Infiltration Trench Design Guidance

Guidance Purpose

The purpose of this design guidance is to aid the designer that does not regularly design infiltration trench facilities. For further details beyond what is provided, refer to the references at the end of this document which contains multiple sources used to help create this guidance. This guidance is meant to be as a good starting point, but leaves some gray area as not all design scenarios can be covered. For more in-depth help and analysis please consult with the DelDOT Stormwater Section. It is presumed that the designer is familiar with the basic theory and methods of analysis and design in both hydrology and hydraulics. Also, the following information will rely heavily on the HydroCAD program and it is suggested that the designer be familiar with this program to better utilize this document.

Trench Design

The design of an infiltration trench involves essentially the same aspects of pond design at its heart, but has the added benefit of infiltrating / discarding runoff back into the surrounding soil, which eventually makes its way back into the groundwater.

In a flow diagram form, the basis of infiltration trench design is roughly:

As can be seen from the above flow diagram, infiltration trench design can be an iterative process; hence, the designer will most likely have to do some trial and error runs until an acceptable final result is obtained.

This document will not go in-depth concerning exploratory studies for infiltration feasibility as that can be found elsewhere. This guidance document presumes that infiltration is acceptable as well as a trench bottom elevation has already been established as per the exploratory studies mentioned above. For more in-depth help and analysis of infiltration feasibility, please consult with the Stormwater Section.

The following is an example of design considerations and processes that would go into the development of an infiltration trench.

Example Problem Statement:
After considering different options, it has been decided that an infiltration trench is the preferred storm water management facility for an upcoming road widening project. The project is located in Sussex County.

Some thoughts that went into selecting this particular facility type were the following:

Infiltration testing showed acceptable infiltration rates as per the “Delaware Sediment and Stormwater Regulations” (DSSR) under “Soil Investigation Procedures for Stormwater Best Management Practices”.
Due to the scope of the project, overall size, and additional new impervious, it was estimated that an infiltration trench would be able to mitigate the additional runoff to the eventual outfall.

Note: An estimated infiltration facility size can be calculated using the “Infiltration Facility Sizing” spreadsheet found in the ES2M Design Guide. This can be useful here as well as if needing a preliminary size for future right of way purposes.

Due to right of way constraints for various reasons, larger facilities, i.e. ponds were not feasible.

The benefit of infiltration is that the runoff quality does not need to be retained for long periods like a pond as well as runoff quantity can be reduced if needed.

The following hydrologic conditions were computed using the HydroCAD program: (the point of analysis (POA) for the trench drainage area was selected during the exploratory studies phase)

Table 1. Subcatchment Data and Calculate Runoff

Hydrologic Condition	Area (ac)	CN	Tc (min)	Runoff Rate (cfs)			Runoff Volume (af)
Hydrologic Condition	Area (ac)	CN	Tc (min)	RPv*	Cv	Fv	RPv*	Cv	Fv
Existing	1.5	76	8	1.43	3.47	7.65	0.151	0.351	0.783
Proposed	2	82	14	1.81	4.27	8.69	0.248	0.563	1.168

* - The RPv (1yr) storm event is used for qualitative management. The designer must change the ‘Ia/S Ratio’ in HydroCAD’s ‘Calculation Settings’ to 0.05 before performing analysis. The initial abstraction (Ia) represents all losses before runoff begins. Selecting 0.05 yields more accurate results for predicting the runoff from small, more frequent storm events as determined from DNREC’s Runoff Reduction Guidance Document.

The Cv (10yr) and Fv (100yr) storm events are used for quantitative management. The designer must ensure the ‘Ia/S Ratio’ in HydroCAD’s calculation settings is set to 0.20 prior to analysis.
Area (drainage area) - is computed from survey and topographic data. Ground truthing (actually visiting site) shall also be done to verify that the survey data is accurately showing the boundaries of the drainage area.
CN – curve number (dimensionless). The CN indicates the runoff potential of a given area as determined by a soil type and cover condition. Sometimes shown as RCN (runoff curve number).
Tc – time of concentration. This represents the time it takes for water to travel from one location to another within a drainage area. The Tc is the sum of all travel times for consecutive segments of the drainage conveyance system and shall start from the hydraulically most distant point. This could take multiple iterations to find the hydraulically most distant point.
Runoff Rate – the rate at which runoff is leaving the drainage area per storm event (denoted in cubic feet per second).
Runoff Volume – the amount of runoff leaving the site as denoted by volume of flow per storm event (this is a total volume over the entire drainage area).
All of these terms are further explained in detail in USDA TR 55.

Routing 1. Example of HydroCAD routing diagram with appropriate hydrographs used to fill in results for Table 1.

In keeping with the above flow diagram, now do the “Estimate Trench Storage” step. What will be shown here as stated before is the ‘quick’ way with HydroCAD. A designer should always have a basic understanding of the calculations involved as well as the inputs and assumptions made to evaluate whether an unusual result is a product of the assumptions or information that has been entered into HydroCAD.

Estimated Storage Volume:
Below is what the routing diagram will look like in HydroCAD (Routing 2). Subcatchment node (1P) input is taken from Table 1. One of the most common outfalls for an infiltration trench is a swale which will be used for this example problem. Obviously, a multitude of outfall types could exist, so make sure to model appropriately. When doing any project, analyze downstream conveyance to make sure if it is a free outfall or if tailwater* conditions will have an effect. For Routing 2, it is assumed that the outflow of the swale is a free outfall and any tailwater effect of the swale itself on the trench will automatically be calculated by HydroCAD via changing the ‘Reach Routing Method’ under ‘Calculation Settings’ to ‘Dyn- Stor-Ind’. The Dyn-Stor-Ind method does not currently allow reaches to respond to tailwater changes, it does allow the overall watershed to be analyzed in a dynamic manner so that ponds may respond to tailwater effects. For further discussion on the HydroCAD Reach and Pond Routing Methods, please refer to the HydroCAD manual.

*Tailwater will encompass a calculated water surface elevation either due to a particular storm depth in a receiving ditch/swale/stream, tidal influence, or if tying into an existing closed drainage system.

Routing 2.

An easy way to calculate the estimated storage for a starting point is to start with the end product of total volume needed for the post-developed RPv runoff or the value as taken from the “Infiltration Facility Sizing” spreadsheet. From Table 1, RPv runoff volume = 0.248 af. Convert this number to cubic feet (cf) = 0.248 af x 43,560 sf/a = 10,802 cf. From the exploratory phase, the trench bottom elevation was already determined to be at 12 ft. The top of the trench will basically be determined from the project’s grading operation, so for this example, use elevation 17 ft, which makes the trench depth equal to 5 ft. All that is left is the preliminary length and width dimensions as an infiltration trench is almost always just an elongated cube (design-wise). The width can be determined usually by what is allowable within right of way for that particular project area, so assume a maximum allowable width of 6 ft. The unknown preliminary length is calculated by: volume / (depth x width) = 10,802 cf / (5 ft x 6 ft) = 360.1 ft ~for simplification just round off to 360’.

