Good afternoon. My name is Rich Lucera, I am with RBF Consulting in the San Diego office. I’m joined here today with Braeden MacGuire to discuss our efforts to determine the numerical influence of geographic variables with respect to adequately mitigating the impacts of hydromodification. The findings from our analysis are the culmination of modeling theoretical development projects across California, as broken down into 3 major zones differentiated by precipitation and other factors.
California covers a vast expanse of varying precipitation, climatic differences, and changing topography. To study everyone combination of these factors would require time and resources well beyond that which was available during our work efforts. Having said that, our analysis was suitable to accurately determine comparative differences between pre and post development conditions, to the extent necessary to establish an accurate BMP sizing criteria. In addition to a description of our methodology, we’ll present a summary of our findings, conclusions, as well as few examples of the resultant sizing criteria that I just mentioned. Before we dive into that, we’d first like to take a moment to review the basic concept of hydromodification.
Hydromodification is a water quality impact that results from artificially accelerated erosion. The otherwise natural process can be accelerated from additional impervious surfaces introduced into a watershed in conjunction with development. These additional impervious surfaces result in greater duration of base flow runoff within a channel and increases the potential for bed movement. Other man made improvements, such as dam construction, can alter the rate of natural sediment transport rate and result in hydromodification. An example of this effect is illustrated here. Increased sediment discharged into the downstream waters is acutely toxic to many naturally occurring aquatic plants fish, and other wildlife.
From a regulatory standpoint, successful mitigation of hydromodification requires matching the pre-development flow duration curve within a critical range of events during which most of the hydraulic work is performed within a stream. In arid and semi-arid climates such as California this window is considered to range from approximately the two year runoff event up to an including the 10 year. In this example covering a 35 year time period, 0.003% (9 hours) of the measured flows within a stream exceeded 7.8 cfs, while 0.04% (123 hours) exceeded 0.74 cfs. As a result of development, those percentages increased to 0.10% (307 hours) and 1.00% (3,066 hours) respectively for the same flow rates. The flow rates in between also experience similar percentage increases.
The goal of our analysis was to develop a simple criteria that could be applied to development projects across the state to accurately predict the required BMP sizing necessary to mitigate hydromodification. The analysis focused on several of the most common BMPs including extended detention, infiltration, and vegetated swales. It required extensive computer modeling to isolate the influence of the most significant factors, only one of which was geographic location.
Our software models calculated (35-50 year) long term continuous simulations based upon hourly rain gauge information from 3 zones considered to reflect the basic variations in precipitation and climate across California. Runoff was determined from an extensive series of HSPF process algorithms, based upon an array of specific pervious land factors – including upper and lower zone storage, infiltration rate, dry weather flow, and evapotranspiration, among others.
The first model considered a 0.36 acre basin, which became our “unit” area. All analysis considered a post development condition of 5% maximum gradient, since the developments were assumed to involve earthwork necessary to establish moderately flat project topography. The model run for the unit area was repeated for a host of pre-development ground cover conditions including “forest”, “grass”, and “urban.” All resulting models were re-run for varying hydrologic soil types ranging from Class “A to Class “D”. The resulting matrix was all re-run again by scaling the unit area by a factor of 10 (3.6 acres) and then again at a factor of 100 times (36 acres). Each of those models was re-run yet again with different assumed geographic locations. In order to maintain a consistent basis of comparison with a post development condition, certain geometric assumptions had to be made about the BMPs used to mitigate the effects……
Extended detention basins were assumed to a have a 3:1 side slope, and a maximum ponding depth of 3’. One foot of additional freeboard was calculated into the stage-storage relationship. Outlet curves were determined by trial and error and were optimized with the given storage. By this, I mean that both the upper, volume driven portion of the curve as well as the lower, outlet dictated portion of the curve represented good matches with the predevelopment. It was assumed that subsequent design efforts would be necessary to design outlet structures that were hydraulically equivalent to the values assumed in the models. Native infiltration based upon soil type was accounted for in the model runs for projects with Type A, B, and C soils.
