Oregon Water Science Center
Water levels at the 0.01 Annual Exceedance Probability at the Glenwood Street and Bybee Boulevard Crossings of Crystal Springs Creek, Portland, Oregon
By Adam Stonewall
In Cooperation with the City of Portland Bureau of Environmental Services
Crystal Springs Creek is a spring-fed creek that flows through southeast Portland, Oregon. Current plans (2014) call for replacing the culverts at the Glenwood Street and Bybee Boulevard crossings to enhance streamflow and fish passage. A hydrologic model was created to estimate the 0.01 Annual Exceedance Probability (AEP) flood at the Bybee/Glenwood area. In addition, a hydraulic model was created to model water-surface elevations associated with the 0.01 AEP flood using both current and proposed crossing infrastructure. Results show that water-surface elevations associated with the 0.01 AEP flood are expected to decrease about 0–1 foot based on current designs.
Crystal Springs Creek flows through a highly urbanized part of southeastern Portland, Oregon (fig. 1, on the left). The creek flows from the largest spring in the Johnson Creek watershed, which is tributary to the Willamette River. The spring-fed nature of the creek keeps flow higher and temperature cooler than in most other parts of the Johnson Creek Basin (Lee and Snyder, 2009). Crystal Springs Creek is habitat for at least three salmonid species listed as threatened under the Endangered Species Act (City of Portland, 2014a). The creek is designated as critical habitat under the Endangered Species Act by the National Marine Fisheries Service (NOAA, 2005).
Figure 1. The Johnson Creek Basin in Portland, Oregon.
As part of the City of Portland’s Grey to Green Initiative (City of Portland, 2014b), the Bureau of Environmental Services is replacing eight culverts that block fish passage along Crystal Springs Creek. Six culverts have been replaced as of this writing (February 2014); the final two, at the Glenwood Street and Bybee Boulevard crossings, are scheduled to be replaced in 2014. The herein named "Firehouse Bridge" (fig. 2), located between the Glenwood Street and Bybee Boulevard crossings, is not slated for replacement.(Top)
This report presents the results of a study to evaluate the hydraulic effects of replacing culverts at the Glenwood Street and Bybee Boulevard crossings, in particular the effect of the proposed stream infrastructure on water-surface elevations associated with the 0.01 Annual Exceedance Probability (AEP) flood at and near the crossings. (A 0.01 AEP flood is the rate of streamflow that has a 1% probability of being equaled or exceeded in any given year. It is commonly known as a “100 year flood.”)
This report describes the methods and results of the following:
This study uses the same hydrologic and hydraulic models created for a Federal Emergency Management Agency (FEMA) Flood Insurance Study (FIS) for Crystal Springs Creek currently underway by USGS personnel. The hydraulic model evaluates water-surface elevations from the outlet at Reed Dam (approximately River Mile 2.0) downstream to the mouth of Crystal Springs Creek.(Top)
During almost all of the year, Crystal Springs Creek streamflow levels are controlled by nearby groundwater levels (Lee and Snyder, 2009). Occasional, small pulses of water result from human activity (construction, flushing of ponds, etc.) along the creek. During intense rainstorms, overland flow and stormwater runoff can also increase streamflow (Dames and Moore, 1998).The existing FIS hydrologic model does not adequately represent the hydrology of Crystal Springs Creek during annual peak flows, which are a function of spring discharge, precipitation, and stormwater infrastructure. This section describes an alternate method for estimating AEP values at the Glenwood Street and Bybee Boulevard crossings.
Streamflow data for Crystal Springs Creek are relatively sparse. For the purposes of this report, "streamflow measurements" are instances where a hydrographer is at the stream and physically measures 20–30 points of velocity and depth according to USGS protocol (Turnipseed and Sauer, 2010). "Streamflow" represents an automated streamflow calculation, typically from a gage through either a stage/discharge relationship or a stream area/velocity equation.
The USGS has made intermittent streamflow measurements at the mouth (54 measurements from 1964 to 2013), at Bybee Boulevard (16 measurements from 1935 to 2001), at 28th Avenue (one measurement in 2013), and just downstream of Reed Lake on the campus of Reed College (44 measurements from 1997 to 2010) (U.S. Geological Survey, 2013a). The Oregon Water Resources Department (OWRD) maintained a streamflow gaging station near the mouth of the creek for a short time, but during moderate or greater streamflow events the gage was in backwater from Johnson Creek, and the stage/discharge relationship was determined to be unusable (Richard Marvin, OWRD, oral commun., 2013). Consequently, the streamflow values at this site were never calculated. OWRD made 41 instantaneous measurements at the gage location from 2000 to 2008, and three historic measurements before the gage was installed (one in 1935 and two in 1964). More recently, the City of Portland Bureau of Environmental Services (BES) has installed a gage measuring stream velocity and gage height in the culvert underneath Bybee Boulevard. Streamflow at this location is calculated using a culvert equation. The velocity and gage height are confirmed during inspections, but independent streamflow measurements are not made (Peter Abrams, BES, oral comm., 2013). The BES gage has been active since October 2011, and has collected natural peak streamflows as high as 33.7 ft3/s, and one anthropogenic peak of 38.3 ft3/s.
