When a drainage report is required, it must be prepared and sealed by a registered engineer, and prepared in accordance with the criteria established by the city. The purpose of the report is to ensure that the development is designed to be protected from flooding, to verify that the development will not improperly divert or increase the stormwater runoff on surrounding properties, and to provide data supporting the design of facilities to be constructed for the management of rainfall runoff. Each drainage report must consider rainfall runoff from storms with a return frequency up to and including a one hundred-year storm. The types of drainage reports required at the various stages of land development shall be as follows, unless waived by the city engineer.
A. Master Drainage Report.
1. A master drainage report is required to accompany the following development applications:
a. Planned area development zone change request;
b. Major amendments to existing planned area developments;
c. Preliminary plat for multi-family, commercial, or industrial developments.
2. Report Requirements. Master drainage report shall address the following aspects of the site drainage:
a. Onsite Flows. Calculating of the amount of developed flows expected for each land use based upon the following runoff coefficients:
i. Commercial, industrial area: 0.80;
ii. Single-family residential: 0.50;
iii. Multi-family; apartments; townhomes, attached dwellings, manufactured home and RV parks: 0.60;
iv. Open space: 0.40;
v. Agricultural: 0.10.
b. Indication of how the developed flows will be retained on site (i.e., shared or individual basins on each site).
c. Indication of how the developed flows from the perimeter and internal streets will be handled.
d. Offsite Flows.
i. Identification of the watershed affecting the site.
ii. Rainfall values for calculating offsite flows shall be as follows:
a) Drainage areas less than twenty square miles: one hundred-year, six-hour storm;
b) Drainage areas between twenty and one hundred square miles: one hundred-year, six-hour and one hundred-year, twenty-four-hour storm;
c) Drainage areas greater than one hundred square miles: one hundred-year, twenty-four-hour storm;
iii. Calculation of the amount of offsite flows entering the site, based upon the same runoff coefficients as for onsite developed flows. For watersheds that are less than one hundred sixty acres tributary to the design point(s), the design engineer shall perform the hydrologic calculations using the rational method to estimate runoff.
Rational Method Runoff Equation (Watersheds 160 Acres or Less):
Q = C*I*A
Q = the peak discharge (cubic feet per second) from a given area
C = a coefficient relating the runoff to rainfall
I = average rainfall intensity (inches/hour), lasting for a Tc
Tc = the time of concentration (hours)
A = drainage area (acres).
Tc = 11.4L^0.5 * Kb^0.52 * S^-0.31 * I^-0.38
Tc = time of concentration in hours
L = length of the longest flow path in miles
Kb = watershed resistance coefficient
S = watercourse slope in feet/mile
I = rainfall intensity in inches/hour
Time of concentration shall not be less than:
1) Ten minutes for residential developments
2) Five minutes for commercial developments
To estimate the resistance coefficient (Kb) and the adjusted intensity for the Tc equation, engineers shall utilize Table 3.1, Figure 3.1, and Equation 3.3 of the Drainage Design Manual for Maricopa County, Volume I, Hydrology.
iv. For watersheds greater than one hundred sixty acres tributary to the design point(s), the design engineer shall perform the hydrologic calculations using a rainfall runoff model. Generally, Hydrologic Engineering Center (HEC-1) of U.S. Army Corps of Engineers, or Drainage Design Management System for Windows (DDMSW) of Flood Control District of Maricopa County (FCDMC) is to be used. Alternative methods of a similar nature may be used when approved by the city engineer.
v. Indication of where the offsite flows currently enter and exit the site.
vi. Indication of how the offsite flows will be carried across the site post-development.
vii. Indication of the location with the city’s master drainage report study.
viii. Indication of compliance with the approved drainage plan of adjacent developments upstream and downstream of the proposed project.
e. Special Flood Hazard Area.
i. Indication of the location of the site on the appropriate FEMA map.
ii. Indication of any special flood hazard area impacting the site.
iii. Description of any modification proposed to special flood hazard areas.
B. Preliminary Drainage Report.
1. A preliminary drainage report shall be required to accompany the following development applications:
a. Site plan/final development plan;
b. Preliminary subdivision plat for all subdivisions.
2. Report Requirements. Preliminary drainage report shall address the following aspects of the site drainage:
a. How the drainage design concepts as set forth in the approved master drainage report or master drainage study for Casa Grande will be implemented.
b. Preliminary design or method of transmitting offsite flows through the site. And indication of how the natural offsite drainage enters flows through and exits the site.
c. Preliminary design for retention of all developed flows generated by the site development. Developed flows shall be calculated based upon the following run-off coefficients:
Retention calculations shall be submitted as follows:
Vr = D (A) (Cw)
12
12
A = Area (square feet or acres)
Vr = Volume required to be retained (cubic feet or acre feet)
D = One hundred-year, one-hour rainfall (inches) = 2.39
Cw = Runoff factor for tributary areas (weighted factors may be required for multiple retention areas and/or special conditions as determined by the city engineer. The weighted runoff coefficient will be estimated by:
Cw = A1C1+A2C2+... +AnCn
A1 +A 2 +... +A n
A1 +A 2 +... +A n
Typical runoff factors:
Buildings, parking lots and similar impervious surfaces: 0.95
Landscape, retention areas open space and undeveloped vacant areas: 0.40
Single-family (lots 20,000 square feet and below): 0.65
Single-family (lots greater than 20,000 square feet): 0.53
d. Calculation of developed flows based upon the following rainfall events:
i. All hydrologic models used for flood control purposes within the city of Casa Grande shall use the NOAA Atlas 14 (version 4.0 or higher). If the storm runoff flows onto the property to be subdivided from adjacent properties, drainage area, calculated peak flows, retention volume requirements and other pertinent runoff data shall be designed to a one hundred-year, two-hour rainfall event. If the flow is in a defined channel, the channel must be improved to carry a one hundred-year flow with a minimum freeboard of one foot.
