X. DIVERSIONS AND CUT-OFF DITCHES

10.1 Diversions

1. Definition and description

These are earth structures constructed across a field to intercept and divert surface run-off from the slope above and drain it to a safe outlet. The structures can be in the form of a trench, a narrow base channel or a hillside ditch.

Criteria for selection

- For gentle slopes (not over 15%) and for machine construction, use the narrow base channel type.

- On steep slopes where small quantities of run-off are to be diverted, use the hillside ditch (terrace) type.

- For diverting large quantities of run-off at foot-hills, use trench type.

2. Objectives

- To protect cultivated lands, roads or farm buildings at the foot of slopes by diverting hillside run-off.

- TO safeguard conservation-treated areas at the foot of slopes.

- To collect water for ponds or other storage schemes.

- To control the development of gully heads.

3. Locations and conditions for use

- In areas where run -off from the slope above endangers low-lying lands, roads or conservation-treated areas.

- Where a gully is developing and especially where its head cutting has progressed to an advanced stage.

- Where a piece of land or a farm above the slope concentrates run-off which is damaging the land below.

- Where there is a need to collect additional water.

4. Design specifications

Details of the design for both narrow base channels and for hillside ditches have been given earlier. However, when designing diversion ditches, it is important that the horizontal gradient is big enough to drain water freely but safely to a protected outlet. The aim should be to carry as much water as possible to the safe outlet. The following design is for the trench type:

Length: Not over 350 m.

Gradients: These can extend to 3%. Horizontal grades can be uniform or variable as long as efficient discharge of run-off is obtained.

Run-off estimation: The quantity of run-off from a drainage area should be estimated using a 10-year return period for protecting agricultural lands and at least a 25-year return period for protecting buildings or homesteads, etc.

Velocity of flow: The maximum velocities for the proposed diversion should not exceed the following:

- Earth only: 0.4 m/sec for sandy soil, 0.6m/sec for other types of soil

- Grass lined: 1.8 m/sec for average conditions

- Ballasted: 3 m/sec

Depth: A freeboard of 10 to 30 cm must be added to the depth as a safeguard. The ditch capacity is usually larger than the designed run-off.

Cross-sections: The cross-sections are determined by the amount of runoff, the designed velocity of the flow, and shape and side slopes of the diversion ditches. The following formula can be used:

Q : quantity of run-off, in cubic m/sec

A : area or cross-section, in square m

V : flow velocity in m/sec

When velocity is calculated by the Manning's formula, within the permitted limit and the total run-off is estimated, the necessary area (A) then can be obtained by the above formula.

Volume: The volume can be calculated by multiplying the designed cross section by the ditch length. Since the trench is formed by cut and fill (approximately equal), only 50% of the cross-section should be accounted for when calculating the volume.

The design of ditch can be adjusted by increasing the cross-section and gradient to accommodate larger amounts of run-off. However, the velocity of the flow should not exceed the safe limits explained earlier. Another design modification is to make the ditch's side slope and gradients less steep on sites where the soils are prone to erosion.

5. Diagrams and tables

Fig. 15 contains a diagram of the trench type of diversion ditch for moderate slopes.

Fig. 22 contains a formula used for calculating the cross-section of a trapezoid trench.

Table 6 lists the cross-sections of trapezoid diversions and their corresponding run-off and safe range of velocities.

Fig. 15. Diagram of a trapezoid-type diversion.

6. Layout and surveying procedures

A diversion should be laid out according to its main objective, ie. just above the area to be protected, at gully head, at the up-slope boundary of a farm where another farm or farms are situated higher up, or in an area where it can intercept as much run-off as possible.

The diversion should be connected to a safe outlet, a waterway, a protected area or a storage area. Surveying must begin from the outlet end to ensure safe drainage or proper storage such as a pond. After staking out the centre line (the non-cut and non-fill line), the side stakes can be set according to the planning width.

7. Construction procedure

By manual labour

Clear all stumps and vegetation from the proposed site. The fill (embankment) at the lower side should be compacted, layer by layer and should be constructed higher than planned to allow for settling (about 10% of the fill). If necessary, extra soil from the lower side can be used to strengthen the embankment. A final check of the bottom gradients and the embankment top is necessary. Any deviation should be put right immediately.

Mechanized construction

For narrow base channels or hillside ditches, the construction procedures are the same as for the broad-base terraces, hillside ditches or bench terraces.

8. Physical output

By manual labour

Diversions entail about 25 to 50% more work than construction of hillside ditches because of the need for deep digging. The output per man-day is from 2.5 to 3 cubic m, depending on soil type.

Mechanized construction

For constructing narrow base channels or hillside ditch type of diversion, refer to earlier physical output sections.

9. Cost relations

For diverting small flows, the hillside ditch (terrace) type is much more economic and effective than the trench type. It is also easier to maintain. Construction costs are much cheaper in areas with few stumps and rocks. Assuming the quantity of the run-off is the same, it is much more expensive to construct a diversion ditch on steep slopes rather than on medium or gentle ones.

