Category: Environmental Management

The main components of sand search investigations include initial layout of survey track line patterns, core site selection, selection of personnel and equipment, scheduling, and cost estimates. Exploration for offshore sand and delineation of potential borrow sites mainly involve geophysical and geotechnical or sediment-sampling programs.

Geotechnical Survey

The geotechnical operations include:

• Surface sampling; Samples of seafloor sediments are required to gain a rough idea of sediment types.
• Jet probes and vibrating coring for subsurface sampling is essential for borrow source identification and evaluation. This is usually accomplished by means of a continuous coring apparatus that can obtain cores 7–13 meters in length.

Geophysical Survey

The geophysical operations include:

• Differential global positioning systems (for geographic location)
• Fathometers or Bathymetric survey (for water depth)
• Side-scan sonar (to show seafloor topography)
• Seismic reflection and sub bottom profilers (to determine stratigraphy, depositional environment, etc.).
• Magnetometer surveys are sometimes required to detect the presence of pipelines and debris buried in the seabed.

Geophysical surveys and geotechnical work is use to establish deposit geometry and the quality and quantity of sand.

Geophysical Methods

Geophysical survey techniques, which use sound waves and high-quality positioning systems on research vessels, gather subsurface geological and geotechnical data in terrestrial and subaqueous coastal environments. Indirect subsurface data, as opposed to the direct sampling procedures (i.e., drilling of boreholes and coring), are obtained during geophysical surveys. Geophysical methods assist in locating and correlating geological materials (i.e., sand deposits) and morphological or topographic features (e.g., coral reefs, sand waves) by determining the acoustic transparency, diffraction patterns, configuration and continuity of reflectors, and apparent bedding patterns.

Acoustic remote sensing provides the only practical means to map and study the surface morphology of the seafloor over large areas and at all depths. Because acoustic waves are generated easily and hardly absorbed in the water column, their reflection off the seafloor relays information that can be used to interpret local morphology.

Acoustic waves are the basis of Sonar. Sonar systems are approximately divided into three categories: Echo sounders, Side-scan sonars, and Multibeam sonars. Echo sounders transmit a single vertical beam, whereas side-scan sonars generally transmit two beams, one on each side. Multibeam sonars transmit several tens of beams on each side. These systems can acquire bathymetry or acoustic imagery or both. Echo sounders and multibeam systems are mostly mounted on the hull of the vessel, whereas sidescan sonars are typically towed as ‘‘fish’’ behind the ship.

A single geophysical method cannot provides sufficient information about subsurface conditions without backup or verification from sediment samples (e.g. vibracores) or additional data from other geophysical procedures that corroborate interpretations. Fathometers (or echo/depth sounders), side-scan sonar, and sub bottom profilers are commonly used to collect geophysical data in marine exploration programs All three systems (Echo/depth sounders, Side-scan Sonar, and Sub bottom profilers) which use electrically powered acoustic devices that propagate acoustic pulses in the water, measure the lapsed time between pulse initiation and return signals that are reflected from features on or beneath the seafloor. These systems are widely deployed to obtain information that is useful in the interpretation of seafloor geomorphology, for delineation of bottom features (e.g., ripple marks, sand waves, rock outcrops), and for estimating the nature (grain size, composition) of underlying rock and sedimentary units

• Acoustic depth sounders are used for bathymetric surveys.
• Side-scan sonar images show the areal distribution of sediments, surface bed forms (e.g., wave or current asymmetrical ripples, plane beds, swaley cross-strata, dunes, bar forms, bed form troughs, low-relief bed forms, sand waves), and macro-morphological features such as shoals and channels.
• Sub-bottom profilers show near-surface stratigraphy (sedimentary layering) below the seafloor.

Echo Sounding System

Echo sounders should be calibrated accurately so that observed (recorded) depths can be related to true water depth by establishing an independent ‘‘true’’ reference. Fathometers (echo sounders) can be calibrated by the bar-check method, which should be performed before and after the fathometer survey in an effort to correct velocity variations and index errors in the echo sounding system.

True depth is measured by placing a flat bar or plate, suspended by two precisely marked lines, to a known depth below the transducer. Variations between true depth and the recorded digital or analog depth are used to correct observed depths. Bar checks, taken at sufficient intervals to develop the variations, thus correct velocity variations, vessel draft variations, and index errors in the echo sounding system.

