2Hydrologist, Redwood Sciences Laboratory, Pacific Southwest Research Station,
USDA Forest Service, 802 N. Main St., Fort Bragg, CA 95437 (firstname.lastname@example.org)
3Assistant Professor, Department of Geosciences, California State University,
Chico, CA 95929 (email@example.com)
Abstract: Storm-induced streamflow in forested upland watersheds is linked
to rainfall by transient, variably saturated flow through several different
flow paths. In the absence of exposed bedrock, shallow flow-restrictive layers,
or compacted soil surfaces, virtually all of the infiltrated rainfall reaches the
stream as subsurface flow. Subsurface runoff can occur within micropores (voids
between soil grains), various types of macropores (structural voids between
aggregates, plant and animal-induced biopores), and through fractures in weathered
and consolidated bedrock. In addition to generating flow through the
subsurface, transient rain events can also cause large increases in fluid pressures within
a hillslope. If pore pressures exceed stability limits of soils and shallow
geologic materials, landslides and debris flows may result. Subsurface monitoring
of pipeflows and pore pressures in unchanneled swales at North Fork Caspar
Creek in the Jackson Demonstration State Forest began in 1985. Four sites have
been established to investigate the effects of timber harvest (K1 and K2) and
road building (E-road) for comparison with an unmanaged control drainage
(M1). Flow through large soil pipes at these sites is highly transient in response to
storm events, reaching peak discharges on the order of 100 to 1,000 L
min-1. Pore pressures at these sites also respond dynamically to transient rain events, but
to date have not exceeded slope stability limits. Most soil pipes cease flowing in
the dry summer period and hillslope soil moisture declines to far below
saturation. The clearcut logging and skyline-cable yarding of the K2 site resulted in
dramatic increases in soil pipeflow and subsurface pore pressures. During the first 4
years after timber harvest, pore pressures increased 9 to 35 percent for the mean
peak storm event in the control M1 site. Peak soil pipeflow response was far
greater, increasing 400 percent in the 4-year postlogging period. These results
suggest that the soil pipes are a critical component of subsurface hillslope
drainage, acting to moderate the pore pressure response. As the subsoil matrix
becomes saturated and pore pressures build, soil pipes efficiently capture excess
water and route it to the stream channel. This logging does not appear to have
impaired the hillslope drainage function. Methods and results at the E-road site are
quite different. Here, the mid-swale road construction and tractor yarding
have resulted in large changes in the pore pressure response. Positive pore
pressures were negligible in the upper portion of this instrumented swale
before disturbance. Subsequent to the road construction in May 1990, there was
little indication of immediate impacts. But, after the completion of felling and
tractor yarding in late summer 1991, dramatic changes in pore pressure response
were observed beginning in hydrologic year 1993 and continuing to date
(1998). Largest pore pressure increases have occurred at sensor locations in and
up-slope of the road prism. Below the road, the response is muted. These data
support previous studies documenting the profound effects of roading and tractor
logging on watersheds and provide special insight into these effects for this region.
response of forested watersheds to rain events
occurs through several interrelated flow processes. Soil
surface conditions determine whether rainfall will run off as surface flow
or whether it will infiltrate and travel through the
subsurface. Infiltration capacities for soils in the coastal redwood region
exceed maximum rainfall intensities common in the region.
Exceptions occur in isolated areas where bedrock is exposed at the land
surface. More widespread are infiltration limitations resulting from
soil compaction associated with road building, landings, and
other constructed surfaces. Over the great majority of forested
landscapes, rainfall infiltrates into the soil and flows through the subsurface
to streams, rivers, and lakes.
Subsurface flow may occur within soil horizons,
regolith (weathered bedrock), or bedrock (fig.
1). The conductive and storage properties of a given earth material as well as the
spatial relations of adjoining materials strongly influence the actual
flow path through the subsurface. For example, water may flow
within soil horizons through the matrix, a porous medium of
individual grains. Pores on the individual grain scale transmit water
very slowly, several orders of magnitude less than surface water
flows. Larger pores (on the order of 1 mm in diameter or larger)
are commonly referred to as macropores, and can conduct
substantial quantities of water at rates approaching surface flow velocities.
