223150 Supplemental Letter 2.pdf

CITY OF SALEM

FINANCE DEPARTMENT

CONTRACTS & PROCUREMENT DIVISION

SUPPLEMENTAL INFORMATION LETTER #2

March 20, 2023

RFP NO. 223150

FOR: Stormwater Code Update

CLOSING DATE: March 24, 2023, AT 4:00 PM (LOCAL TIME)

The following are questions and answers to this RFP:

1. Q) Task 8 of the stormwater update RFP makes reference to the permit’s

Hydromodification Assessment and Stormwater Retrofit Strategy reports.

Would it be possible to obtain copies of these reports?

A) Attached is the City of Salem Hydromodification Assessment dated January 4, 2013.

Shawna Self, NIGP-CPP, CPPB

Contracts & Procurement Manager cc: File

CITY OF SALEM

HYDROMODIFICATION ASSESSMENT

Prepared for January 4, 2013 City of Salem

City of Salem Hydromodification Assessment

October 2012 Page i

TABLE OF CONTENTS

1.0 Introduction

1.1. Hydromodification – Background

1.2. Channel Responses to Hydromodification

2.0 Methods

2.1. Changes in Runoff

2.2. Channel Assessment

2.2.1. Channel Type

2.2.2. Vertical Channel Instability

2.2.3. Lateral Channel Instability

2.2.4. Bank Condition

2.2.5. Geomorphic Stage

2.2.6. Channel Susceptibility

2.2.7. Future Risk

2.3. Data Sources

2.3.1. Spatial Data Sources

2.3.2. Field Data Sources

2.3.3. City Monitoring Data Sources

3.0 Landscape Overview

3.1. Topography, Surficial Geology, and Soils

3.2. Climate

3.3. Land Cover

3.4. Subbasins

3.5. Stream Channels

3.6. Landscape Units

3.7. Alterations

3.7.1. Land Cover Alterations

3.7.2. Channel Alterations

3.7.3. Floodplain Alterations

3.7.4. Wetland Alterations

4.0 Results

4.1. Land Cover Alterations

4.2. Flow Analysis

4.3. Index of Hydrologic Alteration

4.4. Future Land Cover

4.5. Channel Condition by Basin

4.5.1. Battle Creek

4.5.2. Pringle Creek

4.5.3. Croisan Creek

4.5.4. Glenn/Gibson Creeks

4.5.5. Turnage Brook

4.5.6. Pettyjohn and Grey Oak Creeks

4.5.7. Claggett Creek

4.5.8. Little Pudding River

4.5.9. Mill Creek

5.0 Hydromodification Case Study—Waln Creek

6.0 Summary

6.1. Landscape

6.2. Land Cover

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6.3. Channel Assessment

7.0 Implications for Stormwater Management

7.1.1. Recommendations to Continue Hydromodification Assessment

8.0 References

List of Figures

Figure 1. Workflow of this Hydromodification Assessment Figure 2. Simplified schematic of channel evolution in response to changes in water and sediment load Figure 3. Subbasin summary of impervious surface and forest cover Figure 4. Subbasin Results for Impervious surface vs. Road Density Figure 5. Relationship of RB Flash index to contributing area total impervious surface coverage Figure 6. 2011 aerial view of upper Waln Creek. Red line is basin boundary, blue line is flow path Figure 7. April 2012 photo of higher gradient portion of Waln Creek along Holder Street Figure 8. April 2012 photograph of damaged driveway along Waln Creek near Holder and Mallard Streets Figure 9. April 2012 photo of Waln Creek at Woodside Drive

List of Tables

Table 1. Vertical Instability Scores Table 2. Lateral Instability Scores Table 3. Bank Condition Score definitions Table 4. Geomorphic Stage definitions Table 5. Susceptibility Score Definitions Table 6. Future Risk Score Definitions Table 7. City of Salem’s Network of Continuous Monitoring Locations Table 8. Alteration parameters considered for each subbasins within the study area Table 9. Richards-Baker Flashiness Index Values for Selected Gauges Table 10. Index of Hydrologic Alteration score ranges for each parameter Table 11. Subbasins with significant potential for significant increases in future levels of impervious surface based on zoning Table 12. Battle Creek Reach Summary Table 13. Pringle Creek Reach Summary Table 14. Croisan Creek Reach Summary Table 15. Glenn Gibson Creek Reach Summary Table 16. Turnage Brook Reach Summary Table 17. Grey Oak Brook Reach Summary Table 18. Claggett Creek Reach Summary Table 19. Little Pudding River Reach Summary Table 20. Mill Creek Reach Summary

Attachments:

1. Maps 1-15

2. Subbasin Summary table for Land Cover Characteristics

3. Hydrographs for Salem Gauge Data

4. Basin Descriptions

5. Channel Type Examples

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EXECUTIVE SUMMARY

This report presents a Hydromodification Assessment for the City of Salem which has been developed both to inform future stormwater management decisions for the City and to fulfill National Pollutant Discharge Elimination System (NPDES) MS4 permit requirements. Hydromodification is used throughout this report as a blanket term to address how alterations in land use change the hydrologic characteristics of streams.

