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FileNo. 2998-M7-00 Geomorphic Evaluation and Channel Migration Zone Analysis FilE No. 2998-002-00
Aprll 29, 200ö
Prepared for:
Pierce Co¡rnty Water Programs Divlsíon 9850 - 64¡n Street West University Place, Washlngton 9g467
Altentlon: Dennis Dixon
Prepared by:
GeoÊngineers, lnc. 600 Dupont Street Bellingham, Washlngton 98225 (360) 647-1510
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PROJECT OBJECTIVE ...... 1 PROJECT APPROACH AND SCOPE ...... 1 REPORT ORGAN|ZAT|ON...... 2 GEOMORPHTC EVALUATTON ...... 2 |NTRODUCT|ON...... 2 REGTONAL SETTTNG ...... 3 Location ...... 3 Basin Topography ...... 3 Climate...... 3 Basin Hydrology...... 3 GEOLOGTC HTSTORY ...... 5 lntroduction...... 5 Cascades Mountain Range and the Puget Sound Basin...... 5 Continental Glaciation ...... 5 South Prairie Valley Formation...... 5 Osceola Mudflow...... 6 Recent South Prairie Creek Activity ...... 6 GEOLOGTCAND SO|LS UN|TS...... 6 Bedrock...... 6 Orting Formation (Pre-Vashon Stade, Glacial Drift)...... 7 GlacialOutwash (Vashon Stade) ...... 7 Ancestral White River Alluvium ...... 7 Osceola Mudflow ...... 7 South Prairie Creek Riverwash (Alluvium)...... 7 South Prairie Creek Silty Loam (Overbank Alluvium) ...... 7 Distribution of the Geologic Units ...... 8 BASIN DEVELOPMENTAND LAND USE PRACTICES ...... 8 lntroduction ...... I Pre-development Condition...... I Coal Mining...... 8 Logging (Timber Harvesting) ...... 9 Transportation lnfrastructure...... 9 Riparian Habitat...... 9 REACH-SCALE GEOMORPHIC PROCESSES ...... 10 lntroduction ...... 10 Composition of Bank and Bed Material ...... 10 Supply of Sediment and Woody Debris...... 10 Transport Capacity ...... 1l Regionaland LocalTopography ...... 11 CHANNEL M|GRATION ...... 12 Meander Bend Mi9ration...... 12 Avulsion ...... 12 Braided Channel Migration ...... 12 Vertical Migration...... 13
File No. 2998-007-00 Page i GeolxenelnslQ April 29,2005 Trele or Gorurerurs (Cottrlruueo)
Paqe No.
REACH SCALE GEOMORPHTC EVALUAT|ON...... 13 UPPER BASIN ...... 13 REACH 6 (RAVTNE MOUTH TO STA-323) ...... 14 REACH 5 (STA 323-215) ...... 14 REACH 4 (STA 215-195) ...... 15 REACH 3 (STA 195-135) ...... 15 REACH 2 (STA 135-20) ...... 16 REACH I (STA 20-0)...... 17 CHANNEL MTGRATION ZONE (CMZ) ANALYSIS AND RESU1TS...... 17 |NTRODUCT|ON...... 17 CALCULATION OF UNCONFINED MIGRATION RATES ...... 18 cMz DEL|NEAT|ON...... 18 Define the HCOT...... 18 Create CMZ Base Width...... 19 RESULTS...... 19 M|GRATTON POTENTIAL AREA (MPA) DELINEATION ...... 20 |NTRODUCT|ON...... 20 Severe MP4...... 20 Moderate MP4...... 21 Low MPA ...... 21 MPA ADJUSTMENTS...... 21 Modify CM2 Base Width...... 19 RESULTS...... 21
GLOSSARY OF TERMS...... 23
List of Tables Table 1. Flood History of South Prairie Creek Table 2. Summary of Peak-flows on South Prairie Creek (USGS, 1998)
List of Figures Figure 1. Channel Migration Zone Study - South Prairie Creek, Pierce County Figure 2. Sketch of Greater South Prairie Creek Watershed and Major Adjacent Drainages Figure 3. Generalized Geologic Cross-section of South Prairie Valley, Pierce County Figure 4. Extent of the Osceola Mudflow within the South Prairie Valley, Pierce County Figure 5. 1897 USGS Tacoma, Washington Land Classification Map Figure 6. Channel Migration Reaches - South Prairie Valley, Pierce County Figure 7. Abandoned Channels and PotentialAvulsion Routes: Reach 5 Figure 8. Abandoned Channels and PotentialAvulsion Routes: Reaches 3 and 4 Figure 9. Abandoned Channels and PotentialAvulsion Routes: Reaches 1and2
File No. 2998-007-00 Page ii Geo1xetnzeas-1/ April 29, 2005 Tnele or Go¡¡reus (Gotrrruueo)
List of Plates
Plate 1. Channel Migration Zone Boundary and Migration - Potential Areas Channel Migration Zone Study for South Prairie Creek, Pierce County, Washington Plate 2. Reasoning for Migration - Potential area Delineation Channel Migration Zone Study for South Prairie Creek, Pierce County, Washington
APPENDICES
Appendix A - Methods Appendix B - Characteristics of SPC Reaches Appendix B Tables Table B-1. South Prairie Creek CMZ Analysis - Geomorphic Reach Descriptions
Appendix C - Criteria lor CMZ and MPA Delineation Appendix C Tables Table C-1. South Prairie Creek CMZ Analysis - Criteria for CMZ and MPA Delineation
Appendix D - Report Limitations and Guidelines for Use
File No. 2998-007-00 Page iii April 29, 2005 GeoEnetsnenslQ Geouonpnlc EvtluenoN AND Cxlt¡¡¡el Mlcntlot Zone Arualvsls Sourn Pnelnte Cneex Plence GoutttY, Msnl¡¡crou FOR Prence Counrv Wlren Pnoentus Dlvlslot'¡
INTRODUCTION
This report summarizes the results of the Channel Migration Zone (CMZ) Analysis for South Prairie Creek (SPC) prepared for Pierce County Public Works and Utilities, Water Programs Division. It is the fourth in a series of CMZ analyses conducted by GeoEngineers for Pierce County.
The purpose of this project is two-fold: l) map the extent of the channel migration zone (CMZ) for a portion of the South Prairie Creek, and 2) identify severe, moderate and low Migration Potential Areas (MPAs) within the CMZ. The study area, shown in Figure 1, encompasses South Prairie Creek and the surrounding floodplain from just upstream of the town of South Prairie to the confluence of SPC with the Carbon River, a distance of approximately 6 river miles (see Plate l).
PROJEGT OBJECTIVE
Based on observed historic (historic aerial photographs) and present conditions, several sections of South Prairie Creek have been, and are presently, subject to migration. Channel migration is the gradual or abrupt movement of a channel within its floodplain. The CMZ represents the swath of land adjacent to the channel within which the migrating channel could move over a 50-year period of time.
One of the County's principal objectives in delineating channel migration zones with all these projects has been to use the resulting CMZ andMPAs as a decision making tool in the following applications: o Maintenance of existing infrastructure (levees, revetments, roads, bridges) ¡ Floodplainmanagement . Revision of floodplain ordinances o Identification ofpotential future levee setback projects
Because the South Prairie Creek (SPC) study is just one of several CMZ studies to be conducted, the County also wanted to employ an approach and technology that would be consistent with previous CMZ analyses. In order to meet these criteria, as well as the above objectives, the South Prairie Creek (SOUTH PRAIRIE CREEK (SPC) CMZ analysis was conducted for the general condition of an unconfined channel, that is, in the absence of levees, revetments, roads, and other channel confining structures. This approach is consistent with previously completed CMZ analysis, and is needed to meet the intent and long-term goals set by the County, as stated above.
PRo¡ecr AppRoncn Aruo Scops
Our approach to identifying the CMZ boundaries and three (low, moderate, severe) MPAs within the CMZ involved four major elements: 1) Data Collection, Review, and Selection; 2) Geographic Information Systems (GIS) Data Preparation; 3) Geomorphic Evaluation; and 4) CMZ and MPA Delineation. Detailed descriptions of methods and technology used in completing overall project goals are provided in Appendix A. Activities comprising the major scope elements are summarized below.
File No.2998-007-00 Page I GeolxetneznslQ April 29,2005 Collect and review literature and other materials provided in the References section to gain a historical perspective ofthe basin, including channel changes that have occurred in the past, land use history, and the construction of channel constraints.
a Research, collect and evaluate available aerial photographs for the study area.
a Collect and assess existing Geographic Information System (GIS) data and compile a comprehensive, linked GIS data base.
Select 8 sets of aerial photos dating back to l93l to be added to the GIS. A total of 45 aerial photos were selected based on both the quality and extent ofthe photo coverage, and the channel conditions represented in the photos.
Review and evaluate the comprehensive Geographic Information System. The GIS includes primarily topographic, geologic and soils maps, and aerial photographs.
Conduct preliminary stream reconnaissance on South Prairie Creek. Site visits were conducted in June of2004.
Evaluate geomorphic processes; this task includes dividing the main stem channel into geomorphic reaches and measuring the distance of migration between sequential aerial photographs, and calculating rates of migration for each reach using the GIS.
Conduct two additional site visits on July 30 and August 2,2004 to veri$r the results of our data evaluation.
¡ Delineate the Channel Migration Zone (CMZ) for South Prairie Creek based on the results of preceding tasks using GIS tools.
¡ Identify Migration Potential Areas (MPAs) within the CML In coordination with Pierce County staff, we developed criteria by which to delineate MPAs.
Deliverables for this project include (l) this report; (2) GIS layers includingCMZ boundaries, migration potential areas, dated channel locations, historical channel occupation tract (HCOT), and, (3) selected aerial photos for 1961, 1965166,1970,1978,1985, 1988, and 1992 in GIS compatible format. GIS layers and documentation are provided on a CD-ROM, which accompanies this report. Please note that the county has already in its possession orthophotographs, taken in 1998 and 2002, and infrared orthophotographs taken in2002. These images are, therefore, not included in the CD-ROM.
Reponr Oncnruzalon
The following report is presented in two parts: l) the Geomorphic Evaluation and 2) the Channel Migration Zone Analysis. The Geomorphic Evaluation describes basin- and reach-scale characteristics and processes known to influence and/or control channel behavior over time. The Channel Migration Zone Analysis describes the approach, assumptions and results of the CMZ and MPA delineation.
GEOMORPHIC EVALUATION lHrRooucnot
Drainage basin development and channel behavior evolve and develop under the influence of numerous basin- and reach- (or local) scale factors and processes. Because changes in basin and local processes are reflected by changes in channel behavior, migration style and/or rates of migration, delineation of a
File No.2998-007-00 Pøge 2 April 29, 2005 Geo1xenzeaslQ channel migration zone requires a thorough understanding of historic and current geomorphic characteristics and channel forming processes.
Reero¡rnl Serrrruc
This portion of the report focuses on the South Prairie Creek's regional setting and includes sections on the creek's location, the regional topography, climate, basin hydrology, geological history, and land use of the region.
Location
South Prairie Creek (SPC) drains a portion of the greater Puyallup River Basin in eastern Pierce County, Washington (Figure l). The SPC drainage is a tributary to the Carbon River, which flows into the Puyallup River north of Orting, Washington. The SPC basin is approximately 90.2 square miles in area, and extends a total distance of about 30 miles.
Basin Topography
The South Prairie Creek basin is composed of two major topographic elements: an upper basin located in the rugged western slopes of the Cascade Mountain Range, and a lower basin, which extends onto a broad, low relief floodplain.
The upper basin is situated in the steep terrain comprising the northwest face of Carbon Ridge, located on the distal western flank of Mt. Rainier (Figure 2). Carbon Ridge peaks range in elevation from 4,000 to 5,933 feet. The west slope of the ridge contains numerous small lakes and tams, several of which are source lakes for two upper forks of South Prairie Creek and its major tributaries. From these headwater lakes (located between Elevations 4,000 and 4,500 feet) down to roughly Elevation 500 feet, the main stem stream and tributaries descend from 3,500 to nearly 4,000 feet in roughly 22 miles through deeply incised bedrock ravines. The upper basin ravines are generally deep and nanow with steep side slopes. The floors of the ravines contain little or no floodplain area and are typically bedrock confined.
The lower basin extends about 8 miles from the base of Carbon Ridge to the Carbon River confluence. At about Elevation 490 feet, the main stem channel enters a relatively broad valley with low relief terraces and floodplains, South Prairie Valley. The valley is, on average, approximately 1,000 feet wide with steep valley walls that range from 100 to 250 feet in height. The floor of the valley is flat, with the exception of a few asymmetrical terraces that make up less than 10 percent of the valley's surface area. The valley cuts through the Puget Plateau, a relatively flat surface that is roughly 400 to 500 feet above sea level. The Puget Plateau is observed across the majority of the Puget Sound, south of Everett.
Climate
Climate within the South Prairie Creek basin is typically mild and humid, consisting of warm, dry summers and cool, wet winters. The mean annual precipitation in the basin varies from approximately 44 inches near the mouth of SPC to 85 inches near the headwaters. Approximately 80 percent of the precipitation received by the basin falls between October and March (USGS, 1998).
Basin Hydrology
A large portion of the South Prairie Creek basin is located within a transient-snow zone and therefore experiences rain on snow flooding events. In a transient-snow zone, flooding primarily occurs from long-
File No.2998-007-00 Pøge 3 Geolxetxezasl/ April 29,2005 duration rainfall storms augmented by snowmelt contributions. Winter storms produced by regional low- pressure warm-fronts often cause rapid melting of the transient snow pack between elevations 1,500 and 3,500 feet, producing large runoffevents that can cause flooding in the lower basin. These rain-on-snow events are a significant factor in the region's flood patterns and ultimately to channel forming processes.
The South Prairie Creek (SPC) drainage system includes several major tributaries, all of which drain mountainous areas in the upper basin and within the transient snow zone. The five major tributaries are: l) Page Creek, which drains the Puget Plateau south of Buckley ; 2) New Pond Creek, which drains the westem flanks of the northem two peaks of The Three Sisters; 3) East Fork South Prairie Creek, originating on Pitcher Mountain; 4) South Fork South Prairie Creek, draining the north side of Old Baldy Mountain as well as the west flank of the Carbon Ridge; and 5) Wilkeson Creek (and its tributary Gale Creek), which drains Burnt Mountain. Five small unnamed spring-fed tributaries and drainage ditches flow into south Prairie creek within the South Prairie valley in the lower basin.
The SPC basin drainage pattern is one in which all major tributaries in the upper basin collect and convey discharge from their subbasins to the main stem channel above Elevation 500 feet. A consequence of this drainage pattem is the production of relatively Iarge peak flows, and flooding, in the lower basin. These conditions result in relatively frequent episodes of flooding, and dynamic channel forming processes, in the lower basin.
Flooding and Peak Flows. Seasonal flooding on South Prairie Creek (SPC) can occur anytime between October and March. However, flood records indicate that the majority of the floods, roughly 62 percent of all peak flows, have occurred between December and February. Gage data was collected from 1950 to 1979 and from 1988 to present at a USGS gaging station at the town of South Prairie (Figure l). The 10 largest peak-flows between 1950 and 1998 in the South Prairie Creek Basin, are provided in the following table. The flood of record occurred on February 8, 1996.
