Abstraction

Abstraction is the term used to describe the amount of rainfall that doesn’t turn into runoff. There are numerous processes that intercept rainfall. Some rainfall never even reaches the ground, instead being intercepted by leaves on trees. Some rainfall is evaporated back into the atmosphere. Of the rainfall that reaches the ground, some is infiltrated and some just ponds with no outlet.

There are several methods (and lots of equations) for accounting the precipitation than never becomes runoff. StormShed3G™ does not exhaustively account for all runoff. In particular, the program does not address evapo-transpiration.

Curve Number Approach

One of the most common methods of dealing with precipitation losses is the SCS curve number approach. The SCS methodology is popular because it combines infiltration losses with initial abstraction based on the following relationship:

Where

• Q is the accumulated runoff volume
• P is the accumulated precipitation
• S is the maximum soil water retention parameter

S is defined as:

Where CN is the SCS curve number and S is in inches. Based on the above equation, P must exceed 0.2S before any runoff is generated. Incidentally, the 0.2 in the above equation is known as the abstraction coefficient. Many programs, including StormShed3G™ allows for its modification, however, we have not seen any guidance on how to modify the coefficient. In the SCS method, the abstraction coefficient is not limited to a single loss type, but is a catchall for evaporation, interception, infiltration, and surface depression. For the case where the abstraction coefficient is 0.2, the SCS has created a simple chart to quickly determine the amount of runoff in inches based on the site CN number.

Figure 3: Solution to Runoff Equation (TR-55 June 1986)

The SCS has classified most soils into four (4) hydrologic soils groups. The groups are defined are:

• HSG A (low runoff potential). These soils are characterized by high infiltration rates even when thoroughly wetted and consists of deep, well to excessively drained sands or gravels.
• HSG B. These soils have a moderate infiltration rate when thoroughly wetted. They are moderately deep to deep, moderately well to well drained with moderately fine to moderately coarse texture.
• HSG C. These soils have slow infiltration rates when thoroughly wetted. They typically consist of soils with a layer that impeded downward movement or soils with moderately fine to fine texture.
• HSG D (high runoff potential). These soils have a very slow infiltration rate when thoroughly wetted. They consist mostly of clay with high swelling potential. These soils also have high water tables, or are soils with a clay pan or clay layer at or near the surface and shallow soils over nearly impervious material.

The SCS also considers the antecedent moisture condition (AMC) of the soil. The AMC is defined as the amount of rainfall in a period of five to thirty days preceding the design event.

• AMC I – Soils are dry, but not to the wilting point.
• AMC II – Average case.
• AMC III – Heavy or light rainfall and low temperatures having occurred during the previous five days.

Generally, AMC III is used for most detention design application. StormShed3G™ has a database of all the SCS soils types. Simply enter the soils name and the program will return its HSG classification. It should be noted that when the CN number that is entered in StormShed3G™ is the CN number for AMC II. When the program computes the runoff hydrograph, the CN numbers are adjusted for the AMC condition that is specified. The adjustment is made based on the following equations:

There is no adjustment for AMC II.

While the SCS provides the HSG for various soils, there must be a mechanism of converting the HSG to the Curve Number (CN) values that can be used in design. Table 9.1 of the National Engineering Handbook, Section 4 (NEH-4) provides the relationship between land uses and CN. The problem with the table is that the land uses are Fallow, Row Crops, Small Grain, Close-seeded legumes or rotation meadow, Pasture or range, and roads. For the most part, these land use types are of little use for today’s current engineering projects.

Many Municipalities have created their own land use tables. The following is one adopted by Washington State.

SCS Western Washington Runoff Curve Numbers (Published by SCS 1982)
Runoff curve numbers for selected agricultural, suburban and urban land use Type 1A rainfall distribution, 24- hour storm duration
Land Use Description		Curve Numbers by Hydrologic Soil Group
		A	B	C	D
Cultivated land (NEH4 Chapter 9, 1972)	winter condition	86	91	94	95
Mountain open areas	low growing brush and grasslands	74	82	89	92
Meadow or pasture		65	78	85	89
Wood or forest land	undisturbed or older second growth	42	64	76	81
	young second growth	55	72	81	86
Orchard	with cover crop	81	88	92	94
Open spaces, lawns parks, golf courses, cemeteries, landscaping	grass cover on 75% or more of the area	68	80	86	90
	grass cover on 50% to 75% or more of the area	77	85	90	92
Gravel roads and parking lots		76	85	89	91
Dirt roads and parking lots		72	82	87	89
Impervious surfaces, pavements, roofs, etc.		98	98	98	98
Open water bodies	lakes, wetlands, ponds, etc.	100	100	100	100
Single Family Residential (assumes roof and driveway runoff is directed into street/storm system)
Dwelling unit/gross Acre	% Impervious
1.0	15	Separate curve number shall be selected for pervious and impervious portion of the site or basin.
1.5	20
2.0	25
2.5	30
3.0	34
3.5	38
4.0	42
4.5	46
5.0	48
5.5	50
6.0	52
6.5	54
7.0	56
Planned unit developments, condominiums, apartments, commercial business and industrial areas.	% impervious must be computed

StormShed3G™ provides built-in lookup tables that will automatically lookup the CN value based on the land use. The table is fully customizable.