Ammon Fields MPC_Floodplain Study Update Memo_10.4.2022Phone: 208 359-5353 343 E 4th N, Suite 117, Rexburg Idaho 83440-6003 www.grwei.com
Memorandum
To: Tracy Bono, P.E., City of Ammon Engineer and Floodplain Administrator
Nate Hutchinson, BCP Development, Inc.
Pete Evans, BCP Development, Inc
From: Gerald R Williams, P.E.
Date: August 6, 2021, Updated October 4, 2022
Re: Floodplain Assessment and Flood Mitigation Concepts for the Flagship Homes Annexation
Application
This memorandum presents the results of a concept level evaluation of flood risk and mitigation
considerations for the subject property that is bounded on the north by 1st Street, the south by 21st South,
the west by 52nd East, and the east by the foothills, as shown in the map below.
Phone: 208 359-5353 343 E 4th N, Suite 117, Rexburg Idaho 83440-6003 www.grwei.com
Hydrology WEI submitted a LOMR application Case # 16-10-0506P effective November 18, 2016 (2016
LOMR) that resulted in approved hydrology and hydraulic floodplain mapping for all of the Sand Creek
Drainageway and hydrology for Little Sand Creek as far downstream (south) as 49th South. Exhibit 4 of
that report shows flow rates and effective and updated floodplain boundaries and is provided with this
memorandum. The LOMR application included all the upland area floodplain modeling for the purpose of
knowing the flow rate, location, and timing of runoff, but the remapping area was requested to be only the
area between Sunnyside and 49th South. Consequently, FEMA only changed the flow rate in that area with
the LOMR even though to get that flow rate the hydrology was approved from the very top.
The hydrology prepared for the 2016 LOMR is good for the Flagship Homes area with one exception. The
watershed delineation to the north of the Flagship Homes site was based on an old USGS DEM that was
plus or minus 5 feet. Now available is LiDAR DEM which WEI used, and in the process, it was discovered
that all the area north of 3/4 mile north of 49th North does not drain to the Sand Creek Drainageway but
rather to Sand Creek. This was shown and documented in the current WEI LOMR application for Sand
Creek, Little Sand Creek, and Sand Creek Drainageway from 21st South to 81st South, which is LOMR Case
# 21-10-1025P that is tentatively approved and is set for final adoption in February 2022 (2021 LOMR).
Subsequently, WEI also submitted a LOMR application Case # 22-10-0131P, for remapping from 49th
North to Lincoln Road of the Sand Creek Drainageway. This also has been approved and was FEMA
officially adopted August 2, 2022.
None of these LOMRs changed the inflows to the Flagship Homes site and so they remain the same as in
the 2016 LOMR and the 2021 LOMR except for the update to correct the watershed area as described
above. This will, if anything, lower inflows to the site, but that has not yet been determined but will be in
the next phase of development because the model to Lincoln Road will be extended downslope to include
the Flagship Homes site.
Exhibit 1 shows the Flagship Homes site with inflows as per the above 2D LOMR models. Inflows from
the east from Black Canyon and adjacent unnamed watersheds, and also from the south from Euchre
Canyon, are shown. These are not point inflows, but the spreads are relatively short and flow is
concentrated. Flow from the north across 1st Street, however, is spread across the entire northern boundary
of Flagship Homes as shown on Exhibit 2.
Floodwater Reception and Conveyance through and from the Site Exhibits 1 and 2 were furnished to
the project prime engineers LEI who used the information in preparing their Concept Plan which is also
attached to this memorandum. The Concept Plan shows greenbelt channels in dark green.
Inflows from the North At the northeast corner is a channel that would intercept the 250 cfs inflow from
the foothills and convey it, along with intercepting and conveying inflow from the north that crosses 1st
Street, westward towards 55th East, then south and then west to the terrain low point 3000 feet south of 1st
Street.
Black Canyon Inflow An open space area is provided to intercept Black Canyon inflows that would be
spread while overtopping the canal. Runoff would then flow west in a channel along the south side of a
road to a park/open space and continue west with the flow from the north to the east side of 52nd East and
Phone: 208 359-5353 343 E 4th N, Suite 117, Rexburg Idaho 83440-6003 www.grwei.com
then southward the low terrain point 3000 feet south of 1st Street.
