A toolkit for identifying and measuring physical streams features from high resolution DEMs and digital image

G EOMORPHOLOGICAL FEATURES EXTRACTION FROM HIGH RESOLUTION DEM S
C.Cencetti, P.De Rosa, A.Fredduzzi.
OGRS2014::OtaniemiCampus,AaltoUniversity,Espoo::10-12June2014
A toolkit for identifying andA toolkit for identifying and
measuring physical streams featuresmeasuring physical streams features
from high resolution DEMs andfrom high resolution DEMs and
digital imagedigital image
Authors: C. Cencetti
P. De Rosa
A. Fredduzzi

OGRS2014–OtaniemiCampus,AaltoUniversity,Espoo
The problem
● 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.
From Surian & Rinaldi, 2003
The anthropic effects usually
cause a narrowing and incision
of channel leading to a change
of channel type.
Effects:
Change in hydrograph response → flooding

The Challenge
● Stream restoration has emerged as a means to improve aquatic
systems by returning streams to their “pre-disturbed state”.
● So hydraulic work or any operation that affect the river should
not (or al least minimize) the disturb to the system.
● Became fundamental understand the “equilibrium state” of a
river.
– Fluvial geomorphologists are asked to answer the question: is the
river/channel in equilibrium or disequilibrium?
Fluvial experts use indices as sinuosity index, braiding index,
hydraulic discharge, bankfull limits to support their work
comparing this values over time.

How to compare this indices over time?
Which elevation is should use to
derive indices as braiding index
(number of channels), flow discharge .
??
Moreover what happens if we
want to compare such indices
over time remembering that the
cross section change in time.
A reference elevation
is required

In the morphological-sediment study of a water course is useful to define a level (and a
corresponding flow discharge) ”representative” (and responsible) about the shape and size
of river channel. – Formative discharge.
Bankfull discharge
For a river in equilibrium state the formative discharge is the bankfull discharge or better is the
discharge where streamflow spills onto the floodplain. [Leopold et al., 1964].
The flows effectively able to erode and shape
the channel are not the low discharge (they
flows through a channel) and not the highest
discharge as they are capable of eroding the
channel but they occur so infrequently. It is
reasoned, therefore, that exists a range of
intermediate discharges that do most of the
work of shaping the river channel and that
some summary value of these intermediate
flows represents the formative discharge of the
river (Wolman & Miller, 1960)
The formative discharge corresponds to a discharge with a return period between 1 and 3
years.
Bankfull elevation – bankfull discharge

formative discharge – bankfull discharge
Bankfull channel ≠ Active channel
Thalweg
● Reconstructing streams to a pre-disturbed state is a difficult task as we know
the landscape never return to its condition before human-disturbance
● We rely on accurate determination of the modern bankfull channel geometry to
approach stream restoration projects.
Bankfull
limits

The hub for all indices
#1: Bankfull discharge
Bankfull elevation
#2: Number of channels
#3: Bankfull width
#4: Hydraulic conveyance
The tool here presented is developed to extract such morphological feature
directly from an high resolution DEM (LiDAR)
#5: Polygon of channel limits

bankfull elevation
Bankfull location are identified on the following standard field
indicators:
● Break in bank slope corresponding with the active floodplain
● High-flow markers
➢ Limit of bank scour
➢ Roc k staining
➢ Sand/silt deposits
● Vegetation limits
● Bar tops

bankfull elevation
The bankfull definition becomes difficult to apply in riverbeds recently carved or still
in incision.
It may happen that the "active floodplain" is absent and the bankfull discharge could be
identified as a discharge where flow spills onto a “terrace”.
This flow may be associated with return periods that excess the 1÷3 years - does not
correspond to the formative discharge
The bankfull definition becomes difficult to apply in riverbeds recently carved or still
in incision.
It may happen that the "active floodplain" is absent and the bankfull discharge could be
identified as a discharge where flow spills onto a “terrace”.
This flow may be associated with return periods that excess the 1÷3 years - does not
correspond to the formative discharge

Cross section hydraulic magnitude
Vertical scale: horizontal scale = 10:1
hydraulic depth=
wet area
width

The methodology
The idea behind the tool is based
exclusively on the bank/floodplain
slope break.
For each cross section, is graphed the
hydraulic depth as a function of flow
elevation.
The (local) maximum value of this
function indicates a the elevation where
water spills across the floodplain
(approach suggested by McKean
&Wright 2005 as variant of Williams
1978)

The tool
The tool developed is able to:
● graph the hydraulic depth vs the elevation
● investigate such function to locate the local maxima and select the
first local max corresponding to the first break in slope.