By virtue of their geometry, macropores can be shown to conduct
water more rapidly under high moisture conditions than the
"micropores" of the soil matrix. Macropore geometry and type varies with
depth below the land surface arising from various biologic and
soil-forming processes (fig. 1). Interconnected large macropores (on
the order of 2 cm in diameter or larger) are often referred to as
"soil pipes." These features are erosion pathways that extend within
the shallow subsurface horizons as continuous or
interconnected conduits forming complex branching networks (Albright 1992).
An important hydrologic attribute of macropores is that
the surrounding soils must be saturated before water can flow
into these large pores. Thus, the antecedent moisture conditions in
forest soils strongly control the importance of flow through
macropores; and hence, the hydrologic response of a watershed to a
precipitation event. Similarly, fractures in regolith or bedrock may dominate
the flow response under saturated conditions, and thus define
a significant flow path distinct from the soil matrix or macropores.
Figure 1 Hypothetical soil cross-section with characteristic voids and flow path variations.
The movement of water into and through these flow paths
has two consequences of both theoretical interest and
practical application to the management of forestlands. First, surface
runoff in streams is generated on two widely different time scales: (1) on
a seasonal basis and (2) during individual precipitation
events. Runoff volume, timing, and duration affect both water supply
and flood propagation. Seasonal effects of subsurface flows are
manifest in the storage properties of forest soils. During the summer,
water drains from soils and supports perennial streamflow
(baseflow). This drainage creates a water deficit in the soil that must
be replenished before maximum flow through a hillslope can occur.
The second consequence of transient subsurface flow is
directly related to the storm-driven evolution of pore pressures at
the hillslope scale. Gravity is the primary force driving the flow of
water in upland forested watersheds. However, if soil compaction
closes pore spaces and prevents or reduces drainage through
macropores, water pressure may increase such that the strength of the
hillslope is lost and shallow landslides or debris flows may occur.
Mass failures are a significant source of sediment reaching streams
and are generated from background earth surface processes and
from human activities such as road building. Dynamic
interactions between pore pressures, drainage geometry, and the
material properties of soil and bedrock can significantly influence
the stability of slopes and channel heads, as well as sediment releases
to streams (Dietrich and others 1986).
Research investigations at Caspar Creek have explored
these hillslope and subsurface drainage processes with the dual
objectives of identifying impacts associated with logging and road
building and reducing the risk of mass failures associated with timber
harvest activities in the redwood region.
Headwater swales were selected for monitoring in both a
control (MUN) and two designated treatment sub-basins (KJE and EAG)
of the North Fork experimental watershed (Preface, fig. 2,
these proceedings). All study sites are moderately steep zero-order
basins located in the North Fork watershed at an approximate elevation
of 300 m (fig. 2). An almost 100-year-old second-growth
forest occupied these sites at the initiation of these investigations
(Henry, these proceedings). All study swales are drained by one or more
soil pipes with outflow in evidence at the base of the swale axis.
Pipeflow varies seasonally from less than 0.01 L
min-1 to more than 1,000 L
min-1 at individual soil pipes. Most soil pipes are intermittent
or seasonally dry.
Figure 2 North Fork Caspar Creek study swales (2-m contour interval).
The vegetation community is a coniferous forest type with
a closed canopy consisting of coastal redwood (Sequoia
sempervirens (D. Don) Endl.) and Douglas-fir
(Pseudotsuga menziesii (Mirb.) Franco) as the dominant tree species. Although not
measured during this study, the diameter-at-breast height is estimated
to range from approximately 0.3 m to 1.5 m. Forests in the
Caspar Creek watersheds were clearcut and burned in the late 1800's
(Tilley and Rice 1977; Napolitano, these proceedings), and are
generally typical second-growth forests. Other tree species occurring at
this site include grand fir (Abies grandis (Dougl. ex D. Don)
Lindl.), western hemlock (Tsuga
heterophylla (Raf.) Sarg.), and tanoak (Lithocarpus
densiflorus (Hook. and Arn.) Rohn).
The soil at these sites has been classified as a clayey,
mixed isomesic Typic Tropudult described as the Van Damme series
(Huff and others 1985). Surface soils tend to have a loamy texture
and increasing clay content with depth (Wosika 1981).
