Selecting Computational Methods

The design of a water crossing will require the use of a number of analytical methods to assess the design flow rates as well as to complete the hydraulic analysis. It is recognised that the level of detail required in the hydrologic or hydraulic analysis will vary, depending on the type level of structural planning and design that is being undertaken. Therefore, before proceeding review the following.

The following sections provide the details on hydrologic or hydraulic analysis that are applicable to water crossing design proposals. The hydrologic and hydraulic methods specified within this section are accepted industry practices, and can be applied to the appropriate component of the watercourse. The MTO Drainage Management Manual provides the details of the information provided here. The main purpose to providing this information is to establish a link between this document and the MTO Drainage Management Manual.


Level of Detail of Analysis

Where a lower level of detail is proposed for the hydrologic or hydraulic analysis, the Hydrology Report should provide rationale on why the lower level of detail is appropriate. As a guide, review the following land development attributes.

  • The type of water crossing that is proposed in terms of the total span, whether or not the crossing will interfere with the flow in the watercourse or the flood plain.
  • The potential for flooding impacts on surrounding lands and the degree of the impact.
  • The number of upstream and or downstream riparian owners and structures that may be impacted.
  • The requirements of other agencies.
  • Traffic volume.

A more detailed hydrologic or hydraulic analysis may be needed where any of the following may occur (typically, a higher level of detail involves the input of more data):

  • Damage to the property located upstream or downstream of the water crossing;
  • The structural integrity of the structure or the highway right-of-way is threatened; or
  • The safety of the travelling public is threatened.

Having considered the level of detail, review the following sections for details on hydrologic or hydraulic analysis that are applicable to water crossing designs. MTO recognises that the documentation of computational methodology is within the interests of all the regulatory agencies, not just MTO. The hydrologic and hydraulic methods specified within this section are accepted industry practices, and can be applied to the appropriate component of the analysis.

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Flow Rate Calculation

Flows rates are the results of the hydrologic analysis and are typically determined at key locations along the watercourse for a range of frequencies. The results are generally used as input to the hydraulic analysis, which involves the calculation of water surface elevations and flow velocities. Documentation Requirement for the Components of the Drainage System (Table 7) presents the typical range of flow rate frequencies that should be calculated at the proposed location(s) of the water crossing presented in the table.

Inputs to the flow rate calculation include:

The methods that can be used for the flow rate calculation as classified in Figure 2 are:

  1. Non-Hydrographic Methods
    • Method Based on Stream Flow
    • Methods Based on Precipitation Data
  2. Hydrographic Methods
    • Single Event Modeling
    • Continuous Event Modeling

Flow Rate Calculation (Figure 2)

Methods that can be used for the flow rate calculation

Non-Hydrographic Methods

These methods calculate the peak flow rate based on statistical analysis of the stream flow records or precipitation records. Refer to Selecting Precipitation Data for information on the different types of precipitation data input.

The most reliable methods for calculating the stream flow rate at a water crossing are those based on Single Station Frequency Analysis Methods.

Single Station Frequency Analysis

Single Station Frequency Analysis utilizes the records of annual maximum floods at a gauging station to calculate the peak instantaneous flow rates required for design. The ratio of peak instantaneous values to the mean annual can be determined from the data. The analysis then provides the peak instantaneous flow rate values for return periods ranging from 2 to 100 years.

There are a number of assumptions that are usually made to be able to use this method.

  • There are suitable years of record (short term records may lead to unreliable estimates).
  • The upstream land use remains unchanged.
  • The data available is usable.

This analysis may be done by hand, using statistical principles of probability and probability distribution functions. However, this is quite tedious and most likely the analysis will be done using computer programs such as the Consolidated Frequency Analysis (CFA) program.

Depending on the location of the site relative to the gauging station the following options can be considered:

  • The single station stream flow data can be used directly if the station is close to the project site.
  • Interpolating the stream flow data from a distant stream flow gauge station.
  • Transposing data from a stream gauge station on a stream with similar watershed characteristics.

Transposition and interpolation of data from a stream gauge can be done based on the Modified Index Flood method as follows:

Q2 = Q1 [A2 / A1] 0.75
Where:
Q1 = Known peak discharge
Q2 = Unknown peak discharge
A1 = Known basin area
A2 = Unknown basin area

That is, the unknown peak discharge, Q2, is equal to the known peak discharge, Q1, times, open bracket, the unknown basin area, A2, divided by the known basin area, A1, close bracket, to the power of three quarters

If the basins or catchment areas have significantly different hydrologic characteristics transposing the flow rate from another watershed should not be relied on and another method such as the modified index flood method should be used directly.

Limitation of Single Station Frequency Analysis:

The main limitation in using the Single Station Frequency Analysis method is the quality of the stream flow data being relied on. The integrity of the data can be examined by investigating a number of factors. These include:

  • Checking for missing records.
  • Checking for short records compared to the required design return period. Usually a record of a length equal to half the required return period can provide reasonably reliable results. However, in such a limiting case checking the result with other methods, such as using rainfall-runoff modeling may be necessary.

Furthermore, there are four test that should be conducted on the data. These tests check for:

  • Independence of the data - A data value is not dependent on a preceding data value.
  • Randomness - Short and long term cycles around the median, an indication that the data is or is not random.
  • Trends - Gradual changes over a long time. These may be the result of land-use changes, engineering works or other causes. Consideration should be given to possible changes in the future.
  • Data split - Data elements result from similar phenomena (example, no change in land use).
  • These tests are part of the computer program CFA.

Refer to Chapter 5, page 9 in the MTO Drainage Management Manual for more details.

If the quality of the data is found to be questionable or cannot be used, other methods should be considered. Furthermore, it is highly recommended that confirmation of the results obtained using one method should be done using other methods, even if the data quality is reliable.

Regional Regression Methods using Precipitation Data and Watershed Characteristics

The most common methods used to assess the peak flow rates are those based on modelling of the precipitation-runoff process. Some of these methods are empirical. These methods use statistical representations of the precipitation record, from a rainfall gauging station (e.g. Intensity-Duration-Frequency IDF curve), combined with physical parameters representing the catchment (e.g. area, length, slope, and runoff coefficient), to calculate the peak flow rate at a particular location in a catchment area. These methods can be classified into two types:

  • The Rational Method; and
  • Regional Frequency Analysis (Modified Index Flood or Northern Ontario Hydrology Method).

