Storm Hydrograph Tutorial A resource based on an original by David Cuthbertson.

Storm Hydrographs Storm hydrographs are used to measure a river’s flow and show the change in river discharge over a short period of time after a storm.

Learning Objectives To be able to analyse a storm hydrograph To understand the factors that may affect the shape of a storm hydrograph

Hydrograph River Discharge Storm Hydrographs = cross sectional area X Record of River Discharge over a period of time River Discharge = cross sectional area rivers mean (average) velocity X (at a particular point in its course) Storm Hydrographs Show the change in discharge caused by a period of rainfall

Flood Hydrograph 3 2 Discharge (m3/s) 1 0 12 24 36 48 30 72 Basin lag time Flood Hydrograph Peak flow 3 Rising limb Overland flow Recession limb 2 mm Discharge (m3/s) 4 Through flow 1 3 2 Base flow 0 12 24 36 48 30 72 Hours from start of rain storm

3 2 1 Discharge (m3/s) 0 12 24 36 48 30 72 Hours from start of rain storm

Rainfall shown in mm, as a bar graph 3 2 mm Discharge (m3/s) 4 1 3 2 0 12 24 36 48 30 72 Hours from start of rain storm 7

Discharge in m3/s, as a line graph 2 mm Discharge (m3/s) 4 1 3 2 0 12 24 36 48 30 72 Hours from start of rain storm 8

The rising flood water in the river Rising limb The rising flood water in the river 3 Rising limb 2 mm Discharge (m3/s) 4 1 3 2 0 12 24 36 48 30 72 Hours from start of rain storm 9

Maximum discharge in the river Peak flow Peak flow 3 Maximum discharge in the river Rising limb 2 mm Discharge (m3/s) 4 1 3 2 0 12 24 36 48 30 72 Hours from start of rain storm 10

Falling flood water in the river Peak flow Recession limb 3 Falling flood water in the river Rising limb Recession limb 2 mm Discharge (m3/s) 4 1 3 2 0 12 24 36 48 30 72 Hours from start of rain storm 11

Basin lag time Peak flow Basin lag time 3 Time difference between the peak of the rain storm and the peak flow of the river Rising limb Recession limb 2 mm Discharge (m3/s) 4 1 3 2 0 12 24 36 48 30 72 Hours from start of rain storm 12

Normal discharge of the river Basin lag time Peak flow Base flow 3 Normal discharge of the river Rising limb Recession limb 2 mm Discharge (m3/s) 4 1 3 2 Base flow 0 12 24 36 48 30 72 Hours from start of rain storm 13

Storm hydrographs show how a river’s discharge changes after a storm. Depending on a number of factors, the hydrograph can change considerably in shape.

Worksheet Now you have been through the formation of a flood hydrograph, use your new knowledge to fill in your sheet.

There are several different factors which may affect the shape of a hydrograph. These are listed in the table below. Copy the table below and fill in the blanks. FACTOR STEEPER GENTLER 1. Total Rainfall High 2. Intensity of Rain High (runs off) 3. Wetness of Ground Saturated (runs off) 4. Rock Type Impermeable (runs off) 5. Ground Cover Bare Soil (runs off) 6. Slope Angle Steep (runs quickly)

Re-cap