Bioengineering and Bioremediation

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This section covers rock and soil with their classification, the structures and textures of rocks, the geomorphological processes of weathering and erosion, the geology of Nepal through its five zones, river channel morphology, the origin, types and movement of groundwater with porosity, permeability and transmissivity, the aquifer systems of Nepal, and geological hazards with their causes and engineering significance.

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Classification, Structure and Texture Class Origin Structure and texture Examples and engineering character Igneous Solidification of molten magma or lava Massive, crystalline, non-foliated; coarse-grained (plutonic) if cooled slowly at depth, fine-grained or glassy (volcanic) if cooled rapidly at the surface Granite, basalt, dolerite; generally strong and sound, excellent for foundations and aggregate Sedimentary Deposition, compaction and cementation of sediment, or chemical or organic precipitation Bedded and stratified; clastic, chemical or organic texture; fossils occur only here Sandstone, shale, limestone, conglomerate; strength very variable and anisotropic along bedding; shale weak and slake-prone Metamorphi c Alteration of existing rock by heat, pressure or chemically active fluids Foliated (slate, schist, gneiss) or non-foliated (marble, quartzite) Strong across the foliation and weak along it; foliation planes are the controlling weakness • Texture describes the size, shape and arrangement of the mineral grains; structure describes the larger features of the rock mass — bedding, foliation, joints, folds and faults.

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For engineering purposes the structure of the rock mass matters far more than the strength of the intact rock, because failure almost always occurs along a discontinuity rather than through sound rock.

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This is the central point of the topic. • Discontinuities — bedding planes, joints, foliation, faults and shear zones — are described by their orientation (dip and strike), spacing, persistence, roughness, aperture, infilling and water condition.

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A slope is most unstable when the discontinuities dip out of the face at an angle greater than the friction angle but less than the slope angle, which is the classic condition for planar sliding. • Soil, in the engineering sense, is any unconsolidated material overlying bedrock.

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Residual soil has formed in place by weathering of the parent rock; transported soil has been moved and deposited by water (alluvial), wind (aeolian), ice (glacial) or gravity (colluvial).

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Colluvial deposits on Nepal's hillslopes are loose, heterogeneous and prone to failure, which matters greatly for road and canal alignment.

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Weathering and Erosion • Weathering is the in-place breakdown of rock; erosion is the removal and transport of the products.

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Weathering does not involve transport and erosion does not involve breakdown in place — the distinction is examined directly. • Physical (mechanical) weathering disintegrates rock without changing its composition: freeze-thaw, thermal expansion and contraction, exfoliation by pressure release, salt crystallisation and biological wedging by roots.

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It dominates in cold and arid climates. • Chemical weathering decomposes the minerals: hydrolysis (the most important, converting feldspars to clay minerals), oxidation, carbonation (which dissolves limestone and produces karst), hydration and solution.

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It dominates in warm, humid climates, and physical weathering assists it by exposing fresh surface area. • Erosion agents are running water (much the most important in Nepal), wind, glaciers, waves and gravity.

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Forms of water erosion in sequence are splash, sheet, rill, gully and stream bank erosion.

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Controlling erosion at the rill stage is far cheaper than after gullies have formed, which is the practical argument for early intervention. • Nepal's erosion rates are among the highest in the world, because of steep slopes, young and weak rocks, intense monsoon rainfall, active tectonics and pressure on land from cultivation and road construction.

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The Geology of Nepal Zone (south to north) Character Engineering significance Terai Northern edge of the Indo-Gangetic plain; thick Quaternary alluvium of gravel, sand, silt and clay Very productive aquifers, including artesian conditions; flat and easy to build on but liable to flooding and liquefaction Siwalik (Churia) Young, weak Tertiary sandstone, mudstone and conglomerate; poorly consolidated Highly erodible and landslide-prone; the principal source of sediment to the Terai; the Bhabar recharge zone lies at its foot Lesser Himalaya Low-grade metamorphic rocks — slate, phyllite, schist, quartzite, dolomite — much folded and faulted Most of Nepal's roads and settlements lie here; slope instability is the dominant engineering problem Higher Himalaya High-grade gneiss, migmatite and schist with granite intrusions; the high peaks Strong rock but extreme relief, glaciers and rockfall;

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GLOF risk from moraine-dammed lakes Tethys (Tibetan) Himalaya Fossiliferous marine sedimentary rocks north of the high peaks Arid trans-Himalayan region; wind erosion and debris flows • The bounding faults, which must be known with the zones: the Main Frontal Thrust (MFT) separates the Terai from the Siwalik; the Main Boundary Thrust (MBT) separates the Siwalik from the Lesser Himalaya; and the Main Central Thrust (MCT) separates the Lesser from the Higher Himalaya.

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Farther north the South Tibetan Detachment System separates the Higher Himalaya from the Tethys zone. • Nepal lies on the collision boundary of the Indian and Eurasian plates, which converge at roughly 2 cm per year; the whole country is therefore seismically active, and the accumulated strain is released in large earthquakes, as in 1934 and 2015. • River channel morphology: channels are classed as straight (rare and usually short), meandering (single sinuous channel in fine sediment at low slope), braided (multiple shifting channels in coarse sediment at steep slope with high sediment load) and anastomosing.

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Nepal's rivers are typically braided where they emerge onto the Terai, because the sudden reduction in slope causes the coarse bedload to be deposited.

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Sinuosity is the ratio of channel length to valley length, and a channel with sinuosity above about 1.5 is described as meandering.

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Erosion occurs on the outer bank of a meander bend and deposition on the inner bank, forming a point bar — a standard question with direct relevance to bank protection works.

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Groundwater • Origin: almost all groundwater is meteoric, derived from precipitation that has infiltrated.

