DEVELOPMENT AND LAKES: Some Things to Consider

Jeff Dennis, Biologist
Division of Lakes & Biological Studies
Dept. of Environmental Protection

July 1981

This publication is presented for "Classroom Use Only."  Its intended use is to stimulate and aid in discussion and role playing within a classroom setting.

APPENDIX I -1

The purpose of this paper is to make developers and local decision makers aware of the potential impact of residential development on lake water quality and to present some of the methods available to reduce this impact. Hopefully, it will assist local planning boards, selectmen, and town or city councils in administering the municipal subdivision law in lake watersheds.

The Significance of Phosphorus

Good water quality in Maine lakes depends primarily on one factor -- the amount of phosphorus in the lake. Phosphorus is a fertilizer, and, just as the growth of corn is dependent on fertilization of the soil, so the growth of the simple microscopic plants in the lake water, called algae, is dependent on the phosphorus in the lake. The more phosphorus, the more algae. The more algae, the less transparent (clear) the water. Sebasticook Lake in Newport is a prime example of a Maine lake which has received extremely large additions of phosphorus, and, as a result, supports extremely dense, obnoxious growth of algae, called algae blooms. In order to prevent reduced water transparency and ultimately algae blooms in other Maine lakes, it is necessary to minimize any increases in the amount of phosphorus entering these lakes.

Surface Runoff

Most of the phosphorus entering Maine lakes is carried to the lake in water running over the ground surface following rainfall or snowmelt. This water is called surface runoff or stormwater runoff. In addition to the small amounts of phosphorus present in the rainwater, the runoff picks up more phosphorus from the material it comes in contact with as it flows. Much of this phosphorus will be transported to the lake unless the water is absorbed (infiltrated) into the ground first. Although water which enters the ground may eventually reach the lake in ground water flow, most of the phosphorus it contains will either be filtered out at the soil surface or will adhere to small soil particles as it moves through the ground.

It is very important to realize that surface runoff transports phosphorus to a lake not only from near shore areas but from the entire lake watershed which may include areas many miles from the lake.

Land Use Effects

The amount of phosphorus which is carried, or exported, from a particular piece of land by surface runoff is dependent to a large extent on how the land is being used. When evaluating the impact of a new development on a   lake, we must consider the effect that the change from undeveloped to developed land will have on the phosphorus transport.

Forested Land

Most new developments in Maine take place on land that was previously forested. Relatively speaking very little phosphorus is carried in runoff from forested land. There are several reasons for this.

First, the volume of water which runs off forested land is less than for most other land uses. The leaf canopy of the trees intercepts some of the precipitation and stores it until it evaporates. Natural irregularities of the forest floor provide many small depressions which can temporarily store water which would otherwise run off. Given time this water will either infiltrate into the ground water, be taken up by vegetation or evaporate directly into the air.

The second reason that little phosphorus is carried in the surface runoff from forested land is that the concentration of phosphorus in this runoff is very low. This is inpart because the forest does not receive the additions of phosphorus in the form of fertilizers, detergents, and road dust which other land uses receive and which are often carried in the surface runoff. Also, there is very little erosion of soil and organic particles in an undisturbed forest and what erosion takes place is usually filtered out of the runoff as it flows through the organic duff covering the forest floor.

Residential Development

When forested land is converted to residential use both the volume and quality of the surface runoff change. Impervious structures such as buildings, driveways and roads are placed over previously permeable soil. The small scale irregularities of the forest are flattened out for lawns and gardens, thus reducing the surface storage area. Natural drainage ways are straightened out and runoff is concentrated into road ditches. These changes combine to significantly increase the amount of water leaving the site in the surface runoff, sometimes by as much as 40%.

When we consider that this increased volume of water also has a higher concentration of phosphorus because of the fertilizers, detergents, road dust and eroded soil particles which it carries, it is evident that significantly more phosphorus is transported from the residential development than from the former forest. Studies have shown that for typical development this increase ranges from 2 to 10 times depending upon the density of development and the suitability of the land for development.

Development, however, can be designed, constructed, and maintained so as to minimize its impact on a lake. The following is a list of practices which can reduce the increase in phosphorus movement from the developed area.

Practices which Reduce Volume of Runoff

The amount of phosphorus leaving a site in its stormwater runoff is proportional to the volume of the runoff. Any reduction in runoff volume through storage or infiltration will result in a concurrent reduction in phosphorus transport.