The methodology – limitation
ISSUE:
1)We experienced that the
methodology work better in rivers
with floodplain and would be limited
in confined channels that miss
floodplains.
2)The method is less definitive in
channel with multiple terrace,
where could be tricky decide which
terrace and which plateaux in the
hydraulic depth function correspond
to the current bankfull elevation
A specific smoothing procedure
is required to find the
“significant” local maxima
(remove noise)

The QGIS plugin
The tool have been implemented as QGIS plugin ant it have two main steps:
1) Cross sections generation from river axis known longitudinal step and
width
2) Evaluation of bankfull elevation (following the proposed methodology) for
each cross section and deriving of all morphological features.
In tab “Numerical setting” the user select the vertical steps and the minimum hydraulic depth.
1
2

The numerical parameters
Vertical steps: it defines the number of
points in function.
+ High values are more sensitive for
small slope variations
- High computational time
Usually > 500
Minimum hydraulic depth: this setting is to filter
out local maxima below such value.
Usually: 0.5 m

The spline smoothing procedure I
A smoothing procedure, based on splines function, is
required to remove local maxima not significant (“noise”).
● Explore the range 0÷1
for 100 smoothing
parameter (spar).
● To select the best “spar”
parameter a k-fold cross
validation is used.
● For each “spar”
parameter the mean
square error and the 1sd
is computed
Black dots represent
the mean square error
and the grey bar the
1sd
The spar selection

The spline smoothing procedure II
A smoothing procedure, based on splines function, is
required to remove local maxima not significant (“noise”).
● spar parameter with
minimum error
● The corresponding 1sd
● The highest spar
parameter, just smaller
that 1sd, is selected
The spar parameter:
1)non significantly
different from the
minimum
2)Big enough to
smooth data and
remove noise

Evaluation of bankfull elevation
we found the local
maxima looking for
the zero function of
1st and 2nd
derivative of spline
using the
newthon-rampson
method.
The figure shows howThe figure shows how
this approach is ablethis approach is able
to remove small localto remove small local
maxima for a bettermaxima for a better
identification ofidentification of
bankfull elevationbankfull elevation
No selectedNo selected
local maximalocal maxima

Polygon limits for multiple channels
In case of multiple channel the
tool locate the polygon limits to
the channel with wet area max.
Channel limits
Banfull width

The entire workflow

Case study: Paglia River – central Italy
Reach of 2.2 km of length
XS step: 80m
XS width: 400 m

a
Case study: Paglia River – central Italy
Wandering river in a strong
disequilibrium state.
● Narrowing and incision
processes
● Change in channel
morphology from a
braided river to a
wandering
● The old active floodplain
works now as a
morphological terrace
● The active floodplain is
now inside the old
riverbed, before the
narrowing process started.
The blue line is the polygon output of the tool: refers to the
limits of the main channel related to the bankfull elevation.

Results: channel limits in detail

Local incongruence?
Local
incongruence (a)
between main
channel limits and
aerial photo does
not appear in the
shaded map.
There is a shift in
time between
LiDAR survey and
orthofoto date.

Result: number of channels vs bankfull width
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
0
50
100
150
200
250
0
1
2
3
4
5
6
7
bankfull width channels
width(m)
#ofchannels
In a equilibrium
river # of channel
is proportional to
the bankfull width.
The proportion
looks altered.
The floodplain is
going to became a
terrace.

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
0
100
200
300
400
500
600
700
800
900
1000
0
50
100
150
200
250
300
wet area
Bankfull di-
scharge
m3/s
Area(m2)
Result: wet area and bankfull discharge
A good correlation between discharge and wet area.
1)For some cross section the proportion is altered (see 13÷14 and 26÷27) – low bed slope
2)Section 1 has very higher value of area → high discharge
Q =χ⋅S⋅√R j

Cross section #1
● The cross section look carved and in this point the river is
partially confined (the floodplain in right is missing).

Result: discharge variation and water table
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
-2
0
2
4
6
8
10
12
14
16
130
135
140
145
150
155
maxWT
WT linear
egression
variation
As the discharge may be affected by global error, as the DTM LiDAR does not include
the under-water, we should be more interested in variation rather that the discharge
itself.
Potential pitfall
XS # 26 → Increasing of cross section area.
XS #1 → carved cross section.

Conclusions
● Advantages
– The tool/plugin is designed to be extremely simple to use (a DEM
input and two numerical parameters).
– The tool use a robust methodological approach.
– The bankfull limits are correctly located compared to aerial photo.
– Several parameters useful to evaluate the “state” of a river.
– Few tool that extract river morphological features exists already
implemented in a GIS environment
● Disadvantages
– The methodology base the location of bankfull limits only on the
slope/break in slope. Some other collateral indicators, from aerial
photo (?), using image classification procedure could/should be
inserted.
– The tool is less definitive for river that miss floodplain, or in strong
disequilibrium, or for braided rivers.

THANKS FOR YOUR ATTENTIONTHANKS FOR YOUR ATTENTION
The authors acknowledge Luca Scrucca for the collaboration in theThe authors acknowledge Luca Scrucca for the collaboration in the
the statistical smoothing procedure.the statistical smoothing procedure.
Plugin available at:
https://github.com/pierluigiderosa/BankFullDetection

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