Here is a rough sketch of what the post-developed area will look like.

Now input these parameters within HydroCAD under the “Infiltration Trench” node designated “2T”. Right click “2T” and choose ‘Edit’. Now click ‘Detention Pond (or other storage area)’

Now click on the ‘Storage’ tab and double click on the first row, which will bring up the ‘Select New Storage Type’ window. Double click on the ‘Custom Stage Data’ and click on the ‘Stage Voids’ box. Input the storage data for the infiltration trench and for the channel above it. Remember, the void ratio for the infiltration trench is set to 40%, since the trench is filled with stone. The surface area above the trench is 100% void space. Note: the ‘Allow Exfiltration’ checkbox automatically activates when “exfiltration” is entered under the ‘Outlets’ tab.

Now click on the ‘Outlets’ tab and double click on the first row, which will bring up the ‘Select New Outlet Device’ window. Double click on ‘Exfiltration’. In HydroCAD, infiltration and exfiltration mean the same thing. Click on ‘Constant Velocity’ and input the design infiltration rate (not the field measured rate) as determined from the exploratory studies. Note: the ‘Routing’ automatically defaults to ‘Discarded’, which means that is runoff which has been infiltrated and is not subject to any further routing in the model.

Now that the defined loss of runoff due to percolation into the surrounding soil has been established in the model, the next step is to define an outlet for the additional runoff that cannot be infiltrated. Click on the ‘Outlets’ tab and double click on the second row, which will bring up the ‘Select New Outlet Device’ window. Double click on the ‘Custom Weir/Orifice’, which will bring up another window to input the beginning swale dimensions as the overflow for the trench. For this example, the swale invert is set slightly higher than the top of the trench to allow for some ponding above the stone layer.

To define the swale dimensions, right click on the swale node, then left click ‘Edit’. Then update the necessary information on the ‘General’, ‘Section’, and ‘Profile’ tabs as shown below.

For the RPv (Ia/S = 0.05), double click “2T” and the following hydrograph will appear:
(Note: the ‘Time Span’ under ‘Calculation Settings’ was changed. ‘Start Time’ = 1.00, ‘End Time’ = 48.00, and ‘Time Increment’ = 0.01)

As can be seen here, the whole RPv runoff volume is infiltrated and within 48 hrs, which means the stormwater quality component of the DSSR has been met. Also, by looking at the hydrograph, the peak elevation is just below the invert of the swale overflow. If by chance the “Primary” outflow was showing some type of discharge, then the trench would have to have been reconfigured larger, so that the total RPv was contained.

Now change the Ia/S value = 0.20 and check to make sure that the ‘Primary’ component (the rate of flow that will leave via the swale) is at or below the pre-developed Cv and Fv runoff rate. (Note: discarded = infiltration rate and outflow = primary rate + discarded rate)

The Cv is showing an outflow = 2.38 cfs whereas the pre-developed Cv = 3.47 cfs. Since this is at or below the pre- developed rate, this satisfies the DSSR stormwater quantity component.

The Fv hydrograph shows the following:

As per the DSSR, “Infiltration practices shall be designed so that they will 1) infiltrate the Fv within 72 hours or 2) dewater the Fv within 72 hours, or 3) manage the Fv on site with no adverse impact.” This facility drains within 72 hr for the Fv and the post-developed runoff rate of 7.57 cfs is less than the pre-developed runoff rate of 7.65 cfs.

One design alternative to help with multiple facets is to embed a perforated pipe(s) within the stone layer. This can help make the trench shorter and even help to reduce outflows by allowing for a 100% void volume within the 40% void stone space. Note: whenever adjusting the trench dimensions, remember to edit all appropriate data under the ‘Storage’ tab.

For this example, the same trench dimensions will be utilized and a 36” HDPE perforated pipe will be embedded in the trench (to help show a side by side comparison of the reductions noted above). When embedding pipe(s), common practice is to have at least a 1’ stone layer around the whole pipe as well as at the ends. Pipe lengths have to be shorter than the trench length and for ease of construction with HDPE, keep the lengths as a multiple of 5 (and multiples of 10 is even better). Since the trench in this example is 360’, subtract the 1’ of stone needed at each end, which equals 358’, so make the pipe 350’ long. For wider / deeper trenches, multiple pipes even at different diameters and inverts can further optimize storage within the trench. (Note: the ends of the pipe should have solid caps)

In HydroCAD, when embedding pipes within the trench, a defined outer storage volume (total trench volume) must be established first, so that an inner storage volume (pipe volume) can be embedded inside. As a result, the trench storage and the above trench storage have to be separated and placed on different rows (under the ‘Storage’ tab), so that the model can run properly.

Going back to the ‘Storage’ tab, the storage will be divided into three separate rows (devices). Working from the bottom up, the first device will represent the total volume of the stone trench itself. Device 2 will be the pipe that is embedded in the stone trench. And Device 3 will represent the volume above the stone trench.

Screenshots shown below:

Based on the pipe embedding, here are the new hydrographs (RPv, Cv, Fv):

As can be seen, the pipe within the stone area does help the overall functionality, which means if needed, the trench could be shortened as a 360’ long infiltration trench is quite large.

At this point, the overall infiltration trench design should be closely set and other final design procedures can begin. This would include, but not be limited to: construction details, pretreatment areas, observation port(s) placement, sequence of construction finalization, etc.

References

Runoff Reduction Guidance Document (Hawkins Abstraction)

Delaware DNREC Runoff Reduction Guidance Document

Delaware Sediment and Stormwater Regulations Feb 2019

USDA – NRCS - National Engineering Handbook, Part 630, Hydrology Sep 1997

USDA – NRCS – Technical Release 55 June 1986

DNREC – Delaware Post Construction Stormwater BMP Standards & Specifications Feb 2019

HydroCAD Stormwater Modeling System, Owners Manual, Version 10 (Jul 2011)

Wet Pond Design Guidance

The purpose of this design guidance is to aid the designer that does not design wet pond facilities on a daily basis and is not meant to be all encompassing. For further details beyond this guidance, refer to the references at the end of this document which contains multiple sources used to help create this guidance. The steps shown herein are meant to be quick and easy and not get bogged down in over explanation. This guidance is meant to be as a good starting point and will go into some detail, but every gray area will not be able to be covered. For more in-depth help and analysis please consult with the Stormwater section. It is presumed that the designer is familiar with the basic theory and methods of analysis and design in both hydrology and hydraulics. Also, the following information will rely heavily on the use of the HydroCAD program, and it is suggested that the designer be familiar with this program to better utilize this document.