ii. Curbed streets shall be designed and constructed to carry the stormwater runoff from a ten-year storm between curbs.
e. Drainage and grading plan that contains the following minimum information:
i. Show all existing and proposed storm drainage facilities, such as retention basins, catch basins, scuppers, curb and gutter, dry wells, and storm drain pipes, on the grading plans, and identified by type and size.
ii. Show all existing irrigation facilities, lakes and other water features. Use arrows to indicate stormwater flow patterns.
iii. Show all existing and proposed buildings, grass areas, concrete or dessert landscape and other significant structures to remain.
iv. Show existing and finished grade elevations using contour lines or spot elevations.
v. Cross sections must be shown the relative elevation of the building, parking, and retention basins.
vi. Cross sections must be shown for all retention basins with the elevations of the top of slope, bottom of the basin, and the side-slope called out. The depth of ponding in the basin, due to retention of the one hundred-year, two-hour storm event must also be shown on the retention basin cross section.
vii. Show cross section of any drainage channel.
viii. No portion of a retention basin shall encroach within the public utility easement (PUE).
ix. The grading plan of all retention basin must show:
a) The tract number or designation. The same retention basin designation as shown in the drainage report.
b) Show outfall for all individual basins.
c) Show ultimate outfall for entire development.
d) Show volume required/provided per drainage report.
e) Contours shall be one-foot-minimum elevations.
f) The design one hundred-year, high-water elevation, with one foot of freeboard and water elevation.
x. Show construction details for the following items, but not limited to:
a) Catch basins and connector pipes, with profile and showing hydraulic grade line.
b) Scuppers and spillway, showing a longitudinal section, with elevations and slopes.
c) Storm drains, with profile and showing hydraulic grade line.
d) Drainage swales, with grade and spot elevations.
xi. Show the general notes (construction inspection and testing) and grading notes; please refer to city of Casa Grande standard plan notes.
xii. Show city engineer’s approval block and as-built certification block on the cover sheet, and show north arrow, scale, and legend on all sheets.
C. Final Drainage Report.
1. A final drainage report shall be required to accompany the following development applications:
a. Final subdivision plats that contain shared drainage facilities.
b. Site development permit.
c. Public improvement permit that involves the construction of new streets or drainage facilities.
2. Reports Requirements. Final drainage report shall address the following aspects of the site drainage.
a. Final drainage reports are required for specific individual developments to ensure compliance with the approved master and preliminary drainage reports for the subject property, and to identify specific drainage facilities needed to serve the parcel.
b. Sufficient information to determine the path of the water entering and leaving the project property under pre-development and post-development conditions. Sufficient information to show that proposed conditions do not pond water on adjacent properties or discharge at erosive velocities above pre-project conditions.
c. Final design or method of transmitting offsite flows across the property (if any). Drainage area, calculated peak flows, and other pertinent runoff data must be presented. Specific calculation of runoff factor (weighted factor, Cw) is required. If the flow is in a defined channel, the channel must be improved to carry a one hundred-year flow with a minimum freeboard of one foot.
d. A drainage map that shows the following:
i. The discharges at points of concentration and clearly identifies the existing drainage system.
ii. Floodplain area (if any) should be delineated on a drainage map.
iii. Typical cross sections of all streets and average slope.
iv. Establish a bench mark (BM) of known or assumed elevation to which every other elevation is referenced. Show existing grades by a contour lines, especially at property lines and back of sidewalks, and show enough finished grades that drainage direction are clearly evident.
e. Calculation for the proposed stormwater retention basin, showing storage volume required and retention volume provided. Analysis confirming basin draining within thirty-six hours of the end of the design precipitation event is required.
f. Modification to sites which result in increases to unretained developed flows will be required to address drainage retention requirements on the entire site. Exceptions to this requirement may be granted by the city engineer in cases where the lack of retention of existing developed flows are determined to pose no significant flooding problems to surrounding properties or streets.
g. The manner in which the developed flows associated with the adjacent streets will be retained on site or within a shared basins or other appropriate outfall.
h. The amount and manner in which stormwater will be carried in adjacent street. The maximum depth for water flowing in any street shall be eight inches during the peak runoff from a one hundred-year frequency storm. The above requirements imply that in some cases water may flow deeper than normal vertical curb height, and may flow for a short distance over a sidewalk or other back-of-curb areas, but the flow of the water shall always be confined to the road right-of-way or to drainage easements. Particular care must be taken in street sag locations to ensure that these requirements are met. Catch basins, scuppers, or similar facilities, together with the necessary channels, must be provided at appropriate locations to remove water flowing in the streets so as not to exceed the above-described depth limit.
i. Specify the parties that will be responsible for maintenance of all constructed drainage facilities.
(Ord. 1397.15.22 § 2 (part), 2019)