seepage and soil erosion for a clay loam soil

9
Seepage and Soil Erosion for a Clay Loam Soil Chi-hua Huang* and John M. Laflen ABSTRACT Seepage on a hillslope produces an area susceptible to surface erosion, especially near the bottom of the slope. A laboratory study was conducted to quantify the effects of seepage on soil erosion for the Glynwood clay loam (fine, illitic, mesic Aquic Hapludalf). A 5-m-long, 1.2-m-wide soil box with adjustable slope gradient and water table control was used. A plate was installed in the soil box to force water seepage. Erosion from simulated rainfall and concentrated flow was studied. For the rainfall experiment, the soil box was set to 10% slope and exposed to a sequence of three multiple-intensity storms, ranging from 25 to 100 mm h~', every 2 d. For the concentrated flow experiment, five inflow rates ranging from 3.8 to 30.2 L min' 1 were applied to 0.2-m-wide flow channels. Flow experiments were conducted at 5 and 10% slopes and for several different seepage and drainage conditions at the 5% slope. Sediment concentrations under seepage conditions averaged 22% higher than those under free drainage with simulated rainfalls. For concentrated flow conditions, sediment concen- trations under seepage were approximately 81% higher at the 10% slope. At the 5% slope, sediment concentrations were six times higher for a surface under 20 cm seepage pressure compared with a surface drained for 7 d. Visually, it was observed that seepage greatly increased soil erosion because of its effects on headcut development. A process- based erosion prediction model, such as WEPP, should be expanded to predict seepage conditions and their effects on headcutting. S EEPAGE, THE REEMERGENCE OF soil water at the SUf- face, is a common occurrence during periods of excessive soil moisture in fields with an impeding soil layer. The saturated seepage zone sometimes has an exfiltration pressure gradient, causing an outcropping of a shallow surface flow. At the landscape scale, the outcrop of surface water from the seepage zone has been associated with the development of "seepage steps" (Hadley and Rolfe, 1955) and ephemeral gullies (Moore et al., 1988). The develop- ment of these surface features is closely associated with the convergence of water from both surface and subsur- face sources. Seepage is characterized by a zone of saturation with positive pore water pressure and exfiltration gradient. The positive pore water pressure reduces the effective stress between solid contacts, reducing soil strength. The exfiltration gradient works against the gravitational forces, further reducing the contact stress between soil grains. Experimental results showed that soil detachment by raindrops decreases rapidly as soil moisture potential is reduced from saturation to a small tension (or negative pore water pressure) (Cruse and Larson, 1977; Al- Durrah and Bradford, 1982; Schultz et al., 1985). These findings lead to the hypothesis that the erosion rate in the field is also dependent on the moisture regime on a hillslope. Agronomy Dep., Purdue Univ., and USDA-ARS National Soil Erosion Research Lab., 1196 SOIL Bldg., West Lafayette, IN 47907-1196. Re- ceived 20 Mar. 1995. *Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 60:408-416 (1996). Previous work on the effects of seepage on soil erosion is limited. One possible reason is the difficulty of conduct- ing field rainfall simulation studies during the wet period when seepage is active. Stolte et al. (1990) reported a laboratory soil pan study in which the impact of seepage (or return flow) on erosion was investigated for a sand and a loam. They found that the seepage condition increased erosion for the sand, but not for the loam. Their results may be affected by the experimental procedure used to create the return flow condition. We will discuss their study below. This study was initiated from field observations. In the summer of 1993, we conducted a rainfall simulation study on the Glynwood clay loam soil at Blackford Co., IN. All the sloping lands with this poorly drained soil in this county are classified as highly erodible and farmers must develop conservation plans to meet the Food Secu- rity Act requirement. A no-till field with undulating slopes between 5 and 10% was selected for the field study. The field had been in no-till for 7 yr. The surface soil was consolidated and there were ample amounts of crop residues on the surface. During our field visit to select the study site, we found discernable rills near the bottom of the slope at locations where the row direction was up and down the slope. There was a developing ephemeral gully on a long slope. The local Soil Conserva- tion Service officer was concerned about the adverse effects of a gully in a no-till field while promoting conser- vation tillage to reduce erosion. The rainfall simulation study was conducted in July to minimize delay due to natural rainfall. Results from this experiment indicated that the high clay soil is not as erodible as the silt loam soil in the region. On one of the 10-m-long plots, we used a rototiller to breakup the surface soil to create a freshly tilled seed bed condition. Simulated rainfall was applied for 2 h and during the last 20 min of the rain event, the surface started to develop headcuts approximately 1 to 2 m from the downslope end of the plot. After the simulated rainfall was stopped, water continued to seep out from the headcut area for another 20 to 30 min. The long-duration rainstorm caused the seepage flow and the headcut process seemed to be related to the seepage condition. In April 1994, the region received an excessive amount of rainfall over several days. We went to the same no-till field 2 to 3 h after the ram had stopped. The field was saturated. We saw water flowing in small rills on the surface as well as in the ephemeral gully. The water was from seepage because it wasn't raining at that time. As we traced the flowing water in the rill channels upslope toward the crest, we found that water was coming out of the headcuts. Although we were unable to observe headcut development during natural rainfall events, the seepage water flowing in rill channels provided strong evidence that erosion in this clayey soil is associated wim the hillslope seepage condition. Based on these observations, we designed a laboratory 408

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Page 1: Seepage and Soil Erosion for a Clay Loam Soil

