Sixteen Mile Creek Tributaries Meander Belt Width Assessment November 2009

Transcription

1 SIXTEEN MILE CREEK TRIBUTARIES THE CATHOLIC CEMETERIES OF THE DIOCESE OF HAMILTON -MEANDER BELT WIDTH ASSESSMENT - Report to: Attention: From: Director of Cemeteries Catholic Cemeteries of the Diocese of Hamilton 600 Spring Gardens Road Burlington, ON. John O Brien Will Cowlin, EIT., & John Parish, M.A., P.Geo. Report No.: Date: November 2009 (Addendum October 2010) PARISH Geomorphic Ltd. Page 0

2 1.0 INTRODUCTION The Catholic Cemeteries of the Diocese of Hamilton has plans to implement a new cemetery on the 103 acre property located on the northwest quadrant of the intersection of Bronte Road and Lower Baseline Road, in Milton (Figure 1). It is understood that Conservation Halton has requested that a meander belt width study be completed for three Sixteen Mile Creek tributaries to ensure that the proposed land use planning does not fall within hazardous land as per detailed in the Provincial Policy Statement As part of this meander belt width assessment, the following tasks were undertaken: Collect and review available background information, including relevant studies, aerial photographs and mapping; Delineate reaches within the identified study area; Based on aerial photographs, establish preliminary meander belt widths on a reach basis, following the orientation of the valley; Where appropriate and feasible, complete a 100-year erosion analysis to identify the necessary setbacks or belt width allowance; and Complete a field reconnaissance including rapid assessments in order to confirm the existing appropriateness of reach boundaries, 100-year erosion rates and the meander belt width for each tributary PARISH Geomorphic Ltd. Page 1

3 Figure 1: Study areas located at the intersection of Lower Baseline Rd. and Bronte Rd. (Google Imagery, 2005) 2.0 BACKGROUND REVIEW & EXISTING CONDITIONS A review of the previous works conducted for the study areas were evaluated for the purpose of the meander belt width assessment. Past reports provide valuable insight to the historical conditions of the study areas and provide comparable results which may be used to validate geomorphological alterations and/or confirm any inconsistencies found within the collected data. Although, several past reports were evaluated, including many involving Sixteen Mile Creek, the extent of each study did not focus on areas associated with the study site. In such case, aerial photographs and results from the rapid assessments were utilized to characterize the geomorphic conditions of the tributaries and develop the meander belt widths. PARISH Geomorphic Ltd. Page 2

4 2.1 HISTORIC ASSESSMENT LAND USES To understand the current state of a channel, it is important to consider the changes it has undergone in the past, including changes in surrounding land-use. These changes were identified through an historical assessment using the available aerial photos. Photographs used for this investigation were taken from 1972 and 2005 (Google Earth, 2005). Observations obtained from the 1972 aerial photographs of the study area demonstrated similar land coverage as the 2005 photos. The lands surrounding the study area, in 1972, was dedicated to an agricultural land use, with a single dwelling home located at the southwest limit of the property, along Lower Baseline Rd. By 2005, the lands surrounding the study site were still dominated by agriculture but had a few more dwellings situated throughout the surrounding areas. One prevalent difference between the 1972 and 2005 photos was the development of the Rattlesnake Point Golf Club located east of Bronte Rd. 2.2 HISTORIC ASSESSMENT CHANNEL PLANFORM ALTERATIONS Historic migration rates are a useful indicator of planform adjustment and provide a means of quantifying the rate of channel widening or bank erosion over time. Any change to the hydrologic conditions within a catchment can result in a modified flow regime. Long-term, non-local changes such as precipitation patterns result in gradual changes to channel morphology. In the short-term, changes in land use can have more abrupt consequences. In particular, urbanization generally leads to increased peak discharges which ultimately results in increased erosion and, hence, more rapid changes in channel morphology as the system attempts to re-establish a state of dynamic equilibrium. Consequently, the amplitude of meanders may increase as the planform geometry adjusts to the newly imposed flows. Downstream and lateral migration rates were determined for prominent meander bends within the study area using methods outlined in Geomorphic Protocols for Subwatershed Studies (PARISH, 2001). These protocols dictate that channel banks are first delineated and assessed individually to account for channel width changes. Migration rates were calculated from the available 1972 aerial photograph and 2005 aerial imaging, courtesy of Google Earth. Since the 1972 photograph was taken at a large scale, it is therefore prone to error during measurements of migration rates and channel lengths. Measurements were taken manually, PARISH Geomorphic Ltd. Page 3