The design of a pond at its heart is the conservation of mass equation, sometimes referred to as the continuity equation, which is a mass balance and represents storage effects. It states that inflow minus outflow is equal to change in storage between two locations and between two moments in time. Hydrologic flood routing methods use the conservation of mass equation with simplifying assumptions to reduce the conservation of momentum equation to a relation of discharge to storage.

In a flow diagram form, the basis of a pond design is roughly:

As can be seen from the above flow diagram, pond design is an iterative process, because there are more unknowns than equations; hence, the designer will have to do some trial and error runs until an acceptable result is obtained.

The following is an example of design considerations and processes that would go into the development of an extended detention wet pond.

Example Problem Statement:
After considering several options, it has been decided that an extended detention wet pond is the preferred storm water management facility for an upcoming road project. The project is located in lower New Castle County (below the C&D Canal).

Some thoughts that went into selecting this facility type were the following:

Due to clay type soils, infiltration testing showed very low infiltration capacity.
The scope of the project, overall size, and additional new impervious along with changing up some drainage areas, smaller facilities would not be able to facilitate the additional runoff to the eventual outfall.
Since an infiltration BMP is not feasible and no other BMPs are proposed for this example, the pond must provide management of the RPv (Resource Protection or 1-year) storm event in addition to the Cv (Conveyance or 10-year) and Fv (Flooding or 100-year) storm events.
An extended detention dry pond could not be used because the final topography would not support a 2% slope across the entire pond bottom.

The benefit of extended detention is that by utilizing an undersized low flow orifice, it restricts stormwater outflow, so it backs up, becomes stored within the basin, and released at a slower rate. The temporary ponding enables particulate pollutants to settle out and reduces maximum peak discharge to the downstream channel, thereby reducing the effective shear stress on the banks of the receiving stream.

The following hydrologic conditions were computed using the HydroCAD program: (the point of analysis (POA) for the drainage area was selected early in the projects’ development)

Table 1. Subcatchment Data and Calculated Runoff

Hydrologic Condition	Area (ac)	CN	Tc (min)	Runoff Rate (cfs)			Runoff Volume (af)
Hydrologic Condition	Area (ac)	CN	Tc (min)	RPv*	Cv	Fv	RPv*	Cv	Fv
Existing	3	75	12	2.27	5.3	11.7	0.263	0.582	1.272
Proposed	6	81	20	4.37	10.0	20.2	0.654	1.427	2.895

* - The RPv storm event is used for qualitative management. The designer must change the Ia/S ratio in HydroCAD’s ‘Calculation Settings’ to 0.05 before performing analysis. The initial abstraction (Ia) represents all losses before runoff begins. Selecting 0.05 yields more accurate results for predicting the runoff from small, more frequent storm events as determined from DNREC’s Runoff Reduction Guidance Document.

The Cv and Fv storm events are used for quantitative management. The designer must ensure the Ia/S ratio in HydroCAD’s calculation settings is set to 0.20 prior to analysis.
Area (drainage area) - is computed from survey and topographic data. Ground truthing (actually visiting site) shall also be done to verify that the survey data is accurately showing the boundaries of the drainage area.
CN – curve number (dimensionless). The CN indicates the runoff potential of a given area as determined by a soil type and cover condition. Sometimes shown as RCN (runoff curve number).
Tc – time of concentration. This represents the time it takes for water to travel from one location to another within a drainage area. The Tc is the sum of all travel times for consecutive segments of the drainage conveyance system and shall start from the hydraulically most distant point. This could take multiple iterations to find the hydraulically most distant point.
Runoff Rate – the rate at which runoff is leaving the drainage area per storm event (denoted in cubic feet per second).
Runoff Volume – the amount of runoff leaving the site as denoted by volume of flow per storm event (this is a total volume over the entire drainage area).
All of these terms are further explained in detail in USDA TR 55.

In keeping with the above flow diagram, now complete the ‘Estimate Pond Storage’ step. What will be shown here as stated before is the quick way with HydroCAD. A designer should always have a basic understanding of the calculations involved as well as the inputs and assumptions made to evaluate whether an unusual result is a product of the assumptions or information that has been entered into HydroCAD.

Estimated Storage Volume:
Below is what the routing diagram will look like in HydroCAD (Routing 1). For details into what the input parameters are for the separate runs (‘Calculation Settings’), consult with the Stormwater section. Subcatchment node inputs are taken from Table 1. For simplicity, a free outfall is assumed. When doing a project, analyze downstream conveyance to make sure it is truly a free outfall or if tailwater* conditions will have an effect. For the post developed aspect, choose a catch basin, which will have ‘Insignificant Storage’. An outlet must be designated, so a round RCP pipe was initially chosen with an arbitrary diameter, inverts, length, Manning’s number, and entrance loss coefficient. * - Tailwater will encompass a calculated water surface elevation either due to a particular storm depth in a ditch/swale/stream, tidal influence, or if tying into an existing closed drainage system.

Routing 1.

Table 2. Estimate Storage

Storm Event	Estimate Storage * (ac-ft)
RPV** (1 yr)	0.166
Cv (10 yr)	0.395
Fv (100 yr)	0.610

* - To find the ‘Estimated Storage’, double click on the Pond node, a separate window will pop up. Click on the ‘Sizing’ button. Then click on the ‘Table’ button.

Under the first column ‘Desired Peak (cfs)’ find the value that closest corresponds to the existing runoff rate for the storm event being run. Now pick the ‘Required-Storage (acre-feet)’ that closest corresponds to the existing runoff rate and insert that number on the table above for each individual storm event.

** - The RPv must be done in a separate run where Ia/S = 0.05. This can be done in two separate ways. One – change the Ia/S value in the ‘Calculation Settings’ when performing separate runs or Two – have a separate HydroCAD file where the Ia/S values are different. If doing two separate files, remember that any changes beyond the Ia/S value change needs to be reflected in both files.

This ‘Estimated Storage’ corresponds to the amount of storage volume needed to attenuate the proposed inflow to the existing peak runoff value, i.e. storage above the permanent pool.

