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Hillslope Hydrology of Selected Sites in the Eastern Drainage of Caledon Natural Area, Virginia

Brad White

During a period of intense rainfall, Brad White and John Gilreath made observations of channel morphology and runoff processes at the Caledon Natural Area on February 12, 1998 between the times of 3:00 and 4:30 PM. These observations were made to attempt to describe the major processes of channel formation in this area. Horton overland flow, Dunne overland flow, spring sapping, and headward erosion were observed at the Caledon Natural Area. Headward erosion and stream sapping appeared to be the dominant processes of channel initiation in this area.

Study Area

Caledon natural area is located on Rt. 218 east, approximately twenty miles east of the city limits of Fredericksburg, Va. The topography of the area consists of gently sloping hills, with slopes of 5 and 15 percent, depending on location. The relief from the highest elevation in the study area, to the lowest elevation is approximately 170 feet. There are no cliffs, or evidence of bedrock near the ground surface.

The dominant vegetation in the area is white and red oak, maple, poplar, dogwood, and American holly. The underlying vegetation is sparse due to the amount of shade created by the canopy.

The hill slope soils consist mostly of A and O horizons, almost immediately underlain by clay. There appears to be an abrupt transition from the top horizons of organic debris to the underlying clay. There are few rocks visible in the streambed. Those present are pebble size quartzite.

Field Observations

Site #1 appears to be a classic case of headward erosion by the process of spring sapping. Above the cut illustrated in picture 1. Dunne overland flow appears to be the most dominant

Sapping Picture 1

Picture 1

process of water movement. The channel above the headcut here is about one foot wide, and three inches deep. If photo 2 is examined closely enough, a spout of water about the circumference of a thumb can be seen pouring out of the headcut (just below and to the left of the glove). Picture 1c is a close up of the spout coming out of the silty clay in the wall of the headcut.

Sapping Picture 2

Picture 2

This spout is indicative of shallow subsurface flow. Another point to note is the smaller amount of water in the channel above the headcut, and the amount of water in the channel below the headcut. Subsurface flow is a plausible explanation for the visible difference in water volume above and below the headcut.

Site#2 has a stair-step pattern that is not evident in the other sites that we chose. The erosion in the background of this photograph appears to be due primarily to horton overland flow that has eroded through the A and O horizons. There is an abrupt drop in the foreground that is inconsistent with the height of the rest of the stair steps in the stream channel. It appears that erosion has been accelerated here in the very recent geologic past. To the left in the foreground of the picture is a tree root that extends over the pool of water. Presumably, this root was once embedded in soil. This suggests that the accelerated rate was not totally due to overland erosion, and has been aided by the undermining effects of spring sapping. It is also important to note the soil layers directly below the tree with the root extending over the channel. The A and O horizons are clearly visible, and illustrate the amount of topsoil that has been eroded due to a combination of spring sapping, and Horton overland flow when compared to the resistant layer of clay present in the head cut.

Site #3 drew our interest because water could be seen seeping out of the sides of the headcut walls. Although the size of the headcut is fairly small, it appears to be quite active. Another interesting observation about site #3 is its location on the hillslope, and the size of the depression. The slope of the channel is fairly steep in comparison to the short length of the hill. The headcut is located at the top of the hill, and has no discernible channel above it. It appears that the depression is below the elevated water table (due to copious rainfall), that is exhibiting some seepage into the sides of the headcut.

Site #4 is a beautiful example of headward channel erosion by spring sapping. The water in this channel has eroded through the organic layer above the headcut, down to the clay layer forming a channel about a foot and a half wide, and six inches deep. The subsurface flow present in this headcut is testament to the erosive power of spring sapping, which has weakened the clay layer, and eroded a bowl with a volume of about 100 cubic feet.

Interpretation

The headcuts in the observed sites correspond with the processes of headward erosion and spring sapping described by Dunne (1980), and Baker et.al (1990). The spout coming out of the wall of the head cut in site 1 shows undeniable proof of shallow subsurface flow. While no other sites studied in this assignment exhibited visible spouting, water could be seen seeping out of the sides in sites 3 and 4. The spout in site 1 and the seeping in sites 3 and 4 are evidence of spring sapping, and suggest headward erosion. All sites studied were marked by a scalloped formation in the side of the hill above the main channel, similar to a cul-de-sac at the end of a road. Above this scalloped formation, stream flow is the dominant erosive process for all of the sites, but can not account for the sudden absence of soil and clay in the headcut. For example, the channel above the headcut in site one measured about a foot wide, and six inches deep. The headcut below this channel measured about ten cubic feet, or two and a half feet wide and about four feet deep. The most plausible explanation for such a disparity among the top and bottom channels is shallow subsurface flow and spring sapping. Ground water was most likely conducted through the substrata via gravity to the area of the head cut, which at one time was a small indentation in the surface of the ground. Water then collected there below the surface, and built up a pressure gradient, and along with chemical weathering caused the eventual weakening of the cohesiveness in the soil structure. As the soil in this location weakened, it was pushed apart by water pressure, and slumped in on itself. After the initial slumping, a greater indentation was created, and subsurface water was even more attracted to the area, reinforcing this process of slumping. Eventually, a head cut was created, as depicted in the pictures. Figure 1 below is intended to clarify this process.

sketch

Figure 1

References

Baker, V.R., Kochel, R.C. Laity, J.E., Howard, A.D., 1990. Spring sapping and valley network development, in Higgins, C.G. and Coates, D.R.,eds. Groundwater geomorphology; The role of subsurface water in Earth –surface processes and landforms. Boulder, Colorado, Geological Society of America Special Paper 252, p. 235-265.
Dunne, T.L 1980. Formation and controls of channel networks: Progress in Physical Geography, v.4, p. 211-239.

Content Last modified: 01/27/03