Table 1. Flood History of South Prairie Creek
I uscs, rsso 'NHC/FEMA, 2003 provided by Pierce County NR = Not Reoorted
Local flood damage records, over the last 50 years, provided by the 1998 USGS report, County employees, and local accounts provide some insight regarding SPC erosion potential. For example, flood waters washed out an SR-162 bridge in the December 1956 flood. In February 1996 (the flood of record) the creek washed out an existing revetment near Spring Site Road, caused significant bank erosion at several locations, and flooded SR-162 at several locations. Reported flood damages used in this analysis are described in the geomorphic reach description (See Appendix B).
File No. 2998-007-00 Page 4 April 29, 2005 GeoÉxenllnaslQ Modeled Discharge (Annual-Peak-Flow Frequency). A log-Pearson Type III distribution, with log transformation of discharge, was performed by the USGS (1998) to determine annual-peak-flow frequency and to estimate discharges for various annual exceedance probabilities. The 1.25-, 2-,5-, l0-, 25-,50-,100-, 200-, and 500-year peak flows are shown in Table 2 below.
Table 2. Summary of Peak-flows on South Prairie Greek (USGS, 1998)
Recurrence lnterval (years)
Notes: 1uscs,199g 'NHC/FEMA, 2003 provided by Pierce County NR = Not Reported
Geoloe¡c HrsronY lntroduction The geological history of the South Prairie Creek Basin includes tectonic, glacial, volcanic and fluvial processes. Regional tectonics generated the formation of a fore-arc basin (the Puget Sound) and a volcanic mountain range, the Cascade Mountain Range (Cascades). The major geologic events involved in the formation of the basin are described below in chronological order.
Cascades Mountain Range and the Puget Sound Basin The South Prairie Creek Basin includes a portion of the Puget Sound lowlands and the adjacent Cascades. The topography and underlying soils present in the basin are largely a function of the surrounding geologic and structural setting. The Cascades are composed of uplifted Tertiary and Quaternary volcanic and sedimentary rocks (35 million years ago to present). Rocks of Cascade origin are observed in the upper basin. The lower basin is situated within a large structural basin, called the Puget Sound lowlands, that is buried beneath a thick sequence of relatively recent un-weathered glacial and interglacial sediments.
Conti nental G laciation The last continental glacial advance, the Vashon Stade of the Fraser Glaciation, produced much of the existing topography within the study area. As the glacier advanced it overrode and compacted the sediment comprising the Puget Plateau. As the glacier retreated, meltwater streams eroded the trough- shaped South Prairie Creek Valley, shown in Figure 2.
Souúñ Prairie Valley Formation The South Prairie Creek Valley was likely formed by glacial meltwater. From about 14,000 to 5,600 years before the present, the White River flowed through a narrow gorge at the south end of Mud Mountain (located 4 miles east of Buckley) and into South Prairie Valley and the Project Area. At this
File No. 2998-007-00 Page 5 GrcErctxnnslQ April 29,2005 time, the ancestral White River drained an alea three times greater than the present-day South prairie Creek watershed. Thus, peak flows of the ancestral White River were consiàerably higher than those observed in South Prairie Creek today. The ancestral White River carved out the steep bluffs observed along the valley walls of South Prairie Creek and deposited alluvial sediments derived from Mount Rainier across the vallev.
Osceola Mudflow
A partial collapse of Mount Rainier's composite crater triggered a massive lahar, the Osceola Mudflow, approximately 5,600 years ago. The Osceola Mudflow flowed down the northeastem side of Mount Rainier, engulfing the White River Valley and surrounding area from the mountain to about the location of Aubum, Washington. The mudflow plugged the narrow gorge where the ancestral 'White River entered the South Prairie Valley, causing the White River to divert from its historic channel to a new course, which it occupies at present (see Figure 2). The mudflow coursed through the South prairie Valley as far downstream as the Carbon River confluence. The mudflow inundated the gorge and buried much of the South Prairie Valley floor and pre-existing ancestral White River alluvial deposits.
Recent Soufñ Prairie Creek Activity
The Osceola Mudflow redefined the South Prairie Creek (SPC) drainage basin. Prior to the mudflow, the ancestral White River conveyed large volumes of water and sediment (up to boulders in size) from Mt. Rainier glaciers through the South Prairie Valley. The upper basin tributaries and mainstem were probably tributary to the V/hite River. After the mudflow event, the SPC basin was completely cut off from the White River, leaving only the upper basin tributaries and mainstem to collect andconvey water and sediment to the lower basin. The historic gorge is presently drained by Beaver Creek, which formed after the mudflow. The Beaver Creek drainage is a tributary to the modem South Prairie Creek.
The presence of the Osceola Mudflow and the abrupt decrease in basin size have had profound effects on both the fluvial and geomorphic character of the present day SPC mainstem throughout the lower basin. The present day creek utilizes only a fraction of the valley floor and is, by definition, an undersized stream; meaning that SPC lacks the stream power necessary to erode valley walls and the boulders included in the ancestral White River deposits. Over the last 5,600 years the main stem stream has created out its own corridor through the Osceola Mudflow.
Geoloetc AND SotL Ut¡lrs
Seven geologic and surface soil units are present within the South Prairie Creek basin. For discussion purposes, we used the mapping units by Walsh et. al. (1987) unless otherwise noted. These units are described from oldest to youngest:
Bedrock
This unit consist of sedimentary and volcanic rocks from the Eocene (the Puget Group), Oligocene (Ohanapecosh Formation), and Miocene (volcanic deposits). The Puget Group is made up ãf t6. Carbonado and Spiketon Formations, which are indiscemible units of arkosic sandstone, siltstone, and seams of coal. These units were deposited in an environment of fresh to brackish water, such as an embayment receiving fresh water from rivers. The Carbonado and Spiketon Formations are separated by the Northcraft Formation, which is a resistant wedge of andesitic volcanic bedrock.
F¡le No. 2998-007-00 Page 6 April 29,2005 GeoExeneznslQ Ofting Formation (Pre-Vashon Stade, Glacial Drift)
The Orting Formation includes layered silt, sand, gravel and cobbles deposited by glacial meltwater streams. Individual layers are well sorted and may exhibit cross-beds and cross-cutting features. The Orting Formation is well compacted as a result of having been overridden by continental glaciers. These deposits are exposed along portions of the South Prairie Valley walls. In some places outcrops of Orting Formation overlain by Vashon continental glacial till are exposed in vertical cliff faces adjacent to the river. The Orting Formation is moderately resistant to erosion.
Glacial Outwash (Vashon Stade)
This unit consists of sand, gravel and cobbles deposited by glacial meltwater streams. The outwash mantles the older glacial deposits and varies in thickness from l0 Io 120 feet. The deposits are exposed alongside the Orting Valley wall and on the Puget Sound Plateau. Glacial outwash is moderately susceptible to river erosion.
Ancestral White River Alluvium
White River alluvium consists of sand to boulder sized sediment that was transported and deposited by the White River prior to the Osceola Mudflow event (see below). The alluvium is relatively loose and unconsolidated, and therefore susceptible to erosion. However, the largest clasts, which are in excess of two feet in diameter, are too large for the creek to transport and may help to armor the stream banks or channel floor.
Osceola Mudflow
The Osceola Mudflow is composed of an unsofted mixture of sub-angular to sub-rounded volcanic rock fragments (up to 5 feet in diameter), which is suspended in a matrix of sand, silt, and clay. The Osceola Mudflow is the largest in volume of at least l0 mudflows originating from Mount Rainier over the past 14,000 years. It generally exhibits a gradation of sediment-sizes from coarse material at the base to fine material at the top. Carbon-age dating indicates the mudflow is approximately 5,600 years old.
The Osceola mudflow underlies much of the South Prairie Valley, as shown in Figure 4. The mudflow deposit is exposed primarily in the floodplains and along the channel banks. Unweathered mudflow deposits are relatively resistant to erosion and typically form nearly vertical cut faces where exposed. 'Weathered portions of the mudflow are generally friable and are less resistant than the rest of the mudflow to erosion.
South Prairie Creek Rivetwash (Alluvium)
This unit consists of sand to cobble-sized sediment that is transported and deposited relatively recently by the SPC. These deposits are exposed in both active and former SPC riverbeds. Riverwash is generally heterogeneous throughout the project area, but can be moderately well sorted locally. Riverwash is relatively loose and unconsolidated and is, therefore highly susceptible to erosion.
Souúh Prairie Creek Silty Loam (Overbank Alluvium)
This alluvium unit consists primarily of silt and sand with organic detritus deposited by SPC floodwaters. They are commonly reworked by agricultural disking and plowing. Silty loam is relatively loose and unconsolidated, and highly susceptible to erosion.
File No. 2998-007-00 Page 7 GeoEnetxeenslQ April 29,2005 Distribution of the Geologic Units
The spacial distribution of geologic units (described above) throughout the South Prairie Valley support the geological history describe above. The ancestral White River eroded downward through Orting and Vashon deposits (exposed in the valley walls), depositing White River alluvium as it did so. Upon the inundation of South Prairie Valley with Osceola mudflow, the White River was diverted. Recent SPC alluvium and colluvium was deposited on top of older deposits, as shown in Figure 3. Both the surficial soils and SPC channel position are partially dictated by the extent of the ancestral White River alluvium and Osceola mudflow deposits.
The surface of the valley floor is currently composed of terraced mudflow deposits and recent SPC river wash and overbank alluvium. Natural Resource Conservation Service (formerly the Soil Conservation Service) soil studies and surficial geological mapping completed in the 1960s indicate that the valley floor is composed of Osceola Mudflow deposits, which have been eroded or covered by alluvium. The recent alluvium consists of riverwash and silty loam (overbank deposits) deposited by SPC. Figure 4 shows their approximate extent Alluvial sediments derived from the ancestral White River underlie deposits from the Osceola mudflow. This unique stratigraphy is an important factor in the channel behavior.
Basrru DeveloplvrENT AND Lano Use Pnncnces
Introduction
Resource extraction in the upper basin, and development of rural communities and transportation infrastructure throughout the South Prairie Valley have greatly influenced SPC channel behavior over the past several decades. Land use activities in the South Prairie Creek Basin have included coal mining, building stone quarries, railroad operations, timber harvesting, and farming. The construction of transportation infrastructure in support of industry and land use led to construction of channel bank revetments as a means of protecting roads and rails from flooding and erosion, and eventually changed the behavior of the channel over the course of several decades. The development within the basin largely took place from the 1870s to 1930s. Below we describe the regional condition prior to development and the dominant land use activities that took place.
P redevel opment Condition
Historic records indicate that the region was largely forested prior to development in the late 1800s, except one area on the floor of South Prairie Valley, which was apparently maintained as a prairie by Native Americans. Natural prairies are rare in Western V/ashington. A local historian indicated, "fNative Americans] 'routinely bumed the prairies to keep the area clear for hunting purposes." A USGS land classification map surveyed in 1897 confirms the presence of the prairie as treeless areas on the valley floor immediately downstream from the town of South Prairie (see Figure 5).
Coal Mining
Historical maps and accounts indicate that the first development along SPC supported coal mining. Beginning in 1874, coal was mined from the Carbonado Formation. This coal was considered the best coking coal on the West Coast and was used in railroad operations. The underground mines required timbers for working, hence nearby forested hills were cleared. Mining operations were based on gravity; therefore mine tailing and loading operations were based along side or within the creek bed. Mining activity in upper basin of SPC likely increased sediment loads and downstream deposition. Today, layers of fine coal fragments interbedded with sand are observable along SPC cutbanks in several locations,
File No. 2998-007-00 Pøge I April 29, 2005 GeoExetneeaslfi which record the downstream transport of tailings during the coal-mining era, from approximately 1874 to 1960s. The mine waste pilings were likely a sizeable source of sediment to the stream at that time.
Logging (Timber Harvesti ng)
The 1897 land classification map indicates that most of the South Prairie Valley bottom was cleared and replanted by the end of the lgth century (F igure 4). However since that time, the history of logging within in the basin has not been well documented. A USGS study entitled, Flood Potential of South Prairie Creek, Pierce County, Washington by Mastin, MC, 1998, provide a summary of logging within the basin from the mid-1960s to the late 1990s. The report states that logging increased from 1965 to 1990, producing dramatic changes within the basin. The percentage of clear-cut area within the total basin area increased from I1.2%o in 1965 to 34.5o/o in 1990. Accordingly, the total length of timber-harvest roads increased from 119.6 miles in 1965 to 237.0 miles in 1990. The increase in logging activities likely resulted in a greater potential for flooding based on the following factors: l) increased snow accumulation in clear cut areas; 2) increased rates of snowmelt within clear cut areas, creating higher runoff volumes; 3) increased rates of snowmelt during rain events due to exposure to the rainfall; and 4) increased rates of sediment production and erosion associated with clear cutting and road construction.
As described in the USGS report, these activities increased sediment production and delivery to the channel, which in turn, likely decreased channel dimensions and the capacity to contain floods. These conclusions are consistent with other streams in logging areas throughout the northwestem United States, wherein sediment production and delivery to adjacent streams increased in the 1960s and 1970s, and then decreased from the 1990s to the present because of the strengthening of state-mandated sediment management practices.
T ran s po rtatio n I nf rastru ctu re
Along the eastern side of the valley, the former Nofhern Pacific Railway Company constructed a railroad. Completed in 1887, it ran to Buckley and Wilkerson and included a bridge near the town of South Prairie. Where the creek impinged upon the railroad embankment, revetments were constructed to prevent erosion.
Road construction and their protective revetments added to the confinement of the creek, in some areas to a very small portion of the valley. SR-162 (previously Pioneer Way) was constructed sometime between 1897 and 1931. Between 1965-7, the state improved the highway, built new bridges and widened the road. Today, the railroad and SR- I 62 confine SPC to a fraction of the valley floor.
Riparian Habitat
The riparian habitat that existed prior to settlement was largely removed, over 100 years ago. The 1897 land classification map shows the South Prairie Valley was used for agriculture. Today, much of the valley floor is maintained as pasture or fallow open space. Conversely, only a small portion of the valley is currently wooded. The creek's riparian areas commonly consist of narrow bands of Cottonwood and other deciduous trees (from I tree to 30 feet wide).
Recent state regulations and public interest has helped to improve the riparian habitat to protect solmoniods. To date, the riparian areas along the creek have not recovered to pre-development riparian conditions. Mature wood, especially cedar, is not available for recruitment into the stream, which is often cited as a key factor increasing channel complexity and habitat for solmoniods. The availability of wood in a channel is also a vehicle for sediment deposition, which in turn, can increase the potential for channel
File No. 2998-007-00 Pøge 9 GeoEnetxeeeslQ April 29, 2005 migration. A progression of historical photographs from 1961 to lgg2indicates that the size and total number of riparian trees within the project area has increased slightly in the last 22years.
Reacx-ScaLE G EoMoRpHtc PRocESsEs lntroduction
Reach scale channel forming processes include erosion, transport and deposition of sediment and debris within a channel, and flow dynamics through each reach. The extent to which these processes influence channel development are generally determined by one of more of the following controls that can change both spatially (from one reach to the next) and temporally (over time). The relationships between factors that control channel form and channel forming processes can be explained in the following example.
