DRAINAGE INVENTORY AND GENERAL RECOMMENDATIONS REPORT
The information for this inventory was gathered between of August, 2015 and March of 2016. Many aspects of this and other Inventories will be on-going as major features and minor elements of the landscape are changed through the enactment of planning. Of drainage, this will be particularly so, and this inventory will be updated regularly. All areas covered by these inventories will be at least partially re-surveyed on an annual basis, and special attention will be given to how even the slightest change in some aspect of the landscape impacts drainage. The term “survey” typically refers to a non-instrument pedestrian review of site features. Existing drainage maps, drain pipe inventories, and other documents have been used as reference; and the current version of a map showing storm drain locations is included as part of this report.
Recommendations made herein apply to the site conditions at time of survey, and should be verified immediately prior to enacting any drainage solution.
After completing inventories for the wooded areas, common area landscaping, and buildings, it is now hard to imagine when the surface drainage across the entire Parkfairfax community ever functioned completely effectively. The originally extensive system of drain pipes, swales, and slopes has been expanded and improved, but the expansion and improvement has not kept pace with the impedances. The system was obviously designed suchthat water could be effectively conveyed away from the buildings and use areas and toward the network of public storm drains that eventually carries it to Four Mile Run. When one considers the myriad of interruptions and alterations to that conveyance, however, the drain pipes, swales, and slopes have been rendered ineffective to only moderately effective.
Drainage is one of the functions of the infrastructure of Parkfairfax that blurs the lines between public space and private space. Each building has been equipped with at least a minimum number of downspout extensions that carry water out as far as the nearest (clearly defined) common area where they outfall; or they tie into existing drain pipes and yard inlets. Grading, which is closely tied to the function of drainage, often overlaps the line between common area and private area.
When identifying issues with the drainage, one cannot ignore the impact of the residents themselves as they modify grades near their units, build terraces and walkways, install gardens of every type, and even store materials and belongings in various outdoor spaces.
Solutions to drainage issues will have to account not only for the impact of the residents, but for their impact on residents. In all cases, such solutions will be designed with the overall drainage of Parkfairfax as a primary concern. This is the general edict of the Master Plan. The impact of solutions to drainage issues on residents can and will be mitigated by the cooperative efforts of the Parkfairfax Landscape Manager, Parkfairfax maintenance personnel, contractors, and the residents themselves.
It is important to note that previous Master Plans or the bases thereto do not appear to address drainage in any workable form. As various elements included in these Plans were introduced, their impact on the drainage, particularly in narrower common areas between buildings and near frequently-used pedestrian ways, were seemingly ignored. It became obvious as each area within Parkfairfax was reviewed that many small problems have been aggravated into major problems as maintenance crews were caught in a “band-aide application” mode. That is not to say there have not been some very serviceable solutions introduced. This Master Plan hopes to expand on the best ideas in situ.
Prior to 2010, a set of “Flood Watch” maps were done, and there is some documentation to that, but this information was apparently not used to renovate the surface water conveyance system in any meaningful way.
Concerns regarding flooding must be tempered by location. There may be occasions where heavy rain causes temporary standing water, particularly around sump areas and flatter swales. As long as the water does not stand for more than a few hours or enter one of the ground level units, inundation is usually not a problem. It takes some time for any wet area to become a breeding area for mosquitos, and ground water recharge can be a good thing for trees in the vicinity.
A number of documents we have seen tie the amount of water to erosion, suggesting that the sheer volume of water moving through an area can lead to erosion. This is typically not the case. When addressing erosion, it is very important to remember that it is not the volume of water but the speed with which that water is moving that causes erosion. Therefore, if we can identify areas that temporarily flood but are not subject to erosion, we can likely not consider those areas cause for concern, or at least lower them significantly on the concerns list. Likewise, if we identify areas that obviously convey water but for which there is little room for that water to flow and no room for it to dissipate, we should consider those areas highly susceptible to erosion, even if there is not active erosion happening.
The approach of this Master Plan intends to address the negatively self-impacting nature of the continuing development of Parkfairfax such that sustainability, maintainability, high aesthetics are all part of the equation. As we have indicated in other reports for this Master Plan, stewardship and home-owner “buy-in” will be critical.
Similar in arrangement to the other inventories for common area elements within Parkfairfax, the drainage inventory has been done by numbered building areas. This will be a frequently updated document, and has been built to facilitate such updating. The matrix that follows the narrative may include work that is in progress or issues that have recently been addressed.
