Reservoir release decision under an orange alert and conflicting pressure
On 12 November the Water Resources Department's control room at a reservoir in northern Tamil Nadu reports level 43.80 m against an FRL of 45.00 m, live storage 78%, and inflow 3,000 cusecs and rising 8% every three hours. IMD has issued an orange alert for 20 cm of rain in the catchment in the next 24 hours. The reservoir's water-spread area at this level is 12 km². The spillway can pass 60,000 cusecs at FRL, but the downstream river channel's safe carrying capacity through the town is only 12,000 cusecs, and floodwater takes about six hours to reach the three low-lying colonies where 1,200 families live. The rule curve for the date permits a maximum level of 44.20 m. Farmers' associations are demanding that no water be released until the samba crop is harvested, and a viral social media post says the dam is 'about to break'. The Collector must act tonight.
1. Decide, with numbers, whether to release water in the next 12 hours and at what rate. Show the level-rise arithmetic.
Read the worked answer
Convert and project. 3,000 cusecs = 3,000 × 0.02832 = 85 m³/s. In 6 hours that is 85 × 21,600 = 1.836 Mm³, over a 12 km² spread = 0.153 m rise per 6 hours if inflow were steady.
But inflow is not steady: at 8% every 3 hours, after 24 hours inflow = 3,000 × 1.08^8 = 3,000 × 1.851 ≈ 5,550 cusecs. Taking a 24-hour average of about 4,100 cusecs ≈ 116 m³/s, the volume is 116 × 86,400 ≈ 10.0 Mm³, i.e. a rise of 10.0/12 ≈ 0.83 m, reaching about 44.63 m - above the rule curve of 44.20 m and still climbing with rain forecast.
Decision: pre-release now. To hold the level at or below 44.20 m, the excess 0.40 m × 12 km² = 4.8 Mm³ must be released over 24 hours, i.e. 4.8e6/86,400 = 55.6 m³/s ≈ 1,960 cusecs in addition to the inflow.
Indicative schedule: 2,000 cusecs at once; review at 3-hourly inflows and step up to about 5,000 cusecs by 6 hours and 8,000 cusecs by 12 hours, staying below the 12,000 cusecs safe channel capacity. A controlled release now is far safer than an uncontrolled spill of up to 60,000 cusecs later.
2. Who has authority to order the release and the evacuation, and how should the farmers' demand and the 'dam will break' rumour be handled?
Read the worked answer
Authority is split and must be recognised as split. Gate operation is a technical function of the Water Resources Department under the approved reservoir operation rules - the Chief Engineer/Engineer-in-Chief decides and signs the release order. Warning, evacuation, relief and inter-department coordination sit with the Collector as chairperson of the District Disaster Management Authority constituted under Section 25 of the Disaster Management Act 2005, with powers to requisition resources and issue binding directions under Sections 30–34. The Tahsildar/Revenue machinery issues the flood warning; the police enforce evacuation. No single officer should improvise both roles.
Farmers: a crop loss does not override dam safety. The correct response is to explain the rule curve and the projected level publicly, issue the release order in writing, and route crop-loss assessment to the Revenue Department for relief as per norms. Holding water above the rule curve to protect samba would transfer a certain, small, compensated crop loss into an uncertain, uncompensated flood risk for 1,200 families.
Rumour: issue a factual bulletin every three hours - current level, inflow, outflow, rule-curve limit - through official district handles, local FM and cable TV, plus beat announcements in the colonies. File a complaint about the false post, and never repeat the rumour's wording while denying it.
3. At 3 a.m. inflow jumps to 25,000 cusecs and the channel can carry only 12,000. What are your options and your eviction trigger?
Read the worked answer
The arithmetic decides it. 25,000 cusecs ≈ 708 m³/s. The reservoir has no outlet other than the spillway, so the gates must pass the inflow, which is more than double the 12,000 cusecs the town channel can take. Downstream flooding is now unavoidable - the only variable left is how much warning the colonies get.
Trigger: evacuation of the three colonies starts immediately, not on confirmation of a trend. Travel time is six hours and inflow is still rising, so waiting even three hours consumes half the lead time. Sound the siren protocol, open relief centres on higher ground, move the elderly and livestock first, and inform the State Emergency Operations Centre and neighbouring districts in the same hour.
Options, in order: (i) full evacuation with gates passing inflow; (ii) keep the release as steady as possible - a sudden step change creates a surge wave that is more damaging than a gradual rise, so ramp over 30–60 minutes; (iii) suspend all non-essential traffic on the causeway and cut power to low-lying distribution points.
Honest lesson: if a release above safe channel capacity is now forced, the decision that mattered was the pre-release at 6 p.m., not the one at 3 a.m. The post-event review should record that.