OCS STUDY 

 MMS 2004-019 

Preliminary Infrastructure 

Stability Study, Offshore Louisiana 

PREPARED FOR: 

U.S. DEPARTMENT OF THE INTERIOR 

MINERALS MANAGEMENT SERVICE 
OCS Report 

 MMS 2004-019 

Preliminary Infrastructure 

Stability Study, Offshore Louisiana

Authors

Rob Nairn 

Steve Langendyk 

Jacqueline Michel 

March 2004 

Prepared under MMS Contract No. 35-0001-31051 

by

Baird & Associates 

Madison, Wisconsin 

and

Research Planning, Inc. 

Columbia, South Carolina 
i

DISCLAIMER

This report was prepared under contract between the Minerals Management Service (MMS) and 

Research Planning, Inc. This report has been reviewed technically by the MMS, and it has been 

approved for publication. Approval does not signify that the contents necessarily reflect the 

views and policies of the MMS, nor does mention of trade names or commercial products 

constitute endorsement or recommendation for use. It is, however, exempt from review and 

compliance with the MMS editorial standards.
ii

TABLE OF CONTENTS 

1.0 INTRODUCTION ...................................................................................................1 

2.0 BACKGROUND .....................................................................................................2 

 2.1 Proposed Dredging .........................................................................................2 

 2.2 Overview of Borrow Deposits ........................................................................2 

 2.3 Concept of Buffers to Protect Infrastructure...................................................4 

3.0 ANALYSIS..............................................................................................................6 

 3.1 Application of Buffers and Volume Analysis.................................................6 

3.1.1 Discussion of Three Buffer Widths Considered ....................................6 

3.1.2 Volume Analysis....................................................................................6 

 3.2 Topographic Change.......................................................................................7 

4.0 CONCLUSIONS AND RECOMMENDATIONS ................................................11 
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LIST OF FIGURES 

 PAGE

Figure

 2.1 South Pelto (New Cut) and Sandy Point Borrow Areas ........................................12

 2.2 South Pelto Borrow Area Regional Bathymetry....................................................13 

 2.3 Sand Point Borrows Regional Bathymetry............................................................14 

 2.4 South Pelto Borrow Area Infrastructure and Detailed Bathymetry.......................15 

 2.5 Sand Point Borrows Infrastructure and Detailed Bathymetry ...............................16

 3.1 South Pelto Borrow Cells for Dredge Volume Estimates......................................17 

 3.2 Ship Shoal Isopach Map at South Pelto Dredge Area – 5 Dredge Cells ...............18 

 3.3 Sandy Point Borrow Cells for Dredge Volume Estimates.....................................19 

 3.4 South Pelto Borrow 1936 Regional Bathymetry ...................................................20 

 3.5 South Pelto Borrow Bathymetry Comparison Based on 1939 & 1994 

Hydrographic Surveys ...........................................................................................21 

 3.6 South Pelto Borrow Bathymetry Comparison 1994 to 2003 .................................22 

 3.7 South Pelto Borrow 1936 & 1994 Bathymetry Anomalies ...................................23 

 3.8 South Pelto Borrow Bathymetry After 3 Meter Dredge ........................................24 

 3.9 South Pelto Borrow 3D View of Bathymetry After 3 Meter Dredge ....................25 

 3.10 Sandy Point Post Dredge Slope Evolution ............................................................26 

LIST OF TABLES 

 PAGE

Table

 3.1 South Pelto as 5 Dredge Area Cells.............................................................................27 

 3.2 Sandy Point as 2 Dredge Area Cells............................................................................30 
1

1.0 INTRODUCTION 

The Minerals Management Service (MMS) requested a preliminary evaluation of the potential 

impacts of dredging to existing oil and gas infrastructure (primarily pipelines) on the Outer 

Continental Shelf offshore Louisiana. Specifically, the investigation focused on the proposed 

2004 dredging for South Pelto (or New Cut) and Sandy Point Borrow areas. Recommendations 

are required on a provisional buffer to avoid impact to the pipelines either directly or indirectly. 

This report describes the findings of this investigation. 

The methodology for the investigation consisted of the following steps:

1. Collect available information on the most recent bathymetry for the two deposits and 

infrastructure and import to GIS. 

2. Develop a provisional buffer between the limit of dredging and existing infrastructure to 

minimize the risk of erosion in the area of existing infrastructure. This buffer will 

consider positioning accuracy of dredging operations together with a stable slope 

allowance (i.e. considering that the initial dredge cut slope will slump and evolve with 

time to achieve a flatter stable slope and this has the potential to impact infrastructure 

indirectly).

3. Place the buffers in the GIS with bathymetry to show the area available for dredging and 

comment on implications to removing the required dredge amounts for 2004, if any. 

