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Q.10, Short EAF (Part 2) Drainage, Runoff & Flooding

Short Environmental Assessment Form Workbook

Will the proposed action result in an increase in the potential for erosion, flooding or drainage problems?


Floodplain systems store and dissipate flood water, add to our quality of life by providing open space, habitat for wildlife, fertile land for agriculture, and opportunities for fishing, hiking, and biking. Floodplains can be viewed as a type of natural infrastructure that can provide a safety zone between people and the damaging waters of a flood. Adverse impacts on floodplains can occur when there is a decrease in the land's natural ability to store and absorb water. When floodplains are paved over or built upon, floods can be more severe, and damaging to both property and natural systems. Flooding can further lead to more erosion and that ultimately can pollute streams, lakes, and wetlands.

Stormwater runoff causes erosion. Runoff is greater where there are constructed surfaces such as paved streets, sidewalks, parking lots, and rooftops. As stormwater flows overland, it washes and moves chemicals, nutrients, sediments or other pollutants and debris along with it. If this stormwater runoff is not slowed and captured before it flows into lakes, rivers, and wetlands, it can negatively impact water quality and increase flooding. The impact from stormwater runoff can be a very significant problem in urban and developed areas because that is where there are more impervious surfaces.

Related to flooding and erosion are changes in drainage patterns that can result from land disturbances. Drainage can be directed to stormwater drains, storage, and retention areas designed to slow water and allow sediments to settle out. Or drainage that is not handled properly can cause an increase in erosion, changes in stormwater runoff, flooding, and damage to water quality.

Applicable Part 1 Information

Some of the Part 1 questions that should be specifically reviewed when answering this question are:


Some projects may be outside of a 100 year floodplain, but still be in an area with known flooding history. Some projects that disturb more than one-acre may be required to have a stormwater pollution prevention plan (SWPPP) and will need to include engineered or site designed methods to control stormwater.

In order to decide if impacts will occur, the reviewing agency should look at the available information and ask:

  • Is any type of land disturbance planned?
  • Will there be any increase in stormwater discharge from the site, and if so, how much?
  • Is the project site in a 100 year floodplain, or in an area known to have past flooding events?
  • If the project is in a 100 year floodplain, will it alter the flow of water or change drainage patterns into the water body?
  • Will there be a need for new stormwater retention ponds, or other stormwater management practices? If there are stormwater discharges and no existing conveyance system, what are the plans to address stormwater and erosion from the site?
  • If Part 1 indicated that storm water discharges might flow to adjacent properties, where, how much, and what impacts might occur on the adjacent property.
  • If Part 1 indicated that there will be storm water discharges conveyed to established systems, is there enough capacity to handle the extra storm water? Do the proposed plans include any upgrades or expansion to that system?
  • Is the project going to disturb more than one-acre of land and require a SWPPP and coverage under the stormwater general permit? If it does not disturb more than one acre, but still has some land clearing, does the application include any erosion control and runoff controls?
  • Are there any protected water bodies or important surface drinking water supplies nearby that need to be protected from erosion and sedimentation (wetlands, reservoirs, protected streams)?
  • Are any stormwater reduction methods included such as minimizing impervious surfaces, using porous materials, or collecting and reusing stormwater?

Will there be an impact?

Projects that require SEQR but that have no land disturbances will not result in any erosion, flooding or drainage issues.

There is not likely to be any impacts if the proposed project:

  • Is not in a 100 year floodplain and the location is not in an area subject to flooding in the past; and
  • Has no stormwater discharges resulting from paving or construction of other impervious surfaces); or
  • Does not involve any land clearing or grading.

If you determine that the project is such that it will not affect flooding, erosion, or drainage in these ways, then there is not likely to be any impact.

Unless the site is already cleared and developed (such as in a suburb or urban area), many projects that are proposed on a "green" site will disturb at least some ground. This disturbance could cause some erosion and runoff, however small. In this case, you should evaluate whether the potential for erosion is small, or moderate to large.

