WATER QUALITY MODELING
TOOLBOX PROJECT
Assessment of Tahoe Basin Roads:
Road Shoulder Condition, Connectivity,
Compaction, and Pollutant Generation Risk
March 2011
Prepared by:
northwest hydraulic consultants
TABLE OF CONTENTS
1.0 BACKGROUND AND PURPOSE ........................................................................................ 1
2.0 DEVELOPMENT OF GIS PRODUCTS .............................................................................. 2
2.1 ROAD SHOULDER SHAPEFILE ................................................................................................. 3
2.1.1 Road Shoulder Condition............................................................................................... 3
2.1.2 Road Shoulder Connectivity .......................................................................................... 5
2.1.3 Road Shoulder Compaction ........................................................................................... 6
2.2 ROAD RISK SHAPEFILE........................................................................................................... 7
3.0 POTENTIAL USES OF GIS PRODUCTS........................................................................... 8
4.0 REFERENCES ...................................................................................................................... 10
APPENDIX A – UNDERSTANDING SHAPEFILE ATTRIBUTE TABLES...................... 11
List of Figures
FIGURE 1. ROAD SHOULDER CONDITION DECISION PROCESS ...................................... 4
FIGURE 2. DRAFT ROAD SHOULDER COMPACTION DECISION PROCESS .................... 6
FIGURE 3. CONCEPTUAL EXAMPLE INTERPRETING ROAD ATTRIBUTES ................... 9
Tahoe Basin Road Assessment
i
March 2011
1.0 Background and Purpose
The work described in this document was initiated based on the findings in the report: Analysis
of Differences in Pollutant Load Estimates Generated by LSPC and PLRM (NHC and Tetra
Tech, 2010), prepared for the Water Quality Modeling Toolbox Project (SNPLMA Grant PO23).
The previous report investigated functional differences between the Lake Tahoe Watershed
Model (LSPC) and the Pollutant Load Reduction Model (PLRM), and concluded that differences
in output generated by the two models can be reasonably explained based on differences
associated with a few key functions and data inputs. The report noted that while reconciliation
of some of the differences between the models is possible, absolute alignment of the two models
is not a realistic strategy because the models are intended to be applied on different spatial
scales.
The representation of road land uses in LSPC and PLRM was found to cause the most significant
variation in pollutant load estimates between the two models (NHC and Tetra Tech, 2010). The
PLRM is intended for application at a smaller spatial scale than LSPC, and allows more detailed
input of road conditions for estimation of pollutant loads. Key input parameters for PLRM
include road shoulder condition, impervious area connectivity, and road risk (a parameter
associated with potential for pollutant generation). As a step towards model integration and
consistency in model application, road conditions relevant to PLRM inputs were assessed in this
study for all Lake Tahoe Basin roads.
The products developed from this work are Geographic Information System (GIS) shapefiles that
characterize attributes of Tahoe Basin roads important to water quality and which are directly
applicable to load estimation in PLRM. Specifically, the shapefiles define road shoulder
condition, road shoulder connectivity, road shoulder compaction, and the relative risk of
pollutant generation (termed road risk). Products discussed herein were formulated to support
multiple stormwater planning purposes in the Lake Tahoe Basin, including the potential to better
align inputs between the LSPC and PLRM.
Section 2 describes the parameters defined and the procedures used for data collection and
compilation. Section 3 of this document discusses potential uses of the products developed.
Tahoe Basin Road Assessment
1
March 2011
2.0 Development of GIS Products
This section describes the development of the GIS products, which are two GIS shapefiles
containing the attributes noted below. The shapefiles can be downloaded from the PLRM subsite on the TIIMS website (www.tiims.org).
Road Shoulder Shapefile (February 2011): Contains attributes for Road Shoulder
Condition, Connectivity, and Compaction. This shapefile includes two line segments for
each road to characterize attributes of each road shoulder.
Road Risk Shapefile (February 2011): Refines the Road Risk attribute originally
defined in the October 2009 shapefile for Road Risk.
The reader should note the following about the products described:
•
The attributes described are specific inputs for PLRM. The discussion below references
the PLRM and documentation associated with the PLRM.
•
A road shoulder is defined as the area between the edge of the impervious travel way
(cars or bikes) and the edge of the right-of-way.
