Project 2:

Shoaling Analysis

Introduction

The purpose of this shoaling analysis is to identify priority dredging areas in Ocean County.  Priority dredging areas are identified as those areas that currently are in need of dredging and have a high rate of historical shoaling.  Shoaling is the deposition of sediment in open waterways causing a reduction in the local water depth. In recreational waterways, namely navigation channels and marinas, shoaling is a hazard as it impedes the safe passage of boats through limited water depths.  As a result, recreational waterway dredging provides sufficient water depth for safe passage of vessels as well as clear sufficient width for vessels traveling in opposite directions to pass by each other. 

 

To identify the priority dredging areas in this shoaling analysis, past, present, and future conditions were investigated.  Using data provided by the NJDEP, historical shoaling conditions were analyzed using the dredging records of federal, state, municipal, and private dredging projects.  The current conditions were assessed through a field study that collected and analyzed bayfloor sediments from the Ocean County waterways.  The current and future assessment of dredging needs within Ocean County was provided by experienced members of the dredging community including regulatory agency officials, the local marinas and boat captains and other stakeholders.  Using the available information, this shoaling analysis identified priority dredging areas in Ocean County.  Not included in this study are the three federally maintained inlets (Little Egg, Barnegat, and Manasquan).

 

Ocean County Waterways

 

Ocean County is located along the Atlantic coast in central New Jersey (Figure 1).  The county's coastal waterways separate the mainland from a barrier island (Long Beach Island to the south) and Northern Ocean County’s barrier spit to the north.  The channels within Barnegat Bay encompass nearly 80,000 acres of open water over 45 miles of coastline between the Little Egg Inlet and the Manasquan Inlet.  Multiple local rivers flow directly into the Barnegat Bay system and the Manasquan River is connected to the bay via a man-made canal.  Barnegat Bay is linked to the Atlantic Ocean through the Little Egg (south), Barnegat (middle), and Manasquan (north) Inlets which provide a tidal influence to the system.  The shoaling analysis will focus on the navigable channels and marina slips located within this area that service the boating community of Ocean County.



Figure 1.  Ocean County base map.

 

There are a series of navigable channels that are maintained to provide safe passage to boaters through the county.  The main channel that traverses the Barnegat Bay is the NJIWW.  Branching off the NJIWW are multiple state, municipal, and private channels that provide access between the NJIWW and local marinas, private residences and other boat slips.  Maintaining sufficient water depth in these channels and associated boating slips are critical to the boating community in Ocean County.  The frequency of maintenance dredging is dependant upon the shoaling rates within in these locations.  Shoaling within the waterways is dependant upon the sediment characteristics and local hydrodynamics that move the sediments. 

 

A field study was conducted to identify the sediment characteristics of the Ocean County coastal waterways.  65 bay floor sediment samples were collected with grain size analysis preformed on each sample to define the physical characteristics of the surface material.  The location, elevation, and physical characterizes of each sample are provided in the database section.  This data was extrapolated over the entire study area to create a sediment distribution map (Figure 2).  In the figure, areas with lighter shading represents bottom sediments with a higher percentage of sands (less silt and clay) while areas with darker shading represents bottom sediments with a lower percentage of sands (more silt and clay). 

 


Figure 2.  Ocean County bayfloor sediment distribution map.

 

The differing bottom sediment composition lends clues to sediment sources, local currents, and shoaling trends.  In general, the area is largely comprised of sand sediments that are located adjacent to the barrier island/spit.  The fine sediments (silt/clay) are located adjacent to the mainland and are concentrated within the many river and stream channels where they pass through the tidal marshes and low-flow, shallow coves.  Fine sediments often enter these waterways as freshwater containing suspended fines discharges into the tidal portion of these rivers and streams (some examples include Westecunk Creek, Cedar Creek, Gunning River, Forked River, and Toms River).  The presence of salt water enhances the flocculation of the suspended particles by neutralizing the colloidal clay suspension process in the fresh water.  Since these rivers and streams generally have a low flow velocity due to the surrounding low-lying coastal geography, they often transport and deposit the fine sediments along their flow path.  In addition, past dredging has widened and deepened the original natural channel to allow passage of 40-foot recreational vessels, a fact that dramatically alters the shoaling rate because the dredged channel is excessively deep and wide to allow the vessel passage, but too large a hydraulic cross section to permit the transport of the suspended sediment through to the open bay.  The vessel wake and propeller wash also re-suspends bank and mudflat sediments that eventually settle in the navigation channel accelerating the shoaling rate.