Hydromodification impacts to stream channels include direct impacts such as: channelization, fill and development of floodplains, and installation of dams or other impoundments. Indirect source impacts include changes to land cover, largely increased imperious area and decreased vegetation that result in alteration of rainfall-runoff processes that cause stream channels to adjust to new hydrologic regimes. The stream channel adjustments typically involve net erosion of the bed and/or banks, which can degrade water quality, ecological function, and threaten streamside infrastructure.

The overall goal of this report is use landscape level geologic and land cover characteristics in combination with current stream condition to assess channel susceptibility to hydromodification as well as the risk for future channel changes. The following steps were completed:

• Developed a landscape profile that considers the climate, geology, soil, and topographic characteristics of the area to identity the type of stream channels that occur, and how those channel types are likely to respond to hydrologic changes.

• Used spatial land cover data to identify and quantify the range of hydrologic alteration within each drainage basin in the City.

• Surveyed existing stream conditions to determine channel types, overall condition, and susceptibility to change under existing and future conditions.

• Characterized the overall condition and trajectory of stream channels throughout the City. This characterization includes an interpretation of existing conditions to determine each stream reach’s current geomorphic trajectory.

Land cover throughout the City of Salem is typical of developed urban areas in the Willamette Valley and includes a reduction in natural vegetation communities and substantial increases of urban land. The City and surroundings include three general classes of land cover: agricultural areas with relatively low levels of impervious surface and forest cover, single family residential with significant total impervious surface and moderate to low forest cover, and highly developed urban (commercial and industrial) core with the highest total impervious surface cover. The results of the overall land cover analysis indicate that alterations to land cover occur throughout the study area to levels that are likely to impact stream form and functioning. The least altered land cover conditions in the study area are Croison Creek and the upper basins of Battle, Glenn, and Gibson Creeks.

Direct channel modifications and evidence of past and ongoing adjustment were observed throughout the City, indicating significant stream degradation and disruption of channel forming processes. Overall rates of ongoing channel adjustment appear to be relatively slow throughout the study area. This appears to be related to the relatively small drainage area of many of the stream systems, and the strong, cohesive, nature of many of the stream bed and bank materials. The influence of riparian vegetation and floodplain engagement is notable in many of the stream reaches. Stream channels appeared to be generally more stable in areas with greater setbacks and more intact vegetation communities.

These observations and analyses confirm that hydromodification impacts have occurred, and continue to occur throughout Salem’s network of streams. The implications of these impacts varies depending on landscape position, with the most notable adjustments occurring in locations where the bedrock-dominated hills transition to lower, more erodible landscape positions.

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Some key findings and implications for stormwater management include:

• The most active areas of channel adjustment are typically located in the bedrock hills in south and west Salem, often at the transition from bedrock dominated channels to more erodible deposits. These areas often have more recent development in the contributing basin, suggesting that these channels may still be adjusting.

• Areas with less urban development in the upper watersheds, such as Battle, Croisan, Glenn, and Gibson Creeks, are also located in areas with soils that, while shallow in terms of depth to bedrock, will be more amenable to stormwater Best Management Practices (BMPs) that rely on infiltration to avoid exacerbating existing issues and minimize future impacts.

• Flooding issues appear to be coincident with lower gradient reaches that may represent transitory sediment storage within the channel. These flooding issues may be another consideration in how in stream projects are prioritized, to favor projects that can provide greater floodplain storage and/or conveyance capacity along with ecological objectives of riparian corridor structure and diversity.

• The streams draining primarily lacustrine areas are less erosion-prone and occur in areas with less permeable soils. Infiltration-based approaches may be less effective in these areas, where a greater focus on regional detention and high flow bypass pass pipes may be appropriate depending on site conditions.

Flow quantity is still important to consider in these areas, as they do drain through short ravines, which can be destabilized with bed or bank erosion.

The following recommendations are elements the City of Salem could consider for future work to fulfill the NPDES MS4 permit hydromodification requirements. Many of these elements are intended to allow for an efficient link between measures intended to address hydromodifications, stormwater drainage regulations, floodplain management, and habitat restoration.

• To minimize future impacts design standards and development regulations could be updated including:

requiring infiltration and low impact development (LID) wherever feasible.

• Review riparian protections, building setbacks, and floodplain development regulations to reduce the potential repercussions of stream widening and improve channel stability over time.

• Develop a site specific LID sizing tool. This requires an upfront investment by the City, but would facilitate faster implementation by developers.

• Retrofit projects would be required to recover stream processes in areas already developed. These could include floodplain connectivity projects that will improve flow attenuation, improve channel stability, decrease flood risk, and improve habitat.

• Regional detention facilities and strategic high flow bypass pipes can effectively reduce hydromodification and flooding impacts.

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1.0 INTRODUCTION

This report presents the first phase of a Hydromodification Assessment for the City of Salem. The Hydromodification Assessment is being developed to both inform future stormwater management decisions for the City and to fulfill portions of the City’s responsibilities under its National Pollutant Discharge Elimination System (NPDES) Municipal Separate Storm Sewer System (MS4) permit requirements.