If the proposed project is likely to have an impact on flooding, erosion or drainage, then this impact must be evaluated as to its size.

If there is an impact, how big will it be?

If there will be an impact, the reviewing agency must then evaluate the magnitude of that impact. This will depend on the overall scale and context of the proposed project as described in the Introduction to Part 2. The reviewing agency should be reasonable when conducting this review. For additional information on the concept of reasonableness as it applies to SEQR, refer to section F in the Introduction of the SEQR Handbook (PDF) (1.9MB).

Small Impact

A small impact could occur if the project will result in one of the following:

  • The project is not in a 100 year floodplain, but is in an area that has experienced flooding in the past.
  • The project is in a 100 year floodplain, but is a small land disturbance that does not result in a change of floodwaters or drainage to the water body.
  • Stormwater discharges will take place but it will not flow to adjacent properties and the project minimizes stormwater through use of porous materials, or collection and reuse of stormwater?
  • Stormwater discharges will be conveyed to an existing system and that system has the capacity to handle the volume of water without expansions or extensions.
  • The project includes land clearing that disturbs less than one-acre of land and does not require a SWPPP.
  • The project includes some paving or other impervious surfaces, but runoff is either controlled with a SWPPP, or covers a small percentage of the parcel.
  • The project requires and has developed a SWPPP.
Moderate to Large Impact

Moderate to large impacts may occur if:

  • The project is in a 100 year floodplain and is likely to change floodwaters, water flow or drainage to the water body.
  • No SWPPP has been developed, and stormwater discharges will flow to adjacent properties, and negatively impact that property or other natural resources off-site such as wetlands, reservoirs, or protected streams.
  • Stormwater discharges will flow to existing conveyance systems but expansions or extensions of that system are needed to handle the added runoff.

In these scenarios, check "moderate to large impacts may occur" in the Part 2 table and when all questions of Part 2 are completed, proceed to Part 3.

Recording your decision

If you have determined that there are no impacts, or that only a small impact may occur, no further analysis of this topic is needed. Simply check the box under "No, or small impact may occur" next to the question and move on to Question 11. You may choose to include an explanation in Part 3 as to why you decided there were no, or only small impacts, but you are not required to do so.

If you have determined that one or more moderate to large impacts may occur, then additional analysis of this impact will be required in Part 3. You should note what the impacts are, and the reasoning that lead to your decision before moving on to Question 11.


Scenario 1: A 20,000 square foot retail store is proposed in an already built area along a major street.

  • The building is being built on a site that contained other buildings to be demolished.
  • Almost the entire site will be covered with building or parking area.
  • More than one-acre is to be disturbed.
  • An existing stormwater conveyance system exists that has the capacity to handle the discharge.

Then: Because there will be stormwater discharges, there will be an impact. However, because of the SWPPP, site engineering, and the existing conveyance system, there will be only small impacts.

Scenario 2: A small wholesale greenhouse is proposed to be established on a former farm field which is located on lands partially within a 100 year floodplain of a river.

  • The greenhouse will be in the 100 year floodplain.
  • No parking lot is planned.

Then: Because this project is located in the 100 year floodplain, the Planning Board determined that there is the possibility that some impact would occur. After analysis, the Board also determined that due to the nature of the structure, that the impact would be small.

Scenario 3: A structure is proposed to be built on ½-acre of land that has a DEC protected stream running through it.

  • The structure and its septic system are 150' away from the stream.
  • The stream flows into a DEC regulated wetland off-site that is also identified as potentially being the habitat of a threatened plant species.

Then: Because of the slope of the land and presence of highly erodible soils on the site, the drainage and runoff will flow into the stream which could impact the wetland. The Planning Board determined that this could be a moderate to large impact

Back to Part 2 Impact Assessment || Continue to Question 11 - (Part 2)

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