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Rhode Island Stormwater Solutions is getting a new look in 2013!

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What Board Members Need to Know

This section of the website is not yet complete, so please check back for updates! We will be featuring additional resources on topics such as writing ordinances and how to link local actions to water quality.

Understanding Soils provides introductory information about the usefulness of soils data
Interpreting GIS and Understanding Its Accuracy offers a brief overview of the role of Geographic Information Systems  
Reading Maps and Plans a concise, easy-to-understand document to assist with map and plan review 

 


 

Understanding Soils

Soil characteristics are perhaps the single most useful natural feature available to predict water flow and pollutant pathways. For example, soils that are sandy generally allow rapid infiltration of rainwater, unlike silty soils which channel rainwater towards streams and rivers allowing little infiltration. Below is a basic overview of how to interpret soils data.

Introduction

 

This section contains information about interpreting soil symbols and the limitations of soil types.

Table of Soil Hydrogroup Descriptions and Development Risks

 

This downloadable table includes descriptive information on soil hydrogroup as well as possible risks to surface and groundwater associated with development of each soil group.

Summary of Rhode Island Soils Characteristics Table and Soils Fact Sheet

 

This downloadable table and factsheet provide more detailed information about soils.

Using Soils Data in Project Review

 

This section contains information about the approach advocated by RI NEMO for mapping of soils characteristics.

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Soils Introduction

Soils in the United States were first mapped by the Soil Conservation Service, the predecessor to the National Resource Conservation Service (NRCS). The most recent soils data for Rhode Island are contained in the Soil Survey of Rhode Island, published in 1981 but based on field work completed in the 1970's. In this document the soils of Rhode island are mapped and classified into 43 different soil series based on features such as texture and drainage characteristics. Soil series are commonly named for the place where that soil series was first mapped, such as Narragansett silt loam, which was first identified in Narragansett, Rhode Island.

In the Soil Surveys each soil series is categorized in a series of tables that provide information on development suitability, drainage characteristics, how well the soil will support crops, hydrogroup and many other characteristics. Soil hydrogroup is an important consideration for development suitability analysis. There are four hydrogroups: A, B, C and D. These groups indicate soil permeability and the potential for rainfall to either seep into or run-off the ground surface. A is the most permeable and D is the least permeable (usually wetlands soils).

Interpreting Soil Symbols (NaA, NeA, NP, NfB)
The first letter, always a capital, is the initial letter of the soil name. The soil name indicates the soil series. For example, NaA represents Narragansett silt loam, 0-3% slope, while NeA stands for Newport silt loam 0-3% slope. The second letter is a small letter unless the mapping unit includes a broad range of characteristics, then it is a capitol letter. For example, NP is the symbol for Newport -Urban land complex. A soil complex that includes two or more soils that have different characteristics but occur together in such an intricate pattern that they cannot be mapped separately at the scale used in Soils Survey of Rhode Island. The soil name also indicates soil texture and stoniness. For example, Nf B is the symbol for Newport very stony silt loam, 3-8% slopes. The third letter, always a capital A, B,C, D, or E, indicates the slope. The slope categories vary based on soil type but generally range as follows: slope A = 0 to 3 %; B = up to 8%; C = up to 15%; D = greater than 25 %

Limitations of Soil Types
It is important to note the Soil Survey of Rhode Island is designed as a planning tool and is not intended for parcel-level analysis. A site-specific soil survey is needed to determine actual soil conditions on a particular site.

Knowing the proportion and location of soil constraints is valuable in predicting pollution risks and in selecting pollution controls. But the effect of soil type in estimating runoff impacts becomes less important in urban areas with extensive drainage "improvements". Stormwater drainage systems, channelized streams, and artificially drained fields and building sites all bypass natural rainfall storage and infiltration processes quickly diverting runoff to downstream discharge points. These artificial improvements are not identified on soil maps and must be field-inventoried.

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Table of Soil Hydrogroup Descriptions and Development Risks
This downloadable table includes descriptive information on soil hydrogroup as well as possible risks to surface and groundwater associated with development of each soil group. Management actions to mitigate those risks are also included in the table. (Soil Hydrogroup Description and Development Risks Table, PDF)

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Summary of Rhode Island Soil Characteristics and Soils Fact Sheet
The Summary of Rhode Island Soil Characteristics table provides a summary of Rhode Island soil characteristics such as flooding duration and hydrogroup by soil name. (Summary of Rhode Island Soil Characteristics, PDF). The Soils Fact Sheet provides a review of basic soils information as well as an explanation of the codes used in the Summary of Rhode Island Soil Characteristics table. (Soils Fact Sheet, PDF).