Of these methods, the Rational Method is the most suitable method for small land development sites and is therefore, discussed in this section. The Modified Index Flood Method is for watersheds greater than 25 km2 and the Northern Ontario Method is for watersheds between 1 and 100 km2 in area. For information on the use and application of the Modified Index Flood Method and the Northern Ontario Method refer to the "Drainage Management Manual" (MTO 1997), Chapter 8, page 43.

The Rational Method

The Rational Method calculates the peak flow rate at a particular location of a catchment area due to the runoff contributed from the entire upstream area. The Rational Method is represented by the following equation:

Q = 0.0028 C i A
Where:
C = the runoff coefficient (Refer to DMM Design Charts 1.07);
 i = the rainfall intensity (mm/hr) (Refer to DMM Design Charts 1.01(a)-(r)); and
A = the area of the contributing catchment (m²).

That is, discharge, Q, is equal to 0.0028 times the runoff coefficient, C, times the rainfall intensity, i, times the area of the contributing catchment, A.

When applying the Rational Method it is important to demonstrate the applicability of the method. For this purpose, it is important to note the following.

  • The Rational Method is primarily used as a design tool for the design of minor drainage systems such as storm sewers and ditches. Refer to the section Flow in Open Channels for further details.
  • The Rational Method can provide acceptable estimates of peak flow rates in small non-retentive rural watersheds. It is mostly applied to an urban catchment as a design tool to size storm sewers.
  • The present practice in the MTO limits its use to:
    • Rural watershed drainage areas less than 100 ha; or
    • Urban watershed drainage areas less than 50 ha.
  • The catchment area applied to the Rational Method should be that of the entire contributing catchment. The time of concentration is therefore, the time of travel of the flood wave from the furthest point of that catchment to the point of interest (ex. at a culvert or bridge).
  • The applicability of the Rational Method for rural watersheds should be reviewed if there is great variability in soil, vegetation or rainfall.

If the Rational Method is not applicable other methods should be used.

Refer to the DMM, Chapter 8 page 39 for more details and the application of the Rational Method.

Hydrographic Methods

Hydrographic methods calculate the time distribution of flow rate (hydrograph) at any location in a catchment. These methods calculate the response of a catchment to precipitation and snow melt applying mathematical representations of the specific physical hydrological processes in a catchment area, such as infiltration, evaporation and detention.

The two basic types of hydrograph methods are based on the two forms of precipitation data that are available:

  • Single event precipitation record; or
  • Continuous precipitation records.

Refer to the information sheet on Selecting Precipitation Data for more details on rainfall data.

Hydrograph simulation methods are required under the following circumstances:

  • The drainage basin is expected to undergo significant urbanization;
  • The drainage basin will be subject to stormwater management controls or modifications to the drainage system (i.e. hydrograph routing is required);
  • The drainage basin contains reservoirs and watercourses;
  • Peak flow rates or volumes of runoff will be calculated from a historical rainfall or precipitation event (e.g. Hurricane Hazel or Timmins Storm);
  • Different drainage options are to be tested including regulation (i.e. stormwater management controls, modifications to the drainage system etc.);
  • If land use, time of concentration, or soils conditions (e.g. CN) vary significantly across the drainage basin; and
  • The Regional Frequency Analysis method or other empirical methods are not applicable.

When using hydrographic methods it is essential to provide the specific information on the basis of which the modelling was based. This typically includes the following:

  • Catchment areas, slopes and discretization into sub catchments;
  • Imperviousness ratios, land use types, directly connected areas, depression storage, infiltration parameters, soil parameters (e.g. CN number), and the components of the drainage system; and
  • The time to peak of the unit hydrograph, recession constants, computational time step, rainfall event(s) including type, duration and discretization time step, and snow accumulation.

For details refer to Identifying Catchment Inputs for details on the parameters presented above

Single Event Hydrographic Methods

Single event hydrographic modelling simulates the precipitation/runoff process using a short duration precipitation event (i.e. durations ranging from 1hr to a few days). The storm event may be the regulatory storm (Hurricane Hazel, the Timmins Storm or the 100-year storm event) as described in PHY Directive B100.

Single Event Methods are used when:

  • The storm event is a designated design storm (e.g. using a regulatory storm to assess flood line impacts);
  • Stormwater management controls exist or is being proposed;
  • Modifications to the watercourse are proposed;
  • Flow routing in a ditch or storm sewer system may have a major effect on the peak flow; and
  • Impacts to the drainage system must be assessed.

Refer to the DMM, Chapter 8 page 77, for more details.

Single event computer models acceptable to MTO include:

Refer to Evaluation of Drainage Management Software for more additional information on these models, Identifying Catchment Inputs for information on the input parameters to these models, and Selecting Precipitation Data for details on rainfall data.

Continuous Record Hydrographic Method

Continuous event hydrographic methods calculate the flow rate using the entire long-term precipitation record as input. Typical periods of rainfall data range from 10 to 40 years. Continuous simulation is expected to generate runoff with a frequency which best approximates reality; however, calibration is required to achieve accuracy.

Continuous simulation can be an expensive, complex and time consuming process. It is used:

  • When an accurate estimate of peak flow rate return periods is required (e.g. high risk of upstream or downstream impacts, or during legal proceedings);
  • To simulate low flow or base flow conditions; and
  • For water quality analysis (i.e. pollutograph routing).

The following are the most common continuous event hydrographic computer models accepted by MTO:

Refer to the DMM Chapter 4, pages 81 and 86 for more detail. Methods not covered in the DMM may be used if it can be demonstrated, through independent recognised references, that these methods are in agreement with the principles outlined in the DMM and are applicable to Ontario conditions.

Routing the Hydrograph through Channels and Reservoirs

Where the hydrologic analysis involves a hydrographic method, the runoff hydrograph should be routed through the channel and reservoir components of the watercourse or drainage system for the following reasons.

  • Storage within the channel reach or reservoir may result in the attenuation of the hydrograph peak, changing the time to peak and possibly reducing the peak flow.
  • Multiple storage facilities located in the same drainage basin will affect the timing of the hydrograph as it travels downstream. This could increase or decrease peak flows in downstream locations. Coordination of stormwater management detention facilities with other drainage structures, on a watershed or subwatershed basis, is a primary consideration.
  • In catchment areas where natural depressions form part of the drainage system, reservoir routing can be used to determine the possible impacts of any increase in runoff volume to the volume of water stored in the depression areas. For instance, increased runoff volumes could cause a cascading affect, which could result in localised flooding.
  • Where a suitable drainage outlet does not exist and the stormwater runoff is conveyed via sheet flow, channel or reservoir routing may be used to estimate any impacts caused by increased runoff volumes or peak flows (e.g. localised flooding).

