FLOOD05088

• County: Adams
• Community: Arvada
• Stream Name: Hidden Lake, Bates Lake
• Basin: South Platte
• Title: Major Drainageway Planning
• Date: 11/1/1975
• Prepared For: Urban Drainage and Flood Control District, Arvada and Adams County
• Prepared By: Hydro-Triad, LTD.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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111-5

twenty to be consistent with infinite width channel techniques and to
minimize the management of the floodway from the hydraulics standpoint.
Both of the above techniques were used to define the floodway widths

for Hidden Lake and Bates Lake basins. A tabulation of the floodway

widths, as well as the supporting data, is presented in Table 111-7.

The section numbers and the drawing numbers refer to the floodplain

mapping presented in Volume II of this report.

E. Hidden Lake

Part of the proposed improvements for the Hidden Lake basin is the

improvements to the Hidden Lake dam and spillway. To insure the use of

the lake for storm detention, the dam and spillway must be improved to
meet the standards of the State Engineer's office as well as the intended
uses. Therefore, the facility must be designed for a probable maximum
precipitation (PMP) type event. The area downstream of the dam was
classified as a low hazard area by the State Engineer's office, and
therefore, a PMP type Assumption B storm would be used to size the dam

and spillway. The data used for design is as follows:

Hidden Lake Design Data

Total six-hour precipitation:
AMC II, Type Band C soil, average Cn =
Excess precipitation =

5.88 inches
82
3.20 inches (5.88
inches for Hidden
Lake)

0.40 in/hr minimum total loss
Total area =
Total storm volume =
Peak inflow rate =

1,200 acre, tc = 45 min
341 acre-feet
2,043 cfs

From this data and routing of the hydrograph, a fifteen foot wide
broad crested weir was defined giving a maximum discharge of 211 cfs,