Rural System's

RRx

Slopes

Prescription

Study the slopes and their distributions on your ownership. They are used in many analyses for decisions throughout the RRx. Share ideas with RRx staff for applying slope knowledge that may be of specific importance on your area.

Diagnostics

The following is an analysis of the slopes of your area. The following are planned:

These data may be considered in light of the following remarks. They relate to recreational, forestry, fisheries, and other land use potentials. Slope is an example of a non- or abiotic factor of importance in the study of plants and animals and their relations. They also relate to access for emergencies. For some, they characterize land for agriculture, taxation, flood damage and insurance, and human settlement. Slopes suggest the characteristics of some watersheds. Slopes, alone, are important, but take on special meaning when combined with the aspect or compass direction of land seen as if facing downhill.

Here a county-scale map (private land only) suggests slope and aspect categories that can be formed for an individual land ownership. The map is a demonstration only but owners might be interested in a map showing north-facing moderately steep sites for restoring white pine forests.

Many environmental factors influence how viable have been alternative uses of land. Decisions about land use must be made within the context of these factors. This section of the RRx report is about how slope, an integral component of a total ecosystem, influences planning options on the land.

Slope is the angle of incline of the land surface. Slope maps can be readily produced by GIS and often data from them such as slope frequency (O'Neill and Mark 1987) can be more useful in landscape analyses than the map image. Slope steepness is expressed as degrees from the horizontal, as percent (45-degrees being 100-percent), or as a ratio (e.g., a 3:1 slope means 3 units along the horizontal for every 1 unit of measure along the vertical. Steepness is an important factor in rural land management. It effects the utility of land for many purposes, the velocity of running water, and the micro-climate of sites. Slope steepness is a dominant factor in models used to estimate potential soil loss or erosion. It is used as one of many "bands" of information in remote sensing analyses of landscapes. It relates to gravity and the costs of movement. It relates to ground slippage and mass wasting. It relates to solar radiation received within a map unit. Decisions about where to do field work and the impacts of doing so are influenced by knowledge of slope steepness.

Slope and Elevation Classes and Their Diversity

The following tables analyze the proportion of the area in various slope and elevation classes. The emphasis is upon general decisions which often emphasize percentages (or proportions) of areas steeper than ...some specified amount. Decisions often include risk expressed as " ... a high proportion of land in class VI and VII. "

The Shannon Index reported with each is an index of evenness or how evenly distributed the land is in all classes. If there are equal proportions in all classes the index will be at its maximum.

The index will be strongly correlated with indices of surface roughness. It is likely that it will relate to other watershed characteristics but enough studies (e.g., simple plots of suspected relations in different watersheds) have not been done.

The table is simple: Where there were 20 classes of slopes in a table, 4 percent per class (e.g., 0.0 to 3.99, 4.00 - 7.99, ... with the last class being 76.0- 99.99) then the proportions of the area in each class can be presented and the Shannon index shown as a summary statistic (in one case for a large watershed, 1.629).

A table can be created slopes (by degree or class, acres, percent; then aspects can be sparated and the same table presented.

Where slope classes are used, cumulative acres can be presented, first in ascending order, then descending order. These columns can be used to evaluate the percent of the area that is within certain slope constraints. (Constraints are steepness limits beyond which present technology and land protection needs make certain actions inappropriate (e.g., timber harvests with certain machine limits and road cuts.)

A picture of the distribution of slopes can be useful in comparing areas, the cost to an animal population of moving within an area, runoff potentials, etc.

Capability Classes

Fairly well recognized land capability classes are used throughout farming. These are mostly percent-slope classes. (Gross ranges are published but assigning a map pixel to a class requires a precise criterion as shown in the following table.) The classes are of different size, making analyses very difficult. Information on Class V is currently unavailable. A table is created showing the acres (and hectares) in each class with a percentage or proportion of the total area in each class. Perhaps a diversity index computed using these proportions may have meaning for some people.
I 0.0 - 2.0
II 2.1 - 7
III 7.1 - 15
IV 15.1-25
V  
VI 25.1 - 35.0
VII 35.1 - 99.9

Equipment Operability Slope Classes

Logging and other equipment used in rural areas are designed to operate on steep slopes, but there are limits. Typical logging equipment has the following operability classes. A table presenting the acres and proportions of an area in each class suggests "land unsuitable for conventional forestry " and productive potential of areas. Potential for trails, hunting, or other recreational use may be suggested (or the needs suggested for other classes and analyses.) See these classes within the section on Aspect.
I 0.0 - 20.0
II 20.1 - 40.0
III 40.1 - 99.9

Safety

Slope steepness in combination with soil and rock type, moisture, and disturbance influences whether surfaces will fail and rock- or mud-slides occur. Mining stability analyses depend on such knowledge for stability analyses of embankments, hollow-fills, dams, and spoil bank management. Siesmic activity and its consequences are affected by the slopes of the land. Roots and root decay after timber cuts can affect slope stability. Soil strength tends to increase linearly as live root biomass increases (Ziemer 1981).

Construction

Slope, in combination with geologic factors, can determine the suitability of land for constructing such facilities as buildings, roads, and utility lines. Foundation stability, construction costs, accessibility, and immediate and longterrn environmental impacts are also related to slope. Slope thus influences the feasibility of construction.

