Floodplain Memo - Annexation Phase
Phone: 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
Todd Timmons, Development Partner
Nate Brockbank, Development Partner
From: Gerald R Williams, P.E., CFM
Date: December 9, 2021
Re: Floodplain Assessment and Flood Mitigation Concepts for the Ammon Village Annexation
Application
This memorandum presents the results of a concept level evaluation of flood risk and mitigation
considerations for the subject property that straddles 49th South and is west of Ammon Road as shown
below and on Exhibit 1.
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. That is the
current effective floodplain mapping for that area. In July 2021, WEI submitted an another LOMR
application, Case #21-10-1025P, that proposes refinements to the previous application from Sunnyside to
49th South and new hydrology from 49th South to 81st South. That LOMR application is currently under
review. The City of Ammon Floodplain Administrator has a copy of that current application.
This annexation phase floodplain study uses the flood flow rates presented in the current LOMR application
for existing conditions.
Exhibit 2 shows the flows to and from the parcel north of 49th South (North Parcel).
Per the irrigation company manager, the Highline Canal shown with a green dashed line has very little if
any stormwater inflow in major storm events, and thus it would be, during the major flooding season having
rain on snow on frozen ground, empty of irrigation water and reportedly empty or nearly so of stormwater
runoff. Consequently, the canal would intercept runoff and take it to Ammon Road and 49th South where
the main Sand Creek Drainageway runoff converges. As a result on the interception of runoff from the area
north of the North Parcel, there will be no surface water inflow to the site.
There will be onsite generated runoff. This will pond along the north side of 49th South until it overtops the
road, the culvert under the road on the east side of the lone existing house being inadequate to convey the
flow because of the elevated downstream “slough” condition.
Exhibit 3 shows the flows to and from the parcel south of 49th South (South Parcel).
The inflow from the North Parcel is minimal, but Sand Creek Drainageway runoff converges at the
Woodland Hills Elementary School basin and the intersection of Ammon Road and 49th South. Flow will
overtop 49th South from Ammon Road and eastward, some of it crossing the intersection and fanning out
to the southwest which is the source of the north to south flow arrows at the northeast corner of the South
Parcel shown on Exhibit 3. Most of the runoff overflows 49th South and goes into the Woodland Hills
detention basin and channel on the east side of Ammon Road that has substantial capacity but cannot contain
or convey without outflow the 100-year runoff. It has no positive outflow at the south end other than what
gets into the Highline Canal which, at the southwest corner of Woodland Hills (opposite the southeast corner
of Ammon Village), the canal goes east a short distance and passes through a limited capacity culvert.
Modeling this condition, it is apparent that flow would be spilling west over Ammon Road for the entire
half mile going south from 49th South and into the Ammon Village property. Inflow peaks and locations
are shown on Exhibit 3.
Also shown on Exhibit 3 is the outflow to the west. This overland sheet flow depth and flow rate must be
essentially retained unchanged unless the downstream property owner accepts otherwise which could
happen. After all, the more the flow is concentrated at the south end of the property the easier it is for the
west property owner to mitigate flooding on that property.
Phone: 208 359-5353 343 E 4th N, Suite 117, Rexburg Idaho 83440-6003 www.grwei.com
Floodwater Mitigation in the North Parcel The North Parcel only has onsite runoff to mitigate, and for
that a detention basin is currently considered with an outlet to the south, but full retention is also an option.
Floodwater Reception and Conveyance through and from the South Parcel The hydrology section
above presents flow rates to receive, convey, and discharge from the site. Since there is no downstream
offsite channel or large pipe outfall, but only at-grade farmland receiving shallow overland flow, that is
how, without approval otherwise, the developed condition must discharge runoff.
The drainage scheme is to receive the surface flow at grade, convey the water as surface flow more or less
at grade mostly through a system of streets with some greenbelts, and to discharge offsite to the west at
existing grade and more or less at existing rates for a given area.
Preliminary Hydraulic Computations Is there a solution, following the concepts given above, that will
work? Checking this, WEI prepared a cartoon conceptual layout shown on Exhibit 4 that shows inflow from
the east, receiving greenbelt drainage tracts shown in blue and streets, flow going in a southwest direction
per historic conditions, and a spread discharge from the site along the west boundary. Exhibit 4 was never
intended to be the masterplan design, but rather was prepared to show a concept to the developer and
planner.
Exhibit 4 shows there is a design solution that can work. Based on it the planner prepared a rough conceptual
first-round layout. WEI then commented on that version, which comments were mostly addressed in the
second-round layout prepared by the planner which is shown as Exhibit 5.
At this point, WEI applied the aggregate of inflow rates along Ammon Road that are shown on Exhibit 4 to
be intercepted street by street and drainage tract by tract. This was then discussed in a project team Zoom
meeting along with many other issues that would affect the masterplan layout. Thus, a few changes to the
layout shown on Exhibit 5 are still needed which will likely be made for the annexation application, but that
updated version will not likely be ready for us to make it a part of our annexation flood report. Consequently,
changes required for flood mitigation are noted on the second-round masterplan with flood information
overlayed in blue and red as shown on Exhibit 5.
One of the issues discussed in the team Zoom meeting was front yard setbacks only having to be at least
25’ whereas WEI based the conveyance cross section on 30’. Furthermore, it was also discussed in the team
meeting that the ability to have basements was strongly desired. WEI then pointed out that fill or finish
grade to the 100-year “base flood level” (BFE) elevation is required to 20’ out from any basement walls.
Consequently, if basements were used to the front of a house and 25’ of the front yard was a part of the
street and front yard “flood channel,” then the house setback would have to be 45’. The 20’ from basement
walls would also extend to the back as well—not from rear loading garages, but from any basements.
Lot sizes, building sizes, basements, building envelope setbacks, and street/front yard flood conveyance
capacity are all being further considered, and Exhibit 6 was prepared to assist in these considerations.
However, at this annexation phase they are not fully determined—but issues are known and solutions are
feasible.
Phone: 208 359-5353 343 E 4th N, Suite 117, Rexburg Idaho 83440-6003 www.grwei.com
Discharge to Offsite Without approval from downstream property owners, changes due to development
cannot adversely affect downstream property owners or significantly change the flow quantity or form.
Overland shallow sheet flow cannot be changed to concentrated flow at one location, for example.
The North Parcel will discharge to the South Parcel that has the same property owner, so that is not an issue,
but the historic flow pattern from the South Parcel is to the land located to the west as shown on Exhibit 3.
Without approval of the west property owner, discharge from the South Parcel will need to approximate the
history pattern. The drainage scheme shown on Exhibit 5 will allow this to be done. Alternatively, if the
downstream property owner prefers the floodwaters to be more concentrated, the Exhibit 5 layout can allow
for some consolidation of flow more southward.
Final Street-Front Yard “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, but all this will be resolved in FEMA-worthy analyses for a LOMR application.
Supporting Models and Calculations The City recently received from WEI the 2021 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 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.
December 9, 2021