BANK EROSION ASSESSMENT

Maria Α. Lilli Technical University of Crete (TUC)

Summer School 6 September 2019

Riverbank erosion

affects: the river morphology the local habitat

results in:

riparian land loss, damage to property and infrastructures

Steep land slopes…

Degraded erodible soils…

Schist geologic formation…

D

Schists - Surface runoff and flash floods

Temporary Rivers and Flash Floods

Date Duration

(h)

Max hourly flow

(m3/s)

Max hourly flow

/ total flow

Million cubic meters of

water during 12/24-hour

flow

27/10/17 12 115.6 19.0% 2.2

13-14 /2/19 24 136.6 6.4% 7.7

25-26/2/19 24 140.3 4.9% 10.3

Characteristics of the last 3 floods at Koiliaris CZO

Types of river bank failure

• Anavreti tributary Presence of intense erosion A typical example is the flood of 2017 where the flood flow passed through the football field of the area and caused significant erosion on the river bank.

• Main river (Stylos, before connecting with Anavreti tributary) Presence of “mild” but constant erosion

Slight bank slopes

• Main river (Stylos, after connecting with Anavreti tributary) Presence of intense erosion and failure

Vertical/steep bank slopes

Typical photos / Types of bank erosion

Characterization of soil texture

• Particle size distribution - Sieve analysis • Field analysis method of soil texture (NSW

Agriculture, 1998)

𝑭𝒔 = 𝒄′

𝒊 𝑳𝒊 + 𝑺𝒊𝒕𝒂𝒏𝝋𝜾𝒃 − 𝑼𝒊𝒕𝒂𝒏𝝋′

𝒊𝑰𝒊=𝟏

𝑾𝒊𝑰𝒊=𝟏 − 𝑷𝒊

where Fs= Factor of safety Si = the force produced by the suction at the unsaturated portion of the failure surface (kN/m) ci' = effective cohesion of ith layer (kPa); Li = length of the failure plane incorporated within the ith layer (m); f' = effective angle of internal friction (degrees). Wi = weight of the ith layer (kN); Pi = hydrostatic-confining force due to external water level (kN/m) acting on the ith layer; b = failure-plane angle (degrees); a = local bank angle (degrees); I = number of layers

Types of streambank failure

Fs>1.3 “STABLE BANK” 1.0<Fs<1.3 “CONDITIONALLY STABLE BANK”

Fs<1.0 “UNSTABLE BANK”

BANK STABILITY AND TOE EROSION MODEL (BSTEM)

BANK STABILITY AND TOE EROSION MODEL (BSTEM) in HEC-RAS

BSTEM has been incorporated in HEC-RAS version 5.0 and specifically to the Sediment data editor.

DATA INPUT

Data relating to the flood event being studied / Quasi unsteady flow editor tab

The flood event selected was that on 27-10-17. The daily flow Q50 derived from the SWAT model (55.45 m3/s) is converted to the maximum hourly flow when multiplied by the ratio of the maximum hourly flow to the total flow (19.0%) and is equal to 126.1 m3 / s (coefficient 2.3) .

BANK STABILITY AND TOE EROSION MODEL (BSTEM) in HEC-RAS

DATA INPUT

Sediment Data Tabs:

Initial Conditions and Transport Parameters (Tab 1) Boundary Conditions (Tab 2) Bank Stability and Toe Erosion Model (BSTEM) (Tab 3)

BANK STABILITY AND TOE EROSION MODEL (BSTEM) in HEC-RAS

DATA INPUT

Definition of station points for each cross section

Groundwater elevation which is either static or dynamic for the simulation

Soil material for each cross section (left bank and right bank)

PRESENCE OF INTENSE EROSION

Cross section Factor of Safety (Fs) Cross section Factor of Safety (Fs)

