Project 2:

Shoaling Analysis

Introduction

The purpose of this shoaling analysis was to gain information on the dredging needs within Cape May County waters.  Shoaling is the reduction of water depth due to bay floor elevation changes caused by the deposition of sediment.  In navigable waterways, namely navigation channels and marinas, shoaling is a hazard as it impedes the safe passage of boats.  As a result of shoaling, recreational waterways are dredged to provide ample water depth for safe passage of vessels. 

For this shoaling analysis, past, present, and future conditions were investigated in the form of a historical data review, field study, and future assessment.  The historical data review included records of federal, state, municipal and private dredging projects (including permits and past dredging files).  The current conditions were identified through sediment sampling field study.  The future assessment of dredging needs, within Cape May County, was based on discussions with state employees, coastal engineers, scientists, and marina and boat owners and extrapolation of past trends.

Using all the information gathered, the shoaling analysis identified priority dredging areas in Cape May County.  Not included in this study are the five federally maintained inlets (Cape May, Hereford, Townsend, Corson, and Great Egg Harbor).

Cape May County Waterways

Cape May County is the southern-most county in New Jersey.  The intricate system of channels and bays occupy more than 15,000 acres of open water in Cape May County.  The study areas for the shoaling analysis are the tidal channels and bays that extend south from the Great Egg Harbor Bay to the Cape May Harbor (Figure 1).  Located within the meandering channels and bays are federal, state, and local navigation channels as well as numerous public and private marinas and boat slips.  The waterways traverse through extensive tidal saltwater marshlands that are between 2 to 4 miles wide (mainland to barrier island) located behind approximately 30 miles of barrier islands.  The waterways are used by local recreational boaters, local commercial fisherman, and boaters traveling through the area via the NJIWW.  Peak use is during the summer months.  Five inlets (Cape May, Hereford, Townsends, Corson, and Great Egg Harbor) separate the barrier islands and provide access to the Atlantic Ocean.  In addition, a Federally-maintained canal traverses the southern tip of the Cape May County uplands to reach Delaware Bay.  Shoaling within this area is not constant with respect to time and location.  The movement and deposition of sediment relies on a source of sediment and subsequent entrainment, transportation, and deposition. 

Figure - Cape May County study area

Sediment Sources and Transport

There are multiple sediment sources in the study area and these are separated into two groups.  The first group of sources includes sediment that must be transported into the study area via local runoff, upland inflow, or from the nearshore.  Local uplands provide sediments directly to the Cape May County coastal waterways through direct drainage and storm runoff.  Upland inflow sediment sources for this area are the rivers and streams (Tuckahoe River, Middle River, Great Egg Harbor River, and Patcong Creek) that flow into the Great Egg Harbor Bay.  Both local runoff and upland inflow are dependant upon local rainfall to entrain the sediment and transport it to the study area.  Nearshore sediments can also serve as a source of sediment that travels into the tidal bays and channels.  Wave energy and longshore currents work together to suspend and transport sediments to the inlets.  Flood tide currents carry the sediments into the bays and channels to be deposited. This process is usually localized to inlet areas and immediately adjacent waterways.

The second group of sediments consists of previous deposits of marsh muds, organic material, and the bay floor.  Bay floor areas adjacent to navigation channels and marinas are a common source for shoaling sediment as dredged channels tend to maintain a natural equilibrium with adjacent natural contours.  These areas are relatively shallow when compared to the unnatural dredged depths of channels and marinas.  These sediments become resuspended into the water column via currents and wave action. 

Wind waves, boat wake, currents, and propeller scour can cause marsh erosion allowing the marsh sediment to slump into open water and become entrained in the water column.  Once the sediment is entrained into the water column it can be transported and deposited, usually in areas with extremely low current profiles.  More energy is required to transport larger, denser sand particles than compared to smaller, less dense silt and clay particles.  The relationship between energy, in the form of a current velocity, versus grain size is shown in figure 2 which was original to “Hjulstrom, F, 1935, Studies of the Morphological Activity of Rivers as Illustrated by the River Fyris, Bull. Geol. Inst. Uppsala, 25”.