4. Complete a report describing methodology and results to be delivered in hard copy and 

digital form (including GIS files). 

The remainder of the report is divided into the following sections: 

2.0 Background 

3.0 Analysis 

4.0 Conclusions and Recommendations. 
2

2.0 BACKGROUND 

This section includes a description of the proposed dredging, regional context for the two sites, 

and an introduction of the buffer zone concept. 

2.1 Proposed Dredging 

There are plans to remove up to approximately 12,000,000 cubic meters (m3

) of medium sized 

sand from the South Pelto and Sandy Point Borrow Areas commencing in May 2004. The sand 

will be used for land reclamation and beach nourishment projects. 

The water depths range from 7 to 10 m at the South Pelto Borrow Site and 10 to 11 m at the 

Sandy Point Borrow area. The placement sites are located approximately 15 to 25 kilometers 

(km) from the borrow areas. It is likely the depth of cut would be somewhere in the range of 1 to 

3 m. These conditions are ideally suited to Trailing Suction Hopper Dredges (TSHDs) and 

possibly Cutter Suction dredges (though the latter is less likely). TSHDs do not dredge to a slope 

at the edge of the cut and instead cut at the toe and a slope develops with each deepening pass of 

the drag head. 

A concern with the proposed dredging is the potential for impacts to existing oil and gas 

infrastructure that exists in the same areas. The focus of this investigation has been on pipelines 

in the area. At the time of original installation most of the pipelines would have been buried with 

a cover of 3 feet (ft) or approximately 1 m. 

2.2 Overview of Borrow Deposits 

A description of the conditions at the borrow deposits and for the surrounding region has been 

developed through the acquisition of the following information: 

1. GIS information on the location and characteristics of pipelines in the vicinity of the 

borrow deposits was obtained from the website of the MMS, Gulf of Mexico Region, 

Geographic Mapping Data in Digital Format page 

(http://www.gomr.mms.gov/homepg/pubinfo/repcat/arcinfo/index.html). The spatial 

datasets (ArcInfo GIS versions) downloaded included pipelines, platforms, active leases, 

fairways, federal lease blocks, lease term lines, and protraction polygons. The pipeline 

database version was updated 17 November 2003, and the platforms database was 

updated 2 December 2002; 

2. Regional images of the adjacent land are from Landsat 7 images, acquired from the 

Louisiana GIS CD (version 2.0) and from the University of Maryland’s Global Land 

Cover Facility. This satellite provides 30 m resolution images that show the borrow areas 

in relation to features like the Mississippi River delta. 
3

3. Regional bathymetry with 2 m contour intervals was obtained from Louisiana Oil Spill 

Coordinator’s Office (Louisiana Offshore Bathymetry, Geographic NAD83, LOSCO 

(1999)). This dataset was not used for any analysis, only as a large-scale visual reference. 

4. Local and more detailed bathymetry for each borrow area was obtained from multiple 

sources. Historic hydrographic surveys were obtained from the National Oceanic and 

Atmospheric Administration, via the Geophysical Data System for Hydrographic Survey 

Data DVD, version 4.1.18, distributed by the National Geophysical Data Center 

(NGDC). Historic hydrographic surveys were found only for the South Pelto borrow 

area, none were found for Sandy Point borrow areas. The South Pelto historic surveys 

consisted of three 1936 surveys, from maps at 1:40,000-scale, the original survey lines 

are spaced at 500-600 meters with samples taken at 150-280m intervals, and are 

identified as NGDC numbers 03071006, 03071036, and 03071111. Also at South Pelto, a 

1994 hydrographic survey (NGDC# 03091126) was available that covers the west half 

(approx. 60%) of the borrow area. Current bathymetry at South Pelto is represented by an 

August 2003 survey by C & C Technologies of Lafayette, Louisiana referred to as 

‘4037’. Current bathymetry at Sandy Point borrows is represented by a July 2003 survey 

conducted by Coastal Planning & Engineering of Boca Raton, Florida. Only 1-foot 

interval contours were available from this survey. No other bathymetry datasets for the 

Sandy Point borrows area were found or provided. 

5. The outline boundaries of the proposed borrow deposits were created by entering 

coordinate values taken from documents received at Baird. The South Pelto polygon was 

from a document produced by the Louisiana Dept. of Wildlife and fisheries and 

Louisiana DNR Coastal Restoration Division. The two Sandy Point borrow areas were 

from Coastal Planning & Engineering Inc., Pelican Island Restoration (BA-38-1) 

CWPPRA Project Sandy Point. These features were defined using Louisiana South State 

Plane coordinate system in feet, NAD 1983 datum. To support analysis and comparison 

with all of the MMS datasets that are natively defined using NAD 1927 datum, the 

Borrows dataset was projected from NAD83 to NAD27 using ESRI's ArcGIS 

ArcToolbox 8.3 Project Wizard software. 

