thesis road hydrology

7/30/2019 Thesis Road Hydrology

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Forest Road Hydrology: The Influence of

Forest Roads on Stream Flow at Stream Crossings


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AN ABSTRACT OF THE THESIS OF

Elizabeth Myers Toman for the degree of Master of Science in Forest Engineering presented on

April 30, 2004.

Title: Forest Road Hydrology: The Influence of Forest Roads on Stream Flow at Stream Crossings.

Abstract approved:

________________________________________________________________________________

Arne E. Skaugset III

The management of existing forest road systems is an issue of growing importance and

public debate. Roads can alter the hydrologic processes in a watershed especially at stream crossing

culverts where road ditches channel runoff directly into the stream. The objective of this study was

to determine how surface runoff from roads augments natural stream flow at stream crossing

culverts. This study took place within an 824 ha watershed in the foothills of the Oregon Coast

Range approximately three miles west of Corvallis, Oregon. Sixteen stream crossing culverts were

selected for study. Discharge was measured from October 2002 through May 2003 at each stream

and at the adjoining ditch(es). Hydrographs for both stream flow and ditch flow were analyzed for

five storms that occurred during the winter 2002-2003. The interaction of the road with subsurface

flow from the hillslope caused the hydrology of the road segment to be classified as either

intermittent or ephemeral. Peak flow and total runoff at the stream crossing culverts was

compared with the magnitude and timing of peak flow and total runoff in the adjoining ditch(es).

Forest roads were found to alter the flow paths of water through the Oak Creek watershed. The road

altered storm runoff and peak flow at the stream crossing culverts seventy-four times out of seventy-

eight opportunities during five storms. The amount of the change depended primarily on whether or

not the road cutslope intercepted subsurface flow. Contributions of intercepted subsurface runoff to

the stream were greater than contributions of surface runoff by an order of magnitude. In the Oak

Creek watershed, 56 percent of the road cutslopes adjacent to streams intercepted subsurface flow.

Intercepted subsurface flow was more connected in time to stream flow than surface runoff. Ditch

flow, on average, contributed the most volume on the rising limb of the stream hydrograph at the


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culvert. Flow responses at individual culvert locations were highly variable and could not be

predicted using traditional topographic variables.


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Copyright by Elizabeth Myers Toman

April 30, 2004

All Rights Reserved


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by

Elizabeth Myers Toman

A THESIS

submitted to

Oregon State University

in partial fulfillment of

the requirements for the

degree of

Master of Science

Presented April 30, 2004

Commencement June 2004


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Master of Science thesis of Elizabeth Myers Toman presented on April 30, 2004.

APPROVED:

Major Professor, representing Forest Engineering

Head of the Department of Forest Engineering

Dean of the Graduate School

I understand that my thesis will become part of the permanent collection of Oregon State University

libraries. My signature below authorizes release of my thesis to any reader upon request.

Elizabeth Myers Toman, Author


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ACKNOWLEDGEMENTS

First, I would like express my appreciation for those that made this research possible and

allowed me to gain an advanced degree in the process. George Ice and the National Council for Air

and Stream Improvement were instrumental in providing support for the tremendous amount of

infrastructure necessary. I am very grateful for fellowships and funding from the Hoener

Foundation, Gibbet Hill Foundation, Richardson Family Foundation, and the A. & V. Meier

Education Fund. I would like to thank the entire Department of Forest Engineering at Oregon State

University including Rayetta Beall and Yvonne Havill for guidance and help. The biggest thank

you, however, goes to my major professor, Arne Skaugset, who has been a mentor, advisor, and

friend and with whom I look forward to continue working with.

I would also like to thank all who were involved in this project. Kami Ellingson, Debbie

Goard, Erica Marbet, Joseph Amann, Hans Gauger, Richard Keim, Jeremy Appt, Scott Miller,

Kristin Cotugno, Derek Godwin, Terry Luecker, and Hamish Marshall all helped with a part of the

data collection and analysis. A special thanks goes to all the graduate students in the department of

Forest Engineering for their friendship and for conversations and antics that kept me sane.

And last, I want to thank my family; my parents who through example taught me a love of

learning, and Eric whose constant support and encouragement kept me going.


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TABLE OF CONTENTS

Page

1. INTRODUCTION...........................................................................................................................1

2. LITERATURE REVIEW................................................................................................................3

2.1 Forest Hydrology.......................................................................................................................3

2.2 Hillslope Hydrology ..................................................................................................................3

2.3 Road Design and Road Hydrology ............................................................................................6

2.4 Hydrologic Effects of Roads......................................................................................................9

2.5 Conclusions .............................................................................................................................12

3. METHODS....................................................................................................................................14

3.1 Study Area ...............................................................................................................................14

3.2 Site Selection...........................................................................................................................18

3.3 Instrumentation........................................................................................................................20

3.4 Measurement of Ditch Flow....................................................................................................24

3.5 Measurement of Stream Flow..................................................................................................24

3.6 Data Analysis...........................................................................................................................27

4. RESULTS......................................................................................................................................30

4.1 Site, Storm, and Instrumentation Characteristics.....................................................................30

4.1.1 Site Characteristics ...........................................................................................................30

4.1.2 Storm Characteristics........................................................................................................33

4.1.3 Instrumentation Quality....................................................................................................34

4.2 Categorizing Ditch Flow Hydrology .......................................................................................37

4.2.1 Maximum Instantaneous Flow for the Road Ditches........................................................37

4.2.2 Ditch Flow Hydrology......................................................................................................39

4.3 Stream Peak Flow vs. Ditch Flow Comparison .......................................................................43

4.3.1 Maximum Instantaneous Flow for the Culverts................................................................434.3.2 Ditch Flow at Culvert Peak Flow .....................................................................................46

4.3.3 Stream Peak Flow.............................................................................................................47

4.3.4 Increases in Stream Peak Flow.........................................................................................49

4.3.5 Stream Peak Flow vs. Ditch Peak Flow Comparisons......................................................52

4.4 Total Storm Runoff Comparison .............................................................................................54


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TABLE OF CONTENTS (Continued)

Page

4.4.1 Total Runoff During Storms for Ditches ..........................................................................54

4.4.2 Total Runoff During Storms for Streams..........................................................................56

4.4.3 Increase in Total Runoff for the Storms ...........................................................................57

5. DISCUSSION................................................................................................................................60

5.1 Importance of the Road/Hillslope Interaction..........................................................................60

5.2 Flow Normalized for Area.......................................................................................................62

5.3 Cumulative Features of Stream and Ditch Flow......................................................................63

5.4 Influence of Peak Flow Timing ...............................................................................................655.5 Variability within the Watershed.............................................................................................66

5.6 Case Study in Variability.........................................................................................................71

6. CONCLUSION .............................................................................................................................73

7. LITERATURE CITED..................................................................................................................74


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LIST OF FIGURES

Figure Page

1. The different types of road cross-sections .......................................................................................8

2. A conceptual model of runoff transfer between watersheds..........................................................10

3. A conceptual model of a watershed hydrograph with and without the influence of roads.. ..........11

4. Location map of the Oak Creek watershed....................................................................................15

5. A map of the Oak Creek watershed showing watershed boundary, topography, roads, and streams

...................................................................................................................................................16

6. A map of soils within the Oak Creek watershed............................................................................17

7. A picture and schematic of the culvert and ditch installation. .......................................................21

8. A picture and schematic of the capacitance rod installation at a stream crossing culvert..............22

9. A trapezoidal flume installation in roadside ditch. ........................................................................23

10. Stage versus discharge relationship for a 24 inch (61 cm) corrugated plastic culvert .................27

11. Hydrograph comparisons for culverts 14, 34, and 35 ..................................................................35

12. The frequency distribution for maximum instantaneous flows for the ditches for the five storms

selected during the winter of 2002-2003 in the Oak Creek watershed. .....................................39

13. Hydrographs for a road segment that has ephemeral hydrology and one that has intermittent

hydrology through a storm event ...............................................................................................41

14. A graph of runoff ratios for all sixteen culverts during the five storm events..42


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LIST OF FIGURES (Continued)

Figure Page

15. Frequency distribution of maximum instantaneous flows for all 16 culverts for the five storms

selected during the winter of 2002-2003 in the Oak Creek watershed. .....................................45

