Tours 2012 Morphodynamique et transport solide en rivière: du terrain aux modèles

INTRODUCTION

Hydraulic modeling is used to verify

areas of observed erosion and

deposition over the Carpathian and

Subcarpathian reaches (about 50 km in

length) of Prahova River, Romania (Fig.

1).

Like other European rivers, Prahova

has been incising into bedrocks,

narrowing its valley, abandoning its

lateral channels and straightening its

path while crossing the study reach,

mainly during the last 100 years.

METHOD

109 surveyed cross-section profiles

and digitized 1:5000 maps were used to

construct a TIN in Arc-GIS (Fig. 2). By

using the HEC-GeoRAS utility, the

geometry of a 1D hydraulic model was

extracted and exported to HEC-RAS

software (USACE) (Fig. 2).

Prahova has a reduced sediment

inflow along the study reach due to the

absence of main tributaries in the area

(Fig. 3a).

During the last 100 years an intense

decrease of morphometric indexes was

evidenced by diachronic spatial

analysis (fig. 3b). These changes may

be related to a severe anthropic impact

(Fig. 4) such as: gravel mining, river

regulation and channel works, dam

construction, sediment traps, land use

change, population increase, road and

railway construction and afforestation.

Simulations were performed under

steady flow conditions for flow values

between bankfull and the peak

discharge value of typical flood events

with a return period of 20, 50 and 100-

years (Fig. 2b).

OBJECTIVES

1. To analyze and explain the

morphological vertical incision and

planform channel evolution (narrowing

and transition from braiding to sinuous);

2. To relate this pattern change to

hydrodynamic parameters obtained

from numerical simulations performed

with a 1D hydraulic model (HEC-RAS

software);

3. To identify the erosion/deposition

prone areas and their links with human

activities and impacts.

RESULTS

Computed stream power values

show a peak at the limit of Carpathians

with the Subcarpathians. This way the

stream enters the downstream reach

with high energy. Four main areas with

high values of computed shear stresses

and corresponding low values of width

to depth ratio (W/D) were identified (A-

D) along the Subcarpathian reach (Fig.

5a) and b)) through hydraulic modeling.

These areas were found to match the

observed erosion areas, where the river

has incised into bedrock and eroded its

banks (Fig. 7). Other potentially

aggradation areas of low shear stress

values and high W/D (A’-D’) were

confirmed by field observations and

explained geomorphologically.

Maximum values of W/D were found in

cross-sections 4, 17, 36, and 44, where

the tendency of braid-bar development

in the main channel may be observed in

the detail cross-section plots (Fig. 5b).

Long-term planform changes were

evidenced by diachronic cartography

(Fig. 6).

CONCLUSIONS:

The natural, long-term evolutional

morphology of Prahova River channel is

mainly determined by climate, geology,

valley relief, lithology and tectonics,

tributaries and vegetation. On the other

hand, changes over the last 200 years

bear the mark of increased human

interventions.

2. METHOD:

Figure 1. a), b) Prahova watershed in Romania with its Carpathian and

Subcarpathian units as study reach; c) 109 cross-section surveyed along the

study reach (between Predeal and Campina) in 2006

Figure 3. a) Longitudinal profile of entire Prahova River and cumulative

drainage area of tributaries; b) Evolution of morphometric parameters for

the studied reach from diachronic spatial analysis (DIA)

1. SITE AND DATA

Figure 5. Computed a) shear stress values; b) Width / Depth ratio for

Q20-years show 4 areas of potential erosion (A-D) and 4 areas of potential

deposition and development of mid-channel bars (A’-D’). Cross-sections 4,

17, 36 and 44 still showing braiding character of the river

Figure 6. Planform changes along Subcarpathian reach between 1864

and 2005

3. RESULTS:

3.1 Computed shear stress and width / depth ratio, for the

Subcarpathian reach

3.2 Planform changes

3.3 Vertical changes

Daniela NISTORAN GOGOASE*, Iuliana ARMAS**,

Livioara Brasoveanu**, Cristina IONESCU* (*) University Politechnica of Bucharest, Power Engineering Faculty, Department of Hydraulics,

Bucharest, Romania, dnistoran@gmail.com

(**) University of Bucharest, Faculté de Géographie, Bucarest, Romania

USE OF HYDRAULIC MODELING TO

GEO-MORPHODYNAMIC ANALYSIS ALONG A CARPATHIAN

MOUNTAIN RIVER: PRAHOVA, ROMANIA

a)

b) c)

