Hydrological flows

Simon Oakes

Hydrological flows

Overview

•  The hydrological cycle and runoff

•  The storm hydrograph

•  Taking it further: overland flow

•  Throughflow: vertical and lateral components

•  What is ‘the saturated wedge’?

•  Analysing the river catchment response to rainfall

Hydrological flows

The hydrological cycle •  The drainage basin

hydrological cycle is an open system.

•  Inputs of precipitation eventually exit the basin as outputs of river discharge or evaporation and transpiration.

•  Overland flow and throughflow are key processes that transfer water through the system.

•  They are complex processes.

Hydrological flows

Runoff

•  Runoff is ‘all of the rainwater that eventually contributes to river discharge following a rainfall event’.

•  There are three main types of drainage basin water transfer that contribute to runoff: these are overland flow, throughflow and groundwater flow.

•  The balance between these three pathways determines how flashy the river response is and thus the likelihood of flooding.

•  This can be demonstrated using the flood hydrograph.

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The storm hydrograph

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Taking it further: overland flow

There are TWO different overland flow mechanisms.

•  Infiltration-excess overland flow occurs when high-intensity rainfall outstrips the infiltration capacity of the soil. Put another way, it rains too hard for the soil to soak up the soil quickly enough!

•  This sometimes occurs in arid environments, or in the UK when a summer convectional downpour follows a dry period (especially when a baked and trampled soil surface has been left with a low infiltration capacity).

•  Saturation overland flow occurs during a long period of steady rainfall. The amount of water that has already entered the soil builds up to a point where further inputs of water are prevented. With nowhere else to go, water now flows downhill over the soil surface.

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Saturation overland flow

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Taking it further: throughflow

•  Throughflow is ‘water that moves through the soil’.

•  It has a vertical and a lateral component.

•  There is greater lateral movement in the upper soil horizons when lower horizons are compacted or impermeable.

•  Soils with a high proportion of macropores can transmit throughflow at faster rates.

Hydrological flows

What is ‘the saturated wedge’?

•  At the top of any slope, there is obviously no water coming from further upslope, but as we move downslope, drainage from further upslope adds to the depth of the saturated layer of soil.

•  The saturated zone is therefore deepest at the bottom of a slope.

•  This zone is often described as the saturated wedge because, if you draw a diagram to show the saturated zone getting deeper in a downslope direction, it has a triangular or wedge shape.

•  The size of the saturated wedge is variable and increases during a rainfall event of long duration.

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Hydrological complexity

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Can you explain the differences?

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Analysing the catchment response Storm Precipitation Prior

conditions Peak discharge

Storm amount (mm)

Average intensity (mm per hr)

Maximum intensity (mm per hr)

(litres per second)

(mm)

A 11.8 3.05 10.16 57 1034 B 10.7 2.54 3.56 70 694 C 30.5 3.05 4.06 9 1019 D 16.0 1.52 3.56 79 665

Hydrological flows

Key points

•  At AS, you are expected to know the key processes that affect flooding or control drainage-basin water transfers: these are overland flow, throughflow and groundwater flow.

•  If you are presented with an opportunity to write at length about these processes, try to add an extra level of complexity to your response.

•  You can show knowledge and understanding of different types of overland flow.

•  You can demonstrate a more detailed understanding of how throughflow operates.

•  You can introduce the concept of the saturated wedge.