FLOOD07509

• Designation Number: 150
• County: Adams
• Community: Thornton
• Stream Name: Direct Flow Area 0054
• Basin: South Platte
• Title: Flood Hazard Area Delineation - Direct Flow Area 0054 - Including Major Drainageway Planning DFA 0054 Phase A Report 11/79
• Date: 10/1/1979
• Designation Date: 1/1/1980
• Floodplain - Doc Type: Floodplain Report/Masterplan

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with the overland runoff from East 113th Avenue. The flow off 112th Place proceeds
down 112th Way, which terminates in a cul-de-sac. The branch of the storm sewer system
that drains the cul-de-sac is inadequate to convey all but the 2-year runoff, subjecting
several homes in the area to flooding during larger events. Some tile drainage swales have
been constructed to help alleviate damages caused by a flooding, but their effectiveness,
particularly during larger events, is limited.

Storm runoff collects in the vicinity of 112th Avenue and Cherry Drive and
continues in an easterly direction. The earth channel that parallels 112th Avenue has
capacity adequate only to convey approximately 120 cfs. The remaining flow is diverted
overland to 112th Avenue where a 1.5 foot high earthen berm constructed on the south
side of 112th Avenue forces much of the runoff to continue down the street. As storm
runoff proceeds down 112th, overflow is distributed in a "sheet flow" manner over the
berm, exposing large areas to shallow flooding. The flow that does not overtop the berm
or collect in the small depression behind 112th continues east to Holly Street, Sheet 11.

A 3 foot RCP that expands to a 3.5 foot RCP drains the area behind 112th Avenue.
Below 112th and upstream of Holly Street, the drainage channel is cut off by two small
detention ponds which are referred to as the Lower and Upper Medema Ponds, Sheets 3
and 4. The Upper Pond has a capacity of approximately 4.2 acre-feet and drains into the
Lower Pond through a 36 inch CMP. The Lower Pond, with a capacity of approximately
8.3 acre-feet, drains through an 18 inch RCP that passes beneath both Holly Street and
the Colorado Agricultural Canal.

Although the Medema Ponds are both overtopped during the 2-year event, they do
have a limited effect in reducing the peak discharges of all storms. However, no formal
spillway exists for the Lower Pond, and flood waters which overtop the pond also overtop
Holly Street. Since Holly Street itself is only a dirt road with no embankment protection,
it is subject to severe erosion problems whenever it is overtopped. Although it is beyond
the scope of this report to perform a stability analysis of any of these pond and street
embankments, the complete wash-out of Holly Street and consequential rapid draw down
experienced by the downstream slope of the Lower Pond could be critical with regard to
the stability of the slope. This is particularly true with the slow draining clays typical of
the basin area.

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At the Upper Pond, a small rip rap lined spillway has been constructed. However,
it too is undersized, with 5-year events exceeding the spillway capacity, subjecting the
pond embankment to erosion.

The Colorado Agricultural Canal passes beneath Holly Street twice through 36 inch
RCP's. In between, the canal is concrete lined for approximately 270 feet, and forms a
depression that divides the Holly Street embankment and the downstream slope of the
Lower Pond. Overflow from the Lower Pond fills this depression and would flow out both
of the culverts of the canal. Historically, it has been the case that silt quickly fills the
canal and plugs the culverts, rendering the canal useless downstream from this point.
Should the canal be in use when the culverts are suddenly plugged, flow from the canal
would overtop its banks, exposing the area downhill from the canal to shallow flOOding
with irrigation water, Sheet 4.

The usefulness of a leach field belonging to the residence situated just west of
Riverdale Road has been substantially reduced because of persistent trickle flows that
saturate the surrounding soil, Sheet 4. A low flow 21" RCP has been recently installed to
alleviate the problem and has been effective, but any periodic innundation of the
surrounding area caused by runoff quantities in excess of the capacity of the pipe would
again adversely effect the leach field. The residence itself is subjected to flooding from
the 100-year event.

A 12 inch and 18 inch pipe have been constructed beneath Riverdale Road. The
pipes, however, are only sufficient in size to pass very low flows. All excess runoff
overtops the road, exposing Riverdale to the possibility of sustaining damage during major
storms.

Below Riverdale Road, a drainage channel is nonexistent. Runoff, without a
channel to confine and convey it, spreads immediately upon crossing Riverdale, Sheets 4
through 10. In some locations the lOO-year floodplain spreads so wide that it leaves the
drainage swale area and the 100-year floodplain limits cannot be defined. Large areas of
farmland, on which predominently broom barley has been raised, become exposed to flood
hazards. During the early growing season, recently cultivated soil becomes especially
susceptible to erosion. After seeds have germinated and up to the time the crop has
matured, erosion can and .has exposed plant roots, reducing the yield and potentially
destroying the crop. The lower Clear Creek Canal, which traverses the flOOdplain, if not