Hydrology of two forested wetlands that receive pumped agricultural drainage water in Eastern North Carolina

29

HYDROLOGY OF TWO FORESTED WETLANDS THAT RECEIVE PUMPED

AGRICULTURAL DRAINAGE WATER IN EASTERN NORTH CAROLINA

G. M. Cheschcir and R. W. Skaggs Department of Biological and Agricultural Engineering

North Carolina State University Raleigh, NC 27695-7625

J. W. Gilliam Department of Soil Science

North Carolina State University Raleigh, NC 2 7695-7625

R.G. Broadhead Department of Biological and Agricultural Engineering

North Carolina State University Raleigh, NC 27695-7625

Abstract: Wetlands have been recommended for buffering the impact of agricultural drainage water on tidal rivers and estuaries. This two-year study documented the hydrology of two wetland buffer areas in eastern North Carolina that received pumped agricultural drainage water. The hydrology of both sites was documented by monitoring rainfall and pumping rates and by monitoring water surface elevations over a network of stations on each wetland. Water velocities and wetland roughness coefficients were determined by dye tracer studies on the wetlands. The pumping rates of dralnage water onto the wetlands were highly variable, depending on the amount and intensity of rainfall and on the antecedent soil-water conditions on the agricultural area. The two study sites, Laurel Bay (LB) and Northwest Fork (NF), differed in that the wetland at LB was larger in area and received drainage water from a smaller agricultural area. Water depth on the wetland at LB increased by as much as 210 rnm in response to pumping compared to a 380ram maximum increase in water depth atNF. Measured velocities of the pumped water on the LB wetland ranged from 400 to 900 rrdd compared to 1600 to 4600 m/d at NF. Factors causing lower velocities on the LB wetland were lower ground slope from the pumps to the receiving stream, greater resistance to flow, lower pumping rates, and better distribution of the pumped water (facilitated by a canal that distributed pumped water laterally before entering the wetland). Lower velocities will result in greater residence times and greater pollutant removal on the wetland.

30 WETLANDS, Volume 11, No. 1, 1991

INTRODUCTION

Successful agriculture on the low lying organic and high organic mineral soils on the Albemarle-Pamlico Peninsula of eastern North Carolina requires improved drainage. Conventional drainage systems are composed of open ditches about 1.2 m deep and spaced 100 m apart. The land surface is graded to an approximate slope of 0.5 percent toward the ditches to provide good surface drainage. Drainage water from the field ditches is collected in a network of canals and is discharged directly into natural streams or sounds. When the land surface elevation is less than 1.5 m above mean sea level, dikes am constructed and pumps are installed to provide a reliable outlet. Agricultural development of this type increases peak runoff rates, and nitrogen and phosphorus efflux when compared to undeveloped land (Gilliam and Skaggs 1986).

Agricultural drainage water is perceived by many in the region to have adverse environmental effects due to nutrient and sediment loading and because it may lower the salinity of estuarine nursery areas (e.g., Kirby - Smith and Barber 1979). Some investigators have linked declines in fish and shellfish catches in some North Carolina waters to agricultural drainage (Jones and S holar 1981). Nutrient influx from both point and nonpoint sources have resulted in algae blooms causing fish kills and detracting from the area's scenic value. The impact of agriculture on the environment and on the fishing and tourism industries has become a very controversial subject in North Carolina.

The seriousness of the situation and the importance of agriculture, fisheries, and tourism to the North Carolina economy prompted the Governor to appoint a Coastal Water Managcment Task Force to consider water management problems in the Albemarle-Pamlico peninsula and to make recommendations for improve- merit. One of the Task Force recommendations was: "Where pump drainage is necessary, discharge should be into marsh, wooded swamps, or other vegetated land filter areas" (Governor's Coastal Water Management Task Force 1982). It was hypothesized that a wetland buffer would diffuse the pumped water, thcreby reducing its velocity and removing sediments and other pollutants. There were, however, essentially no data to quantify the hydrologic effects of discharging pumped agricultural drainage through wetland areas.

Previous research on the wetland treatment of wastewater has primarily dealt with municipal sewage and urban stormwater runoff. Many of these studies have linked hydrologic factors to pollutant removal in wetlands. The primary mode of pollutant removal is particulate matter deposition, which is largely influenced by flow velocity and residence time of water on the wetland (DeJong 1976, Simpson et al. 1978, Sloey et al. 1978, Tilton and Kadlec 1979, Tchobanoglous and Culp 1980, Martin 1988). Lee etaL (1975) reported that high flow rates through wetlands cause washout of nutrients. Even distribution t~f wastewater over the land surface minimizes downstream erosion as well as enhancing soil contact and phosphorus (P) removal (Tilton and Kadlec 1979,O'Brian and Motts 1980). Hydrologic factors

Chescheir, et al., HYDROLOGY OF TWO FORESTED WETLANDS 31

also have significant effects on dissolved oxygen levels and BOD removal (Sparling 1966, Englcr and Patrick 1974, Engler et al. 1976, DeJong 1976, Simpson et al. 1978). Agricultural drainage water is different from municipal wastewater in that the discharge rate and volume is much greater and pollutant concentrations are much lower.