Seepage and Soil Erosion for a Clay Loam SoilChi-hua Huang* and John M. Laflen

ABSTRACTSeepage on a hillslope produces an area susceptible to surface

erosion, especially near the bottom of the slope. A laboratory studywas conducted to quantify the effects of seepage on soil erosion forthe Glynwood clay loam (fine, illitic, mesic Aquic Hapludalf). A5-m-long, 1.2-m-wide soil box with adjustable slope gradient and watertable control was used. A plate was installed in the soil box to forcewater seepage. Erosion from simulated rainfall and concentrated flowwas studied. For the rainfall experiment, the soil box was set to 10%slope and exposed to a sequence of three multiple-intensity storms,ranging from 25 to 100 mm h~' , every 2 d. For the concentrated flowexperiment, five inflow rates ranging from 3.8 to 30.2 L min'1 wereapplied to 0.2-m-wide flow channels. Flow experiments were conductedat 5 and 10% slopes and for several different seepage and drainageconditions at the 5% slope. Sediment concentrations under seepageconditions averaged 22% higher than those under free drainage withsimulated rainfalls. For concentrated flow conditions, sediment concen-trations under seepage were approximately 81% higher at the 10%slope. At the 5% slope, sediment concentrations were six times higherfor a surface under 20 cm seepage pressure compared with a surfacedrained for 7 d. Visually, it was observed that seepage greatly increasedsoil erosion because of its effects on headcut development. A process-based erosion prediction model, such as WEPP, should be expandedto predict seepage conditions and their effects on headcutting.

SEEPAGE, THE REEMERGENCE OF soil water at the SUf-face, is a common occurrence during periods of

excessive soil moisture in fields with an impeding soillayer. The saturated seepage zone sometimes has anexfiltration pressure gradient, causing an outcropping ofa shallow surface flow.

At the landscape scale, the outcrop of surface waterfrom the seepage zone has been associated with thedevelopment of "seepage steps" (Hadley and Rolfe, 1955)and ephemeral gullies (Moore et al., 1988). The develop-ment of these surface features is closely associated withthe convergence of water from both surface and subsur-face sources.

Seepage is characterized by a zone of saturation withpositive pore water pressure and exfiltration gradient.The positive pore water pressure reduces the effectivestress between solid contacts, reducing soil strength.The exfiltration gradient works against the gravitationalforces, further reducing the contact stress between soilgrains. Experimental results showed that soil detachmentby raindrops decreases rapidly as soil moisture potentialis reduced from saturation to a small tension (or negativepore water pressure) (Cruse and Larson, 1977; Al-Durrah and Bradford, 1982; Schultz et al., 1985). Thesefindings lead to the hypothesis that the erosion rate inthe field is also dependent on the moisture regime on ahillslope.

Agronomy Dep., Purdue Univ., and USDA-ARS National Soil ErosionResearch Lab., 1196 SOIL Bldg., West Lafayette, IN 47907-1196. Re-ceived 20 Mar. 1995. *Corresponding author ([email protected]).

Published in Soil Sci. Soc. Am. J. 60:408-416 (1996).

Previous work on the effects of seepage on soil erosionis limited. One possible reason is the difficulty of conduct-ing field rainfall simulation studies during the wet periodwhen seepage is active. Stolte et al. (1990) reported alaboratory soil pan study in which the impact of seepage(or return flow) on erosion was investigated for a sand anda loam. They found that the seepage condition increasederosion for the sand, but not for the loam. Their resultsmay be affected by the experimental procedure used tocreate the return flow condition. We will discuss theirstudy below.

This study was initiated from field observations. Inthe summer of 1993, we conducted a rainfall simulationstudy on the Glynwood clay loam soil at Blackford Co.,IN. All the sloping lands with this poorly drained soilin this county are classified as highly erodible and farmersmust develop conservation plans to meet the Food Secu-rity Act requirement. A no-till field with undulatingslopes between 5 and 10% was selected for the fieldstudy. The field had been in no-till for 7 yr. The surfacesoil was consolidated and there were ample amounts ofcrop residues on the surface. During our field visit toselect the study site, we found discernable rills near thebottom of the slope at locations where the row directionwas up and down the slope. There was a developingephemeral gully on a long slope. The local Soil Conserva-tion Service officer was concerned about the adverseeffects of a gully in a no-till field while promoting conser-vation tillage to reduce erosion.

The rainfall simulation study was conducted in Julyto minimize delay due to natural rainfall. Results fromthis experiment indicated that the high clay soil is notas erodible as the silt loam soil in the region. On oneof the 10-m-long plots, we used a rototiller to breakup thesurface soil to create a freshly tilled seed bed condition.Simulated rainfall was applied for 2 h and during thelast 20 min of the rain event, the surface started to developheadcuts approximately 1 to 2 m from the downslope endof the plot. After the simulated rainfall was stopped,water continued to seep out from the headcut area foranother 20 to 30 min. The long-duration rainstorm causedthe seepage flow and the headcut process seemed to berelated to the seepage condition.

In April 1994, the region received an excessive amountof rainfall over several days. We went to the same no-tillfield 2 to 3 h after the ram had stopped. The field wassaturated. We saw water flowing in small rills on thesurface as well as in the ephemeral gully. The waterwas from seepage because it wasn't raining at that time.As we traced the flowing water in the rill channelsupslope toward the crest, we found that water was comingout of the headcuts. Although we were unable to observeheadcut development during natural rainfall events, theseepage water flowing in rill channels provided strongevidence that erosion in this clayey soil is associatedwim the hillslope seepage condition.

