefs close to sediment plume sources could inform future land-based sediment remediation activities on this island or other locations across the Hawaiian Islands.

Resuspension repeatedly exposes the same reef area to the same amount of sediment, amplifying the effect of a unit of sediment load by an order of magnitude in some areas [41]. Not only sediment concentration but also sediment duration (i.e., residence time) and frequency of sediment flux events affect responses of reefs to sedimentation [2]. Once land-based sediment reaches the reef, water circulation driven by the interaction among tides, currents, winds, coastline topography, and bottom relief control sediment removal and transport [22,39,43,44,45 46]. We found that sediment movements differed substantially at two locations on the Maunalei reef on Lāna’i Island and that these movements were affected by physical factors, including the combined effects of current flow, tides, and, likely, bottom relief of the reef (for a detailed discussion on the role of waves, see SM, Section 8).

The hydrodynamic regime on the reef flat modulates the sediment removal rate once terrigenous sediment reaches the reef and can strongly affect how rapidly a portion of the reef can recover from sediment influx. The relative difference between sediment removal rates at the different sites is likely to be an underestimate. Addition of a second dimension to the box model, such as constraints on sediment accumulation on the bottom, would improve understanding of the controls on sediment removal rates at these sites. True net accumulation rate can be 20–30 times smaller than the gross accumulation rates estimated by canonical sediment traps [41]. If the ratio of net accumulation rate to gross accumulation rate can be used as a coefficient for residence time of sediment in the water column on a reef that experiences resuspension, the removal time of sediment might be much longer than 35 d, perhaps as much as 30 times longer (or ~18 mo.).

Coral cover and coral assemblages are affected by physical processes and land use [46], and the latter can alter physical and ecological interactions typical of reef systems [47]. Reefs can be exposed to high turbidity and sediment loads even when adjacent to catchments with little current human use [45]. Some coral reefs can persist with high turbidity for weeks to months by increasing their non-photosynthesis-dependent feeding [48 49]. Because the high sediment load and turbidity conditions on Lāna’i are believed to be relatively recent (since the early 20^th century rather than on geologic time scales), we suggest that the Lāna’i reefs are not adapted to such high levels of sediment load and likely will be negatively influenced. However, data are too scarce to identify whether sediment load thresholds exist for many reefs,especially in relation to sediment exposure, which is a function of the relations between the amount of sediment delivered to the reef area and the residence time, which is controlled by currents.

The sediment accumulation values we computed are much higher than the sediment trap gross accumulation rates estimated for a site in Hanalei Bay, Kauai (67–172 g m^–2d^–1) [41]. If the true net accumulation rate is 20–30 times smaller than gross accumulation rates estimated by canonical sediment traps, as suggested by novel sediment traps, SedPods [41], net accumulation rates would be 11.5–17.3 g m^–2 d^–1 for Site A-RF and 8.2–12.3 g m^–2 d^–1 for Site B-RF. These rates also are higher than the net accumulation rates from Hanalei Bay, Kauai (3.5–6 g m^–2 d^–1) [41]. Although these calculations provide information about exposure to sediment, grain size is also relevant to effects of sediment on reef health and function and needs to be considered [50].

Community-level watershed management and efforts to revegetate bare soils have previously improved coastal water quality [51]. On Lāna’i,the Maunalei Community Managed Makai Area, in the context of the sediment remediation project described here, used a volunteer collective to implement Hawaiian cultural resource management practices for sediment remediation by identifying drainage area in the ephemeral stream most prone to erosion and building the gabion dams perpendicular to the water-flow direction. The dams slowed water flow during flash floods, allowed for some water retention in the ground, and trapped about 77 tonnes of sediment during one wet season. The dams are simple in design, but labor-intensive. Community-level initiatives on Lāna’i and in watershed partnerships across the Hawaiian Islands continue to raise awareness about methods for reducing erosion that may maintain and restore Hawaiian ecosystems and food security. High levels of sediment capture in the dams we studied suggest that future efforts to remediate sediment with mesquite gabion dams can focus on constructing fewer dams which would still realize the same efficacy in trapping sediment.

On the basis of our data, we would recommend gabion-dam installation in watersheds adjacent to reef areas that have naturally slow sediment removal rates. Per unit of time, effort, and funds spent on sediment remediation, such a strategy would offer the highest return on investment in terms of increasing the speed of ecological recovery in reef areas prone to sedimentation. We focused on the relations between sediment capture efforts on land and actual removal rates on reefs. We found that gabion dam sediment capture can have a measurable effect and may help guide other small-scale solutions for sediment remediation and erosion control.

The authors declare that they have no competing interests.

We thank the Pacific Islands Ocean Observing System (PacIOOS) program for support with field instrumentation. We acknowledge the substantial efforts of the Maunalei Community Managed Makai Area in building the gabion dams and in aiding with fieldwork for the field deployments of physical oceanography equipment. We thank Jay Carpio for assistance with the installation of the weather station and to Christina Comfort for field assistance in deploying and maintaining in-water instruments. This work was funded through grants from Conservation International Hawai’i to the Community Managed Makai Area and to the University of Hawai’i-Mānoa. We also thank an anonymous reviewer for helpful comments, which improved the manuscript.