The fathometer or Echo sounder and digitizer are calibrated daily by the bar-check procedure at a convenient increment (usually about 1.5 meters) to the project depths. End of the day bar-checks are again conducted to ensure against drift of the fathometer calibration.

Tidal Corrections

Tidal corrections are obtained from tide gauge readings and are applied to the vertical data. Tide data are generally recorded at 15-minute intervals during the entire survey by an offshore stand (with tide staff and stilling well), which is placed seaward of the surf zone. Tidal levels are read manually by an optical level procedure. Tide gauges are normally set onshore on a pier or some other fixed structure

Bathymetric Survey (Echo Sounder)

Bathymetric surveys are required for many studies of geology and geomorphology in coastal waters including offshore sand searches in attempts to define target areas that might eventually become borrows. Fathometers or echo sounders, are most often used to measure water depths offshore. The distance between the sound source and the reflector (seafloor) is computed as velocity of sound in water divided by one-half of the two-way travel time.

Seismic and Side-Scan Sonar Surveys

Successful sand searches rely on sonar imagery of the seafloor and sectional depth views along track lines that show sedimentary layering. Seismic reflection profiling, calibrated to sand searches with the use of vibracore data, is crucial to the delineation of potential sand bodies in terms of depth and lateral extent. Sonar surveys provide useful proxy data that can be interpreted in terms of smoothness or roughness of the seabed, information that is useful for differentiating rock outcrop from unconsolidated sediments.

Side-Scan Sonar Survey

Side-scan sonar is used to distinguish topographic elements on the surface of the seafloor. Acoustic signals from a ‘‘fish’’ towed below the water surface are directed at a low angle to both sides of a trackline, in contrast to downward-directed echo sounder and seismic reflection signals. The resulting  image of the bottom is similar in many respects to a continuous aerial photograph. The image is produced by acoustic beams that are transmitted by the sonar to interact with the seafloor

Seismic Survey/Sub bottom Profiling with the Chirp Sonar System

In geophysical surveys, the distance between the sound source and the reflector is computed as velocity of sound in that medium (rock, sediment, or water) divided by one-half of the two-way travel time. This measurement is converted to an equivalent depth and recorded digitally or printed on a strip chart. A recent development that is extremely valuable to interpretation of bottom-sediment grain size is a signal processing unit that can be interfaced with an echo sounder and used to indicate the size of seafloor sediments in terms of Wentworth or other general classification schemes. This is accomplished by measuring two independent variables, viz., roughness and hardness, from acoustic signals and interpreting these data in terms of sediment type.

The basic principles of sub bottom seismic profiling and acoustic depth sounding are essentially the same. A lower frequency and higher power signal (to penetrate the seafloor) is employed in sub bottom seismic devices. The transmission of the waves through earth materials depends on properties such as density and composition. The signal is reflected from interfaces between sediment layers of different acoustical impedance. Coarse sand and gravel, glacial till, and highly organic sediments are often difficult to penetrate with conventional sub bottom profilers, resulting in poor records with data gaps. Digital signal processing of multichannel data can sometimes provide useful data despite poor signal penetration.

Seismic reflection profiles are roughly analogous to geological cross sections of sub bottom materials because acoustic characteristics are usually related to lithology.

GEOTECHNICAL SURVEYS

In contrast to geophysical surveys that provide proxy data that must be interpreted (usually in terms of sedimentary structure, architecture, grain size, and composition), geotechnical surveys ( vibracores) collect real physical data in the form of sediment samples. Because vibracoring, for example, collects in situ (undisturbed) sediment samples, it is possible to inspect sediment samples as in-hand specimens from split cores and in thin section under the microscope if necessary. Geotechnical surveys help to support geophysical investigations and, in their own right, provide primary data for reconnaissance exploration and detailed proving of potential borrows. Like geophysical surveys, geotechnical surveys are deployed at various scales and at different levels of investigation as tasked by sand search rationale to help establish salient aspects of the coastal geological framework