This paper reports the results of a field study during 1984 and 1985 to characterize the hydrology of two wetland buffer areas receiving pumped agricultural drainage waters. Particular attention was given to hydrologic factors that affect nutrient removal from the water on the buffer areas. These factors include the size of the agricultural drainage area, the size and topography of the buffer area, the resistance to flow on the buffer area, and the presence of a diffuser canal to evenly distribute the water over the buffer area. Some hydrologic information from this field study has been presented in previous papers about wetland water quality (Gilliam et al. 1988), and computer simulation of wetland hydrology (Chescheir et al. 1988). The information presented herein provides a more detailed comparison of the hydrology of the two wetland buffer systems.

METHODS

Site Description

Two sites were selected for study - - Northwest Fork (NF) in Tyrrell County and Laurel Bay (LB) in Dare County (Figure 1). Each site consisted of three main components - - the drained agricultural area, the pumping station, and the wetland buffer area. These sites were selected primarily because their drainage areas contain soils that are representative of most of the soils used for agricultural production in the region. The two sites also represent both a new buffer area and a mature buffer area. The NF pumping station had been in operation for approximately 10 years at the beginning of the experiment, while the LB site had just been cleared and drained for agricultural production.

The buffer areas at both sites were characterized by dense vegetation and thick undergrowth that could be traversed only by manually cutting and clearing a trail. The presence of wetland obligate tree species such as tupelo (Nyssa aquatica), bald cypress (Taxodium distichum), loblolly bay (Gordonia lasianthus), sweetbay (Magnolia virginiana) and swamp blackgum (Nyssa sylvatica var. bifiora) con- firmed that both sites were wetlands (Cowardin et al. 1979). Some tree species were observed to be under stress, probably due to changes in hydrology (Chescheir et al. 1987). The water table was high in both buffer areas with water frequently standing on the surface.

The pumping stations consisted of three pumps each, allowing for a wide range of pumping rates. An 800 m diffuser canal was located immediately downstream of the pumps at LB to distribute the water laterally before entering the


VICINITY MAP L~ /

; •

NORTH CAROLINA ' ~ __ ~ - - ~ ~

)RRITUCK SOUND

WASHINGTON CI

/ , / / - Y R R E L L LB ~ D A R E OREGON

!t ml ~,~ HYDE 2| q ~:~j,i ~ L, L EXPERI,,~EN TA

" / , t. \ ~

SCALE (kin) 11 ~ ' ~ tX~Ni~L'\ , x \ ~ATFER~S O ~ S 10 20 I ~ , \~ f AP" OCRA .\ INLET

\ # X INLET

Figure 1. Location of experimental sites on the Albemarle-Pamlico Peninsula in eastern North Carolina.

buffer area. The NF site did not have a diffuser canal during the first year of the study. This resulted in uneven water distribution, which significantly reduced the effectiveness of the buffer area. A 300 m diffuser canal was designed and constructed in early November 1984 in an attempt to correct this problem. Other characteristics of the sites are given in Table 1.

Both buffer areas were essentially fiat but had significant local variation in ground surface elevations due to fallen trees, stumps, root structures, and short channels. At NF, tile ground elevation varied approximately 0.5 m over the buffer area, although the drop in average elevation between the pumps and Northwest Fork Creek was only 0.14 m. The buffer area at LB did not have a measurable slope within 1000 m of the pumps. Beyond 1000 m, the average elevation of the buffer area fell 0.22 m to Milltail Creek, which was 1700 m from the pumps. The local variation in ground elevation was approximately 0.6 m over this buffer area. The movement of water in the buffer areas was controlled primarily by the water level at the pump outlet, local variations in the ground surface, and the locations of embankments.


Table 1. Hydrologic characteristics of the Laurel Bay and Northwest Fork research sites in eastern North Carolina.

