30 Sep 01
flow exiting the drainage layer
(unit width) (depth of drainage layer).
This method is conservative; however, it is applicable for most cover drainage layer designs. As shown
in Figure B-3, one may determine an appropriate spacing (slope length, L) between drainage collection
pipes, or outlet drains, by having QIN = Q OUT. If the cover system is constructed of high permeability
soils (i.e., rate of infiltration > rate of precipitation), one should compare the infiltration of a storm event,
such as a 6-hour, 25-year storm (or other event) to calculate drainage layer flows.
(b) Computer Models. Several water balance models have been developed for estimating
water movement in and through soil. Some are simplified spreadsheets that account for water balances
each month or day. The Hydrologic Evaluation of Landfill Performance (HELP) model is the one most
commonly used to evaluate the overall performance of a cover system and its individual components.
HELP models the daily hydrologic processes in a landfill system, including surface runoff, infiltration,
evapotranspiration, soil moisture storage, lateral drainage, and leachate production. The HELP model is
used to estimate long-term landfill cover effectiveness, drainage layer flow, head build-up in select fill,
leachate collection system flow, and available water storage for a vegetative cover (wilting point). The
version of HELP currently being used is Version 3. EPA/600/R-94/168a and b provide detailed
guidance on the use of the HELP model. Designers are cautioned that HELP may significantly
underestimate lateral drainage flow rates that are appropriate for drainage layer design (Richardson and
Garrett 2000). This underestimation is most often the result of inappropriate input parameters or using
long-term HELP model results to design short-term, high flow storm events. Lateral drainage flow
values obtained from HELP should always be compared to flow rates calculated by manual methods.
(3) Geosynthetic Drains. Geosynthetic drainage systems have often replaced granular materials
for drainage media, as they require less space and are easier to construct. Geosynthetic drainage
systems typically consist of a three-dimensional drainage core with a geotextile fabric attached to one or
both of its sides. The core, which transmits the flow, must be protected by a fabric that acts as a
filter/separator from the overlying soil. Water passes through the geotextile and into the drainage core.
The geosynthetics industry has developed many drainage core configurations. Some of the drainage
cores that are available include: biaxial extruded geonets, triplanar geonets, and many other
configurations as well. Currently, the most common geosynthetic drainage system used for landfill cover
systems are those that use geonets with attached geotextile filters, commonly called geocomposite
drainage layers. Most commercially available resins used for geonets are made of polyethylene. The
final compound is approximately 97% polyethylene, with 23% carbon black added for ultra-violet
(UV) resistance. An additional 0.51% is additives, such as antioxidants and processing aids. Geonets
typically range from 5.0 to 8.0 mm (0.20 to 0.30 in.) in thickness but can be considerably thicker.
(a) Design Criteria. To design a geocomposite or geonet drainage layer, a required flow rate