Eric L. Peterson

Benthic shear stress and sediment condition

Abstract This article presents a literature review and analysis which finds that the conditions in a shallow waterway are dependent on the underlying shear stress. Circulation and mixing have been recognised as somehow important to the health of the lower water column and sediment of a pond. Benthic shear stress is identified as one of the principal governors of sediment condition. Benthic shear stress regulates the convection of volatile substances such as oxygen across the sediment–water interface. Benthic shear stress is the primary determinant of where particulate matter scours, rolls, saltates, settles, and consolidates. The critical shear stress of very small particles tends to be determined by density rather than size. Excessive shear stress may cause organic material to become buried under a fallout of silt in locations with lower shear stress. Such organic matter trapped in a sludge blanket consumes oxygen faster than it can diffuse through the pores between grains of soil, resulting in anaerobic soils. The objective of pond aeration and circulation systems should be to optimise conditions for the growth of cultured stock. Better management of the epi-benthic habitat of shrimp and prawns would concurrently reduce the use of energy and soil. The finding is that pond sediment condition may be assessed in terms of benthic shear stress. Control of benthic shear stress would enable resuspension and oxygenation of organic detritus without making the situation worse by intermixing organic matter with mineral soil. Desirable benthic shear stresses are expected to exist in the range of 0.003–0.03 N/m2 where organic matter is gently maintained in an aerobic state. Locations having less than 0.001 N/m2 benthic shear stress are subject to burial under accumulating mineral silts. Excessive scour occurs in locations having more than 0.1 N/m2. High stress locations are the source of suspended solids which fallout on locations experiencing lower rates of benthic shear stress. The declining productivity of shrimp mariculture ponds is associated with excessive stocking and feeding because these practices necessitate intensive aeration, which results in high rates of pond circulation. The circulation causes soil to be scoured in some places while fallout covers the bottom in other places. Consequently intensive cropping of a pond causes feed pellets and other organic material to be buried in the soil, creating anaerobic conditions whereby bacterial metabolites are produced and released into the water column.

CFD modelling pond dynamic processes

The parametric pond simulation methodology AUTOPOND was developed as a tool to evaluate the effect of aerators on the sediment quality in an aquaculture pond. The methodology is capable of simulating any combination of paddlewheels and propeller-aspirators in a single pond. Pond bathymetry is modelled with a smooth bottom and a piece-wise series of inclined banks, to generally represent any convoluted shoreline. Simulations predict that a paddlewheel imparts more circulation into a pond than a propeller-aspirator of the same motor horsepower. The propeller-aspirator concentrates an intensive effect in a localized scourhole, surrounded by relatively weak bottom stress. In contrast, paddlewheel simulations predicted a wide swath of high stress, followed by an equally extensive region of moderate stress. A simulation of the combined effect of two paddlewheels and four propeller-aspirators was produced to represent a ‘real world’ earthen mariculture pond stocked with P. monodon. Water currents and sediment conditions reported in the real pond were comparable with the results of the simulation. Given sufficient computational resources, it is now possible to investigate the interactions of pond geometry and aerator operation with a computer. Portions of pond bottoms found to experience high shear stress are now identified for reinforcement to prevent the sedimentation of sludge in otherwise productive zones. This would result in improved feed conversion and reduced ammonia levels. Ponds and aerators can now be objectively coordinated to achieve more productive culturing conditions.

Arrangement of aerators in an intensive shrimp growout pond having a rectangular shape

Simulations have been conducted to suggest general principles for the arrangement of aerators within a rectangular pond used for the growout of marine shrimp such as Penaeus monodon and Penaeus japonicus. Computational fluid dynamic models were produced for three schemes that were identified in a survey of Australian Prawn Farming Association members. These arrangements are ‘in-line’ (series), ‘parallel’ (side by side), and ‘diagonal’ (diverting apart). Model results were assessed on the basis of benthic shear stress by classifying regions of pond bottom as ‘red zone’ (excessive stress), ‘green zone’ (desirable), and ‘dead spots’ (sediment traps). A comparison of results indicates that conventional aerators should be arranged diagonally or in parallel. It is also apparent that low-speed operation would be advantageous. These recommendations are consistent with the long-established practice of establishing pond-wide circulation.