Laurel Bay Northwe st Fork

Total drained land (ha) 1250 Drained land in crop production (ha) 1000 Buffer area (ha) 240 Ratio of total drained land to buffer 5.2 : 1 Ratio of drained crop land to buffer 4.2 : 1 Range of pumping rates (m~/day) 6000 - 18000 Distance from pumps to creek (m) 1700

Soils

Drained land Typic Umbraquult Hyde, Cape Fear, Portsmouth 100 percent

Buffer area Typic Umbraquult Hyde 65 percent

Terric Medisaprist Belhaven, Ponzer 35 percent

2650 1675 145

18.4:1 11.6:1

11000 - 33000 1200

Typic Umbraquult Hyde, Cape Fear 78 percent

Terric Medisaprist Ponzer, Frying Pan 22 percent

Typic Medisaprist Pungo 100 percent

Experimental Procedures

Trails were cut through both buffer areas, and sampling stations were located on each transect (Figures 2 and 3). A survey was conducted to establish tile elevations ofreferencepoints (nails in trees) at each sampling station. The sampling stations were distributed over the entire trail system and were more heavily

34 WETLANDS, Volume 11, No. 1,199t

100

200

300

400

500

600

700

800

go0

1000

1100

1200

Meters 0

AGRICULTURAL DRAINAGE AREA 2650 Ha

V- DITCH

BUFFER AREA 1¢5 Ha

R

I¢.

CANAL

y-,

PUMPS~. DIFFUSER- CANAL AFTER 11//84

101 ~ R D 102

103 104- ' ~ D

_ /

- o R , , l o / 2 1 A e ~ ,,, 11 /

~ 6 07 10a lo9 1 , 0 ~ 1 i = -

149 R -- Recorder V 2 - - D = Dye Study x X

%

A = Automatic Sampler \ R

i i50 151 152

-- NORTHWES'~

Figure 2. Layout of paths cut through buffer area at Northwest Fork site with sampling station locations and numbering. Locations of water level recorders, dye study sites, and automatic samplers are also shown.

concentrated close to the pump outlets. Water-level recorders were installed at various locations on the sites (Figures 2 and 3) and a recording rain gauge was installed near each pump house. The times, rates, and total volumes of water pumped onto the buffer areas were recorded for each pumping event.


U~t~m~ 0 -

1 0 0 -

4 0 0 -

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9 0 0 -

1 0 0 0 -

1100 -

1200 -

1300

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1600

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,~IRIeULII/IIA. I)R~IHA~ 1280 Ha

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Figure 3. Layout of paths cut through buffer area at the Laurel Bay site with sampling station locations and numbering. Locations of water level recorders, dye study sites, and automatic samplers are also shown.

Field data were intensively collected during selectcd pumping events. A pumping event would be selected if enough rain fell to require at least eight hours of continuous pumping. During selected pumping events, the water surface elevation was manually recorded and water quality samples were collected at each station for determination of nitrate nitrogen, total Kjeldahl nitrogen, total phosphorus, chloride, pH, sediment, turbidity, and electric conductivity (only turbidity and electric conductivity data are presented in this paper; results from the other water


quality parameters are presented elsewhere (Gilliam et al. 1988)). Sampling by a four- to six-person crew was repeated as frequently as possible over the duration of a pumping event. Usually data could be collected from each station three times in one day. It was not possible to traverse the wetland to collect data in the dark; therefore, an effort was made to select events that would allow intensive sampling during pump startup (background on the buffer area), while the pumps were running, and after pump shut-down. Sampling continued for at least 12 hours after pump shut-down. Water-level data were collected manually and supplemented by continuous records at the recorder locations (Figures 2 and 3). The experimental sites were visited biweekly for recorder maintenance. When pumping occurred within 48 hours of a maintenance visit, water-level and water-quality data were collected along one transect of the wetland.

The water-level data collected at each station were processed to establish water-surface contours at specific times during a pumping event. Likewise, the water-quality data were used to determine concentration contours. Contour plots of water-surface elevations and concentration levels were used to quantify the movement and storage of pumped water on the site.

Eleven pumping events were intensively sampled during the two year study - - five events at Laurel Bay (LB) and six events at Northwest Fork CNF). The events covered a wide range of pumping rates (Table 2) and included seasonal variations in rainfall patterns, vegetation, and cultivation practices. During events requiting operation of more than one pump, the number of pumps and thus the pumping rates were adjusted periodically according to the water level in the canals. That i s, a maximum pumping rate would be used during the period of highest runoff, then the pumping rate would be reduced as the drainage rate slowed and the water level in the canals receded. The first four events at NF occurred prior to the construction of the 300 m diffuser canal.

Tracer studies using Rhodamine B dye were conducted at selected sites (Figures 2 and 3) to determine water velocity and frictional coefficients. The dye was released from a single point 30.5 m upstream from a sampling line. Samples were taken every 3 m along the 27 m sampling line at time intervals ranging from 1 to 10 minutes until most of the dye had passed the sampling line. The concentration of the fluorescent dye in the samples was determined using a Turner Model 111 Fluorometer. Results were used to calculate the mean water velocity as:

Z v~ - (1) t~

where,

v m = Mean water velocity (m/d)


Table 2. Magnitude and duration of pumping events intensively sampled at the Laurel Bay (LB) and Northwest Fork (NF) research sites.