Typically, channel or reservoir routing is completed as part of the hydrologic analysis. Most hydrologic computer models include channel and reservoir routing options that can be applied with minimal input data. Refer to Evaluation of Drainage Management Software for more details.

  • Channel routing typically requires the input of channel cross-section data and roughness coefficients.
  • Reservoir routing typically requires the input of a storage-discharge relationship for the reservoir. The storage-discharge relationship is a function of the outlet device (i.e. orifice, pipe or weir) and is independent of the inflow rate. The basis for the calculation of the storage-discharge relationship should be provided. Refer to Chapter 8, page 82 for more details. The assessment of parking lot and roof top storage are similar to reservoir in that a stage-storage-discharge relationship should be developed and routing of the hydrograph thought these facilities should be conducted.

Channel routing can also be completed as part of the hydraulic analysis where flow in the channel reach is unsteady (e.g. for very steep or very slopes). Hydraulic channel routing generally requires the use of different computer models. Refer to Evaluation of Drainage Management Software for more details.

Computer Application Errors

Computer application errors can generally be attributed to the following:

  • Incorrectly entered data;
  • Incorrect use of default values intrinsically provided by the program;
  • Incorrect assumptions made in an application;
  • Program misapplication;
  • Incorrect interpretation of modelling results; and
  • Programming errors.

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Identifying Catchment Inputs

Catchment inputs are parameters that are determined at the reference points in the watercourse as noted in Documentation Requirement for the Components of the Drainage System (Table 7), and are used as an input to the flow rate calculation.

Area

  • The total watershed area should include all the land that contributes stormwater runoff to the watercourse. The watershed boundaries should extend far enough upstream and/or downstream to demonstrate that drainage impacts do not occur as a result of the proposed land development.
  • The watershed area should be separated into separate drainage catchment areas so that its downstream boundary will be located at a:
  • The watershed area should be separated into separate drainage catchment areas of similar land use, based on the total imperviousness of the catchment.

Catchment Slope (Slope of Watershed)

  • Slope refers to the representative slope along the longest flow path.
     
  • Two methods to determine catchment slope are presented below.
    1. 85/10 Method
       
      Sw = 100 slope calculation
      Where:
      SW = watershed slope, %
      delta h = difference in elevation, m, between the 85% point and the 10% point obtained from contours, air photos, etc.
      hf = sum of heights of rapids and waterfalls between 10% and 85% points, m
      L = total length of main channel, including the undefined flow path to head of basin, m
      Lf = sum of lengths of rapids and waterfalls, up to 10% of L, m

      That is, watershed slope, Sw, is equal to 100 times, open bracket, the difference of elevation, delta h, minus the sum of heights of rapids and waterfalls, hf , divided by the difference of 0.75 times the total length of main channel, L, minus the sum of lengths of rapids and waterfalls, Lf, close bracket.
    2. Equivalent Slope Method
       
      Sw = 100 slope calculation
      Where:
      Sw = watershed slope, %
      Sn = slope of an individual reach of the channel, m/m
      n = number of reaches of approximately equal length

    That is, watershed slope, Sw, is equal to 100 times, open bracket, divide the number of reaches of approximately equal length, n, by the sum of the slopes of individual reaches of the channel, Sn, raised to the exponent of -0.5, close bracket, raise to the power 2.
  • For further details refer to pages 25 to 27 in Chapter 8 of the "Drainage Management Manual" (MTO 1997).

Total Imperviousness Ratio

  • The total imperviousness ratio defines the amount of paved (i.e. driveways or roadways) and roofed surfaces in a catchment area, as a percentage of area.
  • It is a key input to most hydrologic computer programs and it can directly affect the peak flow of the runoff hydrograph.
  • The total imperviousness ratio of a catchment should be determined from appropriate land use mapping such as Official Plans, Secondary Plans, Draft Plans or Site Plans.
  • General values for generic land uses can be acquired from the planning office of most municipalities.
  • As a guide, the following table can be used.

Percentage Imperviousness for Different Land Use (Table 9)

Land Use % Impervious
Rural 0 to 20
Residential Single Family 20 to 50
Multiple Detached 40 to 60
Multiple Attached 60 to 75
Commercial: Light 50 to 80
Heavy 60 to 90
Industrial: Light 50 to 80
Heavy 60 to 90

 

Directly Connected Imperviousness Ratio

  • Directly connected imperviousness ratio defines the amount of paved (i.e. driveways or roadways) and roofed surfaces that are directly connected to the drainage system, as a percentage of area.
  • If roof leaders are connected to the foundation drains, the directly connected imperviousness ratio will generally be equal to the total imperviousness ratio.
  • Where roof leaders are not connected to the foundation drains, the directly connected imperviousness ratio will be derived from the paved surfaces only (i.e. driveways or roadways).

Depression Storage Values (Table 10)

Land Cover Typical Values
Impervious 2 mm
Pervious: Lawns 5 mm
Meadows 8 mm
Woods 10 mm

 

Infiltration Parameters

Horton Equation - Typical Values (Table 11)

Soil Group Minimum Infiltration
Rate (mm/hr)
Typical Values
A 25 2 mm
B 13 5 mm
C 5 8 mm
D 3 10 mm

* dry soil conditions, Decay Parameter = 2 hr-1

Green-Ampt Method - Typical Values (Table 12)

Soil Group IMD3(mm/hr) Su3 (mm) Ks2(mm/hr)
A (sand) 0.34 250 25
B (silt loam) 0.32 200 13
C (sand clay loam) 0.26 125 5
D (clay) 0.21 180 3

 

Linear Reservoirs

  • For Ontario conditions, the accepted standard is 3.
  • Can be altered based on a model calibration.

Curve Number

  • The curve number, CN, is a common input to computer programs and is used to represent the spoil and land use condition of a catchment area.
  • Three parameters used to define CN are soil type, land use and the antecedent moisture condition.
  • A variation of CN is the CN*. When using CN *, the initial abstraction Ia applied reflects the actual Ia value for the catchment rather than the assumed value of Ia = 0.2S, where S is the potential abstraction. In such a case the CN values are altered as part of a calibration process. As a result the CN* method should be applied with caution, and it will only be accepted by MTO where the computer model has been calibrated and the CN* values can be verified. Before proceeding with the CN* method, contact an MTO drainage representative for further guidance.
  • Refer to the page 20 in Chapter 8 of the "Drainage Management Manual" (MTO 1997), for further details. Soil type, land use, antecedent moisture conditions and the corresponding CN are presented in Charts 1.08, 1.09 and 1.10 of Part 4.

