Chapter 5: Streams and Floods
Flood routing
How can an estimate be made of the level of protection offered by a dam? A flood-control reservoir works by
storing water during high inflows to the reservoir and later releasing this water at a rate much lower than the
highest flood rates. The problem of deducing how outflow from a reservoir is related to its inflows is known as
flooding routing
Solve the surface-water problems using fundamental hydraulics
Propagation of flood waves through reservoirs
Flood routing
Frequency with which a flood of a given magnitude would be expected to occur
Wetlands: protecting life and property from flooding
Wetlands in many locations play an important role in flood protection
Coastal wetlands serve as storm surge protectors when hurricanes or tropical storms come ashore
In the gulf coast area, barrier islands, shoals, marshes, forested wetlands and other features of the coastal
landscape can provide a significant and potentially sustainable buffer from wind wave action and storm surge
generated by tropical storms and hurricanes
Hydrograph
River stage: river depth
Hydrograph: a graph of river stage or discharge v. time at a point
Stage hydrograph
Discharge hydrograph
Rating curve (slide 24)
The discharge hydrograph is not measured directly, but is inferred from the stage hydrograph
Rating curve:
Determine discharge from river state
Often described by a power function
A stream stage should be located
Hydrograph
Discharge is highly variable with time
During and after rainfall events, water moves through the catchment to the stream channel and the discharge
increases
The resulting peak in the hydrograph is termed a flood, regardless of whether the river actually leaves its banks
and causes damage
Each of these floods has characteristic hydrograph, with a steeply rising segment followed by a less steeply falling
segment as the flood passes the station
Background discharge between floods is termed baseflow and is supplied by inflow of groundwater
, Movement of flood waves
Flood propagate downward as waves
Wave speed, peak height, and peak discharge
In an “ideal” channel without frictional resistance of the rough channel bed: a flood wave travels with no
change in form from its point of origin
In a natural channel with fractional resistance of the rough channel bed: magnitude of a flood wave is reduced,
or attenuated, as it travels downstream
Other factors such as water stored in pools and backwaters subsequently released
This attenuation will depend upon several factors
The ability of the channel system to store and release water
The channel roughness
The amount of vegetation in the channel and its floodplain
The relative straightness of the chanel
The number of bridges and other obstructions in the path of the flood wave
The lateral inflow of water from tributaries
Drainage area and flood wave attenuation
In general, discharge increases downstream as drainage area increases
Drainage area and flood wave attenuation in a river or stream network
The normalized hydrographs indicate that the flood wave is attenuated as it moves through the channel
The normalized hydrographs show clearly the time delay between the flood peaks and the reduction in peak
discharge per unit area in the downstream direction
Discharge generally increases downstream as drainage area increases
When normalized by drainage area, peak flood discharge generally does decrease in the downstream direction
owing to channel friction and storage
Flood routing
Flood warning and flood mitigation depending on understanding
How quickly a flood crest travels downstream (discharge)
How the height of the crest (stage) changes as it does so
So solve this problem, you need to predict a downstream hydrograph based on
The upstream hydrograph
An estimate of how fast the flood wave is moving (transient problem)
How much water is being added by inflow
The influence of friction
A complete understanding of the hydrology and hydraulics of the drainage basin
We can simplify the problem by identifying depth and velocity of the flood wave as the two most important
variables
Flood routing
How can an estimate be made of the level of protection offered by a dam? A flood-control reservoir works by
storing water during high inflows to the reservoir and later releasing this water at a rate much lower than the
highest flood rates. The problem of deducing how outflow from a reservoir is related to its inflows is known as
flooding routing
Solve the surface-water problems using fundamental hydraulics
Propagation of flood waves through reservoirs
Flood routing
Frequency with which a flood of a given magnitude would be expected to occur
Wetlands: protecting life and property from flooding
Wetlands in many locations play an important role in flood protection
Coastal wetlands serve as storm surge protectors when hurricanes or tropical storms come ashore
In the gulf coast area, barrier islands, shoals, marshes, forested wetlands and other features of the coastal
landscape can provide a significant and potentially sustainable buffer from wind wave action and storm surge
generated by tropical storms and hurricanes
Hydrograph
River stage: river depth
Hydrograph: a graph of river stage or discharge v. time at a point
Stage hydrograph
Discharge hydrograph
Rating curve (slide 24)
The discharge hydrograph is not measured directly, but is inferred from the stage hydrograph
Rating curve:
Determine discharge from river state
Often described by a power function
A stream stage should be located
Hydrograph
Discharge is highly variable with time
During and after rainfall events, water moves through the catchment to the stream channel and the discharge
increases
The resulting peak in the hydrograph is termed a flood, regardless of whether the river actually leaves its banks
and causes damage
Each of these floods has characteristic hydrograph, with a steeply rising segment followed by a less steeply falling
segment as the flood passes the station
Background discharge between floods is termed baseflow and is supplied by inflow of groundwater
, Movement of flood waves
Flood propagate downward as waves
Wave speed, peak height, and peak discharge
In an “ideal” channel without frictional resistance of the rough channel bed: a flood wave travels with no
change in form from its point of origin
In a natural channel with fractional resistance of the rough channel bed: magnitude of a flood wave is reduced,
or attenuated, as it travels downstream
Other factors such as water stored in pools and backwaters subsequently released
This attenuation will depend upon several factors
The ability of the channel system to store and release water
The channel roughness
The amount of vegetation in the channel and its floodplain
The relative straightness of the chanel
The number of bridges and other obstructions in the path of the flood wave
The lateral inflow of water from tributaries
Drainage area and flood wave attenuation
In general, discharge increases downstream as drainage area increases
Drainage area and flood wave attenuation in a river or stream network
The normalized hydrographs indicate that the flood wave is attenuated as it moves through the channel
The normalized hydrographs show clearly the time delay between the flood peaks and the reduction in peak
discharge per unit area in the downstream direction
Discharge generally increases downstream as drainage area increases
When normalized by drainage area, peak flood discharge generally does decrease in the downstream direction
owing to channel friction and storage
Flood routing
Flood warning and flood mitigation depending on understanding
How quickly a flood crest travels downstream (discharge)
How the height of the crest (stage) changes as it does so
So solve this problem, you need to predict a downstream hydrograph based on
The upstream hydrograph
An estimate of how fast the flood wave is moving (transient problem)
How much water is being added by inflow
The influence of friction
A complete understanding of the hydrology and hydraulics of the drainage basin
We can simplify the problem by identifying depth and velocity of the flood wave as the two most important
variables
