Hydrology, river processes and hazards

The drainage basin as an open system

A drainage basin is the whole area of land drained by a river and its tributaries. Its boundary is the watershed, the ridge of higher ground that separates it from the next basin. Every drop of rain that falls inside that line either leaves through the river's mouth or returns to the atmosphere, which makes the basin a natural unit for studying water and the unit this whole topic is built on.

The basin is an open system, meaning that both matter and energy cross its boundary. Water enters as inputs - precipitation falling as rain, snow or hail - and leaves as outputs: channel flow carried out at the mouth, evaporation from wet surfaces and open water, evapotranspiration from plants, and condensation, as water vapour in the air turns back into cloud droplets. In between, water is held for a time in stores and moved between them by transfers. A store holds water; a transfer is the movement that shifts it from one store to another.

The relationship between the four is a balance. Over a year, inputs equal outputs plus or minus any change in the amount held in store. When rain arrives faster than the transfers can remove it, the stores fill: the soil wets up, the water table rises, and the basin is left with high antecedent moisture. A full store cannot absorb more, so a larger share of the next rainfall becomes overland flow and reaches the channel quickly - which is why the second storm in a wet week usually produces a bigger flood than the first, even from weaker rain.

This is where the systems idea earns marks. Two basins can receive identical rainfall and produce completely different floods, because the water takes different routes between the stores. A fast route such as overland flow delivers water to the channel within minutes; a slow route such as percolation followed by groundwater flow may deliver it months later, in a different season entirely.

• Inputs: precipitation - rainfall, snow and hail. In cool uplands a large share falls as snow and is stored on the ground until it melts.
• Outputs: evaporation, evapotranspiration, condensation (water returning to the atmosphere as vapour and cloud) and channel flow leaving the basin at its mouth.
• Stores: the atmosphere (clouds), vegetation (interception on leaves and stems, and water taken up by roots), the surface (the channel itself and any lake), the soil, and the bedrock, where water is held below the water table.
• Transfers: stemflow, throughfall, infiltration, overland flow (Hortonian and saturation-excess), throughflow, percolation, groundwater flow and baseflow.
• Change in storage is the balancing item: input = output +/- change in store. A basin that is filling is storing water and will flood more readily on the next storm.

Stores and transfers: the route a raindrop takes

When rain lands, the first thing it meets is the vegetation. Interception is rain caught on leaves, branches and stems and held there until it evaporates, and it is the reason a woodland delivers less water to the ground than a ploughed field in the same storm. The amount intercepted depends on the type and density of the vegetation, the season (a deciduous wood intercepts almost nothing in winter), and the size of the storm - a small shower may be intercepted almost completely, while a long heavy storm fills the canopy and then sheds nearly all of it.

Water that is not intercepted reaches the ground by two routes: throughfall, falling directly through gaps in the canopy, and stemflow, running down the trunks and stems to concentrate at the base of the plant. From the surface it enters the soil by infiltration, whose rate depends on the soil texture and structure, the amount of organic matter, the slope, the plant cover and how wet the soil already is. The maximum rate at which a soil can absorb water is its infiltration capacity, and a soil that is already saturated has an infiltration capacity of zero.

Water that cannot infiltrate becomes overland flow, and there are two distinct mechanisms. Hortonian overland flow (infiltration-excess flow) happens when rain falls harder than the soil can absorb it, so water ponds and runs off the surface while the soil below is still unsaturated; it is typical of intense storms on bare, compacted or clay-rich ground. Saturation-excess overland flow happens when the soil is already full, or the water table has risen to the surface, so there is simply nowhere for the water to go; it is typical of valley bottoms and of long, wet winters.

Water inside the soil moves sideways as throughflow, creeping downslope above a less permeable layer and reaching the channel over hours or days; where soils have cracks, root holes and animal burrows (macropores) it can be surprisingly fast. Some water moves downwards instead, by percolation, past the base of the soil into the bedrock, where it becomes groundwater. That groundwater moves slowly as groundwater flow and seeps into the channel as baseflow, the part of the river's discharge that is not supplied by the current storm. The top of the saturated zone in the bedrock is the water table: it rises when recharge exceeds discharge and falls in a drought, and where it meets the surface it produces a spring.