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Connate water is trapped in sediment at the time of deposition and is usually saline; juvenile or magmatic water comes from within the earth and is negligible in quantity. • Porosity is the proportion of voids, and it depends on the sorting, packing, shape and cementation of the grains, but not on the grain size itself — a point regularly examined, since a well-sorted fine sand and a well-sorted coarse gravel can have the same porosity while differing enormously in permeability. • Permeability (hydraulic conductivity) K measures the ease with which water moves through the material, and it does depend strongly on grain size, being controlled by the size of the pore throats.

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Clay has a porosity of 40-60 per cent but a permeability of the order 10−9 m/s, while gravel with a porosity of only 25-35 per cent has a permeability of 10−2 m/s or more.

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High porosity therefore does not imply high permeability, which is the single most examined idea in this part. • Transmissivity T = Kb, the product of permeability and saturated thickness, is the quantity that actually determines the yield of a well. • Rock types: unconsolidated sand and gravel are the best aquifers; sandstone is good if poorly cemented; limestone may be excellent where solution has enlarged the fissures, or poor where it has not; and crystalline igneous and metamorphic rocks have almost no primary porosity, so they yield water only through joints, fractures and weathered zones — which is why secondary porosity governs groundwater in the hills of Nepal. • Aquifer systems of Nepal: the Terai contains the country's principal groundwater resource, in thick Quaternary alluvium.

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The Bhabar zone, a belt of coarse boulder and gravel deposits at the foot of the Siwaliks, is the main recharge area, where rivers lose water to the ground; farther south the finer sediments give confined and often artesian conditions, tapped by deep tube wells, above which shallow aquifers are exploited by hand pumps and shallow tube wells.

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In the hills and mountains groundwater occurs in colluvial and alluvial fans, river terraces and fractured or weathered bedrock, and emerges as springs, which are the principal source of rural water supply.

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The Kathmandu Valley has a distinct basin-fill aquifer system with shallow and deep zones separated by a thick clay layer, and it is severely over-exploited, with a long-term decline of water level.

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Geological Hazards • Floods: in Nepal these include monsoon flash floods in steep catchments, riverine floods in the Terai, and debris floods carrying very large sediment loads.

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Their causes combine intense rainfall with steep terrain, deforestation, encroachment on floodplains and inadequate drainage. • Glacial lake outburst floods (GLOF) occur when a moraine-dammed glacial lake fails suddenly, releasing a large volume of water and debris down the valley.

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The trigger is typically an ice or rock avalanche into the lake producing a displacement wave, or progressive seepage and piping through the moraine dam.

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Glacial retreat is enlarging such lakes, so the hazard is increasing.

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Mitigation is by lowering the lake level through a controlled outlet channel or siphon, together with early warning systems downstream; the Tsho Rolpa lake lowering is the well-known Nepali example, and the 1985 Dig Tsho event caused extensive damage. • Mass movement is classified by the type of movement — fall, topple, slide, spread and flow — and by the material — rock, debris or earth.

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A rotational slide (slump) moves on a curved surface and is typical of deep homogeneous soil; a translational slide moves on a planar surface, usually a bedding or foliation plane, and is the common mode in Nepal's hillslopes; a debris flow is a rapid, water-charged flow of mixed material and is the most destructive because of its speed and reach. • Causes are grouped as those that increase the shear stress — steepening by undercutting or excavation, surcharge at the crest, removal of lateral support, earthquake shaking, and water pressure in tension cracks — and those that reduce the shear strength — weathering, increased pore water pressure, loss of root reinforcement after deforestation, and progressive creep.

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Rainfall is the commonest trigger in Nepal, because it raises the pore water pressure and so reduces the effective stress and hence the frictional strength; this is why landslides cluster in the monsoon and why drainage is the first and cheapest measure in any stabilisation scheme. • Engineering significance: geological conditions govern the choice of alignment for roads and canals, the selection of foundation type and depth, the stability of cuts and embankments, the availability of construction material, the seepage and stability of dams and reservoirs, tunnelling conditions, and the seismic design requirement.

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An adequate geological investigation before design is invariably cheaper than remedying the consequences of its omission.

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This section covers meteorology and the distinction between climate and weather, global wind systems and weather patterns, clouds, storms and their relation to climate, global ocean circulation, El Nino and the Southern Oscillation, greenhouse gases and human emissions, and the outlook for the future.

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Weather, Climate and the Atmosphere • Weather is the state of the atmosphere at a particular place and time — over hours or days; climate is the statistical description of weather over a long period, conventionally thirty years.

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The usual formulation is that climate is what you expect and weather is what you get, and the distinction is examined directly.

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Climate variability is the natural fluctuation about the mean; climate change is a shift in the mean or in the variability that persists for decades or longer. • Layers of the atmosphere, from the ground upward: the troposphere, in which temperature falls with height at an average lapse rate of about 6.5 °C per kilometre and in which essentially all weather occurs; the stratosphere, in which temperature rises with height because ozone absorbs ultraviolet radiation, so it is stably stratified and free of vertical mixing; the mesosphere, where temperature falls again; and the thermosphere.

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The temperature inversion in the stratosphere is what traps pollutants below the tropopause and confines weather to the troposphere — a standard point. • Energy balance: about 30 per cent of incoming solar radiation is reflected (the planetary albedo), and the rest is absorbed and eventually re-radiated as longwave infrared.

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Differential heating between equator and poles drives the whole circulation of the atmosphere and the oceans.

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Global Wind Systems and Weather Patterns • The three-cell model of each hemisphere must be known: the Hadley cell from the equator to about 30°, with rising air at the equator and sinking air at 30°; the Ferrel cell from about 30° to 60°; and the Polar cell from 60° to the pole. • The consequences are the great climatic belts: the Intertropical Convergence Zone (ITCZ) at the equator, where converging trade winds rise, giving heavy convective rainfall and the world's rainforests; the subtropical high-pressure belts near 30°, where the descending air warms and dries, giving the world's great deserts; and the polar front near 60°, where warm and cold air meet, giving the mid-latitude depressions.

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That the deserts lie at about 30° because of descending air is a favourite question. • The Coriolis effect, arising from the earth's rotation, deflects moving air to the right in the northern hemisphere and to the left in the southern.

