Difference between revisions of "What is the pre-development condition"
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==What is the pre-development condition?== | ==What is the pre-development condition?== | ||
<p>When a requirement exists to match runoff rate or volume to “pre-development conditions,” there is a range of options that could be applied to define land cover conditions. This range goes from pre-settlement, which assumes land is in an undeveloped condition, to the land use condition immediately prior to the project being considered, which assumes some level of disturbance in the natural landscape has already occurred. Interpretations of this variation from Scott County, Project NEMO, Dane County (WI), and the USDA-NRCS ere used to lay out the range of approaches that local units can use when applying this criterion. Please note that selection of a pre-development definition should occur only after an evaluation of the hydrologic implications of the choice is performed.</p> | <p>When a requirement exists to match runoff rate or volume to “pre-development conditions,” there is a range of options that could be applied to define land cover conditions. This range goes from pre-settlement, which assumes land is in an undeveloped condition, to the land use condition immediately prior to the project being considered, which assumes some level of disturbance in the natural landscape has already occurred. Interpretations of this variation from Scott County, Project NEMO, Dane County (WI), and the USDA-NRCS ere used to lay out the range of approaches that local units can use when applying this criterion. Please note that selection of a pre-development definition should occur only after an evaluation of the hydrologic implications of the choice is performed.</p> | ||
| − | ===Pre- | + | ===Pre-settlement conditions=== |
| − | + | The most conservative assumption for pre-development conditions is the assumption that the land has undergone essentially no change since before settlement (<span title="A condition that assumes the land has undergone essentially no change since before settlement. In this case, a meadow or woodland in good condition is commonly used to portray a “natural” condition.> '''Pre-settlement conditions'''</span>). In this case, a meadow or woodland in good condition is commonly used to portray a “natural” condition. The following table shows the <span title="The SCS curve number method is a widely used method for determining the approximate amount of runoff from a rainfall even in a particular area. The curve number is based on the area's hydrologic soil group, land use , treatment and hydrologic condition."> '''curve numbers'''</span> used when this situation is applied using TR-55. Similar hydrologic characteristics would be applied when using other models. | |
| − | + | ||
| − | ===Conditions | + | {{:Curve number for Use with pre-settlement conditions}} |
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| − | + | ===Conditions immediately preceding development=== | |
| + | On the other end of the pre-development definition is the assumption that land disturbance has previously occurred with the land use in place at project initiation. This is the definition used under most circumstances by the MPCA in the [https://stormwater.pca.state.mn.us/index.php?title=Construction_stormwater_program Construction General Permit] (CGP). Under this scenario, runoff assumptions after construction need to match those of the land use prior to the development using matching curve numbers or <span title="The runoff coefficient (C) is a dimensionless coefficient relating the amount of runoff to the amount of precipitation received. It is a larger value for areas with low infiltration and high runoff (pavement, steep gradient), and lower for permeable, well vegetated areas (forest, flat land)."> [https://stormwater.pca.state.mn.us/index.php?title=Runoff_coefficients_for_5_to_10_year_storms '''runoff coefficients''']</span>. The new project could possibly improve runoff conditions, if the prior land use did not accommodate any runoff management. That is, implementation of good runoff management to an area that had previously developed without it would likely reduce total runoff amount compared to existing development. Note that the MPCA could alter its definition of pre-development under certain circumstances, such as a <span title="The amount of a pollutant from both point and nonpoint sources that a waterbody can receive and still meet water quality standards"> [https://stormwater.pca.state.mn.us/index.php?title=Total_Maximum_Daily_Loads_(TMDLs) '''total maximum daily load''']</span> (TMDL) established load limit. | ||
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| + | [https://data.nal.usda.gov/dataset/small-watershed-hydrology-wintr-55#:~:text=WinTR%2D55%20is%20a%20single,through%20channels%20and%2For%20reservoirs. NRCS (TR-55)] notes that heavily disturbed sites, including agricultural areas, curve numbers should be selected from the “Poor Condition” subset under the appropriate land use to account for common factors that affect infiltration and runoff. Lightly disturbed areas require no modification. Where practices have been implemented to restore <span title="Soil structure describes the arrangement of the solid parts of the soil and of the pore space located between them. It is determined by how individual soil granules clump, bind together, and aggregate, resulting in the arrangement of soil pores between them."> '''soil structure'''</span>, no permeability class modification is recommended. | ||
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| + | [[Category:Level 2 - General information, reference, tables, images, and archives/General information]] | ||
Latest revision as of 23:27, 21 February 2023
What is the pre-development condition?
When a requirement exists to match runoff rate or volume to “pre-development conditions,” there is a range of options that could be applied to define land cover conditions. This range goes from pre-settlement, which assumes land is in an undeveloped condition, to the land use condition immediately prior to the project being considered, which assumes some level of disturbance in the natural landscape has already occurred. Interpretations of this variation from Scott County, Project NEMO, Dane County (WI), and the USDA-NRCS ere used to lay out the range of approaches that local units can use when applying this criterion. Please note that selection of a pre-development definition should occur only after an evaluation of the hydrologic implications of the choice is performed.
Pre-settlement conditions
The most conservative assumption for pre-development conditions is the assumption that the land has undergone essentially no change since before settlement ( Pre-settlement conditions). In this case, a meadow or woodland in good condition is commonly used to portray a “natural” condition. The following table shows the curve numbers used when this situation is applied using TR-55. Similar hydrologic characteristics would be applied when using other models.
Curve number for use with pre-settlement conditions.
Link to this table
| Runoff Curve Number* | ||
|---|---|---|
| Hydrologic Soil Group (HSG) | Meadow | Woods |
| A | 30 | 30 |
| B | 58 | 55 |
| C | 71 | 70 |
| D | 78 | 77 |
* Curve numbers from USDA-NRCS, Technical Release 55
Conditions immediately preceding development
On the other end of the pre-development definition is the assumption that land disturbance has previously occurred with the land use in place at project initiation. This is the definition used under most circumstances by the MPCA in the Construction General Permit (CGP). Under this scenario, runoff assumptions after construction need to match those of the land use prior to the development using matching curve numbers or runoff coefficients. The new project could possibly improve runoff conditions, if the prior land use did not accommodate any runoff management. That is, implementation of good runoff management to an area that had previously developed without it would likely reduce total runoff amount compared to existing development. Note that the MPCA could alter its definition of pre-development under certain circumstances, such as a total maximum daily load (TMDL) established load limit.
NRCS (TR-55) notes that heavily disturbed sites, including agricultural areas, curve numbers should be selected from the “Poor Condition” subset under the appropriate land use to account for common factors that affect infiltration and runoff. Lightly disturbed areas require no modification. Where practices have been implemented to restore soil structure, no permeability class modification is recommended.
This page was last edited on 21 February 2023, at 23:27.