• Fast routes: overland flow (minutes), stemflow-heavy rainfall on bare ground, flow through artificial drains - these steepen the rising limb.
• Medium routes: throughflow (hours to days) - sustains the falling limb of a storm hydrograph.
• Slow routes: percolation and groundwater flow (weeks to months) - sustains baseflow and keeps the river flowing through dry weather.
• Interception losses are highest in dense, tall, evergreen vegetation and lowest on bare or harvested ground.

Basin characteristics and climate: why basins respond differently

Size and shape. A large basin collects water from a wide area but the water has further to travel and more chance to be stored on the way, so the flood peak is lower per unit area and the lag is longer. Shape matters because it decides whether tributaries deliver their peaks together: a compact, roughly circular basin sends water from all sides at once and produces a sharp peak, while an elongated basin with a long main channel delivers its tributaries in sequence and produces a lower, flatter peak.

Drainage density is the total length of stream channel in a basin divided by its area. A high drainage density means water is collected and delivered by many short channels, so it reaches the main river quickly and the hydrograph is flashy; a low drainage density means water has to travel further over and through the ground before it finds a channel. Density is high on impermeable rock, steep slopes and sparsely vegetated ground, and low on permeable rock with gentle slopes and good vegetation cover.

Porosity and permeability are not the same thing and the distinction is worth a mark. Porosity is the proportion of the rock or soil that is pore space, so it measures how much water can be held. Permeability is the ability to let water pass through, so it measures how quickly water can move. Clay is highly porous but almost impermeable, because its particles are tiny and the pore spaces between them are too small for water to flow through; that combination is why clay soils waterlog and produce overland flow.

By contrast, sand and sandstone are both porous and permeable; chalk is porous and permeable and holds enormous quantities of groundwater; granite is neither porous nor permeable unless it is fractured, and then water moves only through the joints.

Rock type therefore shapes the whole response. A clay catchment has few stores, high drainage density, rapid overland flow and a flashy hydrograph. A chalk catchment absorbs rainfall into a thick groundwater store, has few surface streams, and releases water slowly as baseflow, producing a steady, subdued hydrograph - the chalk streams of southern England are the classic example. Limestone with well-developed joints and bedding planes behaves similarly but can carry water so quickly through enlarged fissures that it offers little real storage, while granite sheds almost all the rain it receives.

Vegetation and land use change interception, infiltration and the speed of delivery: woodland and moorland slow water down, while deforestation, arable fields, compacted pasture and built-up land speed it up. Climate sets the input and the losses: the type of precipitation (snow is stored until it melts), its duration and intensity, the temperature (which controls evapotranspiration) and the antecedent moisture that decides how much room is left in the stores.

Annual hydrographs: the river's year

An annual hydrograph plots discharge, in cumecs, against the months of the year, and it shows the river's regime - the seasonal pattern of high and low flow. Read it in the same order every time: the highest and lowest discharges and when they occur, the size of the range between them, and whether the peaks are sharp or broad. A large range with sharp peaks is a flashy regime; a small range with a smooth curve is a subdued one.

In the British Isles and north-west Europe the pattern is a winter maximum and a summer minimum. Winter brings more rainfall, low temperatures and therefore very little evapotranspiration, and soils that are already wet, so a high proportion of each storm becomes runoff. In summer, high temperatures and active plant growth remove a large share of the rainfall by evapotranspiration, the soil moisture deficit deepens, and the river falls back onto groundwater-fed baseflow.

Other regimes look completely different, and the reasons are the same factors in a different combination. A snowmelt (nival) regime, found in mountain and continental interiors, has a low flow through the frozen winter, when precipitation is held on the ground as snow, and a sharp spring peak when rising temperatures melt it - sometimes reinforced by spring rain. A monsoon regime has a single enormous summer peak produced by months of heavy rain, and a long dry season fed only by groundwater. A glacial regime fed by ice melt peaks in mid to late summer, because that is when ablation is greatest.

The factors the syllabus names for annual variation are the same ones that matter for storms: the type, duration and intensity of precipitation, temperature and therefore evapotranspiration, and antecedent moisture, plus vegetation type and land use. Land use acts through abstraction and regulation as well: irrigation takes water out of the river in the growing season, and a reservoir upstream can hold back winter flow and release it in summer, flattening the natural regime.

