design of slope protection

this is a general design procedure for any bioengineering system. the issues involved in the design of bioengineering schemes can be complex, because of the need to consider:
     - the design of both vegetative engineering systems and civil engineering
       systems; and
     - the relationships between systems in terms of function, physical
       relationships and time. see interaction between plants and civil
       engineering structures.

design aspects of bioengineering systems
     1 site characteristics. make a note of the types of site situations for which
        the system is appropriate. this includes material type, material drainage,
        slope moisture condition, slope angle, failure mechanism and cause of failure.
     2 function. this refers to the main engineering function required of the
        bioengineering system, plus any secondary functions that it may perform.
     3 treatments. these are the bioengineering systems that will be used
        for the application.
     4 hazards are inherent weaknesses of a system which cause
        it to be vulnerable to specific kinds of damage. we must minimise their
        impact in the design of a scheme, if necessary by avoiding the use
        of the preferred system in that situation and selecting an alternative.
     5 site requirements. these are conditions on site that must be present
        in order for the system to be correctly applied, e.g. firm foundation conditions.
     6 interactions. the relationships between the civil engineering components
        and the vegetative engineering components in respect of function, space
        and time.
        the vegetative engineering system must be integrated with the civil
        engineering system in that:
           - if the civil engineering component is a permanent structure, the
             engineering function of the vegetation can be different from that
             of the civil engineering structure;
           - if the civil engineering component is a temporary structure and the
             vegetation is expected to replace it, the civil engineering system must
             last long enough to carry out its engineering function while the plants
             are coming to maturity. In this case, the engineering function of the
             plants must be the same as that of the civil engineering system.

design form for slope protection measures

site charac-teristics	failure mecha-nism	cause	function	treat-ment	hazards	site require-ment	interac-tions
debris. angle < 60° con-solidated. well or poorly drained	erosion	surface water	armour, catch	grass lines, with jute net on long slopes on well-drained material	jute may fail and pull of grass	not too rocky or bouldery	with jute: grass takes over from jute
rocky debris	erosion	surface water	catch	horizontal bolsters with trees below on long, well-drained slopes	bolsters can be under-mined. fitting causes slope distur-bance	as above	structure protects plants at first, then plants support bolster
	slide	surface water	support, drain	herring-bone bolsters with trees below on poorly-drained and wet slopes. bamboos or large trees at base	as above	as above	as above
		ground water	drain	shrub planting		not so rocky that shrubs cannot live	none
hard rock. superfi-cial detach-ment from rock slopes and surfaces	plane failure	weather-ing	limited application; minor failures only: catch	few systems available. wire net, catch fence			none
			armour?	breast wall, render-ing.	cause continues & pushes armour off		none
			Support - for a few situations	Tree propping of boulders			none
soft rock of any kind	erosion	weather-ing plus surface water	No solution yet. Catch debris?	No solution yet. Vegeta-tion cannot live
hard rock	slide	ground water	drain? support	spring catcher. french drain tree prop-ping?	in time, rock may weather and break up around tree	water comes from one spot. ground will accept french drain rock suffi-ciently integral to take propping. soil suitable for growth	none none
perme-able siwalik sand-stone >50°	disinte-gration	weather-ing	no solution yet. (armour or support in short term)	no solution as yet. cut back to stable angle; possibly seed & jute net	cover tends to slide off when mature	probably slope less than 40°	plants take over from jute net
hard & soft rock layers. rock slopes >60°	differen-tial weather-ing	weather-ing and erosion of soft rock, followed by plane failure of hard rock	support (plus armour)	prop walls	perma-nence depends on strength of hard layers	hard beds for good founda-tion. accep-table spacing and orienta-tion of rock planes	none

design of surface and subsurface drainage

design form for surface water drainage measures

site characte-ristics	failure me-chanism	cause	function	treatment	hazards	site require-ment	interac-tions

design aspects of bioengineering surface
water drainage measures

site charac-teristics	failure me-chanism	cause	function	treat-ment	hazards	site require-ment	interac-tions
debris on hill slopes up to 40°	erosion	surface water	drain, catch and armour	earth ditch with bund (<5° side slope). grasses above, on bund and below	erosion, blockage, leakage into the ground. difficult to maintain without damaging ditch	foundation must be firm and stable. ground water must be absent. ground must be soft enough to excavate	plants protect structure
				bound masonry ditch, with grasses above and below, or shrub barrier below	erosion along edges, blockage, cracking, undermining	presence of suitable material for foundations	plants protect structure. plants improve performance of structure by trapping debris above
				ca-scade, with stone pitching outside and grasses beyond, possibly shrubs	erosion along edges, water jumping over cascade	presence of suitable material for foundations	plants protect structure

design form for surface water drainage measures

site characte-ristics	failure me-chanism	cause	function	treatment	hazards	site require-ment	interac-tions

design aspects of bioengineering for sub-surface
drainage measures

site characte-ristics	failure me-chanism	cause	function	treatment	hazards	site require-ment	interac-tions
debris	slide	surface water	drain	gabion bolster with shrubs below.	under-mining, mass move-ment deeper in slope.	almost any site without exces-sive amount of stones, espe-cially colluvial slopes	structure protects plants early in their life. when the plants mature they protect the structure.
		ground water	drain	french drain with grass or shrubs along edges. drain can be made more resistant to disrup-tion by building it in a casing of gabion.	blockage cannot be detected. piping may occur under-ground.	almost any site with slope <45 degrees without excessive amount of stones	plants protect the structure.

