Topic Two (1.2): Discharge Relationships Within Drainage Basins

Introduction

This topic examines how climate and drainage basin characteristics influence storm hydrographs and thus the likelihood of flooding.

You will study:

Components of hydrographs (storm and annual).
Influences on hydrographs.
Climate: precipitation type and intensity, temperature, evaporation, transpiration, evapotranspiration and antecedent moisture.
Drainage basin characteristics: size and shape, drainage density, porosity and permeability of soils, rock type, slopes, vegetation type and land use.

Read and Make Notes

For this topic, you will need to work through pages 5 to 8 of your textbook, completing the Activities at the end of the Section.

Download and Print

Download and print out these resources to accompany your notes:

Top Tips

Make sure that you understand the following key points:

There is a difference between a hydrograph and a river regime – a hydrograph represents a single flood event and a river regime records a whole year of discharge. Both are for a single drainage basin. It is essential you know the difference as exam questions ask about one or other and if you mix them up this could cost you all the marks for that question.
Urban areas have a double whammy effect (two-fold) on river flooding. Firstly the hard impermeable surfaces such as concrete, tarmac, glass and metal do not allow water through so there is no storage beneath these surfaces, and secondly the drainage overflow system for roadside drains is a fast track to the nearest river through designated channels. This means water is collected and dumped in the river in the fastest time possible exacerbating flood risks downstream.

Key Terms

Add the following key terms with definitions to your glossary. You can either add them to your table or make flash cards to help with revision later.

Storm hydrograph shows us how a river changes over a short period, such as a day or a couple of days. Usually it is drawn to show how a river reacts to an individual storm. Each storm hydrograph has a series of parts.
Quickflow or stormflow is the water that gets into the river as a result of overland flow/surface runoff.
Rising limb shows us how quickly the flood waters begin to rise.
Flood is a discharge great enough to cause a body of water to exceed bankfull discharge, overflow its channel and submerge the surrounding land.
Recessional limb is the speed at which the water level in the river declines after the peak.
Peak flow is the maximum discharge of the river as a result of the storm.

Time lag is the time between the height of the storm (not the start or the end) and the maximum flow in the river. (Peak rainfall to peak river flow).
River regime is the annual variation in the flow of a river.

Consolidate