5.0 SEDIMENT BUDGET SITESloslr.ijc.org/en/library/Coastal/02d 2006-12-31 Baird... · sediment...

5.0 SEDIMENT BUDGET SITES

Shoreline erosion is a natural process that occurs throughout the study boundaries, the entire Great Lakes system and many the world’s beaches and bluff coastlines. Although water levels on Lake Ontario have fluctuated significantly in the last 10,000 years following the last glaciation in North America, they have stabilized in the last few thousand years to the present measured range. Consequently, natural processes have been eroding the study shoreline (lake and river) long before European settlement began. Refer to Figure 5.1 for a typical eroding bluff in Northumberland Regional Municipality, north shore of Lake Ontario.

Figure 5.1 Eroding Bluff Shoreline East of Cobourg, Northumberland County (Aug. 9, 2003)

Natural background erosion is vital for the creation of new sand and gravel deposits in the nearshore zone, which ultimately supplies the beach/dune environments and barrier beaches around the lake with new sediment. Refer to the discussion in Section 6.0 of this report on barrier beaches and dunes. Consequently, shoreline erosion is an important natural process on Lake Ontario and it started long before the construction of the Moses Saunders Power Dam. The sediment budget Performance Indicator was created to educate the study participants and riparian community on the benefits of the natural background erosion rate.

When evaluating shore erosion and the creation of new sand and gravel sources in a regional content, the concept of sediment budgets is often used. Like a financial budget, a sediment budget is an accounting system for all the sand and gravel within a defined study boundary (spatial extents). Littoral cells are closed sediment compartments that define the limits of all sand movement, both along the shore and onshore/offshore. Consequently, the limits of a littoral cell make good spatial boundaries for a sediment budget analysis.

Baird & Associates 140 Detailed Study Sites for the Coastal Performance Indicators

The rationale for this performance indicator was a means to describe the beneficial components of shoreline erosion. For example, when storm waves strike a bluff shoreline (such as the conditions in Figure 5.1), the force of the attacking wave energy often exceeds the resisting properties of the soils, resulting in erosion and shoreline recession. If the eroding shoreline belongs to a riparian, there is a loss to the owner, as his total acreage decreases. Generally, shoreline recession is not reversible, and consequently the riparian will not be able to reclaim the lost land. However, for many parts of study area, erosion is a critical natural process that provides new supplies of sand and gravel to nourish barrier beaches and dune environments. The sediment budget for Sites #12 and #13 are described below.

5.1 Site #12 – CND8 Sediment Budget

A sediment budget investigation was completed for the north shore of Lake Ontario classified as CND8 for the purpose of this study. This shoreline unit generally consists of the Northumberland County and the shoreline communities between Port Hope in the west and Prequ’ile Provincial Park in the east. Refer to Figure 5.2.

Figure 5.2 Regional Map of the CND8 and Limits of Sediment Budget Calculations


For the purpose of this analysis, the community of Port Hope was considered the western limits of the littoral cell. Historically, it is possible the cell boundary extended further to the west. Presently, there are three large coastal structures within the boundaries of this sediment budget, including the Port Hope Harbour, Cobourg Harbour and industrial port at Ogden Point. These three structures define three sub-cells in the modern littoral cell. This region is the sediment supply zone of the littoral cell, as shoreline erosion generates new supplies of sand and gravel for the littoral system.

It should be noted that a detailed sediment bypassing analysis was not completed at these three locations. However, for the purpose of the demonstration sediment budget, it is assumed the sediment bypassing these structures. As such, this analysis could also be considered a “pre-settlement” sediment budget between Port Hope and Presqu’ile.

Presqu’ile Provincial Park was the historical downdrift sediment sink for this littoral cell. The present feature associate with the park began as a low crested sand spit when Lake Ontario water levels stabilized in their present range. Refer to Figure 5.3. Sand and gravel transported to the east was deposited between the mainland and bedrock peninsula, and progressively built additional breach ridges. Eventually these ridges attached to the bedrock peninsula and the modern beach was deposited.

original spit

modern beach

bedrock peninsula

Figure 5.3 Progressive Beach Ridges at Presqu’ile Provincial Park


An oblique digital photograph of the beach is presented in Figure 5.4. The tips of the forested beach ridges can be seen in the top right hand corner of the photograph. The modern beach is wide, flat and features a low vegetated dune.

Figure 5.4 Beach and Dunes at Presqu’ile Provincial Park, Looking East (August 9, 2003)

The provincial park protects the valuable dune and marsh habitat, which is used by shore birds and other marsh species of animals and plants. The sandy beach is also a popular summer vacation destination.

Historically, updrift erosion provided new sources of sand and gravel for the littoral system, which was ultimately transported in an easterly direction to the park. The future health of the beach and dune environment at Presqu’ile will require a continuous supply of new sand and gravel from the littoral cell.

5.1.1 Rates of Sediment Supply for Different Regulation Plans

As documented in Section 3.0 of the report, the amount of shoreline recession on Lake Ontario can be influenced by water level regulation. For example, of the legacy plans utilized for the erosion modeling in Section 3.0, pre-project resulted in the highest bluff recession rates at the detailed study site, while the lower new basin supplies and thus lower lake levels associated with the climate change scenarios resulted in the lowest bluff recession rates.