In upstream channel reaches where gradients are steepest, the hydraulic action of fast flowing water results in the net erosion of the streambed, which in turn forms a straight, V-shaped valley wherein the stream channel occupies all or most ofthe valley floor. In contrast, as the channel gradient decreases and the valley becomes wider, a point is eventually reached where the stream behavior changes from one dominated by streambed erosion (vertical down cutting), to more complex behavior including deposition, bank erosion, channel widening, and development of bends, bars and floodplains. Below we review the types of migration we observed and major factors that control channel forming processes for South Prairie Creek, which include the following; a) the composition of bank and bed material, b) the supply of sediment and woody debris, c) the transport capacity (the ability of the stream to move sediment), and d) the regional and local topography.
Composition of Bank and Bed Material
Relative rates of lateral migration are controlled to some extent by the composition of bank and bed soils. As described in the Regional Geology section above, the Orting formation, White River boulders and Osceola Mudflow are relatively resistance to erosion by South Prairie Creek. In contrast, younger alluvium is composed of sediment that was previously transported and deposited by the stream and is therefore susceptible to erosion, transport and re-deposition. The erodibility of the bank material plays an important role in the channel pattem (straight or meandering) that we observed.
Supply of Sediment and Woody Debris
The volume of sediment and woody debris entering the stream from external source areas can greatly influence channel forming processes and, ultimately, channel pattern and behavior. Changes in land use within the South Prairie Creek Basin, particularly mining and logging, significantly increased the volume of sediment in transport compared to pre- development conditions. After mining activity ceased, the volume of sediment entering the main stem channel, and moving through the system, decreased with time.
One result of the abrupt increase and then decrease in sediment influx is the change in channel behavior following the change in sediment flux. For example, as a large slug of sediment enters a stream reach, the typical channel response is channel aggradation coupled with braiding and multi-channel development, widening of the high flow corridor, and active migration. As more sediment is received, rates of migration typically increase. As sediment influx decreases a main channel develops, migration rates decrease, braid bars stabilize and become vegetated, and the main channel becomes more stable.
Another important influence on local and reach-scale fluvial processes is the relative abundance of woody debris in active channels, especially large-diameter wood. The presence of large woody debris (LWD) within the active corridor will typically increase the complexity of local flow dynamics by diverting
File No. 2998-007-00 Page l0 April 29, 2005 Geo1netneenslQ and/or splitting the flow, which in tum, creates chutes, eddies and scour pools, and alters the incidence and character of channel erosion, bar development and, subsequently, channel pattern and behavior. In streams with relatively low channel gradients, bedload sediments accumulate as bars in the immediate vicinity of LWD. Historically, stream reaches subject to high incoming sediment discharge and abundant in-channel LWD tended to aggrade, widen and develop braided channels from bedload deposition, which further contributed to the complexity of in-stream flow and morphology.
Early logging and agriculture within the South Prairie Creek basin reduced or eliminated the historic riparian zone, and thus depleted a large percentage of the large wood available for stream recruitment. As the floodplain was further developed, flood control strategies included the removal of existing large woody material from within the channel as a means of increasing flow velocities, which improved transport capacity (see next section) and subsequently, eroded bars and channel floor sediment causing channel incision and loss of flow and morphologic complexity. New conservation regulations will likely increase the amount of riparian wood available for recruitment by South Prairie Creek.
Transport Capacity
The ability of a stream to transport sediment is referred to its "transport capacity". The transport capacity of a stream is determined by discharge, and channel gradient. Channel gradient is typically used to describe the difference in transport capacity from one reach to the next. For example, an increase in channel gradient, coupled with sparce distribution of channel bars would generally indicate a higher transport capacity. However, signifrcant sediment or water input from either a tributary, spring or groundwater can change the transport capacity downstream ofthat input.
Transport capaci|y, coupled with sediment and woody debris supplied to the system, is a deterministic factor in the development of channel morphology. These three factors affect bar formation and channel form, as well as the shape and dimension of the channel, and ultimately, its behavior and relative migration potential. In river reaches where sediment delivery and transport capacity are well balanced, channel pattern and behavior tend to be relatively stable. In systems where the volume of sediment delivery continually exceeds transport capacity, significant deposition will occur resulting in reach- or valley-scale streambed aggradation and destabilization of the channel. Under these conditions, channel form typically consists of multi-thread braided channels with high width-to-depth ratios in higher gradient areas, or highly sinuous meander bends in lower gradient areas. In contrast, where transport capacity continually exceeds sediment supply, bank erosion and channel incision and entrenchment typically result. Where transport capacity exceeds sediment supply, erosional conditions prevail resulting in an increase in the mean size of bedload materials, lack of bed forms and reduction of channel complexity. Under these conditions, channel pattem is often straight and incised with smaller width to depth ratios.
Regional and Local Topography
The type of channel pattem we observe is partially controlled by topography (both, regional and local) that confines the river channel or protrudes into the channel altering flow direction. Channel pattems are affected where portions of valley walls protrude into a channel, effectively directing the course of the flow away from the feature towards the opposite bank, thus changing the flow dynamic. Changes in flow dynamics often result in changes in the pattems of erosion and deposition, and can lead to the redirection of channel misration.
File No. 2998-007-00 Page lI Geo1netnezeslQ April 29,2005 Ctetruel MtcRtrron
Migration is a reach-scale response of the channel to local and regional processes, as described above. In natural drainage systems, stream channels entering lower gradient reaches are seldom straight except over short distances. Lower gradients usually encourage deposition of a portion of the sediment load in transport, which causes small to large changes in the flow patterns, often resulting in erosion along the outside bank of channel bends. In a state of dynamic equilibrium, erosion along the outside banhof a meander bend and deposition on the point bar at the inside bank of a meander bend occur simultaneously, and at more or less similar rates. A result of this process is the lateral movement, or migration, of the channel across the floodplain.
The type and character of channel migration can vary considerably from one section of a steam to the next. The variation is controlled largely by the combined effect of differing sets of processes; gradient, sediment discharge, bank soils, as well as changes in existing processes. Over the course of the historic aerial photographic record for the project area, three principle types of migration were noted to have occurred most commonly: meander bend migration, channel avulsion, and braided channel migration. A fourth, less often noted type of migration, refened to as vertical migration, is also noted in ihe project area. Following is a description of each migration type.
Meander Bend Migration
Meander bend migration involves erosion of the outside bank of the river bend coupled with concurrent deposition of sediment along the inside bank of the bend. This process results in the lateral movement of the channel, while maintaining consistent channel shape and width. However, the area of most pronounced migration usually occurs where flow converges against the outer bank near the downstream end of a bend, resulting in simultaneous lateral and downstream migration of the bend. prior to confinement of project area channels, highly sinuous meander bends tended to develop in low to moderate gradient areas subject to sandy bedload and erosion prone riverbanks. Highly sinuoui bends are typically the product of both lateral and downstream migration.
Avulsion
Avulsion is the abrupt movement of an active channel to a new location in the river conidor. This process usually occurs in response to sudden deposition and infilling of the active channel by sediment or debris, causing the stream to erode a new channel or reoccupy a formerly abandoned channel. Avulsion is most common in aggrading channel sections, where the active channel may abruptly abandon its location for a new channel area during a single high flow event.
Avulsion is also observed in meander bend areas. Under these conditions avulsion typically occurs as a meander bend cutoff, wherein a highly sinuous, looping bend is pinched off at the neck, thus abandoning the bend and straightening the channel pattern. Avulsion by meander bend cutoffs is common in lower gradient reaches with highly erosive bank soils.
Braided Channel Migration
Braided channel sections typically consist of multi-thread channels separated by gravel and cobble bars. The channels are typically shallow and migrate rapidly within the braided area as bars are eroded and re- deposited within the channel corridor. Braided channel sections tend to develop in areas with a decreasing gradient and where the channel was subject to an influx of large volumes oflsediment.
File No. 2998-007-00 Pøge 12 April 29, 2005 Geo1netneenslQ Highly braided channel migration was not observed within the project area. Historically, short sections of the creek experienced some braided channel migration.
Ve¡tical Migration
Vertical migration involves the downward vertical movement of the channel floor, generally resulting in a deeply entrenched channel detached from its floodplain. Vertical migration usually occurs in channel sections with erosion-resistant bank materials, such as mudflow or glacial till, or where the channel has been held in place by artificial structures such as revetments. Vertically migrating channel sections are generally straight, single thread channels exhibiting virtually no observable lateral migration.
Over time, vertical migration can lead to cut-bank failures and minor channel widening. Many sections of South Prairie Creek show evidence of incision and vertical migration. The evidence includes rip-rap found stranded high on channel banks and undercut root systems ofriparian trees.
Rencx Scele Geouonpuc EvALUATtoN
This section of the report summarizes the results of the geomorphic analysis conducted for the South Prairie Creek CMZ study. The summary is based on detailed descriptions of the geomorphic conditions for stream reaches comprising the SPC basin. The results of the detailed evaluation are provided in Appendix B of this report.
For this evaluation, South Prairie Creek was divided into seven geomorphic sections, or reaches (Figure 6). The following summary includes the upper basin, and six project area reaches, which includes a large portion of the South Prairie Valley. A stationing system was created to assist in locating specific features on the creek. The Stationing STA 0 to STA 360 was created from the DNR stream center line (1998) beginning at the confluence with the Carbon River. Each integer represents 100 feet (i.e. STA 0 begins at the confluence and STA 360 is 36,000 feet upstream of the confluence). Reach delineations are based on the presence and continuity of physical features, and dominant geomorphic characteristics. Reach delineations are included in the GIS CD-ROM package accompanying this report.
The following summary includes information regarding the existing channel condition, historic channel behavior, and potential future behavior.
Uppen Basn
The upper basin is situated in the rugged terrain comprising the northwest face of Carbon Ridge. From the headwater lakes (located between Elevations 3,600 and 4,000 feet) down to roughly Elevation 490 feet the main stem stream and tributaries descend from 3,000 to 3,500 feet in 22 miles through deeply incised volcanic bedrock ravines. All major tributaries enter the mainstem SPC within the upper basin.
Main stem and tributary ravines are generally straight with occasional bends, probably originating from ancient joints and/or other weaknesses contained within the bedrock. The main stem channel gradient is greater than2 percent on average. In general, the walls of the main stem and tributary ravines are steep, and presently, well forested. The bedrock walls of the ravine are overlain by a thin to thick layer of colluvial soil and organic debris. The volcanic bedrock and colluvium are both prone to mass wasting, which can deliver sediment and large woody debris directly to adjacent stream channels. The main stem high flow corridor contains many small alluvial fans and talus cones composed of gravel, cobbles boulders likely derived from erosional processes along the valley walls. Aerial photograph observations suggest that, compared against mass wasted sediment volumes entering the channels, relatively small
File No 2998-007-00 Page 13 Geo1netxezaslQ April 29,2005 volumes of gravel, cobbles and boulders are stored within the high flow corridor. This condition indicates that channel transport capacity greatly exceeds sediment influx throughout the ravine, and that most sediment entering the reach is transported through it to a significant break in slope at the mouth of the mainstem ravine. No evidence of measurable lateral migration was detected within the ravine during this evaluation.
Reacn 6 (Rnvne Mourn ro STA-323)
An alluvial fan extends from the mouth of the main stem ravine (about Elevation 490 feet) to approximately Elevation 440 (Plate 1). Across the alluvial fan in the downstream direction, channel gradients decrease significantly and the valley widens to join the South Prairie Valley. In this zone, the stream is well braided, indicating that a large portion of the sediment load exiting the ravine is depositing in response to the sharp decrease in channel gradient, and associated loss oftransport capacity.
Downstream from the alluvial fan the mainstem enters South Prairie Valley and a much lower channel gradients prevails (1 ro 2 %). Throughout the South Prairie Valley, the main stem channel is bounded by floodplains composed of historic White River alluviam and Osceola mudflow deposits, which bury thã White River alluvium in several areas (see Figure 4).
Rercn 5 (STA 323-2151
Reach 5 defines the upstream end ofthe project area. This reach is characterized by a single, moderately sinuous channel that is relatively well entrenched in flood plain deposits (Figures 6 and 7). Throughout the reach, the channel is relatively narrow compared to its depth, and channel gradients are generally higher than upstream reaches. Roughly 20 percent ofthe stream banks are confined by revetments. The rest of the banks consist of Osceola Mudflow deposits or SPC overbank deposits. Locally deep scour pools and eroded cut banks occur along the outside banks of channel bends. The channel is locally entrenched in the floodplain. The stream bed is composed ofgravel, cobbles and boulders: the boulders inferred to be derived from White River alluvium or Osceola mudflow. However, a few side channel bars or lag deposits were observed at the time of our site visits, indicating that transport capacity exceeds the influx of sediment from upstream sources.
The pattern and position of the main stem channel appears to have changed little since 1961. However, reach dynamics were far more active in the late 1800s, the major bends likely migrated and the stream flowed in a channel that is now abandoned (see Figure 7). Historical maps and dated aerial photographs indicate that a channel avulsion occurred sometime after 1897 and before 1961. The avulsion event significantly altered the flow dynamics of the stream, which in turn caused a meander bend to form downstream between 3TA276 and260.
Since 1965, the channel pattern throughout Reach 5 has evolved to a static, non-migrating and sediment starved channel section. In-channel bars have disappeared or have become vegetated overtime, the channel is locally entrenched, and stream banks, revetments, and road embankments are subiect to erosion during bank full flows.
No meander-bend migration has occurred within Reach 5 since 1965. However, over the last 42 years, very localized episodes of severe bank erosion have been reported. The erosion is typically associated with the recession of a vegetated cutbank, primarily along the outside banks of bends. In most cases. point bar or side channel bar building did not occur concurently with the bank erosion, and the erosional episode occurs but once, with additional erosion detected at the site in successive photo years. For example at STA 297, propefi owners witnessed the recession of a bank durine the Februarv 1996
File No. 2998-007-00 Page 14 April 29, 2005 Geo1xetxeenslQ flooding event. Despite other significant high flow events over the last 8 years no additional, measurable erosion has been reported.
The potential for future migration is relatively low. Over the period or record, the channel has remained stationary and stable, due primarily to the entrenchment of the channel in Osceola mudflow deposits. Localized bank recession in Osceola Mudflow deposits and erosion of alluvial soils will likely continue, but is unlikely to result in migration of channel. The abandoned channel on the north side of the current channel could provide an avenue for avulsion, however, the potential for avulsion here is very low, again due to channel entrenchment.
Rercn 4 (STA 215-195)
This reach is characterizedby a lower channel gradient, and moderately sinuous low flow and secondary channels bounded by large side channel bars (Figures 6 and 8). The low flow channel is relatively wide and shallow, with some pools and riffles between the gravel bars, and sections of localized entrenchment' The main and secondary channels are separated from one another by bars, composed of sand, gravel, cobbles, and a few boulders (probably derived from 'White River alluvium). This condition suggests that Reach 4, has been subject to aggradation, at least within the last several decades.
The right bank is composed of recent alluvium (sand and silt), and the left bank is composed of Osceola Mudflow and alluvial deposits. Roughly 20 percent of the total length of the stream banks are armored by bridge piers and revetments, which are located primarily along the right bank.
