CABRILLO WAY MARINA DREDGING, PORT OF LOS ANGELES, CALIFORNIA, U.S.A. PART 1 - SEDIMENT PLUME MONITORING

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1 CABRILLO WAY MARINA DREDGING, PORT OF LOS ANGELES, CALIFORNIA, U.S.A. PART 1 - SEDIMENT PLUME MONITORING Kathryn Curtis 1, Brent Mardian 2, Ying Poon, D.Sc., P.E 3 ABSTRACT In 2009, the Port of Los Angeles (POLA), along with AMEC Earth & Environmental, Inc. (AMEC) and Everest International Consultants, Inc. (Everest) conducted a dredge monitoring investigation using surrogate technologies to quantify TSS concentrations. The work was undertaken using standard monitoring equipment (OBS and transmissometers) as well as a vessel mounted ADCP and LISST particle size analyzer. Multiple surveys were preformed in the receiving waters of Watchhorn Basin, and the results suggest that of the technology types compared, the LISST showed the greatest correlation with laboratory measured TSS concentrations (R 2 = 0.92). The results of this study indicate that the LISST can be an effective tool for the monitoring of suspended sediment plumes, and offers the added ability to determine particle size distributions. This paper represents the first part in a two part effort to develop a dredge plume model for the LA/LB Harbor, which can ultimately be used by the Port to assess and predict impacts from the resuspension of dredged material. Keywords: Suspended sediment monitoring, sediment modeling, TSS quantification, surrogate monitoring methods. INTRODUCTION The monitoring of suspended sediment plumes that result from dredging activities focuses on quantifying Suspended Sediment Concentrations (SSC). In many permits, water quality criteria are established for TSS, either directly or via surrogate measurements such as light transmittance and turbidity. Criteria are established and often times, exceedances of these criteria can have serious financial implications for the permittee. Depending on the proximity of dredging to natural resources and environmental receptors, the type of material being dredged, and existing water quality criteria; sediment plumes that result from dredging can be very deleterious from an environmental standpoint and difficult to remedy once detected. In the Port of Los Angeles, monitoring routines established by the Los Angeles Water Quality Control Board (LAWQCB) for assessing the impacts from the resuspension of sediment during dredging operations often require the permittee to conduct periodic monitoring for TSS. Water sample collection and laboratory analysis for Total Suspended Solids (TSS) is often required, as well as surrogate electronic measurements such as light transmission and/or turbidity. Along with the collected water samples, Light Transmissometers (transmissometers) and Optical Backscatter (OBS) instruments, have become the standard for dredge monitoring, and are often times the deciding factor for determining water quality compliance. Unfortunately, these monitoring techniques all have limitations in their abilities to provide accurate data for both dredge impact assessment and Best Management Practice (BMP) effectiveness. The most obvious downside to direct TSS measurement is the time lag associated with collection and subsequent laboratory analysis. There is often a 24-hr lag between collection and results. The real time capabilities of the transmissometers and OBS instruments have provided more immediate access for both the regulated and regulators to water quality data, but have both been shown to be influenced by physical properties of the resuspended sediments; such as sediment color and particle size. Sediment color has been shown to decrease the signal of SSC by decreasing the efficiency of scattered light measured by an OBS type instrument (Sutherland et al, 2000). Additionally, both transmissometers and OBS instruments have been shown to be sensitive to fine grain particle sizes (Benns and Pilgrim, 1994). This becomes particularly important given the association of fine grained sediments and contaminant concentrations. 1 Marine Environmental Supervisor, Port of Los Angeles, 425 South Palos Verdes Street, San Pedro, CA 90731, T: (310) , kcurtis@portla.org 2 Senior Marine Scientist, AMEC Earth & Environmental, Inc., 9210 Sky Park Court, Suite 200, San Diego, CA 92123, T: (858) , brent.mardian@amec.com 3 Vice President, Everest International Consultants, Inc., 444 West Ocean Boulevard, Suite 1104, Long Beach, CA 90802, T: (562) , ying.poon@everestconsultants.com 192