16. Frequency distribution of maximum instantaneous flows for all 16 culverts for storm 1 during

the winter of 2002-2003 in the Oak Creek watershed. ..............................................................46

17. Culvert flow and ditch flow hydrographs for culvert 35 during storm 1 in the Oak Creek

watershed...................................................................................................................................47

18. Frequency distribution for percent increases in peak flows at the streams for all sixteen culverts

for the five storms selected during the winter of 2002-2003 .....................................................50

19. The differences in timing of peak flows between streams and ditches. .......................................54

20. Frequency distribution for increases in runoff volume for streams for all 16 culverts for the five

storms selected during the winter of 2002-2003 in the Oak Creek watershed...........................59

21. Graphs of cumulative stream flow and ditch flow during storm four. .........................................64

22. Hydrograph of stream flow and ditch flow at culvert 48 and the difference in timing of peak

flows. .........................................................................................................................................66

23. Scatterplots of instantaneous peak flow graphed against topographic indicators. .......................67

24. Scatterplots of storm runoff volume graphed against topographic indicators..............................69

25. Hydrographs of stream and ditch flow at culverts 31 and 35 during storm four. ........................71


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LIST OF TABLES

Table Page

1. Individual site characteristics for the sixteen stream crossing culvert locations............................19

2. Discharge equations and example variables used..........................................................................29

3. Site characteristics and averages for the sixteen culverts and adjoining road segments in the Oak

Creek watershed. .......................................................................................................................32

4. Characteristics for the five storms selected for analysis during the 2002-2003 winter at the Oak

Creek watershed. .......................................................................................................................34

5. A summary of the data quality for the monitored culvert locations...............................................36

6. Maximum instantaneous flows in the road ditches for 16 road segments in the Oak Creek

watershed during the winter of 2002-2003. ...............................................................................38

7. The runoff ratio for all 16 stream crossing culverts for all five storms .........................................43

8. Maximum instantaneous discharge measured at the invert of the stream crossing culverts for the

five storms selected during the winter of 2002-2003.................................................................44

9. Maximum instantaneous flow for culverts, normalized by area, for the five storms selected during

the winter of 2002-2003.............................................................................................................45

10. Culvert peak flow, ditch flow at culvert peak flow, and calculated stream peak flow for all 16

culverts for the five selected storms during the winter of 2002-2003 in the Oak Creek

watershed...................................................................................................................................48

11. Increases in stream peak flow by culvert and storm for the sixteen culverts and the five storms

selected during the 2002-2003 winter in the Oak Creek watershed...........................................51

12. Descriptive statistics for stream peak flow increases...................................................................52


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LIST OF TABLES (Continued)

Table Page

13. Average differences in lag to peak times for five storms during the 2002-2003 winter in the Oak

Creek watershed. .......................................................................................................................53

14. Statistics for the total runoff during the five selected storms for ditches in the Oak Creek

watershed...................................................................................................................................55

15. Total runoff values for streams during the five selected storms in the Oak Creek watershed. ....56

16. Increases in the total runoff for streams for all 16 culverts for five storms during the winter of

2002-2003 in the Oak Creek watershed. ....................................................................................58

17. Statistics for the increases in total runoff for streams for all 16 culverts for five storms during the

winter of 2002-2003 in the Oak Creek watershed. ....................................................................59

18. Differences between culverts 31 and 35 ......................................................................................72


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LIST OF EQUATIONS

Equation Page

1.....................................................................................................................................................24

2.....................................................................................................................................................25

3.....................................................................................................................................................25

4.....................................................................................................................................................26

5.....................................................................................................................................................28

6.....................................................................................................................................................49

7.....................................................................................................................................................57


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1. INTRODUCTION

As forest management continues to move away from conversion of natural forests to

plantation forestry and harvesting young forests, fewer new roads are required. Roads play an

important role in forest management, including timber harvesting and fire control but are also used

for recreation. The management of existing forest road systems, which includes road maintenance,

reconstruction, and abandonment, is an issue of growing importance and public debate. A key aspect

of road management is understanding the role of individual road segments in the hydrologic

processes of a watershed.

Forest roads have become a focus of concern and debate over the last several decades

because they can disrupt flow pathways and affect hydrologic processes in a watershed. Egan (1999)

stated, Roads are horizontal features in a landscape characterized by vertical processes. Road

surfaces have low infiltration rates relative to adjacent hillslopes and can produce overland flow, a

process that rarely occurs in forested areas (Harr 1977). Roads cut through hillslopes can intercept

subsurface flow and convert it to overland flow (Megahan 1972). Roads can route the intercepted

subsurface flow and surface runoff down roadside ditches and concentrate the water at localized

drainage points.

When roads are connected to the stream network, the potential exists for greater effects on

hydrologic processes. A place where these impacts might occur is at stream crossing culverts. At

these locations, the road crosses the stream perpendicular to it and runoff from the road ditch can

flow directly into the stream. These road segments are hypothesized to be extensions of the stream

network and thus increase the drainage density of the watershed (Wemple 1994).

As a result of the ditch flow, the stream at the stream crossing culvert may have more total

runoff and higher peak flows than if the road was not there. At culverts that cross small, low-order

streams, roads can add a significant amount of runoff to the stream, and cause a peak flow in the

stream that may be due primarily to the presence of the road. Because the road-related runoff is

generated by a different runoff mechanism than stream flow, peak flows at the culvert influenced by

the road may not be synchronized with peak flows in the stream at the culvert.

The goal of this study was to determine how surface runoff from roads augments natural

stream flow at stream crossing culverts. The first objective was to measure the change in stream

flow at stream crossing culverts due to roads during storm events. The second objective was to


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determine how any increase in runoff due to roads corresponds in time to natural stream flow. To

meet these objectives ditch flow and stream flow were measured at stream crossing culverts where

road ditches drained directly into streams. The timing and magnitude of ditch flow and stream flowwere compared.

Results of this study will allow forest planners and managers to better design the placement

of roads to minimize hydrologic impacts due to stream crossings. It will also help identify road

segments that have the greatest hydrologic impact and allow for remediation, such as the installation

of additional drainage structures. This research will also be of interest to hydrologists that wish to

further understand the influence of roads on watershed hydrology.


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2. LITERATURE REVIEW

2.1 Forest Hydrology

Forest watersheds function differently than urban or agricultural watersheds. The presence

of perennial, large vegetation, both alive and dead, is one of the major reasons. Vegetation cover

intercepts precipitation, and organic material on the soil surface and in the soil leads to high

infiltration (Ice and Sullivan 1993). Water travels through a forest watershed as surface flow in

stream channels and as subsurface flow.

Topographic features and geology determine forest watershed and sub-catchment

boundaries. The outlet of the watershed is where the stream network leaves a watershed and is the

point where all water within the watershed boundaries drains (Gordon et al 1994). This is the same

for catchments. The length of streams within the watershed divided by the total basin area is the

watersheds drainage density.

At a watershed or catchment outlet, a hydrograph depicts water discharge against time. A

storm hydrograph represents a basins response to a storm event. Hewlett and Hibbert (1967)

devised a way to separate a storm hydrograph into storm or quick flow and delayed or base flow.

Storm flow directly results from a rainfall or snowmelt event, and delayed flow is from ground

water aquifers, or flow that is left after quick flow has been separated. The rate at which

precipitation becomes storm flow is dependent on the drainage efficiency of the watershed (Gordon

et al 1994).

The magnitude of peak flows for large infrequent storms currently is used to size the

openings for bridges and culverts. The magnitude of predicted and historical peak flows also guides

prediction of the extent of downstream flooding during high return interval events (Harr et al 1975).

2.2 Hillslope Hydrology

There are three ways that precipitation gets to streams: channel interception, overland flow,and subsurface flow. Channel interception, where the precipitation falls directly on the stream and

associated saturated areas, is the most direct flow pathway (Brooks et al 1997).

Overland flow can be separated into Horton overland flow and saturated overland flow.

Hortonian overland flow occurs when rainfall intensity exceeds the infiltration capacity of the soil

(Horton 1933). This process is rare in forests and does not contribute significantly to storm runoff


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(Hursh 1936; Harr 1977; Hewlett and Hibbert 1967). Saturated overland flow occurs when a water

table intersects the ground surface in variable source areas adjacent to streams (Dunne and Black

1970). Dunne and Black (1970) found that saturated overland flow occurred in hollows on thehillslope where there was an impeding soil layer and a shallow water table. Saturated overland flow

provided an important contribution to the storm hydrograph at Sleepers River Experimental

Watershed in Vermont (Dunne and Black 1970).

