IV. - HYDROLOGY
Hydrology is the study of precipitation. Policy makers and engineers must study and understand hydrology because they are interested in designing and building structures and systems to safely convey and discharge precipitation runoff while minimizing the potential of flooding. They must determine how much water should be collected and conveyed or stored, how fast this process must take place, how much can be safely discharged without adversely impacting surrounding properties, and what are other effects of the development being considered. The following sections discuss specific parameters and methods to be used in analyzing proposed developments in the City's service area.
Storm Frequency
All drainage improvements shall, at the minimum, be designed for the following storm frequencies. The return intervals listed here are minimums, and the individual design engineer or the City may choose to exceed these minimums given site specific requirements or constraints.
Type of Facility Return Interval Storm Closed Conduit Storm Sewers (for new developments) 3-year Ditch Culverts (serving less than 50 acres) 5-year Ditch Culverts (serving 50 to 100 acres) 25-year Ditch Culverts (serving 100 acres or more) 50-year Bridges crossing City Ditches 100-year Major Ditches and City Channels 100-year Detention Facilities 100-year Peak Storm Runoff Rates
The Rational Method can be used for determining peak runoff flow rate for both existing and proposed conditions. These peak runoff rates are used to estimate the impact of development and the conveyance requirements for drainage improvements. This method is applicable for small to medium drainage areas (generally less than 200 acres) where the flow domain is typically overland sheet flow or shallow surface ditch flow. Other methods should be used to estimate peak runoff rates for larger areas or those served by well defined channels where flow routing in defined channels may be significant. The Rational Method takes the following form:
Q = C f * (C * I * A)
Where:
Q = Peak Runoff Flow Rate (cfs)
C = Runoff Coefficient, See ATTACHMENT A
C f = Frequency factor (the product of C f and C should not exceed 1.0)
A = Area of drainage basin being studied (acres)
I = Rainfall Intensity of the design storm (inches/hour)
Frequency Factor (C f )
The Frequency Factor is used in the Rational Method to scale the magnitude of the peak runoff in relationship to the return interval of the storm consistent with observed runoff data. This adjustment factor is used to account for the effects of antecedent moisture conditions that are generally associated with the less frequent storms. Appropriate values of C f are presented in the following table.
Storm Frequency Frequency Factor (C f ) 10 1.00 25 1.10 100 1.25 The product of C f and C used in the Rational Method should not exceed 1.0.
Basin Time of Concentration (T c )
The storm rainfall Intensity used in Rational Method will be selected based upon the return interval of the storm to be used (specified in the Storm Frequency Table above), and the duration of the storm to be used (based on the study basin's time of concentration). Time of Concentration (T c ) is defined as the length of time it takes a drop of water to travel from the most hydraulically remote portion of the drainage basin to its outlet. T c is a property of the drainage basin reflective of its area, shape, surface gradient, land use, land cover, and soil type. T c (in minutes) may be estimated from the following equation:
T c = Length/(Velocity * 60) + 10
Where:
Length = Flow distance (feet)
Velocity = Flow velocity (fps) [see following table]
Flow Condition Representative Velocities Shallow overland flow in undefined channels 0.25 to 0.50 fps Flow in street curb & gutter or road ditches 0.75 to 1.25 fps Flow in shallow ditches 1.5 to 3.0 fps Flow in defined channels 2.0 to 4.0 fps Flow in closed conduit storm sewers 3.0 to 5.0 fps The constant value of 60 in this equation is used to convert seconds to minutes and 10 is used as an estimate of initial delay between the start of rainfall and development of actual surface runoff. This method can be applied fairly accurately to large and small basins with either undeveloped or developed surfaces. However, the designer must specify the flow condition and estimated flow velocities for each flow domain on the site (i.e., the first 100' is overland flow followed by 250' in a gutter followed by 400' in closed conduit, etc.) and estimate time of concentration as the sum of all these individual flow conditions. The flow path used as the basis of this calculation should be clearly denoted on the plans with the associated design calculations.
Another method that can be used to estimate time of concentration for developed areas (i.e., storm sewer projects) is in the following form:
T c = 10 * (A) 0.1761 + 15
Where:
A = Drainage Basin area (acres)
This method accurately estimates T c for sewered projects; however it tends to underestimate actual T c for basins with significant overland flow or open ditch flow, and therefore may overestimate peak runoff flow rates for these basins.
Alternative methods for estimating the basin's time of concentration will be accepted for reviewed by the City, and may be allowed for use if the method's applicability to a specific situation warrants its use over the methods presented.
Storm Intensity (I)
For small watersheds and individual developments, the storm intensity should be based upon the time of concentration of the basin being analyzed. For example, in the design of a detention facility serving a basin with a 2-hour time of concentration, an Intensity for a 100-year, 2-hour storm should be selected for use in the analysis.
For large watersheds and regional studies, use a 24-hour duration storm for the analysis and design. Appropriate design storm intensities are shown in ATTACHMENT C for various return interval storms.