FLOOD10070

• County: Larimer
• Community: Fort Collins
• Basin: South Platte
• Title: Master Drainage Plan Update Colorado State University
• Date: 1/1/1998
• Prepared For: Fort Collins
• Prepared By: Ayres Associates
• Floodplain - Doc Type: Floodplain Report/Masterplan

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REVIEW OF THE PROPOSED MASTER PLAN

Improvements proposed in the 1996 master plan were run using the SWMM hydrologic
model with the 1997 storm data, The purpose of this simulation was to determine the
performance of proposed improvements for an event the magnitude of the 1997 storm.

Tables 3 and 4 summarize the results of the simulation. These tables compare existing
elevations to 100-year and 1997 elevations. The results show that proposed improvements
lower elevations experienced in the 1997 event. Although elevations were lowered, this
would probably not reduce inundation significantly.

Table 3. Summary Elevation Results for the Simulation of the July 28-29, 1997 Storm
with Master Plan Improvements.
Existing
SWMM Conditions Master Plan Differ.
HWM Model Location Simulation Conditions From
Element (Elevation) (Elevation) Existing
#15&16 14 College Gym 4986.2 4986.1 -0.1
#17-20 15 Heating Plant and Oval 4993.3 4992,2 -1.1
#21-2 224 Engineering Parking Lot 4993.9 4993.4 -0.5
#22 53 Lagoon NW 01 Student Center 5004.8 5004.7 -0.1
#31 259 Athietic Fields - Football Practice 5024.1 5024.5 0.4

Table 4, Summary Discharge Results for the Simulation of the July 28-29, 1997 Storm
with Master Plan Improvements.
Existing Differ, %
SWMM Conditions Master Plan from Reduction
Model Location Simulation Conditions Existing from Existing
Element (cIs) (cIs) (cfs)
14 College Gym 353 267 -86 24
15 Heating Plant and Oval 322 251 -71 22
224 Engineering Parking Lot 872 674 -198 23
53 Lagoon NW 01 Student Center 1495 1204 -291 19
259 Athletic Fields - Football Practice 1886 1886 0 0
45 Prospect & RR 513 435 -78 15
299 Lake Street 513 435 -78 15

The major reason for the limited effect of the proposed improvements was due to the
magnitude of the storm event and the volume of water required to be handled by the
drainage facilities, The original plan relied upon water detention to minimize the damages,
therefore the overwhelming volume of the event would have overtopped even 100-year
detention facilities.

As a result the overall improvement plan was studied to identify future modifications that
could be made to reduce damages such as in the 1997 event.

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Ayres Associates