Time to Peak (of the unit hydrograph)

  • The time to peak (of the unit hydrograph) is a key input of most hydrologic computer programs. It is a very sensitive parameter that directly affects the peak flow. For instance, a small time to peak (of the unit hydrograph) will result in a runoff hydrograph with a high peak flow and a short peak time, while a higher value will result in a runoff hydrograph with a lower peak flow and a longer time to peak.
  • There are three primary methods of calculation. For details refer to page 71 in Chapter 8 of the "Drainage Management Manual" (MTO 1997).
    1. HYMO/OTTHYMO
      • Rural watershed with watershed slope less than 2%.
         
        tp = 0.0086  A0.422  S -0.46  [L/W] 0.133

        That is, time to peak, tp, is equal to 0.0086 times the drainage area, A, to the power of 0.422, multiplied by the watershed slope, S, to the power of -0.46, multiplied by, open bracket, the length of the watershed, L, divided by the width, W, close bracket, raise to the power of 0.133.
      • Rural watershed with watershed slope greater than 2%.
         
        tp = 0.016  A0.31  S -0.50

        That is, time to peak, tp, is equal to 0.016 times the drainage area, A, to the power of 0.31, times the catchment slope, S, to the power of -0.5.
      • Urban watershed.
         
        0.5 tp (using above methods)
        Where:
        tp = time to peak (of the unit hydrograph), hours
        S = catchment or watershed slope, m/m
        A = drainage area, ha
        L/W = length to width, dimensionless

    2. MIDUSS
       
      tp =  0.6 tc  +  0.5 t
      Where:
      tp = time to peak (of the unit hydrograph), hours
      tc = time of concentration, hrs, using MIDUSS method
      t = computational time step, hrs

      That is, time to peak, tp, is equal to 0.6 times the time of concentration, tc, plus 0.5 times the computational time step, t.
    3. Time to Peak Calculated Using Time of Concentration
       
      tp =  0.67 tc
      Where:
      tp = time to peak (of the unit hydrograph), hours
      t = computational time step, hrs
      That is, time to peak, tp, is	equal to 0.67 times the time of concentration, tc

Recession Constant

  • The recession constant controls the shape of the hydrograph, after the time to peak has passed.
  • Two approaches can be used.
    1. HYMO/OTTHYMO
       
      • Rural watershed with watershed slope less than 2%:
         
        K =  0.0095A 0.231  S -0.777  [L/W] 0.124

        That is, the recession constant, K, is equal to 0.0095 times the drainage area, A, raised to the power 0.231, times the catchment or watershed slope, S, to the power of -0.777, times, open bracket, length, L, divided by width, W, close bracket, to the power 0.124.
      • Rural watershed with watershed slope greater than 2%:
         
        K =  0.00316A 0.24  S -0.84

        That is, the recession constant, K, is equal to 0.00316 times the drainage area, A, raised to the power 0.24, times the catchment or watershed slope, S, to the power of -0.84.
      • For urban watersheds:
         
        K =  0.5K (using above methods)
      Where:
      K = recession constant, hours
      S = catchment or watershed slope, m/m
      A = drainage area, ha
      L/W = length to width, dimensionless

       
    2. Hydrograph Method
       
      Qt = Q0  Kt
      That is, discharge at time after the peak, Qt, is equal to discharge at the start of recession, Q0, times the recession constant, Kt.or
      log Kt  =   (log Qt - log Q0)
      divided by
      (tt - t0)
      Where:
      Qt = discharge at time t after the peak, m3/s
      Q0 = discharge at the start of recession, m3/s
      Kt = recession constant
      tt = time after peak
      t0 = initial time

      That is, logarithm of recession constant, Kt, is equal to, open bracket, logarithm of discharge at time after the peak, Qt, minus the logarithm of discharge at the start of recession, Q0, close bracket, divided by the remainder of time after peak minus initial time
  • For details refer to page 72 in Chapter 8 of the "Drainage Management Manual" (MTO 1997).

Time of Concentration

  • The time of concentration measures the total time that it takes a drop of rain to travel the longest flow path in a catchment area.
  • When the time of concentration is reached, the entire catchment is contributing to the flow at the catchment confluence point.
  • Three methods to calculate time of concentration are listed below.
     
    1. Bransby William Formula
      Used if Rational Method runoff coefficient is greater than 0.40.
       
      tc  =   0.057 L
      divided by
      Sw0.2 A0.1

      That is, time of concentration, tc, is equal to the product of 3.26 times, open bracket, 1.1 minus the rational method runoff coefficient, C, close bracket, times the catchment length, L, to the power of 0.5., all divided by the catchment slope, Sw to the power of 0.33.
    2. Airport Equation
      Used if Rational Method runoff coefficient is less than 0.40.
       
      tc  =   3.26 (1.1 - C) L0.5
      divided by
      Sw0.33
      Where:
      tc = time of concentration, minutes
      C = Rational method runoff coefficient
      L = catchment or watershed length, m
      Sw = catchment or watershed slope, %
      A = catchment or watershed area, ha

      That is, time of concentration, tc, is	equal to 0.057 times the catchment length, L, divided by the product of the catchment slope, Sw, to the power of 0.2 and catchment area, A, to the power of 0.1.
    3. MIDUSS
       
      tc = calculation ieff-0.4
      Where:
      tc = time of concentration, minutes
      k = 6.989 for metric units
      L = flow length (m)
      n = Manning's roughness coefficient
      S = slope of catchment or watershed, m/m
      ieff = effective rainfall (mm/h)
      That is,

Computational Time Step

  • The computational time step is the incremental period of time that a computer program will use when convoluting a rainfall distribution into a rainfall hydrograph.
  • Typically, the computational time step is set to be equal to 1/5 of the time to peak (of the unit hydrograph). It should be small enough to ensure that the peak of the rainfall distribution is captured in the convolution of the hydrograph rainfall time step. However, some computer models limit the amount of hydrograph co-ordinates that can be stored in memory: so if the time step is too small, the amount of hydrograph co-ordinates derived in the convolution might exceed the memory storage limit, and the hydrograph output could get truncated.
  • For urban applications, the computational time step should be less than 10 minutes.

Rational Method Runoff Coefficient

  • The runoff coefficient, C, is used in the Rational Method.
  • Refer to the DMM, Design Chart 1.07 in Part 4.