Subsurface flow is precipitation that flows vertically and horizontally through the

subsurface to the stream. The mechanisms of this process have been studied intensively and yet are

still poorly understood. Early work on subsurface flow was carried out at the Coweeta Hydrologic

Laboratory where Hursh and Brater (1941) concluded that storm water moving through soil,

colluvial fill, and talus slopes could reach the stream in time to contribute to the storm hydrograph.

(Hibbert and Troendle 1988; Hursh 1936; Hursh 1944).

Hewlett and Hibbert (1963) used an artificial soil slope and Weyman (1970) used a natural

hillslope to show that saturated and unsaturated subsurface flow are important components of storm

flow. They showed that after saturation of a soil the initial drainage from the soil came from

saturated subsurface flow. After a period of 5 to 6.5 days, drainage from the soil came from

unsaturated subsurface flow that contributed to a narrow saturated zone at the base of the slope

(Hewlett and Hibbert 1963; Weyman 1970). Weyman (1970) concluded that water moves vertically

in unsaturated soil until there is a reduction in permeability, i.e. a soil/bedrock interface, and then it

flows laterally until it reaches a saturated area where discharge from the slope occurs. The saturated

zone is supplied by unsaturated subsurface flow from upslope (Weyman 1970). A study of

subsurface flow in a steep watershed in the Oregon Cascades supports these concepts (Harr 1977).

During storms at the study watershed, over 38 percent of the watershed contributed to watershed

response and the lower 10 to 15 m of the slope were continuously saturated (Harr 1977).

The concept of the variable source area model for runoff generation is based on the

contracting and expanding of streamside saturated zones or source areas. The model is based on the

concept that as precipitation continues, stream flows increase due to increased runoff from

expanding variable source areas (Hewlett and Hibbert 1967; Harr 1977). TOPMODEL, a watershed

model, was developed to predict the distribution of variable source areas in a watershed and the

runoff produced (Beven and Kirkby 1979; Quinn et al 1995).

A flow pathway hypothesized by Hewlett and Hibbert (1967) to explain the rapid response

of runoff with subsurface flow is called translatory flow. Hewlett and Hibbert hypothesized that as

new water, in the form of precipitation, infiltrates it displaces old water stored as soil moisture,

which in turn displaces other water downslope until water is released into the zone of saturation.


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This translatory flow process may be regarded as a pulse in soil moisture that will migrate slowly

downhill (Hewlett and Hibbert 1967). In recent years the old/new water concept has been verified

using stable isotopes and other tracers. McDonnell (1990) used stable isotope tracers to separate oldand new water and found that pre-event water dominated storm runoff. Anderson et al (1997b) used

solutes in runoff to determine that new water contributed only 16 to 23 percent of the total storm

runoff.

Megahan (1972) observed that water infiltrated into the soil surface and flowed vertically

until it reached the bedrock surface where the water was added to a saturated layer and traveled

laterally downslope. During research using stable isotope tracers, McDonnell (1990) found that

subsurface flow did not move uniformly through the hillslope but moved through preferential flow

paths like soil macropores or pipes, which occurs more rapidly than unsaturated subsurface flow.

Other studies have shown the importance of preferential flowpaths in the contribution of subsurface

storm flow to storm runoff. A study in California found that soil pipes played an important role in

hillslope drainage especially as the soil and subsoil become saturated (Keppeler and Brown 1998).

Studies at Mettman Ridge in the Oregon Coast Range found that nearly all storm runoff from the

catchments passed through a layer of fractured bedrock (Montgomery et al 1997; Anderson et al

1997a). After traveling downhill in the bedrock, the subsurface flow exfiltrated into the overlaying

colluvium near the channel head creating a subsurface variable source area (Anderson et al 1997a).

Other research suggests that unsaturated subsurface storm flow can occur as matrix flow in

response to pressure rather than through preferential pathways. In a dye-tracer study, precipitation

on the soil surface created a head gradient that produced a pressure wave through the soil column,

increasing hyraulic conductivity and the delivery rate of water to the underlying saturated zone

(Torres et al 1998).

Subsurface flow can also be driven by differences in topography (Woods and Rowe 1996;

McDonnell 1990). Freer et al (2002) found that bedrock topography was the primary control on

patterns of storm runoff at a site in Atlanta, Georgia. Montgomery et al (1997) found spatially

discontinuous areas of saturation on a hillslope in bedrock hollows. The influence of topography and

slope on subsurface flow processes is intuitive, but Montgomery and Dietrich (2002) suggest that

for subsurface storm runoff, the hydrologic response of a catchment is insensitive to slope because

of the controlling timescale of the vertical unsaturated flow.

Knowledge regarding hillslope hydrology has increased in recent years, however the

contribution of individual subsurface flow pathways still cannot be predicted because of widespread

variability in subsurface conditions. McDonnell (2003) suggests moving away from the variable

source area concept and thinking of a watershed as a series of reservoirs. Regardless of how


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subsurface flow travels, we know that water does move through the subsurface, sometimes rapidly,

and does contribute to storm runoff.

2.3 Road Design and Road Hydrology

Forest roads are an essential part of forest management and are also used for recreation and

fire access. They may be constructed using a cut-and-fill approach, where road spoil material is

excavated and used to fill in low portions of the road bed, or a full-bench endhaul approach, where

all the road spoil is excavated and transported to a stable location. The road surface is constructed

using one of three cross-sections: 1. inslope, where the entire road surface is sloped toward the

hillslope, 2. outslope, where the entire road surface is sloped toward the fill slope, and 3. crown,

where the road surface is sloped outward from the middle of the road (Figure 1). Crowned and

insloped roads are constructed with a ditch on the cutslope side of the road. Surface water travels

along the ditch until is reaches a drainage structure such as a waterbar, culvert, or ditch-out.

Tuncok and Mays (2001) define culverts as closed conduits in which the top of the

structure does not form part of the roadway. Culverts that cross under the road are the most

common way to move water from the road ditch to the fill side of the road (Piehl et al 1988). The

design of a culvert installation is influenced by cost, hydraulic efficiency, fish-passage, and the

topography at the proposed culvert site (Debo and Reese 1995). Where a forest road intersects a

stream, a stream crossing culvert is installed. In a survey of 285 miles of forest road in Oregon,

Skaugset and Allen (1998) found 2,810 drainage points and 498 or 18 percent were stream crossing

culverts. They also found that 78 percent of all the road segments were crowned.

Forest roads are hypothesized to increase the effective drainage density in a watershed

because the roads are directly connected to streams. The ditches adjacent to forest roads can deliver

runoff directly to streams thus some argue they extend the stream network by the length of road

directly connected to the stream network (Wemple 1994). Road segments can also be connected to

the stream network through gullies that form at the outlet of ditch-relief culverts and extend to the

stream. Reid and Dunne (1984) found that 75 percent of the road drainage structures drained directly

to the streams in the Olympic Mountains of Washington. This number is high compared to other

studies. Wemple et al (1996) found 57 percent connectivity in the Oregon Cascades, Bilby et al

(1989) found 34 percent connectivity in southwest Washington, and Skaugset and Allen (1998)

found 31 percent in western Oregon. The degree of connectivity is dependent on drainage density

(higher drainage density, higher connectivity). Thus differences in connectivity may be due to

differences in catchment drainage densities. Differences in the percent connectivity found in these


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studies may also be related to the time period that the road networks were constructed. The road

segments studied by Reid and Dunne (1984) were constructed before 1968 while some of the road

segments in Wemple et al (1996), Bilby et al (1989) and Skaugset and Allen (1998) had beenconstructed during the 1980s. Croke and Mockler (2001) estimated a 6 percent increase in drainage

density in an Australian watershed when adjoining road lengths and gully-channel linkage were

included.

The road surface is highly compacted by design. Compaction increases bulk density and

decreases hydraulic conductivity. Ziegler and Giambelluca (1997) found that the saturated hydraulic

conductivities in the forest in northern Thailand were more than 200 times greater than on the road

surface. Because of higher bulk densities (reduced pore space), forest roads have lower infiltration

rates than the surrounding hillslopes and thus can produce Horton overland flow during storms

(Johnson and Beschta 1980; Ziegler and Giambelluca 1997). Ziegler and Giambelluca (1997)

observed surface runoff during all rainfall intensities during simulated rainfall tests on road surfaces.

