FLOOD05894

• Designation Number: 342
• County: Jefferson
• Community: Arvada
• Stream Name: Moon Gulch Watershed
• Basin: South Platte
• Title: Flood Hazard Area Delineation - Moon Gulch Watershed
• Date: 8/1/1992
• Designation Date: 9/1/1992
• Floodplain - Doc Type: Floodplain Report/Masterplan

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based on land use, soil type, and basin physical features including size, length, slope, and

surface characteristics and the rainfall parameters. Existing land use for the study basin was

established by site visits and review of aerial photography, while future land use was

developed by coordinating information received from tbe City of Arvada Planning
Department and Jefferson County Zoning.

Sub-basin runoff was established by CUHP as indicated above, then routed using UDFCD's

Storm Water Management Model (UDSWM2-PC), In accordance with District policy, no

credit was given to detention facilities which are not publicly owned and maintained.

Basin modeling results show that the detention pond located at the Ralston Valley

development and 72nd Avenue, east of Eldridge Street, only controls runoff under an

existing two year design frequency. Less frequent events resulted in the detention pond

being overtopped. The lOO-year design frequency analysis indicated little attenuation of

flows passing through the detention pond. Table 1 indicates peak discharge at several key

locations in the basin for the frequencies studied.

TABLE I
SUMMARV OF PEAK DISCHARGES
DISCHARGE (cr.)
Location
Detention
Qua.ker St.1 Highline Canal Croke Canal Pondl
Fr~aue.cv Church Ditch North S2.Y1h North South Eldrid2e St. nod Ave ConOuence
2-yr. Existing 88 15 90 18 86 104 56 68
Future 297 61 340 11 343 426 438 475
5-yr. Existing 221 72 281 86 289 392 409 485
Future 526 127 632 149 645 824 864 1019
lO-yr. Existing 319 114 427 140 447 618 671 846
Future 692 172 840 205 865 1110 1184 1423
50-yr. :Existing 591 225 821 279 869 1213 1341 1723
Future 1274 324 1558 397 1615 2117 2181 2135
lOO-yr. Existing 839 320 1167 399 1239 1742 1832 2416
Future 1487 381 1836 470 1904 2502 2572 3312
SOO-yr. .Exisling 1040 410 1520 470 1620 2180 2360 3160
Future 2000 510 2400 600 2570 3240 3410 4450
.Log-normal Probability Distribution Extrapolation

The existing 9.0 acre-ft detention pond, located at tbe Ralston Valley development located

immediately upstream of 72nd Avenue, is included in tbe model. Existing depressions

located above the Croke Canal and Highline Canal are not modeled as it would be in

violation of UDFCD policy. In addition, modeling does not assume flow discharged into

the existing irrigation ditches in accordance to UDFCD policy.

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