Master Concept Drainage Report

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Table of Contents  
Page 
1.0 
SCOPE 
2 
2.0 
SITE LOCATION 
2 
3.0 
DESCRIPTION AND PROPOSED DEVELOPMENT 
3 
4.0 
FIRM CLASSIFICATION 
3 
5.0 
OFFSITE DRAINAGE 
4 
6.0 
ONSITE DRAINAGE 
4 
7.0 
REFERENCES 
8 
 
EXHIBITS 
Exhibit A – Vicinity Map 
Exhibit B – Aerial Map 
Exhibit C – FIRM Map 
Exhibit D – Phasing Plan 
Exhibit E – Future Road Network 
 
APPENDICES 
 
None

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1.0 Scope 
The objectives of this drainage master report are: 
1. To research and identify the current offsite drainage impacts associated with the 
property. 
2. To determine the hydrologic and hydraulic conditions for the proposed 
development. 
3. To create a drainage concept which adequately conveys site-generated discharges 
and offsite flow, if applicable, through and/or around the parcel in accordance 
with City of Mesa and Maricopa County drainage design standards. 
4. To ensure that proposed finish floor elevations are free from the 100-year storm 
event in accordance with City of Mesa and Maricopa County drainage design 
standards. 
This study has been performed utilizing an aerial-generated topography map and field 
reconnaissance conducted by Rick Engineering, authorized to perform a topographic 
study by Optimus Civil Design Group.  This drainage report was prepared in 
accordance with the current City of Mesa and Maricopa County Floodplain 
Ordinance, design criteria, regulations, and policies.  
 
2.0  Site Location 
The project site is located on the northeast corner of Williamsfield Road and 
Ellsworth Road in the City of Mesa, Arizona (see Exhibit A). 
More specifically, the project site is located within a portion of the Southwest and 
Northwest Quarter of Section 27, Township 1 South, Range 7 East of the Gila and 
Salt River Base and Meridian, Maricopa County, Arizona.

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3.0  Description and Proposed Development 
The site is currently on an approximately 210-acre parcel that resides at the northeast 
corner of Williams Field Road and Ellsworth Road. The improvements will consist of 
multi-family residential, mixed-use development, general office space, and hotel. The 
site is currently undeveloped desert and slopes from east to west at approximately 
0.4% and drains into a set of box culverts at Ellsworth Road that connects to the 
Ellsworth Channel on the west side of the street.  
The project is planned to have a core phase that will be built out immediately, and the 
rest of the site has a general use plan, as identified by the Legacy Park Community 
Plan, that does not yet have a determined site use and will be built out by separate 
developers at a later time. For these later developments, a conservative use case will 
be used when analyzing drainage impacts to the site. See Exhibit D for the phasing 
plan. 
To the south, the site is bound by the Legacy Sports Complex and future Gateway 
Crossing. The north and eastern sides of the site are bound by Arizona State Route 
24. The west side is bounded by the future Gateway East, a 273-acre development 
used for manufacturing, industrial, retail, office, and hotel space. 
4.0 FIRM Classification 
The Maricopa County, Arizona and Incorporated Area Flood Insurance Rate Map 
(FIRM) map number 04013C2770L dated October 16, 2013, 04013C2760L dated 
October 16, 2013, and 04013C2790L dated October 16, 2013 shows that the project 
site is currently located within flood hazard Zone “X” (see Exhibit C).  
Zone “X” (shaded) is defined by FEMA as:  areas of 500-year flood; areas of 100-
year flood with average depths of less than 1 foot or with drainage area less than 1 
square mile and areas protected by levees from 100-year flood.

