JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION VOL. 35, NO.2
AMERICAN WATER RESOURCES ASSOCIATION
APRIL 1999
HYDROLOGIC DESIGN CONSIDERATIONS OF CONSTRUCTED WETLANDS FOR URBAN STORMWATER RUNOFF' Tonja Koob, Michael E. Barber, and Wade E. Hat hhorn2
1989). The popularity of wetlands can be traced to their low maintenance requirements. Unfortunately,
ABSTRACT: The successful design of constructed wetlands requires a continuous supply of water or vegetation that can withstand drought conditions. Having a constant water source is the best alternative to insure species diversity throughout the season. Consequently, detention structure designs should be based on times
robust design criteria have been slow in coming due to
the inherent variability in governing factors such as hydraulic and environmental loading rates, temperature, vegetation, fauna, exposure and pollutant types (Kadlec and Knight, 1996). Moreover, this lack of
between events as well as on hydrologic return periods, since between events is when most evaporation and infiltration losses are likely to occur. In arid or semi-arid environments, this is a difficult process because of long interevent times and seasonal changes in precipitation patterns. This discussion is predicated on the assumption that phytoplankton, epiphytic algae, and emergent vegetation require moist conditions to be effective at removing nutrients, metals and other pollutants. There are drought tolerant species of veg-
understanding has led to oversimplification and
extrapolation of limited amounts of data to "similar" situations (Baker, 1993; Kadlec and Alford, 1989; Knight, 1990). Several attempts have been made to
etation that can be used in constructed wetlands but it may take several days to re-establish the attached bacteria communities necessary for optimum pollutant removal. This paper examines a stochastic framework to examine the probability of extended dry periods based on historic rainfall data. The number of consecutive
transfer wastewater treatment wetland criteria to wetlands for stormwater runoff. These approaches, however, fail to recognize the startling differences between the two systems in terms of hydrologic uncer-
tainty, especially in semi-arid climate regions or in areas that experience dramatic differences in seasonal weather patterns such as southern California or
dry days is selected for a specified level of assurance. By multiplying this value by the sum of daily system losses, an overall pond volume can be determined that ensures a minimum depth of water. To illustrate the utility of the approach, the method is applied to a site in Spokane, Washington. (KEY TERMS: best management practices; urban runoff; pollutant removal; stochastic; cumulative density function.)
the Pacific Northwest.
Much of this uncertainty lies in the prediction of design flows and the variability of daily runoff quantities. Unlike wetland systems developed for municipal
wastewater treatment which only must be designed for daily fluctuations, wetlands constructed to treat stormwater from urban sources (such as highways, roofs and roads) are subject to flow rates that are much more variable. In fact, the stormwater wetland may be exposed to extended periods without inflows.
INTRODUCTION
A number of textbooks and articles have document-
ed the success of using constructed wetlands as a
treatment for municipal wastewater, acid mine
As a result, the design process is far less clear. Several hydraulic design schemes have been imple-
drainage, and urban stormwater runoff (Hammer,
mented throughout the United States. Given uncertainties in both hydrologic and biologic performance requirements, it is possible that systems could perform adequately using any one of these designs. The
1989; WSDOT, 1995; Kadlec and Knight, 1996). The use of surface-flow constructed wetlands for environ-
mental mitigation of urban stormwater runoff has continued to gain widespread popularity (Hartigan,
1Paper No. 97078 of the Journal of the American Water Resources Association. Discussions are open until December 1, 1999. 2Respectively, Graduate Research Assistant and Assistant Professor, Department of Civil and Environmental Engineering, Washington State University, Pullman, Washington 99164-2910; and Consulting Engineer, Economic and Engineering Services, 4380 S.W. Macadam Ave., Suite 365, Portland, Oregon 97201 (E-Mail/Barber:
[email protected]). JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION
323
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Koob, Barber, and Hathhorn
intent of this paper is not to suggest one design for all
occasions; rather it is to suggest an alternative that may improve the wetland design in some instances. To help improve the hydrologic design process, this paper focuses on the design criteria of surface-flow
wetlands used to treat urban stormwater runoff. Specifically, the paper presents a stochastic method for enhancing designs to incorporate the reliability that the wetland will remain moist enough to support vegetative growth. In doing so, the paper examines three common hydraulic loading design procedures to suggest a method whereby local precipitation patterns
and temperature data could be factored into the design scheme. For demonstration purposes, an example of the proposed design criteria is applied to a highway runoff project near Spokane, Washington.
