HED Manual – 1300 Investigation and Model Study


Investigation for coastal structures, especially for fishing harbours, consists of two parts. (1) Engineering Data collection and (2) Fishery data collection
Engineering data collection involves topographic and hydrographic surveys, coastal data collection, sub soil investigations etc.
Fishery data collection on the other hand involves collection of data about (a) types and quantity of fish catch, (b) types and number of fishing crafts, (c) beneficiary fishermen population, (d) their current fishing practices and location of fish landings, (e) benefits of proposed project etc. For this, data published by Fisheries department and other related agencies can be very much useful along with interaction with stake holders like local fishermen, boat owners, commission agents etc
The items involved in the Engineering Data collection are listed below.

Reconnaissance Survey: The proposed site of development should be visited and a preliminary reconnaissance should be conducted to ascertain the ground situations. Locations of permanent points and other parameters can be decided during the reconnaissance survey.
Permanent Points and Location Control: Permanent points should be erected at site, before the investigation starts. The locations of these points are to be in such a manner that they are inter-visible and are sufficiently safe distance from line of probable sea erosion. The geo coordinates of these points are to be found out and these act as basis of all survey work. Total stations and GPS are available now for the same.
Transferring G.T.S Benchmark: The value of nearby G.T.S bench mark should be transferred to a permanent point fixed at the site and this should be shown clearly in the site plan. The details like location and RL values of nearby bench marks can be obtained from SOI on request with a fee. If the RL value of nearest GTS Bench mark is already known, the topographic surveys should be based on this. If some delay is expected to get the details of nearby GTS bench marks, the topographic surveys can be started with an arbitrary value assigned to the permanent points erected at site, and later updated with the actual value.
Topographic Survey: The site plan should contain permanent points, existing structures, departmental land if any, acquisition of land if any and tentative proposal for the development. Topographic contour lines are to be clearly shown in the plan. The topographic survey should cover all the above aspects. Normally 2km of coastline on either side of project is surveyed. The extent to the landward side depends largely on the type of project and the peculiarity of the site. It is preferable that at least a distance of 200m is covered in the topographic survey, especially the road and rail network and other natural bodies like rivers, lakes etc.
Hydrographic Survey: Hydrographic survey is usually carried out by Hydrographic Survey Wing of Port department with the help of DGPS and Echo sounders. Yearly requirement of hydrographic surveys are informed to them and they in turn conduct the surveys. Usually the surveys are conducted twice in a year – pre and post monsoon. It is often found that HSW conduct surveys beyond -1.0m CD contour only and hence, the details within the above depths may have to taken by the department. These surveys can be conducted with fair accuracy with the help of GPS and sounding ropes. Only high end echo sounders are able to measure the depths less than 3m accurately and hence, conventional sounding ropes with weights are the only option.

Physical and computational models are widely used in Coastal Engineering to finalize various design and layout parameters. Much experience has been gained evaluating harbour layouts by means of physical models. Computational models are also available for assessing the effects of different layouts. They can also be used to determine the qualitative effects of different layouts but do not necessarily give sufficiently detailed information for all aspects required. Current best practice for major projects is to use both physical and computational models. Early collection of site data to enables the model to be constructed and the test programme planned and completed in time. The physical models referred to above are constructed to a small scale in a wave basin covering the whole harbour area. However, the long-crested seas generated are not fully representative of the multidirectional and short-crested waves which occur in nature, particularly in deep water. In order to study such effects on a structure it is necessary to generate multidirectional waves in a specially designed basin. Such a procedure is difficult and has rarely been used in breakwater engineering.

Two dimensional physical models in flumes are used to study breakwater structure stability. These physical hydraulic model testing is the most efficient and reliable way of determining the stability of a breakwater design and recent developments in laboratory techniques enable most hydraulic aspects of stability to be investigated. A comprehensive series of model tests should be carried out to refine the design and determine the safety of the structure under extreme conditions. Because of limitations of time and cost it is rarely possible to test all options and the test programme should be carefully prepared to obtain the greatest benefit from testing and to assist interpretation of the results. With reliable data it is reported that rock armored rubble mound sections in a flume give almost complete agreement between model and prototype. The primary object of testing is to check the stability of the breakwater up to and exceeding the design state and its hydraulic performance in respect of run-up, overtopping, wave transmission and reflection. On the other hand, computational models have not yet been developed for examining the hydraulic stability of breakwater elements.

