Hydrology & Fluvial Questions

Define evaporation

Water turning into gas (water vapour) due to heat.

Define percolation.

Downward movement of water through soil/bedrock after infiltration.

What is a flood recurrence interval?

The probability that a certain flood level will be equalled or exceeded in any given year, e.g., 1-in-100 years = 1% chance.

Describe and explain the formation of deltas.

When a river enters a lake or the sea, its velocity rapidly decreases, and it loses energy, therefore its capacity and competence to transport sediment is reduced.

As a result, deposition begins, starting with the heaviest materials which form the foreset beds.

Lighter particles settle further out to sea, forming bottomset beds.

Over time, finer sediments deposited nearer the river mouth form topset beds, which are laid over the foreset beds.

Additionally, flocculation occurs, especially with clay particles, causing them to bind together and settle more quickly.
Distributaries develop as sediment blocks the main channel, which leads to the river splitting and depositing sediment over a wider area.

The shape of the delta depends on the balance between river deposition, marine currents, and tidal processes – for example, arcuate deltas (like the Nile) form where wave action smooths the coastline, whereas bird’s foot deltas (like the Mississippi) form when river deposition dominates.

Why does the minimum velocity needed for sediment erosion vary (based on the Hjulström curve)?

Sediments smaller than sand (like clay) require higher velocities to erode because of their cohesive nature.

Therefore, even though clay is light, its particles stick together, resisting erosion.
On the other hand, sediments larger than sand require higher velocities due to their greater mass and weight, which increases the force needed to dislodge them.

This variation creates a U-shaped curve on the Hjulström diagram.

Explain two reasons for the variation of deposition along a river channel.

• Velocity change – As a river enters a lake or sea, velocity drops, leading to deposition, especially when flocculation causes fine particles to settle.

• Channel shape and friction – In meanders, water moves slower on the inside bend, causing point bar deposition.

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  Friction is also greater in the upper course, reducing competence and promoting deposition in turbulent areas.

Why might precipitation not reach the river channel?

Some is intercepted by vegetation, then lost to evapotranspiration.

Some infiltrates into soil or bedrock, feeding groundwater or aquifers.

Other water may evaporate, be stored in lakes, or used by humans.

In cold regions, precipitation may fall as snow, temporarily stored as ice.

What two factors influence braided channel formation?

• Variable discharge (e.g., snow/glacial melt) causes alternating periods of high and low energy, leading to sediment build-up.

• High sediment load means more deposition during low flow, forming islands or eyots, splitting the channel.
  Easily erodible banks also help the river split into multiple channels.

How can soft and hard engineering prevent river flooding?

Soft engineering approaches aim to work with natural processes. For example, reforestation in upper drainage basins increases interception and infiltration, which reduces surface runoff and lowers peak discharge, making flooding less likely. Floodplain zoning restricts development in areas most likely to flood; this does not stop flooding but reduces the impact on people and property.

Therefore, soft engineering is often low-cost and environmentally sustainable, although it may be less effective in high-magnitude floods.

On the other hand, hard engineering involves large-scale, man-made interventions. Dams and reservoirs store water during periods of high rainfall and release it slowly, controlling river discharge. Levees increase the height of riverbanks, allowing the channel to hold a greater volume of water, thereby reducing the chance of overflowing. In some cases, river straightening is used to increase the velocity of water flow, helping it exit vulnerable areas faster. However, this can cause flooding downstream if not carefully managed.

Explain how changes in land use might cause a river to flood.

Changes such as urbanisation lead to the construction of impermeable surfaces (e.g. roads, pavements), which reduce infiltration and increase surface runoff, making the hydrograph more “flashy.”

Similarly, deforestation removes trees that would intercept rainfall and absorb water via roots. This reduces interception and infiltration, increasing overland flow.

In farming, replacing forests with crops or leaving soil bare can also increase runoff and flood risk downstream.

Describe how throughflow occurs on slopes.

After infiltration, water percolates laterally downslope through the soil, under gravity. It moves parallel to the surface, faster on steeper or more permeable soils.

Explain how slopes affect the shape of a storm hydrograph.

Steep slopes increase surface runoff, creating a shorter lag time, higher peak discharge, and a more flashy hydrograph.

In contrast, gentle or vegetated slopes promote infiltration, producing a flatter hydrograph with lower peak and longer lag time.

Describe and explain the formation of gorges.

Gorges are steep-sided valleys formed when vertical erosion exceeds lateral erosion.

One common process involves the retreat of a waterfall. As water plunges, it undercuts the soft rock at the base, causing collapse of the harder rock above. Over time, this causes the waterfall to retreat upstream, leaving a gorge behind.

Alternatively, gorges form due to rejuvenation when tectonic uplift or a fall in sea level increases river energy. The river cuts rapidly downward to reach a new base level.

Additionally, mass movement like rockfall can help steepen gorge walls, while abrasion and hydraulic action deepen the channel.

Thus, both fluvial and slope processes work together to create a gorge over time.