Based on this finding, the shoreline classification for CND8 was used in conjunction with erosion rate modifiers developed from the detailed sites to quantify the impacts of water


level regulation on sediment supply to the littoral zone of Lake Ontario. The principal steps in the investigation are summarized below:

• Erosion rate modifiers were developed for each of the legacy plans and climate change scenarios based on the results from the detailed study sites discussed in Section 2.0. For example, if the pre-project hydrograph doubled the recession rate compared to 1958DD, then the modifier for pre-project would be 2.0. For the climate change scenarios, the multipliers were all less than 1.0, since they resulted in less bluff erosion compared to 1958DD;

• The long term AARR for the 1 km reach classification was extracted from the FEPS database for the littoral cell boundaries. The AARR was modified for each of the legacy plans and climate change water levels based on the erosion rate modifiers;

• Since the lakebed in Northumberland is shelving limestone bedrock, the primary source of sediment input is the eroding cohesive shoreline above Chart Datum. The 1 km shoreline classification was used to determine the type of shoreline geology, percentage of sand in the soil, the average height of the bank or bluff and the percentage of shoreline protection for each 1 km reach. This information was used in conjunction with the AARR for each regulation plan to calculate the annual volume of sand and gravel entering the nearshore zone from shoreline recession.

The results are summarized in Figure 5.5. Shoreline recession associated with pre-project generated over 11,000 m /yr of sand and gravel on average, while the current regulation plan, 1958DD, produced 7,300 m /yr. The climate change water levels produced significantly less, especially the 2090 water levels, which rarely exceed chart datum and thus result in very little shoreline recession.

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The results from the sediment supply calculations summarized in Figure 5.5 provide two very important pieces of information when considering historic, present and future sediment budgets for Lake Ontario. First, it is quite possible the long term recession rates for the cohesive shorelines around the lake were higher prior to the regulation of Lake Ontario. Higher rates mean more sediment was produced from the natural background recession rate. Conversely, since regulation began in the 1960s, less sand and gravel is generated from shoreline erosion and thus available for the nearshore sediment budget. In other words, there is less sand available to maintain and build beaches.

In the future, if supplies of water to the Great Lakes watershed and thus by extension lake levels decrease due to the impacts of global warming, the rates of shoreline erosion for the cohesive bank and bluff sites around the lake will also decrease. This will result in


less sand and gravel entering the littoral system and thus available to build new beaches

and maintain existing ones.

5.1.2 Economic Function to Quantify the Value of Eroded Sediment

In order to assess the economic implications of regulation plans that alter the long term background recession rate, a preliminary economic function was developed. The function assumes the sand and gravel supplied to the nearshore zone from erosion is an important commodity and has value for the nearshore environments of Lake Ontario. Specifically, the sand is required to maintain existing beaches, such as the one at Presqu’ile Provincial Park.

The function assumes the value of the eroded sediment can be quantified by the cost to truck this material to the shoreline from upland sources. For example, if a particular parcel of land contributes 10 m of new sand and gravel a year and it costs $20/ m to ship the equivalent amount of sand to the site from an upland quarry, then the value of the eroded sediment is $200 per year.

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Figure 5.5 Hypothetical Annual Inputs of Sand and Gravel for Different Regulation Plans

This non-market assessment technique is depicted graphically in Figure 5.6. In this example, we assume Plan A generates 10 m of sand and gravel a year for a given stretch of shoreline and the cost to truck the sand to the lakeshore is $20/ m or $200/truckload. Plan B causes twice as much shoreline recession and thus new sand and gravel generated is worth $400.


There are some limitations to this performance indicator if applied to the entire lake without consideration to the local conditions. For example, if the new sand and gravel generated from bluff recession is deposited into a navigation channel, it may represent a cost for a local port authority, since dredging would be required to remove the sediment.

Figure 5.6 Economic Function for Sediment Budget Performance Indicator

For the case study described in Section 5.1, we have assumed that all of the eroded sediment between Port Hope and Presqu’ile has a beneficial economic value, since ultimate this sediment built the historical sand spit and modern beach at the Provincial Park.

The volume of sediment generated for each of the legacy plans and climate change scenarios was converted to a dollar amount using a unit cost of $20/ m . These dollar amounts were then normalize to 1958DD, since it is the baseline for the comparison as the present regulation plan. The results are presented in Figure 5.7. Pre-project generates an average annual benefit of approximately $100,000, while the benefits from 1958D without deviations represent approximately $35,000 in benefits.

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The climate change scenarios generate significantly less sediment per year, which is expressed as a cost in Figure 5.7. These annual costs range from $67,000 to $170,000 per year. As mentioned previously, climate change lake levels and reduced sediment supplies will have a negative impact on sand and gravel beaches, particularly sites that feature a cohesive or bedrock lakebed. The preliminary economic function developed for

5.1.3 Benefits and Costs of Regulation for the Port Hope to Presqu’ile Littoral Cell


the sediment budget performance indicator could be used to quantify the costs of this reduction.

Figure 5.7 Economic Benefits and Costs of Eroded Sediment for Legacy Plans and Climate Change Scenarios (results normalized to 1958DD)

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5.2 Site #13 – Eastern Lake Ontario Sediment Budget

The Eastern Lake Ontario site is a dynamic coastal environment that features bedrock headlands, eroding drumlins, barrier beaches, modern and relic dunes, and shifting inlets, such as the one presented below in Figure 5.8. In many locations, natural shoreline processes are constrained and often negatively impacted by shoreline development. For example, refer to the residential development constructed on top of a former barrier beach in Figure 5.9. Shoreline armoring is required to protect the homes from flooding and erosion hazards, and thus the barrier beach can no longer function as a dynamic system.

