In addition to the estimates of dissolved P concentrations
in runoff at the site location, further information was required:
- Peak runoff flow rate
- Average annual flow volumes and dissolved P load
- Hydraulic head
The average annual flow volume and peak flow rate was calculated by using site information required for estimating the NRCS curve number (ftp://ftp.wcc.nrcs.usda.gov/wntsc/H&H/training/runoff-curve-numbers1.pdf). This required watershed information regarding soil type (used to determine hydrologic soil group), ground cover, greatest length of flow, and slope. Each of these parameters, except for soil type and flow length (determined by soil survey and contour maps) were determined via site visit. The wonderful NRCS folks at the local Stilwell office (Chad Kacir, Andy Inman, and others) were responsible for conducting the site survey for the curve number method and also calculations for peak flow rate and average annual flow volumes.
Peak runoff flow rate estimate
The curve number is used in conjunction with the
precipitation depth for the design storm.
In our case, we wanted to design our structure for a 2yr-24hr storm,
which produces about 4 inches of rainfall as estimated by standard USDA
rainfall tables (http://www.nws.noaa.gov/oh/hdsc/PF_documents/TechnicalPaper_No40.pdf).
For our site, the curve number (CN) was 78; the cover was mostly
pasture.The curve number method resulted in a runoff depth of 1.97
inches for our 9 acre watershed, predicted for a 2yr-24hr storm. Next, the runoff depth is used to calculate the
peak flow rate through use of the Soil-Cover-Complex method and “time of
concentration”. The time of
concentration is calculated using the curve number and the previously
determined greatest length of flow via curve number method. For our site, the greatest flow length to our
proposed structure location was 1050 ft, which resulted in an estimated time of
concentration of 24 minutes. Based on
this time of concentration, the estimated peak discharge was 0.9 cfs/acre-inch
runoff.For our scenario:
0.9 cfs/acre-inch runoff
* 9 acres * 1.97 inches runoff = about 16 cfs peak runoff rate.
Therefore, our goal was to design a structure that could
handle a flow rate of at least 16 cfs so that it would be able to treat all of
the runoff produced from a 2 yr-24hr storm.
Average annual runoff volume and P load estimate
Next, the annual flow volume is necessary in order to determine what the annual load of dissolved P is. To achieve this, the runoff coefficient method was used. An example is shown here: http://watershedmg.org/sites/default/files/docs/wmg_calculating_runoff_worksheet.pdf ) This calculation is simply based on cover for estimating the runoff coefficient, watershed area, and average annual rainfall depth. For our site, the annual rainfall depth was 44 inches. Based on our site, the average annual runoff volume is 12 inches/yr, or 9 acre-ft.
Our grab samples indicated dissolved P concentrations between
1 and 2 mg/L. Using the higher value of
2 mg P/L for over-design, this resulted in an estimated load of 22 kg of P/yr
transported in runoff from the watershed.
Therefore, our estimates for the required mass of PSM for this site will be partly based on the predicted value of 22 kg dissolved P lost in runoff per year.
Hydraulic head
Hydraulic head is critical for achieving the desired flow rate through the P removal structure. As discussed in the previous post, the runoff water must flow through the structure in order for P removal to occur. Simply put, the hydraulic head is the force that “pushes” the water through the structure. The hydraulic head can be estimated by conducting an elevation survey of the proposed structure location.
Based on the elevation survey, we will have an appreciable
amount of hydraulic head to achieve the desired flow rates (i.e. 16 cfs for a
2yr-24hr storm).
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