LONG-TERM EFFECTS OF TIMBER HARVESTING ON
SEDIMENT DELIVERY AT SEVERAL TIMESCALES
Study Synopsis
Objective
The
objective
of this study is to quantify the long-term
effects
of two levels of timber harvest on suspended sediment delivery
at
several
timescales:
within
an
individual storm, from storm
to
storm within a year, and from year to year.
Study design and history
This
study
utilizes
the
paired
watershed
approach.
Three
different treatments will be compared: a 100% clearcut
watershed
without roads (WS 1), a 25% harvested patch-cut watershed with
6%
of area in roads (WS 3), and a forested control (WS
2).
Continuous measurement of discharge was initiated in 1953
and
sampling of suspended and bedload sediment was initiated in
1957
on all three watersheds.
Watershed 1 was harvested over
the
period 1962-1966 and slash was broadcast burned in a hot fire
in
1966. Roads were constructed in WS 3 in 1959 and three
small
areas were clearcut in 1963. Watershed 3 experienced
multiple
debris slides and flows during the 1964 storm. Watershed 1
also
experienced several smaller slides which did not become
debris
flows during the period 1964-1972.
All three sites have
been
monitored through 1988, although the frequency of storm
sampling
has steadily diminished during the 1980's.
Suspended sediment was grab sampled during storms and at
irregular
intervals during non-storm periods. One sample per timepoint
per
watershed was taken. Most but not all major storms were
sampled.
Number of samples per storm varied from 1 to 16 averaging
from
3-4. The sediment contained in each sample was dried, weighed
and
reported as a concentration (milligrams/liter).
The
primary
basis for comparing the three watersheds is in
terms
of their sediment rating curves, a relation between
the
measured
sediment
concentration and the discharge at the time the
sediment
sample was taken.
We hypothesize that
both
the
slope
and
the
intercept
of
this
relation
will vary as a function of the
time
within a storm (before or after the peak),
the
sequence
of
the
storm within the year (early fall storms will have
correspondingly
higher sediment concentrations for the same discharge than
later
storms), from year to year (there will be a decline with time
in
the concentration of sediment for a storm of similar magnitude
and
sequence) and as a function of treatment type (patch-cut
versus
clearcut).
Exploratory data analysis will be conducted to develop
predictive
models between discharge and sediment discharge with the
least
variation at these different timescales.
This analysis
will
involve looking at alternative approaches to characterizing
the
sediment samples in terms of their timing and sequence within
a
storm or from storm to storm. Initially, we will concentrate
on
developing these relations for WS 2 (the control) with
the
intention of using the same form of the relations for
the
treatment watersheds.
Data structure
The data consists of the primary hydrology
(Table 1) and
sediment
(Table 2) data sets and two secondary data sets derived from
these
primary sets: a storm hydrology data set (Table 3) and a
sediment
sample
data
set
(Table
4).
The
storm hydrology data set
was
generated through an analysis of the complete hydrology
data
set
consisting
of the complete 35 year discharge record.
Storms
were
identified based on several criteria, including the
magnitude
of
the
secondary
hydrograph
rise
and the time since previous rise;
peaks within a given storm were also identified.
Each
storm
was
numbered
by
year
and watershed; storms were cross-correlated
by
number
across
watersheds
so
that
the
same
storm
could
be
identified at all three sites.
Variables describing the storm
are
given in the attached table.
Using the storm data set, a secondary sediment sample data set
was
derived.
Each
sediment
sample was classified as either a
storm
sample, if it was taken during an identified storm
period,
or
a
non-storm sample.
For storm sample points, a set of
additional
variables were derived (Table 4). The secondary sediment
sample
set will be cross-referenced to the storm data set through
the
storm number.
Questions to be addressed:
1.
What multivariate techniques might be available
to
identify
the
most
important
variables
influencing
the
discharge/sediment discharge relationship at each
timescale?
2.
What is the most efficient way of
classifying
the
sediment
sample points with respect to their storm attributes?
3.
What
is
the appropriate structure for the statistical
tests
for
comparing
the
response
of
the
same
watershed
over
different
timescales
or
different watersheds over the
same
timescales?
4.
Would a type of
time
series
analysis
be
appropriate
for
looking at the changes in sediment discharge with time?
Table 1.