5 except in the case of the 2005 photograph where measurements were completed in Google Earth. For this study, several measurements were taken along visible meander bends, to quantify both lateral and downstream channel migration. The results of the assessment are summarized in Table 1. Table 1: Lateral and downstream migration rates for Reach SMT-1 & SMT-2 ( ). Migration Rates Reach SMT-1 ( ) Reach SMT-2 ( ) Lateral (m/yr) Downstream (m/yr) Lateral (m/yr) Downstream (m/yr) Max Min Mean Upon completing the migration rates for the two reaches, results demonstrate that the channels have remained quite stable for the last 33 years. It should be noted that, based on the poor resolution of the historical air photos, the error associated with the measurements was anticipated to have a similar value compared to the magnitude of migration observed between 1972 and Visual observations of the channel planform over the historical record indicate that, indeed, the watercourses had not undergone any major adjustments over the past 33 years. One significant alteration in the channel planform was observed along reach SMT-1, however, it was hypothesized that the degree of change was uncharacteristic of the watercourse s natural geomorphological capabilities and was concluded to be an affect of anthropogenic activities. As a result, these sections of reach SMT-1 were not used to within the migration rate analysis. Reach SMT-3 was not included within the migration rate analysis because its channel planform was not observable within the 1972 aerial photos; hence, no comparison could be made with the 2005 aerial photographs. PARISH Geomorphic Ltd. Page 4

6 2.3 REACH DELINEATION The characteristics of the flow or channel materials can change along a creek or stream. In order to account for these changes, channels are separated into reaches normally several hundred meters to several kilometers in length. A reach displays similarity with respect to its physical characteristics, such as channel form, function, and valley setting. Delineation of a reach considers sinuosity, gradient, hydrology, local geology, degree of valley confinement, and vegetative control using methods outlined in the Belt Width Delineation Procedures (PARISH Geomorphic Ltd, 2004). Figure 2: Reach delineations for the study area. (Google Imagery, 2005) Reach SMT-1 Reach SMT-1 extends from its confluence with reach SMT-2, immediately upstream of the Bronte Rd. culvert crossing, and proceeds upstream to where the land uses change from agricultural to the manicured lawn of the single dwelling home located along Lower Baseline PARISH Geomorphic Ltd. Page 5

7 Rd. The reach was delineated based on its confluence with reach SMT-2 which represents an alteration to the local hydrology and the change in local land uses between agricultural and residential areas. Reach SMT-1 displayed a straightened meander planform which could be described as a swale feature, however, had a more defined channel planform within the vicinity of the Bronte Rd. culvert crossing. Land cover within the proposed study area consisted primarily of agricultural practices dedicated to a corn crop. An open scrub meadow buffered the swale through the extent of the cornfield. A single residential dwelling was located along Lower Baseline Rd approximately 420m west of the intersection of Bronte Rd. and Lower Baseline Rd Reach SMT-2 Reach SMT-2 extends from its confluence with reach SMT-1, immediately upstream of the Bronte Rd. culvert crossing, and proceeds upstream to the north limits of the cornfield. The reach was delineated based on its confluence with reach SMT-1 which represents an alteration to the local hydrology and the change in the channel planform as the channel becomes undefined along the north limits of the study area. Reach SMT-2 displayed a straightened meander planform which could be described as a swale feature, however, had a more defined channel planform within the vicinity of the Bronte Rd. culvert crossing. The bank conditions of the channel were steepened and the land cover within the proposed study area consisted primarily of agricultural practices dedicated to a corn crop. An open scrub meadow buffered the swale through the extent of the cornfield. Instream vegetation was present within the channel closer to the Bronte Rd. culvert Reach SMT-3 Reach SMT-3 historically extended from the culvert crossing of Lower Baseline Rd., approximately 110m west of the Bronte Rd. and Lower Baseline Rd. intersection, to its confluence with reach SMT-1. The reach was delineated based on its former confluence with reach SMT-1 which represents an alteration to the local hydrology and the channel disturbance created by the Lower Baseline Rd. bisection. Reach SMT-3 displayed no meandering planform or a defined channel configuration through the cultivated land, in which it was situated. PARISH Geomorphic Ltd. Page 6

