hydrographs

I have a general stormwater question with respect to quality. I've read and heard that the general convention is to treat the runoff from the first inch of rainfall. This dictates the sizing of BMP's as far as volume is concerned. As far as the effective treatment rate of the BMP's, how do you know what the intensity should be? That is, the intensity of the first inch of a 25 yr. storm would be different than the intensity during the first inch of a 2 yr. storm.

In determining these quantities, I'm assuming I would initially generate a 1 hr unit hydrograph. Then upon determining/deciding what the duration should be, I would develop the unit hydrograph for that duration (using the S-curve method for example).

Again, I'm new to the stormwater management world and I appreciate any and all help even remotley related to this question. Also, if someone has a good Excel spreadsheet for hydrographs and the convolutions that you are willing to share, feel free to do so. I have one from my hydrology class but I'm not sure how good it is. Thanks.

Jeff Thompson
JThompson@HOBBSUPCHURCH.COM

Jeff,

BMPs can be sized by volume or by flow. If say a sand filter is being sized by volume then it will be sized to hold the particular volume of water (say 1/2-inch across the impervious acreage of the site) prior to the sand media with the rest of the storm event left to bypass the filter. The flow of water through the sand media is so slow that routing calculations are not typically performed through the media. Although the intensity will be important for conveyance it can be overlooked for the actual BMP sizing.

Other BMPs, like hydrodynamic BMPs, can be sized by using a flow rate. This is typically done by using a storm that represents maybe 80% of storm events in a year for the local. In Alexandria, VA they use a 3-month storm event. By the Rational Method (or other method) a peak flow rate can be calculated and used as the design flow rate for the BMP. The assumption here is that by capturing the more frequent storm event the BMP will capture completely most all of the runoff producing storms throughout the year and for those events not entirely captured the BMP will at least capture the early portion of the storm (typically the dirtiest portion) will be captured completely.

Depending on flow capacity of the BMP a combination of the two methodologies can be used by routing calculations through the BMP.

Bill Hicks, P.E.
e-mail: bhicks@novaregion.org

Jeff,

The single event rainfall criteria is simply an extrapolation of a flood control design criteria applied to water quality. As you are finding out, determining a single duration and intensity profile for a storm that is representative of a water quality problem is a difficult thing to do. In flood control, we pick storms that are conservative (i.e. large volume, short duration and high peak intensities) because flooding (typically we hope) is caused by infrequent events. While many people believe that water quality problems stem from all events, and not just the infrequent events, the single event criteria is still used for two reasons:

1. It is simple to use 2. If a water quality control measure is designed for a large event, it is believed that all the smaller events will be treated more effectively (this depends).

Since you posted to SWMM, I would be reticent if I didn't say that many people think (including myself) that continuous simulation is a better methodology to design water quality measures since it is more representative of the distribution of intensities and storm duration and, perhaps, a more scientific approach to water quality enhancment. SWMM is well suited for this.

Enough of the methodology debate, California has allowed the use of the 90th percentile rainfall intensity for flow through BMPs. I tend to like this approach since is somewhat of a bridge between the two methodologies (single and continuous) and you still have to do an assessment of the continuous rainfall record to determine the 90th percentile value.


Graham Bryant
www.envirocalm.com