However, not all precipitation that falls on the road surface becomes runoff. Marbet (2003) found

that in spite of low infiltration capacities for forest roads in the Oregon Coast Range (


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Figure 1. The different types of road cross-sections. Figure A represents an insloped road;

figure B, an outsloped road; and figure C, a crowned road. The arrows indicate the flow

direction of surface runoff.

A.

B.

C.


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In addition to creating and directing surface runoff to the streams, roads can intercept

subsurface flow, which alters the hydrologic pathways of water in a watershed. Subsurface flow can

be intercepted at the road cutslope and directed down the road ditch into the stream. Many studieshave documented the occurrence of interception of subsurface flow by the road cutslope (Megahan

1972; Megahan 1983; Megahan and Clayton 1983; Harr et al 1975; King and Tennyson 1984;

Wright et al 1990; Wemple 1994). Parizek (1971) attributes this phenomenon to road cutslopes that

extend below the water table. Other researchers have expanded this concept to include when

transient water tables or discontinuous saturated zones rise above cutslopes (Dutton 2000; Wemple

1998; Tague and Band 2001).

The location and amount of intercepted subsurface flow is variable throughout a watershed.

Wemple and Jones (2003) found that hillslope length, soil depth, and cutslope height explained

much of the variability in the amount of subsurface flow intercepted by cutslopes. La Marche and

Lettenmaier (2001) found no relationship between peak runoff and cutslope height of adjoining road

segments. Gilbert (2002) found no relationship between spatial variability of subsurface interception

and topographic indicators in the Oregon Coast Range. Gilbert (2002) and Marbet (2003) used the

terms intermittent and ephemeral to distinguish ditch flow at road segments where the cutslope

did or did not intercept subsurface flow.

2.4 Hydrologic Effects of Roads

Through re-routing subsurface flow and directing surface runoff, roads can change the

apparent size of the contributing area upslope of a stream crossing. When a road cuts across an

adjacent watershed, it may intercept runoff from one watershed and route it via the road ditch to a

point in the next watershed (Figure 2). The effects of water transfer from one watershed to another

are not likely to be noticeable at the higher order watershed level but at the first order watershed

scale an increase in storm runoff may be realized. Parizek (1971) states that for a small watershed, a

diversion like this may be a significant portion of the discharge from the watershed. There are other

ways that roads may change the ground-water flow of a hillslope. Parizek (1971) describes a

beheaded aquifer effect where a road intercepts groundwater that previously would have charged

an aquifer below the road. This may diminish a downslope water supply.


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Figure 2. A conceptual model of runoff transfer between watersheds. The forest road in this

illustration transfers surface runoff and subsurface flow that has been intercepted by the

cutslope from outside the watershed directly to a stream within the watershed via a stream

crossing culvert. The original watershed boundaries are defined by the solid line and the

additional drainage area is outlined with the dashed line.

One effect of roads on watershed hydrology that is debated is the effect on peak flows.

Wemple et al (1996) presented a conceptual model describing how roads may increase the

magnitude of peak flows and change the timing. Wemple et al (1996) hypothesized that peak flows

increase and move forward in time because roads expand the stream network, which converts slow

subsurface drainage to rapid surface runoff. The conceptual effects on the basin hydrograph are

shown in Figure 3. Research investigating this conceptual model has had mixed results that include

increases in peak flow, no change, to decreases in peak flow (Beschta et al 2000; Wright et al 1990;

King and Tennyson 1984; Springer and Coltharp 1980). Even when the same data set is used,

different analysis methods have produced different results (Jones and Grant 1996; Thomas and

Megahan 1998; Beschta et al 2000; Jones and Grant 2001; Thomas and Megahan 2001).

Outlet of watershed at streamcrossing culvert

Forest road

Subsurface flow is

intercepted by the roadcutslope and routed with

surface runoff down the

roadside ditch


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Figure 3. A conceptual model of a watershed hydrograph with and without the influence of

roads. Recreated from Wemple et al 1996.

The Alsea Watershed was one of the earliest studies where changes in peak flows were

hypothesized to be linked with roads. Harr et al (1975), based on data from the Alsea Watershed

study, hypothesized that if more than 12 percent of the area of a watershed was occupied by roads

then peak flows would be increased by as much as 30 percent. Another result of the study was that

although peak flows increased, there was no change in total storm runoff volume (Harr et al 1975).

Results from other studies could not confirm this result. In the South Fork of Caspar Creek no

significant changes in peak flows and total runoff volumes were detected even though 15 percent of

the watershed area was heavily compacted by roads and skid trails (Ziemer 1981; Wright et al1990).

King and Tennyson (1984) studied six watersheds with less than five percent of the

watershed area in roads with mixed results. The only significant findings regarding peak flows were

an increase in one watershed and a decrease in another. They hypothesized that the effects of roads

on peak flows were related to the timing of road related runoff entering the stream. This led to the

conclusion that even with a small percent of a watershed area in roads, the hydrologic behavior of

the watershed could be increased when peak flows from the road and stream are synchronized (King

and Tennyson 1984).

Road location has also been stressed as a factor affecting the magnitude of the influence ofa road on watershed hydrology. Wemple et al (2001) hypothesized that mid-slope roads were the

road segments that influenced peak flows the most because these segments are most likely to

intercept subsurface flow. Jones (2000) provided support for this hypothesis by relating the

magnitude of increases in peak flows observed in seven basins to the density of midslope roads

within these basins.

Time

Discharge

With Roads

Without Roads


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12

Jones and Grant (1996) hypothesized that the hydrologic changes due to roads increase

with the increase in watershed size. They found higher peak discharges and earlier hydrograph begin

times following road construction in the small watersheds, although not statistically significant.Jones and Grant (1996) also suggested that a combination of forest harvesting and roads could

increase peak flows by 100 percent in the large basin pairs. Using the same data but a different

statistical analysis, Thomas and Megahan (1998) reported that forest roads did not alter peak flows

in one of the large basin pairs and that the analysis for the other two large basins was statistically

invalid. They also reported that in the small watershed with roads, peak flows were increased up to

40 percent for the small storms, but peak flow increases decreased as flow event size increased

(Thomas and Megahan 1998). A third analysis of the data also suggested that peak flow increases

were a function of flow event size. Beschta et al (2000) did not differentiate between the effects of

roads and harvesting but reported that in the small basins, peak flow increases depended on the size

of the peak flow: the smaller peak flows had large increases in peak flow due to harvesting

procedures that included roads. In recent commentaries, it is apparent that the issue of the influence

of roads on increases in peak flow is not resolved. Further, it is complicated by the influence of

other harvesting activities (Jones and Grant 2001; Thomas and Megahan 2001; Luce and Wemple

2001).

2.5 Conclusions

A summary of the current theories from literature that pertains to this study is as follows:

The processes that convert precipitation to storm runoff include channel interception,

Horton overland flow, saturated overland flow, and saturated and unsaturated subsurface

storm flow. Of these processes, saturated overland flow and saturated and unsaturated

subsurface storm flow are considered to be the greatest contributors to storm flow in an

undisturbed forested watershed.

There are many concepts to describe how subsurface flow travels to the stream, but the

predominate concept is that it infiltrates vertically to an impermeable layer where it travels

laterally as a transient, perched water table.

Stream crossing culverts are located where roads intersect streams. At these locations,

roads are directly connected to streams.

Most forest roads in Oregon were constructed with a crowned cross section that directs

surface runoff into roadside ditches.


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13

The road surface is compacted and can produce Horton overland flow during storms.

Road cutslopes can intercept subsurface flow and convert it to surface runoff.

Forest roads can change the area and shape of upslope contributing areas at low-orderstream crossing culverts.

Forest roads may change the hydrology of watersheds. This may change the timing and

magnitude of peak flows especially for low order streams. Understanding and predicting

peak flow is important for designing adequate culverts, bridges, and road infrastructure and

for interpreting channel response to management.


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14

3. METHODS

3.1 Study Area

This study took place in Oak Creek, an 824 hectare watershed near Corvallis, Oregon. Oak

Creek is part of the McDonald/Dunn Research Forest, which is the school forest for the College of

Forestry at Oregon State University (Figure 4). Elevations within the watershed range from 140 to

over 600 meters and hillslope gradients range from 20 to over 60 percent. The study watershed is

the drainage area above a concrete structure located near the southwest entrance to the Research

Forest. The Research Forest is an actively managed forest.

