Hydraulic Analysis
New flood hazard delineations were not completed with this study. Hydraulic analyses for Leyden Creek were assembled from past reports and studies, and served as the basis of evaluating proposed improvements. Upstream of Leyden Lake, a HEC-RAS model, developed by ICON Engineering, as part of Colorado Water Conversation Board’s, Clear Creek Watershed Risk Map Project was utilized. Downstream of Leyden Lake, hydraulic modeling from the 2006 Mapleleaf Subdivision Filing No. 2 LOMR (06-08-B422P) and the 2005 LOMR model based on the 2004 Boyle FHAD study were used for the hydraulic evaluations.. All modeling was performed using HEC-RAS version 4.1.0.
The steady flow files used within these models were updated with new discharges developed for this study, as discussed in Section 3.0 Hydrologic Analysis.
4.1 Evaluation of Existing Facilities
Existing infrastructure along Leyden Creek was evaluated to determine capacity with future conditions discharges.
Several changes were made to the previous modeling. First, while repairing flood damage from the September 2013 event, the Indiana Street crossing was improved to triple 36” corrugated metal pipes (CMP)’s. The Alkire Street crossing was changed from a 36” CMP in the effective model to a 48” CMP based on information provided by the City of Arvada.
Existing conditions floodplains can be seen in APPENDIX C - Hydraulic Analysis as part of the Problem Area Identification Map. The HEC-RAS cross sections can be seen in APPENDIX C - Hydraulic Analysis.
An inventory of structures in the existing floodplain can be found below.
Table 4-1 : Buildings in Existing 100-yr Floodplain
| Reach | Location | Total Structures | Residental |
|---|---|---|---|
| 1 | Woodland Valley Filing No. 3 | 1 | 0 |
| Woodland Valley Filing No. 7 | 8 | 8 | |
| 2 | Downstream of West 75th Place | 1 | 1 |
| Alkire Street to West 75th Place | 2 | 0 | |
| 3 | Upstream of Indiana Street | 2 | 2 |
| 4 | Upstream of Quaker Street | 4 | 2 |
| Total | 18 | 13 |
Most of the roadway crossings were found to be vastly undersized to convey the 100-yr discharge without overtopping. In many cases, capacity was estimated to be less than the 2-year event. A summary of existing infrastructure and estimated capacity is presented below.
Table 4-2 : Existing Facilities Table
| Location | Cross Section | Structure Type | Estimated Structure Discharge (cfs) | 2014 MDP 100-yr Future Flow (cfs) | Return Period |
|---|---|---|---|---|---|
| Simms Street | 502.5 | (4) 12' x 5' RCBC | >2157 | 2157 | > 100-yr |
| W. 72nd Ave | 505.5 | Bridge | 590 | 2083 | < 10-yr |
| W. 75th Place | 513.5 | 36" CMP | 60 | 1416 | < 2-yr |
| Alkire Street | 518.5 | 48" CMP | 71 | 1249 | < 2-yr |
| Croke Canal | 527.5 | 36" CMP | 59 | 665 | < 2-yr |
| Indiana Street | 529.5 | (3) 36" CMP | 180 | 665 | < 5-yr |
| Leyden Lake | -- | 36" CI | 224 | 665 | < 100 yr |
| Church Ditch | 4030 | 84" CMP | 367 | 5380 | < 2-yr |
| Quaker Street | 5865 | 66" CMP | 207 | 5212 | < 2-yr |
| W. 82nd Ave | 19390 | (2) 50" x 36" RCP | 142 | 2453 | < 5-yr |
4.2 Flood Hazards
Existing flood hazards closely resemble those presented by the effective FEMA flood hazard information and proposed updates associated with the FEMA RiskMap project. A summary of these flood hazards is presented in the Drainage Problem Areas Figure (Figure C-1) in APPENDIX C - Hydraulic Analysis.
4.2.1 Flood Hazards of Reach 1
Upstream of the confluence with Ralston Creek, to Simms Street, no buildings are located within the 100-year floodplain boundary. The four cell, 12 feet by 5 feet reinforced concrete box culvert (RCBC) at Simms Street conveys the 100-year discharges without overtopping.
Problem Area 1A - Woodland Valley Filing No. 3
Due to the overtopping of West 72nd Avenue, the 100-year floodplain is
widespread through the Woodland Valley Filing No. 3 subdivision.
Although the residential homes are generally not inundated according to
the FEMA Flood Insurance Study, split flows from West 72nd Avenue
surround many of the buildings. The First United Church of Arvada is the
only building inundated from this flooding. Flood problems in this area
relate to adding capacity at West 72nd Avenue without worsening
flooding potential within the Woodland Valley subdivision, as well as
improving flood conditions for First United Church.
Problem Area 1B - West 72nd Avenue Bridge Crossing
The bridge roadway crossing at West 72nd Avenue has a conveyance
capacity less than the 10-year event. Flows in excess of the bridge
capacity divert water east flowing across West 72nd Avenue as
described in Problem Area 1A. The City of Arvada would like to maintain
100-year capacity at all major crossings.
Problem Area 1C - Woodland Valley Filing No. 7
Upstream of West 72nd Avenue, eight residential homes are impacted by
the 100-year floodplain due to a lack of channel conveyance capacity
downstream of Leyden Creek Park.
4.2.2 Flood Hazards of Reach 2
Within Reach 2, one residential home and two additional buildings are impacted by the 100-year regulatory floodplain.
Problem Area 2A - West 75th Place
One residential home is located within the 100-year floodplain boundary
within Reach 2. The structure is located shortly downstream of West
75th Place. The road crossing at West 75th Place is undersized 36”
CMP, with a conveyance capacity less than the 2-year discharge.
