Storm Hydrographs Yr12IB Drainage Basins

The Storm (flood) Hydrograph A = Discharge B = Lag time C = Peak discharge D = Storm flow E = Base flow F = Falling limb G = Rising limb

Match the label to the correct letter Watershed c Confluence Tributary b Source d a

What affects the shape of a hydrograph? Hydrographs Hydrographs are graphs which show discharge (the amount of water passing a particular point in a river at a particular time). Geology and soil Gradient of the valley sides What affects the shape of a hydrograph? Type and amount of precipitation Land use

How do trees reduce flooding? trees intercept the rain evapotranspiration reduces the amount of water that reaches the river water is taken through the roots and so less enters the river

Hydrographs Peak discharge Falling limb Rising limb storm flow normal (base) flow Peak rainfall Lag time = Peak discharge (hrs) - Peak rainfall (hrs)

Hydrographs Rising limb Falling limb 1) What type of flow (choose from surface runoff, throughflow and groundwater flow) causes the rising limb? 2) Which is the last type of flow to reach the river? 3) What is the lag time?

Hydrographs b a The hydrographs ‘a’ and ‘b’ have been produced from the same storm event but from different drainage basins. Which of the following river basin descriptions are more likely to have produced hydrograph ‘a’ and which are more likely to produced hydrograph ‘b’? Chalk bedrock Impermeable bedrock Urban area Clay soil Deforested river basin Rural area Gentle valley gradients

Hydrographs Compare these two hydrographs. Which one has the largest lag time? Explain your answer.

Hydrographs Imagine that these two areas have had the same amount of rain over a 24-hour period. Draw a possible hydrograph for each of these areas. A B

Flood hydrographs

In some drainage basins the discharge of a river can rise slowly or quickly in response to a period of rainfall. This can be shown on the Flood Hydrograph. When a storm begins, discharge does not rise immediately as only a little of the rain will fall directly into the channel. The shape of the hydrograph will very much depend on a number of factors. Key: Definitions: Discharge-Amount of water passing a given point at a given time. Lag Time- The difference between peak rainfall and Peak discharge Rising Limb – Discharge rising in the river (i.e. water level rises) Falling Limb – Water level falls in the river.

Which conditions give a Flashy Hydrograph Which conditions give a Flashy Hydrograph? Which will give a lower, later peak discharge?

Heavy storms Well distributed rainfall Permeable Rock Impermeable Rock Dense Vegetation Little Vegetation Long & thin Basin Short & round Basin Steep slopes Gentle slopes Flood control scheme No river management Rural area Urban area