There are 4,877 meters of stream and 4,572 meters of road within the watershed resulting

in a drainage density of 5.92 m/ha and a road density of 5.55 m/ha. A majority of the roads were

constructed during the 1950s and 1960s using cut-and-fill methods, however, the drainage on the

road system has been significantly upgraded in recent years. The roads are, on average, five meters

wide, have unpaved crowned surfaces, and roadside ditches. There are 99 drainage structures within

the study watershed that are currently part of the road network. Of these structures, 23 (23.2%) are

stream crossing culverts and the remaining 76 (76.8%) are drainage relief culverts (Figure 5). A

stream crossing culvert is defined as a culvert that ran surface water at least part of the year and was

directly connected to the stream channel as evidenced by a defined channel for at least 10 meters

upslope of the culvert invert and a defined channel below the culvert outlet that converged with

another stream channel.

The parent material for the McDonald-Dunn Research Forest is the Siletz River Volcanics,

a basalt formation (Knezevich 1975). The Siletz Volcanics underlie the Jory, Price, Ritner, Witzel,

Dixonville, and Philomath soil series. In the northwest corner of the watershed, the Flourney

Formation, a Tyee sandstone, is the base for the Dupee and Hazelair soil series. Recent alluvium in

the valley bottom forms the basis for the McAlpin and Abiqua soil series. Figure 6 shows the

location of these ten soil series within the watershed. These soil series range from coarse-grained

mineral to highly organic soils. The most common soil texture is silty clay loam with an average soil

depth of 125 centimeters. Soils have high bulk densities, 1.10 - 1.30 g/cm3 and low porosities

(Knezevich 1975).


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j 15

Figure 4. Location map of the Oak Creek watershed.

McDonald-Dunn

Research Forest

Oregon, USA


30/92

N

I ,L e j e n d

S t u d y S i t e sC u l v e r t s* R a i n G a u q e s- S t r e a m s- R o a d s

3 0 m C o n t o u r s

16

Figure 5. A map of the Oak Creek watershed showing watershed boundary, topography,

roads, and streams. The location of stream crossing culverts included in this study are labeled

with numerals. The locations of rain gages are shown with stars.


31/92

L e g e n d- 3 t r e a r i s- O I L T Y P EA b i q u aD i x o n v i l l

D u p e eH a z e l a i rJ U r yM c . A l p i nP h o r r L t hP r i c ert J n k n o p

W t z e lS l o p e F a i l u r e

C

17

Figure 6. A map of soils within the Oak Creek watershed. See the Benton County Soil Survey

(Knezevich 1975) for soil descriptions.


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18

The Oregon Coast Range and the Oak Creek watershed in particular experience cool, wet

winters and warm, dry summers. Mean annual precipitation is approximately 1400 mm falling

mainly between October and May. Rainfall intensities for the Oregon Coast Range are low with arecorded maximum 2-year return interval 1-hour intensity from a thirteen-year record of 16.5 mm/hr

(Marbet 2003). Snow occurs at higher elevations within Oak Creek but melts quickly and does not

persist for more than days. Gilbert (2002) documented a transient snow zone in the Oregon Coast

Range at 650 meters (Gilbert 2002).

Overstory vegetation in Oak Creek is Douglas-fir (Pseudotsuga menziesii) with some

Grand fir (Abies grandis), Oregon white oak (Quercus garryana), and bigleaf maple (Acer

macrophyllum). Red alder (Alnus rubra) and Oregon Ash (Fraxinus latifolia) are located along

streams. Understory species are of California hazelnut (Corylus cornuta var. californica), trailing

blackberry (Rubus ursinus), oceanspray (Holodiscus discolor), common snowberry

(Symphoricarpos albus), vine maple (Acer circinatum), poison oak (Rhus diversiloba), sword fern

(Polystichum munitum), and bracken fern (Pteridium aquilinum).

Forest management within the watershed varies by location. There are no-harvest zones

along the riparian area and in upper elevation spotted owl (Strix occidentalis caurina) habitat. The

northwest area of the watershed is in even-aged, long-rotation management. The remainder of the

watershed is in even-aged stands but is managed for uneven-aged stands with overstory trees that

have diameters 91 to 107 centimeters (36 to 42 inches) at breast height (DBH).

3.2 Site Selection

There are twenty-three stream crossing culverts in the Oak Creek watershed and sixteen

were chosen for study. The most important variables that guided selection were hydraulic control,

road maintenance, and road use. Only inlet controlled culverts could be used to facilitate discharge

calculations. The maintenance of the road surface influences the amount of surface runoff that runs

into the roadside ditch and thus was considered a selection criterion. Of the seven stream crossing

culverts that were not chosen for study, one was outlet controlled, five were located on abandoned

roads, and the last stream crossing culvert did not have a roadside ditch. The culverts were not

selected randomly and thus, inferences outside of Oak Creek will not be possible, however as

individual case studies, the culverts characterize the variability within this watershed.

The location of each culvert was found using a Trimbel hand-held Global Positioning

Systems (GPS) unit. These locations were placed on a 6-meter Digital Elevation Model (DEM)

developed from Laser Altimetry (LIDAR) data. The elevation, adjoining ditch length, road gradient,


33/92

19

and upslope contributing area for each site were generated using ArcView GIS 3.2 (Environment

Systems Research Institute Inc.). The results are shown in Table 1. Culvert characteristics that

included diameter and construction material were recorded in the field. Fourteen of the culverts hadeither 24 or 18 inch (61 or 46 centimeter) diameters and eleven of the culverts were constructed of

corrugated plastic.

Table 1. Individual site characteristics for the sixteen stream crossing culvert locations.

Site

#

Elevation

(m)

Adjoining Ditch

Length (m)

Contributing

Upslope Area (ha)

Road Gradient

(%)

2 150 68 11.3 2.2

4 162 114 8.3 3.811 195 40 4.3 11.6

14 204 153 34.7 9.5

31 368 61 3.0 4.4

34 383 122 5.5 10.2

35 395 86 3.4 3.6

48 199 97 2.4 11.0

50 236 121 1.4 11.5

51 251 27 13.3 6.9

56 183 143 14.2 8.1

63 220 192 112.0 9.6

74 172 36 5.7 0.8

84 501 210 5.2 1.0

89 498 55 2.7 6.7

97 464 67 2.0 0.5

Avg. 286 100 14.3 6.3


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20

3.3 Instrumentation

Capacitance rods were installed at the invert of the sixteen stream crossing culverts. A

trapezoidal flume was installed in the adjoining ditch just uproad from the culvert (Figure 7). The

road dips at culvert 14 so flumes were installed in the ditches on both sides of the stream crossing

culvert. Four tipping-bucket rain gages were located throughout the watershed: a southern one in a

lower elevation meadow, a northern one in a mid-slope clear-cut, and one each on watershed peaks

in the east and west. The locations of the stream crossing culverts selected for this study and the rain

gages are shown in Figure 5.

Half-meter capacitance rods (TruTrack Ltd., New Zealand) were used to measure and

record water level at the culverts. Polyvinyl chloride (PVC) casings with slits that allowed for water

entry were attached to fence posts at the invert of each culvert. The capacitance rods hung from thetop of the casings. Figure 8 shows a picture and a schematic of a culvert installation. The

capacitance rods recorded water height at 10-minute intervals throughout the winter and were

downloaded once a month using a portable computer. When water was running in the culverts the

water level in the culvert was measured by hand using a ruler and compared with the water height

measured by the capacitance rod for quality assurance.


35/92

21

Figure 7. A picture and schematic of the culvert and ditch installation.

Ditch

Fillslope

Road

Cutslope

Culvert

Capacitance

Rod

Flume


36/92

22

Figure 8. A picture and schematic of the capacitance rod installation at a stream crossing

culvert.

Culvert

Capacitance rod insidecasing


37/92

23

Large 60 V-notch trapezoidal flumes (Tracom Incorporated, Alpharetta, Georgia) as

described by Robinson and Chamberlain (1960) were installed in the road ditches at each culvert.