Figure 2 - Erosion-deposition criteria for different grain sizes (http://www.slideshare.net/geographyalltheway/ib-geography-drainage-basins-hjulstrom-curve)

The dominant means of transportation of sediment in Cape May County coastal waterways is from tidal currents.  The ocean tide, a cycle of ebb and flood, propagating through the inlets and into the bays and channels, creates tidal currents.   The flow rate at which the tide propagates is dependant upon the local water depth.  As tides pass in and out of the inlets, the water levels of the channels and bays rise and fall with an associated lag.  The direction and magnitude of the currents is highly dependant upon the location and morphology of the bays and channels as well as the tidal stage.  At the period of high and low tide there is slack water where velocity is zero.  Currents develop during the transition between flood and ebb tide.  In general the maximum rate of ebb-tidal flow occurs late in the ebb-tidal cycle when the water level is below tidal mud flats and is therefore, only found in the main, deep channels leading to the inlet.  Channels and bays located closer to an inlet will generally have a greater magnitude current.  Areas with relatively narrow, well defined cross sectional areas such as tidal channels will have greater current velocities when compared to larger open areas such as bays.  In addition to the tidal currents, there is a fluvial influence at the northern part of the study area (Ocean City) from upland flow into the Great Egg Harbor Inlet.  The fluvial inflow into the bay skews the tidal currents creating larger ebb tidal current and smaller flood tidal currents when compared to the surrounding area.  Using engineering judgment and local grain size data (discussed later) estimates of tidal current magnitude and direction were developed.  The expected flood tide currents are shown in Figure 3.

Figure 3 - Predicted flood tidal currents

Deposition of entrained sediments occurs in low energy areas.  The amount of deposition depends on location and time.  The energy threshold required for sediment deposition is dependant on the sediment grain size and density.  This accounts for higher concentrations of sands located near inlets and in areas of stronger tidal currents.  Silts and clays are often found in the shallow back bays and mid island channels where energetic conditions are minimized.  This sediment distribution can be seen in Figure 4 (see discussion below).  In addition, sharp turns and bends (meanders) in the channels also are regions for sediment deposition because of decreasing rates of flow that develop toward the inside curve of the meander, causing deposition of sediment along the inside bend of turns.  Canals and cuts that do not shorten the flow distance usually shoal.  Man made cuts acting as substantial short-cuts frequently accelerate flow rates and produce channel widening or deepening with consequent shoaling where the flow returns to the natural channel.    Flows through low velocity cuts are usually quiescent and cannot support sediment movement through the cut.   This decay in flow rate is also the prime reason for rapid, fine-grained deposition in marina harbors because they too, are areas of open water where tidal flow has no natural reason to exist.  As a result, suspended sediment falls onto the marina bay floor at times of slack water (four times each day). 

In addition to transport of entrained sediments, shoaling may also occur via bedload transportation.  Bedload transportation is the movement of sediment along the sediment bed without entraining the sediment into the water column.  The primary indicator that bedload transport is occurring is the presence of bottom features such as ripples or larger features known as dunes or sand waves.  These features move down current as the water flows over the bed.  Fluid shear is the mechanism rolling the sand particles into the bedforms present on the bed.  Size and spacing is proportional to current velocity.  Bedload transportation from relatively shallow areas into deeper channels and marinas creates shoals. The differing bottom sediment composition lends clues to sediment sources and local currents.   

Physical Properties of Sediment in Cape May County

During the field campaign in 2007, sediment samples throughout the study area were collected and their physical properties analyzed, namely grain size.  Table 1 provides the location, elevation, and physical properties of the samples collected in Cape May County.  This data was extrapolated over the entire study area to create Figure 4.

Table1 - Sediment sample data summary

Figure4 - Cape May County grain size distribution

In general, the study area is largely comprised of sand-sized sediments with isolated locations having a high percentage of fine silts and clays.  In the above figure, lighter areas represent bottom sediments with a higher percentage of sands (less fine silts and clays) while darker areas represent bottom sediments with a lower percentage of sands (more fine silts and clays).  Locations of higher flow (inlets and flow through channels) have a higher percentage of sands while the low flow, low energy location have a higher percentage of silts.  The areas with a higher percentage of silts and clays include; the west end of Cape May harbor, Sunset Lake, Post Creek Basin, Jenkins Sound, Pennsylvania and Princeton Harbor, Corson Sound, and Upland Thorofare.  Most of these locations are shallow open body areas with limited flow regimes that allow the fines to be deposited.  Fine silts and clays have a limited beneficial reuse.

Historical Dredging Data

A thorough review of available historical dredging data in Cape May County provided an overview and identified trends of past dredging projects.  The major sources of the data include a 2003 U.S. Army Corps of Engineers Report (New Jersey Intracoastal Waterway Preliminary Draft Feasibility Report & Integrated Environmental Assessment), NJDEP Office of Sediment Technology permit data, and NJDEP Bureau of Coastal Engineering (BCE) information.  Each set of data is discussed below.

NJDEP Permit Data

In order to legally dredge in the waterways of New Jersey, a Waterfront Development permit must be filed with the NJDEP.  All dredging activity is subject to USACE examination; however, for the purpose of this project, USACE permits were not reviewed because they duplicate the NJDEP process.   Permitted dredging projects include maintenance dredging of boat slips and state and local channels as well as dredging for new waterfront developments.  These records were provided by the NJDEP Office of Dredging and Sediment Technology (ODST) for the entire state for permits filed between 1998 and 2008.  Not all of the information provided in the database was complete.  Missing information was gathered by requesting to review specific permits through the NJDEP Office of Public Record (OPRA). 