6. Isopachous coverage of Ship Shoal, as constructed from the available vibracore data and 

high-resolution seismic profiles, was extracted from the Ship Shoal: Sand Resource 

Synthesis Report (Kulp et. al., 2001). The isopach contours were used to construct a 

surface that was clipped to the South Pelto Borrow Area extents and used to derive a total 

sand volume estimate. 

7. The use of a 1000-ft setback distance from infrastructure is described in the draft of the 

MMS Negotiated Noncompetitive Lease for Sand, Gravel and Shell Resources on the 

Outer Continental Shelf, Lease number OCS-G 25379, for the removal of materials from 

within the Sandy Point Northwest and Sandy Point Southeast Borrow Areas: 

“STIPULATION NO.5 – Avoidance of Oil and Gas Structures – During all dredge 

operations, a minimum “no dredge” setback distance of 1000 ft will be established from 

existing pipelines and all other oil and gas-related infrastructure.” 
4

The location of the South Pelto and Sandy Point Borrow Areas relative to the coast of Louisiana 

is shown in Figure 2.1. 

The South Pelto Borrow Area is located at the east end of Ship Shoal as shown in Figure 2.2. 

Ship Shoal is a relict submerged barrier island. The Sandy Point Borrow Areas are located 

immediately on the west side of the main delta of the Mississippi River (see Figure 2.3). The 

shallower water on the west side of the delta suggests this is the location of a former distributary 

mouth of the Mississippi River. 

Figures 2.4 and 2.5 show the existing pipeline areas and the designated borrow areas (the latter 

determined from geophysical investigations) for the South Pelto and Sandy Point Borrow areas, 

respectively. Five individual borrow cells are created by the pipeline network at South Pelto 

while the two smaller Sandy Point Borrow Areas are not bisected by existing infrastructure. 

2.3 Concept of Buffers to Protect Infrastructure 

In a recent project for MMS, buffer requirements for archeological resources were reviewed (see 

Research Planning, Inc. et al., 2004). Buffer zones are also stipulated to protect hard bottom 

habitat from dredging impacts. There are several reasons that buffers or avoidance/exclusion 

zones need to be stipulated and these fall into two groups: a) to avoid direct impact of the dredge 

with the resource; and b) to avoid indirect impacts related to subsequent changes to the sea bed 

and the possibility of erosion or scour at the resource. These same reasons also apply to the oil 

and gas infrastructure and are described below: 

Avoidance of Direct Impacts 

• To address the possibility of position inaccuracies by the dredge vessels and the drag arm 

or cutter head (although these are low at +/- 5 m); 

• To allow for the possibility of power loss, influence of storms or human error in the 

navigation and operation of the dredge vessel; 

• Possible inconsistencies related to common geographic coordinates for the infrastructure 

and the dredge vessel position. 

Avoidance of Indirect Impacts 

• The slope at the edge of the borrow area (which will generally become 1 to 3 m deeper 

than the surrounding areas outside the borrow area) will adjust with time usually 

becoming flatter than the initial slope, thus potentially expanding the area of bed 

lowering;

• If dredging occurs on either side of a pipeline (such as the case may be at South Pelto – 

see Figure 2.4) a berm may develop with a potential height of 3 m above the adjacent sea 
5

bed. It may be possible that this berm (which is left in place to protect the pipeline) could 

begin to migrate, eventually exposing or undermining the pipeline. 

• The sea bed in these depths is dynamic and particularly the South Pelto Borrow Areas at 

the east end of Ship Shoal. Therefore, there will be natural erosion and accretion with 

time. It is possible that the dredged borrow areas may change the natural 

morphodynamics (patterns of sea bed change). 

Direct impacts to the infrastructure would result in damage or breakage of the infrastructure, 

such as pipelines. Indirect impacts could result in uncovering or undermining (creation of spans) 

of pipelines. Therefore, a buffer is required to address the potential for direct dredge impacts 

mostly associated with potential errors in the relative position of the dredge and infrastructure in 

addition to a buffer to address the indirect changes to the sea bed. The two buffers are not 

additive as the indirect changes will develop with time. The larger of the two buffers is selected. 

In the MMS Archeological Survey (Research Planning, Inc. et al., 2004) several dredging 

companies in the US and Europe were canvassed for their recommendations on buffer strips. 

