The SCS Runoff Method is described in the SCS National Engineering Handbook – Section 4 (NEH-4), with the input attributes described below.
Pervious Curve Number – The runoff curve number for the pervious fraction of the Catchment, as described in NEH-4 Chapter 9. Typical values vary from 20 for regions with high infiltration and interception capacities to 98 for impervious areas. It is a dimensionless number depending on hydrologic soil group, cover type, treatment, hydrological condition and antecedent moisture conditions. This number has a valid range from 0 to 100, with typical values from 60 to 90 and 98 for impervious surfaces.
When the Use Land Uses/Soil Types layers box is ticked, the Pervious Curve Number is calculated from weighted averages of Land Uses and Soil Types.
Pervious and Impervious Curve Number
The Pervious CN characterises the fraction of the Catchment that is not impervious (area derived from the Percentage Impervious value). The impervious fraction of the Catchment is assigned a default CN of 98.
The Curve Number Calculator can be used when the Use Land Uses/Soil Types layers box is unticked. The calculator can be accessed using the button to the right of the field. This provides a quick and easy way to select the appropriate curve number.
Time of Concentration – The time of concentration as described in NEH-4 Chapter 15 (in mins) is defined in two ways; the time for runoff to travel from the furthermost point of the watershed to the point in question, and the time from the end of excess rainfall to the point of inflection on the trailing limb of the unit hydrograph. Tc can be estimated from several formulas, such as Kinematic-wave. For a constant excess rainfall can be described as:
Tc=C(n^0.6 L^0.6/i^0.4 S^0.3)
in which L is the distance from the upper end of the plane to the point of interest (usually the inlet) n is the Manning resistance coefficient, i is the excess rainfall rate, S is the dimensionless slope of the surface and C is a constant that depends on units of the other variables. For tc in minutes, i in inches/hr and L in feet C equals 0.938. For tc in minutes i in mm/hr and L in meters C equals 6.99.
Another formula for determining tc is the lag equation
Tc = L/0.6 where L= l^0.8(S+1)^0.7/1900Y^0.5
Tc = time of concentration in hours
L = lag time in hours
I = hydraulic length of the watershed in feet
Y = Average land slope in per cent
S = Potential maximum retention in inches
CN = weighted Curve Number
For ease, the Time Of Concentration Calculator can be accessed using the button to the right of the field. This allows a set of popular methods to be used to determine the Time of Concentration.
Shape Type – Select from the available shape types from the drop-down list.
Shape Factor – The shape factor or peak rate factor as defined in NEH-4 Chapter 16. The typical value is 484 for a hydrograph where the volume under the falling side of the triangular unit hydrograph is equal to 1.67 times the volume under the rising limb of the curvilinear unit hydrograph. Actual values may vary from 100 in very flat swampy country to 645 in steep rocky terrain.
The Soil Conservation Service has determined the hydrograph shape factor to be 484 for most watersheds. This was the result of analysing many watersheds of various sizes and geographic locations. It is used in the formulation of peak discharge and the peak of the unit hydrograph:
Qp=484A/tp
in which Qp is the peak discharge in cfs, A is the drainage area mi^2 and tp is the time to peak in hr. Several other values have been used for shape factor within Florida and other areas of the country for wetlands, sandy soils and steep terrain. Typical values in Florida include 323 or 256, and values as low as 100. Effectively, the hydrograph shape factor will lengthen the time base of the unit hydrograph and lower the peak. If you are using real rainfall data, this parameter can be used for calibration. For example, the falling limb time base of the hydrograph will be doubled when using a shape factor of 300 instead of 484.
A curvilinear or triangular shape may be chosen by selecting the appropriate button. Both shapes are supported for the entire range of hydrograph shape factors.
Initial Abstraction – The initial abstraction from the precipitation may be represented as an absolute number. That is the total depth of precipitation that is less (in mm or inches) or as a fraction of the amount of precipitation (between 0 to 1).
Initial Abstraction, Ia, contains all the loss terms prior to beginning of runoff. Although it was found to be highly variable for many small agricultural watersheds, it was found that
Ia = 0.2 × S
Where S = (1000/CN)–10
S is the potential maximum abstraction.
Percent Impervious – Pervious portion of the Inflow area. When the Use Land Uses/Soil Types layers box is ticked, the Percentage Impervious is calculated from weighted averages of Land Uses.
Composite Curve Number – The weighted CN used by the simulation. It is calculated as follows:
CN = (PIMP / 100 * 98) + ((1 – (PIMP / 100)) * PACN)
where:
PIMP = Per cent Impervious
PACN = Pervious Area Curve Number
Urban Creep – This scales the impermeable area by the amount specified. This can be used to take the increase of urban areas, or other factors, into account. This value is only enabled (and used) when the analysis creep of the Analysis Criteria is set to "Use Catchment Areas".
References
James 1998, Editor "Modelling the Management of Stormwater Impacts" Volume 6, Chapter 23, Authors: Ashok Pandit and Joanie Regan, "What is the Impervious Area Curve Number". Publisher: CHI Guelph, Ontario, Canada.
Viessman W. et al 1977, "Introduction to Hydrology", Harper & Row Publishers, New York
USDA National Resources Conservation Service, 1986, Urban Hydrology for Small Watersheds, Technical Release 55.
WEF 1992, Manual of Practice FD-20 "Design and Construction of Urban Stormwater Management Systems" ASCE and WEF ISBN 0-87262-855-8