Not all waterway areas are dredged in Cape May County as some areas are naturally relatively deep.  Dredging projects generally occur where channels or marinas are shoaling or where new areas are to be developed.  The NJDEP permit data for Cape May County was divided into five regions separated by inlets.  These regions are shown in Figure 5 and a summary of the region is presented in Table 2. 

Figure5 - NJDEP dredging permit region identification

Table2 - NJDEP dredging permit region summary

Dredging permit data located in each region was grouped to determine the dredging demand and estimate the shoaling rate.  The dredged material from these projects is placed in an upland area either a CDF or an on-site facility.  A summary of the dredging permit information for each region in Cape May County is shown in Table 3.

Table3 - NJDEP dredging permit data summary

USACE NJIWW report

The U.S. Army Corps of Engineers published a report "Preliminary Draft New Jersey Intracoastal Waterway Draft Feasibility Report & Integrated Environmental Assessment" (USACE 2003) in which they evaluated the maintenance dredging needs of the NJIWW.  The NJIWW is a series of channels that runs from the Great Bay to Cape May in NJ (Figure 6).  The original maintenance depth of the channel was 12 feet.  However, in 1991 the depth limit was reduced to 6 feet mean low water.  The USACE-Philadelphia District is responsible for maintaining the channels, which includes maintenance dredging.  The state is responsible for providing placement options for the dredged material.  Historically, maintenance dredging has been limited to specific reaches of the NJIWW where shoaling is the greatest.  These reaches in Cape May County are identified in Figure 6.  Reach 1 (R-1), located in the Cape May Canal is not part of the study area. 

In the Cape May County Shoaling Analysis study area, there are 13 distinct maintenance dredging reaches, Reach 2 to 14 (Figure 6).  A summary of dredging activities in these reaches are summarized in Table 4.  These reaches, totaling 30.3 miles, represent approximately one half of the total NJIWW in Cape May County.  To compare the reaches, approximate shoaling rates were calculated assuming a channel width of 100 feet.  In general, yearly shoaling rates are not linear but are a function of time and the difference between the native bed elevation and dredged bed elevation.  Shoaling rates tend to decrease in time as the differential between the two bed elevations decreases.  The calculated shoaling rates indicate the variance in shoaling between the areas which is based on the channel location, type, and sediment characteristics of the reach.  The surrounding sediment varies by location, usually related to distance from one of the inlets.  Using nautical charts and sediment data, the reaches are characterized according to location, reach distance, sediment, and channel (Table 5).    

Figure6 - NJIWW NJDEP reach designation (USACE 2003)

Table 4 - NJIWW dredging data summary

                                                                                                      Table 5 - NJIWW reach descriptions

NJDEP BCE State Dredging Locations

The NJDEP Bureau of Coastal Engineering is responsible for all state sponsored dredging activities.  Based on interviews with BCE personnel and a review of available BCE project files, several areas of Cape May County that have been dredged regularly were identified.  These locations are highlighted with yellow in Figure 7.

Figure7 - Reoccurring state dredging locations

Historical Dredging Trends

Historical dredging data is very valuable for identifying dredging trends in Cape May County and predicting future dredging needs.  Historically, funding and placement options were available for dredging projects.  Therefore, once an area had shoaled to a certain elevation it would be dredged.  Each dredging project is different with respect to material quantity, location, area, dredge depth, reoccurrence interval, and placement.  Using the available historical information, priority dredging areas were identified based on approximated shoaling rates and historical dredging demand relative to other projects within Cape May County.   Although there are many areas throughout the county with dredging trends the following areas have the highest predicted dredging demand from historical sources and are shown in Figure 8 :

·        Cape May Harbor

·        Ludlam Bay

·        Crook Horn Creek

Figure8 - Priority dredging location from historical trends

The area in and around Cape May Harbor is the main priority dredging location based upon historical dredging trends.  Cape May Harbor serves as a major navigational waterway for recreational and commercial boats.  It has the highest shoaling rate and dredging demand from all the historical sources.  The NJIWW channel that passes through the harbor shoals at a rate of nearly 2 ft per year and has a 20 year dredging demand of 1,200,000 cy.  In addition to the NJIWW there are three channels that are routinely dredged; Devils Reach, Schellenger Creek, and Spicer's Creek as well as multiple large marinas.  A majority of the sediment dredged from these locations are placed in the USACE Cape May Canal Disposal area as well as at the Bree-Zee-Lee Marina (Linger Point #1) and Utsch's Marina private CDFs.  The private CDF have capacity for approximately 10 years (NJDOT OMR 2008).  The sediments in this area range from sands closer to the inlets to fine silts and clays in the back of the harbor (sediment samples #1, 52, 53).  The Cape May Harbor is located just south of the Cape May (Cold Springs) Inlet and is connected to the Delaware Bay through the Delaware Canal.  During the flood tide, water fills the Cape May Harbor and flows westward through the canal to the Delaware Bay.  Due to the convergence at the canal entrance there is a low flow zone where shoaling occurs.  Another location of shoaling in this area is at the transition between the inlet and the harbor.  Sand from the nearshore is transported in during a flood tide and deposited at these flood-tidal shoals.  

