Alewife Study Group: Impact of Development on Wetlands and Flooding

Lew Weitzman, a member of the Alewife Study Group, provides a valuable discussion of wetlands issues and regulatory approaches in “Cambridge’s wetland protection laws – ideas for improvement” (1999). It would be a shame if excessive infill development in Northampton led to the problems experienced in Cambridge. Here are selected passages from Weitzman:

Background to the Massachusetts Wetlands Act

…Wetlands mitigate the dangers from toxic chemicals in the environment and act as buffer zone limiting the effects of flood damage during major storms.

…The Wetlands regulations are very specific about which ‘resource areas’ fall within the jurisdiction of the Conservation Commissions. Resource areas are generally those that are near bodies of water such as rivers, pond or the ocean and/or contain wetlands habitats such as cattail marshes… 100-year flood plains (and other lands subject to flooding) are also within the jurisdiction of the conservation commissions…

The Need for Conservation Bylaws in Cambridge

…As a direct result of development on the Alewife flood plain and its low elevation, there is periodic and significant flood damage in the surrounding communities. A 1981 study by the MDC found property damage in the Alewife area to be the highest of any portion of the Mystic River watershed of which it is a part.

During flood events at Alewife, the Mystic River backs up into the Alewife Brook and Little River areas. Flood waters from Belmont also empty into this area during storm events. During the flood of October, 1996, the Alewife Brook flooded over the Alewife Brook Parkway and into North Cambridge neighborhoods. East Arlington neighborhoods adjacent to Alewife were also inundated with flood water. The Arthur D. Little parking lot was covered by several feet of flood water.

The flood event of 1996 was estimated to be a ’30-40 year’ event by the national weather service. The ‘100-year’ event would be significantly worse and could even pose a threat to the city drinking supply at fresh pond.

Development pressures will continue to pose an immediate threat to Cambridge’s remaining wetland and flood plain areas. For all of these reasons Cambridge is in desperate need of conservation bylaws….

Expanding Habitat Protections

The wetlands regulations as written provide almost no protection for wildlife habitats outside of actual wetlands. For example wildlife habitats within the 100-year flood plain are virtually unprotected. The Bedford conservation bylaw has corrected this by requiring that construction in their 100-year flood plain not “alter the ability of the land to provide breeding habitat, escape cover, or food for wildlife.” Duxbury’s conservation bylaw emphasizes the significance of their flood plain to their adjacent wetlands. Dunstable’s conservation bylaw adds new protections to their 100-year flood plain and protects its ‘aesthetic character’…

Construction in the Flood Plain

…Even with the best of compensatory storage schemes it is clear that flooding continues to increase where construction occurs. Therefore, it is the recommendation of FEMA that no construction occur in the 100-year flood plain and this is also the policy of the Cambridge emergency response plan. The state Zoning Act (G.L. Ch. 40A) also encourages municipalities to prohibit or regulate development on flood plains…

Compensatory Storage

In the past developers have been able to create large parking lots without having to compensate for its effect on the volume and velocity of floodwater. Meadows and wetland areas absorb floodwater and slow down its speed during a major storm. Paved parking lots do not do this.

Bedford has recognized this phenomenon by requiring no more than 25% impervious surfaces in the 100-foot buffer zone to wetlands. Bedford also requires that construction in their flood plain “neither decrease the flood storage capacity nor decrease the groundwater infiltration rate”.

The weight of built structures has a significant effect on flood capacity and ought to be a factor in calculating compensatory storage. The weight of a building will compress soil so that it can no longer hold as much water.

A loophole has allowed developers to excavate compensatory storage from ground areas that already contain groundwater. Clearly these types of flood storage areas would add nothing to total flood storage capacity and should not be accepted as meeting the intent of the law. The Danvers conservation bylaw has closed this loophole by requiring that “storage capacities shall be based on the volume of active storage above maximum seasonal groundwater levels”.

And, finally, there has been no consideration given to area-wide flooding conditions. Until now a developer has been allowed to focus exclusively on their own contribution to flooding without an analysis of the general conditions in their area. The Danvers conservation bylaw has corrected this oversight by requiring that “hydrologic analysis shall be based on a reasonable estimate of developed conditions within the entire watershed”.