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Selecting Precipitation Data

Flows rates are calculated as part of the hydrologic analysis and are typically determined at key locations along the watercourse for a range of frequencies, as noted in Documentation Requirement for the Components of the Drainage System (Table 7). Since precipitation data will serve as an input to the flow rate calculation, the frequency of the selected precipitation event must correspond to the flow rate frequency that is that is to be calculated. Key characteristics of the precipitation data include the type and duration. The precipitation data may be one of the following depending on the type of analysis required and the data available.

Rainfall Intensity from a Representative IDF Curve

Rainfall intensity from a representative IDF curve is applicable when using the Rational Method for calculating the peak flow rate. A representative IDF curve includes one of the following:

  • MTO district IDF curve (refer to the DMM Part 4, Design Charts 1.01(a) - (r) for this data);
  • AES IDF curve for a meteorological station closest to the catchment; and
  • Municipal IDF curve.

Single Representative Storm Event from the Historical Record

A single representative storm from the historical record, usually the regulatory storm, is applicable when using a single event modelling technique for assessing the impact of the regulatory storm (Hurricane Hazel, Timmins Storm or the 100-year event). PHY Directive B100 specifies the applicable regulatory storm based on the geographic location of the catchment area under investigation.

Synthetic Storm Events

Synthetic storm events are typically used to assess impacts to the drainage system or in the design of mitigative works. A synthetic storm is produced by distributing the total precipitation volume over the duration of the storm based on a defined mathematical distribution (e.g. Chicago, AES or SCS distributions). Refer to the DMM, Chapter 8 page 10, for more details and to Example 8.1 page 11 for the method of developing a Chicago Storm. Input parameters include the following:

  • The precipitation volume should be obtained from a representative Intensity-Duration-Frequency (IDF) curve for the appropriate duration.
  • The storm duration has traditionally been chosen to be at least equal to the basin time of concentration. The time of concentration will be longer for basins with significant storage (as in rural catchments), and short for catchments with high imperviousness (as in urban catchments). Where a drainage basin is serviced by a stormwater management detention facility, a long duration storm event should be used. The magnitude of the synthetic storm will vary with the applied storm return period.
  • The storm event return period, which can vary from 2 to100 years, and is selected based on the flow rate frequency that is that is to be calculated. Refer to Documentation Requirement for the Components of the Drainage System (Table 7) for more details.
  • The rainfall time step should be small enough to ensure that the peak of the rainfall distribution is captured in the convolution of the hydrograph. However, some computer models limit the amount of hydrograph co-ordinates that can be stored in memory: so if the time step is too small, the amount of hydrograph co-ordinates derived in the convolution might exceed the memory storage limit, and the hydrograph output could get truncated.

Table 13 provides the acceptable synthetic storm events, the applicable storm duration and rainfall time step for each of these storm events based on land use. These parameters should always be provided to support the selection of a storm event.

Synthetic Storm Events (Table 13)

Storm Event IDuration Time Step Land Use1Applicability
Chicago (Keifer & Chu) Variable (usually 3hr or 4hr) Variable Urban
SCS Type II 6hr, 12hr or 24hr 15 min Rural
AES (30%) - 1 hr 1 hr 5 min Urban
AES (30%) - 12 hr 12 hr 15 min Rural
AES/Hydrotek 1 hr 5 min
Note: 1 Urban > 20% impervious area, Rural < 20% impervious area

Continuous Storm Record

A continuous storm record for a representative meteorological station is used when performing continuous hydrological modelling. Typical periods of rainfall data are 10 to 40 years. An alternative to using the entire continuous storm record is the use of a series of individual historical storm events. Each event is analyzed statistically and a frequency analysis of the results is then performed. Refer to the "Drainage Management Manual" (MTO 1997), Chapter 3 Appendix 3A for more details.

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Culvert Analysis

When designing a new culvert, culvert analysis is conducted to calculate the headwater level upstream of a culvert based on the flow rate, selected culvert characteristics and tailwater elevation. Refer to Finalizing Design and Construction Issues (Table 6) for culvert characteristics that are required to complete the culvert analysis. Refer to the DMM, Chapter 5, page 17 and Chapter 8 page 134, for details on culvert analysis.

For existing culverts, culvert analysis is conducted to check the capacity of the culvert. This will be necessary if a number of changes have taken place upstream. These changes may be associated with changes in the watershed, highway improvement or the need for culvert replacement, to name a few.

The Peak flow rates used in the analysis are determined from the hydrologic analysis and are calculated at the upstream section of the culvert for the range of frequencies specified in Documentation Requirement for the Components of the Drainage System (Table 7).

Water Surface Elevations (headwater level)

A culvert will operate in either inlet control or outlet control depending on the magnitude of the flow rate. For each flow rate, the headwater depth is computed for both the inlet and outlet conditions. The headwater level is determined from the condition that governs (i.e. the condition yielding the higher headwater level).

Tailwater Elevation

In the case of outlet control, an accurate assessment of tailwater elevation is essential as it has a significant effect on the headwater level. The two main methods of calculating the tailwater elevation are as follows:

  • The Manning equation, which calculates the normal depth downstream of the culvert. This method is valid only if it can be demonstrated that the flow in the downstream channel is steady and uniform.
  • Backwater analysis, which calculates the water level downstream of a culvert, based on the known water level at another point. This method is used if the flow in the channel is steady uniform or gradually varied.

If the calculated tailwater elevation is below the top of the culvert outlet the governing tailwater level will be the greater of the following two levels:

  • the calculated tailwater elevation; and
  • (dc+D)/2 (where dc is the critical depth and D is depth of the culvert opening).

Method of Analysis

Culvert analysis can be conducted either using hand calculation methods or computer models.

  • Hand calculations: refer to the DMM, Chapter 5, page 17 and Chapter 8 page 134. Culvert analysis nomographs are presented in Part 4 of the DMM, Design Charts 5.39 to 5.49.
  • Computer models: accepted by MTO are HEC2 and HEC-RAS. Refer to Evaluation of Drainage Management Software web document for more information on these models. The user manuals of these models should be consulted for details on their application. Other models, not mentioned in this document or the DMM, may be used provided it can be demonstrated, through independent recognised references, that these methods are in agreement with the principles outlined in the DMM.

Checking Culvert Capacity

Where the capacity of the highway culvert is being checked, the analysis need only be completed for the design flow frequency.