Stream elevation data are more readily available in the watershed, although still somewhat sparse. In 1994, the City of Portland installed a staff plate approximately 0.4 miles upstream of the mouth of Crystal Springs Creek, and water levels were recorded by a citizen volunteer. During periods of normal streamflow, the gage height of the creek at this location ranged between 0.75 and 1 ft (above an arbitrary datum). During the February 1996 flood, which at the time was the second largest flood in 66 years of record at nearby USGS station 14211500 (Johnson Creek at Sycamore, OR) and resulted in widespread flooding across much the State, a gage height of up to 1.80 ft was recorded at the citizen volunteer staff plate. In November of 1996, an even larger flood occurred, and the gage height at the staff plate read 2.60 ft. After the peak, while creek levels were still elevated, removal of aquatic vegetation lowered the water elevation between 1.1 and 1.5 ft.
Water levels were also recorded near the mouth at the OWRD gage from 2003 to 2008. Typical water levels were between 0.75 and 1.0 ft (above an arbitrary datum). During flooding, when experiencing backwater from Johnson Creek, gage height sometimes exceeded 5 ft.
In a previous FIS (Federal Emergency Management Agency (FEMA), 2010) for Crystal Springs Creek, a U.S. Army Corps of Engineers rainfall-runoff model HEC-HMS (U.S. Army Corps of Engineers,1998) was used to estimate AEP streamflows. The model used the Muskingum-Cunge channel flow routing scheme. Attempts to obtain a working copy or write-up of the previous model inputs were unsuccessful, so the exact methodology is unknown. However, due to the extent of the urban stormwater infrastructure within the basin and the importance of the underlying baseflow conditions in streamflow peaks, a rainfall-runoff model would likely prove inaccurate for estimating AEP streamflows at Crystal Springs Creek. Most of the precipitation falling within the watershed is captured by the stormwater infrastructure, which moves water into and out of the watershed (Dames and Moore, 1998).
The hydrologic model for the FEMA study (FEMA, 2010) reports an initial 0.01 AEP streamflow of 40 ft3/s at the headwaters. The streamflow increases considerably around river miles 1.5 and 1.0 with a peak value of 212 ft3/s. Streamflow then decreases considerably to 70 ft3/s before reaching the confluence with Johnson Creek. According to the model, the 0.01 AEP streamflow in the Glenwood/Bybee area is 70 ft3/s (at river mile 1.0). Based on historical measurements made in the creek, streamflow has either been consistent or increased as it moves downstream (Dames and Moore, 1998). In addition, the highest of the 150+ total streamflow measurements made in the creek is 21.6 ft3/s. Furthermore, institutional knowledge (based on historical high water marks and professional judgment) of those who have lived and worked in and around Crystal Springs Creek during flooding suggest that streamflows greater than 40–60 ft3/s are unlikely in the Glenwood/Bybee area.
Dames and Moore (1998) analyzed previous measurements along Crystal Springs Creek and evaluated the hydrology of the watershed. Their report details the history of the combined sewer system and dry wells in the basin. Sumps are designed to be able to handle a 10-year event, after which direct runoff into Crystal Springs Creek would occur. Analysis of individual streamflow measurements shows that streamflows at Bybee Boulevard and near the mouth are very similar. Of 12 streamflow measurements made at both locations on the same day, the average difference between the two locations was less than 1%. Streamflow at the mouth ranged from 7% lower than at Bybee Boulevard, to 5% higher. Most measurements at both locations are rated good, indicating a confidence in in accuracy of +/- 5%.
Although rainfall-runoff models are commonly used in this type of analysis, independent streamflow measurements, the BES stream gage, and previous investigations (Dames and Moore, 1998; Lee and Snyder, 2009) show that the creek is unresponsive to small and moderate rainfall events. Consequently, the rainfall-runoff model was deemed unsuitable for the hydrologic conditions at Crystal Springs Creek.
AEP streamflow values (table 1) were also evaluated using the USGS StreamStats tool (U.S. Geological Survey, 2013b) with the most recent published western Oregon flood frequency regression equations (Cooper, 2005) and local urban flood-frequency equations (Laenen, 1983), which are available in the USGS National Streamflow Statistics tool (Ries, 2006). However, both methods are intended for streams with more typical responses to rainfall events, and not for streams dominated by groundwater discharge. As a result, streamflow estimates calculated using both techniques were well beyond reasonable values.