Plywood boards that were at least 15 cm deeper than the bottom of the flume and 15 cm wider thanthe ditch were fastened to the upstream side of the flumes. When the flume was installed, the

headboard was buried in the walls and bottom of the ditch so that the flume lined up with water

travel in the ditch and the bottom of the flume was level with the ditch surface. The flumes were

leveled and secured with fence posts. A polyvinyl chloride stilling well was attached to the flume

and a capacitance rod was installed in it. A picture of a flume installation is shown in Figure 9. By

design, water levels in the stilling wells were equal to water levels at the discharge calculation point

within the flume. This was verified in the laboratory and in the field by comparing hand

measurements of water level to capacitance rod measured water levels within the stilling well.

Capacitance rods were again set on 10-minute recording intervals and downloaded monthly.

Sediment that had deposited within the flume was removed during the monthly downloads.

Tipping-bucket rain gages with 8-inch (20 cm) openings (NovaLynx Corporation, Grass

Valley, California) were located at four spots throughout the watershed. These locations varied by

elevation and aspect. Hobo data loggers (Onset Computer Corporation, Bourne, Massachusetts) in

the rain gages recorded each tip, which measured 0.01 inches (0.0254 cm) of precipitation.

Precipitation data were downloaded monthly.

Figure 9. A trapezoidal flume installation in roadside ditch.


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24

3.4 Measurement of Ditch Flow

During the winter of 2001-2002, trapezoidal flumes were installed in the adjoining ditches

at five culverts. During the summer of 2002 trapezoidal flumes were installed in the adjoining

ditches at the remaining eleven culverts. Data were collected at all sixteen flumes through the winter

of 2002-2003 (October through May). Discharge in the flumes was calculated using the equation:

58.255.1 hQ =

Equation 1

where Q is discharge (m3/s) andh is water height in the flume (m) (Robinson and Chamberlain

1960). This equation was provided by the flume manufacturer as the discharge rating curve for the

flume (Tracom Incorporated, Alpharetta, Georgia).

3.5 Measurement of Stream Flow

Stage or water level was measured at the stream crossing culverts by the capacitance rods

throughout the winters of 2001-2002 and 2002-2003 (October through May). Discharge was

measured using three methods to develop stage versus discharge relationships for the stream

crossing culverts.

The first method used to measure discharge was a bucket and stopwatch. A bucket was

held at the outlet of a culvert for a specified length of time and the volume of water was measured.

Discharge was calculated by dividing the volume of water collected by the collection time.

Discharge was correlated with the water level measured by a capacitance rod at the invert of the

culvert at the time the discharge measurement was made. This method was feasible only for

discharges less than 10 liters/second. Water height to discharge data points measured with this

method are labeled as manual in Figure 10.

For the second method of measuring discharge, a trapezoidal flume was installed

temporarily in a roadside ditch immediately above a culvert. The culvert was located near a stream

where water could be pumped from the stream to the ditch. A capacitance rod was installed to

measure water level at the invert of the culvert. Once the equipment was positioned, water was

pumped into the road ditch at several discharges and the flume was used to quantify the discharge.

These discharge values were correlated with the water level measured by the capacitance rod for

that flow rate. This method was used on seven culverts. These culverts included; two 24 inch (61

cm) corrugated plastic pipes, three 18 inch (46 cm) corrugated plastic pipes, one 18 inch (46 cm)


39/92

25

smooth plastic pipe, and one 15 inch (38 cm) corrugated plastic pipe. The data were used in the

general rating equation and are shown in Figure 10, labeled as flume data. This method was flow-

limited by pump capacity that varied based on pumping distance and elevation head. The testingtook place in the summer while the streams at the two 24 inch (61 cm) culverts were flowing. The

other five culverts remained dry through the testing period. At the 24 inch culverts, measurements of

stream discharge (without pumped water) were taken periodically during the pumping tests with a

bucket and stopwatch at the outlet of the culvert. Stream discharges remained constant during the

pumping tests and they were added to the discharges from the pumping tests.

The third method used to measure discharge was a tracer dilution method (Rantz 1982).

Sodium (salt) was used as the tracer. This technique was used to measure discharge at rates higher

than 6 liters/second. A discrete volume of dissolved salt (NaCl) was injected into the stream

upstream of a culvert. Electrical conductance was measured at the culvert outlet using a conductance

probe (YSI Incorporated, Yellow Springs, Ohio). Specific conductance was recorded at 1-second

intervals until the conductance values returned to background. A calibration curve was developed

between electrical conductance (in S/cm) and NaCl concentration (in mg/l) allowing electrical

conductance to be used as a surrogate for NaCl concentration. Discharge was calculated using the

equation:

( )

=

0

11

dtCC

CVQ

b

Equation 2

The denominator, ( )

0

dtCCb , is approximated by the equation:

( )( )=

+ N

i

iibi ttCC

1

11

2

Equation 3

In these equations, Q is the discharge of the stream, V1 is the volume of the tracer injected, C1 is theconcentration of the tracer injected, Cis the measured tracer concentration at time t at a downstream

sampling site, Cb is the background concentration in the stream, tis time, i is the number of the

sample,Nis the number of samples, andti is the time when a sample, Ci was obtained (Rantz 1982).

Discharge was calculated and then correlated with the water level measured by the capacitance rod

at the culvert invert.


40/92

26

The objective of the stream flow measurement was to develop a single rating equation for

culverts that would allow discharge for any sized culvert to be calculated based on the water level at

the culvert invert. Rather than develop individual rating curves for each culvert monitored in thisstudy, the hope was to take advantage of the fact that all of the culverts that were monitored had

inlet-controlled culverts. Thus, the possibility existed to treat the culvert as a weir and develop a

single rating equation.

The first attempt at a generalized equation was the result of an empirical approach by the

Federal Highway Administration (FHA) (Normann et al 1985). In general, this method

overestimated discharge and at high flows drastically overestimated discharges (Figure 10). The

Chezy equation (Chanson 1999) and Mannings Equation (Mott 1994) were also used to predict

discharge at the culverts. Both equations, as expected for an inlet control condition, underestimated

discharge and were not used.

Finally, an empirical equation developed by Henderson (1966) was used. The form of

Hendersons equation used in this analysis was found in Chanson (1999) and takes the form:

6.09.1432.0 DHgQ =

Equation 4

where Q is discharge in m3/s, g is the acceleration due to gravity in m/s2,His the height of the water

above the culvert invert in m, andD is the culvert diameter in m. This is an empirical equation and

is valid when the water depth to culvert diameter ratio is less than 0.8 and the slope of the culvert

barrel is less than 36.1 percent (Henderson 1966). Table 2 shows sample calculations for a 24 inch

(61cm) culvert using the equations of Henderson, Manning, Chezy, and the Federal Highway

Administration. Discharge was calculated using Hendersons equation and water levels measured at

the culvert invert using capacitance rods. The calculated discharge values were graphed versus the

measured discharge values (Figure 10). Discharge values predicted using Hendersons equation

matched the actual values the best.


41/92

27

0

50

100

150

200

250

300

350

400

450

0 20 40 60 80 100

Discharge (l/s)

StageHeight(mm)

Chezy

Manning

Henderson

FHWA

Flume Data

Tracer

Manual

Figure 10. Stage versus discharge relationship for a 24 inch (61 cm) corrugated plastic culvert.

The graph shows the measured discharge values from three methods of measurement (flume,

tracer, and manual) and the predicted discharge values using four equations (Chezy,

Manning, Henderson, and FHA).

3.6 Data Analysis

Stream flow was defined as flow in the stream just upstream from the stream crossing

culvert. Ditch flow was defined as flow in a roadside ditch that was measured by a trapezoidal flume

before entering the stream at the stream crossing culvert. Culvert flow was defined as flow measured

at the culvert invert. Culvert flow included stream flow and ditch flow.

The four rain gages in the Oak Creek watershed recorded cumulative precipitation in

increments of 0.01 inches (0.0254 cm). These data were binned into 10-minute, half-hour, and hour

time intervals using a Microsoft Excel macro written by Richard Keim and graphed as

hyetographs (precipitation through time). The storms that produced the largest flows in Oak Creek

were selected from the precipitation record. This was accomplished by determining the largest peak


42/92

28

flows from the winter 2002-2003 Oak Creek hydrograph and matching the storm hydrographs to

precipitation. Storm duration, total storm precipitation, and maximum storm intensities for 15-

minute and 1-hour intervals were calculated.Five storms during the 2002-2003 winter were selected. Hydrographs of stream flow and

ditch flow were prepared at all culvert locations for each selected storm. Discharge from the stream

at the stream crossing culverts was matched in time to the road runoff from the adjacent roadside

ditches. Ditch flow was subtracted from culvert flow to give an estimate of the discharge in the

stream without the influence of the road.