Based on these observations, we designed a laboratory408

Page 2: Seepage and Soil Erosion for a Clay Loam Soil

HUANG & LAFLEN: SEEPAGE AND SOIL EROSION 409

study to examine the effects of hillslope seepage conditionon soil erosion for the Glynwood soil. The objective ofthis study was to quantify the effect of seepage on soilerosion under both simulated rainfall and concentratedflow conditions for the clay loam soil.

MATERIALS AND METHODSSurface soil materials from a Glynwood clay loam (from

Blackford Co., IN, with 22% sand, 49% silt, and 29% clay)were collected from a no-till field. The soil was collected fromthe top 10 cm of the profile. Except natural drying duringstorage, no additional mechanical disturbance, such as sieving,was done on the soil material until the experiment.

A 5-m-long, 1.2-m-wide, and 0.3-m-deep soil box with arunoff collector trough at the outlet end was fabricated (Fig.1). The outlet end was 5 cm lower than adjacent sides. Thesoil depth in the box was approximately 25 cm. The slope ofthe soil box was adjustable, up to 40%. A plate was installed1.2m from the outlet end of the box to force seepage to occur

at this location. The top of the plate was approximately 5 cmbelow the soil surface. Twenty five rows of holes, at 0.2 mapart with three holes per row, were drilled at the bottom ofthe box for water entrance into or drainage from the box. The75 watering ports were connected to water supply troughs viaflexible tubes. The watering system consisted of two 10-m-diam. polyvinyl chloride (PVC) pipes suspended by threadedrods mounted on three support brackets. These two pipes canbe separately adjusted to different positions and slopes. Onepipe supplies water to the upper 3.8-m section of the soil boxand the other one to the lower 1.2-m section. Rectangularbaffle plates, 0.2 m apart, were inserted halfway into thePVC pipe to create a series of baffled chambers. Each baffledchamber in the PVC pipe was connected via flexible tubes toa corresponding row of holes in the soil box. Water waspumped from a supply tank to the long watering trough first(Fig. Ib). After flowing through 19 baffled chambers for theupper 3.8-m section, water was routed to the short trough forthe lower 1.2-m section of the soil bed. The excess water was

(a) Schematic of the 5-m Soil Bed

ThreadedRod Watering/Drainage

PortsBaffle Plate

V V B VTubing to Soil Pan

(b) Water Circulation System

Dv V v v V v VTubings to upper 3.8m section

( o p r p r ; 0

h

i 1 /V V VV_f To lower 1.2m

section^ — • ———— ̂

Pump

—— 0 ———— - —————

f^-^__— -

1V

/

Supply Tank

^ —————— -x

Fig. 1. Experimental setup showing (a) the 5-m-long soil box and two watering troughs, and (b) the water circulation system for seepageconditions.

Page 3: Seepage and Soil Erosion for a Clay Loam Soil

410 SOIL SCI. SOC. AM. J., VOL. 60, MARCH-APRIL 1996

cycled back to the supply tank. A pump ran continuously tomaintain a constant water level in each of the baffled chambers.

The 5-m-long soil box was placed under a set of threeprogrammable rainfall simulators equipped with oscillatingVeeJet nozzles (Part no. 80100, Spraying Systems Co.,Wheaton, IL).1 The distance between the upslope end of thesoil box and the rainfall nozzle was approximately 2.5 m. Therainfall simulator was programmed to produce rainfalls at 25,50, 75 and 100 mm h^1.

Prior to the filling of the soil box, a 2-cm layer of sandwas laid at the bottom of the soil box. The rest of the boxwas filled with the soil sampled from the field. Large clodsat the surface were broken up to 3- to 4-cm sizes using a handtrowel. The surface was prepared to look homogeneous undervisual inspection.

After the soil box was prepared, a 30-min rainstorm at 50mm rr1 was applied with the box level. This initial rainstorm,as part of the preparation process, was used to wet down thesoil surface and to consolidate the loose aggregates to form auniformly sealed, wet surface. The initial storm was expectedto reduce the variability of the surface preparation.

Three experiments were conducted: (i) simulated rainfall at10% slope, (ii) concentrated flow at 10% slope, and (iii)concentrated flow at 5% slope.

Experiment 1 - Simulated Rainfall at Ten Percent SlopeThe treatment was for both seepage and free-drainage condi-

tions. For seepage conditions, the watering trough for theupper section was set at 5% slope. The water level of theupper watering trough was approximately 2 cm above the soilsurface at the location of the impermeable plate. The slope ofthe watering trough for the lower 1.2-m section was 1 % andthe water level of the trough was at the surface level at theoutlet end of the box. Water was circulated through the wateringtroughs continuously to create seepage conditions in the soilbox.

For drainage conditions, all tubes connected to the watersupply troughs were disconnected and the soil pan was allowedto drain freely.

Two days after the initial rain, a 100-min rainstorm wasapplied. The storm consisted of four intensity sequences: 50mm h~' for 60 min, 25 mm h~ ' for 20 min, 75 mm h~ ' for10 min, and 100 mm h~' for 10 min. Runoff samples werecollected in 8-L (2-gallon) buckets every 4 min during the twolower intensity rains and every 2 min during the two higherintensity rains. Time to fill the bucket was recorded and thebucket was weighed immediately to obtain the runoif rate.Sediment in the buckets was transferred to autoclavable bottlesand oven dried to obtain the sediment weights.

The 100-min rainstorm was applied three times at 2-d inter-vals. After the third erosive rainstorm, the soil box was driedunder a fan for 3 to 5 d. The top 1 to 2 cm of the dried surfacematerial was removed and the soil turned over, new soil added,and the box prepared for the next run.

