The selection of the location and alignment of a water crossing shall take into account the following factors:
(a) the stability of the channel;
(b) road geometrics and road user safety; (c) geotechnical conditions;
(d) the effect of the crossing on adjacent structures and property; (e) the effects of adjacent dams, bridges, and other structures; (f) the interests of waterway users; and
(g) any known future developments.
1.9.4 Estimation of scour
1.9.4.1 Scour calculations
Depths of general scour, local scour, degradation, and artificial deepening of the channel shall be estimated at all structure sites.
Scour calculations shall be prepared for all potentially critical conditions, including maximum depth of flow, maximum velocity, and extreme ice conditions.
If abnormal flood conditions can occur at the site, a design flood discharge based on the lowest downstream water level likely to coincide with the design flood shall be considered.
1.9.4.2 Soils data
The properties of the material below the estimated depth of maximum scour shall be examined to reduce the possibility of large errors in scour predictions.
1.9.4.3 General scour
1.9.4.3.1 Average depth
The average depth of general scour shall be calculated using the competent velocity method or another Approved method and shall be referenced to the original stream bed.
1.9.4.3.2 Maximum depth
The maximum depth of general scour at any point in a structure opening shall be determined by redistributing the average depth of general scour as described in the Transportation Association of Canada’s Guide to Bridge Hydraulics or using other Approved methods. The maximum depth of general
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The depth of local scour at a pier, abutment, or other obstruction shall be measured below the anticipated depth of general scour and shall be calculated using Approved methods. The possibility of local scour being caused or aggravated by ice jams or trapped debris shall be considered.
1.9.4.5 Total scour
The total depth of scour at a point across a structure opening shall be taken as the sum of the maximum depth of general scour and the depth of local scour.
1.9.4.6 Degradation
The probable depth of stream bed degradation during the design life of a structure shall be estimated by investigating the site and the characteristics and history of the channel.
1.9.4.7 Artificial deepening
The amount of artificial deepening of a channel anticipated during the design life of a structure shall be obtained from drainage plans, if available. Otherwise, it shall be estimated from an investigation of existing structures on the same channel.
1.9.4.8 Allowance for degradation or artificial deepening
The ultimate bed elevation at structures on channels subject to degradation or artificial deepening shall be taken as follows:
(a) for degrading channels not having a concrete or steel invert that complies with Clause 1.9.5.7, the expected amount of degradation plus one-half the total scour; and
(b) for channels likely to be artificially deepened, the expected amount of deepening plus the total scour.
1.9.5 Protection against scour
1.9.5.1 General
Scour protection requirements for structure foundations shall be determined on the basis of the normal design flood and shall be modified if necessary to ensure that structural failure will not occur as a result of the check flood.
1.9.5.2 Spread footings
1.9.5.2.1 Depth of footings
The following shall apply to depth of footings:(a) Minimum depth: except as specified in Items (b) to (e), the bottom surfaces of spread footings that could be exposed to stream flow shall be placed at the lowest of the following elevations:
(i) a depth below the original bed not less than the following: (1) abutments other than arches: 1.50 m; and
(2) piers and arch abutments: 2.00 m;
(ii) a depth below the original bed not less than 1.7 times the estimated total depth of scour; and (iii) a depth not less than 0.50 m below the lowest level of existing or past scour.
(b) Bedrock: spread footings may be founded on scour-resistant durable bedrock at a higher elevation than that specified by Item (a) if the depth is sufficient to ensure that they remain unaffected by scour, freezing, weathering, degradation, or artificial deepening during the design life of the structure.
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November 2006complying with Clause 1.9.5.7, the footing depth specified in Item (a)(ii) shall not be taken as less than the expected amount of degradation plus the estimated total depth of scour.
(e) Artificial deepening: on channels likely to be artificially deepened, the footing depths specified in Items (a)(i) and (a)(ii) shall be measured below the expected stream bed elevation after deepening.
1.9.5.2.2 Protection of spread footings
Spread footings shall not be founded at a depth less than that specified in Clause 1.9.5.2.1 unless (a) the structure opening has a concrete or steel invert complying with Clause 1.9.5.7; or
(b) the footings are protected against undermining by sheet piling along the inside face and ends of the footings or by other Approved means.
Spread footings adjacent to the stream channel shall not be founded at an elevation higher than the stream bed on material other than bedrock or rock fill unless protected by a concrete revetment complying with Clauses 1.9.5.7and1.9.9.3.