Culvert Flow = Stream flow + Ditch flow

Solving for stream flow:

Stream flow = Culvert flow Ditch flow

Equation 5

Instantaneous maximum discharge in the streams and the corresponding values in the

ditches were compared as well as total runoff. Total runoff for the streams during storms was

calculated using the hydrograph separation technique described by Hewlett and Hibbert (1967).

Total runoff for the ditches was calculated as the volume of all water that passed through a ditch

flume during a storm event.The differences in timing between the peak flow at the stream and the peak flow at the

ditch were compared. Lag to peak, as described by Montgomery and Dietrich (2002), is the time

elapsed between when half the storm rainfall has fallen to the peak discharge. Lag to peak was

analyzed at each culvert and lag to peak times in the ditches were compared to lag to peak times in

the corresponding streams.


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29

Table 2. Discharge equations and example variables used. Example is a 24 in (61 cm)

corrugated plastic pipe at a 3% gradient running water at a stage height of 6 in ( 15.2 cm).

K,M:

Empirically derivedconstants 0

.519,

0.64

A2: Cross sectional area of

culvert (m2) 1.915

C:

Chezy co-efficient 54

S:

Slope of the channel (%) 3 3

R:

Hydraulic radius; area

divided by the wettedperimeter (m) 0

.088

0.088

A1: Cross sectional area of

water at the culvert invert

(m2) 0.055

0.055

n: Mannings roughness

value 0.027

D:

Diameter of culvert (m)0.61

0.61

H,

Hw:

Water height at culvert

invert (m) 0.152

0.152

VariablesUsed

g:

Gravity (m/s2)9.81

EquationForm:

Q=0.432g0.5H

1.9D

0.6

Q=n-1A1R2/3S1/2

Q=A1C(RS)1/2

Q=(HWD-1A2DM/2)1/M

EquationName:

Henderson

Manning

Chezy

FederalHighway

Administration


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30

4. RESULTS

All inlet-controlled stream crossing culverts on roads in the Oak Creek watershed and their

adjoining ditches were monitored from October 2002 through May 2003. Five storms that occurred

during that time period were analyzed. The interaction of the road with subsurface flow from the

hillslope caused the hydrology of the road segment to be classified as either intermittent or

ephemeral. Peak flow and total runoff at the stream crossing culverts were compared with the

magnitude and timing of peak flow and total runoff in the adjoining ditch.

4.1 Site, Storm, and Instrumentation Characteristics

At each stream crossing culvert some physical characteristics were measured in the field

and some were calculated in ArcView GIS 3.2 (Environment Systems Research Institute Inc.)

using a 6-meter grid developed from LIDAR data. Storm characteristics were generated from

tipping-bucket rain gage data. Data from a rain gage located in a lower-elevation meadow within the

Oak Creek watershed (see Figure 5) were used for all storm analysis because this rain gage

consistently produced reliable data. Capacitance rods recorded data throughout the winter of 2002-

2003 but instrument malfunction at some culvert locations created gaps in the data set.

4.1.1 Site Characteristics

Within Oak Creek, 16 stream crossing culverts and their adjoining ditches were monitored.

At one culvert location, culvert 14, the road on both sides of the stream crossing culvert drained to

the culvert, thus at that culvert ditch flow was measured in both ditches. Characteristics of the

culvert locations, which include elevation, length of the adjoining road and ditch, average road

gradient, upslope contributing area for the stream, upslope contributing area and contributing road

surface area for the adjoining ditch, average hillslope gradient, average soil depth, average cutslope

height, and topographic index, are listed in Table 3. The elevations for the stream crossing culverts

averaged 286 m and ranged from 150 to 501 meters. The length of the adjoining road and ditch

averaged 100 m and ranged from 27 to 210 meters. The gradient of the contributing road averaged

6.3 percent and ranged from 0.5 to 11.6 percent. Twelve of the sixteen adjoining road segments had

gradients that were less than 10 percent. Drainage area for the stream crossing culverts averaged

13.4 ha and ranged from 0.9 to 110.7 hectares. The contributing area for the ditch included the

contributing surface area of the adjoining road and the upslope contributing area. Values for the

contributing area of the ditch averaged 1.3 ha and ranged from 0.30 to 3.89 hectares. Hillslope

gradient ranged from 4.5 to 55 percent with an average of 30 percent. The average soil depth for the


45/92

31

sites ranged from 0.4 to 1.5 meters. Cutslope heights averaged between 0.4 and 3.2 meters. The

topographic index is calculated as the area of the hillslope contributing to the stream at the stream

crossing culvert in hectares divided by the gradient of the hillslope in degrees (Beven and Kirkby1979). The topographic index for the study sites ranged from 0.03 to 6.96.


46/92

To

Average

CutslopeHeight (m)

0.42

1.01

0.86

3.21

2.05

2.25

1.39

0.57

0.61

1.18

1.48

2.38

0.50

0.75

0.64

0.47

1.24

Average

Soil Depth(m)

1.52

0.76

1.27

1.27

0.76

0.76

0.76

0.41

0.41

1.27

1.37

1.52

0.41

1.27

0.41

1.52

0.98

Average

Hillslope

Gradient(%)

41.7

44.8

39.0

44.8

20.5

13.3

12.2

18.2

14.8

13.1

33.8

28.5

12.7

54.5

33.3

55.3

30.0

Upslope

Contributing

Area forDitch (ha)

1.16

2.63

1.16

2.04

0.79

0.61

0.48

0.50

0.60

0.74

1.44

1.40

0.30

3.89

1.28

1.18

1.26

Upslope

Contributing

Area forStream (ha)

10.2

5.7

3.1

32.8

2.2

5.0

3.0

1.9

0.9

12.6

12.8

110.7

5.4

5.2

1.4

0.9

13.4

Average

Road

Gradient(%)

2.2

3.8

11.6

9.5

4.4

10.2

3.6

11.0

11.5

6.9

8.1

9.6

0.8

1.0

6.7

0.5

6.3

Contributing

Road and

Ditch Length(m)

68

114

40

153

61

122

86

97

121

27

143

192

36

210

55

67

100

Elevation(m)

150

162

195

204

368

383

395

199

236

251

183

220

172

501

498

463

286

Culvert

2

4

11

14

31

34

35

48

50

51

56

63

74

84

89

97

Average


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4.1.2 Storm Characteristics

Five storms from the winter of 2002-2003 were analyzed. The storms were chosen by first

displaying the annual hydrograph for the Oak Creek watershed within the McDonald-Dunn

Research Forest. The largest storms on the hydrograph for the watershed were isolated and then

were matched to precipitation data from the watershed. The hydrographs for these storms were

isolated from the runoff record for each of the stream crossing culverts that was being studied. The

recurrence interval for each storm was estimated by matching peak flows at the Oak Creek

watershed to peak flows at nearby USGS (United States Geological Survey) gauged watersheds.

These watersheds are the Marys River near Philomath, Oregon (USGS site number 14171000) and

the Luckiamute River near Suver, Oregon (USGS site number 14190500). Cumulative frequency

distribution curves for peak flows were developed for the watersheds based on 44 years of data for

Marys River and 81 years for the Luckimute (Wellman et al 1993). Comparison of the five peak

flows from Oak Creek during the winter of 2002-2003 with the frequency analysis for Marys River

and the Luckimute River showed that the five storms during 2002-2003 that were studied were sub-

annual events with nearly a 100 percent chance of occurring annually.

