HYDROSPHERE 2

DRAINAGE BASIN HYDROGRAPHS HYDROSPHERE 2 DRAINAGE BASIN HYDROGRAPHS Read page 7 of the booklet. It introduces you to two rivers that run side-by-side yet have different reactions to the same amount of precipitation. In Standard Grade, you learned about the drainage basin of a river, and some associated terms. In this section, we will be studying a typical drainage basin and learning how to ‘read’ the water flow in its streams. These slide-shows are all on the Prepwork folder if you wish to copy any notes from them; we will not be stopping in class for you to do this

Construction and Analysis of hydrographs HYDROSPHERE 2 Construction and Analysis of hydrographs A hydrograph is a special kind of graph that records the speed that a river system removes the water that enters it. It consists of a small bar graph showing the precipitation and a line graph showing the amount of water flowing past a point on the river. Read about it on page 8 of the booklet. When you look at a hydrograph, you should ask yourself certain questions- see the next slide.

2. When it started was it heavy or light? HYDROSPHERE 2 2. When it started was it heavy or light? 1. When did the precipitation start? 5. How long was it after the highest ( peak) precipitation until the river was at its highest? 3. How long did it precipitate for? 4. Did it stop suddenly or tail away? 7. How quickly did it return to base ( normal) flow? 6. How quickly did the river rise? 8. What was peak river flow?

This is what a hydrograph looks like. HYDROSPHERE 2 Basin lag time Peak flow This is what a hydrograph looks like. 3 Rising limb Overland flow Recession limb 2 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

Hours from start of rain storm HYDROSPHERE 2 The next slides will demonstrate the hydrographs elements. With each new element there is a term and its definition to copy. 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 HYDROSPHERE 2 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

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

The rising flood water in the river HYDROSPHERE 2 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

Maximum discharge in the river HYDROSPHERE 2 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

Falling flood water in the river HYDROSPHERE 2 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

HYDROSPHERE 2 Basin lag time Basin lag time Peak flow Time difference between the peak of the rain storm and the peak flow of the river 3 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

Normal discharge of the river HYDROSPHERE 2 Basin lag time Peak flow 3 Base flow 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

+ = Overland flow* + Through flow* Storm Flow 3 2 Discharge (m3/s) 1 HYDROSPHERE 2 Basin lag time Peak flow (of storm flow) Overland flow* 3 + Through flow* Rising limb + Overland flow Recession limb = 2 mm Storm Flow Discharge (m3/s) 4 Through flow 1 3 2 Base flow 0 12 24 36 48 30 72 Hours from start of rain storm *See next slide for definitions

Volume of water reaching the river from surface run off HYDROSPHERE 2 Through flow Overland flow Volume of water reaching the river from surface run off Volume of water reaching the river through the soil and underlying rock layers

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

HYDROSPHERE 2 Analysis

Interpretation of Storm Hydrographs HYDROSPHERE 2 Interpretation of Storm Hydrographs Basin lag time You need to refer to: Peak flow 3 Rising Limb Rising limb Overland flow 2 Recession limb mm Recession Limb Discharge (m3/s) 4 Through flow 1 3 Lag time 2 Base flow Rainfall Intensity 0 12 24 36 48 30 72 Hours from start of rain storm Peak flow compared to Base flow Recovery rate, back to Base flow Copy this list

HYDROSPHERE 2 Read and discuss the next few slides as a class. The following are some theoretical interpretations of influencing factors BUT………… when interpreting hydrographs all factors must be considered together !

HYDROSPHERE 2 Area Large basins receive more precipitation than small therefore have larger runoff Larger size means longer lag time as water has a longer distance to travel to reach the trunk river Shape An elongated basin will produce a lower peak flow and longer lag time than a circular one of the same size- can you explain why?

HYDROSPHERE 2 Slope Channel flow can be faster down a steep slope therefore steeper rising limb and shorter lag time Rock Type Permeable rocks mean rapid infiltration and little overland flow therefore shallow rising limb, less channel-fill and a long lag time. Impermeable rocks mean faster channel-fill, steeper rising limb and a shorter lag time.

Drainage Density ( number of stream channels) HYDROSPHERE 2 Soil Infiltration is generally greater on thick soil, although less porous soils (eg. clay) act as impermeable layers The more infiltration occurs the longer the lag time and shallower the rising limb Drainage Density ( number of stream channels) A higher density will allow rapid overland flow

Land Use Tidal Conditions HYDROSPHERE 2 Land Use Afforestation - intercepts the precipitation, creating a shallow rising limb and lengthening the lag time Urbanisation - concrete and tarmac form impermeable surfaces, creating a steep rising limb and shortening the lag time. Tidal Conditions High spring tides can block the normal exit for the water, therefore extending the length of time the river basin takes to return to base flow

The effects of urbanisation on lag times. HYDROSPHERE 2 The effects of urbanisation on lag times.

HYDROSPHERE 2 Compare the lag times of the two land uses receiving the same rainstorm!

Precipitation and temperature HYDROSPHERE 2 Precipitation and temperature Short intense rainstorms can produce rapid overland flow and steep rising limb Snow on the ground can act as a store producing a long lag time and shallow rising limb. Once a thaw sets in the rising limb will become steep If there have been extreme temperatures, the ground can be hard (either baked or frozen) causing rapid surface run off NOW TRY THE EXERCISE ON THE NEXT SLIDE !

Read pages 9 and 10 of the booklet. HYDROSPHERE 2 Read pages 9 and 10 of the booklet. Draw up a table like the one shown below- take a whole page, layout in landscape. Fit the phrases from the handout into the correct place in the table. This should take no more than half a lesson! Gives reduced discharge gives short lag time gives longer lag time The items down the side are the headings of the factors in the booklet

There is a homework exercise to do for one week today. HYDROSPHERE 2 There is a homework exercise to do for one week today. Take copies of the handouts and listen to the instructions carefully! 4

Influence each other Change throughout the rivers course HYDROSPHERE 2 Remember; these influencing factors will- Influence each other Change throughout the rivers course 5

HYDROSPHERE 2 END OF PART TWO