Haakanson cone apparatus

Rapid measure of physical sediment character and bottom dynamics (erosion, transportation, accumulation)

 

The fall cone technique yields a simple, inexpensive and rapid measure of physical sediment character and bottom dynamics (erosion, transportation, accumulation) in all types of aquatic environments in an objective, numerical and reproduceable manner. The basic idea is to determine the sediment type by means of an instrument consisting of two or more cones of different shape and weight, whose tips are zero adjusted at the sediment surface (in situ or in a sampler) (Haakanson 1982). The cones are then allowed to penetrate the sediments for a certain period of time, whereafter the penetration depth of the cones is measured. Thus, the instrument yields rough but objective data on the sediments.

Fig. 1 and 2 illustrate the construction of the in situ cone apparatus. Fig. 2 shows that the cone apparatus consists of a bottom plate with three holes for the three cones, a direction plate for the cone axis (1 m long) and a suspension axis for the whole apparatus. The diameter of the apparatus is 20 cm and the total weight 7,5 kg. The use of the cone apparatus can be described by the following steps:

1. Lowering.

The cable attached to the suspension axis is taut, which keeps the stop/release wedge in up-position. The wedge presses the stop/release disk with its rubber stoppers against the cone axis so that they are kept still.

2. At bottom contact.

When the cable slackens the stop/release wedge attains down position (accentuated by the spring - see fig. 3). Then the pressure of the rubber stoppers against the cone axis is released and the cones are free to penetrate the sediments. The widest and lightest cone (L1) will fall the shortest distance.

3. Retrieval.

When the cable is taut for retrieval, the stop/release wedge again attains up-position, whereby the rubber stoppers hold the cone axis in the position attained at the sediment penetration. When the apparatus is retrieved, the penetration depth of the three cones can be directly read. Repeated empirical tests and theoretical calculations have demonstrated that the depth of penetration equilibrium is reached after 1 - 3 s depending on cone type and sediment type. After this short time span the cones have obtained a practically fixed position.

To give stability when the bottom is reached and to minimize the penetration of the apparatus into the sediments, three foldable arms are attached to the bottom plate.

 

The cones, L1/2/3:

The widest and lightest cone, (L1) (top angle 90°, weight 250 g, height 3,0 cm) will penetrate a comparatively short distance in the sediments. A distance primarily depending on the physical character of the top deposits. It has a register from 0, on flat, hard bottoms, to about 5 cm, on very loose bottoms.

Cone 2, (L2) acute angle 30°, weight 300 g, height 3,0 cm, will have a register down to 80 cm in the depicted version. (The length of the cone axis can, of course, be altered).

L3 which is the heaviest and most pointed (angle 30°, weight 500 g, height 1,5 cm) will have a register down to 80 cm in the depicted version. (The length of the cone axis can, of course, be altered).