Discontinuous argillic horizons have been observed in scattered soil pits
(Dahlgren 1998). Soil thicknesses range from 1.0 m along the ridges to 1.5 m
in the swales (Wosika 1981). The parent material below this depth
range is a highly weathered layer of fractured regolith derived from
the underlying graywacke sandstone of Cretaceous age known as
the Franciscan Assemblage (Huff and others 1985). Geologically
recent tectonic forces (1 my b.p.) acting along the San Andreas fault
system just offshore have contributed to a gradual uplift of up to 200
m (Jenny 1980). Field estimates of hydraulic conductivities were
made using slug tests in piezometers in EAG, KJE, and MUN
swales. Saturated hydraulic conductivity estimates for the regolith above
the hard bedrock contact range on the order of
10-8 to 10-4 m
Local climate is heavily influenced by the site's proximity to
the coast (approximately 10 km to the west). Like most of
coastal California, the large majority of the rainfall occurs during late fall
and winter months. The mean annual rainfall for this area is 1190
mm (46.85 in). Relatively little rainfall occurs between the months
of April and October, but coastal fog may supply moisture to the
soils via fog drip. Air temperatures range from a January mean of 7 °C to
a high of about 15 °C in July.
The M1 site is the designated control site and thus
retains continuous second-growth forest cover. This 1.7-ha swale
(fig. 2a) is the largest subsurface study site. The terrain slope varies from 20
to 50 percent. One large and several small soil pipes drain the
swale. These soil pipes were fitted for instrumentation in 1986
(Ziemer and Albright 1987). The large 80 cm (height) by 60 cm (width)
pipe, M106, has discharged the highest pipeflow peak recorded in
the North Fork watershed1,700 L min1 on January 20, 1993.
This pipe occurs at the interface between the upper soil and an
argillic horizon (Albright 1992). Two transects of piezometers, denoted
A (three instruments) and C (four instruments), were installed
to bedrock (at depths of up to approximately 6.0 m) on the side
slopes above the piping gage station (Brown 1995). A nest of
piezometers was installed at the confluence of the two subswales.
Two piezometer nests were installed at the confluence of the
subswales (piezometers B1 and B2) and just upslope of the swale at the
bottom of the C transect (piezometers C1 and C2). Two
additional piezometers were installed to bedrock, one in each of the two
upper tributary swales (Brown 1995). On the basis of the soil
borings excavated during piezometer installations, a geologic
cross-section was prepared across the A-C transects
(fig. 3). Soil horizons and regolith thicknesses were fairly uniform throughout both slopes.
Figure 3 Cross-section of soils, geology, soil pipe, and
piezometer installations at the M1 site.
A second pipeflow site, K1, was developed near the KJE
stream gaging station in 1986 (Preface, fig. 2, these proceedings). This
1.0-ha swale (fig. 2b) is drained by several soil pipes within the
upper 0.5 m of the soil with diameters ranging from 10 to 20 cm
(Albright 1992). Most are flashy and ephemeral, yielding significant
flows only during storm events. Pipeflow, surface flow, and matrix flow
at the soil face were gaged at this second site, but no subsurface
pore pressure measurements were made. The site was clearcut
and skyline yarded from the ridge in 1989 as part of the Caspar
East timber sale unit K (Henry, these proceedings). No slash burning
or other site preparation was done in this unit after timber harvest.
This 0.8-ha zero-order swale (fig. 2c) was first instrumented
for pipeflow measurements in 1986. Three soil pipes were gaged at
this site. The largest soil pipe, K201, is 50 cm in diameter and
emerges from the exposed soil face at a depth of less than 1.5 m from
the ground surface (Albright 1992). In 1987, a network of
piezometers and tensiometers was established along five hillslope
transects (Keppeler and others 1994) that were aligned perpendicular to
a west-facing K2 hillslope. To prevent excessive disturbance of
this steep 70 percent slope, a system of ladders and catwalks was
built before instrument installation. Hillslope installations include:
31 bedrock piezometers, 27 1.5m-deep piezometers, and
25 tensiometers at depths of 30, 45, 60, 120, and 150 cm. Three
of these instrument transects (A, B, C) are about 20 m in length
and extend from near the swale axis to mid-slope positions. The
other two transects (D and E) extend nearly to the ridge. Two
additional bedrock piezometers are installed in the swale axis. After
two winters of data collection, the K2 site was clearcut and
skyline yarded from the ridge during August 1989 (Henry,
these proceedings). No slash burning or other site preparation was
done in this unit following timber harvest.