Figure 5.8 Inlet at Lakeview Wildlife Management Area, August 6, 2003

Figure 5.9 Development on Barrier Beach at South Pond, August 6, 2003


A regional map of Shore Unit US7, which roughly corresponds to the Eastern Lake Ontario (ELO) site, is presented in Figure 5.10. The 1 km shoreline reaches are also plotted on Figure 5.10, along with a summary of the classification in the legend. For example, the shoreline along the open lake and inside the major ponds is classified as 32% baymouth barrier, 33% open shoreline wetlands (mostly associated with interior ponds and rivers), 31% low bank and 4% coarse cobble/gravel beaches.

The majority of the lake bottom is classified as sandy (85%), with exposed bedrock and silty/mucky organic sediment comprising the remaining 15%. When shoreline protection structures were evaluated, 75% of the 1 km shoreline reaches were completely unprotected, while the remaining 25% of the reaches had some percentage of protection, such as armor stone revetments.

A second map of the study area is presented in Figure 5.11 and includes the place names and geomorphic features that will be discussed throughout the report.

The studies completed to investigate the regional sediment budget for the ELO site will be summarized, including a review of the post glacial history, modern geology and geomorphology, sediment sources, sediment sinks, transport patterns, harbor bypassing, and the overall sediment budget findings. The analysis at this detailed study site will conclude with a discussion of water level regulation impacts on the ELO site.


Figure 5.10 US7 Shore Unit Map and 1 km Reach Classification


Figure 5.11 Eastern Lake Ontario with Place Names (from Woodrow et al., 2002)


5.2.1 Post Glacial Evolution at ELO Site

The Eastern Lake Ontario site has been evolving since the retreat of the Pleistocene Glaciers over 12k YBP (years before present). For example, following the final retreat of the glacial ice, Lake Iroquois formed on top of the modern Lake Ontario Basin when the ice sheet stalled at Brockville. Water levels of Lake Iroquois were over 100 m higher than the present conditions and the lake drained southeast through Syracuse and Rome. Once the continental glacier retreated further north, Lake Ontario was cutoff from further meltwater sources. Lake Erie drainage via the Niagara River had not yet been established. Therefore, the level of Lake Ontario decreased dramatically below the modern condition.

Between 10k and 6k YBP, the level of Lake Ontario slowly started to increase. The present drainage into and out of Lake Erie was evolving and slowly the supply of water to Lake Ontario increased. By 5k YBP, the level of Lake Ontario breached the sills on the St. Lawrence River and began draining down its modern outlet to the Gulf of St. Lawrence.

Lake levels higher than the modern condition may have occurred approximately 4k YBP during a period known as the Nipissing Transgression. This high water period was short in duration, with levels then falling below the present range. From 4k YBP to present, there has been a slow rise in lake levels to the modern condition.

5.2.2 Modern Geology and Geomorphology

Section 5.2.2 will review the modern geomorphic features within the study area and the geologic properties of the sediments and soils.

Regional Littoral Cell

On a regional scale, the Eastern Lake Ontario barrier complex is a very large pocket beach that is controlled or anchored by the Stony Point bedrock headland in the north and Nine Mile Point in the south. In the north, the bedrock outcrops just north of Stony Creek, which drains Black Pond and the tributaries that feed the marsh. A picture of the shelving bedrock recorded during the August 2003 aerial survey of the shoreline is presented in Figure 5.12.


Figure 5.12 Shelving Bedrock Immediately North of Black Pond and Stony Creek

A photograph of the eroding bedrock shoreline in the Nine Mile Point area, east of Oswego, is presented in Figure 5.13. At this particularly location, there are no beaches, just vertical eroding bedrock cliffs. In other locations, the bedrock in the nearshore zone is shallower and gravel beaches are present along the shoreline.

Figure 5.13 Eroding Bedrock Cliff Near Nine Mile Point

These two bedrock zones define the spatial boundaries of the ELO littoral cell. All sediment supply is limited to sources within the cell and movement is confined within its boundaries.


Lakebed Geology

Extensive surveying of the lake bottom (surface and sub-surface) was completed for the Eastern Lake Ontario Sand Transport Study (Woodrow et al., 2002), which was completed by Woodrow et al (2002). One of the products of this study, which is reproduced in Figure 5.14, is a regional map of the lakebed sediment distribution.

In the Nine Mile Point area, while bedrock is exposed along the waterline, it is capped on the lake bottom with a broad sheet of glacial till. This till sheet extends north to approximately the mouth of the Salmon River. From the Salmon River north to Black Pond and Stony Creek, the lake bottom is covered with a broad sand sheet that extends offshore 4-5 km. The thickness of the sand sheet ranges from 1 to 5 m.

Offshore of the sand sheet, laminated silts and clays were identified. Presumably this clay plan continues onshore and is the foundation under the nearshore sand, the dunes and marshes.

North of Black Pond and Stony Creek, the bedrock foundation of Stony Point was recorded and the spatial extent is noted in Figure 5.14.


Figure 5.14 Surficial Lakebed Mapping for ELO (from Woodrow et al, 2002)

Several sources for the large sand sheet on the bed of the lake at the ELO site were proposed in Woodrow et al (2002), including glacial processes and post-glacial beach ridges when Lake Ontario was at different elevations. Within the littoral cell, new sand sized sediment is introduced to the littoral system when the drumlins found along the shoreline erode, however, this is not thought to be a major source. In summary, the


majority of the sand has been in this littoral cell for thousands of years and the position of the sandy barriers and dunes is modified waves, currents and wind.

