Chapter 3 SCS Method

CONVERTING RAINFALL TO RUNOFF Runoff = Rainfall – Abstractions Q = P - I

Calculating Runoff Abstractions From Precip Surface storage Infiltration Evapotranspiration Bank Storage Interception

Surface Storage Accounts for what is stored on surface and does not contribute to runoff Also have surface detention due to roughness, but an outlet exists to get the flow to the watershed exit. Detention contributes to travel time or time lag, not to surface storage.

Surface Storage Typical Values

Infiltration Water that flows through the surface into the soil matrix Very important component of the hydrologic cycle Can be modeled with empirical or theoretical relationships Most widely used is empirical methods

General Abstraction Model NRCS Curve Number Q P IaIa S

CN = fctn(soil type, land use, antecedent moisture)

A soil can have different CNs as a result of: High Water Table Impervious Layers

CN = fctn(soil type, land use, antecedent moisture)

Unit hydrographs definition Unit hydrographs and runoff hydrographs A unit hydrograph is the runoff hydrograph from one inch of rainfall excess falling at a constant rate during a duration D distributed uniformly over the watershed.

Unit hydrographs terminology Unit hydrographs and runoff hydrographs D=Duration of Rainfall Excess t L = Lag Time t p =Time to Peak t p =Time Base q p =Peak Discharge V=Volume = 1 in.

Rainfall Excess V = 0.75 in Intensity Time Flow Rate p q Unithydrograph V = 1in q0.75 p Runoffhydrograph V = 0.75in Unit hydrographs and runoff hydrographs Unit hydrographs

Unit hydrographs and runoff hydrographs Runoff hydrographs 0.2*UnithydrographV = 0.2 in 0.35*UnithydrographV = 0.35in0.15*UnithydrographV = 0.15 in UnithydrographV = 1in Runoffhydrograph V = 0.7in Intensity Time Flow Rate Total Rainfall Excess V = 0.7in DD DD

Time of Concentration –Most often used timing parameter –Time for water to flow from most hydraulically remote point to watershed outlet. Unit hydrograph timing parameters Unit hydrographs and runoff hydrographs L i = length of flow path i V i = velocity in flow path i

Lag Time –Frequently used on larger watersheds –Time from midpoint of rainfall excess to peak discharge of unit hydrograph –NRCS Equation Unit hydrograph timing parameters Unit hydrographs and runoff hydrographs Lag Time

q p = q u AQF p –q u unit peak discharge (Figure 3.40) csm/in –A = area in square miles –Q = runoff volume in inches –F = Pond and swamp adjustment factor (Table 3.25) Not quite as simple as Rational Finding widespread use TR55 Peak Discharge

Example Go through the hydrology for the Edmonds property Look at spreadsheets for SCS storm and peak flow