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It is zero at the equator and greatest at the poles, which is why tropical cyclones never form within about five degrees of the equator — another standard question. • Surface winds are the trade winds, blowing from the subtropical highs towards the equator and deflected to become north-easterly in the northern hemisphere; the prevailing westerlies of the mid-latitudes; and the polar easterlies. • The jet streams are narrow bands of very fast westerly wind near the tropopause, and their position steers mid-latitude weather systems. • The monsoon, which governs Nepal's climate, is driven by the differential heating of land and sea: in summer the Asian landmass heats faster than the Indian Ocean, creating low pressure that draws in moist south-westerly air, which is forced to rise over the Himalaya and yields heavy orographic rainfall; in winter the pattern reverses and dry north-easterly air flows out.

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Nepal receives roughly 80 per cent of its annual rainfall in the four monsoon months from June to September, and rainfall decreases from east to west and increases with altitude up to a maximum on the southern flank of the mountains, with a pronounced rain shadow beyond.

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Clouds, Storms and the Ocean • Cloud formation requires moist air to be cooled to its dew point, usually by being lifted, together with condensation nuclei.

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The four lifting mechanisms are convective (surface heating), orographic (forced ascent over ground), frontal (warm air riding over cold) and convergence. • Clouds are classified by height and form: high cirrus, cirrostratus and cirrocumulus, composed of ice; middle altostratus and altocumulus; low stratus, stratocumulus and nimbostratus; and the vertically developed cumulus and cumulonimbus.

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Cumulonimbus is the thunderstorm cloud and the only one that produces hail, thunder and lightning; nimbostratus gives steady prolonged rain. • Clouds and climate embody the largest single uncertainty in climate projection: low clouds reflect sunlight and cool the surface, while high thin clouds trap outgoing infrared and warm it, so whether the net cloud feedback is positive or negative depends on how the balance between them changes. • Tropical cyclones require sea surface temperature above about 26.5 °C through a depth of some 50 m, low vertical wind shear, pre-existing disturbance, and sufficient distance from the equator for the Coriolis effect to induce rotation.

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They are called hurricanes in the Atlantic and eastern Pacific, typhoons in the western Pacific and cyclones in the Indian Ocean — the same phenomenon under three names. • Global ocean circulation has two parts: wind-driven surface currents, organised into gyres that rotate clockwise in the northern hemisphere and anticlockwise in the southern; and the deep thermohaline circulation, driven by differences in density arising from temperature and salinity.

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The thermohaline circulation — the global conveyor belt — sinks in the North Atlantic and around Antarctica where the water is cold and salty, and upwells elsewhere, taking of the order a thousand years to complete a circuit.

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It transports an enormous quantity of heat polewards, and the Gulf Stream is the reason north-west Europe is far milder than its latitude would suggest. • Upwelling, where wind drives surface water away from a coast and cold nutrient-rich water rises to replace it, supports the world's most productive fisheries.

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El Nino and the Southern Oscillation • Under normal conditions in the tropical Pacific, the trade winds blow from east to west, piling warm water in the western Pacific and allowing cold nutrient-rich water to upwell off South America.

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Rainfall is therefore high over Indonesia and low over the eastern Pacific. • In an El Nino event the trade winds weaken or reverse, the warm pool migrates eastward, upwelling off South America is suppressed and the sea surface there becomes unusually warm.

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The consequences are drought in Indonesia, Australia and often South Asia; heavy rainfall and flooding on the coast of Peru and Ecuador; collapse of the anchovy fishery through loss of upwelling; and a small rise in global mean temperature.

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El Nino years are often, though not invariably, associated with a weaker South Asian monsoon. • La Nina is the opposite phase, with unusually strong trade winds and cold eastern Pacific water, generally bringing the reverse pattern of effects. • The Southern Oscillation is the atmospheric component — the see-saw in sea-level pressure between Tahiti and Darwin — and the combined phenomenon is called ENSO because the oceanic and atmospheric parts are two aspects of a single coupled system.

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That ENSO couples ocean and atmosphere, rather than being purely oceanic, is the point examined.

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Events recur irregularly every two to seven years and last about a year.

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Greenhouse Gases and the Outlook • The greenhouse effect is natural and essential: without it the mean surface temperature of the earth would be about −18 °C instead of about +15 °C.

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The concern is the enhanced greenhouse effect caused by human emissions, not the greenhouse effect itself — a distinction examined directly. • The mechanism is that greenhouse gases are largely transparent to incoming shortwave solar radiation but absorb outgoing longwave infrared radiation, re-emitting part of it downward.

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Gas Principal human sources Global warming potential (100 yr, CO₂ = 1) Notes Carbon dioxide (CO₂) Fossil fuel combustion, deforestation, cement manufacture 1 The reference gas; by far the largest contributor because of its quantity and long persistence Methane (CH₄) Livestock, rice paddies, landfills, leakage from gas and coal About 28 Far more potent per molecule but much shorter-lived, about a decade Nitrous oxide (N₂O) Nitrogen fertilizers, manure, some industry About 265 Very long-lived; also depletes stratospheric ozone Fluorinated gases (HFCs, PFCs, SF₆) Refrigeration, electrical switchgear, industry Hundreds to over 20 000 Small in quantity but extremely potent and very persistent Water vapour Not directly controlled by emissions — The largest natural contributor, but it acts as a feedback rather than a forcing • Water vapour is the most abundant greenhouse gas but is treated as a feedback rather than a forcing, because its concentration is set by temperature rather than by emissions — a point worth holding. • Observed changes include a rise in global mean surface temperature of roughly 1.1 °C above pre-industrial levels, rising sea level from thermal expansion and the melting of land ice, retreat of glaciers and sea ice, ocean acidification from the absorption of carbon dioxide, and an increase in the frequency and intensity of many extreme events. • Impacts of particular concern in Nepal: accelerated retreat of Himalayan glaciers and the growth of glacial lakes with the consequent GLOF hazard; changes in the timing, intensity and reliability of monsoon rainfall; more intense short-duration rainfall and hence more floods and landslides; shifting of agricultural and ecological zones upslope; effects on the dry-season flow of rivers that depend on snow and ice melt; and stress on water supply, agriculture and hydropower. • Response: mitigation reduces the emission of greenhouse gases or enhances their removal — renewable energy, efficiency, afforestation, carbon capture; adaptation adjusts systems to the changes that occur or are expected — drought-resistant crops, flood defences, early warning, water storage.

