DES 606 : Watershed Modeling with HEC-HMS Module 4 Theodore 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

HEC-HMS Hydrologic Cycle Components in HEC- HMS (circa 2008) Land Surface and Vegetation ChannelsReservoirs Infiltration Loss Snowpack Rainfall, P(t) Snowfall Snowmelt Runoff Percolation Loss Evapo- transpiration Discharge, Q(t)

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.