Wetlands Replication

The Wetlands Act allows the destruction of up to 5,000 square feet of wetlands so long as they are replicated elsewhere in the watershed. This policy is now being reconsidered by many municipalities.

The Dover conservation bylaw concludes that “wetlands replication is generally unsuccessful”. The Chicopee conservation bylaw goes further and states that “the quality and quantity of Chilcock’s (a significant wetlands in the town) wetlands shall be increased and restoration and replacement shall undo past damage.”

See also:

UK Blog: Floods and Development, 7/23/07
The events of the last week have been tragic and frightening for many residents in the borough, although thankfully Twickenham and Teddington have not been the worst affected areas in the country, and my thoughts go out to those people in the Midlands who still do not have access to power or running water…

…the LibDem Councillors, at a recent Cabinet meeting produced documents promoting an increase in “high density developments” in Teddington, Whitton and Twickenham. This is an issue I will come back to as a separate debate on planning, but given the flood risks in the area, should we really be concreting over our green spaces and further reducing drainage potential and increasing flash flood risk?

Kohl’s Condo Proposal: Where Things Stand and What You Can Do
Kohl Construction proposes to build 31 condo units in the forest between North Street and the bike path. This development, with its access roads and 66 parking spaces, will claim 5.49 acres of land…

A wetland centered on Millyard Brook runs through the heart of the forest. The buffer area around a wetland is important in itself, an edge zone of high species diversity that helps regulate water flows into the wetland. These areas are fragile, can take a long time to establish, and are easily disturbed by development. The consequences of development around wetlands are often increased flooding in wet periods and more severe dryness during droughts…

Conservation Commission to Take Up Kohl’s Wetlands Delineation on August 23

…Some of the condo units appear to encroach within 35 feet of the wetland boundary as drawn by Kohl.

Intermittent Streams Merit a 100-Foot Buffer Zone in Hopkinton
Creeks and streams, including intermittent streams, are important for storm damage prevention, flood control, ground water protection, wildlife habit
at, and recreation values. During spring, summer, and fall these streams disperse snowmelt and storm runoff across the landscape thereby preventing dangerous volumes and flows from spilling over roadways and property. This broad dispersal also allows for larger volumes of water to infiltrate into the ground, recharging groundwater supplies…

Some animals, such as pickerel frogs and eastern spotted newts, rely heavily on intermittent streams for movement. For these reasons the upland areas surrounding intermittent streams are heavily utilized by wildlife for living space, breeding, feeding, migrating, dispersal, and security. Accordingly, this Bylaw protects streams of all forms (Bylaw section 206-3 and Regulation 11.9) and the buffer zone within 100 feet of those streams. [emphasis added]

EPA: Do Stormwater Retention Ponds Contribute to Mosquito Problems? [emphasis added]
…Reducing our reliance on stormwater ponds for runoff control is another way to reduce potential mosquito breeding habitat. More people are turning to alternative non-structural techniques, such as rain gardens, bioinfiltration, infiltration, and vegetative swales, that slow down water and help it infiltrate without extended periods of ponding. These techniques are successfully minimizing or eliminating the need for stormwater ponds or significantly reducing the pond size requirements. Care must be taken to ensure that these alternative controls drain all standing water as designed over the years.

Similarly, efforts to reduce the amount of impervious surface in communities can reduce the need for stormwater ponds. Narrower streets, sidewalk-less communities, and elimination of cul-de-sacs are just a few of the ways that communities are now reducing the need for stormwater controls. That is not to imply that stormwater ponds can be eliminated easily. Retention/detention ponds use less space than many other types of stormwater controls and are often found to be the best and cheapest way to control runoff–especially when flooding is a concern.

Mosquito proliferation in stormwater ponds is a concern, especially when so many wet and dry ponds are in place and continue to be installed across the country. Many ponds are not properly maintained, particularly in cases where they are installed in subdivisions and other developments where the entity responsible for long-term maintenance is not clearly defined once the construction is complete…