The smallest and most recent Caspar Creek subsurface
monitoring site is the E-road swale. This 0.4ha swale is located in the EAG
sub-basin and cutblock E of the North Fork (Preface, fig. 2,
these proceedings). This north-facing swale is drained by
two instrumented soil pipes. A single 44m-long transect consisting
of six bedrock and two shallower piezometer installations
extends through the swale axis from the soil pipe excavation to a position
38 m from the ridge (fig. 2d). Piezometer depths range from < 1.5 m
at the lower end of this transect to almost 8 m at the top. The
terrain slope along this transect averages 35 percent. This site
was instrumented in fall 1989 to evaluate the impacts of
road construction on hillslope drainage processes.
Predisturbance monitoring of pore pressures and pipeflow occurred during
the winter 1990. In June 1990, a seasonal road was built across
this swale to a yarder landing on the unit divide. The road
centerline crosses the instrument transect at the R4P2 piezometer. The
grade of this 30m road segment averages 19 percent. The fill depth is 3
m at its maximum, 2 m at the centerline, 1.6 m at R3P2, and <1 m
at R2P2 (near the base of the roadfill). This haul road was rocked
for use during October and November of 1990 when a portion of
the Unit E cutblock was harvested using a cable skyline yarder at
the end of this spur. In late-summer 1991, the timber not cut during
the road right-of-way felling was harvested using tractor yarding
above and long-lining below the road. Broadcast burning of the
unit occurred in late November. Because of the north-facing aspect
of this swale, fuel consumption was incomplete.
Field investigations were undertaken first to identify the
most upslope occurrence of gullying or sinkholes associated
with pipeflow outlets at each study swale. At these existing
collapses, handcrews excavated a near-vertical soil face to facilitate the
capture of pipeflow and soil matrix discharge. Soil pipes ranging in
diameter from 2 to 60 cm and occurring within 2 m of the soil profile
were instrumented. Flow from individual sources (pipes, overland
flow, and soil matrix flow) was captured by first driving metal
flashing collectors into the excavated soil profile, then connecting
these collectors to PVC (polyvinyl chloride) pipe, and finally routing
the flow into an upright PVC standpipe container. Drainage holes
were drilled into these standpipe containers and a laboratory
calibration was done to establish the relationship between stage in
the container and discharge. Containers were designed with a variety
of drain hole diameters and placements to accommodate a wide
range of discharges. Using electronic pressure transducers and
data loggers, container stages were recorded at 10min intervals
during the winter season and at 30min intervals during the lowflow
season. Frequent manual discharge measurements were made at
these pipeflow sites to verify and refine the standpipe
container calibrations (Ziemer and Albright 1987).
To measure the pore pressure response along selected
transects in these study swales, piezometer wells were installed by
hand-augering 10-cm-diameter holes through the soil profile. A PVC
pipe (38 or 51 mm diameter) was then cut to extend from the base of
the hole to several centimeters above the ground surface. The lower
15-cm length of this pipe was slotted with a hack saw. Plastic
mesh screen was wrapped around the slotted portion of the pipe
before the pipe was placed in the augered hole. The hole was
backfilled first with pea gravel for about 25 cm of the depth, then 15 to 20
cm of bentonite, and finally, with natural soil. Hillslope
instruments were assigned a transect identifier and numbered beginning
with the base of the slope and progressing up the hill. P2 indicates
a "bedrock" piezometer, and P1 indicates a shallower installation.
Bedrock installations were augered to the physical limit of
the hand auger device. At some sites, rock fragments in the lower
saprolite prevented the auger from reaching competent bedrock.
Shallower piezometers were installed at certain sites where a
low-permeability clay layer (argillic horizon) was encountered. Finally, a
few piezometers were installed into competent bedrock using a rock
drill. Water levels (pore pressures) were monitored using a combination
of techniques. Manual measurements were made at all piezometers
at least weekly using an electronic water surface detector.
Electronic pressure transducers connected to a data logger sensed
piezometer water levels at 15min intervals during the winter and less
frequently during the lowflow periods at the K2 and E-road swales (Keppeler
and Cafferata 1991, Keppeler and others 1994). Accuracy of
these measurements was generally within a 0.05-m tolerance. At the
M1 site, a comparable transducer/data-logger combination
provided water level heights with a design accuracy of approximately 0.01
m along three transects (A, B, and C). Only very rarely did the
electronic data differ from hand measurements by more than 0.02 m.