•
The Road Shoulder shapefile characterizes attributes as observed in 2010 (or in some
cases 2007 or 2009 when Google Street View was used). The designations of an attribute
may not be appropriate for a TMDL Baseline Condition pollutant load estimate, which
requires assessment of road conditions using the period of October 1, 2003 – May 1,
2004 as a basis. Note that:
o Roads within known water quality improvement project (WQIP) areas occurring
after 2004 are flagged in the Road Shoulder Shapefile.
o This flag is present to warn a user that it may be incorrect to assume that the
attributes defined in the Road Shoulder Shapefile are similar to the TMDL
Baseline Condition.
•
All products are initial estimates developed during a rapid survey of the entire Tahoe
Basin. Individual users and jurisdictions should review and revise the products when
more detailed analyses support modifications to improve accuracy.
Tahoe Basin Road Assessment
2
March 2011
2.1 Road Shoulder Shapefile
Attributes in the Road Shoulder Shapefile were defined through visual inspection of road
shoulders while applying the decision processes discussed in this section. Visual inspection was
primarily completed in the field. In some cases, Google Street View was used when significant
snow accumulation hindered field assessment. Hard copy maps were used to define road
shoulder attributes in the field. Break-points between road shoulder attributes were created when
the dominant characteristic of an attribute changed along a road shoulder. The hard copy maps
were digitized in GIS. The final GIS shapefiles were quality assured to check that the attributes
assigned in the field were correctly digitized in GIS.
While the decision processes described below provide a standard method to define road shoulder
attributes, subjective elements to each decision process remain. To promote consistency in the
final products, personnel tasked with assessing road shoulder attributes initially characterized the
same road shoulders, compared results, and discussed interpretations of each decision process.
This process was repeated until personnel conducting the road shoulder assessment consistently
interpreted each decision process in the same manner and assigned the same designation for each
road shoulder attribute.
The following subsections define the terms used to characterize each attribute and describe how
each decision process was interpreted.
2.1.1 Road Shoulder Condition
The PLRM uses four designations for Road Shoulder Condition: 1) Erodible; 2) Protected; 3)
Stable; and 4) Stable and Protected. Below are brief definitions for each designation. Expanded
definitions and examples can be found in Section 6.1.2 of the PLRM User’s Manual.
Stable – The road shoulders have physical improvements or other elements that either: 1)
promote sheet flow and avoid concentration of storm water and conveyance along the
road shoulder; or 2) stabilize the conveyance of storm water and inhibit erosion.
Protected – The road shoulders have physical features (e.g. structures, vegetation, etc) or
policies (parking ordinances) that discourage or minimize disturbance by automobiles
and snow plows. Defining a road shoulder as Protected means that the majority of the
pervious portion of road shoulder will remain undisturbed in the typical case.
Stable and Protected – The road shoulders meet the definitions of Stable and Protected
as discussed above.
Erodible – The road shoulders do not meet the definitions of Stable or Protected as
defined above
Tahoe Basin Road Assessment
3
March 2011
Road Shoulder Conditions were designated using a decision process modified from the PLRM
User’s Manual (Figure 1).
Question 1: Could storm water
runoff collect along the
road shoulder and have the
potential to cause erosion?
Yes
Question 2: Can automobiles and/or
snow plow activity disturb a significant
portion of the unpaved road shoulder
in the typical case?
No
Question 2: Can automobiles and/or
snow plow activity disturb a significant
portion of the unpaved road shoulder?
Yes
Erodible
No
Yes
Protected
Decision Point
Road Shoulder
Condition
No
Stable and
Protected
Stable
Figure 1. Road Shoulder Condition Decision Process
The following key points regarding the interpretation of the Road Shoulder Condition decision
process (Figure 1) were followed when designating this attribute:
•
Question #1: Storm water that collects in a flat, dirt road shoulder should be considered to
“have the potential to cause erosion”. Visual evidence of conveyance is not a
requirement to make this determination.
•
Question #1: If the “and” statement in the question is not satisfied the response is “No”.
For example:
o Storm water runoff that sheet flows through the road shoulder has the potential to
cause erosion; however, it is not collecting along the road shoulder. In this case
the answer to Question #1 is “No”.
o Storm water in curb and gutter is collecting along the road shoulder; however, the
curb and gutter prevents erosion. In this case, the answer to Question #1 is “No”.
•
Question #2: The statement “in the typical case” requires interpretation.
For example:
o In a rural residential neighborhood where development is dispersed, road
shoulders without visible signs of disturbance may be considered Protected
because parking disturbances are usually infrequent (typical case).
Tahoe Basin Road Assessment
4
March 2011
o Well vegetated road shoulders could feasibly be parked on. However, most
drivers will avoid parking on well established vegetation (typical case) and
therefore the road shoulders may be considered Protected.
