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CLOSE THIS BOOKRoadside Bio-Engineering - Site Handbook (DFID, 1999, 160 p.)
VIEW THE DOCUMENT(introduction...)
VIEW THE DOCUMENTAcknowledgements
VIEW THE DOCUMENTIntroduction
VIEW THE DOCUMENTSafety Code of Practice for Working on Slopes
Section One - Stabilising slopes with civil and bio-engineering
Section Two - Civil engineering techniques
Section Three - Bio-engineering techniques
Sector Four - Production of bio-engineering plants
Section Five - Maintenance of bio-engineering
VIEW THE DOCUMENTAnnex A - Site assessment pro forma
VIEW THE DOCUMENTAnnex B - Full lists of species for bio-engineering in the road sector
VIEW THE DOCUMENTAnnex C - Nursery registers
VIEW THE DOCUMENTGlossary
VIEW THE DOCUMENTBack cover

Annex A - Site assessment pro forma

Complete one pro forma per site. Use additional forms if there are more than three segments on the site

Site location and chainage:




Date of assessment:

Assessor's name:



Sketch of site



[Label segments]













Scale:


Orientation:


Segment number

(1)

(2)

(3)

(a) Erosion and failure processes




(b) Other factors




(c) Slope angle(s)




(d) Slope length




(e) Material drainage




(f) Segment moisture




(g) Altitude




Assessment criteria. See Roadside Bio-engineering Site Handbook, Annex A, pages 127-129 for details

(a)

Erosion and failure processes.

List the erosion or failure processes. State their size and severity.

(b)

Other factors.

List any physical factors that might affect the site. State their size and severity.

(c)

Slope angle(s).

Measure and place in one of 3 classes: <30°, 30 - 45°, or >45°.

(d)

Slope length.

Measure and place in one of 2 classes: <15 metres or >15 metres.

(e)

Material drainage.

Assess and place in one of 2 classes: good or poor.

(f)

Segment moisture.

Assess and place in one of 4 classes: wet, moist, dry or very dry.

(g)

Altitude.

Determine: ± 100 metres. Use an altimeter, map or site drawing.

Figure A1: The main erosion and failure processes

MECHANISM

DESCRIPTION

Erosion on the surface

Rills and small gullies form in weak, unprotected surfaces. Erosion should also be expected on bare or freshly prepared slopes.

Gully erosion

Gullies that are established in the slope continue to develop and grow bigger. Large gullies often have small landslides along the sides.

Planar sliding
(translational landslide or debris slide)

Mass slope failure on a shallow slip plane parallel to the surface. This is the most common type of landslide, slip or debris fall. The plane of failure is usually visible but may not be straight, depending on site conditions. It may occur on any scale.

Shear failure
(rotational landslide)

Mass slope failure on a deep, curved slip plane. Many small, deep landslides are the result of this process. Large areas of subsidence may also be due to these.

Slumping or flow of material when very wet

Slumping or flow where material is poorly drained or has low cohesion between particles and liquefaction is reached. These sometimes appear afterwards like planar slides, but are due to flow rather than sliding. The resulting debris normally has a rounded profile.

Debris fall or collapse

Collapse due to failure of the supporting material. This normally takes the form of a rock fall where a weaker band of material has eroded to undermine a harder band above. These are very common in mixed Churia strata.

Debris flow

In gullies and small, steep river channels (bed gradient usually more than 15°), debris flows can occur following intensive rain storms. This takes the form of a rapid but viscous flow of liquefied mud and debris.

(a) Erosion and failure processes

A number of erosion and failure processes are to be found. The types of erosion and slope failure found in Nepal are given in Figure A1. All sites have a combination of these mechanisms at work on them. During site assessment, you should check for these.

Figure A2: The main physical factors affecting slopes

POTENTIAL FACTOR

DESCRIPTION

Fault lines

Small fault lines may cause differential erosion in parts of the site.

Springs

There may be seasonal springs within the site, which cause localised problems of drainage or slumping.

Slip planes

The main plane of failure may not be the only one. Many sites have secondary, smaller slip planes additional to the main failure mechanism

Large gullies

Large gullies nearby may erode backwards and damage the site. Alternatively, they may discharge on to the site, causing deposition there.