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Nepal contributes a negligible share of global emissions but is highly vulnerable to the effects, so adaptation is the greater priority nationally while mitigation remains a global obligation. • The international framework is the UNFCCC of 1992, the Kyoto Protocol of 1997, and the Paris Agreement of 2015, which commits parties to holding the increase in global mean temperature well below 2 °C above pre-industrial levels and to pursuing efforts to limit it to 1.5 °C, through nationally determined contributions.

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The IPCC assesses the science but does not conduct research itself — a point often asked.

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Projections, targets and the current state of commitments change, so up-to-date sources should be consulted for any specific figure.

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This section covers the fundamentals of disaster risk reduction, the relationship between environment, development and disasters, disaster typology and classification, the concepts of hazard, risk, vulnerability, disaster, mitigation and adaptation, climate change and variability and their implications for disaster risk, climatic extreme events, disaster risk mitigation and management, disaster preparedness, and the mainstreaming of climate change adaptation with disaster risk reduction.

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Core Concepts • These definitions carry a large share of the marks in this section and must be exact: • Hazard — a process, phenomenon or human activity that may cause loss of life, injury, damage, disruption or environmental degradation.

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It is the potential event. • Exposure — the people, property, systems and assets located in hazard-prone areas. • Vulnerability — the conditions that increase the susceptibility of a community or asset to the impact of a hazard.

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It has physical, social, economic and environmental dimensions. • Capacity — the combination of strengths and resources available to reduce risk. • Risk — the potential loss, conventionally expressed as Risk = Hazard × Exposure × Vulnerability / Capacity.

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Risk is zero if any of hazard, exposure or vulnerability is zero, which is why reducing any one of them reduces risk. • Disaster — a serious disruption of the functioning of a community involving losses that exceed its own ability to cope using its own resources.

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The last clause is the operative one: an event is a disaster only when the community cannot cope unaided, so an identical earthquake may be a disaster in one place and merely an incident in another.

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This is the most examined idea of the section. • A hazard is not a disaster: an earthquake in an uninhabited desert is a hazard event but not a disaster, because there is no exposed vulnerable population. • Resilience — the ability of a system to resist, absorb, accommodate and recover from a hazard in a timely and efficient manner, including preserving and restoring its essential functions.

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Typology and the Disaster Cycle Category Sub-type Examples Natural — geophysical Earth processes Earthquake, volcanic eruption, landslide, tsunami Natural — hydrological Water Flood, flash flood, GLOF, debris flow Natural — meteorological Short-term weather Storm, lightning, hailstorm, cold and heat wave Natural — climatological Long-term climate Drought, wildfire, glacial retreat Natural — biological Living organisms Epidemic, pandemic, insect infestation Technological (man-made) Industrial and transport Chemical release, fire, explosion, structural collapse, transport accident Complex / socio-natural Human influence on natural processes Landslide caused by road cutting, flood worsened by encroachment, conflict-related displacement • The category most often asked about is the socio-natural, because it captures the point that the distinction between natural and man-made disasters is not clean: a landslide triggered by an unstable road cut and a flood made worse by building on the floodplain are natural processes amplified by human decisions. • By speed of onset, disasters are sudden (earthquake, flash flood, GLOF) or slow (drought, desertification, sea level rise); slow-onset events cause enormous cumulative loss but attract far less attention and funding, which is itself a recognised problem. • The disaster risk management cycle, which must be known in order: mitigation (reducing the hazard or vulnerability before the event) → preparedness (planning, training, early warning, stockpiling) → response (immediate relief, search and rescue, emergency services) → recovery (rehabilitation and reconstruction).

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Mitigation and preparedness are pre-disaster and response and recovery post-disaster.

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Recovery should feed back into mitigation, which is the principle of building back better — rebuilding to a higher standard rather than recreating the vulnerability that caused the loss. • Structural mitigation uses physical construction — embankments, check dams, retaining walls, seismic strengthening, safe building design.

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Non-structural mitigation uses land use planning and zoning, building codes and their enforcement, insurance, public awareness, training and legislation.

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Non-structural measures are usually far cheaper and often more effective, but they require sustained institutional capacity, which is why they are frequently neglected. • Preparedness covers early warning systems, contingency and evacuation planning, drills, emergency stockpiles, search and rescue capability, and community-based disaster preparedness.

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An early warning system has four elements — risk knowledge, monitoring and warning service, dissemination and communication, and response capability — and it fails if any one of them is missing.

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A technically perfect forecast that does not reach people who know how to act on it saves no one, which is the point of the four-element formulation.

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Environment, Development and Disasters • The relationship runs in both directions, and stating it in both directions is what the examination expects: • Environmental degradation increases disaster risk: deforestation increases runoff and landslide frequency; wetland reclamation removes natural flood storage; mangrove clearance removes coastal protection; unplanned quarrying and road cutting destabilise slopes; and over-abstraction of groundwater causes subsidence. • Disasters damage the environment: through landslide scars, chemical release, debris, and the pressure placed on natural resources by displaced populations. • Unplanned development creates risk: settlement on floodplains and unstable slopes, construction without regard to codes, and infrastructure that itself becomes a hazard, as with a badly aligned hill road. • Conversely, well-planned development reduces risk, and ecosystem-based disaster risk reduction — protecting forests, wetlands and natural drainage — is often cheaper and more durable than engineered protection. • Disasters set back development, destroying accumulated assets and diverting resources from investment to relief.

