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

I

I.

0611\40892\PREP-FHAD\S4.DOC 4f5/05 cje

4-5

I

mil

I

I

The 100-year floodplain north of 104th Avenue on the west side of the channel is confined to the channel and small
portions of the west overbank; however, the floodplain extends up to 4,000 feet across the east overbank (station
602+70 to 480+00). The floodplain contains mostly past and present mining operations and vacant property.
Although a residential area, sited northwest of the intersection of 104th A venue and Brighton Road, is near the 100-
year floodplain boundary, very few buildings are located within the floodplain. The reach from 104th Avenue to
Brantner Ditch (station 602+70 to 513+50) includes a berm and temporary stockpiles from mining operations that
function as an island between the channel and the east floodplain. Because of the length of this embankment, the
hydraulics between the channel and floodplain flows become disconnected, resulting in a split flow situation with
different water surface elevations in both areas.

Changes have occurred since the previous FHAD where topography has been altered, gravel mining has taken
place, and degradation of the riverbed has resulted in larger islands. In addition, several new hydraulic structures
have been constructed since the publication of the previous FHAD. This section summarizes the areas where the
floodplain has changed significantly.

4.4.5 Floodplain Comparisons

'I

I

I

Downstream of 88th Avenue the floodplain is no longer confined to the channel, widening rapidly to 104th
Avenue (station 602+70). Natural topography, rather than levees or other structures, largely defines the floodplain
limits in this reach. Active gravel mining after the date of mapping may result in changed topography on the east
and west banks from 88th to 104th Avenue. Although the floodplain is relatively wide, the areas of flooding are
mostly past and present mining operations, with few structures. The SACWSD Williams-Monaco Treatment Plant,
located northeast of McKay Road (station 665+00), is situated on a floodplain island, protected by constructed
levees. Problems associated with flooding in this reach include nuisance flooding at McKay Road and potential
bypass of the Fulton Ditch diversion structure by the river channel during a flood event.

The elevation contours used to map the 100- and 500-year floodplain, along with the one- and half-foot floodway,
are the same elevation contours used in the Master Plan. Updated model segments such as the E-470 and SACWSD
LOMRs, MWRD Reaeration Structure RS4, and the Henderson and York Street grade control structures were,
however, developed using more recent topographic data. In addition, there are numerous topographical changes
throughout the Study Area resulting from modifications to or removal of temporary gravel stockpiles. In these
cases, the floodplain was mapped based on hydraulic model output and engineering judgment based on
knowledge of existing conditions along the river and, thus, the floodplain may not follow the contours exactly.

4.4.4 Mapping

I

The flooded area to the east of the channel upstream of 88th Avenue is
floodplain downstream of 88th Avenue extending back upstream.

I

I

The 100-year event downstream of Clear Creek (station 887+50) to Steele Street (859+00) is largely confined to the
river channel. From Steele Street to 88th Avenue the west bank water surface elevations were calculated assuming
the east bank levee. In the 1977 FHAD, a split flow of 13,500 cfs was predicted along the east embankment between
the UPRR Bridge and the 88th Avenue Bridge (station 780+29 through 855+46). The FHAD model and the Master
Plan model agree within 0.5 feet throughout this reach and the channel has sufficient capacity to carry the 100-year
flow without overtopping.

due to the backwater effect from the

The flood way is the area of the floodplain reserved for reasonable passage of the 100-year flood, which will convey
the base flood with not more than a specified rise in the water surface elevation at all locations along the study
reach. In this study, two flood ways were delineated through the Study Area; a one-foot rise flood way, as defined
by FEMA and several adjacent municipalities, and a half-foot rise flood way, as defined by UDFCD and Adams
County. Both definitions assume that there will be an equal degree of encroachment extended for a significant
segment on both sides of the watercourse. Flood way widths that represent the maximum limits of encroachment
into the floodplain are tabulated in Table A-I. From Franklin Street (station 1058+60) to 88th Avenue (station
779+40), the floodway is confined to the channel, except for the west overbank of the York Street split flow reach
(station 1051+00 through 1054+00 to 1012+50). Below 88th Avenue, the flood way extends onto the overbanks.

4.4.3 Floodway

I

I

I

4.4.1 Flood Characteristics

The floodplain upstream of Clear Creek (station 887+50) is primarily confined to the river channel. Exceptions
occur at the upstream boundary of the Study Area between stations 1058+00 to 1040+00, on the west overbank in
an industrial area, and a small area southwest of the 1-270 Bridge. Between 1-76 and 1-270 areas on the west bank
are subject to shallow flooding with an average depth less than 3 feet. From stations 944+40 to 1011 +70, the average
depth of shallow flooding on the west bank decreases from less than 3 feet to less than 1 foot. The split flow
between Franklin Street and York Street is predicted to be approximately 2,235 cfs. In comparison, the 1985 FHAD
states 3,550 cfs

is expected

to split.

Road crossings and bridges modeled in HEC - RAS are listed in Table 4-3. These crossings were modeled using the
energy approach for low flow conditions (typical for perched bridges); the pressure/ weir flow solution method
was used for high flow conditions (i.e., when the water surface elevation is greater than the low chord or the high
chord is overtopped). None of the crossings were overtopped by the 100-year flood event, although the water
surface does extend to the low chord of the Baseline Road (168th Avenue) Bridge, the 160th Avenue Bridge, the
104th Avenue Bridge, and the upstream side of the 88th Avenue Bridge, resulting in pressure flow situations.

4.4.2 Bridges

I

,I

,I

I

I

4.4 Results

The updated existing conditions model was used to simulate the 10-,50-,100-, and 500- flow events. Half- and one-
foot flood way analyses for the 100-year event were also modeled. The modeling results are presented in Appendix
A, Table A-1, included at the end of this report. The floodplain for the 100-year event is shown on the plan and
profile drawings in Appendix B of this document. The 10- and 50-year floodplain limits are not shown; however,
the 10-year flow was contained within the existing channel except for minor breakouts near the diversion dams of
the Brantner and Brighton Ditches. The 50-year floodplain is predicted to extend outside of the channel below 88th
Avenue, though to a lesser degree than the 100-year floodplain. Further results are discussed below.

4/8/05

Downstream of Brantner Ditch to Baseline Road, the river is generally sinuous and the floodplain unconfined,
defined by the natural topography with no significant embankments. The floodplain consists predominantly of
agricultural and gravel mining operations. From stations 281 + 30 upstream to 259+40, the topography on the west
bank has been altered after the date of mapping due to E-470 construction. Upstream from stations 265+00 to
118+20 areas on the east bank are subject to flooding due to potential levee failure. Areas on the west bank between
the same two stations are subject to flooding assuming the east bank levee remains intact. Active gravel mining
after the date of mapping has changed the topography of the east and west banks of the river downstream of
Brantner Ditch to Baseline Road. Some future and existing residential areas as well as the Brighton Wastewater
Treatment Plant are located within close proximity to floodplain boundaries. A flow split exists upstream of
Brighton Ditch from station 226+27 to 127+42 between the east side of the floodplain and the main channel.
Another flow split exists from stations 291 +60 upstream to 74+60 on the west bank. The floodplain is not confined
to the channel with the west bank floodplain extending up to 3,500 feet from station 56+ 10 to 168th Avenue.

Section 4
Hydraulic Analyses