HEC-HMS employs a dual-pronged approach to manage sediment transport within the reach element. First, it calculates the transport capacity for each grain size, determining the sediment available for subsequent routing within each sub-reach. This computation factors in the transport potential and sediment sources on the reach bottom. The subsequent step revolves around computing sediment routing for the available sediment between sub-reaches, employing a range of sediment routing methods.

Sediment Transport Potential Methods

The Sediment Transport Potential specifies which method to use to calculate the stream flow sediment carrying capacity for non-cohesive sediments. Different methods have been proposed for calculating the transport potential. Each method has been developed for a particular sediment grain-size distribution and environmental condition. The selected transport potential method will be used at all reaches within the Basin Model. The available choices are shown in the table below. 

Method

Type

Method

Reference

Ackers and White

NC

SP

Ackers and White, 1973

Engelund-Hansen

NC

SP

Engelund and Hansen, 1967

Laursen-Copeland

NC

ES

Laursen, 1958; Copeland and Thomas, 1989

Meyer-Peter Müller

NC

ES

Meyer-Peter and Müller, 1948

Toffaleti

NC

RE

Toffaleti, 1968

Wilcock-Crowe

NC

ES

Wilcock and Crowe, 2003

Yang

NC

SP

Yang, 1984

Krone and Parthenaides Methods

CO

̶

Krone, 1962; Parthenaides, 1962

Sediment Delivery Ratio (only for HEC-HMS)

NC/CO

̶

Pak and Lee (2012) 

Notes: non-cohesive (NC) or cohesive (CO). Method is excess shear (ES), stream power (SP), or regression (RE).

A cohesive transport potential method (Krone Parthenaides) can also be selected. When selected, transport of cohesive sediment is computed in addition to the non-cohesive sediment. More information can be founded in HEC-HMS manual (Cohesive Transport).
Transport potential functions for calculating the amount of sediment that can be carried by the stream flow. 

For a more comprehensive understanding, including information on algorithms that translate hydrodynamics into transport, please refer to the "HEC-RAS Sediment Manual" by Gibson and Sánchez (2020) (Sediment Manual). This reference is relevant because HEC-HMS shares the same Sediment Transport engine as HEC-RAS. 

Sediment Delivery Ratio (SDR)

The integration of the SDR Transport Potential method is geared towards managing erosion and deposition functions by relying on fundamental hydrological principles. Its specialized application is tailored specifically for simulating debris flow within the HEC-HMS framework and it's important to note that this particular SDR option is not incorporated within HEC-RAS.

The SDR method itself serves as a valuable technique in hydrology and sediment transport studies. It functions by estimating the proportion of sediment generated in an upstream area, typically a watershed or catchment, that ultimately reaches a downstream location, such as a river or reservoir. This estimation sheds light on the effectiveness of sediment retention and trapping mechanisms operating within the watershed.

The SDR method is conventionally expressed as a fraction or percentage. A high SDR value signifies that a substantial portion of the generated sediment successfully reaches the downstream location, while a low SDR indicates effective sediment trapping, resulting in reduced sediment delivery downstream.

It's important to note that the SDR method necessitates the consideration of ratios for each grain class, such as Clay, Silt, Sand, and Gravel when the Grade Scale is selected as "Clay Silt Sand Gravel." When the Grade Scale is set to "AGU 20," it includes Clay, Silt, Sand, Gravel, Cobble, and Boulder. In this context, a Ratio greater than 1 signifies an erosional scenario, a Ratio less than 1 signifies a depositional scenario, and a Ratio equal to 1 represents an equilibrium situation.

\begin{aligned} & \mathrm{O}_{\mathrm{sed}}=\operatorname{SDR} \times \mathrm{I}_{\mathrm{sed}} \\\\ & \text O_{\text{sed}} = \text { Sediment Outflow }  \\ & \operatorname {SDR}=\text { Sediment Delivery Ratio }  \\ & \text I_{\text{sed}} = \text { Sediment Inflow }  \\ \end{aligned}

Sediment Routing Methods

HEC-HMS necessitates supplementary sediment transport mechanisms within its unique reach configuration. In contrast to HEC-RAS, which relies on intricate cross-section dividers, HEC-HMS employs inner rough sub-reaches within the reach element. Consequently, these sediment routing options are pivotal in improving sediment transport between these sub-reaches within the reach element.

There are a total of five diverse sediment routing methods at your disposal within HEC-HMS. These methods encompass:

  • Fisher's Dispersion
  • Linear Reservoir
  • Muskingum
  • Uniform Equilibrium
  • Volume Ratio

The following sections detail their unique concepts and uses.