Particles

Particles

Up to 5 particle fractions can be specified for each case. Input columns with blank particle name values are ignored.

Particle parameters are specified on the 'Edit Particles' screen.

To delete a particle fraction, just delete the particle name. All other particle fractions will be shifted. At least one particle fraction must be defined.

See discussion of reading/saving particle data .

Default particle files calibrated are provided (see below). These can be used in a design context with the assumption that site particle characteristics are similar to those measured under the Nationwide Urban Runoff program. Site-specific particle files can be developed if sufficient data are available.

See sample input files.

Accumulation/Washoff Parameters for Impervious Surfaces

Accumulation Rate - buildup of particles on impervious surfaces
Decay Rate - removal via non-runoff processe
Washoff Coefficient - used to compute washoff = SWMM "RCOEFX"
Washoff Exponent - used to compute washoff = SWMM "WASHPO"
Sweeper Efficiency - % removed in one pass of street sweeper Imperv.
Runoff Conc - in addition to accumulation/washoff

Pervious Runoff Parameters

Concentration - Runoff Conc (ppm) at Runoff Intensity of 1 in/hr
Exponent - Slope of Log(Conc) vs. Log(Intensity) relationship

Particle Class Parameters

Settling Velocity - rate of sedimentation in treatment device
First-Order Decay - rate of decay via first-order processes
2nd-Order Decay - rate of decay via second-order processes
Filtration Efficiency - % of particles removed from infiltrating flows

Typical Particle Settling Velocities

Urban Runoff (used in original calibration, 50th percentiles)

Percentile
50th Percentile

Rate (ft/hr)
Particle Class Percent of TSS

0 0.0 P0% (dissolved) 0%

10 0.03 P10% 20%

30 0.30 P30% 20%

50 1.50 P50% 20%

80 15.0 P80% 40%

Source: Driscoll (1983), USEPA(1986), based upon nationwide NURP data

Settling Velocity vs. Soil Type

Soil Type Rate(ft/hr)

Fine Silt             1.5
Medium Silt         5.5
Coarse Silt          7.2
Fine Sands          360
Medium Sands    1440

Source: McCuen (1982)

See Sample Input Files for description of other particle files distributed with the program.

Figure 26 of model documentation for the DOS version of P8 relates settling velocity to particle diameter & specific gravity, based upon Stokes Law.

Initial Calibrations (Walker, 1990)

Washoff parameters for particle fractions P10% - P80% are contained in particle files NURP50.P8P, NURP90.P8C, & SIMPLE.P8C have been calibrated as follows:

Accumulation Rate = 1.75 lbs/ac-day (P10%,P30%,P50%), = 3.5 lbs/ac-day (P80%) calibrated to provide median EMC = 100 ppm Total Susp. Solids ; using Providence Airport weather data.
Accum. Decay Rate = .25 1/day; assumes buildup on impervious surfaces reaches 90% of steady-state after 10 days of dry weather without sweeping
Washoff Exponent = 2; provides intensity-dependent washoff, as in SWMM (Huber et al., 1988)
Washoff Coefficient = 20 calibrated so that load/volume relationship for impervious watersheds saturates at ~1 inch of rainfall; provides 92% washoff for a 1-inch, 8-hour storm.
Impervious Runoff Conc = 0; buildup/washoff dynamics are used to predict impervious runoff conc.

Pervious runoff concentration parameters contained in particle files NURP50.P8C, NURP90.P8C, & SIMPLE.P8C have been calibrated as follows:

Model:
CONC = a RUNOFF ^b

Variables:
CONC = concentration in pervious runoff (ppm)
RUNOFF = runoff intensity from pervious areas (inches/hr)

Parameters:

a = intercept = conc @ runoff intensity of 1 in/hr = 100 ppm; calibrated so that flow-weighted mean TSS EMC from pervious watersheds = 100 ppm; calibration period = 1983-1987; Curve Number = 74; Providence Rhode Island Rainfall.

b = exponent = 1; linear log(c) vs. log(q) relationship; typical of stream sediment rating curves (Huber & Dikinson,1988)

Percentile	50th Percentile Rate (ft/hr)	Particle Class	Percent of TSS
0	0.0	P0% (dissolved)	0%
10	0.03	P10%	20%
30	0.30	P30%	20%
50	1.50	P50%	20%
80	15.0	P80%	40%