hydrology

Drainage basin

The total land area drained by a river system and all of its tributaries, bounded by a watershed.

Watershed

The elevated ridge or highland boundary that separates one drainage basin from another.

Reason a drainage basin is defined as an open system

Both energy (such as solar radiation) and matter (such as precipitation and river discharge) freely cross its boundaries.

Drainage basin water balance equation

$P = Q + E \pm \Delta S$, where $P$ is precipitation, $Q$ is channel discharge/runoff, $E$ is evapotranspiration, and $\Delta S$ is change in water storage.

Interception store

Precipitation that is caught and temporarily retained on plant leaves, branches, and stems before reaching the soil.

Stemflow vs. Throughfall

Throughfall is precipitation dripping through or falling directly between vegetative foliage, whereas stemflow is water flowing down plant stems and tree trunks to the ground.

Soil moisture store

Subsurface water retained within the unsaturated pore spaces of soil above the phreatic zone (water table).

Groundwater store

Water stored under hydrostatic pressure in saturated rock layers and sediment beneath the water table.

Infiltration vs. Percolation

Infiltration is the initial absorption and downward entry of surface water into the topsoil, whereas percolation is the deeper vertical movement of water through soil into underlying permeable rock.

Throughflow

The lateral downslope transfer of water through unsaturated soil layers toward a river channel.

Hortonian overland flow

Surface runoff that occurs when rainfall intensity exceeds the infiltration capacity of the soil, despite the soil not necessarily being fully saturated.

Saturation-excess overland flow

Surface runoff that occurs when prolonged rainfall completely fills all pore spaces in the soil profile, forcing subsequent water to flow over the surface.

Porosity vs. Permeability

Porosity is the percentage of total pore or void space in a rock or sediment, whereas permeability is the measure of how easily fluid can transmit through interconnected pores.

Aquifer vs. Aquitard

An aquifer is a permeable rock layer that readily stores and yields groundwater, whereas an aquitard is a low-permeability rock layer that severely restricts groundwater movement.

Consequences of excessive groundwater abstraction

Lowering of the water table, reduced baseflow to surface streams, land subsidence, and saltwater intrusion in coastal aquifers.

Storm hydrograph lag time

The time interval between the point of maximum rainfall intensity and the point of peak river discharge.

Rising limb vs. Recession limb

The rising limb marks the rapid increase in river discharge following rainfall, whereas the recession limb reflects the gradual post-peak drop in discharge back toward baseflow.

Environmental factors creating a flashy hydrograph

Steep relief, impermeable rock, thin or saturated soils, high rainfall intensity, deforested catchments, circular basin shape, and dense urban tarmac with stormwater drains.

Environmental factors creating a subdued hydrograph

Gentle valley gradients, permeable geology, deep dry soils, dense forest canopy interception, elongated basin shape, and extensive natural wetland storage.

Hydraulic action

Fluvial erosion caused by the sheer mechanical force and turbulence of water dislodging particles and compressing air within bank fissures.

Abrasion (corrasion)

The mechanical gouging and wearing away of the river bed and banks by rock particles carried within the moving water.

Attrition

The progressive rounding and reduction in size of sediment particles as they collide with one another during transport.

Solution (corrosion)

The chemical dissolving of soluble minerals and rock types (such as calcium carbonate in limestone) by mildly acidic river water.

River competence vs. Capacity

Competence is the maximum sediment particle diameter a river can move at a given velocity, whereas capacity is the total volume or mass of sediment a river can transport.

Four mechanisms of fluvial sediment transport

Traction (rolling large bedload), saltation (hopping sand/gravel), suspension (turbulent floating of silt/clay), and solution (dissolved ionic load).

Conditions causing fluvial deposition

A drop in stream velocity and energy resulting from decreased gradient, channel widening, entry into a standing body of water, or floodwaters spilling over banks.

Hjulström curve: clay entrainment anomaly

Clay requires surprisingly high water velocity to erode despite its fine size because of strong electrostatic bonds and cohesion between clay platelets.

Bradshaw model: downstream changes in channel width, depth, and discharge

Channel width, hydraulic depth, and discharge all progressively increase downstream as tributaries join and the river approaches base level.

Formation of a river waterfall and plunge pool

A resistant caprock overlies weaker bedrock; differential erosion undercuts the weaker layer to create an overhang and a plunge pool, which eventually causes caprock collapse and upstream retreat.

River gorge

A narrow, steep-sided rocky valley formed by the sustained upstream migration and retreat of a waterfall over time.

Meander mechanics: outer bend vs. Inner bend

Helicoidal flow directs fast current to the outer bend, producing lateral erosion and a river cliff, while slower water along the inner bend deposits bedload to form a point bar (slip-off slope).

Formation of an oxbow lake

Continuous lateral erosion narrows a meander neck until high flood discharge breaches it directly; sediment deposition subsequently cuts off the old loop, isolating a curved body of standing water.

Natural levee formation

During overbank flooding, water leaving the channel experiences an immediate drop in competence, depositing coarse alluvium directly along the channel edges to form raised parallel ridges.

River rejuvenation and characteristic landforms

A drop in base level or tectonic uplift that stimulates renewed vertical downcutting, producing river terraces, incised meanders, and knickpoints.

Conceptual flood risk equation

$$\text{Flood Risk} = \text{Hazard} \times \text{Exposure} \times \text{Vulnerability}$$

Jubilee River flood relief channel

An approximately $11.6\,\text{km}$ artificial flood relief channel opened in 2002 to divert high flows from the River Thames around Maidenhead, Windsor, and Eton.

Flood control dams: major trade-offs

They regulate downstream peak discharge and store water, but involve high construction costs, trap sediment, disrupt natural ecosystems, and risk catastrophic failure.

Soft engineering flood mitigation approaches

Floodplain land-use zoning, afforestation, wetland preservation, soil management, and predictive flood warning and evacuation systems.