TR55 Presentation

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Soil Conservation Service Runoff

Equation

S I P

I P

Q a

a

)(

)( 2

whereQ = runoff (in.)

P = rainfall (in.)

Ia = initial abstraction (in.)

S = potential maximum retention after runoff begins (in.)



Initial Abstraction, Ia

S I a 2.0

Ia is all losses before runoff begins. It includes water retained insurface depressions, water intercepted by vegetation, evaporation,

and infiltration. Ia is highly variable but generally is correlated with

soil and cover parameters.



Soil Conservation Service Runoff

Equation becomes:

)8.0(

)2.0( 2

S P

S P

Q

101000 CN

S

where CN = curve number



Curve Numbers

The major factors that determine CN are the hydrologic soil

group (HSG), cover type, treatment, hydrologic condition,

and antecedent runoff condition (ARC). Another factorconsidered is whether impervious areas outlet directly to the

drainage system (connected) or whether the flow spreads

over pervious areas before entering the drainage system

(unconnected).



Group A soils have low runoff potential and high infiltration rates even when thoroughly

wetted. They consist chiefly of deep, well to excessively drained sand or gravel and have a highrate of water transmission (greater than 0.30 in/hr). Soil Texture: Sand, loamy sand, or sandy

loam

Group B soils have moderate infiltration rates when thoroughly wetted and consist chiefly of

moderately deep to deep, moderately well to well drained soils with moderately fine to

moderately coarse textures. These soils have a moderate rate of water transmission (0.15-0.30in/hr). Soil Texture: Sil t loam or loam

Group C soils have low infiltration rates when thoroughly wetted and consist chiefly of soils

with a layer that impedes downward movement of water and soils with moderately fine to fine

texture. These soils have a low rate of water transmission (0.05-0.15 in/hr). Soil Texture:

Sandy clay loam

Group D soils have high runoff potential. They have very low infiltration rates when

thoroughly wetted and consist chiefly of clay soils with a high swelling potential, soils with a

permanent high water table, soils with a claypan or clay layer at or near the surface, and shallow

soils over nearly impervious material. These soils have a very low rate of water transmission (0-

0.05 in/hr). Soil Texture: Clay loam, silty clay loam, sandy clay, si lty clay, or clay

Hydrologic Soil Groups



101000 CN

S

.28.3

))33.3(8.00.6(

))33.3(2.00.6(

)8.0(

)2.0( 22

in

S P

S P Q

.33.31075

1000 inS

.83.47)()175(.)28.3(.12

.1 ft acre xacres xinV in

ft



Time of Concentration and Travel Time

• Travel Time (Tt) is the time it takes water to travel

from one location to another in a watershed. It is

typically a component of Tc.

• Time of Concentration (Tc) is the time for runoff totravel from the hydraulically most distance point of

the watershed to a point of interest in the watershed.



Factors Affecting Tt and Tc

• Surface roughness

• Channel shape and flow patterns

• Slope



Water Movers through a Watershed as:

• Sheet flow

• Shallow concentrated flow

• Open channel flow, or

• A combination of these.

V

LT t

3600

where:

Tt = travel time (hr)

L = flow length (ft)

V = average velocity (ft/s)

...4321 T T T T T T t c



Sheet Flow

• Shallow flow depth (< 0.1 ft) over plane surfaces

• Only for flows up to 300 feet

4.05.0

2

8.0

)()(007.0

s P nLT t

where:


n = manning’s roughness coefficient (table 3-1)L = flow length (ft)

P2 = 2-year, 24-hour rainfall (in)

s = slope of hydraulic grade line (land slope, ft/ft)



Shallow Concentrated Flow

• After a maximum of 300 feet, sheet flow usually

becomes shallow concentrated flow.

V LT t

3600

where:


L = flow length (ft)

V = average velocity from Figure 3-1 (ft/s)



Open Channel Flow

• Based upon Manning’s Equation

n

sr V

2/13/249.1

where:V = average velocity (ft/s)

r = hydraulic radius (ft) and is equal to area/wetted perimeter

s = channel slope (ft/ft)

n = Manning’s roughness coefficient for open channel flow

V

LT t

3600

Then plug V and L

into this equation:



Example

Segment AB: Sheet flow; dense grass; slope (s) =0.01 ft/ft; and length (L) = 100 ft.

Segment BC: Shallow concentrated flow; unpaved;

s = 0.01 ft/ft; and L = 1,400 ft.

Segment CD: Channel flow; Manning’s n = .05;

flow area (a) = 27 ft2; wetted perimeter (pw) =

28.2 ft; s = 0.005 ft/ft; and L = 7,300 ft.



Segment AB:

Sheet flow; dense grass; slope (s) = 0.01 ft/ft; and length

(L) = 100 ft.



Segment BC:

Shallow concentrated flow; unpaved; s = 0.01 ft/ft;

and L = 1,400 ft.



Segment CD:Channel flow; Manning’s n = .05; flow area (a) = 27 ft2;

wetted perimeter (pw) = 28.2 ft; s = 0.005 ft/ft; and L =

7,300 ft.



Graphical Peak Discharge Method

• The peak discharge equations used is:

pmu p QF Aqq

where:

q p = peak discharge (cfs)

qu = unit peak discharge (csm/in)

Am = drainage area (mi2)

Q = runoff (in)

F p

= pond and swamp adjustment factor



Rational Method(used for areas of less than 50 acres)

Q = C i A where:

Q = Peak rate of runoff in cubic feet per second

C = Runoff coefficient, an empirical coefficientrepresenting a relationship between rainfall

and runoff

i = Average intensity of rainfall for the time ofconcentration (Tc) for a selected design storm

A = Drainage area in acres



Lets convert In-Ac/Hr to CFS:

[(1ft / 12 In) x (43,560 ft2 / Ac)] / 3600 Sec / Hr =

1.008 CFS / In-Ac / Hr

The degree of accuracy doesnot warrant a units conversion

factor (constant) of

1.008!!

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