3 MARCO REFERENCIAL DE LA INVESTIGACION
3.2 MARCO CONCEPTUAL
The surface area, expressed as a percentage of the site, is then compared to the percentage of site area required for water quality control. The volume requirement for stormwater management quality control is based on the requirement to treat the first ½ inch of runoff (approximately 1,800 cubic feet per acre) from impervious areas. This volume is translated to a percent of the site area by assuming a storage depth of 6 inches. The procedure for calculating the site area required
Appendix A-11
Low-Impact Development: An Integrated Environmental Design Approach
Figure A.4. Procedure to determine percentage of site area required for IMPs to maintain predevelopment runoff volume and peak runoff rate.
Appendix
A-12
Example A.2
Determining Site Area Required to Maintain Volume (CN) Using Chart Series A: Storage Volume Required to Maintain the
Predevelopment Runoff Volume Using Retention Storage
Given:
Site Area is 18 acres Existing CN is 60 Proposed CN is 65 Design storm is 5 inches
Design depth of IMP is 6 inches
Solution:
Use Chart Series A: Storage Volume Required to Maintain Runoff Volume or CN.
0.35 inch of storage over the site is required to maintain the runoff volume.
Therefore: if 6-inch design depth is used, 1.1 acres (18 acres x 0.35 / 6) of IMPs distributed evenly throughout the site are required to maintain the runoff volume, or CN.
Additional Considerations:
1) Account for depths other than 6 inches:
Site of IMP Area = 1.1 acres, if 6-inch depth is used Depth of IMPs = 4 inches
Site of IMP Area = 1.1 x 6 in./4 in. Site of IMP Area = 1.65 acres
2) Account for infiltration and/or evapotranspiration (using Equation A.4)
If 10% of the storage volume is infiltrated and/or reduced by evaporation and transpiration.
Site of IMP Area = (storage volume) x (100 - X) / 100 Site of IMP Area =1.1 x (100-10)/100
Appendix A-13
Low-Impact Development: An Integrated Environmental Design Approach
for quality control is provided in Example A.3. The greater number, or percent, is used as the required storage volume to maintain the CN.
From the results of Example A.3, 0.1 of storage is required for water quality using retention; from Example A.2, 0.35 of storage is required to maintain the runoff volume using retention. Since the volume required to maintain the runoff volume is larger, in this case 0.35 of storage over the site should be reserved for retention IMPs.
Step 3: Determine storage volume required to maintain
peak stormwater runoff rate using 100 percent retention.
The percentage of site area or amount of storage required to maintain the predevelopment peak runoff rate is based on Chart Series B: Percentage of Site Area Required to Maintain Predevelopment Peak Runoff Rate Using 100% Retention (Exhibit B). This chart is based on the relationship between storage volume, ∀
∀ s r , and discharge, i o Q Q ,
to maintain the predevelopment peak runoff rate.
Where: Vs = volume of storage to maintain the predevelopment peak runoff rate using 100% retention;
Vr = postdevelopment peak runoff volume; Qo = peak outflow discharge rate; and Qi = peak inflow discharge rate. Example A.3
Calculation of Volume, or Site Area, for Water Quality Control
Given:
Site area is 18 acres
Impervious area is 3.6 acres (20%) Depth of IMP is 6 inches
Solution:
The water quality requirement is to control the first ½ inch of runoff from impervious areas (18 acres x 20%) x 0.5in. / 18 acres = 0.1 inch storage for water quality 0.1 inch is less than 0.35 inch (from example A.2). Therefore, use storage for runoff volume control to meet the water quality requirement.
Appendix A-14
The relationship for retention storage to control the peak runoff rate is similar to the relationship for detention storage. Figure A.5 is an illustration of the comparison of the storage volume/discharge relation- ship for retention and detention. Curve A is the relationship of storage volume to discharge to maintain the predevelopment peak runoff rate using the detention relationship from Figure 6-1 (SCS, 1986) for a Type II 24-hour storm event. Curve B is the ratio of storage volume to discharge to maintain the predevelopment peak runoff rate using 100 percent retention. Note that the volume re- quired to maintain the peak runoff rate using detention is less than the requirement for retention. This is graphically demonstrated in
Figure A.6.
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Hydrograph 2 represents the response of a postdevelopmentcondition with no stormwater management IMPs. This hydrograph definition reflects a shorter time of concentration (Tc), and
increase in total site imperviousness than that of the
predevelopment condition. This resultant hydrograph shows a decrease in the time to reach the peak runoff and discharge rate and volume, and increased duration of the discharge volume.
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Hydrograph 8 illustrates the effect of providing additional deten-tion storage to reduce the postdevelopment peak discharge rate to predevelopment conditions.
Figure A.5. Comparison of retention of storage volumes required to maintain peak runoff rate using retention and detention.
Appendix A-15 Low-Impact Development: An Integrated Environmental Design Approach
∀1 is the storage volume required to maintain the
predevelopment peak discharge ratio using 100% detention storage. The combination of ∀1 and ∀2 is the storage volume required to maintain the predevelopment peak discharge rate using 100% retention storage.
The following calculations apply to Design Chart Series B:
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The Tc for the postdevelopment condition is equal to the Tc forthe predevelopment condition. This equality can be achieved by techniques such as maintaining sheet flow lengths, increasing surface roughness, decreasing the amount and size of storm drain pipes, and decreasing open channel slopes. Chapter 2 of this manual provides more details on these techniques.
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The depth of storage for the retention structure is 6 inches. For other depths, see Example A.2.If the Tc is equal for the predevelopment and postdevelopment conditions, the peak runoff rate is independent of Tc for retention and detention practices. The difference in volume required to maintain the predevelopment peak runoff rate is practically the same if the Tcs for the predevelopment and postdevelopment conditions are the same. These concepts are illustrated in Figure A.7. In Figure A.7, the
difference in the required IMP area between a Tc of 0.5 and a Tc of 2.0
Figure A.6. Storage volume required to maintain peak runoff rate
Appendix A-16
is minimal if the predevelopment and postdevelopment Tcs are maintained.