1. Keeney Cove Reconstruction
depth (cm)
Irene – finest grained, highest K/Zr
Main Points:
1. Sediment reconstructions from the Connecticut River watershed suggest that Tropical Storm Irene caused more severe erosion than any event of the historic record.
2. Conditions that caused the event were unique not in terms of rainfall rate or totals, but rather occurrence during a period of anomalously wet background conditions.
3. This is consistent with a regional trend towards increased annual precipitation over the twentieth century based on an analysis of instrumental data.
4. Results suggest a four-fold increase in the frequency of severe erosion events due to moistening background conditions, even in the absence of more intense or more frequent extreme precipitation.
100
200
300
400
500
600
mud with coarse event layers
interbedded sand and mud
River bedload
varves
K/Zr
2.5 5 10 20
137Cs
Onset
1954
Hg and Pb Onset
~1900
Pb peak
~1973
Cove forms
~1692
1955
1936, 1938 – coarse grained, decreases in K/Zr
1927 – coarse grained, increase in K/Zr 50
100
percent >38 μm

2. Uplands: Amherst Lake, Plymouth, VT
K/Zr 5 10
GS (um)
Irene 10
depth (cm)
White River
Saxtons River 20
~1954
137Cs
onset 30
1927 40 50

3. Antecedent Precipitation10 12 14 16 18 20 22 2 4 6 8
event P (cm)
Antecedent P (cm)
Irene
1927 A
𝛾 30
Independent probabilities of
1/200 year storm
AND
95th centile antecedent precipitation
𝛾 30 =95%
𝑃 𝑎𝑛 𝛾>19 cm = → 𝑃 𝑑 =1.4𝐸=−5
𝑃 𝑑 𝛾 30 >17.5 cm =0.05
Irene P was a 1 in 200 year event. Which equates to a small daily probability.
Assume independence between P30 and P since precip is distributed evenly across the year.
High antecedent P leads to rapid Q increase and increased pore pressure, reduced effective stress in bank media.
𝑃 𝑑 𝛾 30 >17.5 𝑐𝑚 ∩𝛾>18.5 𝑐𝑚 = 𝑃 𝑑 𝛾 30 >17.5 𝑐𝑚) × 𝑃 𝑑 (𝛾>18.5 𝑐𝑚
𝑃 𝑑 =7𝐸=−7 or 1 in 4000 yr

4. Changing distributions for antecedent precipitation
Increasing Background Precip
Changing distributions for antecedent precipitation
0.02
0.04
0.06
0.08
1970
2020
1920
𝑃 𝑑𝑎𝑖𝑙𝑦 𝛾 30 >17.5 𝑐𝑚
Quadrupled between 1923 and 2013
𝑃( 𝛾 30 >17.5 𝑐𝑚) 7 8 9
1960
2020
1920 16 18 20
median
95th centile
𝛾 30 (cm)
The probability of having 17.5 cm P in preceding month was much greater today than 100 years ago.
𝛾 30 (cm)