HomeMy WebLinkAboutWATERGLEN PUD - FINAL - 71 93B - CORRESPONDENCE - CITY STAFF (6)14. Please include plots of the water surface profiles in the floodplain analyses.
15. Sheet 54 of 68; the top "Matchline See Sheet 56 of 68" should read "See Sheet 55 of
68"
Comments on HEC-2 model proposed conditions
16. Contraction and expansion coefficients were held constant throughout the model at 0.1
and 0.3 respectively. Between sections 8200 and 8550 there are abrupt changes in
channel geometry with regard to contraction and expansion that may warrant higher
values.
17. Cross -sections 7799, 7800, 7900, 8300, 8400, 8500, 9100 had to be "extended"
(special note 3280) by the HEC-2 program in order to calculate the hydraulic
properties of the cross -sections. Please modify the input files so that the
(surveyed/proposed) input ground elevations are above the calculated water surface
elevation. Please revise the floodplain delineation maps accordingly.
18. Divided Flow at sections 8400, 8500, 8550 occurs due to buildings. Additional cross -
sections are needed to define the water surface profile withyrega`rd to structures in the
cross-section geometry. Please address and revise.
19. In general, sections 8300 through 8600 are problematic with regard to divided flow,
conveyance, minimum specific energy, and assumption of critical depth special notes.
As discussed above, additional cross -sections are needed to better define the water
surface elevation profile and account for energy losses at these sections with regard to
regulation of existing and proposed structures and floodplain delineation.
cc: Glen Schlueter
certification by a professional engineer. If structures are floodproofed the applicant
must comply with all requirements with regard to structural components capable of
resisting hydrostatic and hydrodynamic loads and effects of buoyancy.
5. National flood insurance is required for all structures located in the proposed
floodplain. Once the City of Fort Collins constructs the proposed Box Elder Creek
Master Plan improvements east of I-25, which will effectively remove Boxelder Creek
overflows from the Waterglen development site and eliminate the floodplain, flood
insurance may no longer be required.
6. Sheet 54 of 68; The floodplain delineation should be extended to the north through the
overflow swale on the eastern property boundary.
7. Sheets 53 and 54 of 68; Please provide and plot base flood elevation data for the
proposed and existing floodplains.
8. Page 3, Storm Drainage Section; states that rock riprap erosion protection will be
provided at all locations where velocities warrant armoring for proposed and existing
structures which will be elevated a minimum of 1-1/2 feet above the 100-year flood
levels. The applicant refers to Sheet 67 of 68 and the "Hydraulic section" for
calculations, however, it is unclear to this reviewer what is being referenced. Please
identify all locations requiring armoring on a map and provide design detail.
9. Swale summary table, Hydraulic Structures Calculations section and cross -sections,
Sheet 59 of 68; Please include tabulated velocity and Froude number data for all
swales.
10. Please provide a map with all swales labeled. Sheet 54 of 68 and the Overall Drainage
Facilities Plan in the Storm Drainage section only depicts a few swales.
11. Swale calculations section; swales 11,12, and 14 are designed with Froude numbers
(1.17, 1.1; and 1.0) indicating critical and slightly supercritical flow . Please redesign
these swales for subcritical flow regime since flow at or near the critical state is
unstable. A minor change in the specific energy at or near critical will cause a major .
change in depth and affect the channel design stability.
12. Swale 2, which conveys flows from the Boxelder overflows to detention pond E, has a
design velocity of 7.61 fps. The proposed design exceeds maximum permissible
velocities recommended for grass -lined channels and should be redesigned according to
the City design criteria and standards.
13. Cross -sections are required to show elevated buildings on fill in their proposed final
configuration for the proposed floodplain. Please modify all cross -sections as needed
on Sheet 59 of 68 and modify/rerun the HEC files to reflect proposed floodplain
conditions.
TO: Basil Hamden
FROM: Mike Grimm, Floodplain Administrator
DATE: October 24, 1994
SUBJECT: Waterglen P.U.D. Floodplain Comments
The Waterglen P.U.D. project is located in the Cooper.Slough basin. The attached FIRM
(County Panel 080101 0180D, effective date 11/17/93) shows the location of the proposed
project in relation to detailed study areas for the Boxelder overflow channel and the
headwaters of Cooper Slough. Although the detailed study area of the FIRM floodplain with
determination of base flood elevations (BFE's) has not been extended into the waterglen site,
the proposed project site is considered to be within the regulatory floodplains of the Boxelder
Creek and Cooper Slough.
City Code Section 10-52 and 44 CFR 60 requires that base flood elevation data shall be
provided for subdivision proposals and other proposed developments which contain at least
fifty (50) lots or five (5) acres, whichever is less. The applicant has provided a delineation of
the existing and proposed 100-year floodplain.
The applicant has proposed a grassaed waterway to convey the Boxelder overflows through a
detention pond. A result of the channelization is a substantial reduction in the 100-year
floodplain boundary (Sheets 53 and 54 of 68), however several proposed and existing
structures remain ni the proposed toodplain_, -:._,
My comments are as follows;
1. The cover letter and page 2 of the Floodplain Section states that the applicant is
requesting a variance from floodplain regulations with regard to street overtopping at
Waterglen Drive and Glenloch Court. The applicant should direct the variance request
at the Storm Drainage Design Criteria and Construction Standards rather than
floodplain requirements. .
2. All channel and floodplain improvements must be completed p for t�uingbuilding
permits in the development to reduce the flood hazard and comply with floodplain
regulations.
'3. The applicant is required to obtain floodplain use permits for all proposed structures
located in the proposed 100-year floodplain.
4. All structures, existing or proposed, must be elevated or floodproofed to a minimum of
18" above the base flood elevation. The applicant is required to submit elevation or
floodproofing certificates for all such structures. Elevation and floodproofing requires