Salem’s NPDES MS4 permit includes condition 5 which outlines requirements for hydromodification assessments. Condition 5 and its objects are:

5. Hydromodification Assessment:

a. Collect and maintain information that will inform future stormwater management decisions related to hydromodification based on local conditions and needs;

The permittee must conduct an initial hydromodification assessment and submit a report by November 1, 2012 that examines the hydromodification impacts related to the permitee’s MS4 discharges, including erosion, sedimentation, and/or alteration to stormwater flow, volume and duration that may cause or contribute to water quality degradation. The report shall describe existing efforts and proposed actions the permittee has identified to address the following objectives:

b. Identify or develop strategies to address hydromodification information or data gaps related to waterbodies within the permittee’s jurisdiction;

c. Identify strategies and priorities for preventing or reducing hydromodification impacts related to the permittee’s MS4 discharges;

and,

d. dentify or develop effective tools to reduce hydromodification.

This report addresses objectives a and b while providing the background for decision making to address objectives c and d which will be completed in the next phase of the project.

The City of Salem has decided on a watershed approach for the hydromodification assessment to focus on how existing stream channels function, how they have been altered, and how susceptible they may be to changes in the future. A watershed approach considers a broad range of watershed and stream functions that influence impacts of hydromodification thereby providing a framework for a broader suite of strategies to address these impacts in a cost effective way.

Hydromodification is used throughout this report as a blanket term to address how alterations in land use change the hydrologic characteristics of streams. These hydrologic changes drive physical changes in stream channels as the channel adjusts to altered rates of water and sediment delivery. Note that this assessment does not specifically include the Willamette River, which is influenced by factors that function at a much larger spatial scale.

The report is organized to:

1. Provide an overview of hydromodification processes;

2. Detail data sources and methods;

3. Present the landscape context for the City’s streams;

4. Provide results at a landscape-scale analysis regarding the extent of land use changes that can influence rainfall-runoff relationships;

5. Provide results of a rapid channel reconnaissance and other channel data collection that identifies existing status and trends of channel condition throughout the City of Salem; and

6. Conclude with an overall summary of conditions and discussion of implications for potential strategies and approaches to address hydromodification.

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1.1. Hydromodification – Background

Hydromodification is defined as the alteration of the hydrologic characteristics of coastal and non-coastal waters, which in turn could cause degradation of water resources (EPA, 2007). In the City of Salem, changes to hydrological characteristics include both changes in how rainfall becomes surface water runoff and direct manipulation of stream channels. Urban development alters land cover by reducing vegetation and increasing impervious area. These changes increase the volume of stormwater runoff by reducing infiltration, interception, and evapotranspiration. Runoff from urban areas is captured from impervious surfaces and piped directly to streams, concentrating flow and raising flood peaks while interrupting natural overland flow paths that historically delivered coarse sediment to the streams. The ability for streams to adjust their bed and banks is increased by both reduced coarse sediment supply and increased flows to streams. Historically, urban development often filled in natural wetlands and floodplains further reducing water storage and flow attenuation functions. Straightening stream channels to accommodate development increases the channel gradient and stream power which often results in incision.

Urban development can also reduce sediment supply to streams, which has a similar effect to increasing the volume of runoff. Impervious surfaces and landscaping tend to cap sediments, directly reducing coarse sediment delivery to streams. Streams affected by hydromodification typically have increased cross sectional area (width and depth) to accommodate the increased flows or reduced coarse sediment input. These physical changes to the stream channel influence both the extent and type of aquatic and riparian habitat, often for decades and sometimes permanently (Henshaw and Booth, 2000). Stream erosion can also threaten infrastructure near a stream such as bridges and structures on the stream bank. Bridges and culverts can be undermined and sewer pipes exposed by erosion. Streamside properties can be threatened as banks become unstable.

This Hydromodification Assessment employs a watershed characterization approach to assess the susceptibility of stream channels to erosion from the perspective of stormwater management. The risks of hydromodification are focused on the potential degradation of water quality and stream ecology with an acknowledgement that erosion may also threaten infrastructure. Watershed characterization was performed by considering the process controls of climate and geology, existing and historical land cover, and reach scale channel alterations. The extent of hydromodification impacts are influenced by the natural watershed context and the influence of stressors caused by urban development such as increased impervious surfaces and pollutants. Stressors from outside the watershed, such as climate change and atmospheric pollutants, are not included in this assessment.

The goal of this work is to develop a holistic understanding of how changes in the landscape have influenced the physical characteristics of streams throughout the City and to document current conditions of those streams. The overall workflow of this study is illustrated in Figure 1.