Observations of pond hydrodynamics.

This paper provides a detailed observation of pond hydrodynamics. Bathymetry and aerator deployment largely control the hydrodynamic regime. Aerators drive pond circulation, which is resisted by bottom and bank friction. Hydrodynamic friction is characterized by sediment grain size. One particular pond is offered as a proxy representative of Australian mariculture ponds in general: Pond X. The pond had been stocked with P. monodon at densities in the range 25–35 m−2 about ten times during the period from 1989 to 1995. Observations were made in the year 1996. The site was on the east coast of Australia at 18° south latitude. The location was sheltered by a mountainous offshore island, such that winds effecting the pond averaged 2.3 m s−1. Water exchange occurred at a rate of about 8.3% of pond capacity per day. Pond X was found to be rectangular, 120 m long and 87 m wide. The banks were found to slope between 1:1.5 and 1:7.6, with an average slope near 1:3. The pond was shallowest in one corner, where depth was 834 mm, and drained diagonally with 1872-mm maximum water depth. Aeration was provided by a total of six 1.5-kW machines, comprised of two of the paddlewheel type and four of the propeller-aspirator type. Each machine was found to deliver 200 N of horizontal thrust into the pond water column. The combined effect of the aerators created a circulation current averaging about 11 cm s−1 around the pond periphery. The central region of the pond was found to have a fluctuating velocity of less than 1 cm s−1. The mass-average size of pond soil and sediment was found to vary from 25 μm in the central region to 250 μm at the banks and where the bottom was swept by paddlewheels.

Assessing sediment removal capacity of vegetated and non-vegetated settling ponds in prawn farms

Sediment removal capacity is assessed for a constructed mangrove wetland, and a non-vegetated settling pond that are both used for filtering water in tropical aquaculture. The assessment is performed through sediment budget analysis using data of suspended sediment concentration collected from optical backscatter sensors. The sensors were deployed at the ponds inlet and outlet. These data sets provide a measure of trapping efficiency of each pond with different flow regimes and settling areas. The tides influenced flow in the wetland but none was felt in the settling pond. The average trapping efficiency obtained for the vegetated and the non-vegetated ponds was (40±33) and (70±36)%, respectively. The deposition rate calculated for the vegetated and non-vegetated pond ranges between 13–174 g/m2 per h (average=63 g/m2 per h) and 10–19 g/m2 per h (average=14 g/m2 per h), respectively. The efficiency of vegetated and non-vegetated ponds is likely to be improved by decreasing the aspect ratio (length/width) from the current value of 6 to 1 and of 5 to 1, respectively.

Effect of speed on Taiwanese paddlewheel aeration

Aerators are generally used in Australian aquaculture ponds day and night at full speed without regulation. This situation is untenable in view of climate change, as energy conservation becomes an essential issue for all industries, including aquaculture. Variable speed performance curves were developed for the paddlewheel aerators that have been employed on Australian marine aquaculture ponds so that speed may be actively adjusted to match pond water quality requirements. Results show that speed of rotation is a significant factor effecting the performance of a paddlewheel aerator. Of particular note was the observation of backsplashing when kinetic energy (pumping head) was greater than the radius of a paddlewheel. The process of backsplashing is readily identified when whitewater is seen flying above a paddlewheel. It is hypothesised that backsplashing dilutes the oxygen-starved water entering a paddlewheel, thereby degrading the operational efficiency. Backsplash breakpoint speed is related to paddlewheel diameter. Aerator users can reduce backsplash by changing mechanical gearboxes or using a variable frequency drive (VFD inverter).