FLOOD01419

• County: Statewide
• Title: Colorado Association of Storm Water and Floodplain Managers 7th Annual Conference
• Date: 9/18/1996
• Prepared For: State of Colorado
• Prepared By: CASFM
• Floodplain - Doc Type: Educational/Technical/Reference Information

In order to estimate the total surface area of all suspended particles in a given volume of stormwaler, it was
assumed that all such particles are spherical. This allowed for calculation of a particle's surface area based
on the diameter of thai unique particle.

Figure 4. South Walnut Creek TSS Particle Size Distribution (Station SW023)

100
90
~
;; 80
..
:ii
1::
" 70
0-
e
..
.l:
0> 80
e
"
S
~ 50
OJ
E
" 40
"
"
"
E
~ 30
C
..
l: 20
..
Q.
10

2000

4000

6000

BOOO

10000

12000

14000

16000

180CO

2CCOO

Particle Diameter (1 x 10" m)

. Assumption: Total suspended sediment particles are spherical.

Building on this assumption. the surface area.to volume ratio can be calculated for all particle sizes.
Assuming that the density of the various size particles is equal, the surface area to volume ratio represents a
normalized surface area to mass ratio. The particle size distribution, based on mass, can then be ca~ve",d
to a distribution of total surface area of all particles. Because the smaller diameter particles have a larger
surface area to mass ratio than do the larger particles (based on the assumption of spherical pa:::c:e
geometry) the majority of the surface area. of all the suspended particles combined. is located on the sm.2.ilec
diameter particles. A surface area distribution versus particle diameter relationship was developed and
plotted for suspended particles collected at Station SW023.

Particle Settling Velocity

The settling velocity of a particle in water is commonly calculated using Stokes' Law, which relates the
settling velocity as proportional to the particle's density and the square of the particle's cross-sectionai
diameter (Thomann, 1987). Other research has found particle settling velocities to be an order of
magnitude higher, or more rapid, than Stokes' Law (Thomann, 1987). Yet another study developed a m.ting
curve where settling velocities were nearly one order of magnitude lower, or slower, than the Stokes' Law
rate (Reynolds, 1982). In order to use the most conservative approach, the latter of these studies wa.:; used
to determine settling velocities for different sizes of particles. By choosing the most conservative settling