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5.0 Offsite Drainage 
There are existing culverts on the west side of the site that cross under Ellsworth 
Road that used to convey offsite flows from the north and east. However, due to the 
construction of SR-24, the offsite flows are being intercepted by a new channel on the 
northeast side of SR-24 and routed away from the project site to Powerline Floodway 
per ADOT’s report “Final Design Concept Report, SR-24, Ellsworth Road to 
Ironwood Drive Interim Phase II, 2017” and the corresponding Appendix C of that 
report, “Initial Drainage Report in support of the interim Phase II DCR Plans, 2016”. 
There are no significant offsite flows that cross through the site. 
There is also an existing 30” private HDPE storm drain pipe that routes to the west 
underneath SR-24 just north of the property’s northern extents. This pipe then routes 
to the south and connects in to an existing lateral that crosses Ellsworth Road. There 
is no additional remediation needed as part of this project apart from acknowledging 
that the pipe exists and should be considered as design progresses. 
The north of the existing Williams Field Road pavement edge is a swale that currently 
drains to the existing culverts that outlet under Ellsworth Road. In the proposed 
condition, this swale will be eliminated, and the half-street volume will be captured 
by storm drain inlets and retained on site. 
The eastern half-street portion of Ellsworth Road currently drains to existing storm 
drain inlets that ultimately discharge to the western side of Ellsworth Road. In the 
proposed condition, this storm drain system will stay intact and the half-street volume 
of water will not be retained on site but rather continue to be pushed west to 
Ellsworth Channel. 
6.0  Onsite Drainage 
The site is currently undeveloped desert land and flows from east to west. The flow is 
captured in earthen channels on the eastern side of Ellsworth Road and the northern 
side of Williams Field Road and ultimately discharged under Ellsworth Road through 
the existing culverts, proceeding north away from the site on the western side of 
Ellsworth Road.

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In the proposed condition, the onsite drainage will no longer be conveyed offsite, but 
instead be retained onsite, per City of Mesa standards, for the 100-year, 2-hour storm 
event.  
The specific retention requirements for each parcel will be calculated at the time the 
drainage report is put together for that site plan. This master drainage report only acts 
as a framework for the Legacy Park development. 
The internal roadway network throughout the site will be treated as public roadways. 
As a result, the half-street runoff will be routed onto its respective development parcel 
through a series of storm sewer inlets and retained within that parcel. For the parcels 
that aren’t being developed in the first phase, temporary basins will be installed for 
the half street retention. 
Retention basins, where used, will be a maximum depth of 3’, measured from the 
bottom elevation of the basin to the high-water elevation. An additional 6” depth for 
freeboard will be provided. All other developments throughout the site will utilize 
underground storage for retention. 
The following formula will be used to determine the required storage volume: 
  
V = C * (P/12) * A 
  
 
 
Where: 
V = Required retention volume (cubic feet) 
 
 
 
 
C = Runoff Coefficient  
 
 
 
 
*P = Precipitation Depth (inches) = 2.20” 
 
 
 
 
A = Drainage Area (square feet) 
*The greater of 2.20” or the mean of the 90% confidence interval of the current 
NOAA Atlas 14 point precipitation frequency estimates shall be used. 
The runoff coefficients “C” for use on the project are listed below: 
Land Use 
C 
Commercial/Mixed Use
0.90
Undeveloped
0.50
Water
1.00
 
Pursuant to COM 2025 Engineering and Design Standards Section 806, this project 
proposes to retain all onsite flows for the 100-year 2-hour event onsite including the

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half-street of all adjacent public roads. The only exception to this is Ellsworth Road, 
which currently drains to Ellsworth Channel to the west, and it is proposed to 
continue doing so. 
This project, due to its urban nature and density, will be using underground retention 
tanks to store the design volume, typically using 96-inch or 120-inch diameter pipes. 
The bleed-off method for this arrangement is the use of drywells, as is typical 
throughout Maricopa County and other municipalities.  
This project, as a general rule, will vary from section 806.20 to allow the use of 
drywells as a preferred method of water disposal within the development, with the 
exception of parcels that are fronting Ellsworth Road. Where feasible and space 
allows, open-bottom arch pipe retention systems will be analyzed for use, which 
directly percolate into the ground with minimal drywell usage. 
All parcels adjacent to Ellsworth Road will be required to check the feasibility of a 
bleed-off into the existing drainage system, either across Ellsworth Road into the 
channel or tying into the existing storm drain system within the roadway. If this 
disposal method is not attainable by gravity, the City will approve alternate methods 
of disposal on a case by case basis. 
There are a number of items that are driving this variance, included below: 
o 
Due to the nature of this development and the platting, there will be users 
in the form of large office users, commercial retail, resort development, 
and multi-family which will be doing their own onsite retention. As such, 
it would serve the best interest of this development and the City to let 
them control their own storm disposal method, with the exception of all 
parcels fronting Ellsworth Road. 
o 
The majority of this development is not being built out during Phase 1, 
therefore routing a bleed-off line to each undeveloped parcel and sizing 
adequately is incredibly difficult given the unknowns of how a developer 
plans to layout their site and dispose of storm water. 
o 
Committing to storm sewer layout with a single outfall in the corner of the 
site creates challenges for all future sewer and storm drain crossing 
locations. As many of these developments will be built out in phases and

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are not being designed as part of Phase 1, flexibility in utility placement, 
both horizontal and vertical, is crucial to the success of this site. 
 