Site Event Date Time Duration Max Pump Rainfall Volume Number Pumping Pumping Rate Pumped

Began Began hrs m3/hr cm m 3 X 1000

LB 1 FEB-13-84 0900 8.8 6,000 1.19 53 LB 2 MAR-22-84 0130 8.5 6,000 1.30 51 LB 3 MAY-09-84 0700 11.7 6,000 2.87 70 LB 4 MAY-30-84 0630 82.5 18,000 9.68 890 LB 5 NOV-05-85 1210 48.0 18,000 6.55 783

NF 1 JAN-12-84 0800 8.6 11,000 NA 98 NF 2 MAR-14-84 2200 12.0 11,000 1.39 137 NF 3 APR-23-84 0540 13.0 11,000 4.28 148 NF 4 MAY-30-84 0830 49.0 33,000 8.97 880 NF 5 FEB-12-85 1 I45 28.0 11,000 2.90 308 NF 6 AUG-08-85 1500 41.0 22,000 6.93 630

Z = Distance from dye drop point to sample point (m)

j t f tC(t) dt t m = Mean time that dye passed sample point, tin= 0j,rc(t)

dt

t = Time since dye release (d) tf = Time of last detectable dye concentration at the sample point. C = Concentration (ppb) of dye measured at sample point as a function of time

Integrals were solved numerically using the trapezoid rule. Manning's roughness coefficient, n and the frictional coefficient, o~,

(Kadlec et at. 1981) were calculated from the tracer data. Kadlec's equation may be written as:

v = t~d9 ~ dh (2) s dz

where:

v = Superficial velocity (m/d) v = v m S


S = Wetland porosity (assumed equal to 0.80) B = Frictional exponent (assumed equal to 2) t~ = Frictional coefficient (m/d/m ~) D = Depth of water on wetland (m) h = Water-surface elevation (m) z = Position on spatial grid (m)

Manning's roughness coefficient (n) equation for wide channels.

was calculated from Manning's

1 l l - Ah -o~ v - n D°'67 L-~z ] (3)

v n = Manning velocity (m/s), v = vJ86400

RESULTS AND DISCUSSION

The wetland buffer system is made up of three components - - the drained agricultural fields, the pumps, and the wetland. Precipitation on the agricultural area provides the majority of the input to the system, resulting in system input that is highly variable and unpredictable. The runoff in response to precipitation on the agricultura~ lands depends on the soil type, crop conditions, drainage system design, and antecedent soil water conditions. The pumping rates are dictated by the need to remove water from the drainage area to prevent crop damage. The hydrology of the agricultural area and the pump components of the system results in highly variable inputs to the wetland component; consequently, the hydrology of the wetland is very transient with periods of high flow and periods of no flow. The residence times of the water pumped on the wetland are variable.

Hydrology of the Agricultural Drainage Area

The pumps were operated (manually at LB and automatically at NF) in response to the level of the water in the canals. Increases in canal water level were caused by surface and subsurface drainage from the agricultural fields during and after rainfall events. Surface drainage resulted in a rapid increase in canal water level requiring pumping immediately after an event, while subsurface drainage resulted in a slow increase in canal water level causing a need for intermittent pumping for several days after an event. The maximum daily amounts of water that can be removed from the drainage area were 35 mm at LB and 30 mm at NF. These

Chescheir, et al . , HYDROLOGY OF TWO FORESTED WETLANDS 39

maximum rates usually could notbe sustained for more than a few days due tobuild- up of debris on the pump grates. Thus, removal of water from large events could take several days of pumping at various rates.

The amount of surface and subsurface drainage was controlled by the magnitude and intensity of a rainfall event and by the soil water conditions in the drainage area prior to the event. Consequently, these controlling factors affected the volumes of drainage water pumped in response to rainfall events. For example, a 29 mm event at LB (Figure 4) on days 129 and 130 that occurred with moist antecedent soil conditions resulted in pumping of 14 mm of drainage water from the watershed over the next week, while a 29 mm event on days 176 and 177 that occurred with dry antecedent soil conditions resulted in the drainage of only 2 mm of water. On days 151 and 152, a 97 mm rainfall on wet soil conditions resulted in

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1 2 2 1 3 7 1,52

DAY OF Y E A R

167 t87_

,,< ~E

U3

¢r" LtJ

Figure 4. Relationships between rainfall, pumping volume and water surface elevations on the Laurel Bay buffer area. Daily pumped volume is expressed as the depth of water removed from the 1250 ha drained agricultural area. The water surface elevations were recorded at 100 m (STA 121) and 850 m (STA 127) from the pumps.


