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# Computing Subbasin and Reach Characteristics

Last Modified: 2023-11-09 08:23:36.419

Software Version

HEC-HMS version 4.12 beta 3 was used to create this tutorial. You will need to use HEC-HMS version 4.12 beta 3, or newer, to open the project files.

## Overview

This tutorial will guide you through the steps of computing subbasin and reach characteristics within HEC-HMS.

## Compute Subbasin Characteristics

- Open the
**Punx.hms**project.- Select the Basin Model →
**Sep2018**. **Compute subbasin characteristics**by selecting**Parameters→ Characteristics→ Subbasin**.- HEC-HMS will calculate various characteristics for each subbasin in the basin model.

- Select the Basin Model →
- For a quick overview of each of the subbasin characteristics, see the
**Basin Characteristics**page of the HEC-HMS Users Manual.

## Compute Reach Characteristics

- In a similar fashion as above,
**compute reach characteristics**for the Punx model.- Select
**Parameters→ Characteristics→ Reach**. - HEC-HMS will calculate a few reach characteristics for each of the reaches in the basin model. In this simple model, there is only one reach.

- Select
- For a quick overview of the above reach statistics, see the
**Basin Characteristics**page of the HEC-HMS Users Manual.

## Questions

When comparing the longest flowpath, centroidal flowpath, and 10-85 flowpath slopes, which slope is the steepest? Why might this be?

For most subbasins, the

**longest flowpath slope**is typically the steepest. Since longest flowpaths extend from the subbasin outlet all the way to the subbasin divide, the higher elevations near the divide can result in a steeper computed slope that is not necessarily representative of the other portions of the flowpath. For this reason, the 10-85 slope is often more representative of the general flowpath slopes as seen throughout most of the subbasin.Which of the subbasin characteristics is an indicator of the general shape of a subbasin?

**Elongation Ratio**. Elongation Ratio = (Area^{0.5} / Length) * (2/ \pi^{0.5}). Given a circle with the same area as the subbasin of interest, the elongation ratio can be thought of as the diameter of the circle (D_{C}) divided by the length of the longest flowpath (L_{LFP}).Subbasin A and Subbasin B both experience the same uniform rain event. Given only the below characteristics, which subbasin would likely experience the higher peak discharge?

Longest Flowpath Length (MI)

Relief Ratio Elongation Ratio **Subbasin A**13.9 0.013 0.52 **Subbasin B**14.0 0.009 0.91 Based on the given information,

**Subbasin B**would likely experience the higher peak discharge although arguments could also be made for Subbasin A.Arguments for Subbasin B → The

*Elongation Ratio*for Subbasin B is very close to 1 meaning that the shape of Subbasin B closely resembles a circle. Circular basins often have compact, organized tributary networks that drain water to the main stem and outlet at nearly the same time thus leading to higher peak flows. For Subbasin A the*Elongation Ratio*indicates that the subbasin is elongated. Elongated subbasins can experience very different arrival times at the outlet depending on where the rain hits within the subbasin; this can lead to a more attenuated hydrograph at the outlet. Furthermore, given the similar*Longest Flowpath*lengths but very different*Elongation Ratios*between Subbasins A and B, one could assume that the drainage area of Subbasin B is substantially larger than Subbasin A. If exposed to the same uniform rain event, Subbasin B would receive more precipitation volume overall.Arguments for Subbasin A → The

*Longest Flowpath*is slightly shorter for Subbasin A meaning that water has a shorter path to travel to get from the most hydraulically remote part of the watershed down to the outlet. The*Relief Ratio*is larger for Subbasin A meaning that it has a greater relative elevation difference from the subbasin divide to the subbasin outlet.

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