Historic maps suggest that migration was probably more active prior to 1965. The 1874 GLO and 1897 USGS maps indicate that two channels occupied the reach in the late 1800s and early 1900s. The 1874 GLO map shows the two channels running separately for more than 3000 feet; the 1897 USGS map indicates somewhat shorter channel lengths. One of the channels was situated in the present day channel location and the other channel was situated along the foot of the northem valley wall (see Figure 8). The northern channel was abandoned some time between 1897 and 196l'
Historical aerial photographs indicate that the main channel prevailed as a single meandering channel from 196 1 To 2002. During the February I 996 flood, however, a portion of the creek flow was directed across the Spring Site Road and along the north side of SR 162 (Reach 3), nearly resulting in the formation of a second channel and possible avulsion (personal communication with Pierce County staff). In spite of the 1996 flood event, the aerial photograph record indicates that the low flow channel width has steadily decreased with time. The decrease in width suggests that l) the rate of migration has decreased and2) sediment influx has likely decreased and the channel has become entrenched.
Based on historic and recent trends of migration, the potential for future migration is relatively high. Channel gradients, coupled with the potential for the influx of sediment arriving from upstream source areas and erosion of sensitive bank soils, form excellent conditions under which migration and continue to occur. Future redevelopment ofthe riparian zone throughout the reach could also lead to an increase of woody debris in the active and the high flow channel corridor, which would, in tum, contribute to bar building and erosion associated with migration.
Reecn 3 (STA 195-135)
Reach 3 bears great similarity to Reach 5. The reach can be characterizedby a single, moderately sinuous channel entrenched into the valley floor (Figures 6 and 8). Throughout the reach, the channel is relatively narrow; on average 50 feet wide, and 2 to 5 feet deep at the bank full condition. Channel gradients are
File No. 2998-007-00 Page I5 GeolrctneeaslQ April 29,2005 generally higher than the upstream reach. The channel floor is well armored with relic boulders from the White River alluvium and/or Osceola Mudflow deposits, and only a few gravel/sand bars are present throughout the reach. The stream banks are generally composed of Osceola Mudflow deposits oi valley wall soils. Roughly ll%io of the total stream bank length within the reach has been reveted to date.
Reach 3 has been subject to major modifications, which have significantly altered channel conditions. Historically, the channel occupied a different alignment; the 1897 USGS map indicates that the upper half of the creek was located in the middle portion of the valley and the lower half parallel to the train tracks on the left side of the valley, as shown in Figure 8. State Route 162 was built in the 1950s and it is not clear when or how (by human intervention or natural avulsion) the creek came to occupy its present channel location. Presently, the upper one-third of the channel within the reach is confined between SR 162 and the abandoned railroad bed (See Figure 8), where the banks are well reveted with riprap. Along the lower two thirds of Reach 3, the channel crosses from one side of the valley to the other in largé sweeping bends. However, the channel is either entrenched or semi-confined by the presence of the Orting Formation or Osceola Mudflow exposed in the channel banks.
The pattern and position of the main stem channel appears to have changed little since 1961. However, the channel is still able to respond somewhat to extemal stimuli. For example, the 1978 photograph shows a sediment mass (presumably a landslide deposit) at the toe of the left valley wall and within the active flow corridor (STA 147 in Figure 8). In response to the sediment mass, the channel migrated approximately 25 feet to the north. Subsequent photographs show that the creek migrated back to its original course as the mass of sediment diminished. As described in the Hydrology Section above, flooding in 1996 resulted in a portion of the creek flow being directed across the Spring Site Road (within Reach 4) and down the north side of SR 162 within Reach 3. The flow re-entered the current creek channel at STA 187 and 175 (see Figure 8).
Reach 3 is subject to localized episodes of significant erosion and bank recession. Similar to Reach 5, small isolated channel movements were observed over the last 42 yearc. In all cases a tree or clump of trees lining the bank was missing and presumed to be carried downstream. The majority of the problèms occurred between 1970 and 1978 (presumably during a l0-year flood event in 1975) and sometime within the 1996 flood season. Field observations confirm that in some areas local vegetation is being undercut along the cut bank side of the river.
Sustained channel migration has not been a dominant geomorphic characteristic in the reach since the channel was moved to its present location. Therefore, the potential for future migration is limited to periodic episodes of bank recession, which do not comprise lateral movement of the channel across the floodplain. In the event of an avulsion in Reach 4, the would likely occupy a low lying floodplain north ofSR 162 andthe creek.
Rercr 2 (STA 135-20)
Reach 2 is characterized by a highly sinuous single channel with several active meander bends and multiple abandoned channels. The high flow corridor includes numerous large bars and secondary channels (Figures 6 and 9). The low flow channel is generally broad (about 70 feet wide on average). It is bounded by gravel bars and is relatively complex, compared to upstream reaches. Channel complexity consists of channel pools and riffles, buried woody debris, a few small log-jams, and extensive off- channel and riparian habitat. Woody debris that is entrained in the active channel may be associated with local bar building and aggradation ofthe channel floor.
File No. 2998-007-00 Page 16 April 29, 2005 GrcExenzznslQ The streambed consists of gravel, sand and cobbles in the upper reach and medium to fine sand in the lower reach. The floodplains and streambanks are composed of either; SPC alluvium, Osceola Mudflow deposits or Orting Formation. The majority of Reach 2 is unconfìned; up to 10 percent of the total bank length within the reach has been hardened by bridge crossings, and revetments placed to protect the railroad and the highway.
Reach 2 has been subject to relatively active migration from 1874 to 2002. Oxbows located within the flood plains indicate that the channel was, historically, capable of very of dynamic movement. More recently sequential photographs (from 1961 to 2002) indicate that channel behavior is still quite active. These photographs identifr many aggrading, and therefore, unstable channel sections whose bends migrate both laterally and in a downstream direction. The most active migration appears to have occurred in aggrading channel sections, in particular those sections where woody debris is present.
Similar to upstream reaches, however, migration appears to have slowed over the last 22 years. Aerial photographs indicate that the channel has moved smaller distances or less frequently. In additions, the size of in-stream bars appears to have decreased over the same pèriod, suggesting that sediment influx has decreased.
The potential for future migration within Reach 2 is high. Based on the results of the geomorphic evaluation, this reach will likely remain active with respect to lateral migration of the main and secondary channels and downstream migration of channel bends.
Reacn 1 (STA 20-0)
Reach I joins the Carbon River. The reach is a sinuous, single channel, whose last 500 feet flows in a former Carbon River channel which is seasonally inundated by Carbon River flood flows (Figures 6 and 9). The channel is generally 60 feet wide and 2-4 feet deep at bank full conditions.
The streambed is composed primarily of sand, however, the lower channel at the confluence is armored with cobbles and boulders likely deposited by either the ancestral White River or the Carbon River. The low-lying floodplains on either side of the creek consist of low relief fields. No evidence of former SPC channel migration, such as abandoned channels or relict bars, are preserved in the floodplains.
No appreciable migration is likely within Reach l. Historical and recent records indicate that this reach is dominated by Carbon River hydraulics and sedimentation.
CHANNEL MIGRATION ZONE (CMZ) ANALYSIS AND RESULTS lnrnooucron The principal objective of this project is the delineation of 50-year channel migration zones in the absence of levees, revetments and infrastructure, which presently confine portions of the channel. This objective requires evaluating the channel from the perspective of a freely migrating system that is subject to existing reach-scale channel conditions and geomorphic processes. Problematic to this approach is the fact that revetments, roadway and railroad embankments, and bridge foundations have hardened sections of stream banks. Although most of these structures are discontinuous in nature, their presence clearly modifies both the character and long term rates of local migration. Therefore to achieve our project objectives, the character and rates of migration were documented at sites within each reach that have not been confined by levees and revetments, and that reflect observed reach-scale migration type and character.
File No. 2998-007-00 Page I7 GaoExetneeaslQ April 29, 2005 Our approach to delineating the probable extent of migration assumes that, in the absence of existing channel constraints, the future rate and character of migration will be similar to that of the past, given similar water and sediment discharge conditions. Subsequently, delineating the zone of potential future migration requires a thorough understanding of the cause/effect relationships between local topography, sediment influx, transport capacity and the type and character of migration occurring in each reach, as well as the characteristic response ofthe channel to changes in those relationships. Based on the results of our basin and reach-scale geomorphic analysis, those elements and relationships having the greatest influence on the directional trends and rates of migration include l) the influence of the Osceola mudflow deposit on the long term rates of migration, 2) the influence of the South Prairie Valley walls on channel pattern and migration, and 3) the potential for channel avulsion into ancient and historic abandoned channels.
GllcuurroN OF Ur.lconnteD MtcRAloN RATES
Average rates of migration were calculated for each reach by averaging the distance of migration observed between dated aerial photographs over the number of years between the photographs. Measurements were made directly from channel positions digitized from dated aerial photographs scanned and rectified into our comprehensive GIS database. Migration distances were measured at sites deemed representative of the dominant type and character of channel migration observed throughout the reach. Both lateral and downstream migration were noted throughout the study area. Where present, the trend of downstream bends migration were documented within each reach.
Whenever possible, two migration rates (long and short-term) for a section of the creek were calculated and recorded. A period of 41 years represents the longest term period available for this project; as defined by dated aerial photographs from l96l to 2002;this timeframe includes the 1996 flood of record. Older maps were not used in the calculation due to scaling and accuracy considerations. Shoft term migration rates were averaged over the period of two sequential aerial photographs. The short-term calculation accounts for abrupt surges of migration observed in some sequential photographs. The maximum long term migration rates for each reach were used in delineating the CMZ width.
CMZ DeUnEATIoN
Our approach in delineating the CMZ assumes that in the absence of channel constraints, the future rate and character of migration will be similar to those of the past, given similar water and sediment discharge conditions. We delineated the CMZ by 1) defining the HCOT,2) ueating a base width of the CMZ from calculated migration rates and 3) modifoing the base width based on reach specific conditions (for example an erosion resistant bank).
Define the HCOT
The HCOT identifies the documented zone that the stream has occupied over the period of record. The HCOT is relevant to the CMZ analysis because it is composed of alluvial sediments that are generally quite susceptible to erosion, depending on the extent of plant and tree growth on bars. For this study, therefore, it is assumed that the main channel can migrate anywhere within the HCOT in very short periods of time, perhaps in the period of single storm event. The outside boundary of the HCOT is used in this project as the line of origin from which the width of the CMZ is measured.
File No.2998-007-00 Page l8 April 29, 2005 GtoEnetneeeslQ Create CMZ Base Width
The base width of lhe CMZ was delineated to identiff the distance the channel could travel in a single direction over a period of 50 years in the absence of confining structures. The width and shape ofthe CMZ arc determined by measured lateral and downstream migration rates and trends. The CMZ was delineated in two steps; 1) establish the width of the CMZ by multiplying the maximum annual rate of migration for each reach by 50 years in a single direction, and 2) modiff the outside boundary of the HCOT to account for the trend of downstream migration (where applicable).
The shape of the CMZ was defined primarily by the shape of the HCOT and the projected direction of downstream migrating bends. The results of reach-specific migration analysis indicates that measured rates of downstream migrating bends are roughly 1.5 times greater than the lateral rate of migration for the same bend. This finding suggests that the future location of such bends could shift downstream and possibly increase the width and modify the shape of the CMZ. Therefore, to accommodate the impact of downstream migrating bends on the CMZ, the boundary of the CMZ was adjusted as necessary within the reach.
The migration rates used in calculating the base width of the CMZ are included in Appendix C.
Modify CMZ Base W¡dth
The CMZ base width was modified to compensate for geomorphic processes and migration character not accounted for in the process described above. The base width of the CMZ was modified for the following conditions: . The presence of the erosion resistant Osceola Mudflow deposit. Typically, this condition results in a reduction in the CMZ base width. The CMZ boundary will generally follow the mudflow deposit. In cases were the 1998 high flow channel was at the mudflow boundary, Íhe CMZ boundary is located 25 feet landward of the mudflow outcrop.
o The presence of a valley wall within the base CMZ. The South Prairie Valley wall is composed of Orting formation soils that are generally more resistant to erosion than floodplain soils. This condition also represents a reduction in the CMZ base width. The CMZ boundary follows the break in slope marking the toe of the valley wall. In cases were the 1998 high flow channel was positioned at the toe of the valley wall, the CMZ boundary is located 25 feet landward of the valley wall. o The potential for channel avulsion into ancient and historic abandoned channels. Ancient channel tracks were identified using aerial photos and topographic depressions in the valley floor. Professional judgment on the placement of the CMZ boundary was made on a case-by-case basis by a geomorphologist. However, in a few cases where an ancient abandoned channel was deemed to have the potential for capturing stream flow the CMZ was placed outside of the ancient abandoned channel track. A detailed discussion regarding the criteria and exceptions for placing the CMZ boundary are included in the metadata associated with the SPC channel_migration_zone boundary.shp shapefile. The criteria used to delineate the CMZ boundary are provided in Appendix C. Resulrs
The results of our CMZ delineations for the South Prairie Creek (SPC) are shown in Plate I and are provided in the GIS CD-ROM package, which accompanies this report. These results clearly show the broad variation of migration potential throughout the project area. Reach-scale variations of the CMZ are
File No. 2998-007-00 Page 19 GzoExetnzzeslQ April 29, 2005 a result of the local maximum rates of migration estimated for each reach and the influence of downstream migration bends. Some of the results of our CMZ delineation are summarized below: o Migration rate measurements show clearly that SPC migration is highly variable. Migration rates are highest in Reach 2, where relatively high influxes of sediment and low channel gradients create local aggrading sections. As a result the CMZ width is greatest in Reach 2.
. Reaches 3 and 5 were dominated by periodic surges in bank erosion and recession. Only small isolated channel movements were observed over the last 42 years, resulting in a relatively nanow CMZ.
. Reach I results clearly indicate that the channel has not migrated appreciably over the period of record. The Reach I CMZ boundary is positioned 25 feet from the edge of the existing high flow channel. This distance 25 feet is approximately equal to the positional error possible over the sequence of historical photos we reviewed.
o The potential for channel avulsion outside the HCOT was evaluated with respect to abandoned channels or topographic lows throughout the project area. Four potential sites of avulsion are present Reaches, 2, 4 and 5 (see Figures 9, 8, and 7). The CMZ was widened to include the abandoned channel or topographic low at these sites.
MTGRATTON POTENTTAL AREA (MpA) DELTNEATTON hrnoouclo¡¡
The MPA delineation involved identifring severe, moderate and low migration potential areas within the delineated CMZ. Our MPA delineation approach is similar to that employed in our CMZ analysis; that future rates and character of migration will be similar to those of the past for similar water discharges, sediment influx, and debris entrainment conditions. This analysis was also based on the absence of levees, revetments and other confining structures. The width of each MPA was measured, based on delineation criteria developed specifically for this project, and then adjusted to accommodate geomorphic conditions not accounted for in the maximum migration rates. Criteria developed for mapping severe, moderate and low MPA are provided in the following paragraphs:
Severe MPA
The severe MPA includes the area lying inside the HCOT, and an area immediately outside the HCOT boundary equivalent to a distance the channel could travel in a specified period. The extent of the Severe Migration Potential Area outside the HCOT boundary is determined by two criteria. The first criterion is the distance the outside channel edge could travel in l0 years of steady lateral migration away from the HCOT boundary (Maximum lateral migration rates multiplied by a ten- year period). The second is defined by the distance the outside channel edge could travel in storm single event (i.e. maximum ovemight rate) from the current channel position (2002). The landward most boundary ofthe two criteria defines the Severe Migration Potential Area. The CMZ boundary will serve as the severe migration potential area boundary at sites where the distance between the HCOT boundary and the CMZ boundary is less than l0 years of steady lateral migration. In some places, the width of the Severe Migration Potential Area may be increased based on geologic interpretation and professional judgment.
