FLOOD09085

• Designation Number: 97
• County: Adams
• Community: Thornton
• Stream Name: Grange Hall Creek
• Basin: South Platte
• Title: Major Drainageway Planning - Grange Hall Creek, Phase B, Thornton, Adams County
• Date: 12/1/1977
• Designation Date: 1/19/1978
• Floodplain - Doc Type: Floodplain Report/Masterplan

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of the hydrologic design locations were plotted and are presented in Figures 111-3 to 111-9.

The Muskingum method of channel routing was used in this analysis between the various

hydrological points. The Modified Puis method of flood routing through a reservoir or an

inadvertent detention area was also used for this analysis. Detention routing was

performed at the following locations: Grange Hall Creek at Colorado Blvd., the Union

Pacific Railroad, the Union Pacific Railroad Spur, the Woshington Street pond, and the

Croke Reservoir; Tributary S at 104th Avenue pond, Washington Street, and 1-25;

Tributary N at the Union Pacific Railroad embonkment; and the unidentified tributary at

the Union Pacific Railroad embankment.

One of the common effects of urbanization on natural drainageways is the

separation of flow or flow splitting, which is caused by the inability of a drainoge facility,

such as a storm sewer or culvert, to pass the entire flood peak or by splitting of flows ot a

street intersection. The result of the flow splitting is that portions of the runoff are

carried in more than one direction. Generally, the condition is insignificant when

performing a major drainogeway study; however, two flow splitting conditions in Grange

HolI Creek basin were considered. One location is at Irma Drive ond 112th Avenue, where

the 48" RCP storm sewer in Irma Drive was insufficient to carryall the storm runoff and

the flows in excess of the capacity of the pipe seporoted and flowed eost along I 12th

Avenue. This condition was analyzed by computing both the storm sewer hydrograph and

the overflow hydrograph based upon the characteristics of the total hydrograph.

The second flow splitting condition anolyzed occurred in the upper reaches of the

Grange Hall Creek basin where the Niver Conal enters Sub-basin G-3 at Pecos Street (see

Figure III-I). The hydrographs for Sub-basins G-I, G-2, and G-22 were combined and

routed to the design point, ond the peak flow was compored to the estimated capacity of

the canal ( 200 cfs). The flows in excess of 200 cfs were considered to overtop the canal

and go into the Big Dry Creek basin. The remainder of the flows were added to the runoff
in the Grange Hall Creek basin.

The flood routing for the Phase B analysis consisted of routing the future

development hydrographs through the improved drainageway as proposed by the selected

alternative plan (see Selected Alternative Table in the Synopsis). The effects of the

proposed improvements on the flood profiles and inundation areas are presented on the

Plan and Profile Drawings for Grange HolI Creek and the Tributaries S, N, NE, and SE.

E. Floodway Determination

One of the key factors In the administration of a flood plain regulation is the

subdivision of the 100-year flood plain (Flood Regulatory District) into a Floodway

District and a Flood Storage District. The Floodway District is defined as that area

required for the reasonable passage or conveyance of the 100-year flood which is

choracterized by hazardous and significant depths ond velocities. The Floodwoy District

would have strict regulations governing its use. The Flood Storage District is defined as

the fringe portion of the Flood Regulatory District in which flows are characterized by

shallow depths and low velocities. This area has less stringent regulations and is

developable under certain conditions.

The floodway definition that was used for this analysis and accepted for use by

Adams County, Northglenn, Thornton, and U.D. and F.C.D. is based on the criteria that

the water surface elevotion in the Floodwoy District would increase by no more than 0.5

feet with filling in the Flood Storage District.

In addition, the following criteria was recommended and accepted for defining the

floodway:

I. The floodway width is not defined less than the width of the main channel due

to high velocity flow. The main channel is defined as that portion of the

channel carrying 50% or more of the 100-year flood.

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