The soil box was replicated three times for both treatments.The seepage and free-drainage conditions were maintainedthroughout the entire duration of a particular run: after theinitial packing rain to the end of the third erosive rainstorm.

To show changes in microtopography from seepage-inducederosion, microtopography near the seepage zone after eachrainfall event was digitized using a laser scanner developedby Huang and Bradford (1990).

Experiment 2 - Concentrated Flow at Ten Percent SlopeFor the concentrated-flow experiment, the soil pan was

prepared in a fashion similar to the rainfall experiment. Metalplot borders were inserted into the soil box to form six 0.2-m-wide, 5-m-long flow channels. The flow channels were pre-pared to ensure a uniform roughness. After the plot preparation,including the 30-min 50 mm rr1 initial rain, the soil box wasset to 10% slope and water was applied to create the seepagecondition for 24 h. Slopes and positions of the two wateringtroughs were identical to those used in the simulated rainfallexperiment.

The concentrated flow experiment was conducted with addedwater inflow at the top end of the flow channel. Five flowrates, 3.8, 7.6, 15.1, 22.7, and 30.2 L min-', were set usingan in-line flowmeter. The run started with the lowest flow rateand incremented to the highest rate. Runoff samples werecollected after inflow water had reached the collection trough.Runoff samples were collected every 2 min for the two lowflows, every 1.5 min for the mid-level inflow, and every minutefor the two high inflow rates. Time to fill the 8-L bucket wasrecorded and the sample buckets were processed in the samefashion as for the rainfall experiment.

The experiment was conducted first on three flow channelswith seepage conditions. After runs under seepage conditions,the soil box was drained for 4 h and the flow experiment wasconducted on the three remaining channels under free-drainageconditions.

Differences in surface roughness, seepage, and drainageconditions caused different initial times to runoff at the firstinflow (3.8 L mur'), but the timing sequence after the initiationof runoff was maintained identical for all the flow experiments.

Experiment 3 - Concentrated Flow at Five Percent SlopeA total of six runs were conducted at a 5 % slope to examine

the effects of different seepage and drainage conditions on soilerosion by concentrated flow (Table 1). Seepage was set forthe location of the baffle plate at the 3.8-m mark. The seepagepressure is defined as the height of the water surface in thesupply trough above the soil surface. Runs 3A, 3C, and 3Erepresented an increasing level of seepage pressure or exfiltra-tion gradient. Runs3A-3B, 3C-3D, and 3E-3F were conductedas seepage-drainage pairs. These drainage runs also representdifferent levels of infiltration gradients and surface moistureconditions.

Each seepage-drainage pair experiment was conducted simi-lar to the runs at the 10% slope, first on three flow channelsunder seepage conditions and later on the remaining threechannels after the soil bed was drained.

For all seepage runs, the watering trough for the lower1.2-m section was set at 1% slope and the water was at the

Table 1. Seepage and drainage conditions for flow experimentsat the 5% slope.

RunSeepage pressuret, cmUpper trough slope, %Seepage zonei, m

RunFree drain time (after the run with water table)

Runs with water table3A 3C 3E2 10 201 3 4

0.05-0.1 1 3Runs under free drain-

age3B 3D 3F4 h 12 h 7 d

1 The use of brand names is for identification purposes only and doesnot constitute endorsement by the USDA.

t The seepage pressure is the water head in the upper supply trough atthe 3.8-m mark.| The seepage zone is the visible zone of saturation on the surface above

the 3.8-m mark.

Page 4: Seepage and Soil Erosion for a Clay Loam Soil

HUANG & LAFLEN: SEEPAGE AND SOIL EROSION 411

Fig. 2. Surface microtopography of a 0.64 by 0.64 m area near theseepage zone after three rainfall events. The plot is drawn usingidentical scales for all axes to present a realistic view of the surface.Headcuts were evident.

soil surface level at the outlet end of the box. For the free-drainage runs, the entire soil box was drained.

RESULTSSimulated Rainfall Experiment

at Ten Percent SlopeDuring the sequence of three 100-min rainfall events,

the soil pans with seepage developed headcuts near theseepage zone. These headcuts advanced gradually up-slope. After the third rainfall event, all three replicateruns showed a seepage step on the surface with waterflowing out of the headcuts. The seepage step is shownin Fig. 2. Under free drainage, there were no apparentheadcuts near the impermeable plate area.

Results of the simulated rainfall experiment are pre-sented in Fig. 3 and Table 2. Runoff and sedimentdelivery rates shown in Fig. 3 are averages of the lastfour sample buckets for each rainfall intensity. Runoffand sediment data presented in Table 2 were averagesof all three replicates. Results of a Mest for seepage

effects on sediment delivery were annotated. Sedimentconcentrations were calculated from average runoff andsediment data.

In Fig. 3, curves were drawn through the data to showthe nonlinear trend between sediment delivery and runoffdischarge. A similar trend was also observed for differentsoils from meter-scale interrill plots except at a lowersediment delivery rate (Huang, 1995). This similarityindicated the possibility of deriving a scaling relationshipfor sediment delivery under rainfall situations as thespatial scale is changed.

Sediment delivery rate decreased for subsequent rain-storms. The decrease is due to the combined effects ofsurface seal development and removal of easily detachedand transported sediments. This rainfall experiment wasconducted on wet surfaces without drying, other studieshave shown that drying may cause an increase in sedimentdelivery, especially for soils containing swelling clays(Zhang, 1993; Huang and Bradford, 1993).