January 2013 Page 7

Figure 1. Workflow of this Hydromodification Assessment

1.2. Channel Responses to Hydromodification

This Hydromodification Assessment is focused on how stream channels in Salem have responded to changes in land cover and current stream condition to determine the susceptibility of existing conditions and risk of stream adjustment to future development. This requires that the stream networks be put in context of how they have formed in the past to assess and predict how they are likely to respond to hydromodification. This section includes an overview of channel forming processes, the controlling factors in those processes, and how

Page 8 January 2013 hydromodification alters both stream form and process. In general stream power (the ability of the flowing water to mobilize bed and bank sediments) is the driving force of hydromodification and is increased with an increase in channel slope or flow quantities. The resisting forces of stream channels are the amount and size of sediment material and stream side vegetation. Therefore, some urban activities that increase hydromodification impacts include:

• Increased runoff from impervious area (increased flows)

• Removal of vegetation (reduced resistance)

• Straightening channels (increased slope)

• Filling floodplains and wetlands (loss of storage)

• Reduced coarse sediment supply (reduced resistance)

At the basin scale, channel networks display characteristic forms that vary from the headwaters to the receiving water body. These forms relate to the overall water and sediment loads at that region of the basin. Urban areas often reduce the delivery of coarse sediment (sands and gravels) while increasing the delivery of fine sediment to streams. Fine sediment is much harder to remove from surface water runoff and often contributes to degraded water quality and habitat. Coarse sediment on the other hand protects streams by dissipating stream power and providing habitat for fish and macroinvertabrates. A watershed can be generalized into regions of the stream that supply, transfer, or store sediment.

In headwaters areas at the upper parts of the basin, supply processes dominate as water and sediment are delivered to small streams. These upper basin areas typically have steeper gradients and more rapid weathering processes that provide sediment to streams where it can be transported downstream. In Salem, the main supply areas consist of the upper reaches of streams that drain the bedrock hills to the west and south. Mill Creek includes supply areas that extend well outside of the City’s jurisdiction.

The transfer zone is dominated by conveyance of flow and sediment as tributary streams grow in size and come together. Sediment is stored in the stream temporarily, often in the form of gravel and sand bars. The duration of storage relates to frequency of large flow events which will move the temporarily stored sediment downstream. .

The storage zone occurs when streams reach the low gradient lowlands with higher volumes of stored alluvium.

Storage zones often occur where smaller streams empty into larger rivers or tidal waters. In Salem, this occurs where smaller streams flow into the Willamette River floodplain. Floodplains provide a more critical function in the storage where additional capacity for high flows is attenuated and fine sediment can be deposited. In Salem, many of the streams in the lacustrine areas function as storage zones, but with generally low sediment supply.

These lacustrine areas are low slope channels incised into cohesive sediments, and lack the stream power to erode bed and banks to contribute sediment to the system.

Channel response to alterations can be quite different in these zones. In supply and transfer zones, increased flows can accelerate erosion processes, resulting in larger capacity channels over time especially when the available coarse sediment is limited. This can often occur in urban settings where flow is increased in combination with bank armoring (such as riprap and walls) and land cover conversions reduce coarse sediment supply. In storage zones, channel response may not be as dramatic due to less overall channel slope, but flooding and inundation duration can increase in response to greater runoff volumes.

At a more local scale, channel adjustments occur in both the vertical and lateral dimensions. These adjustments are based on local stream power, bank strength and condition, and bed materials and composition. Researchers have identified general stages of channel response to changes in the controlling variables of water and sediment delivery (Hupp and Simon, 1986; Schumm, 1993). It is important to note that not all channels respond the same way as indicated in these simplified conceptual models. Increased flows or decreased coarse sediment load in channels with adjustable beds starts a sequence of channel adjustments that begins with channel incision (downcutting).

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Vertical adjustments in the channel profile can have broad-ranging implications for overall channel stability.

Channel incision has a positive feedback cycle, as the lowering of the channel elevation traps additional flow and erosive force within the banks, hastening downcutting to a new base level (Figure 2). Incision typically results in channel banks that are oversteepened, resulting in a second stage of overall channel widening until the banks re-stabilize. The third stage is typically the reestablishment of riparian vegetation within the inset floodplain.

Figure 2. Simplified schematic of channel evolution in response to changes in flow quantity, channelization, and sediment load.

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2.0 METHODS

This Hydromodification Assessment was developed using the following approach:

1. Landscape overview was developed to characterize the primary geologic, climatic, and geomorphic processes that influence streams in the study area;

2. Existing alterations to rainfall and runoff processes were quantified using spatial land cover and land use data;

3. Potential future alterations were investigated using zoning information; and

4. Local channel conditions were cataloged in terms of geomorphic and stream bank conditions.

These factors were taken together to assess the overall degree of existing hydromodification, the potential for future hydromodification, and susceptibility and risk to continuing and future hydromodifications.

2.1. Changes in Runoff

Existing alterations to rainfall runoff patterns were assessed primarily using GIS analysis of land cover characteristics such as impervious surface, forest cover, and road density in each subbasin. While there is no direct correlation between these land cover characteristics and local channel condition, these parameters suggest general magnitudes of alterations to rainfall runoff processes. These land cover parameters were then combined to develop an Index of Hydrologic Alteration for each subbasin. The extent of impervious area within floodplains was also assessed to suggest locations where infrastructure is in conflict with natural overbank flooding processes.

Percentage of remaining wetlands was also used as an indicator of the magnitude of changes that has occurred to the stream system.

Stream gauging data were also used to assess the influence of land cover conversion on runoff processes.

Available flow data were analyzed to calculate the Richards-Baker (R-B) ‘flashiness’ index which identifies systems with more rapid fluctuations in hydrograph (Baker et al. 2004). The R-B index was calculated by the equation:

R-B Index = ∑ 0.5 (|𝑞𝑖+1− 𝑞𝑖|+|𝑞𝑖− 𝑞𝑖−1|) 𝑛 𝑖=1

∑ 𝑞𝑖𝑛 𝑖=1

Where qi is the mean daily discharge in cubic feet per second (ft3/sec) and n is the number of days of flow record.

