Natural Environmental Impact Evaluation


Wetlands are areas where water either covers the soil or is near the soil surface all year long or for a significantly long period. Soil, water, plants, and other living organisms acting together create a unique wetland ecosystem.

The unique wetland ecosystem plays a critical role in human society. It acts as a filter for water. When water from high ground flows and drains to a lower elevation basin, it brings sediment, nutrients, and other natural or human-made substances such as fertilizers and pesticides with it. This low-lying basin is the wetland ecosystem. Here microbes absorb nitrogen, sulfur, and other nutrients, break down other compounds, and form the base of the food chain. Further along the food chain, organisms such as fish, amphibians, shellfish, and insects grow and consume these nutrients. Many larger organisms, such as birds, turtles, and alligators, also rely on wetlands for food, water, and shelter.

Section 10 of the Rivers and Harbors Act, Section 404 of the CWA, Emergency Wetlands Resource Acts, and Executive Order 11990 Protection of Wetland, all have statutory requirements to protect wetlands. Unlike many other environmental protection regulations that are only applicable to Federal actions, wetland impact analysis and mitigation are required for all projects, including private projects.

Wetland impact evaluation starts by identifying the type, quantity, quality, and function of the wetlands involved. This preliminary identification often begins with the digital National Wetland Inventory imagery map. By overlaying the wetland inventory map over an engineering design alternative, the quantity (acres) of wetland impacted and types of wetland (e.g., riverine, slope depressional, flat, and fringe) impacted can be analyzed. Additionally, the primary function of the wetland involved and the relative importance of the impacted wetland can be compared with other wetlands in the area (same watershed).

During the alternative development process, coordination with the U.S. Army Corps of Engineer (COE), local district, and a States environmental resource agency should occur as early as possible. An early buy-in on a highway agency’s design concept from the COE and the state’s environmental resource agency is critical to ensuring a viable and acceptable alternative for the project (Figure 5-3).

One of the last critical steps in wetland impact analysis is the official wetland boundary determination. The legal boundary defining where a wetland begins and ends is called a jurisdictional boundary. The jurisdictional boundary is determined from consensus work between state highway agency’s wetland experts and the U.S.

Sample wetland inventory map. (Drawn from USFWLS National Wetland Inventory CIS data at html.)

Figure 5.3 Sample wetland inventory map. (Drawn from USFWLS National Wetland Inventory CIS data at html.)

Army COE wetland experts in the field through both soil profiles and plant species analysis. Once a boundary is established, the exact amount (acreage) of impact can be computed.

The COE offers voluntary COE wetland delineation training and certification. Those who pass the COE examination are credentialed as the U.S. Army COE- certified wetland delineators.

While design engineers should avoid and minimize wetland impacts, there are situations where wetland impact is unavoidable. In these situations, minimizing impact is practiced, and mitigating the unavoidable impact is carried out.

There are many ways of mitigating unavoidable wetland impact. Creating a new piece of wetland next to the impact one, creating a single large piece of wetland to mitigate all impacted wetland from all projects in the same watershed area, and creating a large contiguous wetland to mitigate all impacts from all projects occurred in a region are all potential mitigation options.

Historically, a single piece of wetland was created next to the impact site (on-site wetland) for mitigation purposes. However, it was discovered that this newly created isolated wetland does not function well pertaining to the wetland functions. The current approach is to create large contiguous wetlands in a region. These large wetlands created for mitigation purposes are called wetland mitigation banks. States or private businesses can voluntarily create (typically restore) large acreage of functional wetlands. These newly created wetlands act like bank deposits (credit). As a deposit (credit), it can be drawn to meet mitigation needs. Also, these deposits can be sold on the marketplace to those who need to mitigate impacts. A purchaser can acquire such wetland instead of creating his or her own wetland.

On the Federal level, the U.S. Army COE regulates wetland.

When a piece of wetland is impacted (filled), it is unlikely that the replacement wetland (mitigation) will be the same size as the original (1:1 ratio). In most cases, for every 1 acre of wetland impacted, more than 1 acre of replacement is needed. This ratio depends on the quality of the impacted wetland. The higher the quality of the impacted wetland, the higher the mitigation ratio is. This ratio is a negotiated result between a State highway agency’s wetland permitting professionals and an environmental resource agency’s personnel.

For example, a state highway agency’s project impacts a total of 2.1 acres of wetland. Through negotiation with environmental resource agencies, it is determined that the mitigation ratio is 1:1.8. The state will need to create 2.1 X 1.8 = 3-78 acres of new wetland. The state can purchase this 3-78 acres of wetland from a wetland mitigation bank to satisfy its mitigation obligation.

During the preliminary engineering and EIE stage, wetland impact and mitigation analyses are not the final conclusions given that the design plan is still preliminary. In the Preliminary Engineering Report and the EIE document, a state transportation department commits to mitigating wetland impact as the project is moved forward.

Before construction can begin, a state highway agency must have the wetland impact permit (also known as Section 404 permit) in hand.

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