With seepage, sediment delivery rates were consis-tently higher than those obtained under free drainage(Table 2). Test statistics showed a 0.001 significancelevel for most cases except for the 25 mm h~' eventduring Rains 1 and 2, which were significant at the 0.05and 0.01 levels, respectively. Since runoff rates weresimilar for both cases, the higher sediment delivery isattributed to the higher detachment caused by seepage.Using the sediment concentration as the measure of theseepage effect, we found that seepage conditions pro-duced a 15 to 30% increase in sediment concentrationand the average increase for all intensities and stormevents was 22%.

Concentrated Flow Experimentat Ten Percent Slope

After the inflow runs, the 0.2-m-wide, 5-m-long flowchannels developed headcuts at two areas: near the plot

|2

'•o* .I/) 1

Rainfall Runs Under Seepage Condition

Eventooooo First"Ai-A* Second+ ±t*.+ Third

\7CNE

Oa:£-4-

*a

•a<u .</> 1

20 100 120

Rainfall Runs Under Free Drainage Condition

Evento o o o o First".**.*? Second+ ± + +_+ Third

~2cT ^tT ~r60

~T~80

Runoff, mm/h100 12040 60 80

Runoff, mm/hFig. 3. Sediment delivery rate as a function of runoff from three successive rainfall events under seepage and drainage conditions. The slope of

the soil bed was 10%.

Page 5: Seepage and Soil Erosion for a Clay Loam Soil

412 SOIL SCI. SOC. AM. J., VOL. 60, MARCH-APRIL 1996

Table 2. Data summary for simulated rainfall experiments at 10% slope.

Rainfall

Rain 1

Rain 2

Rain 3

Intensity

mm h~'502575

100502575

100502575

100

Duration

min602010106020101060201010

discharge

mm h~'50.925.079.3

106.351.125.179.9

107.952.325.681.0

108.1

Free drainage

Sediment

Delivery

kg m~2 h-'1.720.513.425.341.240.382.674.181.210.372.704.08

Concentration

gkg-'33.820.343.250.324.315.233.338.723.114.533.337.8

Runoffdischarge

mm h"'50.124.379.4

106.551.925.581.5

108.254.927.883.9

110.8

Seepage condition

Sediment

Delivery Concentration

kg m-2 h-1 g kg-'1.94*** 38.70.56* 23.14.00*** 50.46.32*** 59.31.62*** 31.30.47** 18.43.56**5.41**1.56**0.47**3.44**

43.750.028.516.941.0

5.15** 46.5

cone, ratio

1.141.141.171.181.291.211.311.291.231.171.231.23

*, **, *** Significant seepage effects for sediment delivery at the 0.05, 0.01, and 0.001 probability levels, respectively.

end and at the seepage zone. Headcuts near the plot end,caused by the exit condition, are common to all flowchannels and are unrelated to their drainage conditions.They extended about 0.2 to 0.4 m from the plot end.With seepage, there was a significant development ofheadcuts at the seepage zone. The headcuts extended 1.5to 2 m upslope from the seepage zone. They were 0.1to 0.2 m wide and roughly 0.05 m deep at the head.Under free-drainage conditions, no headcut was evidentnear the impermeable plate.

Runoff and sediment delivery data are summarized inTable 3 and the average sediment rate as a function ofdischarge rate is plotted in Fig. 4. Data in Table 3 showa consistent transient behavior, a sharp rise and rapiddecline, of the sediment delivery rate after each stepincrease in the inflow rate. The transient behavior iscommon in concentrated-flow experiments and we donot believe that there is a formal analysis of this unsteady

phenomenon. It demonstrates the importance of main-taining an identical time schedule among duplicate inflowruns to enable data averaging and comparison.

Table 3 also showed a slightly higher discharge underseepage conditions compared with drainage conditionsas the inflow rate was increased. This increase can becaused by either errors in setting the inflow or the additionof seepage water from the headcut areas or the combina-tion of both. The timing sequence used in this experimentdid not allow us to check the inflow rate during the run.

The effects of seepage conditions on sediment lossunder concentrated flow were more pronounced thanthose observed under simulated rainfall. This is demon-strated by the concentration ratios calculated for eachsampling period. Increases in sediment concentration dueto seepage range from 50 to 250%. Results of a f-testshowed significant seepage effects at the 0.01 level orbetter (0.001 level).

Table 3. Data summary for concentrated flow experiment at 10% slope.

Inflow

L min"1

3.8

7.6

15.1

22.7

30.2

Sample

12341234123412341234

Average

Flowdischarge

L min"1

3.53.43.43.47.57.47.47.3

15.214.914.714.822.021.821.621.430.529.930.129.615.5

Free drainage Seepage condition

Sediment

Delivery

kgm-2h- !

4.81.71.10.99.73.92.22.2

22.313.110.69.4

34.129.427.024.361.843.336.336.418.7

Concentration

gkg"1

22.98.05.44.6

21.48.86.06.0

24.514.712.010.625.922.620.818.933.724.120.120.520.1

Flowdischarge

L min"1

3.53.53.43.58.48.18.18.2

16.615.715.715.723.324.023.723.532.131.531.931.516.6

Sediment

Delivery

kg m"2 h"1

8.6**4.0**2.2**2.1**

23.9**10.5**8.5**7.4**

55.8***26.5**24.9**21.2**67.0**63.3**45.6**43.3***

100.3**68.8***71.4***70.1**36.3

Concentration

gkg-1

40.919.110.610.247.321.617.615.055.928.126.522.448.044.232.130.752.136.437.337.136.4

Seepage/drainagecone, ratio

1.792.381.962.232.212.463.523.032.281.922.202.121.861.%1.551.621.541.511.851.811.81

**, *** Significant seepage effects for sediment delivery at the 0.01 and 0.001 probability levels, respectively.

Page 6: Seepage and Soil Erosion for a Clay Loam Soil

HUANG & LAFLEN: SEEPAGE AND SOIL EROSION 413

90

.60 -o>15oe 50 -

' 40 -

c0)E 20 -

0>10 -

Sediments from Concentrated Flow Channels

x'Free Drainage

5 10 15 20 25 30 35Discharge, L/min

Fig. 4. Average sediment delivery rate as a function of flow dischargefrom concentrated-flow channels at the 10% slope under seepageand drainage conditions.