File No. 2998-007-00 Page 20 April 29, 2005 Gzo1xenezaslQ Moderate MPA
The moderate MPA includes the area extending from the outside boundaries of the severe migration potential area. The width of the moderate migration potential area is determined by the distance the outside channel edge could travel in l0 years of steady lateral migration beyond the outside edge of the severe migration potential area. The CMZ boundary will serve as the outside edge of the moderate migration potential boundary at sites where the distance between the severe migration potential boundary and the CMZ boundary represents less the l0 years of steady lateral migration. Moderate migration potential areas are not included at sites where the outside edge of the severe migration potential area is determined by the location of the CMZ boundary. The rate of migration used in the calculation is the maximum average rate of migration for each geomorphic reach (measured as described above). In some places the width of the Moderate Migration Potential Area may be modified based on geologic interpretation, professional judgment.
Low MPA
The low MPA includes areas adjacent to the outside edge of the moderate migration potential area. The extent of the Low Migration Potential Area beyond the moderate migration potential boundary will be determined by CMZ boundary, as determined by our CMZ evaltation. Low migration potential areas will not be included at sites where the outside edge of either a severe or moderate migration potential area is determined by the location of the CMZ boundary.
MPA Ao¿uSTMENTS
The most common adjustments typically involved widening the moderate MPA to include ancient abandoned channels deemed capable of arresting main stem flow in an avulsion event. Other common Moderate MPA adjustments involved increasing or decreasing the base width to accommodate the following conditions: o The presence of native erosion resistant bank materials, such as the Osceola Mudflow. Typically, this condition represents a reduction in the MPA base width.
. Local downstream or oblique direction of meander bend migration. This condition is typically treated by adjusting the shape of the CMZ to account for the downstream or oblique movement of channel bends.
A detailed definition of on how The CMZ boundary was made and what exceptions were included is included in the metadata associated with the CMZ.shp shapefile. The criteria used to delineate the CMZ boundary per reach is included in Appendix C, and the method that was used to delineate each segment of the MPA boundaries is show in Plate II.
Resulrs
The resulting migration potential areas are provided on the GIS CD-ROM package accompanying this report. Reach-scale variations in the calculated width of severe and moderate zones are results of the following factors: . The local maximum rate of migration measured for each geomorphic river reach, . The proximity of the channel corridor to valley walls and/or hard, erosion resistant geologic units. . The presence ofabandoned channels in the floodplain.
File No.2998-007-00 Pøge 21 Geo1xeneeeslQ April 29, 2005 REFERENGES
Abbe, T.8., 2000, Pattems, Mechanics and Geomorphic Effects of Wood Debris Accumulations in a Forest River System, Ph.D. dissertation, University of Washington, Seattle, Washington.
Crandell, D.R., 1963, Surfical Geology and Geomorphology of the Lake Tapps Quadrangle Washington, Geological Survey Professional Paper 388-4, U.S. Geological Survey.
Cadastral Survey Plats: Washington [electronic resource], 2000, United States Bureau of Land Management, General Land Office, Portland, Oregon, distributed by Allied Vaughn, CD-28, cD-29.
Federal Emergency Management Agency, 1989, Pierce County, Washington, Flood Insurance Studies, Volumes I and2.
Gard, L.M., Jr., Bedrock Geology of the Lake Tapps Quadrangle, Pierce County, Washington, 1968, USGS P-388-8.
Henshaw, F.F., and Parker, G.L., 1913, Water Powers of the Cascade Range, Part2 - Cowlitz, Nisqually, Puyallup, White, Green, and Cedar Drainage Basins, Water Supply Paper 0313, U.S. Geological Survey.
James M. Montgomery Consulting Engineers Inc, May 1 991 , Puyallup River Basin Comprehensive Flood Control Management Plan, Pierce County Department of Public Works, River Improvement Division.
Mastin, MC, 1998, Floodpotential of SouthPrairie Creek, Pierce County, Vy'ashington, U.S. Geological Survey WRI98-4009.
Mastin, M.C., 1999, Real-time Flood Alert and Simulation of River Flood Discharges in the Puyallup River Basin, Washington. Water Resources Investigation 98-4226, U. S. Geological Survey.
Montgomery, DR, and Buffington, JM, 1998, Channel Processes, Classification, and Response, in River Ecology and Management, Naiman, R and Bilby, R, eds, Springer-Verlag, New York, p 13-42.
Pierce County, GIS Department, 1998 Orthophotographs of Pierce County
Pierce County, GIS Department,2002Infra-red Orthophotographs of Pierce County
Schumm, S.4., 1986, Alluvial River Response to active Tectonics, in Active Tectonics, Geophysics Study Committee, Geophysics Research Forum, Commission on Physical Sciences, Mathematics, and Resources, National Research Council, National Academy Press, Washington, DC, p 80-94.
Sisson, T. 1995, History and Hazards of Mount Rainier, Washington: USGS Open-File Report 95-642
United States Geological Survey, 1897 Land Classification Map, Tacoma, 1900
United States Geological Survey,2002 Orthophotographs, Puget Sound, Washington,2003.
United States Geological Survey, Buckley, Washington 7.5 Minute Topographic Map, 1968, United States Geological Survey.
United States Geological Survey, Buckley, Washington 7.5 Minute Topographic Map, 1973, United States Geological Survey.
File No 2998-007-00 Page 22 Geo1rctnezeslQ April 29, 2005 United States Geological Survey, Buckley, Washington 7.5 Minute Topographic Map, 1993,
United States Geological Survey, Orthophotographs, Puget Sound, Washington,2002.
United States Geological Survey, Orting, Washington 7.5 Minute Topographic Map,1973, United States Geological Survey.
United States Geological Survey, Tacoma, Washington l5 Minute Topographic Map, 1900, United States Geological Survey.
United States Geological Survey, Tacoma, Washington 15 Minute Topographic Map,1944, United States Geological Survey.
United States Geological Survey, Wilkerson, Washington 7.5 Minute Topographic Map, 1973, United States Geological Survey.
Walker and Associates, 1970, Aerial Photographs of the South Prairie Creek.
Walsh, T.J., Korosec, M.4., Phillips, W.M., 1987, Geologic Map of Washington - Southwest Quadrant, GM-34, Washington State Department of Natural Resources.
Walters, K.L, Kimmel, G.8., Ground-Water Occurrence and Stratigraphy of Unconsolidated Deposits, Central Pierce County, Washington, Washington State Department of Ecology, ECOL WSB-22, 1968.
Washington Department of Natural Resources, 1964-65, Aerial Photographs of the South Prairie Creek.
Washington Department of Natural Resources, 1978, Aerial Photographs of the South Prairie Creek.
Washington Department of Transportation, 1985, Aerial Photographs of the South Prairie Creek.
Washinglon Department of Transportation, 1988, Aerial Photographs of the South Prairie Creek.
Washington Department of Transportalion, 1992, Aerial Photographs of the South Prairie Creek.
GLOSSARY OF TERMS
active channel The wetted portion of a channel at the time of observation.
aggradation The process of building up a surface by sediment deposition.
avulsion The process by which a stream abandons an active channel for a new one. The new channel may be created at time of the avulsion event or a previously abandoned channel.
bank full conditions The staee at which the elevation of the water surface of a stream is fl owing-at channel capacity.
braided channel migration A process where a stream forms an interlacing network of shallow, short lived channels separated by gravel and cobble bars.
File No. 2998-007-00 Pøge 23 GeoErcweaeslQ April 29, 2005 high flow channel The wetted channel width that the stream can, or has recently, occupied during high flow events. As observed on aerial photographs, the high flow channel consists of the low flow channel as well as active and exposed gravel bars.
channel capacity The maximum flow that a given channel is capable of transmitting without overtopping its banks.
channel gradient The degree of inclination of a river channel. In this report gradient is averaged over the length ofriver reaches.
channel migration The lateral or downstream shifting of a river channel within a river valley.
CMZ Channel Migration Zone
Electron mudflow A 500 year old volcanic deposit consisting of consolidated mud, rock and debris that covered much of the Puyallup River Valley.
entrenched channel A channel that has cut vertically downward into streambed deposits, resulting in a channel that is much deeper than it is wide.
FEMA Federal Emergency Management Agency
GIS Geographic Information Systems
HCOT Historical Channel Occupation Track
GLO Govemment Land Office
lag deposits Coarse grained material deposited in the channel during waning storm flows. These deposits are typically devoid of finer sediment.
Mainstem A channel identified as carrying the majority of the flow.
meander bend migration The lateral or downstream movement of a sinuous curve in a stream within a river valley.
meander scar A crescent shaped cut in a bluff or valley wall produced by the sideward cutting of a meandering stream, indicating the former location ofthe channel.
MPA Migration Potential Areas multi-threaded channels A section of a stream that is characterized by several discemable shallow channels separated by sediment bars.
Osceola mudflow A 5,600 year old volcanic deposit, consisting of consolidated mud, rock and debris that originated in the White River system and is found in portion of the South Prairie Creek Valley.
File No. 2998-007-00 Page 24 April 29,2005 GtolxaneznslQ Puget Plateau A remnant of the most recent continental glaciations in the Puget Sound area. It consists ofa relatively flat landscape,400 to 600 feet above sea level, dissected by steep walled valleys. rectification The operation of matching a scanned photograph or map with projected electronic data (I.e. parcel lines or an Orthophotograph). In this report the scanned data was rectified to a 1998 Orthophotograph. river reaches A length of a river channel that is uniform with respect to discharge, gradient, channel shape, bank composition, valley shape, sediment supply and other factors. stade A substage of a glaciation marked by the re-advance of a glacier. transport capacity The ability of the river system to transport sediment. under fìt stream A stream that appears too small to have eroded the valley through which it flows.
USGS United States Geological Survey (Department of Interior)
WDNR Washington Department of Natural Resources
X-year flood A predicted size ofa storm event; based on a model that uses historical data to estimate the size of a flood based on is probability to reoccur (i.e. a 100-year flood, has a one percent chance of occurring in any given year).
File No.2998-007-00 Page 25 GeolnetnezeslQ April 29,2005
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x o F ts o o N Ë o o) Ìt x E .9. Note: a) This figure is not to scale. E) o b) Cross section represents the South Prairie Valley at STA 270 in Reach 5 of the South Prairie Creek. N c) This draw¡ng is for informational purposes lt is intended to ass¡st @ in showing features d¡scussed in an attached documenL o) No ô,t o .õ-0)
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Ø ë It ¡s unlawful to part personal (õ copy or reproduce all or any thereof, whether for use or resâle. w¡thout oermiss¡on GENERALIZED GEOLOGIC CROSS.SECTION OF d¡ SOUTH PRAIRIE VALLEY, PIERCE COUNTY oö G¡oEucrNE ERIJ/ Office: BAM Path: S:GlS/projecU2l2998007l00lgis/mxd/reporU2998007fig4.mxd Map Revised : January 21,2004 t No Scale
Note: This drawing is for informational purposes lt is intended to assist in show¡ng þatures discussed in an attached document
It is unlawful to copy or reproduce all or any part thereof whether for personal use or resale, without permission
Damaged Revetment along Road
Data Sources: Reach boundaries created by GeoEngineers. Active drannel digitized by GeoEngineers frcm 2002 orhophotographs with permission/assisbnce of P¡erce County GlS. Historical dhannel occupation tract and anc¡ent abandoned channel tracts digitized by GeoEngineers from rectified aerial photographs and maps. GEI dreek stat¡oning created by GeoEngineers from Wash¡ngrton Department of Ecology 1998 stream centerlines (3/04). The Background ¡s a 2002 Ortho-¡nfared photograph provided by Pierce County GlS.
Notes: 1) The locations of all features shown are approximate. 2) All data is produced at a scale o1 1:12K unless othemise noted. 3) Map Poection: Lambert Conformal Conic, Washington State Plane South (feet), North American Datum 1983
EXPI-ANATION , PotenialAwlsionRoutes 1.-. Rbandon Channels (prior to 1961) /-r*, Act¡ve Ghannel (2002) Historical Channel Occupation Tract (f 961-2002) ^--,./ Reacñ Boundaries Note: Th¡s drawing is for informational purposes. lt is intended to assist ,20 eer Creek Stat¡on¡ng in showing features discussed in an attached document. ç2 U) It is unlawful to copy or reproduce all or any part thereof wheher for personal use or resâle, w¡thout perm¡ssion o ABANDONED CHANNELS AND POTENIAL o G¡oEtrcrNE ERSJ/ AVULSION PATHWAYS: REACH 5 oö SOUTH PRAIRIE CREEK CIIIZ ANALYSIS Boulders in Channel: Potenial Avulsion Point
EXPI-ANATION , Potenial Avulsion Routes 1.-, Abandon Channels (prior to'1961) /-\, Act¡ve Channel (2002) 1z tt¡storical Channel Occupation Tract (1961-2002) ./ Reach Boundaries 20' GEI Creek Stationing
Note: Th¡s drawing is for informational purposes. lt is intended to ass¡st ¡n show¡ng features discussed in an attached document. Please see figure7 for a list ofdata sources. ïö It is unlawful to copy or reproduce all or any part thereof, whether for personal use or resale, without perm¡ss¡on. o (L ABANDONED CHANNELS AND POTENIAL AVULSION PATHWAYS: REAGHES 3 AND 4 d¡ G¡oEucrNE ERIJ/ CMZ ANALYSIS oö SOUTH PRAIRIE CREEK ü EXPI.ANATION Potenial Avulsion Routes Abandon Channels (prior to 1961) Active Channel (2002) Historical Channel Occupation Tract (1 961-2002) ^*-,/ Reach Boundaries 20' GEI Creek Stationing
ABANDONED CHANNELS AND POTEN¡AL G eoE rucrN EER AVULSION PATHWAYS: REAGHES I AND 2 SOUTH PRAIRIE CREEK CMZ ANALYSIS
GeoEne weenslQ
Appeuox A MernoooLoGY APPENDIX A METHODS lnrnooucro¡¡
The scope of this project required the project team to work with large volumes of historical information dating back to the 1860s. The information included historical and recent aerial photographs, printed topographic and geologic maps, and digitized data, all of which are published in different scales and formats. Our evaluation necessitated bringing the information into a common coordinate system in order to facilitate seamless comparison and review of the river reaches at variable scales. In order to achieve the flexibility required, our project approach utilized Geographic Information Systems (GIS) technology for review and evaluation of the database, and as an analytical tool. GIS also provided the tools for generating the CMZ boundaries that were further delineated into low, moderate and severe migration potential areas (MPAs).