2 BACKGROUND The Port is currently underway with a wharf revitalization project focusing on Cabrillo Way Marina (Figure 1). The Cabrillo Way Marina Project represents the second phase of improvements of the West Channel/Cabrillo Beach Recreational Complex by the POLA. Phase I of the overall improvements was constructed in Phase II development is located on the eastern side of the West Channel in the Watchorn Basin. Dredging and land-side construction started in the spring of 2009, with the first phase of construction continuing over six months. The waterside improvements associated with the Cabrillo Way Marina Phase II project include: 1) Dredging within Watchorn Basin 2) Creation of a new water area through excavation of an existing land area 3) Creation of a new land area through filling of an existing water area 4) Removal of existing rock/rubble slope protection 5) Demolition and removal of existing marina docks and concrete and timber piles 6) Construction of new sheet pile bulkhead and rock slope protection waterfront perimeters 7) Construction of new docks and installation of new steel piles. This project requires the dredging of approximately 78,000 m 3 (102,000 cy) of sediment from Watchorn Basin. In addition, approximately 134,000 m 3 (175,000 cy) of soil would be excavated from existing land, creating new water areas. Pursuant to the waste discharge requirements (WDR) issued by the Los Angeles Regional Water Quality Control Board s Order No. R , the monitoring of receiving waters is required throughout the duration of the Cabrillo Way Marina dredging operations. WDR monitoring requirements rely on transmissivity as criteria to determine impacts from dredging operations and to trigger the requirement for chemical testing. This standard water quality monitoring was conducted by the Port of Los Angeles Harbor Laboratory on a weekly schedule, with accelerated chemical testing based on light transmittance attenuation (%) between the monitoring sites 300 meters downstream from the dredging operation and a reference location. An additional requirement was set forth in the supplemental EIR for the project. Mitigation Measure WQ 1.1, which called for additional monitoring using new and novel approaches to sediment plume tracking. AMEC and Everest worked together to develop a survey plan to qualitatively and quantitatively characterize particle size and suspended sediment plume distribution in and around the Cabrillo Way Marina dredge area. This study focused on the use of surrogate instruments for indirect measurement TSS. Through the use of these specialized instruments and multiple real-time vessel based surveys, this innovative approach to dredge plume monitoring satisfies the mitigation and monitoring requirements as outlined in the CEQA document, and furthers the Port s ongoing efforts to be proactive in its ability to mitigate impacts from dredge activities and mitigate future impacts from suspended sediment plumes. 193

3 Figure 1. Cabrillo Way Marina project area 194

4 MATERIALS AND METHODS During suspended sediment monitoring, a series of water quality profiles were conducted to assess the ambient water quality and turbidity within the Cabrillo Way Marina study area. A specialized package of water quality sensors was developed for this study to allow concurrent water quality measurements. The water quality package was constructed using the following water quality sensors: 1) Conductivity, temperature, and depth transmissometer (CTD), 2) Optical backscatter (OBS), 3) Grab sampler, 5) Transmissometer, and 6) Laser in situ Scanning and Transmissometry (LISST) (Figure 2). A Sea Bird Electronics (SBE) 19 CTD was used to measure temperature, ph, dissolved oxygen (DO), and conductivity (PSU). The CTD was also configured with a Wet Labs Eco-Flour OBS sensor and a transmissometer for quantifying turbidity (NTU) and light transmittance (%), respectively. The CTD was connected via a Kevlar cable to a SBE SeaCat 21 optically isolated NMEA deck box for real-time communications and GPS integration. A Van Dorn type grab sampler was affixed to the CTD cage. The Van Dorn was triggered via a line and weighted messenger. This allowed for total suspended solid (TSS) grab samples to be taken concurrently as instrument readings. Scan numbers and times were recorded in CTD cast logs at the precise moments the Van Dorn bottle was triggered, resulting in less than a 1-sec. differential between instrument readings and collected samples. Figure 2. Water quality survey package Specialized Instruments Beyond the transmissometer and OBS sensors, the water quality package also included additional instruments for the detection and measurements of suspended sediment concentrations. A Sequoia Scientific LISST-100X was added to the CTD cage for in situ particle size determination and total volume concentration (µl/l). The LISST- 100X uses laser scattering to accurately determine suspended particle size distributions, providing a high resolution dataset of particle bins ranging from 2 µm to 500 µm, as summarized in Table 1. These bins can then be categorically grouped in standard size classes (silt, clay, very fine grain sand, fine grain sand, medium grain sand) and analyzed throughout the water column. The sum of the 32 bins yields a total volume concentration expressed as µl/l. Conversion of LISST measurements to TSS does not require calibration for sediment color, which is the case for turbidity measurements. 195