Shoreline Conditions

The shoreline conditions for the ELO site are described with the assistance of the oblique digital photographs captured on August 6, 2003. Additional relevant site photographs are provided in Sections 6.2 and 6.3.

The mouth of the Salmon River is protected with a long armor stone jetty that extents in a northwest direction from the south shore of the inlet. A smaller east-west jetty was constructed along the north banks of the outlet, as seen in Figure 5.15. Small fillet beaches have developed north and south of the jetties. The beach material is primarily sand, with trace deposits of pebbles and cobbles.

Figure 5.15 Salmon River Jetties and Adjacent Beaches

The Rainbow Shores community is located north of Deer Creek. Much of this development is located on top of an eroding drumlin, which separates the Deer Creek marsh from South Pond. Refer to Figure 5.17.

The southern half of North Pond is presented in Figure 5.18. The southern barrier beach features some large relic dunes, as does the north barrier. These dunes are heavily vegetated and have been developed with seasonal and permanent residences. Shore protection has been constructed in some locations to protect from flooding and erosion hazards.

North of the Salmon River jetties, a broad marsh is sheltered from Lake Ontario waves by a barrier beach and dune ridge, as seen in Figure 5.16. This area is protected as part of the Deer Creek Wildlife Management Area.


Figure 5.16 Marsh and Barrier Beach at the Deer Creek Wildlife Management Area

Figure 5.17 Portion of Rainbow Shores Development Located on Top of an Eroding Drumlin

Figure 5.18 Eroding Relic Dunes Along the Southern Barrier Beach at North Pond


The inlet to North Pond is presented in Figure 5.19. Since this inlet was created, a series of successive interior beach ridges have formed along the margins of the inlet. The reduced channel width and sediment is a problem for recreational boaters. Maintenance dredging is now routinely required to provide safe access to Lake Ontario.

Figure 5.19 Modern Inlet to North Pond

The Montario Point community is located between Cranberry Pond and South Colwell Pond. Refer to the oblique photograph in Figure 5.20. The higher elevations between the two marshes is associated with the drumlin field, however, there is no drumlin present at the waterline as at Rainbow Shores. The beaches in the community consist of pebbles, cobbles and boulders.

Figure 5.20 Montario Point Community and Cranberry Pond


North of Montario Point, the shoreline alternates between natural areas and shoreline development. The marshes are sheltered from Lake Ontario by barrier beaches and drain via sandy inlets. A picture of the shoreline development located on the dunes north of Southwick State Park is presented in Figure 5.21. The natural conditions at the inlet to Black Pond are presented in Figure 5.22.

Figure 5.21 Development on the Dunes North of Southwick Beach State Park

Figure 5.22 Black Pond Inlet and Barrier Dunes


Formation of Barrier Beaches, Dunes and Marsh

Carbon dating of sediment cores extracted from the lake bottom offshore of Eastern Lake Ontario indicate the formation of the sand sheet began approximately 8,400 years ago (Woodrow et al, 2002). As lake levels increased to their modern range, it is possible barrier beaches and dunes existed at locations now occupied by the lake bottom.

Peat samples were extracted from the base of the Rainbow Shores marsh immediately above the clay/mud surface for carbon dating. The results indicate the marsh began to form approximately 2,400 years ago. Since some form of a protective barrier would be required for the accumulation of organic material in the marshes and ultimately the formation of peat deposits between the high ground associated with drumlins, it can be inferred that the barrier beaches were also forming at this time. The barrier beach dunes in Figure 5.18 are an example of a very old relic dune at ELO.

Carbon dating of plant debris within the dunes at Black Pond suggest the material was approximately 1,300 YBP. Organic material in the dunes at Southwick Beach State Park, which is a younger dune system, dated approximately 350 YBP. The dunes along the inlet to Lakeview in Figure 5.8 are a good example of a young dune system.

5.2.3 Sediment Sources and Sinks

Three potential sources of new sand and pebbles/cobbles within the littoral cell are discussed based on the data collected and generated for this study, including: beach and dune erosion, lakebed erosion and isostatic rebound. Sediment sinks, such as inlet migration, fillet beaches and aeolian transport into the dune systems will also be reviewed.

Beach and Dune Erosion

A historical 1960s aerial photograph was geo-referenced against the 2002 detailed orthophotograph collected for this study. This region of the lake was a difficult area to register photographs, since the vast tracks of open natural areas don’t provide suitable low level ground control points, such as road intersections. In some locations, it was simply not possible to register the 1960s photographs with acceptable error limits (horizontal registration error less than predicted change in the shoreline change reference feature, such as the toe of dune).

Once registered, the toe of the dune in 1960 was digitized. This feature is commonly the edge of active vegetation for stable or accreting beaches or the base of the eroding dune scarp. Since the lake level difference between the two photographic series was only 0.13 m, the lines were not corrected for water level differences. For example, if we


assume a 1:10 (V:H) beach slope the 0.13 m difference in lake levels only corresponds to a horizontal positional shift of 1.3 m or 0.03 m per year.

The toe of the dune in 2002 was provided by the mapping contractor that generated the 2002 orthophotograph. Shore perpendicular transects were drawn with Baird Shoretools to measure shoreline change rates and then annualized for the 42 year period. The individual transects were grouped together into the 1 km shoreline reaches and are presented in Figure 5.23. A positive shoreline change rate (SCR) indicates dune erosion, while a negative SCR indicates dune accretion. Place names are added to the figure to provide reference.