Water Supply

Slope directly influences the local amount of runoff, stored surface water, and ground water recharge. If there are constant soil characteristics, the runoff of precipitation which will eventually enter a lake or stream will increase with increasing slope. Conversely, as the degree of slope decreases, more water is stored at the surface and percolates into the groundwater system. These concepts are important in planning projects such as those for water supplies, or in building and regulating ponds and storm water control structures.

Slope classes are frequently used but they tend to obscure information already avaialble. When there are appropriate limits and comparisons need to be made with past studies, slope classes may be appropriate. Classes are usually based on other factors such as interest in water movement. Whether slope classes, land form, or drainage classes may be a good question asked for the following table:
Number Soil Drainage Classes Position and Slope
1 Very poorly drained Flood plain < 2 %
2 Poorly drained Toe slope - equal or greater than 6 %
3 Somewhat poorly drained Toe slope and side - <6 %
4 Moderately well drained Upper and lower side - Shoulder
5 Well drained Side
6 Somewhat excessively drained Side and steep shoulders
7 Excessively drained Ridges
From information in column three about map cells, the other two columns can be estimated and the condition mapped.

Waste Disposal

Slope is a significant factor in planning both solid and liquid waste disposal facilities. Slope must be included in decisions for locating landfills since too steep an area is not economically feasible. In addition, the seepage of pollutants may contaminate local water supplies.

The efficiency of drainfield sewage disposal systems is correlated to slope. A steeper area loses efficiency in sewage effluent renovation due to the decreased retention time. Therefore, a relatively flat area makes a better location for a drain field.

Roads and Other Transportation Systems

Both the location of road networks and their extent are largely determined by slope. Accessibility decreases, and construction costs increase with increasing slope. The necessity of grading steeper slopes may greatly increase erosion by disturbing natural vegetation and drainage patterns. The necessity of cutting and filling steep grades adversely affects natural scenery. The removal of snow is less essential and usually more easily accomplished on gentler slopes. Recreational area parking lot construction is less expensive and less environmentally disruptive in areas of minimal slope.

Similarly, the degree of slope restricts the development and scale of air, rail, and even water transportation systems.

Streams

Slope directly affects the velocity of running water and therefore streamflow characteristics. Gradient is the fall in feet per mile, and is usually greater at the headwaters than at the stream mouth. The rate of peak stream discharge and the volume of sediment that a stream can carry increase as the slope or gradient increases. Erosion of the stream bed, both vertically and laterally, is greater where the water velocity is higher due to steep gradients. Many flat areas have that characteristic because of past flooding and the influences of water and beaver dams.

In addition to these properties, slope is also related to many other stream characteristics such as stream bed substrate, water temperature, the type of fishery that is possible in a stream, and stream ecosystem energy relationships.

Life Spaces

Varying slopes produce a variety of microclimates and other microsite characteristics. Temperature, moisture, and wind are affected by slope, as are soil particle size distribution and the resulting soil characteristics. The energy coming into a site and that leaving at night and available water are also influenced by slope. The microsite conditions created by these factors strongly influence which plant and animal species will exist on the site, the more energy is required for mammals and reptiles to live there. Less steep areas typically have more wildlife but these same areas have other uses, thus conflicts occur.

Rayburn (1972) showed how slopes were used in estimating the energy requirement of white-tailed deer to ive in an area (the steeper, the more energy-costly).

Recreation

Steep slopes are essential for some forms of recreation and may enhance or detract from others. Skiing requires steep slopes and trail bikers and 4-wheel drive enthusiasts seek out steep, rugged terrain. The visual contrasts of varying slopes enhance visitor experiences on hiking trails and scenic drives. However, steep slopes also limit the types and location of developed public recreation areas by raising their construction and maintenance costs.

Resources

Excessive slopes restrict mining, agriculture, and forestry by increasing costs or limiting accessibility for certain types of machinery. Although very steep areas can be made accessible, the environmental costs may be very high at current technological levels.

Basic Research

Basic or not, scientists have been interesting in landscape analyses and descriptions and in modeling the land form. Concave profiles may be cycloidal or of exponential form. In one area, the cycloidal form seems associated with erosional processes and the exponential form with depositional processes (Bridge and Beckman 1977).

From The Roanoke Times, June 3, 2006 with permission; from "Report #466 of the APA Planning Advisory Service" in an article by Olshansky. U.S. Geological Survey (Professional Paper 351)."

Using the Topographic Abney Level
a hand-held surveying device

To find E
E = A x B

To find A
A = C / slope distance for 1 chain on the horizontal



Related Questions and Areas for Exploration by RRx staff and Others

  1. Are we using the best available algorithm for slope analysis (then Topo-II by Grender of the Va Tech Geology Dept.) to give as accurate slope information as is available from the sampled elevations?
  2. Can the computational costs be reduced?
  3. Are there alternatives (data structure and storage) (Wong 1980- a MS thesis of the Computer Sci. Dept.)?
  4. What is the effect of changing cell size on the algorithms? On accuracy?(Mathur in a MS thesis in 1990 of the Geography Dept at Va Tech explored some of these relations)
  5. Can resulting data and its form be useful or does the GIS system and use constrain the algorithm?
  6. Are alternative approaches feasible and useful in gaining cost effectiveness such as: