Pre-development
The drainage area to PC1 was composed of mature hardwood forest during the entire pre-development period of monitoring (3/03-3/04). Runoff during this period was low with only four of 31 significant storm events (rainfall > 0.3 in) producing measurable runoff. All four of the storms had a rainfall accumulation of more than 1.5 in (fig ?). A hydrograph from one of the storms is shown in the figure ?. The peak 30-minute rainfall intensity, average rainfall intensity, peak discharge, and average discharge are computed for each storm event to facilitate comparisons between pre-, during-, and post-development hydrology. Summaries of the pre-, and during-development (construction) data are shown in Table 1. As shown, there were more runoff producing storms during the construction period mainly because the average accumulation of storm required to produce runoff decreased from about two to about one inch. The decrease in the average volume of runoff is mostly the result of a decrease in the drainage area to the monitoring station. As part of the erosion and sediment control plan, a diversion was installed across the drainage area to divert runoff to a large sediment pond at the corner of the site. This reduced the drainage area from 2.48 ac. to 1.48 ac. with all of the reduction in area coming from the land under development. After stabilization runoff will be directed through the storm drain system, which will restore some of the drainage area to the monitoring station.
During-development (construction)
The construction phase of development began in May, 2005 when tree stumps were removed (fig 7). Tree stumps and other woody remains were ground up and stockpiled on-site for future use (fig 8). Major grading of roads and lots, which was minimized to attempt to mimic pre-development hydrology, was completed in the Fall of 2005 along with most of the storm drains. A riparian buffer along the downslope side of the property was left undisturbed to provide a natural area and undisturbed land for stormwater treatment.
Monitoring results for the pre-construction (3/16/03-3/31/04) and during-construction (5/12/05-2/4/06) periods are shown in Table 2. Runoff increased for the construction period as was expected when clearing occurs and impervious surfaces are introduced into the drainage area. The export of organic (TKN) and inorganic (NO3-N) nitrogen and total phosphorus (TP) from the site increased substantially during the construction period. Sediment (TSS) export increased dramatically despite the presence of a sediment trap (figure 9) and other erosion and sediment control measures in the drainage area to this monitoring site. Runoff and sediment export from the other site, PC3, was also considerable. The drainage area to this site was typical of constructions sites consisting of exposed soil and house roofs thereby creating a high erosion potential.
Rain |
Peak |
|||||
| Period | No. | Rain accum. |
Peak 30 min. (in) |
Duration (hr) |
Intensity (in/hr) |
Runoff (gal) |
| Pre-construction | ||||||
| Runoff producing | 4 | 1.96 | 0.22 | 12.8 | 0.16 | 40247 |
| No sign runoff | 27 | 0.76 | 0.16 | 10.3 | 0.23 | 3 |
| All storms | 31 | 0.92 | 0.17 | 10.6 | 0.22 | 5369 |
| During Construction (to 12/05) | ||||||
| Runoff producing | 9 | 1.08 | 0.21 | 3.9 | 0.51 | 6574 |
| No sign run | 13 | 0.85 | 0.31 | 7.4 | 0.66 | 1 |
| All storms | 22 | 0.94 | 0.28 | 5.8 | 0.63 | 3089 |
| Site | Begin | End | Rain | Runoff | Run/Rain | TKN | HN3N | NO3N | TP | TSS |
|---|---|---|---|---|---|---|---|---|---|---|
in |
in |
------------ kg/ha-yr -------------- | ||||||||
| PC1 | 3/16/03 | 3/31/04 | 28.4 | 2.29 | 0.08 | 0.18 | 0.05 | 0.01 | 0.02 | 3.1 |
| PC1 | 5/12/05 | 2/4/06 | 22.4 | 2.62 | 0.12 | 0.52 | 0.01 | 0.07 | 0.33 | 2,437 |
| PC3 | 5/10/05 | 2/4/06 | 23.9 | 18.31 | 0.77 | na | na | na | na | 32,3501 |
1 The actual drainage area was difficult to determine, but 3.6 ac was used in the calculations.