Primary hydrologic data
*************************************************************************
*
HF04
Variable Format and Definitions
FSDB
05/24/89
*************************************************************************
*
Format: 1
HJ
Andrews
watersheds
complete
summary
-----------------------------------------------Variable
-------DATACODE
SITECODE
COYOTE
Coded
--Y
Format
------A1
A1
Columns
------1-1
Units
-------
2-2
hydrograph
Definition
---------S = STREAMFLOW CODE
H = H.J. ANDREWS,
C
=
CREEK, F = FOX CREEK
WSNUMBER
I2
3-4
WATERSHED NUMBER
WATRYEAR
I2
5-6
WATERYEAR
MONTH
I2
7-8
CALENDAR MONTH
YEAR
I2
9-10
CALENDAR YEAR
DAY
I2
11-12
TIME
F5.2
13-17
HH.DD
DECIMAL TIME
STAGE
F5.3
18-22
FT
STAGE
A1
23-23
ESTCODE
PROPORTIONAL
CALENDAR DAY
E
= ESTIMATE, S =
SAMPLE
CFS
F8.3
24-31
CFS
CUBIC FEET PER SECOND
CFSM
F7.3
32-38
CFSM
CUBIC FEET PER SQUARE MILE
INTERVAL
INTERVAL
F6.2
39-44
HRS
HOURS IN STREAMFLOW
MEANCFS
STREAMFLOW
F8.3
45-52
CFS
AVERAGE
CFS
IN
INTERVAL
MEANCFSM
F7.3
53-59
CFSM
AVERAGE CFSM IN INTERVAL
INCHES
F7.5
60-66
INS
INCHES OF WATER WATERSHED
AREA
II.
Table 2.
Primary sediment data
*************************************************************************
*
HS03
Variable Format and Definitions
FSDB
05/24/89
*************************************************************************
*
Format: 2
HJA
with
streamflow
------Variable
-------STCODE
samples
susp
sediment
conc
-------------------------------------------Coded
---
FORMAT
SITECODE
grab
Y
SAMPLENO
Format
------A4
Columns
------1-4
Units
-------
Definition
---------STUDY CODE
A1
5-5
FORMAT
A3
6-8
SITE CODE
1X,A4
10-13
SAMPLE ID NUMBER
1X,A2
15-16
ID NUMBER OF STREAM
DATE
1X,A6
18-23
YYMMDD
DATE SAMPLE TAKEN
TIME
1X,A4
25-28
HHMM
TIME SAMPLE TAKEN
1X,F5.3
30-34
FT
GAGE HEIGHT (USUALLY HOOK
1X,A1
36-36
STREAMNO
Y
GAGEHT
GAGE)
STATUS
HYDROGRAPH
Y
TREND
OF
(RISING,FALLING,STEADY)
TEMP
CFS
SECOND
CFLW
SED1
VALUE OF
1X,I2
38-39
DEG F
F8.3
40-47
CUBIC F
F8.2
48-55
ACRE FE
F7.1
56-62
mg/l
AIR TEMPERATURE
CUBIC FEET OF WATER PER
CUMULATIVE FLOW
FIRST
SUSPEDIMENT
WATER SAMPLE
SED2
SEDIMENT VALUE
F7.1
63-69
mg/l
SECOND SUSPENDED
OF WATER SAMPLE
III.
Table 3.
Secondary hydrologic data: storm identification
*************************************************************************
*
HF06
Variable Format and Definitions
FSDB
05/24/89
*************************************************************************
*
Format: 1
HJA
WS
1,2,3 listing of storm begin, peak,
& flow
----------------------------------------------------------Variable
-------STCODE
Coded
---
FORMAT
BASIN
Y
WATERSHD
Format
------A4
Columns
------1-4
Units
-------
end
times
Definition
---------STUDY CODE
A1
5-5
FORMAT
A3
6-8
BASIN
1X,I2
10-11
WATERSHED NUMBER
WATERYR
I2
12-13
WATER YEAR
STORMID
WATER YEAR
1X,I2
15-16
# OF STORM
WITHIN
CORRELATED
TO
WATERSHEDS 1 AND
2
IDCODE
RECORDED
A1
17-17
IF STORM IN ONE WS WAS
AS
TWO IN ANOTHER, EACH
OF THE
TWO GETS A LETTER TO
DESIGNATE
THAT
THEY
WERE COUNTED
AS ONE
STORM IN THE FIRST WS.
