Summary of Discharge Measurements

Flood of July 28, 1997 in Fort Collins, Colorado
 

Mark E. Smith

U.S. Geological Survey, Denver, Colorado

Introduction


 


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.
 



Fairbrooke channel and Dorset Drive

 


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

Type of Measurement: Three-section slope-area (Tate and Benson, 1967).

Computed Discharge: 530 ft3/s.

Measurement Quality: Fair to poor (+/- 20%).

Hydraulic Properties:

Cross section
* Station distance, ft
Peak water elevation, ft
Average velocity, ft/s
Top width,
ft
Average depth, ft
Maximum depth, ft
1
860
5080.78
5.9
45
2.0
4.6
2
816
5080.35
7.3
37
2.0
3.3
3
782
5079.88
7.4
39
1.8
3.4
* Distance upstream from guard rail on west side of Hampshire Road.


Fairbrooke channel and Dorset Drive

Dorset Drive

Type of Measurement: Two-section slope-conveyance (Tate and Benson, 1967).

Computed Discharge: 1220 ft3/s.

Measurement Quality: Poor (+/- 25% or greater).

Hydraulic Properties:

Cross section
* Section location
Peak water elevation, ft
Average velocity, ft/s
Top width,
ft
Average depth, ft
Maximum depth, ft
2
1918 Dorset
5093.00
7.2
96
1.8
2.6
1
1900 Dorset
5092.23
7.9
87
1.8
2.7
* Street address on Dorset Drive.

Site Summary

Total peak discharge: 1750 ft3/s (assuming peaks occurred simultaneously).

Overall Quality: Poor (+/- 25% or greater).

Design discharge: 326 ft3/s (City of Fort Collins, written commun.).

(100-year event)

Drainage Area: 0.834 mi2.

The 100-year design discharge (326 ft3/s) reflects the combined discharge for flow in
the Fairbrooke channel and overflow along Dorset Drive, assuming the peaks occurred
simultaneously. The Fairbrooke drainage area (0.834 mi2) does not include all outlying
areas drained by irrigation canals that might have overtopped and contributed to the measured flood peak.



Spring Creek downstream from Shields Street

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

Type of Measurement: Step-backwater computation (Davidian, 1984).

Computed Discharge: 6100 ft3/s.

Measurement Quality: Fair to poor (+/- 20%).

Hydraulic Properties:

Cross section
* Station distance, ft
Peak water elevation, ft
Average velocity, ft/s
Top width,
ft
Average depth, ft
Maximum depth, ft
2
87
5006.47
7.6
186
4.3
7.9
Approach
36
5005.65
9.6
175
3.6
7.4
crest
0
5004.60 (computed)
10.4
180
3.3
6.1
* Distance upstream from crest of drop structure. Wallenburg Drive Type of Measurement: Three-section slope-area (Tate and Benson, 1967).

Computed Discharge: 1300 ft3/s.

Measurement Quality: Poor (+/- 25% or greater).

Hydraulic Properties:

Cross section
* Station distance, ft
Peak water elevation, ft
Average velocity, ft/s
Top width,
ft
Average depth, ft
Maximum depth, ft
3
0
5003.87
5.9
101
2.2
3.3
2
132
5003.47
5.6
102
2.3
3.4
1
262
5002.81
6.3
95
2.2
3.4
* Distance downstream from 1904 Wallenburg.


Spring Creek downstream from Shields Street

Overflow in Hill Pond subdivision

Type of Measurement: One-section slope-conveyance (estimate).

Computed Discharge: 850 ft3/s.

Measurement Quality: Poor (+/- 50% or greater).

Hydraulic Properties:

Cross section
* Station distance, ft
Peak water elevation, ft
Average velocity, ft/s
Top width,
ft
Average depth, ft
Maximum depth, ft
1
--
5003.22
1.5
554
1.0
3.8
* Vicinity of 638 Gilgalad St.

Site Summary

Total peak discharge: 8250 ft3/s (assuming peaks occurred simultaneously).

Overall Quality: Poor (+/- 25% or greater).

FEMA Regulation: 2135 ft3/s (100-year flood); 3325 ft3/s (500-year flood).

Drainage Area: 6.23 mi2.

The site drainage area (6.23 mi2) includes the drainage area of the main Spring Creek channel (4.87 mi2) and the drainage area of the canal importation channel (1.36 mi2).  The site drainage area does not include any outlying area drained by other irrigation canals that might have overtopped and contributed to the measured flood peak.



Burlington-Northern Railroad embankment and trailer parks

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):
 

Culvert structure
* Distance,
ft
Upstream invert elevation, ft
Downstream invert elevation, ft
Downstream high-water elevation, ft
Concrete box culvert

(14-ft by 12-ft)

815
4977.34
4976.74
4992.5
North concrete pipe culvert

(7-ft diameter)

1602
4980.35
4979.69
4990.7
Middle concrete pipe culvert

(7-ft diameter)

1615
4979.09
4978.62
4990.7
South concrete pipe culvert

(7-ft diameter)

1625
not

surveyed

4979.69
4990.7
Bike path culvert

(9-ft diameter pipe-arch)

1837
4989.42
4987.93
4990.5

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

CLICK ON FIGURE TO ENLARGE



 


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

Click on Figure to enlarge.