5 Table 1. LISST 100X particle size bins Bin Particle Size (µm) LISST Sediment Class Clay Silt Very Fine Sand Fine Sand Medium Sand In addition to instantaneous measurements with the AMEC water quality package, an Acoustic Doppler Current Profiler (ADCP) was also deployed from the side of the sampling vessel for performing acoustic transects for further sediment plume delineation (Figure 3). An ADCP determines current speeds by emitting an acoustic ping, listening for the return, and then processes the Doppler shift of the return signal and translating that to current speed. How this instrument functions (i.e., acoustic return) is also useful for monitoring suspended sediment, primarily because the ADCP is not measuring the water directly, but rather particles in the water. For suspended sediment applications, the strength of the returned acoustic ping, or echo intensity, is a qualitative measurement of suspended sediment concentrations, and can be used to determine plume characteristics such as plume velocity and direction. All ADCP surveys were preformed in real-time and integrated with a Trimble XRT-Pro Differential GPS receiver with external antenna for ± 1 meter accuracy. ADCP data acquisition was accomplished by using RDI Teledyne s VmDas software application which enabled real-time viewing and vessel navigational track information to be merged together into one output file. Figure 3. Real-time ADCP survey 196

6 Sampling and Monitoring Sampling and monitoring was conducted during active Cabrillo Way Marina dredging operations. Surveys were conducted eight times during the dredge period, with both transect type surveys and spot monitoring completed during surveys. All acoustic data was collected in real time using RDI s software package VmDas and integrated with the incoming NMEA 0183 text string generated by a Trimble XPS Differential GPS. Raw data was stored on the host computer. A backup file was stored separately on non-volatile flash type memory. Survey speed was maintained during the course of the surveys, and other boat traffic was generally at a minimum. Acoustic Transects Figure 4. Suspended sediment survey area Acoustic suspended sediment transects using a vessel mounted ADCP and DGPS were performed within the Watchorn Basin area. Transects were run from the entrance to Watchorn Basin in the south to the marina terminus to the north (Figure 4). Vessel speed was kept below 2.06 m/s during survey operations. As dredging was conducted on a 24-hr schedule, both day and night surveys were completed as part of this effort. The ADCP-measured current profiles will be used for testing the potential of using the Port s Water Resources Action Plan (WRAP) Model to predict the transport and dispersion of suspended sediment plumes generated by dredging. The WRAP Model is a three-dimensional hydrodynamic and water quality model developed by Everest (2009) for the Port of Los Angeles and Port of Long Beach to support the development of the joint Port Water Resources Action Plan. The use and integration of the ADCP data is discussed in Part 2 of this paper. 197

7 Spot Sampling Random stations were spot sampled using the water quality and ADCP instruments. The packages were lowered at a constant rate through the water column. During the down cast portion of the cast, the package was stopped at a random depth to assess variability in measurements and water column readings. It was during this time in the cast that a water sample was collected with a horizontally mounted Van Dorn bottle. Detailed logs of specific CTD scan number and time of water collection were maintained throughout the individual spot sampling. The ADCP was deployed simultaneously to obtain concurrent velocity and echo intensity measurements. Spot sampling was conducted periodically within the suspended sediment survey area and throughout the Harbor area. Sampling occurred on May 20, May 28, May 29, June 3, and June 12, All sampling times are reported in Coordinate Universal Time (UTC). Data collected in the vicinity of dredging operations at Cabrillo Way Marina included combinations of ADCP, LISST, TSS, CTD, and OBS spot measurements. Sampling locations by data type are shown in Figure 5. In the figure, locations of concurrent ADCP, LISST, TSS, CTD, and OBS measurements are shown in blue. Locations of concurrent LISST, TSS, CTD, and OBS measurements are shown in purple. Concurrent measurements of LISST, CTD, and OBS data are indicated by orange; and concurrent ADCP and LISST measurements are represented by green. Figure 5. Spot sampling locations 198