The data collection requirement for each project is dependent on the nature of studies proposed. Data requirement also varies from different model study agencies and hence it is important that the investigation programme should begin only after getting the concurrence of model study agency. Mathematical model studies are usually undertaken to find the layout of breakwaters considering (1) wave tranquility, (2) littoral drift, (3) tidal hydrodynamics and (4) siltation aspects. In rare cases, as explained above, physical modeling is resorted to, but the time and cost of conducting physical modeling techniques may be prohibitive in several cases.

Important note: programme of investigation listed below is only indicative and concurrence from model study agency is most preferred (for the location/ extent, duration, frequency etc) before the data collection is started. Any other data as required by the model study agency, in addition to the listed ones, has also to be collected.

Numbers of tidal data collection locations are dependent upon the site conditions and accordingly one, two or more locations may be required. The observations should be started two days before new moon and should continue for at least 34 days. This should be conducted during non-monsoon season so that the error due the freshest discharge can be avoided (in case of estuarine projects). The C.D may be fixed based on those observations. This should be lower than the lowest low water. The level of C.D should be connected with G.T.S bench mark. Tidal measurements can be made with automatic tide gauges or with the help of tide poles.

Following factors should be considered, while selecting the location of tidal observation:
a) The gauge should be in sufficiently deep water to avoid drying out;
b) It should be sheltered as far as possible from the effect of sea and swell;
c) It should not be in a position where water is impounded as the tide drops;
d) It should be sheltered from accidental damage by vessels and should not be mounted or fixed on members that are subject to settlement.

Permanent points on the shore are fixed up to 2 km on either side of the estuary at 200 m intervals and levels of these points are fixed. Cross- section from every permanent points starting from 5 m back of the point and extending up to -3 m contour are taken and the contour are plotted in the shore line map in a scale 1:5000. The high water mark also is to be plotted in the contour map. The shore lines may be superimposed once in every 3 months. The shore line survey should be conducted once in every month preferably on the same day during every month.

Gut cross section is to be taken during every month (preferably on the same day of every month), whereas the river cross sections need to be taken once during monsoon and once during non-monsoon periods. The cross section of all the branches of river confluencing at the estuary should be taken at every 100 m, up to 1 km upstream and thereafter at every 300 m up to the limit of tidal influence. Sounding taken in the river should be connected to C.D.

Bed samples are to be collected both from river and sea, if the project is an estuarine one. Bed samples from about 20 different locations are to be taken from the river. Each samples may be analysed for grain size distribution, salinity etc and the result are to be submitted along with a location map. Similarly samples are to be collected from sea once in every 2 months along alternate beach cross sections. Four samples viz., from the shore, from the wetted beach, from the breaker zone and from -3 m contour, or in other words +2 m, 0.0 m, -1 m & -3 m contours should be taken from beach cross sections. Time of sampling, location, date, line and position on beach etc should be noted on each sample. The samples are to be analysed for particle size distribution. The details may be tabulated, plotted and submitted along with a location map.

The observations are to be carried out at 2 points in the sea, one pint at the gut and 2 points inside the backwater, in case of estuarine projects. This is to be taken during monsoon and non-monsoon for both spring and neap tides. The observations are to be started in the morning and carried out for 27 hours. Current meter readings at depths 0.2 D, 0.6 D & 0.8 D are to be taken at 1 hour interval. Latest advancement in the current measurement has produced equipments like ADCP (Acoustic Doppler Current Profiler) which automatically record the current profile across the depth of water.