Pressure heads in the piezometers were logged at 15-min intervals
during storm periods, and at 2-hr intervals between storms.
Soil tensiometers were installed at some sites to provide
a measure of soil moisture in unsaturated conditions and to
indicate when the shallower soil horizons became seasonally
saturated. These devices are commonly used for assessing
agricultural irrigation needs. Our tensiometers consist of a porous ceramic
cup connected to a closed tube and a vacuum gage. The cup is buried
in the soil and the tube is filled with water. As the soil moisture
tension equilibrates with the water tension in the tube, a vacuum is
created and indicated on the gage. At field capacity, this tension is 33 cb.
As the soil drains, tensions exceeding 85 cb may be recorded.
These gages were read manually at weekly intervals and, in some
cases, connected to a data logger via a pressure transducer allowing
for frequent readings and recordings. Keppeler (these
proceedings) reports summer soil moisture changes at these sites.
Ziemer (1992) evaluated changes in peak pipeflow after the
logging of the K1 and K2 swales using data from hydrologic year
1987 through 1991. Regression analysis was used to develop
a relationship between individual soil pipes at the K1 and K2
sites and the M1 site control, as well as total pipe discharge per site.
A second set of regressions was developed using the
postlogging pipeflow peaks. Chow's test (Chow 1960) was used to
detect differences between these regression lines
(p < 0.05). For this report, additional peak pipeflow data through hydrologic year 1993
from the K201 and M106 sources were analyzed using this
regression approach. This analysis included 38 prelogging and 41
postlogging storm peak pairs from K201 and M106.
Keppeler and others (1994) evaluated the piezometric
response to logging in the K2 site. Regression analysis was used to define
the prelogging relationship between peak pore pressures along
selected K2 transects and peak discharge
(log10) at the M106 soil pipe. Postlogging regressions were then developed for storm
peaks occurring during hydrologic years 1990-1993. Zar's test
for comparing regression lines (Zar 1974) was used to detect
differences between the calibration and postlogging relationships
(p < 0.05). A similar procedure was applied to evaluate piezometric
pressure heads during nonstorm periods.
Initial analysis of the pore pressure response to road
building was done nonstatistically by comparing E-road piezometric
peaks and ranges before and after road construction and tractor
logging. In addition, further analysis was attempted using E-road
piezometer peaks regressed on peak discharge
(log10) at the M106 soil pipe. Only preliminary screening of other factors relating to the
E-road subsurface response has been performed.
Results and Discussion
Increased peak pipeflow was detected at the fully clearcut K2
site during the first winter after logging (1990), but larger
increases were observed one year later (Ziemer 1992). During 1990 and
1991, peak pipeflow at K2 (pipe K201) was 370 percent greater
than predicted by the calibration relationship with M1 (pipe
M106). Extending this analysis to include peak discharges through
1993 provides further insight into the pipeflow response to logging.
With 38 peaks ranging up to 525 L min1 (M106) in the prelogging
data set, a linear regression provides a very good fit to the data
(r2 = 0.96) as evident in figure
4b. The postlogging data set contains 41
peaks, with all but two of the M106 peak discharges less than 300 L
min1. Those two large peaks exceed the prelogging data by a
substantial margin (fig. 4a), and present an interesting complication
to evaluating treatment effects. The largest storm produced a peak
at M106 of 1700 L min1 on January 20, 1993, triple the size of
the largest M106 peak in the prelogging data set. The return interval
for this peak is approximately 8 years based on the 35-yr North
Fork peakflow record. The postlogging relationship between M106
and K201 is much more variable than the prelogging
relationship. Although pipe K201 yields maximum discharges of up to 500 L
min1, it appears to be capable of unrestricted discharge only until
about 250 L min1. The recurrence interval of the comparable North
Fork streamflow peak is approximately 0.3 years. K201 discharge
appears to be restricted by pipe capacity above a discharge of about 250 L
min1, whereas M106 can pass discharges of at least 1,700 L
min1. The other instrumented soil pipes also exhibit peak
discharge restrictions at even lower discharges. In contrast to open
channel conditions, pipeflow is limited by the physical capacity of the
pipe. The cross-sectional area of pipe K201 is much less than M106;
thus, discharge capacity at K201 is more limited than M106.
