Stormwater Calculations - Mathematics and Statistics for the Environment - page 770

Computer advances have greatly reduced the analysis time and related expenses associated with
continuous models. It can be expected that future models, which combine some features of continu-
ous modeling with the ease of single-event modeling, will offer quick and more accurate analysis
procedures. The hydrologic methods discussed in this text are limited to single-event methodolo-
gies, based on historic data. Further information regarding the derivation of the IDF curves and the
SCS 24-hour rainfall distribution can be found in NEH, Section 4, Hydrology.
25.4 RUNOFF HYDROGRAPHS
A runoff hydrograph is a graphical plot of the runoff or discharge from a watershed with respect to
time. Runoff occurring in a watershed flows downstream in various patterns which are influenced
by many factors, such as the amount and distribution of the rainfall, rate of snowmelt, stream chan-
nel hydraulics, infiltration capacity of the watershed, and others, that are difficult to define. No two
flood hydrographs are alike. Empirical relationships, however, have been developed from which
complex hydrographs can be derived. The critical element of the analysis, as with any hydrologic
analysis, is the accurate description of the watershed's rainfall-runoff relationship, flow paths, and
flow times. From this data, runoff hydrographs can be generated. Some of the types of hydrographs
used for modeling include
• Natural hydrographs are obtained directly from the flow records of a gauged stream.
• Synthetic hydrographs are obtained by using watershed parameters and storm characteris-
tics to simulate a natural hydrograph.
• Unit hydrographs are natural or synthetic hydrographs adjusted to represent one inch of
direct runoff.
• Dimensionless unit hydrographs are made to represent many unit hydrographs by using
the time to peak rates as basic units and plotting the hydrographs in ratios of these units.
25.5 RUNOFF AND PEAK DISCHARGE
Despite its simplification of the complex rainfall-runoff process, the practice of estimating runoff
as a fixed percentage of rainfall has been used in the design of storm drainage systems for many
years. It can be accurate when drainage areas are subdivided into homogeneous units, and when the
designer has enough data and experience to use the appropriate factors. For watersheds or drain-
age areas comprised primarily of pervious cover such as open space, woods, lawns, or agricultural
land uses, the rainfall/runoff analysis becomes much more complex. Soil conditions and types of
vegetation are two of the variables that play a larger role in determining the amount of rainfall that
becomes runoff. In addition, other types of flow have a larger effect on stream flow (and measured
hydrograph) when the watershed is less urbanized. These factors include
1. Surface runoff occurs only when the rainfall rate is greater than the infiltration rate and the
total volume of rainfall exceeds the interceptions, infiltration, and surface detention capac-
ity of the watershed. The runoff flows on the land surface, collecting in the stream network.
2. Subsurface low occurs when infiltrated rainfall meets an underground zone of low trans-
mission and travels above the zone to the soil surface to appear as a seep or spring.
3. Base flow occurs when there is a fairly steady flow into a stream channel from natural stor-
age. The flow comes from lakes or swamps, or from an aquifer replenished by infiltrated
rainfall or surface runoff.
In watershed hydrology, it is customary to deal separately with base flow and to combine all other
types of flow into direct runoff. Depending upon the requirements of the study, the designer can
calculate the peak flow rate , in cfs (cubic feet per second), of the direct runoff from the watershed,