Figure 4 is a plot of the average sediment rate as afunction of discharge. Numbers plotted in this figure wereobtained from averaging all the discharge and sedimentsamples for each inflow rate. Seepage conditions causedan average 81% increase in sediment delivery fromfree-drainage conditions when compared at equal dis-charge rates.

Concentrated Flow Experimentsat Five Percent Slope

Runs at 5% slope were designed to demonstrate thesensitivity of the flow detachment to a range of hydrologicconditions including three seepage and three drainageconditions. These different hydrologic conditions canrepresent a hillslope section at different locations as wellas at different times.

Different seepage pressures caused different degreesof surface saturation. For Run 3A, under 2-cm seepagepressure, a saturation zone of 5 to 10 cm was visibleprior to the addition of the first inflow. At 10-cm seepagepressure, the saturation zone extended approximately1 m beyond the impermeable plate. The soil surface hada continuous flow of seepage water. Under 20-cm seepagepressure, the seepage flow started approximately 1 mfrom the upslope end of the box. The background seepageflow was measured prior to the run and was used tocorrect discharge rates.

Drainage conditions also represented three situations:a short 4-h drainage from a low seepage pressure, 12-hdrainage from a 10-cm seepage pressure, and a long 7-ddrainage. Under these drainage conditions, soil surfaceswere still moist but displayed different degrees of watersaturation as indicated by cracking patterns. The surfaceprior to Run 3B, drained 4 h after Run 3A, had cracks1 to 2 mm wide in the top half of the box. The surfacecracks were less visible prior to Run 3D, which was

drained for 12 h from a 10-cm seepage pressure. Slightfinger probing, to minimize surface disturbance, in thepreviously saturated area indicated high moisture contentbefore Run 3D. Apparently, the combination of highwater table and low hydraulic conductivity of the clayeysoils caused the seepage zone to have a higher watercontent than the surface prior to Run 3B. In this case,the 12-h drainage was probably not sufficient to drainand consolidate the soil in the seepage zone. The surfaceafter 7 d of free drainage prior to Run 3F was moistand consolidated, and had cracks 5 to 10 mm wide.

After the runs, the flow channels ranged from severelyeroded to hardly eroded. Run 3E eroded most. The extentof the headcut after Run 3E was similar to the run at 10%slope under the low-seepage condition. The headcutsextended 1.5 to 2 m upslope from the seepage plate.They were 0.1 to 0.2 m wide and 5 to 7 cm deep at thehead position.

The least erosion was from Run 3F. During the run,we observed an excessive amount of water dripping fromdrain holes. Water flowing through the surface cracksproduced the lowest discharge among all runs. The sur-face after the run showed incised pits from scouring.These scouring pits coincided with the surface cracks.The scouring from the cracked area can be attributed tothe lowered soil strength along the surface crack andenhanced turbulence from the surface discontinuity.

Average sediment and discharge data are given inTables 4 and 5. The transient nature of the sedimentdelivery rate after each step increase in flow rate wasobserved. To simplify comparisons, these values wereaveraged for each inflow rate and plotted in Fig. 5.

Sediment delivery increased as the seepage pressurewas increased. The most significant feature of Fig. 5 isthe wide range of sediment delivery rates under differentmoisture conditions from the same set of inflow rates.To demonstrate the range of variability, we calculatedthe average sediment concentration for each run fromthe average sediment and discharge rates and comparedit to the least erodible Run 3F. The relative concentrationratios for Runs 3A through and 3F are: 2.1, 1.4, 2.8,2.2, 5.7, and 1, respectively. The three seepage runsproduced approximately two, three, and six times thesediment of the run after 7 d of free drainage.

Sediment delivery for the different drainage conditionsshows the effect of prior moisture history. Sedimentdelivery rates from Run 3D were slightly lower thanthose obtained from its seepage counterpart (Run 3C).They were in the same magnitudes as the low-seepage-pressure Run 3A. For this case, the low hydraulic conduc-tivity of the clayey soil has kept the seepage zone athigh moisture saturation even after 12 h of drainage.

DISCUSSIONThe experimental data clearly demonstrate that seepage

conditions caused increased erosion under both simulatedrainfalls and concentrated flows for a clay loam soil.We also showed the variability of concentrated flowdetachment under different moisture regimes at 5 % slope.Contrary to our finding, Stolte et al. (1990) concluded

Page 7: Seepage and Soil Erosion for a Clay Loam Soil

414 SOIL SCI. SOC. AM. J., VOL. 60, MARCH-APRIL 1996

Table 4. Data summary for concentrated flow experiments under seepage conditions at 5% slope.

