Storm Water Flow

Actual flow is a function of statistical frequency of design storm, distribution of the storm in time (hyetograph), previous conditions, season of the year and physical design of the collection system

There are methods which using for handling the storm water flow. These are The Rational Method, Hydrograph Techniques and Computer Simulation Techniques

The Rational Method

The simplest method used for storm sewer design.
Valid for areas < 3 km2
Q = CiA
Q = Total volume of water
A = Area
i = rainfal intensity
C= runoff coefficient

Run off Coefficent;
Fraction of precipitation that appears as runoff depends on landuse, moisture, soil characteristics, ground slope.
Use literature values for various surfaces and land uses.
C increases as urbanization level increases
Cave=Sum(Ai*Ci)/Sum(Ai)

Time of concentration: Time required for a particle of water to flow hydraulicly from the most distant point in the watershed to the outlet or design point.

Frequency-Duration-Intensity:
Used as input to many hydrologic design methods
Compiled for most locations
Most often used by entering with the duration and frequency to find the intensity.

Hydrograph Techniques

Unit Hydrograph: Hydrograph of surface runoff resulting from an effective rain falling for a unit of time.
Unit of time > time of concentration
Based on the observation that all single storms on a watershed which have equal duration will produce runoff during equal lengths of time.
Constructed from existing records of rainfall and stream flow

Computer Simulation Techniques

HEC-1:
Hydrologic/ hydraulic computer model
U.S. Army Corps
Urban Drainage Systems
Data required: Hyetograph, infiltration/detention function, sufficient hydraulic detail to permit routing by selected technique

STORM:
Pollutant loadings resulting from urban runoff
Continuous simulation model of multiple storm events
Simpler than HEC-1

SWMM:
Hydrologic/ hydraulic computer model
EPA
Rainfall is introduced in the form of hyetographs which can be varied for different subareas
Abstractions: Detention storage and infiltration
Overland flow is calculated from Manning’s Equation
Overland flow is routed using continuity equation in storage form