Catchment Hydrology Overview

Alluvial: Sensitive to changes in water and sediments and adjusts easily and frequently


Rock-Cut: Requires a high amount of energy to erode the bed and adjust its morphology, so changes only after a high energy event

HUMID

Channel Morphology

SEMI-ARID

Channel Morphology

Perennial Streams

  • Precipitation > Evapotranspiration
  • Flows all year round (constant baseflow)
  • High WT, stream is at or below the WT
  • Water from ground to channel

Ephemeral Streams: BSh 🌄

  • Precipitation < Evapotranspiration
  • Flows for only short periods after episodic rainfall (flash floods)
  • Low WT, stream is above the WT
  • Water from channel to ground

Intermittent Rivers: BWh 🏜

  • Discontinuous, much of the river would dry up leaving only ponds in the channel depressions, even leading to complete drying if dry conditions persist

Discharge & Channel Shape

Discharge & Channel Shape

Slope Profile

  • Long channel length and concave profile (steeper to gentler gradient)
  • Due to greater erosion and downstream fining where finer sediments are deposited downstream

Slope Profile

  • Short channel length and convex profile (gentler to steeper gradient)
  • Limited erosion and lack of downstream fining

Channel Pattern

Channel Pattern

  • Before a rainfall, there is no discharge so there is no channel pattern
  • During a rainfall, single straight channel appears due to high surface runoff and steep rising limb. During high discharge, sediments get transported as load and might overflow causing braiding / anabranching

  • After rainfall ceases, pattern is braiding or extensive anabranching depending on erodibility of channel banks


    • Braiding occurs for highly erodible and unconsolidated banks with huge mid-channel bars
    • Anabranching occurs for hardened channel banks that are resistant and limits channel widening, so it overflows easily during high discharge
      (may be relicts associated with larger discharges in past climates 🐨)

  • Meandering channel patterns can occur when the source of the river is located outside of the drainage basin

  • Such rivers have high energy for deflection in helicoidal flows (highly unlikely in arid)
    EG: Indus River which flows through drylands though source is from China and Zin River in Israel

🌧 AF

  • Bimodal distribution with two clear peaks during the equinoxes
  • Only one bankfull discharge but frequently occurs
  • Discharge ↑ downstream due to contribution by tributaries, throughflow and baseflow
  • Exhibits channels of a single size ; proportional depth and width with high hydraulic radius (narrower & deeper)
  • Large flood may temporarily enlarge the channel but after time, channel would revert


🌦 AM / AW

  • High ppt during summer but rest are dry seasons

  • Streamflow α rainfall


  • Affects channel efficiency (high flow, ↑ HR, ↑ efficiency)

  • Several, less frequent bankfull discharge levels due to rainfall seasonality (normal flows to floods in wet seasons)
  • Thus, nested channel shape, a macrochannel consisting of other channels of different discharge levels
  • Width many times larger than depth, so hydraulic radius varies with flood events
    EG: Auranga River

Fluvial & Runoff Processes

Deposition

Transport

Erosion

Water Balance:
Af: Positive
Aw/ Am: Fluctuates between + or - during different seasons


  • Frequency of bankfull discharge is not more than once or twice a year
  • Discharge decreases downstream due to high evaporation rates, absence of tributaries and large transmission losses
  • Flood discharges of varying magnitudes due to past wetter climates 🐨 , producing nested channels called wadis
    EG: Todd River

Water Balance
Negative as low ppt < storage + output , but after a rainfall event, can be positive

Fluvial & Runoff Processes

Af : Low Drainage Density
Am/Aw: Moderate

Arid : Low/0 Drainage Density
Semi-Arid: Moderate

Transport

Erosion

Deposition


  • Continuous erosion of high frequency where discharge is constantly high
  • Abrasion more common due to large supply of available load while hydraulic action occurs mostly during a rainfall event where channel has high energy
  • Smaller extent of erosion than arid as humid's sediment load is smaller and finer
  • In seasonally humid, erosion and transport is dominant during wet seasons
  • Continuous frequent transport due to constant high discharge from regular high rainfalls
  • Commonly saltation and suspension
  • Channel load is larger in numbers and finer (fine sits/clay/fine sands) due to downstream fining and dominant CW
  • Coarser load transported by traction during large rainstorms
  • Sediments transported further downstream
  • Sediment size decreases with increasing discharge downstream
  • Af: transport is frequent and regular while Am/Aw: all load is transported during wet seasons
  • More frequent deposition in seasonally humid where there are regular seasonal rainfalls and high discharge
  • Deposited load is coarser and larger due to higher settling velocities, so mostly bedload is deposited while suspended load remains in suspension due to lower settling velocities and constant discharge and river energy
  • Deposition occurs throughout channel
    • 🌧 Inner banks of meanders as point bar deposits
    • 🌦 Large mid-channel bars , especially in dry seasons where stream capacity and competence ↓
  • Rare infrequent erosion due to irregular rainfall
  • Erosion only occurs during episodic rainfalls, usually hydraulic action due to high stream energy and abrasion due to large calibre load
  • Larger extent of erosion than humid because of highly erodible banks with sparse vegetation to hold them together and prevent erosion
  • Infrequent transport due to irregular discharge
  • During rainstorm where discharge is high from large surface runoffs, channel competency and capacity ↑, and all load is suspended
  • Load is coarser and smaller in number due to dominant PW and lack of downstream fining , transported only a short distance downstream [every fluvial process in the arid occurs upstream as due to transmission losses, there is rarely a downstream part of a channel as discharge is insufficient]
  • After the intense rainfall where river energy, competence and capacity falls, all load is deposited in temporary mid-channel bars
  • Compared to frequency in humid, arid has lesser deposition as materials deposited rarely gets picked up to be deposited again
  • Deposition occurs upstream, only in the middle of the channel as large temporary bars

Channel Load & Deposits

Channel Load & Deposits

Af: Meandering

  • High discharge and fine bedload so river has capacity and competence
  • Mid channel deposition is unlikely (not braided)

Am/Aw: >1 pattern straight or meandering (wet) , braided (dry)

  • Due to rainfall seasonality and distinct changes in discharge
  • Large presence of coarse bedload that is deposited due to reduced capacity and competence forms mid-channel bars and a braided pattern
  • Large sediment loads due to active weathering and mass movement
  • 🌧 Finer load (silt/clay)
  • 🌦 Coarser load where load transport is highest in the wet season
  • Large sediment load of mainly coarse sediments (sand/gravel) that are transported only during rainfall events
  • Presence of large mid-channel bars and deposits outside of the channel as stable islands and ridges