FLOOD10493

• County: Adams
• County: Arapahoe
• Community: Aurora
• Stream Name: Upper Box Elder Creek
• Basin: South Platte
• Title: Upper Box Elder Creek Outfall Systems Planning Study - Alternative Evaluation Report
• Date: 12/1/1994
• Prepared For: Aurora, Arapahoe County, Adams County, UDFCD
• Prepared By: CH2M Hill
• Floodplain - Doc Type: Floodplain Report/Masterplan

3.3 Streams

Section 3.0
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Streams (major drainageways) draining 130 acres or more of subwatershed area were
defmed as shown in Appendix F. To provide an orderly system of managing stream
information and hydrologic results, streams (above each confluence point) were designated
with an identification number. Streams were numbered consecutively from downstream to
upstream. Coyote Run was designated as Stream 100. Thirty-eight tributaries were
identified in the Coyote Run watershed (Streams 101-138). Box Elder Creek within the
study area was designated as Stream 200. Fifty-one tributaries were identified in the Box
Elder Creek portion of the study area (Streams 201-251). In all, 91 separate streams were
identified within the study area. In addition, Box Elder Creek upstream of the study area
was given the designation Stream 400 and one tributary upstream of the study area was
identified with the designation Stream 401.

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The study area was divided into five major watersheds and modelled separately, to man-
age the size of the models. Watersheds 1 and 2 pertain to downstream and upstream
Coyote Run. Watersheds 3, 4, and 5 pertain to lower, middle, and upper Box Elder Creek
within the study area. In addition, the watershed upstream of the study area was modelled
in order to provide representative inflow hydrographs at the upstream study limit. This
upstream watershed was designated as Watershed 6. The Watershed 6 analysis was
conducted with less detail than the analysis of the five watersheds within the study area.

Watersheds

Major

3.1

Each stream shown in Appendix F was divided into a series of adjoining conveyance
elements. The conveyance elements were designated with even numbers, increasing
consecutively in an upstream direction. Design points (direct routing elements) were
coded into the hydrologic models between each pair of conveyance elements. The design
points were designated with odd numbers that fall appropriately between each pair of even
conveyance element numbers.

Some of the streams shown in Appendix F have been identified with names and numbers
on UDFCD Drainage Basin Description Maps (UDFCD, 1990). Table 3-2 provides a
cross reference between the study stream numbers and the equivalent UDFCD numbers
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Each of the five watersheds in the study area were divided into subwatersheds as shown
in Appendix D. The subwatersheds were numbered consecutively from north to south and
limited to a maximum area of 130 acres. Watershed 6 (upstream of the study area) was
divided into subwatersheds with areas no greater than 10 square miles. A work map of
Watershed 6 is shown in Appendix E. As shown in Table 3-1, the study area was divided
into a total of 615 subwatersheds with an average area of 78 acres and Waters~ed 6 was
divided into 18 subwatershecls with an average area of 7 square miles.

3.2 Subwatersheds

Variations exist between stream delineations shown in the Drainage Basin Description
Maps and the streams shown in Appendix F. The streams shown in Appendix F represent
major drainageways (streams draining 130 acres or more), which include streams not
shown on USGS mapping, and do not include USGS stream reaches that extend far
upstream into 130 acre subwatersheds.

3-1

Hydrologic Models

The UDFCD's Colorado Urban Hydrograph Procedure (CUHP) (UDFCD, 1985a) model
was used to develop runoff hydrographs for each subwatershed. The runoff hydro graphs
were routed through the existing stream network using UDFCD's Storm Water
Management Model (UDSWM2) (UDFCD, 1985b). The UDSWM2 model does not have
a channel infiltration component; however, channel infiltration, even though it occurs, is
not expected to significantly affect peak discharges during extreme flood events.

3.4

I Table 3-1 I
Subwatershed Distribution
Watershed Number Area Number of
(Sq. mi.) Subwatersheds
1 (Coyote Run downstream) 12 94
2 (Coyote Run upstream) 17 136
3 (Box Elder downstream) 11 98
4 (Box Elder middle) 18 156
5 (Box Elder upstream) 17 131
Subtotal, Study Area 75 615
Average Subwatershed Area (acres) 78 --
6 (Box Elder upstream of study area) 127 18
Average Subwatershed Area (square miles) 7
I Total I 202 I 633 I

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