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Using Soils Data in Project Review
Soils data are available in electronic format from both the Rhode Island Geographic Information System (RIGIS) as well as the Soil Survey Geographic (SSURGO) database. The RIGIS soils coverages are based on the SSURGO data and are a rich resource for development suitability analysis. The basic RIGIS soil data table alone describes nineteen different characteristics ranging from slope to farmland value to hydrologic characteristics. These data are appropriate for an initial site review, but a site specific soil survey should always be undertaken before development because the RIGIS/SSURGO data are not extremely accurate at the scale of a small development. STATSGO (State Soil Geographic) data is also available from the NRCS, however STATSGO data are designed for large-scale planning activities making these data a poor choice for looking at projects on the county or local level.

The approach advocated by RI NEMO for mapping of soils characteristics is associated with the MANAGE method for site assessment. For more information about the MANAGE method please click here (MANAGE). Soils mapped using the MANAGE approach are classified based on Seasonal High Water Table (SHWT) depth and hydrogroup. When soils are mapped in this manner likely pathways for water flow and pollutant movement are revealed. Sandy soils (Hydrogroup A) with deep water tables are zones of groundwater recharge where pollutants can easily infiltrate to deeper groundwater. In contrast, slowly permeable soils (hydrogroup C and D) have low infiltration rates and tend to have high water table. These areas naturally collect surface water and generate runoff. High water table areas are almost always connected to small streams, wetlands and intermittent drainage ways, forming an extended drainage network. As a result of these connections, pollutants generated in these areas can move rapidly to surface waters or to shallow groundwater. Septic systems constructed in these slowly permeable soils are also more likely to fail, especially where a dense compacted "hardpan" soil layer restricts downward flow of water.

MANAGE Soil Risk Map Legend and Description

The legend provided below is the standard legend used to describe soils in the MANAGE method. This legend highlights soils with the greatest potential for transporting pollutants to surface water bodies (slow and slow/wetland soils) or to groundwater (Very rapid soils).

The legend above is based upon the soil characteristics exhibited in the table below:

Soil Characteristics Description MANAGE Legend Description
Hydro-Group A, Deep Water Table Sandy or loose soils, very permeable (Very Rapid, >6 ft)
Hydro-Group B, Deep Water Table (>6') Moderate texture and permeability; few development constraints; may have excessively permeable subsoil (Moderate, >6 ft)
Hydro-Group B, Shallow Water Table (1.5’ – 3.5’) Moderate texture and permeability, seasonal high water table (Moderate 1.5-3.5)
Hydro-Group C, Deep Water Table (>6') Silty soils slowly permeable with restrictive layer (Slow >6 ft)
Hydro-Group C, Shallow Water Table (1.5’ – 3.5’) Silty soils slowly permeable with restrictive layer and seasonal high water table (slow 1.5 – 3.5)

Hydro-Group C, Very Shallow Water Table (0 – 1.5’)

Hydro-group D, Very shallow Water Table (0 – 1.5’)

Silty soils with restrictive layer and seasonal very high water table

Mostly hydric soils, essentially wetlands

(slow/wetland 0-1.5)

Variable and unknown   Variable and unknown

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Interpreting Geographic Information Systems (GIS) and Understanding Its Accuracy

The two self-guided Power Point presentations provided below are meant to serve as brief introductions for those who are unfamiliar with Geographic Information Systems (GIS). If you would like additional information about mapping and GIS, please visit the link to Mapping Links and Resources.

What is GIS?
This brief Power Point presentation provides an overview of what Geographic Information Systems are and some of their capabilities (graphics and some text provided by Paul Jordan, RIDEM Office of Management Information Systems).

An Easy Way to Access Rhode Island GIS Data
This brief Power Point presentation provides information on websites where Rhode Island GIS data are available for use without specialized software. A tutorial on the use of the Rhode Island Environmental Resources Interactive Data Viewer is included.

Interpreting the Accuracy of Scale
Scale is one of the most often misunderstood aspects of GIS and maps in general. All map data is created at a specific scale; in the case of RIGIS mostly 1:24,000 or 1:12,000. For example, a scale of 1:24,000 means that 1 inch on the map equals 24,000 inches (or 2,000 feet) on the ground. When we zoom in to a larger scale (ie: 1:10,000), the data become less accurate and obvious errors begin to appear. This is because at 1:24,000 scale one inch equals 2,000 ft but at the 1:10,000 scale, one inch equals only 10,000 feet, incorrectly implying that there is more data then at the 1:24,000 scale. GIS and other non site-level data are useful for general assessment but the user must keep scale limitations in mind and request more refined data when needed.

A good way to visualize error is "variation around a line” as shown below. For example, when a road section is mapped there is some error inherent in the equipment used to map the road. The errors will fall randomly along either side of the line making the line appear wider then it really is. These types of inaccuracies become more apparent as the user zooms in.

