Employing Runoff Reduction Techniques

Benefits of Reducing Imperviousness

Reducing imperviousness may provide some of the following benefits:

• Increased infiltration and decreased rate and volume of site runoff.

• Decreased WQCV and, in turn decreased size of required WQCV facilities.

• Decreased 2-year and 5-year peak runoff rates and volumes for downstream conveyance and detention facilities.

• Reduced need for irrigation.

• Less curb and gutter.

• Smaller storm sewer systems.

• Decreased pavement.

• Decreased runoff rates and volumes further downstream in watershed, especially if MDCIA is used on a widespread basis.

BMPs for Minimizing Effective Imperviousness

Described next are structural BMPs that minimize effective imperviousness.

Grass Buffer (GB)

Uniformly graded and densely vegetated area of turf grass. This BMP requires sheet flow to promote filtration, infiltration, and settling to reduce runoff pollutants.

Grass Swale (GS)

Densely vegetated drainageway with low-pitched side slopes that collects and slowly conveys runoff. Design of longitudinal slope and cross-section size forces the flow to be slow and shallow, thereby facilitating sedimentation while limiting erosion.

Modular Block Porous Pavement (MBP)

Modular block porous pavement consists of open void concrete slab units underlain with gravel. The surface voids are filled with sand. This BMP is intended to be used in low traffic areas to accommodate vehicles while facilitating stormwater infiltration near its source. A variation of this BMP is termed stabilized-grass porous pavement, consisting of plastic rings affixed to filter fabric underlain with gravel. The surface voids are filled with sand and grass sod/or seed.

Applying MDCIA to a Site

Minimizing directly connected impervious area requires a basic change in land development design philosophy. This change seeks to reduce paved areas and directs stormwater runoff to landscaped areas, grass buffer strips, and grass-lined swales to slow down the rate of runoff, reduce runoff volumes, attenuate peak flows, and encourage filtering and infiltration of stormwater. Traditional land development practices do not focus on water quality enhancement. Instead, they promote runoff from rooftops, parking lots, driveways, and roads to quickly flow to a curb and gutter and to a formalized stormwater conveyance system. This practice concentrates runoff quickly, which results in a fast responding system and relatively large peak runoff rates during small storms.

Minimizing DCIAs can be made an integral part of landscape and drainage planning for any development. Roof collection systems can direct flow to landscaped areas, infiltration areas, grassed buffer strips, and to grass swales. In some proposed developments, portions of curb and gutter may be eliminated. In others, the use of slotted/intermittent curbing, along with stabilized grass shoulders and swales, may be feasible. Residential driveway runoff can be redirected from flowing directly into the street. Large parking lots can reduce DCIAs by using modular block or stabilized grass porous pavement in less used portions of the lot to encourage local infiltration or storage.

Site slopes should be capable of directing stormwater runoff by gravity in a sheet flow away from buildings, roads, and parking lots toward grass-covered or porous pavement covered areas. The runoff then needs to flow as a sheet over these porous surfaces before it reaches swales, storage, stormwater collection, and stormwater conveyance systems. As a result, in areas of high permeability soils, Hydrologic Soil Class A and B soils, the ground can provide for infiltration of large portions of surface runoff. (Hydrologic Soil Classes range from A to D, and are based on the minimum annual steady ponded infiltration rate for a bare ground surface. Class A soils have the highest infiltration rate and class D the lowest.) Where less permeable soils are present, significant runoff losses can also be achieved, while the use of sand trenches with underdrains under grass swales can be used to prevent the nuisance of standing water.

Steep sites with average terrain slopes exceeding 4 percent may not lend themselves well to implementing some aspects of this BMP. Some of the difficulties can be dealt with by using terracing and retaining walls. Nevertheless, most sites with general terrain slopes flatter than 4 percent should be suitable for this BMP; the flatter the better.

Minimizing DCIAs can be implemented in varying degrees. Two general levels associated with minimizing DCIAs have been identified and are described below:

•  Level 1 . The primary intent is to direct the runoff generated by impervious surfaces to flow over grass-covered areas, and to provide sufficient travel time so as to encourage the removal of suspended solids before runoff leaves the site, enters a curb and gutter, or enters another stormwater collection system. Thus, at Level 1, all impervious surfaces are made to drain over grass buffer strips before reaching a stormwater conveyance system.

•  Level 2 . As an adjunct to Level 1, this level replaces street or internal curb and gutter systems with low-velocity grass-lined swales and pervious street shoulders. Conveyance systems and storm sewer inlets will still be needed to collect runoff at downstream intersections and crossings where stormwater flow rates exceed the capacity of the swales. Small culverts will be needed at street crossings and at individual driveways until inlets are provided to convey the flow to a storm sewer. The elimination of public street curb and gutter will only be considered in special developments: residential areas with low traffic volumes and developed drainage flows; such drainage flows must be adequately handled through construction of other improvements and be acceptable from a maintenance standpoint, as determined by City Engineering.

Calculating Effective Imperviousness

The first step in estimating the magnitude of runoff from a site is to first estimate the site's imperviousness. The total imperviousness of a site is the weighted average of individual areas of like imperviousness. For instance, paved streets (and parking lots), drives and walks have an imperviousness of 100 percent, roofs have an imperviousness of 90 percent, and lawn areas have an imperviousness of 0 percent. The total imperviousness of a site can be determined taking an area-weighted average of the imperviousness of the street, walk, roof, and lawn areas.

Structural BMPs for minimizing imperviousness impact this calculation in two ways. First, the use of modular block porous pavement reduces the imperviousness associated with parking areas and drives built using modular block pavement from 100 percent to 35 percent (assuming the use of underdrains). Second, the use of grass buffers and grass swales provides a reduction in imperviousness according to Figure ND-1. This figure represents the reduction in imperviousness associated with Level 1 and Level 2 MDCIA as discussed above. Grass buffers and/or grass swales are to be configured according to the design procedure documented in section 4.2, New Development BMP Factsheets.

Hydrologic Modeling

The current hydrologic methodology in the City/El Paso County Drainage Criteria Manual is inadequate for the MDCIA method. If such a BMP is proposed, a proven hydrologic modeling method must be utilized and agreed to by City Engineering.

Application Examples

The following figures provide a number of illustrations of how the principle of MDCIA can be applied to development sites. Figure ND-2 shows an example of MDCIA for a residential and commercial site. Figure ND-3 shows an example for a multi-family residential site. Figure ND-4 shows typical application examples of modular block porous pavement.

The Total Percent of Watershed Imperviousness for the traditional residential layout in Figure ND-2 is approximately 47 percent. Using porous pavement and a grass swale, as shown at the bottom of the figure, reduces the Total Percent of Watershed Imperviousness to 34 percent. This shows that the inclusion of BMPs can significantly reduce total imperviousness. Additional BMP benefits are achieved when the user determines the Impervious Percent to Use with WQCV in Figure ND-1 because the MDCIA layout allows the use of the Level 2 MDCIA curve. The resulting Impervious Percent to Use with WQCV values for the traditional residential layout and the residential MDCIA layout are 47 percent and 20 percent, respectively.

FIGURE ND-1
Imperviousness To Use With Water Quality Capture Volume (WQCV)

FIGURE ND-2
Examples of Minimizing Directly Connected Impervious Areas - Residential and Commercial

FIGURE ND-3
Examples of MDCIA for Multi-Family Residential Development

FIGURE ND-4
Typical Application of Modular Block Porous Pavement