Now onto step 4 of the above flow diagram, the grading plan. As taken from the DNREC “Delaware Post Construction Stormwater BMP Standards and Specifications (Feb 2019)” as well as some DelDOT practices based on past experience, the following are some highlighted pond geometry criteria needed for this step:

Hydrologic Soil Group (HSG) A and some HSG B soils may require that the pond have a liner. Infiltration testing can help prove if a liner may be necessary. Keep in mind where the water table and seasonal high water table are located as well. Consult with the Stormwater section if need more information about infiltration testing and water table features.
A forebay shall be provided at each inflow that provides 10% or greater of the total RPv volume. The forebay volume can be included with the overall pond volume when it comes to doing the stage-storage calculation.
Forebays shall be sized to contain 10% of the volume of runoff from the contributing area to that forebay for the RPv storm event. The forebay shall have a 3’ maximum depth.
Minimum pond depth = 4’, maximum pond depth = 8’. Try to design for an overall depth of 6’ – 7’. A minimum of 50% of the pond area should have the minimum depth of 4 feet.
Side slopes may be no steeper than 3h:1v.
A 10’ wide flat aquatic bench will always be required 1’ below the permanent pool elevation. A 10’ wide safety bench is required 1’ above the permanent pool elevation unless the side slopes are 4h:1v or flatter.
Minimum width at top of slope/embankment = 10’.
Minimum width of access road = 15’, with a profile grade equal to or flatter than 10h:1v. The access road has to be able to reach the forebay(s) and outlet structure.
The total length of the pond flow path compared to the linear length from inlet to outlet, should strive to be a minimum ratio of 2:1. This helps promote an irregular shape as well as better overall sediment trapping capabilities. The ratio of the shortest flow path through the pond (due to an inlet located near the outlet) to the overall length shall be at least 0.5:1. The drainage area served by any inlets located less than a 0.5:1 ratio should constitute no more than 20% of the total contributing drainage area (CDA).
An earthen emergency spillway designed to convey the Fv shall be cut in natural ground or, if cut in fill, shall be constructed and stabilized with methods to prevent erosion and structural failure. An earthen emergency spillway is not required if using a weir wall configuration as it is built into the weir wall hydraulics. For earthen spillways, the emergency spillway invert shall be no higher than one (1) foot above the Fv storm event peak elevation.
The total storage should not extend more than 5 feet above the permanent pool unless specific design enhancements to ensure side slope stability, safety, and maintenance are identified and approved as per the Stormwater section.

Given all the information above, as well as taking into consideration the topography of the area where the pond will be located*, the designer can now develop a preliminary pond layout and grading plan. During the trial and error phase (step 7), this is one of the elements that may change up slightly. As seen from the flow diagram and from the undertaking here, it is much easier to change up the outlet structure components than to modify all the grading; hence, this facet can be changed if needed, but is realized as more of a secondary versus a primary change. * - Other features to keep in mind are: property/parcel boundaries, utilities, environmental (natural and cultural) resources, existing structures, potential cut/fill volumes, etc.

There are three ways to start an initial grading plan. It can be started from the top and work downwards to a desired depth, start at the bottom and grade upwards, or a combination of the two. The choice is going to depend on all the factors shown above as well as an ultimate invert outlet if grading a pond in cut (preferred) versus designing an embankment pond. Don’t forget to incorporate the bench(es) and forebay(s) into the grading plan. This can be one of the more challenging steps in figuring out with all the constraints, but a decision on which one to do could come down to the following:

Start at bottom and grade up if no major constrictions and outlet invert would be easily accessible, i.e. open field area that would drain to an exposed outfall. If doing a wet pond, start at an elevation which would provide a 6’-7’ depth to the pond outfall invert and make the bottom surface area equal to the RPv estimated storage area. Our example has 0.166 ac-ft, so discount the “ft” part and start at elevation 16 with 0.16 ac (rounded down from 0.166 for convenience). From there, keep working upwards with a 3h:1v slope, start incorporating the benches, and eventually grade to the top of the pond where there would be a 10’ wide area that encircles the facility (minimum top width). If the facility surface area at the top starts becoming too large, then start again by decreasing the bottom elevation surface area and grade upwards again. If still too large, maybe decrease the permanent pool height from 6’ to 5’ and start at elevation 17. Remember, this is all an iterative process until final design.
Start at the top and grade down if there are potential major constrictions at the highest water surface elevation or is needed to keep it inside a certain boundary. This will probably be the more common method as large open areas are not always available. Taking into account the 10’ wide access around the facility and keeping the ultimate water surface elevation at least 1’ below the edge of shoulder and etc., establish a starting elevation. Grade downwards from the starting elevation until reaching the desired depth. Remember, the estimated storage areas shown in Table 2 are above the permanent pool. When at the bottom of the facility, i.e., lowest elevation, check to make sure the criteria listed above is met. If not, then reevaluate.
The combination method, which might be the most practical, would begin with the pond outfall invert (or slightly higher) as basically the “starting line” and would also be the normal pool elevation. Working upwards from this line, look at the Fv storage needed from Table 2 (0.61 ac-ft). With knowing the storage needed (volume-wise), estimate a height that would likely be able to hold this volume. The height is going to depend on such things as, what is the invert of the incoming flow, is this elevation above the road edge, would that height cause the pond to become an embankment pond, etc. For this example, let’s say that it can have a 2’ depth with no real problems. That would now make the normal pool surface area roughly = 0.61 ac-ft / 2ft = 0.305 ac x 43560 sf /ac ~ 13,300 sf. From this number, plot out a rough length and width of the normal pool. Once this is established, grade upwards at a 3h:1v or flatter slope (if 4h:1v or flatter, then the safety (upper) bench is not required). With the upward grading, the original 2’ depth assumption will be less in reality because of the side slopes, i.e. the 2’ depth assumption was considering vertical sides. Also starting from the rough normal pool length and width, grade downward at 3h:1v, include the 10’ wide aquatic bench (1’ below the normal pool), then 3h:1v downward until you reach your desired depth of hopefully 6’ – 7’.

Now that an initial grading plan is completed, then an elevation – surface area table can be filled out. The surface area at each elevation should be able to be calculated by the designer’s CAD program.

Table 3. Elevation-Surface Area (this table is abbreviated, use as many rows as needed to complete entire facility)

Elevation (ft)	Surface Area (ac)	This information can then be input into Hydrocad. Right click on the Proposed Pond. Left click 'Edit'. On the 'General' tab, change the 'Pond Type' to 'Detention Pond'. Now click on the 'Storage' tab and then click 'Edit Storage' button on bottom right. Click on 'Custom Storage Data' and then click 'OK'. A 'Custom Stage Data' table should pop up and then copy the information from Table 3 into HydroCAD. None of the settings already marked should need changing, then click 'OK'. Also, under 'Edit', click on the 'Advanced' tab and enter the permanent pool elevation under 'Starting Elevation'. Click 'OK'.
16	Very Bottom Elevation
17
18
18.99	Use these two elevations to account for the 10' aquatic bench.
19
20
21

Outlet Structure Configuration (step 5):
Design of the principal spillway is completed to accomplish two primary objectives:

Control the peak flows so that proposed peak flows <= existing peak flows
Provide water quality benefits via extended detention (48 hr)

In order to accomplish these objectives, the principal spillway is usually designed in one of two different ways. One is to size a weir wall with assorted weirs and the other is to use an outlet structure (normally a standard size catch basin) that will usually contain an orifice and assorted weirs. A weir wall is the preferred option as it is easier to maintain, as well as stated before, the emergency spillway can be designed within the weir wall. Normally, the low flow portion will be handled by a V-notch weir (but could be an orifice) with assorted larger rectangular weirs above that. Favorable conditions for utilizing a weir wall include but are not limited to direct connection to an outfall (via usually a short run swale) and a moderately sized wall (sometimes the design has walls getting overly long and/or high). Discussions need to occur to discern when a weir wall would not be practical.

