Dam Design and Flood Management: Hydraulic and Hydrological Methods
Flood Wave Hydrograph
This graphic representation shows the flow evolution in a channel section during a flood. Flow redistribution, due to temporary water storage, decreases peak flow and increases hydrograph base time. Relocation, due to flow travel time, shifts the hydrograph in time. Both phenomena occur simultaneously.
Hydraulic Methods
These methods solve complex unsteady flow equations in open channels, requiring extensive data. They are used in studies needing precise water level information.
Hydrological Methods
Based on the continuity equation and a simplified dynamics equation, these methods offer a simpler approach with easier solutions.
Rolling in a Reservoir
Based on two hypotheses:
- Constant water level across the reservoir surface at any given time.
- Outflow depends solely on the reservoir level (stored volume).
Equations:
- Continuity equation: A function of storage.
- Output volume depends only on stored volume (both expressible in terms of reservoir height).
Maximum output hydrograph occurs when inflow (I) equals outflow (Q). Maximum outflow occurs at maximum storage (S), where Q = Q(S) and dS/dt = 0.
Ward Normal (NMN)
Sufficient for draining floods. Less than superelevation due to wave action (including earthquakes).
Minimum Ward (NAP)
Concerning the NAP. Less than superelevation due to wave action in avenues. Considers Extreme Avenue (NAE).
Cota coronation = Maximum (NMN + normal Ward, NAP + minimum Ward)
Normal Ward = Max (Wave Ward, Earthquake Ward)
Minimum Ward = Swell Ward
Avenue to Extreme Exhaustion Dams
- Unsupported by wind waves: NC > NAE + Wave Receipt by wind.
- Gravity Dams: Allow accidental discharges. NC > Max (NAP + Wave Receipt by wind, NAE).
Loose Material Dams
Advantages
- Economical in cost and execution time.
- Suitable for closed-open valleys with low bearing capacity.
- Utilize locally available materials.
Disadvantages
- Safety concerns: No discharge at coronation, leading to erosion and potential rupture.
- Spillways required on closed slopes, increasing cost.
Homogeneous Dams
The embankment consists of a single material providing both structure and impermeability. Barrier drains create preferential paths for filtered water, reducing saturation and increasing stability. The downstream face needs protection against erosion (e.g., vegetation), and the upstream face needs riprap protection. Berms, built every 10-20m, facilitate construction and improve stability.
Heterogeneous Dams
The embankment provides structure, while a core or screen provides impermeability. Filters between layers prevent material washing and siphoning. An impermeable vertical screen reduces under-dam pressure. A plinth supports the screen, increasing stability.