Input data can easily be modified for sensitivity analysis. The program may be used for design, reviewing calculations and comparing results. Applications include: bridges, culverts, storm sewers, dams, and flood plain analysis.

Peak Flow Calculation Methods

Environment Canada Index Methods

Index equations determine peak flows for two(2) year return periods. Index equations are included for both instantaneous and daily peak flows. The daily flows and are converted to instantaneous peak flows by Fuller's formulae. Flows for other return periods are produced from conversion curves. The equations and conversion factors are unique to each geographical region. Converted daily flows only apply to Southern Ontario.

General Form of Equation:

Q₂=C.Aⁿ

Q₂ = 2 year return period flow in m3/s, A = drainage area in km2, C = constant, n = exponent

Environment Canada Regional Equations

Two sets of equations are included for each return period and geographical area. Not all watershed parameters are needed, depending on the geographical area. The Base Flow Index parameter only applies to southern and central Ontario.

All results include an expected probability adjustment.

General Form of Equation:

log(Q_T) = a₀+a₁.log(A)+a₂.(BFI)^1/2+a₃.(slp)^1/3+a₄.(ACLS)^1/2+a₅.(slp)+a₆.log(MAR) +a₇.(MAR)+a₈.log(ACLS+1)+a₉.(MAP)+a₁₀.(shp)

Q_T = return period flood, a₀, a₁, etc., are constants; BFI, MAR, ACLS, MAP are watershed parameters as defined elsewhere; slp = slope (m/km); shp = shape factor (L²/A), L = length of main channel (km), A= watershed area (km²)

Ministry of Natural Resources Regional Equations

Equations are included for various return periods and geographical area. Results are provided by direct computation of instantaneous peak flows and by converting instantaneous peak flows from daily flows using a peaking equation.

All flows results include an expected probability adjustment

General Form of Equation:

Q_T = a₀.A^a₁.(MAR).^a₂(ACLS).^a₃.s^a₄

Q_T = return period flood; a₀ , a₁, etc., are constants; MAR, ACLS are watershed parameters defined elsewhere; s = slope (m/m), A= watershed area (km2)

The peaking factor to convert daily flows to instantaneous peak flows is:

PF = 1+a₀.A^a₁.(ACLS) ^a₂.s^a₄(MAS)^a₅

PF = peaking factor, MAS = Mean Annual Snowmelt

Ministry of Transportation Modified Watershed Index Method

This method uses an index equation to calculate the 25 year return period flow. The equation uses a watershed index number. The watershed index number is based on SCS (Soils Conservation Service) Curve Numbers (#) for southern Ontario and on the storage percentage (%) for the Canadian Shield. The watershed index for southern Ontario is adjusted for shape, storage, precipitation index, and slope. Two equations are included for Canadian Shield areas. One for storage areas that are predominantly lakes and one for storage areas comprising of both lakes and swamps.

Flows for other return periods are determined from conversion factors.

General Form of Equation:

Q₂₅ = C.A^0.75

Where Q₂₅ is the 25 year return period flood, C is the watershed index (constant), and A is the watershed area in km².

Ministry of Transportation Preliminary Equations

Equations for southern Ontario and for Canadian Shield areas are included for each return period. The 25 year return period is used instead of the 20 year return period.

General Form of Equations:

Southern Ontario

Q25 = a₀.A^a₁.CN^a₂.st^a₃.s^a₄

Canadian Shield

Q₂₅ = a₀A^a₁.st^a₂

Where Q₂₅ is the 25 year return period flood, A is the watershed area in km², CN is the SCS curve number, st is watershed storage, %, s is channel slope in m/m, and a₀, a₁, etc., are coefficients

Rational Method

The rational method uses average rainfall intensities for each return period based on rainfall intensity curves. The user can select from 112 curves depending on the location. The duration for determining the average rainfall intensity is the time of concentration (tc) for the watershed.

The Bransby Williams equation is used for calculating times of concentration where the runoff coefficient is greater or equal to 0.4. The Airport formula is used for calculating times of concentration for lower runoff coefficients.

Parameters - needed to use this method: Area (km^2), Length (km), Slope (m/km), and Runoff Coefficient, Storage (%) is optional and may be specified to modify results.

Usual form of equation

q = 0.0028.A.I.C

Where q = flow (m3/s), A = area (ha), I = rainfall intensity (mm/h), C = Runoff Coefficient

Hydrograph Calculations

The user may select a triangular shaped hydrograph or the "Hymo" hydrograph. The triangular hydrograph rising limb, time to peak (t_p) is calculated as 0.6t_c + 0.5 hours. Where t_c is the time of concentration by the Bransby Williams formula.

The Hymo hydrograph has a rising limb shape based on a gamma function. The recession limb has two parts, both parts using exponential decay functions. The recession constant for the second part of the recession limb is double the recession constant for the first part (2k versus k ) producing a more gradual drawdown.

The peak flow is generated by summing individual storm runoff hydrographs. Various storms may be selected and the runoff is determined by SCS (Soil Conservation Service) loss procedures.