Maps corresponding to the areas as listed below can be seen in the tabs following the matrix. Each map indicates major drainage patterns and fixtures, references the potential flood areas, and indicates specific areas of concern.
In spite of being on the highest ground in Parkfairfax, the 100s are not without drainage issues. An apparent spring – more likely an artificially created false aquifer outcrop – outfalls behind building 103. The impact of this can be seen on the sidewalk along Gunston Road. In late fall and early spring, the sidewalk is often wet, and could potentially ice over. Inadequate swales run behind building 104, 110 and 112. Water tends to pool behind these buildings, and means of outfall are the narrow areas between buildings. Grades flatten out in the rear common areas among buildings 111 – 117 and among buildings 112 – 116.
Beyond these issues, the majority of the other problems appear to be caused at least in part by grading, terracing and landscaping done by residents. The terrace behind the northern-most end unit of building 116, for example, appears to have a good deal of siltation. This would indicate a sump has been formed on the terrace, and water cannot escape into the sloped common area and toward the existing drain inlets behind 110 and 112. There is a practically impenetrable (by water anyhow) screen of bamboo along the common area/private area line of this unit. Root mass has built up to the point where water cannot move away from the building, and the bamboo is encroaching on the common area, impacting the roots zones of other more desirable plants.
Heavy foot traffic has caused a low area to form between buildings 106 and 109. This is the main access to the play structure, so it is typically a muddy mess for some time after each rain. Gravel (read “sand”) has been placed (read “pounded”) over the area (read “down this rathole”) a number of times, but there have been no attempts at a permanent solution.
1. Implement a rain garden/vegetated swale between buildings 102 and 103. The upper portion of this swale can be micro-graded through the downhill side of the wooded area behind buildings 102 and 104 to the natural existing high point. Water from the aquafer outcrop can be directed towards this rain garden through a pipe or series of pipes under the proposed trail (see site plans).
2. Grade a small diversion along the back of building 104 to direct water between buildings 104 and 106.
3. Implement a rain garden behind building 110. Grade open areas in the rear green among buildings 108 – 116 to direct water to this rain garden; and remove invasive bamboo behind building 116 to allow relief of water toward it.
4. Implement reforestation in open areas behind buildings 101 – 107 and behind buildings 117, 110 and 116.
The 200s run along the military crest of a ridge that peaks on Crestwood Drive. There is a significant drop off from the area near building 230 towards Preston Road and toward Valley Drive. The storm drain system along Martha Custis Drive reflects these elevation changes.
Apparently due to a combination of age, inadequate maintenance, soil compaction, and attrition in the form of trees failing, there are large areas of erosion between many of the buildings and the boundary areas. Some of the drainage in these areas is toward the buildings, some of it away from the buildings, and some is parallel to the rear faces of the buildings. There are smaller areas of rutting and sumps between the buildings.
Significant areas of concern include a variety of issues:
- The inlet behind building 204 has been silted in (and cleaned out) a number of times. It is very close to the terraces of the middle units of 204, and due to the large drainage area being pushed to this inlet in combination with the siltation, the terraces frequently become inundated.
- The extremely wet area behind building 208 and the lack of positive drainage towards existing area drains behind buildings 204 and 210. A “beehive” style yard inlet was installed behind 208 to provide relief, but the drain was installed too high, and the lip if the inlet sits above the surrounding grade. The drainage area coming to the back of 208 is several hundred square feet of mostly open turf on highly compacted ground. The end unit of 208 (on the building 210 end of 208) has experienced some flooding on their terrace.
- Apparently due to uneven settling of the ground along the wooded area at the Parkfairfax boundary with the neighboring community water tends to stand in the open areas between the woods and the ends of buildings 228, 230, 229, 227, and 225.
- Compaction has led to large areas near the ends of buildings 211, 215, and 217 being little more than exposed clay. Slumping has made for uneven ground and there is poor turf cover.
1. Relocate and lower the beehive inlet behind building 208. It should be moved away from the building and any nearby mature trees and lower by 9 to 12 inches. There is adequate grade in either direction – that is around either end of building 208 – to allow the drain pipe leading from this inlet to maintain functional slope to the next downhill inlet or manhole.
The grade at the top of the inlet should be set to insure that there is positive drainage away from building 208, but no less than 9 inches lower than it is now. A high point/ low point (or saddle) can be established in the open area behind and between 208 and 206.
2. The large dead tree should be removed from behind building 206. If we don’t have to account for the root system of this tree, we can more efficiently grade the area behind these buildings. Also, a new tree or two could be planted once grades are set. Newer trees will actively use water more efficiently than a dead or dying tree.