Field observations indicate that upslope of the M1, K2, and K1
gaging sites; several ungaged pipe outlets produce significant
discharge volumes during larger storm peaks. These "overflow"
features provide further evidence of the hydraulic limitations of these
main soil pipe pathways.
Figure 4 A comparison of the peak pipeflow response from M106
(unlogged control) and K201 (clearcut in 1989) and regression lines. The solid line is
the linear fit to the prelogging data. The dashed line is the locally fitted
regression of the postlogging data and is approximated by a second-order polynomial
fit to a log-log relationship. Plot (a) includes all data. Plot (b) excludes the
larger M106 peaks where K201 begins to exhibit capacity limitations.
Because of these physical limitations, a linear
regression analysis of the postlogging K201 and M106 peakflow data is
not appropriate for moderate to high peak discharges. However, it
is clear that a substantial increase in K201 pipeflow occurred
after logging (fig. 4). When the postlogging data are fit by a
locally weighted regression (Cleveland 1993), it is evident that the
greatest departures from pretreatment data occur at discharges of less
than 200 L min1 at M106. Above this level, K201 peaks begin to level
off. When the M106 discharges exceed 500 L
min1, it is not possible to detect any postlogging change in K201 discharge peaks.
The prelogging regression equation predicts that at the mean
M106 peak pipeflow of 118 L min-1 the expected K201 peak is 51 L
min1, but the postlogging locally weighted regression predicts a peak
of 143 L min-1 a 280 percent increase
The maximum postlogging increase at K201 was more
than 300 L min-1 for two moderate storm events that occurred January
7, 1990 and December 10, 1992 (fig. 4b). These storms
produced discharges at North Fork Caspar with return intervals of 1.7
times per year. The largest proportionate increases in pipe
peakflow occurred during two minor storms in February 1991, when
winter rainfall totals had been far below normal. These were the
first stormflow responses at M1 for that year indicating that
antecedent soil moisture conditions were just reaching saturation, whereas
K2 soils were more fully saturated. As previously explained, the soil
in the vicinity of the pipe pathway must be saturated before water
can flow through these conduits. Ziemer's evaluation of Caspar
Creek streamflow peaks (these proceedings) states that the
largest increases in peak discharges occur when the greatest differences
in soil moisture exist between the logged and forested watersheds.
At K1, peakflow from instrumented soil pipes did not show
a significant increase (Ziemer 1992). However, an additional pipe
outlet located about 30 m upslope of the pipeflow gaging
instrumentation began to discharge storm flows. This source flowed rarely
before logging, but regularly during storm events after logging,
suggesting that the capacity of the K1 soil pipes was quite limited in
comparison to either K201 or M106. When the discharge from this source
is added to that of the other instrumented K1 pipes, the K1
peakflow increase approximates the increase observed at K201 (Ziemer 1992).
Keppeler (these proceedings) reports increases in
minimum summer pipeflow, as well. The duration of these
postlogging increases has yet to be documented.
M1 Pore Pressures
The water table throughout the entire monitoring period
was observed only along the regolith-hard bedrock interface. A
typical water table profile across the A-C transects during late
February 1994 is shown in figure 3. On the basis of field observations of
the soil pipes emerging at the pipeflow gages, it appears that the
pipes in the swale bottom occur at depths where the water table
often fluctuates into and around the pipe zone. As the winter
progressed, the piezometers responded more rapidly to larger rain events.
This behavior supports the findings of Ziemer and Albright (1987)
who observed a strong dependence of pipeflow on soil
moisture conditions. Piezometric responses in undisturbed drainages
will generally mirror pipeflow responses because both are
dependent on flow through macropores. Soil pipes are simply the largest
size-class of macropores. The peak piezometric response was noted for
a mid-February 1994 storm with peak rainfall occurring over an
18-hr period (fig. 5). Piezometric responses on the two
side-slope transects were fairly similar to each other. The lag between
the rainfall and the peak piezometric response for A and C
transects generally exceeded the lag for the B-nest piezometers.
The convergence of flow in the B subswales could explain the
difference in lag times with the parallel side-slopes.
Figure 5 M1 pipeflow and piezometric response to a moderate storm event during
February 1994. Note the similarity between pipe discharge and subsurface pore pressures responses to
this discrete storm event.
