3.4 SLOPE ANALYSIS

Slope and aspect analysis helps in understanding the topography of the region. Slope analysis provides the slope ranges available in the region using the contour data thus determining the compatible and incompatible slopes for urban development. Slope analysis, along with surface geology and soil data, will determine the most appropriate sites for various land uses.

The slope of an area is the ratio of the difference between the highest and lowest altitude points of the place (rise) to the horizontal distance measured between the points (run). The larger rise and the shorter run, the steeper is the slope. Normally slope is represented in terms of slope angle (???or ?in terms of the slope percentage (%).

The slope angle, or degree of slope ???is determined by tanø=rise / run.
The slope percentage is determined by the formula slope % = (rise/run) X 100.
The slopes are classified into five categories based on the steepness of the slope.
They are:
1. Flat terrains (0 – 5%)
2. Gradual slopes (5 – 10%)
3. Moderate slopes (10 – 20%)
4. Steep slopes (20 – 30%)
5. Very steep slopes (greater than 30%)

The above graphics demonstrate the various slope categories represented as color codes in the slope analysis map.

The slope analysis map provides the range of slope percentage for the valley. The colors ranging from yellow to dark green represent the flat, gradual and moderate slopes. Steeper slopes are represented by darker shades of red and violet.

The analysis determines that most of the valley floor area falls under the first three categories of slope ranges and is best suitable for urban development. But when we overlay the slope analysis map with the geological data it reveals that most of the flat terrains (0 – 5% slopes) are Lower River Terraces, which are potent in ground water and have rich soil resources. So mainly those areas, which are not Lower River Terraces, are suitable for development, such as those found in Hejo and Jongshina.

The areas above 30% slopes are not suitable for development due to the following reasons
• These slopes are prone to soil erosion and landslides.
• Development on these slopes involves cutting of slopes, clearing of vegetation thus damaging the slope characteristics, affecting the stability of the soil and slopes. This in turn accelerates the rate of soil erosion and increased surface runoff. Larger parts of the 30%, and above, slopes are within forest areas except a few areas, like the slopes above (east of) the Lungtenphu-Simtokha road, upper Babesa and the upper Jongshina areas. These areas are not suitable for urban development.

View of the city showing various slope ranges

3.4.1 OTHER ISSUES

• The potential slope ranges (0 - 5% and 5 - 10%) for urban development in terms of densification and the ease of providing infrastructure, are occupied by low density institutional land uses like India House, IMTRAT, RBP, RBA, RBG, the proposed Diplomatic Enclave area and other institutions, which can be accommodated in moderate and steep slopes (10 - 20% and 20 - 30%), or even out of the city limits.
• Development on very steep slopes (30% and above) should be restricted or even banned, as development on these slopes could cause soil erosion, thus increasing the surface runoff, which could make these slopes prone to landslide.

Note: Please refer Table No.3.2, Slope Analysis, for various slope categories, locations that fall under these categories, characteristics and the significance of theses slope ranges and the land use suitability for each of the slope categories.

Hill slopes above Lungtenphu – Simtokha road which fall under the very steep slope category has already started showing signs of soil erosion due to lack of vegetative cover.