**Stormwater Design
****Example: Infiltration
Trench**

INFILTRATION TRENCH DESIGN EXAMPLE

This example describes in detail the design of an infiltration trench to treat stormwater runoff from the Brown Community Center (Figure 1). The trench must meet the following design criteria:

- Provide Recharge
- Treat the Water Quality Volume

__Step 1. Determine if Infiltration Is Feasible__

The table below summarizes the requirements that need to be met to successfully implement infiltration practices. On this site, infiltration is feasible, with restrictions on the depth and width of the trench.

*This table presents examples for infiltration feasibility criteria. Specific criteria may vary. See
Selection Criteria for more details.*

__Step 2. Compute Water Quality Volume:__

WQ_{v} previously determined to be 6,752 cubic feet.

__Step 3. Compute Recharge Volume__

The recharge volume was previously calculated as 1,688 cubic feet. The infiltration trench will automatically meet the recharge requirement because it infiltrates the entire water quality volume which is greater than the recharge volume.

__Step 4. Compute WQ _{v} peak discharge (q_{p})__

Calculated as 2.6 cfs (__see Sandfilter
Design Example, Step 4__).

__Step 5. Size the infiltration trench.__

The area of the trench can be determined by using the following equation:

* This equation is borrowed from the Maryland
Department of Natural Resources 1984 Publication
"Maryland Standards and Specifications for
Stormwater Management Infiltration Practices"*

Where:

A = Surface Area

n = porosity

d = trench depth (feet)

k = percolation (inches/hour)

T=Fill Time (time for the practice to fill
with water), in hours

Assume that:

n= 0.32 (see Chapter 3; porosity for a stone
reservoir)

d= 5 feet (see above; feasibility criteria)

k= 1 inch/hour (see above; site data)

T= 2 hours

Therefore:

*A* = 3,800 *ft*^{2}

Since the width can be no greater than 25' (see above; feasibility), determine the length:

*L = *160 *feet*

Assume that of the runoff from the site drains
to Point A and drains to Point B.

Use an L-shaped trench in the corner of
the site (see Figure 2 for a site plan view).

The surface area of the trench is proportional
to the amount of runoff it drains (e.g., the portion draining from Point A is
half as large as the portion draining Point B).

__Step 6. Size the flow diversion structures__

Since two entrances are used, two flow diversions are needed.

For the entire site:

Q_{10-year} = 13.97 cfs (See site data)

Peak flow for WQ_{v }= 2.6 cfs.
(Step 4).

For the first diversion (Point A)

Assume peak flows equal of their value for the entire site.

Thus, Q_{10-year} = 4.7 cfs

Peak flow for WQ_{v }= 0.9 cfs

Size the low flow orifice to pass 0.9 cfs with
1.5' of head using the Orifice equation.

Q=CA(2gh)^{ ½} ; 0.9 cfs = 0.6A(232.2
ft/s^{2} 1.5')^{ ½
}A=0.15 sq. ft. = d^{2}/4;
d = 5.3"; use 6"

Size the 10-year overflow at 22'. Use a concrete weir to pass the 10-year flow (4.7 cfs.) Assume 1 foot of head to pass this event. Size using the weir equation.

Q = CLH^{1.5}; L= Q/(CH^{1.5})

L = 3.8 cfs/ (3.1)(1)^{1.5 }= 1.2';
__use 1.5'__

Size the second diversion (Point B) using the same techniques.

Peak flows equal of their volume for the entire site. Thus:

Q_{10-year} = 9.3 cfs

Peak flow for WQ_{v }= 1.7 cfs

Using the same calculations as above, and substituting
the higher flows, d=7.3";

Use 8" for low flow orifice diameter.

For the 10-year overflow, L=2.4'. Use 2.5'.

__Step 7. Size pretreatment.__

Must treat ¼ of the WQ_{v}. Therefore, treat 1,688 ft^{3}.

* In
general, infiltration practices should have significant pretreatment volumes
to prevent clogging. (See Design Criteria - Infiltration)*

For pretreatment use a plunge pool, a grass channel and a pea gravel filter layer with filter fabric.

*Pea Gravel Filter
* The 2" pea gravel filter layer covers
the entire trench. Assuming a porosity of 0.32, the water quality treatment
in the pea gravel filter layer is:

WQ

*Plunge Pool*s

Use a 5'X10' plunge pool at Point A and
a 10'X10' plunge pool at Point B with average depths of 2'.

WQ_{pool}= (10 ft)(10+5 ft)(2 ft^{2})
= 300 ft^{3}

*Grass Channel
*Thus, the grass channel needs to treat
at least (1,688-213-300)ft

Use a Manning's equation nomograph or software to size the swale.

The channel at point A should treat one third of 1,175 ft^{3} or 392 ft^{3}

- Assume a 4' channel bottom and a Manning's n value of 0.15. Use a nomograph to size the swale; assume a 1% slope.
- Use a peak discharge of 0.9 cfs (Peak flow for one third of WQ
_{v}) - Compute velocity:

V=0.5 ft/s

- To retain the 1/3 of the WQ
_{v}(2,250 ft^{3 }) for 10 minutes, the length would be 300 feet. - The swale only needs to treat 25% of the water
quality volume minus the treatment provided by the plunge pool and the gravel
layer, or:

2,250 ft^{3 }(0.25) - (300 ft^{3}/3) - (213 ft^{3}/3) = 392 ft^{3}

Therefore, adjust length:

L= (300 ft)(392 ft^{3}/2,250 ft^{3})
=52 feet. __Use 55 feet.__

Size the swale at point B using the same method.
Q=1.7 cfs Slope = 0.5%

__L=55 feet.__