Problem Area 2B - Cameo Estates Unit No. 2
Upstream of West 75th Place, Problem Area 2C and 2D lead to the Zone X
Shaded shallow flooding seen in Cameo Estates Unit No. 2. The shallow
flooding flows spilling over Alkire impact several residential
buildings, although these structures are not within the regulatory
100-year floodplain boundary. There are two structures within the
regulatory floodplain between Alkire Street and West 75th Place.
Problem Area 2C - Alkire Street Culvert Crossing
Alkire Street is overtopped for the 2-year and larger discharges. As
shown by past studies, and as part of the 2013 flood, the low point in
Alkire Street does not occur at the culvert location, but approximately
600 feet south of the culvert. Flooding from the low point in this area
results in widespread shallow flows across Alkire Street to the east,
within the Cameo Estates Unit No. 2 subdivision. The City of Arvada
wishes to manage a higher level of capacity for the crossing at this
location to reduce flooding potential downstream.
Problem Area 2D - Upstream of Alkire Street
Upstream of Alkire Street, Leyden Creek spills out of its right bank
across the Youth Memorial Sports Complex entrance road diverting flows
south spilling over Alkire Street. Any flows that are diverted as part
of Problem Area 2D contribute to the flood hazards mentioned in Problem
Area 2C.
4.2.3 Flood Hazards of Reach 3
Problem areas within this reach relate to the impacts of private ponds in the floodplain areas in addition to the buildings and residences located within the floodplain. A total of three structures, two residential homes are inundated by the 100-year floodplain within Reach 3.
Problem Area 3A - Cameo Estates
Although not impacted by the 100-year regulatory floodplain, many homes
near this area were impacted by the flooding in September 2013. Flooding
was determined to be related to diversion from Leyden Creek near ponds,
fence lines, and other obstructions, as well as overflows from the canal
systems located in proximity to Cameo Estates. This area was also
determined to be subject to flooding potential for events exceeding the
100-year event, such as the September 2013 flood.
Problem Area 3B - West of Cameo Estates
Upstream of Cameo Estates, private ponds, fence lines, and other
obstructions have potential to divert flows towards residential
development, away from the natural flow paths. This was evident in
September 2013.
Problem Area 3C - Croke Canal
The 36” CMP crossing of Leyden Creek under the Croke Canal is undersized
leading to flows spilling into the Croke Canal. These comingled flows
exceed the design capacity of the canal, jeopardizing the structural
integrity of the canal. This was experienced with the September 2013
flooding where the canal bank eroded and eventually breached near West
78th Place and Eldridge Street.
Problem Area 3D - Indiana Street
Even though it was improved since the 2013 flood, the crossing at
Indiana Street still remains undersized compared to the 100-year event.
Upstream of Indiana Street there are two residential homes inundated by
the 100-year floodplain.
4.2.4 Flood Hazards of Reach 4
Problem Area 4A - Leyden Lake
As demonstrated by the updated hydrology in this report, Leyden Lake
overtops during the 100-year and more significant events even though the
original design of this facility was intended to provide 100-year level
of protection. Flows that are discharged over the Leyden Lake spillway
may be intercepted by the Farmers’ High Line Canal and can lead to other
potential damages.
Problem Area 4B - Church Ditch
The Leyden Creek crossing with the Church Ditch lacks capacity to convey
the flow without overtopping the canal. The canal embankment was eroded
during the September 2013 flood due to the lack of capacity. This
ultimately diverted flows from the Church Ditch into Leyden Lake. No
structures are present in the vicinity of the Church Ditch crossing.
Problem Area 4C - Quaker Street
The existing crossing at Quaker Street has less than a 2-year capacity,
causing excess flows to overtop the road. This lack of conveyance
capacity also contributes to the flooding associated with Problem Area
4D.
Problem Area 4D - Leyden Townsite
Upstream of the Quaker Street crossing, within the Leyden Townsite
subdivision, four structures are at risk within the 100-yr regulatory
floodplain, including two homes. The source of the flood hazards in the
area are due to the lack of channel conveyance capacity and the
overtopping at Quaker Street.
Problem Area 4E - Pattridge Open Space
Through Pattridge Open Space, steep longitudinal slopes have led to high
velocities, causing erosion of the banks of Leyden Creek. As further
development occurs upstream and flows increase, grade control is
warranted to address future degradation.
Problem Area 4F - West 82nd Avenue The culvert crossing at West 82nd Avenue currently has less than a 5-year capacity. The City of Arvada would like to maintain 100-year conveyance capacity for all major roadway crossings.
Problem Area 4G - Blue Mountain Estates
During the September 2013 flood, flows from the Blue Mountain Estates
area eroded the shoulders of Blue Mountain Road and diverted water into
the Leyden Creek watershed. Not only did this require repair from the
City of Arvada, but it also limited access to Blue Mountain Estates
during the flood. The City of Arvada wishes to maintain a higher level
of capacity for Blue Mountain Road and reduce the potential for future
damages in the area, as seen in the Drainage Problem Area Figures in
APPENDIX C - Hydraulic Analysis.
4.3 Previous Analysis
Previous analysis of the Leyden Creek watershed consists of the Ralston/Leyden Creek Major Drainageway Planning by Wright-McLaughlin in 1977 and the Flood Hazard Delineation Ralston Creek - Leyden Creek in 2004 by Boyle Engineering. The 1977 MDP identified the road crossings throughout the watershed to be improved from West 82nd Avenue (Leyden Road in 1977 MDP) to Simms Street. The 2004 FHAD identified the Alkire Street crossing as “inadequately sized to convey more than nuisance flows and results in overtopping of the street and shallow flooding to the east of Alkire Street during larger events.” The water surface elevations downstream of the Leyden Lake to W 75th Place have increase due to increased discharges from the dam due to inaccurate elevations used in the rating curve in previous studies.