FLOOD09316

• County: Teller
• Community: Cripple Creek
• Stream Name: Pony Gulch, Poverty Gulch
• Basin: Arkansas
• Title: Drainage Master Plan
• Date: 4/1/1992
• Prepared For: Cripple Creek
• Prepared By: GMS
• Floodplain - Doc Type: Floodplain Report/Masterplan

A'" drainage area in acrc,;

Cf", frequency factor related to the design storm
return frequency,

The Rational Method is based on the following assumptions:

a. The peak rate of runoff at any point is a direct function of the average
uniform rainfall intensity for the estimated runoff time of
concentration to that point.

b. The frequency of the peak discharge is the ,ame as the frequency of
the average rainfaU inten,ity.

0,

The runoff time of concentration is the time required for runoff to
become e,tablished and flow from the most hydraulically remote part
of tbe drainage area to the design point under consideration. This
u\,surnption applies to the runoff most remote in time, not necessarily
in distance.

d,

l11e peak discharge per unit area generaUy decreases as the drain agc
area increases, and the intensity of rainfall decreases as its duration

increases.

1.

Design Storm Frequency

Selection of a ruinf:!lI event for design of drainage facilities is done on the
basis of economic considerations. Cost associated with the magnitude and
lrequency of damage caused by storm water quantities in excess of system
capacity must be balanced v.ith the capital af\d maintcn<mcl;: costs uf the
drainage ,y,tem. Discussion with City of Cripple Creek officials have led to

IV-5

!be following design criteria for ,izing uf drainage facilities within the
corporate limits of Cripple Creck. A tOO-year uesign storm event shall be
u~ed for all facilities which would convey in excess of 300 CFS a~ a result of
thetDO-yearevcnt. A JQ.yeardesign storm event shall be used for facilities
which wuuld convey less than 100 CFS as a result of the lOO-year event.
Facilities which would convey between tOO CFS and 300 CFS as a result of
the tOO-year event shall be ~ized based on site specific conditions and the cost
associated with potential damage from flOOding. Where drainage facilities are
de~igned to convey runoff from the to-year de~ign storm event, runoff in
excess of that from the to-year slOrm mUSl be confined within the public
right-of-way or drainage easements, encroachment upon private property will
not be acceptable.

It is important for anyone using this Drainage Master Plan to understand that
!beto-yearortOo-yearfloOd~llilteventsthatoccuron1yeveryl0orlOO
years. The terminology, "to-year design storm event" or "lOQ..year design
~torm event,' refers to the reciprocal of the annual probability of occurreoce.
Theprobabilityofoccurrenceofa to-yearde,ignstormeventoccurringinany
given ycar would be 10% and the probability of occurrence of the ]()().year
design storm event occurring in any une year would be 1 %. This is not to say
that 2 or more to-year or greater design storm events could not occur in the

same year.

2,

Runoff Coefficient

Thc runuff cudficient, C, is a variable of the Rational Method which is least
susceptible to precise determination and provides the designer with a degree
of latitude to exercise his independent judgement hased upon known effecting
plHnmeters. The following discussion is illlen(kd to provide a guide to
promOle the unifnrmselectionandapplicatinnnfrunoffcoefficicOls.

[VOl)