87 mm of drainage water pumped onto the buffer area over the following week. This was 6.2 times as much drainage water for 3.3 times as much rain when compared to the day 129 and 130 event. The volume and rate that water is pumped onto the wetland depends on the hydrology of the drained agricultural lands.

The hydrology of drained agricultural lands in eastern North Carolina has been extensively studied (Gilliam etal. 1978, Skaggsetal. 1980, Evans etal. 1989). Drainage volumes and rates reported herein and in other studies are similar in that they are variable depending on rainfall events and that the transient high water table conditions existing on these lands make the drainage volume and rate particularly sensitive to antecedent moisture conditions. Past research has also shown that the volume and rate of water draining from these lands is dependent on soil type, crop conditions, and drainage system design. Models for simulating the hydrology of drained agricultural lands have been developed and verified (Skaggs 1980, 1982). These models could be used in the planning and management of the wetland buffer systems.

Hydrology of the Wetland Buffer Area

Water-surface elevations in the wetlands were greatly affected by pumping. Water-surface elevations 100 m from the pumps at LB rose sharply in response to pumping, while the response further from the pumps was delayed and attenuated (Figure 4). Increases in water levels for many of the small pumping events were not detected at a distance of 850 m from the pumps. After the pumps were shut down, the water surface equilibrated rapidly and then receded slowly as water was removed by evapotransporation and slow drainage.

The water-surface elevations along a line between the pumps and the river (Figure 5) at LB varied with pumping rates. When the pumps are not operating, the water-surface profiles are fiat over the first 1000 m reach of the buffer. Beyond 1000 m, the gradient increases toward Milltaii Creek, which is I700 m from the pumps. Water stands on the site for the first 1000 m due to a small rise in ground elevation at that location. This rise was not obvious from site survey data because of the local variations in ground elevations. Operation of the pumps creates a gradient over the first 1000 m from the pumps. With one pump running, the water surface ranged from 70 to 100 mm higher than the water surfaces with no pumps running. With three pumps running, the water surface ranged from 140 to 210 mm higher than the water surface with no pumps running. Recall that the drained area at LB is 1250 ha and the buffer area is 240 ha. Therefore, the maximum daily pumped drainage of 35 mm from the watershed is equal to 182 mm spread evenly over the buffer area.

The water-surface profiles at NF (Figure 6) differed from those at LB. When pumps were not running at the NF site, the water-surface profiles had a constant gradient from the pumps to the river. Much less water was observed on the NF buffer between pumping events than was observed on the LB buffer, indicating


1.75

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-"----20 I - l l l - - I - - i - - I I ~ l - - - - I I1~1\1~____ I

i 1 1 I

0 300 600 g00 1200 1500

DISTANCE FROM PUMPS ( m )

Figure 5. Water elevations vs. distance from pump at various pumping rates at Laurel Bay (Stations 119 - 130). See Table 4 for detail of pumping record.

1.30

Z 1,20

1.10

1,00

~J 0,90

0.80

0.70

• -- • NO PUWAPS

I]/2 O-- 0 1-PUMP B k ~ - - ~ 1-PUMP A

D, I~ -- I-I 3-PUMPS 0 O--D.~_ "

O ~ ° r l ~

i t - - i ' m - re..----.--m__,._..._ m ~ ~ 0

... i ~ ~ _ l l I l

0 200 400 600 800 1 000 1200

DISTANCE FROM PUMPS (m)

Figure 6. Wamr elevations vs. distance from pump at various pumping rates aT Northwest Fork (Stations 3 - 13). SeeTable 4 for detail of pumping record. ! -PUMP B is one pump running before diffuser canal, l -PUMP A is one pump running after diffuser canal. 3-PUMPS is three pumps running before diffuser canal.


better drainage of the NF buffer. Prior to the construction of the diffuser canal at the NF site, large gradients occurred near the pumps when the pumps were running. This is easily explained since flow was restricted by the small pond that served as the pump outlet. With one pump running, the water surface ranged from 110 to 263 mm higher than the water surfaces with no pumps running. With three pumps running, the water surface ranged from 190 to 380 mm higher than the water surface with no pumps running. Gradients near the pond were reduced after construction of the diffuser canal; however, gradients were still greater at NF than at LB. With one pump running, the water surface ranged from 110 to 170 mm higher than the water surfaces with no pumps running. The drained area at NF is 2650 ha and the buffer area is 145 ha. Therefore, the maximum daily pumped drainage of 30 mm from the watershed is equal to 552 mm spread evenly over the buffer area.