Storm hydrographs: reading the flood

A storm hydrograph plots discharge against time for a single storm, usually with a bar chart of rainfall underneath it. The baseflow is the discharge before the storm arrives, supplied by groundwater. The rising limb is the steep climb as the storm water reaches the channel, the peak discharge is the highest flow, the lag time is the delay between the peak of the rainfall and the peak of the discharge, and the falling (recession) limb is the slower decline as throughflow and then groundwater drain away. A steep rising limb, a short lag and a high peak describe a flashy basin; a gentle rise, a long lag and a low broad peak describe a subdued one.

Every factor in this topic can be turned into a sentence about the lag and the peak, and the marks are for the sentence, not the label. Urbanisation gives a short lag and a high peak because roofs, roads and pavements are impermeable, so infiltration is almost zero, and because gutters, drains and culverted channels remove the water faster than a natural channel would - the water that used to soak away now arrives at the river within minutes. Deforestation does the same by removing interception and the root channels that help water into the soil, and by compacting the ground with machinery.

Rock and soil work the same way in reverse. On clay, infiltration is slow, the soil store fills quickly and overland flow delivers water fast; on chalk, rainfall disappears into the groundwater store and comes back as baseflow weeks later. Steep slopes shorten the time water takes to run off and lengthen the rising limb's steepness; a small catchment has short travel distances; and frozen ground behaves like an impermeable surface, which is why a rapid thaw on frozen soil is a classic flood combination.

The rainfall itself matters as much as the basin: a short, intense burst produces a sharp peak, a long, gentle spell produces a low, drawn-out one, and rain that falls late in a storm - when the stores are already full - runs off more efficiently than the same rain at the start. Antecedent moisture is the factor students most often forget, and it is often the one that decides whether a storm becomes a flood.

Channel processes: erosion, transport and deposition

A river erodes in four ways, and each needs to be named and explained rather than just listed. Hydraulic action is the sheer force of flowing water prising rock particles out of the bed and banks, and driving air and water into cracks under pressure. Abrasion (corrasion) is the load itself acting as a tool, scraping and grinding the bed and banks as stones are dragged along.

Solution (corrosion) is chemical: slightly acidic water dissolves soluble minerals such as calcium carbonate from limestone and chalk. Cavitation happens in very fast, turbulent flow, when tiny air bubbles collapse against the rock surface and send out shock waves that pit and weaken it - a mechanism that matters most at waterfalls and in flood conditions.

The load is carried in four ways, and size decides which. Traction is the rolling and dragging of the largest boulders along the bed. Saltation is the bouncing of pebbles and coarse sand in short hops as they are lifted and dropped by turbulent water. Suspension carries the finest silt and clay, often high in the water column, which is why a flooded river looks brown.

Solution carries the dissolved load, which is invisible and is what makes the water hard. Capacity is the total amount of load the river can carry; competence is the size of the largest particle it can move.

The relationship between velocity, discharge and load is what links the whole topic. Discharge is the volume of water passing a point each second: discharge = cross-sectional area x mean velocity, measured in cumecs. Velocity rises with discharge because a fuller channel is deeper and has proportionally less contact with its rough bed and banks, so friction is lower. As velocity rises, competence rises very steeply - roughly with the sixth power of velocity in theory, so doubling the velocity increases the largest particle the river can move many times over - and capacity rises with discharge, so a flood can carry both coarser and far more sediment.

As the flood falls, competence falls first: the coarse load is dropped, and the fine load stays in suspension longest and settles last, far downstream or out on the floodplain.

Deposition therefore happens wherever the river loses energy: on the inside of bends, where the channel widens and the flow slows; behind obstacles; at the foot of a steep slope where the gradient flattens; where a tributary slows as it enters a larger river; and on the floodplain where water spreads out and stops. Sediment that is deposited in these places is sorted by size, coarse nearest the channel and fine furthest away, and that sorting is the evidence you can read in a deposit.