Assessing Impacts to the Drainage System

Where headwater levels or flow velocities are being determined for the range of flow rate frequencies specified in Documentation Requirement for the Components of the Drainage System (Table 7), a separate culvert analysis should be completed for each flow rate. The results can be presented on a culvert performance curve, which plots the headwater level against the flow rate (refer to pg 144 in Chapter 8 of the DMM for more details on performance curves). Where relief flow occurs, weir flow will occur and the performance curve should reflect this condition.

Flow Velocity at the Outlet

Flow velocities should be determined for each headwater level determined in the culvert analysis. The exit velocity from a culvert should not result in erosion downstream of the culvert, otherwise, erosion protection should be provided. Refer to the section "Designing Erosion Protection Measures" for more details.

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Bridge Analysis

Bridge analysis is completed to determine the size and shape of the bridge opening and to assess the associated backwater effect.

Peak flow rates are determined from the hydrologic analysis and are calculated at the upstream section of the bridge for the range of frequencies specified in Documentation Requirement for the Components of the Drainage System (Table 7). The results will serve as an input to the analysis.

Bridge analysis is used to calculate the headwater level upstream of a bridge based on the flow rate and bridge characteristics. Refer to Finalizing Design and Construction Issues (Table 6) for bridge characteristics that are required to complete the bridge analysis. Refer to the DMM, Chapter 5, page 17 and Chapter 8 page 134, for details on bridge analysis.

Water Surface Elevations (headwater level)

Method of Analysis

The Two basic methods of bridge analysis are:

  • hand calculations using the equation presented in the discussion below; or
  • using computer program such as HEC2 or HEC-RAS.

Hand Calculations

When analysing a bridge structure, the flow through the bridge should be checked for the following cases:

  • Constricted open channel flow occurs when channel flow is conveyed through a bridge cross-sectional flow area that is less than the stream channel cross-sectional area immediately upstream. The result can be an increase in elevation of the water surface profile upstream of the structure. The head loss at a waterway may be expressed as:
     
    hT = [ KT a2 + a1 {(A2 / A4)2 - (A2 / A1)2}] * [V22/2g]
    Where:
    hT = Total head loss, m
    A = Cross-section area perpendicular to flow, m2
    a = Velocity head coefficient
    V2 = Velocity at entrance, m/s
    KT = Total loss coefficient
    KT = Kb + Kp + Ke + Ks
    Kb = Base coefficient
    Kp = Pier coefficient
    Ke = Eccentricity coefficient
    Ks = Skew coefficient

    That is, total head loss, hT, is equal to, open first bracket, total loss coefficient, KT, times Velocity head coefficient, a2, plus velocity head coefficient, a1, times, open second bracket, open third bracket, cross-section area perpendicular to flow, A2, over cross section area perpendicular to flow, A4, close third bracket, to the power of 2, minus, open fourth bracket, cross-section area perpendicular to flow, A2, over cross-section area perpendicular to flow, A1, close fourth bracket, to the power of 2, close second bracket, close first bracket, multiplied by the quotient of the velocity at entrance, V2, to the square exponent, over, 2 times the gravitational constant, g.The number subscripts refer to section locations. For a subcritical flow analysis, the calculation should start from the downstream end and proceed upstream. Refer to the DMM, Chapter 5 Page 12, for more details.

  • Pressure flow can occur when the water surface profile is above the maximum soffit elevation on the upstream side of the bridge. A difference in head must exist between the water surface elevations at the upstream and downstream faces of the structure, to force the flow of water, under pressure, through the waterway opening.

    Where a bridge soffit is fully submerged, pressure flow, Qp, through waterway openings, may be analyzed using the following equation:

    Qp = Cd * A * (2gH)0.5
    Where:
    Qp = flow rate, m3/s

    That is, flow rate, Qp, is equal to weir coefficient, Cd, times area, A, times, open bracket, 2 times the gravitational constant, g, times the height of upstream water surface, H, close bracket, to the exponent of 0.5. 
  • Weir flow occurs if there is relief flow above the top of the roadway. Refer to the DMM, Chapter 3 page 27, for a discussion on relief flow.

    Assuming that the roadway performs like a broad-crested weir, using the following equation:

    Qw = C * L * H1.5
    Where:
    Qw = Weir discharge, m3/s
    C = Weir coefficient
    L = Length of weir, m
    H = height of upstream water surface above weir crest, m

    That is, weir discharge, Qw, is equal to weir coefficient, C, times the length of weir, L, times the height of upstream water surface above weir crest, H, to the power of 1.5.Refer to the DMM, Chapter 5 page 12 for more details.

Computer Programs

Computer models accepted by MTO are HEC2 and HEC-RAS. Refer to Evaluation of Drainage Management Software for more information on these models. The user manuals of these models should be consulted for details on their application. Other models, not mentioned in this document or the DMM, may be used provided it can be demonstrated, through independent recognised references, that these methods are in agreement with the principles outlined in the DMM.

Checking Bridge Capacity

Where the capacity of the highway bridge is being checked, the analysis need only be completed for the design flow frequency.

Assessing Impacts to the Drainage System

Where headwater levels or flow velocities are being determined for the range of flow rate frequencies specified in Documentation Requirement for the Components of the Drainage System (Table 7), a separate bridge analysis should be completed for each flow rate. The results can be presented on a bridge performance curve, which plots the headwater level against the flow rate (refer to pg 144 in Chapter 8 of the DMM for more details on performance curves). Where relief flow occurs, weir flow will occur and the performance curve should reflect this condition.

Flow Velocity at the Outlet

Flow velocities should be determined for each of headwater levels determined in the bridge analysis. The velocity through a bridge and the exit velocity from a bridge should not result in erosion downstream of the bridge, otherwise, erosion protection should be provided. Refer to the sections The Potential for Scour and Channel Erosion for more details.

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Flow in Open Channels

Open channel flow analysis is used to calculate the depth of flow and flow velocity in the stream channel or roadside ditch, when water is flowing under the influence of gravity with a free water surface. Refer to Finalizing Design and Construction Issues (Table 6) for stream channel or roadside ditch characteristics that are required to complete the analysis.

Peak flows rates are determined from the hydrologic analysis and are calculated at the upstreamsection of the open channel for the range of frequencies specified in Documentation Requirement for the Components of the Drainage System (Table 7). The results will serve as an input to the analysis. Channel routing may be required as part of the analysis. Refer to Routing the Hydrograph through Channels and Reservoirs for more information on flow routing.

Water Surface Elevation

The water surface elevation in an open channel can be determined using the Manning's equation or other similar method such as the Chazy formula if the flow is steady and uniform.

