STREAM BED MOBILITY IN A LOW-GRADIENT, THIRD-ORDER PERENNIAL STREAM

As the interface between surface water and ground water, the streambed (hyporheic zone) is vital to the stream ecosystem as a habitat or refuge for many aquatic macroinvertebrates. Additionally, the sedimentary structure (e.g. grain-size distribution, sorting, and packing) controls the flux of fluids and solute between surface and ground water.  Thus, entrainment and transport of bed sediment can have significant implications on the structure and function of the hyporheic zone.  This research shows that mathematically, low discharge events in a low gradient stream should exceed the sediment transport threshold.

Sediment mobility was evaluated using a comparison of critical shear stress and basal shear stress based on measured stream velocities and stream bed grain size analysis. The tractive force available for particle entrainment, basal shear stress, is:

t_b=g_w h S

 where g_w is the weight density of water (9,800 N/m³), h is water depth in m, and S is water slope. Within the stream of interest, S is 0.002, and water depth ranges from a minimum of 0.10 m to 3.0 m at bankfull conditions. These values generate basal shear stresses of 1.96 to 58.8 N/m².

The tractive force at which erosion begins to occur, critical shear stress, is: 

t_c=q (g_s - g_w) d

 where q is the Shield’s Parameter (0.044 for fully turbulent flows typical of most streams), g_s is the weight density of sediment (26,000 N/m³ for most common sediments), g_w is the weight density of water (9,800 N/m³), and d is the particle diameter in m. Substitution into the above equations simplifies to: t_c=713d with units of N/m². Thus, t_c becomes a function of particle size.

When t_b/t_c > 1, the sediment transport threshold has been exceeded for the particle size in question.  With a minimum t_b of 1.96 N/m², the stream is capable of entraining a 0.0027 m particle.  Within the streambed, the mean particle size is 0.0021 m ± 0.0007 m (n=25 grain size analyses) suggesting that the streambed of a low-gradient third-order stream is not stable and is continuously changing at the lowest measured flows.  At bankfull conditions (t_b of 1.96 N/m²), a 0.0824 m (87.4 mm) particle could be entrained.  Since greater than 90% (by weight) of the streambed is smaller than 0.0824 m, the streambed is entirely mobilized during bankfull conditions which occur at least once per year.