8 2.4 RAPID ASSESSMENTS In order to provide insight regarding existing geomorphic conditions, a site reconnaissance was conducted in October, Rapid assessment techniques (RGA and RSAT) were applied to determine the dominant geomorphic processes affecting each site. A Rapid Geomorphic Assessment (RGA) documents has shown 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 values that indicate whether the channel is stable/in regime (score <0.20), stressed/transitional (score ) or adjusting (score >0.41). A RSAT provides a broader view of the system by also considering the ecological functioning of the stream (Galli, 1996). Observations include in-stream 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. RSAT scores rank the channel as maintaining a low (<20), moderate (20-35) or high (>35) degree of stream health. Table 2 provides a summary of the rapid assessment results, while Appendix A provides a photographic record of site conditions at the time of survey. Table 2: Summary of RGA and RSAT scores for the study area. Reach/Crossing RGA Condition RSAT Stability SMT-1 & SMT In Regime 20 Moderate Reach SMT-1 Bankfull widths along this section of channel ranged from 1-2m and had bankfull depths that ranged from m. Substrate throughout the reach consisted of a mixture of silt fine sands and clay. No defined pool-riffle sequences were notable along the reach and the much of the channel was vegetated with tall grasses and cattails. RGA results indicated that the channel was In Regime ; however minor evidence of aggradation and planimetric form adjustment was present. PARISH Geomorphic Ltd. Page 7

9 2.4.2 Reach SMT-2 Bankfull widths along this section of channel ranged from 1-2m and had bankfull depths that ranged from m. Substrate throughout the reach consisted of a mixture of silt fine sands and clay. No defined pool-riffle sequences were notable along the reach and the much of the channel was vegetated with tall grasses and cattails. RGA results indicated that the channel was In Regime ; however minor evidence of aggradation and planimetric form adjustment was present Reach SMT-3 Historically, reach SMT-3 conveyed flows through the east corner of the study area. The channel has since been disturbed by the agricultural practices conducted on the land which has caused the channel to loose its natural channel planform and configurations. Based on observations obtained during the field reconnaissance, the historical flow regime conveyed by reach SMT-3 seems to have been re-routed to an alternate drainage ditch located to the west. The alternate drainage ditch begins approximately 110m west of the Bronte Rd., at a Lower Baseline Rd. culvert, and connects, mid-reach, with SMT-1. The straight channel corridor was well vegetated and was bordered by the surrounding cultivated crops. The low flow channel within the corridor had a meandering planform and was fairly entrenched within the channel corridor. Evidence of erosion was present along the outside meanders. 3.0 MEANDER BELT WIDTH DELINEATION In support of both the Provincial Policy Statement and the Valley and Stream Corridor Management Program, the Toronto and Region Conservation Authority (TRCA) has produced a detailed document which outlines Belt Width Delineation Procedures (PARISH Geomorphic Ltd., 2004) for confined and unconfined systems. This document provides a process-based methodology for determining the meander belt width for watercourses within the jurisdiction of the TRCA based on background information, historic data (including aerial photography), degree of valley confinement and channel planform. PARISH Geomorphic Ltd. Page 8

10 Initial delineation of the meander belts were undertaken as recommended by Leopold and Wolman (1960). As per this method, tangential lines are drawn along the outside bends of the laterally extreme meanders within the reach, following the meander axis. The distance between these two lines represents the meander belt width. By using this approach, preliminary belt widths were determined for two reaches within the study area. Refer to Table 3 for the preliminary belt width results determined from the belt width assessment. Table 3: Summary of the Preliminary Belt Width Results. Reach/Crossing Preliminary Belt Width (m) SMT-1 12 SMT-2 10 Due to the geomorphic conditions of reach SMT-3, a meander belt width assessment could not be conducted for the reach. With little evidence of a defined channel planform or configurations, any recommended meander belt width would be inaccurate and may falsely represent the geomorphic capabilities of the area YEAR MIGRATION RATE/EROSION SETBACK Considering the migrations rates for each reach (Section 2.2) were minimal and the error associated with each measurement potentially exceeded the measured distance of migration, it was determined that introducing a factor of safety would provide a more conservative representation of any future migration. In order to apply a factor of safety to the meander belt width, a 20% factor of safety was applied to the preliminary belt width on a reach basis (10% to be applied to either side of the watercourse). This dimension essentially represents the hazard limits associated with stream erosion within the study area. The erosion setbacks and final belt width dimensions have been presented in Table 4. Figures 3 and 4 demonstrated the preliminary and final belt width delineations for each of the three study areas. Table 4: Summary of the Meander Belt Width Results. Reach/Crossing Preliminary Belt 20% Factor of Final Belt Width (m) Width (m) Safety SMT SMT PARISH Geomorphic Ltd. Page 9

