Watershed Planning for Improved Drainage

Water has been widely identified as one of the greatest nemeses of roadway longevity and how to deal with it has always been an issue. In CLRP’s Roadway and Roadside Drainage Workshop, the three main topics are “Drainage!, Drainage!, and Drainage!” Getting water away from your roads will extend the life and durability of those roads for many years.

Photo of man walking on a road badly damaged by a 2006 flood
Road Damage from Flooding in 2006

As part of roadway drainage we need to examine ditching, underdrains, culverts, and even the quality of the backfill and pavement materials as a means of removing excess moisture from the vicinity of the roadway. Proper drainage can be both a proactive and a reactive effort, often at the same time, and neither approach may solve every issue that could occur in the future. To expand the concept of drainage and develop more effective ways of dealing with excess water, many regions are working together to evaluate their current issues on a larger scale, from the perspective of a watershed.

This approach focuses on evaluating the physical attributes of a watershed, which directly affect the volume and velocity of the water that is discharged to a given point. To better understand this approach it is necessary to understand what a watershed is. According to the New York State Department of Conservation (NYSDEC) a watershed is “an area of land that drains into a body of water, such as a lake, river, reservoir, estuary, sea or ocean”. Watersheds are “separated from adjacent watersheds by high points such as ridges, hills, and mountains.” We all live in a watershed.

NYS map showing 17 main watersheds
The 17 major watersheds of New York State
Source: New York State Department of Environmental Conservation (NYSDEC)

In New York there are 17 major watersheds, each draining either directly into a “body of water” or contributing to a larger watershed. The Chemung River and the Susquehanna River watersheds in New York, for example, are the upper reaches of the Chesapeake Bay Watershed which include drainage areas in Pennsylvania, Delaware, Maryland, Virginia, West Virginia, and Washington D.C. The Chesapeake Bay Watershed is the second largest watershed east of the Mississippi. The largest is the Ohio River drainage basin, which drains into the Gulf of Mexico. Parts of its headwaters, the Allegany River Watershed, can be found in Cattaraugus, Chautauqua and Allegany counties in the western part of New York.

Obviously, when evaluating the impact of development or land use changes in a specific area looking at the entire ‘major’ watershed does not make sense. In these situations it makes better sense to evaluate the contributing area and the immediate downstream region. This smaller area, often referred to as a sub-watershed or a drainage area, often involves multiple jurisdictions. By proceeding with an evaluation of a sub-watershed there is an increased ability to see how various upstream factors related to stormwater can influence structures, residents, and communities downstream. Taking into account all of these factors can facilitate the reduction in erosion and structural damage on municipal and residential land and structures.

Stormwater as defined by the NYSDEC is “water from rain or melted snow that doesn’t soak into the ground but runs off into waterways.” Calculating the amount of this stormwater runoff can often be a bit overwhelming and if done incorrectly can result in problems in the future. In many cases the local Soil and Water Conservation District (SWCD) may be available to assist in these calculations.

Factors that matter when evaluating a watershed and its associated runoff include:

The Area of Influence is the area of the watershed contributing to the point of concern, the upper sub-watershed. A large area or one with steep terrain will have higher flows; everything else being equal. Rainfall varies from region to region and from storm to storm. Understanding and applying your regional rainfall data can greatly influence the success of any stormwater evaluation. It is also important to realize that rainfall patterns have changed in recent years, resulting in shorter more intense storms, not specifically represented in the historical rainfall data.

Land Use refers to how the land is used, i.e.: forest, agriculture, residential and commercial. How much stormwater runoff from a rainfall has a lot to do with the land use. For example, if you pour a bucket of water on a sandy soil, the water will soak into the ground very quickly. On the other hand, if you pour a bucket of water on a non-pervious paved surface the water will flow in a path of least resistance and be directed to another place since it cannot be absorbed into the surface. That which is not captured, absorbed or evaporated is the runoff that needs to be calculated to correctly size your culvert, stream bank protection or bridge support structure. There are multiple Land Use factors that influence the amount of stormwater that can affect drainage ditches and culverts. Several key factors that are typically evaluated are:

Impervious Areas are surface areas that do not absorb moisture and include roof tops, road surfaces, driveways, sidewalks, etc. The result of an increase in the amount of impervious surface is a direct increase in the amount of runoff discharged from that area and an increase in the discharge velocity of that runoff.

Vegetation quantity and quality also affect runoff; the more dense and better established the vegetation, the greater amount of runoff that is held on the site. Allowing the runoff to pass through vegetation decreases the velocity of the flow and helps filter out contaminants and sediment that can be washed downstream. This supports the need for re-establishing vegetation after a site is disturbed or ditches are cleaned.

When calculating the amount of runoff from a specific type of land use, a factor referred to as the Coefficient of Runoff is used. The Coefficient of Runoff represents the portion of rainfall which actually results in runoff. From the table on the previous page typical coefficients for various types of land use and vegetation are represented. Note that the coefficient for forested land is low, in the range of 0.1 to 0.4, while for paved development the coefficient is high, in the range of 0.8 – 0.9. The portion of rainfall that results in runoff is much greater for a development with impervious areas.

Coefficient of Runoff for different types of land use
Coefficient
of Runoff (C)
Land type
0.1 – 0.2
Parks and cemeteries
0.1 – 0.4
Forested land
0.2 – 0.4
Pastures
0.2 – 0.5
Cultivated land
0.3 – 0.4
Suburban residential areas
0.3 – 0.5
Single–family residences
0.7 – 0.9
Downtown business districts
0.8 – 0.9
Paved development

Slopes of the contributing land area and the slopes of ditches and culverts affect the velocity of the runoff. Also, a steeper slope will decrease the time it takes for the flow to get to a critical point, thereby increasing the maximum runoff volume in many cases.

Surface Roughness will also affect the flow velocity; a coarse flow channel will have a lower velocity due to the greater amount of friction; a smooth flow surface has less friction, therefore having a greater flow velocity. This is why check dams and stone cobbles are used in steeper drainage courses.
Retaining and detention areas can slow down the flow and reduce the maximum runoff volume. When retained, much of the runoff may be absorbed into the ground, evaporated into the air or discharged at slower rate than the initial flow would have allowed.

Because we all live in a watershed, looking at the entire contributing drainage area is an important step in managing the effects from stormwater runoff. Developing a Watershed Management plan and working together with adjacent communities, is an important step in protecting and restoring property and infrastructure, and preserving and improving the water quality of an area. Whether you are an upstream or downstream community, working with other jurisdictions within your drainage area can work to benefit everyone. Instead of increasing culvert sizes to allow runoff to pass beyond a municipality’s boundary, focusing on storm runoff at its point of discharge reduces the effects downstream and contributes to the improvement of local aquifers, streams, ponds, lakes and rivers. Watershed Management and regional planning can provide a means for evaluating and enhancing the economic benefits found in growth and development without the additional costs associated with excessive runoff.

Resources

Roadway and Roadside Drainage, Orr, David, PE. Cornell Local Roads Program, CLRP#98-5, Updated 2003 (PDF)

NYS Soil and Water Conservation Districts contact website

New York State Department of Conservation (NYSDEC)

Stormwater Management Guidance Manual for Local Officials, NYSDEC/Department of State, 2004

NYSDEC Stormwater website

NYSDEC Watershed Management

Center for Watershed Protection

Summer 2012

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This work by the Cornell Local Roads Program (CLRP) is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License.