TYPESTRM
Y
1X,A1
19-19
TYPE OF STORM
ENDCAUSE
Y
A1
20-20
CAUSE OF STORM'S END
DURATN
TIMEBGND
BEGINNING
1X,F5.1
22-26
HOURS
1X,A6
28-33
YYMMDD
DURATION OF STORM
DATE
OF
STORM'S
(YYMMDD)
TIMEBGNT
BEGINNING
1X,A4
35-38
HHMM
TIME
OF
STORM'S
(HHMM)
GHTBGN
BEGINNING OF
1X,F5.3
40-44
FEET
GAGE
HEIGHT
AT
STORM
CFSBGN
CUFLWBGN
STORM
1X,F6.3
46-51
CUBIC F
CFS AT BEGINNING OF STORM
1X,F7.2
53-59
ACRE FE
CUMULATIVE FLOW AT OF
TIMEPKD
(YYMMDD)
1X,A6
61-66
YYMMDD
DATE OF STORM'S PEAK
TIMEPKT
(HHMM)
1X,A4
68-71
HHMM
TIME OF STORM'S PEAK
GHTPEAK
STORM
1X,F5.3
73-77
FEET
GAGE HEIGHT AT PEAK OF
CFSPEAK
1X,F6.3
79-84
CUBIC F
CFS AT PEAK OF STORM
CUFLWPK
STORM
1X,F7.2
86-92
ACRE FE
CUMULATIVE FLOW AT PEAK
TIMEENDD
(YYMMDD)
1X,A6
94-99
YYMMDD
DATE OF STORM'S END
TIMEENDT
1X,A4
101-104
HHMM
TIME OF STORM'S END (HHMM)
GHTEND
STORM
1X,F5.3
106-110
FEET
GAGE HEIGHT AT END OF
CFSEND
1X,F6.3
112-117
CUBIC F
CFS AT END OF STORM
1X,F7.2
119-125
ACRE FE
CUFLWEND
STORM'S END
IV.
CUMULATIVE FLOW AT
Storm data codes
*************************************************************************
*
HF06
Variable Code Definitions
FSDB
05/24/89
*************************************************************************
*
HJ Andrews WS 1,2,3 storms
Variable: BASIN
HJA
H J ANDREWS EXPERIMENTAL FOREST
Variable: WATERSHD
1
2
3
WATERSHED 1
WATERSHED 2
WATERSHED
3
Variable: TYPESTRM
P
S
AFTER
PRIMARY STORM
SECONDARY STORM
(STORM
BEGINNING
12-24
HOURS
PREVIOUS
STORM)
HAS
SLIGHTLY
LOWER
STORM
STORM
12-24
QUALIFYING
CRITERIA THAN PRIMARY STORM
Variable: ENDCAUSE
HRS
N
STORM ENDED BY BEGINNING OF NEW
D
P
PEAK
STORM ENDED BY TIME: 24 HRS FROM PEAK
STORM ENDED BY TIME:12 HRS FROM SEC PEAK AND >
G
FROM PRIM PEAK
STORM ENDED BY GAGE HEIGHT: GAGEHT DROPPED TO
FROM
24 HRS
W/IN
20%
OF INIT GAGEHT
V.
Table 4.
Variable
-------WATERSHD
Secondary sediment sample data set
Coded
---
Format
------1X,I2
Columns
------10-11
WATERYR
I2
12-13
STORMID
WATER YEAR
1X,I2
15-16
Units
-------
Definition
---------WATERSHED NUMBER
WATER YEAR
#
OF
STORM
WITHIN
CORRELATED TO WATERSHEDS
1
AND
2
IDCODE
RECORDED
A1
17-17
IF STORM IN ONE WS WAS
AS TWO IN ANOTHER, EACH
OF
THE
TWO
GETS A LETTER TO
DESIGNATE
THAT THEY WERE COUNTED
AS
ONE
STORM IN THE FIRST WS.
STORM
STORM SAMPLE POINT (Y OR
N)
LPEAKHRS
(HRS)
LPEAKCFS
STATUS
FALLING POINT
TIME SINCE LAST PEAK
DISCHARGE AT LAST PEAK
RISING,
(R,P,F)
PEAK, OR
TYPESTRM
PRIMARY OR SECONDARY
PROPEAKP
CFS
PROPORTION OF CFS TO
PEAK
PEAK
(PRIMARY)
PROPEAKS
PEAK CFS
PROPORTION
OF
CFS TO
(SECONDARY)
SSAMPNUM
STORM (I.E.
NUMBER IN SAMPLE IN
1 OF 5)
SSAMPTOT
CFLW
DISCHARGE
TOTAL SAMPLES FOR STORM
CUMULATIVE
(ACRE-FEET)
CFS
SEDCONC
CONCENTRATION
DISCHARGE (CFS)
AVERAGE SEDIMENT
(PPM)