Euchre Canyon Inflow A channel along the north side of 21st South would intercept and convey inflow and
take it around the southwest site boundary to 52nd East and then north to the terrain low point 3000 feet
south of 1st Street.
The channel widths shown on the concept plan vary. This is because they are not just a schematic, but the
width is set based upon preliminary hydraulic calculations so that the Concept Plan would more realistically
show features proportionately to scale.
Preliminary Hydraulic Computations The following is taken from a basis of design document prepared
by LEI.
The channels were designed based on Manning’s Equation for an open trapezoidal channel. An n-
value of 0.027 was used for the channels which represents a vegetated state of short grass and weeds.
This should represent the final vegetated conditions.
A maximum depth of 3-feet was used in the design of the channels and 3:1 side slopes were used.
The channels were designed to provide a maximum velocity of 7.0 fps to minimize erosion of the
grassed channel...
No freeboard was added to the preliminary channel sizing calculations. Freeboard will be planned
and accounted for in the final design of these channels. The freeboard area may incorporate the
adjacent road areas with berms on the development sides of the channels.
There are multiple points where the runoff will need to be conveyed under roads. In these locations
box culverts will be used. The design of these crossings will be completed with final designs.
Conceptual sizing has been provided on the [Concept Plan] based on the average ground slope and
the anticipated flows at the respective crossings.
In order to be representative of a pre-development flow condition as the water is transported
downstream there will be a detention basin constructed on the east side of 52nd East. Historically
the runoff has been conveyed as sheet flow to 52nd East where it overtops the road and flows west.
In order to route the upstream runoff through the development it will be channelized, which will
increase the velocity and therefore [reduce] the time of concentration at the crossing of 52nd East.
To match the post-development time of concentration to the pre-development conditions this
detention basin will be used. The design of this basin as well and pre and post development
conditions will be detailed in future storm drain reports and designs.
Discharge to Offsite Runoff generated onsite and that passes through the site would, under current
conditions, sheet flow across 52nd East over a considerable distance as shown by the floodplain limits on
Exhibit 4 from the 2016 LOMR that accompanies this memorandum. Unless there is approval otherwise
from the downstream property owner, post-development flow from the site cannot be concentrated but must
cross 52nd East similarly to what it would do under pre-development conditions. This is the “change
Phone: 208 359-5353 343 E 4th N, Suite 117, Rexburg Idaho 83440-6003 www.grwei.com
nothing” option that is available for the development and as such is the approach used and described by LEI
above. At the final stage of design, this would mean designing the channels enroute to the detention basin,
from the north and south, to overtop laterally across the road and retain essentially the same overflow depth
and spread, or at least not exceed these, the same as is being done at 21st South with the DL90 project. Some
flow would reach the linear detention facility that mitigates other issues mentioned by LEI.
An option to spreading runoff flow crossing 52nd East, if the downstream property owner would cooperate,
is to allow concentrated flow to cross 52nd East and channelize the flow to the DL90 channel. This would
remove much of the downstream property from the floodplain.
The downstream channel option may be investigated, but the option shown on the Concept Plan and
assumed in channel sizing is based on maintaining status quo which is a viable and allowed solution.
Final Channel Sizing Through the design process there will be changes. However, at some point or points,
channel design will be evaluated in the 2D model where actual backwater calculations are performed which
may require upsizing the channels, but all this will be resolved in FEMA-worthy analyses for a LOMR
application.
Supporting Models and Calculations The City has received from WEI all the above-mentioned LOMR
application documents on which the hydrology is based, but not the actual hydrological and hydraulic model
files. These and other supporting computations can be provided upon request.
Conclusion Much of the site is in a FEMA mapped zone AE and X Shaded floodplain. Runoff that creates
the floodplain must be accounted for, received, conveyed, and discharged to downstream property owners
in an acceptable manner, which means that any adverse change in quantity, form, or location of discharge
to other property must be avoided unless there is approval otherwise by the property owner. This flood
mitigation concept report acknowledges flows and provides for acceptable solutions.
October 4, 2022