 

Historical Dredging Data

 

A review of available historical dredging projects for Ocean County was used to identify shoaling trends within the county.  The historical data sources include permit dredging information from the NJDEP Office of Dredging and Sediment Technology (ODST) and state dredging projects from the NJDEP Bureau of Coastal Engineering (BCE).  These resources were used to identify areas of increased dredging in Ocean County.

 

NJDEP ODST Permit Data

 

A NJDEP issued permit is necessary to dredge in the waterways of New Jersey.  Permitted dredging projects include maintenance dredging of existing channels and boat slips as well as new work dredging (locations or depths that have not been previously dredged).  Using these records, areas of significant historical dredging were identified.  Without major changes in the sediment characteristics or hydrodynamics it is expected that these trends will continue in the future.  It is noted that although dredging projects and their permits are dependant on a need to provide ample water depth, they are also dependant upon other parameters such as public funding and Dredged Material (DM) placement site availability which cannot be accounted for in this analysis.

 

The NJDEP Office of Dredging and Sediment Technology provided a database of historical dredging permits that were submitted between 1998 and 2008.  This database contains multiple fields of information including; application number (identifies municipality), applicant, permit type, brief project description, NJDEP project manager, quantity, placement destination, date received, date completed, and the decision to permit.  Not all of the information was provided for each permit and only those records with a municipal application number, quantity, and date were used in this analysis.  The complete list of Ocean County dredging permits can be found in the database section.

 

After filtering the database, a total of 203 dredging permits between 1998 and 2008 were used in the analysis.  Together these permits accounted for over 300,000 cy of dredged material.  The majority of these permits were for relatively small dredging projects (less than 1,000 cy) which account for less than 10% of the total volume.  These small scale dredging projects are commonly located at small marinas and individual boat slips.  The remaining 90% of the dredging volume is accounted for in 38 large scale permits (greater than 1,000 cy).  These relatively large scale dredged projects are commonly located in large marinas and navigation channels.  A summary of the NJDEP ODST dredging permit data by municipality and scale is provided in Table 1. 

 

According to the NJDEP permit data the three municipalities that contribute most of the permits and volume are Little Egg Harbor Township, Lacey Township, and Brick Township.  These municipalities have large areas of open water and their water frontage is highly developed.

 

 Table 1.  - NJDEP ODST Permit Data Summary (1998-2008)

 

NJDEP BCE Project Files

The NJDEP Bureau of Coastal Engineering provides professional services for State-sponsored dredging activities.  A summary record of historical state dredging activities was provided by the NJDEP BCE.  An analysis of the state dredging projects that date back to the 1960's identified three locations that were repeatedly dredged.  These three areas are Ocean Township, Point Pleasant Beach and Lacey Township.   

A summary of the location and dates dredged are shown in Table 2.  The complete NJDEP BCE dredging project listing can be found in the database section.

 

The repetition of state dredging activities at these locations is an indicator that these channels have a high shoaling rate.  Although the volume dredged from each channel was not provided, it is assumed that these projects were relatively large scale projects (greater than 1,000 cy).

 

Table 2 - NJDEP BCE repeated state dredging activity

 

Historical Dredging Summary

The historical dredging activity in Ocean County is summarized in Figure 3.  From the two historical data sources that were provided by the NJDEP ODST and BCE, Lacey Township has a high rate of historical dredging due to extensive waterfront development that often requires dredging.  These dredged materials were historically placed in the CDFs located along Forked River and Cedar Run Creek.  Information on these CDFs can be found in Project 1, Ocean County Sites.