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Figure 5.23 Shoreline Change Rates from 1960 to 2002 (based on toe of dune measurements)

Before the results are described, it should be mentioned that there are many challenges when measuring SCR at dynamic sandy environments such as ELO. For example, when measuring rates of change based on the active edge of dune vegetation, factors other than erosion and accretion trends can affect the presence or absence of dune vegetation (e.g. human disturbances). Also, the embryo dunes where the vegetation is most commonly found can inflate (grow) in volume very quickly, especially during low water periods. Conversely, one storm at high lake levels could completely erode an embryo dune stabilized with vegetation. Therefore, the antecedent conditions prior to the aerial photography are very important. Fortunately, for this analysis the lake levels were very similar for both photographic series.

The ideal method for measuring rates of change for a dynamic sandy environment such as ELO would be a full 3D volumetric analysis. For example, a 3D surface of the nearshore zone, beach, dunes and backslope and/or swale would be generated for both time periods. GIS software would be used to calculate volumetric changes along the shoreline,


expressed as m /m, for example. This methodology would be ideal for tracking changes in the shoreline due to inlet migration, dune blowouts and depositional features due to aeolian processes. Unfortunately, there was insufficient information in the 1960s dataset to complete this type of analysis.

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The results in Figure 5.23 will be described from north to south. Reaches 625 and 626 correspond to the Jefferson Park, Sunset Bluff and Eastman communities. The long term trend for the beach and dunes in this region is a slow recession rate. Southwick Beach State Park featured a small accretion rate of 0.07 m/yr.

The trend for reaches 628 to 652 is an accretion rate of 0.42 m/yr. This region corresponds to the Lakeview Wildlife Management Area. The natural setting for this stretch of coastline plus the lack of development make it an idea location for embryo dune growth and a healthy barrier beach system. Reach 653 and 654 represent the wide barrier system in front of North and South Colwell Pond. This natural area also features a healthy accretion rate of 0.47 m/yr since 1960.

Reach 662 to 664 correspond to the northern barrier beach and dunes that shelter North Pond. There is a progressive increase in the long term recession rate for this region, from 0.13 to almost 3.0 m/yr (increasing in a southerly direction). The most extreme rates at Reach 664 are related to the current inlet location. In 1960 the inlet was located further south. With the development of the modern inlet, the toe of the dune has migrated significantly inland, which is captured in the 2002 orthophotograph.

In 1960, the inlet to North Pond was located at Reach 691. This feature has since filled with beach sediment creating a triangular wedge of sand south of the modern inlet. The toe of the dune at Reach 691 has migrated lakeward 1.2 m/yr since 1960.

The rate of shoreline change for Sandy Island Beach, South Pond and Rainbow Shores is captured in the results for Reaches 693 to 697. The trend between 1960 and 2002 is recession and it decreases from 0.55 m/y in the north to 0.10 m/yr in the south at Rainbow Shores. There are several possible reasons for this long term trend of shoreline recession, including disruption of the natural barrier beach processes by the housing developments, construction of shoreline protection structures and the natural tendency for shoreline erosion along this drumlin section of the shoreline. The impacts of the regional sediment transport patterns may also influence the long term shoreline change rates along this stretch of shoreline in ELO. Refer to Section 5.2.5 for additional details.

Reach 698 represents the Deer Creek WMA. The long term trend for this stretch of shoreline is a slow accretion rate (0.14 m/yr). At Brennen Beach, the accretion trend increases to 0.46 m/yr. The shoreline position and dune stability at Brennen is partially influenced by the north jetty at the Salmon River. South of the south jetty at the Salmon River, the growth of a small fillet beach in captured with the rates at Reach 721, which documents 0.37 m/yr of accretion since 1960. In Reach 722, which does not appear to be influenced by the jetties, the long term trend is accretion of 0.2 m/yr.


In summary, the beaches south of the Salmon River jetties are a sediment sink, as are the conditions for a few kilometers north of the river. The eroding drumlins and barrier beaches from Rainbow Shores north to South Pond are a source of new littoral sediment. However, it should be mentioned that only a fraction of the eroded material in the drumlins remains on the beaches. The percentage of sand is unknown but is generally in the range of 10 to 20 percent for a glacial till. The eroding drumlins also supply the rounded pebbles, cobbles and boulders found on the beaches throughout ELO. For example, refer to the conditions 5.24.

Figure 5.24 Eroding Drumlin at Rainbow Shores (note pebble/cobble sized material in the till)

The deposition at Reach 691 along the southern barrier at North Pond represents a long term sediment sink for the littoral cell. Although the shoreline change rates document erosion for the northern barrier, this does not translate into new sediment for the littoral cell, since the eroded sediment is deposited in the pond and used to build the flood delta


inside the marsh (the arrow head shaped depositional feature at the inlet). This is discussed further in the section on inlet migration below.

At Montario Point, the shoreline has been very stable since 1960. It appears the beach and nearshore are well armored with the cobles and pebbles eroded from the glacial till. It is also interesting to note that these cobbles do not appear to migrate significantly north to the beach at North and South Colwell Pond or to the south along the northern barrier beach at North Pond. This cobble deposit is either very isolated on the beach or the net direction of longshore drift is very close to zero at this location.

From South Colwell Pond north to the limits of Southwick Beach State Park, including Lakeview WMA, the dunes have been accreting or inflating since 1960. This dune growth and shoreline stability is at least partly attributed to the natural state of the shoreline, which is capable of responding dynamically to periods of high and low lake levels. Plus, since the dam became operational in 1960, the regulation plan has worked to eliminate the natural high lake level conditions that would have occurred under the pre-project scenario. These natural highs combined with storm events may have lead to more dune erosion events, which would have been followed by recovery during average and low levels. Collectively this region is a net sediment sink for the littoral cell.