11 Figure 3: Preliminary and final meander belt width delineation for reach SMT-1. PARISH Geomorphic Ltd. Page 10

12 Figure 4: Preliminary and final meander belt width delineation for Reach SMT-2. PARISH Geomorphic Ltd. Page 11

13 4.0 CONCLUSION The Catholic Cemeteries of the Diocese of Hamilton has plans to implement a new cemetery on the 103 acre property located on the northwest quadrant of the intersection of Bronte Road and Lower Baseline Road, in Milton. It is understood that Conservation Halton has requested that a meander belt width study be complete for three Sixteen Mile Creek tributaries to ensure that the proposed land use planning does not fall within hazardous land as per detailed in the Provincial Policy Statement Results of the geomorphic assessment established preliminary meander belt widths of 12 m and 10 m for reaches SMT-1 and SMT-2, respectively. A historic review and 100-year erosion rate analysis resulted in the establishment of 2.4 m and 2.0 m erosion setbacks to be applied to the respective belt widths of each watercourse, resulting in final belt width dimensions of 14.4 m and 12 m. PARISH Geomorphic Ltd. Page 12

14 5.0 REFERENCES Galli, J., Rapid stream assessment technique, field methods. Metropolitan Washington Council of Governments. 36 pp. Leopold, L.G. and M.G. Wolman River meanders. Bulletin of the Geological Society of America, 71: Ministry of Environment Revised Stormwater Management Guidelines Draft Report. PARISH Geomorphic Ltd Belt Width Delineation Procedures. Submitted to: Toronto and Region Conservation Authority. PARISH Geomorphic Ltd Urban Stream Crossing Guidelines. Phase 2, Submitted to: Toronto and Region Conservation Authority. PARISH Geomorphic Ltd. Page 13

15 Addendum Recommended Structure Span October 2010 ADDENDUM RECOMMENDED STRUCTURE SPAN A risk-based approach is typically undertaken when evaluating the recommended span of a crossing structure in terms of the local geomorphic conditions (Figure A-1). This risk assessment protocol provides a site-specific context whereby an appropriate crossing structure size can be evaluated and determined from a geomorphic perspective, based on the processes that are operative at the proposed crossing location. Further integration of these recommendations with design considerations and constraints identified through the overall study will then establish whether the geomorphic requirements are the governing factor for structure span. The risk assessment process involves the following parameters: a) Channel Size: The potential for lateral channel movement and erosion tends to increase with stream size. Headwater streams tend to exhibit low rates of lateral migration due to the stabilizing influence of vegetation on the channel bed and banks. Erosive forces in larger watercourses tend to exceed the stabilizing properties of vegetation and result in higher migration rates. b) Valley Setting: Watercourses with wide, flat floodplains and with low valley and channel slopes tend to migrate laterally across the floodplain over time. Watercourses that are confined in narrow, well defined valleys are less likely to erode laterally but are more susceptible to downcutting and channel widening, particularly where there are changes to upstream land use. Typically the classification of the valley will fall into one of three categories: confined; partially confined; and, unconfined. c) Meander Belt Width: The meander belt width represents the maximum expression of the meander pattern within a channel reach. Therefore, this width/corridor, covers the lateral area where the channel could potentially occupy over time. This value has been used by regulatory agencies for corridor delineation associated with natural hazards and the meander belt width is typically of a similar dimension to the regulatory floodplain. The use of the meander belt width for structure sizing has been a criterion from some of the agencies and certainly represents a very conservative approach. d) Meander Amplitude: The meander amplitude and wavelength are important parameters to ensure that channel processes and functions can be maintained within the crossing. For the purposes of this study, the meander amplitude of the watercourse was measured in vicinity of the crossing and used as a guide to determine the preliminary crossing structure span. e) Rapid Geomorphic Assessment (RGA) Score: An RGA score is essentially a measure of the stability of the channel. Channels that are unstable tend to be actively adjusting and thus more sensitive to the possible effects of the proposed crossing. Accordingly, there is more risk associated with unstable channels. While the actual RGA score will be PARISH Geomorphic Ltd. Page i