Float observation also gives the velocity of currents in the area of interest. Float observations are conducted once each for spring and neap tides during monsoon and non-monsoon season. Observations for flood and ebb floats are to be done separately. The flood floats are released at one and three hours after low water. A minimum of 4 floats should be placed in the sea and its movement towards the estuary is to be traced. The ebb floats are released one and three hours after high water. They are placed at different locations inside the estuary and the movements of the floats are traced with the help of GPS attached to them or more conventionally by theodolites or compass by intersection method. The observations are to be made either for three hours or till the float beyond visibility.

These are done once during monsoon and once during non-monsoon at low water, high water, 3 hours after high water and 3 hours after low water for 7 consecutive days. Water samples at 0.2 D, 0.6 D & 0.8 D are collected from different locations and analysed for silt and salinity. The sit and salinity observations are to be made at about 10 locations in the sea and 10 locations inside the estuary. Standard water samplers are to be used for collection of water samples.

This component can be conducted in conjunction with the silt and salinity observations mentioned above. The tidal prism observations are done once for spring tide and once for neap tide during non-monsoon period. An entrance section which is free from waves is chosen. This is divided into three or four sectors. Velocity observations in the middle of each sector are made using current meter. Reading are taken at 0.2 D, 0.6 D & 0.8 D. simultaneous tidal observations are also made, sediment samples may also be collected at different depths. Observations are to be made simultaneously in all sections at half an hour interval for 27 hours.

Wave observations can be done visually as well as instrumentally. Model study agencies use ship observed data on waves, published by standard agencies for obtaining the deep water wave parameters (if instrumentally observed deep water data is not available). Wave rider buoys and directional wave recorders are used for this. If done manually, wave observations are done 3 times a day continuously for one year at depth beyond which the wave breaks. Wave period, wave height and wave direction are to be observed by erecting a wave pole. This should be continued for at least one year. Wind observations are to be carried out during the period of observation of wave, and whenever possible, measurements are made at least at 10 m height to obtain an exposure clear of surrounding obstacles.
The monsoon flood is likely to carry huge quantities of sediments. The quantity of sediments carried by the rivers can be calculated by taking monsoon water samples and analysing it for sediments. Water samples at depths 0.2 D, 0.6 D & 0.8 D are collected from all the branches of the rivers confluencing at the estuary.

The discharge from the all the river branches confluencing at the estuary can be collected from PWD or the gauging stations established by CWC. Discharge data should contain the average daily discharge in 3 sections and peak discharge for about 25 years.

Note: The following step (geotechnical investigation) can be started only after the alignments of breakwaters are obtained from the model study and the location of jetty is finalized.

Soil investigations are to be conducted at least at three points along the alignment of each breakwater and at one point along the alignment of wharf/ jetty. Standard penetration tests are to be conducted at the location where structures like breakwater, wharf, water tank etc are to be constructed. Some undisturbed soil samples may also be collected and tested. The S.P.T test should be conducted as given in IS 2131. Probing or augur boring may be conducted along the alignment of channel and at different locations where dredging is to be done. A plate load test may also be conducted, if found necessary. Jet probing may be resorted to some cases, in order to identify the locations of shallow hard strata in the area of interest.

As mentioned earlier, both the Engineering and Fishery data collection are to be completed before the Techno Economic Feasibility Report (TEFR) is prepared. However a Detailed Project Report (DPR) may be prepared once the model studies are over for the project, incorporating the detailed estimate of each component. This DPR will be used as the basis of EIA studies, CRZ clearance and for obtaining consent from pollution control board etc.

Coastal projects require several statutory clearances before the administrative sanction is issued by the authorities. They are listed below:
1. Consent to establish from State Pollution Control Board
2. CRZ status report (issued by CESS, Thiruvananthapuram)
3. EIA studies, which have to carried out only by empanelled consultants approved by MoEF
4. CRZ / EIA clearance, as the case may be. For this the department has to apply in prescribed form to the state coastal zone management authority in the prescribed format.

Other coastal projects such as Shore protection measures etc require similar investigation programme but may not be of the scale mentioned above. Suggestions and recommendations of the model study agency are of prime importance in the details to be collected at site.

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