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River Study: Riera de Ribes

How and why does the flood risk vary between the north and south of the Sant Pere de Ribes area of Barcelona?

Background

Sant Pere de Ribes, a small town to the west of Barcelona, is delimited to the north and west by the Riera de Ribes, a river with a typical Mediterranean regime. This river is normally dry during the summer months, but is subject to flash flooding which is most common during the Autumn. A notch on a river cliff to the north of the town provides evidence of a past flood level. The hydraulic radius, gradient and Manning n can be used to calculate the discharge for the flood event and to analyse the flood risk posed by the river, both in the vicinity of the river cliff and further downstream. The risk to recent housing and camp site developments on the river floodplain can be assessed together with the effectiveness of river management schemes. Other factors that affect the flood risk, such as increased urbanisation, forest fires, changing agricultural land use, drainage density, drainage basin shape, reforestation, etc. can be analysed with the aid of secondary data.

Tasks: Site 1

1. Measure the river cross-section using the flood notch as the water level. Use a topochaix line secured with ranging poles at both ends, levelled (using an abney level) with the flood notch.  This line marks the flood level. Measure the channel depth (using a calibrated surveying pole and plumb line) between the river bed and this line at 5m intervals and at every change of slope. Distances across the river should be measured using a measuring tape. See diagram below:

Site 1: measuring the river channel cross-section using the flood notch

Record on Data Chart 1.

2. Identify the n value for Manning's co-efficient. This measures friction or the resistance of the channel to river flow. This depends upon the nature of the bed and bank material, vegetation cover and sinuousity of the channel. Circle which one of the categories below best your river cross section: only circle one value.

Stream description

Major streams <30m wide at bank-full

Channel state (sinuousity)

bad fair good best

Regular reach with no boulders or weed

0.060

0.050

0.035

0.025

Irregular and rough section

0.100

0.080

0.050

0.035

 

 

 

 

3. Channel Gradient

Record the channel gradient using an abney level as shown in the photograph below:

Using an abney level to measure river channel gradients

Record on Data Chart  2

4. Field Sketch

You will need to draw a field sketch looking up the river channel towards your cross-section study site. This will require clear and detailed annotation as follows:

  • Evidence of management/flood prevention
  • Land use adjacent to the river channel
  • Vegetation size and type
  • Bed load (shape/size)
  • River landforms: river cliff, terraces
  • Rock type

Record on Data Chart 3

5. Land Use Maps

Annotate the land use map using the following categories:

  • Residential
  • Agricultural
  • Recreational
  • Transport/communication
  • Industry
  • Flood prevention schemes
  • Natural vegetation
  • Current building works

 

 

Tasks: Site 2

1. Measure the river cross-section using bank-full as the water level. Use a topochaix line secured with ranging poles or vegetation at both ends, levelled (using an abney level). with the top of the river bank. This line marks bank-full. Measure the channel depth (using a calibrated surveying pole and plumb line) between the river bed and this line at 5m intervals and at every change of slope. Distances across the river  should be measured using a measuring tape. See diagram below:

Site 2: measuring the river channel cross-section using bank full

Record on DATA CHART 1

 

 

2. Identify  the n value for Manning's co-efficient. This measures friction or the resistance of the channel to river flow. This depends upon the nature of the bed and bank material, vegetation cover and sinuousity of the channel. Circle which one of the categories below best your  river cross section: only circle one value.

Stream description

Minor streams <30m wide at bank-full

Channel state (sinuousity)

bad fair good best

Lowland

Clean, straight, no rifts or deep pools

0.033

0.030

0.0275

0.025

Some weeds or stones

0.040

0.035

0.033

0.030

Clean, winding, some pools or shoals

0.045

0.040

0.035

0.033

More stones

0.060

0.055

0.050

0.045

Sluggish reaches, weedy, deep pools

0.080

0.070

0.060

0.050

Very weedy reaches

0.150

0.125

0.100

0.075

Upland (no vegetation in channel except at bank-full; banks steep)

Gravel, cobbles and some boulders on bed

0.050

0.040

0.035

0.030

Cobbles and large boulders on bed

0.070

0.060

0.050

0.040

 

 

3. Channel Gradient

Record the channel gradient on Data Chart 2 using the same method as for site 1.

4. Field Sketch

Record on Data Chart 3 using the same method as for site 1.

5. Land Use Maps

Annotate the land use map using the same method as for site 1.

DATA CHART 1: RIVER CROSS-SECTION DATA

Site 1 Total Width:
Distance from bank

  0 m

(river cliff)

                   
Depth                      
Distance from bank                      
Depth                      
Distance from bank                      
Depth                      
 

 

Site 2 Total Width:
Distance from bank

  0 m

(river bank)

                   
Depth                      
Distance from bank                      
Depth                      

 

 

DATA CHART 2: CHANNEL GRADIENTS

Site 1

Site 2

Gradient Reading 1   Gradient Reading 1  
Gradient Reading 2   Gradient Reading 2  
Gradient Reading 3   Gradient Reading 3  
Gradient Reading 4   Gradient Reading 4  
Average   Average  

 

 

 

DATA CHART 3: FIELD SKETCHES

SITE 1 (Looking Upstream)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

SITE 2 (Looking Upstream)