PART D - DRAINAGE

2.2 DITCHES* AND DRAINS*

DEFECT: OBSTRUCTIONS

Main Causes- vegetation growth, bushes, fallen trees, debris, loose

silt, loose rocks.

Development, if neglected- blockage of ditch.

Remedies- clearing and cleaning.

DEFECT: SILTING

Main Causeslnvert* slope is too flat, the water cannot flow atsufficient speed.

Development, if neglected:- Blockage of the ditch.

Remedies

- deepen ditch (desilting), and/or provide new mitredrains (turnouts),

where deepening or turnouts are not possible becauseof topography, the construction of a new culvert* with adrop-inlet may be possible, in order to discharge wateronto the other side of the road.

* See List of Terms, Volume I.

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PART D - DRAINAGE

DEFECT: PONDING IN DITCH AND ON SHOULDER

Main Causes- the ditch cross-section is too small,- the ditch gradient is too flat.

Development, if neglectedThe shoulder material becomes soft and can easilyerode. The pavement* can also be flooded andthereby weakened.

Remedies- deepen ditch,- provide new mitre drain.

DEFECT: DITCH CROSS-SECTION IS DESTROYED(UNLINED DITCH)

Main Causes- vehicular or animal traffic, cave-in.

Development, if neglectedPartial silting will result if the ditch sides havecollapsed. Erosion can start where water flow passesthe blocked section.

Remedies- reshape/regrade ditch, line drain.

* See List of Terms, Volume I.

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PART D - DRAINAGE

DEFECT: INVERT* AND SIDES OF DITCH* AREERODED

§ Main Causes

Invert slope is too steep.

§ Development, if neglected

The water flows at high speed and starts erodingthe soil. The ditch becomes deeper (ravine). Thesides then cave-in, the road shoulder and even partof the carriageway* can be washed away.

§ Remedies

Erosion control:

- Regrade/realign drains,- provide repair scour protection,- line drain slopes and invert,- construct cascade.

* See List of Terms, Volume I

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PART D-DRAINAGE

DEFECT: DITCH* LINING IS DAMAGED

§ Main Causes

- poor construction workmanship,- soil settlement, erosion of soil under ditch lining,- poor alignment or sudden change in flow direction.

§ Development, if neglected

When flowing water reaches the soil protected by thelining,. erosion starts. The amount of soil washedaway increases, the lining is further damaged by lossof support, leading to complete destruction of thelining.

§ Remedies

Erosion control:

- repair lining,- realign drain.

* See List of Terms, Volume l

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PART D - DRAINAGE

DEFECTS: EROSION AT DRAIN OUTFALL*

Main Causes

- flow too fast,- flow too concentrated,

for the soil at the outfall to resist.

Development, if neglected

Erosion will continue back into the ditch andincrease in the area of the outfall. The erosion mayeventually threaten the road as well as thesurrounding land.

Remedies

Reduce water flow and speed by:

- realign drain to flatter gradient,- provide new mitre drain, upstream from existing.

Reduce impact at outfall by:

- construct cascade,- construct flow spreader.

Erosion Control for the soil:

- turfing,- wattling,- stone pitching.

* See List of Terms, volume I

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