Reach Element

Overview

The Reach element represents a channel or conduit that conveys and routes flow from upstream to downstream. Reaches model the translation (delay) and attenuation (peak reduction) of hydrographs as they travel through the drainage system. HydraLink supports four routing methods, each suited to different situations.

When to Use

  • Between any two elements where flow is conveyed through a channel, pipe, or overland flow path
  • When hydrograph attenuation and translation through a channel segment is important
  • For modeling the effect of channel geometry on hydrograph shape

Routing Method Comparison

Method Complexity Attenuation Best For Requires
Lag Low None Short reaches, quick estimates, preliminary analysis Lag time only
Muskingum-Cunge Medium Yes Open channels where geometry is known and calibration data is unavailable Channel geometry (length, slope, Manning's n, cross-section)
Modified Puls Medium Yes Reaches where storage-discharge relationship is known Storage-discharge table
Kinematic Wave Medium Yes Steep channels where other methods may be unstable Channel geometry

Input Parameters

Every reach shows its upstream element(s), read-only, and a Drains To selector for the downstream element. New reaches default to the Lag method.

Lag Method

Parameter Units Description Notes
Lag Time minutes Time delay applied to the hydrograph Hydrograph shape is preserved unchanged; only shifted in time
Regional Consideration: The Lag method provides no attenuation: it translates (time-shifts) the hydrograph without reducing the peak flow. The outflow hydrograph is identical to the inflow hydrograph, just shifted by the lag time. This approach is typically used for relatively short reaches where the travel time through the reach is small relative to the overall hydrograph duration, and where storage effects within the reach are negligible. Regional design criteria and the engineer’s judgment should guide whether lag routing is appropriate for a given application. For longer reaches or reaches where channel storage may significantly affect hydrograph shape, a routing method that accounts for attenuation (such as Muskingum-Cunge or Modified Puls) may be more appropriate.

Muskingum-Cunge Method

Parameter Units Description Notes
Length ft Reach length Measured along the channel centerline
Slope ft/ft Channel bed slope Positive value, typically 0.001–0.05
Manning's n dimensionless Roughness coefficient See standard tables; typical 0.03–0.05 for natural channels
Cross-Section — Trapezoidal, Rectangular, Triangular, Circular, or Eight-Point Circular asks for a diameter; Eight-Point is entered as station–elevation pairs via Edit Section…
Bottom Width ft Width of channel bottom Applies to the shape chosen
Side Slope H:V Horizontal to vertical side slope ratio e.g., 3 means 3 ft horizontal per 1 ft vertical
Trapezoidal channel cross-section with dimension labels

How it works: Muskingum-Cunge is a variable-parameter scheme. The inflow peak sets how many subreaches the reach is divided into; then at each time step and subreach it solves normal depth for the current flow, computes the wave celerity from the section's discharge–area relationship, and derives K = Δx / c and X = 0.5 × (1 − Q / (T × c × S × Δx)). If X has to be clamped, the run log warns that the reach may be too short or the time step too large.

Note: Muskingum-Cunge derives routing parameters directly from measurable channel properties (geometry, slope, roughness), eliminating the need for calibration data. It is widely used for open channel routing applications. However, the appropriate routing method for any given project depends on the engineer’s judgment, local design criteria, and site conditions.
Applicability Considerations: Muskingum-Cunge may not be appropriate for: backwater-affected reaches, very flat slopes (S < 0.0001), tidally influenced areas, or reaches with significant floodplain storage. In these cases, other methods such as Modified Puls with a known storage-discharge relationship may be more suitable. The engineer should evaluate the site conditions and select the routing method that best represents the physical system.

Modified Puls Method

Parameter Units Description Notes
Subreaches — Number of subreaches Divides storage-discharge among subreaches
Storage-Discharge Table acre-ft vs cfs Paired storage and discharge values Defines the relationship between reach storage and outflow

The Modified Puls method uses the storage-indication approach: (2S/Δt + O) is plotted against O. For each time step:

SIj+1 = Ij + Ij+1 + SIj − 2Oj

Outflow is interpolated from the SI curve.

Kinematic Wave Method

Parameter Units Description Notes
Length ft Reach length Same geometry inputs as Muskingum-Cunge
Slope ft/ft Channel bed slope Works best for steep slopes
Manning's n dimensionless Roughness coefficient
Cross-Section — Trapezoidal, Rectangular, Triangular, Circular, or Eight-Point Same shape choices as Muskingum-Cunge
Bottom Width ft Channel bottom width Applies to the shape chosen
Side Slope H:V Side slope ratio
Note: Kinematic Wave assumes a simplified wave propagation model. It works best for steep channels where the Froude number is high and diffusion effects are minimal. HydraLink warns when the slope is below 0.2% or when the time step is poorly matched to the reach length.

Results

Output Units Description
Peak Inflow cfs Maximum inflow to the reach
Peak Outflow cfs Maximum routed outflow
Volume In / Volume Out acre-ft Inflow and routed outflow volumes; the difference is the attenuation stored in the reach
Peak Time h:mm Time of peak outflow
Routed Hydrograph cfs vs. time Full outflow hydrograph

Tips & Best Practices

  • Muskingum-Cunge is commonly used when channel geometry is known and calibration data is unavailable. The appropriate routing method depends on site conditions and local design criteria.
  • Modified Puls is often used when you have survey data defining the storage-discharge relationship of a reach (e.g., a wide floodplain section).
  • Reach length should be measured along the channel thalweg, not as a straight-line distance.