Often people talk about large and small scale maps. This comparison is a statement regarding the ratio of the map units. A map with a scale of 1:100,000 inches (1 inch equals 100,000 inches or 1/100,000) is smaller scale then a map with a scale of 1:5,000 (1/5,000) inches. A large scale map is more detailed and generally shows a smaller area then a small scale map, generally used to provide an overview.

Examples of different scale maps:

Larger Scale Map
This map of a subdivision is a larger scale than the map of the Town of Cumberland, Rhode Island

Large Scale Map
This map of the Town of Cumberland Rhode Island is much larger sclae than the map of the entire State of Rhode Island (shown to the right)
Small Scale Map
This map of the State of Rhode Island exhibits very little detail and is a much smaller scale than the map of the Town of Cumberland (shown to the left)

Site plans typically use equivalent scales such as 1" = 400'. Switching from one type of scale to the other can be confusing. The table below presents corresponding ratio and equivalent scales.

Ratio Scale
Equivalent Engineer's Scale
 
1:24,000
1" = 2000'
Most RIGIS data are designed for use at this scale
1:12,000
1" = 1000'
 
1:6,000
1" = 500'
 
1:4,800
1" = 400'
 
1:2,400
1" = 200'
 
1;1,250
1" = 100'
 

GIS data are produced at many different scales, each with its own level of error. A good rule of thumb is to treat all GIS/general data as approximate. It is acceptable to use these different scale data together, but be aware that inaccuracy can be compounded. Accuracy is related to the Minimum Mappable Unit – the smallest area that was collected as data. For example, if the minimum mappable unit for wetlands is ¼ acre then the smallest amount of wetland that will be placed on the map will be ¼ acre. Smaller areas of wetlands will not be collected. Metadata for each RIGIS data layer are extremely useful in determining the accuracy of data.

In regards to accuracy, data for Rhode Island fall roughly into seven groups as presented in the table below. Please note that this table does not include all RIGIS data but is solely an overview.

Data Type Base Map/Date of Production Error
1: 24,000 RIGIS Data, Topographic Maps, and NOAA Navigational Charts(1) USGS 7.5 Minute Topographic Quadrangle/ NOAA Charts (various years) + or - 50’ Note: Land use/cover has1/2 acre min. mappable unit.
1: 15,840 Soils (RIGIS/SSURGO) (2) Soil Survey of Rhode Island (1976-81) + or - 40’ (delineation boundaries may also have error) Note: 1/4 acre min. mappable unit.
1:24,000 Wetlands (RIGIS/ NWI) (3) Aerial Photos (1988) + or - 75’ Note: 1/4 acre min. mappable unit.
1: 12,000 Orthophotography (4) Aerial Photos (1995) + or - 33’
1: 5,000 RIGIS Roads, Hydrography, and Orthophotography (4) Aerial Photos (1997) + or -15’
Low Quality Parcel Data (5) Scanned or digitized plat maps + or - 100-200’
High Quality Parcel Data Scanned or digitized plat maps based on high resolution planimetric base + or - 5-50’

Notes

  1. The 1: 24,000 USGS Topographic map has 10’ contours and is of limited value for site assessment. A detailed site engineering survey with 2’ contours is much more useful.
  2. The 1988 wetlands data were created from aerial photographs at a scale of 1: 24,000. Standard error associated with this scale data (+ or - 50’). But because of the uncertainty associated with remote delineation, potential error is considered + or - 75’. In addition, only wetlands ¼-acre and larger were mapped. This means that the wetlands line on a GIS-derived map should only be considered a general indicator- never an accurate delineation. Field delineation should be done as early as possible.
  3. RIGIS soils data were created at a scale of 1: 15,840. While adequate for getting a general sense of soil conditions, it is not as accurate as a site-specific soil survey. The soil series boundaries can easily be 40’ off. A site-specific soil survey will be needed to confirm that soils are accurately mapped and to identify soil features where the map unit is actually a complex of soils with dramatically different characteristics. If initial evaluation eveals very complex soil constraints, it may be prudent to have the developer commission a site-specific soil survey early in the review process.
  4. Orthophotographs are aerial photographs that have been processed for use with GIS data. They are available at 1: 5,000 (half-meter pixel) and 1: 12,000 (one-meter pixel) scales. This means that each pixel on the map is equal to 1 meter for the 1:12,000 scale. The smaller the pixel size the more accurate the photograph and the closer you can zoom in before the picture becomes fuzzy.
  5. Parcel data vary greatly because of the different quality base maps used to produce it. Consult metadata to determine intended scale.

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Reading Maps and Plans

Have you ever found yourself wondering what some of those symbols and bars mean?

Reading Maps and Plans (available as a PDF, 3051 KB) is a concise look at the fundamentals of those types of documents.

Map Reading 101: Using and Reading Maps and Plans (available as a Power Point Slide Show, 23,252 KB) is a PowerPoint style presentation that takes the user through a quick tour of how to read and use maps.

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