This assessment also considered future land cover using current zoning and ‘full build out’ assumptions. This analysis used the existing city zoning and assumptions about urban land cover for each development type (e.g., more impervious surface for industrial areas than single family residential). These results were then compared to existing land cover to determine the potential for future alterations to runoff processes. This approach does not provide resolution for areas outside the City’s zoning layer, which also include many of the less developed areas outside the Urban Growth Boundary (UGB). These areas could be developed in the future if the areas are annexed, so this potential is accounted for in the assessment of susceptibility and risk.

These landscape level alteration parameters were then combined to create an Index of Hydrologic Alteration which indicates relative levels of change to rainfall-runoff processes within the study area.

2.2. Channel Assessment

The assessment of channel condition was based primarily on field observations and included the following steps:

1. Reach breaks identification. General reach breaks were identified in GIS using LiDAR, geologic, and other spatial data. Reach breaks identify different processes that occur at a broader landscape scale.

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These reaches are intended as a first cut to capture likely changes in channel type and condition to target data collection.

2. Rapid stream reconnaissance. All channels were visited and most were walked to observe condition and record signs of instability and overall bank condition. These reaches were selected for access points and focused on transitions within the channel. The reconnaissance was also focused on identifying locations where significant channel adjustment is likely under current conditions.

3. Data collection- specific alterations. The City of Salems’ stream crew collected data regarding specific alterations within the channel such as bank armoring, headcuts, and extensive bank erosion. This data set was used to refine reach breaks, document areas where significant channel adjustment is more likely, and is incorporated into the overall channel susceptibility rating. This step was carried out by the City’s stream cleaning crew.

The rapid reconnaissance investigated and inventoried overall channel type, bank condition, signs of vertical instability, signs of lateral instability, and the severity of ongoing trends changes. These indicators were then used to assign a score for both the existing geomorphic trajectory of the channel and the overall susceptibility of the channel. The assessment for each channel condition parameter is further described below.

2.2.1. Channel Type

Channel types were collected primarily using the Montgomery-Buffington (1993) typology, with two additions:

1) Streams that are highly modified were included in a “channelized” category that includes portions of streams that had been straightened along road and railroad corridors, and 2) A ‘swale’ channel type was also included to capture channels with little or no typical morphology such as headwater streams at the upper part of drainage basins, in lacustrine areas, and in the lower Willamette River floodplain. The dominant channel types throughout the study area were channelized, swale, pool-riffle, and step-pool. Examples of each channel type are included in Attachment 5.

Swale morphology dominated the upper reaches of the streams draining lacustrine areas such as Claggett Creek and the, Little Pudding River. These swales were often also channelized, so we identified streams as swales if they had a meandering plan form, and channelized if they had been straightened for a significant distance.

Pool-riffle streams dominated the lower reaches of the larger systems, often as the stream gradient decreased and the alluvial valley width increased. Lower Pringle and Mill Creeks exhibit the most distinct pool-riffle structure.

Some of the incised streams exhibited muted pool-riffle methodology at low flow because the bedforms were shrouded with fine sediments following the high flow season.

Step-pool morphology was typically more prevalent in the higher gradient bedrock reaches. The morphology was often subtle, similar to pool-riffle because of the removal of large wood and channel incision. However, it was still the most appropriate channel type for these areas. Steps were observed to be reforming with limited coarse sediment load.

2.2.2. Vertical Channel Instability

Vertical adjustments in the channel profile can have broad-ranging implications for overall channel stability.

Using the theories of channel evolution discussed in Section 1, channel incision (downcutting) is typically the first phase of channel adjustment to increased runoff or reduced coarse sediment supply. Channel incision has a positive feedback cycle, as the lowering of the channel elevation traps additional flow and erosive force within the banks, hastening downcutting to a new base level. Incision typically results in channel banks that are oversteepened, resulting in overall channel widening until the banks restabilize, often within a new inset floodplain.

This process is evident in a number of Salem’s streams. In some cases it occurs naturally as the stream flows through a ravine between two geomorphic surfaces. In many cases, it is likely that channel downcutting has occurred following changes in land cover in the contributing area. As with many urban systems, the extent to which channel incision has occurred is sometimes moderated by road crossings, weirs, and other artificial grade

Page 12 January 2013 control structures. These factors, along with relatively shallow depth to bedrock in many of the higher gradient reaches, has limited overall incision throughout the City. It should be noted that road and pipe crossings often provide temporary bed control but are often not designed as such and could eventually fail, resulting in high costs.

Vertical Instability scores were assigned ratings of 1 through 4, with 1 representing more stable and 4 representing less stable situations. The rationale for each score is described in Table 1.

Table 1. Vertical Instability Scores

Score Definition Notes

1 Stable No signs of recent incision, low entrenchment ratio. Includes bedrock control on bed.

2 Minor Instability Bed is adjustable and shows signs of adjustment that are much smaller than bankfull depth and are not active.