8 Harbor Sampling As part of this monitoring effort, previously established water quality sampling stations were sampled using the same instrument configuration as the Cabrillo Way Marina sampling. Beyond the benefits of an additional survey at these established locations, by surveying these locations with the LISST instrumentation, the Port expands upon this historical water quality dataset. The water quality stations, shown in Figure 6, were sampled using the aforementioned spot sampling approach. Profiles were preformed concurrently with ADCP surveys. Concurrent ADCP, LISST, TSS, CTD, and OBS data were taken at all EWQ locations for three separate occasions on May 20, May 28, and June 12, Figure 6. Enhanced water quality sampling locations 199

9 Additional Sampling The receiving water monitoring as stipulated in the waste discharge requirements (WDR) was conducted by the Port s Test Laboratory. Samples are required to be collected at four stations on a weekly basis, as summarized in Table 2. Required water quality monitoring includes dissolved oxygen, light transmittance, ph, and suspended solids. Parameters are measured throughout the water column at 2-meter increments, with the exception of suspended solids, which is sampled at mid-depth. Additional sampling and analysis are required based on a 30% difference in light transmittance at stations C and D (refer to Table 2 for station locations). Light transmittance at stations C and D are compared at near surface, mid-water, and near bottom depths. If the difference of light transmission is greater than 30%, mid-depth or depth of maximum turbidity requires additional analysis for various constituents identified in the sediment characterization report. In the case of the Cabrillo Way Marina project, this analysis included trace metals, DDTs, PCBs, and PAHs. Also, water quality sampling is required for three consecutive days following the day with light transmission exceedance of 30% or greater. Station A B C D Table 2. Port WDR permit monitoring locations Description 30.5 meters (100 feet) up current of the dredging operations, safety permitting meters (100 feet) down current of the dredging operations, safety permitting meters (300 feet) down current of the dredging operations. Control site (area not affected by dredging operations) RESULTS Five different types of data were collected, each with a different instrument, over varying durations depending on the sampling locations. In general, spot samples were taken for durations of 5 to 15 minutes. Each instrument was set up with varying sampling rates. ADCP measurements were made at one to two second intervals, while LISST measurements were taken at an interval less than one second; resulting in datasets with thousands of readings. The data analysis required organizing and processing of the data prior to comparing or correlating the different data types. The multiple data sets were merged by date and time, and processed to determine the instrument measurements at specified water depths. The processed ADCP, LISST, TSS, CTD, and OBS data are shown graphically in Figures 7 to 10, with each figure showing the results from one of the four sampling days. In the figures, the red squares indicate the sampling locations, with a box next to each location summarizing the data collected at that location. The box shows the station name, the data types collected at that location for that day, and the processed results for different water depths. In addition, the dredging limits are also shown in the figures by the blue shaded area and the Port permit sampling locations are indicated with yellow stars to provide a reference of the dredging location on that day. Per permit requirements, station A is located approximately 30.5 m (100 ft) up-current of the dredge site and stations B and C are located about 30.5 m (100 ft) and 91.4 m (300 ft) down-current of the dredge site. Hence, the dredge location is somewhere midway between station A and station B. Results of the data collected on May 28, 2009 are shown in Figure 7. For this day, TSS, LISST, OBS, and ADCP Echo Intensity (Echo) were collected at Cab-ss_1 and TSS, OBS and Echo were collected at Cab_ss_2. These data were collected approximately two and a half hours after the Port permit data were collected. Coordinate information for station A on this day was found inaccurate and hence not shown in the figure. At both locations, OBS and Echo were higher near the surface indicating higher sediment concentration near the surface. The data also shows that, in general, there is a correlation between OBS and Echo such that the higher the OBS results, the higher the Echo intensity. This correlation between OBS and Echo will be further explored in the next section. In addition, both the OBS and Echo values are higher at Cab_ss_2 which is closer to the dredge location than at Cab_ss_1 farther away. 200

