WATERSHEDS - Water Quality Engineering in Natural Systems - page 211

1320 mm, and it is estimated that 50% of the annual
rainfall does not produce any runoff. Assuming that the
EMC distribution of suspended solids is normal, esti-
mate the median and 90th percentile load of suspended
solids contained in the runoff.

The EMC approach to estimating pollutant loads in
urban runoff should be used with caution, since storm
drainage systems in some urban watersheds contain dry
weather flow that does not originate from rainfall runoff
and yet accounts for a significant portion of the annual
pollutant load discharged by the system. The dry weather
flow in a storm drainage system typically originates
from such sources as groundwater inflow, permitted dis-
charges, illicit connections, excess irrigation, automobile
washing, and other residential and commercial uses.
In contrast to using only Equation (6.2) to estimate
pollutant loadings in urban runoff, a composite mass
loading approach can be utilized to predict the contami-
nant load in urban runoff. The following example illus-
trates this approach.

Solution

From the NURP data in Table 6.3, the median sus-
pended solids (SS) concentration in runoff from a resi-
dential area is 101 mg/L, and the coefficient of variation
is 0.96. Since the suspended solids EMC distribution is
normal, the mean, µ , and standard deviation, σ , of the
EMC are given by

µ= 101mg/L

EXAMPLE 6.2

σ= 0 96 101 . ( mg/L ) = 97 mg/L

A watershed contains a variety of land uses and soils,
and the pollutant loadings for total phosphorus are as
follows:

The 90th percentile frequency factor for a normal dis-
tribution is obtained from Appendix C as 1.28, and
hence the 90th percentile EMC for suspended solids,
EMC 90 , is given by

EMC = + µ 1 28 . σ = 101 mg/L + 1 28 97 . ( mg/L ) Total P

(kg/ha/year)

90

= 225 mg/L

Soil Group

For an imperviousness of 40%, the runoff coefficient,
C , can be estimated from Figure 6.3 as 0.34. From the
data given, P o = 1320 mm, P j = 0.50, and hence the
effective annual precipitation, P , is given by Equation
(6.6) as

Land Use A B C D

Open space 0.09 0.09 0.09 0.09

Meadow 0.09 0.09 0.09 0.09

Newly graded 3.47 5.26 6.27 7.39

Forest 0.09 0.09 0.09 0.09

Commercial 1.79 1.79 1.79 1.79

P P P j = = ( 1320 0 50 )( . ) = 660 mm .

o

Industrial 1.46 1.51 1.51 1.46

Residential
0.05 ha or less

Equation (6.2) gives the median (= mean) annual load
of suspended solids as

1.79 1.79 1.90 1.96

0.10-0.13 ha 1.01 1.23 1.23 1.23

0.2-0.4 ha 0.78 1.01 1.06 1.06

load kg/ha ( ) = 0 01

0 01 0 34

.
.

C P × × EMC

0.8-1.6 ha 0.27 0.37 0.37 0.37

Smooth surface 2.24 2.24 2.24 2.24

= ( . ) × ( 660 ) × ( 101 )

= 227 kg/ha

and the 90th percentile load as

The watershed area is predominantly Type B soil and
consists of 24.3 ha of residential land with 0.4-ha lots,
8.9 ha of commercial area, 4.1 ha of industrial area, and
2.0 ha of open space. Estimate the annual loading of TP
at the catchment outlet.

load kg/ha ( ) = 0 01

0 01 0 34

.
.

C P × × EMC

90

= ( . ) × ( 660 ) × ( 225 )

= 505 kg/ha

These estimated pollutant load statistics reflect a typical
urban watershed and could be significantly different,
depending on local conditions, such as the amount of
ongoing construction within the area.

Solution

Using the given data, the annual phosphorus loading
from each type of area in the watershed is calculated