 

Figure 3 - Ocean County Historical Dredging

Current Dredging Needs

 

Current dredging needs have been identified through meetings with the NJDEP BCE, local municipalities, and local marina owners.  These locations are currently in need of maintenance dredging due to a lack of channel depth and are identified in the table and figure below. Neglected maintenance dredging is commonly due to a lack of funding and absence of adequate placement options which allows the channels to continue shoaling.  Without recent hydrographic survey information the dredging demand cannot be quantified, but field experience at low tide at both Westecunk Creek and Parkers Run provided first-hand information on the lack of sufficient depth for a modest-sized vessel (21-foot open, center console outboard).

Table 3  - Locations of needed dredging

 

 

 

Figure 4 - Locations of dredging needs (2008)

Priority Dredging Areas

 

In general most of the Ocean County coastal waterways are in need of some degree of dredging.  Priority dredging locations are those which are predicted to need or are currently in need of dredging.  To identify these areas, data was collected on historical dredging throughout Ocean County.  Current priority dredging areas are those that have the highest dredging need which were identified through meetings with state and municipal employees and marina owners.  A summary of the priority dredging areas is shown in Figure 5.

 

From the available data that was used in this analysis, the auxiliary channels that provide access to the NJIWW and the local marinas and slips in Lacey Township, Little Egg Harbor Township, and Brick Township have been identified as priority dredging areas.  These areas have a high historical rate of shoaling and are in need of dredging.  Specifically those channels located near the rivers and streams that flow into the bay appear to have the highest rate of shoaling.  These areas have been identified to have a large percentage of fine sediments from the sediment sampling investigation.  It is difficult to beneficially reuse dredged material that contains a large percentage of fines and are often placed in upland confined disposal facilities.  These channels are extensively recreationally utilized, but the growth in both the vessel size and power commonly purchased by local bay-front property owners combined with the need to enlarge these small hydraulic cross sectional streams to accommodate the vessels, means that dredging will be a repetitious activity with a 10-year repeat interval as a maximum.

 

 

Figure 5 - Ocean County Dredging Summary

 

Findings

It was found that the NJIWW dredge material, since it is located near both the two major inlets and runs immediately landward of both ocean barrier beaches, consists of at least 70% sand.  The proximity to barrier beach storm overwash and other ocean-based processes have meant that the sediment source is most likely sand.  The open water of Barnegat Bay also means that sand is the dominant sediment type of the NJIWW.  State and local channels, on the other hand, tend to have higher silt and mud contents.

From Figure 5, several conclusions can be made:

 

  • The NJIWW has “flow through” channels (no dead ends) that keep fine sediments moving and the bed is usually sand.  The NJIWW is also just west of the developed bay shoreline of the barrier island/spit and is located where pre-history episodes of major storm overwash carried sand into Barnegat Bay (supported by the CDF core data).  The last historical storm to do so occurred in 1962.
  • The NJIWW near an inlet has high velocities associated with tidal flushing (the water volume that moves into the bay through the inlet with the tide).  Fine sediments in this region keep moving and are deposited deep in the bay or on the marshes.  Sand forms depositional features at the seaward and bay side ends of the inlet channel as the ebb- and flood-tidal deltas.  These always occur at inlet openings and can provide convincing evidence from shape, and sedimentology contained within them for former inlets, now closed.  These features are especially prominent at both Barnegat and Little Egg Inlets.
  • State channels that extend toward the barrier island/spit are short and usually end at major recreational vessel berthing areas.
  • The only major State channel leading to a commercial use area is between Barnegat Inlet and the commercial fishing berths on the bayside of Barnegat Light Borough.
  • The State maintains all the channels within the tidal “estuary” portion of the freshwater streams and rivers draining Ocean County such as the Toms River, Metedeconk River or Tuckerton Creek and Westecunk Creek. 
  • These State channels located on such drainage channels have high percentages of fine material due to destruction of the suspension properties of a colloid of silt when fresh water mixes with salt water and the demand by vessel owners for a much deeper and wider pathway than needed by the natural river discharge to reach Barnegat Bay.
  • Most private channels are within the plethora of “lagoon” communities that were built between 1950 and 1970 along the upland margins of Ocean County on the fresh water drainage pathways leading to the Barnegat Bay.  These contain 90% silt and organic-rich muck that has very little re-use potential.
  • The State maintains a few dredged channels through the open waters of Barnegat Bay, associated with the southern portion of the bay near the entrance to Tuckerton Creek.