Restoring Runoff to Pre-developed Conditions:
Fact or Fiction?
By Michael K. Glazner
Executive Director of Haestad Methods, Inc.
January 2001
Abstract
Many areas in the United States have established stormwater detention pond ordinances that require
the post-developed outflow from a site to discharge at a rate equal to or less than the pre-developed
peak flow rate. This may sound like restoring the site to its pre-developed runoff state, but is that
really the case?
This paper discusses the impacts of increased stormwater runoff volume due to development, and
reviews how this phenomenon may impact downstream areas. An example progresses throughout the
different sections of this article to demonstrate these concepts. For simplicity, only one rainfall event
(24-hour, 8.636 cm or 3.4 inches) is reviewed.
Example Scenario
Figure 1 shows the pre- and post-developed (not detained) hydrographs computed for a 24-hour,
8.636 cm (3.4 inch), rainfall event. These hydrographs were calculated using the SCS Unit
Hydrograph Method and the SCS Type II, 24-hour rainfall distribution.
Pre-developed Data: Area = .80937 km (200 acres);
Post-developed Data: Area = .80937 km (200 acres);
SCS CN = 61;
SCS CN = 85;
Tc=0.75 hours
Tc=0.50 hours
Figure 1: Pre- and Post-developed Hydrographs
To conform to a peak outflow criteria, the engineer must design a detention pond that restricts
the maximum post-developed outflow to the pre-developed peak rate. Figure 2 compares the
pre-developed hydrograph with the final pond design’s post-developed pond inflow and outflow.
Based on the pre-developed peak flow criteria, this pond would pass review for this 8.636 cm (3.4
inch) rainfall event, even though the post-developed runoff volume increased 365%.
Figure 2: Review of Final Design
The Difference: Pre vs. Post
Even if the peak outflow is attenuated to match the pre-developed peak, at least three differences
typically occur:
1) The total volume of outflow is increased. In this example, the post-developed outflow
volume was more than three times the pre-developed total.
Volumes for this example:
Pre-developed Runoff = 10,865 cubic meters ( 8.81 acre-ft)
Post-developed Inflow = 39,668 cubic meters (32.16 acre-ft)
Pond Outflow = 39,668 cubic meters (32.16 acre-ft)
2) The time to peak is shifted (earlier or later). In this example, the peak flow rate from the
pond is delayed .75 hours compared to pre-developed conditions.
Times to peak (Tp) for this example:
Pre-developed Tp = 12.40 hours
Pond Inflow Tp = 12.20 hours
Pond Outflow Tp = 13.15 hours
3) Increased volume causes a pronounced receding limb on the pond outflow hydrograph.
Figure 2 shows that the engineer successfully attenuated the peak outflow rate. However, what about
the shape and volume of the outflow hydrograph? Note that the overall shape and volume of the predeveloped and pond outflow hydrographs are different. In this example, the post-developed volume
was more than triple the pre-developed volume!
The detained post-developed hydrograph is shifted in time and has an overall greater volume, which
results in a pronounced receding limb compared to the pre-developed hydrograph. Therefore, the
attenuated pond outflow hydrograph is more likely to coincide with other downstream higher flow
levels than did the pre-developed conditions.
Therefore, even though the pond may be mitigating peak rates immediately downstream of the site, it
may not be effective in terms of the overall regional watershed peak rates. The true downstream
impacts are better approximated by including the pond design site as a subset of an overall larger
model of the watershed.
The Impacts
Perhaps the most crucial difference in post-developed hydrographs is the increase in total runoff
volume due to development. Figure 3 shows graphically that the pond’s outflow volume is
significantly more than the pre-developed runoff hydrograph.
Figure 3: Increased Runoff Volume
For most detention ponds (ones that completely drain after a storm), all of the increased volume is
eventually passed downstream. Therefore, the pond mitigates the peak discharge effect, but not the
increase in volume.
From Figure 3, we can see the level of outflow is above pre-developed rates for almost the entire
storm. Therefore, the downstream channels are encountering higher depths of flow for longer periods
of time, even if technically not “flooding.”
Some effects of increased runoff volume:
 Prolongs rise in water surface downstream, which may affect the slope stability of channels.
 The receding limb of the pond outflow hydrograph is prolonged, which affects the way the
hydrograph will add up downstream, changing the downstream flow rates compared to predeveloped conditions.
 The increase in volume represents the amount of groundwater recharge that is no longer being
absorbed for that size event (the runoff is now being discharged into the downstream
conveyance system).
