FLOOD08330

• County: Gilpin
• Community: Central City
• Stream Name: Gregory Gulch, Nevada Gulch
• Basin: South Platte
• Title: Flood Hazard Identification Report - Gregory Eureka Gulch and Nevada Gulch
• Date: 2/1/1987
• Prepared For: Central City
• Prepared By: CWCB
• Floodplain - Doc Type: Floodplain Report/Masterplan

I
I
I
II
,

\1
I
I
I
I
I
I
I

I
~I
I
I

Hydraulic Analysis

An analysis of the hydraulic character __ICS of Gregory and Nevada
Gulches was performed in order to determine water surface elevations for
the 10-year and 100-year floods on each stream. Cross-section data was
taken from a 2-foot contour map of Central City (Ref. 16). Building
locations were determined from an aerial photo of the town (Ref. 4) and
were then "blocked out" of the cross-sections effectively preventing flow
through them. This cross-section data became input data in the U.S. Army
Corps of Engineers HEC-2 step-backwater computer program (Ref. 18).
Bridge and culvert elevations and sizes of openings were surveyed and
measured in the field by the Colorado Water Conservation Board (Ref. 7).

A comparison of bridge cross-section elevations surveyed by CWCB
with those obtained from the 2-foot contour map led to the conclusion
that there was a problem with the map. Elevations of the map varied with
sma 11 (i. e. 1 ess than 2-3 feet) differences from true mean sea 1 eve 1
datum according to benchmarks in town. Because of these differences, it
was decided to use the 2-foot contour map as the datum in this study
instead of mean sea level datum due to the fact that it was the map that
flood boundaries would be delineated on. Elevations surveyed by CWCS
were adjusted to conform to the map.

Roughness coefficients (Manning's "n" va lues) were also used as
HEC-2 input. They were obtained in the Colorado Water Conservation Board
field survey. Roughness values for Gregory Gulch varied from 0.030
(streets) to 0.040 for the channel and from 0.030 to 0.040 for the
overbank areas. Roughness value for Nevada Gulch was 0.030 for the
channe 1 (street).

At both Prosser Street bridge crossings over Gregory Gulch, the
4-foot diameter culverts were assumed to be partially plugged (25
percent) with debris due to their small diameters. No plugging was
assumed at the D Street and Lawrence Street crossings of Gregory Gulch
due to the large sizes of the openings. Because of its significant
storage potential, the D street crossing of Gregory Gulch was examined
for the possibility of it acting as a detention basin thereby attenuating
(lowering) the peak flow downstream from the street; it was found however
that the peak flow downstream from this "reservoir" would be the same as
the peak flow upstream.

II

Hand calculations were performed at two Gregory Gulch culverts to
determine the maximum flow of water that could enter them. One of these
cu 1 verts was the very long cu lvert (fl ume) that runs under the famous
opera house; the other culvert was the one that begins just downstream
from the flume (just downstream from Spring St. in the central downtown
area). Because of their large diameters, these culverts were assumed to
be unplugged. Hand calculations were made due to the virtual
disappearance of the stream from sight. Floodwaters that could not enter
into these culverts would run down the town's streets; the streets then
became the apparent channel. In the vicinity of these two culverts the
cross-sections were aligned perpendicular to the streets themselves and
the depth of flow in the streets was determined by the amount of water
that did not enter into the culverts. The storm grates on Eureka Street
were assumed to be plugged because of their small openings.

,I
I
I