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The 2015 Gorkha earthquake is the obvious Nepali illustration of a single event erasing years of development gain.

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Climate Change and Disaster Risk • Climate change alters disaster risk in four ways that should be listed: it changes the frequency and intensity of extreme events; changes their geographical distribution and seasonality; creates new hazards, such as the growth of glacial lakes and the GLOF risk; and interacts with existing vulnerabilities, so that its effects fall most heavily on those least able to cope. • Climatic extreme events of concern in Nepal are intense short-duration rainfall causing flash floods and landslides; prolonged drought and delayed monsoon onset; heat waves in the Terai and cold waves in winter; hailstorms and windstorms damaging crops; and forest fire in the dry season. • The distinction between adaptation and mitigation must be exact, because it is examined constantly: mitigation addresses the causes of climate change by reducing greenhouse gas emissions or enhancing sinks; adaptation addresses the consequences by adjusting systems to the changes that occur or are expected.

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Planting a forest to absorb carbon is mitigation; planting a drought-resistant crop variety is adaptation. • Maladaptation is an adaptation measure that increases vulnerability elsewhere or in the longer term — as when an embankment protecting one bank increases flooding on the other, or when air conditioning raises the emissions that drive the warming. • Nepal's instruments include the National Adaptation Programme of Action (NAPA), the Local Adaptation Plans for Action (LAPA), which bring adaptation planning to the local level, the National Adaptation Plan (NAP) and the Climate Change Policy.

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Nepal's share of global emissions is negligible, so adaptation is the national priority. • Mainstreaming CCA and DRR means integrating climate change adaptation and disaster risk reduction into ordinary development planning, budgeting and sectoral programmes, rather than treating them as separate projects.

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The argument for it is that the two fields share the same objective of reducing vulnerability to climate-related hazards, they address overlapping risks, and running them as separate silos duplicates effort and wastes scarce capacity.

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The obstacles are institutional fragmentation, different time horizons — DRR looking at present risk and CCA at future change — separate funding streams and limited capacity. • International frameworks: the Sendai Framework for Disaster Risk Reduction 2015-2030, which succeeded the Hyogo Framework for Action, with its four priorities — understanding disaster risk, strengthening disaster risk governance, investing in disaster risk reduction for resilience, and enhancing preparedness for effective response and to build back better.

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Sendai's shift of emphasis from managing disasters to managing risk is its defining feature. • In Nepal, the Disaster Risk Reduction and Management Act, 2074 (2017) replaced the Natural Calamity (Relief) Act, 2039 and established the National Disaster Risk Reduction and Management Authority, shifting the national approach from relief to comprehensive risk management.

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Institutional arrangements are still evolving under federal restructuring, so current sources should be consulted.

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This section covers integrated watershed management, the problems that arise on slopes, the functions of bioengineering, the calculation of the relative shear strength contributed by root growth, nursery design criteria, and the green road concept in Nepal.

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Integrated Watershed Management • A watershed or catchment is the area draining to a common outlet, and it is the natural unit for planning land and water management because everything done in the upper catchment affects the lower. • Integrated watershed management is the coordinated management of land, water, vegetation and other resources within a watershed, taking account of the interactions between them and involving the people who live there.

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Its distinguishing features are that it takes the watershed rather than an administrative boundary as the planning unit, treats upstream and downstream as one system, and depends on community participation — since measures imposed without local agreement are not maintained. • Its objectives: to control erosion and sedimentation, conserve soil and water, improve land productivity and rural livelihoods, moderate flood peaks, sustain dry-season flow and maintain the ecological condition of the catchment. • Measures are grouped as agronomic (contour cultivation, strip cropping, mulching, cover crops, agroforestry), mechanical (terracing, contour bunds, check dams, gully plugs, diversion drains) and vegetative or bioengineering (grass, shrub and tree planting on slopes).

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Treatment should begin in the upper catchment and work downward, because treating the lower reaches while the sediment source above is untreated achieves little — a standard examination point.

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Slope Problems and the Functions of Bioengineering • Problems on slopes include surface erosion by rain splash, sheet, rill and gully action; shallow planar failures in the weathered mantle; deep-seated rotational landslides; debris flows; rockfall and toppling; undercutting by streams; and instability of cut and fill slopes along roads.

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On engineered slopes the commonest causes are inadequate drainage, over-steep cutting, and spoil tipped loosely on the slope below. • Bioengineering is the use of living vegetation, either alone or with small civil engineering structures, to arrest and prevent slope failure and erosion. • The five engineering functions of vegetation must be known as a set, since questions ask for them by name: • 1.

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Catch — vegetation intercepts material falling or rolling down the slope from above. • 2.

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Armour — a dense cover protects the surface from the erosive action of rain and runoff. • 3.

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Reinforce — roots permeate the soil and increase its shear strength. • 4.

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Anchor — deep, strong roots pin the surface layer to more stable material beneath. • 5.

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Support — a substantial buttress of vegetation holds back the material upslope of it. • Hydrological effects are separate and also act in both directions: vegetation intercepts rainfall, reduces the velocity of runoff, and removes water from the soil by transpiration, all of which lower pore water pressure and help stability; but it also increases infiltration and adds surcharge weight, and tall trees transmit wind load to the slope.

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On balance grass and shrubs are beneficial on steep unstable slopes, while large trees can be a liability on a marginally stable one — a point often asked. • The limitations are as important as the benefits: bioengineering is effective only within the rooting depth, typically one to two metres, so it stabilises shallow failures and surface erosion but cannot arrest a deep-seated landslide, which requires civil engineering measures.

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It also takes one to three growing seasons to become fully effective, so it must be combined with civil works where immediate protection is needed. • Its advantages are that it is cheap, uses local materials and labour, requires no heavy equipment, is self-repairing and grows stronger with time — the opposite of a concrete structure, which deteriorates — and is environmentally and visually appropriate.