K2 Pore Pressures
The posttreatment response along two transects (C and E) has
been evaluated through hydrologic year 1993. Regression analysis
results indicate increased peak piezometric responses after logging. All
six postlogging regressions were significantly different than
the prelogging regressions (p < 0.05). The postlogging intercept
terms were greater than the prelogging coefficients and, in some cases,
the slopes of the postharvest regression lines were reduced
(fig. 6). Unlike the stream discharge peaks (Ziemer, these proceedings)
and the pipeflow peak response just described, increases in peak
pore pressures were detectable for both large and small storms, as well
as for antecedent moisture conditions ranging from a relatively dry
to a fully saturated soil profile. Between storms, piezometric
water levels remained higher in the postharvest period than
before harvest. Sidle and Tsuboyama (1992) state that pore
pressure responses tend to be less variable at the base of the hillslope than
at upslope positions because of higher soil moisture content and
the presence of preferential pathways in the saturated zone. These
K2 data support that hypothesis. Greater variation and
larger magnitude increases were observed in the upslope
piezometers (C3P2 and the E transect). At the mean M106 peak discharge,
pore pressures were 9 to 35 percent greater than those predicted by
the preharvest relationship.
Figure 6 Regression comparisons of peak piezometric response at the K2 and E-road sites before and
after clearcutting. C1P2, C2P2, and C3P2 are located in the lower portion of the K2 slope. E1P2, E2P2, and E3P2
are mid-slope K2 locations. R1P2, R2P2, and R3P1 are located along the E-road swale axis between pipeflow
outlet and the new road. Elevations are relative to the pipeflow outlet at each swale.
E-Road Pore Pressures
With only a single year of pretreatment data, regression
analysis was only marginally successful in illuminating changes in
pore pressure response at the E-road site. Before road construction,
pore pressure responses at this site were minimal. Although the
bore holes for the two most upslope piezometers were the
deepest installations at this swale (5.7 and 7.7 m, respectively),
positive pore pressures were not detected before road building and
tractor logging. During the first winter after road building, these
upslope piezometers remained dry; however, some changes were
observed at the lower instrument sites (R1P2 and R2P2). There were
brief spikes in pore pressures of less than 0.5 m, reflecting
individual precipitation events superimposed on a more extended
pore pressure response of about half that magnitude indicative
of seasonal effects (fig. 7). Hydrologic year 1991 was also the
second-driest year on record at Caspar Creek, with annual
precipitation totaling only 716 mm. This lack of rainfall made first-year
changes difficult to detect.
Figure 7 Piezometric response at the E-road site (between the base of the road fill and the
pipeflow outlet) for three winter periods: 1990 (predisturbance), 1991 (post-roadbuilding) and 1992
(postlogging). Elevations are relative to the pipeflow outlet.
After tractor logging was completed late in 1991, a series
of normal and abovenormal rain years ensued. The event-driven
pore pressure spikes continued at R1P2 during 1992 and 1993.
The regression analyses of the predisturbance pore pressure peaks
on the M106 peak pipeflows (log10) were fairly successful at
explaining the variations in response at the downslope installations both
before and after logging. The r2 values for the prelogging regressions
were greater than 0.80 for R2P2 and R3P1, and 0.49 for R1P2. Similar
r2 values resulted from the postlogging regressions of these
piezometer peaks. The postlogging regressions indicate increased peak
pore pressures at R1P2, R2P2, and R3P1 that are similar to
those observed at the K2 site (fig. 6). However, there was no
significant relationship between peak pipeflow at M106 and the pore
pressure response at the upslope E-road piezometers. These results
suggest the upslope E-road pore pressure response was quite different
than the response below the road and the response in the
undisturbed M1 swale.
However, after road construction and logging, a clear
and dramatic increase in pore pressure response at the
above-road installations is evident. Not only did peak pore pressures
increase in response to a discrete storm event, but also there was
a progressive increase in piezometric water levels related
to cumulative seasonal precipitation (fig.
8). At R4P2, the dry-season recession was particularly slow. By late fall of 1994 and 1996,
the pore pressure level remained higher than it had been at the onset
of the preceding hydrologic year. Pore pressures at this
bedrock installation located directly under the road centerline have yet
to return to predisturbance levels. At this same location, a
second piezometer, installed at the time of road construction at
the interface between the fill and the original ground surface,
never showed a positive pressure response. However, this
pressure transducer failed in 1996 and was not replaced.