16 Addendum Recommended Structure Span October 2010 reported, there are three levels of stability: is stable; 0.21 to 0.40 is moderately stable; >0.40 is unstable. f) 100-year Migration Rates: Using historical aerial photographs, migration rates were quantified (where possible) for each crossing location. A higher migration rate indicates a more unstable system and higher geomorphic risk. Essentially, it is a risk assessment where, based on the condition of the channel, migration rates and trends, existing planform and valley configuration, an appropriate opening size from a geomorphic perspective can be developed. SMT-1 Based on the findings of the historic assessment, field evaluation, channel planform and valley setting, the risk associated with a crossing structure along this reach was deemed low. From a reach-based perspective, the channel was poorly defined (bankfull widths in the range of 1-2 m) and highly vegetative-controlled. It displayed a straight planform due to the low order nature of the stream, as well as historic land use practices. Given the altered nature of the stream, a governing meander amplitude could not be applied to establish the preliminary structure span from a geomorphic perspective. Although the meander belt width stipulated a 12 m preliminary width consideration, the scale and stable nature of the drainage feature provided justification to refine this dimension. In order to maintain the geomorphic form and function of the existing channel, a 5 m span was deemed appropriate. This dimension refers to a single-span culvert (preferably with an open footing) that is oriented optimally (90 degrees) to the direction of flow. It also represents a dimension that is equivalent to roughly three times the average bankfull width which provides an additional factor of safety in order to account for any potential future channel planform adjustment or widening due to shading effects within the structure. Table A-1 provides a summary of the risk-based considerations and recommended crossing span for SMT-1. Table A-1. Geomorphic parameters and recommended structure size for SMT-1. Preliminary Meander Belt Width Erosion Setback (Risk) 12 m 1.2 m (Low) Meander Amplitude Bankfull Width Valley Setting m Unconfined RGA Score 0.10 In Regime (Aggradation) Crossing Span 5 m PARISH Geomorphic Ltd. Page ii

17 Addendum Recommended Structure Span October 2010 Figure A-1. Geomorphic risk-based assessment protocol for span recommendations. PARISH Geomorphic Ltd. Page iii

18 Addendum Recommended Structure Span October 2010 SMT-2 Based on the findings of the historic assessment, field evaluation, channel planform and valley setting, the risk associated with a crossing structure along this reach was deemed low. From a reach-based perspective, the channel was poorly defined (bankfull widths in the range of 1-2 m) and highly vegetative-controlled. It displayed a straight planform due to the low order nature of the stream, as well as historic land use practices. Given the altered nature of the stream, a governing meander amplitude could not be applied to establish the preliminary structure span from a geomorphic perspective. Although the meander belt width stipulated a 10 m preliminary width consideration, the scale and stable nature of the drainage feature provided justification to refine this dimension. In order to maintain the geomorphic form and function of the existing channel, a 5 m span was deemed appropriate. This dimension refers to a single-span culvert (preferably with an open footing) that is oriented optimally (90 degrees) to the direction of flow. It also represents a dimension that is equivalent to roughly three times the average bankfull width which provides an additional factor of safety in order to account for any potential future channel planform adjustment or widening due to shading effects within the structure. Table A-2 provides a summary of the risk-based considerations and recommended crossing span for SMT-2. Preliminary Meander Belt Width Table A-2. Geomorphic parameters and recommended structure size for SMT-2. Erosion Setback (Risk) 10 m 1.0 m (Low) Meander Amplitude Bankfull Width Valley Setting m Unconfined RGA Score 0.10 In Regime (Aggradation) Crossing Span 5 m SUMMARY A risk-based geomorphic assessment was conducted on a reach basis to establish the recommended crossing span from a geomorphic perspective within the study area. Within the context of the meander belt width assessment, the low risk and stable nature of both Reaches SMT-1 and SMT-2 provided justification to refine this dimension and resulted in a PARISH Geomorphic Ltd. Page iv

19 Addendum Recommended Structure Span October 2010 recommended span of 5 m for both reaches. This span refers to a single-span culvert that is oriented optimally to the direction of flow. PARISH Geomorphic Ltd. Page v

20 Appendix A Site Photographs

21 Photo 1: Upstream view of the vegetated corridor of reach SMT-1. Photo 2: Upstream view of reach SMT-1, demonstrating the channel conditions.

22 Photo 3: Downstream view of the culvert crossing Bronte Rd. Confluence of reaches SMT- 1 and SMT-2. Photo 4: Upstream view of the vegetated corridor of reach SMT-2

23 Photo 5: View of the bank erosion occurring within reach SMT-2. Photo 6: Upstream limits of reach SMT-2. Reach entering cultivated area.

24 Photo 7: Upstream limits of reach SMT-3. Undefined channel planform was observed with a large pooling area. Photo 8: Reach SMT-3 had no channel planform except for standing water, mid-reach.

25 Photo 9: Photo demonstrates the land depression in the cultivated field where standing water was located in reach SMT-3.