FLOOD00831

• Designation Number: 320
• County: Jefferson
• Community: Arvada
• Stream Name: Lower Ralston/Van Bibber and Leyden Creeks
• Title: Major Drainageway Planning - Lower Ralston/Van Bibber and Leyden Creeks - Phase B
• Date: 6/1/1986
• Designation Date: 5/1/1992
• Floodplain - Doc Type: Floodplain Report/Masterplan

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IV-3

140 span. To minimize associated channel costs at the time of con-
struct ion, the invert beneath the bri dge was 1 eft at its pri or e 1 ev at ion.
This resulted in a bridge with a 140 foot width and a vertical clearance of
approximately 3.5 feet. The abutments and foundations for this bridge are
sufficiently below grade to allow deepening of the bridge opening. The
hydraulic analysis shows that at the present time, the bridge opening is
inadequate to pass the 100-year event, however, with future channel deepen-
ing it will pass the 100-year event without overtopping.

Leyden Creek
Reach L-1. This reach of Leyden Creek extends from the confluence of Ral-
ston Creek to the city limits of Arvada. This reach passes through areas
al ready developed with homes and areas planned for development in the near
future. The crossing at West 72nd Avenue creates a backwater which diverts
flow from the channel and into downstream properties. A development pro-
posed at the confluence has been shown on the drawings in Appendix B and is
included in the hydraulic model of the improved channel. The proposed
channel was planned to pass the 100-year flood event.

Reach R-2. This reach begins at the Pierce Street crossing and extends
upstream to the Ralston Creek confluence with Van Bibber Creek. Downstream
of the Wadsworth bypass (SH 121), the channel and the floodplain are char-
acteri zed by a grass channel with an apartment compl ex in both overbanks.
SH 121 presents a severe constriction to flow as a result of undersized box
cul verts. Irnmedi ately upstream of the SH 121, to approximately Wadsworth
Boulevard, the channel is dominated by the backwater effects of the SH 121
box culvert constriction.

Reach L-2. This reach extends from the city limits of the City of Arvada
to Alkire Street. The channel is small, but clearly defined and more
closely resembles the natural state. Floodplain development consists of
rural residenti al homes and some greenhouses in the vicinity of Alkire
Street. The Alkire Street crossing is inadequately sized to convey more
than nui sance flows and results ins i gni fi cant overtoppi ng of the street
during larger events.

From Wadsworth Boul ev ard through to the confl uence wi th Van Bi bber Creek,
the channel fl ows in a well mai ntai ned park-l i ke grassed waterway. There
are minor instances of flooding which exceed this channels capacity and
floods adj acent streets and homes. In thi s reach, Carr Street presents a
constriction to the flow. The bridge can convey the 100-year flood without
overtopping Carr Street, however, it requires the development of head
against the low chord on the upstream side of the bridge.

Reach L-3. This reach of Leyden Creek extends from Alkire Street to Leyden
Lake. The channel in this reach has steep grades with resulting high velo-
cities. In general, the channel meanders across a relatively undisturbed
floodplain. Just above Alkire Street is the North Jeffco Parks and Recrea-
tion District's sports complex which has encroached into the floodplain.
There has been I ittle increase in the backwater at that crossing as a
result of filling in the floodplain. In this reach, Leyden Creek crosses
the Croke Canal and the Farmers Highline Canal.

Reach R-3. Thi s reach of Ral ston Creek extends from the confl uence with
Van Bibber Creek upstream to the Ral ston Creek confl uence with Leyden
Creek. The hydraulic capacity of this reach is severely constricted by
encroachment into the floodplain by old residential development. In areas
where encroachment has not occurred on both sides of the channel, the open
space areas have been developed as parkland.

Leyden Lake is the upstream limit of this reach. In its current condition,
the Leyden Lake spillway is capable of conveying only 970 cfs prior to dam
embankment overtopping. This is due in part to the settlement of the em-
bankment and to the inadequate size of the spillway. The State Engineer of
Colorado has imposed a hold order limiting the maximum storage pool eleva-
tion to 4 1/2 feet below the crest of the spillway. The hydrologic analy-
sis used to develop the flows for this master plan assumed that Leyden Lake