Two patterns of flow matter for the landforms that follow. Laminar flow is smooth, in parallel layers, and occurs in slow, deep, regular channels; turbulent flow is chaotic and eddying, occurs at high velocity and over rough beds, and is what keeps fine sediment in suspension and mixes oxygen into the water.

Helicoidal flow is a corkscrew motion that spirals along a meander bend: the surface water is thrown outwards towards the outer bank, dives down it, and returns along the bed towards the inner bank. That single pattern explains why the outer bank is eroded and the inner bank is built up. The line of fastest flow is the thalweg, which swings towards the outer bank on a bend and stays near the centre in a straight reach.

Landforms of erosion and deposition

Meanders are the clearest example of erosion and deposition working in the same channel at the same time. Helicoidal flow throws the fastest water against the outer bank, where hydraulic action and abrasion undercut it to form a steep river cliff, while the slower water on the inside drops its load and builds a gently sloping point bar (slip-off slope). The cross-section becomes asymmetric - deep and steep on the outside, shallow and gentle on the inside - and the bend migrates outwards and downstream over years.

When two bends migrate towards each other until only a narrow neck of land separates them, a flood may cut straight across it; the old bend is abandoned, deposition seals both ends, and an oxbow lake is left on the floodplain. It silts up over time, becomes a marsh, and finally a curved hollow - a meander scar. The lower Mississippi has produced many oxbow lakes, and the meander scars around it mark former courses.

Riffle and pool sequences are the alternating shallow and deep sections of a channel. A riffle is shallow, turbulent, and floored with coarse gravel and cobbles, because the flow accelerates over it and the finer material is washed away; a pool is deep, smooth and floored with sand and silt, because the flow slackens and drops what it carries. They alternate roughly every five to seven channel widths, and they matter both as a habitat, because riffles are oxygen-rich and pools hold water in a drought, and as a way of dissipating the river's energy.

Waterfalls and gorges are produced by differential erosion. Where a band of resistant rock overlies softer rock, the river erodes the soft layer more quickly, undercutting the hard cap until it collapses. The fall retreats upstream, leaving a steep, narrow gorge downstream of it, and the plunge pool at its foot is deepened by abrasion and cavitation. High Force on the River Tees is the classic British example: the river drops over the resistant dolerite of the Whin Sill, which caps softer rocks beneath, and the gorge below it has been cut by the fall's retreat upstream.

Bluffs are the steep slopes that mark the outer edge of a floodplain, standing above it like a low cliff. They are cut by the lateral migration of meanders, which sweeps the whole meander belt across the valley floor and shaves the valley side each time the outer bank reaches it. Floodplains are the flat alluvial floors the river has built: the meander belt deposits point bars and abandoned channel fill, and every overbank flood adds a thin layer of fine sediment across the whole surface, which is why floodplain soils are deep, fertile and stone-free.

Levees are natural embankments built by the river itself. When a river overflows, the water at the channel edge slows abruptly and drops its coarsest load first, so each flood adds a ridge of sediment along the bank. Repeated floods build it higher and make it coarser than the floodplain behind, so a natural levee is highest and coarsest next to the channel. Artificial levees copy the same principle but on a much larger scale - which is why they can fail catastrophically when a flood overtops them.

River floods: causes, impacts and vulnerability

A flood is simply a discharge that exceeds the channel's capacity, so anything that increases the volume or speed of water reaching the channel, or reduces the channel's ability to carry it, is a cause. The physical causes start with precipitation: prolonged rainfall over days saturates the stores so that later rain runs off almost entirely, and short intense rainfall can exceed infiltration capacity on its own. Snowmelt and ice melt release a winter's accumulated water in a few warm days, especially when the ground is still frozen, and a rain-on-snow event is the worst combination. Basin characteristics then decide how much of that water arrives at once: impermeable rock, clay soils, steep slopes, small steep catchments, high drainage density and a low-lying flat floodplain all make flooding more likely, as does a high tide or storm surge backing water up at the mouth.

