Michigan Trout Unlimited - Headwaters Chapter

1. Introduction

Sediment has been identified as one of the primary pollutant threats to our water quality.  Road/Stream crossings can become a conduit for this pollution when excessive soil from roads and/or eroding banks at the culvert placement, flow into a tributary. These road/stream crossings range from 18-inch culverts to two lane highway bridges. The cumulative effects of sedimentation are an area of concern in stream systems, as they can directly affect diverse fish and riparian dependent wildlife. Sediment can cover aquatic spawning beds and clog fish gills as well as impair water quality. As part of the critical area evaluation for the Sturgeon Watershed Plan an inventory of all public road/stream crossings within the watershed was conducted. The purpose of this inventory was to identify and document all of the road crossing sites on the numerous tributaries of the Sturgeon River Watershed. A total of 55 sites were located and documented during this inventory.

a. Methods

On site field evaluations were performed to inventory each potential crossing. A Road/Stream Crossing Field Data Form was completed at each site. A series of photographs were also taken to document existing conditions at each crossing. Each site was visited to assess potential problems that may contribute nonpoint source pollution and impact water quality. Data collected at the crossings included detailed information about the location, road characteristics (width, shoulder, drainage, approaches, surface), culvert condition, erosion and runoff problems. Stream characteristics such as width, depth, current and substrate were also recorded.
At each crossing, soil erosion was evaluated in terms of existing and potential conditions; additionally, various physical measurements were made and each site was documented with an inlet and an outlet photograph. This information was compiled into a database for evaluation.
One of the key functions of an inventory is to aid in the prioritization of sites for improvement. Each crossing was assigned a “score” along with a corresponding severity ranking category: Minor, Moderate or Severe. The ranking is designed to reflect the relative severity of existing and potential erosion conditions at each site. In general, the severity ranking will be one of several considerations for improvement decisions. Point scores were calculated using the scoring work sheet and the sites were assigned the severity rankings as follows:

Point Score Total Severity Category

0-15 Minor
16-29 Moderate
>30 Severe

Severity rankings are useful as a quick reference to sites that fall within a specific category. However, it is expected that resource managers will carefully review candidate sites for improvement by paying closer attention to individual scores before selection of sites for implementation.

Definitions of the terms used in data collection and severity ranking are provided below:

Adjacent Landowners: Ownership was determined from county plat book maps.  Recent ownership may not be reflected and should be re-checked prior to any improvement work.
Corrective Measures/Drainage Control Features: Any best management plan measures used to correct site-specific erosion problems; generally these include diversion outlets, erosion blankets, and sediment basins.
Embankment: The area surrounding the culvert. The slope associated with the inlet and outlet of a corrugated metal pipe or box culvert.
Extent of Erosion: This category provides a subjective assessment of the observation of sand deposition, gullies, or similar conditions at the sites. It does not reflect erosion potential.
Fish Passage Problem: This refers to the flow through a culvert and whether or not fish can move through the culvert in either direction. Certain obstructions have the potential to impede passage such as a perched culvert.
Flow Through Culvert: This is an indication of obstruction to flow. Obstructed flow is generally associated with large debris accumulations such as beaver dams or due to large sediment inputs associated with run-off or grading.
Fill: Refers to the amount of material (e.g. sand, soil, gravel, etc.) over the culvert.
Length of Approaches: The downward slope of a road approaching a stream crossing where typically the stream is located at the low point.
Recommended Treatment: One or more best management practices recommended for each crossing. The practices are selected based on proven ability to reduce sedimentation and are generally accepted by road and water resource professionals.
Runoff Pathway: The course of runoff to a stream channel. This may be via two general routes, the road or ditch/shoulder. Typically, roads with a surface of either gravel or sand result in runoff traveling down the road.
Slope of Approaches: The ratio of an increase in height over the distance of a given road and is usually expressed as a percentage.
Stream Current: Average upstream and downstream current was observed and classified as slow, medium, or fast.
Vegetation: Defines the presence, absence, and relative abundance/condition of existing vegetation on the embankments of a given crossing. Generally, vegetation that is at all disturbed by access or road grading is considered to be partial.
Wetlands: Any stand of vegetation that is typical of an area of land that is at least partially inundated by water for part of the year.
Visible Down Cutting: This indicates the distance or drop from the base of the culvert outlet to the surface of water.

b. Results

Of the 55 road stream crossing sites that were identified within the Sturgeon River Watershed, 2 sites ranked severe, 11 sites were ranked as moderate, and 42 sites ranked as minor. A number of common factors contributed to the severity of those sites scoring 30 or more points. The sites ranked as severe were found to have extensive erosion on the embankments. The sites ranked as moderate are generally heavily trafficked areas, and also tend to occur at the older bridges.