2.1.2 Road Shoulder Connectivity
Roads are designated as either directly or indirectly connected impervious area in PLRM. This
work assumes that road shoulder connectivity can be used to infer impervious connectivity of
each side of a road (assuming the road is crowned). Below are brief definitions for impervious
area connectivity in PLRM. Additional guidance can be found in Section 7.2 of the PLRM
User’s Manual.
Directly Connected Impervious Area (DCIA) – an impervious surface draining through
a hydraulic connection to a surface water drainage system. A surface water drainage
system could be storm drain, a stream channel, a storm water treatment facility, or any
receiving water.
Indirectly Connected Impervious Area (ICIA) – impervious surface draining to a
pervious surface that promotes sheet flow, infiltration, or storage prior to overflow
discharging into a surface water drainage system.
The following key points regarding the interpretation of the Road Shoulder Connectivity
definitions were followed when designating this attribute:
•
•
•
•
Road shoulders were classified as DCIA if they contributed runoff to an earthen
(pervious) channel when the channel didn’t appear to be engineered to infiltrate, or didn’t
appear to be maintained to infiltrate.
When it appeared that runoff from the road produced shallow concentrated flow over a
compacted pervious area, as evident from the presence of a ditch or swale, the road
shoulder was typically classified as DCIA.
Road shoulders were classified as ICIA if they are normally not connected, although they
may become connected at high flows. For such areas, an assumption of DCIA is
sometimes used in event-based modeling of peak flows to support drainage design.
However, this assumption will typically over-estimate runoff volumes and pollutant
loading for long-term continuous simulation modeling.
The ultimate discharge point of a surface drainage system was not assessed in this work.
Therefore, a road segment identified as DCIA should not be assumed to be directly
connected to a receiving water (e.g., curb and gutter to a storm drain was defined as
DCIA even if the storm drain and associated conveyance terminated in a forested area).
Tahoe Basin Road Assessment
5
March 2011
2.1.3 Road Shoulder Compaction
While road shoulder compaction is not currently a PLRM input, future PLRM improvements
may incorporate new algorithms to refine infiltration characteristics of road shoulders based on
observed disturbance and compaction. In anticipation of this PLRM improvement, relative road
shoulder compaction was designated using the following categories:
Highly Disturbed – The road shoulders are not protected from disturbance and visual
evidence suggests the soil is significantly compacted from frequent parking or other
urban disturbances.
Moderately Disturbed – The road shoulders are not protected from disturbance and
visual evidence suggests the soil may be disturbed by occasional parking or other urban
disturbances.
Protected from Disturbance – The road shoulders have physical features (e.g.
structures, vegetation, etc) or policies (parking ordinances) that discourage or minimize
disturbance by automobiles and snow plows. Defining a road shoulder as Protected from
Disturbance means that the majority of the pervious portion of road shoulder will remain
undisturbed in the typical case.
Designation of a disturbance category was assigned using the decision process developed in:
Measuring Road Shoulder Saturated Hydraulic Conductivity to Inform and Refine PLRM
Algorithms (NHC, unpublished). Figure 2 illustrates the Road Shoulder Compaction decision
process.
Question 1: Is the majority of the
pervious portion of the road
shoulder Protected
from disturbance?
Yes
Protected from
Disturbance
No
Question 2: Qualitatively, what
level of disturbance do you
estimate for the pervious portion
of the road shoulder?
Occasional
Notes:
1. The “pervious portion of the road
shoulder” is the area within the
right-of-way.
2. Occasional – some evidence of
disturbance
3. Frequent– a large degree of
disturbance is evident
Moderately
Disturbed
Decision Point
Condition
Designation
Frequent
Highly
Disturbed
Figure 2. Draft Road Shoulder Compaction Decision Process
Tahoe Basin Road Assessment
6
March 2011
Recommended input values to simulate infiltration in PLRM related to the disturbance categories
described above have not been finalized. When established, recommended input values will be
published in a revised version of the PLRM User’s Manual along with guidance on their
appropriate application.
2.2 Road Risk Shapefile
Road Risk is a concept unique to PLRM that categorizes the relative potential of a road to
generate pollutants. Road Risk is defined for a road segment as High, Moderate, or Low. The
designation of High Risk signifies the poorest potential water quality in storm water runoff and
the designation of Low Risk signifies the best potential water quality in storm water runoff.