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Root Reinforcement and Relative Shear Strength • The Mohr-Coulomb shear strength of soil is τ = c′ + σ′ tan φ′, where c′ is the effective cohesion, σ′ the effective normal stress and φ′ the effective friction angle. • Roots add an apparent cohesion cR, so the reinforced strength becomes τ = (c′ + cR) + σ′ tan φ′.

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The critical point, examined directly, is that roots increase the cohesion term and do not change the friction angle, because they act as flexible tensile fibres bridging the shear surface rather than altering the interparticle friction. • The Wu-Waldron model gives cR = k TR(AR/A), where TR is the tensile strength of the roots, AR/A the root area ratio — the proportion of the shear plane occupied by root cross-section — and k a factor of about 1.2 allowing for the orientation of the roots to the shear plane. • The root area ratio is the governing variable, and it decreases rapidly with depth, which is why the reinforcement is concentrated near the surface. • Relative shear strength is expressed as the factor of safety of the vegetated slope divided by that of the bare slope, or equivalently as the percentage increase in shear strength attributable to the roots.

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Typical values of cR range from about 1-5 kPa for grasses to 5-20 kPa for shrubs and trees, which is a substantial proportion of the strength of a weak shallow soil. • Species selection matters accordingly: the best species have deep, dense, strong and well-distributed root systems, rapid establishment, tolerance of poor and dry soil, resistance to drought and grazing, ease of propagation from cuttings or slips, and a local usefulness that gives the community a reason to protect them.

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Nepali practice relies heavily on Amliso (broom grass), Napier and Khar grasses, and on shrubs and trees such as Bamboo, Utis (alder), Bakaino, Sisau and Khayer.

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Locally available and locally valued species should always be preferred to introduced ones, because they survive better and are maintained by the people who benefit from them.

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Nurseries and the Green Road Concept • Nursery design criteria, which are asked for as a list: a gentle, well-drained, level or slightly sloping site; a reliable, clean water supply available through the dry season; good deep loamy soil or access to suitable soil for the mixture; full sunlight with provision for shade when required; shelter from strong wind; easy access to the planting sites and to a road, to limit transport damage; protection from grazing animals; a secure and available labour supply; and adequate area for beds, paths, composting, storage and future expansion. • Layout: seedbeds, transplant beds, polythene bag or pot areas, hardening-off area, compost heap, water storage, tool store and access paths.

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Seedlings are hardened off by gradually reducing shade and watering before dispatch, so that they can withstand field conditions — an important step that is often omitted. • The green road concept is Nepal's approach to low-cost rural hill road construction, and it is examined directly.

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Its principles are: • Labour-based construction using local people, hand tools and locally available materials, rather than heavy machinery. • Environment-friendly alignment following the contour, avoiding unstable ground, steep slopes, active landslides and, where possible, crossing streams at safe points. • Balanced cut and fill so that the excavated material is used to form the road platform, with no side-tipping of spoil down the hillside.

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Uncontrolled tipping of spoil is the single largest cause of environmental damage from hill roads, because it destabilises the slope below and delivers sediment directly to the rivers. • Construction spread over several dry seasons, with a monsoon between the initial cutting and the final completion, so that the slopes can settle and be stabilised before the surface is finished. • Extensive use of bioengineering to protect the cut and fill slopes, with adequate cross-drainage and side drains, since water is the principal agent of road-slope failure. • Community participation in planning, construction and maintenance, so that the road is owned and looked after locally. • The advantages are low cost, local employment and income, and much less environmental damage; the disadvantages are slow construction, a geometry unsuited to heavy traffic, and dependence on continued local maintenance.

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This section covers the different walls and structures used in bioengineering, grass plantation, brush layering, palisades, turfing, and the selection of optimum techniques.

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Small Civil Structures Structure Description Function and use Dry stone masonry wall Stone laid without mortar, battered back into the slope Toe support and retention of small slips; free-draining, flexible and easily repaired with local material Gabion wall Rock-filled wire mesh baskets laid in courses Retaining and toe protection; flexible, permeable and tolerant of settlement, so well suited to weak foundations and river banks Cement masonry wall Stone in cement mortar Where a rigid, higher wall is needed; must be provided with weep holes and a filter, or water pressure will build behind it Check dam Small barrier across a gully, of stone, gabion, brushwood or bamboo Reduces the gully gradient and traps sediment, allowing vegetation to establish; built in series from the top of the gully downward Catch wall and catch drain Wall or ditch above the road to intercept falling debris or runoff Protects the road from material and water arriving from above Surface and cascade drain Lined channel, often stepped on steep ground Removes water safely; the cascade dissipates energy so the drain is not scoured Revetment Facing of stone, gabion or vegetation on a slope or bank Armours the surface against erosion by water Toe wall Low wall at the base of a slope Prevents undercutting and supports the material above • Drainage is the governing consideration for every structure in this list.

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Any wall that retains soil must be free-draining or provided with weep holes and a granular filter, because water pressure behind a wall is the commonest cause of failure and can easily exceed the earth pressure it was designed for.

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This is why dry stone and gabion structures, which are inherently permeable, are so widely used in hill bioengineering. • Gabions deserve particular note: they are flexible, so they deform without failing when the foundation settles; permeable, so no water pressure develops; and made from local stone with only the wire imported.

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Their weakness is corrosion and abrasion of the wire, especially in rivers carrying coarse bedload, which is why PVC-coated mesh is used in aggressive conditions. • Check dams work by reducing the gully bed gradient, so that the flow velocity and hence the erosive power fall, and by trapping sediment behind them, which creates a flat wet area in which vegetation can establish.

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They must be built in series starting from the head of the gully, keyed well into both banks and the bed, and provided with a spillway so that water passes over the centre and not around the ends — outflanking is the commonest mode of failure.

