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.