3 Instability Bed is adjustable and shows signs of active adjustment that are smaller than bankfull depth.

4 Major Instability Bed is actively adjusting and has incised to depths greater than bankfull depth.

Channel incision is common in portions of the City where the higher gradient bedrock channels transition into the lacustrine flats. The most significant reaches where substantial incision has occurred are Battle Creek between the golf courses and West Middle Fork Pringle Creek downstream of Kuebler Road. Incision in Battle Creek is the most significant, but appears to have slowed in recent years, with 15 to 20 year old alder trees establishing on the lower banks. No streams received a ‘major instability’ rating of vertical instability.

Portions of Pringle Creek and Clark Creek show signs of past incision, but the impacts have been limited by frequent road crossings and shallow bedrock. Claggett Creek has recent vertical instability in its ravine reach directly west of Interstate 5. The streams draining the lacustrine areas appear to be vertically stable, with minor incision of low flow channels into larger swales.

2.2.3. Lateral Channel Instability

Lateral instability scores were assigned ratings of 1 through 4, with 1 meaning more stable and 4 meaning less stable situations. The rationale for each score is described in Table 2.

Table 2. Lateral Instability Scores

Score Definition Notes

1 Stable No signs of recent bank erosion, low entrenchment ratio, banks in good condition.

2 Minor Instability

Very minor bank erosion, typically related to local hydraulics or natural channel migration. Bank erosion is limited in longitudinal length to less than two bankfull widths.

3 Instability Systemic bank erosion is apparent with oversteepened banks. Bank erosion is not significantly changing cross-sectional area, or overall channel plan form.

4 Major Instability

Systemic bank erosion is apparent with active erosion that is changing cross-sectional area or overall channel plan form.

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Lateral instabilities were noted in a number of the streams that drain bedrock dominated hills. These instabilities were typically related to oversteepened banks following vertical incision. There were two instances (Gibson Creek at the upstream City boundary, and West Middle Fork Pringle north of Battle Creek Drive) where lateral movement appeared to be related to channel meandering induced by removal of riparian vegetation and potentially grazing.

2.2.4. Bank Condition

Bank condition information was classified using a simple 1 to 4 rating, with 1 representing more and 4 representing less stable situations. The rationale for each score is described in Table 3.

Table 3. Bank Condition Score definitions

Score Definition Notes

1 Stable – not armored

Bank shows no recent evidence of retreat or erosion that is not related to natural channel migration. Riparian vegetation is present, and appears to have sufficient root strength to assist with bank stability.

2 Stable -armored Bank shows no recent evidence of retreat or erosion, due to sufficient armoring.

3 Unstable -minor

Bank has either insufficient vegetative root mass or unengineered armoring to resist erosion.

Bank shows signs of erosion, but overall cross-sectional change is low.

4 Unstable -major

Bank has either insufficient vegetative root mass or unengineered armoring to resist erosion.

Bank shows signs of erosion, and overall cross-sectional change is significant.

2.2.5. Geomorphic Stage

The vertical instability, lateral instability, and bank condition scores were used to assess the overall geomorphic stage of the channel, using a simplified typology described in Table 4.

Table 4. Geomorphic Stage definitions

Score Definition Notes

1 Stable Channel shows no signs of systemic adjustment.

2 Incising Channel is downcutting into its bed. Headcuts, exposed roots, undercut infrastructure and other field indicators are apparent and recent.

3 Widening Channel is laterally adjusting post-incision, or strongly meandering.

4 Widening and incising

Channel is adjusting both laterally and vertically, with lateral being the primary dimension of adjustment.

5 Aggrading Channel bed is aggrading. Sediments sufficiently raise the bed profile with coarse sediments able to resist some high flows. Fine sediments in the channel bed that would be transported during high flows do not necessarily indicate an aggrading system.

Throughout Salem, streams experiencing vertical changes were less frequent than those in a widening phase. For streams with apparent incision, there were many indicators to suggest limited further adjustment due to road crossings or other artificial bed controls.

Of the many streams that are likely in a widening phase, the rate of channel adjustment is typically slow. This appears to be due to the fine grained, cohesive, sediments that tend to hold a near vertical angle for some time.

Riparian vegetation is also prevalent throughout most of the streams, adding to the high bank soil strength.

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2.2.6. Channel Susceptibility

Channel susceptibility information was classified using a 1 to 4 rating, with 1 meaning less susceptible and 4 meaning more susceptible. The rationale for each score is described in Table 5. These scores integrate considerations of landscape position and existing channel condition.

Table 5. Susceptibility Score Definitions

Score Definition Notes

1 Less Susceptible

- natural Existing channel geomorphic state is in a natural condition and is dynamically stable.

2 Less Susceptible

- enforced

Bed and bank configuration is stable due to armoring. Local erosion is possible, but armoring is sufficiently extensive to preclude significant channel adjustment.

3 Susceptible Bed and/or bank are adjustable, are in a landscape position where channel response is more likely, and active change is apparent.

4 Highly Susceptible

Bed and/or banks are currently in a period of adjustment, so any changes would prolong the time to re-equilibrate, or is a reach located in a sensitive landscape position with low hydrologic alteration score for the contributing basin.