Human activity makes floods more likely by speeding water up and taking away storage. Deforestation removes interception and the roots and litter that keep infiltration rates high, and the machinery used compacts the soil; the rain that was once held and transpired now runs off within hours. Changing agricultural systems work the same way - larger fields, the removal of hedges and ditches, the ploughing of grassland, heavy machinery compacting the soil, field drains that deliver water straight to the channel, and river straightening to speed drainage - while irrigation can add water to a catchment and raise the water table. Urbanisation is the strongest of all: roofs, roads and pavements are impermeable, gutters and storm drains carry water to the river within minutes, and building on the floodplain removes the very land that used to store floodwater.

Impacts are best sorted by time and by who or what is affected. In the short term floods kill and injure people, cut off roads and railways, cut power, water and communications, close schools and businesses, destroy crops and livestock, and contaminate drinking water, which is what turns a flood into a disease outbreak. In the long term they destroy homes and livelihoods, make insurance unaffordable or unavailable, depress property values and local economies, leave psychological trauma, and deposit sterile sand and gravel over fertile fields or scour away the soil altogether. Environmental impacts include the destruction of river habitats and the spreading of pollutants and sewage, but floods also recharge floodplain wetlands and deposit fresh fertile silt, which is why some farming communities depend on them.

Vulnerability decides how bad the disaster is, and it has both physical and human sides. The scale and duration of the event set the ceiling: a flood that covers a whole region for weeks overwhelms every local response. Population density decides how many people and how much property are in the way, and densely settled floodplains are common because the land is flat, fertile and well connected.

Infrastructure decides whether people can be warned, evacuated and supplied - a flood that takes out a bridge, a power station or a hospital multiplies its own impact. Economic development decides whether a country can afford defences, warnings, insurance and rebuilding, and whether people have somewhere else to go; the same flood that is an expensive inconvenience in a rich country can be a catastrophe in a poor one.

Managing flood risk: prediction, engineering and response

Flood risk can be predicted before it happens and forecast as it happens, and the two are different skills. Prediction uses the record: the recurrence interval (return period) of a flood of a given size is the number of years of record divided by the number of times a flood of that size has been recorded, so a flood recorded four times in a hundred years has a recurrence interval of 25 years. A one-in-a-hundred-year flood has a 1 per cent chance of happening in any single year, which is not the same as happening once a century. Hazard mapping uses gauging records, historic flood outlines, detailed elevation data and computer models to map which land floods in events of different sizes, and that map then underpins land-use planning, insurance and evacuation plans.

Forecasting is real time. Gauging stations measure river level (stage) and, through a rating curve, discharge; rain gauges and rainfall radar measure what is falling; and in cold catchments snow depth and temperature are monitored to forecast melt. Those data feed hydrological models that predict the level at downstream towns hours or days ahead, and the forecast becomes a warning issued through television, radio, websites, sirens and mobile alerts. The weakness is that a forecast is only as good as its data and its model, and a warning only saves lives if people understand it, trust it and have somewhere to go.

Hard engineering controls the river with structures. Dams store water and release it slowly, cutting the peak downstream, and may also supply water and power - but they are extremely expensive, displace people and drown land, trap the sediment that would have replenished the delta and floodplain, and create a catastrophic risk if they fail.

River straightening speeds water through a town and lowers the local level, but it does so by passing the problem downstream faster and by removing the storage and habitat of the natural channel. Artificial levees keep water out of the floodplain for events up to their design height, but they raise the water level between them, encourage people to build behind them, and produce a violent, unexpected flood when they are overtopped or breached. Diversion spillways and relief channels carry water around a town altogether, but they need somewhere to put it, they can raise levels where they rejoin, and they are costly.

• Afforestation and vegetation management: increases interception, evapotranspiration and infiltration, and slows overland flow - but takes years to work and needs to be in the right part of the catchment.
• Wetland conservation and restoration: wetlands store water and release it slowly, and a single hectare can hold a great deal of floodwater - but only if the wetland is upstream of the town at risk.
• Riverbank conservation: fencing, replanting and allowing the channel to meander naturally slows flow and reduces erosion - cheap, but it needs land taken out of production.
• Floodplain management and zoning: keeping new development off the floodplain is the cheapest defence of all, and the hardest to enforce where land is scarce.
• Emergency response and recovery: evacuation, temporary barriers, sandbags, rescue, shelters, restoring services, compensation and rebuilding - essential, but it acts after the damage.