BACKGROUND
Rodgers and Dunn (1993) proposed a method for predicting pesticide concentrations from agricultural runoff also based indirectly on the retention time. They used the pesticide's retention time in combination with its half-life to create: —O.693--.
runoff for the storm event and the percentage of impervious area in the watershed. The problem associated with each of these single storm event designs can be partially seen by examin-
ing the precipitation data collected by Mancini and Plummer (1986). As shown in Table 1, there exists a great deal of variability in rainfall patterns between sites. While many locations have storm durations that
between storms of approximately three days, there are numerous exceptions. Oakland, California, for example, has an average time between storms of nearly 14 days. It is important to note that the actual
time between storms often may be substantially greater than this reported average. This is especially true in areas subject to seasonal variations in weather patterns. If this is not accounted for in the design,
attraction of straight-forward design techniques, argue that these single event approaches are unacceptable and "totally inadequate for final design." Their philosophy entails a design which addresses ten separate areas of concern, including the concept of an
acceptable level of risk. Perfecting a single design which successfully encompasses all of these items is an impossible task given the current level of understanding. By focusing only on the hydrologic factors, one can begin to see some of the complexity and constraints of single event methods. Any hydrograph method may be used to determine the amount of stormwater runoff, from as simple as the Rational Method to computer modeling programs such as WEPP or SWMM (USDA, 1995; Huber and Dickinson, 1988). The level of complexity is left for
the designer to determine based on the needs of the specific design project. Even when complex hydrograph methods are used for design, other hydrologic factors sometimes must be taken into consideration. For example, the Washington State Department of Transportation (WSDOT) uses the Santa Barbara Urban Hydrograph (SBUH) method with 2-year, 10-year and 100-year return frequencies to determine
(2)
where C0 is the initial concentration, F is the specific base of the decay function, ® is the pesticide residence time, and t112 is the half-life of the pesticide. In this design, the basin volume is selected using a retention time desired to achieve a minimum pollutant concentration. JAWRA
eter used. Consequently, the wetland basin surface area is sized as a percentage of the total watershed area. The only information required is the amount of
Kadlec and Knight (1996), while recognizing the
0.30 m (0.98 ft), Kadlec and Knight (1996) report total phosphorus removal is projected to be approximately 45 percent.
11/2
design capture volume. This is the only design param-
between extended wet and dry periods.
of a 90th percentile event, and A is the watershed area. By dividing by a user-defined depth (typically between 0.15 m and 0.45 m; 0.49 ft and 1.47 ft), the area of the wetland can be determined. In this design, wetland performance is indirectly assumed to be governed by the hydraulic retention time. For a depth of
r
wetland area is to limit the surcharge depth for the
the vegetation may not be able to sustain itself
(1)
where C is a runoff coefficient, p is the rainfall depth
C(t) = C0 *
depths for a design capture volume should be 0.30 m to 0.60 m (0.98 ft to 1.97 ft) once emergent vegetation has become established. Their approach for sizing the
are clustered around six hours and have times
Schueler (1992) proposed a rule-of-thumb, singlenumber method for determining the wetland surface area required for treatment of stormwater runoff. He proposed determining the runoff volume, V, using the 90th percentile storm event and a modified "rational method" approach whereby:
V=C pA
Another rule-of-thumb approach is based on the maximum depth of inundation of wetland plants. Urbonas and Stahre (1993) suggest that surcharge
hydraulic flood control requirements (WSDOT, 1995). 324
JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION
Hydrologic Design Considerations of Constructed Wetlands for Urban Stormwater Runoff
However, when designing environmental best management practices (BMPs), WSDOT determined an
TABLE 1. Average Storm Durations and Intervals (after Mancini and Plummer, 1986).
