FLOOD10336 ‎(2)‎

• County: Adams
• Community: Brighton, Commerce City and Thornton
• Stream Name: South Platte River
• Title: Flood Hazard Area Delineation, South Platte River, Adams County
• Date: 4/1/2005
• Prepared For: UDFCD; Adams County; Brighton, Commerce City, Thornton, Metro Wastewater
• Prepared By: CDM
• Floodplain - Doc Type: Floodplain Report/Masterplan

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4-1

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The hydraulic model developed for the Master Plan was based on the 1977 FHAO
Adams County, extending from Baseline Road to Franklin Street, and
upstream portion of the river from 1-270 to Franklin Street. Both

the 1985 FHAD update that covers
documents were developed by UOFCD

for the South Platte River in
the

In

To properly define channel geometry, it was necessary to obtain the channel-specific data from other sources. The
MWRD channel survey information provided greater detail, with survey elevations taken at grade breaks. These
data were available for cross-sections 26 through 48. For cross-sections upstream and downstream of the surveyed

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4.2 Prior Studies
4.2.11977 and 1985 FHAD Models

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The regulatory floodplain as defined by FEMA is the extent of flooding at a water surface elevation resulting from
river flows having a 1 percent chance of occurring in any 1 year. The regulatory floodplain is subdivided into
floodway and flood fringe areas. FEMA defines a designated flood way as the area encroached upon resulting in no
more than a 1.0-foot rise in the 100-year flood event. Local jurisdictions, however, may implement more rigorous
requirements. Since the South Platte River in Adams County actually flows through multiple jurisdictions, both a
1.0- and O.5-foot rise flood way needed to be computed. In both cases, the flood way was delineated by removing
equal conveyance areas from both sides of the floodplain in increments until the target water surface elevation
increase was achieved. The encroached-upon floodplain areas outside of the flood way are considered to be the
flood fringe areas. Any modifications or work within the flood way, which includes the river channel, cannot
increase the regulatory water surface elevation (i.e., base flood elevation [BFE])

The updated mapping and channel surveys were used to regenerate the FHAO cross-sections with data that were
both more current and provided greater detaiL Ground elevations across each cross-section were extracted from
the updated mapping by using AutoCAO SoftOeskTM or BOSS RMSTM software. The elevation increments in the
model were input at a minimum of 2 feet, as determined by vertical accuracy of the mapping.

Cross-section locations from the 1977 and 1985 FHAOs were transferred onto 1998 Adams County topographic
mapping obtained from UOFCD and the MWRO, maintaining their original locations to the maximum extent
practicaL Slight modifications in orientation were made where obvious changes in flow direction have occurred.
Detailed channel surveys were conducted in 1997 and 1999 under COM's direction related to preliminary design
work for the MWRO reaeration structures on the South Platte River. These surveys cover the main channel reach
from 88th Avenue to Henderson Road (COM 1999). Detailed channel information for cross-sections that fall
outside the MWRO survey was obtained from the UDFCD Geomorphic Assessment (Stevens 1996)

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flooding in the Study Area.

To develop the FHAO model, the HEC-2 Master Plan model was first imported into HEC-RAS, then the split flow
reaches were added to the main stem model, and, finally, channel modifications since the preparation of the
Master Plan model were incorporated. The HEC-RAS water surface profiles for this FHAO were compared to the
Master Plan profiles throughout model development. Due to computational differences between HEC-2 and
HEC- RAS, and new information added to the FHAD model, discrepancies in computed water surface elevations
were expected. Comparing the water surface profiles was used to identify discrepancies. The resulting FHAO
model was then used in conjunction with the topographic mapping for Adams County to determine the extent of

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Cross-sections with new
Channel geometry
Distances between cross-sections
Roughness coefficients
Bridge and ditch diversion data
Ineffective flow areas
Split flow areas

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topographic information

In order to create the Master Plan model, the following update modifications were made to the 1977/1985 FHAO
duplication model:

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The HEC-RAS computer program calculates steady and unsteady flow water surface profiles for a full network of
natural or manmade channels. A steady-flow model was developed for this study. HEC-RAS can model
subcritical, supercritical, and mixed flow regime water surface profiles as well as the effect of various floodplain
encroachments, such as bridges, culverts, and weirs. The one-dimensional energy equation is used to calculate the
water surface profiles from one cross-section to the next, using an iterative method known as the Standard Step
Procedure. The program is designed for floodplain management purposes, including evaluation of floodplain
encroachments, and the delineation and designation of regulatory floodplains.

To produce the FHAD duplication run, the 1977/1985 FHAD models were merged and regulatory water surface
elevations regenerated. The respective HEC-2 input files that contained the needed information were obtained
from UDFCO as hardcopy for the 1977 FHAO and in electronic format for the 1985 FHAD. A digital copy of the
1977 FHAD input file was created from the hardcopy version and the models were merged.

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The purpose of this FHAD study modeling effort was to adapt the hydraulic modeling for the 2002 Master Plan
Phase B Report (Master Plan) (UOFCO 2002) to a model that complies with regulatory standards developed by the
UOFCO and FEMA for floodplain management purposes. The primary task in this conversion effort was the
migration of the Master Plan model from the USACE HEC-2 program to the more recent HEC-RAS v3.1.1. The
FHAO report also required modeling a 500-year flood and one-foot rise flood way, which were not a part of the
Master Plan modeling effort.

4.2.2 Master Plan Model

The Master Plan model, published in 2002, involved first preparing and running the 1977/1985 FHAO models and
duplicating those model predictions. This duplicate model was the starting point for the Master Plan modeling
work (UDFCD 2002).

4.1 Introduction

conjunction with local sponsors, including Adams County, City of Commerce City, City of Brighton, and City of
Thornton. The 1977 FHAO and 1985 update HEC-2 models form the basis for the regulatory floodplain that was in

effect at the time of this study.

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Hydraulic Analyses

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Section 4

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4/8/05