APPENDIX 1
QUANTITY AND COST ESTIMATION OF GULLY CONTROL WORK
To estimate the costs and determine the quantity of gully control work, first survey the gully's longitudinal profile and its cross-sections. Structure designs and rate analysis should be prepared later.
1.1 Surveying the longitudinal profile of the gully
Lightweight instruments such as the clinometer (0 to 90 degrees), handcompass (0 to 360 degrees), and 50 m measuring tape can be used to survey the longitudinal profile of the gully. A simple field survey book can be used for this purpose (Fig. Al).
The gully's longitudinal profile is an open traverse between the proposed first and last check dams. Proceedings from the bottom to the top of the gully channel, the horizontal angles, vertical angles, and the ground distances between station points are measured with a hand compass, clinometer and 50 m tape, respectively. While surveying the open traverse, materials available at the site should be noted, as this will play a part in determining what kind of check dams to build.
Horizontal and vertical distances between station points are computed first as indicated in Fig. Al; the longitudinal profile is drawn later as shown in Fig. A2. The number of check dams needed for the gully or gully portions is calculated according to the total horizontal and vertical distances, average and compensation gradients, and the average effective height (from ground level) of the dams.
1.2 Surveying the cross-sections of the gully
The cross-sections representing each portion of the gully bed can be measured with surveying rods (Fig. A3).
The average width of a gully portion is determined first; then the cross-section at this point is measured as indicated in Fig. A4.
One worker holds the rod vertically in the centre of the gully bed (the lowest point of the cross-section). This rod is scaled in centimetres on both sides. Another holds the other rod horizontally by using a carpenter level so that one end of the rod touches the ground. He reads the horizontal distance between the first touching point and the centre of the gully bed. At the same time, the first man reads the vertical distance between the centre of the gully bed and the first touching point. A third man records both readings in a notebook by drawing a sketch of the gully cross-section (Fig. A4). While moving to the first touching point, both the first and second man measure the vertical and horizontal distances between all the touching points in the same way.
When the sum of the vertical distances becomes greater than the height of the proposed check dam, the two men return to the starting point at the centre of the gully bed, and survey the second part of the cross-section in the same way.
1.3 Design of check dams
The cross-section for each portion of the gully bed is drawn to scale, usually 1:20, 1:50, 1:100 or 1:200. Then the check dam is designed according to cross-sections based on the dimensions of the spillway, the thickness of the crest and base.
1.4 Quantity estimation
After designing check dams for each portion of the gully, the check dam's body volume and the volume of the soil excavated for the foundation are calculated. These volumes are multiplied by the number of check dams for that portion of the gully to determine the total volume.
1.5 Rate analysis
Rate analysis is used to compute the unit price of each construction (gabion, dry stone, excavation, etc.). If a standard rate analysis document is not available, unit prices indicated by field experience can be used to estimate costs.
1.6 Cost estimation
Multiply the unit price by the quantity of work to determine total cost. A "cushion" of 10 percent should be added to the total cost to cover contingency expenses.