appropriate level of risk can be obtained using a
Storm Duration
6-month, 24-hour storm event. This event produces only 64 percent of the 2-year event runoff volume
Location
(WSDOT, 1995), indicating WSDOTs acceptance of a higher level of risk. Of course, simple statistics such as those presented
in Table 1 or other averages may not convey the entire hydrologic story. The total rainfall depth for a 24-hour duration and 2-, 10- and 100-year recurrence intervals for Spokane, Washington, are 3.56 cm (1.4 in), 5.08 cm (2.0 in), and 6.6 cm (2.6 in), respectively
(WSDOT, 1995). For the 2-year storm event, the 6-month wetland pond design event for environmental mitigation would be 2.29 cm (0.9 in), 64 percent of the 2-year event, as previously described. The potential problem with this value can be seen in Figures 1
(hours)
Atlanta, Georgia Boston, Massachusetts Chicago, Illinois Detroit, Michigan Gainsville, Florida Greensboro, South Carolina Memphis, Tennessee New Orleans, Louisiana
8.0 6.1
New York, New York
6.7 3.6 5.9 6.1
70.0 76.0 89.0 77.0 93.0 80.0 77.0
9.1 4.3 3.2 8.0 7.8
144.0 320.0 286.0 127.0 133.0
6.0 13.3
15.5 21.5
83.0 101.0
3.5 4.2
Tampa, Florida Washington, D.C. Zanesville, Ohio
and 2. The active growing season for the Spokane area excludes the five average wettest months of the
Denver, Colorado
Oakland, California Phoenix, Arizona Rapid City, South Dakota Salt Lake City, Utah
year (November-March). In fact, summarizing nearly 43 years of record collected at the Spokane airport, Figure 1 illustrates that July, August, and September each average less than 2.03 cm/month (0.8 in/month).
Portland, Oregon Seattle, Washington
Maintaining a fully functional wetland, thus poses the problem of sustaining plant growth in extended
Time Between Storm Midpoints Hours Days
5.7
4.4 7.6 5.0
6.9 6.9
94.0 68.0 72.0 57.0 106.0
3.9 2.8 3.0 2.4 4.4 2.9 3.2 3.7 3.2 3.9 3.3 3.2
11.9
5.3 5.5
periods of low or no runoff. 2.5
Airport Information Spokane International (1948-1990)1
2.0 0 .0 U)
fl.5 1.0
0 0) 0IC)
0.5 0.0 Jan
Feb
Mar
Apr
May Jun
Jul
Aug
Sep
Oct
Nov Dec
Time Figure 1. Average Rainfall Amounts for Spokane, Washington, Airport (1948-1990). JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION
325
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Koob, Barber, and Hathhorn
16.0
Spokane WSO Airport Information 14.0 DNumber of Rainy Days P >0.01 inches
E
[mber of Rainy Days P > 0.05 inches
12.0 C
>
10.0
8.0 ___ '4..
6.0
'4-
0 4.0 I. 0
'
IT
___
Jan Feb Mar Apr May Jun
Jul Aug Sep Oct Nov Dec
Time Note: storms where P>0.05 shown for growing season only Figure 2. Average Number of Rainfall Events per Month at Spokane, Washington, Airport (1948-1990).
Further, Figure 2 shows that for the same months (July-September), over the same period of record, the number of storms averaged approximately 4, 5, and 6, respectively. Since an event is defined as any measurable precipitation equal to or greater than 0.03 cm (0.01 in), the number of storms that produce runoff is even lower. In fact, by selecting events which produce daily rainfall amounts equal to or greater than 0.13 cm (0.05 in), the average number of storms over the July to September time period falls to approximately 2, 3, and 4, respectively. The growing season precipitation events greater than 0.13 cm (0.05 in) are also
seasonality. The selection of the design storm yields a
basin volume which is intended to capture the extreme runoff event (especially during the winter months). Since there are extended dry periods which produce little or no runoff, such a basin is simply too large for the vast majority of the year. The result is a basin which, more often than not, is dry and will not support vegetation growth.
HYDROGRAPH-VOLUME BASED APPROACH
shown in Figure 2.