Water flow over the buffer area at LB was fairly consistent and predictable during a pumping event. Measured water surface and electric conductivity contour plots (Figures 7 and 8) indicate that the pumped water was evenly distributed over the buffer; however, the electric conductivity contours indicate one dimensional linear flow, while the water-surface contours suggest a tendency toward radial flow especially near the pumps. Since a smalI addition of pumped drainage water in the buffer area had a much larger effect on the electrical conductivity than it did on the water level, the conductivity measurements were more sensitive. Consequently, the contours shown in Figure 8 probably give a more accurate indication of the distribution of pumped water than those in Figure 7.

Flow patterns and the distribution of water on the NF buffer prior to the construction of a diffuser canal was significantly different from Laurel Bay (Figures 9 and 10). The more confined outlet (discharge into a small pond as opposed to a diffuser canal) caused the water to enter the buffer area at higher velocities, as indicated by the steep gradients near the pumps. The high velocities caused scouring of the ground surface near the outlet pond, resulting in the formation of channels and preferred flow paths. From the water-level contour plot (Figure 9), the water seems to spread fairly evenly over the site, but it was clear from site observation that the flow down the north side of the buffer was much greater than on the rest of the site. The electrical conductivity data (Figure 10) also indicated more flow down the north side of the buffer area. Vegetation was thick over much of the site, with the exception of along the north embankment where undergrowth was more sparse. Although it was not obvious from our measured water-level contours, there was preferential flow along this boundary.

The water-surface gradients and velocities near the pumps decreased greatly after the construction of the diffuser canal (Figure 11). The distribution of the pumped water over the site (Figure 12) was also improved; however, it was observed that water still had a tendency to flow in the old preferred paths. The distribution of water over the site may have been better had the diffuser canat been constructed before the pump station went into operation.


M e t e r = 0

100

ZOO

300

4O0

500

6OO

7OO

800

900

1000

1100

1200

1300

1400

1500 -

1 6 0 0 - -

1 7 0 0 - -

I / ;- DIFFUSER CANAL PUMPS ll~ =

, "= 1 . 5 0 9

~ 1,494

_ ~ ~ 1.463 ~ 1.448

N

m

• - - ' - " ~ I L L T A I L

J 1.433

c - - - - . . . .

Figure 7. Water-level contours at Laurel Bay 7 hours after pumping began. All elevations are in meters.

The velocities, frictional coefficients (ct), and Manning's roughness coefficients (n) calculated from dye study data collected at various locations on both wetland sites reflected the differences between the buffer areas (Table 3). Friction coefficients calculated for NF were 2 to 3 times greater than those calculated for LB.


M e t e r s

°t 100

2 O O

300 - i

~,00 -

500 -

6 0 0 -

700 -

800

900

1000

t100

1200 -

1300

1 ~ 0 0 -

1500

1 5 0 0 - -

1700 --

/ l PUMPS ~, ~ D I F F U S E R C A N A L

0.525 NN,,.. 0.5oo

0.475 ~ , - - 0 . 4 5 0 x - - 0.425

~ ~ 0.400

" - " - ~ 0 375

x N

- Z2 ~ ' ~ L T A I L C R E - -

Figure 8. Contour plot of electrical conductivities at Laurel Bay 7 hours after pumping began. Conductivity measurements are in decisiemens per meter (dS/m).


Mete rs 0 - - - -

100 - -

200

300 - -

4 0 0 - -

5 0 0 - -

6 0 0 - -

7 0 0 - -

800 - -

900 - -

I000~

1 1 0 0 - -

1200 -

PUMPS $ ~ ! . 0 3 6 1.006

0 .975 0 . 9 4 5

0 . 8 5 3

0.914

0 . 7 9 2

0 . 7 6 2

J

Figure 9. Water-level contours at Northwest Fork 11.75 hours after pumping began during sampling event #2. All elevations are in meters.

Manning's roughness coefficients calculated forLB were 2 to 3 times greater those calculated for NF. Higher Manning's coefficients and lower frictional coefficients indicate greater resistance to flow; therefore, resistance to flow was greater at LB. This was consistent with field observations that a thick understory of wax myrtle (Myrica cerifera) existed at LB that did not exist at NF.