In areas where a weir wall is not feasible (as determined by the designer), then a multiple stage riser would be the next most likely design option. This option will involve an outlet pipe coming from the structure to a stabilized outfall along with the structure itself potentially having multiple orifices and weirs.

For our example, we will use a multiple stage riser, since it could be more complicated than a weir wall. Since the outlet structure is required to safely convey the Fv event (as per DSSR), we can first size the outlet pipe for the existing Fv peak flow. For simplicity, start at the pipe outfall location and set the invert at an elevation which would maintain it as a free outfall, i.e. no or negligible tailwater conditions. Next, look at the location where the outlet structure would be located and determine a length and slope. Most all outlet pipes will be round reinforced concrete, but other shapes (i.e. elliptical) and types of material may have to be used depending on site conditions. Again, this is a designer decision. There are multiple ways to determine the proper pipe size. For this example, use HydroCAD to determine the size. Going back to Routing 1, add a pond node (catch basin with no storage) downstream of the existing drainage area {remember we are just sizing a pipe for the existing Fv peak flow}.

Routing 2.

Assume free/open channel flow (non-pressure), no tailwater, a Ke factor of 0.20 (groove end with headwall), Manning’s number of 0.12, pipe length of 48’ {tip: try to use whole sections of pipes for a length when possible, RCP comes in 8’ lengths}, outlet invert of 15, and inlet invert of 16. This information will all be entered under the ‘Outlet’ tab of the Pond node (right click ‘Proposed Fv Pipe’ pond node, then left click ‘Edit’).

Left click ‘Edit Outlet’, select ‘Culvert’, then click ‘OK’

Input the above parameters for this example, but the designer’s project parameters, most especially the inverts and length will be different. Under ‘Diameter’, just put 15” as that would be an absolute smallest size for our purposes. Click ‘OK’

Click ‘OK’ on ‘Edit Pond Pipe’ window. Left click once on the ‘Proposed Fv Pipe’ pond node (its border will be bolded/highlighted). Now click on the ‘Calculation Settings’ button up top and go to the ‘Resize’ tab. Click on the ‘Catch Basin Culverts’, set the ‘Maximum Elevation’ to ‘Culvert Crown or Pipe Full’, change the ‘Size Increment’ to 3 and the ‘Minimum Size’ to 15. Click ‘OK’. HydroCAD will automatically calculate the proper size pipe needed for that node only. Now we have the principal spillway pipe size (Go back to ‘Edit Outlet’. New pipe size will be shown under ‘Diameter’.) Use this information to update the ‘Proposed Pond’ outlet.

In an extended detention facility for water quality treatment, the RPv storage volume is detained and released over a specified amount of time (48 hours as per the DSSR). The release period starts with inflow into the facility until the entire calculated runoff volume drains out of the basin. The extended detention outlet (orifice) can be sized using multiple methods, but what is shown here will use multiple HydroCAD runs to let it do all the work versus calculating on paper.

Calculate an average discharge rate by dividing the total runoff value (convert af to cf) by 48 hours and convert to cfs.

Calculate a maximum allowable discharge (MAD) of 5x the average discharge rate.
The HydroCAD file for the actual wet pond should be set up now with an initial grading for the facility and an outlet structure with the primary outfall. That was the pipe size determined above.
Now add on a 3” orifice on the structure at an invert elevation set to the normal pool for the wet pond. Remember only the RPv is being calculated at the moment, so make sure the Ia/S value is set to 0.05 in the ‘Calculation Settings’.

Double-click the pond outlet and click the ‘Summary’ tab on the screen that popped up.
Compare the “Primary” rate to the MAD rate from Step 2. Also, double check the ‘Peak Elev’ to make sure that it is a reasonable height within the pond. NOTE: the peak elevation shown here is super high because this is still being looked at as a catch basin, i.e. an elevation - surface area table was not completed for this example.

If the ‘Primary’ rate is higher than the MAD number, then go back into your pond outlet node, make the orifice smaller, and rerun. If the ‘Primary’ rate is smaller than the MAD number, then make the orifice bigger.
Keep redoing step 7 above until there is an orifice size that closely corresponds the ‘Primary’ and MAD numbers. These do not have to exactly match but should be really close. Plus, make sure to mind the significant digits for the orifice size. A 3.176842” orifice size would not make any sense from a constructability standpoint. A realistic orifice size should not go beyond the tenth’s digit.
To double check to make sure that 48-hr detention is achieved, do the following:

After coming up with an acceptable orifice size (from Step 8 above)
Double click the pond node to bring up the hydrograph.
On the right side bottom, click on ‘Table’ and click off ‘Shrink’.
This will bring up an expanded table. Take note of the time when the ‘Inflow’ first starts and the time when the ‘Primary’ ends. The amount of time in between is the total detention time, for which 48 hours is the minimum needed. Longer than 48 hours is okay.

If not achieving the required 48 hour extended detention, then go back and make the orifice smaller until the minimum 48 hour time limit is obtained.

Now that the “low flow” orifice (or maybe even a V-notch weir) is completed, the Cv and Fv outlets can be computed. At this stage, the designer has two important parameters to keep in mind. One is post developed peak discharge when comparing to the predeveloped peak discharge. The second is to be mindful of how much storage volume is available in the pond, i.e. keep an eye on the elevations. The Cv weir opening invert will start at the maximum RPv peak elevation (shown on the RPv hydrograph). The Cv weir dimensions can be determined very similar to the RPv process (note: Ia/S value should be set at 0.20). Keep trying different weir dimensions in the structure until the primary rate is at or below the “Existing Runoff Rate for Cv” as shown in Table 1. Once you have the Cv weir dimensions set, start the same type process for the Fv. The Fv weir opening invert will start at the maximum Cv peak water surface elevation. The Fv could be an extension of the Cv weirs to the top of the structure or the Cv could be an enclosed weir, with the Fv relying more on the top of the structure being open. Again, this is all an iterative process that depends on different factors and no one size fits all. As part of the iterative process, a larger structure might have to be designed to obtain a certain weir length needed. Although not required by the DSSR, it is highly preferred that the Fv be completely drained within 72 hrs. That can be checked on the ‘Table’ similar to how the RPv 48hr extended detention was checked, except now there should be 0 cfs flow after the 72 hr mark. So, short summary is that the RPv will probably just be an orifice (or v-notch), the Cv will most likely be one or more weirs in the sides of the structure and the Fv could be an extension of those weirs and/or rely more on the top of the structure being open. In referencing back to the original flow chart, if the outlet structure design is not working with other topographic aspects, then a resizing of the initial grading plan may be necessary.