The Manning Equation is given by:

Q = (1/n)  R2/3 S1/2 A
Where:
n = the Manning roughness coefficient
A= the area of the contributing catchment (m2)
R = the hydraulic radius (m2/m)
S = the channel slope (m/m)

That is, discharge, Q, is equal to 1 over the Manning roughness coefficient, n, times the hydraulic radius, R, to the power of two thirds, times the channel slope, S, to the power of half, times the area of the contributing catchment, A.However, due to variations in channel cross-section, slope and meander pattern, uniform flow condition cannot be assumed. In such cases, gradually varied flow condition will govern and backwater analysis would have to be conducted.

Backwater Analysis

Backwater analysis applies the continuity and energy equations in assessing the water surface elevation at each cross-section starting from a point of known water level, which will become the starting water surface elevation.

When assessing drainage impacts, backwater analysis should be conducted for a distance upstream or downstream where there will be no appreciable difference between the pre-development and post-development water surface elevations. Refer to Performing The Hydraulic Design for further details.

Method of Analysis

Backwater analysis can be conducted either using hand calculation methods or computer models.

  • Hand calculations: the most widely used method for conducting backwater analysis for natural and artificial channels is the Standard Step Method. This method is based on the application of the continuity and energy equations. Refer to the "Drainage Management Manual" (MTO 1997), Chapter 8 Page 130, for a worked example on the application of this method.
  • Computer models accepted by MTO are HEC2 and HEC-RAS. Refer to Evaluation of Drainage Management Software for more information on these models. The user manuals of these models should be consulted for details on their application. Other models, not mentioned in this document or the DMM, may be used provided it can be demonstrated, through independent recognised references, that these methods are in agreement with the principles outlined in the DMM.

Checking Capacity

Where the capacity of the highway roadside ditch is being checked, the analysis need only be completed for the design flow frequency.

Assessing Impacts to the Drainage System

Where water surface levels or flow velocities are being determined for the range of flow rate frequencies specified in Documentation Requirement for the Components of the Drainage System (Table 7) a separate analysis should be completed for each flow rate. Water surface levels, storage volume, and flow rates can be presented on stage-storage and stage-discharge curves for both the predevelopment and post-development scenario(s).

Flow Velocities

Flow velocities should be determined for each of the water levels determined in the analysis. The velocity in a stream channel or roadside ditch should not result in erosion, otherwise, erosion protection should be provided. Refer to Channel Erosion for more details.

Channel Stability

If changes to a channel slope, shape or meander patterns are being considered, channel stability should be assessed. If a channel becomes unstable, it will attempt to return to an equilibrium state through the processes of aggradation and degradation. The result of these processes will be erosion or sedimentation that can occur at the immediate location of the channel or anywhere within an appreciable distance upstream or downstream. For more details, refer to the "Drainage Management Manual" (MTO 1997), Chapter 9 or to a reference on natural channel design.

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Channel Erosion

An assessment of channel erosion can be completed as part of Performing the Hydraulic Design. The purpose of the analysis is to determine if the velocities within the vicinity of the proposed crossing will increase such that erosion will occur. Erosion will occur if the streambed and bank material is inadequate to resist the increase in flow velocity.

The flow velocities calculated as part of the hydraulic analysis for culverts, bridges, and open channels, and serve as an input to the channel erosion analysis. Refer to Finalizing Design and Construction Issues (Table 6) for erosion protection characteristics that are required to complete the analysis.

The susceptibility of a channel to erosion can be assessed using one of the following methods:

  • Maximum permissible velocity.
  • Maximum permissible tractive force.

The assessment of scour at a stream crossing is discussed in "Assessing the Potential for Scour".

Maximum Permissible Velocity

Maximum permissible velocity is the maximum flow velocity that a channel can withstand without serious deformation of the channel bed or bank. The maximum permissible velocity depends on a number of factors including the bed material, flow depth, sediment load, channel alignment and vegetation.

When checking the lining of an existing channel the flow velocity should be less than then maximum permissible velocity. When designing a channel the hydraulic radius (R) should be less than the hydraulic radius corresponding to the maximum permissible velocity calculated using the Manning equation.

Refer to the "Drainage Management Manual" (MTO 1997), Chapter 5 page 111, for design details and to Design Chart 2.17, for typical maximum permissible velocities for different lining materials.

Maximum Permissible Tractive Force

Tractive force is the shear force exerted by the flow on the wetted channel surfaces. Tractive stress (N/m2) is the tractive force (N) per unit area (m2). If the tractive force caused by the flow is greater than the resistive forces holding the material, erosion will occur. The tractive stress varies along the bed and sides of a channel.

The following equation can be used to determine the maximum tractive stress along the bed:

Equation: the maximum tractive bed stress, tb max, is equal to tractive force (bed) coefficient, Kb, times the unit weight of water, gamma, times the hydraulic radius, R, times the channel slope, S.
Where:
tau b max = maximum tractive bed stress, N/m2
Kb = tractive force (bed) coefficient
gamma = unit weight of water, 9810 N/m3
R = hydraulic radius, m
S = channel slope, m/m

The maximum tractive stress along the side of a channel can be calculated using the following equation:

Equation: maximum tractive bank stress, ts max, is equal to tractive force (bank) coefficient, Kbk, times the unit weight of water, gamma,  times the hydraulic radius, R, times the channel slope, S
Where:
tau s max = maximum tractive bank stress, N/m2
Kbk = tractive force (bank) coefficient
gamma, R, S as above

Refer to the "Drainage Management Manual" (MTO 1997), Chapter 5 page 112, for more details and to Design Chart 2.25, for permissible unit tractive force values for different soil types.

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Ice and Debris Requirements

The formation of ice and flow of debris in a watercourse can be a governing factor in the establishment of the soffit elevation of a water crossing. As a result an assessment of this potential has to be done in order to determine if it is a factor in the design of a structure and to establish the appropriate soffit elevation.

Assessing Ice Requirements:

In Ontario, ice cover will form during the winter months in streams with continuous flows. Water will continue to flow under the ice. In the case of intermittent watercourses, water may freeze to the channel bed with thin layers of water flowing over the ice and eventually freezing.

Ice problems are usually associated with spring melt. Some of these problems are:

  • Ice jamming
  • Channel icing

In all these conditions, ice exerts forces on the channel and any structure in its path that can be significant. These forces cause great stresses resulting in some cases of structural failure.