Using the average July precipitation and four
The stormwater volume, V, can be determined by integrating the area under the hydrograph which is
events suggests that the wetland should be designed to collect less than 0.51 cm (0.2 in) of precipitation every eight days. If the 0.13 cm (0.05 in) threshold
equivalent to:
were used for initial abstraction (a more realistic
n
approach), the system would receive approximately
V=
0.61 cm (0.24 in) of precipitation every 12.4 days. This
At
(3)
i=1
amount of precipitation is substantially below the where n is the number of increments in the hydrograph, Q1 is the hydrograph ordinate at each time increment, and the overbar symbol represents the
estimated design storm of 2.29 cm (0.9 in). The data used to establish the design storm en com-
pass all storm events without taking into account JAWRA
326
JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION
Hydrologic Design Considerations of Constructed Wetlands for Urban Stormwater Runoff
average value for the time interval At. Runoff hydrographs may be generated by any number of methods depending on local regulations and practice (Chow et al., 1988).
Evapotranspiration (ET) represents the water lost
in the cycle due to evaporation and transpiration through the leaves and stems of the wetland vegetation. The runoff volume should be reduced by the average daily ET multiplied by the number of days between events and the average surface area. Most attempts to predict ET have focused on determining an appropriate ratio of ET to empirical open water evaporation equations (Chow et al., 1988). However, ET rates have been found to be quite variable depend-
radiation, G is the heat flux, c is the specific heat of moist air, a and b are empirical coefficients, U2 is the wind speed 2 m (6.6 ft) above the ground surface, eas is the saturated vapor pressure, ed is the vapor pressure, ra is the aerodynamic resistance, and r is the canopy resistance. Another potential loss of water for unlined wetlands can be through groundwater infiltration. Down-
ward infiltration into an initially dry soil occurs
2.16 for Typha angustata, and 1.75-3.67 for water
primarily because of the soil suction gradient and the ponding head (Hillel, 1980; Ferguson, 1994). Unlike natural wetlands, constructed wetlands may be located in areas where the soils are not conducive to water retention, and, thus, infiltration may be substantial. Marble (1992) explains and identifies the significance often groundwater site selection and design features. The underlying soils category is listed as a high priority item.
hyacinth (Abtew, 1996). Based on a year long study in Orlando, Florida, Dolan et al. (1984) reported an aver-
The significance of soil type can be seen when using the ponded Green-Ampt equation where the
ing on meteorological conditions and vegetation type(s) and density. These ratios have typical ranges from 0.84 for lily pads, to 1.62 for Typha latifolia, to
age daily ET rate of 3.6 mm/day (0.14 in/day) for a mixed emergent aquatic macrophyte community contaming arrowhead, pickerel weed, panic grass and
infiltration rate, f, for a homogeneous soil after ponding is given by (Chow et al., 1988):
marsh hibiscus. The research site was a marsh
((p+h \A® °'
approximately 32 ha (79 ac) in size adjacent to a
f=K
small lake and the Palatlakaha River. Abtew (1996) examined six widely used equations for predicting ET from cattails, mixed marsh vegetation and open water/algae locations in southern Flori-
F
+1
(6)
and the cumulative infiltration, F, depth given by:
da. His study indicated that the Penman-Monteith equation produced results which best fit the measured data for cattail and mixed marsh vegetation,
F—F—('.P+h0)A®ln
while the Penman Combination equation seemed best
, + h01A0+
F
(P+h0)A®+F
('i)
suited for open water/algae site conditions. These equations can be as follows:
where K is the hydraulic conductivity, P is the soil suction head, h0 is the ponded water depth, t, is the time to ponding, F is the cumulative infiltration at the ponding time, and A® is the change in moisture
Penman-Mo nteith
ET =
A(R —G)+pcp(eas d)
/
=K(t—t
content. K, A®, and P are all functions of the soil clas-
1
r
sification.