 The additional runoff increases the total volume feeding into existing (and future)
downstream detention ponds.
The Solution
The ultimate solution (outside of zero development) in terms of mitigating the increase in runoff
volume downstream would be to design a pond that yields an outflow hydrograph of the exact shape
and volume of the hydrograph for pre-developed conditions. That would ensure that the water
flowing from the site is impacting downstream reaches in a fashion identical to pre-developed
conditions.
A variation of this idea would be to use on-site detention to mitigate the initial peak discharge rates
and capture the initial runoff pollutants. Then, use regional detention to hold back the increased
volume in water for the overall watershed, while also reshaping the outflow hydrograph to closely
match the pre-developed state for the overall watershed. Perhaps these regional ponds could be
incorporated in parks and golf courses and even be used for irrigation if the water quality is good
enough.
A Tall Order
Think carefully before rushing in to vote for a new drainage ordinance that requires the postdeveloped outflow hydrograph to exactly match the pre-developed hydrograph in shape and volume.
Here are just a few of the difficult challenges that are involved with accomplishing this concept:
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It will be very time-intensive to design grading plans and channel slopes to decrease or
increase time of concentration and reach routing times so that the time to peak on the routed
pond outflow matches the pre-developed time to peak.
The shape of the outflow hydrograph (as well as the peak) will have to be considered when
designing outlets for the pond.
The most common way to keep the additional volume from releasing downstream is to hold it
on-site. This may mean a significant increase in required storage, with the ability to evaporate
or infiltrate the additional volume back into the groundwater.
If the increased volume is handled on a per-lot basis, underground infiltration may cause “wet
spots,” or localized drainage problems in adverse soils.
Extra safety precautions may be necessary because of permanent pools if the increased
volume is held on-site via a wet pond or separate evaporation and infiltration pond.
If this additional volume is infiltrated back into the groundwater, care must be taken not to
introduce stormwater pollutants into the groundwater.
Even if the pre-developed hydrograph shape is matched, the conditions may change from predeveloped non-point (sheet flow) outflow to a single discharge point (pond outlet structure).
The Impossible Dream?
In practical terms of today’s design and construction practices, it would be very difficult, if not
impossible, to achieve a perfect match to the pre-developed state. There are many complex issues,
including water quality (surface and groundwater), that interplay with this idea and must be
considered before establishing drainage criteria. Until we collect much more research data, this
hydrograph matching concept is more like a “compass” that points in the direction we may hopefully
one day achieve.
Research Needed
Here are just a few research areas that could help provide engineers with the necessary tools for
better detention design:
 Establish much more “life-like” rainfall distributions that better model the real world for both
pre- and post-developed conditions.
 Understand the effects of stormwater recharge on groundwater quality infiltration (through
drainage surfaces, ponds, infiltration chambers, etc.).
 Study the environmental impacts of using detention water for irrigation purposes.
 Develop new ideas for detaining water on a lot-by-lot basis to better emulate pre-developed
groundwater recharge throughout the site.
 Find new methods for removing storm water pollution from detention so water quality
matches the pre-developed state.
 Study impacts of stormwater pollution on clogging soil pore space, which affects the
infiltration rates through a pond bottom.
 Analyze groundwater mounding effects on detention.
 Develop better outlet rating curve data for typical outlet structures.
 Design new outlet devices that can help shape the outflow hydrograph for different types of
storms.
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Find new ways of shaping sites to better emulate the runoff response times of the predeveloped conditions.
Establish tolerance limits for determining if the pre-developed and post-developed
hydrograph shapes match “closely enough” by performing a curve fit test, ordinate by
ordinate, to determine if the overall hydrographs match within an acceptable range.
Executive Summary
This paper discusses how the current approach for using the pre-developed peak flow rate as the
criteria for post-developed target peak outflow rate does not address the issue of increased runoff
volume due to development. An increase in developed runoff volume can have significant impacts
downstream. This phenomenon is discussed in the following ways:
 The current state-of-practice for detention pond design is typically to attenuate the postdeveloped runoff rate to be less than or equal to the pre-developed peak flow rate. The change
in runoff volume is usually not considered.
 Unless post-developed stormwater is held on-site, there will usually be an increase in the total
runoff volume that is passed downstream of the pond, which may result in adverse effects.
 A duplication of the pre-developed state can only be accomplished if the post-developed
outflow hydrograph matches both the volume and shape of the pre-developed conditions.
 Without significant research, the concept of closely matching pre-developed and postdeveloped conditions may not be possible in the near future.