A.4 Water management futures

IDBM is defined by two features: it treats the whole basin as a single interconnected system, and it balances multiple (economic, social, environmental) needs across all stakeholders, not just one group or one purpose.

The option stating 'holistic approach that coordinates the competing economic, social, and environmental demands of all stakeholders across an entire river basin' matches both features — whole basin scope and multi-stakeholder, multi-objective coordination.

'Focuses exclusively on flood prevention' describes a single-purpose strategy, not IDBM. 'Prioritises hydropower' again is single-purpose. 'Primarily to regulate irrigation supply from large dams' reduces IDBM to one function and one infrastructure type, contradicting its holistic nature.

IDBM is distinguished from single-purpose schemes precisely because it optimises across the entire basin and multiple goals simultaneously, making the holistic, multi-stakeholder definition the only accurate match.

The question asks for the definition of IDBM, so the correct answer must capture both its scale (whole basin) and its scope (multiple objectives and stakeholders).

'Focuses exclusively on flood prevention within a single country's river network' contradicts IDBM by implying a single purpose and a single-country scope — IDBM spans the entire basin and often multiple countries.

Prioritising one water use (hydropower) over all others is the opposite of IDBM's balanced, multi-objective approach; this describes a single-purpose dam scheme.

This option confines management to irrigation from dams — a single function — and ignores the coordinated, basin-wide, multi-stakeholder nature that defines IDBM.

'A holistic approach that coordinates the competing economic, social, and environmental demands of all stakeholders across an entire river basin' is the only option that captures both whole-basin scope and multi-objective, multi-stakeholder coordination.

$\rho =1000$ kg/m³, $g=9.81$ m/s², $Q=150$ m³/s, $h=80$ m, $\eta =0.90$.

$P=\rho \times g\times Q\times h\times \eta$ $P=1000\times 9.81\times 150\times 80\times 0.90$

$1000\times 9.81=9810$; then $9810\times 150=1,471,500$; then $1,471,500\times 80=117,720,000$; then $117,720,000\times 0.90=105,948,000$ W.

$105,948,000$ W $\approx$ 105.9 MW, matching the second option. Note that power depends on both head and discharge together, not either variable alone.

We need $P=\rho gQh\eta$ with the given values. Any option that does not equal approximately 105.9 MW results from a calculation error.

84.4 MW would result from using $\eta =0.72$ instead of 0.90, or omitting part of the head. With $\eta =0.90$ and $h=80$ m the true answer is higher than 84.4 MW.

94.2 MW corresponds to $Q=120$ m³/s rather than the stated 150 m³/s — an error in applying the given flow rate.

118.4 MW would result from rounding g to 9.87 or using $\eta =1.0$ (100% efficiency), ignoring the stated efficiency of 0.90.

The correct calculation gives $\approx 105,948,000$ W $\approx$ 105.9 MW, confirming this option is correct.

A dam controls flooding through flow regulation: the reservoir intercepts high-discharge events, storing the peak inflow and then releasing water slowly over subsequent days or weeks at a rate the downstream channel can safely handle.

During the summer monsoon, operators deliberately keep reservoir levels below maximum to create flood-storage capacity. Incoming peak discharge is absorbed into the reservoir, and controlled, gradual release reduces the magnitude of the flood peak reaching downstream floodplain cities on the middle Yangtze.

The outcome is a smoothed hydrograph downstream — a longer-duration but lower-magnitude discharge event — rather than the sudden, high-peak flood pulse that would occur without the dam.

'The reservoir stores excess discharge during peak flow events and releases it gradually, smoothing the flood hydrograph downstream' accurately describes this mechanism of flow regulation.

The question asks specifically about the mechanism of flood risk reduction — how the dam physically achieves lower downstream flood risk, not just that it reduces it.

Dams do not transfer water to other basins; they regulate discharge within the same river system. This option describes an engineering approach that does not represent how the Three Gorges Dam or any comparable dam operates.

The dam raises water levels upstream in the reservoir (improving navigation there) but does not increase channel depth downstream. Downstream reaches may actually receive less sediment and altered flow, not deeper channels.

Dams reduce seasonal extremes but do not eliminate seasonal variation entirely — reservoir management still follows seasonal patterns, and some flow variation is maintained for ecological and practical reasons.

Only 'the reservoir stores excess discharge during peak flow events and releases it gradually, smoothing the flood hydrograph downstream' correctly describes the flow-regulation mechanism of flood control.