The time to pon ding, and the associated cumulative infiltration, are quite small in a wetland application where infiltration is being examined over days or
(4)
A+1I 1+.r
ra)
weeks as opposed to hours. Consequently, Equation (7) can be reduced to:
Penman-Combination
ET =
1
A(R. —G)+?6.43(a +bwU2)(eas —ed)
(P+h0)A® j F_(+hO)A®ln[+h00+F
(5)
(8)
As demonstrated in Ferguson (1994), use of the suction head term in the Green-Ampt equation pri-
where A is the slope of the vapor pressure curve, X is
manly affects the initial infiltration rate. As the
the latent heat of vaporization, p is the atmospheric density, y is the psychrometric constant, R is the net JOURNAL OF THE AMERICAN WATER RESOURCES AsSOcIATIoN
= Kt
transmission zone lengthens over time, the effect of the suction head, P, decreases leaving gravity as the 327
JAWRA
Koob, Barber, and Hathhorn
an event as any daily precipitation event which produced at least 0.13 cm (0.05 in) of rain, the mean maximum time between storms was 39.6 days and the variance was 203.86. The smallest and largest maximum annual dry periods were 15 and 82 days, respec-
controlling force and causing the total hydraulic gradient to approach a lower value of 1.0. Moreover, since
h0 decreases as F increases, it can be seen from Equa-
tion (6) that the infiltration rate approaches the hydraulic conductivity. As such, the cumulative infiltration function in Equation (8) begins to parallel K*t. Because of the difficulty in using the Green-Ampt formula, Ferguson (1990) proposed the following simplified infiltration equation for detention basins:
tively.
In examining the statistical character of the data,
the Extreme Value Type I distribution was selected as
a functional candidate. Made famous by Gumbel (1958), this distribution is commonly used to repre-
V = K*Aflr * S.F + 0.7 * K *
sent the probability of random annual maximum
(9)
hydrological events (Stedinger et al., 1993). The probability density function (PDF), p(x), for the Extreme Value Type I distribution (a.k.a. Gumbel distribution) can be written as (Haan, 1977):
where V is the volume of infiltration in m3, S.F. is a
"safety factor," K is the hydraulic conductivity in are the surface area of the pond floor and the side areas in ha, respectively. The term "safety factor" is really a correction factor that Ferguson set to 0.5 to match some pond experiments. This exposes one weakness of Equation (8) and Equacmlhr, and Afloor and
1 [—(x—f3)1' (—(x—) Px(X)=P a k a —expi[ a j)ii
tion (9) with respect to matching measured values. Over time, small soil particles will tend to settle out in wetlands. As a consequence, the soil is no longer homogeneous and, depending on the assumed thick-
(10)
where a and 3 are scale and location parameters, respectively. The correspon ding cumulative di stribution function (CDF), P(x), is given by (Vogel, 1986):
ness and composition of the sediment layer, the hydraulic conductivity can vary by several orders of magnitudes. The wetland size needs to be able to initially store the desired stormwater runoff volume and ultimately keep the wetland sufficiently moist during extended dry periods. Such a design requires an analysis of the trade-off of depth versus area, as prescribed by the various abstractions. Here, the depth of the pond can be adjusted such that the surface and bottom areas limit the volume of water lost via ET and infiltration over a "reliable" dry period to the amount initially stored. However, to maintain an emergent wetland plant community, the water level should not exceed a depth which would prohibit plant growth due to limited light penetration. If emergent plants are not a part of the wetland design, the pond water level can be greater. In addition, a safety factor, in terms of depth, can be added to account for a designer's "risk accep-
P (x) =
exp [ ex[_(J]J
(11)
Method of moments estimators for the parameters a and f3 can be made using: = f3 + O.577a
(12)
a2 = 1.645 a2
(13)
where t and a2 are the mean and standard deviation of the sampled data.
Rearranging Equation (10) to form the inverse CDF produces (Vogel, 1986):
-
x=P1(x)= a ln(-ln(P(xfl)
tance level."
1
(14)
a
Crucial to this design is the assessment of the so-
called "reliable" dry period. This period can be obtained from a stochastic analysis of the hydrologic records, as outlined in the following section.
To investigate our hypothesis that the data (i.e., annual maximum number of consecutive dry days) was Gumbel distributed, a test of fit was performed using Filliben's probability plot correlation coefficient (Filliben, 1975). In conducting the test, the first step
Stochastic Dry Period Design
is to rank the data (in ascending order) and assign a CDF value using Gringorten's plotting position formula (Gringorten, 1963):
In highlighting our approach, precipitation data from 42 years of record (1948-1990) at the Spokane, Washington, Airport were examined to determine the maximum annual dry period which occurred during the peak growing season of May-September. Defining JAWRA
-
P(x)= 328
i—0.44 0. 12
(15)
JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION
Hydrologic Design Considerations of Constructed Wetlands for Urban Stormwater Runoff
where n is the total number of observations and i is the rank order. Cunnane (1978) found Equation (15)
simply the set of observed data (with corresponding
to be superior to other plotting position formulas
are found using Equation (13) for each of the same
CDF values given by the plotting position) and the M1
when using the Gumbel distribution.
values of the CDF.