2.2.7. Future Risk

Future risk was assessed using the future build out analysis described above. The future risk possibility was classified using a 1 to 3 rating, to capture less to more risk. The rationale for each score is described in the table below. These scores integrate the potential for future build out and susceptibility due to landscape position.

Table 6. Future Risk Score Definitions

Score Definition Notes

1 Less Risk Existing channel geomorphic state is in a natural condition and is dynamically stable. Future development does not have the potential to significantly increase contributing area alteration.

2 Moderate Risk

Alterations in future runoff are possible in channel sections that are susceptible to channel movement. Includes reaches that are currently adjusting.

3 More Risk Significant alterations to channel stability are possible. These reaches have a contributing basin that is likely to develop and channel characteristics that are susceptible to adjustment and/or are already adjusting.

2.3. Data Sources

These analyses relied on a number of spatial and field data sources, summarized here.

2.3.1. Spatial Data Sources

Spatial data sources were used to assess landscape level characteristics throughout the City. The GIS analysis for the Hydromodification Assessment utilized a number of spatial data sources provided by the City of Salem (see Appendix A) as well as publically available data. Primary data sets and their sources included:

1. Basin boundaries (City of Salem)

2. LiDAR Digital Elevation Model (DOGAMI)

3. NRCS soils (Williams, 1972; Knezevich, 1982)

January 2013 Page 15

4. Geology (1:500,000 scale provided by the City of Salem)

5. Stream mapping (City of Salem)

6. 2011 aerial photography (City of Salem)

7. Level 6 hydrologic unit Codes (US Geological Survey (USGS))

8. Local wetland inventory data (City of Salem)

9. Riparian shade areas (based on PHS, 2009)

10. Tree canopy (based on 2009 Natural Agriculture Imagery Program (NAIP) imagery AMEC, 2010)

11. Vegetation areas (based on 2009 NAIP imagery)

12. Impervious surfaces (based on 2009 NAIP imagery)

13. Land cover (2009 GAP analysis)

2.3.2. Field Data Sources

ESA staff, accompanied by City staff, performed a reconnaissance level review of all of the subbasins within the City during the wet season from April 2 to 4, 2012. The reconnaissance focused on reviewing assumptions about land cover, reach (and finer) scale channel modifications, and appropriateness of initial thresholds for hydrologic alteration. The January 2012 substantial runoff event was also considered. The event generated 6.1 inches of rain over a 48 hour period, as recorded at Salem Airport, a record for that location.

Stream surveys were performed August 21 to 24 and September 17 to 19, 2012. These visits included in-channel reconnaissance of most of the larger tributaries. The City also provided spatial data for specific channel alterations captured by the Stream Cleaning Crew in September and October 2012.

2.3.3. City Monitoring Data Sources

The City of Salem maintains a monitoring network of continuously recording gauges and collects routine water samples in streams throughout the City. Data collected at each monitoring point include stage and flow, temperature, pH, turbidity, specific conductivity, and dissolved oxygen (DO). Each continuous monitoring point and period of record is summarized in Table 7, with locations shown on Map 1. (See Geosyntec 2008 for a full discussion of monitoring results up to that date).

Table 7. City of Salem’s Network of Continuous Monitoring Locations

Station Identifier Identifier Time Period

Clark Creek 1 – Bush Park CLK 1 10/2006 to present

Clark Creek 12 – Ewald Avenue SE CLK12 10/2006 to present

Glenn Creek 12 – Hidden Valley Road NW GLE12 8/2007 to present

Glenn Creek 3 – Wallace Road NW GLE3 8/2007 to present

Mill Creek 12 – Turner Road SE MC12 10/2006 to present

Mill Creek 3 – North Salem High School MC3 02/2007 to present

Pringle Creek 12 – East Fork Pringle Creek PRI12 10/2006 to present

Pringle Creek 3 – Pringle Park PRI3 10/2006 to present

Battle Creek 12 – Lone Oak Road SE BAT12 10/2008 to present

Battle Creek 3 – Commercial Street SE BAT3 10/2008 to present

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3.0 LANDSCAPE OVERVIEW

The City of Salem covers approximately 49 square miles (68 square miles including the UGB) in the central Willamette Valley, Oregon. The City includes the mainstem Willamette River and all the streams in the City eventually drain to the river either directly or via the Molalla River system (Map 2). The geomorphic landscape is the cumulative result of eruptions of Columbia River basalt flows, Missoula Floods, and the migration of the Willamette River.

The landscape level attributes of this area are further discussed below, and are used to identify areas within the City with similar characteristics that would influence rainfall-runoff patterns, stream channel response, and potential stormwater management strategies.

3.1. Topography, Surficial Geology, and Soils

Landforms within the City have been influenced by a mixture of remote and local processes which in turn influence the types of channels that have formed. Bedrock underlying the City and forming the western and southern hills is part of the Columbia River Basalt Group (CRBG) representing nearly the furthest extension of these flows into the Willamette Valley. The CRBG eruptions occurred in a number of phases ranging from 17.5 to 6 million years ago (mya). The rocks below Salem represent the Grande Ronde eruptive phase from approximately 16 mya. These rocks have been weathered and tectonically altered over time and are exposed close to the surface in the west and south hills. Soils that developed over these hillsides are typically coarser grained than in the lowlands where bedrock is overlain with thicker deposits of finer grained materials.

