Lecture 9: Understanding and managing flood risk

Question 1

Event basis runoff generation

Answer 1

Event runoff generation occurs through 5 mechanisms, but the “hydrological crossroads” of infiltration versus runoff (infil excess) essentially depends upon the rate of rainfall relative to the rate of soil hydraulic conductivity =f (moisture, pore size distribution, organic matter macropores).

Hydraulic conductivity is higher in wet soils (

Question 2

Infiltration excess

Answer 2

Many rainfall intensities (RI) Ksat infiltration excess occurs.

*(Ksat=saturated hydraulic conductivity)

Question 3

Infiltration excess 2

Answer 3

During an event as larger pores fill, Infil rate declines

This leads to progressive increase in infil excess (runoff), especially by larger contributing areas.

Question 4

Annual runoff regime

Answer 4

Discharge or Runoff (Q) per unit rainfall (P) is called the runoff ratio (Q/P)

Changes in the short term according to filling and draining of stores (eg canopy, soil, groundwater).

Canopy store must fill before rainfall is effective.
In the long term stores are in equilibrium so Q/P reflects losses (canopy interception, deep percolation)

If events are small and climate warm then interception losses can be high and runoff ratio therefore low.

Long term changes may take place because of land use change or rainfall characteristics change.

Question 5

River long profiles

Answer 5

River characteristics and geomorphology change along profile

River cross section adapts to increasing flow

Discharge, cross-sectional area, velocity all increase downstream

Bedload roughness, turbulence and friction decrease downstream

Question 6

Stream order

Answer 6

A stream with no tributaries (headwater stream) is considered a first order stream. A segment downstream of the confluence of two first order streams is a second order stream.

Thus, a nth order stream is always located downstream of the confluence of two (n-1)th order streams.

Question 7

Stream order 2

Answer 7

River characteristics and geomorphology change along profile

River cross section adapts to increasing flow from upstream cumulation

Discharge, cross-sectional area, velocity increases downstream (up order)
Bedload roughness, turbulence and friction decreases downstream (up order)

Question 8

Drainage density and morphometry

Answer 8

Drainage density is length of streams per unit area (km/km2)

High drainage densities on impervious terrain

Low or no drainage density on highly porous terrain eg. limestones

Morphometry depends on geological structure

Determines hydrograph response and flow concentration