Generally, the dredging contractors recommended buffers of 100 to 300 m. 
6

3.0 ANALYSIS 

3.1 Application of Buffers and Volume Analysis 

The analyses performed for this preliminary assessment consisted of developing some reasonable 

and conservative buffers and applying these to the two borrow areas to determine the 

implications to the size and yields (in terms of quantities) of the borrow areas. In addition, the 

eventual form of the sea bed topography was evaluated to assess the potential for indirect 

impacts, and particularly migration of berm features along the pipeline corridors. 

3.1.1 Discussion of Three Buffer Widths Considered 

Three different buffer widths were applied to the pipelines in the vicinity of the borrow areas: 

150 m (corresponding to the 500 ft buffer presented in a EIS for an OCS Lease Sale in the Gulf 

of Mexico), 200 m (corresponding to the suggested 630 ft buffer by the MMS Gulf of Mexico 

Region a recent dredge test lease issued by MMS within South Pelto Block 13), and 300 m (the 

upper end suggested by dredging contractors and consistent with the Draft Stipulations for the 

Sandy Point Borrow Areas). The largest buffer of 300 m also corresponds to an excavation of 3 

m with a potential ultimate slope of 1:100 (vertical:horizontal) at the edge of the borrow area or 

1:300 with a 1m excavation – which are relatively conservative and would likely take many 

years to develop. 

The sea bed gradients have been calculated at a variety of positions representing steepest 

gradients and existing gradients perpendicular to proposed protective berm corridors along 

pipelines and range from 1:285 to 1:1,800. These slopes are all flatter than our recommended 

assumption of 1:100 to develop a buffer for a 3m excavation or 1:300 for a 1m excavation. 

Therefore, in the long term, based on slope alone, the assumption of a 1:100 to 1:300 slope is not 

overly conservative. 

3.1.2 Volume Analysis 

Figure 3.1 illustrates the dredge cells created by a buffer width of 300 m for the South Pelto 

deposit. An initial slope of 1:3 was assumed to be created at the dredge boundary, ultimately 

forming a 1:100 slope in the long term. (Interestingly, the slope of 1:3 creates a slope volume to 

total volume ratio constant of 0.00374.) Table 3.1a presents the quantities of sand available from 

each of the created blocks in the South Pelto Borrow area for dredge cuts of 1, 2 and 3 m 

corresponding to a 300 m buffer on either side of the pipelines, with the first set of values 

representing the maximum volume assuming an equivalent dredge depth, and then a second set 

of values incorporating the allowance of an initial 1:3 slope at the dredge cell boundaries. Tables 

3.1b and 3.1c present the quantities for buffers of 200 m and 150 m, respectively, and are 

presented for comparison to the 300 m buffer to show the comparative volumes. For the largest 

300 m buffer, the five South Pelto cells yield approximately 15,000,000, 31,000,000 and 

46,000,000 m3 for dredge cuts of 1, 2 and 3 m, respectively. 
7

It is interesting to note that the assumed total reserve of the South Pelto Borrow Area, without 

consideration for the need to protect infrastructure, is estimated to be 67,500,000 m3

. This was 

calculated based on the information presented in the Ship Shoal: Sand Resource Synthesis 

Report (Kulp et al., 2001), specifically Figure 19 which depicts an isopach map of the entire 

Ship Shoal sand body at an estimated map scale of 1:500,000. According to the report, the 

isopach contours were “constructed from the available vibracore data and high-resolution 

seismic profiles.” For the South Pelto Borrow Area, the isopach described in the report has a 

thickness of up to 4 m across an east-west ridge at the north of the Borrow Area, diminishing to a 

thickness of at least 1 m across the southern portions of the Borrow Area. 

The isopach contours defined by Kulp et al. (2001) were used to construct a surface that was 

clipped to the South Pelto Borrow Area extents (as shown in Figure 3.2) and used to derive a 

sand volume estimate (that incorporates the initial slope of 1:3), also presented on the right-side 

of Table 3.1a. These volumes incorporate an allowance for the initial slope of 1:3 using a slope 

volume to total volume ratio constant of 0.00374 that was derived from values in the earlier step 

of slope volume allowance. Incorporating these estimated isopach limits, the volume potentials 

are reduced for the deeper cuts as the depth of sand is limited: 

1 METER – There is at least 1 m sediment thickness over the entire Borrow Area, resulting in no 

volume reduction and the same value of approximately 15,000,000 m3.

2 METER – At the 2 m thickness, about half of cell SP-A is not available, resulting in a volume 

estimate of 23,200,000 m3, compared to 30,565,000 m3 before accounting for sediment thickness 

limitations. 

3 METER – At the 3 m thickness, only portions of cells SP-B, SP-C & SP-D have any 

significant coverage, resulting in only an additional 3.5 million m3 of material, totaling 

26,704,000 m3, significantly less than the 45,848,000 m3 as would be available if 3 m thickness 

was available over the entire 5 cells.