3. Wide swales should be can be formed behind the buildings from 208 to 204 and from 219 to 211. There appears to be adequate grade change to facilitate positive drainage away from these buildings. All graded area can be sodded or planted with trees.
4. Smaller swales should be graded to push water to the existing inlets between buildings 225 and 227, between 229 and 230, and between 226 and 228. These graded areas may be also sodded or planted with trees.
5. The open areas along the woods from building 211 to building 220, and from building 202 to 212 should be reforested. These areas are not level enough or wide enough to facilitate recreational play, and have become a maintenance albatross. Once grading is completed, larger trees can be used nearer the buildings, and smaller trees as you approach the boundary lines.
6. Landscaping – whether it has been installed by the Community or by unit owners – that goes beyond the established common area/private area line and is interfering with positive drainage should be removed or relocated such that the flow of surface water, particularly during storm events, can be facilitated.
The 300s have relatively few major drainage issues. The open areas between the buildings and the boundary at Quaker Lane slopes continuously from building 302 towards building 305. Some of the few areas where water tends to stand are behind building 306, near the top of the sloped area, and behind buildings 303 and 305 at the low end of the slope. Inlets follow the long slope, so minor swales and diversions should be adequate to push water to these inlets.
There is a small area of significant erosion behind building 313. This issue has been compounded by grading and hardscape done by residents. A naturalized area exists in the common area among buildings 311, 313, and 315.
No large scale recommendations are hereby made. Smaller, localized issues will be addressed as they arise.
For the erosion behind building 313, we recommend that a series of water bars be installed across and mostly perpendicular to the centerline of the rut that has formed along the back of the building. These water bars can be timbers, rocks, or mounding, but should be placed to direct water across the wooded naturalized area and towards a functioning storm drain inlet behind building 311. The wooded area should be cleaned out such that the leaf litter and debris are less than 3” deep so water can flow easily across the area and the ground plane can still be protected. A low dike can be installed along the top of the rut between it and building 315 at the Martha Custis end of building 315 to direct water away from building 313 and into the naturalized area.
With approximately less than 20 vertical feet of fall between building 405 and building 401, and less than 25 vertical feet over the entire area, the 400s are on fairly flat ground. The most imposing element in the landscape is the 30’ high retaining wall for the 395 corridor. It haa concrete v-section ditch at its foot, and much of the water along the boundary line and from the open areas behind the buildings is directed to this ditch.
Some of the unit owners in the 400s have over-extended their private landscaping to the point it could impede water from adequately draining away from the buildings.
1. The area near the concrete ditch that is on Parkfairfax property should be kept clear. For areas that have tree cover near the ditch, such as that closest the volley ball court and behind building 405, a 4’ wide strip of river rock could be installed as a maintenance-free solution to keeping the area clear.
2. Additional trees could be added in areas where the grade has simply subsided over time. Any landscaping should be installed such that positive drainage is maintained away from buildings.
3. Landscaping – whether it has been installed by the Community or by unit owners – that goes beyond the established common area/private area line and is interfering with positive drainage should be removed or relocated such that the flow of surface water, particularly during storm events, can be facilitated.
From the high point at approximately 104 near building 528 to the low point of approximately 39 near building 513 – the lowest point in the whole of Parkfairfax – there is generally continuous fall across the 500s area. There is generally continuous fall from the high point along Gunston Road in both directions – toward Valley Drive and toward Martha Custis Drive, and then along Valley Drive and Martha Custis Drive as well.
There are several smaller drainage issues within the 500s. most of them can be attributed to the landscaping installed by unit owners. The landscape maintenance personnel have done a great job (as of the date of this report) managing the solutions to these issues, but this is one of the many ways in which we need to look at the bigger picture. We need to establish standards for the common are/private area line, and hold to those standards.
Building 531 is a great example. With barely 20 feet between buildings 531 and 535, there is very little room for drainage facilities, and with a 5’ vertical grade difference between the first floors of those two buildings, pushing water between them is difficult. A combination of overgrown shrubs, a large terrace behind the end unit of 531, a broken-down chain link fence and heavily compacted soil all come together within that small area, further aggravating our ability to get water through. Grades going around the other end of 531 tend to flatten out a bit too much and it would be counter-productive to raise grades along the back of the entire building. A retaining walls have been constructed to facilitate a terrace behind the end unit on the 535 end of 531, and this wall has since been added to and lengthened where it meets the neighboring unit. The older section of the wall that is immediately behind the end unit (1414 Martha Custis Dr) is poorly constructed, has no weep holes, and is falling apart.