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GEOMORPHIC SOLUTIONS

T • 416 • 213 • 7121 F • 905 • 890 • 8499 www.geomorphicsolutions.ca

141 Brunel Road Mississauga, ON L4Z 1X3

Geomorphic Sciences Land Development Engineering Land Development Planning Municipal Engineering Services Transportation & Transit Planning Utility Infrastructure Design Water Resources Engineering

November 21, 2006 AMEC Earth and Environmental 104 Crockford Blvd. Scarborough, Ontario Canada, M1R 3C3 Attention: Mr. S. Irshad Hassan, M. Eng., P.Eng. Dear Mr. Hassan: Re: Sixteen Mile Creek Geomorphic Assessment and 100-year Erosion Setback Milton Heights Neighbourhood Town of Milton Project No. 06316.450 INTRODUCTION A geomorphic and hazard assessment, including evaluation of a suitable meander belt width and 100-year erosion limit was performed for the portion of Sixteen Mile Creek associated with the proposed Milton Heights Development. The properties in question are located between Highway 401 and Steeles Avenue and are bounded on the east and west by Peru Road and Tremaine Road respectively. It is our understanding that the development is proposed for the tableland and that along most section of the creek a valley wall confines the river. As such, the main concern is potential migration of meander bends near the valley wall and the associated hazard. The 100-year erosion limit represents the potential valley wall erosion associated with channel migration. In conjunction with a geotechnical assessment of the stable top of bank, erosion limits provide an appropriate hazard set back for the combined stable top of slope and erosion set back allowance. In areas where the channel in unconfined or partially confined a meander belt width was also calculated. The assessment had two main objectives: delineate hazard lands associated with channel migration and bank erosion and quantify a suitable 100-year erosion setback limit. Quantifying rates of adjustment provides insight into the active processes and systematic adjustments along the study reach. Specifically, this assessment included the following tasks:

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AMEC Earth and Environmental Mr. S. Irshad Hassan November 21, 2006

• Identify reaches using aerial photographs and topographic mapping; • Provide information on active processes and systematic adjustments

along the reach through a historical assessment; • Determine the necessary set back based on channel and valley

characteristics, specifically an appropriate 100-year erosion set back and/or meander belt width; and

• Complete field reconnaissance to identify systematic adjustments and channel stability and confirm the appropriateness of the desktop assessment conclusions.

REACH DELINEATION AND STREAM CORRIDOR CHARACTERIZATION A background review was completed using recent topographic mapping, surficial geology mapping, aerial photographs (black and white: years 1954, 1974 and 1979; colour digital: year 2005) and reports to understand existing and historic channel form. Channel form is a product of the channel flow regime and the availability and type of sediments within the stream corridor. The ‘dynamic equilibrium’ of these inputs dictates the channel form. These factors are influenced on smaller systems by land cover, physiography and riparian vegetation. In order to account for all these factors, channels are separated into reaches. Each reach displays similarity with respect to its physical characteristics such as channel form, function, and valley setting. Delineation of a reach considers planform, gradient, hydrology, local surficial geology, physiography, and vegetative/land cover control (Montgomery et al., 1997; Richards et al., 1997). The underlying geology influences the rate of channel change (e.g. migration), sediment input (i.e. amount and type), and channel geometry. Sixteen Mile Creek reworks its own alluvial deposits of dominantly Halton Till origin. Bank sediments are dominated by these cohesive clay loam deposits that are offer inherent stability to the channel banks. The parent material is only slightly stony (1-2%) so many of the clasts within the channel are likely derived from outwash gravels atop the Niagara Escarpment (Chapman and Putnam, 1984). Where the clayey and silty deposits are in contact with the creek, they provide increased resistance to channel migration. Within the study area the Halton Till lies atop the dense and weakly erosive Wentworth Till (correlative to Newmarket Till) and this provides a measure of resistance to channel down cutting. Reach analysis was completed utilizing historical aerial photographs, surficial geology mapping and field reconnaissance. Based on planform, gradient and riparian vegetation the section of Sixteen Mile Creek within the study area was divided into five reaches (Table 1). Reach 1 was defined as a section of channel from Peru Road to immediately downstream of the pond. Channel length and sinuosity for Reach 1 is 55 m and 1.06, respectively. Reach 2, with a channel length of 184 m and a sinuosity of 1.05 was defined as a section of channel immediately upstream

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of the pond. There was a distinct difference in channel dimensions upstream of Reach 2. Reach 3 is wider and shallower than Reach 2 and has a channel length of 223 m and sinuosity of 1.01 m. Reach 4 is characterized by a sinuous section of channel adjacent to a manicured lawn. This change in riparian vegetation appears to be associated with erosion along the outside meander bends as well as sand bar formation. Channel length and sinuosity for Reach 4 is 142 m and 1.27 respectively. Channel length and sinuosity for Reach 5 is 628 m and 1.08 respectively. Table 1: Reach summary of reach length and sinuosity.

