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

I
I
I
I
.
I
I
I
I
I
I
I
I
I
I
I
I
I
I

2. In the channel, where the side slopes are 2: I ar steeper (grass or earth banks),

a floodway narrower than the flood plain wos not defined due to the potential

erosion hazards of filling a channel on such 0 steep slope.

3. A floodway equal to the flood plain width is defined for the reach between 1-25

and the Railroad Spur on Grange Hall Creek. This is because future building

development is unlikely in the park areas owned by Northglenn.

A tabulation of the floodway widths, as well as the supporting data, is presented in

Tobie 111-5. The section numbers and the sheet numbers refer to the flood plain mapping

presented on the master plan drawings of this report.

F. Erosion

Ninety-five percent of the soils in the Grange Hall Creek bosin range from loam to

clay, with the remoining five percent consisting of alluviol soils and some gravel deposits.

For the surfoce soil layer (0-9 inches) over 70% of the soils pass the No. 200 sieve. This

very fine-grained soil is susceptible to erosion, which can be witnessed by inspection of

the steep, highly eroded banks of Grange Hall Creek. This erosion potential presents a

problem from the standpoint of bank stabilizotion for channels or sedimentation from

construction ond from the standpoint of water quality.

G. Groundwater Recharqe

Due to the loamy, clay type soils in the Grange Hall Creek basin, the soils have a

low permeability (0.06 to 2.0 in/hr range), but have a good water holding capacity (0.14 to

0.20 in/hr). Therefore, the groundwater recharge is minimal in the steep channel area.

The existing stock water ponds have very minimal effect on the groundwater recharge due

to the sealing effect of the fine-grained soils on the pond bottom area. The bedrock

geology of the basin as discussed in Chapter II does not favor significant amounts of deep

aquifer recharge.

CHAPTER IV
MAJOR DRAINAGEWAY IMPROVEMENTS

A. General

The description of the major drainageway improvements which wos selected by the

participating agencies to reduce the flood domages was presented in the Summary Table in

the Synopsis. The design criteria used for the improvements and the detailed description

of the master plan drawings is presented in this chapter.

8. Design Criteria

The criteria used for the preliminary design of the improvementsNas obtained from

the Urban Drainage Criteria Manual and the requirements of Adams County and the cities

of Northglenn and Thornton. Specifically, the criteria for design is os follows:

I. Open Channels

Where natural chonnels of adequate capacity to pass the 100-year storm exist no

improvements were recommended. Where modifications of the channel section were

required the channel section which minimizes right-of-way acquisition and erosion

potential was recommended. All modified channel sections are to be lined with natural,

drought resistant grasses where the average channel velocities are less than approximately

7 fps and a concrete low flow or trickle channel is included. The exact velocity is

dependent upon soil conditions, type of grass, and flow frequency. Channel side slopes ore

a maximum of 4: I for the grass-lined portion. Since open channels are highly susceptible

to erosion before a good stand of grass is achieved, all the seeded channel slopes should be

stabi Iized by an erosion protection blanket.

Trickle flow is considered to be the runoff from activities such as snowmelt, lawn

watering, and short duration, low intensity summer storms. Trickle flows of 10 to 15 cfs,

12