All Drywells shall be dual chamber and will be maintained by each parcel user via a 
set of CC&R’s/REA agreements. All Dry wells will be registered with ADEQ so they 
can be tracked. All dry wells will have a maintenance schedule for inspection and 
cleaning. 
 
Finished floor elevations will be a minimum of 14” above the elevation of the site 
ultimate outfall, and the hydraulic grade lines of storm drains will remain 1’ below 
the grate elevation at all inlets. 
All onsite overland and underground flow drainage facilities will be sized for the 10-
year and 100-year event per the rational method at the time of final construction 
documents. Per the Drainage Design Manual for Maricopa County, the proposed 
depressed curb and grated inlets will be sized using a reduction factor of 0.80 and 
0.50 respectively. 
In conclusion, the proposed conceptual drainage design is shown to keep the 
proposed improvements free and clear of the 100-year, 2-hour storm event. The 
design parameters and methodologies for the site are in accordance with City of Mesa 
and Maricopa County Flood Control District drainage design standards.

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7.0 References 
1. City of Mesa Engineering & Design Standards, August 2025. 
2. Drainage Design Manual for Maricopa County, Arizona, Volume 1 Hydrology, 
December 14, 2018. 
3. Drainage Design Manual for Maricopa County, Arizona, Volume 2 Hydraulics, 
August 22, 2018. 
4. Final Design Concept Report SR-24, Ellsworth Road to Ironwood Drive Interim 
Phase II, January 31, 2017

EXHIBITS

4650 E. COTTON CENTER BLVD.
SUITE 140
PHOENIX, AZ  85040
PH: (602) 286-9300      FAX: (602) 286-9400

4650 E. COTTON CENTER BLVD.
SUITE 140
PHOENIX, AZ  85040
PH: (602) 286-9300      FAX: (602) 286-9400
SR-24
E PECOS ROAD
PROJECT
ELLSWORTH ROAD
E RAY ROAD
PROPOSED GATEWAY
CROSSING (BY OTHERS)
ARIZONA ATHLETIC
GROUND

4650 E. COTTON CENTER BLVD.
SUITE 140
PHOENIX, AZ  85040
PH: (602) 286-9300      FAX: (602) 286-9400

4650 E. COTTON CENTER BLVD.
SUITE 140
PHOENIX, AZ  85040
PH: (602) 286-9300      FAX: (602) 286-9400

4650 E. COTTON CENTER BLVD.
SUITE 140
PHOENIX, AZ  85040
PH: (602) 286-9300      FAX: (602) 286-9400

OF
JOB NO.:
DRAWN:
DESIGNED:
CHECKED:
DATE:
DRAWING NO
TJM
TJM
JDB
07/2026
NO.
REVISION
DATE
4650 E. COTTON CENTER BLVD.
SUITE 140
PHOENIX, AZ  85040
PH: (602) 286-9300      FAX: (602) 286-9400
Contact Arizona 811 at least two full
working days before you begin excavation
Call 811 or click Arizona811.com
251461
MESA, AZ
LEGACY PARK
PHASE 1 PLAN

OF
JOB NO.:
DRAWN:
DESIGNED:
CHECKED:
DATE:
DRAWING NO
TJM
TJM
JDB
04/2026
NO.
REVISION
DATE
4650 E. COTTON CENTER BLVD.
SUITE 140
PHOENIX, AZ  85040
PH: (602) 286-9300      FAX: (602) 286-9400
Contact Arizona 811 at least two full
working days before you begin excavation
Call 811 or click Arizona811.com
251461
MESA, AZ
LEGACY PARK
FUTURE ROAD NETWORK