At this point, the overall pond design should be closely set and other final design procedures / checks can begin. This would include, but not be limited to: forebay volume checks, final outlet structure design (amount of reinforcing, base material specs, etc.), trash racks, phreatic line, possible addition of anti-seep collars, foundation cut off core trench, sequence of construction, pond liner, Class A bedding for an outlet pipe, etc.

Embankment ponds are sometimes necessary to be able to impound water properly within a given topography. They could have an embankment built up around the whole facility or sometimes just one side of the facility needs this feature. A facility is considered an embankment pond if the depth of the water impounded against the embankment at the spillway elevation is 3 feet or more. The depth of water impounded is defined as the difference between the spillway inlet elevation (normal pool elevation) and the channel elevation at the point of discharge. In laymen’s terms, if the normal pool is 3 feet or more above natural ground, i.e. material had to be brought in to build up (embankment) the area needed to hold the permanent pool, than this facility would now be considered an embankment pond. For this situation, a foundation cut off core trench will need to be installed wherever the embankment exists. Information and details concerning the cut off core trench can be obtained from the Stormwater section.

A phreatic line is defined as a top flow line of a saturated soil mass below which seepage takes place. Hydrostatic pressure acts below the line whereas atmospheric pressure exists above the phreatic line. This line should be shown on the plans for any outlet structure that has a pipe outfall through an embankment and will help determine if anti-seep collars are required. The phreatic line is determined by projecting a line at a slope of 4h:1v from the point where the normal pool meets the upstream slope (near the outlet structure) and would go down through the pipe to the invert. Anti-seep collars would be required in this saturation zone of embankment to help prevent unwanted seepage along the pipe. For anti-seep collar placement, sizing, and spacing, contact the Stormwater section for further details.

References

Runoff Reduction Guidance Document (Hawkins Abstraction)

Delaware DNREC Runoff Reduction Guidance Document

Delaware Sediment and Stormwater Regulations Feb 2019

Delaware Stormwater Management Design Manual Draft for Public Comment, Module 1 August 4, 1993

USDA – NRCS - Agriculture Handbook Number 590 Ponds – Planning, Design, Construction

USDA – NRCS - National Engineering Handbook, Part 630, Hydrology Sep 1997

Georgia Stormwater Management Manual, Volume 2: Technical Handbook 2016 Edition

Virginia DOT BMP Design Manual of Practice April 2013

New Jersey Stormwater Best Management Practices Manual, Chapter 5 Feb 2004

USDA – NRCS – SCS TP 149 – A Method for Estimating Volume and Rate of Runoff in Small Watersheds Revised Apr 1973 (WARNING: Clicking this link will download a PDF from NRCS)

USDA – NRCS – Technical Release 55 June 1986

USDA – NRCS – SCS – TR 20 Computer Program for Project Formulation Hydrology Feb 1992

DNREC – Delaware Post Construction Stormwater BMP Standards & Specifications Feb 2019 (WARNING: Clicking this link will download a PDF from DNREC)

DNREC – Delaware Urban Runoff Management Model (DURMM) v2.51 Excel Spreadsheet (WARNING: Clicking this link will download a PDF from DNREC)

USDA – SCS – Pond Code 378 May 2011

HydroCAD Stormwater Modeling System, Owners Manual, Version 10 (Jul 2011)

Bioretention Area Design Guidance

The purpose of this design guidance is to aid the designer that does not regularly design bioretention area facilities. For further details beyond what is provided, refer to the references at the end of this document which shows sources used to help create this guidance. This guidance is meant to be a good starting point, but leaves some gray area as not all design scenarios can be covered. For more in-depth help and analysis please consult with the Stormwater Section. It is presumed that the designer is familiar with the basic theory and methods of analysis and design in both hydrology and hydraulics. Also, the following information will rely heavily on HydroCAD and it is suggested that the designer be familiar with this stormwater modeling program to better utilize this document.

The design of a bioretention area involves essentially the same aspects of pond design at its heart, but has the added aspect of filtering (and sometimes infiltrating) the runoff before it is released (or infiltrated back into the surrounding soil).

In a flow diagram form, the basis of bioretention area design is roughly:

As can be seen from the above flow diagram, bioretention area design can be an iterative process; hence, the designer will most likely have to do some trial-and-error runs until an acceptable final result is obtained.

This document will not go in-depth concerning exploratory studies for infiltration feasibility as that can be found elsewhere. A bioretention area bottom elevation is presumed to have already been established as per exploratory studies, which again can be found elsewhere. For more in-depth help and analysis of infiltration feasibility, please consult with the Stormwater Section.

The following is an example of design considerations and processes that would go into the development of a bioretention area.

Example Problem Statement:

After considering different options, it has been decided that a bioretention area is the preferred storm water management facility for an upcoming intersection upgrade project. The project is located in Kent County.

Some thoughts that went into selecting this particular facility type were the following:

Infiltration testing showed unacceptable infiltration rates as per the “Delaware Sediment and Stormwater Regulations” (DSSR) under “Soil Investigation Procedures for Stormwater Best Management Practices”.
Due to the scope of the project, overall size, and additional new impervious, it was estimated that a bioretention area would be able to effectively mitigate the water quality requirements while also providing some limited water quantity mitigation.
Due to right of way constraints and a smaller overall drainage area, larger facilities, i.e. ponds were not feasible.

The following hydrologic conditions were computed using the HydroCAD program: (the point of analysis (POA) for the bioretention area drainage area was selected during the exploratory studies phase)

Table 1. Subcatchment Data and Calculated Runoff

Hydrologic Condition	Area (ac)	CN	Tc (min)	Runoff Rate (cfs)			Runoff Volume (af)
Hydrologic Condition	Area (ac)	CN	Tc (min)	RPv*	Cv	Fv	RPv*	Cv	Fv
Existing	1.0	76	8	1.00	2.47	5.50	0.095	0.222	0.499
Proposed	1.75	82	14	1.67	3.99	8.18	0.206	0.470	0.980

* - The RPv (1yr) storm event is used for qualitative management. The designer must change the ‘Ia/S Ratio’ in HydroCAD’s ‘Calculation Settings’ to 0.05 before performing analysis. The initial abstraction (Ia) represents all losses before runoff begins. Selecting 0.05 yields more accurate results for predicting the runoff from small, more frequent storm events as determined from DNREC’s Runoff Reduction Guidance Document.