Historical trends identify Ludlam Bay, located west of Sea Isle City, as a secondary priority dredging area.  The state has identified this location as a reoccurring dredged project and the USACE report (2003) estimates a 20-year demand of approximately 300,000 cy with a calculated shoaling rate of 0.33 ft/yr in the channel.  Sediment within the bay is mostly sand 9(sample #39) and is placed in the Ludlam Thorofare #2 CDF.  This CDF has less than 75,000 cy of capacity remaining.

Historical trends identify Crook Horn Creek, located west of the northern Ocean City, as a secondary priority dredging area.  The state has identified this location as a reoccurring dredged project and the USACE report (2003) estimates a 20 year demand to be about 750,000 cy with a calculated shoaling rate of ~0.3 ft/year in the channel.  Sediment within the channel transitions from a high percentage of silts and clays in the northern part of the channel (sample #9) to a mix of sands and fines in the southern part of the channel (sample # 13).  The city owns one of the primary CDF’s (Crook Horn Creek #1) located 100 yards north of the former rail link to Ocean City. 

Current Dredging Needs

Current dredging needs have been identified through meetings with the NJDEP BCE, local municipalities, and local marina owners.  These locations are identified in Figure 9.  Neglected maintenance dredging is commonly due to a lack of funding and placement options.  Dredging has not been conducted regularly and channels have been allowed to shoal.  All anecdotal information points to a serious need for maintenance dredging on a large scale at most main channels, many minor waterways, and at most major public and private back barrier island recreational vessel mooring and berthing areas. 

Figure9 - Current dredging needs

One foreseeable dredging project will occur in the Stone Harbor waterways.  A combination of recent development along Stone Harbor Boulevard and continued shoaling in the Stone Harbor Canal and Scotch Bonnet waterways running adjacent to Stone Harbor Boulevard have resulted in the need to dredge the area.  This waterway runs 3 miles and consists of the channels running adjacent to Stone Harbor Boulevard and Scotch Bonnet which intersects the NJIWW at Great Channel. Using recent soundings collected by the State of New Jersey, March 2008, an estimated dredged volume of 100,000 cubic yards was calculated assuming that the channel will dredged to a depth of 5 feet MLW and have a width of 50 feet.  If this area were to be dredged it would exceed the Sanctuary Island CDF, which has been recently rehabilitated and used for recreational slip sediments.  The previous placement option for this site, Nummy Island CDF, is currently filled to capacity (Figure 10).

In general dredging projects are located no more than 14,000 feet from a CDF in order to eliminate the need of booster pumps which increase dredging costs.  From the data, there are recurring dredging needs in the surrounding channels with an 2-3 year interval that would average around 15,000 cy of material per event.  If this trend continues into the future, either another CDF location must be identified for this material or the current two sites must be emptied to handle new sediment derived from this set of waterways.  Several projects and studies have been preformed to test the feasibility of the beneficial reuse of dredged material in order to increase CDF capacity.

Figure10- Dredged material placement at Nummy Island

Priority Dredging Locations Conclusions

In general, most of the Cape May County coastal waterways are in need of some degree of dredging.  Priority dredging locations are those which are currently in need of dredging based on meetings with state and municipal personnel and selected marina owners (Table 6).  Historical priority dredging areas are based upon the shoaling rate and predicted dredging demand.  Historical dredging data and existing channels with clearly identifiable dredging needs provide the evidence to identify the priority dredging sites throughout Cape May County.  A summary of the priority dredging areas is indicated below.

Table 6 - Cape May County priority dredging areas

All available sources indicate the Cape May Harbor and the waters that flow into it is Cape May County's primary priority dredging location (Figure 11).  This body of water supports a large number of boating traffic both recreational and commercial.  Historically this region has been dredged the most from both the NJIWW study data and the dredging permit data.  Additionally, stakeholders have identified that this region needs to be dredged.  The material in this area varies.  High percentage of sand sediments are located near the Cape May Inlet, high percentage of fine silts and clays are located in the western portion of the Cape May Harbor and a mix of sediments are located between.  Placement options for materials dredged from this location include USACE Cape May Canal CDF and two private CDFs (Linger Point #1 and Linger Point #2).  It is estimated that at least 10 years of dredged material can be placed within these sites.

Figure11 - Cape May County summary