ADCP & Current Monitoring Surveys
Current monitoring is essential in a variety of environments from near-shore coastal regions to offshore locations where structures are to be built & monitored or where marine traffic is required to navigate.
The measurement of current has seen a resurgence in recent years with the increase in resource assessment for potential offshore renewable energy devices.
Coastal zone engineering projects, including the construction of bridges, docks, piers, etc. require engineers to monitor fluctuating tide levels and any project involving the demolition or movement of large structures must take into account any known fluctuations in water levels during the project life cycle.
Increasing environmental awareness on many projects also require monitoring works, particularly habitat restoration projects where an accurate knowledge of tide and current conditions is required.
ADCP [Acoustic Doppler Current Profiler] surveys are an established method of recording tidal flows and charting the results. The data sets typically provide information on changing tidal stream directions and resultant features.
ADCP surveys can be undertaken either as part of an on-going data gathering programme, to provide accurate information on the tidal current profile from the sea bed to the surface, or as standalone bespoke projects. The data is collected over a pre-determined period – usually anywhere from 1 day to 30 days, if a full lunar cycle period is required.
Equipment can either be mounted in bespoke frames and situated on the seabed, held in place with large clump weights and marked on the surface with a lit buoy, or mounted onto a vessel if information is required over a transect as opposed to from a single location.
If the equipment is laid on the seabed, a camera or tilt sensor is lowered with the ADCP such that the situation of the unit can be confirmed at time of deployment.
Long Period Seabed Mounted Observation
Where it is necessary to gain a full understanding of the current in an area, it is normal to carry out long period observations over a period of at least a full tidal cycle through both Neaps and Springs tides, or for a full lunar period of 30 days.
This is normally undertaken using seabed mounted current meters which are capable of measuring and outputting current readings at resolutions of up to 0.5m above the sensor, to the sea surface in pre-determined ‘binned’ data sets.
Information can be downloaded via a data link mid-way through the data capture period to any enable early analysis to be completed and potential characteristics identified.
Short Period Vessel Mounted Observation
Where there is a requirement to understand the current at a geographic point, or over a wider area, but where it is not critical to have a highly-detailed data set it is possible to obtain relevant data sets by means of vessel mounted observations over a period of hours as opposed to days.
This approach provides data over the full water column in 0.5m intervals and is usually achieved by completing observations throughout either a 13 hour or 25 hour period, over both a Spring and a Neap tide, so that the extremes of the current to be encountered in an area can be recorded and analysed.
Long or Short Period Point observations
Using electromagnetic sensors such as the S44 current meter, it is possible to deploy equipment at a single depth for a specific period of time. This method provides high quality current data at a single point within the water column, over a pre-determined period of deployment.
It is also possible, if less detail is required over the water column, to provide analogue current measurement at a point. This equipment is more commonly used inland, in rivers or at outfall sites where a single measurement or continual feed of the current is required.
Current and Dispersion Studies
Measurement of current and dispersion rates can also be carried out using drogue tracking techniques. Within this methodology a drogue or number of drogues are positioned in the water at a point of interest and then followed over a period of time to ascertain current speed and direction. Drogues are routinely fitted with RTK GPS trackers so that an accurate record of their movement over time can be provided.
This approach is routinely used to determine how effluent from an outfall will disperse within a near-shore region given the incumbent currents.
Data processing is completed in-house and issued in a number of formats which are compatible with industry standard hydraulic modelling software, enabling designers, engineers of environmental bodies to accurately estimate water levels, tidal heights, the likely impact of any structure being placed in the area and potential flood extents for coastal regions, rivers and watercourses.
