Hydrologic Terms

Key Hydrologic Terms in Stream Ecology

Overview

  • The lecture briefly covers key hydrologic terms related to streams to prepare students for papers in stream ecology and discussions with professionals.

  • The content is organized into two parts for better digestion.

Learning Objectives

  • Articulate and understand the following:

    • Shear stress

    • Shield numbers and their ecological significance

    • Calculation components related to shear stress

    • Grain size distribution, specifically d50 and d80

    • Reynolds number and its influence on organism mobility in water.

Shear Stress

  • Definition: Shear stress is the force moving over the surface of the bed per unit area.

  • **Components in Calculation:

    • p:** Fluid density (e.g., density of water, honey, syrup).

    • g:** Gravitational acceleration (constant value).

    • r:** Hydraulic radius = Channel cross-sectional area / Wetted perimeter.

    • s:** Slope of the channel (high gradient vs low gradient).

  • Approximation for Natural Channels: For wide shallow streams, the hydraulic radius is approximately equal to the mean flow depth.

    • Example: A stream 1 meter deep and 100 meters wide gives an approximate r of 0.98, while in narrower streams, the r is significantly different.

Shields Number

  • Definition: Shields number is a dimensionless number representing the ratio of boundary shear stress in the stream to submerged weight of sediment grains per unit area.

  • Importance: It indicates the threshold for initiating sediment movement in a fluid.

  • Formula: \tau{cr} = \frac{\tau}{\rhos - \rho_w} Where:

    • \tau** = Shear stress

    • \rho_s** = Density of sediment

    • \rho_w** = Density of water

  • Significance in Ecology: Movement of stream sediment acts as a major disturbance affecting organisms in or on substrates, influencing community dynamics and the successional processes of allelic communities.

Grain Size Distribution

  • Definition: Grain size distribution (dX) provides insight into sediment sizes present in a stream system.

  • Examples:

    • d50 signifies the median grain size.

    • Cumulative percent distributions show percentage of grains less than specified sizes.

  • Example of Cumulative Distribution:

    • Less than 20 mm: 8% of grains.

    • Less than 100 mm: 50% of grains.

Biological Relevance of Shear Stress

  • Drift Propensity: The likelihood of macroinvertebrates being found in the drift vs on the benthic substrate.

  • Immigration Ratio: The ratio of organisms drifting into vs out of a stream reach.

    • Hypothesis Testing: Correlate drift propensity and immigration ratios with Shields number.

  • Ecological Implications:

    • High drift propensity observed in high mobility streams indicates low community presence on benthos, affecting feeding dynamics in fish.

    • In contrast, stable substrates (bed mobility) promote higher retention of macro invertebrates in the benthic community.

Categories of Substrates

  • Established sizes for substrates in stream studies include:

    • Boulders: > 256 mm

    • Cobbles: Various sizes categorized into large and small

    • Gravel: Various categories

    • Sand and Silt: Smaller sizes.

  • Methods to categorize substrates: Measured either by individual grain sizes or obtained through binning techniques.

Bed Load and Mobility

  • Bed Load: Refers to the portion of sediment transported by intermittent contact with the stream bed ( rolling, sliding, bouncing)

    • Movement initiated when the Shields number is exceeded.

    • Only a subset of sediments move as bed load; larger particles experience lesser movement.

Reynolds Number

  • Definition: The Reynolds number is a measure of inertial forces compared to viscous forces.

  • Formula: Re = \frac{v \times L}{} Where:

    • v: Velocity (meters per second)

    • L: Length of the object

    • : Kinematic viscosity of the liquid.

  • Physical Implications: Different organisms experience different Reynolds numbers based on size, directly affecting their mobility.

Effects of the Reynolds Number on Organisms

  • Comparison of different organisms:

    • Results in distinctive interactions with water, e.g., macroinvertebrates and fish.

    • Small organisms experience higher effects of viscous forces, while larger organisms experience higher inertial forces, making mobility experiences distinct.

  • Research Observations: Dust particles with low Reynolds numbers can travel vast distances due to their very low inertia, affecting nutrient dynamics in ecosystems like Caribbean forests receiving Saharan dust.

Drift Propensity and Reynolds Number Relationship

  • Key Finding: Higher drift propensity correlates with a higher Reynolds number; larger macroinvertebrates exhibit increased drift tendencies in streams with higher velocities.

  • Understanding this relationship is vital for evaluating ecological dynamics concerning feed resources for fish and other aquatic organisms.