2024-05-27

Funding Agency: Government of Nepal
Client: Department of Hydrology & Meteorology, Nepal

A.       Discharge Measurement:

The overall scope of work is to extensively measure the flow of specified Snow Stations during low flow (Dec-May), medium flow (Oct-Nov) and High flow (Jul-Sept), Cross section survey as well as low flow measurements shall be carried out during low flow season (Mar-Apr). It shall be confirmed that

• Preliminary field survey shall be done during low flow season to confirm
• The equipment used should be well calibrated and guaranteed for sound performance in order to minimize the error associated with it. The format for recording the Calibration Coefficient Data in the case of the Salt Dilution Method is described in Annex 1
• The technician shall be well-trained and experienced in measuring discharge in different field conditions.

B. Cross-section Survey

The main purpose of the cross-section survey is to carry out the bathymetry of the river and cross-sectional survey across the bathymetry section covering the highest flood level from the history and the past year's maximum flood level in order to estimate the flow for the extreme floods. The scope of work is defined as enlisted below.

• Prepare a site plan for the stream gauging station showing staff gauges’ site, discharge measurement site, survey section, bridges and other important features.
• Measure the Latitude, Longitude, and Elevation of the station (at benchmark) on MSL, and link with the existing staff gauge.
• Arbitrary BM shall be established in the vicinity of the gauge station at the permanent structure where staff gauge is not available during the survey.
• Take a photograph of the station showing location, staff gauges, arbitrary BM if any river bed, and other Features.
• Take 2 (Two) no. of discharge measurements in the vicinity of the gauging site during the time of the survey.
• Take a cross-section survey at three locations up to 5m above the flood mark (mountain region) or 200 m from banks (terai region) whichever is less than one at the gauging site, two GON
• at d/s of the site or two at u/s of the site or one d/s and one u/s of the site (each having 100m apart).
• For the survey work. The standard total station should be used.

C.  Estimate Peak Flood using indirect method (Slope-Area Method)

• Compute Manning's n from the measured discharges.
• Compute Cross-section area (A), Wetted perimeter (P), Hydraulic radius (R), and Discharge (Q) at every 0.2 m water level rise up to 5m above the Highest Flood Level (HFL) with table format as described in Appendix-2
• Prepare a plot of water level (H) vs AR2/3 and H vs AR1/2
• Compute Peak Flood Discharge using the slope-area method.
• Prepare a plot of the stage-discharge curve (rating curve) from the table in Appendix 2.
• Prepare records including all above-mentioned things as well as information on equipment, observers, and the history of the station - Appendix 3