Reach Reach Length Sinuosity 1 55 1.06 2 184 1.05 3 223 1.01 4 142 1.27 5 628 1.08

The historical aerial photographs were used to examine changes in channel patterns and land use/cover. Within the area of interest, there was no significant land use change between 1954 and 2005. The tablelands have remained largely agrarian, with the exception of a pocket of land along the north side of Reach 5 that has become a successional forest. There was limited evidence of planform modification and channel migration, with the exception of a meander cut-off (oxbow) in Reach 3 visible in the 1954 and 1978 aerial photographs. The online pond located immediately upstream of Peru Road, was smaller in 1954 compared to 2005. MIGRATION RATES AND EROSION SETBACKS Migration rates for tracked meander bends were calculated from measurements extracted from the 1954 black and white aerial photographs and the 2005 aerial photographs. This provides 51 years of record, exceeding the 25-year minimum suggested by MNR Guidelines. The 1974 and 1979 aerial photographs were manually examined to temporally constrain obvious channel adjustments (oxbows, avulsions) and quantify land use changes over a longer time scale. Aerial photographs were geo-referenced in ArcGIS 9.1 using 4 to 8 control points with a first order polynomial correction to minimize image distortion. Control points were selected from known points between photographs. Control points surrounding the areas of importance were selected to minimize rectification errors. Errors between corrected control points were averaged to provide a correction error. In this case the error was 0.5 m between the 1954 and 2005 aerial photographs and 1.2 m between the 1978 and 2005 aerial photographs. It can be assumed that the error in channel position was smaller, as the channel position was within the selected control points. This method is less error prone, provides a measure of potential

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rectification/scaling inaccuracy, and is likely more reproducible than manual measurements. Distances between inflection points on the outside bends of individual meanders were tracked to calculate migration rates. These measures were made relative to the valley trend to provide a lateral migration rate. Four meanders contact the valley wall though only three of these could be tracked. Meanders within the valley proper and approaching the valley wall were tracked to observe the difference in their behaviour. Generally, lateral migration rates were found to be low. Downstream and lateral migration rates for tracked meanders are presented in Table 2. The average lateral migration rate was 0.04 m/yr. Migration rates are likely low due to the cohesiveness of the Halton Till, the well-vegetated stream banks and the dissipation of energy in transporting outwash gravels. Since portions of the creek are unconfined, specifically Reach 4, a meander belt width of 55 m was calculated. From the historical assessment this Reach has been the least impacted by agricultural or other activities and has the most natural planform. Other reaches, in contrast, appear to have been straightened and lack the sinuous pattern expected for natural systems. As such, Reach 4 is considered a good surrogate of the Creeks historic, and perhaps future, scale of channel migration. Table 2: Migration rates for prominent meanders (See Appendix A).

Years

Period (yr)

Downstream

Migration Rate (m/yr)

Lateral

Migration Rate (m/yr)

Valley Toe Contact*

(y/n) Meander 1 1954-2005 51 0.06 0.05 Y Meander 2 1954-2005 51 Negligible 0.05 Y Meander 3 1954-2005 51 0.05 0.03 Y

FIELD ASSESSMENT Field observations were collected on September 6, 2006 to field truth the desktop assessment, identify active geomorphological processes, and assess channel stability. During field reconnaissance, Reaches 1 to 5 were walked, evidence of active processes noted and areas of active erosion identified. The main tributary flowing from the south east was also surveyed, as well as, the small tributary that flows north east across Peru Road. A photographic inventory was collected and is summarized in Appendix B. Rapid assessments were completed for Reaches 1 to 5 (e.g. Rapid Geomorphic Assessment and Rapid Stream Assessment Technique). A Rapid Geomorphic Assessment documents observed indicators of channel instability (MOE, 1999). Observations are quantified using an index that identifies channel sensitivity based on evidence of aggradation, degradation, and channel widening and planimetric adjustment. The index produces

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values that indicate whether the channel is stable/in regime (score <0.20), stressed/transitional (score 0.21-0.40) or in adjustment (score >0.41).