When the pumps run at a constant rate for a Iongperiod of time, water-flow conditions on the buffer approach steady state. The water-surface elevations shown in Figures 5 and 6 represent those during flow conditions as near as possible to


Meters 0 - - - -

100 - -

2 0 0 - -

3 0 0 --

4 0 0 --

500 ~ -

6 0 0 - -

7 0 0 - -

8 0 0 - -

9 0 0 - -

1 0 0 0 - -

1100 - -

1 2 0 0 - -

P U M P S * * *

/ /

I /

,¢ /

/ t

/ I

/ /

I !

v" /

/

/ 0 , 1 8 5 /

/

/

0.180

0 . 1 6 0

0 . 1 5 0

NORT H~NES~ [ ~u

. /

Figure 10. Contour plot of electrical conducfivities at Northwest Fork 11.75 hours after pumping began during event #2. Conductivity measurements are decisiemens per meter (dS/m).

steady state for the given pumping rate. To verify the coefficients presented in Table 3, water depths and water-surface gradients from Figures 5 and 6 were used to calculate the Manning roughness coefficients, n (Eq. 2) and friction coefficients, et (Eq. 3). The results of these calculations (Table 4) were consistent with Table 3, verifying that the resistance to flow at LB was greater than that at NF.

The Manning's roughness coefficients obtained in this study for the LB buffer were well within the range (0.16 to 0.55) reported by Shih and Rahi (1982) for a subtropical marsh and slightly below the range (0.43 to 2.76) reported by


M e t e r s P O M P S • • , 0 . 9 1 4 0 - - - - I n /

1 0 0 - -

2 0 0 - -

3 0 0 - -

i

4 0 0 - -

5 0 0 - -

6 0 0 - -

7 0 0 - -

8 0 0 - -

900 - -

I000 --

I 1 1 0 0 - ~

1 2 0 0 - -

0 . 9 7 5

f 3 . 9 4 5

0.884

\

NORTHWES'~

/ 1

Figure 11. Water-level contours at Northwest Fork 22.2 hours after pumping began during sampling event #5. All elevations are in meters.

Rosendahl (1981) for an Everglades marsh. Manning's roughness coefficients obtained for the NF buffer were below these ranges.

Water velocities calculated from dye study data were higher for NF than forLB (Table 3). Velocities at NFranged from 1600 to 4600 m/d compared to 400 to 900 m/d at LB. Higher velocities observed at NF resulted from the combined effects of higher pumping rates, steeper gradients and lower resistance to flow. Since the velocity was slower and the distance between the pumps and the receiving stream was greater, the residence times of pumped drainage water on the wetland at LB was greater than at NF. The dye studies were conducted at NF after the


Meters 0 -

100 -

200 -

300 -

400 -

500 -

600 -

7 0 0 -

800 -

900 -

1000 " -

1100 -

1200 - -

PUMPS * * *

t , o

o

275

,b

125

NORTHWP-..

J

Figure 12. Contour plot of turbidity values at Northwest Fork 22.2 hours after pumping began during event #5. Turbidity measurements are in nephelometric turbidity units (NTU).

construction of the diffuser canal. Water velocities near the pumps before construction of the diffuser canal were probably greater than those reported in Table 3.

Residence times for drainage water pumped onto the wetlands were highly variable since the pumped volumes depended on the transient drainage from the agricultural area. Water pumped onto the wetland buffers during small events did not reach the receiving stream and was effectively stored in numerous depressions and potholes until being displaced by subsequent pumping events. Water pumped during these frequent small events had long residence times, probably greater than 5 days. Much shorter residence times occurred during large pumping events when


Table 3. Velocities and friction coefficients calculated from dye study data collected at the Laurel Bay and Northwest Fork research sites.

Station Rate Slope Depth Mean Vel Friction Manning's m3/hr percent m m/day coef ct n

Laurel Bay

120 6,000 0.0045 0.27 516 1.30 x 108 0.467 121 6,000 0.0057 0.19 712 3.17 x 108 0.303 121 18,000 0.0040 0.39 893 1.20 x 108 0.327 39 6,000 0.0044 0.34 717 1.15 x 108 0.389 28 6,000 0.0049 0.22 799 2.72 x 108 0.276 42 6,000 0.0038 0.22 541 2.26 x 108 0.359

126 6,000 0.0037 0.15 408 3.85 x l0 s 0.364

Northwest Fork

NP 11,0017) 0.0112 0.16 2031 5.50 x 10 ~ 0.133 NP 22,000 0.0122 0.20 4621 8.16 x 108 0.071 8 22,000 0.0136 0.18 1707 3.21 x 108 0.188

107 11,000 0.0107 0.12 1610 7.62 x 108 0.135 107 22,000 0.0101 0.15 1638 5.59 x 10 s 0.150

the pumped water completely traversed the buffer before pumping ceased. These minimum residence times were probably in the range of 6 to 12 hours at NF and 18 to 36 hours at LB. Determination of a mean or effective residence time for wetland buffer areas receiving agricultural drainage water wouId require models for simulating the transient drainage from the agricultural lands and models for simulating the transient overland flow and storage of water on the wetland. An analysis of this type was conducted for the LB wetland predicting that over 53 percent of the water pumped onto the wetland over a 20 yr period would have residence times greater than 5 days. (Chescheir et al. 1988).