Prahova watershed

Carpathian

unit

Subcarpathian

unit

Predeal

Comarnic

Campina

Figure 2. a) TIN of the study area with cross-section lines to extract the

profiles in HEC-GeoRAS utility (USACE); b) Boundary conditions for the

hydraulic model in HEC-RAS (USACE)

W / D for the 20-year flood peak

0

20

40

60

80

100

0 2,000 4,000 6,000 8,000 10,000 12,000 14,000 16,000 18,000

Distance from downstream confluence with Doftana River (km)W

/ D

A'B'

C'

C

B

A

Campea tributary

Erosion Deposition

D

17

4

44

36

D'

100 200 300 400

404

406

408

410

mo delare Pra hova Pl an: P lan 0 2 06 .07.2 011 Pr ofil 41 nou

Station (m)

Ele

vatio

n (B

lack S

ea) (m

)

Legen d

EG 1%

W S 1%

Crit 1%

EG PF 5

W S PF 5

Cr it PF 5

Ground

Bank Sta

.07 .045 .07

0 20 40 60 80 100 120 140498

499

500

501

502

503

504

River = Prahova Reach = A RS = 44 Profil nr 14 nou Hydraulic model Prahova River

Station (m)

Ele

va

tio

n (

Bla

ck S

ea) (

m)

Legend

WS 10% Bf

Ground

Bank Sta

.045 .07

0 20 40 60 80 100 120

467

468

469

470

471

River = Prahova Reach = A RS = 36 Profil 22 nou Hydraulic model Prahova River

Station (m)

Ele

va

tio

n (B

lack S

ea) (m

)

Legend

WS 10% Bf

Ground

Bank Sta

.045 .07

0 50 100 150 200 250 300368

369

370

371

372

373

374

mo delare Pra hova Pl an: P lan 0 2 06 .07.2 011 Pr ofil 54 nou

Station (m)

Ele

vatio

n (B

lack S

ea) (m

)

Legend

EG 1%

WS 1%

EG PF 5

WS PF 5

Ground

Bank Sta

.07 .045 .07

Photo 3 Photo 2

49

57

43

3719

316

11

12 32

48

Photo 1

Belia tributary

b) c)

b)

Figure 4. Anthropic impact within the last 200 years along study reach;

Evolution of characteristic lengths

0

10

20

30

40

50

60

70

80

1840 1860 1880 1900 1920 1940 1960 1980 2000 2020

Time (years)

Mo

rph

olo

gic

al

pa

ram

ete

rs

L along thalwegL along straight lineL anabranchesTotal L

Evolution of Sinuosity and Braiding Indexes

0

0.5

1

1.5

2

2.5

3

1840 1860 1880 1900 1920 1940 1960 1980 2000 2020

Time (years)

Sin

uo

sit

y a

nd

Bra

idin

g

ind

exes

Sinuosity index

Braiding index

a)

Qbankfull,

Q20-years,

Q50-years

Q100-years

0 100 200 300 400 500370.5

371.0

371.5

372.0

372.5

373.0

373.5

374.0

374.5

River = Prahova Reach = carp_subcarp RS = 1082.73* SH Campina SH Campina Hydraulic model Prahova River

Q Total (m3/s)

Legend

Obs RC GS Campina

b)

0

100

200

300

400

500

600

700

800

900

1000

1100

0 20 40 60 80 100 120 140 160 180 200 220

Distance from source (km)

Ele

va

tio

n r

ela

tiv

e t

o B

lac

k S

ea

le

ve

l

(m)

0

500

1000

1500

2000

2500

3000

3500

4000

Dra

ina

ge

are

a (

sq

km

)

thalweg line

drainage area

Piedmont plain reach

Subcar-

pathian

reach

Carpathian

reach

1

2

Main tributaries:

1 - Doftana

2 - Teleajen

Lowland plain reach

Busteni GS

Campina GS

a)

b)

b)

Figure 7. Incision

observed along

the downstream

reach

a) erosion area A

(Photo 1);

b) erosion area B

(Photo 2);

c) Erosion area C

(Photo 3).

0

50

100

150

200

250

300

350

400

0 2 4 6 8 10 12 14 16 18 20

Be

d s

he

ar s

tre

ss

(N

/m2

)

Distance from downstream confluence with Doftana River (km)

Bed shear stress for the 20-year flood peak

C

Campea tributary

D'

B A

A'B'

Degradation/Erosion Aggradation/Deposition

D

C'

49

5743

37

31

196

11 1217

Belia tributary

CAMPINA

CORNU - BREAZA

BREAZA - NISTORESTI

BELIA, COMARNIC

32 36

44

49

a)