Inflow

Lmin-'3.8

7.6

15.1

22.7

30.2

Sample

12341234123412341234

Average

Flowdischarge

L rain-'3.23.43.43.48.18.08.08.0

15.315.314.915.122.923.222.622.631.331.531.231.216.1

Run: 3A

Sediment

Delivery

kgm- 2 h" '0.850.340.230.223.272.451.320.96

11.337.305.644.77

23.4817.2112.6813.2141.1131.8623.3923.9811.28

Concentration

gkg"'4.41.71.11.16.85.12.72.0

12.37.96.35.3

17.112.49.49.7

21.916.912.512.811.7

Flowdischarge

L min"1

4.34.64.64.78.68.58.58.5

16.816.516.616.623.923.623.423.932.132.132.031.817.1

Run: 3C

Sediment

Delivery

kgm" 2 h- '1.010.610.480.434.642.491.681.38

16.058.905.856.05

24.8922.3022.0516.5850.9647.2846.9035.8015.82

Concentration

gkg-3.92.21.71.59.04.93.32.7

15.99.05.96.1

17.415.715.711.626.524.524.418.715.4

Flowdischarge

L min"1

4.03.94.24.28.58.48.48.4

16.516.716.516.724.224.223.724.433.333.533.833.117.3

Run: 3E

Sediment

Delivery

k g m - 2 h - >1.891.240.910.988.894.563.813.15

29.3424.8026.3528.4953.1547.2344.7753.7785.1488.7886.1074.9333.41

Concentration

gkg- 1

7.95.33.63.9

17.49.17.66.3

29.624.826.628.436.632.631.536.742.644.142.537.732.1

that the return flow (seepage) condition did not have anyeffect for a loam soil under simulated rainfall. This isprobably because they applied the seepage pressure dur-ing the rainfall simulation run at the beginning or midwaythrough the rainstorm. When the return flow was intro-duced at the beginning of the rain storm, it was switchedoff midway during the storm to simulate the infiltrationcondition. In the other case, the rainstorm began underthe infiltration condition and seepage was introducedmidway through the rainstorm. The duration of the rain-storm was relatively short, ranging from 20 to 40 mindepending on the rainfall intensity. They attributed the

lack of response of the loam to the low hydraulic conduc-tivity of the soil. We believe that the application of anupward flow gradient from the bottom of the soil boxduring the rainstorm may have caused an air entrapmentproblem, which could hinder the development of seepageconditions. For their experiment, there was no assurancethat seepage had been established at the surface during therainstorm. In our simulated rainfall experiment, seepageconditions were applied for at least 48 h prior to therun. The seepage condition was assured.

Soil detachment is controlled by the balance betweenhydraulic stresses from the rainfall and surface flow and

Table 5. Data summary for concentrated-flow experiments under free drainage at 5% slope.

Inflow

L min"'3.8

7.6

15.1

22.7

30.2

Sample

12341234123412341234

Average

Flowdischarge

L min"1

3.33.43.43.57.47.47.47.4

14.814.815.014.722.522.822.422.430.930.630.630.815.8

Run: 3B

Sediment

Delivery

kg m-2 h"1

0.290.200.160.102.180.820.580.487.525.513.803.52

16.0310.879.788.99

22.8016.6216.3020.43

7.35

Concentration

gkg-1

1.61.00.80.54.91.91.31.18.56.24.24.0

11.87.97.36.7

12.39.08.9

11.07.8

Flowdischarge

Lmin-1

2.12.32.52.66.46.66.66.7

14.214.114.114.221.821.721.721.930.029.529.830.214.9

Run: 3DSediment

Delivery

kgm- 2 h" 1

0.210.160.120.102.201.230.880.79

10.768.136.074.26

18.2014.7714.9015.2232.7332.0031.4629.9311.21

Concentration

gkg-1

1.71.20.80.75.73.12.22.0

12.79.67.25.0

13.911.411.411.618.218.117.616.512.5

Rowdischarge

L min"'1.01.21.41.55.05.15.15.3

11.811.811.911.919.119.019.018.627.728.027.927.813.0

Run: 3F

Sediment

Delivery

k g m - 2 h - 1

0.0230.0170.0160.0190.770.480.390.386.933.652.552.06

10.916.195.544.18

16.7812.398.515.904.38

Concentration

gkg"'0.40.20.20.22.61.61.31.29.85.13.62.99.55.44.93.7

10.17.45.13.55.6

Page 8: Seepage and Soil Erosion for a Clay Loam Soil

HUANG & LAFLEN: SEEPAGE AND SOIL EROSION 415

Run, Conditionooooo 3A, WT-2 cmoeooo 3B, FD-4 h

3C, WT-10 cm3D, FD-12 h3E, WT-20 cm

*x-x-x 3F, FD-7 d

Discharge, L/min

Fig. 5. Average sediment delivery rate as a function of flow dischargefrom concentrated-flow channels at the 5% slope under differentseepage and drainage conditions. Runs 3A, 3C, and 3E had seepagepressures (WT) of 2, 10, and 20 cm. Runs 3B, 3D, and 3F werefree drained (FD) for 4 and 12 h and 7 d.

the soil strength against these erosive stresses. Detach-ment begins when erosive stresses exceed soil strength.Detachment rate increases as the difference between ero-sive stress and soil strength is increased. The exfiltrationgradient associated with the seepage condition exerts astress on the soil material. This outward stress worksagainst the cohesive and gravitational forces that are partof the soil strength components keeping the aggregatestogether. Therefore the soil strength is lowered as theseepage pressure is increased. As the water drains fromthe soil, surface tension or capillarity pulls aggregatesclose together. Therefore, drainage increases the cohe-sive strength of the soil. During the run, water on thesurface produced an infiltration gradient. At the seepagezone, this infiltration gradient is counterbalanced by theexfiltration gradient. After studying erosion from irriga-tion furrows, Brown et al. (1987) concluded that theinfiltration gradient enhanced the adsorption of fine parti-cles on the wetted perimeter of the flow channel (seealso Kemper et al., 1985). The thin layer of absorbedsediments reduced water intake and maintained both themoisture tension gradient across the surface boundaryand the cohesion of the surface layer. Therefore theerosion rate decreased. Once the surface seal layer isbreached, a much higher permeability subsoil is exposed.This causes a rapid reduction in the tension gradientsurrounding the pitted area. Vortices from the surfacepits and the reduced cohesion enhance the expansion ofthe pitted area and cause the development of headcuts.

