OGRS 2018 conference. The stream power variation in a GIS environment as an index to evaluate the most 'sensitive' points of a river

1. The stream power variation
in a GIS environment as an
index to evaluate the most
'sensitive' points of a river
Authors: Pierluigi De Rosa, Corrado Cencetti, Andrea Fredduzzi
9-11 OCTOBER, 2018 / SUPSI LUGANO
5TH OPEN SOURCE GEOSPATIAL
RESEARCH AND EDUCATION
SYMPOSIUM

2. Changes in river morphology
 A river is a natural
system
characterized by
continuous
changes (in
space and in
time).
 In most
developed
countries, over
the past
decades, the
morphology of
most rivers have
suffered huge
changes, mainly
due to anthropic
interventions.
Effects:
Change in
hydrograph
response → flooding
The anthropic
effects usually cause
a narrowing and
incision of channel
leading to a change
of channel type.

3. The stream
power
 As water flows downslope, that
potential energy is gradually
convertedinto kinetic form, whence it
is used to perform erosional and
transportational work once a critical
level has been reached.
 The adjustment of channel form
depends on such work, it is relevant to
ask how energy is distributed in the
fluvial system.
𝛺 = 𝛾 𝑄 𝑆
where γ is the specific weight of water (=9810 N m−3), Q is water discharge
(m3s−1), and s is energy slope (m m−1, which may be approximated by the
slope of the channel bed).
The total stream power (TSP)
as index

4. The stream
power
From https://extension.umass.edu/riversmart/sites/extension.umass.edu.riversmart/files/fact-
sheets/pdf/Task_Force_StreamPower.pdf
The aim of this work is to develop
a GIS tool able to calculate the
TSP along a river axis and explore
the relation between TSP and river
morphology

5. The study area – Topino Basin
River Topino – main
features:
• Located in centre of
Italy
• About 250 km²
• Length 35 km
• Elevation drop 430 m

6. The stream power
calculation
 Recent DEM allow the calculation of channel gradient
and consequently stream power with a finer spatial
resolution, opening promising and novel opportunities to
investigate river geomorphical processes and forms.
 Slope is a difficult task. The evaluation should be done
as much local as possible (Robert, A. River processes: an
introduction to fluvial dynamics. Routledge, 2014).
Stream network ordered by
Horton – Strahler
Basin delineationfor a
specificoutlet
𝛺 = 𝛾 𝑄 𝑆
Slope maps
𝑄 ∝ 𝐴 𝐿𝑒𝑛

7. Methodology: the discharge
Flood estimation
handbooks
River Tiber Basin
Autority
Intensity-duration-
frequency (IDF) curves
•a,b k parameters
Time of concentration
•Lenght of main channel,
evevationdrop,area basin
Rainfall excess
•Curve number
a b k
CN
Discharge in m3/s
INPUT
For each point in
basin is possible to
obtain a discharge
once the returning
time is defined
For TSP the
returning time
is 2 years
(median food)
ℎ = 𝑎 ∙ 𝑓(𝑘) ∙ 𝑑 𝑏

8. The discharge
The discharge increase in any
(big) confluence, showing the
relation with the area basin
60.00
70.00
80.00
90.00
100.00
110.00
120.00
130.00
140.00
0.15
0.75
1.35
1.95
2.55
3.15
3.75
4.35
4.95
5.55
6.15
6.75
7.35
7.95
8.55
9.15
9.9
Discharge(m3/s)
distance (km)

9. Methodology: slope
The TSP «greatly» depends
on local slope
Local (energy) slope can
be approximated by the
bed slope
DEM can provide an
approximationof local bed
slope
•25 m spatial
resolution
•2.9 m v ertical
accuracy
EU-DEM
version
1.1
•Local gradient
based on D8
method
•OUTPUT: Local
gradient
based on drop
upstream
(200m,
500,1km,2km)
OUTPUT:
r.stream_power: a grass gis extension have
been developed (used matrix segmentation for
fast calculation)
➢ Discharge using the flood estimation
handbook
➢ Slope according to the D8 and local upslope
Tool available here:
https://github.com/pierluigiderosa/r.stream.power
Local D8 slope
250 m asl
320 m asl
420 m asl
480 m asl
Upstreamslope

10. The slope determination
Local D8 slope
250 m asl
320 m asl
420 m asl
480 m asl
Upstreamslope
0.000
0.020
0.040
0.060
0.080
0.100
0.120
18.0
20.0
22.0
24.0
26.0
28.0
30.0
32.0
34.0
36.0
local upslope D8
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
0.040
0.045
0.050
17.5
19.5
21.5
23.5
25.5
27.5
29.5
31.5
33.5
35.5
Upstream slope
upslope 200 upslope 500 upslope 1km
Added a
numerical
control in case
of zero slope

11. The stream power variation
0
20000
40000
60000
80000
100000
120000
18 20 22 24 26 28 30 32 34 36
Streampower
TSP D8
Zone 1
Zone 2
Zone 3
0
5000
10000
15000
20000
25000
30000
35000
40000
45000
50000
18 20 22 24 26 28 30 32 34 36
Streampower
TSP 200
TSP 500
TSP 1000
Zone 1
Zone
2
Zone 3
Zone 4

12. 0
5
10
15
20
25
30
35
40
45
50
18 20 22 24 26 28 30 32 34 36
Streampower
Thousands
TSP 200 4 Perc mobile mean
The stream power variation
0
5
10
15
20
25
30
35
40
45
50
18 20 22 24 26 28 30 32 34 36
Streampower
Thousands
TSP 200 TSP 500 TSP 1000
Zone 1 Zone
2
Zone 3
Zone 4

13. Zone 1
Upper part of studied basin.
The reach is located in a narrow valley affected
by regressive erosion due to a previous
sediment supply of the tributaries downstream
• High value of TSP depends on gradient and
not on discharge.
• All plots (slope) shows this reach as critical

14. Zone 1 –
Lateral slide

15. Zone 2
The TSP increase is caused by:
• Gradient increase
• Discharge increase(10 m3/s)
The TSP calculatedusingslope with upstream
reach bigger than 200 read the increase
downstream

16. Zone 3 – transfer zone
The TSP increases for the local gradient
(downstream a narrow valley a small
floodplain is present)
All the graphs shows this feature even if the
D8 slope location is more accurate

17. Zone 4
In this zone the river arrive into the floodplain
(slope decreases) and the TSP is influenced
by the confluenceof Menotre River (100
km2 — half basin)
The TSP calculatedwith the D8 method here
fails

18. Conclusions
Advantages
•The GRASS GIS python script
implemented is simple to run and
fast.
•Future development the script is
ready for parallel processing
•The TSP calculated with D8 slope
method should be used in
association with other methods to
verify if the increase of TSP is due to
discharge or steep slope
•The TSP can be used for the
investigation of morphological
features due to fluvial dynamics
(zone 1 – regressive erosion)
Limitations
•The slope D8 method provide
locations more accurate (zone 3) bat
can fails in large floodplain (zone 4)
•Minimum evaluable slope
•vertical accuracy/cellsize = 2.9/30 =
0.11
•The method is preferable for small
mountain basin (high slopes and
small width od riverbed)
•The TSP calculated upstream slope
bigger than 500m is smoothed
(maybe too much).