Navigating the NHD Flow Network

This tutorial was created using ArcGIS version 9.3.1. but may be completed in newer versions of ArcGIS.

Introduction

The NHD is a powerful tool that can be used in modeling surface water drainage. The NHD provides an important component for mapping purposes, but is more than just blue lines due to its flow network and reach addressing system.

Goal:

Explore NHD features that enable the flow network. Use the Utility Network Analyst toolbar to trace a contaminant’s path downstream, and find upstream accumulation to see the potential upstream impact of an invasive species.  

Objectives:

• Explore NHD features that enable the flow network.

• Use the Utility Network Analyst toolbar to trace a contaminant’s path downstream.

•  Find upstream accumulation to see the potential upstream impact of this contaminant on

an invasive species. 

Understanding the NHD Flow Network

This information is also available as a video!+

   Navigating the NHD Network

What is the NHD Flow Network?

The NHD Flow Network allows for navigation up and dowstream within a GIS.

Only NHDFlowline feature class features participate in the NHD  network. Other NHD feature classes  do not participate.  Flow within the NHD is based on the HydroNetJunctions feature class, and is a Node (Junction) to Node (Junction) navigation system. For NHDFlowline features to participate in the network, they must be connected. There can be no gaps, breaks,  or missing features.  NHDFlowline features should all have the correct flow direction. All NHDFLowline features must have the ‘FlowDir’ attribute set to ‘WithDigitized’ to participate in network.  ‘UnInitialized’ will be left on features where flow direction or other information is incomplete.

The graphic below shows an example of an NHDFlowline attribute table. This graphic highlights the FlowDir field, which enables NHDFlowline features  to participate in the network

What determines Feature flow direction?

Flow direction in the NHD is initially determined by direction of vector collection.

Flow direction is from the ‘Start’ junction to the ‘End’ junction. If flow direction is incorrect,  NHDFlowline features may be ‘flipped’ to correct the flow direction.

This  graphic shows an example of a feature with incorrect flow direction. This feature would impact the Flow Network.

A gap between NHDFlowline features  would also impact the NHD Flow Network.

The graphic below shows an example of a disconnected network. Oftentimes, these disconnected networks are legitimate. The one here may be the result of a StreamRiver  flowing underground via a sinkhole.

Complete the tutorial on Navigating the NHD Flow Network to see  how the NHD Flow Network is used in action.

Navigating the Network: Part 1