The mechanism proposed by Brown et al. (1987) mayhelp explain the transient behavior of the concentrated-flow data. Here, we arbitrarily define the sediment ratioas the ratio of the sediment delivery rate between thefirst and the fourth sample and use the fourth sample as

20 40 60 80Final Sediment Rate, Kg/m2/h

100

Fig. 6. Ratio of sediment delivery between the first and last samplesvs. sediment delivery rate from the last sample from concentrated-flow experiments at the 5% slope.

the final sediment delivery rate. A high sediment ratioindicates a rapid decline from the first to the fourthsample. A sediment ratio around 1 indicates constantsediment discharge. Figure 6 is a plot of the sedimentratio against the final sediment rate for the 5% slopeusing all the data. Despite the scatter, Fig. 6 shows ageneral trend of decreasing sediment ratio as the finalsediment rate is increased. At low sediment rates, theinitial flush of sediments was followed by a rapid decline.A plausible explanation is given below.

A step rise in inflow rate causes a wavefront movingdownstream. The flood-like water wave is called a mono-clinal rising wave. The velocity of the wavefront, orcelerity, is greater than the mean velocities of its upstreamand downstream stages. A detailed analysis of the risingwave velocity is given in Chow (1959, Ch. 18 and 19).Therefore, associated with the step rise in the inflowrate, there is a velocity surge, which may have causedthe initial high sediment detachment. The decline insediment detachment rate can be attributed to the combi-nation of (i) the reduced velocity behind the wavefront;(ii) the reestablishment of a surface seal from the finesediments after the initial scour; and (iii) the adjustmentof channel geometry to the new flow regime to minimizethe energy dissipation. For the low sediment rates, thescour-type shear detachment is probably the dominantmechanism.

At high sediment rates, the sediment ratio is ap-proaching 1, indicating a constant detachment rate (Fig.6). In our experiment, the headcut process induced bythe seepage conditions contributed to the high sedimentrate. At low slopes, seepage flow alone is insufficientto cause any sediment detachment. Only under the addi-tion of hydraulic stresses does the headcut process be-come significant.

Recent work has concentrated on quantifying hydraulicconditions for rill erosion, especially to define a threshold

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416 SOIL SCI. SOC. AM. J., VOL. 60, MARCH-APRIL 1996

condition for rill initiation. Examples are works by Obie-chefu and Morgan (1994), Gilley et al. (1993), andSlattery and Bryan (1992). The importance of surfaceconditions that change the soil strength is less recognized.Although shear strength has been used as a measure ofresistance against rilling (Torri et al., 1987; Rauws andCovers, 1988), the concept of credibility is still largelyconceived of as an inherent soil property that can berelated to other inherent properties, such as texture, claytype, chemical adsorption ratio, ion-exchange capacity,etc. We have shown that for the same soil, a change inthe moisture condition, which aifects the soil strength,can also change the erosion rate greatly. Therefore,the hillslope erosion processes depend not only on thehydraulic stress of the erosive agent but also on thehydrologic condition of the surface.

Although created in the laboratory, the seepage anddrainage conditions do represent some realistic situationsin the field. During the wet period, the hillslope canhave both seepage and drainage conditions at differentslope positions. The seepage and drainage conditions canalso represent the temporal changes at a fixed location.Therefore, the experimental setup allowed us to examineboth spatial and temporal variabilities as part of thehillslope erosion process.

CONCLUDING REMARKSWe have conducted a set of laboratory experiments

to show the effects of seepage conditions on soil erosionunder both simulated rainfall and concentrated flow fora clay loam soil. Hillslope seepage caused an increasein sediment discharge and the sediment delivery wasincreased as the seepage pressure or exfiltration gradientwas increased. For a 5% slope, we showed a sixfoldincrease in sediment delivery rate from a 7-d drainageto a 20-cm seepage pressure.

These results demonstrate the importance of having aproper hillslope hydrology model in the development ofa erosion prediction model. The hydrology model needsto be able to quantify the return flow hi the profileand predict the intermittent seepage face during the wetseason. Much effort has been invested in the developmentof a process-based erosion model. A hydrology modelthat can account for the hillslope seepage condition wouldbe expected to improve the performance of the erosionmodel.

Another result of this study is that subsurface drainageis probably an important erosion control measure forhillslopes with seepage areas.

ACKNOWLEDGMENTSThe authors would like to thank Fay Earnhart, former Dis-

trict Conservationist at Blackford Co., IN, for guiding numer-ous field trips to identify study sites and to observe seepageconditions and other related conservation problems. We alsothank Perry Clamme, farmer, for the use of his field and forallowing us to collect a large amount of surface soil for thisstudy. We thank Steve Parker, Dan Boyd, Scott Gabbard,and Theresa Hofmeister for their efforts in the design andconstruction of the 5-m soil bed and carrying out the experi-ments. We also would like to thank Dr. Joe Bradford and Dr.Doral Kemper for stimulating discussions and encouragementduring the course of this study.