This report summarizes discharge measurements made in Fort Collins by the U.S. Geological Survey (USGS) after the flood of July 28, 1997; discharge hydrographs for the Cache la Poudre River upstream and downstream from the confluence of Spring Creek also are included. Measurements were made at the following sites:

1. Fairbrooke channel and Dorset Drive.

2. Spring Creek downstream from Shields Street.

3. Burlington-Northern Railroad embankment and trailer parks (high-water marks).

4. Spring Creek downstream from Riverside Avenue.

5. Plum channel upstream from Taft Hill Road.

6. Clearview channel downstream from Taft Hill Road.

7. Fossil Creek at South Lemay Avenue.

8. Cache La Poudre River.

The summary data include measurement location, type of measurement, computed discharge, measurement quality, hydraulic properties of the flow (including peak water- surface elevation), and contributing drainage area (provided by the City of Fort Collins). If available, regulatory discharges for the 100-year and 500-year recurrence intervals are listed for comparison with measured flood discharges (Federal Emergency Management Agency, FEMA; 1996).

Cross sections for each measurement are ordered upstream to downstream. Measurement depths and velocities have been rounded to the nearest tenth of a unit (0.1). Peak water elevations are referenced to the National Geodetic Vertical Datum (NGVD) of 1929. An adjustment of +0.34 foot was made to the NGVD by the National Geodetic Survey in 1984; this adjustment is incorporated in all reported elevations.

The flood measurements were made using standard techniques documented by the USGS. Measurement quality ranged from fair (+/- 15 percent) to poor (+/- 25 percent or greater), depending on conditions at each site (Benson and Dalrymple, 1967). Post- flood discharge measurements can be subject to substantial uncertainty, depending on the quality and variability of high water marks, channel hydraulic conditions, and the effects of debris carried by the flow. All of these factors were evaluated to assess the uncertainty associated with each measurement.

Figures showing the location of each measurement site and of specific cross sections are available from the City of Fort Collins.
 

The Fairbrooke drainage is south and west of the intersection of Prospect Road and Taft Hill Road. Two runoff components contributed to flooding in this area:

1. Primary runoff is carried by the Fairbrooke channel, which runs west to east through
the Fairbrooke subdivision.

2. Runoff west of the Pleasant Valley and Lake (PV&L) canal, which flows north to
south, is collected by a detention pond at the Fairbrooke Heights Planned Unit
Development (PUD). During the flood, substantial overflow occurred from the pond
and the canal, causing street flooding along Dorset Drive.

Flows from the Fairbrooke drainage eventually are routed into Spring Creek via the canal importation channel, which joins Spring Creek downstream from Shields Street.

Fairbrooke channel

Dorset Drive

The flood reached its largest peak discharge at this site. Three flow components were included in this measurement: 1) flow in the main channel of Spring Creek; 2) street flow down Wallenburg Drive, resulting primarily from overflow of the canal importation channel; and 3) overflow south of the main channel in the Hill Pond subdivision, resulting from overflow across Shields Street at Rolland Moore Park.

Main channel

Overflow in Hill Pond subdivision

Railroad embankment

The locations and elevations (referenced to NGVD) of high-water marks were surveyed in the vicinity of the Burlington-Northern Railroad embankment; surveying was done July 30, August 8, 14, and October 8, 1997. No discharge computations were made.

The high-water elevation of the detention pond was determined to be 4997.6 feet (+/- 0.1 foot). This determination was based on a line of high water marks on telephone poles and fence posts on the upstream side of the embankment. The highest marks were of good quality (fine debris) and were fairly consistent from north to south across the entire ponded area.

Railroad-grade elevations (top of west rail) were surveyed from Prospect Road to the south side of the ponded area; surveyed elevations reflect the final condition of the railroad embankment after post-flood repairs were completed by the railroad company. Surveyed limits of erosion indicated that the length of the embankment overtopped during the flood was approximately 235 feet. Figure 1 shows details of the survey.

All culvert systems through the embankment (fig. 1) were surveyed for dimensions, location, and invert (bottom) elevations. Culvert details and downstream high-water elevations are shown below (the upstream high-water elevation at all culverts was 4997.6 feet):
 

* Approximate distance south from Prospect Road (edge of red-tile walkway over
railroad tracks).
 
 

Railroad embankment

Two cross-section profiles were surveyed to characterize the shape of the embankment (fig. 2). Cross section 1 was surveyed in the overtopped section of the embankment. Cross section 2 was surveyed at the middle 7-ft diameter concrete pipe culvert.

A summary of the railroad-grade and location survey follows. Culvert locations are referenced to the structure centerline. Distance is approximate, measured from the south side of Prospect Road (edge of red-tile walkway over railroad tracks).
 

