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Dredging Methodology

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  Technical Description on the Methodology of Topographic and Bathymetric Survey (Pipisik River, Gumaca Quezon Province) 1.   Establishment and Observation of Project Control Establishment of Project control was conducted with the use of NAMRIA’s Philippine  Active Geodetic Network (PAGeNet), a network of permanently-installed, continuously operating geodetic reference stations that utilize signals from Global Navigation Satellite Systems (GNSS) to provide real-time corrections, high-precision geographic position data to users via the Internet. Through PAGeNet, a continuous reference station in Sta Cruz Laguna was used to established a set of temporary control point within the project area. Figure 1 : Location of Temporary Control Points 2.   Reference System and Projection To have a single reference system during the conduct of topographic and bathymetric survey, and at the same time eliminate the errors due to satellite observation, all survey were observed in World Geodetic System of 1984, Universal Transverse Mercator Zone 51 North. The elevation is referred from the mean sea level through the GPS ellipsoid acquired from NAMRIA, the Philippine Geoid Model 2018, (PGM 2018). All final data are then transformed to Philippine Plane Coordinate System - Philippine Reference System 1992 Zone III for the finalization and preparation of prescribed output. Figure 2 and Figure 3 show the location of the Philippines in WGS 1984 and PRS 1992 Projections    Figure 2 : World Geodetic System 1984 - Universal Transverse Mercator Zone 51 North Figure 3 : Philippine Reference System 1992 - Zone III The transformation variable between the two reference systems used are as follows: System Projection Zone Central Meridian Origin Latitude Scale Factor False Easting False Northing WGS 1984   UTM (Universal Transverse Mercator)   51 N   E 123 ° 00′ 00″  N 00° 00′ 00″ 0.9996 500,000 0 PRS 1992 PTM (Philippine Transverse Mercator) III E 121 ° 00′ 00 ″  N 00° 00′ 00″  0.99995 500,000 0  3.   Topographic and Bathymetric Survey Topographic and bathymetric survey with the use of Global Navigation Satellite System (GNSS) equipment starts with the occupation and observation of established project control points. A GNSS receiver is set up at one of the established project control and serves as the GNSS Base station. The base station act as a point of reference and send correction to a GNSS Rover using the Real Time Kinematic (RTK) technique. The rover is configure to accept and record points with a prescribed positional accuracies of ±2 cm for the horizontal and ±3 cm for vertical. Figure 4 show the positional diagram for the Satellite positioning survey setup. Figure 4. Diagram of the GNSS setup for Bathymetric and Topographic Survey Figure 5. Setup of GNSS Receiver as Base Station Topographic Survey     For topographic survey, the profile for the project area the cross-section of the river easement are survey for the determination of sectional view of the river and the flood plain. Cross-sections are surveyed perpendicular to the riverbanks and extended with an easement width of 50 meters beyond both banks. The width is extended to determine the ground elevation of main flood prone area. The cross-section line follow the path of the nearby road or goat trails with a 5 meter interval for each point measurement. Each cross-section is identified sequentially from downstream to upstream direction. Meanwhile, profile surveys are conducted to obtain the upper and lower banks of the river in a longitudinal extent.  A profile survey is consists of the Left Upper Bank (LUB) and Left Lower Bank (LLB), Right Upper Bank (RUB) and Right Lower Bank (RLB). An interval between successive profile points is approximately 10 meters. A buffer of at least 20 meters from the project area are covered. For profile and cross-section survey, additional points are gathered to describe apparent change in elevation along the path. Bathymetric Survey To extract the information of riverbed, bathymetric survey is performed by the integration of a survey-grade single beam echo sounder capable of logging time-stamped depth value in centimeter together with a dual frequency GNSS receiver using the RTK technique. The integrated system is attached to a survey boat/vessel for data acquisition while navigating the river with at most speed of 14 m/s. Data acquisition is logged at one second intervals both for GNSS positions (X,Y,Z) and echo sounder depth reading. For portions of the river that is not navigable by boat due to shallow waterless than a meter, riverbed is acquired through manual bathymetric survey using the GNSS rover receiver .
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