The characteristics of the storms are listed in Table 4. Storm 1 began at 18:11 on December

29th, 2002 and ended at 23:06 on December 30th, 2002; a duration of 28.9 hours with 55.9 mm of

total precipitation. The peak 1-hour intensity was 5.8 mm/hr and the peak 15-minute

intensity was 7.1 mm/hr. Storm 2 began on February 16th, 2003 at 08:13 and lasted 54.6 hours with

49.0 mm of precipitation. The peak 1-hour intensity was 4.5 mm/hr and the peak 15-minute

intensity was 6.1 mm/hr. Storm 3 was the longest storm with a duration of 70.1 hr and had the

highest total precipitation with 79.5 mm. This storm began at 11:18 on March 5th, 2003 and ended at

09:24 on March 8th, 2003. Peak intensities for the storm were 6.0 mm/hr for the 1-hour intensity and

8.8 mm/hr for the 15-minute intensity. Storm 4 produced the highest 15-minute peak intensity at

10.1 mm/hr. This storm began March 20th, 2003 at 23:38 and lasted 61.5 hours. Total precipitation

for storm 4 was 53.6 mm and the peak 1-hour intensity was 5.3 mm/hr. Storm 5 was the smallest

and the shortest of the storms. It began on April 5th, 2003, at 08:44, lasted 19.7 hours, and produced

26.4 mm of total precipitation. Peak intensities were also the lowest for this storm. The peak 1-hour

intensity was 3.6 mm/hr and the 15-minute peak intensity was 5.5 mm/hr.


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Table 4. Characteristics for the five storms selected for analysis during the 2002-2003 winter

at the Oak Creek watershed.

StormCharac teris t ics Sto rm 1 Sto rm 2 Sto rm 3 Sto rm 4 Sto rm 5

Start Time 12/29/02 18:11 2/16/03 8:13 3/5/2003 11:18 3/20/03 23:38 4/5/2003 8:44

End Time 12/30/02 23:06 2/18/03 14:50 3/8/2003 9:24 3/23/03 13:04 4/6/2003 4:26

Duration (hr) 28.9 54.6 70.1 61.4 19.7

Total Precipitation

(mm) 55.9 49.0 79.5 53.6 26.4

Peak Intensities

1 hour (mm/hr) 5.8 4.5 6.0 5.3 3.6

15 min (mm/hr) 7.1 6.1 8.8 10.1 5.5Storm Return

Interval (yr)


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selected storms. The rain gages are not designed to handle snow and so these data were not accurate.

Also, two of the higher elevation gages had battery failures that occurred intermittently.

Precipitation data for all analysis were taken from the lower-elevation meadow rain gage, whichproved to be the most consistent and accurate throughout the selected storms.

Figure 11. Hydrograph comparisons for culverts 14, 34, and 35. The top panels show

precipitation hyetographs with millimeters of rainfall per hour (y-axis) for the given time

period and the bottom panels show the stream hydrographs for culverts 14, 34, and 35 with

stage in millimeters (y-axis) through time (x-axis). The left panels show a two day stormduring January 2002. Notice that the stream at all three culverts react similarly in response to

precipitation. The right panels show the same culverts over a six day period in March 2003.

Although culverts 14 and 34 seem to track each other, the hydrographs do not seem to

respond to precipitation and are remarkably different from the hydrograph for culvert 35.

0

50

100

150

200

250

300

350

1/24/02 1/25/02 1/26/02

14

34

35

0

2

4

6

8

0

20

40

60

80

100

120

140

160

180

3/7/03 3/9/03 3/11/03 3/13/03

14

34

35

0

2

4

6

8

Stage(mm)

Precipitation(mm/hr)


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Table 5. A summary of the data quality for the monitored culvert locations. The shaded areas

are where data were removed from analysis, or where data do not exist.

Site

Storm 1

12/29/02

Storm 2

2/16/03

Storm 3

3/5/03

Storm 4

3/20/03

Storm 5

4/5/03

culvertditch

culvertditch

culvertditch

culvertditch

culvertditch No Flow

culvertditch

culvertditch

culvertditch Data Lost

culvertditch

culvertditch

culvert Bad Dataditch

culvertditch

culvertditch

culvert Data Lost Data Lostditch Data Lost No Flow

culvertditch

culvert No Flow Data Lost No Flowditch No Flow No Flow

Included

Instrument malfunction for entire winter

2

4

11

14

31

34

Instrument malfunction

35

48

50

51

56

63

74

84

89

97

no flow occuredremoved because of lost data


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4.2 Categorizing Ditch Flow Hydrology

Runoff from the road surface and from the interception of subsurface flow was measured

using trapezoidal flumes in the road ditches that were adjacent to the stream crossing culverts. The

maximum instantaneous flow for the ditch for each site during each storm was determined. Based

on findings from Gilbert (2002) and Marbet (2003) the ditch flow hydrology was characterized as

either ephemeral or intermittent.

4.2.1 Maximum Instantaneous Flow for the Road Ditches

Ditch flow hydrographs through each storm were produced and analyzed. The greatest

discharge measured at ten-minute intervals in the storm hydrograph was the maximum

instantaneous flow for the ditch or peak flow for the ditch. This value varied from no flow (the

ditches at culverts 31 and 84 during storm 5 and the ditch at culvert 97 during storms 2 and 3) to

14.4 l/s (Table 6).

The values of maximum instantaneous flows for the ditches measured in the Oak Creek

watershed are consistent with values of ditch peak flows in other studies. In the Oregon Coast

Range, Gilbert (2002) and Marbet (2003) also used trapezoidal flumes to measure ditch flow.

Gilbert measured ditch peak flows from 0.1 to 7.0 l/s for six sites over four storms during the 1999-

2000 winter. The sites had similar physical characteristics as the sites in the Oak Creek watershed

and the storms studied did not exceed a 2-year recurrence interval (Gilbert 2002). Marbet used one

of the same sites and measured ditch peak flows for five sites in the Oregon Coast Range that

ranged from 0.0 to 1.0 l/s. During storms 3 and 4 over 50 percent

of the maximum instantaneous flows for the ditches fell in this range (9 of 16 ditches for storm 3

and 8 of 15 ditches for storm 4). There were four road segments that had maximum instantaneous

flows for the road ditches of over 10 l/s. Three of these peak flows occurred during storm 1 and the

fourth occurred during storm 2 at culvert 50. Maximum instantaneous flows for the ditches for

storm 1 ranged from


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ditches for storm 4 ranged from


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0

1

2

3

4

5

6

78

9

10

0.0 0.1 -

1.0

1.1 -

2.0

2.1 -

3.0

3.1 -

4.0

4.1 -

5.0

5.1 -

6.0

6.1 -

7.0

7.1 -

8.0

8.1 -

9.0

9.1 -

10

>10

Peak Flow (l/s)

Frequency

Storm 1

Storm 2Storm 3

Storm 4

Storm 5

Figure 12. The frequency distribution for maximum instantaneous flows for the ditches for the

five storms selected during the winter of 2002-2003 in the Oak Creek watershed.

4.2.2 Ditch Flow Hydrology

Based on the results of Gilbert (2002) and Marbet (2003), the first step during the analysis

of flow data was to characterize the hydrology of the ditch flow. Gilbert (2002) and Marbet (2003)classified the hydrology of ditch flow as either ephemeral or intermittent. Hydrographs of ditch

flow that exhibit ephemeral hydrology are flashy with steep rising and falling limbs (middle panel

Figure 13). Flow in these ditches is hypothesized to be dominated by road surface runoff, and flow

occurs in direct response to precipitation and ceases when precipitation ceases. Ditch flow classified

as intermittent also responds to precipitation, but the hydrograph has more gentle slopes and

attenuated falling limbs. Flow for these road ditches is hypothesized to be dominated by the

interception of subsurface flow. Flow in ditches with intermittent hydrology continues after rainfall

has ceased (bottom panel Figure 13).

An objective way to classify ditch flow as either ephemeral or intermittent is to calculate a

runoff ratio (Marbet 2003). The runoff ratio is an index used to compare the hydrology of road

segments. The runoff ratio is calculated by dividing total ditch flow during a storm by the volume of

rainfall that fell on the contributing area of the road surface. Hypothetically, a runoff ratio of 1.0

represents a road segment where all of the precipitation that fell on the road surface became surface

runoff and was measured as ditch flow. This, of course, cannot occur because precipitation that


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infiltrates and flow that is not directed into the ditch (flow down a tire rut that bypasses the ditch and

culvert) are not measured. A runoff ratio of less than 1.0 implies runoff losses and a runoff ratio

greater than 1.0 implies that there are sources of flow to the ditch other than runoff from the roadsurface.

Runoff ratios were calculated for each road segment for all five storms and the road

segments were classified as having either intermittent or ephemeral hydrology (Figure 14). At the

road segments with intermittent hydrology, total storm runoff greatly exceeded the volume of

thesis road hydrology

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