A stage-area discharge (streamflow) rating was constructed from USGS measurements near Bybee Boulevard and the gage height record from BES. Due to the lack of variability in streamflow and varying levels of aquatic growth on the stage-discharge control for the reach measured by the staff plate, measurements do not plot consistently near the rating curve. The rating was extended to the highest recorded gage height from the November 1997 flood (2.6 ft). This event resulted in an AEP of approximately 0.04 (25-year return period) at the nearby Johnson Creek at Sycamore gage (14211500). Depending on how the rating curve is extended, the estimated streamflow derived from the stage-area rating is about 34 +/- 4 ft3/s.
Finally, an attempt was made to statistically model the BES Crystal Springs Creek stream gage using independent meteorological and hydrologic parameters. Using this model, the peak annual floods were estimated for the water years 1999–2013 and analyzed using Bulletin 17B (U.S. Interagency Advisory Committee on Water Data, 1982).
Measurements made on the same days at the mouth and near the BES Bybee gage show that there is not a significant difference in streamflow between the two locations, so the 0.01 AEP streamflow at the BES Crystal Springs Creek gage can be used as a surrogate for the Bybee Boulevard crossing. Correlation between streamflow at the BES Crystal Springs Creek gage and a number of transformed and untransformed independent variables were considered (table 2).
Regression models were optimized to fit high flows (>20 ft3/s). In addition, precipitation and local streamflow records were analyzed to screen for high flows resulting from human activity. High flows occurring without at least 0.05 inches of precipitation over a 3-day period and in which nearby gaging stations did not show an increase in flow were eliminated from consideration.
The resulting statistical model for estimating streamflow at the mouth of Crystal Springs Creek is:
A plot of the observed streamflow at Crystal Springs Creek versus the estimated streamflow from the multiple linear regression model can be seen in figure 3. The coefficient of determination (r2) value for this multiple linear regression is 0.90. Average and median error rate are 3.2% and 2.1%, respectively. However, these error metrics are understated by the large intercept coefficient of the regression equation and resulting high fit at the low end of the regression. If only the top 10 streamflow values are considered, the r2 value drops to 0.81, and average and median error rates increase to 4.6% and 3.2%, respectively.
Figure 3. Relation between measured and estimated streamflow at Crystal Springs Creek at Bybee Boulevard, Portland, Oregon
The resulting multiple linear regression equation model was used to augment the BES Crystal Springs Creek instantaneous streamflow gage record back to the start of water year 1999 by using the streamflow data at Johnson Creek at Regner Road and the precipitation data at the Harney Street precipitation gage. Using those estimated flows, the peak annual flows were then calculated for water years 1999–2013 (table 3). Finally, the 0.01 AEP streamflow was calculated according to Bulletin 17B , with the skew value computed using the systematic record (table 4).
The final result was a 0.01 AEP streamflow of 45.0 ft3/s. This value is consistent with the history of individual streamflow measurements made in Crystal Springs Creek and local knowledge of creek behavior. In addition, the 0.04 AEP streamflow was estimated to be 37.8 ft3/s using Bulletin 17B. This value compares well against 0.04 AEP streamflow derived using the BES staff plate, estimated to be about 34 ft3/s.(Top)
Geospatial field data were acquired using a Trimble® R8 GNSS System and a Trimble 5600 Total Station. All surveying was performed using Real-Time Network (RTN) surveying techniques with the Oregon Real-time GPW network, or was geospatially tied to RTN-surveyed points using the Total Station.
Channel cross-section data were surveyed above and below each of the three crossings: Bybee Boulevard, Firehouse Bridge, and Glenwood Streets. In addition, structural geometry data on and around the bridge and culverts were obtained. Data collected were compared against LiDAR data for accuracy, and against points with established elevations (table 5). Results suggest that the surveyed data were within acceptable limits. The root mean squared-error for all benchmarks was around 0.10 foot.
Cross sections were primarily clustered around creek crossings (bridges and culverts), and added as needed between long stretches without crossings. A total of 102 crossings were evaluated for the approximately 2-mile reach of Crystal Springs Creek from Reed Dam to the mouth.