In more recent geologic times, substantial volumes of fine grained materials were deposited throughout the Willamette Valley. These deposits occurred within lakes formed after floodwaters from the Missoula Floods1

The Willamette River has been the most active geomorphic agent in the vicinity in the last several thousand years.

The river has migrated within its valley, incising into the lacustrine deposits and developing an anastamosing (multiple active channels separated by vegetated islands) channel plan form. Dams now regulate flows on the upper Willamette River. This change in combination with land use changes for agriculture have resulted in channel simplification, reduced channel migration rates, and altered channel-floodplain interactions. The Willamette River’s floodplain through the City varies between 1,200 and 6,000 feet wide. Mapped surficial geologies for the vicinity of Salem are shown on Map 3, simplified into bedrock, lacustrine, and alluvial groups.

The alluvial group includes former river terraces that are no longer active under the current hydrologic regime of the Willamette River.

inundated the Willamette Valley approximately 15,000 years ago, creating broad lakes with associated deposition of lacustrine (lake) sediments. These deposits are topographically flat and typically have finer grained soils than the higher elevation bedrock areas. These soil types have high runoff potential due both to the fine grained matrix, and high groundwater tables.

The south and west Salem hills extend up to approximately 800 feet above sea level, dropping to the Willamette floodplain near elevation 200 feet (Map 4). The steepest slopes occur in the southwest hills near the northern transition to the Willamette River floodplain. The slopes in West Salem are also prominent; with similar morphology to the southwestern hills. Based on regional mapping of landslide hazards, there are limited areas in Salem that are subject to landslides. The mapping identified landslide hazard areas focused in the steepest sections of the west and south hills (Hofmeister et al., 2002).

Map 5 shows the hydrologic soil groups throughout Salem. These are broad hydrologic soil groups based on surveys conducted by the National Resource Conservation Service (NRCS). These categories range from A soils

1 The Missoula Floods were a series of giant outburst floods that repeatedly erupted from glacial lakes in Western Montana, flowing through the Columbia River valley at the end of the Pleistocene (Approx 15 to 13,000 years ago) (Allen et al. 1997).

January 2013 Page 17 with high infiltration rates to D soils with very fine grained soils with low infiltration rates, or soils with a high groundwater table. Salem includes primarily B and C soils, with expected higher infiltration rates on soils developed over bedrock and the lower infiltration soils in the lacustrine flats.

Much of the bedrock dominated area is mapped within the B soil hydrologic group, suggesting relatively rapid infiltration. Field observations in the summer indicate that these areas support significant stream baseflows, including many seeps evident in channel side slopes. This is likely due to water storage within the soil profile perched above bedrock. This process supports a good supply of relatively cold water through the summer for many streams that drain the bedrock hills.

Soils types have a strong influence on stormwater management strategies. Infiltration strategies are most effective in uncompacted A and B soils. Hydromodification impacts are often most severe where A and B soils are capped with impervious surfaces, since this situation presents the greatest change from the pre-disturbance condition (i.e., eliminating soil storage that had provided significant infiltration).

3.2. Climate

Climatic conditions in Salem are typically consistent with the broad patterns found throughout the Willamette Valley. Rainfall occurs throughout the year, but typically more than half falls from November through February (Western Regional Climate Summary). Average annual rainfall at Salem (Salem McNary Field Airport KSLE Station) is 40.57 inches. Winter months have typical low temperatures in the 30s (degree F) and highs in the 40s and low 50s. Summer high temperatures can reach over 100 but are typically in the 80s.

Regional precipitation data are available as part of the PRISM dataset (PRISM Climate Group, 2004) represented for Salem on Map 6. The City also maintains a number of continuously recording rain gauges. Both sources confirm that greater amounts of precipitation are likely at higher elevations, notably in the west and south hills.

While these variations exist and likely play a significant role at the scale of a single storm event, the total variation in annual precipitation is modest. Therefore, at this point in the analysis, climate conditions are assumed to be the same throughout the City.

This climate has the following potential implications for hydromodification impacts and strategies;

1. Extended wet winter months result in extended periods of high flows, high groundwater tables, and saturated soils.

2. Water capture and reuse is limited to what can be consumed on site during the winter.

3. Riparian vegetation is more easily maintained and established than drier regions and can be a significant stream bank stabilization feature.

3.3. Land Cover

Salem is within the Willamette Valley ecoprovince (Franklin and Dyrness, 1973). This portion of the valley was historically dominated by oak woodlands, transitioning into the riparian forest along the Willamette River. These oak woodlands were influenced by managed fire regimes, and disturbance regimes of channel migration and volcanism from the Cascades.

Land cover within the City has been highly altered to include substantial infrastructure. Early phases of development included river-dependent uses such as woolen mills. These uses caused substantial direct alterations to channels to move water through turbine systems and similar equipment. Additional information on earlier phases of development is described in Hemesath and Nunez (2002).

Today, the City is dominated by residential, commercial, and agricultural land covers with associated transportation and utility infrastructure.