WETLANDS - Water Quality Engineering in Natural Systems - page 295

particles removed by the wetland, and is defined by the
relation

Q
A

(8.2)

q =

where Q is the average flow rate through the wetland
(L 3 T −1 ), and A is the surface area of the wetland (L 2 ).
Loading rates for domestic wastewater are typically in
the range of 0.7-5 cm/d (0.3-2 in/d).

Detention Time. The detention time , τ (T), measures the
characteristic travel time through the wetland, and is
defined by

τ= Vn

Q

(8.3)

Figure 8.11. Wetland outlet weir. Source : Emmons and Oliver
resources, Inc. (2005).

where V is the volume of water in the wetland (L 3 ), n is
the porosity of the wetland (dimensionless), and Q is
the average flow rate through the wetland. The porosity,
n , is defined by the relation

and deepwater areas devoid of emerging vegetation
offer habitats for fish (e.g., Gambusia affinis, , the
mosquito-eating fish). Fluctuating water levels can
provide needed oxidation of organic sediments and
can, in some cases, rejuvenate a system to higher levels
of chemical retention. Water levels are typically con-
trolled by inflow and outflow structures, such as feed
pumps, gates, and weirs. A typical outflow weir pro-
tected by a trash rack is shown in Figure 8.11. During
the startup period, low water levels are needed to avoid
flooding newly emerged plants. Startup periods for the
establishment of plants may take 2-3 years, and the
development of an adequate litter sediment compart-
ment may take another 2-3 years (Mitsch and
Gosselink, 2007).

n = water volume
total volume

(8.4)

where n = 0.9-1 for FWS wetlands, depending on the
growth density of vegetation. In cases where the wetland
inflow and outflow rates are different, such as might be
caused by significant precipitation and/or ET during the
travel time, the water recovery fraction, R (dimension-
less), is defined by

Q
Q

o

R = (8.5)

i

where Q o is the outflow rate (L3T−1) 3 T −1 ) and Q i is the
inflow rate (L3T−1). 3 T −1 ). F or cases in which 0.5 < R < 2, the
average flow rate, Q , used in calculating the detention
time, τ , yields an error of less than 4% in τ (Kadlec
and Wallace, 2009). For R < 0.5 or R > 2, the detention
time, τ , can be estimated using the relation (Chazarenc
et al., 2003)

Seasonal Pulses. Storms and seasonal patterns of
floods can significantly affect the performance of wet-
lands designed for the control of nonpoint surface
runoff. Highest nutrient loadings from agricultural
sources occur during the first storms after fertilizer
application, and a good wetland design should take
advantage of these pulses for system replenishment,
and provide for excess wet-weather storage if nutrient
retention is a primary objective. Infrequent floods
and droughts are important for dispersing biological
species to the wetland and adjusting resident species
composition.

ln( )
1

R

(8.6)

τ τ =

i

R −

where τ i is the detention time (T) calculated using the
inflow rate.
The optimum detention time is in the range of 5-14
days for treatment of municipal wastewater. The deten-
tion time is sometimes called the retention time ; however,

Hydraulic Loading Rate. The hydraulic loading rate , q ,
measures the characteristic sedimentation velocity of