Cross sections (fig. 4) of Crystal Springs Creek at the Glenwood Street and Bybee Boulevard crossings were modeled with the Hydrological Engineering Center’s River Analysis System (HEC-RAS) version 4.1.0 (U.S. Army Corps of Engineers, 2010). HEC-RAS is a one-dimensional hydraulic-flow model designed for use in flood-plain management. It is commonly used for flood-insurance studies and to simulate estimated flood inundation (Christiansen and Eash, 2008). The HEC-RAS model was used to evaluate the 0.01 AEP flood water-surface elevations at the Bybee and Glenwood crossings using both current infrastructure and design plans for replacement infrastructure.
Figure 4. Cross-section of Crystal Springs Creek at River Station 5,706 near Glenwood Street, Portland, Oregon, showing example output from the HEC-RAS model. The River Station number represents the distance from the confluence with Johnson Creek. The bank station is the survey point nearest the flood level. The energy grade line (also known as the "energy head") shows the simulated potential energy (as elevation) at the cross section. The water surface is the simulated stream stage, which is the energy head minus friction losses due to channel roughness.
A Geographic Information System (GIS) coverage of 2-foot elevation contours of the study area, and the designs for the Bybee and Glenwood replacement culverts and surrounding areas were provided by the City of Portland. Cross sections were extracted from the 2-foot elevation contours provided by the City of Portland using HEC-GeoRAS software (U.S. Army Corps of Engineers, 2011). HEC-GeoRAS utilizes a set of tools, procedures, and utilities to process geospatial data in ArcGIS (ESRI®, Redlands, CA). The program has a graphical interface, and can be used to prepare geometric data for import into HEC-RAS and process streamflow simulation results exported from HEC-RAS. When necessary, USGS personnel modified HEC-GeoRAS results to provide a logical transition between land surface contour data and modeled cross sections.
At and around the Glenwood Street, Bybee Boulevard and Firehouse Bridge crossings, the 0.01 AEP flood water-surface elevations associated with the proposed crossing plans are between 0.01 ft higher (downstream of Bybee Boulevard) and 0.98 ft lower (upstream of Glenwood Street) than those associated with the existing creek infrastructure (table 6). Water-surface elevations are projected to be essentially unchanged below the Bybee Boulevard crossing. Between the Bybee Boulevard and Glenwood Street crossings, water-surface elevations associated with the 0.01 AEP flood are projected to be from 0.62 to 0.68 ft lower with the proposed new culvert geometry. Surface water elevations are projected to be almost a foot lower just upstream of the proposed Glenwood crossing.
The planned addition of logs for fish habitat in the creek resulted in a rougher stream bed, which was represented by an increase in Manning’s ‘n’ value (raised from 0.40 to 0.42). This increase in stream roughness in turn resulted in a slight increase in water-surface elevations. However, the increase in stream roughness was more than offset by the proposed increase in cross-sectional area at both the Bybee Boulevard and Glenwood Street crossings. The net result was significantly lower surface-water elevations associated with the 0.01 AEP streamflow behind both the Bybee and Glenwood crossings.Crystal Springs Creek is not projected to spill overbank between the Glenwood Street and Bybee Boulevard crossings during the 0.01 AEP flood with current stream infrastructure or proposed infrastructure (fig. 5), assuming no culverts are blocked and there is not a significant increase in channel roughness.
Figure 5. Plan view of inundation levels of Crystal Springs Creek associated with the 0.01 Annual Exceedance Probability (AEP) flood in Portland, Oregon under (A) current infrastructure, and (B) proposed infrastructure.(Top)
The 0.01 AEP streamflow for the Bybee/Glenwood crossing at Crystal Springs Creek was computed using a statistical model. The resulting 0.01 AEP streamflow of 45 ft3/s was consistent with expected values on the basis of historical streamflow measurements and local knowledge.
Flood and associated water-surface elevations were computed for Crystal Springs Creek near the Glenwood and Bybee Boulevard crossings in Portland, Oregon, for existing conditions and proposed post-culvert replacements. Differences in 0.01 AEP flood water-surface elevations were found to be essentially unchanged below the Bybee Boulevard crossing, and from 0.62 to 0.98 ft lower above the Bybee crossing.(Top)
The author gratefully acknowledges the support of the City of Portland Bureau of Environmental Services, in particular Greg Savage and Ali Young; and USGS personnel Matthew Whitehead, Chad Ostheimer, Glen Hess, Jonathan Haynes, and Katherine Breen.
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Conversion Factors and Datums
Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88).
Stonewall, Adam, 2014,Water levels at the 0.01 annual exceedance probability at the Glenwood Street and Bybee Boulevard crossings of Crystal Springs Creek, Portland, Oregon: U.S. Geological Survey Webpage, http://dx.doi.org/10.5066/F7ZK5DP0
For more information, contact:Director, Oregon Water Science Center
U.S Geological Survey
2130 SW 5th Ave
Portland, OR 97201
Phone: (503) 251-3200