Distance, in Elevation of

feet south from top of rail, in

Prospect Road feet (NGVD) Remarks
 
 

6 4996.60

167 4996.04

281 4995.97

392 4996.32

483 4996.64

493 4996.61

520 4996.75 North limit of overtopping (approximate)

564 4996.88

634 4996.99

717 4997.14

730 4997.17 Embankment cross section 1 (fig. 2)

755 4997.22 South limit of overtopping (approximate)

814 4997.38

815 14-ft by 12-ft concrete box culvert

883 4997.58

1047 4998.09

1235 4998.89

1313 4999.28

1501 5000.78

1602 North 7-ft diameter culvert

1615 Middle 7-ft diameter culvert

1615 5001.81 Embankment cross section 2 (fig. 2)

1625 South 7-ft diameter culvert

1650 5002.14

1773 5003.52

1833 5004.17

1837 Bike path (9-ft diameter pipe-arch culvert)

1922 5005.06

1944 5005.29

High-water marks in the vicinity of the trailer parks

High-water-mark elevations (NGVD) surveyed in the vicinity of the trailer parks downstream from the railroad embankment are summarized as follows:

Elevation Description

1. 4996.93 West side of shopping center, second door from north end of building:

70 feet south of West Prospect Road, 50 feet east of railroad tracks
(111 West Prospect Road).

2. 4991.06 Southwest corner of Discount Tire Company (1751 College Avenue).

3. 4991.20 West side of Discount Tire Company (1751 College Avenue).

4. 4991.26 South side of Breakpoint Aerobic and Weight Training (1739 College
Avenue).

5. 4991.21 West side of Fort Collins Guitar Center (1793 College Avenue).

6. 4991.05 Southwest side of Creative Airs Balloons and Flowers (1795 College
Avenue).

7. 4991.50 North side of Tae Kwon Do building (1801 College Avenue).

8. 4990.75 Northwest corner of Tae Kwon Do building (1801 College Avenue).

9. 4990.22 Southwest corner of Dairy Queen (1805 College Avenue).

10. 4990.19 Southeast corner (glass) of Dairy Queen (1805 College Avenue).

11. 4989.11 Garage door of Fort Collins Muffler (2003 College Avenue).

12. 4989.44 Wash line on garage door of Fort Collins Muffler (2003 College Avenue).

13. 4989.09 Car lift at Fort Collins Muffler building (2003 College Avenue).

14. 4990.49 West side of building 1807, Kelly C. Brown Ice, Inc. (in trailer park).

15. 4990.45 South side of tan/brown block building, north of building 1807 (in
trailer park).

16. 4990.77 Southeast corner of car wash building (in trailer park).
 

This measurement site was selected to characterize the flood peak downstream from the Burlington-Northern Railroad embankment and College Avenue. The flow was contained completely by the trestle bridge downstream from Riverside Avenue (south of Sutherland?s Lumber Company).

The contributing drainage area at this site is 9.52 mi², but the flood peak was substantially affected by runoff storage at the Burlington-Northern Railroad embankment.

Substantial flooding occurred in the Plum channel drainage west of Colorado State University. The measurement site is at the west end of Orchard Place (and west of Taft Hill Road) in a new addition of the Jefferson Commons Apartments; a concrete box culvert (three 8-ft by 3-ft barrels) at the road crossing southeast of apartment building 7 was used to measure the peak discharge. Although runoff increased dramatically downstream from Taft Hill Road, no acceptable downstream measurement site was found.

Substantial flooding occurred in the Clearview channel drainage west of Colorado State University. The measurement site is downstream from Taft Hill Road at Avery Park. Four cross sections were surveyed through the park, but the upstream cross section was not used in the measurement because of inadequate high-water-mark definition.

Flooding occurred in the Fossil Creek drainage in south Fort Collins. Peak discharge was measured at the South Lemay bridge crossing, near the intersection of South Lemay and Muirfield (and about 1 mile south of Harmony Road). Because of the dimensions of the conduit (length greater than width) the measurement was computed as flow through a box culvert.

Figure 3 shows discharge hydrographs of the Cache la Poudre River upstream and downstream from the confluence with Spring Creek, recorded for the period July 28-30, 1997. The hydrographs are for upstream USGS streamflow-gaging station 06752260, Cache la Poudre River at Fort Collins (Lincoln Street), and downstream USGS streamflow-gaging station 06752280, Cache la Poudre River above Box Elder Creek near Timnath.

Contributing drainage area is 1127 mi² at streamflow-gaging station 06752260, and 1245 mi² at streamflow-gaging station 06752280.