The Jefferson Park, Sunset Bluff and the Eastman communities featured an average long term recession rate of 0.15 m/yr. In many locations, these homes were constructed on top of the dunes, permanently disrupting the natural function of the sand system. This reach of shore is a small net provider of sediment and may have supplied some of the material for the dune accretion further to the south.

2001 SHOALS Lakebed Profiles

The SHOALS bathymetry collected in 2001 provided complete coverage for the ELO nearshore environment, from the waterline to depths often greater than 10 m. A total of 36 shore perpendicular profiles were extracted from the 3D bathymetry grid. The reach profiles will be described from 619 in the north to 732 in the south. Collectively, they provide a vivid story of the lake bottom geology and morphology.

Figure 5.25 plots five profiles that are strikingly different in their shape and geology. Reach 619 corresponds to the southern limits of the bedrock at Stony Point. The profile is very flat and features a slope of approximately 1:100 (H:V) from the waterline to the 6 m depth contour. Reach 622 is located just south of black pond and the profile morphology suggests this is still a bedrock profile with a shallow nearshore sand deposit. Between 800 and 1,500 m on the x-axis a shoal is observed for the Reach 622 profile with a crest elevation of 7.0 m. Interestingly, this shoal is also observed in the profiles for Reaches 623 and 624, however at a higher elevation (2.2 to 3.6 m below Chart Datum). It is not known whether these shoals are bedrock or glacially features, such as drumlins. The field observations, aerial photographs and digital oblique photographs all


categorize the shoreline for Reaches 622 to 624 as sandy with wide beaches and dunes. Therefore, the bedrock must dip to the east and south, where it is eventually buried by glacial sediments and the large ELO sand sheet.

The final profile in Figure 5.25 represents the conditions for Reach 625, which extends offshore of the Eastman residential community. The profile geometry is dramatically different than the adjacent reaches, since this profile records the full extent of the Eastern Lake Ontario sand sheet. When compared to the adjacent reaches (623 and 624), it appears the sand between the 2 and 6 m depth contour is anchored by the shoal for profiles 623 and 624. This feature prevents sediment from leaking (moving) in an northerly direction out of the littoral cell when wave generated currents are moving in a northerly direction.

Figure 5.26 presents the profiles for Reaches 626 to 631, which corresponds to Southwich Beach State Park and the Lakeview WMA. All the profiles feature a prominent nearshore bar and trough feature, are very flat, and strikingly similar in shape from the waterline to the 10 m depth contour. They appear to be in equilibrium with the local wave climate.

The lakebed profiles between North and South Colwell Pond and the north barrier beach at North Pond are presented in Figure 5.27. Again, a prominent nearshore bar is documented and the lakebed morphology is very similar for this group of profiles. The origin of the depression for Profile 633 at 600 m on the x-axis is unknown (i.e. physical feature or surveying error).

Figure 5.28 presents nine profiles for the shoreline from the southern barrier beach at North Pond to Brennan Beach. Although the general morphology is similar for this group of profiles, there is considerably more variability than the profiles to the north. Also, the nearshore conditions are much deeper, particularly for profiles 695 and 696, which correspond to the Rainbow Shores development. Shoreline protection has been constructed to protect many of the homes along with stretch of shoreline. Reach 699, which is located at Brennan Beach, appears to record the glacial till lakebed below the 6 m depth contour (approximately 700 m on the x-axis). This area marks the transition from the sandy lakebed to glacial till (refer to Figure 5.14).

The lakebed profiles for a ten kilometer stretch of shoreline south of the Salmon River Jetties are presented in Figure 5.29. They capture the variability associated with the glacial till lake bottom for this region of Eastern Lake Ontario.


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Figure 5.25 2001 SHOALS Lakebed Profiles from Stony Point to Jefferson Park Community


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Figure 5.26 2001 SHOALS Lakebed Profiles from Southwick State Park to Inlet at Lakeview WMA


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Figure 5.27 2001 SHOALS Lakebed Profiles from Inlet at Lakeview WMA to Inlet to North Pond


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Figure 5.28 2001 SHOALS Lakebed Profiles from South Barrier Beach at North Pond to Brennan Beach (adjacent to north jetty at Salmon River)


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Figure 5.29 2001 SHOALS Lakebed Profiles from South Fillet Beach at Salmon Inlet to 7.5 km south of Salmon River


Lakebed Change

A bathymetric survey of ELO was completed in 1948 by NOAA. These soundings were compared to the 2001 SHOALS survey to document over 50 years of lakebed evolution at the site. Plus, the findings help identify zones of lakebed erosion and thus sediment supply regions for the littoral cell.

Starting in the north, Figure 5.30 compares the 1948 NOAA soundings to the SHOALS survey at Reach 628, which corresponds to the Lakeview WMA. Since a large survey vessel was likely deployed in 1948, the soundings were generally limited to depths below the 1.0 m contour. Between the 2 and 6 m depth contour in Figure 5.30, the lakebed has been stable for the last 50 years, with no significant deposition or erosion areas.

The profile offshore of Montario Point, presented in Figure 5.31, documents a different trend. Although the lakebed has been stable from the 4 to 7 m depth contour, between the 4 and 2 m depth contour the data documents downcutting or lakebed erosion. This may be related to the permanent erosion of the glacial sediments that outcrop at the waterline at Montario Point.