Figure 8 Upslope piezometric response at the E-road site after road construction and tractor
logging. Note the seasonal increases in pore pressure heads as the rainy season progresses and the
discrete storm response most evident at R5P2. Elevations are relative to the pipeflow outlet. Gaps in
traces indicate missing data.
Table 1Annual maximum pore pressures (m) at E-road piezometers. Road constructed in May 1990 with
road centerline at R4P2. Tractor logging of swale occurred September through October 1991. "NR" indicates that
no positive pressure head was observed during that year. Base elevation (m) is the bottom of piezometer well relative
to the elevation of the pipeflow outlet. Maximum pore pressures for all years are shown in bold.
For all installations at the E-road site, the
post-road construction and logging annual pore pressure peaks exceeded
the predisturbance annual peak (table 1). To explore this difference,
a variable reflecting the storm rank through all years (1990 to
1995) was evaluated. This regression was more significant in
explaining the peak responses at R5P2
(r2 = 0.23) and R6P2 (r2 = 0.61), but
not significant for the belowroad installations. Above the road,
the apparent trend in pore pressures levels is one of increased
peak levels over time since logging (fig. 9). However, this may be
a reflection of above-normal rainfall totals in 1993 and 1995,
rather than the isolated impact of road construction in this swale.
More work remains to be done to model the pore pressure response
at this Eroad site. Pipeflow data from this site has yet to be
evaluated. This future analysis will provide an important indication of
the integrity of the macropore flow mechanism at this site after
road construction and tractor logging.
Figure 9 Peak pore pressure response upslope of the new road at the E-road
site. Fitted lines connecting median pore pressure peaks suggest a trend of increased
pore pressures over the 6-year period since road construction. Elevations are relative to
the pipeflow outlet.
Subsurface flow is the dominant process by which rainfall
is delivered to stream channels in the coastal redwood region.
Several different flow paths exist within soils and bedrock, and they
interact on both rain-event and seasonal time scales. As the soil and
subsoil become saturated, the soil pipes play an extremely important
role in hillslope drainage. The combined water storage and
transmissive properties of shallow earth materials are such that
headwater watersheds produce significant storm runoff and dynamic
changes in fluid pressures that are an important factor in the stability
of hillslopes. Management activities such as timber harvesting
and road construction can alter the subsurface flow and pore
pressure response to rain events. Increased subsurface flow from the loss
of rainfall interception and transpiration after timber
harvesting increases peak pipeflow and may accelerate scour erosion
within the soil pipes. This form of subsurface erosion can lead to
the expansion of discontinuous gullies within the unchanneled
swales and increased sediment loading to surface channels such as
has been observed in some of the Caspar Creek cutblocks (Ziemer
1992; Lewis, these proceedings). Further, subsurface drainage may
be impeded by the felling and yarding of logs in these zeroorder
swales if matrix and macropore flows are reduced by soil compaction
or shallow pipe collapses, thus accelerating gully erosion.
Timber harvesting increases peak pore pressures, but
whether these fluid pressures pose significant risks to slope stability is
highly dependent on local hillslope conditions. At those sites most
prone to failure because of inherent geologic and soil conditions,
timber harvest activities may tip the delicate balance of hillslope
stability towards failure. However, such failures are expected only
in response to relatively extreme rainfall events occurring at
roughly 5-year return periods (Cafferata and Spittler, these
proceedings). Thus far, the data from the North Fork phase of the Caspar
Creek study suggest that the frequency of large landslides has
not increased owing to the timber harvest activities between 1989
and 1991. The road location and design used in the North Fork
logging demonstrate a tremendous improvement in the application of
the principles of subsurface hydrology to minimize the risks
of aggravating slope instabilities. However, it is too early in the
post-harvest history to draw definitive conclusions concerning
slope stability. Large landslides occur relatively infrequently; thus, it
is necessary to evaluate failure rates over a long time. One
caution suggested by the findings in the M1 swale and previous research
is that convergent topography will amplify pore pressure
responses and there should be special attention and analysis when
planning operations in these areas. Designated crossings are an
effective safeguard, provided that the designator understands the
principles of subsurface hydrology as they relate to erosion control.
Road construction can have a very significant impact on the timing
and magnitude of pore pressure responses as exemplified by the
E-road site. Additional work is needed to further elucidate more
general relationships between road construction and pore
pressure evolution, as well as to better understand site-specific
subsurface conditions as they affect slope stability.
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