The next step is to compute Filliben's probability plot correlation coefficient test statistic, R, defined as the product moment correlation coefficient between the observed data, x1, and the order statistic medians,
Once an R-value is computed, the test statistic is then compared to critical values established for varying degrees of Type I error (i.e., significance levels). Here, a null hypothesis was formed that assumed the test data were Gumbel distributed. At the 0.10 signifi-
M. R can be expressed as (Filliben, 1975):
cance level with a set size of 42, a computed R of 0.9901 compared favorably to the critical value of 0.9697 (see Vogel, 1986). Therefore, the null hypothe-
_)(M —M) 1/2
[(x — _)2 (M1 —
sis could not be rejected. Hence, it was assumed the data was sufficiently Gumbel distributed. The results of the stochastic analysis are shown in Figure 3. Here, the CDF is plotted against the maximum annual number of days between events. Data from the 42 complete years of rainfall events are also shown plotted on the figure as a rough indication of statistical fit. A designer in Spokane who wanted to be 80 percent sure that the wetland would never be
(16)
)2]
where and M are the means of the observed data and the order statistic medians, respectively. In computing R, each element of the vectors x1 and M are determined at fixed values of the CDF. The x1 are
1.0
0.9 0.8 0.7
. (5
0.6
2
a. (5
> (5
0.5 0.4
E C.)
0.3 0.2 0.1
0.0
0
40
20
60
80
100
Maximum Time Between Events (Days) Figure 3. Cumulative Density Function for Maximum Interovent Times. JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION
329
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Koob, Barber, and Hathhorn
dry, would need to plan for approximately 50 days between rainfall events. By multiplying the average daily ET and ground water infiltration rates by the
number of days between events, an approximate
The total evaporation and infiltration loss is 13.7 mm/day (0.54 in/day) over the surface area of the wetland. This information is used in conjunction with the CDF curve shown in Figure 3 to determine the mini-
water loss depth can be determined. The minimum depth of water in the wetland should be greater than or equal to this value to maintain 100 percent saturation of bed soils at all times. If 100 percent saturation is not favorable for some desired plant species, the
mum depth of water in the wetland. For example, if the designer wishes to insure that the pond will contain water at least 70 percent of the time, the pond must hold 44 days of water. At an average loss rate of
minimum depth of water should be less than the determined value. The designer must take this into
to hold at least 60 cm (23.6 in) of water. Of course, vegetation on the sides of the pond would be exposed much sooner. As mentioned previously, vegetation requirements should be matched against the possible fluctuations in water surface elevations within the pond. This analysis also indicates the site would not be suitable for a constructed wetland since the depths
consideration when specifying types of vegetation. Final wetland design should examine the trade-off between depth and surface/bottom areas to determine how much, if any, additional water depth should be incorporated into the design.
13.7 mm/day (0.54 in/day), the pond must be designed
of flow in wetlands are generally less than 45 cm (17.7 in).
The same general conclusion was reached by
Application to Spokane Area Highway Runoff
WSDOT, who decided to construct a wet pond at the
The Sunset Hill stormwater detention facility is located in Spokane, Washington, along Highway 90 which connects Spokane to Seattle. The drainage area consists of approximately 9.87 ha (24.4 ac) of pavement and 6.35 ha (15.7 ac) of soil within the highway department right-of-way limits. The facilities collect highway runoff from both directions of approximately 1.6 km (1 mi) of eastJwest traffic and runoff from the
I-90/H-195 interchange. Except for some small springs, no other non-highway sources of pollution drain to the facility. The average daily traffic volume for this site is currently 49,400 based on an estimate from 1992 actual counts.
