Runoff Hydrology 101

Hydrology 101 Runoff Hydrology 101

Legislation (c) The agency shall develop performance standards, design standards, or other tools to enable and promote the implementation of low-impact development and other stormwater management techniques. For the purposes of this section, "low-impact development" means an approach to storm water management that mimics a site's natural hydrology as the landscape is developed. Using the low-impact development approach, storm water is managed on-site and the rate and volume of predevelopment storm water reaching receiving waters is unchanged. The calculation of predevelopment hydrology is based on native soil and vegetation. Minnesota Statutes 2009, section 115.03, subdivision 5c Runoff Hydrology 101

The Water Cycle

Hanson, 1994 Runoff Hydrology 101 Development Impacts on the Water Cycle Runoff Hydrology 101 Runoff Hydrology 101

Rate Impacts of Development 1 Existing Hydrograph

2 Developed, conventional CN, no controls Vegetation Removal

Pre-development Soil Compaction Peak Runoff Volume Rate Q

Drainage Alteration 2

Impervious Areas 1 Area under the curve = Volume

T Runoff Hydrology 101 Hydrograph Scenarios Rate 1 Existing

2 Developed, conventional CN, no control. 3 Developed, conventional CN and control.

MIDS Pre-development 4 Peak Runoff Volume Rate Q

3 2

1 4

T Runoff Hydrology 101

Factors Affecting Q Runoff

• Precipitation • Antecedent moisture • Soil permeability • Watershed area • Ground cover • Storage in watershed • Time parameters Runoff Hydrology 101

Runoff Equations Runoff Hydrology 101

Runoff Curve Numbers Commonly used approach to determine runoff

Based on land cover and soils

Simple regression model that is useful for quickly assessing stormwater management practices and assessing impacts of land use changes Runoff Hydrology 101

Hydrological Soil Group:

Soil groups which are classified according to their drainage potential. Group A soils absorb a lot of water and are deep, well-drained, and composed of sand or gravel. Conversely, Group D soils do not absorb as much water and have a high runoff potential, and have a layer of high clay content near the surface or are shallow soils over bedrock or other material which does not absorb water. Hydrologic Soil Groups Runoff Hydrology 101

Hydrologic Condition:

A rating (good, fair, poor) that is based on a combination of factors that affect infiltration and runoff, including: • density and canopy of vegetative areas • amount of year-round ground cover • amount of grass or close-seeded legumes in rotations • percent of residue cover on the land surface (good is more than 20%) • degree of surface roughness Runoff Curve Numbers for urban areas Runoff Hydrology 101 Initial Abstraction all of the losses that occur before runoff begins, including interception, evaporation, and infiltration.

S is the potential maximum retention after runoff begins Runoff Hydrology 101 Development Impacts on the Water Cycle

Ia represents all of the water that hits the ground that is unavailable for runoff.

Some of the water from rainfall is absorbed by plants, some of it sits in puddles or lands right in a lake or pond, some of it evaporates back into the atmosphere, and some of it soaks into the ground. Runoff Hydrology 101 Runoff Hydrology 101

Ia = .223”

Ia = .666” Ia = 1.636” Runoff Hydrology 101 Rainfall Distribution

4 3.9 3.8 3.7 3.6 3.5 3.4 3.3 3.2 3.1 3 2.9 2.8 2.7 2.6 2.5 2.4 2.3 2.2 2.1 2 CN = 55 Ia = 38.77” 1.9 1.8 1.7 1.6 1.5 1.4 1.3 1.2 1.1 1 0.9 CN = 75 0.8 0.7 0.6 0.5 0.4 CN = 90 0.3 Ia = 28.54” 0.2 0.1 0 1 10 19 28 37 46 55 64 73 82 91 100 109 118 127 136 145 154 163 172 181 190 199 208 217 226 235 244 253 262 271 280 289 298 307 316 325 334 343 352 361 Rosemount, MN 2002 – 43” Average – 34.6”