Figure 3.3 shows a 300 m buffer for the Sandy Point Borrow Areas and Table 3.2 presents the 

corresponding quantity of dredge material for 1:3 slope down to a cut depth of 45 ft below the 

original surface. The irregular outline shape of the Sandy Point Borrow Areas (not rectangular 

like South Pelto), and the significantly deeper dredge depth, meant that the slopes would meet 

opposite facing slopes at narrow areas of the borrow. To accurately represent this surface, a 

series of 14 concentric internal buffers were created. Incorporating these factors, the combined 

quantity of dredge material available at the two Sandy Point Borrow Areas is 8,634,000 m3.

3.2 Topographic Change 

As noted in Section 2.3, there is concern for possible indirect impacts related to shifting of 

created sea bed features (such as berms created by dredging along either side of a pipeline) and 

related to changes to the natural morphodynamic development that may be caused by the 

dredging.

The first step required to investigate these possibilities is to evaluate the historic change in the 

areas of proposed dredging. At the South Pelto Borrow Area, both historic and recent bathymetry 
8

data were available to facilitate a bathymetric change analysis. Insufficient data were available to 

complete a similar analysis at Sandy Point. 

As indicated earlier, the South Pelto site is located at the east end of the Ship Shoal feature. Ship 

Shoal features a steeper shoreward slope and flatter seaward slope and this asymmetry typically 

indicates that the feature is migrating shoreward in the long term (see Figure 3.4). Due to the size 

of Ship Shoal (even at this extreme east end of this feature the “crest width” of the feature is 1.5 

to 3.0 km which is large compared to the height of the feature of approximately 5 m above the 

surrounding sea bed) landward migration rates will be very slow. Figures 3.5 and 3.6 show 

comparisons of bathymetric change between 1936 and 1994, and 1994 and 2003, respectively. 

Figure 3.5 shows erosion on the seaward flank and accretion on the shoreward flank that is 

compatible with slow long-term shoreward migration of the shoal, although this might be 

exaggerated as a result of the limits of the 1936 hydrographic surveys. The two individual 1936 

hydrographic surveys that contributed to this surface are pretty sparse (lines spaced at 500-600 

m, samples taken at 150-280 m intervals) so there are some artifacts of the surface interpolation 

process that show circular patterns in the contour lines at the bottom of the shoreward slope, and 

a closer inspection reveals north-south linear troughs in the top face seaward slope of the shoal 

(as evident in the 0.25 m contour lines). This figure also shows widespread erosion of the crest 

of the feature in the range of 0.25 to 1.25 m between 1936 and 1994 (which may be related to a 

vertical datum discrepancy – the 1936 vertical datum is “mean low water” while the 1994 

vertical datum is “mean lower low water” and “aveTide = 0.4”).

Figure 3.6 shows comparisons of bathymetric change between 1994 and 2003, illustrating 

accretion in some areas (up to 0.5 m) and erosion in others (up to 0.75 m – the accretion value of 

1.25 m is the result of a shallow spike data anomaly in the 1994 survey). Looking at the 1994 

surface, a much more dramatic and distinctive north-south ridge in the bathymetric map is 

visible, with a subtle rise in the surface measuring about 1 km across and with depths about 0.5 

m shallower than the areas to the west. This cannot be a result of surface interpolation because 

the 1994 survey is much more dense than the 1936 survey. It is likely a trough has formed in the 

middle of the 1994 survey, but not visible in the other surveys (1936 or 2003). This makes 

visible a distinct band of high erosion at the easterly limit of the 1994-2003 comparison area as 

the 0.5 m subtle rise is eroded away. 

Figure 3.7 illustrates the data anomalies of the 1936 and 1994 hydrographic surveys. 

As a general comment, these are significant changes both in comparison to the depth of burial of 

the pipelines (approximately 1 m) and relative to the proposed dredge cut depths (1 to 3 m). It is 

possible that the pipelines may have been exposed in places by this degree of erosion depending 

on when they were constructed. More information is required on the current depth of burial and 

whether there are any existing instances of spanning. This information may result in a revision to 

the recommended buffer requirements. 

With regard to the potential impact of dredging on the stability of the east end of Ship Shoal, the 

shoal only extends about 5 m above the adjacent seabed (from –12 m up to –7 m). Therefore, the 
9

removal of 3 m would essentially significantly lower the feature. However, with the stipulations 

of buffers to protect pipelines, only part of the feature will be lowered significantly. The 

influence of the buffering is discussed in the remainder of this section. 

With all five cells at South Pelto dredged to 3 m deep with 300 m buffers along the pipelines, the 

resulting form of the shoal is shown in Figure 3.8. This extent of dredging would yield of 

46,000,000 m3 sediment (but only 26,700,000 m3 after including limits of the isopach estimates). 