CONCLUSIONS

Drainage volumes from the agricultural areas were highly variable, depending on rainfall amounts and antecedent soil water conditions. These drainage


Table 4. Manning roughness coefficients and friction coefficients for the Laural Bay (LB) and Northwest Fork (NF) buffer areas calculated from water depths and water-surface gradients shown in Figures 5 and 6.

. . . . . . . . . Pumping Record . . . . . . . . . Max number Duration Rate Gradient Depth Width

of pumps hr m3/hr percent m m I1 a

Lal)r~l B~V

1 9.5 11400 .0019 .27 1150 16.0 5700

2 15.0 12300 .0016 .31 1150 3.0 6100 4.0 11900

.361 2.4X 108

.196 4.2X 108

3 24.0 18300 .0055 .36 1150 .308 1.2X10 ~

1 Before Canal

1 After Canal

Nor~;hwqst Fork

10.0 11000 .0190 .13 660 .104 7.0X 108

22.0 11000 .0184 .14 660 .106 6.8X 108

3 6.0 22700 Before 6.0 7500 Canal 18.0 30200

1.0 18900

.0187 .21 660 .077 5.3 X 108

At LB, gradient was calculated from Sta 119 - 128. Width was the estimated average width between these two stations. At NF, gradient was calculated from Sta 7 - 13 (downstream of effect of small outlet before diffuser canal). Width was the estimated average width between these two stations.


volumes were the major inputs to the wetland and caused the hydrology of the wetland to be very transient, with random periods of high flow and periods of no flow. As a result of the transient hydrology of the wetland, the residence times of the water pumped on the wetland were variable.

Water velocities on the LB buffer were slower than those observed on the NF buffer. These lower velocities were caused by a variety of factors: 1) better distribution of water over the buffer area, 2) lower water-surface elevation gradients from the pumps to the receiving stream, 3) greater resistance to flow as measured by frictional coefficients and Manning's roughness coefficients, 4) lower pumping rates, and 5) larger buffer area. Lower velocities and greater buffer length at LB resulted in higher residence times of the water on the buffer.

The presence of a diffuser canal to distribute the pumped water laterally before it entered the buffer area improved the effectiveness of the buffer. The LB site was equipped with a diffuser canal and water entering the buffer was evenly distributed. Prior to construction of a diffuser canal on the NF site, the pumped drainage water was not well distributed, with much higher velocities near the discharge point. This resulted in scouring of the ground surface and formation of channels near the pumps. A diffuser canal constructed during the study greatly decreased the velocities near the pumps, but overall flow distribution was still not as good as that at LB.

ACKNOWLEDGMENTS

The research was supported in part by a grant from the Water Resource Research Institute of the University of North Carolina. Additional funding was provided by the North Carolina Agricultural Research Service. Use of trade names in this paper does not imply endorsement by either the North Carolina Agricultural Research Service or the Water Resource Research Institute of the products named, nor does it imply criticism of similar products not mentioned. We gratefully acknowledge the technical and field assistance of Toney Jacobs, Bertha Crabtree, Lydia Nelson, Wilson Huntley, OctavioReis, Vic Lilley, Scott Fassnacht, and Steve Barnes. The suggestions and contributions of those who reviewed this paper are also greatly appreciated.

LITERATURE CITED

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Chescheir,

Cowardin,

DeIong, I.

Engler, R.

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Shth, S. F. and G. S. Rahi. 1982. Seasonal variations of Manning's roughness coefficient in a subtropical marsh. Transactions of the American Society of Agricultural Engineers 25:116-119.

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Manuscript received 26 September 1989; revision received 19 November 1990, accepted 24 November I99(1.


WETLAND PHOTOGRAPH BY JAMES E. MEEKER

PHOTO 2. Northern wild rice (Zizania palustris L.) wetlands of the Kakagon Sloughs, Chequamegon Bay, Lake Superior. The low topographic gradient and regular seiches of 30 cm amplitude result in reverse currents as far as 7 km upriver on the meandering channel. The reverse currcnts may increase the residence time of nutrients within the wetland. Wild rice ~s an annual species that requires substantial nutrient uptake, so it may benefit from the longer retention time. (Photo: James E. Meeker, Northland College, Ashland, WI 45806).

Documents

Hydrology of two forested wetlands that receive pumped agricultural drainage water in Eastern North Carolina