Transcript Erosion in Forests: Measurements for Management

Forest activities and Water
Quality
What can we confidently say after 30 years of
research?
DOES LOGGING IMPACT
UPON WATER QUALITY?
• $64,000 question that largely remains
unanswered
• The question itself is not
CONSTRUCTIVE - it offers no
solution and no practical help to the
industry
TRADITIONAL APPROACH
• Paired catchment monitored before
and after the period of major
disturbance.
• Estimates of suspended solids or
turbidity are used to infer increased
sediment delivery to streams
IMPORTANT LIMITATIONS
• Black-Box ie you can NOT relate the
increase in suspended load at the
catchment outlet to activities on the
slopes
• Tells us nothing about where the
sediment comes from and hence how
to manage it
NEW APPROACHOPEN UP THE BLACK BOX
For water pollution to occur, you need 3
key ingredients;
• a source of sediment,
• delivered directly to the stream
• complete failure of any sediment
control strategies or BMP’s
CAN WE BE CONFIDENT?
• BROAD GENERALISATIONS
• WELL ESTABLISHED
• LIMITED EVIDENCE
• SPECULATIVE
SEDIMENT AND NUTRIENT
SOURCES
Well Established
 Unsealed forest roads are the major sources
of sediment in managed forests.
 Road usage is a critical factor in explaining
sediment production rates on roads.
Feeder Access
Secondary Access
Sediment concentration
(mg/L)
100000
Dump Access
5 yr snig track
(Reid and Dunne 1984)
10000
1000
100
Abandoned
10
10
100
1000
Culvert discharge (mL/s)
10000
SEDIMENT SOURCES
Sediment
Flux (g/m/s)
100
Road
10
Snig Track
1
Hillslope
0.1
0.01
0.001
0
500
1000
1500
Time (secs)
2000
2500
SEDIMENT AND NUTRIENT
SOURCES
 Sediment production rates on roads and
tracks decline within the time frame of 2to 5 years
 Sediment yeilds from managed forests are
lower than those from other landuses
primarily agriculture
RECOVERY OVER TIME
Snig
Track
Yield (kg/ m2)
1.20
0.80
0.40
Metasediment
0.00
0.5
1
2
3
4
5
6
Red Granite
Age (years)
Light Granite
Hillslope
Yield (kg/m2)
0.04
0.03
0.02
0.01
0.00
0.5
1
2
3
Age (years)
4
5
6
SEDIMENT AND NUTRIENT
SOURCES
 Nutrient concentrations in streams
draining forested catchments are
considerably lower than those reported
for other landuses, primarily agriculture.
• The dominant cause of increased
nutrients in streams if observed, is due to
the effects of prescribed burning and
wildfire.
SEDIMENT AND NUTRIENT
SOURCES
Limited Evidence
 The GHA is not a significant source
of sediment due to limited
sediment availability, high retention
of vegetation cover and spatially
variable infiltration rates.
Metasediment soil
Red granite soil
0.25
0.26
2.31
t/ha
0.69
1.62
0.35 t/ha
GHA
11.08
t/ha
Snig
0.05 t/ha
GHA
Snig
SEDIMENT DELIVERY
PATHWAYS
• The degree to which the source
is connected or linked to the
receiving waters
Bridge
Partial channel
linkage
Infiltration
No channel
linkage
Direct linkage
Full channel
linkage
DELIVERY PATHWAYS
Well Established
 Channelised pathways forming at road
drainage outlets form the most efficient
conduit for sediment and nutrient
delivery to streams.
 Sediment delivery ratios are closely
associated with the size composition of
the in-situ and eroded soil.
Full Channel
CULVERT
Partial Channel
100
90
Percentage
80
70
60
50
40
30
20
10
0
MITRE DRAIN
Culverts Mitres Push Outs
Follow the Red Line
Major area of Impact
N
2km
The impact can be widespread
• The impact of channelised pathways
extends beyond a single point or
particular road drain and can be
catchment-wide
• They also become permanent
features of the landscape
DELIVERY PATHWAYS
Well Established
 Channelised pathways forming at road
drainage outlets form the most efficient
conduit for sediment and nutrient
delivery to streams.
 Sediment delivery ratios are closely
associated with the size composition of
the in-situ and eroded soil.
Full Channel
CULVERT
Partial Channel
100
90
Percentage
80
70
60
50
40
30
20
10
0
MITRE DRAIN
Culverts Mitres Push Outs
Follow the Red Line
Major area of Impact
N
2km
The impact can be widespread
• The impact of channelised pathways
extends beyond a single point or
particular road drain and can be
catchment-wide
• They also become permanent
features of the landscape
Road travelway gradient
Drain discharge hillslope gradient
2.5 5.0 7.5 10 15 20 25 45
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
- 110 95
155 110 95
155 110 95
150 110 95
125 110 95
100 100 95
90 90 90
80 80 80
70 70 70
65 65 65
60 60 60
55 55 55
50 50 50
45 45 45
40 40 40
40 40 40
90
90
90
90
90
90
90
80
70
65
60
55
50
45
40
40
85
85
85
85
85
85
85
80
70
65
60
55
50
45
40
40
80
80
80
80
80
80
80
80
70
65
60
55
50
45
40
40
75
75
75
75
75
75
75
75
70
65
60
55
50
45
40
40
75
75
75
75
75
75
75
75
70
65
60
55
50
45
40
40
Maximum contributing
length
95m
for road gradient of
5 degrees
and DHG 7.5
200
100
80
60
50
40
30
20
2 year event for T = 30mins
10 year event for T = 30mins
10
8
6
5
100 year event for T = 30mins
56
8 10
20 30 405060 80100
Inter-bank length (m)
200
EFFECTS OF BEST
MANAGEMENT PRACTICES
Well established
 BMP’s play a significant role in the
reduction of adverse effects in forested
catchments.
 Forest buffer strips are an effective
measure in reducing the volume of
surface water and sediment/nutrients
delivered to a stream.
EFFECTS OF BEST
MANAGEMENT PRACTICES
Limited Evidence
 The best location and design of
buffer strips in forested catchments
of varying topography and landuse.
 The specific role and effectiveness
of BMP’s on the hillslope compared
with those in the near-stream area.
CONCLUDING REMARKS
• The Most Dramatic Impacts On Water
Quality Will Come About In Response To
The Most Dramatic Landuse Impact• Broad Scale Slash And Burn- Large Scale
Conversion To Pasture