A perspective view of one of the berms (between cells A and B- see Figure 3.1) is shown in 

Figure 3.9. With a dimension of 3 m in height and 600 m wide, these berms would be susceptible 

to landward migration, similar to Ship Shoal. This would be particularly true for those berms 

with an east-west axis (i.e., between A and B and D and E). Numerical modeling would be 

required to determine the potential rate of migration. 

If only 1 m were removed from each of the five cells the berms created by the buffers would be 

very subtle features and likely not susceptible to migration. As noted in Section 3.1.2 this would 

yield 15,000,000 m3 of sediment. At this time this would appear to be the best option to provide 

the sediment required. It would result in the least change to the feature and produce relatively 

subtle berm features. However, one implication of initially limiting the dredge depth to 1 m is 

that this would influence the efficiency of large cutter suction dredges that have a full production 

cut depth of 1.6 to 2 m. There would be no negative implications to the operations of trailing 

suction hopper dredges. If it is important to have a project that is suitable to both cutters and 

hoppers (e.g., broadening the market for the project) then another possibility would be to allow 

1.5 to 2 m dredge cut in Cell A only producing a little over 12,000,000 m3. This would avoid the 

creation of isolated berms along the pipeline corridors. However, this option should be further 

investigated with numerical modeling. 

At the Sandy Point Borrow Area, the removal of 3 m would create pits on the sloping shelf off 

the southwest flank of the delta. While this may not be an issue with respect morphologic 

change, it may become a water quality issue and/or these pits would fill with fine sediment 

discharged from the Mississippi River. It is likely this area of the delta is a depositional area.

Generally underwater stable slopes are around 1:3 immediately following dredging. However, 

the ultimate stable slope will depend upon many influencing factors such as sediment size, 

sediment transport, wave climate, etc. A 1:3 slope is relatively steep considering the existing 

grades in the Sandy Point Borrow Area are around 1:1,500.

How the slope adjusts and evolves after dredging depends on the local sediment transport 

patterns. There is anticipated to be preferential deposition of regional suspended sediment (from 

the significant loads of the Mississippi River) into deeper pits, reducing the affect of the slopes 

eroding and/or slumping to a gentler stable slope and thereby reducing the risk of impacts to 

infrastructure. Post-dredge slope evolution of a 6 m (20 feet) dredge pit could develop under the 

following three scenarios (as illustrated in Figure 3.10):
10

A. No infilling of the dredge pit occurs and the slope continues to erode back towards the 

pipeline infrastructure. This would require eroding back some 318 m to ultimately create a slope 

of 1:53.

B. Eroded slope material settles into the dredge pit, creating a complete balance of the sediment 

locally. This mid-depth scenario would create an ultimate slope of 1:106.

C. Infilling of pit will be heavily influenced by sediment loads transported from the Mississippi 

River. If we assume 3 m of infilling and slope adjustment from the point where the infill 

intersects the original 1:3 slope, then the ultimate slope that would interact with the pipeline 

would be 1:106 again. What is unclear without further analysis is the timeline for infilling of the 

pit.

The question is “Do slopes of 1:53 to 1:106 give an adequate margin?” While it is not overly 

conservative, the erosion and evolution of the dredge slope would be a slow process, possibly 

spanning many years (though this needs to be confirmed by modeling), which would allow time 

for monitoring. This long time scale of adjustment would allow for mitigating efforts to be 

implemented, in response to the results of monitoring, such as filling the dredge pit well before 

any adverse impacts to the pipelines.

The removal of 15 m, the deepest proposed cut, would result in more rapid infilling. The rate of 

infilling could be estimated through additional analysis or numerical modeling. The rate of pit 

infilling will influence the ultimate slope development beyond the dredge area. With no pit 

infilling and with a 15 m cut plus 300 m buffer, the ultimate slope for scenario A would be 1:23 

from edge of pipe to toe of pit, which is considerably steeper than average slopes in the area. 

Therefore, with no pit infilling a larger buffer would be prudent for a 15 m cut. It is possible that 

this additional buffer could be applied as a berm at the 20 ft cut depth if a deeper dredge cut was 

planned as a second phase (see Scenario D in Figure 3.10). If it is necessary to dredge to 20 m 

(45 ft) immediately, we would recommend a buffer of 600 m. 

4.0 CONCLUSIONS AND RECOMMENDATIONS 

A preliminary assessment of the potential impacts to oil and gas pipelines of dredging the South 

Pelto and Sandy Point was completed. 

With respect to buffer widths, at this time 300 m on either side of the pipeline is proposed. This 

buffer width is more than sufficient to prevent any accidental direct impact due to positional 

errors of the dredging operation but is required to prevent indirect impacts related to subsequent 

sea bed changes. 

