Session 4: Reservoir flood routing

Question 1

Flood routing:

Answer 1

calculation procedure that follows
propagation of flood hydrograph
from upstream to downstream.

Question 2

reservoir or storage routing

Answer 2

determines outflowing flood
hydrograph arising from inflowing
flood to storage basin by considering
change in storage in reservoir

Question 3

river routing

Answer 3

determines flood propagation from
an upstream to downstream section
in a river reach

Question 4

engineering purpose of reservoir routing procedures

Answer 4

flood forecasting, sizing spillways and determining required dam
embankment or weir crest height, river basin studies, catchment/river
basin plans such as location of water supply structures
and regional flood control measures.

Question 5

Reservoir characteristics

Answer 5

• Water level/surface elevation in reservoir
• Volume of water stored in reservoir
– How are water level and volume related?
• Inflow from catchment and upstream river
• Outflow from reservoir over spillway or
through orifice or sluice gates

Question 6

Reservoir storage terminology

Answer 6

• Reservoir yield:
– constant rate of supply that can be maintained from reservoir through
historic drought
• Active storage capacity:
– Provision for yield during period of
drought
• Flood storage capacity:
– Extra volume provision for storage
of flood waters
• Dead storage capacity:
– Volume that cannot be drained by gravity, volume provision for sediment
settlement and deposition
• Compensation water:
– Continuous release from direct supply reservoir to meet in-stream (e.g.
ecology) requirements of river channel downstream of dam

Question 7

Continuity equation

Answer 7

Rate of change in reservoir storage S
equal to difference between inflow (I)
and outflow (O) rates:
Inflow (I) – Outflow (O) = Change in storage (deltaS)

Question 8

Types of storage in river reach

Answer 8

• Total storage S: sum of prism Sp
and wedge Sw storage:
 S = Sp + Sw
• Prism storage (Sp
): dependent on downstream stage
only (i.e. ignores water surface slope)
downstream stage uniquely related to outflow (O)
Sp = f1
(O)
• Wedge storage (Sw): exists due to difference between
inflow (I) and outflow (O)