Landslides

Nearby landslides may extend headwards or sideways, or may supply debris on to the site.

River flooding

A large river below the site may flood badly, damaging the site by either erosion or deposition, or a combination of both.

River cutting

Rivers below the site may move in floods, undercutting the toe of the site.

Catchments

If there is an extended catchment area above the site, it could lead to a large discharge, which causes bad damage by erosion or deposition.

Drain discharge

The discharge of drainage water must be safeguarded to avoid causing erosion or mass failures. Poorly sited or inadequately protected discharge points can cause severe problems.

Khet and kulos

Khet (rice paddy) land or a kulo (irrigation leat) above a site usually means a large volume of water infiltrating into the slope, with a greater potential for failure or large-scale erosion.

Construction activities

Construction activities on or near the site may lead to undermining through excavations, or surcharging through spoil disposal in the wrong places.

(b) Other factors

In addition to erosion and failure mechanisms, other factors may affect the site. Some are internal (e.g. springs) while others are external (e.g. river undercutting). During site assessment, you must check for signs of any of the potential damaging factors listed in Figure A2.

(c) Slope angle(s)

Measure the average slope angle of the slope segment and place it in one of three classes:

< 30º,
30 - 45°, or
> 45°.

If there is more than one dominant slope, record all main slope angles.


Figure

(d) Slope length

Measure the average length of the slope segment and place it in one of two classes:

< 15 metres or
> 15 metres.

Figure A3: Common features indicating soil drainage characteristics

MATERIAL DRAINAGE
CHARACTERISTICS

TENDENCY TOWARDS GOOD DRAINAGE

TENDENCY TOWARDS POOR DRAINAGE

Overall drainage

Freely draining material; dries quickly after rain storms

Slowly draining material; tends to remain wet for long periods after rain; behaves like firm dahi

Soil particle size

Coarse textures; loams and sandy soils

Fine textures; clays and silts

Porosity

Large inter-connecting pores

Small pores

Material types

Stony colluvial debris; fragmented rock; sandy and gravelly river deposits

Residual soils of fine texture; debris from mud flows, slumps, etc. rato mato

Slope types

Fill slopes; cut slopes in stony debris (colluvium)

Cut slopes in original consolidated ground

(e) Material drainage

Assess and place in one of two classes: good or poor (see Figure A3).

Figure A4: Environmental factors determining site moisture

SITE MOISTURE FACTOR

TENDENCY TOWARDS DAMP SITES

TENDENCY TOWARDS DRY SITES

Aspect

Facing N, NW, NE and E

Facing S, SW, SE and W

Altitude

Above 1500 metres; particularly above 1800 metres

Below 1500 metres; deep river valleys surrounded by ridges

Topographical location

Gullies; lower slopes; moisture accumulation and seepage areas

Upper slopes; spurs and ridges; steep rocky slopes

Regional rain effects

Eastern Nepal in general; the southern flanks of the Annapurna Himal

Most of Mid Western and Far Western Nepal

Rain shadow effect

Sides of major ridges exposed to the monsoon rain-bearing wind

Deep inner valleys; slopes sheltered from the monsoon by higher ridges to the south

Stoniness and soil moisture holding capacity

Few stones; deep loamy* and silty soils

Materials with a high percentage volume of stones; sandy soils and gravels

Winds

Sites not exposed to winds

Large river valleys and the Terai

Dominant vegetation

e.g. utis, katus, chilaune, amliso, nigalo, bans, lali gurans

e.g. khayer, babiyo, khar, dhanyero, salla, imili, kettuke

* Loam is the name given to a soil with moderate amounts of sand, silt and clay, and which is therefore intermediate in texture and best for plant growth.

(f) Segment moisture

Assess and place in one of four classes:

Wet:

permanently damp sites (e.g. north-facing gully sites).

Moist:

sites that are reasonably well shaded or moist for some other reason.

Dry:

generally dry sites.

Very dry:

sites that are very dry; these are usually quite hot as well (e.g. south-facing cut slopes at low altitudes).

See Figure A4.

(g) Altitude

Determine: ±100 metres. Use an altimeter, map or site drawing.

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