10 Figure 7. Cabrillo Way Marina data collected 5/28/09 201

11 Data from the May 29, 2009 sampling event, as shown in Figure 8, were collected at three locations. At each location, a set of TSS, LISST and OBS data was first taken, followed by a set of LISST and Echo measurements. These data were taken at about the same time period as the Port s permit sampling. As expected, the data shows higher TSS, LISST, and OBS values at Cab3_2, which is nearest to the dredging site. Similar to the data collected on May 28, both the LISST and OBS data indicated that the dredge plume showed higher sediment concentrations near the water surface. Figure 8. Cabrillo Way Marina data collected 5/29/09 202

12 TSS, LISST, and OBS data were sampled at two locations and LISST and OBS were taken at five locations on June 3, As shown in Figure 9, these data were collected away from the dredge location which was located at the north end of the West Channel. In general, the results show that the near surface sediment concentrations became lower farther away from the dredge location. Contrary to the observations of the two prior sampling events, which showed higher sediment concentration near the water surface, data collected on June 3 in general showed higher sediment concentration in deeper water. This may be because on June 3, data were collected farther away from the dredge location and re-suspended sediments may have started to settle to deeper water at this distance away from the dredge location. This also indicates the complex structure of the sediment plume generated by dredging. Figure 9. Cabrillo Way Marina data collected 6/3/09 203

13 TSS, LISST, OBS and Echo data were collected at seven locations on June 12, The results are summarized in Figure 10. Similar to the data collected for the three earlier sampling events, higher TSS, LISST, and OBS data were measured at sampling locations closest to the active dredging area. In addition, the same trend was observed where sediment concentrations near the dredge location tended to be higher near the water surface compared to deeper in the water column. Figure 10. Cabrillo Way Marina data collected 6/12/09 204

14 TSS Correlations One major objective of the Cabrillo Special Study was to test the use of emerging technology (LISST and ADCP) to monitor water column sediment concentrations during dredging operation, as well as to compare these new measuring techniques with traditional monitoring methods (OBS and transmissivity). To achieve this, LISST, ADCP echo intensity, OBS or transmissivity data that were collected simultaneously with TSS from the Cabrillo Way Marina and EWQ stations were analyzed to seek correlations between the various methods. For the analysis, all non-detect TSS values were set to half of the minimum detection limit (MDL) of 0.95 mg/l. LISST data are compared with TSS values in Figure 11a. In the figure, data collected at the Cabrillo Way Marina locations are shown as blue diamonds, and data collected at the EWQ stations are shown as red squares. As expected, TSS values collected at Cabrillo Way Marina stations where dredging was taking place are in general higher than those collected at the Port s established water quality sampling stations. The correlation between the LISST and TSS is indicated by the linear-fitted line (black line) shown in each of the panels and the corresponding R 2 value (square of the sample correlation coefficient). The R 2 value is a measure on the strength of the relationship between two variables; R 2 value close to one indicates strong correlation between the two variables and R 2 value close to zero indicates the two valuables are unrelated. The R 2 value between LISST and TSS is 0.93, indicating strong correlations between LISST and TSS. Similarly, the TSS correlations with the OBS, echo intensity, and transmissivity are shown in Figures 11b 11d, respectively. The correlation between TSS and OBS shows a correlation of The R 2 value of 0.24 indicates a poor correlation between TSS and echo intensity. Recent studies (e.g. Aardoom 2006) showed that in order to use ADCP echo intensity to measure suspended sediment concentration, the raw echo signal needs to be corrected for the effects of salinity, temperature and water depth. In addition, since acoustic attenuation is dependent on particle size distribution, the ADCP echo intensity needs to be calibrated with other data such as OBS or LISST. For TSS and transmissivity, the R 2 value was Figure 11. TSS correlation with LISST, OBS, echo intensity, and transmissivity 205