10. Operation and management

In order for diversions to be operational and effective, the upslope area should be protected with vegetation cover or other conservation measures in the case of cultivated fields. This may not always be possible when the use of the upslope land is controlled by other people. One or more rows of thick tall grass should be grown and maintained above the cut bank in order to stop soils coming down into the trench or channel.

11. Protection and maintenance

Protection

The trench or channel should be planted with a rhizome-type low grass or can be partially ballasted, depending on designed velocities. The embankment facing downslope should also be planted with rhizome grass for protection purposes.

Maintenance

Deposits in the trench or channel must be cleared regularly to ensure that the water flows freely all the time. Any large plants or bushes should be removed from the embankment, trench or channel.

Table 6. Different sizes of diversions and flows

Note: The column to the left of the heavy line indicates a safe range of velocity.

10.2 Cut-off ditches

1. Definition and Description

This is a fairly deep trench constructed across a field to reduce seepage damage or to intercept sub-surface flow in particular. The ditches can be made of earth, loose rock or other material depending on resources and needs.

Criteria for selection

- For a large quantity of sub-surface flow, use a drainage or a cut-off trench.

- For small but constant seepage, use a filling-in trench or a cut-off ditch.

2. Objectives

- To intercept primarily subsurface flow.

- To reduce seepage damage, slope failure or land slips caused by constant wetness.

- To protect against the development of a gully head by perched ground water.

3. Locations and conditions for use

- On sites where seepage damage is serious.

- Where the foothills or downslope lands need to be protected from constant or intermittent sub-surface flows.

- Above a landslip area or at the head of a gully.

- Immediately below a water source area.

4. Design specifications

Drainage type: The depth of the trench or ditch should be deep enough to intercept the sub-surface flow. The whole of the trench or just the up-slope wall and the bottom can be built with loose rock or stones to prevent it sliding backwards down the slope. In all other respects, the design is similar to that for the trench-type of diversion.

Filling-in type: These can be much narrower than the drainage type. trench or ditch should be cut deep enough to prevent water seeping through. Material such as clay, rocks with cement and, if necessary, concrete can be used to fill in the trench to cut off seepage. The depth of filling is usually higher than the seepage line. The gradient of the trench or ditch is smaller than for the drainage type since not much water will be drained. A gradient of less than one percent is usually appropriate. Any collection of surface run-off must be drained properly.

The trench volume for stone ditches can be obtained by calculating the cross-section and multiplying by the length as explained earlier. The dimension of rough cut is usually larger than designed in order to accommodate a stone layer (15 to 20 cm). The volume of the stone lining can be obtained by estimating the size of the bottom and the side(s) and then multiplying by a unit area (Table 7) to obtain the total material. If only the up-slope wall is lined with stones the size of that wall is used for calculating the material. The volume calculations for earth ditches are the same as for stone ones except the actual designed dimensions should be used when cutting the cross-section.

5. Diagrams and tables

See Fig. 16.

Table 7 shows different types of stone ditches and the material required for constructing one square metre.

6. Layout and surveying procedures

The same principles and procedures as for other types of ditches or trenches. The depth of cutting needed for intercepting seepage, ,etc., can be determined by a field survey using a soil auger.

7. Construction procedures

Earth trench and filling-in type: The earth trench can be dug either by machine or by hand. The dug-out soil should be placed immediately downslope to form an embankment. Filling-in must be done layer by layer and should reach a height above the seepage line.

Stone ditch and drainage type: A larger cross-section than designed should be dug to accommodate the stone lining. The stones must be cleaned beforehand and only solid, uniform-sized ones should be used. Lay the stones layer by layer beginning from the bottom of slope. The longitudinal side should be away from the ditch and the narrower sides against each other (Fig. 16).

Use small pebbles to fill in the cracks and to stabilize the layer. If the stones are lined with concrete, they should be embedded in the foundation and must always be wetted before they are placed in position. If the ditch is lined with concrete, small holes or pipes should be used for releasing the water.

8. Physical output

The earth moving part is about 2 to 3 cubic m per man-day depending on the depths of the trench and type of soil. The output of the stone lining operation depends on available material and the skills of the operator. Three men can complete 15 square m a day.

9. Cost relations

An earth trench is much cheaper than a stone-lined one. For the drainage type, it may be more economical and practical to line only the up slope and/or the bottom. An earth trench is usually sufficient for the filling in type. Its cost depends largely on the kind of filling material and the method used.

10. Operation and management

Any deposits, obstacles or overgrowth must be removed from the trench. Any slipping of the banks should be repaired promptly.

Fig. 16. Drainage-types of lined, cut-off ditches.

Table 7. Different types of stone ditches and required materials for construction of one square m.

The amount of materials required for making the concrete or cement and sand mixture is as follows:

* each weighs 50 kg