TOPIC 1:

HYDROLOGY

Interpreting the Hydrograph

Approach SegmentIt shows the discharge of the river before

the storm – antecedent flow rate.At the time when the storm begins, the

river’s response is negligible for although some of the rain does fall directly into the channel, most falls elsewhere in the basin and takes time to reach the channel.

Interpreting the Hydrograph

Rising Limb However, when the initial surface runoff and

later the throughflow eventually reach the river there is an increase in discharge.

This is indicated by the rising limb. Once overland flow begins (after interception

storage and the infiltration rate are exceeded) and later, throughflow, the river’s discharge begins to increase.

The steeper the rising limb the faster the water is reaching the channel from the drainage basin.

Interpreting the Hydrograph

Lag Time The period between maximum precipitation

and peak discharge is referred to as lag time. Lag time varies according to conditions within

the drainage basin. Examples; soil and rock type, slope and size

of the basin, drainage density, type and amount of vegetation and water already in storage.

In river basins with a short lag time there is often a high peak discharge and the river is more prone to flooding.

Interpreting the Hydrograph

Falling or Receding Limb It is the segment of the graph where discharge

is decreasing and the level of the river is falling.

This segment is usually less steep than the rising limb because throughflow is still being released into the channel.

By the time, all the water from the storm has passed through a given point in the channel.

The river will have returned to its baseflow level – unless there has been another storm within the basin.

Interpreting the Hydrograph

Base Flow Base flow is very slow to respond to a storm

but by continually releasing water from the lower ground it maintains the river’s flow during period of low precipitation.

Indeed, baseflow is more significant over a longer period of time than an individual storm and reflects seasonal changes in precipitation, snow melt, vegetation and evapotranspiration.

Interpreting the Hydrograph

Bankfull DischargeIt is the point when the level of water has

reached the top of its channel and any further increase in discharge will result in flooding of the surrounding land.

Flood Hydrograph

Flood hydrograph is also known as Storm hydrograph.

What do you understand by flood hydrograph? The hydrograph is a means of showing the

discharge of a river at a given point over a short period of time.

The discharge is the amount of water originating as precipitation which reaches the channel by surface runoff, throughflow and baseflow.

Discharge is therefore the water not stored in the drainage basin by interception, as surface storage, soil moisture storage, groundwater storage or lost through evapotranspiration.

Measuring Discharge

Discharge is the velocity or speed of the river, measured in metres per second, multiplied by the cross-sectional area of the river, measured in square meters. This gives the volume in cubic metres per second or cumecs and can be expressed as:

Q = A x Vwhere Q = discharge

A = cross-sectional areaV = velocity

Questions:How you recorded lag time? The length of time between peak precipitation and peak discharge.

What is meant by peak discharge? The maximum flow of the river during a rainstorm.

What is meant by peak rainfall? The time when the maximum amount of rain was falling.

What is meant by rising limb? The rapid increase in the river discharge as overland flow and

throughflow reach the river channel.

What is meant by falling or recession limb? The fall in river discharge as overland flow ceases and throughflow

decreases.

Lists as many as you can the influences of flood hydrographs.

Basin size, shape and relief.Types of precipitation.Temperature.Land use.Rock types (Geology).Soil type.Drainage density.Tides and storm surges.Urbanisation.

Factors That Influencing Flood Hydrograph

(1) Drainage Basin Size.

Small Size Large Size

It is likely that rainfall will reach the river or

channel more rapidly.

It is likely that rainfall will reach the river or

channel is more slowly.

Water has short distance to travel.

Water has long distance to travel.

Lag time is shorter Lag time is longer

Factors That Influencing Flood Hydrograph

(2) Drainage Basin Shape.

Circular Basin Elongated Basin

It is more likely to have a shorter lag time and a

higher peak flow.

It is more likely to have a longer lag time and a

lower peak flow.

Factors That Influencing Flood Hydrograph

(3) Rock Type.Permeable Rock Impermeable Rock

Rocks that allow water to pass through them.

Two types of permeable rock:1. porous (sandstone and chalk) – allow water to fill

the many pores in the rock.2. pervious (carboniferous

limestone) – allowing water to enter bedding planes

and joints.

Rocks that do not allow water to pass through them such as

granite (impermeable).