Road Risk is combined with other inputs and algorithms in PLRM to estimate the characteristic
runoff concentration (CRC) from a road in a specific condition.
The premise behind Road Risk is that physiographic characteristics influence the relative
magnitude of potential pollutants generated from a road based on the magnitude of winter road
abrasive applications and/or impacts from adjacent land uses. For example:
•
•
A school bus route is a physiographic characteristic that typically leads to a High Risk
designation because school bus routes are typically heavily sanded roads in the winter.
Roads adjacent to commercial or industrial areas are physiographic characteristics that
typically result in elevated Road Risk designations because the road shoulders are more
commonly and continuously disturbed.
The initial GIS Road Risk layer (2009) was created by automated algorithms that used slope,
adjacent land use, and traffic volumes to infer designations of High, Moderate, or Low Risk
(NHC et al., 2009b). However, this automated process did not adequately capture all
physiographic characteristics influencing winter road operations. For example, the GIS
algorithms couldn’t identify heavily sanded roads due to school bus routes or significant
problems with ice formation.
To improve upon the initial GIS Road Risk layer, each Tahoe Basin jurisdiction was provided
the opportunity to revise the initial Road Risk layer to incorporate their sanding practices and
knowledge of land use impacts on road shoulders. Engineers and maintenance personnel from
each jurisdiction were contacted, and many jurisdictions participated in the effort to refine the
Road Risk layer to better represent actual conditions and winter road operations. The
information gathered from each jurisdiction was digitized in GIS to produce a revised Road Risk
layer.
Tahoe Basin Road Assessment
7
March 2011
While the Road Risk layer produced through this work may be more representative of actual
conditions and operations among jurisdictions, Road Risk remains a subjective concept and more
work is needed to ensure Road Risk is equitably characterized among Tahoe Basin jurisdictions.
Critiques and revisions to the current Road Risk layer produced through this work are allowable
and encouraged.
3.0 Potential Uses of GIS Products
1) Potential to better align LSPC Watershed Model and PLRM inputs (if desired):
The data sets produced characterize all Tahoe Basin roads (roughly 700 miles of roads)
and therefore can inform differing spatial scales of analyses. The LSPC Watershed
Model estimates loads at the subwatershed scale and the PLRM estimates loads at the
water quality improvement project (WQIP) scale.
The attributes in the data sets can be used to directly inform the current version of PLRM.
For the LSPC Watershed model, a secondary GIS analysis could be conducted using the
attributes within the data sets to estimate characteristic runoff concentrations predicted by
PLRM algorithms at the subwatershed scale. These estimates could provide estimates
runoff concentrations for roads by subwatershed to inform the LSPC Watershed Model.
Additionally, the data sets could be assessed to estimate impervious connectivity of roads
at the subwatershed scale. This information could be used to develop subwatershed
specific inputs and algorithms of connectivity for urban areas at the subwatershed scale to
inform the LSPC Watershed Model.
2) Default data set providing inputs for PLRM to characterize roads:
The data sets provide a consistent and comprehensive starting place to develop inputs to
characterize Tahoe Basin roads to inform PLRM analyses. The data sets can be used to
support development of a baseline pollutant load estimate at the jurisdictional scale, or to
estimate an existing conditions load for a WQIP analyses.
3) Improved basis for storm water planning for Tahoe Basin roads:
The data sets and associated attributes provide a basis to perform spatial analyses of
Tahoe Basin roads and their relative potential to generate pollutants. Results from
detailed analyses could inform WQIP prioritization and road maintenance operations to
improve water quality. Figure 3 is a conceptual example of how attributes in the GIS
shapefiles could be combined and interpreted to estimate relative pollutant potential for
Tahoe Basin roads. In Figure 3, the highest pollutant potential would be identified from a
road characterized as High Road Risk / DCIA / Erodible. In Figure 3, the lowest
pollutant potential would be identified from a road characterized as a Low Risk Road /
ICIA / Stable.
Tahoe Basin Road Assessment
8
March 2011
Highest
Pollutant Potential
Road Risk
High
Mod
Low
Stable
Lowest
Pollutant Potential
Erodible
Condition
.
Figure 3. Conceptual Example Interpreting Road Attributes
Tahoe Basin Road Assessment
9
March 2011
4.0 References
California Regional Water Quality Control Board, Lahontan Region (LRWQCB) and Nevada
Division of Environmental Protection (NDEP). 2009. Lake Tahoe Total Maximum
Daily Load.