Ice Forces

Flowing ice may result in the following types of forces on structures:

  • Horizontal forces acting on piers, abutments and embankments due to impacts of ice sheets. The forces may act longitudinally, transversely or obliquely. The forces are generally horizontal, however, a vertical component may be generated on sloping surfaces. These forces may be static, caused by contraction and expansion of ice sheets or dynamic due to impact caused by moving ice sheets.
  • Vertical forces caused when water levels vary and ice sheets are frozen around piers. Lifting of superstructure from ice contact due to insufficient clearance. Ice impact may also result in undesirable vibrations in bridges.

There are two ice related design aspect that need to be completed in order to determine the required soffit elevation and any training work needed to accommodate the ice flow. These tasks are:

  • Assess River Ice Conditions
  • Estimating Design High Ice

Estimating High Ice Conditions

Design high ice conditions are estimated to ensure that the waterway opening of a bridge crossing is large enough to pass flowing ice and to incorporate mitigative measures to handle adverse effects. Specifically, an estimate of design high ice may be used for:

  • Establishing the minimum soffit elevation for a proposed structure to provide adequate clearance above the estimated high ice elevation.
  • Determining the magnitude and elevation of dynamic ice forces caused by ice floes crushing against pier(s).
  • The maximum ice force would occur in a channel generally at or below the bank-full condition. For flows greater than bank-full, ice sheets may move onto the flood plain and be stored, thereby resulting in a reduced ice build-up.
  • Designing counter measures to resist or reduce ice forces on a proposed structure.

Refer to the MTO Drainage Management Manual (DMM, Chapter 5 page 83 for details.

Assessing Ice Jam Condition

Ice jams are a local phenomenon and information on ice jams for one reach cannot necessarily be transposed to another reach.

Analytical techniques applicable to ice jams are much less developed compared with the hydraulics of open water flow, therefore, there is a greater need for site data to project and verify predictions. The height of a potential ice jam may be estimated using the Equilibrium Ice Jam Estimating Method.

Refer to the Drainage Management Manual Chapter 5 page 68 for more details.

Assessing Debris Requirement

Rivers and streams, when flooded, may carry debris. The bulk of debris consists of tree material and other vegetation that is floated by the flow and/or uprooted by erosion undercutting the stream banks and carried downstream.

Some debris may not float and travel downstream but may obstruct the flow path. The amount of debris flow will be dependent on the carrying capacity of the channel. Streams with a relatively wide channel or flood plain and greater velocity of flow generally carry large debris, such as logs. Narrow channels, sharp bends, channel bars or islands and waterway constrictions, such as bridges or culverts may cause deposition of floating debris.

Refer to the MTO DMM, Chapter 5 Page 93 for details.

Impact of Debris

Flowing debris results in the following types of adverse impacts to channel and water crossings.

Reduces Conveyance Capacity - Floating debris may cause blockage and therefore reduce the conveyance capacity of a water crossing. This may increase upstream flooding and accelerate channel erosion in the vicinity and downstream.

Structural Problems - Flowing debris may cause blockage and excessive erosion in the vicinity of a structure, including abrasion of embankments. Excessive erosion may result in undermining of structure foundations. Debris blockage may also transfer forces of flow to a bridge superstructure, tending to dislodge it.

Controlling Debris

As discussed above, by studying the watershed and stream channel, it may be possible to assess the potential for debris flow at a site of interest. However, the occurrence and severity of debris relative to a flood event is hard to predict. In practice, the problem of debris is generally handled by avoiding difficult sites, by providing flow deflectors to facilitate the passage of debris or by providing a larger waterway opening than would otherwise be required. Debris racks for culverts have been used with a limited success on streams with light to mild debris, as they require periodic cleaning.

Refer to the MTO DMM, Chapter 5 Page 95 for details.

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Assessing the Potential for Scour

An assessment of scour is completed as part of the hydraulic analysis of the bridge or culvert. Scour may undermine the foundations of a structure, possibly leading to its failure. Particular attention should be given to the natural stream characteristics. A stream may be unstable due to meandering, degradation or aggradation.

The flow velocities calculated as part of the hydraulic analysis for culverts and bridges serve as an input to the scour analysis. Refer to Finalizing Design and Construction Issues (Table 6) for culvert or bridge characteristics that are required to complete the analysis.

Scour in a stream channel is defined as the lowering and/or widening of the streambed due to erosive forces exerted by flowing water. Flowing water in a stream channel exerts force in the direction of flow on the channel boundary surface. If the boundary force due to flow exceeds the resisting force of the boundary material, bed material particles are dislodged, resulting in scour of the streambed. Refer to the "Drainage Management Manual" (MTO 1997), Chapter 5, pages 43-65 and Chapter 9 (Basic Stream Geomorphology for Highway Applications) for more information.

Types of Scour

Natural Scour - The occurrence of scour in the absence of any structural interference is commonly referred to as natural scour. A stream channel goes through progressive bank and bed scour over time due to naturally occurring flows and stream processes, resulting in sediment transport and channel adjustment.

General Scour - The local lowering of a channel bed in the vicinity of a structure waterway opening is called general scour.

Local Scour - Bed degradation that is generally localized around an obstruction, such as piers or groins is called local scour. The depth of local scour is in addition to the depths of natural or general scour in the vicinity.

Factors Affecting Channel Scour

The main purpose of completing a scour analysis is to assess the change in scour vulnerability of a stream channel bed at a water crossing and the adequacy of the existing or proposed scour protection works. When conducting a scour analysis the following are the main factors to be considered are:

  • design flow rate;
  • stream characteristics, including: width, depth, slope, and meanders;
  • constriction/obstructions in channel;
  • water clarity and evidence of scour activity;
  • design and check flow velocity;
  • bed material (particle size and cohesiveness);
  • structure type, configuration and alignment relative to the stream channel; and
  • scour protection works.

Estimating General / Natural Scour

The methods for predicting scour depth are empirical and based on experience and judgement. These methods are:

  • The Competent Velocity Method;
  • The Mean Velocity Method;
  • The Regime Method; and
  • The Laursen Method.

These methods are not universally applicable. Therefore, as a minimum, when assessing the scour at a bridge crossing or in a channel these methods should be used to assess the potential for scour. When assessing the applicability of the different methods, the following aspects should be considered:

  • Various methods should be considered and the results compared;
  • The limitations of each method should be reviewed;
  • Scour depths resulting from any analysis should be compared with soil stratigraphy at that depth; and
  • Selection of the method most suited to a particular site requires experience and judgement; the results from various methods may vary.

Field Measurements

Scour may be determined from field measurements such as probing affected areas, surveying the streambed and underwater sounding. Despite extensive research and development, methods for measuring scour in the field are not exact.

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