Give rapid or high infiltration, thus there is little surface

run-of and limited number of streams

Give less infiltration, thus produce more run-off and a greater number of streams.

Factors That Influencing Flood Hydrograph

(4) Soil Type.

The type of soil controls the rate of infiltration, the amount of soil moisture storage and the rate of throughflow.

Sandy Soil with Large Pore Spaces

Clays with Smaller Pore

Spaces

Rapid (rate and volume) infiltration and do not

encourage flooding

Less (rate and volume) infiltration and

encourage surface run-off. Thus, increase the

risks of flooding.

Infiltration refers to the amount of water that passes through the soil surface in terms of depth (inches) in a given time period.

Lighter textured soils (such as sands or sandy loams) have desirable infiltration rates.

This is important in the fact that the turf can have water penetrate into the soil relatively quickly so no runoff or puddling occurs.

'Heavy' textured soils which have a lot of clay and/or silt often have poor water penetration (infiltration), because the space that the soil occupies is relatively dense.

Organic matter helps infiltration because of the soil aggregation that occurs with organic matter (makes for larger soil particles).

The addition of turf grass to the turf-system generally increases infiltration rates.

Turf

Factors That Influencing Flood Hydrograph

(5) Temperature.

Higher Temperature(Very Hot and Dry in

Summer)

Lower Temerature(Very Cold in Winter)

The rate of evapotranspiration is higher

The rate of evapotranspiration is lower

Restrict infiltration Restrict infiltration

Less water flow into river. More water flow into the river when there is a lot of water.

Less water flow into river.

Explain how types of precipitation can influence the flood hydrographs and drainage basin.

Amount

Long period of rainfall are a frequent cause of flooding. The soil becomes saturated and infiltration is reduced.

This generates large quantities of surface runoff causing rapid rises in the river discharge.

Types of Precipitation

(A)Prolonged Rainfall Flooding most frequently occurs following a long period of heavy rainfall when

the ground has become saturated and infiltration has been replaced by surface runoff (overland flow).

(A)Intense Storms Example; Convectional Thunderstorms.

When heavy rain occurs, the rainfall intensity may be greater than the infiltration capacity of the soil (example; in summer in Britain, when the ground

may be harder). This resulting large quantities of surface runoff and produce a rapid rise in river

levels (flash floods) or rapid rise in river discharge.

(A)Snowfall Heavy snowfall means that water is held in storage and river level drop. [Heavy

snow is intercepted and stored. This lead to a fall in river levels]. When temperatures rise rapidly, meltwater soon reaches the main river.

[melting occurs the water is released]. It is possible that the ground will remain frozen for some time, in which case

infiltration will be impeded (hinder). This may generate large quantities of surface runoff, which reach river channel

quickly especially if the ground remains frozen restricting infiltration. Rainfall is also intercepted but may infiltrate quickly if the soil is unsaturated.

The rate at which it reaches the river channel depends upon interception, infiltration and overland flow.

Explain how land use can influence the flood hydrographs and drainage basin.

(1) Vegetation• It may help to prevent flooding by intercepting rainfall (storing

moisture on its leaves before it evaporates back into the atmosphere).

• Estimates suggest that tropical rainforests intercept up to 80% of rainfall (30% of which may later evaporate) whereas arable land may intercept only 10%.

• Interception is less during the winter in Britain when deciduous trees have shed their leaves and crops have been harvested to expose bare earth.

• Plant roots, especially those of trees, reduce throughflow by taking up water from the soil.

• Flooding is more likely to occur in deforested area, e.g. the increasingly frequent and serious flooding in Bangladesh is attributed to the removal of trees in Nepal and other Himalayan areas.

• In area of afforestation, flooding may initially increase as the land is cleared of old vegetation and drained, but later decrease as the planted trees mature.

(2) Urbanisation• Urbanisation has increased flood risk. Water cannot

infiltrate through tarmac and concrete, and gutters and drains carry water more quickly to the nearest river.

• Small streams may be either canalized so that (with friction reduced) the water flows away more quickly, or culverted (a transverse and totally enclosed drain under a road or railway), which allows only a limited amount of water to pass through at one time.

(3) Land use & Human Activity• Deforestation, afforestation, urbanization and agriculture

all have major impacts on the storage capabilities of a drainage basin.

• In addition to these ‘accidental’ impacts, the construction of dams and reservoirs strongly influences hydrograph.