Dlrn CollecnoN, REVIEW, At¡o Selecloru
Information collected for this project falls into three categories, l) written reports, 2) maps and aerial photographs, and 3) GIS data layers. The information includes historic and recent aerial photographs and orthophographs, published and unpublished topographic, soils, and geologic paper maps, technical reports, and papers and GIS electronic data. The data sources included, but were not limited to: Pierce County, Washington Department of Natural Resources (WDNR), Washington Department of Transportation (WSDOT), US Geological Survey (USGS), Federal Emergency Management Agency (FEMA), and GeoEngineers' files.
We reviewed written reports to identify key information with regard to geology, flooding, topography, soils, land use, roads and planning. The information provided a framework for evaluating channel changes, land use changes and construction of channel constraints (e.9., revetments).
We reviewed all available maps and photographs dating from the 1870s through 1998. During our review we conducted a preliminary evaluation of progressive channel conditions. To manage the volume of available data for the analysis, we selected aerial photographs and maps suitable to project goals for conversion to our GIS database via electronic rectification. Our criteria for selection and inclusion of aerial photographs and maps in the GIS data base include 1) obtaining the earliest and most recent flight year coverage,2) the available extent of coverage, 3) changes in channel position or other significant features, and 4) condition and visual quality of photographs and maps. Based on these criteria, we selected and scanned photographs from flight years 1961, 1965-66,1970 and 1978 for the complete study area. We also selected aerial photographs covering specific channel sections from flight years 1931, 1985, 1988, and 1992 to capture observed geomorphic changes important to the analysis. A total of 56 photographs were rectified into the GIS.
GIS data layers were obtained from several different data sources including Pierce County GIS, USGS, FEMA, WSDOT and the WDNR. These GIS data layers were reprojected to the Washington State Plane coordinate system and added to the GIS.
GIS Drrr Developuerur Three major types of GIS data were developed; l) digital rectified aerial photographs, 2) digitized features from historical data sets, and 3) delineated HCOT, CMZ and MPA boundaries. All GIS data development and analysis were completed using ESRI's ATcGIS version 8.2 software.
File No. 2998-007-00 Page A-I GeoEncnzzeslQ April 29,2005 The aerial photographs were scanned and rectified to 1998 orthophotography. Due to inherent distortions when rectifring older photographs to current orthophotographs, our target Route Mean Square (RMS) enor of all the control points was equal to or less than 14 feet. In some cases, only a portion of the photograph was rectified. Some aerial photographs were highly distorted. In these cases, the photograph was rectified with the lowest RMS error possible, and the RMS error was documented for reference. Additional information regarding specific RMS errors and data processing is available in the associated metadata included on the CD-ROMs.
The development of GIS data included digitizing and attributing points, lines and polygons with critical information from scanned aerial photographs. Digitizing was completed at a scale of approximately l:12,000 (or I inch: 1000 feet). Critical information was digitized into four GIS shape files, also referred to as GIS layers: l) active channel locations; 2) high flow channels; 3) historical channel occupation tracts (HCOTs); 4) CMZ boundaries, and, 5) Migration Potential Areas (MPAs).
Actìve channels are defined as the wetted channel width observed on aerial photographs of a selected year, and are generally correlative with the "low flow" channel. Active channels are attributed within the GIS to distinguish the different aerial photograph year set (1961, 1965, 1970, 1978, 1985, 1998 and 2002).
The active channels are mapped from scanned and rectihed photos. The rectification process has a small but measurable amount of distortion, as compared with other data formats (i.e. topography and other photo-years). The active channels were not adjusted to account for any distortion and, therefore, may not match precisely with other GIS layers (HCOT,CMZ and MPA).
High flow channels are dehned as corridor width most likely occupied by recent high flow events, as observed on dated aerial photographs. The high flow channel generally includes the active channel, exposed alluvial side bars, and channel scour features. High flow channels are attributed within the GIS to distinguish the different aerial photograph year set (1961, 1965,1970,1978, 1985, 1998 and 2002).
HCOT is defined as the zone within which the active channel has been located between 196l and 2002. The width of the HCOT is typically equal to or greater than the width of any single high flow channel. Reaches displaying overlapping high flow channels and HCOT lines typically identify reaches where the channel position has not moved appreciably over the period ofrecord.
HCOT boundaries have been adjusted with respect to the topographic contours to account for distortion from photos and, therefore, may not precisely match digitized active and high flow channels. HCOT boundaries are also adjusted to account for circumstances where levies were constructed within the high flow corridor. In such cases. channel scares located outside of the levies have been included in the HCOT.
CMZ boundaries are defined as the distance the channel edge could migrate laterally away from the HCOT at a measured rate of erosion in 50 years. (Please see CMZ Analysis in Appendix C).
MPAs include severe, moderate or low migration potential areas and are defined as the distance the channel edge could migrate laterally away from the HCOT at a measured rate of erosion in a specific period of time (please see MPA Analysis in Appendix C).
Geomonpxrc EvALUATtoN
A key element of our CMZ analysis included identiffing the geomorphic processes operating throughout the project area, and evaluating the affects of those processes on migration. Channel migration is a
F¡le No. 2998-007-00 Page A-2 Geo1netneeaslQ April 29,2005 dynamic process driven by the interaction of physical characteristics and geomorphic processes operating at both local and watershed scales. Physical characteristics include topography, geology, regional and local channel gradients, channel dimensions, and the composition of riverbank and stream bed materials. Principle geomorphic processes include local and reach-scale flow dynamics, sediment supply and delivery, sediment transport capacity, and erosion and deposition within the channel.
A principle objective of our geomorphic evaluation was to identifl the type and character of migration operating at both local (river reach) and regional scales. This task was complicated by changes in channel character resulting from the placement of discontinuous levees and revetments, and in some reaches, relocation of the channel within the floodplain to accommodate roads and railways. Our approach involved the following steps; l) define reach boundaries, 2) conduct a geomorphic evaluation on a reach scale, 3) calculate the river gradient per reach, 4) measure maximum migration rates for each reach, and 5) evaluate reaches for regional geomorphic changes.
For the purposes ofour evaluation, we divided each river channel into reaches, based on the presence and continuity of specific physical features, geomorphic characteristics and channel conditions. Physical features and geomorphic characteristics used in the delineation include the following: . Valley and floodplain conftguration, ¡ Dominant channel pattem (meander, braided, straight channel), o Average channel gradient, . Size and abundance ofsand and gravel bars, ¡ Rate and continuity of lateral channel movement, o Floodplain features indicative ofhistoric/ancient channel activity (abandoned channels).
The features and characteristics summarized above were identified from aerial photographs, topographic maps, geologic maps and field reconnaissance. A complete list the characteristics used to describe and interpret the condition of the reaches is provided in Appendix B. Reach delineations are shown on Figure 6 and included in the GIS database, both of which accompany this report.
File No.2998-007-00 Page A-3 GtoEncwezeslQ April 29, 2005 GeoEncrNEER
Appeuotx B Cuenncrgntsrtcs oF Souru Pnnrue CneaxRencngs c H A RA cr E R r s r r c s o FTjSIHD'Jå, *, - c R E E K R EA c H E s
Geouonpnlc Reecn Evnlurloru
This Appendix contains the results of the reach-scale geomorphic evaluation for the South Prairie Creek (SPC) project area. This evaluation is a key element of the CMZ analysis in that it provides the context for evaluating fluvial and geomorphic processes operating throughout the project area, as well as the effects of those processes on channel migration.
For this project, the project area was divided into six geomorphic reaches (Plate I and Figure 6). A geomorphic reach is defined by the presence and continuity of physical characteristics (such as channel gradient), channel pattem, sediment influx, transport capacity, and migration character. The features and characteristics used in the evaluation were identified from a dated sequence of aerial photographs, topographic maps, geologic maps and field observations. Many of the data collected are provided in Appendix Table B-1. Reach delineations are included in the GIS CD-ROM package accompanying this report.
Of the six reaches described below, Reaches lthrough 5 lie within the project area. Reach 6 is located upstream of the CMZ project area and is included here to provide a context for observed project area conditions. For each reach we summarize the following. ¡ Existing channel conditions, o Historicobservations. . channel forming processes o Potential for future channel migration.
The following reaches are presented in the downstream direction, beginning with Reach 6. Channel features and conditions are referenced to stationing (STA X) developed for this project from the WDFW stream net layer (included in the GIS CD-ROM). The stationing convention is such that each integer represents 100 feet (i.e. STA 0 begins at the confluence and STA 360 is 36,000 feet upstream of the confluence).
Reach 6 Existing Condition. An "accreationary zone" extends from the mouth of the upper basin ravine (at about Elevation 490 feet) to approximately Elevation 440. Across the alluvial fan in the downstream direction, channel gradients decrease significantly and the valley widens to join the South Prairie Valley. In this zone the stream is well braided, indicating that a large portion of the sediment load exiting the ravine is depositing in response to the sharp decrease in channel gradient, and associated loss of transport capacity .
Downstream from the alluvial fan the main stem enters South Prairie Valley, where lower channel gradients prevail. Throughout the South Prairie Valley, the main stem channel is bounded by floodplains deposited by the historic White River and Osceola mudflow deposits, which mantle the floodplains in several areas (see Figure 4).
Reecn 5 (STA 323-2151 Existing Channel Conditions
Reach 5 is characterized by a single, moderately sinuous channel bounded on both sides by the floodplain (Figure 7). Compared to the upstream reach, the channel gradient is relatively high, and transport capacity through the reach typically exceeds sediment influx. The channel position has not moved
File No. 2998-007-00 Page B-1 GeolxeneeeslQ April 29, 2005 appreciably over the period ofrecord and several channel sections throughout the reach are relatively well entrenched.
Channel dimensions throughout the reach vary from 35 to 85 feet wide and from 2 to 5 feet deep at bank full conditions, although, individual bank heights can reach 5 feet or greater. Deep pools are rare but, where present, can reach depths of7 feet or greater. The deepest pool observed at the time ofthe site visit is located along the outside of the bend at STA 271. The streambed is generally composed of gravel, cobbles and boulders. Few active gravel bars are present within the reach, and no sand bars were present at the time of the site visits. The median grain size of the streambed (8-inch cobbles) is much greater than that of the bars (gravel), indicating that White River Alluvium, and possibly boulders derived from Oseoloa mudflow deposits are exposed on the streambed
Natural terraces, flood plain deposits, Osceola Mudflow deposits, and man-made revetments confine the majority of SPC to about l0 percent of the valley floor. Roughly 20 percent of the total bank length within the reach is confined by revetments for a total distance of about 4,300 feet. County roads confrne the creek between STA 3l0 and 270. The revetments have been eroded at two sites, STA 307 and STA 289, exposing embankment fill and road bed materials. Unprotected stream banks are generally moderately eroded. The eroded sections are slightly to moderately undercut, and nearly vertical faces exposing Osceola Mudflow deposits, which is typically erosion resistant.
We identified 8 transportation structures that contribute to bank hardening within reach 5 (Plate 1). The structures include the following: an abandoned railroad bridge at STA 315, the SR 162 bridge #5 at STA 100, a private log bridge at STA 220,two former bridges (foundations and pilings) which confine the active channel at STA 297 and275, a small irigation pump (STA 282), a waste water inlet (STA 296) and an engineered stormwater facility for South Prairie Road (STA 274).
No major tributaries enter SPC in this reach. The flood plains are drained by a floodplain drainage channel and two ditches, which collect and convey surface water runoff from the valley floor to the creek. The two ditches drain the north floodplain and enter SPC at STAs 290 and270. The drainage channel flows across fields on the south floodplain and enters SPC channel at approximately STA 225. None of the drainages appear to contribute significant flow or sediment to the creek.
Historic Observations
The most notable historical observations include: 1) late 1800s channel avulsion and a single bend migration, 2) loss of channel bars, limited bend migration, and local channel incision over the last 20 years, 3) periodic bank erosion surges during large storms, and 4) damage to revetments and road grades.
The 1874 GLO and 1897 USGS maps show SPC flowing in a channel that is now abandoned; Figure 7 shows the approximate location of the channel. A channel avulsion occurred sometime after 1897 and before 1961. The avulsion took place somewhere near STA 305 and the two channels rejoined near STA 270. The avulsion event changed the downstream direction of flow, which in tum caused a meander bend to form between STA 276-260 (Plate 1). By 1965 the meander bend had migrated to its current configuration. The point bar deposited during the bend migration is presently a stranded, vegetated floodplain terrace.
No meander-bend migration has occurred within Reach 5 since 1965. However, over the last 42 years very localized episodes of significant bank erosion have been noted. The erosion is typically associated with the recession of vegetated cutbanks, primarily along the outside banks of bends. In most cases, point bar or side channel bar building have not occurred concurrently with the bank erosion, and the erosional
F¡le No.2998-007-00 Page B-2 GeoEnetxeeaslQ April 29, 2005 episodes appear to occur at each site once only, with no additional erosion detected at the site in successive photo years. For example at STA 297, property owners witnessed the recession of a bank during the February 1996 flooding event. Despite other significant high flow events over the last 8 years no additional, measurable erosion has been reported.
Three important historical observations \^/ere identified at STA 297,260 and225. Woody debris tends to pile up near a former bridge at STA 297. The first South Prairie Road had two bridge crossings, STA297 and260. Both bridges were removed prior to l96l; however some of the bridge pilings at STA 297 inthe middle of the channel still catch woody debris. The constriction at STA297 caused the left bank to erode were the creek now takes an abrupt right-hand tum. At STA 260, the right bank bridge abutment is being undercut and the bank is eroding. Finally, the revetments and pilings on the left bank upstream of the bridge at STA 220 are severely undercut and the channel floor appears incised by approximately 5 feet.
Channel Forming Processes
The composition ofbank materials and the local topography appear to control channel form and behavior within Reach 5. The streambanks are composed of erosion resistant Osceola Mudflow deposits or protected from erosion altogether by revetments. Subsequently, the volume of sediment derived from bank erosion is likely small, compared to incoming sediment from upstream sources. Channel dimensions and the near absence ofgravel bars and sandy lag deposits indicate that transport capacity is much greater than sediment influx. Native streambank soils have limited migration in the past, and will likely continue to do so in the foreseeable future.
Potential for Channel Migration
The potential for future migration within Reach 5 is relatively low. Over the period or record, the channel has remained stationary and stable, due primarily to the entrenchment of the channel in Osceola mudflow deposits. Localized bank recession in Osceola Mudflow deposits and erosion of alluvial soils will likely continue, but this activity is unlikely to result in future channel migration. The abandoned channel on the north side ofthe channel could offer an avenue for avulsion; however, the potential for avulsion here is very low, again due to channel entrenchment.
Rencn 4 (STA 215-195) Existing Channel Conditions
This reach is characterized by decreasing channel gradient, and moderately sinuous low flow and secondary channels bounded by large, side channel bars (Figure 8). The channel gradient through the reach is lower than in Reach 5 and sediment influx through the reach appears, typically, to exceed transport capacity. Reach 4 has been subject to aggradation, at least within the last several decades, and the reach has migrated actively both historically, and over the period of record.
The high flow corridor consists of a main channel and several secondary channels, all of which are separated by gravel bars. Channel pattem and dimensions have fluctuated somewhat over the period of record; however at present, the main channel averages about 65 feet wide, and the thalweg varies from I to 4 feet deep depending on the presence ofdeep pools and shallow chutes. The channel floor and bars are composed ofsand, gravel, cobbles, and a few relic boulders.
