Building Marron Ponds Guidelines for well-constructed ponds

1. The site for construction of a group of marron ponds should be, or tend to be, a uniformly and gently sloping piece of land and not an undulating and steep valley slope. The cost of pond construction is related to the volume of earth moved, which is determined by the type of excavation needed to reach clay and the need to level the site, or, at least, have each row of adjacent ponds at one level, or perhaps two. Figure 1 shows the layout of rows of ponds in relation to land contours (slope), the long axis of ponds, the pond outlets and the site's internal and perimetral drains. The bank tops need to be horizontal (as well as level), particularly on the long axis, with very small free-board (<=0.5m) between water surface and bank top. The site slope needs to be small so that it is not difficult to construct pond floors with a slope of not more than 1 in 100 down the long axis to the outlet.

2. The immediate subsoil horizons of the pond site need to be examined at test holes over the site for presence of clay from the surface and the suitability of the clay (permeability, compaction, ...) by a qualified civil engineer (see 9 below). Presence of the clay from the surface determines the degree to which the ponds can be perched and drain depths; cost increases and construction practicability decreases with the need for deeper excavation and ground level reduction to give banks with the required small free-board. For a realistic annual water budget, the seepage loss of water must be minimised to about 1 metre by use of a site with suitable clay and by specified compaction during construction.

3. A detailed layout of the ponds, banks, perimetral and internal drains, effluent settling and nutrient stripping ponds and access tracks should be prepared by the site civil engineer to scale on a contour plan, determined by a qualified surveyor (the attached plan shows a generalised layout). The pond area will need to be adequately drained by perimetral deep drains, both for the harvest draining of ponds but also to prevent a perched (winter) water table and to divert both surface and sub surface drainage from passing through the pond site from uphill. Bank widths and access to ponds by farm vehicle, for spreading food and harvesting, must be adequate ( at least 3m width). Allowance should be made also for bank poles to carry overhead bird netting and underground power lines for pond aerators.

Figure One

4. The design of a marron pond, which is purpose-built for semi-intensive high yielding farming, should be clearly distinguished from a farm stock, excavated tank dam (the type used for yabbies in the wheatbelt). The later can be used only for an extensive (low yielding) crayfish harvesting level of culture. The pond constructor should be provided with professionally prepared engineering plans for both the general layout of the ponds and for the specification of an individual pond(s). The latter plan, and accompanying specifications, is particularly important since many constructors are experienced only in building to a rough farm dam design, and they will do so unless otherwise briefed. The resulting ponds will be too great in depth, they will have banks which are much too high (too much freeboard) and pond batters and floors which are too uneven and insufficiently compacted.

5. For a pond of the currently accepted commercial area of 1000m2 the length and width of the water area are usually about 40 and 25 m, resp... Ponds should not be square or too rectangular; 50 x 20m is the most acceptable extreme.

6. An earthen, and particularly crayfish, pond should not have a flat floor with steep, near vertical batters near the shoreline. Close attention should be given to batter design for the width and length cross-sections. Batter slopes of not more than 1 in 3 allow proper compaction to give long term stability, minimizing bank slip.

7. Presence of a 1000m2 pond down the centre line of the long axis (length) should increase from about 1 m to about 1.5-2 m at the outlet, so as to provide a draining slope of 1 in 100 down the length of the pond to the outlet.

8. Each pond must have a bottom drain outlet for control of a constant water level and minimum freeboard, for bottom water flushing, for quick and complete draining for harvesting and for prolonged drying out of pond sediments for rehabilitation of a pond. The outlet pipe (pvc) diameter must be of sufficient capacity to drain the pond rapidly; maximum time to drain should be 10 hours. Pipe of 140 mm diameter is needed for a 1000m2 pond. The outlet pipe to the internal drain line should be keyed into the end bank of a pond with a concrete collar. The push-in pvc water level standpipe can be surrounded by a larger diameter pvc pipe with bottom cutouts for overflow water to be taken from the bottom of the pond. At drain harvest the outlet is surrounded by a circular screen (1.5-2 m dia.), which is unnecessary at other times. The outlet surround basin should be a 3 x 3 m concrete pad, with an extension keyed into the near end bank to prevent bank washout.

Aerators are used to maintain water quality in this type of marron pond, so that the amount of pond overflow or effluent water is very low. Tests have shown that even during pond draining for harvest, the nutrient and mud loading of effluent is very low until the pond sediment is disturbed. The annual water budget for the farm uses a "topping up" rate of water supply, by ball cock to offset evaporative and seepage losses, and a high rate (75%) of reuse of drained water. However, hosed out pond sediments are extremely polluting. The pond layout plan attached includes a deep settling pond for mud, followed by a shallow macrophyte pond for nutrient stripping.

9. Following the basic forming of the ponds by use of a large bulldozer, the banks, batters and floors of the ponds must be finished off. These aspects are smoothed and tidied up by removing excess loose soil with a back blade on a tractor or with a bobcat. The ponds are then compacted, using up a tractor-drawn sheep's-foot roller for good clays and up to eight passes with a vibrating sheeps-foot for poorer clays. Compaction to specification based upon the initial soil tests may require wetting during summer construction. The floor of the pond must be consolidated with gravel (to 75-100 mm depth) despite the appearance of hardness of the dry floor of a new pond. This dry, firm appearance is totally misleading and, without gravel compaction, most ponds develop unworkable soft floors later after use. Later, when a pond is drained for harvest, the floor must be even (no potholes) and firm under foot. Road limestone should be used instead of gravel if the water supply is low in calcium (<10 mg/L). Bank tops may need to be gravelled and consolidated , too, remembering that vehicle access for drain harvesting is needed during the winter wet season.

Some of the original site top soil can be replaced on banks and freeboards to encourage grass cover for reducing rain-runoff erosion. The freeboard water edges of ponds may be planted with lawn-type grass; strips of "instant turf" are ideal, but no true aquatic plants should be used, ie, rooted plants that grow out into the water such as sedges and rushes.

10.The whole pond area should be caged to exclude bird and other predators. Overhead netting should be at a sufficient height to permit people to work standing erect on the banks and in the ponds and to allow the movement of a work vehicle.

11. A high rate of marron pond production is dependant upon a high rate of feeding which, in turn, must be supported by an aerator installed in each pond. A mains power, time-switched watermill/paddlewheel is the most effective type of aerator for large shallow ponds. There should be a central control for the aerators and not an individual control at each pond.

12. Water inlet delivery should be sufficient to rapidly refill a drained pond to a sufficient depth to cover newly stocked marron; during growout only a topping up rate of supply by a ball-cock valve is required.

Figure 2

Figure 3

Figure Three

Figure 4

Produced September 1994, Revised by Dr N.M. Morrissy & Mr G.S. Cassells February 1995, Research Services Division, the Department of Fisheries