Calvert County, MD Estimation of Runoff

The Rational Method is provided as follows:

Q = CIA

Where
Q	=	Quantity of runoff in cubic feet per second
C	=	Coefficient of runoff (ratio of runoff to rainfall).
I	=	Rainfall intensity in inches per hour for a given storm frequency and time of concentration
A	=	Drainage area in acres.

(2)

The runoff coefficient, C, is a percentage factor which represents the portion of the total quantity of water falling on the area that remains as runoff.

(a)

The runoff coefficient used in computing flow to a point under consideration shall be a composite of the C factors for all the areas contributing to this point.

(b)

In areas where the nature of future developments is uncertain, the engineering professional representative shall consider the future development in accordance with the plan for the County and the Zoning Maps of the County using the C factor that reflects the highest runoff coefficient. Runoff factors for various types of soil, ground cover and ground slopes are shown in Plate SD-1 of the Calvert County Construction Standards for Roads, Streets, and Incidental Structures.

(3)

Rainfall Intensity, I, shall be determined from the rainfall intensity curves shown in Plate SD-2, Rainfall Intensity Chart, of the Calvert County Construction Standards for Roads, Streets, and Incidental Structures. These curves indicate maximum intensities for times of concentration from five minutes to two hours, occurring with a frequency of two, five, 10, 20, 50, and 100 years. Storm frequency used in design shall be as follows:

(a)

A ten-year storm frequency shall be used for all drainage systems for which the ten-year design discharge is less than 100 cubic feet per second.

(b)

A twenty-five-year storm frequency shall be used for all sump areas.

(c)

A fifty-year storm frequency shall be used for design of major cross drains, box culverts and channels for which the ten-year runoff exceeds 100 cubic feet per second. The design shall be checked for a one-hundred-year storm frequency to establish the limits for a one-hundred-year floodplain and review the effects of a one-hundred-year design flow.

(d)

A one-hundred-year storm frequency shall be used for the design of cross culverts that have a drainage area of over 400 acres or lie within a one-hundred-year floodplain. Appropriate rainfall studies may be used at the Engineering Division Chief's discretion.

(4)

The time of concentration, TC, shall be the total time required for runoff to travel from the farthest point of the drainage area to the point of concentration.

(a)

Overland flow time shall be estimated using Plate SD-3 of the Calvert County Construction Standards for Roads, Streets, and Incidental Structures, Overland Sheet Flow Chart, in areas that are ultimately nonenclosed, and shall be the sum of the duration of flow for all applicable types, such as overland flow, swale flow, ditch flow and pipe flow.

(b)

Overland flow through most undeveloped land shall become channel flow within 400 feet.

(c)

The length of overland flow within subdivisions shall vary depending on the lot size and grading, but it usually becomes swale flow within 75 feet to 100 feet.

(d)

Stream or channel flow time should be computed by estimating the average velocity in each type of course and dividing it into the total length of flow through that course.

(e)

The time for pipe flow can be established using pipe flow charts for partial or full depth flow, whichever is applicable.

(f)

In no instance shall a time of concentration of less than five minutes be used for any drainage computation.

(g)

In drainage areas where more than 60% of the land usage is or shall be commercial, or any type of development where more than 50% of the area is impervious, a maximum time of concentration of seven minutes shall be used to the first inlet or structure for a closed drainage system.

(h)

The maximum time of concentration to the first inlet or drainage structure of a closed drainage system for residential developments shall be as indicated on Plate SD-1 of the Calvert County Construction Standards for Roads, Streets, and Incidental Structures.

(5)

All areas delivering runoff to the point under consideration shall be included in the drainage area, A. The extent of the drainage area should be determined using the highest order of information available from one or a combination of the following sources:

(a)

Photogrammetic maps.

(b)

USGS Quadrangle Maps.

(c)

Maryland Geological Survey Topographic Maps.

(d)

Field observations in conjunction with the above maps. The engineering professional representative shall certify that the topography has been field verified.

While the Rational Method for computing runoff is an accepted, trusted method, it should not be considered standard. When working with homogenous hydrological units, the Rational Method is acceptable; however, whenever hydrological units are to be nonhomogenous, other methods of computing runoff should be used.

For drainage areas greater than 100 acres but less than 2,000 acres, the Runoff Curve Method as adopted by the United States Department of Agriculture/NRCS described in Chapter 2 of its Engineering Field Manual or TR-55 is to be used to estimate runoff.

Whenever it becomes necessary to make a transition from use of the Rational Method of computing runoff to the Runoff Curve or other approved method for drainage areas, the discharge, Q, shall not be reduced as the result of computations made with the new method. The Q shall remain constant until it increases as the result of computations made with the new method. In computations involving fringe areas of 100 acres, the Rational and the Runoff Curve Method shall be computed, and the governing method shall be used. When proposed upstream development and existing downstream development share a common storm drain line, the approved discharge at the common point of the existing development shall be used as the maximum allowable discharge, Q, at the common point for the proposed upstream development.