Similar assumptions regarding side slope were made for the infiltrations basins. Outlet curves were also established by trial and error, and maximum ponding depth was limited to 2’. This change in assumption was done in the interest of preventing potential vector control issues. Native infiltration rates varied with soil type and were modeled based upon recommended values published by the Natural Resource Conservation Service.
We established a “Northern” zone which included the coastal northwest portion of the state, as well as the Sierra Region, coastal portions of the Bay Area, the central coast, and some minor mountainous areas of the southern California. It was defined by mean annual precipitation falling within a range of approximately 40-80”, although some very small isolated pockets within the Klamath Mountains receive as much as 200.” The Northern zone includes 2 primary climatic regions…the first includes the coldest mountain and intermountain areas. Winter time lows average 0 – 11 degrees Fahrenheit, while summers days are relatively mild with chilly evenings. Snowfall is common. The landscape is rich with evergreen conifers and tough deciduous trees. The second primary climatic zone is found in coastal mountains dominated by the effects of marine air and is situated in cold air basins. Winters are chilly with lows ranging typically from 21-28 degrees Fahrenheit. The summer days experience consistent nagging afternoon winds.
Our “central” zone consisted primarily of high Modoc Plateua in the north eastern portion of the state, the eastern Bay Area, and a portion of the central valley. Mean annual precipitation ranged from 15-40 inches. The region consists of climatic regions defined in some instances by thermal belts, while others by cold air basins. Summer daytime temperatures are hot and sunshine is almost constant. Winter is marked by occasionally piercing north winds and tule fog, which hugs the ground during the evening.
The “southern” zone encompassed the driest portions of the state, including portions of the central valley, the southern coastal areas, and the desert southeast. Average annual precipitation within this zone is less than 15 inches. The climatic variation includes high desert and lower sub tropical deserts, with wide temperature swings. Summer is marked by hot days with mild evenings. Over 100 days of the year experience temperatures in excess of 90 degrees. Winter days can reach temperatures near 60 degrees, with evening lows typically hovering near freezing. The southern zone also includes cold winter and thermal belt areas influenced by the southern California coast. Winters are mild here with evening lows rarely dipping below 28 degrees. Summers are also mild by occasionally experience hot dry Santa Ana winds, which originate from points inland.
Good afternoon folks, I’m going to walk us through our results and conclusions. So, without further ado… The results clearly indicated that for BMPs subject to the same geometric constraints (depth, side slope, and outlet configuration) different volumes are required to provide hydromodification mitigation. An extended detention basin in our northern region required up to 180% more volume than a extended detention basin in the central region, and extended detention basins in the southern region required up to 100% more volume. For infiltration basins, the pattern was the same, with Northern region basins requiring up to 60% more volume than those in the central region; southern basins required up to 25% more volume. The precise cause of the regional differences was not identified, but we believe the difference is the result of varying climatic and soil conditions. For instance, northern California receives rainfall over a longer period of the year than southern California. And, southern California receives rainfall as thunderstorms during a short wet season and little to no snow, whereas northern California can be subject to longer periods of rain and drizzle and snowfall occurs in the mountains. It is also important to note that hydromodification analysis is NOT event-based, so commonly receiving rainfall over a period of days would definitely increase the volume required to mitigate for hydromodification. In these cases, the antecedent moisture condition of the soil is important… as are other soil characteristics – upper and lower zone storage capacities and hydraulic conductivity to name a few. <click mouse> The typical outlet structure that we modeled consisted of a riser pipe with an outlet orifice located at the basin invert. As you can see, we found that we required orifice diameters ranging from approximately 0.2inches up to 3.75inches. For reference, under 3feet of head, a 0.2inch orifice would convey less than one one-hundredth of a cubic feet per second (approximately 0.007cfs), while a 3.75inch orifice would convey 2.5cfs. Keep in mind, we’re talking about a 3.75inch diameter orifice to mitigate a 36acre drainage area… for small drainage areas, the resulting orifice size would likely result in constructability and clogging issues, so an equivalent configuration would need to be designed. <click> The good news is that on average the footprint of our hydrmodification mitigation BMPs accounted for only 5-7 percent of the contributing drainage area. This was the case for both extended detention and infiltration basins. <click>