The right bank is composed of recent alluvium (sand and silt) and the left bank is composed primarily of Osceola Mudflow deposits (Figure 4), which is exposed in the cut banks. Revetments line roughly 20% of the channel in Reach 4. Where unprotected, the streambanks are eroded. At the downstream end of the reach, the channel makes a left bend and flows beneath SR 162 bridge and turns abruptly to the west.
File No 2998-007-00 Page B-3 GtolxetneeaslQ April 29, 2005 No major tributaries enter the main channel in Reach 4. Spring water flowing through an abandoned SPC channel enters the creek from the north at STA 205. The channel is presently drowned by a beaver dam.
Historic Observations
Observations from historic maps show two channels in the late 1800s and early 1900s. The 1874 GLO map identifies two separate channels extending more than 3000 feet. The 1897 USGS map shows somewhat shorter channel lengths, indicating channel movement. One of the 1897 channels was situated in what is now the existing channel location; the other channel was situated along the foot of the northern valley wall (see Figure 8). The northern channel was abandoned some time between l897and 1961.
Historical aerial photographs from 196l to 2002 show the single, sinuous channel of today. Although the sequence of photos indicates that channel sinuosity and bar development have fluctuated over the period of record, they also show that channel width and bar growth have steadily decreased with time.
The historical photos also indicate that just upstream of SR 162 Bridge # 4 (STA 195), the right streambank has been subject to several episodes of erosion and possible avulsion. A revetment was first identified at this site on the 1961 photo, the year the bridge was undergoing construction. Since then, the revetment has been repaired several times. Much of the damage occurring at this site is a result of migration, or possibly avulsion, activity. Just prior to 1978 the river moved laterally about 75 feet, toward Spring Site Road. The revetment was damaged again during a significant flooding event in 1996; Pierce County officials reported that during and just following the flood of record in February 1996, flood waters were redirected across the Spring Site Road and down the north side of SR 162 (Reach 3). The flow re-entered the channel at STA 187 and STA 175 (Figure 8). We understand that Pierce County reconstructed the revetment to withstand similar future erosional events.
Channel Forming Processes
The combination of decreasing channel gradient and fluctuations in the volume of incoming sediment from upstream reaches appear to exert the greatest controls on channel forming processes. Based on historic channel pattems and apparent bar building, the volume of sediment entering the reach appears to have exceeded transport capacity throughout the reach, resulting in channel aggradation, formation of multiple channels, and migration or avulsion of the channels.
Changes in sediment volumes entering the reach are a likely result of land use practices, such as logging, known to episodically generate large volumes of sediment. As the sediment moves into a reach, channel forming processes often respond with streambed aggradation, bar building, bank erosion and channel migration. As incoming sediment supply decreases, channels and bars become subject to erosion, eventually resulting in channel entrenchment and the reduction or stranding ofbars.
The supply of woody debris in the reach may also play a minor roll in channel form and process. However, development of a major log jam in the future could have a big impact. Log jams may increase the channel complexity and could trigger an avulsion. Historically, woody debris has collected upstream of Bridge #4.
Potential for Channel Migration
Based on historic and recent trends of migration, the potential for future migration is moderately high. Over the period of record, the channel has become entrenched, at least locally, and bar sizes have decreased, indicating a reduction in sediment influx. However, channel gradients, coupled with the potential for periodic increases in sediment arriving from upstream, form excellent conditions under which migration can continue to occur. Redevelopment of the riparian zone throughout the reach could
File No. 2998-007-00 Page B-4 GeoEnøxeeaslQ April 29, 2005 Potential for Channel Migration
Based on historic and recent trends of migration, the potential for future migration is moderately high. Over the period of record, the channel has become entrenched, at least locally, and bar sizes have decreased, indicating a reduction in sediment influx. However, channel gradients, coupled with the potential for periodic increases in sediment arriving from upstream, form excellent conditions under which migration can continue to occur. Redevelopment of the riparian zone throughout the reach could also lead to an increase of woody debris in the active and the high flow channel corridor, which would, in tum, contribute to bar building and erosion associated with migration.
Reecn 3 (STA 195-135) Existing Channel Conditions
Reach 3 is similar to Reach 5; the stream flows in a single, moderately sinuous channel bounded on both sides by the floodplain (Figure 8). The channel gradient is slightly steeper than the upstream reach and transport capacity appears to exceed sediment influx. The channel position has moved only slightly over the period ofrecord, and many channel sections are presently entrenched.
Throughout the reach the channel is narrow, averaging 50 feet wide and 2 to 5 feet deep at bank full conditions, although bank heights can extend 2 to 3 times higher. The channel floor is well armored with relic boulders from the White River Sediments and/or the Osceola Mudflow. Few active bars are present within the reach. The bars are composed of sand and gravel; the D-50 of the bars is much smaller than the D-50 of the channel floor sediments (or armor), further suggesting the channel floor is grounded on White River alluvium.
Extensive floodplains are found on both sides of Reach 3. A natural levee separates the right side of the creek from the adjacent low-lying floodplain, where water collects along the valley wall. Similar to Reach 5, the floodplains, which are composed of Osceola Mudflow and silty overbank deposits, confine the channel throughout the reach. Revetments line roughly 11 percent of the stream banks in Reach 3. The upper one-third ofthe channel is presently confined between SR 162 and the abandoned railroad bed (See Figure 8), where the banks are well reveted with riprap. Along the lower two thirds of Reach 3, the channel crosses from one side of the valley to the other in large sweeping bends. The channel is also confined by SR 162 Bridge # 3 aT STA 175, the base of the right valley wall from STA 155 to 149 (Figure 8), and by SR 162 Bridge #2 at STA 145. Unprotected bank sections are slightly to moderately undercut or stand vertically, exposing Osceola Mudflow deposits.
No major tributaries enter SPC within Reach 3. The north floodplain is drained by 3 ephemeral drainage networks, which enter SPC at STA 187, STA 175, and STA 155. None of the drainages appear to contribute significant flow or sediment to SPC.
Historic Obseruations
The major historical observations within Reach 3 includes: 1) an early 1900s channel avulsion or man- made channel relocation, 2) a small debris flow that modified the channel by approximately 40 feet, 3) periodic surges in bank erosion during larges storms, and 4) a temporary high-flow channel formed on the north side of the highway between STA 200 and STA 175 in 1996.
The 1897 historical map indicates that SPC followed a different alignment; the upper half of the creek was situated in the middle of the valley, and the lower half was situated parallel and adjacent to the railroad embankment onthe left side of the valley (Figure 8). State Route 162 was built inthe 1950s;
File No 2998-007-00 Page B-5 GeoÊnetn*es1Q April 29, 2005 wall adjacent to the creek, although no obvious evidence of a mass failure is observable on the aerial photographs.
Reach 3 is subject to localized episodes of significant erosion and bank recession. Similar to Reach 5, small isolated channel movements were observed over the last 42 years. In all cases, a tree or clump of trees lining the bank was dislodged and presumably carried downstream. The majority of the erosion occurred between 1970 and 1978, most likely during a lO-year flood event in 1975, and sometime within the 1996 flood season. Field observations confirm that in some areas local vesetation is beins undercut along the cut bank side ofthe river.
As described above, flooding in 1996 resulted in the flow of flood waters across the Spring Site Road (within Reach 4) and down the north side of SR 162 within Reach 3. The flow re-entered the current creek channel at STA 187 and 175 (see Figure 8). Pierce County constructed a revetment to protect the bank and discourage similar future flows.
Chan nel Form i ng Processes
The composition of bank materials and the increased channel gradient appear to control channel form and behavior within Reach 3. The streambanks are composed of either erosion resistant Osceola Mudflow deposits or protected from erosion altogether by revetments. The volume of sediment derived from bank erosion is therefore small compared to incoming sediment from upstream sources. Channel dimensions and the near absence ofgravel bars and sandy lag deposits indicate that transport capacity is much greater than sediment influx. Native streambank soils have in the past, and will likely continue to limit migration ofthe bends throughout the reach.
Potential lor Channel Migration
The potential for future migration within Reach 3 is relatively low. Sustained channel migration has not been a dominant geomorphic characteristic in the reach since the channel was moved to its present location. Therefore, the potential for future migration is limited to periodic episodes of bank recession, which have not, over the period of record, resulted in lateral movement of the channel across the floodplain. In the event of an avulsion in Reach 4, the channel would likely occupy a low lying portion of the floodplain north ofSR 162 and the creek.
Rercr 2 (STA 135-20) Existing Channel conditions
Reach 2 is characterizedby a relatively complex system which includes a highly sinuous single channel, several active meander bends, and multiple active and abandoned secondary channels (Figure 9). The channel gradient decreases through the reach and sediment influx exceeds transport capacity. This reach is subject to aggradation, and has migrated actively both historically, and over the period of record.
The channel pattem is relatively complex and includes pools, riffles, and bar complexes which separate the main and secondary channel. . Buried woody debris and a few small log-jams increase the channel complexity. Portions of the reach are aggrading, along with recent meander bend migration and channel avulsion/abandonment. V/oody debris actively modifies the channel conditions, and the reach has the most extensive off-channel and riparian habitat within the project area.
Similar to Reach 4, channel dimensions have fluctuated over the period of record. At present, the main flow channel is roughly 70 feet wide, on average, and the thalweg is 1-4 feet deep with occasional pools up to 7 feet deep and riffles between the gravel bars.
File No.2998-007-00 Page 8-6 Geo1xetxezaslQ April 29, 2005 The channel pattem is relatively complex and includes pools, riffles, and bar complexes which separate the main and secondary channel. . Buried woody debris and a few small log-jams increase the channel complexity. Portions of the reach are aggrading, along with recent meander bend migration and channel avulsiorVabandonment. Woody debris actively modifies the channel conditions, and the reach has the most extensive off-channel and riparian habitat within the project area.
Similar to Reach 4, channel dimensions have fluctuated over the period of record. At present, the main flow channel is roughly 70 feet wide, on average, and the thalweg is l-4 feet deep with occasional pools up to 7 feet deep and riffles between the gravel bars.
The streambed is composed gravel, cobbles and sand in the upper portion of the reach, grading to primarily sand in the lower portion of the reach. Bars throughout the reach are composed of gravel and cobbles with sand.
The floodplains are composed of SPC alluvium, Osceola Mudflow deposits and Orting Formation sediments (in the valley walls). Floodplain topography includes low relief terraces, abandoned channels, and low-lying topography. Linear sections of the adjoining floodplain are lower than the cunent channel floor. These areas are separated from the high flow corridor by natural levees and the SR 162 embankments near STA 070. Elevated terraces located in the upper portion of the reach are composed of the Osceola Mudflow deposits (Figure 4). Abandoned oxbow and secondary channels are found adjacent to Stations 720, 710, 045, 037, and 035.
Compared to upstream reaches, Reach 2 is relatively unconfined; only l0 percent of the total bank length within the reach (approximately 2,200 feet ) is presently hardened by two bridges and revetments placed to protect the railroad and highway. Revetments are sporadically located from STA 104 to STA 095, and at SR 162 Bridge #1 (STA 063). The main channel is also confìned between STA 037 and STA 028, where the stream is situated along the base of the north valley wall.
Throughout the reach, unprotected stream banks are subject to erosion. Two sites warrant particular note; one site is located adjacent to the SR 162 embankment at STA 070, the other extends from STA 106 to STA 085, adjacent to the Veteran's RV park.
Historic Observations
Reach 2 has been subject to relatively active migration from 1874 To 2002. Oxbows located within the flood plains indicate that the channel was, historically, capable of very dynamic movements. The 1874 GLO Map and 1897 USGS map show very different channel patterns than what is observed today. At least two sections of the historic channels were abandoned either by channel avulsion or by the construction of the precursor to SR 162 and Bridge #l (STA 065, shown in Figure 9).
More recently sequential photographs (from 1961 to 2002) indicate that channel behavior is still quite dynamic. Sequential aerial photographs show many channel sections whose bends migrate both laterally and downstream. Over the past 40 years numerous episodes of meander bend migration, avulsion, and meander bend cut-off avulsions are observable. The most active migration appears to have occurred in aggrading channel sections. Of particular importance to this reach is the role of woody debris in channel aggradation and migration. Aerial photographs indicate a close association between logjam or snag location, aggradation and the occurrence of significant channel migration or avulsion.
Over 50 large-scale lateral and downstream migration measurements were documented over the period of record. Aerial photographic analysis indicates that three sections to be particularly unstable. The first section extends from Stations 130 to ll0; this section experienced an avulsion and continued meander
File No.2998-007-00 Page B-7 GwlxeweeaslQ April 29, 2005 bend migration. The second section extends from Stations 085 to 065; in this section log jams appear to accelerate aggradation and migration rates. The third section extends from Stations 055 to 035, wherein a large scale meander bend cut-off occurred sometime 1970 and 1980.
Similar to upstream reaches, however, migration appears to have slowed over the last 22 years. Aerial photographs indicate that the channel has moved smaller distances or less frequently. In addition, the size of in-stream bars appears to have decreased over the same period, suggesting that the volume of sediment entering the reach has decreased.
Chan nel Form i ng Processes
Decreasing channel gradient, the volume of incoming sediment from upstream reaches, and the relative supply ofwoody debris appear to exert the greatest controls on channel forming processes throughout this reach. Based on historic channel pattern and bar building, sediment influx appears to greatly exceed transport capacity, both historically and over the period of record. The presence of wood in the channel has likely increased long term rates of sediment deposition and migration, and possibly the frequency of avulsion.
As mentioned above, migration rates appear to have slowed and bar sizes have decreased over the last 22 years. These changes are likely linked to a basin-scale reduction in sediment volumes available for downstream transport. Changes in sediment volumes entering the reach are a likely result of land use practices, as discussed in the Reach 4 description.
Resistant bank soils and local topography also exert control over some sections of Reach 2. The presence of Osceola Mudflow deposits in floodplains and the presence of erosion resistant soils in the valley walls will likely limit migration in those areas. The elevated road prism of SR 162 and the railroad, will also likely affect channel pattern.
Potential for Channel Migration
The potential for future migration within Reach 2 is high. Based on the results of the geomorphic evaluation, this reach will likely remain active with respect to lateral migration of the main and secondary channels and downstream migration of channel bends.
Reecn 1 (STA 20-0) Existing Channel conditions
Reach 1 joins the Carbon River. The reach is a sinuous, single channel, whose last 500 feet flows in a former Carbon River channel which is seasonally inundated by Carbon River flood flows (Figure 9). The channel has not moved appreciably over the period ofrecord.
Channel dimensions are generally 60 feet wide and 2-4 feeT deep at bank full conditions. The streambed is composed primarily of sand, however, near the confluence, the channel floor is armored with cobbles and boulders likely deposited by either the ancestral White River or the Carbon River.
The low-lying floodplains on either side of the creek consist of low relief fields. The stream banks consist of sand, silt and cobbles, and are relatively well vegetated. In the lower portion of the reach, the right bank, from STA 008 to STA 006, is bounded by a levee that maintains separation from the Carbon River. Over the lower 500 feet of the reach, the creek flows into a relatively steep channel just upstream
File No. 2998-007-00 Page B-8 Geo1xetxweslQ April 29, 2005 of the Carbon River confluence. No evidence of former SPC channel migration, such as abandoned channels or relict bars, are observable in the floodplains.