15 Correlations of TSS with OBS and transmissivity were also determined using the Port WDR-required monitoring data. Contrary to the results shown above using the data collected for this study, the Port WDR data showed very poor correlations between OBS and TSS with a R 2 value of Correlations between TSS and transmissivity using the Port WDR data are better but the R 2 value of 0.40 is still poor compared to the data collected for this study which shows a R 2 value of Such a great difference between the data collected for this study and the Port WDR data may be the result of how data were being collected in the field. Indirectly, the poor correlation between TSS and OBS for the Port WDR data reflects the complex structure and transport of the sediment plume caused by dredging such that the sediment concentrations (TSS) have substantial spatial and temporal variations. This agrees with previous studies within the Port, that have shown OBS measurements were not the most reliable method for detecting suspended sediment (MBC 2003). SUMMARY The Cabrillo Special Study was conducted to collect suspended sediment data during dredging operations of the Cabrillo Way Marina Project as an initial assessment of alternative ways to monitoring sediment plumes resulted from dredging activities. In addition to employing traditional methods of collecting grab samples to measure TSS, using a transmissometer for transmittance, and measuring OBS for turbidity, this study included the use of the Laser in situ Scanning and Transmissometry (LISST) and ADCP for acoustic backscatter (echo intensity). Random spot samples of TSS, transmissivity, turbidity, sediment volumetric concentration (LISST), and ADCP echo intensity (Echo) were taken for four days between May 28 and June 12, 2009 at the Cabrillo Marina Way while dredging was ongoing. Although these spot samples are unable to fully capture the complex three-dimensional sediment plume structure resulting from the dredging operations, the collected data do provide some insight into the characteristics of the sediment plume. In general, as expected, suspended sediment concentrations were higher closer to the dredge location. In addition, near the dredge location, sediment concentrations were in general higher near the water surface compared to deeper in the water column, indicating that dredging operations can bring suspended sediments all the way to the water surface. Farther away from the dredge location, the overall sediment concentrations became lower, and characteristics of the sediment concentration profile also changed. Whereas near the dredge location sediment concentrations were higher near the water surface, sediment concentration profiles at locations father away showed lower sediment concentrations near the water surface compared to concentrations deeper in the water column. This shows that farther away from the dredge location, in addition to the effect of dilution of the sediment plume concentrations, the near surface sediments also started to settle, resulting in higher concentrations in deeper water then near the water surface. The collected data were analyzed to evaluate the correlation between LISST, OBS, echo intensity, transmissivity and TSS measurements. The results are summarized in Table 3. As shown in the table, TSS shows the best correlation with LISST measurements, much better than correlation between TSS and transmissivity, the latter of which is currently the basis of water quality regulations for monitoring dredging operations. The strong correlation between LISST and TSS suggests that the LISST could be a powerful tool for dredge monitoring once it has been calibrated, since the LISST can provide real time in-situ sediment concentrations profiles. Moreover, with further calibration and testing, the LISST may also be able to provide particle size distribution in addition to total suspended sediment concentrations. The correlation between OBS and TSS is also fairly good for the data collected for this study. However, for the Port WDR data, no correlation between TSS and OBS was found. The lack of correlation between the TSS and OBS data for the WDR data is probably due to the fact that the TSS and OBS data were not collected at the same location or at the same time. Table 3. TSS correlation summary Data Data Source Correlation with TSS (R 2 ) LISST Cabrillo and EWQ 0.93 OBS Cabrillo and EWQ 0.82 Echo Intensity Cabrillo and EWQ 0.24 Transmissivity Cabrillo, EWQ, and Port WDR

16 In summary, this first attempt for the Port of Los Angeles to test emerging technologies for dredge monitoring was successful demonstrating the potential use of LISST to provide real time in-situ measurement of suspended sediment concentrations (TSS). In addition, the LISST is a more reliable method to measure TSS compared to more traditional methods such as OBS and light transmittance. REFERENCES Aardoom, Jeroen, (2006). Quantification of Sediment Concentrations and Fluxes from ADCP Measurements. IXémes Journées Nationales Génie Civil Genie Côtìer, September 2006, Brest. Sutherland T.F., P.M. Lane, C.L. Amos, and John Downing. (2000). Calibration of Optical Backscatter Sensors for Suspended Sediment of Varying Darkness Levels. Marine Geology 162(2000), pp Benns, E.J. and Pilgrim, D.A. (1994). The Effect of Particle Characteristics on the Beam Attenuation Coefficient and Output from an Optical Backscatter Sensor. Netherlands Journal of Aquatic Ecology 28(1994), pp MBC Applied Environmental Sciences (MBC), (2000). Turbidity Issues in Relationship to Dredging, Port of Los Angeles 207