Now we’ll take a look at some of the intuitive trends that emerged from our analysis. For starters, the volume required to mitigate for hydromod is directly related to drainage area. As drainage area increases, expect to increase the volume of your BMP. The graph on your left clearly shows that BMP volume increases with drainage area. The graph on the right indicates that as the drainage size increases, we achieve some economy of scale… the overall percentage of the contributing drainage area required to mitigate for hydromod actually decreases with increasing drainage area. These observations held true across all geographic zones modeled. The graph on the right really highlights the power infiltration as a tool to mitigate hydromod. Simply put, infiltration basins required less volume than detention basins. When water is infiltrated from your basin, it is essentially lost from the system, which results in a corresponding decrease in the volume required for hydromod mitigation. It is important to make one last note about infiltration from our modeling work – very little infiltration is not zero infiltration. Even Type D soils will infiltrate some miniscule amount of flow, and in long term continuous simulation that tiny bit of flow can have an enormous impact on the volume required to mitigate for hydromod. So, take advantage of what your site gives you, even if it doesn’t seem significant at first glance.
I found this next point to be very interesting. The volume required to mitigate for hydromodification decreases with pre-development conditions that promote runoff. So, Type D soils tend to require less volume to mitigate than Type A soils. Urban or grassy areas require less volume to mitigate than forested areas or shrublands. Steeper slopes require less volume to mitigate than flat slopes. This was true for all geographic zones. We can see these trends in this table – moving across the table from left to right we see a decrease in volume. We see the same trend as we travel down each column. This trend was in evidence for both extended detention and infiltration basins in all geographic zones.
Alright, so the goal was to create simplified sizing criteria that might be used for designing BMPs given various soil conditions, land uses, slopes and drainage areas. For a detention basin located on a site in southern california with Type B soils, these curves would provide a fairly accurate estimate of the volume necessary to mitigate for hydromodification. If we look at the graph we see a trend that we just talked about: urban and grass land uses require smaller volumes to mitigate hydromodification than a site that was developed on forested lands. If we look closely, we can also see the economy of scales… we increase drainage size 100fold from 0.36acres to 36acres, but our requisite BMP volume only increases by a factor of 89.
We can develop a similar set of curves for sizing our orifice and riser pipe. As you can see by the pattern of the curves, drainage area is directly correlated to the maximum orifice discharge that we can allow from our extended detention or infiltration basins. Smaller drainage areas require smaller a orifice, while larger areas require a larger orifice. Each of the stage-discharge curves shown correlates to a specific orifice diameter. This next point is very important - If we exceed the maximum allowable orifice discharges shown above, we fail to mitigate hydromodification. A larger orifice will pass too much flow to our receiving waterbody. A smaller orifice will inevitably cause our BMP to fill too quickly, causing it to top the riser pipe and discharge too much flow to our receiving water body. This also causes us to fail to mitigate hydromod. Similarly, undersizing the basin volume will cause the basin to fill too quickly… we’ll either overtop the riser more often than planned OR we’ll provide too much head on our orifice too often – either way, we will increase flows and/or durations to our receiving water body and fail to mitigate for hydromodification. In the end, sizing an infiltration basin or extended detention basin for hydromodification requires us to balance the discharge from our orifice with the volume of our basin. The two are not independent.
We did note that geographic location has an impact upon sizing a BMP to mitigate for hydromodification. All things being equal, a detention basin designed to mitigate for hydromodification in San Diego will need to be larger than one that provides mitigation in Berkeley. A detention basin in northern California would be larger than either. The results across all geographic areas appear to fall within a fairly well-defined range for any given drainage area. This promotes the hypothesis that simple regional curves may be developed. It is important, however, to keep in mind that mitigating hydromodification with detention or infiltration basins requires that we size the BMP outlet structure as well as the volume.
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