Practice	Rationale
1. Leave natural, undisturbed wooded areas, called buffer strips, between developed areas and any lakeshores, streambeds, natural or manmade drainageways, or even road ditches. The width of the buffer should depend on the slope of the land and the size of the developed area which drains into it. Buffers in flat permeable soils need only be 25' wide but on steeper slopes they should be 50' to 100'. It may be necessary to define the buffer zone as the entire streambank or gully.	1. Buffer strips intercept runoff from disturbed areas and provide storage for eventual infiltration and/or evaporation of much of the runoff.
2. Restrict the amount of impervious surfaces allowed. This can be done by either putting a square footage limitation on buildings, driveways and roads or by using permeable materials instead of pavement.	2. Reduction in impervious areas increases the opportunity for on sitestorage and infiltration of precipitation.
3. Limit the size of the developed area, including areas cleared and graded for lawns and gardens. This can be done on a lot by lot basis, or by best design of the entire development.	3. This will keep as much area as possible in a relatively natural state, thus reducing increases in runoff azd also providing incidental buffer zones adjacent to developed areas.
4. Provide on-site detention basins which will store and slowly release water to flat downslope infiltration areas. This can be done on a large scale for drainage from an entire development or on a lot by lot basis. Diversion of up slope natural runoff around disturbed area may be necessary to reduce size required for detention areas.	4. Detention areas provide storage for the initial influx of runoff from a storm and slow release allows time for soil recovery and thus greater infiltration. They will also act as a sediment trap.
5. Attempt to disperse concentrated runoff into flat areas. For instance, culvert outlets can be designed so they disperse flow into flat wooded areas.	5. Once runoff is channelized most of it will reach the lake unless it is physically intercepted and dispersed. Dispersion to flat areas provides additional opportunities for infiltration and evaporation.
6. Use trapezoidal, not V-shaped, road ditches.	6. Trapezoidal (flat-bottomed) ditches provide maximum contact of runoff with soil surface and therefore greatest opportunity for infiltration. They also have a lower erosion potential.

Practices which Reduce Phosphorus Contamination of Runoff

The amount of phosphorus leaving a site in its stormwater runoff is also proportional to the phosphorus concentration in the runoff. Any reduction in phosphorus contamination of the runoff will result in a concurrent reduction in phosphorus transport.

.

Practices which Prevent Erosion and Sedimentation

Much of the phosphorus leaving a developed site is attached to soil particles which have been eroded and are being carried downstream in the stormwater runoff. This is particularly true during the construction phase.

Practice	Rationale
1. Do not develop on steep slopes (20%). Leave them in as near to natural condition as possible.	1. Steep slopes are very vulnerable to erosion.
2. Immediate vegetative or mechanical (i.e. rip rap) stabilization of any disturbed soil.  It may be necessary to limit the area of soil exposed at any one time to accomplish this.	2. Most unsodded soils are very easily eroded. This erosion is not continous but is a catastrophic occurrence during major, unpredictable and usually infrequent storm events. The best way to be prepared for these events is to minimize the area of disturbed soil exposed at any one time.
3. Placement of hay bales in drainage ditches below construction sites.	3. Properly installed hay bales will filter out the coarser sediments. They have only limited utility, however, since most of the phosphorus will be attached to the finer particles. Hay bales also serve as velocity reducers.
4. Install filter fabric fences on down slopes below construction sites, preferably where runoff will be intercepted before it is concentrated into channelized flow.	4. Filter fabric fences are effective in removing fine as well as coarse soil particles from the runoff.
5. Construct sedimentation basins on drainageways below major construction sites.	5. Properly designed and maintained sedimentation basins settle out coarse and medium-sized soil particles and also provide for temporary storage and controlled release of runoff.
6. Diversion of natural upslope runoff around construction sites.	6. Diversions limit the runoff flowing over a site to that derived from rainfall falling directly on the site.
7. Manmade waterways and road ditches should be seeded, sodded, or rip-rapped, depending on the steepness of the grade and should have the capacity to handle any likely flows. On steep slopes road ditches and drainageways should have velocity-reducing structures or should be discontinuous. Culvert outlets should have drop pools to reduce velocity and trap sediment.	7. Underdesigned waterways and ditches will be washed out during major storms and will become chronic erosion problems.
8. Make sure that natural drainage ways and streambeds can handle the increased volume of runoff from the developed area. It may be necessary to artificially stabilize sensitive streams beds well downstream of the developed area.	8. Overloading of natural streambeds and drainages can result in catastrophic erosion downstream of the developed area.

NOTE: Information on proper design and construction of 5 through 8 above can be found in:

1. Environmental Quality Handbook - Maine. Erosion and Sediment Control on Commercial, Industrial, Residential, Recreation, Governmental Construction Sites, 1974. Available from: Maine Soil and Water Conservation Commission, Department of Agriculture, State House, Augusta, Maine 04333.

2. Urban Hydrology for Small Watershed, Technical Release #55. Engineering Division, Soil Conservation Service, USDA 1975.

These practices should be designed to handle peak flow and runoff volume from a 25 yr. 24 hr storm event.

The preceding list is by no means complete. There are many other methods available to minimize the impact of development, especially from erosion and sedimentation. One obvious addition is strict adherence to the Maine Plumbing Code. This should guarantee that any new wastewater will be properly treated in the soil. Many of the above practices may be implemented with deed covenants or restrictions.

We are not suggesting that all of the practices listed are necessary on all developments. The practices must be fitted to the needs of the site and the need of the particular lake in question. A development on highly permeable flat soils may have little impact even if none of these techniques are used.

Conversely, development on steep, shallow soil may require use of many controls.

Some lakes are more vulnerable to small additions of phosphorus than are others. Generally speaking, large deep lakes and lakes with large watersheds and, hense, fast flushing rates are the least sensitive. Small shallow lakes with small watersheds, however, can be extremely vulnerable.

If you wish information on the vulnerability of a particular lake or assistance in planning or reviewing development in a lake watershed, please contact the Division of Lakes and Biological Studies at the Department of Environmental Protection, Station #17, State House, Augusta, Maine 04333 or by calling 1-800-452-1942 or 289-2591.