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Vegetative Techniques Technique Method Function Best use Grass planting (slip planting) Rooted grass slips planted in lines across the slope Armour, and some reinforce The basic and most widely used technique on cut and fill slopes Turfing Sods of established turf laid directly on the prepared surface and pegged Immediate armour Where instant protection is needed, as in drain linings and small areas; costly and needs a turf source Brush layering Live woody cuttings laid in shallow trenches across the slope, tips protruding, then covered Reinforce, catch and support Shallow slips and eroding slopes; the cuttings root and form a reinforced layered mass Palisade Live cuttings driven vertically in a close line across the slope Catch and support Traps material moving downslope and forms small terraces Live check dam Palisade or brush construction across a small gully Catch, with sediment trapping Small gullies where a masonry check dam is not justified Fascine (live wattle) Bundles of live branches laid in shallow trenches along the contour Armour, catch and drain Surface erosion control and interception of runoff Live pole and stake Cuttings driven through an existing cover or into fill Anchor and reinforce Pinning surface material to firmer ground beneath Tree and shrub planting Nursery-raised seedlings planted in pits Anchor, support and long-term catch Longer-term stabilisation of large areas, above the road line Jute netting and geotextile with seeding Biodegradable net pegged over a seeded surface Temporary armour while vegetation establishes Steep bare surfaces where seed and soil would otherwise wash away • Brush layering deserves particular attention, since it is the technique most often examined.

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Woody cuttings are laid in shallow trenches excavated across the slope at a slight downslope angle into the hillside, with the tips protruding a short distance, and the trench is backfilled and compacted.

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The cuttings both reinforce the soil immediately, acting as tensile elements across potential shear surfaces, and then root to provide permanent reinforcement.

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The layers also break the slope into short lengths, reducing surface runoff velocity and catching debris.

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Brush layering is therefore one of the few bioengineering techniques that gives some immediate mechanical benefit rather than only a delayed one. • The distinction between a palisade and brush layering is examined directly: a palisade consists of cuttings driven vertically into the ground in a close line, whereas brush layering lays them horizontally in trenches across the slope. • Turfing gives immediate protection but is expensive and requires a source of turf; grass slip planting is far cheaper but takes a season to become effective. • Planting time is critical: in Nepal bioengineering planting is done at the start of the monsoon, in June and July, so that the plants have the whole wet season to establish before the dry months.

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Planting immediately before the dry season wastes the material.

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Cuttings are taken during the dormant season and planted the right way up — planting a cutting inverted is a common practical failure.

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Selecting the Optimum Technique • Selection depends on a systematic assessment rather than on preference, and the factors should be listed: • The nature and depth of the problem — surface erosion, shallow failure or deep-seated instability, since the last cannot be treated by vegetation at all. • The engineering function required — catch, armour, reinforce, anchor or support; the technique is chosen to supply that function. • The slope angle — grass and jute netting are used on very steep slopes; brush layering and palisades on moderate slopes, generally below about 45 degrees; and tree planting on gentler ground. • The soil and site conditions — depth, fertility, drainage, aspect and exposure. • The climate and altitude, which determine which species will survive. • The availability of plant material, labour and funds, and the accessibility of the site. • The urgency of protection — where immediate effect is needed, civil structures or turfing must be used, with vegetation added to take over the function in the longer term. • The general rules that the examination looks for: treat the cause before the symptom, and drainage before anything else, since water is the underlying agent in most slope failures; work from the top of the slope downward, because treating the bottom while the top continues to supply material is wasted effort; combine civil and vegetative measures rather than relying on either alone; use the simplest and cheapest technique that will perform the required function; and provide for maintenance from the start, because bioengineering works fail more often from neglect than from bad design.

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This section covers the introduction to bioremediation, the micro-organisms and contaminants involved, the strategies of microbial degradation and bioremediation, the bioremediation of organic and inorganic pollutants, and the available remediation technologies.

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Principles and Requirements • Bioremediation is the use of living organisms — chiefly micro-organisms, but also plants and fungi — to degrade, transform or immobilise contaminants in soil, water or air, converting them to less hazardous or non-hazardous forms. • Its advantages: it is generally much cheaper than physical or chemical methods, destroys rather than merely transfers the contaminant, can often be carried out in place with minimal disturbance, and is publicly acceptable because it is perceived as natural. • Its limitations, which are equally examined: it is slow, taking months or years; it works only on biodegradable contaminants; it is ineffective at very high contaminant concentrations, which are toxic to the organisms; it is highly sensitive to temperature, pH, moisture and oxygen; the products of partial degradation are sometimes more toxic than the parent compound; and the results are difficult to predict and to demonstrate to a regulator. • The requirements for successful bioremediation must be listed: a suitable population of micro-organisms capable of degrading the contaminant; a contaminant that is biodegradable and bioavailable; an electron acceptor, usually oxygen; nutrients, particularly nitrogen and phosphorus, in adequate proportion; suitable environmental conditions of temperature, pH (usually near neutral), and moisture (commonly 40-60 per cent of water-holding capacity); and the absence of inhibitory substances such as heavy metals at toxic concentration. • Bioavailability is the concept most often missed: a contaminant strongly sorbed to soil particles or trapped in a non-aqueous phase is not accessible to the organisms, so it will not be degraded however capable they are.

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This is why ageing contamination is harder to treat than a fresh spill, and why surfactants are sometimes added to increase availability.

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Micro-organisms and Contaminants • The principal organisms are bacteria — Pseudomonas being the best known and most versatile genus, along with Bacillus, Mycobacterium, Rhodococcus, Alcaligenes and Sphingomonas — together with fungi, notably the white rot fungus Phanerochaete chrysosporium, whose lignin-degrading enzymes attack a wide range of persistent aromatic compounds, and algae and some protozoa. • Aerobic degradation uses oxygen as the electron acceptor, is faster and more complete, and suits hydrocarbons; anaerobic degradation uses nitrate, sulphate, iron or carbon dioxide instead, is slower, and is essential for highly chlorinated compounds such as PCE and TCE, which resist aerobic attack.

