FLOOD05940

• Designation Number: 325
• County: Teller
• Community: Cripple Creek
• Stream Name: Pony Gulch, Poverty Gulch
• Basin: Arkansas
• Title: Drainage Master Plan - Cripple Creek, Final Report
• Date: 4/1/1992
• Designation Date: 7/1/1992
• Floodplain - Doc Type: Floodplain Report/Masterplan

1.

Replace existing stfUClllre no. 19 (dual 40.inchstee] culverts)....ith a structure
capable of conveying runoff from the lClO-year design storm event. 65l! CFS.
An ll'S"x7'3" arch culvert complete with headwall, and wingwalls would
convey approximately 675 CFS when set at a headwater depth of
approximately!l.5'.

2.

Replace existing structure no. 18 (dual 36-inch CMP) with a structure capable
of conveying runoff from the lOO-year design storm event. 679 CFS. An
11'5"x7'3" arch culvert with headwall and wingwalls would convey
approximately 675 CFS when set at a headwater depth of approximately 8.5'.

3.

The existing channel between existing structure numbers 19 and 18 is grass
lined, has a relative flat slope and has inadequate capacity to convey the
lOO-year design storm runoff of 679 CFS. This channel should be replaeed
with a riprap lined channel similar to the typical channel seetion shown in
Figure 4 of Appendix A A channel with a bottom width of approximately 5
feet and a flow depth of approximately 4 feet would adequately convey the
design flow. Approximately 950 feet of this channel lining will be required.
Provisions should be made for access from the school buildings to thc foO!hall
field and track on the opposite side ofthc channel.

4.

Existing structure numbers 11, 12, 13, 14 and 15 are all at road crossings of
Pony Gulch and are all severely undersized and incapable of conveying the
runuff from ll)()-year design storm event which varie, from 713 CFS to
767 CFS. For example, existing struCliJrc no. 11 (dual 36-inch CMPs with
approximately 5 fectofavailable headwater) husa totul capaeityof 110CFS
while a design capacity of 767 CFS is required. Also. the existing channel
between these Slructures is very poorly developed and a pOlcntial hazarduut"
to near vertical side slopes. Severa! alternatives werc revicwcd for
replucementoftheexi>tingstructuresandchannclbetwccnStruC!Urenumhcr~

VI.3

11 and 18. Land acquisition costs and eoSlS ussociatetl with the largerequiretl
structure ~izes at each street ero~sing made use of an open channel cost
prohibitive. Large structures are required at roatl crossings of open channels
/Jue to the pondingdepth requiretl to force flow into the culvert. Once flow
is ina~'lIlvert,pipesizecan bereduceda~thereare nnad/Jitionalhea/Jlos>es
associated with entrance conditions. Hydraulic calculations indicate that a
72-inch RCP installed at a slope of approximately 3.5 percent between existing
structure nos. 18 and 11 which would adequately convey the lOO-year design
flow. Approximately 1,400 lineal feet of n.inch RCP would bc requiretl. In
atldition, a means of intercepting the small overlot flows within basin PG-C
is necessary. Preliminary design indicates that construction of a shallow ,wale
over the new 72-inch Rep and installation of five strategically placed inleH
will adequately intercept these flows and convey them into the new 72-inch
outfall.

It is important to note that existing cnnditions result in significant f!oodingof
privatc prnpcrty during a design runoff event. BStrcctisgradetlhigherthan
surrounding land, thus directing excess runoff out of thc road right-of-way
onto adjacent land.

5.

Improvements to the channel below existingstmcture no. I] (Bennett Avenue
crossing) are not recommended at this time as that channel passes through
pres<:ntly undevelop<:d property and the potential for f!()oding damage is
minimal. As tha.t property develops, it will be the r.:sponsibility of th.:
developer to eliminate any encrn..ehment of development upnn the !lood
plain as uefio.:d in the SCS Roo(] Plain Management Study or develup the
channel as required to reduce the limits of the floo/J plain.

VI.4