Hydrological observation and Flood Forecasting (HO & FF) network of 89 sites is maintained through Mahi Division & Tapi Division.
Water level or stage of the River is measured as its elevation above the GTS datum. Water level measurement was conducted by reading non-recording gauges. A series of vertical staff gauges as per the specifications laid down in is 4080-1977 have been fixed at three sections at each site i.e. Upstream, station gauge and downstream. The gauge posts are of RCC/wooden/ metallic with cut and edge waters and are fixed securely in position by installing them in m-150 concrete blocks of suitable size. Enameled gauge plate with marking in metric unit is fixed on the gauge posts with least count 0.005 m. Out of the three gauge lines the central line is used as station gauge line and readings of the other two lines are used for calculating the surface slope. The gauges were read hourly during Monsoon season and three hourly i.e. 08.00, 13.00 and 18.00 Hrs during non Monsoon season at station gauge line.
Discharge observation is conducted once a day at 08.00 Hrs, at all the sites by area velocity method except on Sunday and holidays in non Monsoon period. However additional observations were conducted during floods to cover different stages, irrespective of holidays. The River width is generally divided into 15 to 25 segments based on the degree of accuracy as outlined in is: 1192-1981. The width of the River is measured by steel/metallic tape or wire rope stretched across the River with segment markings indicated thereon, when the River width is quite small and the flow depths permit wading. For larger width and deeper flow conditions and in unmanageable flood conditions segment points vertically are located by measuring the navigation craft with reference to pivot point and segment blocks constructed at sites.the depth measurement is carried out by using sounding rod for depths up to 3 meter and by using long bamboos for depths between 3 meter and 6 meter. For depths exceeding 6 meter sounding reel measurements at segment points are resorted to, and in some cases, the depths are measured by echo sounder or are computed from the most recent x-sections of the River.
The velocity is measured as per is 3918 - 1976 by using a cup type current meter conforming to is 3910 - 1976. The current meter is lowered to the requisite depth i.e. 0.6 of total depth down the vertical at every segment point by suspension equipment as specified in is 6064 - 1981 and where the depth is less than 0.3 meters, the velocity is observed just below the water surface. In medium and high stages with significant flow velocities, boats fitted with power engines are used. Measurements of velocity are sometimes carried out from the bridges when the River flow condition does not permit the boat to be kept stable for velocity observation. When none of the above procedures are possible, the velocity is measured by float observations.
Rain Fall Observation
There are 16 rain fall stations are maintained under Hydological Observation Circle. Daily rainfall data recorded at sites equipped with wireless is transmitted to Sub Division on real time basis daily and if required, hourly during the floods. Considering judiciously heavy rainfall warnings, QPF, movement of depression and rainfall data, flood forecast / inflow forecast is formulated more accurately and well in advance for various forecasting Sites.
Sediment observations are carried out at 16 stations under the jurisdiction of Hydrological Observation Circle, Narmada Tabi Basin Organisation, Gandhinagar. These stations were opened under three schemes viz. National Network (NNW), 80-key hydrological stations (80-key) and flood forecasting (FF) Scheme. These stations are located on ten west flowing river basins in the States of Madhya Pradesh, Maharashtra, Gujarat and Rajasthan.
Suspended sediment observations are conducted simultaneously with discharge observation once a day starting at 08:00 hours except on Sundays and holidays. The observations are conducted at station gauge line under normal conditions. Sediment samples are collected at 0.6 depth from each vertical where velocity observation is done for computation of discharge, provided depth of flow is greater than 0.3m. The samples thus collected from each vertical are grouped in 1, 2, 3, 4 or 5 composite groups depending on the width of river in such a way that each composite group discharge is almost nearly equal or within the limit of average discharge +10 % of the total discharge. Punjab type bottle sampler is commonly used for collection of water sample for suspended sediment analysis.