Right Left Right Left

Κ.Δ7 70000 4.68 Κ.Δ66 2.18 1.1

Κ.Δ37 1.08 0.02 Κ.Δ67 1.05 0.98

Κ.Δ38 0.22 1.01 Κ.Δ68 0.47 1.09

Κ.Δ39 1.01 1.06 Κ.Δ69 0.01 1.22

Κ.Δ40 1.12 1.03 Κ.Δ70 1.08 1.24

Κ.Δ41 1.08 0.08 Κ.Δ75 0.99 1.02

Κ.Δ42 0.97 1.03 Κ.Δ76 0.01 1

Κ.Δ43 0.44 1.11 Κ.Δ77 1.04 1.34

Κ.Δ44 1 1.04 Κ.Δ78 1.11 1.09

Κ.Δ45 1.25 1.13 Κ.Δ79 0.91 1.01

Κ.Δ46 1.01 1 Κ.Δ80 0.2 1.07

Κ.Δ47 12.34 1.18 Κ.Δ81 0.18 1.05

Κ.Δ49 1.15 1.52 Κ.Δ82 1.01 3.41

Κ.Δ50 0.94 2.5 Κ.Δ83 1.01 1.36

Κ.Δ52 0.88 1.02 Κ.Δ84 0.55 1.02

Κ.Δ53 1.06 1.12 Κ.Δ85 0.22 1.05

Κ.Δ54 1.02 1.02 Κ.Δ86 1.09 1.06

Κ.Δ55 0.05 1.09 Κ.Δ87 1.01 1.2

Κ.Δ56 1.2 1.19 Κ.Δ88 1.07 1.07

Κ.Δ57 1.06 1.22 Κ.Δ89 1.14 1.54

Κ.Δ58 0.01 0.99 Κ.Δ90 2.33 2.11

Κ.Δ59 0.49 1.02 Κ.Δ91 1.69 0.01

Κ.Δ60 0.91 1.1 Κ.Δ92 1.89 1.62

Κ.Δ61 1.28 1.07 Κ.Δ108 1 1.1

Κ.Δ62 1.03 1.01 Κ.Δ109 1.04 1.1

Κ.Δ64 0 1.05

ANALYSIS OF SIMULATION RESULTS (MAIN RIVER, KOILIARIS)

ANALYSIS OF SIMULATION RESULTS ANAVRETI TRIBUTARY

KERAMIANOS TRIBUTARY

Cross section Factor of Safety (Fs)

Cross

section

Factor of Safety

(Fs)

Right Left Right Left

AN.Δ11 0.01 0.01

AN.Δ55 10000000

0 1.03

AN.Δ12 0.11 0.04 AN.Δ57 1.01 0

AN.Δ13 0.26 0.76 AN.Δ58 0.99 1.11

AN.Δ14 0.37 0.02 AN.Δ59 22.08 0.99

AN.Δ21 1.13 0.80 AN.Δ62 1.03 0.14

AN.Δ22 1.01 11.59 AN.Δ63 0.73 0.34

AN.Δ23 0.06 0.56 AN.Δ64 1.07 0.17

AN.Δ24 1.14 0.57 AN.Δ65 1.73 1.02

AN.Δ25 0.09 1.84 AN.Δ66 0.96 0.97

AN.Δ26 16.26 0.86 AN.Δ67 1.00 0.98

AN.Δ53 21.54 23.33 AN.Δ68 0.91 1.01

AN.Δ54 100000000 1.01

AN.Δ69 0.03 0.98

Cross section Factor of Safety (Fs)

Right Left

ΚΕ.Δ1 100000000 0

ΚΕ.Δ2 0 0

ΚΕ.Δ32 100000000 0

ΚΕ.Δ37 1.01 1.69

ΚΕ.Δ38 1.13 2.11

RESTORATION INTERVENTIONS FOR BANK EROSION CONTROL IN KOILIARIS CZO

1) Use of boulders in areas with presence of intense erosion and failure (vertical/steep bank slopes)

2) Use of rip-rap in areas with presence of slight bank slopes for the protection of the existing vegetation

3) Bed formation in areas with presence of intense erosion 4) River bank strengthening with vegetation and creation of a riparian forest

BANK STABILITY AND TOE EROSION MODEL (BSTEM)

Example Use of Excel version 5.2

The Bank Stability and Toe Erosion Model is a physically-based model. It represents two distinct processes, namely, the failure by shearing of a soil block of variable geometry and the erosion by flow of bank and bank toe material. The effect of toe erosion, vegetative treatments or other bank and bank toe protection measures can be illustrated by calculating the actual Factor of Safety (Fs) of the bank.

BANK STABILITY AND TOE EROSION MODEL (BSTEM) Version 5.2

1) Open Excel file “BSTEM-5.2” 2) Click on “Input Geometry” sheet 3) Select EITHER Option A or Option B to input bank geometry 4) Enter Bank-layer Thickness 5) Enter channel and flow parameters

BANK STABILITY AND TOE EROSION MODEL (BSTEM) Version 5.2

6) Enter “View bank geometry”

7) Enter “Run Bank Geometry Macro”

BANK STABILITY AND TOE EROSION MODEL (BSTEM) Version 5.2

8) Enter Bank-material Properties: “Bank Material” sheet

𝝉𝒄 = 𝟎. 𝟎𝟒𝟒 × 𝟏𝟔. 𝟐 × 𝑫𝟓𝟎, 𝑫𝟓𝟎 < 𝟐𝒎𝒎

𝑘 = 0.1 × 𝜏𝑐−0.5

Critical shear stress (Pa):

Erodibility coefficient (cm3/Ns):

BANK STABILITY AND TOE EROSION MODEL (BSTEM) Version 5.2

9) Select “Toe Model Output” sheet and Click on “Run Toe-Erosion Model” 10) Export New (Eroded) Profile into Model (Returned to “Input Geometry” sheet)

BANK STABILITY AND TOE EROSION MODEL (BSTEM) Version 5.2

11) View Bank Geometry and then Run “Bank Geometry Macro”

Eroded profile

BANK STABILITY AND TOE EROSION MODEL (BSTEM) Version 5.2

12) Click on “Bank Model Output” sheet; Set water-table depth and Click “Run Bank Stability Model”

13) Save file under different name

BANK STABILITY AND TOE EROSION MODEL (BSTEM) Version 5.2

14) Open file and Click on “Bank Vegetation and Protection” sheet Run root-reinforcement Model Select the species, specify plant age and percent contribution to assemblage

BANK STABILITY AND TOE EROSION MODEL (BSTEM) Version 5.2

15) Click on “Bank Model Output” sheet