The Cv (10yr) and Fv (100yr) storm events are used for quantitative management. The designer must ensure the ‘Ia/S Ratio’ in HydroCAD’s calculation settings is set to 0.20 prior to analysis.
Area (drainage area) - is computed from survey and topographic data. Ground truthing (actually visiting site) shall also be done to verify that the survey data is accurately showing the boundaries of the drainage area.
CN – curve number (dimensionless). The CN indicates the runoff potential of a given area as determined by a soil type and cover condition. Sometimes shown as RCN (runoff curve number).
Tc – time of concentration. This represents the time it takes for water to travel from one location to another within a drainage area. The Tc is the sum of all travel times for consecutive segments of the drainage conveyance system and shall start from the hydraulically most distant point. This could take multiple iterations to find the hydraulically most distant point.
Runoff Rate – the rate at which runoff is leaving the drainage area per storm event (denoted in cubic feet per second).
Runoff Volume – the amount of runoff leaving the site as denoted by volume of flow per storm event (this is a total volume over the entire drainage area).
All of these terms are further explained in detail in USDA TR 55.

Routing 1. Example of HydroCAD routing diagram with appropriate hydrographs used to fill in results for Table 1.

In keeping with the above flow diagram, now do the “Estimate Bioretention Storage” step. A designer should always have a basic understanding of the calculations involved as well as the inputs and assumptions made to evaluate whether an unusual result is a product of the assumptions or information that has been entered into HydroCAD.

Estimated Storage Volume:
Below is what the routing diagram will look like in HydroCAD (Routing 2). Subcatchment node (1P) input is taken from Table 1. One of the more common outfalls for a bioretention area is a catch basin which will be used for this example problem. Obviously, a multitude of outfall types could exist, so make sure to model appropriately. When doing any project, analyze downstream conveyance to make sure if it is a free outfall or if tailwater* conditions will have an effect. For Routing 2, the outflow of the catch basin will go into a pipe which in turn outfalls into a swale. It is again assumed that the swale will not have any tailwater effects, i.e. free outfall and any tailwater effect of the swale itself on the pipe and catch basin will automatically be calculated by HydroCAD via changing the ‘Reach Routing Method’ under ‘Calculation Settings’ to ‘Dyn-Stor-Ind’. The Dyn-Stor-Ind method does not currently allow reaches to respond to tailwater changes, it does allow the overall watershed to be analyzed in a dynamic manner so that the bioretention area may respond to tailwater effects. For further discussion on the HydroCAD Reach and Pond Routing Methods, please refer to the HydroCAD manual.

* - Tailwater will encompass a calculated water surface elevation either due to a particular storm depth in a receiving ditch/swale/stream, tidal influence, or if tying into an existing closed drainage system.

Routing 2.

An easy way to calculate the estimated storage for a starting point is to start with the end product of total volume needed for the post-developed RPv runoff. From Table 1, RPv runoff volume = 0.206 af. Convert this number to cubic feet (cf) = 0.206 af x 43,560 sf/ac = 8,973 cf. From the exploratory phase, the trench bottom elevation was already determined to be at EL = 12’. The top of the trench will basically be determined from two parameters, the minimum depths of stone and biosoil required for a bioretention area (as per the DSSR) and the project’s grading operation. For this example, use EL = 17’, which makes the total bioretention area depth equal to 5 ft. (NOTE: as per the DSSR, the biosoil mix depth needs to be 2 ft minimum and the stone layer must have a 2 ft sump below the underdrain as well as a 3 in stone layer above the underdrain. The underdrain itself is a 6 in diameter pipe. That would make the minimum top of the biosoil be at EL = 16.75’, but was rounded off to EL = 17’ for easier constructability purposes.) All that is left is the preliminary length and width dimensions as the bioretention area is basically an elongated cube, design-wise. From a landscape architecture aspect, bioretention areas should flow with the surrounding contours and not be perfect rectangles, but rather elliptical or curved shaped. The width can be determined usually by what is allowable within right of way for that particular project area, so assume an allowable width of 25 ft to start. The unknown preliminary length is calculated by: volume / (depth x width) = 8,973 cf / (5 ft x 25 ft) = 71.78 ft ~ for simplification just round off to 72’.

Here is a rough sketch of what the post-developed area will look like:

Design Storage Volume
Now input these parameters within HydroCAD under the “Bioretention Area” node designated “1B”. Right-click “1B” and choose ‘Edit’. Now click ‘Detention Pond (or other storage area)’

Now click on the ‘Storage’ tab and double click on the first row, which will bring up the ‘Select New Storage Type’ window. Double click on the ‘Custom Stage Data’ which will bring up a new screen ‘Custom Stage Data Storage’. Click on the ‘Stage Voids’ box. Input the storage data for the bioretention area and for the open area above it. Start at the bottom elevation and work upwards. The void ratio for the bioretention area is set to 40% for both the stone and the biosoil mix areas. The surface area above the bioretention area is 100% void space. Note: the ‘Allow Exfiltration’ checkbox automatically activates when “exfiltration” is entered under the ‘Outlets’ tab.

For the ‘Custom Stage Data Storage’, since this example has an underdrain, that is the starting point for the storage area as the stone layer beneath the underdrain cannot be counted as per DNREC technical guidance. If the bioretention area were an infiltration facility, i.e. no underdrain, then the starting point would be the invert of the facility itself. Even though an underdrain is being utilized, the 2’ stone layer below the underdrain is still required in the overall facility’s construction. Also, there is a three-inch (3”) stone layer required to be above the underdrain; hence, that will need to be shown on the plans, but does not necessarily need to be called out specifically in this file. The reason for not needing to show that separation line is that the stone and biosoil mix are each designed with a 40% void ratio. The easiest way to think about it is to start where the runoff would outlet, i.e. underdrain or facility invert. In HydroCAD, the first row is required to have a 0% void space.

Outlet Design:
Now click on the ‘Outlets’ tab and double click on the first row, which will bring up the ‘Select New Outlet Device’ window. Referring back to Sketch 1 when thinking through the inlets and outlets for the catch basin in this example, the pipe flowing into the swale would be the overall basin outlet (primary). The open area on top of the basin flows to the pipe and the underdrain also flows to the pipe via an orifice on the side of the catch basin. The flow rate (exfiltration) going through the biosoil mix is set at 2.83 in/hr as per the DSSR*. Also, as per DNREC guidance, the maximum ponding depth above the surface layer of the biosoil mix is 12” for the RPv (18” for the Cv and 24” for the Fv).

The routing under the ‘Outlets’ tab would be:
Device #1 = Pipe Outfall / Culvert (Routing = Primary)
Device #2 = 6” vertical orifice at EL = 14’ (Routing = Device #1)
Device #3 = Exfiltration of 2.83 in/hr at all elevations (Routing = Device #2)
(Note: For ease of routing, start the elevation from the underdrain invert even though the biosoil mix doesn’t start till 3” above the top of the underdrain.)
Device #4 = Open top of catch basin at EL = 18’ (Routing = Device #1)

*NOTE – If this were an infiltration facility (no underdrain), the lesser of 2.83 in/hr or the design infiltration rate should be used and obviously no orifice in the side of the catch basin. In HydroCAD, infiltration, and exfiltration mean the same thing. Click on ‘Exfiltration’ and input the design infiltration rate (not the field measured rate) as determined from the exploratory studies. The routing automatically defaults to ‘Discarded’, which means the runoff has been infiltrated and is not subject to any further routing in the model.