An RSAT provides a broader view of the system by also considering the ecological functioning of the stream (Galli, 1996). Observations include instream habitat, water quality, riparian conditions, and biological indicators. Additionally, the RSAT approach includes semi-quantitative measures of bankfull channel dimensions, type of substrate, vegetative cover, and channel disturbance (see Table 3). RSAT scores rank the channel as maintaining a low (<20), moderate (20-35) or high (>35) degree of stream health. Reaches 1, 3 and 4 are in regime, while Reaches 2 and 5 are in transition/stress. Reaches 1, 3 and 4 had RGA values less than 0.20 with evidence of widening. Evidence included fallen/leaning trees and exposed tree roots (Appendix B). Reaches 2 and 5 both had an RGA value of 0.23 with evidence of widening. Evidence included fallen/leaning trees, occurrence of large organic debris, exposed tree roots and basal scour along the inside of meander bends. The RSAT values ranged from 24.5 to 31.5 and indicate that the reaches are in good biological condition. Table 3: Summary of rapid assessment results.

RGA RSAT Reach Score Condition Form(s) of

Adjustment Score Condition Limiting Feature(s)

1 0.17 In Regime Widening 28.5 Good Riparian Habitat Conditions

2 0.23 In Transition Widening 31 Good Physical Instream Habitat / Riparian Habitat Conditions

3 0.18 In Regime Widening 29 Good Physical Instream Habitat / Riparian Habitat Conditions

4 0.18 In Regime Widening 24.5 Good Riparian Habitat Conditions

5 0.23 In Transition Widening 31.5 Good Riparian Habitat Conditions

Along all reaches, channel substrate consists of small cobble, gravel and sand in the pools and small to medium cobble and gravel, in the riffles. Bank materials consist predominantly of clayey silt. However, the RGA scores clearly indicate that the dominant process is channel widening, as the channel banks are less resistant to erosion compared to the channel bed. Along Reaches 1 and 2 the bankfull width is 3.5 to 5.0 m and bankfull depth ranges from 0.3 to 0.4 m (Table 4). Along Reach 3, the bankfull width increases to 5.0-6.0 m with a bankfull depth ranging from 0.5-0.6 m. Along Reaches 4 and 5 the bankfull width is 4.0 to 5.0 m and bankfull depth ranges from 0.3 to 0.6 m.

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Table 4: Summary of existing conditions within each reach.

Reach Bankfull width(m)

Bankfull depth(m)

Wetted width(m)

Wetted depth(m) Riparian vegetation

1 3.5 0.3 – 0.4 3.0 0.1 – 0.2 Mature Willow, continuous

shrub

2 4.0 – 5.0 0.3 – 0.4 4.2 0.3 – 0.4

Mature Pine, Willow, Maple and continuous

shrub

3 5.0 – 6.0 0.4 – 0.5 3.2 0.2 – 0.3 Mature Cedar and Willow.

Continuous shrub layer

4 4.0 – 5.0 0.3 – 0.4 3.5 – 4.0 0.2 – 0.3

Hydrophytes on right bank, turf grass on left

bank

5 5.0 0.5 – 0.6 3.5 – 4.5 0.2 – 0.4 Primary succession forest,

continuous shrub SUMMARY AND DISCUSSION • Following the MNR Policy Statement a detailed assessment of migration

rates was completed to define an erosion setback. This was based on a 51-year record, which is greater than the 25-year minimum suggested by MNR. Measured lateral migration rates were between 0.03 m/yr and 0.05 m/yr, with an average of 0.04 m/yr.

• Where the channel is confined, an erosion setback of 5 m is proposed. This 100-year erosion setback should be measured from the toe of slope and addresses the hazard associated with channel migration. Although lower migration rates were observed in some meanders, this may be due to backwater effects caused by wood debris jams, which may change in the future. As such, the proposed setback is a conservative estimate.

• Channel conditions included active erosion and till dominated sediments. The MNR Hazard Policy suggests a 5-8 m setback from the channel bank for these observed conditions.

• A meander belt width of 55 m was calculated based on the meandering planform of the least disturbed reach. Following standard protocols, the average lateral migration rate was added to this value. Therefore, where the channel is unconfined a meander belt width of 59 m should be applied to define hazard limits with respect to erosion.

Should any point require further clarification, or I can be of additional assistance please contact the undersigned. Respectfully submitted, GEOMORPHIC SOLUTIONS

<digitally signed> Paul Villard, P.Geo., Ph.D. Senior Geomorphologist, Associate

<digitally signed> Bradley Wright, B.Sc., M.E.S. Geomorphic Systems Technician

REFERENCES Chapman, L.J. and Putnam, D.F. 1984. The Physiography of the Southern Ontario, 3rd Edition, Ontario Geological Survey, Special Volume 2. Galli, J. 1996. Rapid stream assessment technique, field methods. Metropolitan Washington Council of Governments. 36pp. Ministry of Environment. 1999. Revised Stormwater Management Guidelines Draft Report. Montgomery, D.R and J.M. Buffington, 1997. Channel-reach morphology in mountain drainage basins. Geological Society of America Bulletin, 109 (5): 596-611. Richards, C., R.J. Haro, L.B. Johnson, and G.E. Host. 1997. Catchment and reach-scale properties as indicators of macroinvertebrate species traits. Freshwater Biology, 37: 219-23.