Historic Observations
The oldest available aerial photographic coverage forthis reach, dated 1931, indicates channel traces and bars on the floodplain adjacent to SPC. The size and scale of the channel features indicate they were generated by the Carbon River. We understand that the Carbon River continued to modify the lower portion of Reach I (STA 015 to STA 000) until the levee was constructed just prior to 1961. No observable changes to the lower SPC channel has occurred since 1961.
Chan nel Form i ng Processes
Channel forming processes and sediment deposition in Reach I are dominated by Carbon River hydraulic conditions.
Potential for Channel Migration
No appreciable migration is likely within Reach l. Historical and recent records indicate that this reach is dominated by Carbon River hydraulics and sedimentation.
Fite No. 2998-007-00 Page B-9 GrcEncneenslQ April 29, 2005 APPENDIX TABLE B-1 SOUTH PRAIRIE CREEK CMZANALYSIS GEOMORPHIC REACH DESCRIPTIONS SOUTH PRAIRIE CREEK PIERCE COUNTY. WASHINGTON
Reach lD Reach I Reach 2 Reach 3 Reach 4 Reach 5 3asic Name Backwater of Ca¡bon River Rapidly Migrating Section Single Channel Depositional Section Single Channel liver Section: (by WDFW fiþoostsl 0.3 2.4 35 3.9 5.8
River Sectbn: (by WDFW STA) o-20 2ù135 135-195 195-215 21$.323
Length: (in Feet) 2,036 11,442 6,019 2,003 10,750 Length: (in llÞs) 0.3 21 1.1 0.4 1.9 Eþvation (beglnning) 2E9.3 293.8 325 351.3 371.1 Êrad¡ent 0.27 0.33 0.46 0.80 0.60 úalby Shape or Conflguratlon: Flat bottomed (1,500' to 2,000' Flat terrace bottomed (1,300' to 2,000' Flat bottomed (800' to 'l,800' Gently sloping bottom (1,000' Flat bottomed valley (1,80C wide), steep valley walls, SPC wide), steep valley walls (with landslide uide), steep valley walls, SP( wide), no steep walls to 3,000'wide) river is runs on the right-side, Carbon scarps), SPC shifts to each side of the shifts to each side of the (laidback), SPC runs in middle incised with a few historica River in the middle valley valley of valley river teraces iloodpbin and Ter¡aces : Broad cpnnected floodplain Small, discontinuous connected Broad connected fl oodplains Right valby connected, þft ValÞy ñoor is largely lpresence, and character) (right) floodplains (on both s¡des), flat tenace disconnected (lahar) disconnected from river on the f¡rst 1/3 of the right side of SPC) loodplains (hhar) -l(x)oplarn tse¡¡tuaes: Former Carbon River Channel Channel Scars in HCOT, flat foodplain I/t/eüand on right side of t reüand and a slde channel Complex set of tenaces, scars, thick overbank deposits with thick floodplain deposits channel (colþcb water from scars, tributary is dammed by and drainage feature frorn wetlands form in some oHer channel valley wall) active beaver colony active lahar, surface runoff positions (collects water from valley spring in the hillside, and wall) down+utting from he SPC
3ravel Bar Frequency wlthln None Many, active and stranded side and Few small stranded side bars Moderate Sporadic :he tlgration Tract: channel bars lhannel Pattern: SingÞ straight tread Single, meandering thread, with Single channel Single fairly straight channel S¡ngle fa¡rly straight occasional side bar or anastomising channel, two section have channel miqratinq sections lVidth ol Aclive Corridor to 711æo/o 2ù1æo/o 6G100% 25-1OOYo 5G100o/o figration Tract: lhanrpl lligration: None Frequent, avulsionary Sporadic Episodic, historic avulsion Sporad¡c, histor¡c avulsion trclston l.lA No Yes. varies 0-2 feet l.¡A Yes. varies G5 feet \ ggregatlon/Depositlon No/ltlA Maybefles No/No No/Yes No/Small secl¡ons only ¡loûes Theory of a hydraulic dam Depositional, migrating bends and Cut bends are rip-raped Avulsion M¡grating bends caused be Carbon River anostomising APPENDIX B (Continued)
Reach lD Reach I Reach 2 Reach 3 Reach 4 Reach 5 fributaries: {one Jnnamed watercourse, STA 125 (left) Unnamed ditch (right) drains Unnamed ditch (left) drains Jnnamed ditch drains Jnnamed wetland collects surface and sTA 192.5 STA 206 najority of upper valley ;pring from right valley wall, STA 63 Unnamed wetland collects Wetland and creek fed by iloor. STA 220 Jnnamed watercourse, STA 62 (right) ¡urface and spring from right spring, STA 2O1 and211 vallev wall, STA 150 (riqht) lriohtl Prlmary Eanl(s soll: Ight bank sand and gravel rnds and gravels Lahar 3ilty sand Colluvium Silty sand teft bank rnds and gravels Silty sand -ahar Lahar lransport Reg¡me: (Bedload iize Range) iediment Source A¡ea: Upstream, field runoff Upstream, field runoff Upstream, field runoff Jpstream, field runoff Jpstream, field runoff iristing Gonstraints None SR bridge 1 SR bridges 2 and3 ìR bridge 4, dairy bridge 3ridge 5, rail-trail bridge mpediments to migration general): ight bank Private road, valley wall SR road prism, valley slope Earthen levee, lahar, valley ey slope (terrac€) ìoad prism, valley wall (terrace)/wall wall 'ett bank Carbon river revetments and SF RaiFtrail, County road prism, valley wall County/SR road prism, rail- ar, SR road prism -ahar, SR road prism road orism lrail- vallev slooe lterracel lioarian loenerall: 'lght bank teftå¿,nk Redml00\Finals\299800700AppendixB.xls G¡oEna weenslQ
Appe¡tøx C CNnemnFoR CMZ Iuo MPA DELINEATIioN APPENDIX TABLE C.1 SOUTH PRA¡RIE CREEK CMZANALYSIS CRITERIA FOR CMZAND MPA DELINEATION SOUTH PRAIRIE CREEK PIERCE COUNTY. WASHINGTON
Short-term Guldellnes for dellneating the boundaries per reach Max (overnight Long-term Values RMS ratel errol Lateral Lateral Downstream Reach Severe MPA Moderate MPA IO*RLMPY cMz ôvarninhf RLMAÞY PLTIAÞY 5O*RLiiPY 6 83 2.7 6.3 135 27 (HCOT+50.RLMPY) or )vernight rate, (HCOT+1O-RLMPY) or (Mud-flow/ Severe MPA + 5 (Mud-flow/ valley wall ralley wall boundary with 25-foot buffer where the IO*RLMPY) orAbandonec 14 55 24 þ.Þ 120 24 boundary) Vludflow is location along the active channel) channel boundary (HCOT+50"RLMPY) or )vernight rate, (HCOT+10.RLMPY) or (Mud-flow/ Severe MPA + 4 (Mud-flow/ valley wall ralley wall boundary with 2S-foot buffer where the 1 O"RLMPY) or Abandonec 14 70 2.7 6.3 135 27 boundary) Mudflow is location along the active channel) channel boundary (HCOT+50*RLMPY) or )vernight rate, (HCOT+1O'RLMPY) or (Mud-flow/ Sevefe MPA + 3 (Mud-ñow/ valley wall ralley wall boundary with 2S-foot buffer where the 1O.RLMPY) orAbandonec 14 20 0.5 0 25 5 boundary) Vludflow is location alonq the active channel) channel boundarv 'HCOT+50*RLMPY) or )vernþht rate, (HCOT+1O.RLMPY) or (Mud-flow/ Severe MPA + 2b Mud-flod valley wall ralley wall boundary with 2S-foot buffer where the 10.RLMPY) orAbandoned 14 271 4.3 14.9 215 43 ¡oundary) Vludflow is location along the active channel) channel boundary IHCOT+S0'RLMPY) or Jvernight rate, (HCOT+1O.RLMPY) or (Mud-flow/ Severe MPA + 2a iMud-flow/ valley wall ralley wall boundary with 2S-foot buffer where the 1O-RLMPY) orAbandoned 14 140 28 1s.1 140 28 ¡oundary) Vludflow is location along the active channel) channel boundary {s defined by the Carbon ¿b-lool Mrnrmum accuracy se¡þacK NIA or as definecl by the 1 ìiver CMZ analysis Carbon River CMZ 14 NA Ì.lA NA NA NA analvsis
Notes: * = outs¡de project area NA = not applicatable RLMAPY = Rate of Long-term Unconfined Migration Averaged Per Year within a given reach. Overnight channel movements = The amount of movement (measured in feet both laterally and downstream) a channel has moved between two consecut¡ve photo years. l.e. 1961 1992 and 1998. Eye witness accounts indicate that in some cased movement occuned during one flooding or high flow event. Overnight rate = Maximum channel movement observed between sequential photographs (two consecutive photo years) within a given reach. Mudflow boundary = Approximate Mudflow boundary were indentied with the NRCS soil survey. The location of the boundary was refined by field mapping and photogri interpreation of brakes in slope. Valley wall boundary = The approximate valley wall boundary is determined by the placement of brakelines used in development of orthorectified surveys in 1998 by Titan, survey brakelines were not available GEI located the line using orthophotographs and USGS DEMS. Resistant unit boundary layer = A shapefile that includes both the valley wall and mudflow boundries. SPC = South Prarie Creek CMZ = Channel Migration Zone MPA = Miqration Potenial Area GroEnctNEER
AppeNox D Reponr Ltunertows AND Gwoeu¡tes FoRUse REpoRr Lr M trA' ottPsPATS'ðu?o r., * Fo n us er =s This appendix provides information to help you manage your risks with respect to the use of this report.
GeorectucAl SERV¡cES ARE PeRroRrueD FoR Speclnc Punposes, PeRsot'ts Atto Pno¡ecrs
This report has been prepared for the exclusive use of Pierce County Department of Public Works and Utilities and their authorized agents. This report is not intended for use by others, and the information contained herein is not applicable to other sites.
GeoEngineers structures our services to meet the specific needs of our clients. For example, a geotechnical or geologic study conducted for a civil engineer or architect may not fulfill the needs of a construction contractor or even another civil engineer or architect that are involved in the same project. Because each geotechnical or geologic study is unique, each geotechnical engineering or geologic repofi is unique, prepared solely for the specific client and project site. Our report is prepared for the exclusive use of our Client. No other party may rely on the product of our services unless we agree in advance to such reliance in writing. This is to provide our firm with reasonable protection against open-ended liability claims by third parties with whom there would otherwise be no contractual limits to their actions. Within the limitations of scope, schedule and budget, our services have been executed in accordance with our Agreement with the Client and generally accepted geotechnical practices in this area at the time this report was prepared. This report should not be applied for any purpose or project except the one originally contemplated.
A GeorecHNtcAL Er.lclteenll'lc On Geolocrc REpoRT ls BASED ON A UHloue Ser Or Pno.¡ ecr-S PEcrFrc Frcrons
This report has been prepared for the Upper Cowlitz River and Rainey Creek. GeoEngineers considered a number of unique, project-specific factors when establishing the scope of services for this project and report. Unless GeoEngineers specifically indicates otherwise, do not rely on this report if it was: o not prepared for you, o not prepared for your project, . not prepared for the specif,rc site explored.
SuesuRrrcE CoNDrroNs CAN Gnlnoe This geotechnical or geologic report is based on conditions that existed at the time the study was performed. The findings and conclusions of this report may be affected by the passage of time, by manmade events such as construction on or adjacent to the site, or by natural events such as floods, earthquakes, slope instability or groundwater fluctuations. Always contact GeoEngineers before applying a report to determine if it remains applicable.
Mosr GeorecnucAL AND Geoloclc Flt¡or¡¡cs Ane PRoresstoNAL Opltllotls
Our interpretations of subsurface conditions are based on field observations from widely spaced sampling locations at the site. Site exploration identifies subsurface conditions only at those points where subsurface tests are conducted or samples are taken. GeoEngineers reviewed held data and then applied our professional judgment to render an opinion about subsurface conditions throughout the site. Actual
I Developed based on material provided by ASFE, Professional Firms Practicing in the Geosciences; www.asfe.org .
File No. 2998-007-00 Page D-L GeoEnetneenslQ April 29,2005 subsurface conditions may differ, sometimes significantly, from those indicated in this report. Our report, conclusions and interpretations should not be construed as a warranty ofthe subsurface conditions.
A GeorecHNrcAl Erucrrueenrnc On GEOLOGIC RepoRr Goulo Be Sue¡ecr To MtslnrenpRETATIoN
Misinterpretation of this report by others can result in costly problems. You could lower that risk by having GeoEngineers confer with appropriate members of the design team after submitting the report. Also retain GeoEngineers to review pertinent elements of the design team's plans and specifications. Contractors can also misinterpret a geotechnical engineering or geologic report. Reduce that risk by having GeoEngineers participate in pre-bid and preconstruction conferences, and by providing construction observation.
Rero Txese PRovrsrot¡s Gt-oseuv
Some clients, design professionals and contractors may not recognize that the geoscience practices (geotechnical engineering or geology) are far less exact than other engineering and natural science disciplines. This lack of understanding can create unrealistic expectations that could lead to disappointments, claims and disputes. GeoEngineers includes these explanatory "limitations" provisions in our reports to help reduce such risks. Please confer with GeoEngineers if you are unclear how these "Report Limitations and Guidelines for Use" apply to your project or site.
GeorecnucAL, GEoLoctc AND ErvrnorueNTAL REpoRTs SHoULD Nor Be lrurencnANcED
The equipment, techniques and personnel used to perform an environmental study differ significantly from those used to perform a geotechnical or geologic study and vice versa. For that reason, a geotechnical engineering or geologic report does not usually relate any environmental findings, conclusions or recommendations; e.g., about the likelihood of encountering underground storage tanks or regulated contaminants. Similarly, environmental reports are not used to address geotechnical or geologic concerns regarding a specific project.
File No 2998-007-00 Page D-2 GeoEnøxeeaslQ April 29,2005
CHANNEL MIGRATION ZONE BOUNDARYAN MIGRATION POTENTIAL AREAS CHANNEL MIGRATION ZONE STUDY FOR SOUTH PRAIRIE CREEK, PIERCE COUNTY, WASHINGTON PLATE 1
COMPILED BY COMPLETED FOR PIERGE COUNTY PUBLIC WORKS AND UTILITIES ENVIRON MM ENTAL S ERVICES, WATER PROGRAMS DIVISION Map Revised: April 25, 2005
REASONING FOR MIGRATION POTENTIAL AREAS DELIN EATION CHANNEL MIGRATION ZONE STUDY FOR SOUTH PRAIRIE CREEK, PIERCE COUNTY, WASHINGTON PLATE 2
COMPILED BY COMPLETED FOR PIERGE COUNTY GToEUGTNEER PUBLIC WORKS AND UTILITIES ENVIRON MMENTAL SERVIC ES, WATER PROGRAMS DIVIS¡ON Map Revised: April 25,2005