Water Dynamics: Hydrological Cycle, Basins, and Erosion

Posted on Jan 4, 2025 in Geology

Water’s Role in Shaping Landscapes

Water, a dominant agent of landscape alteration, erodes terrain and transports sediments. The total water in the hydrosphere is approximately 1,360 million cubic kilometers. This water is constantly moving, interacting with the hydrosphere, atmosphere, solid earth, and biosphere. This continuous movement is known as the hydrological cycle.

Hydrographic Basins

A hydrographic basin is an area of land drained by a single natural drainage system. It is defined by the section of a river and is delimited by the line of the summits, also called the divide. The main characteristics of a basin are:

  • Curve-bound surface: This characteristic gives the first indication of the basin’s hydroelectric potential.
  • Form coefficient: Provides preliminary indications of the type of flood wave it can generate.
  • Branch coefficient: Also gives preliminary indications regarding the type of flood wave.

Basin characteristics can either intensify or mitigate flooding. Important aspects include:

  • Area
  • Shape
  • Slope
  • Lithology
  • Soil saturation level
  • Drainage network
  • Morphology of channels

The Iberian System

The development of infrastructure and the geology of the Iberian Peninsula show that the western block forms the main part, while the eastern borders form the Iberian system. The Iberian system acts as the water divide of the peninsula. From this system, waters flow towards the Atlantic and Mediterranean basins. The Iberian Peninsula is characterized by large mountains and a marked coastline. The relief divides the water basins.

Runoff and Infiltration

Water runoff initially flows in thin sheets over the soil, known as sheet runoff. The amount of water that runs off instead of infiltrating depends on the infiltration capacity of the soil.

  • The infiltration capacity is controlled by factors such as:
    • Intensity and duration of precipitation.
    • Initial soil moisture.
    • Soil texture.
    • Terrain slope.
    • Vegetation cover.

Water can flow in two ways:

  • Laminar flow: Water particles move in straight paths parallel to the riverbed, without mixing. This is only possible when water moves very slowly through a smooth channel.
  • Turbulent flow: Water moves erratically, characterized by turbulent eddies. This occurs when the current velocity is high or the channel changes abruptly.
Erosion and Sediment Transport

A current’s capacity to erode and transport is directly related to its velocity. Even slight variations in velocity can cause significant changes in the sediment load that water can carry.

  • Several factors determine the velocity of flows, which control the amount of erosive work:
    • Channel gradient or slope.
    • Shape, size, and irregularity of the channel.
    • Flow.

The gradient is expressed as the vertical drop of a current over a given distance. The most effective channel is the one with the smallest perimeter. Flow is the quantity of water that passes a point in a given time, measured in cubic meters per second. It is calculated by multiplying the cross-sectional area of the current by its velocity.

A useful way to study water currents is to review their longitudinal profile, which is a section of the current from its source (headwaters) to its mouth (where it empties into another body of water). The base level is the lowest elevation to which a current can deepen its track. The ultimate base level is the ocean, a lake, or another stream. There are also local base levels such as lakes or resistant rock layers.

Currents erode their channels in three ways:

  • Removing weakly consolidated grains.
  • Abrasion.
  • Dissolution.

Sediment is transported in three ways:

  • Solution (dissolved load).
  • Suspension (suspended load).
  • Along the bottom of the channel (bed load).

Most of the dissolved load comes from groundwater. Most currents transport a suspended load, which is controlled by the sedimentation velocity and the water velocity. Coarser grains move along the bottom as bed load through rolling, sliding, and saltation.

Capacity is the maximum load a current can transport. Competence is the maximum grain size a stream can transport. The stronger the flow, the larger the grains that can be transported.

Alluvial Fans and Deltas

Alluvial fans develop where high-gradient streams leave mountain valleys and enter a plain. The coarsest sediment is deposited at the apex of the fan, while finer sediment is transported towards the base. Deltas form when a current enters a lake or ocean. They have three layers:

  • Frontal layers: Coarse particles deposited immediately upon entering the water body.
  • Top layers: Thin, horizontal layers deposited during flooding.
  • Base layers: Fine silts and clays deposited at a distance from the mouth.
River Courses

Upper course: Characterized by narrow valleys and waterfalls. Middle course: Meandering channels. Lower course: Deltas or estuaries.

Notable features include rapids and waterfalls. A gorge is a structure where a river flows through a narrow passage with steep walls.

Flooding

Common types of flooding include:

  • Regional floods.
  • Flash floods.
  • Ice jam floods.
  • Dam failure floods.

Regional floods can be seasonal, caused by snowmelt or heavy rainfall. Flash floods are influenced by the intensity and duration of precipitation, and topographic conditions. Mountainous areas are particularly susceptible due to steep slopes. Engineering efforts to control flooding include:

  • Artificial dikes.
  • Flood control dams.
  • Channelization.