Sediment yield in a small mountain basin during extreme events Giovanna Grossi 1 & Paolo Caronna 1 FRIEND project - MED group UNESCO IHP-VII (2008-13) 4 th International Workshop on Hydrological Extremes From prediction to prevention of hydrological risk in Mediterranean countries University of Calabria, 15-17 September 2011 1 Department of .Civil Engineering, Architecture, Land and Environment, University of Brescia, Italy, via Branze 43
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Sediment yield in a small mountain basin during extreme events
Giovanna Grossi1 & Paolo Caronna1
FRIEND project - MED groupUNESCO IHP-VII (2008-13)
4th International Workshop on Hydrological Extremes From prediction to prevention of hydrological risk in Mediterranean countries
University of Calabria, 15-17 September 2011
1Department of .Civil Engineering, Architecture, Land and Environment,University of Brescia, Italy, via Branze 43
Objectives, methodologies, case study-------------------------------------------------------
Hec‐Ras hydraulic model
Application of a ‘lumped type’ sedimenttransport flow equation
GIS implementation of the RUSLE equation
2) Sediment yield in the river network
3) Sediment transport
1) Water erosion on the hillslope
Area (A) 30.9 km2
Contour (P) 33.3 kmMain stream length (L) 10.6 km
Minimum elevation (Hmin) 195 m s.l.m.Maximum elevation (Hmax) 1131 m s.l.m.
Mean elevation (Hmedia) 643 m s.l.m.Max elevation range (∆Hmax) 1136 m
Mean elevation range(∆Hmedio) 448 m
Guerna watershed (BG – North Italy)-------------------------------------------------------
N
Lithology & Land use-------------------------------------------------------
Permeable rocks (karst)
Gravity sediments
Fields
Woods
Lithology and land use – maps were used for the GIS application
‐ Revised version of USLE: Universal Soil Loss Equation (Wischmeier e Smith, 1978)
PCSLKRA ⋅⋅⋅⋅⋅=
Erosion: Revised Universal Soil Loss Equation (RUSLE) (Renard et al.1991)-------------------------------------------------------
Erosion estimate: R and K factors-------------------------------------------------------
yearPR ⋅+= 48.346.38 (“Actual Erosion in the Alpine Space” , European Soil Bureau)
mmPyear 1260=yhha
mmMJ⋅⋅
⋅ 4423.26=R
K
Van Der Knijff et al. (2000)
t ha hha MJ mm
⋅ ⋅⋅ ⋅
.
0.035=K
Erosion estimate: L and S factors-------------------------------------------------------
β the slope
( )03.0sin8.1013.22
+= ⎟⎠⎞
⎜⎝⎛ β
λ m
LS
( )05.0sin8.1613.22
−= ⎟⎠⎞
⎜⎝⎛ β
λ m
LS
ff
m+
=1
( )56.031
0869.0sin
sin 8.0 +⋅=
ββf
if tanβ < 0.09
if tanβ ≥ 0.09
Being:
Renard et al. (1991) write:
Topographic factors L and Swere computed on the basis of a20 m resolution digital elevationmodel.
Erosion estimate: C and P factors-------------------------------------------------------
The coltural factor C is computed as the ratio between the soil loss under actualconditions to losses experienced uder the reference conditions.
Tables depending on crops and crops rotation
Description C factorUrban area 0.003
Unproductive soil 0.36Old and new forest 0.003
Woodland 0.451Lawn 0.04
Sown ground 0.4Wild vegetation 0.003
The P factor is defined as the ratio between the soil loss with contouring and/orstripcropping to that with straight row farming up‐and‐down slope.
Obtained results: Sediment yield at the end of the simulation period
At the outlet in the Oglio river (section 0) sediment yield is 1192 t (γs=2600 kg/m3
Vs~460 m3)
Estimate of sediment transport: Meyer-Peter Müller equation-------------------------------------------------------
The sediment transport rate qs is the sediment volume discharge per section widthunit.
(Meyer‐Peter Müller, 1948)
The index Φ, is the non dimensional form for qs :
∆⋅⋅=−⋅=Φ
gd
qsc 3
50
5.1)(3.13 φφ
6/190
23
50
26' ; '1
; 047.0 ; −⋅=⎟⎟⎠
⎞⎜⎜⎝
⎛⋅
⋅⎟⎟⎠
⎞⎜⎜⎝
⎛−
⋅==
−=∆ dk
kk
d
iRS
S
S
SC
S
ρρ
φφρρρ
ρ : water unit mass
ρs : sediment unit mass
φC : Shields number
R : hydraulic radius
i : bed slope
ks : Strickler roughness due to bed grain and shapek’s : Strickler roughness due only tograinsd50 : 50% of sediment weight is lower insized90: 90% of sediment weight is lower insize
3°On the basis of Meyer‐Peter Müller equation alone, sedimenttransport rates would be very low and their distribution wouldbe much different from those obtained through the detailedhydraulic model.