1.9.5.3 Piles
1.9.5.3.1 General
In sands and other highly erodible soils, piles shall be used in preference to spread footings to provide better protection against the effects of scour.
1.9.5.3.2 Penetration and strength
The penetration and structural strength of piles shall be sufficient to ensure their stability with the stream bed at its ultimate bed elevation.
1.9.5.3.3 Abutments supported on piles
The bottom of footing elevation of abutments exposed to flowing water shall be set at least 1.0 m below the ultimate stream bed elevation. This requirement may be waived if footing protection meeting the requirements of Clause 1.9.5.2.2 is provided.
1.9.5.4 Sheet piling
When required for scour protection, sheet piling shall comply with the following requirements:
(a) the piling shall be securely attached to the footing or otherwise anchored to prevent movement; and (b) the penetration and structural strength of the piling shall be sufficient to ensure stability of the
structure and piling with the stream bed at its ultimate bed elevation.
1.9.5.5 Protective aprons
Flexible aprons used for protecting piers and abutments against local scour shall comply with the
recommendations of the Transportation Association of Canada’s Guide to Bridge Hydraulics or those of the Regulatory Authority.
Rip-rap stone sizes for aprons shall be determined by designing for a velocity 1.5 times the average velocity of the normal design flood discharge through the structure opening. The thickness of rip-rap aprons shall be not less than 1.5 times the median size of the stone.
1.9.5.6 Paved inverts and revetments
Concrete and steel inverts and revetments that are needed to stabilize a channel at a structure shall have cut-off walls of sufficient depth and strength to prevent undermining. The cut-off walls shall be integral with or securely attached to the invert or revetment.
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the maximum depth of degradation likely to occur during the design life of the structure or up to the time of scheduled stream bed maintenance.
(b) An apron, energy dissipator, or other device to control erosion caused by the discharge from the structure.
(c) If the invert is lower than the adjacent upstream bed, a sill, weir, or other effective control at the inlet.
1.9.6 Backwater
1.9.6.1 General
Backwater shall be calculated using Approved methods. Backwater shall be limited so as to preclude damage to upstream property and buildings during the design flood. Unless otherwise Approved, the design flood shall be the normal design flood.
1.9.6.2 High-water level
The high-water level used in the backwater calculations shall be as specified in Clause 1.9.1.6. If the crossing is subject to abnormal flood conditions, the highest downstream water level likely to occur together with the design flood shall be assumed.
1.9.6.3 Assumed depth of scour
Unless otherwise Approved, the depth of scour assumed to have occurred in backwater calculations shall not be more than one-half the average depth of general scour calculated in accordance with
Clause 1.9.4.3.
1.9.6.4 Waterway modification
The enlargement of the natural channel cross-section at a bridge to reduce backwater shall be of such a nature as to provide a stable channel and preclude progressive sedimentation and growth of undesirable vegetation.
1.9.6.5 Reduction of backwater by relief flow
Backwater calculations shall include relief flow over the roadway where allowed by Clause 1.9.1.2.
1.9.7 Soffit elevation
1.9.7.1 Clearance
The clearance between the soffit of the structure and the high water level determined in accordance with
Clause 1.9.7.2 shall be sufficient to prevent damage to the structure by the action of flowing water, ice floes, or debris, and unless otherwise Approved shall not be less than 1.0 m for freeways, arterial roads, and collector roads and not less than 300 mm for other roads. The clearance for a structure with an arched soffit shall be based on site-specific considerations.
Vertical clearance for a structure on a navigable waterway shall be measured from the highest water level at which usual navigation is likely to occur. This level, together with the vertical and horizontal clearances, shall be determined in accordance with the Government of Canada’s Navigable Waters Protection Act.
Unless otherwise Approved, the clearance between the lowest point of the soffit and normal water level shall be at least 1.0 m.
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November 2006Subject to Clause 1.9.7.2.2, and unless otherwise Approved for the site, the high-water level for establishing the minimum soffit elevation shall be the higher of
(a) the high-water level determined in accordance with Clause 1.9.6.2; and
(b) the high-water level caused by ice jams and having a return period comparable to that of the design flood.
1.9.7.2.2 Flooding and relief flow
Use of the regulatory flood as the design flood to ensure that flooding of upstream property will not occur shall not require the soffit elevation to be higher than that required for the normal design flood, provided that adequate relief flow, as permitted by Clause 1.9.1.2, can occur.