General steps in sand search surveys

I. Literature review & sequencing of exploration programs

1. Review of existing literature and data

2. Preparation of design and sequence of exploration program

a. Incorporate available data, information on a regional base map

b. Enumerate various tasks involved in exploration

II. Reconnaissance geological (geotechnical) and geophysical surveys

3. Reconnaissance geological and geophysical surveys

a. Positioning

b. Bathymetric survey (on regional scale)

c. Seismic survey, sub-bottom profiling (on regional scale)

d. Preliminary sampling—mainly grab samplng and a few vibracores

4. Identification of potential target areas for detailed exploration

III. Detailed geophysical surveys

5. Detailed geophysical investigation

a. Bathymetric survey

b. Seismic survey, sub-bottom profiling

IV. Detailed geotechnical survey, data reduction, and determination of sand volume in borrows

6. Detailed geotechnical investigation

a. Detailed surface sampling

b. Detailed subsurface sampling (vibracores)

7. Evaluation of geotechnical data

a. Laboratory analyses

b. Delineation of borrow areas

8. Hazard and archaeological assessment survey

a. Sidescan sonar survey

b. Magnetometer survey

9. Borrow area selection, calculation of the available sand volume.

V. Geotechnical and Geophysical Report preparation

10. Geological interpretation and analysis, raw data in hard copy and digital data in GIS format

VI. Hazards & Archeological Assessment Survey and Report (Cultural Resources)

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Sand Search is the process of investigating and exploring water bodies with the aim of locating areas with sand deposits in commercial quantity to be exploited by dredging for reclamation of marshy terrain or stockpiling for sale to construction companies.

Determination of the quantity of sand excavated from the seabed or riverbed is a major factor that cannot be underestimated in all dredging activities as the process and cost of dredging is often based on the quantity of the dredged materials.  Sand search survey with accurate report is the first step towards carrying out a sand dredging  & stockpiling.

The proper approach foremost is to determine the availability of sand within the proposed dredging site, the quality of such material and the distance from the points of mining to the point of discharge. Such information determines to a large extent the profitability of dredging.

A systematic approach to sand search surveys involves:

• Hydrographic surveying:  This is the determination of the depth of the river/creek bed by a technique known as echo sounding. Hydrographic survey is also facilitated by carrying out a detailed survey of the water body and its adjoining man-made or natural features. In addition, topographic survey is employed to obtain the spot heights of areas on land that are above water level, while the drill borehole positions are selected.
• Geotechnical survey along the identified river channel in search of sand availability. Boring for sample collection and  determining the dimensions (length and breadth) of the intended borrow pit, the depth of water, volume of sand as well as the quality with sieve analysis (physical characteristics )

Local or Manual Sand search methods for shallow water depth.

Local or manual methods include drilling borehole by water jet which is limited to shallow water depths. The results from the borehole logs, showing various depth interval of deposits with the X Y Z coordinates recorded, make it easy for the volume of sand to be calculated by simple geometry and arithmetic formula with given dimensions of length, breadth and depth. The project area if very large is divided into blocks with accurate mapping of the project area using land survey methods or the more recent satellite mapping technique using the Global Positioning System (GPS). The map so produced is an essential tool for sand mining project.

Accurate positioning of the borehole points and measurement of the distance between such points and the stockpiling site on land while giving considerations for slack needed for the dredger to swing. Dredging companies need to know the distances for exploitation of the sand deposits as this will enable them ascertain the length of their floating hoses needed for that particular project. Furthermore the co-ordinates of the center point of proposed underwater borrow pits must be known for accurate positioning of the dredger on the sand pit. Pegs are usually placed on such points after investigations.

In conclusion therefore, a good sand search survey report must among other things provide information on the following:

• Quantity or volume of sand available for dredging can be calculated from the results of borehole logs showing the various layers of deposits with given dimensions of length, breadth and depth.
• The quality of sand determined by laboratory sieve analysis for particle size range of the samples
• A bathymetric map of the project area depicting depths for smooth navigation of the dredger to the sand pit and as a base for determining changes in the sub-bottom topography after dredging.
• The exact co-ordinates of the center point and radius of borrow pits.
• The distances of the borrow pits to the stockpiling site and a recommendation on the length of dredger ladder required for maximum mining of the deposit from the borrow pits.

Professional Hydrographic Surveyors and Geologist should be used for this exercise to ensure accurate data acquisition methods.

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Whether you’re at Land, Swamp barge drilling Rig and a campsite. You are starting an eco-friendly campsite in the middle of nowhere, there’s always the need to sort out your septic system. Affordable, sewage treatment plants may be your only option if you plan to discharge your wastewater into a stream or ditch so that it has minimal negative impact on the environment.

What’s the difference between septic tank and a sewage treatment plant?

Understanding the basic differences between the options available is the first step to getting just what you need.