Reach 664 corresponds to the location of the modern inlet to North Pond. Refer to the profile comparison in Figure 5.32. There has been significant lakebed erosion between the 1 and 5.5 m depth contour (below Chart Datum). This erosion may be partly associated with the changes to the barrier for the modern inlet.

Reach 693 is located along the southern barrier at North Pond. Figure 5.33 presents the profile comparison between 1948 and 2001. Lakebed erosion is focused between the 2 and 4 m depth contours.

The lakebed conditions at the Deer Lake WMA are represented by the profile comparison findings for Reach 698, as plotted in Figure 5.34. At this location, the lake bottom below the 2 m depth contour has been stable for the last 50 years.


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Figure 5.30 1948 to 2001 Lakebed Profile Comparison for Reach 628 (Lakeview WMA)


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Figure 5.31 1948 to 2001 Lakebed Profile Comparison for Reach 661 (Montario Point)


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Figure 5.32 1948 to 2001 Lakebed Profile Comparison for Reach 664 (North Barrier for North Pond)


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Figure 5.33 1948 to 2001 Lakebed Profile Comparison for Reach 693 (South Barrier for North Pond)


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Figure 5.34 1948 to 2001 Lakebed Profile Comparison for Reach 698 (Deer Lake WMA)


Isostatic Rebound

During the last glacial period, the weight of the ice sheets that covered the Lake Ontario Basin depressed the underlying bedrock. When the continental glaciers melted and migrated in a northerly direction, the underlying bedrock slowly began to bounce back. This process is known as isostatic rebound. The measured rate of rebound for the eastern end of Lake Ontario is 2.3 mm/yr. Over a century, this translates into a rebound rate of 0.23 m.

Without the erosive action of waves and currents in Lake Ontario, in theory isostatic rebound would raise the lake bottom along ELO by 0.23 m per century. However, as the profile comparisons have shown, the lake bottom at ELO is in a state of dynamic equilibrium with the wave climate. In other words, over time the shape of the sand sheet has evolved in response to lake levels and waves. This observation is at odds with isostatic rebound, since the nearshore environment should be slowly getting shallower.

If the rebounding lake bottom is eroded by waves and currents, this process may represent a sediment sink for the littoral cell. For example, consider the following volumetric calculation. The ELO sand sheet is approximately 28 km in length from the Salmon River Jetties to the outlet at Black Pond. Waves and currents are generally focused on a zone of the lake bottom between the 6 m depth contour the and the waterline. This region is approximately 1 km in width. Collectively, this zone of active sediment dynamics is 28,000,000 m . When the annual rebound rates is applied (2.3 mm/yr), approximately 64,000 m /yr of new sediment is added to the littoral zone. In other words, for the profile morphology to maintain its equilibrium form, waves and currents must erode this sediment and this process generates sediment for the littoral cell.

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Over periods of multiple decades, sand is eroded from the lake bottom to maintain the equilibrium profile shape and this new sediment is transported alongshore and onshore to build beaches and dunes. Therefore, in a regional context, the rebounding lake bottom represents a source of sediment for the littoral cell.

Sediment Supply from the Salmon River

An oblique view of the Salmon River Jetties was provided in Figure 5.15. The mouth of the river has been modified by the armor stone jetties. Upstream dams have modified the natural flow regime and sediment delivery from this watershed. Prior to these modifications, when the river drained naturally to the lake, the watershed may have been a source of new sand sized sediment for the coastal zone. The gradients of the river and nature of the surficial sediment has not been investigated to determine the feasibility of the watershed as a sediment source prior to anthropogenic modifications.


Sediment Deposition Associated with Inlet Migration

An image of North Pond and the modern barrier beach is provided in Figure 5.35. On the pond side of the barrier, the multiple arrow head features record a long history of inlet migration and sediment. The implications of these processes and the associated depositional features is investigated.

Figure 5.35 1948 to 2001 Lakebed Profile


For example, the former inlet is located to the south of the modern entrance to North Pond. From the aerial photograph it is clear channel sedimentation and aeolian processes have deposited a significant volume of sand into North Pond to fill the old inlet. If the present inlet was to close and a new channel opened further to the north, over time a similar feature would develop.

To investigate the implication of this channel migration and evolution on the regional sediment budget, two historical bathymetry datasets were compared to the 2001 bathymetry collected for this study. Profile B runs parallel to the back side of the barrier ridge and documents the history of inlet migration below lake level.

Figure 5.36 Profile B Comparison from 1878 to 2001 at North Pond (0 on the x-axis corresponds to south)

It should be noted that 0.0 m on the x-axis in Figure 5.36 corresponds to the southern corner of North Pond. In most cases, the 1878 bathymetry records the maximum depth of the pond. Based on the contours of the lake bottom, it appears the flood shoal for the inlet in 1878 was between 2,500 and 3,000 m on the x-axis.


Some time between 1878 and 1948, the inlet was located along the southern half of the barrier, likely just north of the large relic sand dunes along the current south barrier. This inlet location is documented by the flood shoal in Figure 5.36 between 800 and 1,300 m on the x-axis.

In 1948, the inlet was located between 2,000 and 2,500 m on the x-axis in Figure 5.36. In Figure 5.35 this corresponds to the word “Profile” along the line for Profile B. Once the new inlet is opened, sediment is driven into the pond during storm events, particularly events at high lake levels or storms that feature a large storm surge. This sediment accumulates in the flood shoal in the pond and is permanently lost from the littoral system. In other words, there is no physical processes capable of transporting the sediment back out into the nearshore zone of the lake.

