Silverstream Research Station of the National. Institute of Water ... Engineer Waterways Experimental Station (Kraus et al. 1994) and ...... Rotterdam, Brookfield,.
New Zealand Journal of Marine and Freshwater Research, 1997, Vol. 31: 435—448 0028-8330/97/3104-0435 $7.00/0 © The Royal Society of New Zealand 1997
435
On stream periphyton-turbulence interactions
V. I. NIKORA D. G. GORING B. J. F. BIGGS National Institute of Water and Atmospheric Research Ltd P. O. Box 8602 Christchurch, New Zealand Abstract A set of experiments was carried out to determine what effects periphyton communities could have on near-bed hydraulic fields. We analysed velocity distribution, skewness and kurtosis coefficients, Reynolds stresses, relative turbulence intensity, coefficient of eddy diffusivity, velocity spectra, and turbulence scales at two flows with, and without, diatom-dominated periphyton on the bed. We found that turbulence was affected by the periphyton mat selectively. The largest influence occurred for the velocity distribution, Reynolds stress, coefficient of eddy diffusivity, and velocity cross-spectra. Changes as a result of the periphyton mat were revealed in the large-scale turbulence structure. The periphyton affected not only the region near the bottom but also the entire logarithmic layer. The periphyton mat increased roughness length by a factor of =5 whereas the integral resistance to flow (reciprocal Chezy coefficient and Manning's roughness coefficient) was increased by 2 0 - 2 5 % . The microscale turbulence structure was investigated using values of Kolmogorov's microscale 1)^- These appeared to be commensurate with the characteristic scales of periphyton filaments. This allowed us to hypothesise that the mechanism of the "periphytonturbulence" interaction is connected, to a certain degree, with viscous effects.
M97005 Received 21 February 1997; accepted 1 July 1997
Keywords periphyton; open-channel flow; turbulence INTRODUCTION Periphyton are considered to be the dominant source of energy for higher trophic levels in unshaded streams (Minshall 1978). Factors controlling periphyton growth include nutrient mass-transfer, light, and temperature. Factors controlling loss include high shear stress/turbulence (in both temporal and spatial dimensions), sediment instability, and invertebrate grazing (Biggs 1996). During periods of summer low flow when bed sediments are most stable, water velocities are low, and temperatures and nutrient levels are relatively high, losses are minimised, and growth processes dominate community dynamics. At such times, periphyton can form extensive mats in streams and this can have major consequences on stream environments. One of these effects is to strongly alter flow fields (Reiter 1989; Dodds 1991) which can then cause major changes in the hydrophysical habitat for other stream life. Only one study has specifically investigated such phenomena of periphyton alteration of flow fields in streams (Reiter 1989). In this, Reiter used simple hydraulic measures (which reflect some integral flow properties, but not its internal structure) in small laboratory flumes to demonstrate that growth of a periphyton mat resulted in increases in friction velocities, roughness height, and mixing lengths. The height of the mat also appeared to be limited by a maximum velocity or shear, and was equal among flumes. These results confirmed that major changes in nearbed hydraulic parameters occur with periphyton development in streams, and that the degree of change may be community specific. The importance of the interaction between hydrophysical factors and biological communities is also widely recognised in other environments (e.g., Nowel & Jumars 1984; Leonard & Luther 1995).
436
New Zealand Journal of Marine and Freshwater Research, 1997, Vol. 31
With the recent availability of robust and nonintrusive high resolution measuring technology such as the Acoustic Doppler Velocimeter (ADV), we believe that it is now possible to make much more progress on investigating the hydrophysical interaction between natural streambed periphyton (and other plants) and their associated flow fields. Furthermore, it facilitates the collection of data at scales relevant to what the organisms "feel". In the following study, we used such an ADV system to measure velocity time series in three dimensions down vertical profiles to within 2 mm of the bed in a large concrete-lined outdoor flume. The measurements were carried out at two flows and for each flow they were repeated with, and without, a periphyton mat. Our objectives were to see how the mat was influencing near-bed turbulence properties of the stream and whether this effect varied under different flow conditions.
BACKGROUND To investigate the effects of periphyton on flow structure we used the following hydraulic characteristics: velocity distribution, skewness and kurtosis coefficients, Reynolds stresses, relative turbulence intensity, coefficient of eddy diffusivity, velocity spectra, and turbulence scales. In our measurements we used the right-handed coordinate system: X-axis was oriented along the main flow ({/-velocity component), y-axis was oriented to the left bank (F-velocity component), and Z-axis was pointing toward the water surface (W-velocity component). The following describes our characteristics in more detail. The skewness (Skj) and kurtosis (Kiij) coefficients (Panchev 1971; Bendat & Piersol 1986) characterise the shape of the probability distribution of the i-th velocity component (index i = 1 denotes longitudinal U, i = 2 denotes vertical W, and i = 3 denotes transverse V velocity components) for a given measuring point. To estimate their values we used the relationships Ski = U}3 la] and Kut = U-4 l
