Types of Slope MeasurementSeveral methods exist for calculating slope steepness. The appropriate method, slope categories, and scale and accuracy of mapping used depend on the intended use of the information. It is also important to understand that different techniques of slope steepness calculation might give different results. Three common methods of calculating slope are identified below (Table 1.A).Table 1.A - Types of Slope Measurement
Type of Measurement
Site Specific Slope Map
This consists of measuring the horizontal distance between each set of parallel contours on a topographic map, and mapping the results in various slope categories (e.g., 0 to 15 percent, 15 to 25 percent, etc.). The resulting map is a mosaic, identifying the slope steepness at any point within a given geographic area. The precision of the slope map is dependent upon the scale and contour interval of the topographic map being used. For example, a slope map prepared from a topographic map with a scale of 1"=100' with contour intervals of 5 feet will be far more accurate than using a topographic map with a scale of 1"=2,000' and a contour interval of 20 feet.
Cross-Sectional Slope Map
A (typically) 100- to 200-foot cross-section line is drawn on a site plan map through the middle of a building site at right angles to the contour lines, and the overall slope of the cross section is measured. Often, the steepness of several cross sections is measured to define an average slope. While significant changes in grade a re taken into account, this method tends to minimize the steepness of rolling lands.
Average slope defines the average steepness of a given geographical area, and is expressed by the following formula:
S = Average slope percent
A = Total acreage of the area being measured
L = Length of each of the contours, in feet (with appropriate scale), within the area being measured
I = Contour intervals, in feet
While average slope is a useful concept to describe generalized steepness of a given geographical area, it can mask actual conditions in that area. For example, moderately rolling hills may have the same average slope as a flat plain that is cut by a deep canyon. Thus, even though the flat plain is readily accessible and easy to develop, the use of average slope to define developability might understate the potential of that property.
How Steep Is Too SteepAs noted above, many communities use slope steepness to define developability. The basic premise of this approach is that, as slopes become steeper, grading and provision of infrastructure becomes more difficult and expensive, and the extent of landform modification, loss of the aesthetic appeal of natural hillsides, and environmental degradation becomes greater. Based on a review of several hillside ordinances and regulations from communities in the western United States, slopes are generally categorized and treated in the manner described in Table 1.B.Table 1.B – Typical Slope Categories Used in the Western United States
Slope Problems and General Management Criteria
Less than 15 percent
These lands are not generally considered to be hillside for management purposes due to the absence of problems related to slope, although rolling land with slopes greater than 5 percent are commonly considered to be hills. Permitted land uses and density/intensity of development are generally determined by underlying zoning.
15 to 25 percent
At about 15 percent, slopes begin to create problems for development. Above 12 to 15 percent, roads must run diagonal rather than perpendicular to slopes, as 12 to 15 percent roads are the steepest that are comfortable and safe to drive on, except for very short distances. Grading becomes difficult at this steepness, as roads and buildings require extensive cut and fill of earth. The more restrictive hillside ordinances prohibit development on slopes in excess of 20 percent, or limit the maximum allowable density to 1 dwelling unit (du)/10 acres (ac), while other ordinances limit density to approximately 1 du/ac or 50 to 75 percent of comparable flat land densities within the underlying zone.
25 to 35 percent
Slope becomes a critical factor in development at this steepness. Large parcels are required because of the extensive cut and fill needed to create roadways and level building pads. Access between roadways and building pads is difficult, and the design of individual lots and buildings to fit site-specific terrain conditions is important. Moderately restrictive ordinances begin prohibiting development or limit ing density to 1 du/10 or 20 ac. Where development is permitted, uses are generally limited to single family residential. Other ordinances limit density to approximately 1 du/2 or 3 ac or 25 to 50 percent of comparable flat land densities within the underlying zone.
Over 35 percent
Slope of this steepness is extremely critical. Allowable steepness of cut and fill slopes (50 percent) approaches or exceeds natural slopes, resulting in very large cuts and fills or the need for retaining walls for creating even small flat areas. By and large, roads should not penetrate terrain of this steepness.
Because of grading problems, individual sites and homes need to be custom designed. Grading in slopes exceeding 50 percent can result in extreme disturbances, and are generally considered to be undevelopable. Only natural building sites within large areas in this steepness category can be developed without extreme disturbance and high costs. Development is prohibited or highly restricted (1 du/40 ac), and uses are limited to single family residential and open space. Where development is permitted, density generally is restricted to 1 du/5 ac or 25 percent of comparable flat land density.(Source: www.ci.glendora.ca.us)
Friday, April 24, 2009
Hill cutting raises concerns, questions
Hill cutting continue unabated in Penang following the collapse of BN state government. The protest by TBRA and members of civil society seem so far to have fallen on deaf ears. The hills in penang are gift of nature. Rampant and indiscriminate hill cutting to create flat land sites for new housing and for other uses is destroying these. The need to incorporate a hillside development ordinance into the Town and Country Planning Act has become urgent as the guidelines for hillsides are not good enough to deter the present state government and the vested interest from stretching their greedy grid to our hills.
A series of landlides on engineered slopes have occurred in which lives were lost. Of these the High Land Tower and Bukit Antarabangsa tragedies probably received the most publicity. When tradegy occurred our politicians were quick to response with call to stop hillside development, but immediately after the dust have settled the same politicians choose to ignore our concern by giving approval after approval to development on hill slope with gradient even more than 35 degree. Lim Guan Eng's administration choose to ignore it's obligation to preserve hillside areas and the need to protect the safety of surrounding communities by blaming the previous state government for the approval of hillside development and do nothing about it.
We need to develop sensible and enforceable ordinances and regulations regarding hillside development to balance the desire and need to perserve hillside areas with a recognition of private property rights.
First we need to define steep slope "Rule of Thumb" to determine what sort of topography is developable. Then we should define the types of slope measurement that are applicable in determining the slope steepness instead of leaving the decision on slope steepness to the state bureaucrat's discretion.
The state government rely on slope steepness as the primary or sole determinant of developability. A slope is defined as the vertical change in elevation over a given horizontal distance. It can be measured as a percentage, a ratio, or as an angle, as illustrated in Figure 1.1.
Figure 1.1 -Grades
We cannot allow massive hill cutting to go on without taking consideration of the impact on our environment. The current housing development practise on hill slopes involves a lot of hill cutting and earth filling. Using heavy excavating equipment to carry out massive cutting into the hill slope and removing the earth to make the hill slope less steep is how the vested interests committed atrocity against our nature. This type of practise have serious environmental impact which might lead to landslide especially during rainy season. We need to incorporate the definition of slope categories as use in US in hillside development ordinance to prevent indiscriminate hill cutting from carry out. We need to make the state government to accountable for it's policy and position on hillside development.