The 2001 bathymetry records the location of the modern flood shoal between 3,400 and 3,800 m in Figure 5.36. In 1878 to 1948, the surveys record depths in the pond ranging from 2.5 to 5.0 m below chart datum. Today much of the flood shoal is at chart datum or slightly below. A significant volume of sand has accumulated inside North Pond in this depositional feature.

A 3D volumetric comparison of the lake bottom in 1878 and 2001 was completed in GIS. It should be noted that in addition to sedimentation associated with the flood shoals, deposition of fine sediment, such as silt and clay has also occurred in the pond since the 1878 survey. This deposition is thought to be more concentrated in the eastern (back) half of the pond.

The annualized rate of sediment deposition in North Pond is approximately 58,000 m /yr. Since some of this deposition is associated with the riverine transport and deposition of fine silts and clays, it is estimated 40,000 to 50,000 m /yr of sand has been deposited in the pond, on average, since 1878. This represents a large sediment sink for the littoral cell.

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Fillet Beaches

There are three zones of deposition associated with the Salmon River Jetties: a small north fillet beach, sand bars inside the jetties next to the navigation channel and the south fillet beach. Using a the 1960s vegetation line as a basis for comparison with the 2002 orthophotograph, a series of preliminary calculations were completed to estimate the volume of sediment in these three sinks. Assuming the deposition has occurred over a vertical depth of 4.0 m, the estimated accumulation is 100,000 to 150,000 m . 3

Dune Blowouts and Aeolian Transport

Dune blowouts and aeolian processes can transport large volumes of beach sand inshore of the foredune ridge, particularly when the dunes are destabilized by natural or artificial processes. Once the sand is deposited landward of the foredune ridge, it represents a permanent sink for the sediment budget. An example of a large aeolian deposit at Sandy Island Beach is seen in the background of Figure 5.37. The cottage in the figure is located on the backside of the dunes for the south barrier at North Pond.

An estimate of the volume of sediment transported inshore of the foredune and thus permanently removed from the littoral system was not completed for this investigation due to insufficient historical information.

Figure 5.37 Large Depositional Lobe of Sand on the Backside of Sandy Island Beach


5.2.4 Regional Sediment Transport Patterns

Longshore sediment transport estimates for the ELO site were completed with the COSMOS model to assess regional trends and rates. A sediment grain size of 0.4 mm was used for the calculations. The historical wave climate and lake levels from 1981 to 2000 were used as model input. Profile conditions at over 30 of the 1 km shoreline reaches were used.

A sample of the predictive capabilities of the COSMOS model for a 2D beach profile is presented in Figure 5.38 for the profile at Lakeview WMA. All the profiles extended from the dune crest to the 10 m depth contour. The model predicts the distribution and magnitude of the transport across the profile. As waves break across the nearshore bar and on the beach, the resulting longshore currents transport sediment. Thus, the magnitude of the transport is greatest in the shallow nearshore zone and across the bar.

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Figure 5.38 Cross-shore Distribution of Longshore Sediment Transport (Lakeview WMA)

A Lakeview, the shoreline orientation and wave climate result in a large southerly component, which is mapped in red in Figure 5.38. The northerly component features a similar cross-shore distribution of LST but the annual volume is significantly less. Thus, the next direction of LST at Lakeview is to the south.


The results from the regional sediment modeling are summarized graphically in Figure 5.39. The LST rates are annualized and plotted as the northerly, southerly and net volumes. The shore perpendicular azimuth is plotted on the Y2 axis in Figure 5.39 to highlight the influence of shoreline orientation. It should also be mentioned that these are actual LST rates and they assume the full supply is available to be transported.

In the Jefferson Park and Lakeview area, the dominant direction of LST is to the south, although there is a small northerly component. Consequently, the net direction is to the south or center of the Eastern Lake Ontario site. The rate decreases towards the center of ELO and reaches zero at North Pond. It is also worth noting that since the northern limits of the littoral cell are bounded by bedrock, the potential supply from the north is zero or very close to zero. Therefore, although the net LST rate predicted with COSMOS is approximately 200,000 m /yr, the supply is very close to zero. Therefore, although the net direction is to the south, the actual volume of sediment transported in a southerly direction is likely much closer to zero than 200,000 m /yr.

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South of the Salmon River Jetties, the net direction for LST is to the north. However, as discussed previously, the lake bottom is glacial till and much of the shoreline is hardened with shoreline protection. Therefore, the actual supply from the south is likely very close to zero. From the Salmon River to North Pond, the net LST direction is to the north and decreasing. In other words, there is a decreasing gradient to the north, with the net LST rate reach zero at North Pond.

In summary, longshore sediment transport at Eastern Lake Ontario converges from the north and south at North Pond. Along an open coastline, the convergence of LST from two directions or a decreasing gradient in LST generally results in the formation of a depositional feature. However, at ELO the sediment transported to the center of the site is transported into North Pond via the present and historical inlets and stored in the flood shoals.

A significant volume of sediment has also accumulated in the barrier beach at North and South Colwell Pond. Based on a comparison of the modern and 1878 shoreline, approximately 970,000 m of sand has accumulated in this feature or 7,900 m per year. This barrier beach is also a significant depositional zone.

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Figure 5.39 Regional Longshore Sediment Transport Estimates for Eastern Lake Ontario (average annual rates)

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