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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Probably the largest flood recorded in the last few years occurred on May 2, 1979,
as this report was in preparation. This event was documented by the Urban Drainge end
Flood Control District and estimates were made of the discharge at Holly Street,
Riverdale Road, and 112th Avenue based on data gathered during the event. Flows over
the roads were calculated to be approximately 53 cfs and the culverts were rated at from
12 to 14 cfs, which sums to a total flow of about 65 cfs.

The major problems associated with the runoff of May 2, 1979 were: overtopping
and erosion of the Holly Street embankment, figures 2 and 3; overtopping of Riverdllle
Road and flooding of the fields to the east, figure 4; and sedimentation of the existing
detention ponds, irrigation canals and local drainage culverts. Nearly all of the sediment
appeared to be coming from residential construction projects adjacent to the channel
which had undergone severe erosion, see figure 5.

The problems described are aggravated by the presence of the irrigation canals
which traverse the basin from south to north. These canals are usually full of irrigation
water during the flood season, and since there are no wasteway flow separation structures
where they intersect the drainageways, the canals intercept the storm water and carry it
away from the drainage chanrlel. This usually results in the canals being overburdened
with water and overflowing at points downstream, causing additional flood damage. The
heavy silt loads of the storm water coming from DF A 0054 also clog the irrigation canals
necessitating regrading and dredging to keep the canals operable. These problems were
also present in the storm of MaY 2, 1979; see figure 6.

The floods described above have all occurred on the southernmost tributary which
drains the developing parts of the City of Thornton to the west of Holly Street. There is
no recorded information on floods which have occurred on the other tributaries which
drain agricultural and rural lands west of East 112th Avenue.

2. Holly Street overtopping and erosion - May 2, 1979

3. Holly Street overtopping and erosion - May 2, 1979

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