design of gully protection works

location of check dams

i     1. check dams are used to prevent gully erosion. discuss with your
          partner how they do this and write down your answer.
      2. examine the drawing of the gully floor profile shown below. decide
          on suitable locations for check dams for this gully.

gully cross-section before check dam

produce a sketch design for a check dam elevation for this gully cross-section.
you do not have to provide exact dimensions, nor do you have to choose the type of check dam.
be specific about the shape of the upper edge of the check dam, the dimensions of the key into the gully sides and the depth of the foundation.

longitudinal profile of gully head

produce a sketch design for a civil engineering solution to stabilise this gully head profile.
you do not have to provide exact dimensions. no specific type of structure is required if you choose to incorporate on.
you must be specific about the type of solution you propose.

vegetative engineering design for gullies

     1 the gully cross-section, after a checkdam has been installed, is shown
        below. produce a sketch design for bioengineering measures to keep
        this new cross-section stable.
     2 working with your proposal for civil engineering works to stabilise
        the gully head:
                 a   prepare a sketch design for supplementary bioengineering
                      measures to keep the whole gully head stable;
                 b   note the structural engineering functions being performed
                      by the vegetative works;
                 c   briefly describe the interactions, if any, between the vegetative
                      works and the civil engineering works.

design principles for bioengineering systems in gullies

1 longitudinal profile of gully
over the 100 m section of gully, there are three favourable locations for check dams. these are shown in the sketch below.

location A is a 'nick point' in the gully floor, a point where the floor drops down suddenly after a relatively gentle gradient. a check dam positioned here will hold up material for a long way upstream and further reduce the gradient. this particular nick point is composed of material, which is softer than the sandstone bed. the slope below the check dam would need to be very well protected with an apron to prevent scour of the nick point and undermining of the check dam. the higher the check dam, the greater is the need for scour prevention.
location B is a deposit of debris lying in the bed of the gully. flowing water will move this downstream. a check dam placed at the front of this debris pile will prevent it from moving further. this will have the effect of reducing the gradient of the gully floor and protecting it from scour. the danger then is that the stream will wander sideways and scour the bank, as discussed under the gully cross-section, below.
location C is an outcrop of a harder bed of rock in the gully floor, producing another nick point. a check dam can be built here for the same reason as at location A. however, an alternative would be to take advantage of the presence of a good foundation and simply armour the nick point against scour with a masonry lining on the gully floor

2 cross section of gully
a check dam should have a top profile that encourages the water to flow through the centre, not near the ends, where it will scour the gully sides and possibly expose the extremities of the check dam. the ends should be keyed at least 0.5 m into the bank at each side, and a similar depth into the floor. masonry check dams should have weep holes at the base as well as all the way up.

note that water flowing over a check dam to the left or right of centre can strike the sidewalls of the gully on the downstream side. this will cause undercutting and scour. planting and stone pitching should be laid to prevent this.
debris halted in its path by a check dam often comes to rest in a convex heap, causing the stream to flow along the edges of the debris, against the bank. the stream can scour the bank, widening the gully and bringing very large quantities of debris into the bed. this situation is even more difficult to deal with than a V-notch cross section. however, it is a consequence of installing check dams and it must be considered in the design of schemes. bioengineering measures must be added along the lower gully sides. resilient shrubs such as simali (Vitex negundo) are probably best for this, as they resist partial burial and battering by rocks. they have good root systems to resist scour, by armouring the surface, and to support the bank, and they trap or catch debris. they should be planted when the check dams are being installed, to allow them to grow as large as possible before they are called upon to resist moving debris. if the stream starts to undercut the banks very actively, guide walls can be considered in addition to a programme of planting.
the top of the gully sides can be smoothed off into the surrounding hill side and the whole planted with grasses. the gully side slopes can be protected with shrubs and small trees, especially at the base, where support will be needed. grass can be used on the middle part of the slope.

3 stabilisation of a gully head
gully heads tend to fall in periodically, bringing large amounts of debris into the channel. the steep headwall can be reduced by rounding it off into the hillside and building a check dam to retain the debris and keep the gradient low. the check dam must be very substantial, and properly drained. trees, shrubs and grasses can be planted to minimise erosion. trees placed at the foot of the original slope should provide support at that point.
the area below the check dam is vulnerable. if the volume of water is low, the gully floor can be planted with shrubs to reduce scour and trap debris, forming a natural apron. if scour is likely to be strong, a dry pitched stone or masonry apron should be installed, with plants below on the downstream edge.

sketch design of civil engineering works to stabilise gully head



bioengineering scheme to stabilise gully sides



sketch design for bioengineering scheme to stabilise gully head