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The general rule that heavily chlorinated compounds are degraded reductively under anaerobic conditions while lightly chlorinated and non-chlorinated compounds are degraded oxidatively is examined directly, and it is why sequential anaerobic-aerobic treatment is used for chlorinated solvents. • Co-metabolism is degradation of a contaminant by an enzyme produced for another purpose, giving the organism no energy or carbon benefit, so a primary substrate must be supplied to sustain growth — the standard example being the degradation of trichloroethylene by methanotrophs supplied with methane. • Biodegradability falls in the general order straight-chain alkanes > branched alkanes > small aromatics > cyclic alkanes > polycyclic aromatics > highly chlorinated compounds.

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Persistence increases with molecular weight, with the degree of halogenation, with branching and with the number of aromatic rings, and these four factors are what make a compound recalcitrant. • Persistent organic pollutants (POPs) — organochlorine pesticides, PCBs, dioxins and furans — are persistent, bioaccumulative, toxic and subject to long-range transport, and are controlled under the Stockholm Convention.

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Strategies Strategy Description Note Natural attenuation (intrinsic bioremediation) Relying on the indigenous organisms and natural processes, with monitoring to confirm progress Cheapest option; must be monitored, and is correctly called monitored natural attenuation, not 'do nothing' Biostimulation Adding nutrients, oxygen or other electron acceptors to stimulate the organisms already present Usually more effective than adding organisms, because the native population is already adapted to the site Bioaugmentation Adding cultured micro-organisms, native or engineered, to the site Useful where no competent population exists; introduced organisms often compete poorly and die out Bioventing Supplying air at low flow through the unsaturated zone to stimulate aerobic degradation in place Distinguished from soil vapour extraction, which aims to strip the contaminant rather than degrade it Biosparging Injecting air below the water table Treats the saturated zone and the groundwater Land farming Spreading contaminated soil in thin layers and tilling it periodically Simple and cheap; needs large area and a lined base; suits petroleum-contaminated soil Composting Mixing contaminated soil with bulking agents and organic amendments Raises temperature and microbial activity; suits explosives and some pesticides Bioreactor (slurry phase) Treating excavated soil as a slurry in a controlled vessel Fastest and most controllable, but the most expensive Phytoremediation Using plants to extract, degrade, stabilise or volatilise contaminants Cheap and attractive, but slow and limited to the rooting depth • The in situ versus ex situ distinction is examined directly: in situ techniques treat the contamination where it lies, avoiding excavation and transport, which makes them cheaper and less disruptive but slower and harder to control and monitor; ex situ techniques excavate the material and treat it elsewhere, giving faster, more controllable and more verifiable treatment at much greater cost and with the risk of exposure during handling. • Biostimulation is generally preferred to bioaugmentation, because the indigenous organisms are already adapted to the site's conditions and to the contaminant, whereas introduced cultures must compete with them and usually fail to persist.

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Bioaugmentation is reserved for cases where the necessary degrading capability is genuinely absent. • Phytoremediation mechanisms should be known by name: phytoextraction, in which the plant takes the contaminant up into its harvestable tissue; phytodegradation, in which the plant metabolises it; rhizodegradation, in which micro-organisms in the root zone degrade it, stimulated by root exudates; phytostabilisation, in which the contaminant is immobilised in the root zone rather than removed; phytovolatilisation, in which it is taken up and released to the atmosphere; and rhizofiltration, in which roots absorb contaminants from water.

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Note that phytostabilisation does not remove the contaminant at all — it only prevents its movement — and that phytovolatilisation transfers it to the air rather than destroying it, so neither is a complete solution.

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Organic and Inorganic Pollutants • Organic pollutants can be mineralised — broken down completely to carbon dioxide, water and inorganic ions — which is the great strength of bioremediation.

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Petroleum hydrocarbons are the classic and most successful application, and bioremediation was used extensively after the Exxon Valdez spill.

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Chlorinated solvents require the sequential anaerobic-aerobic approach; polycyclic aromatic hydrocarbons are degraded slowly and the heavier ones very slowly; and explosives such as TNT and RDX respond to composting. • Inorganic contaminants — heavy metals and radionuclides — are fundamentally different, and this is the most important distinction in the section: metals are elements and cannot be destroyed by any biological or chemical process.

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Bioremediation can only change their oxidation state, their mobility or their location. • The mechanisms available for metals are therefore: biosorption, in which metals bind to the surface of living or dead biomass; bioaccumulation within cells; bioleaching, in which micro-organisms solubilise metals so that they can be recovered, as in the commercial bioleaching of copper; biotransformation of the oxidation state, the most important practical case being the reduction of highly toxic and mobile hexavalent chromium to the far less toxic and much less mobile trivalent form, and similarly the reduction of soluble uranium(VI) to insoluble uranium(IV); biomethylation, which must be treated with caution because the methylation of mercury by bacteria produces methylmercury, which is far more toxic and bioaccumulative than the inorganic form; and biomineralisation, in which the metal is precipitated as a sulphide or phosphate. • The methylmercury case is a standing warning that a biological transformation can make a contaminant more dangerous rather than less, and it is examined for exactly that reason.

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Other Remediation Technologies • Bioremediation is one option among several, and the alternatives should be known for comparison: • Physical: excavation and disposal, containment by capping and slurry walls, soil vapour extraction, pump and treat for groundwater, air sparging, thermal desorption, and solidification and stabilisation. • Chemical: soil washing and flushing, chemical oxidation with permanganate, persulphate or Fenton's reagent, chemical reduction with zero-valent iron, and permeable reactive barriers placed across the flow path of a plume. • Thermal: incineration, which destroys organics completely but is costly and destroys the soil structure, and thermal desorption, which volatilises them for separate treatment. • Selection depends on the contaminant type and concentration, the soil and hydrogeological conditions, the volume of material, the required clean-up standard and the time available, the future use of the site, and the cost.

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Pump and treat, once the standard approach for contaminated groundwater, is now recognised to be slow and often incapable of reaching the clean-up standard, because contaminant slowly diffuses back out of low-permeability layers — the phenomenon of tailing and rebound, and permeable reactive barriers and in situ treatment have largely displaced it.