At the South Pelto site it is recommended that the dredging depth be limited to 1 m in any of the 

five cells at this time. An implication of this stipulation is that large cutter suction dredges would 

not be able to operate at full production rates (i.e., because a cut of 1.6 to 2 m is required), thus 

narrowing the market on tendering the project. As a less desirable alternative to allow both 

cutters and trailers to work the project at South Pelto, dredging could be restricted to the largest 
11

Cell A with a 2 m maximum cut depth. For either this second alternative or subsequent 

consideration of increased dredge depths, a numerical modeling assessment of the stability of the 

shoal and the created berms should be undertaken. It is also recommended that the conditions of 

the pipelines around and through the proposed borrow areas be evaluated with respect to existing 

cover and invert elevations and, specifically, for any existing instances of spanning. 

Of the two Sandy Point Borrow Areas, only the northwest deposit is trimmed by the 

implementation of 300 m buffers. Here the only concern with a 3 m dredge cut is the potential 

for the pits to fill with fine sediment, possibly becoming anoxic. While these changes do not 

have morphologic implications they may have implications to the change of habitat. Three 

scenarios for infilling of a 6 m cut show that slumping and sedimentation would create slopes of 

1:53 to 1:106 over a 300 m buffer that could take many years to develop (the slope evolution 

should be closely monitored). A 15 m cut would result in a very deep pit that fills in quite rapidly 

(the rate of infilling could be estimated through additional analysis). The rate of infilling would 

need to be determined to evaluate the eventual slope development beyond the dredge cut area 

and impact for pipelines. Without the benefit of numerical modeling a 600 m buffer is 

recommended for a 15 m cut adjacent to pipelines. For both the 6 m and the 15 m cut plans 

additional analysis is required to better understand slope evolution processes at this site and to 

develop a site specific monitoring plan. 

All of these recommendations have been developed on the basis of limited investigation and are 

therefore preliminary in nature. However, it is noted that in the event that the slopes evolve 

more quickly than expected (i.e. towards the pipelines), mitigation measures are possible and 

could consist of filling in the pits through dredging. These recommendations have also assumed 

that the pipelines are currently buried and that there are no significant spanning problems at 

present.

5.0 REFERENCES CITED 

Kulp, M., S. Penland, and K. Ramsey, 2001. Ship Shoal: Sand Resources Synthesis Report. 

Prepared by the Coastal Research Laboratory, Department of Geology and Geophysics, 

University of New Orleans, La. 70 pp. + appendices 

Research Planning, Inc., Tidewater Atlantic Research, Inc., and Baird & Associates Ltd., 2004. 

Archaeological Damage from Offshore Dredging: Recommendations for Pre-Operational 

Surveys and Mitigation During Dredging to Avoid Adverse Impacts. U.S. Department of the 

Interior, Minerals Management Service, Sand and Gravel Unit, Leasing Division, Herndon, 

VA. OCS Report MMS 2004-005, 75 pp. + appendices.
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Table 3.1a South Pelto as 5 Dredge Area Cells

300m Buffer from Pipelines

Cell Name Area, Feet2 Area, Meters2

SP-A 66,483,098 6,176,482SP-B 24,806,912 2,304,638SP-C 12,118,921 1,125,885SP-D 43,003,618 3,995,167SP-E 18,701,967 1,737,470Total Area: 165,114,517 15,339,641

Assumed Equivalent Dredge Depth Incorporate Initial 1:3 slope at Perimeter Incorporate Isopach Limits

Dredge Depth Potential Volume Area Area Volume Total Volume

Cell Name Meters Meters3 Feet2 Meters2 Meters3 Meters3

SP-A 1 6,176,482 332,390 30,880 15,440 6,161,042 1m thickness is available over the entire area

SP-B 1 2,304,638 241,472 22,433 11,217 2,293,421 Area, feet2 165,114,516

SP-C 1 1,125,885 150,600 13,991 6,996 1,118,889SP-D 1 3,995,167 288,851 26,835 13,418 3,981,749 Volume, meters3 15,339,640

SP-E 1 1,737,470 220,445 20,480 10,240 1,727,230 Account for slope, m3 -57,310

Total 15,339,641 57,310 15,282,331 Volume at 1m cut: 15,282,331

SP-A 2 12,352,964 664,020 61,689 30,845 12,322,119 2m thickness is available over about half the area

SP-B 2 4,609,275 481,807 44,761 22,381 4,586,894 Area, feet2 85,546,805

SP-C 2 2,251,769 300,402 27,908 13,954 2,237,815 Volume, meters3 7,947,558

SP-D 2 7,990,334 576,747 53,582 26,791 7,963,543 Account for slope (0.00374), m3 -29,724

