DES 606 : Watershed Modeling with
HEC-HMS
Module 4Theodore G. Cleveland, Ph.D., P.E
29 June 2011
Module 4: Storage
• Surface Storage – Treated as abstractions:
• Canopy (interception) storage• Depression storage
• Reservoir Storage– Treated as hydrologic/hydraulic elements
• Reservoirs (regulated and unregulated)• Detention basins• Certain stormwater BMPs
Rainfall-Runoff Process
• Watershed– Losses
– Transformation
– Storage
– Routing
• Precipitation– Meterology, Climate
• Runoff– Fraction of precipitation
signal remaining after losses
Canopy and Depression Storage
Reservoir Storage
HMS – Basin Component
Canopy (Interception) Storage
Canopy (Interception) Storage
• Interception is precipitation that does not reach the soil, but is instead intercepted by the leaves and branches of plants and the forest floor.
• The intercepted water generally evaporates and leads to loss of that precipitation for the drainage basin.
~ excess precipitation
~ loss
Canopy (Interception) Storage
• In HMS part of sub-basin properties.– Simple Canopy– Gridded Canopy
• HMS “gridded” implies a GIS type interface– This course does not
directly use gridded methods, although introduced in last module.
Canopy (Interception) Storage
• All precipitation intercepted until storage capacity satisfied.– Excess precipitation then
directed to surface (depression) storage if any.
– Then excess to runoff component.
• Also considers potential evapo-transpiration (PET) as part of the hydrologic cycle.
Canopy (Interception) Storage
• Sophisticated hydrologic abstraction and likely uncommon in typical engineering hydrological applications, esp. because of the PET feedback.– Utility in “scientific investigation”– Measurements are likely non-existent.
• While the process undoubtedly occurs, would not be commonly used in Texas, except perhaps East Texas Piney Woods
Surface (Depression) Storage
• Depression storage. The volume of water contained in natural depressions in the land surface, such as puddles. (After Horton, 1935, p. 2) – In the Green-Ampt model, water ponds at non-zero
depth; hence depression storage is arguably important for such infiltration models.
– The interaction of depression storage and infiltration is the basis of Hortonian overland flow
Surface (Depression) Storage
• In HMS part of sub-basin properties.– Simple Surface– Gridded Surface
Surface (Depression) Storage
• Initial storage (depth)• Maximum storage
(depth)– Storage is satisfied.– Excess can become
runoff.
Rainfall-Runoff
• As a process diagram:
Loss Model(s)
Precipitation
Depression Storage Excess Precipitation
Interception Storage
Infiltration
Evapotranspiration
Sub-basin properties
Meterologic properties
Reservoir Storage
• Reservoir – A pond, lake, or basin, either natural or
artificial, for the storage, regulation, and control of water.
• Regulated reservoir– Outflow controlled by moveable gates and valves.– Head, and valve settings determine outflow.
• Unregulated reservoir. – Outflow controlled by fixed weirs and orifices.– Head and constructed weir height determine outflow.
Reservoir Storage
• In HEC-HMS reservoirs (and detention basins) are treated as a hydrologic element in the basin model
Reservoir Storage
• Accounts for storage• Flows are “routed”
through a reservoir– Level pool routing– Orifice flow– Weir flow
Reservoir Storage
Pond with storage, orifice and weir flow.
Orifice flow; energy loss model
Weir flow; critical depth model
Image from ftp://ftp.crwr.utexas.edu/pub/outgoing/Robayo/HECHMS.../HEC-HMS.ppt
Reservoir Storage
• Storage Representations– Storage vs. Discharge– Storage vs. Elevation– Surface Area vs. Elevation
• Discharge Representations– Spillways, Weirs– Orifices, Sluice gates– Pumps– Dam Breach
Image from ftp://ftp.crwr.utexas.edu/pub/outgoing/Robayo/HECHMS.../HEC-HMS.ppt
Reservoir Storage
• The storage relationships are usually developed external to HEC-HMS– Like rainfall and external hydrographs, use
external tools to develop the storage-discharge relationships
HEC-HMS
• Example 4 – Illustrate Reservoir Storage Data Entry– Ash Creek Watershed– Use the GA runoff generation model
• Will use canopy storage and surface storage to illustrate the effects of these components.
– Pretend we will place a small detention facility at the outlet
• Develop the storage-discharge curves in Excel• Enter into HEC-HMS, examine effects.
Summary
• Storage in HEC-HMS is of two types:– Abstraction: Canopy and Depression– Hydrologic/Hydraulic: Reservoir
• Abstraction storage is a sophisticated concept, hard to estimate parameters for engineering practice – its is uncommon.
• Reservoir storage is common, if not fundamental in watershed models– Detention facilities– BMPs
Summary
• Example 4 illustrates the data entry activities associated with both kinds of storage.