 

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).
 



Spring Creek downstream from Riverside Avenue

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.

Type of Measurement: Width contraction (Matthai, 1967).

Computed Discharge: 5860 ft3/s.

Measurement Quality: Fair (+/- 15%).

FEMA Regulation: 2187 ft3/s (100-year flood); 2920 ft3/s (500-year flood).

Drainage Area: 9.52 mi2.

Hydraulic Properties:

Cross section
* Station distance, ft
Peak water elevation, ft
Average velocity, ft/s
Top width,
ft
Average depth, ft
Maximum depth, ft
Approach
264
4922.12
4.6
254
5.0
12.8
Bridge
0
4918.82
10.5
85
6.6
15.2
* Distance upstream from bridge exit (contraction).
 


Plum channel upstream from Taft Hill Road

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.

Type of Measurement: Type I flow through culvert (Bodhaine, 1968).

Computed Discharge: 370 ft3/s.

Measurement Quality: Fair (+/- 15%).

Design discharge: 356 ft3/s (City of Fort Collins, written commun.).

(100-year event)

Drainage Area: 0.319 mi2.

Culvert dimensions: Box culvert, three 8-ft by 3-ft barrels. Length, 59 feet;

upstream invert elevation, 5073.77 feet;

downstream invert elevation, 5073.46 feet.

Hydraulic Properties:

Cross section
* Station distance, ft
Peak water elevation, ft
Average velocity, ft/s
Top width,
ft
Average depth, ft
Maximum depth, ft
Approach
55
5076.64
3.5
50
2.1
3.1
Culvert
0
5075.71
7.9
24
1.9
1.9
* Distance upstream from culvert entrance.
 


Clearview channel downstream from Taft Hill Road

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.

Type of Measurement: Three-section slope-area (Tate and Benson, 1967).

Computed Discharge: 2400 ft3/s.

Measurement Quality: Poor (+/- 25% or greater).

Design discharge: 532 ft3/s (City of Fort Collins, written commun.).

(100-year event)

Drainage Area: 0.414 mi2.

Hydraulic Properties:

Cross section
* Station distance, ft
Peak water elevation, ft
Average velocity, ft/s
Top width,
ft
Average depth, ft
Maximum depth, ft
2
196
5030.40
5.4
260
1.7
5.2
3
402
5029.10
4.9
284
1.7
4.9
4
676
5027.40
4.3
309
1.8
4.8
* Approximate distance downstream from intersection of Clearview Drive and Meadowbrook Drive (measured from light post with attached street signs).
 



Fossil Creek at South Lemay Avenue

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.

Type of Measurement: Type 3 flow through culvert (Bodhaine, 1968).

Computed Discharge: 1800 ft3/s.

Measurement Quality: Fair (+/- 15%).

50-year discharge: 2030 ft3/s (City of Fort Collins, written commun.)

100-year discharge: 2520 ft3/s (City of Fort Collins, written commun.)

Drainage Area: 14.2 mi2.

Culvert dimensions: Concrete box culvert (natural bottom). Length, 90 feet;

width perpendicular to flow, 32 feet; height, 16.4 feet;

upstream invert elevation (minimum), 4894.94 feet;

downstream invert elevation (minimum), 4894.85 feet.

Hydraulic Properties:

Cross section
* Station distance, ft
Peak water elevation, ft
Average velocity, ft/s
Top width,
ft
Average depth, ft
Maximum depth, ft
Approach
164
4904.17
6.2
67
4.3
10.8
Culvert exit
0
4903.79
6.8
32
8.3
8.9
* Distance upstream from culvert exit (east side South Lemay Avenue).
 



Discharge hydrographs of the Cache la Poudre River

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.

Click on figure to enlarge

References cited


  Bodhaine, G.L., 1968, Measurement of peak discharge at culverts by indirect methods: U.S. Geological Survey Techniques of Water-Resources Investigations, book 3,
chap. A3, 60 p.

Davidian, Jacob, 1984, Computation of water-surface profiles in open channels: U.S. Geological Survey Techniques of Water-Resources Investigations, book 3, chap. A15, 48 p.

Federal Emergency Management Agency, 1996, Flood insurance study,
City of Fort Collins, Colorado, 36 p.

Matthai, H.F., 1967, Measurement of peak discharge at width contractions by indirect methods: U.S. Geological Survey Techniques of Water-Resources Investigations, book 3, chap. A4, 44 p.

Tate, T., and Benson, M.A., 1967, General field and office procedures for indirect discharge measurements: U.S. Geological Survey Techniques of Water-Resources Investigations, book 3, chap. A1, 30 p.

Tate, T., and Benson, M.A., 1967, Measurement of peak discharge by the slope-area method: U.S. Geological Survey Techniques of Water-Resources Investigations,
book 3, chap. A2, 12 p.