Northwest Hydraulic Consultants (nhc), Geosyntec Consultants, and 2NDNATURE. 2009a.
PLRM User’s Manual. Prepared for Lake Tahoe Basin Storm Water Quality
Improvement Committee. www.tiims.org.
Northwest Hydraulic Consultants (nhc), Geosyntec Consultants, and 2NDNATURE. 2009b.
PLRM Model Development Document. Prepared for Lake Tahoe Basin Storm Water
Quality Improvement Committee. www.tiims.org.
Northwest Hydraulic Consultants (NHC) and Tetra Tech. January 2010. Analysis of Differences
in Pollutant Load Estimates Generated by LSPC and PLRM. Prepared for SNPLMA
Grant PO23.
Northwest Hydraulic Consultants (nhc). Unpublished. Measuring Road Shoulder Saturated
Hydraulic Conductivity to Inform and Refine PLRM Algorithms.
Tahoe Basin Road Assessment
10
March 2011
Appendix A – Understanding Shapefile Attribute Tables
Appendix A provides a key to interpreting the attributes and fields defined in the Road Shoulder
Conditions shapefile (February, 2011), which can be downloaded from the PLRM sub-site on
www.tiims.org.
QuickID – This field provides a unique identifier for all three attributes in the shapefile: road
shoulder connectivity, condition, and compaction. QuickIDs should be interpreted as follows:
QuickID
Road Shoulder
Connectivity
Condition
Compaction
D,E,H
DCIA
Erodible
High
D,P,H
DCIA
Protected
High
D,S,H
DCIA
Stable
High
D,SP,H
DCIA
Stable & Protected
High
D,E,M
DCIA
Erodible
Moderate
D,P,M
DCIA
Protected
Moderate
D,S,M
DCIA
Stable
Moderate
D,SP,M
DCIA
Stable & Protected
Moderate
D,E,P
DCIA
Erodible
Protected
D,P,P
DCIA
Protected
Protected
D,S,P
DCIA
Stable
Protected
D,SP,P
DCIA
Stable & Protected
Protected
D,SP,NA
DCIA
Stable & Protected
Not Applicable
I,E,H
ICIA
Erodible
High
I,P,H
ICIA
Protected
High
I,S,H
ICIA
Stable
High
I,SP,H
ICIA
Stable & Protected
High
I,E,M
ICIA
Erodible
Moderate
I,P,M
ICIA
Protected
Moderate
I,S,M
ICIA
Stable
Moderate
I,SP,M
ICIA
Stable & Protected
Moderate
I,E,P
ICIA
Erodible
Protected
I,P,P
ICIA
Protected
Protected
I,S,P
ICIA
Stable
Protected
I,SP,P
ICIA
Stable & Protected
Protected
I,SP,NA
ICIA
Stable & Protected
Not Applicable
Gated
Gated Community - No Data Obtained
NoData
Data Not Obtained
Tahoe Basin Road Assessment
11
March 2011
Condition – This field identifies the condition of the road shoulder for pollutant generation.
Defined as: Erodible, Stable, Protected, or Stable and Protected.
Cond_Imp – This field identifies the impervious connectivity of the road shoulder. Defined as:
Directly Connected Impervious Area (DCIA) or Indirectly Connected Impervious Area (ICIA).
Cond_ID – This field is a placeholder that can be populated by the individual user to merge
designations of Road Risk with Road Condition. This merger is needed because the PLRM
requires the input of Road Condition by Road Risk Category (see PLRM User’s Manual Section
6.1). Recommended identifiers to populate this column are as follows:
Cond_ID
H_E
H_P
H_S
H_SP
M_E
M_P
M_S
M_SP
L_E
L_P
L_S
L_SP
Description
High Risk Erodible
High Risk Protected
High Risk Stable
High Risk Stable and Protected
Moderate Risk Erodible
Moderate Risk Protected
Moderate Risk Stable
Moderate Risk Stable and Protected
Low Risk Erodible
Low Risk Protected
Low Risk Stable
Low Risk Stable and Protected
Compaction – This field identifies the compaction category of the road shoulder as Highly
Disturbed, Moderately Disturbed, Protected, or Not Applicable (typically used to denote paved
road shoulders).
WQIP_Flag – This field identifies road shoulders within a known water quality improvement
project (WQIP) that occurred after the year 2004. The flag is present to warn a user that it may
be incorrect to assume that the attributes defined in the Road Shoulder Shapefile are similar to
the TMDL Baseline Condition.
Tahoe Basin Road Assessment
12
March 2011