APPENDIX A GEOMORPHIC ASSESSMENT AND 100-YEAR EROSION SETBACK

MEANDER 1

MEANDER 2

LATERAL MIGRATION 0.05 m/yr



1

2

34

5

MEANDER BELT WIDTH55 m

Tarlock Sandhu

Milton MeadowsProperties Inc. 1321387 Ontario Inc.

LEGEND

1979 CHANNEL

KEY MAPN.T.S.

GEOMORPHIC ASSESSMENT AND

DRAWN BY: B.W.

PROJECT No. 06315 APPENDIX A

AERIAL PHOTOGRAPH 2005

DATE: NOV. 2006

MILTON HEIGHTSNEIGHBOURHOOD

1954 CHANNEL

2005 CHANNEL

REACH BREAKVALLEY WALL CONTACTGPS POINT

TRACKED MEANDER

´

Peru Road

100-YEAR EROSION SETBACK

MEANDER BELT WIDTH

Tremaine Road

PROPERTY BOUNDARY(Approximate - for information only)

(Dashed line indicates valley wall contact)

Note: Final meander belt width = 59 m (meander belt width + 100 year migration distance)

0 60 120 180 24030Meters

APPENDIX B PHOTOGRAPHIC INVENTORY

Photographic Record

Page 1 of 8 GEOMORPHIC SOLUTIONS

Photo 1. Downstream view of the Tremaine Road crossing showing extensive armouring of the outside bank.

Photo 2. Upstream view of a small tributary exiting the West Vit Estates property.

Photographic Record


Photo 3. View upstream of a small tributary draining Andrin Properties and crossing Peru Road.

Photo 4. GPS 1 Upstream view of Sixteen Mile Creek at the Peru Road crossing.

Photographic Record


Photo 5. GPS 2 South fork of the pond outfall. Ad hoc armouring has been applied to the banks to mitigate damage to adjacent residential properties.

Photo 6. GPS 3 North fork of pond outfall. Banks have been armoured with concrete slabs. Riparian conditions shown in the photo are typical for Reach 1.

Photographic Record


Photo 7. GPS 4 Upstream view of online pond. Through flow of Sixteen Mile Creek does not appear to have been greatly impacted. The online pond was machine enlarged during the late 1950’s in order to provide a private park with swimming.

Photo 8. GPS 5 Downstream extent of Reach 2 (view is upstream). Note the debris jam (GPS 6) that almost completely blocks the channel near the mid point of the photo.

Photographic Record


Photo 9. GPS 8 Active erosion scar on outside meander of Reach 2. Banks are generally low but the undulating topography resultd in intermittent areas of steep banks. The banks are composed of cohesive Halton and Wentworth Till and have good inherent stability.

Photo 10. GPS 9 Downstream extent of Reach 3. The left bank has been armoured with concrete rubble. The right bank is generally low and stable. Note the health of the riparian vegetation.

Photographic Record


Photo 11. GPS 10 Cobble medial bar near midpoint of Reach 3. The channel bed is dominantly cobbled throughout all studied reaches.

Photo 12. GPS 12 Erosion on the left bank of West Vit Estates. Material is cohesive Halton Till (clay silt till) that is failing as sub-angular blocks and is generally stable. The channel is not confined at this location.

Photographic Record


Photo 13. GPS 13 Active erosion scar on right bank of Reach 5. A scour pool occurs immediately upstream. At this location the channel is confined. Note the exposed roots and undercut tree.

Photo 14. GPS 14 Upstream view of Reach 5. Riparian vegetation is dominated by mature hardwood and continuous shrub cover.

Photographic Record


Photo 15. GPS 15 Valley wall contact and an active erosion scar on the right bank of Reach 5. This is Meander 1 of the Geomorphic Assessment. The bank is ~3m high and root depth extends to >1m. The bank is generally stable due to the cohesiveness of the bank materials. Of note is a visible facies change from Halton Till to the denser underlying Wentworth Till near the base of the exposure.

Photo 16. GPS 16 Valley wall contact on right bank of Reach 5. The erosion scar is semi-stable as evidenced by the establishment of a discontinuous vegetation cover.

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