SP-E 2 3,474,939 439,620 40,842 20,421 3,454,518 volume from 1m thick 15,282,331Total 30,679,281 114,391 30,564,890 Volume at 2m cut: 23,200,165SP-A 3 18,529,446 994,885 92,428 46,214 18,483,232 3m thickness is limited to portions of SP-B, SP-C & SP-D

SP-B 3 6,913,913 721,002 66,983 33,492 6,880,421 Area, feet2 37,852,783

SP-C 3 3,377,654 449,407 41,751 20,876 3,356,778 Volume, meters3 3,516,639

SP-D 3 11,985,501 863,685 80,239 40,119 11,945,381 Account for slope (0.00374), m3 -13,152

SP-E 3 5,212,409 657,516 61,085 30,543 5,181,866 volume from 1+2m thick 23,200,165Total 46,018,922 171,243 45,847,678 Volume at 3m cut: 26,703,651

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Table 3.1b South Pelto as 5 Dredge Area Cells

200m Buffer from Pipelines

Cell Name

Area, Feet2 Area, Meters2

SP-A 74,140,578 6,887,885SP-B 31,676,426 2,942,836SP-C 15,695,249 1,458,136SP-D 49,299,020 4,580,029SP-E 21,821,843 2,027,316Total Area: 192,633,117 17,896,202

Assumed Equivalent Dredge Depth

Dredge Depth Potential Volume

Cell Name Meters Meters3

SP-A 1 6,887,885SP-B 1 2,942,836SP-C 1 1,458,136SP-D 1 4,580,029SP-E 1 2,027,316Total 17,896,202SP-A 2 13,775,770SP-B 2 5,885,673SP-C 2 2,916,273SP-D 2 9,160,058SP-E 2 4,054,631Total 35,792,404SP-A 3 20,663,655SP-B 3 8,828,509SP-C 3 4,374,409SP-D 3 13,740,087SP-E 3 6,081,947Total 53,688,607

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Table 3.1c South Pelto as 5 Dredge Area Cells

150m Buffer from Pipelines

Cell Name Area, Feet2 Area, Meters2

SP-A 79,493,665 7,385,203SP-B 35,169,089 3,267,315SP-C 17,888,867 1,661,930SP-D 52,761,167 4,901,673SP-E 23,563,924 2,189,160Total Area: 208,876,711 19,405,281

Assumed Equivalent Dredge Depth

Dredge Depth Potential Volume

Cell Name Meters Meters3

SP-A 1 7,385,203SP-B 1 3,267,315SP-C 1 1,661,930SP-D 1 4,901,673SP-E 1 2,189,160Total 19,405,281SP-A 2 14,770,406SP-B 2 6,534,631SP-C 2 3,323,860SP-D 2 9,803,346SP-E 2 4,378,320Total 38,810,563SP-A 3 22,155,609SP-B 3 9,801,946SP-C 3 4,985,790SP-D 3 14,705,018SP-E 3 6,567,480Total 58,215,844

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Table 3.2 Sandy Point as 2 Dredge Area Cells

300m Buffer from Pipelines

Cell Name Area, Feet2 Area, Meters2

Northwest Borrow 3,571,489 331,802

Southeast Borrow 4,953,933 460,235

Total Area: 8,525,422 792,038

Assumed Equivalent Dredge Depth Incorporate Initial 1:3 slope at Perimeter

Dredge Depth Potential Volume Area Area Volume Total Volume

Cell Name Meters Meters3 Feet2 Meters2 Meters3 Meters3

Northwest Borrow 1 331,802 140,199 13,025 6,512 325,290

Southeast Borrow 1 460,235 141,758 13,170 6,585 453,651

Total 792,038 778,940Northwest Borrow 2 663,604 279,582 25,974 25,974 637,630

Southeast Borrow 2 920,471 282,515 26,247 26,247 894,224

Total 1,584,075 1,531,855Northwest Borrow 3 995,406 414,478 38,506 57,759 937,647Southeast Borrow 3 1,380,706 422,270 39,230 58,845 1,321,861

Total 2,376,113 2,259,508

(Depth of 20 feet)

Northwest Borrow 6.096 2,022,666 815,162 75,731 230,828 1,791,838Southeast Borrow 6.096 2,805,595 835,532 77,623 236,596 2,568,999

Total 4,828,261 4,360,837

For the 45 foot calculation, a cut TIN surface was

(Depth of 45 feet) created and these values derived from a direct

Northwest Borrow 13.716 4,550,998 comparison with the original surface.Southeast Borrow 13.716 6,312,590 Feet3 Meters3

Total 10,863,588 Total: 304,932,022 8,634,713

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