Using Equation (5), the Penman-Combination equation, climatological data, and typical weather values for July in Spokane (Chow et al., 1988; van der Leeden et al., 1991), an average estimated precipitation loss of 3.7 mmlday (0.15 in/day) over the entire surface area of the wetland was computed. July was assumed to be the critical period for evaporation losses in the Spokane area due to high temperatures and general lack of humidity.
described as detention systems comprised of "a permanent water pool, a temporary storage area above the permanent pool, and a littoral zone planted with native aquatic vegetation" (Livingston, 1989). The basic elements of a wet pond include a sediment forebay, emergency spiliway, outlet structure, pool area and buffer strip (GKY and Associates, 1996). Wet ponds differ somewhat from wetlands in that there may be little or no emergent vegetation within a wet pond and the depth of the permanent water pool is greater — on the order of 0.91 m (3 ft) or more, compared to 30.5 cm to 45.7 cm (12 in to 18 in) in a wet-
land (Pressley and Hartigan, 1991). These ponds remove soluble pollutants through plant growth, principally algae, and through the settling of particles to
which soluble compounds have adsorbed. The
Spokane wet pond is designed to operate at a depth of 1.22 m (4.00 ft) and to hold approximately 2,000 m3 (70,629 ft3) of water immediately after a storm event
has drained from the pond. Because this pond is 1.22 m (4.00 ft) deep, bottom vegetation would be able to survive at least 89 continuously dry days.
Because the facility is located in the highway cloverleaf, the surface area of the pond is limited.
The bottom surface area is 1,094 m2 (11,776 ft2) and the side area is an additional 1,080 m2 (11,625 ft2). If not for sedimentation, the sandy-loam type soil at the site would produce a hydraulic conductivity of approximately 1.0 cm/hr (0.39 in/br). However, following the example of Ferguson (1994), a conservative estimate for infiltration loss of 0.05 cmlhr (0.20 in/br) was used in conjunction with Equation (9) to produce an infiltration loss of 15.6 m3/day (551 ft3/day). Divided over the average surface area of the pond, this flux results
CONCLUSIONS
It has been demonstrated that seasonal variations in precipitation patterns should be accounted for in the design of constructed wetlands for storm water treatment. The use of even long-term averages can result in misleading design criteria if the hydrologic pattern is not matched against the growing season. A stochastic framework for design has been presented that addresses the possibility of extended dry periods
in an additional water surface elevation change of approximately 10 mm/day (0.39 in/day). JAWRA
site rather than a wetland. Wet ponds can be
330
JOURNAL OF THE AMERICAN WATER RESOURCES AssociATioN
Hydrologic Design Considerations of Constructed Wetlands for Urban Stormwater Runoff
and permits designers to set a level of acceptable risk. Precipitation records from Spokane, Washington, airport were used to generate a Gumbel probability distribution which could be used to predict the maximum number of days between precipitation events during the wetland growing season. Combining this informa-
Gumbel, E. J., 1958. Statistics of Extremes. Columbia University Press, New York, New York.
GKY and Associates,
1996. Evaluation and Management of High-
way Runoff Water Quality. Office of Environment and Planning, Publication No. FHWA-PD-96.032, Washington, D.C.
Haan, C. T., 1977. Statistical Methods in Hydrology. Iowa State University Press, Ames, Iowa. Hammer, D. A., 1989. Constructed Wetlands for Wastewater Treat. ment. Lewis Publishers, Chelsea, Michigan.
tion with evapotranspiration and infiltration equa-
tions permitted minimum wetland depths to be
Hartigan, J. P., 1989. Basis for Design of Wet Detention Basin
determined. Based on an acceptable level of risk, a designer can
BMP's. In: Design of Urban Runoff Quality Controls, L. Roesner,
B. Urbonas and M. Sonnen (Editors). ASCE, New York, New
now determine the amount of dead storage that is needed in a wetland. This can lead to improved
York. Hillel,
D., 1980. Applications of Soil Physics. Academic Press, New
York, New York.
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ACKNOWLEDGMENTS
The authors wish to thank the Washington State Department of Transportation for their assistance in providing information, data, and partial funding for this project.
Manual for the Puget Sound Basin. Washington Department of Ecology, Public Review Draft, Publication No. 90.73, Olympia, Washington.
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