Below is what the ‘Outlets’ tab would look like when using constant velocity exfiltration followed by screenshots for each outlet device. The 24in pipe diameter outlet was randomly chosen for now. The top of the catch basin is based on a standard-sized catch basin (for simplicity). A method for sizing the outlet pipe with HydroCAD will be shown later and the catch basin size can be changed for quantitative management if needed.

Another outlet needs to be added that would potentially handle any overflow of the bioretention area that could not be handled by the catch basin or if the catch basin and or pipe were to become clogged. This overflow would go directly into the swale. This is commonly referred to as an emergency spillway. Since the maximum ponding guidance for the Fv is 24 in, the invert will be set at EL = 19’. The dimensions of the spillway should be what would best fit within the grading of the area to direct flow to the swale. This example will use the spillway dimensions of a 3 ft weir length, 3:1 side slopes, and 1 ft depth. Screenshots are shown below:

The reason that the emergency spillway is being routed to ‘Secondary’ is that it would flow to the downstream swale directly and not through the outlet pipe, which is designated as the ‘Primary’. When the secondary routing is chosen, a red circle will appear below 1B (the ‘Bioretention Area’ triangle). Left-click and hold on top of the red circle and then drag that to the 1S ‘Swale’ and then release. HydroCAD will now route the emergency spillway flow directly to the swale in the model.

Right-click on node ‘1S’ and select ‘Edit’ to input the outlet swale dimensions. Then update the necessary information on the ‘General’, ‘Section’, and ‘Profile’ tabs as shown below.

For sizing the outlet pipe utilizing HydroCAD, this step could be done pretty much at any time because it involves only using the existing drainage area node ‘1E’. Refer to Routing 3 to see the setup.

Route 3. Determining outlet pipe size

First, open the ‘Calculation Settings’ and change the routing method to ‘Stor-Ind’. This pipe sizing method only works when using ‘Stor-Ind’. After this overall procedure is completed, go back and change to ‘Dyn-Stor-Ind’. Also, make sure the rainfall event is set to the ‘100-year’.

Right-click on the ‘Pipe’ node and select ‘Edit’. Check the ‘Catch Basin (or pond with insignificant storage)’ box.

Click on the ‘Outlets’ tab, double click the first row and select ‘Culvert’ on the next box and then click ‘OK’

Fill out the appropriate information for the outlet pipe and choose a 15” RCP, since that is the minimum recommended size as per the DelDOT Road Design Manual, then click ‘OK’.

Left-click on the ‘Pipe’ node once (that node will now have a bolded border). Open the ‘Calculation Settings’ and click on the ‘Resize’ tab. Click on the ‘Selected Nodes’ box. Set the ‘Size Increment’ to ‘3’ and the ‘Minimum Size’ to ‘15’. Then click ‘OK’.

Right-click on the ‘Pipe’ node and select ‘Edit’. Go to the ‘Outlets’ tab and double-click on the 1st row. The pipe diameter has now changed to what it can safely handle for an existing 100-year (Fv) storm event. This is the size that will need to be updated in the ‘Bioretention Area’ node ‘1B’.

Open the ‘Calculation Settings’ and change the routing back to ‘Dyn-Stor-Ind’. Update the culvert size in node ‘1B’ to 18”.

Bioretention Routing:
For the RPv (Ia/S = 0.05), double click “1B” and the following hydrograph will appear:
(Note: the ‘Time Span’ under ‘Calculation Settings’ was changed. ‘Start Time’ = 1.00, ‘End Time’ = 48.00, and ‘Time Increment’ = 0.01)

As can be seen here, almost the whole RPv runoff volume is filtered and emptied within 48 hours, which means the stormwater quality component of the DSSR has almost been met. Looking at the hydrograph, the peak elevation is just above the invert of the catch basin overflow. Granted, the height is just barely above the catch basin top invert, but in order to stay within the DNREC guidance of a maximum of 12 in of ponding for the RPv, the facility will be made slightly larger (i.e., increase the storage component) to help bring the peak elevation down. This could be accomplished a couple of different ways, (i.e., making the whole facility larger or maybe just expanding the part of the facility above the biosoil mix area) but it is the designer’s prerogative on how this should be accomplished and will most likely depend on right of way and grading characteristics. For this example, the whole facility will be made slightly larger, so the ‘Custom Stage Data Storage’ will have to be recomputed and reentered. The new hydrograph with the updated storage information is shown here. Notice the storage is larger than previous as well as the peak elevation is below the top of the catch basin invert. This was just the first larger iteration tried, but at times multiple iterations may have to be done; hence, part of the trial-and-error process as mentioned in the very beginning. The dimension of the biosoil mix area has increased from 72’ x 25’ to 80’ x 30’. Since this project is also trying to achieve some water quantity reduction by having the post-developed flow rate be less than or equal to the existing flow rate, the top of the catch basin invert will be moved up to EL = 18.5’. This is the maximum ponding allowed for the Cv. If water quantity does not need to be abated, then the top of the catch basin could be placed just above the RPv water surface elevation at EL = 17.6’.

Now change the Ia/S value = 0.20 and compare the ‘Primary’ component (the rate of flow that will leave via the swale) to the existing Cv and Fv runoff rates. For the Cv, the peak elevation is slightly above the DNREC recommended 18” of ponding, as well as the outflow rate, is less than the existing Cv.

For the Fv, the peak elevation is below the DNREC recommended 24” of ponding, but the outflow is greater than the existing Fv. As per the DSSR, the bioretention area has to be infiltrated or drained for the Fv within 72 hours and this parameter is fulfilled as shown on the hydrograph table. Since the Cv and Fv are showing some parameters slightly greater than required and recommended, the facility and/or the outlet structure could be resized and recomputed, but at this point it would be best to share the findings with the Stormwater Engineer and discuss. If the Cv and Fv limitations were to be compliant with the elevation, outflow, and time to drain, then proceed forward with the next steps.

At this point, the overall bioretention area design should be closely set and other final design procedures can begin. This would include, but not be limited to: construction details, pretreatment areas, cleanout / observation port(s) placement, sequence of construction finalization, outlet pipe scour protection, etc.

References

Delaware DNREC Runoff Reduction Guidance Document

Delaware Sediment and Stormwater Regulations Feb 2019

USDA – NRCS - National Engineering Handbook, Part 630, Hydrology Sep 1997

USDA – NRCS – Technical Release 55 June 1986

DNREC – Delaware Post Construction Stormwater BMP Standards & Specifications Feb 2019

HydroCAD Stormwater Modeling System, Owners Manual, Version 10 (Jul 2011)