Septic Tanks

Septic tanks are multi-chambered and are able to treat and discharge the liquid part of the sewage. Waste enters the first tank where gravity separates the liquids from the solids. The liquid effluent flows out of the tank and discharges underground, where it is cleaned as it percolates through the soil. The solids sink to the bottom, where some of the ‘sludge’ is broken down by natural bacteria, but the rest will need to be taken away by lorry. Septic Tank is most suitable for a single house or a small development. Your ground condition should be porous enough to allow the liquid effluent to discharge.

Advantages: Relatively low installation and running costs as they only require emptying (otherwise known as ‘de-sludging’) once or twice a year.

Disadvantages: Only suitable if your ground is porous enough to allow the waste to percolate through.

Maintenance: Breaking down the waste relies on natural bacteria, which can be killed off by the bleach or harsh chemicals in today’s wastewater. Septic tank treatments can help keep the bacteria healthy – and your septic tanks running costs low.

Sewage Treatment Plants

There are domestic units as well as large-scale, commercial units. They all work in the same way. Sewage treatment plant is most suitable foreverything from single domestic dwellings right up to large developments. The only option if you want to discharge your treated waste to a ditch or stream.

Advantages: Affordable, clean and sewage treated to higher standard so that it has minimal negative impact on the environment.

Disadvantages: Requires an electricity supply and regular maintenance and, while the volume of solid matter is greatly reduced, it’ll still need pumping into a lorry for disposal.

Maintenance: With more moving parts than septic tanks or cesspools, sewage treatments are more prone to wear and tear, so will require regular maintenance.

Septic tanks vs sewage treatment plants: Which one is better?

Sewage treatment plants are fast becoming the preferred option. The following may help if you’re torn between getting a septic tank and a sewage treatment plant.

Environmental friendly

The sewage treatment plant is environmentally friendly. Septic tanks make a highly polluting effluent high in ammonia which cannot be discharged into a watercourse. Instead, it can only be discharged to a soak away for further treatment of the pollutants by the natural aerobic soil bacteria.

A sewage treatment plant, however, produces a clean, non-polluting effluent. In fact, as it leaves the final waste chamber to be discharged, the effluent can be as much as 95% clean, posing no threat to the environment.

Electricity

Most sewage treatment plants do require a power source because they work by pumping in compressed air or by rotating discs. However, you don’t normally need a very large supply – sometimes just the amount of power it takes to run a 60 watt light bulb. But where will you get this power if you’re off-grid? It normally comes from the main circuit board in your house or a generator.

Maintenance

Septic tanks have previously been a great option because they’re incredibly low maintenance – they only need to be emptied once or twice a year and can last for over 20 years.

Sewage treatment plants may require a little more upkeep as they have more moving parts, yet they produce such little sludge that they may need emptying even less frequently (although it’s recommended that you do de-sludge once a year to make sure no solids can build up and damage the treatment chamber).

How do sewage treatment plants work?

The job of a sewage treatment plant is very simple. It cleans all the wastewater a building produces (think showers, baths, toilets, dishwashers and sinks) so that it can then be discharged safely into a river or stream.  

All sewage treatment plants work in similar ways. First, the wastewater flows into the primary chamber, where gravity separates any solids from the liquid. The solids sink to the bottom to form what’s known as a sludge which will be evacuated at a later date.

The remaining liquid flows into the second chamber for treatment. In this biological zone, compressed air is pumped in, and this added oxygen encourages the naturally occurring aerobic bacteria to flourish. Some sewage treatment plants also have rotating discs which give the bacteria a larger surface area to grow on.

At this point, the treated effluent goes into the final part of the tank – a calm area that allows the bacteria to settle at the bottom (for removal back into the first tank) while the clean liquid can flow out either to a watercourse (subject to Environment Agency consent) or to a ground soakaway field or drainage mound.

Do sewage treatment plants still need emptying?

Yes, just as with a septic tank, the sludge still needs emptying by tanker because the job of the sewage treatment plant is really just to clean the water. Most manufacturers will recommend you empty them once a year to keep sludge from building up – and remember that some low-budget options may need emptying more frequently than this.

Where can I discharge the wastewater?

Your wastewater can either be discharged to a watercourse like a river or stream, or to a soak away treatment system such as a drainage mound. If you’re planning on discharging to water, your effluent needs to abide by certain rules laid down by the Environment Agency.

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