Drainage Design Manual, Hydrology - Flood Control District of ...

Drainage Design Manual for Maricopa County
Hydrology: Unit Hydrograph Procedures
5
5 UNIT HYDROGRAPH PROCEDURES
TABLE OF CONTENTS
5 UNIT HYDROGRAPH PROCEDURES
5.1 GENERAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1
5.2 CLARK UNIT HYDROGRAPH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-3
5.3 LIMITATIONS AND APPLICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-11
5.4 DEVELOPMENT OF PARAMETER ESTIMATORS . . . . . . . . . . . . . . . . . . . . . . . . 5-11
5.5 ESTIMATION OF PARAMETERS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-12
5.5.1 Time of Concentration............................................................................... 5-12
5.5.2 Storage Coefficient.................................................................................... 5-17
5.5.3 Time-Area Relation ................................................................................... 5-18
5.6 S-GRAPHS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-21
5.6.1 Limitations and Applications...................................................................... 5-24
5.6.2 Sources of S-Graphs................................................................................. 5-25
5.6.3 S-Graphs for Use in Maricopa County ...................................................... 5-25
5.6.4 Estimation of Lag ...................................................................................... 5-28
5.7 PROCEDURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-30
5.7.1 Clark Unit Hydrograph............................................................................... 5-30
5.7.2 S-Graph..................................................................................................... 5-31
5.1 GENERAL
Rainfall excess can be routed from a watershed to produce a storm discharge hydrograph at a
downstream location (concentration point) by one of two methods: 1) hydraulic routing involving
the complete or some simplified form of the equations of motion (i.e., the momentum equation
plus the continuity equation); or 2) hydrologic routing involving the application of the continuity
equation. Kinematic wave routing, as available in HEC-1, is an example of simplified hydraulic
routing. Hydrologic routing is usually accomplished by either direct application of the equation of
continuity (Equation (5.1)), or a graphical procedure such as the application of the principles of
the unit hydrograph.
I – O=
dS -----
(5.1)
dt
where:
I = Inflow
O = Outflow
dS
-----
dt
= Change in storage per change in time.
August 15, 2013 5-1